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UBC Theses and Dissertations

Electrochemical oxidation of phenol for waste water treatment 1979

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ELECTROCHEMICAL OXIDATION OF PHENOL FOR WASTE WATER TREATMENT by V I V I A N SMITH de^SUCRE B . S c . U n i v e r s i d a d Simon B o l i v a r , 1975 A THESIS SUBMITTED I N P A R T I A L FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF A P P L I E D SCIENCE i n THE FACULTY OF GRADUATE STUDIES Depar tmen t o f C h e m i c a l E n g i n e e r i n g We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d THE UNIVERSITY OF B R I T I S H COLUMBIA A u g u s t , 1979 0 V i v i a n S m i t h de S u c r e , 1979 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced degree a t the U n i v e r s i t y o f B r i t i s h C olumbia, I agree t h a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y purposes may be gr a n t e d by the Head o f my Department o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department o f ^UxCcAL &0&IM£IUA)G The U n i v e r s i t y o f B r i t i s h Columbia 2075 Wesbrook P l a c e Vancouver, Canada V6T 1W5 DE-6 BP 75-51 1 E ABSTRACT The e l e c t r o c h e m i c a l o x i d a t i o n o f p h e n o l f o r w a s t e t r e a t m e n t a p p l i - c a t i o n s was i n v e s t i g a t e d on l e a d d i o x i d e p a c k e d - b e d a n o d e s . The e l e c t r o - l y t i c c e l l was o p e r a t e d i n b o t h b a t c h and c o n t i n u o u s modes w i t h f e e d s t r e a m s up t o 1100 mg /1 p h e n o l d i s s o l v e d i n aqueous s o l u t i o n s o f N a 2 S 0 i + and H 2 S L \ o r NaOH. E l e c t r o d e p o s i t e d l e a d d i o x i d e was f o u n d t o be a b e t t e r anode f o r p h e n o l o x i d a t i o n , t h a n the l e a d d i o x i d e o b t a i n e d by a n o d i z i n g l e a d s h o t . R e s u l t s showed t h a t a l l t h e p h e n o l i n s o l u t i o n c o u l d be r e a d i l y o x i d i z e d b u t c o m p l e t e t o t a l o r g a n i c c a r b o n ( T . O . C . ) r e m o v a l was more d i f f i c u l t . R a t e s o f p h e n o l o x i d a t i o n were s i m i l a r i n d i v i d e d and u n d i v i d e d c e l l s . The o x i d a t i o n o f p h e n o l was f a v o u r e d by an a c i d i c p H , b u t an a l k a l i n e pH i m p r o v e d t h e f u r t h e r o x i d a t i o n o f i n t e r m e d i a t e p r o d u c t s . I n d i v i d e d c e l l s , an a n i o n i c membrane, w h i c h a l l o w e d m i g r a t i o n o f h y d r o x y l i o n s , p r o v e d t o be s u p e r i o r t h a n a c a t i o n i c membrane f o r T . O . C . r e m o v a l . The p e r c e n t o f p h e n o l o x i d i z e d i n c r e a s e d w i t h i n c r e a s i n g c u r r e n t d e n s i t y , and d e c r e a s e d as i n i t i a l p h e n o l c o n c e n t r a t i o n , e l e c t r o l y t e f l o w r a t e , and anode p a r t i c l e s i z e were i n c r e a s e d . C o m p a r i s o n s o f t h e e x p e r i m e n t a l r e s u l t s w i t h a mass t r a n s f e r m o d e l a r e p r e s e n t e d f o r t h e b a t c h e x p e r i m e n t s , and a s i m p l i f i e d m o d e l i s p r o - posed t o i n t e r p r e t t h e r e s u l t s f rom c o n t i n u o u s e x p e r i m e n t s i n t e rms o f r e l a t i v e mass t r a n s f e r and e l e c t r o c h e m i c a l r e a c t i o n r e s i s t a n c e s . i i TABLE OF CONTENTS ABSTRACT i i L I S T OF TABLES v L I S T OF FIGURES v i ACKNOWLEDGMENTS v i i i C h a p t e r 1 INTRODUCTION 1 1.1 P h e n o l s as p o l l u t a n t s 1 1.2 Methods o f t r e a t m e n t o f p h e n o l i c w a s t e s 2 2 BASES OF THE ELECTROCHEMICAL PROCESS 6 2 . 1 G e n e r a l c o n c e p t s 6 2 . 2 L i t e r a t u r e r e v i e w on t h e e l e c t r o c h e m i c a l o x i d a t i o n o f p h e n o l 12 2 . 2 . 1 R e a c t i o n p r o d u c t s 12 2 . 2 . 2 P r o p o s e d r e a c t i o n mechanisms 13 2 . 2 . 3 E l e c t r o d e m a t e r i a l s t e s t e d 18 2 . 2 . 4 E f f e c t o f c u r r e n t d e n s i t y 20 2 . 2 . 5 E f f e c t o f n a t u r e o f t h e e l e c t r o l y t e . . . . 21 2 . 2 . 6 E f f e c t o f pH 24 2 . 3 The l e a d d i o x i d e e l e c t r o d e 25 3 OBJECTIVES 29 4 EXPERIMENTAL APPARATUS AND METHODS 31 4 . 1 A p p a r a t u s 31 4 . 1 . 1 C e l l d e s i g n 31 4 . 1 . 2 F l o w d i a g r a m o f t h e a p p a r a t u s 39 4 . 2 E x p e r i m e n t a l methods 43 4 . 2 . 1 B a t c h e x p e r i m e n t s 43 4 . 2 . 2 C o n t i n u o u s e x p e r i m e n t s 45 4 . 3 A n a l y t i c t e c h n i q u e s 46 4 . 3 . 1 P h e n o l a n a l y s i s 46 4 . 3 . 2 T o t a l o r g a n i c c a r b o n a n a l y s i s 47 4 . 3 . 3 L e a d a n a l y s i s 48 i i i 5 RESULTS AND DISCUSSION 49 5 . 1 E l e c t r o d e m a t e r i a l s . 49 5 . 2 E f f e c t o f pH u s i n g t h e d i v i d e d c e l l 55 5 . 3 E f f e c t o f c u r r e n t u s i n g t h e d i v i d e d c e l l 64 5 .4 C o m p a r i s o n s o f membrane p e r f o r m a n c e s 65 5 . 5 E f f e c t o f pH u s i n g t h e u n d i v i d e d c e l l 68 5 . 6 E f f e c t o f c u r r e n t u s i n g t h e u n d i v i d e d c e l l . . . . 69 5 .7 C o m p a r i s o n s o f d i v i d e d and u n d i v i d e d c e l l s . . . . 75 5 . 8 E f f e c t o f c o n d u c t i v i t y o f t h e e l e c t r o l y t e 76 5 .9 E f f e c t o f i n i t i a l p h e n o l c o n c e n t r a t i o n 78 5 . 1 0 E f f e c t o f e l e c t r o l y t e f l o w r a t e 83 5 . 1 1 E f f e c t o f p a r t i c l e s i z e 86 5 . 1 2 C o m p a r i s o n s o f e x p e r i m e n t a l r e s u l t s w i t h m a t h e m a t i c a l mode l s 88 5 . 1 2 . 1 B a t c h e x p e r i m e n t s 88 5 . 1 2 . 2 C o n t i n u o u s e x p e r i m e n t s 89 5 . 1 3 C u r r e n t e f f i c i e n c i e s , e n e r g y r e q u i r e m e n t s and ene rgy c o s t s f o r p h e n o l o x i d a t i o n 95 5 . 1 3 . 1 B a t c h e x p e r i m e n t s 95 5 . 1 3 . 2 C o n t i n u o u s e x p e r i m e n t s 96 5 . 1 3 . 3 C o s t c o m p a r i s o n s 96 6 CONCLUSIONS 99 7 RECOMMENDATIONS 101 NOMENCLATURE 104 BIBLIOGRAPHY 107 APPENDIX 1 S p e c i f i c a t i o n o f a u x i l i a r y equ ipmen t and m a t e r i a l s . . . I l l 2 E x p e r i m e n t a l d a t a 116 3 M a t h e m a t i c a l mode l s 158 4 C a l c u l a t i o n s 171 5 R e l e v a n t p h y s i c a l d a t a 181 i v L I S T OF TABLES T a b l e 1 R a t e s o f p h e n o l o x i d a t i o n on d i f f e r e n t e l e c t r o d e m a t e r i a l s 19 2 E f f e c t o f c u r r e n t d e n s i t y and t y p e o f e l e c t r o l y t e on C . O . D . r e m o v a l 21 3 E f f e c t o f t y p e o f e l e c t r o l y t e on p h e n o l o x i d a t i o n . . . . 23 4 F u n d a m e n t a l s p e c i f i c a t i o n s o f t h e e l e c t r o l y t i c c e l l . . . 37 5 C o m p a r i s o n s o f d i v i d e d and u n d i v i d e d c e l l s 76 6 T y p i c a l c u r r e n t e f f i c i e n c i e s , ene rgy r e q u i r e m e n t s and e n e r g y c o s t s i n b a t c h e x p e r i m e n t s w i t h u n d i v i d e d c e l l . 95 7 T y p i c a l c u r r e n t e f f i c i e n c i e s , e n e r g y r e q u i r e m e n t s and e n e r g y c o s t s i n c o n t i n u o u s e x p e r i m e n t s w i t h u n d i v i d e d c e l l 96 8 O p e r a t i n g c o s t s o f v a r i o u s t r e a t m e n t m e t h o d s , e s t i m a t e d f o r 1974 f o r a c a t a l y t i c c r a c k e r e f f l u e n t c o n t a i n i n g 700 mg/1 p h e n o l 97 A p p e n d i x 1 A - l Summary o f t y p i c a l p r o p e r t i e s o f IONAC membranes . . . . 113 A p p e n d i x 2 E x p e r i m e n t a l d a t a t a b l e s f o r : Run 1-1 t o Run 1 -9 : D i v i d e d c e l l , b a t c h e x p e r i m e n t s w i t h a n o d i z e d l e a d 1 1 9 - 1 2 3 Run 2 -1 t o Run 2 - 1 1 : D i v i d e d c e l l , b a t c h e x p e r i m e n t s w i t h e l e c t r o d e p o s i t e d Pb02 124-134 Run 3-1 t o Run 3 - 1 5 : U n d i v i d e d c e l l , b a t c h e x p e r i m e n t s w i t h e l e c t r o d e p o s i t e d P b 0 2 135-149 Run 4 - 1 t o Run 4 - 8 : U n d i v i d e d c e l l , c o n t i n u o u s e x p e r i m e n t s w i t h e l e c t r o d e p o s i t e d P b 0 2 150-157 A p p e n d i x 4 A - 2 T h e o r e t i c a l p h e n o l f r a c t i o n a l c o n v e r s i o n v s t i m e f o r a mass t r a n s f e r — c o n t r o l l e d b a t c h s y s t e m 173 A - 3 C a l c u l a t i o n o f e x p e r i m e n t a l , mass t r a n s f e r and r e a c t i o n r a t e c o n s t a n t s f rom e x p e r i m e n t s 4 - 1 , 4 - 2 , 4 - 3 175 A - 4 C a l c u l a t i o n o f e x p e r i m e n t a l , mass t r a n s f e r and r e a c t i o n r a t e c o n s t a n t s f o r e x p e r i m e n t 4 -4 176 A - 5 C a l c u l a t i o n o f e x p e r i m e n t a l , mass t r a n s f e r and r e a c t i o n r a t e c o n s t a n t s f o r e x p e r i m e n t 4 -8 177 A - 6 pH o f s o l u t i o n s o f NaOH and H ^ O ^ a t 2 0 ° C 181 A - 7 C o n d u c t i v i t i e s o f aqueous s o l u t i o n s o f NaOH, H ^ O ^ and N32S01+ a t 20°C 181 A - 7 % p h e n o l i o n i z e d v s pH 182 v L I S T OF FIGURES F i g u r e 1 V o l t a g e components i n a d i v i d e d e l e c t r o l y t i c c e l l . . . . 9 2 R e a c t i o n p r o d u c t s 12 3 H a l f - w a v e p o t e n t i a l v s p H , f o r t h e o x i d a t i o n o f 4 x I O - 4 p h e n o l 24 4 S i d e v i e w o f t h e g e n e r a l d i v i d e d - c e l l a r r angemen t . . . . 32 5 F r o n t and s i d e v i e w s o f t h e anode chamber f o r t h e < a n o d i z e d l e a d e l e c t r o d e 34 6 F r o n t and s i d e v i e w s o f t h e anode chamber f o r t h e e l e c t r o d e p o s i t e d Pb02 e l e c t r o d e 35 7 D e t a i l o f t h e i n l e t o r o u t l e t c o n n e c t i o n a d a p t e d on t h e e l e c t r o d e p o s i t e d Pb02 on g r a p h i t e anode . . . . 36 8 D e t a i l o f t h e mechanism u s e d t o h o l d t h e c e l l 38 9 F l o w d i a g r a m o f t h e a p p a r a t u s 40 10 E f f e c t o f t y p e o f l e a d d i o x i d e e l e c t r o d e a t 10 A and i n i t i a l pH - 9 . 4 w i t h IONAC MC-3470 membrane 52 11 S c a n n i n g e l e c t r o n - m i c r o g r a p h s o f t h e e l e c t r o d e p o s i t e d Pb02 p a r t i c l e s a f t e r u s e 53 12 S c a n n i n g e l e c t r o n - m i c r o g r a p h s o f t h e a n o d i z e d l e a d p a r t i c l e s , a f t e r u se 54 13 E f f e c t o f c u r r e n t on pH and % T . O . C . o x i d a t i o n w i t h IONAC MC-3470 membrane 56 14 E f f e c t o f c u r r e n t on % p h e n o l o x i d a t i o n a t i n i t i a l pH = 9 . 4 w i t h IONAC MC-3470 membrane 59 15 E f f e c t o f c u r r e n t on p H , % T . O . C . o x i d a t i o n and % p h e n o l o x i d a t i o n w i t h NAFION-127 membrane 60 16 E f f e c t o f pH on % T . O . C . and % p h e n o l o x i d a t i o n a t 20 A w i t h NAFION-127 membrane 62 17 Type o f membrane-pH e f f e c t on % T . O . C . and % p h e n o l o x i d a t i o n a t 20 A . . . . . . . . . . . . . . . . . . 63 18 C u r r e n t e f f e c t on % T . O . C . and % p h e n o l o x i d a t i o n a t pH = 2 . 5 w i t h IONAC MC-3470 66 19 Type o f c a t i o n i c membrane e f f e c t on % T . O . C . and % p h e n o l o x i d a t i o n a t 20 A and pH = 2 . 5 67 20 pH e f f e c t on % T . O . C . and % p h e n o l o x i d a t i o n a t 10 A i n an u n d i v i d e d c e l l 70 21 pH e f f e c t on % T . O . C . and % p h e n o l o x i d a t i o n a t 20 A i n an u n d i v i d e d c e l l 71 22 E f f e c t o f pH on % T . O . C . and % p h e n o l o x i d a t i o n a t 30 A i n an u n d i v i d e d c e l l • . . 72 23 C u r r e n t e f f e c t on % T . O . C . and % p h e n o l o x i d a t i o n a t pH - 2 . 5 i n . an u n d i v i d e d c e l l 73 24 E f f e c t o f c u r r e n t on % T . O . C . and % p h e n o l o x i d a t i o n a t i n i t i a l pH = 1 2 , i n an u n d i v i d e d c e l l 74 v i 25 E f f e c t o f e l e c t r o l y t e c o n d u c t i v i t y a t 20 A ( i n a l k a l i n e and a c i d m e d i a ) 77 26 E f f e c t o f e l e c t r o l y t e c o n d u c t i v i t y a t 10 A and i n i t i a l pH = 12 79 27 E f f e c t o f e l e c t r o l y t e c o n d u c t i v i t y a t 10 A and i n i t i a l pH - 2 . 5 80 28 % p h e n o l o x i d i z e d v s t i m e f o r v a r i o u s i n i t i a l p h e n o l c o n c e n t r a t i o n s a t 10 A and pH = 2 . 5 81 29 P h e n o l c o n c e n t r a t i o n e f f e c t on p h e n o l c o n c e n t r a t i o n v s t i m e a t 10 A and 2 . 5 pH 82 30 E f f e c t o f f l o w r a t e on t h e s i n g l e p a s s % p h e n o l o x i d a t i o n a t (a) 10 A , (b) 20 A 85 31 E f f e c t o f anode s u r f a c e a r e a - p a r t i c l e s i z e on t h e % p h e n o l o x i d i z e d i n a s i n g l e - p a s s v s f l o w r a t e . . . 87 32 - J l n ( l - X ) v s (u) --*- f o r t h e c a l c u l a t i o n o f e x p e r i m e n t a l r a t e c o n s t a n t s i n s i n g l e p a s s e x p e r i m e n t s 91 A p p e n d i x 3 A - l P a c k e d bed r e a c t o r i n p l u g f l o w 159 A - 2 P o t e n t i a l d i s t r i b u t i o n i n a p a r t i c u l a t e e l e c t r o d e . . . 161 A - 3 S c h e m a t i c r e p r e s e n t a t i o n o f e q . A - 1 4 164 A - 4 S c h e m a t i c r e p r e s e n t a t i o n o f e q . A - 1 5 166 A - 5 S c h e m a t i c r e p r e s e n t a t i o n o f a b a t c h r e c i r c u l a t i o n s y s t e m 168 v i i ACKNOWLEDGEMENTS I w o u l d l i k e t o t h a n k my s u p e r v i s o r P r o f . P a u l W a t k i n s o n f o r h i s a d v i c e and encouragement t h r o u g h o u t t h e w h o l e o f t h i s w o r k . I am a l s o g r a t e f u l t o P r o f . C o l i n Oloman f o r h i s s i n c e r e i n t e r e s t i n t h e p r o j e c t and f o r t h e many u s e f u l d i s c u s s i o n s we had t o g e t h e r . Thanks a r e a l s o due t o my h u s b a n d , G u s t a v o S u c r e f o r h i s s u g - g e s t i o n s , p a t i e n c e , and u n d e r s t a n d i n g . I w i s h t o e x p r e s s my a p p r e c i a t i o n t o t h e C h e m i c a l E n g i n e e r i n g s t a f f f o r t h e i r c o o p e r a t i o n and a s s i s t a n c e and t o t h e p e r s o n n e l o f t h e E n v i r o n m e n t a l E n g i n e e r i n g L a b o r a t o r y i n t h e C i v i l E n g i n e e r i n g Depa r tmen t f o r t h e i r gene rous h e l p i n t h e o p e r a t i o n o f a n a l y t i c a l a p p a r a t u s . A l s o a c k n o w l e d g e d a r e M r s . R ima K a p l a n f o r h e r t r a n s l a t i o n s f rom R u s s i a n p a p e r s , M r s . M o n i c a G u t i e r r e z f o r t h e d r a f t i n g o f f i g u r e s , and M r s . N i n a T h u r s t o n f o r t y p i n g t h e m a n u s c r i p t . F i n a n c i a l s u p p o r t f rom t h e V e n e z u e l a n Government t h r o u g h FONINVES (Fondo p a r a l a I n v e s t i g a c i o n en M a t e r i a de H i d r o c a r b u r o s ) i s g r a t e f u l l y a p p r e c i a t e d . v i i i CHAPTER 1 INTRODUCTION 1.1 P h e n o l s as p o l l u t a n t s " P h e n o l s " i n w a s t e w a t e r t r e a t m e n t t e r m i n o l o g y i n c l u d e s n o t o n l y p h e n o l (CgHsOH), b u t a l l t h o s e d e r i v a t i v e s o f t h e a r o m a t i c r i n g t h a t c o n - t a i n one o r more h y d r o x y l g r o u p s . P h e n o l s a r e c o n s t i t u e n t s o f many i n d u s t r i a l w a s t e w a t e r s t r e a m s . The ma jo r s o u r c e s o f p h e n o l i c w a s t e s a r e o i l r e f i n e r i e s and c o k e p l a n t s . P h e n o l s a r e f i n d i n g i n c r e a s i n g use i n c o a t i n g s , s t r i p p i n g a g e n t s , s o l - v e n t s , p a i n t v e h i c l e s , p l a s t i c s , e x p l o s i v e s , r u b b e r s u b s t i t u t e s , f e r t i l i z e r s , wood p r e s e r v a t i v e s , and d r u g s . G i v e n t h e u s e f u l n e s s o f p h e n o l i c compounds , t h e y w i l l u n d o u b t e d l y c o n t i n u e t o be a ma jo r p r o d u c t o f t h e c h e m i c a l i n d u s t r y . U n f o r t u n a t e l y , some o f t h e c h e m i c a l c h a r a c - t e r i s t i c s t h a t make p h e n o l s so u s e f u l , a r e a l s o r e s p o n s i b l e f o r t h e i r h i g h p o l l u t i o n p o t e n t i a l . C h l o r i n e u sed i n d r i n k i n g w a t e r combines w i t h p h e n o l s t o f o r m c h l o r o p h e n o l s w h i c h a r e p e r s i s t e n t p o l l u t a n t s , s i n c e t h e y a r e n o t e a s i l y d e g r a d a b l e i n t h e e n v i r o n m e n t . C o n c e n t r a t i o n s as l o w as 5 u g / 1 o f p h e n o l s w i l l i m p a r t o b j e c t i o n a b l e t a s t e s and o d o u r s t o d r i n k i n g w a t e r s when p h e n o l s a r e combined w i t h c h l o r i n e ( 1 ) . F o r t h i s r e a s o n t h e U . S . P u b l i c H e a l t h S e r v i c e has s e t t h e a l l o w a b l e c o n c e n t r a t i o n o f p h e n o l s i n d r i n k i n g w a t e r s a t 1 y g / 1 ( 2 ) . P h e n o l s a r e t o x i c t o f i s h a t l e v e l s above 2 m g / 1 , b u t c a n c a u s e t a s t e i n f i s h f l e s h a t c o n c e n t r a t i o n s f a r 1 2 b e l o w t h e t o x i c l e v e l ( 3 ) . The c h e m i c a l o x y g e n demand, C . O . D . o f p h e n o l s i s r e l a t i v e l y h i g h ( t h e o r e t i c a l l y 2 . 4 mg 0 2 / m g p h e n o l ) and i n s u f f i c i e n t c o n c e n t r a t i o n c a n d e p l e t e t h e oxygen o f a r e c e i v i n g body o f w a t e r c a u s i n g t h e d e a t h o f v e g e t a b l e and a n i m a l s p e c i e s . P e r m i s s i b l e l e v e l s o f p h e n o l s have been e s t a b l i s h e d by t h e U . S . E n v i r o n m e n t a l P r o t e c t i o n Agency ( E . P . A . ) f o r d i f f e r e n t i n d u s t r i a l w a s t e s . These g u i d e l i n e s g e n e r a l l y l i m i t e d p h e n o l i c c o n c e n t r a t i o n s t o 0 . 1 mg/1 i n 1977 and p r o j e c t a s t a n d a r d o f 0 . 0 2 mg/1 f o r 1983 ( 3 ) . 1.2 Methods o f t r e a t m e n t o f p h e n o l i c w a s t e s The c o n c e n t r a t i o n o f p h e n o l i n i n d u s t r i a l e f f l u e n t s v a r i e s w i d e l y as do t h e e f f l u e n t f l o w r a t e s ( 4 , 5 ) . G e n e r a l l y r e c o v e r y i s o n l y a p p l i c a b l e f o r w a s t e s o f a t l e a s t 2000 mg/1 o f p h e n o l and f l o w s i n e x c e s s o f abou t 50 G . P . M . ( 2 ) . P h e n o l s may be r e c o v e r e d by l i q u i d - l i q u i d e x t r a c t i o n p r o c e s s e s u s i n g o r g a n i c s o l v e n t s s u c h as b e n z e n e , b u t y l a c e t a t e , o r b u t y l a l c o h o l . These methods show e f f i c i e n c i e s o f r e c o v e r y up t o 99 .7%, bu t t h e c o n c e n - t r a t i o n s r e m a i n i n g i n t h e aqueous phase a f t e r r e c o v e r y a r e s t i l l s i g n i f - i c a n t f r o m t h e p o l l u t i o n c o n t r o l p o i n t o f v i e w . T h e r e f o r e t h e w a s t e s t r e a m r e q u i r e s f u r t h e r t r e a t m e n t b e f o r e b e i n g d i s c h a r g e d . The c h o i c e be tween r e c o v e r y o r d e s t r u c t i o n o f t h e p h e n o l i c c o n t e n t o f a g i v e n s t r e a m i s made on t h e b a s i s o f e c o n o m i c s . S o l v e n t e x t r a c - t i o n , f o r e x a m p l e , has been f o u n d t o be an e x t r e m e l y e x p e n s i v e a l t e r - n a t i v e i n many c a s e s ( 1 ) . T h e r e a r e s e v e r a l c o n v e n t i o n a l methods f o r t r e a t i n g p h e n o l i c w a s t e s t h a t c anno t be e c o n o m i c a l l y r e c o v e r e d . These i n c l u d e a d s o r p t i o n , 3 i n c i n e r a t i o n , b i o l o g i c a l t r e a t m e n t , and c h e m i c a l o x i d a t i o n . C a r b o n a d s o r p t i o n i s a p p l i c a b l e f o r r e l a t i v e l y l o w (100-200 mg/1) p h e n o l i c c o n c e n t r a t i o n s . Thus i t may be n e c e s s a r y t o p r e t r e a t o r d i l u t e t h e w a s t e s t r e a m b e f o r e i t i s a p p l i e d t o t h e c a r b o n beds ( 3 ) . The m a i n d i s a d v a n t a g e o f t h e a c t i v a t e d c a r b o n p r o c e s s i s t h a t t h e c a r b o n has a f i n i t e c a p a c i t y f o r r e m o v i n g p h e n o l s ( 0 . 0 9 - 0 . 4 g p h e n o l / g a c t i v a t e d c a r b o n ) and e v e n t u a l l y t h e bed becomes f u l l y l o a d e d . To make t h e c o s t o f t h e o p e r a t i o n r e a s o n a b l e t h e c a r b o n must be r e - a c t i v a t e d and r e - u s e d . C h e m i c a l and t h e r m a l r e g e n e r a t i o n s a r e p o s s i b l e . The f i r s t p r o d u c e s a more c o n c e n t r a t e d p h e n o l s t r e a m and t h e s e c o n d d e s t r o y s t h e a d s o r b e d p h e n o l c o m p l e t e l y . V e r y h i g h t e m p e r a t u r e s a r e r e q u i r e d f o r t h i s p u r p o s e ( 9 0 0 ° C ) and c a r b o n l o s s e s o f 5-10% c a n r e s u l t f rom t h e o p e r a t i o n . O p e r a t i n g c o s t s o f t h e a c t i v a t e d c a r b o n p r o c e s s have been compared w i t h t h o s e o f o t h e r p r o v e n t r e a t m e n t methods i n a r e c e n t s t u d y (1) and i t was f o u n d t h a t a c t i v a t e d c a r b o n was t h e most e x p e n s i v e . I n c i n e r a t i o n t e c h n i q u e s a r e o n l y a p p l i c a b l e t o c o n c e n t r a t e d w a s t e s . I n t h e c a s e o f d i l u t e p h e n o l s o l u t i o n s , t h e c o s t o f e n e r g y t o e v a p o r a t e l a r g e amounts o f w a t e r w o u l d be p r o h i b i t i v e . T y p i c a l o p e r a t i n g t e m p e r a - t u r e s f o r c o m b u s t i o n o f p h e n o l t o c a r b o n d i o x i d e and w a t e r a r e as h i g h as 8 0 0 ° C . B i o l o g i c a l t r e a t m e n t s f o r t h e d e g r a d a t i o n o f p h e n o l i c w a s t e s a r e a p p l i c a b l e f o r c o n c e n t r a t i o n s up t o s e v e r a l t h o u s a n d s m g / 1 . Many b i o - l o g i c a l p l a n t s r e p o r t t r e a t e d e f f l u e n t s i n t h e r a n g e o f 0 . 1 mg/1 f o r i n f l u e n t l o a d s o f abou t 1000 mg/1 ( 6 ) . D i f f e r e n t b i o l o g i c a l t r e a t m e n t f l o w schemes , s u c h as a l t e r n a t i n g a c t i v a t e d s l u d g e , t r i c k l i n g f i l t e r , o r l a g o o n c a n be u s e d , b u t t h e 4 a c t i v a t e d s l u d g e s y s t e m i s t h e most common. A v e r y c r i t i c a l a s p e c t i n t h e s u c c e s s o f b i o l o g i c a l t r e a t m e n t i s t h e c o n t r o l o f s h o c k l o a d s t o t h e s y s t e m , b e c a u s e t h e m i c r o o r g a n i s m s a r e o n l y a d a p t a b l e t o a c e r t a i n r a n g e o f p h e n o l c o n c e n t r a t i o n and s t a b l e c o n d i t i o n s o f pH and t e m p e r a t u r e . T h e r e f o r e , i n many c a s e s i t has been n e c e s s a r y t o p r o v i d e an e q u a l i z a - t i o n b a s i n b e f o r e t h e b i o l o g i c a l t r e a t m e n t . O p e r a t i n g c o s t s o f b i o - l o g i c a l t r e a t m e n t a r e r e l a t i v e l y l o w , b u t g e n e r a l l y l a r g e l a n d a r e a s a r e r e q u i r e d w h i c h may r e s u l t i n s u b s t a n t i a l c a p i t a l c o s t s . C h e m i c a l o x i d a t i o n o f p h e n o l s i n c l u d e s t r e a t m e n t by h y d r o g e n p e r o x - i d e , p o t a s s i u m p e r m a n g a n a t e , o z o n e , and c h l o r i n e d i o x i d e . D e p e n d i n g on t h e d o s e o f o x i d i z i n g agen t t h e p h e n o l c a n be c o m p l e t e l y o x i d i z e d t o c a r b o n d i o x i d e and w a t e r o r o n l y p a r t l y c o n v e r t e d t o c e r t a i n i n t e r m e d - i a t e , l e s s h a r m f u l o r g a n i c compounds. I n t h e l a t t e r c a s e , a d d i t i o n a l t r e a t m e n t may be r e q u i r e d t o r e d u c e t h e t o t a l o r g a n i c c a r b o n ( T . O . C . ) o r t h e c h e m i c a l o x y g e n demand ( C . O . D . ) o f t h e w a s t e t o a c c e p t a b l e l e v e l s . O x i d a t i o n by h y d r o g e n p e r o x i d e c a n p r o v i d e 99% p h e n o l r e m o v a l and abou t 40% C . O . D . r e m o v a l when u s i n g a r a t i o o f 2 g H £ 0 2 / g p h e n o l ( 3 ) , b u t when s u b s t i t u t e d p h e n o l s a r e p r e s e n t t h e amount o f p e r o x i d e n e c e s - s a r y c a n i n c r e a s e t o abou t 4 g H 2 0 2 / g s u b s t i t u t e d - p h e n o l . The p r e s e n c e , o f a m e t a l c a t a l y s t i s r e q u i r e d i n t h e o x i d a t i o n r e a c t i o n w h i c h i n c r e a s e s s i g n i f i c a n t l y t h e o p e r a t i n g c o s t s , and f o r t h i s r e a s o n p o t a s - s i u m pe rmangana te i s a more d e s i r a b l e o x i d i z i n g a g e n t . P o t a s s i u m pe rmangana te r e q u i r e s a h i g h e r o x i d a n t t o p h e n o l w e i g h t r a t i o f o r t h e o x i d a t i o n r e a c t i o n — t h e o r e t i c a l l y 1 5 . 7 g K M n O ^ / g p h e n o l . The m a i n d i s a d v a n t a g e i s t h a t t h e r e a c t i o n p r o d u c e s a p r e c i p i t a t e o f manganese d i o x i d e t h a t has t o be r e m o v e d , c o m p l i c a t i n g t h e o p e r a t i o n and i n c r e a s i n g i t s c o s t ( 1 ) . 5 Ozonization can be very e f f e c t i v e i n the destruction of phenols. For example, s t a r t i n g at 2500 mg/1, 99% removal can ba achieved i n 60 min, when using a r a t i o of 1.7 g ozone/g phenol. Ozone can ox i d i z e the phenol completely to CO2 and water but the usual p r a c t i c e i s to p a r t i a l l y o x i d i z e the phenol to organic compounds more e a s i l y biodegradable and then use a b i o l o g i c a l treatment. This i s done since at low phenol con- centrations the necessary ozone to phenol r a t i o i s too high to be econ- omical. I n i t i a l costs are r e l a t i v e l y high because the ozone generating system has to be i n s t a l l e d . The products of the oxidation of phenol by c h l o r i n e dioxide are very dependent on pH. At near n e u t r a l pHs, i n the range 7-8, phenol i s o x i - dized to benzoquinone with a t h e o r e t i c a l requirement of 1.5 g Cl02/g phenol and above pH 10 the products are maleic and o x a l i c acid r e q u i r i n g a weight r a t i o of 3.3. Chlorophenols are not produced by t h i s process because the benzene r i n g i s completely destroyed. An economic study made on the oxidation of phenolic coking wastes by C I O 2 indicated that the process was excessively expensive unless the o x i d i z i n g agent was already being produced on s i t e (3). New methods f o r t r e a t i n g phenolic wastes are being nought, because of the importance of the p o l l u t i o n problem and the highly r e s t r i c t i v e future p o l l u t i o n c o n t r o l standard. Increasing i n t e r e s t i s being shown i n methods such as Gamma I r r a d i a - t i o n (7), wet a i r , and c a t a l y t i c oxidation (8), u l t r a v i o l e t oxidation (9), and electrochemical oxidation, which i s the subject of the present study. In the following chapter the fundamental bases of the electrochemical process are presented along with a l i t e r a t u r e review on previous attempts at electrochemical oxidation cf phenol. CHAPTER 2 BASES OF THE ELECTROCHEMICAL PROCESS 2 . 1 G e n e r a l c o n c e p t s F o r any e l e c t r o c h e m i c a l r e a c t i o n j o f t h e f o r m B + z e " Z A t h e r e v e r s i b l e e q u i l i b r i u m p o t e n t i a l i s w r i t t e n as T j - * j " I ' » where t h e a c t i v i t y c o e f f i c i e n t o f e a c h s p e c i e s ( i . e . , f ^ = a ^ / C ^ ) i s e q u a l t o u n i t y ( 1 0 ) . The e l e c t r o d e p o t e n t i a l i s d e f i n e d as t h e d i f f e r e n c e b e t w e e n t h e p o t e n t i a l o f t h e m e t a l o f t h e e l e c t r o d e and t h e p o t e n t i a l o f t h e s o l u t i o n a d j a c e n t t o t h e e l e c t r o d e ( F i g . 1 ) . T h u s , Anode p o t e n t i a l = v = d> - cb [2] r a ma s a Ca thode p o t e n t i a l = V = <j> - <j> [3] * c mc s c The r a t e o f an e l e c t r o c h e m i c a l r e a c t i o n i s a f u n c t i o n o f t h e o v e r - p o t e n t i a l , o r d i f f e r e n c e be tween t h e e l e c t r o d e p o t e n t i a l and t h e e q u i l i - b r i u m p o t e n t i a l f o r r e a c t i o n j . The a n o d i c and c a t h o d i c o v e r p o t e n t i a l s f o r r e a c t i o n j , a r e r e s p e c t i v e l y , n . = v * - v . [4] aj a 3 The current density ( i ) i s defined as the amount of current passing p e r u n i t area of the electrode, and may be r e l a t e d to the ov e r p o t e n t i a l , e i t h e r l i n e a r l y at low overpotentials (n a i ) or through the T a f e l equa- t i o n at high ov e r p o t e n t i a l s , H. - a. + b. log i [6] 3 3 3 In a given e l e c t r o l y t e , the T a f e l constants a and b have s p e c i f i c values f o r each electrochemical r e a c t i o n j occurring on a given electrode at determined conditions of pH and temperature. These have been reported f o r common electrode reactions on d i f f e r e n t electrodes (10,11). A side r e a c t i o n w i l l occur i f the p o t e n t i a l of the electrode i s equal to the t o t a l p o t e n t i a l required to drive the side r e a c t i o n (equilibrium poten- t i a l plus o v e r p o t e n t i a l ) . Major side reactions associated with e l e c t r o - l y t i c processes i n aqueous solutions are the reactions of water e l e c t r o - l y s i s , that i s , the anodic formation of oxygen and the cathodic formation o f hydrogen. Depending on the pH of the e l e c t r o l y t e and p o t e n t i a l of the el e c t r o d d i f f e r e n t water e l e c t r o l y s i s reactions, may occur, (Standard reduction Oxygen evolution r e a c t i o n s : p o t e n t i a l s ) 2 0H~ t % 02 + H 20 + 2e~ V° = 0.4010 [Rl] H 20 t % 0 2 + 2H + + 2e" V° = 1.2290 [R2] Hydrogen evolution reactions: H 20 + e~ t hE2 + OH"' V° =-0.8277 [R3] H + + e~ t h H 2 V° =0.0000 [R4] Side reactions w i l l compete with the desired electrochemical reac- t i o n f o r current so that the applied current density w i l l be the sum of the p a r t i a l current d e n s i t i e s supporting each r e a c t i o n . Considering 8 t h e e l e c t r o c h e m i c a l r e a c t i o n (A -* B + ze ) , t he c u r r e n t e f f i c i e n c y f o r t h e o x i d a t i o n o f A i s d e f i n e d as t h e r a t i o be tween t h e t h e o r e t i c a l amount o f e l e c t r i c i t y needed t o o x i d i z e one e q u i v a l e n t o f A , and t h e a c t u a l amount o f e l e c t r i c i t y p a s s e d p e r e q u i v a l e n t o f A o x i d i z e d . T h u s , an e x p r e s s i o n f o r t h e p e r c e n t o f c u r r e n t e f f i c i e n c y i s : % C . E . = x 100 [7] w h e r e , m = number o f m o l e s o f A o x i d i z e d . I f t h e o x i d a t i o n o f A i s t h e d e s i r e d e l e c t r o c h e m i c a l r e a c t i o n , somet imes i t i s n e c e s s a r y t o s u p p r e s s t h e r e v e r s e r e d u c t i o n r e a c t i o n ( d e p e n d i n g on t h e r e l a t i v e r e a c t i o n r a t e s ) . I n o r d e r t o a v o i d c o n t a c t b e t w e e n t h e o x i d i z e d s p e c i e s and t h e c a t h o d e , o r t o p r e v e n t m i x i n g o f a n o l y t e and c a t h o l y t e w i t h p o s s i b l e r e a c t i o n an i o n exchange membrane o r a d i a p h r a g m c a n be u sed t o s e p a r a t e t h e anode and c a t h o d e chambers i n a d i v i d e d c e l l . A d i a p h r a g m a l l o w s t h e t r a n s p o r t o f i o n s and m o l e c u l e s . An i o n exchange membrane c a n be e i t h e r a n i o n s e l e c t i v e o r c a t i o n s e l e c t i v e , i n w h i c h c a s e o n l y a n i o n s o r c a t i o n s r e s p e c t i v e l y w i l l be t r a n s p o r t e d t h r o u g h t h e membrane. Due t o t h e s e l e c t i v e p r o p e r t i e s o f i o n exchange membranes , t h e y c a n a l s o be u sed t o c o n t r o l t h e pH o f t h e a n o l y t e and c a t h o l y t e , s i n c e t h e t r a n s p o r t o f [OH ] o r [ H + ] i o n s w i l l be d e t e r m i n e d by t h e t y p e o f membrane u s e d . F o r t h e g e n e r a l c a s e o f p l a n e e l e c t r o d e s and two s e p a r a t e c h a m b e r s , t h e d i f f e r e n t p o t e n t i a l d r o p s t h r o u g h t h e c e l l a r e i l l u s t r a t e d i n F i g . 1. 9 i g . 1. V o l t a g e components i n a d i v i d e d e l e c t r o l y t i c c e l l . I f K , K , and K a r e t h e e l e c t r x c a l c o n d u c t i v i t i e s o f t h e a n o l y t e , e e, e . • J a d c d i a p h r a g m , and c a t h o l y t e , and a r e u n i f o r m , t h e Ohm's l a w c a n be w r i t t e n as f o l l o w s : K I A<p I K I A<j> , I K I A<p I e 1 s a 1 e ' d ' ° 1 r<=^i i = e ' s c 1 c S. a 'd c T h e r e f o r e , t h e t o t a l o h m i c d r o p i s g i v e n b y : S . S - . £ AV . . = A(J> + A<}), + A<(> = i ( - ^ + — S - + — - ) . ohmic Y s a Y d Y s c K K K ' e e , e a d c When t h e a n o l y t e and c a t h o l y t e have a p p r e c i a b l y d i f f e r e n t c o m p o s i t i o n s , an e x t r a p o t e n t i a l d r o p may e x i s t , c a l l e d " l i q u i d j u n c t i o n p o t e n t i a l " ; 10 (10) b u t u s u a l l y i t i s r e l a t i v e l y s m a l l . The t o t a l e l e c t r o l y s i n g v o l t a g e f o r a c u r r e n t d e n s i t y i w o u l d b e , AV = V a * + | V c * | + i + [8] e e., e a d c The t o t a l e l e c t r o l y s i n g v o l t a g e i s o f i m p o r t a n c e s i n c e t h e o p e r a t i n g c o s t o f t h e e l e c t r o l y t i c p r o c e s s w i l l depend on i t s power r e q u i r e m e n t s w h i c h i s d i r e c t l y r e l a t e d t o t h e t o t a l v o l t a g e d r o p t h r o u g h t h e c e l l a t a g i v e n c u r r e n t d e n s i t y . P r a c t i c a l l y , t h e t o t a l e l e c t r o l y s i n g v o l t a g e i s e a s i e r t o measure t h a n t h e p a r t i c u l a r e l e c t r o d e p o t e n t i a l s b e c a u s e t o measure o r V c a r e f e r e n c e e l e c t r o d e has t o be c o n n e c t e d a t t h e s u r f a c e o f t h e anode o r c a t h o d e t o d e t e c t t h e d i f f e r e n c e i n p o t e n t i a l be tween t h e m e t a l and t h e s o l u t i o n . The r a t e a t w h i c h an e l e c t r o c h e m i c a l r e a c t i o n o c c u r s depends s t r o n g l y on t h e e l e c t r o d e p o t e n t i a l as w i l l be shown b e l o w . C o n s i d e r a s i n g l e r e v e r s i b l e r e a c t i o n t h a t o c c u r s on an e l e c t r o d e (anode o r c a t h o d e ) , 1 A X B + ze 1: o x i d a t i o n r e a c t i o n 2 2 : r e d u c t i o n r e a c t i o n The n e t r a t e o f e l e c t r o c h e m i c a l r e a c t i o n o v e r t h e e l e c t r o d e i s t h e modu lus o f t h e d i f f e r e n c e be tween t h e r a t e s o f o x i d a t i o n and r e d u c t i o n o f t h e r e v e r s i b l e r e a c t i o n m 2 m i - = | K r 2 C B - K r i C A | [9] s s where C . and C„ a r e t h e c o n c e n t r a t i o n s o f A and B a t t h e s u r f a c e o f A B s s t h e e l e c t r o d e , mi and m 2 a r e t h e o r d e r s o f t h e o x i d a t i o n and r e d u c t i o n r e a c t i o n s r e s p e c t i v e l y , and K r i and K r 2 a r e t h e e l e c t r o c h e m i c a l r e a c t i o n r a t e c o n s t a n t s , w h i c h c a n be e x p r e s s e d i n t e rms o f t h e e l e c t r o d e p o t e n - t i a l b y u s i n g an A r r h e n i u s t y p e o f r a t e c o n s t a n t - a c t i v a t i o n e n e r g y r e l a t i o n s h i p , K r 1 = K r ° exp 1— K r = K r ! exp 2 2 RT J f ( l - a ) z F |v*r RT [10] [11] H e r e Kr£ and K r ° a r e r a t e c o n s t a n t s r e f e r r e d t o a p a r t i c u l a r e l e c t r o d e p o t e n t i a l a t s t a n d a r d c o n d i t i o n s , and a i s a c o n s t a n t known as t h e c h a r g e t r a n s f e r c o e f f i c i e n t . These e q u a t i o n s i m p l y t h a t a f r a c t i o n o f t h e • *• e l e c t r o d e p o t e n t i a l c t |V | d r i v e s t h e f o r w a r d r e a c t i o n and t h e r e m a i n d e r (1 -a) | V I d r i v e s t h e r e v e r s e r e a c t i o n . The f a c t t h a t t h e e l e c t r o c h e m i c a l r a t e c o n s t a n t depends e x p o n e n - t i a l l y on t h e e l e c t r o d e p o t e n t i a l and n o t j u s t on t h e t e m p e r a t u r e as i n t h e c a s e o f a p u r e c h e m i c a l r e a c t i o n , i l l u s t r a t e s t h a t t h e r e a c t i o n r a t e c a n b e v a r i e d b y o r d e r s o f m a g n i t u d e by s i m p l y a d j u s t i n g t h e p o t e n t i a l . , A t a g i v e n r e a c t i o n r a t e , t h e c o n c e n t r a t i o n o f r e a c t a n t a t t h e s u r f a c e o f - t h e e l e c t r o d e w i l l be r e l a t e d t o t h e r a t e o f mass t r a n s f e r . . . f r o m t h e b u l k o f t h e s o l u t i o n t o t h e e l e c t r o d e s u r f a c e , Mass t r a n s f e r f l u x = K ( C . - C . ) [12] . m - \ A s w h e r e K i s the. mass t r a n s f e r c o e f f i c i e n t , c h a r a c t e r i s t i c o f t h e p a r t i c - m u l a r e l e c t r o d e c o n f i g u r a t i o n and f l u i d d y n a m i c s . E m p i r i c a l and t h e o r - e t i c a l e x p r e s s i o n s f o r t r a n s f e r c o e f f i c i e n t s , s u i t a b l e f o r d e s i g n p u r p o s e s a r e a v a i l a b l e i n s t a n d a r d t e x t s ( 1 2 ) . 12 2 . 2 L i t e r a t u r e r e v i e w on t h e e l e c t r o c h e m i c a l o x i d a t i o n o f p h e n o l A s u b s t a n t i a l l i t e r a t u r e e x i s t s r e l a t e d t o t h e e l e c t r o c h e m i c a l o x i d a t i o n o f p h e n o l . H o w e v e r , o w i n g t o t h e c o m p l e x i t y o f t h e o x i d a t i o n r e a c t i o n s and t h e v a r i e t y o f o p e r a t i n g c o n d i t i o n s u s e d i n e a c h s t u d y , mechanisms h a v e b e e n p r o p o s e d , some o f w h i c h a p p e a r h i g h l y s p e c u l a t i v e . I n o r d e r t o c o n s i d e r p o s s i b l e r a t e d e t e r m i n i n g f a c t o r s , a r e v i e w o f t h e l i t e r a t u r e i s p r e s e n t e d h e r e and some o f t h e c o n t r a d i c t i o n s a r e d i s - c u s s e d . 2 . 2 . 1 R e a c t i o n p r o d u c t s The a n o d i c o x i d a t i o n o f p h e n o l was e x t e n s i v e l y s t u d i e d by F i c h t e r and. c o - w o r k e r s ( 1 3 - 1 5 ) d u r i n g t h e e a r l y p a r t o f t h i s c e n t u r y . They r e p o r t e d t h a t when p h e n o l i s o x i d i z e d a t a l e a d d i o x i d e e l e c t r o d e i n s u l p h u r i c a c i d m e d i a t h e p r o d u c t s shown i n F i g . 2 a r e i n v o l v e d . r e p o r t e d f i n d i n g s a r e somet imes c o n t r a d i c t o r y . Many d i f f e r e n t r e a c t i o n OH QH 0 I! 0 CH-COOH CH-COOK rt OH p h e n o l h y d r o q u i n o n e p - b e n z o q u i n o n e m a l e i c a c i d OH OH e t h e r o f p y r o c a t e c h o l 2>4' d i h y d r o x y d i p h e n y i 4 , 4 ' d i h y d r o x y d i p h e n y i F i g . 2 . R e a c t i o n p r o d u c t s . 13 It was also found that i f benzene (13) is electrochemically oxidized at a platinum electrode in sulphuric acid solution some of the same products were encountered, e.g., hydroquinone, p-benzoquinone, catechol, maleic, and oxalic acids. It was suggested (11,13) that probably phenol was f i r s t produced as an intermediate in the oxidation of benzene, even though phenol had not been isolated from the reaction mixture. Thus, information concerning benzene electrooxidation can be useful for the , present study. 2,2.2 Proposed reaction mechanisms a) Hydroxylation The formation of hydroquinone and catechol was attributed to the introduction of hydroxyl groups into the aromatic ring, by the action of anodically generated oxygen. By the same mechanism phenol would be produced i f benzene was the starting substrate (13). However, this assumption is contradicted i n a more recent paper (16) where the oxida- tion of benzene to p-benzoquinone is reported at 100% current efficiency at potentials below those at which oxygen evolution occurs. But a clear explanation of the mechanism is not given in the paper. b) Nuclear linkage Fichter explained the formation of diphenyi derivatives by supposing that a linkage of two aromatic nuclei was brought about by a bond of oxygen, [R5] A similar mechanism would produce a l l the diphenyi compounds indicated in Fig. 2. Those compounds were found when lead peroxide anodes were 14 employed a t a r e l a t i v e l y l o w c u r r e n t d e n s i t y ( 2 5 A / m 2 ) . F i c h t e r a l s o r e p o r t e d t h a t t h e n u c l e a r l i n k a g e , . i s e v e n more p r o n o u n c e d w i t h p h e n o l homologues l i k e o - C r e s o l due t o t h e p r e s e n c e o f an e l e c t r o n d o n a t i n g g r o u p . The d i p h e n o l s w e r e f o u n d s u s c e p t i b l e t o f u r t h e r o x i d a t i o n upon c o n t i n u i n g t h e e l e c t r o l y s i s o r upon i n c r e a s i n g t h e c u r r e n t d e n s i t y . I n more r e c e n t p u b l i c a t i o n s ( 1 7 - 2 2 ) t h e p r e s e n c e o f d . i p h e n y l d e r i v a t i v e s h a s n o t b e e n r e p o r t e d among t h e p r o d u c t s o f o x i d a t i o n o f p u r e p h e n o l s o l u t i o n s . G e n e r a l l y t h e r e p o r t e d p r o d u c t s a r e h y d r o q u i n o n e , b e n z o - q u i n o n e , m a l e i c a c i d , and c a r b o n d i o x i d e . I t i s p o s s i b l e t h a t u n d e r t h e r e p o r t e d c o n d i t i o n s t h e d i p h e n y l d e r i v a t i v e s e i t h e r a r e n o t f o r m e d , o r a r e f u r t h e r o x i d i z e d t o t h e above m e n t i o n e d e n d - p r o d u c t s . H o w e v e r , i n t h e c a s e o f s u b s t i t u t e d p h e n o l s , t h e p r e s e n c e o f c o u p l e d p r o d u c t s i s a l w a y s r e p o r t e d i n r e c e n t s t u d i e s ( 2 3 , 2 4 ) w h i c h i s i n agreement w i t h F i c h t e r ' s f i n d i n g s . c ) • P r i m a r y e l e c t r o n , t r a n s f e r mechan i sm I n t h e o x i d a t i o n o f p h e n o l , two d i f f e r e n t , f i r s t - s t e p mechan i sms of e l e c t r o n t r a n s f e r have b e e n p r o p o s e d ( 2 3 , 2 5 ) , due t o t h e a b i l i t y o f p h e n o l s t o e x i s t i n t h e i o n i z e d o r u n i o n i z e d f o r m d e p e n d i n g on the pH o f t h e s o l u t i o n , - r OH 0 4- H + u n i o n i z e d f o r m p h e n o x i d e ion [R6] A t l o w pH i n aqueous s o l u t i o n s p h e n o l s w i l l tend t o be i n the u n i o n i z e d f o r m , and a t h i g h pH v a l u e s w i l l tend t o be as a p h e n o x i d e ion. The f o l l o w i n g mechanisms have been p r o p o s e d , f o r t h e f i r s t s t e p o f t h e o x i d a t i o n : I n a c i d i c s o l u t i o n s t h e i n i t i a l step i n v o l v e s two e l e c t r o n s where the 1 5 electrophilic attack of the aromatic nucleus produces the "phenoxonium ion", - 2e H" 0 -y U - ^ phenoxonium ion (mesomeric) [R7] In alkaline solutions the primary anodic reaction of phenoxide ions i s a one-electron transfer with the formation of a phenoxy free radical that i s very reactive. • • - Ie. phenoxi radical [R8] In appendix 5, the percentage phenol ionized as a function of pH is calculated from the dissociation constant for phenol at 20°C ( K , = 1 . 2 8 x 10~ 1 0). d d) The divided or undivided c e l l and the reaction mechanism- A great deal of information regarding the electrochemical oxidation of phenol exists because of commercial interest in the production of hydroquinone or" p-benzoquinone (.19-22). Covitz studied the electro- chemical oxidation of phenol for hydroquinone production at lead dioxide, anodes in an undivided c e l l in acid media. He showed that the reaction can be controlled to produce hydroquinone at over 90% yield. The simplified mechanism for the electrolytic process proposed is (26, p. .157): OH I t i s o f i n t e r e s t t o n o t e t h a t i n t h e a n o d i c r e a c t i o n , w a t e r i s u t i l i z e d t o i n t r o d u c e a n o x y g e n a tom i n t o t h e s t a r t i n g p h e n o l m o l e c u l e . I n t h e u n d i v i d e d c e l l p - b e n z o q u i n o n e i s r e d u c e d a t t h e c a t h o d e t o p r o d u c e h y d r o - q u i n o n e . F rom t h i s r e a c t i o n scheme i t i s o b v i o u s t h a t i f t h e p r o c e s s i s c a r r i e d o u t i n a d i v i d e d c e l l , by u s i n g a membrane o r a d i a p h r a g r c . p - b e n z o q u i n o n e w o u l d n o t c o n t a c t t h e c a t h o d e and t h e r e f o r e w o u l d n o t be r e d u c e d b a c k t o h y d r o q u i n o n e . C o v i t z r e p o r t e d (19) t h a t when u s i n g a s e m i p e r m e a b l e membrane, t h e o n l y m e a s u r a b l e p r o d u c t i n t h e a n o l y t e was p - b e n z o q u i n o n e . A n o t h e r p o s s i b l e r e a c t i o n i n an u n d i v i d e d c e l l i s t h e o x i d a t i o n c f h y d r o q u i n o n e b a c k t o p - b e n z o q u i n o n e w h i c h w o u l d compete w i t h t h e p h e n o l f o r o x i d a t i o n a t t h e a n o d e , t h u s l o w e r i n g t h e c u r r e n t e f f i c i e n c y f o r p h e n o l o x i d a t i o n . e) E l e c t r o l y t i c a c t i o n o f l e a d d i o x i d e Some a u t h o r s ( 1 6 , 2 3 , 2 6 ) s u p p o r t t h e h y p o t h e s i s o f e l e c t r o c a t a l y t i c o x i d a t i o n o f p h e n o l on l e a d d i o x i d e . I n o t h e r w o r d s , p h e n o l i s o x i d i z e d c h e m i c a l l y by l e a d d i o x i d e and t h e r e d u c e d l e a d s p e c i e s so formed a r e r a p i d l y o x i d i z e d b a c k t o Pb02 by a c h a r g e t r a n s f e r o r e l e c t r o l y t i c s t e p . 1) P b 0 2 + ORGANIC + PRODUCT + P b + 2 [R12] T h i s mechan i sm i s s u g g e s t e d as an a l t e r n a t i v e e x p l a n a t i o n t o e l e c t r o n t r a n s f e r f rom t h e o r g a n i c m o l e c u l e . E x p e r i m e n t s have been c a r r i e d o u t t o d e t e r m i n e t h e o x i d a t i v e a b i l i t y o f P b 0 2 i n t h e a b s e n c e o f c u r r e n t . C l a r k e and c o - w o r k e r s (16) made s t u d i e s w i t h g r a n u l a r P b 0 2 i n s t i r r e d benzene e m u l s i o n s . The a n a l y s i s o f t h e p r o d u c t s showed t h a t b e n z o q u i n o n e and m a l e i c a c i d were r a p i d l y f o r m e d . H o w e v e r , . n u m e r i c a l r e s u l t s were n o t p r o v i d e d . As d i s c u s s e d p r e v i o u s l y , t h e same p a p e r c o n t r a d i c t s t h e h y p o t h e s i s o f h y d r o x y l a t i o n as a mechan i sm o f i n t r o d u c t i o n o f o x y g e n i n t o t h e o r g a n i c m o l e c u l e . Thus i f P b 0 2 i s supposed t o be t h e oxygen c a r r i e r i t w o u l d be n e c e s s a r y t o r e p l a c e t h e o x y g e n l o s t i n r e a c t i o n [R12] by r e a c t i o n [ R 1 3 ] . The u l t i m a t e s o u r c e o f o x y g e n , i s o f c o u r s e , t h e w a t e r , w h a t e v e r i s t h e p r e v a i l i n g m e c h a n i s m . f ) F u r t h e r o x i d a t i o n o f i n t e r m e d i a t e s The a v a i l a b l e i n f o r m a t i o n c o n c e r n i n g t h e l a s t s t a g e s o f t h e e l e c t r o - l y t i c o x i d a t i o n o f p h e n o l t o open c h a i n o r g a n i c compounds o r e v e n t u a l l y t o c a r b o n d i o x i d e i s v e r y l i m i t e d , p r o b a b l y b e c a u s e o n l y a few i n v e s t i g a - t i o n s have been c o n c e r n e d w i t h t h e t o t a l d e s t r u c t i o n o f t h e o r g a n i c s u b s t r a t e f o r w a s t e t r e a t m e n t a p p l i c a t i o n s ( i . e . , 1 7 , 1 8 , 2 5 ) . A mechan i sm f o r t h e l a s t - s t a g e r e a c t i o n s has n o t e v e r been p r o p o s e d . H o w e v e r , i n t h e e a r l y i n v e s t i g a t i o n s (14) i t was shown t h a t i n t e r m e d i - a t e s were s u s c e p t i b l e t o f u r t h e r d i s i n t e g r a t i o n . C a t e c h o l , f o r e x a m p l e , was w e l l known as an e a s i l y o x i d i z a b l e s u b s t r a t e , and p - b e n z o q u i n o n e , w h i c h o f f e r e d a h i g h r e s i s t a n c e t o c h e m i c a l o x i d a t i o n , was shown t o be r e a d i l y b r o k e n down by e l e c t r o c h e m i c a l means ( 2 7 ) . F i c h t e r e s t a b l i s h e d t h a t t h e d e c o m p o s i t i o n p r o c e s s o c c u r s f a s t e r a t h i g h c u r r e n t d e n s i t i e s , when t h e a r o m a t i c n u c l e u s i s s a t u r a t e d w i t h e l e c - t r o l y t i c o x y g e n . He r e p o r t e d l e s s c h a r a c t e r i s t i c f i n a l p r o d u c t s s u c h as o x a l i c a c i d , f o r m i c a c i d , and c a r b o n m o n o x i d e , w h i c h i s i n agreement w i t h some o f t h e p r o d u c t s r e p o r t e d by G l a d i s h e v a ( 1 7 ) . A p a r t i c u l a r l y i n t e r e s t i n g c o n t r o l l e d p o t e n t i a l s t u d y i s p r e s e n t e d f o r t h e e l e c t r o l y t i c o x i d a t i o n o f benzene (16) where i t i s shown t h a t as t h e p o t e n t i a l i s i n c r e a s e d above t h a t o f o x y g e n e v o l u t i o n , f r a g m e n t a t i o n o f t h e a r o m a t i c r i n g o c c u r s and t h e c u r r e n t e f f i c i e n c y f o r m a l e i c a c i d and c a r b o n d i o x i d e p r o d u c t i o n i n c r e a s e s . A n o d i c a l l y g e n e r a t e d o x y g e n i s known as one o f t h e most p o w e r f u l o x i d i z i n g a g e n t s , and seems t o be r e s p o n s i b l e f o r t h e f u r t h e r o x i d a t i o n o f t h e i n t e r m e d i a t e s , even t h o u g h t h e e x a c t r e a c t i o n mechan i sm has n o t b e e n d e t e r m i n e d . 2 . 2 . 3 E l e c t r o d e m a t e r i a l s t e s t e d L e a d d i o x i d e has b e e n t h e most commonly u sed e l e c t r o d e i n t h e e l e c t r o c h e m i c a l o x i d a t i o n o f p h e n o l and i n s e v e r a l c a s e s i s recommended as t h e e l e c t r o d e m a t e r i a l o f c h o i c e . H o w e v e r , i t i s o f i n t e r e s t t o r e v i e w and compare t h e p e r f o r m a n c e s o f d i f f e r e n t e l e c t r o d e m a t e r i a l s . I n t h e p a p e r by G l a d i s h e v a and L a v r e n c h u k (17) s e v e r a l anode m a t e r - i a l s were t e s t e d : n i c k e l , smooth p l a t i n u m , g r a p h i t e , and l e a d d i o x i d e e l e c t r o d e p o s i t e d on a n i c k e l b a s e . The e x p e r i m e n t s showed t h a t u n d e r t h e same o p e r a t i n g c o n d i t i o n s , t h e h i g h e s t o x i d a t i o n r a t e o c c u r r e d on t h e l e a d d i o x i d e e l e c t r o d e . The r e s u l t s a r e shown i n T a b l e 1 where t h e r a t e o f o x i d a t i o n i s g i v e n a t two p h e n o l c o n c e n t r a t i o n s and two c u r r e n t d e n - s i t i e s . The c h e m i c a l s t a b i l i t y o f t h e d i f f e r e n t e l e c t r o d e s t e s t e d was TABLE 1 RATES OF PHENOL OXIDATION ON DIFFERENT ELECTRODE MATERIALS (17) E l e c t r o d e I n i t i a l p h e n o l R a t e o f O x i d a t i o n (mg/min] m a t e r i a l c o n e , (mg/1) i = 50A/ m 2 i = 1000 A / m 2 E l e c t r o d e p o s i t e d 200 1.0 3 .7 l e a d d i o x i d e 1000 9 . 2 2 1 . 6 G r a p h i t e 200 0 . 7 2 . 4 1000 6 . 3 1 7 . 0 Smooth p l a t i n u m 200 0 . 4 2 . 4 N i c k e l 200 0 . 2 2 . 9 a l s o d i s c u s s e d . T h e - g r a p h i t e anode was f o u n d r e l a t i v e l y s t a b l e a t c u r - r e n t d e n s i t i e s be tween 50 -250 A / m 2 b u t a t h i g h e r c u r r e n t d e n s i t i e s t h e g r a p h i t e s t a r t e d t o b r e a k down, f o r m i n g s m a l l p a r t i c l e s t h a t were d i f f i - c u l t t o remove by f i l t r a t i o n . N i c k e l e l e c t r o d e s were u n s u i t a b l e s i n c e a t pH = 10 n i c k e l d i s s o l u t i o n o c c u r r e d p a r a l l e l t o p h e n o l o x i d a t i o n , c o n s u m i n g a s i g n i f i c a n t amount o f c u r r e n t , and d e s t r o y i n g the e l e c t r o d e . The smooth p l a t i n u m e l e c t r o d e w a s , o f c o u r s e , e l e c t r o c h e m i c a l l y s t a b l e b u t t h e r a t e s o f o x i d a t i o n o f p h e n o l were much l o w e r t h a n e x p e c t e d , c o n s i d e r i n g t h a t p l a t i n u m has a h i g h o x y g e n o v e r p o t e n t i a l . T h i s f a c t was e x p l a i n e d by t h e f o r m a t i o n o f a t a r f i l m on t h e s u r f a c e 20 o f t h e anode w h i c h d i d n o t d i s s o l v e i n a l k a l i n e o r a c i d s o l u t i o n . How- e v e r , t h e p r e s e n c e o f s u c h a f i l m was n o t m e n t i o n e d i n t h e c a s e o f t h e l e a d d i o x i d e e l e c t r o d e . I n t h e s t u d y o f e l e c t r o c h e m i c a l o x i d a t i o n o f p h e n o l t o q u i n o n e by F i o s h i n e t a l (22) t h e same r e s u l t was o b t a i n e d when c o m p a r i n g t h e p l a t - inum and l e a d d i o x i d e e l e c t r o d e s . The c h e m i c a l y i e l d t o q u i n o n e was 33% on t h e l e a d d i o x i d e anode , whereas i t was o n l y 5% on p l a t i n u m , a l t h o u g h i t i s known t h a t t h e o v e r p o t e n t i a l o f t h e s e e l e c t r o d e s i n a c i d i c s o l u t i o n s a r e p r a c t i c a l l y t h e same. The r e a s o n s u g g e s t e d was t h e d i f f e r - en t a d s o r p t i v e powers , o f t h e two e l e c t r o d e s t o w a r d s t h e same o r g a n i c s u b s t r a t e . H o w e v e r , t h e l o w e r q u i n o n e y i e l d on p l a t i n u m c o u l d a l s o have been c a u s e d by f u r t h e r d i s i n t e g r a t i o n o f t h e q u i n o n e . T h i s p o s s i b i l i t y was n o t s u g g e s t e d and o t h e r p r o d u c t s a n a l y s e s were n o t p e r f o r m e d . 2 . 2 . 4 E f f e c t o f c u r r e n t d e n s i t y S t u d i e s have been c a r r i e d o u t o v e r a w i d e r a n g e o f c u r r e n t d e n s i t i e s . F o r e x a m p l e , i n r e f e r e n c e (17) t h e c u r r e n t e f f e c t on p h e n o l o x i d a t i o n was s t u d i e d i n t h e r ange o f 50 -2000 A / m 2 . I t was c o n c l u d e d t h a t among t h e v a r i a b l e s s t u d i e d , t h e c u r r e n t d e n s i t y was t h e s t r o n g e s t d e t e r m i n i n g f a c t o r i n t h e r a t e o f e l e c t r o c h e m i c a l o x i d a t i o n o f p h e n o l . The r e s u l t s a r e g i v e n i n T a b l e s 1 and 2 . I n T a b l e 1 i t c a n be o b s e r v e d t h a t on t h e Pb02 e l e c t r o d e , a t t h e i n i t i a l c o n c e n t r a t i o n o f 1000 mg/1 o f p h e n o l , t h e r a t e o f p h e n o l o x i d a t i o n a p p r o x i m a t e l y d o u b l e d when t h e c u r r e n t d e n s i t y was i n c r e a s e d 20 t i m e s . T a b l e 2 shows t h a t i n t h e s o d i u m s u l p h a t e e l e c t r o l y t e , when s t a r t i n g a t 466 mg/1 o f c h e m i c a l oxygen demand ( C . O . D . ) a t a c u r r e n t d e n s i t y o f 50 A / m 2 , t h e f i n a l C . O . D . was 420 mg/1 a f t e r 5 h , whe reas a t 2000 A / m 2 t h e C . O . D . d r o p p e d t o 30 mg/T i n o n l y 1 h . 21 TABLE 2 EFFECT OF CURRENT DENSITY AND TYPE OF ELECTROLYTE ON C . O . D . REMOVAL (17) Time o f C u r r e n t d e n s i t y E l e c t r o l y t e E l e c t r o l y s i s F i n a l C . O . D . A / m 2 t y p e (h) (mg/1 o f 0 2 ) 50 I 5 307 I I 5 420 500 I 3 90 I I 5 120 1000 I 1 30 I I 2 75 2000 I 0 . 5 0 I I 1 .0 30 N o t e s . I n i t i a l p h e n o l c o n c e n t r a t i o n = 200 mg/1 I n i t i a l C . O . D . c o n c e n t r a t i o n = 466 mg/1 o f 0 2 E l e c t r o l y t e I - 1 g / 1 N a C l , 1.5 g / 1 Na 2 S0i t E l e c t r o l y t e I I - 3 g / 1 N a 2 S O ^ 2 . 2 . 5 E f f e c t o f n a t u r e o f t h e e l e c t r o l y t e I n s e v e r a l s t u d i e s on t h e e l e c t r o x i d a t i o n o f p h e n o l f o r w a s t e t r e a t m e n t , c h l o r i d e s a l t s were u sed as e l e c t r o l y t e s ( 1 7 , 1 8 , 2 5 , 2 8 - 3 0 ) . I n s u c h m e d i a t h e o x i d a t i o n o f p h e n o l f o l l o w s t o t a l l y d i f f e r e n t r e a c t i o n p a t h s . When u s i n g N a C l o r C a C l 2 as e l e c t r o l y t e s , t h e f o l l o w i n g r e a c t i o n s have been p r o p o s e d (17) 1. E v o l u t i o n o f c h l o r i d e a t t h e a n o d e , 2 C l " - 2e~ -» C l 2 [R14] 22 2 . a) F o r m a t i o n o f h y p o c h l o r i t e f o l l o w e d by c h e m i c a l r e a c t i o n w i t h p h e n o l , C l 2 + H 2 0 H C l + HCl© [R15] 8 HClO + C 6 H 5 O H •> CH - CO-OH II + 2 C 0 2 + 8 H C l + H 2 0 CH - CO-OH [R16] b) C h l o r i n a t i o n o f - p h e n o l by m o l e c u l a r C l 2 p r o d u c i n g 2 , 4 d i c h l o r o p h e n o l and 2 , 4 , 6 t r i c h l o r o p h e n o l As c a n be s e e n , t h i s p r o c e s s does n o t r e p r e s e n t p u r e e l e c t r o c h e m i c a l o x i d a t i o n o f p h e n o l , bu t i n s t e a d i t i s e q u i v a l e n t t o t h e e l e c t r o l y t i c p r o d u c t i o n o f c h l o r i n e and h y p o c h l o r i t e f o l l o w e d by a c h e m i c a l o x i d a t i o n o f p h e n o l . I t a l s o g i v e s r i s e t o u n d e s i r a b l e c h l o r i n a t i o n p r o d u c t s . A l t h o u g h t h e s e a r e c l a i m e d t o be c a p a b l e o f f u r t h e r o x i d a t i o n , t o g i v e p r o d u c t s o f t h e q u i n o n e t y p e , t h e r e m o v a l i s n e v e r 100%. T h i s was shown i n r e f e r - ence (25) where e x p e r i m e n t s were c a r r i e d ou t s t a r t i n g w i t h p - c h l o r o p h e n o l , 2 , 4 d i c h l o r o p h e n o l and 2 , 4 , 6 t r i c h l o r o p h e n o l and t h e p e r c e n t r e m o v a l o f c h e m i c a l oxygen demand ( C . O . D . ) were 82%, 79%, and 58% r e s p e c t i v e l y . K n o w i n g t h a t c h l o r i n a t e d p h e n o l s a r e more o b j e c t i o n a b l e t h a n p h e n o l i t s e l f , t h e a d d i t i o n o f c h l o r i d e s a l t s t o t h e e l e c t r o l y t e does n o t appea r t o be a good s o l u t i o n t o t h e p o l l u t i o n p r o b l e m even i f t h e r a t e o f p h e n o l o x i d a t i o n i s h i g h e r t h a n when u s i n g an i n e r t s u p p o r t e l e c t r o l y t e s u c h as s o d i u m s u l p h a t e . The p e r f o r m a n c e o f a m i x t u r e o f N a C l and N a 2 S 0 i t and p u r e N a 2 S 0 i + e l e c t r o l y t e s have been compared (17) i n t e rms o f f i n a l C . O . D . a f t e r t r e a t m e n t . The r e s u l t s a r e a l s o shown i n T a b l e 2 where i t i s o b s e r v e d t h a t a t t h e f o u r c u r r e n t d e n s i t i e s u s e d , t h e f i n a l C . O . D . was a l w a y s l o w e r when t h e e l e c t r o l y t e c o n t a i n e d N a C l t h a n when t h e e l e c t r o - l y t e was p u r e N a 2 S 0 t f . Howeve r , an a n a l y s i s o f c h l o r i n a t e d p h e n o l s w h i c h 23 may have been p r o d u c e d was n o t p r o v i d e d , t h u s i t i s d i f f i c u l t t o d e c i d e w h i c h e l e c t r o l y t e i s more s u i t a b l e . O t h e r e l e c t r o l y t e s , s u c h as Na2Bi t 0y, NH3 and H^SO^ were t e s t e d u s i n g a p a c k e d bed g r a p h i t e e l e c t r o d e ( 1 8 ) . The r e s u l t s a r e shown i n T a b l e 3 . TABLE 3 EFFECT OF TYPE OF ELECTROLYTE ON PHENOL OXIDATION (18) E l e c t r o l y s i s F i n a l p h e n o l E l e c t r o l y t e pH t i m e (h) c o n e , (mg/1) 0 . 1 M s o d i u m b o r a t e 9 .7 3 . 5 420 0 . 1 M ammonia 1 0 . 0 5 . 5 400 0 . 1 M s u l p h u r i c a c i d 1.1 3 .7 400 5% s o d i u m c h l o r i d e 5 .7 20 -0 10 I n i t i a l p h e n o l c o n e . = 1000 mg/1 Volume o f e l e c t r o l y t e = 700 m l C u r r e n t = 0 . 6 A (on g r a p h i t e anode) W i t h t h e f i r s t t h r e e e l e c t r o l y t e s p h e n o l was removed f rom 1000 mg/1 t o abou t 400 -420 mg/1 i n e l e c t r o l y s i s t i m e s f r o m 3 t o 5 h . The f i n a l p h e n o l c o n c e n t r a t i o n r e a c h e d 10 mg/1 o n l y when u s i n g a s o d i u m c h l o r i d e e l e c t r o l y t e and a t t h e e x c e s s i v e l y l a r g e t i m e o f 20 h . The d i f f e r e n t e l e c t r o l y s i s t i m e s a l l o w e d makes t h e c o m p a r i s o n be tween e l e c t r o l y t e p e r f o r m a n c e s d i f f i c u l t . I n t h i s c a s e t h e c u r r e n t a p p l i e d was r e l a t i v e l y l o w and an e s t i m a t i o n o f t h e e l e c t r o d e a r e a was n o t p r o v i d e d . 24 2.2.6 Effect of pH The effect of pH on the oxidation potential of phenol has been reported (23). The results of this polarographic study are represented in Fig. 3. > r 1 1 1 1 r 1 < 02 1 1 1 1 1 1 : 1 1 X 0 A 8 12 pH Fig. 3. Half-wave potential vs pfi, for the oxidation of 4 x 10" 4 M phenol (23). The half wave potential i s defined as the potential on a polaro- graphic curve when the current i s equal to one half the mass transfer limiting current (31). It can be observed that the half wave potential decreases when going from acid to basic solutions and eventually stabilizes at a constant value for a pH equal to the pK^ (logarithmic of the dissociation constant of phenol). At pH = pK^ a l l the phenol w i l l be in the ionized form, or in other words, protonation w i l l be negligible. This means that a high pH makes the phenol more easily oxidizable as far as potential require- ments are concerned. The effect of pH on the rate of phenol oxidation i s not well docu- mented. In reference (17), i t was concluded that the velocity of oxidation was practically independent of pH of the solution in the range 25 o f pH 6 t o 9 , i n ab sence o f c h l o r i d e i o n s . H o w e v e r , when t h e pH was changed t o 1 1 . 9 a r a p i d i n c r e a s e i n t h e o p t i c a l d e n s i t y o f t h e s o l u t i o n was r e p o r t e d , w h i c h was e x p l a i n e d by an i n c r e a s e i n t h e c o n c e n t r a t i o n o f h y d r o q u i n o n e . Howeve r , p h e n o l o r C . O . D . a n a l y s e s were n o t r e p o r t e d i n t h i s c a s e . I n t h e s t u d y by T a r j a n y i e t a l (18) t h e e f f e c t o f pH can n o t be i s o l a t e d f rom t h e d a t a ( T a b l e 3 ) . No d e f i n i t i v e r e s u l t s c o u l d be f o u n d i n t h e r e v i e w e d l i t e r a t u r e abou t t h e e f f e c t o f pH on t h e f u r t h e r o x i d a t i o n o f i n t e r m e d i a t e p r o d u c t s . F i c h t e r (13) s u g g e s t e d t h a t i n a l k a l i n e s o l u - t i o n s t h e p r i m a r y p r o d u c t s o f t h e o x i d a t i o n w o u l d p r o b a b l y be t h e same as i n a c i d m e d i a , b u t t h a t p - b e n z o q u i n o n e w o u l d be u n s t a b l e a t h i g h pH and more e a s i l y o x i d i z a b l e by t h e a t o m i c o x y g e n . 2 . 3 The l e a d d i o x i d e e l e c t r o d e Two t y p e s o f l e a d d i o x i d e commonly e x i s t . These d i f f e r a c c o r d i n g t o t h e c r y s t a l s t r u c t u r e . a-Pb02 i s o r t h o r o m b i c and B-Pb02 i s t e t r a g o n a l . E a c h v a r i e t y can be p r e p a r e d s u b s t a n t i a l l y f r e e f rom t h e o t h e r u n d e r c a r e f u l l y c o n t r o l l e d c o n d i t i o n s ( 3 2 ) . L e a d d i o x i d e n e v e r con fo rms e x a c t l y t o t h e s t o i c h i o m e t r i c Pb02 f o r m u l a , an o x y g e n d e f i c i e n c y i s a l w a y s d e t e c t e d ( 3 3 ) , w i t h a-Pb0 2 t e n d - i n g t o show a l o w e r oxygen c o n t e n t . C o n t r a r y t o most m e t a l o x i d e s , l e a d d i o x i d e i s a good e l e c t r o n i c c o n d u c t o r , and i s b e t t e r i n f a c t t h a n l e a d i t s e l f ( 2 6 ) . I t i s b e l i e v e d t h a t t h e h i g h e l e c t r i c a l c o n d u c t i v i t y may be c o n n e c t e d w i t h t h e o x y g e n d e f i c i e n c y i n t h e l e a d d i o x i d e s t r u c t u r e ( 3 2 ) . P o t e n t i a l - p H d i a g r a m s ( o r P o u r b a i x d i a g r a m s ) show t h e r e g i o n s o f t he rmodynamic s t a b i l i t y o f l e a d and l e a d compounds ( 3 4 ) . From t h e r m o - dynamic p r e d i c t i o n s l e a d c a n be u s e d as an anode a t h i g h e l e c t r o d e 26 p o t e n t i a l s f o r pH v a l u e s be tween 0 and 12 w i t h o u t a p p r e c i a b l e c o r r o s i o n . Under s u c h c o n d i t i o n s t h e m e t a l w i l l be c o v e r e d w i t h a l a y e r o f P b 0 2 . D e l a h a y e t a l (35) have c o n s t r u c t e d t h e p o t e n t i a l - p H d i a g r a m f o r l e a d i n t h e p r e s e n c e o f s u l f a t e i o n s (1 g - i o n / 1 ) . Many e l e c t r o d e r e a c t i o n s a r e t h e r m o d y n a m i c a l l y p o s s i b l e a t d i f f e r e n t p o t e n t i a l s and p H s . From t h e s e , t h e most s t u d i e d r e a c t i o n s a r e t h o s e t h a t f o r m t h e b a s i s o f t h e u n i v e r s a l l y u sed l e a d a c i d s t o r a g e b a t t e r y , P b 0 2 + 2e~ + 4 H + + S O u ~ 2 t PbSOi* + 2 H 2 0 V° = 1 .685 [R17] Pb + SOLT 2 t PbSOtt + 2e~ V° = - 0 . 3 5 6 [R18] The r e a c t i o n s o c c u r i n t h e i n d i c a t e d d i r e c t i o n d u r i n g d i s c h a r g e and i n t h e o p p o s i t e d i r e c t i o n d u r i n g c h a r g i n g ( 2 3 ) . I t i s w e l l known t h a t t h e mech - a n i s m o f d i s c h a r g e o f P b 0 2 i n t h e p r e s e n c e o f e x c e s s s u l f a t e i n v o l v e s b l o c k i n g o f t h e P b 0 2 s u r f a c e w i t h a d e p o s i t o f PbSOLv (32) . These r e a c - t i o n s may a l s o be o f i m p o r t a n c e i n t h e o x i d a t i o n o f p h e n o l i n s u l p h u r i c a c i d m e d i a . The s u i t a b i l i t y o f l e a d d i o x i d e as an anode m a t e r i a l has been known f o r many y e a r s . I t i s c l e a r t h a t l e a d d i o x i d e i s a b l e t o w i t h s t a n d p r o l o n g e d h i g h a n o d i c p o t e n t i a l s more e f f e c t i v e l y t h a n g r a p h i t e ( w h i c h u n d e r g o e s d e g r e d a t i o n ) . A l s o , l e a d d i o x i d e p o s s e s s e s a r e l a t i v e l y h i g h o x y g e n o v e r v o l t a g e ( 1 0 , 3 6 ) o f t h e same o r d e r o f m a g n i t u d e as p l a t i n u m and i s much c h e a p e r . P r e p a r a t i o n . L e a d d i o x i d e c a n be p r e p a r e d as a c o a t i n g on l e a d by a n o d i z a t i o n , o r d e p o s i t e d o n t o o t h e r m e t a l s by e l e c t r o d e p o s i t i o n . a) A n o d i z a t i o n . The most c o n v e n t i o n a l a n o d i z a t i o n method i s t o p u t t h e l e a d i n c o n t a c t w i t h an aqueous H 2 S 0 t | e l e c t r o l y t e and p r o v i d e a f l o w o f c u r r e n t u n t i l oxygen e v o l u t i o n i s p l a i n l y v i s i b l e , and t h e g r e y l e a d has a c q u i r e d t h e c h a r a c t e r i s t i c b l a c k d e p o s i t o f l e a d d i o x i d e ( 2 6 , 3 3 ) . I t i s w e l l known t h a t o x y g e n i s e v o l v e d f rom a l e a d anode o n l y when a l a y e r o f l e a d d i o x i d e has been l a i d down ( 3 2 ) . A f t e r a n o d i z a t i o n , t h e e l e c t r o d e s h o u l d n o t be l e f t i n c o n t a c t w i t h s u l f a t e i o n s t o a v o i d l o s s e s by r e d u c t i v e p r o c e s s e s ( R e a c t i o n R17) and s h o u l d be u sed as soon as p o s s i b l e ( 2 6 ) . Some a u t h o r s ( 3 7 , 3 8 ) have p o s t u l a t e d t h a t a s h o r t c o m i n g o f t h e p r e p a r a t i o n o f Pb02 by a n o d i z a t i o n o f Pb i s t h a t a s o l i d phase r e a c t i o n o c c u r s be tween t h e Pb02 and t h e u n d e r l y i n g Pb t o p r o d u c e t h e l e s s c o n - d u c t i v e PbO, P b 0 2 + Pb t 2 PbO [R19] Howeve r , a n o t h e r ' s t u d y (39) r e p o r t s t h a t t h e o n l y p r o d u c t s o f t h e a n o d i - z a t i o n o f l e a d i n H2SO4 o b s e r v e d by x - r a y d i f f r a c t i o n , were c t -Pb02, B - P b 0 2 and PbSOt+j b u t no PbO was d e t e c t e d , even a f t e r s e v e r a l weeks o f s t o r a g e o f t h e e l e c t r o d e i n t h e d r y s t a t e . I n t h e same s t u d y a mechan i sm i s p r o p o s e d f o r t h e a n o d i z a t i o n o f l e a d . F i r s t , PbSO^ i s formed f rom Pb a t t h e P b / P b S O ^ p o t e n t i a l , and l a t e r when t h e p o t e n t i a l r i s e s t o t h e o x y g e n o v e r p o t e n t i a l v a l u e , t h e PbSOij f i l m t r a n s f o r m s t o 8-Pb02 and t h e u n d e r l y i n g g r i d m e t a l i s c o n v e r t e d d i r e c t l y t o a - P b 0 2 - I t has been r e p o r t e d (36) t h a t a n o d i z e d l e a d c a n n o t t o l e r a t e t h e p r e s e n c e o f c h l o r i d e i o n s w h i c h cause i t t o d i s i n t e g r a t e . b ) E l e c t r o d e p o s i t i o n . T h e r e a r e s e v e r a l methods f o r t h e e l e c t r o - d e p o s i t i o n o f Pb02 on i n e r t m e t a l s f r o m e l e c t r o l y t e s c o n t a i n i n g l e a d . Some o f t h e methods a r e summar ized i n r e f e r e n c e ( 3 2 ) . Pb02 has been d e p o s i t e d on n i c k e l , t a n t a l u m , p l a t i n u m , c a r b o n , o r g r a p h i t e . M o s t o t h e r m e t a l s a r e u n s u i t a b l e b e c a u s e o f t h e i r i n h e r e n t l y easy o x i d a t i o n ( 2 6 ) . S e v e r a l t y p e s o f e l e c t r o l y t e s have been u s e d f o r t h e d e p o s i t i o n o f P b 0 2 , and o f t h e s e , l e a d n i t r a t e has been f o u n d t o g i v e 28 t h e . b e s t d e p o s i t s ( 4 0 ) . The l a r g e s t p r o d u c e r s o f c o m m e r c i a l e l e c t r o d e p o s i t e d l e a d d i o x i d e anodes i n t h e w o r l d a r e P a c i f i c E n g i n e e r i n g and P r o d u c t i o n C o . o f Nevada and Sanwa C h e m i c a l C o . L t d . , o f T o k y o , J a p a n . S i n c e t h e b r e a k t h r o u g h by P a c i f i c w i t h a l e a d d i o x i d e c o a t e d g r a p h i t e a n o d e , t h e g r e a t e s t i n t e r e s t f o r P b 0 2 f o r m a t i o n has been shown i n t h e e l e c t r o d e p o s i t i o n p r o c e s s ( 3 6 ) . The e l e c t r o d e p o s i t i o n on g r a p h i t e u s e s a l e a d n i t r a t e e l e c t r o l y t e i n a c i d m e d i a as d e s c r i b e d by G i b s o n ( 4 1 ) . The t e t r a g o n a l 3 - P b 0 2 i s t h e f o r m found i n t h e c o m m e r c i a l anodes p r o d u c e d f rom a c i d l e a d n i t r a t e b a t h s . The a - f o r m i s l e s s common and c a n be d e p o s i t e d f rom a l k a l i n e s o l u t i o n s ( 2 6 , 3 6 ) . U n l i k e t h e a n o d i z e d l e a d , t he e l e c t r o d e p o s i t e d P b 0 2 c a n o p e r a t e e f f e c t i v e l y i n c h l o r i d e c o n c e n t r a t i o n s c l o s e t o s a t u r a t i o n . I n f a c t t h e l e a d d i o x i d e anodes p r o d u c e d by P a c i f i c and Sanwa C o . a r e i n use f o r p e r c h l o r a t e m a n u f a c t u r e . ' CHAPTER 3 OBJECTIVES The a i m o f t h i s work was t o s t u d y t h e e l e c t r o c h e m i c a l o x i d a t i o n o f p h e n o l f o r w a s t e t r e a t m e n t a p p l i c a t i o n s . A p a c k e d bed anode was s e l e c t e d b e c a u s e i t p r o v i d e s l a r g e r e l e c t r o d e s u r f a c e a r e a s p e r u n i t c e l l v o l u m e compared t o a s i m p l e f l a t p l a t e e l e c t r o d e . T h i s i s p a r t i c - u l a r l y i m p o r t a n t where d i l u t e s o l u t i o n s a r e t o be t r e a t e d . The r e s e a r c h r e p o r t e d h e r e i n c l u d e s t h e d e s i g n and c o n s t r u c t i o n o f equ ipmen t t o c a r r y ou t t h e p r o c e s s and an e x p e r i m e n t a l s t u d y o f t h e e f f e c t o f i m p o r t a n t o p e r a t i n g v a r i a b l e s . These v a r i a b l e s i n c l u d e t y p e o f l e a d d i o x i d e anode ( a n o d i z e d l e a d v e r s u s e l e c t r o d e p o s i t e d l e a d d i o x i d e ) , c e l l c o n f i g u r a t i o n ( d i v i d e d o r u n d i v i d e d c e l l ) , t y p e o f i o n - s e l e c t i v e membrane ( a n i o n i c o r c a t i o n i c ) , c u r r e n t a p p l i e d , pH o f t h e e l e c t r o l y t e , c o n d u c t i v i t y o f t h e e l e c t r o l y t e , p h e n o l c o n c e n t r a t i o n , f l o w - r a t e , and p a r t i c l e s i z e . L e a d d i o x i d e was s e l e c t e d as t h e anode m a t e r i a l t o c a r r y ou t t h i s s t u d y f o r t h e r e a s o n s g i v e n i n C h a p t e r 2 . The p e r f o r m a n c e o f a n o d i z e d l e a d and e l e c t r o d e p o s i t e d l e a d d i o x i d e i s compared i n t e rms o f p h e n o l o x i d a t i o n b u t a l s o some t e s t s were made t o compare them i n t e rms o f c o r r o s i o n r e s i s t a n c e . The use o f a d i v i d e d o r u n d i v i d e d c e l l i s o f i m p o r t a n c e , s i n c e i t may c o m p l e t e l y change t h e r e a c t i o n mechan i sm f o r t he f u r t h e r o x i d a t i o n o f i n t e r m e d i a t e s . T h e r e i s l i t t l e i n f o r m a t i o n i n t h e l i t e r a t u r e on s u c h e f f e c t s . 29 The e l e c t r o l y t e s t o be u s e d f o r t h e o x i d a t i o n o f p h e n o l c o n s i s t o f m i x t u r e s o f Na2S0i + and H^SO^ o r Na2S0L; and NaOH, t o be a b l e t o v a r y i n d e p e n d e n t l y pH and c o n d u c t i v i t y o f t h e e l e c t r o l y t e s . From t h e p o i n t o f v i e w o f w a s t e t r e a t m e n t , i t i s o f i n t e r e s t t o d e t e r m i n e what f r a c t i o n o f t h e p h e n o l i s c o n v e r t e d t o c a r b o n d i o x i d e , o r how much o r g a n i c c a r b o n r e m a i n s i n s o l u t i o n a f t e r t h e e l e c t r o c h e m i c a l t r e a t m e n t . T h e r e f o r e , t h e e f f e c t o f t h e i m p o r t a n t v a r i a b l e s i s r e p o r t e d n o t o n l y i n t e rms o f p h e n o l o x i d a t i o n b u t a l s o on t h e t o t a l o r g a n i c c a r b o n ( T . O . C . ) o x i d a t i o n . I n t h e r e v i e w e d l i t e r a t u r e no T . O . C . a n a l y s e s have b e e n r e p o r t e d . I n some c a s e s c h e m i c a l o x y g e n demand ( C . O . D . ) h a v e been r e p o r t e d . B u t t h i s i s n o t an a d e q u a t e t e c h n i q u e o f a n a l y s i s i n t h e c a s e o f a r o m a t i c compounds ( 4 2 ) . R e l a t i v e l y l o w p h e n o l c o n c e n t r a t i o n s a r e u s e d i n t h i s s t u d y (up t o 1100 mg/1) i n o r d e r t o i n v e s t i g a t e t h e p r o c e s s u n t i l p r a c t i c a l l y t o t a l p h e n o l o x i d a t i o n i s a c h i e v e d f o r t h e r a n g e o f o p e r a t i n g c o n d i t i o n s o f t h e e x p e r i m e n t s . A b a t c h - r e c i r c u l a t i o n s y s t e m was s e l e c t e d as t h e o p e r a t i n g mode t o s t u d y t h e e f f e c t o f some o f t h e v a r i a b l e s . Once t h e s y s t e m was b e t t e r u n d e r s t o o d and c o n t r o l l e d , some e x p e r i m e n t s were p e r f o r m e d i n t h e c o n t i n - uous mode. F i n a l l y , t h e e x p e r i m e n t a l f r a c t i o n a l c o n v e r s i o n s o f p h e n o l a r e com- p a r e d w i t h t h e mass t r a n s f e r m o d e l f o r a b a t c h r e c i r c u l a t i o n o p e r a t i o n , and a s i m p l i f i e d m o d e l i n c l u d i n g e l e c t r o c h e m i c a l r e a c t i o n c o n t r o l i s p r e s e n t e d i n o r d e r t o a n a l y z e t h e d a t a f rom t h e c o n t i n u o u s e x p e r i m e n t s and compare t h e mass t r a n s f e r and e l e c t r o c h e m i c a l r e a c t i o n r e s i s t a n c e s . CHAPTER 4 EXPERIMENTAL APPARATUS AND METHODS 4 . 1 A p p a r a t u s 4 . 1 . 1 C e l l d e s i g n The e l e c t r o l y t i c c e l l c o n s i s t s o f a s t a c k o f e l e m e n t s a r r a n g e d i n s e r i e s and compres sed by a c lamp m e c h a n i s m . T h i s f l e x i b l e d e s i g n i s u s e d b e c a u s e i t p e r m i t s t h e a s s e m b l y o f d i f f e r e n t a r r a n g e m e n t s ( d i v i d e d o r u n d i v i d e d c e l l ) and s i m p l i f i e s w o r k w i t h e l e c t r o d e m a t e r i a l s , a) D i v i d e d c e l l A s i d e v i e w o f t h e d i v i d e d c e l l a r r angemen t i s shown i n F i g . 4 . B a s i c a l l y , t h e c e l l c o n s i s t s o f two f l a t p l a t e s , t h e anode and c a t h o d e c u r r e n t f e e d e r s , w h i c h a r e i n c o n t a c t w i t h t h e a n o d i c and c a t h o d i c p a c k i n g s . B o t h p a c k i n g s a r e c o n t a i n e d i n 3 mm t h i c k s l o t t e d n e o p r e n e g a s k e t s and a r e s e p a r a t e d f r o m e a c h o t h e r by an i o n - s e l e c t i v e membrane w h i c h p r e v e n t s t h e m i x i n g o f t h e a n o l y t e and c a t h o l y t e . The c a t h o d i c p a c k i n g i s u sed t o p r e v e n t t h e membrane f rom s a g g i n g due t o t h e w e i g h t o f t h e a n o d i c p a c k i n g . The a n o l y t e and c a t h o l y t e i n l e t s a r e l o c a t e d a t t h e b o t t o m o f t h e c e l l and t h e o u t l e t s a t t h e t o p , t o f a c i l i t a t e t h e e x i t o f t h e gases t h a t w i l l be p r o d u c e d d u r i n g t h e e l e c t r o l y s i s . Two d i f f e r e n t k i n d s o f l e a d d i o x i d e anode p l a t e s a r e u sed i n t h i s s t u d y : a n o d i z e d l e a d s h e e t and e l e c t r o d e p o s i t e d l e a d d i o x i d e on g r a p h i t e p l a t e . I n t h e c a s e o f t h e a n o d i z e d l e a d e l e c t r o d e t h e c u r r e n t f e e d e r p l a t e 31 F i g . 4 . S i d e v i e w o f t h e g e n e r a l d i v i d e d - c e l l Legend a = 1.6 mm t h i c k neoprene i n s u l a t o r b = 1.6 mm t h i c k c a t h o d i c f e e d e r p l a t e ( s . s . 316 p l a t e ) c = 3 mm t h i c k s l o t t e d neoprene g a s k e t c o n t a i n i n g c a t h o d i c p a c k i n g d = i o n s e l e c t i v e membrane a g a i n s t p r o t e c t i v e p l a s t i c s c r e e n ( v a r i a b l e t h i c k n e s s ) e = 3 mm t h i c k s l o t t e d neoprene g a s k e t c o n t a i n i n g a n o d i c p a c k i n g f = a n o d i c c u r r e n t f e e d e r : 3 mm t h i c k l e a d p l a t e , o r l e a d d i o x i d e c o a t e d g r a p h i t e p l a t e 3 cm t h i c k gement . (no s c a l e ) 33 was cut from a 0.3 cm t h i c k lead sheet. The d e t a i l e d front and side views of the anode chamber when using such electrode are shown i n F i g . 5. A neoprene gasket determines the cross s e c t i o n a l area of the feeder p l a t e that w i l l be transporting the current. The side view shows that an extra s t a i n l e s s s t e e l p late i s used mainly to f a c i l i t a t e the welding of the i n l e t and out l e t connectors to the c e l l and also to give more strength to the lead sheet. The front and side views of the cathode chamber are the same as the anode chamber, except that the thickness of the s.s. 316 cathode feeder was 0.16 cm. The electrodeposited lead dioxide on graphite p l a t e was obtained from P a c i f i c Engineering Co. The t o t a l thickness of the p l a t e i s 3 cm and the thickness of the lead dioxide coating on each side of the graphite i s 0.2 cm. Some modifications had to be made to the o r i g i n a l commercial electrode to adapt i t to the c e l l design being used. The f i n a l front and side views of the electrodedeposited Pb02 anode are shotvn i n F i g . 6. To avoid p o s s i b l e cracking of the Pb02 coating, the electrode was l e f t with i t s o r i g i n a l width of 15 cm. A mo d i f i c a t i o n had to be made i n order to introduce the e l e c t r o l y t e flow through the graphite coated p l a t e . A d e t a i l of the connection adapted i s shown i n F i g . 7. The e l e c t r o l y t e never comes i n contact with the graphite base p l a t e because the nylon connection was in s u l a t e d by means of a neoprene washer. This type of connection prevents corrosion .. of the graphite base and eventual d e t e r i o r a t i o n of the lead dioxide l a y e r . Some fundamental s p e c i f i c a t i o n s o f the d i f f e r e n t elements of the c e l l are given i n Table 4. The. dimensions of the anode and cathode chambers were never changed, but d i f f e r e n t s i z e s of anodic packings were used. The:cathodic packing consisted of several s t a i n l e s s steel-304 screens Legend a = l e a d s h e e t anode f e e d e r b = s l o t t e d neoprene g a s k e t c = e l e c t r o l y t e o u t l e t d = anode p a c k i n g ( l e a d s h o t ) e = e l e c t r o l y t e i n l e t f = neoprene i n s u l a t o r g = s t a i n l e s s s t e e l 316 p l a t e (where c o n n e c t o r s a r e we lded ) F i g . 5 . F r o n t and s i d e v i e w s o f t h e anode chamber f o r t he a n o d i z e d l e a d e l e c t r o d e . (no s c a l e ) . Legend a = u n c o a t e d g r a p h i t e s e c t i o n o f t h e anode f e e d e r b = e l e c t r o l y t e o u t l e t c = anode p a c k i n g d = s l o t t e d neoprene g a s k e t e = e l e c t r o d e p o s i t e d Pb02 s e c t i o n o f t h e f e e d e r p l a t e f = e l e c t r o l y t e i n l e t g = neoprene i n s u l a t o r F i g . 6 . F r o n t and s i d e v i e w s o f t h e anode chamber f o r t h e e l e c t r o - d e p o s i t e d PbC>2 e l e c t r o d e , (no s c a l e ) 2.54 cm A 6.3 mm Legend a = n y l o n c o n n e c t i o n b = s t a i n l e s s s t e e l mesh c = neoprene washe r d = l e a d d i o x i d e l a y e r e = g r a p h i t e base p l a t e f = c o m p r e s s i n g nu t ( t h r e a d e d ) F i g . 7 . D e t a i l o f t h e i n l e t o r o u t l e t c o n n e c t i o n a d a p t e d on the e l e c t r o d e p o s i t e d Pb02 on g r a p h i t e anode . (no s c a l e ) u> ON TABLE 4 FUNDAMENTAL SPECIFICATIONS OF THE ELECTROLYTIC C E L L D i m e n s i o n s o f t h e anode and c a t h o d e c h a m b e r s : L e n g t h = 38 cm W i d t h = 5 cm T h i c k n e s s = 3 mm A n o d i c p a c k i n g s : P a r t i c l e s i z e (mm) l e a d s h o t ( t o a n o d i z e ) 2 e l e c t r o d e p o s i t e d l e a d d i o x i d e 1 . 7 - 2 . 0 0 0 . 7 - 1 . 1 C a t h o d i c p a c k i n g : s t a i n l e s s s t e e l - 3 0 4 s c r e e n s (20 x 20 mesh) c r o s s s e c t i o n a l a r e a o f t h e s c r e e n s (38 x 5)cm Membranes : c a t i o n i c : IONAC MC 3142 IONAC MC 3470 NAFION 127 a n i o n i c : IONAC MA 3475 P r o t e c t i v e s c r e e n s : s a r a n p o l y p r o p y l e n e 2 mm 2.54 cm Jl E o O OJ C: -6 cm E o CM . 6 E o •3- m c\j G: CLAMP WELDED TO SQUARE TUBES 0 i G= -5 cm- l a ) P L A N VIEW . F i g . 8 . D e t a i l o f t h e mechan i sm used to h o l d the c e l l . ( b ) FRONT VIEW (no s c a l e ) 39 (20 mesh) c u t t o t h e s i z e o f t h e c a t h o d e chamber (5 x 38 c m 2 ) and j o i n e d so t h a t t h e t o t a l t h i c k n e s s o f t h e p a c k i n g was 0 . 3 cm. The d i f f e r e n t i o n s e l e c t i v e membranes t e s t e d i n t h i s s t u d y a r e l i s t e d i n T a b l e 4 , a l o n g w i t h t h e p r o t e c t i v e p l a s t i c s c r e e n s . The p r o p e r t i e s and c h a r a c - t e r i s t i c s o f t h e s e m a t e r i a l s as s u p p l i e d by t h e m a n u f a c t u r e r s a r e g i v e n i n A p p e n d i x 1. A d e t a i l o f t h e mechan i sm used t o compress t h e v a r i o u s p a r t s o f t h e c e l l i s g i v e n i n F i g . 8. I t c o n s i s t s o f f o u r m i l d s t e e l s q u a r e t u b e s w h i c h a r e w e l d e d t o s i x C - c l a m p s ( J o r g e n s e n , S t y l e 8 1 ) . The c e l l i s i n t r o d u c e d t h r o u g h t h e u p p e r p a r t o f t h e p r e s s mechan i sm, and once t h e C-c l amp s c r e w s a r e t i g h t e n e d , t h e f o u r s q u a r e t u b e s compress t h e neop rene g a s k e t s , p r o v i d i n g an e f f e c t i v e s e a l f o r t h e c e l l . T h i s v e r s a t i l e p r e s s d e s i g n p e r m i t s v a r i a t i o n s i n t h e t h i c k n e s s and w i d t h o f t h e c e l l m a t e r i a l s w i t h i n a c e r t a i n r a n g e , and a l s o c a n be r a p i d l y opened and c l o s e d . b) U n d i v i d e d c e l l A s i d e v i e w o f t h e u n d i v i d e d c e l l a r r angemen t i s s i m i l a r t o t h a t r e p r e s e n t e d i n F i g . 4 , e x c e p t t h a t t h e i o n s e l e c t i v e membrane and t h e c a t h o d i c p a c k i n g a r e e l i m i n a t e d and t h e i n l e t and o u t l e t o f t h e c a t h o d e s i d e a r e c l o s e d by u s i n g a c a t h o d e f e e d e r p l a t e w i t h o u t h o l e s . I n t h i s c a s e , o n l y a p l a s t i c s c r e e n ( s a r a n o r p o l y p r o p y l e n e ) i s p l a c e d be tween t h e a n o d i c bed and t h e c a t h o d i c f e e d e r p l a t e . 4 . 1 . 2 F l o w d i a g r a m o f t h e a p p a r a t u s F i g u r e 9 i s t h e s c h e m a t i c f l o w d i a g r a m . Equ ipmen t s p e c i f i c a t i o n s a r e g i v e n i n A p p e n d i x 1. Two m a i n f l o w c i r c u i t s e x i s t . A t t h e r i g h t hand s i d e o f t h e e l e c t r o - J l y t i c c e l l i s t h e a n o l y t e f l o w c i r c u i t . Pump P U - 1 d e l i v e r s t h e a n o l y t e F i g . 9 . F l o w d i a g r a m o f the a p p a r a t u s . o 41 Legend f o r F i g . 9 . P . S . ) Power s u p p l y ( D . C . ) V) V o l t m e t e r E . C . ) E l e c t r o l y t i c c e l l T - l ) A n o l y t e t a n k ( c o n t a i n s p h e n o l s o l u t i o n ) T - 2 ) A n o d i z a t i o n t a n k T - 3 ) C a t h o l y t e t a n k T - 4 ) W a s h i n g t a n k P U - 1 ) A n o l y t e pump P U - 2 ) C a t h o l y t e pump R - l ) A n o l y t e r o t a m e t e r R - 2 ) C a t h o l y t e r o t a m e t e r P - l ) A n o l y t e p r e s s u r e and t e m p e r a t u r e gauges P - 2 ) C a t h o l y t e p r e s s u r e and t e m p e r a t u r e gauges F - l ) A n o l y t e f i l t e r F - 2 ) C a t h o l y t e f i l t e r G L - 1 ) G a s - l i q u i d s e p a r a t o r f o r t h e a n o l y t e G L - 2 ) G a s - l i q u i d s e p a r a t o r f o r t h e c a t h o l y t e V - 1 ) A n o l y t e t a n k s h u t - o f f v a l v e V - 2 ) A n o d i z a t i o n t a n k s h u t - o f f v a l v e V - 3 ) C a t h o l y t e t a n k s h u t - o f f v a l v e V - 4 ) W a s h i n g t a n k s h u t - o f f v a l v e V - 5 ) A n o l y t e f l o w c o n t r o l v a l v e V - 6 ) C a t h o l y t e f l o w c o n t r o l v a l v e V - 7 ) Ca thode chamber p r e s s u r e - c o n t r o l v a l v e V - 8 ) L i q u i d s amp le v a l v e V - 9 ) L i q u i d l e v e l c o n t r o l v a l v e i n G L - 1 V - 1 0 ) L i q u i d l e v e l c o n t r o l v a l v e i n G L - 2 42 f r o m t a n k s T - l o r T - 2 t o t h e anode chamber . The l i q u i d f l o w r a t e i s c o n t r o l l e d by a d j u s t i n g v a l v e V - 5 and i s measured w i t h r o t a m e t e r R - l . P r e s s u r e and t e m p e r a t u r e o f t h e a n o l y t e a t t h e e n t r a n c e o f t h e a n o l y t e chamber a r e measured i n P - l . F i l t e r F - l i s l o c a t e d a t t h e o u t l e t o f t h e anode chamber , t o c o l l e c t s m a l l p a r t i c l e s t h a t m i g h t be w i t h d r a w n f r o m the c e l l , t h u s p r o t e c t i n g t h e pump f rom damage. T h i s g l a s s - w o o l f i l t e r s e r v e s a l s o t o a g g l o m e r a t e s m a l l gas b u b b l e s p r o d u c e d i n t h e e l e c t r o l y s i s i n t o b i g g e r ones t o f a c i l i t a t e t h e gas l i q u i d s e p a r a t i o n i n G L - 1 . The g a s - l i q u i d m i x t u r e e n t e r s a t t h e b o t t o m o f t h e g a s - l i q u i d s e p a r a t o r G L - 1 i n w h i c h a bed o f g l a s s beads p r o v i d e s e x t r a a g g l o m e r a t i o n s u r f a c e f o r t h e gas b u b b l e s . V a l v e V - 9 c o n t r o l s t h e l i q u i d l e v e l a t t h e o u t l e t o f G L - 1 , t o e n s u r e t h a t gas b u b b l e s a r e n o t c a r r i e d ou t w i t h t h e l i q u i d f l o w . T h i s w o u l d r e s u l t i n a p r o g r e s s i v e a c c u m u l a t i o n o f gas i n t h e a n o l y t e l i n e w h i c h may a f f e c t t h e r e s u l t s o f t he e x p e r i m e n t s . The gas i s t h e n r e l e a s e d a t t h e t o p o f G L - 1 and t h e l i q u i d f l o w s t o w a r d s t h e f e e d t a n k s ( T - l o r T - 2 ) t o be r e c y c l e d t o t h e c e l l . The d o t t e d l i n e r e p r e s e n t s t h e r e c y c l e l i n e when t h e a n o d i z a t i o n t a n k T - 2 i s i n u s e . V a l v e V - 8 s e r v e s t o c o l l e c t l i q u i d s amp le s a f t e r p a s s a g e t h r o u g h t h e c e l l . The f l o w d i a g r a m c o r r e s p o n d i n g t o t h e c a t h o l y t e c i r c u i t i s b a s i c a l l y a n a l o g o u s t o t h e a n o l y t e c i r c u i t above d e s c r i b e d . V a l v e V - 7 s e r v e s t o c o n t r o l t h e p r e s s u r e i n t h e c a t h o l y t e chamber , p r o v i d i n g p r e s s u r e e q u a l - i z a t i o n a t b o t h s i d e s o f t h e membrane, t h u s a v o i d i n g t o o h i g h p r e s s u r e d i f f e r e n c e s be tween t h e a n o l y t e and c a t h o l y t e chambers t h a t may r e s u l t i n membrane b r e a k i n g . When t h e c e l l was a s s e m b l e d w i t h o n l y one chamber , o n l y t h e a n o l y t e c i r c u i t was u s e d . The c a t h o l y t e c i r c u i t was e l i m i n a t e d by c l o s i n g t h e 43 c a t h o l y t e i n l e t and o u t l e t t o t h e c e l l . The c e l l was powered by a 1 KVA D . C . power s u p p l y ( A p p e n d i x 1 ) . The c e l l c u r r e n t was r e a d f rom t h e p o w e r - s u p p l y m e t e r , and t h e v o l t a g e d r o p a c r o s s t h e e l e c t r o d e s was measured i n d e p e n d e n t l y . 4 . 2 E x p e r i m e n t a l methods 4 . 2 . 1 B a t c h e x p e r i m e n t s The e x p e r i m e n t a l p r o c e d u r e i s d e s c r i b e d f o r t h e more c o m p l i c a t e d two c h a m b e r s - c e l l o p e r a t i o n , s i n c e the one-chamber c e l l o p e r a t i o n can be c o n s i d e r e d as a p a r t i c u l a r c a s e o f t h e f i r s t , a) A n o d i z a t i o n p r o c e s s B e f o r e each e x p e r i m e n t , t h e l e a d e l e c t r o d e was a n o d i z e d by e l e c t r o - l y s i s i n 20% l ^ S O ^ ( 4 3 ) , t o e n s u r e t h a t t h e anode was e q u a l l y a c t i v e b e f o r e e v e r y r u n . V a l v e s V - 1 and V - 4 were s h u t o f f and t a n k s T - 2 and T - 3 were f i l l e d w i t h a 20% H2S01+ s o l u t i o n . The D . C . power s u p p l y was t u r n e d o n . V a l v e s V - 2 and V - 3 were t h e n opened and b o t h pumps, P U - 1 and P U - 2 , were a c t i v a t e d a t t h e same t i m e . A b o u t 2 & o f s o l u t i o n coming o u t f rom t h e g a s - l i q u i d s e p a r a t o r s was w i t h d r a w n a t each s i d e t o pu rge t h e s y s t e m b e f o r e t h e a n o l y t e and c a t h o l y t e f l o w s were r e c y c l e d t o t a n k s T - 2 and T - 3 r e s p e c t i v e l y . F i v e l o f r^SOtj s o l u t i o n r e m a i n e d i n e a c h t a n k f o r t h e a n o d i z a t i o n p r o c e s s . I m m e d i a t e l y t h e c u r r e n t was a d j u s t e d t o 10 A ( c . d . = 5 2 6 . 3 A / m 2 ) and t h e l i q u i d p r e s s u r e s i n b o t h chambers were e q u a l i z e d by a d j u s t i n g v a l v e V - 7 . When l e a d was t o be a n o d i z e d f o r t h e f i r s t t i m e , a 12 h a n o d i z a t i o n t i m e was a l l o w e d , b u t f o r s u c c e s s i v e e x p e r i m e n t s t h e s t a n d a r d a n o d i z a t i o n t i m e was 1 h . ( C h o i c e o f t h e a n o d i z a t i o n t i m e was j u s t i f i e d e x p e r i m e n - t a l l y , as shown i n C h a p t e r 5 . ) A f t e r a n o d i z a t i o n , b o t h pumps were 44 s i m u l t a n e o u s l y t u r n e d o f f and v a l v e s V - 2 and V - 3 were c l o s e d . T h e n , t a n k s T - l and T-4 were f i l l e d w i t h d i s t i l l e d w a t e r and c o n n e c t e d t o pumps P U - 1 and P U - 2 r e s p e c t i v e l y . The c e l l was washed u n t i l t h e c u r r e n t d r o p p e d p r a c t i c a l l y t o z e r o and t h e p o t e n t i a l d i f f e r e n c e t h r o u g h t h e c e l l i n c r e a s e d i n d i c a t i n g t h a t e s s e n t i a l l y no e l e c t r o l y t e was c o n t a i n e d i n t h e c e l l . b) P h e n o l e l e c t r o c h e m i c a l o x i d a t i o n p r o c e s s A f t e r t h e c e l l was t h o r o u g h l y w a s h e d , 8 I o f a n o l y t e s o l u t i o n were p r e p a r e d i n t a n k T - l . The c o n c e n t r a t i o n o f p h e n o l , t h e pH and t h e c o n - d u c t i v i t y o f t h e a n o l y t e were s e t t o t h e d e s i r e d l e v e l s by a d d i n g t h e n e c e s s a r y v o l u m e s o f s t o c k s o l u t i o n s o f p h e n o l , NaOH o r ^ S O ^ , and N32S01+, w h i c h had been p r e v i o u s l y p r e p a r e d . The t a n k was w e l l a g i t a t e d b e f o r e t h e i n i t i a l s ample was t a k e n , and pH and c o n d u c t i v i t y were measured and r e a d j u s t e d i f n e c e s s a r y . An e q u a l v o l u m e o f c a t h o l y t e s o l u t i o n was t h e n p r e p a r e d i n t a n k T - 3 and c o n d u c t i v i t y and pH were m e a s u r e d . A n o l y t e and c a t h o l y t e d a t a a r e r e c o r d e d f o r e a c h e x p e r i m e n t i n A p p e n d i x 2 . Tanks T - l and T - 3 were c o n n e c t e d t o t h e c o r r e s p o n d i n g pumps and f l o w r a t e s o f a n o l y t e and c a t h o l y t e were s e t by a d j u s t i n g v a l v e s V - 5 and V - 6 r e s p e c t i v e l y . P r e s s u r e e q u a l i z a t i o n was p r o v i d e d a d j u s t i n g v a l v e V - 7 . I m m e d i a t e l y t h e c u r r e n t was s e t a t t h e d e s i r e d v a l u e . As t h e o p e r a t i n g c o n d i t i o n s were b e i n g s e t 3 i o f t h e s o l u t i o n s c o m i n g o u t f r o m t h e gas l i q u i d s e p a r a t o r s were d i s c a r d e d i n o r d e r t o p u r g e t h e s y s t e m and a l s o t o p r o v i d e some t i m e f o r f l o w s and c u r r e n t s t a b i l i z a - t i o n . A t t h e moment t h e l i q u i d s were r e c y c l e d t o t a n k s T - l and T - 3 , 5 H o f e l e c t r o l y t e r e m a i n e d i n e a c h t a n k . The e l e c t r o l y s i s t i m e was measured f r o m t h e moment when t h e a n o l y t e was r e c y c l e d t o t a n k T - l . T h i r t y m l s a m p l e s w e r e t a k e n i n i n t e r v a l s o f 15 m i n f o r p h e n o l a n a l y s i s by o p e n i n g v a l v e V - 8 . When t h e u s u a l e l e c t r o l y s i s t i m e o f 2 h was com- p l e t e d , t h e c e l l was washed w i t h d i s t i l l e d w a t e r w h i l e t h e c u r r e n t was s t i l l f l o w i n g t o a v o i d r e d u c t i o n o f t h e Pb02 a n o d e . The s a m p l e s w e r e f i r s t a n a l y z e d f o r p h e n o l and T . O . G . and l a t e r pH and c o n d u c t i v i t y w e r e m e a s u r e d , t o a v o i d p o s s i b l e c o n t a m i n a t i o n o f t h e s a m p l e s when i n t r o d u c i n g t h e pH and c o n d u c t i v i t y p r o b e s . 4 . 2 . 2 C o n t i n u o u s e x p e r i m e n t s Some e x p e r i m e n t s w e r e c a r r i e d o u t i n t h e c o n t i n u o u s mode ( w i t h o u t r e c y c l e ) , t o t e s t t h e e f f e c t o f v a r y i n g f l o w on p h e n o l o x i d a t i o n i n a s i n g l e p a s s t h r o u g h t h e u n d i v i d e d c e l l . The e l e c t r o d e p r e t r e a t m e n t o r a n o d i z a t i o n was c a r r i e d o u t by t h e . s t a n d a r d method o f e l e c t r o l y s i s w i t h 20% H2SO4 a t 10 A f o r 1 h . I n t h i s c a s e t h e p h e n o l s o l u t i o n t o be t r e a t e d was p r e p a r e d i n t h e same manner d e s c r i b e d p r e v i o u s l y b u t t h e t o t a l v o l u m e o f t h e e l e c t r o l y t e i n t a n k T - l was 20 L A f t e r t h e i n i t i a l e l e c t r o l y t e s amp l e was t a k e n , and c o n d u c t i v i t y and pH w e r e measu red and a d j u s t e d t o t h e d e s i r e d v a l u e s , t h e D . C . power s u p p l y was t u r n e d o n , and t h e e l e c t r o l y t e was f e d t o t h e c e l l . A l i q u i d f l o w r a t e was t h e n f i x e d by a d j u s t i n g v a l v e V - 5 , and t h e d e s i r e d c u r r e n t was s e t . F o u r I o f e l e c t r o l y t e w e r e w i t h d r a w n a t t h e o u t l e t o f G L - 1 b e f o r e t h e f i r s t s a m p l e was t a k e n , t o e n s u r e s t a b i l i z a t i o n o f t h e p r o c e s s . A d i f f e r e n t f l o w r a t e was t h e n s e t up and t h e same p r o c e d u r e r e p e a t e d , u n t i l t h e l i q u i d f l o w r a t e r a n g e p r o v i d e d b y r o t a m e t e r R - l was c o v e r e d . I n A p p e n d i x 2 t h e e x p e r i m e n t s a r e d i v i d e d b y g r o u p s a c c o r d i n g t o c e l l a s s e m b l y and o p e r a t i n g mode. 46 4 . 3 A n a l y t i c t e c h n i q u e s The s ample s t a k e n a t t h e o u t l e t o f t h e c e l l we re a n a l y z e d f o r c o n - c e n t r a t i o n s o f p h e n o l , t o t a l o r g a n i c c a r b o n , and i n some c a s e s , l e a d . 4 . 3 . 1 P h e n o l a n a l y s i s P h e n o l c o n c e n t r a t i o n s i n t h e samples were d e t e r m i n e d by gas c h r o m a - t o g r a p h y u s i n g a f l a m e i o n i z a t i o n d e t e c t o r . The a n a l y t i c equ ipmen t s p e c i f i c a t i o n s and o p e r a t i n g c o n d i t i o n s u sed a r e g i v e n i n A p p e n d i x 1. S t a n d a r d p h e n o l s o l u t i o n s r a n g i n g f rom 0-116 mg/1 were p r e p a r e d by p i p e t t i n g f rom t h e same p h e n o l s o l u t i o n u sed t o p r e p a r e t h e e l e c t r o l y t e f o r t h e e x p e r i m e n t s . Copper s u l f a t e was added t o t h e s t a n d a r d s t o an a p p r o x i m a t e c o n c e n t r a t i o n o f 1 g /1 t o p r e s e r v e them f rom p o s s i b l e b i o - l o g i c a l d e g r a d a t i o n ( 4 2 ) . P h e n o l peaks were t h i n enough so t h a t t h e e s t i m a t i o n o f t h e a r e a b e l o w t h e peak was no t n e c e s s a r y , and peak h e i g h t s c o u l d be u s e d . B e f o r e t h e a n a l y s i s o f t h e samples f rom each r u n , t h e p h e n o l s t a n d a r d s were a l w a y s i n j e c t e d and t h e c a l i b r a t i o n c u r v e o f peak h e i g h t s v s p h e n o l c o n c e n t r a t i o n was c o n s t r u c t e d . S t a n d a r d s and s ample s were i n j e c t e d u n t i l t h e v a r i a t i o n i n peak h e i g h t was n o t more t h a n 2-3%. T h i s means t h a t when t h e r e c o r d e r f u l l s c a l e c o r r e s p o n d e d t o abou t 100 mg/1 t h e maximum a l l o w e d v a r i a t i o n r e p r e - s e n t e d ± 2 m g / 1 , and t h e r e c o r d e r d e t e c t a b i l i t y was 1 m g / l / d i v i s i o n . No peaks o t h e r t h a n t h e s o l v e n t and p h e n o l peaks were o b s e r v e d , i n o t h e r words no i n t e r f e r e n c e o f t h e p h e n o l o x i d a t i o n p r o d u c t s was d e t e c t e d i n t h e G . C . a n a l y s i s . P h e n o l d e t e n t i o n t i m e was a p p r o x i m a t e l y 30 s e c o n d s . A l l t h e samples were a n a l y z e d on t h e day o f t h e e x p e r i m e n t , e v e n t h o u g h i t was shown t h a t t h e c o n c e n t r a t i o n o f p h e n o l i n t h e sample d i d 47 n o t v a r y a f t e r one week o f s t o r a g e . F o r t h o s e e x p e r i m e n t s p e r f o r m e d a t p h e n o l c o n c e n t r a t i o n s h i g h e r t h a n 100 mg/1 t h e samples were d i l u t e d by t h e n e c e s s a r y f a c t o r i n o r d e r t o w o r k w i t h t h e same s t a n d a r d s and d e t e c t a b i l i t y . 4 . 3 . 2 T o t a l o r g a n i c c a r b o n a n a l y s i s A l l t o t a l o r g a n i c c a r b o n a n a l y s e s were c a r r i e d ou t on a Beckman a n a l y z e r i n t h e C i v i l E n g i n e e r i n g D e p a r t m e n t . B a s i c a l l y , t h e t o t a l o r g a n i c c a r b o n a n a l y z e r c o n s i s t s o f two f u r n a c e s : t h e t o t a l c a r b o n f u r n a c e and t h e i n o r g a n i c c a r b o n f u r n a c e . The t o t a l c a r b o n f u r n a c e o p e r a t e s a t a t e m p e r a t u r e o f 1 0 0 0 ° C t o c o n v e r t a l l t h e c a r b o n c o n t a i n e d i n t h e sample t o c a r b o n d i o x i d e . The i n o r g a n i c c a r b o n f u r n a c e o p e r a t e s a t 1 5 0 ° C t o c o n v e r t o n l y the i n o r g a n i c c a r b o n c o n t a i n e d i n t h e sample t o c a r b o n d i o x i d e . The amount o f c a r b o n d i o x i d e t h u s p r o d u c e d i s d e t e c t e d i n an i n f r a r e d a n a l y z e r ( 4 4 ) . The t o t a l o r g a n i c c a r b o n ( T . O . C . ) i n t h e sample i s c a l c u l a t e d by s u b t r a c t i n g t h e i n o r g a n i c c a r b o n f rom t h e t o t a l c a r b o n , t h e r e f o r e two d i f f e r e n t c a l i b r a t i o n s a r e r e q u i r e d , one f o r e a c h c h a n n e l . The s t a n - d a r d s u s e d f o r t h e t o t a l c a r b o n a n a l y s i s were t h e same p h e n o l s t a n d a r d s u s e d t o c a l i b r a t e t h e c h r o m a t o g r a p h . Ca rbon c o n t e n t r a n g e d be tween 0 -90 m g / 1 . B e f o r e t h e a n a l y s i s o f t h e samples f r o m e a c h r u n , t h e t o t a l c a r b o n s t a n d a r d s were i n j e c t e d and t h e c a l i b r a t i o n c u r v e peak h e i g h t v s mg/1 c a r b o n c o n s t r u c t e d . V a r i a t i o n s i n peak h e i g h t i n s u c c e s s i v e i n j e c - t i o n s o f a c e r t a i n sample n e v e r exceeded 2%. The i n o r g a n i c c a r b o n s t a n d a r d s were p r e p a r e d f rom a s t o c k s o l u t i o n o f Na2C03 and NaHC03 c o n t a i n i n g 1000 mg/1 o f c a r b o n , so t h a t t h e i n o r g a n - i c c a r b o n i n t h e s t a n d a r d s a l s o r a n g e d be tween 0 -90 m g / 1 . A f t e r t h e c a l i b r a t i o n c u r v e o f mg/1 i n o r g a n i c c a r b o n v s peak h e i g h t was c o n s t r u c t e d , 48 t h e samples were i n j e c t e d i n t h e i n o r g a n i c c h a n n e l . A g a i n maximum v a r i - a t i o n s i n peak h e i g h t f o r a same sample were o f t h e o r d e r o f 2%. K n o w i n g t h a t p h e n o l c o n t a i n s 0 .7657 g C / g p h e n o l , i t i s p o s s i b l e t o compare t h e p h e n o l c o n c e n t r a t i o n o b t a i n e d by gas c h r o m a t o g r a p h y w i t h t h e c o n c e n t r a t i o n c a l c u l a t e d f rom t h e T . O . C . a n a l y s i s , f o r t h e i n i t i a l s a m p l e . D e v i a t i o n s be tween b o t h a n a l y s i s were u s u a l l y i n t h e r ange o f 2-3% w h i c h g i v e s c o n f i d e n c e i n t h e a n a l y t i c a l r e s u l t s . A l l t h e s p e c i f i c a t i o n s and o p e r a t i n g c o n d i t i o n s o f t h e T . O . C . a n a l - y s i s a r e g i v e n i n A p p e n d i x 1. The p h e n o l s t a n d a r d s f o r T . C . a n a l y s i s showed no a l t e r a t i o n a f t e r one month o f s t o r a g e when compared w i t h f r e s h s t a n d a r d s , and t h e samples f rom one e x p e r i m e n t d i d n o t show v a r i a t i o n i n t h e T . O . C . a n a l y s i s a f t e r one week . 4 . 3 . 3 L e a d a n a l y s i s To t e s t e l e c t r o d e c o r r o s i o n , t h e s ample s f rom some o f t h e e x p e r i m e n t s were a n a l y z e d f o r l e a d u s i n g a t o m i c a b s o r p t i o n s p e c t r o p h o t o m e t r y . L e a d s t a n d a r d s were p r e p a r e d by d i l u t i n g a s t o c k s o l u t i o n c o n t a i n i n g 1000 mg/1 o f l e a d . The r a n g e o f c o n c e n t r a t i o n s o f t h e s t a n d a r d s was c h o s e n a c c o r d i n g t o t h e l e a d c o n t e n t o f t h e s a m p l e s . The s p e c i f i c a t i o n s o f t h e a t o m i c a b s o r p t i o n a p p a r a t u s u sed and i t s o p e r a t i n g c o n d i t i o n s a r e g i v e n i n A p p e n d i x 1. CHAPTER 5 RESULTS AND DISCUSSION 5 . 1 E l e c t r o d e m a t e r i a l s Some p r e l i m i n a r y e x p e r i m e n t s were p e r f o r m e d t o t e s t p o s s i b l e e l e c t r o d e m a t e r i a l s b e f o r e s t a r t i n g t o i n v e s t i g a t e o t h e r v a r i a b l e s a f f e c t i n g t h e p r o c e s s . R e s u l t s a r e g i v e n i n t h e t a b l e f o r each r u n i n A p p e n d i x 2 . The a n o d i z e d l e a d e l e c t r o d e was t e s t e d i n e x p e r i m e n t s g roup N o . 1. Runs 1-1 and 1-2 were c a r r i e d ou t t o t e s t t h e e f f e c t o f a n o d i z a t i o n t i m e on T . O . C . r e m o v a l . The r e s u l t s showed t h a t 1 h and 12 h a n o d i z a t i o n t i m e s p r o d u c e d a p p r o x i m a t e l y t h e same % T . O . C . removed v s t i m e . T h i s i n d i c a t e s t h a t 1 h a n o d i z a t i o n t i m e i s p r o b a b l y s u f f i c i e n t t o c o a t t h e l e a d s h o t w i t h a l a y e r o f Pb02- A f t e r e a c h o f t h e s e r u n s the e l e c t r o d e showed a f a i r l y u n i f o r m brown c o a t i n g . I t i s p r o b a b l e t h a t t h e c o a t i n g i s t h i c k e r f o r t h e l o n g e r a n o d i z a t i o n t i m e . A f t e r t h e a n o d i z a t i o n p e r i o d and w h i l e t h e c e l l was b e i n g washed w i t h d i s t i l l e d w a t e r , t h e l i q u i d a t t h e o u t l e t t o o k on a brown c o l o u r as i f some f i n e p a r t i c l e s f rom t h e e l e c t r o d e s u r f a c e were e n t e r i n g t h e w a t e r . T h e r e f o r e i t was c o n s i d e r e d n e c e s s a r y t o t e s t f o r l e a d c o n c e n t r a t i o n i n s o l u t i o n . A s e r i e s o f t e s t s were c a r r i e d ou t t o i n v e s t i g a t e what g o v e r n e d t h e p r e s e n c e o f l e a d i n t h e a n o l y t e . E x p e r i m e n t s 1-4 t o 1-8 were p e r f o r m e d u n d e r d i f f e r e n t e l e c t r o l y t e c o n c e n t r a t i o n s and p H s . The r e s u l t s a r e summar ized on p . 1 2 2 . i n a l l 49 50 t h e s e r u n s t h e c o n c e n t r a t i o n o f l e a d i n t h e a n o l y t e t e n d e d t o d e c r e a s e w i t h t i m e , i n d i c a t i n g t h a t t h e c o r r o s i o n r a t e i s p r o b a b l y h i g h e r when t h e e l e c t r o d e f i r s t conies i n c o n t a c t w i t h t h e s o l u t i o n . A l s o , t h e l e a d i o n s a r e a b l e t o p a s s t o t h e c a t h o l y t e chamber t h r o u g h t h e c a t i o n - m e m b r a n e t h e r e f o r e r e d u c i n g t h e l e a d c o n t e n t o f t h e a n o l y t e . (The pH changes i n t h o s e r u n s o c c u r r e d s p o n t a n e o u s l y as w i l l be e x p l a i n e d i n t h e n e x t s e c - t i o n . ) I n Run 1-4 t h e e l e c t r o l y t e (5 g / 1 NaOH) was r e c i r c u l a t e d t h r o u g h t h e c e l l w i t h o u t an a p p l i e d c u r r e n t and i t p r o d u c e d t h e h i g h e s t maximum c o n - c e n t r a t i o n o f l e a d i n s o l u t i o n (140 m g / 1 ) . L a t e r t h i s r u n was r e p e a t e d , e x c e p t t h a t a p o t e n t i a l d i f f e r e n c e was a p p l i e d b e f o r e t h e e l e c t r o l y t e was f e d . t o t h e c e l l . The maximum amount o f l e a d i n s o l u t i o n was 2 . 7 m g / 1 . T h i s o b s e r v a t i o n i s i n agreement w i t h t h e p o t e n t i a l - p H d i a g r a m f o r l e a d w h i c h i n d i c a t e s t h a t Pb02 i s t b e r m o d y n a m i c a l l y s t a b l e a t p o s i t i v e o r a n o d i c e l e c t r o d e p o t e n t i a l s w i t h r e s p o c t t o t h e n o r m a l h y d r o g e n e l e c t r o d e . Runs 1-5 and 1-6 showed t h a t a t t h e same i n i t i a l pH o f 9 .8 t h e maximum l e a d c o n c e n t r a t i o n was 4 .2 mg /1 when w o r k i n g a t 30 g / 1 N a 2 S 0 i + , and i t was 1 .7 mg /1 when w o r k i n g w i t h a 5 g / 1 N a 2 S 0 i t e l e c t r o l y t e . A t t h e same c o n c e n t r a t i o n o f Na2S0i | (30 g / 1 ) t h e amount o f l e a d i n s o l u t i o n was h i g h e r a t h i g h e r i n i t i a l p H - ( R u n s 1 - 6 , 1-7 and 1-8 r e s p e c - t i v e l y ) . A n a t t e m p t was made t o s t u d y t h e r e s p o n s e o f t h e a n o d i z e d l e a d e l e c t r o d e u n d e r a 5 g / 1 N a C l e l e c t r o l y t e (Run 1 - 3 ) . I t was n o t p o s s i b l e t o f i n i s h t h e e x p e r i m e n t b e c a u s e t h e e l e c t r o d e c o a t i n g began t o d i s s o l v e r a p i d l y , and as a r e s u l t t h e a n o l y t e p r e s s u r e i n c r e a s e d and t h e v o l t a g e d r o p d e c r e a s e d . When t h e c e l l was opened t h e e l e c t r o d e had l o s t i t s b r o w n c o a t i n g , s h o w i n g t h e u n d e r l y i n g g r e y l e a d , and t h e membrane was c o v e r e d w i t h a d e p o s i t o f some l e a d compound. 51 The p e r f o r m a n c e s o f a n o d i z e d l e a d and e l e c t r o d e p o s i t e d Pb02 c a n be compared f rom Runs 1-9 and 2 -2 r e s p e c t i v e l y . B o t h e x p e r i m e n t s w e r e p e r - fo rmed unde r t h e same e x p e r i m e n t a l c o n d i t i o n s , t h e same c a t i o n i c membrane (IONAC MC-3470) and a p p r o x i m a t e l y e q u a l v o i d a g e f r a c t i o n ( 0 . 6 ) . The r e s u l t s a r e r e p r e s e n t e d i n F i g . 1 0 . The c u r v e f o r % p h e n o l o x i d i z e d v s t i m e when u s i n g t h e e l e c t r o d e p o s i t e d P b 0 2 e l e c t r o d e i s above t h a t c u r v e c o r r e s p o n d i n g t o t h e a n o d i z e d l e a d . The % T . O . C . removed v s t i m e c u r v e s a r e p r a c t i c a l l y c o i n c i d e n t a l f o r b o t h e l e c t r o d e s , b u t t h e maximum c o n - c e n t r a t i o n o f l e a d i n s o l u t i o n was abou t t e n t i m e s h i g h e r i n t h e c a s e o f t h e a n o d i z e d l e a d . S c a n n i n g e l e c t r o n m i c r o g r a p h s o f t h e e l e c t r o d e p o s i t e d P b 0 2 p a r t i c l e s a r e shown i n F i g . 1 1 . L e a d a n a l y s e s a r e r e p o r t e d i n many o t h e r e x p e r i - m e n t s , ( e x p e r i m e n t s Groups 2 and 3) where e l e c t r o d e p o s i t e d P b 0 2 was u s e d . I n each c a s e t h e maximum l e a d c o n c e n t r a t i o n was l o w e r t h a n 0 . 4 m g / 1 , and t e n d e d t o z e r o t o w a r d s t h e end o f t h e r u n . The o n l y e x c e p t i o n was f o u n d i n Run 2 - 3 where no c u r r e n t was a p p l i e d and t h e l e a d c o n c e n t r a t i o n b u i l t u p , r e a c h i n g 2 mg/1 a f t e r 90 m i n u t e s . S c a n n i n g e l e c t r o n m i c r o g r a p h s o f t h e a n o d i z e d l e a d p a r t i c l e s ( F i g . 12) show t h a t f l a k i n g o f t h e P b 0 2 f i l m o c c u r r e d . Once t h e e l e c t r o d e f l a k e s a r e c a r r i e d ou t f rom the c e l l t h e y w o u l d d i s s o l v e more e a s i l y i n t h e a b s e n c e o f t h e a p p l i e d a n o d i c p o t e n t i a l . The a n o d i z e d l e a d e l e c t r o d e c o u l d become more r e s i s t a n t t o c o r r o - s i o n a f t e r s u c c e s s i v e a n o d i z a t i o n s o r a f t e r l o n g p e r i o d s o f u s e . A l s o i t i s p o s s i b l e t h a t i t s r e s i s t a n c e t o c o r r o s i o n c o u l d be i m p r o v e d by a n o d i z i n g t h e l e a d v e r y s l o w l y u s i n g more d i l u t e H2SO4 s o l u t i o n s , l o w e r c u r r e n t d e n s i t i e s and l o n g e r a n o d i z a t i o n t i m e s . . T h i s m i g h t p r e v e n t t h e f o r m a t i o n o f c r a c k s (where c o r r o s i o n p r o b a b l y s t a r t s ) t h a t r e s u l t f rom a 52 d 20 d H i o 0 100 80 Q LU 60 X o _J Z 40 UJ X CL 8" =8* - A " -o- 20 0 -A -O KEY RUN N2 ELECTRODE O 1-9 ANODIZED LEAD A 2-2 E L E C T R O - DEPOSITED P b 0 2 I i l . I 30 60 00 TIME (min) F i g . 1 0 . E f f e c t o f t y p e o f l e a d d i o x i d e e l e c t r o d e a t 10 A and i n i t i a l pH = 9 .4 w i t h IONAC MC-3470 membrane. F i g . 1 1 . S c a n n i n g e l e c t r o n - m i c r o g r a p h s o f t h e e l e c t r o d e p o s i t e d PbC>2 p a r t i c l e s a f t e r u s e . ( p a r t i c l e s i z e s be tween 1 . 7 - 2 . 0 0 mm) F i g . 12. S c a n n i n g e l e c t r o n - m i c r o g r a p h s o f t h e a n o d i z e d l e a d p a r t i c l e s , a f t e r u s e . ( p r e p a r e d f r o m 2 mm l e a d s h o t ) 55 p e r h a p s t o o s t r o n g a n o d i z i n g a c t i o n . A n a t t e m p t was made t o t e s t a p a c k e d b e d n i c k e l e l e c t r o d e a t pH = 12 and 10 A , b u t i n two h o u r s t h e v o l t a g e d r o p t h r o u g h t h e c e l l i n c r e a s e d c o n s i d e r a b l y and a f t e r o p e n i n g t h e c e l l , i t was o b s e r v e d t h a t a p r e c i p - i t a t e was p l u g g i n g t h e bed and c o v e r i n g t h e membrane. T u n g s t e n c a r b i d e (WC) p a r t i c l e s w e r e a l s o t e s t e d a t pH = 12 and 10 A . The a n o l y t e t o o k a g r e y c o l o u r i n d i c a t i n g d i s s o l u t i o n o f t h e e l e c - t r o d e and t h e t o t a l c a r b o n t e s t r e v e a l e d i n c r e a s i n g amount o f c a r b o n i n s o l u t i o n . F rom t h e s e p r e l i m i n a r y t e s t s i t was c o n c l u d e d t h a t t h e e l e c t r o - d e p o s i t e d Pb02 was t h e most c o n v e n i e n t c h o i c e t o c a r r y o u t t h e r e m a i n d e r o f t h e e x p e r i m e n t s b e c a u s e o f i t s b e t t e r c o r r o s i o n r e s i s t a n c e and i t s h i g h e r % p h e n o l o x i d a t i o n when compared t o t h e a n o d i z e d l e a d . 5 . 2 E f f e c t o f pH u s i n g t h e d i v i d e d c e l l B e c a u s e o f t h e l a c k o f i n f o r m a t i o n i n t h e l i t e r a t u r e abou t p h e n o l e l e c t r o o x i d a t i o n i n a l k a l i n e e l e c t r o l y t e s , t h e e f f e c t o f a b a s i c pH r a n g e was i n v e s t i g a t e d f i r s t u s i n g t h e d i v i d e d c e l l . B e f o r e d i s c u s s i n g t h e e f f e c t o f pH on t h e e l e c t r o o x i d a t i o n o f p h e n o l , i t i s n e c e s s a r y t o c o n s i d e r some i m p o r t a n t o b s e r v a t i o n s abou t t h e pH b e h a v i o u r i n t h e d i v i d e d c e l l . U s i n g t h e c a t i o n membrane MC-3470 w h i c h i s s u i t a b l e f o r a l k a l i n e e l e c t r o l y t e s , t h e a n o l y t e p h e n o l c o n c e n t r a t i o n was s e t a t 100 mg/1 and t h e pH was a d j u s t e d t o 9.4. I t was o b s e r v e d t h a t a f t e r 15 m i n t h e pH o f t h e sa taple a t t h e o u t l e t o f t h e c e l l had d r o p p e d t o abou t 3, when w o r k i n g at. 10 A (Run 2 - 2 , F i g . 1 3 ) . L a t e r , t h e same e x p e r i m e n t a l c o n - d i t i o n s as i n Run 2 - 2 w e r e r e p e a t e d b u t w i t h o u t p h e n o l b e i n g p r e s e n t and  t h e same pH d r o p was o b s e r v e d . T h u s , t h e pH d r o p a p p e a r s t o be due t o s i d e r e a c t i o n s o f oxygen e v o l u t i o n , and n o t t o p r o d u c t i o n o f an a c i d f rom p h e n o l o x i d a t i o n . What p r o b a b l y happens i s t h a t t h e h y d r o x y l i o n s a r e d i s c h a r g e d f i r s t a t t h e e l e c t r o d e , and t h e n t h e mechanism o f o x y g e n e v o l u t i o n changes ( R e a c t i o n s R I o r R2) p r o d u c i n g h y d r o g e n i o n s . I f t h e r a t e a t w h i c h t h e h y d r o g e n i o n s a r e p r o d u c e d i s h i g h e r t h a n t h e r a t e o f t r a n s - p o r t t h r o u g h t h e membrane, t h e n e t e f f e c t i s a p r o g r e s s i v e i n c r e a s e i n t h e c o n c e n t r a t i o n o f h y d r o g e n i o n s , w h i c h w o u l d r e s u l t i n t h e o b s e r v e d pH d r o p . T h i s w o u l d a l s o e x p l a i n why t h e e l e c t r o l y t e c o n d u c t i v i t y i n c r e a s e s t o w a r d s t h e end o f t h e r u n and as a r e s u l t t h e v o l t a g e d rop t h r o u g h t h e c e l l d e c r e a s e s (Run 2 - 2 ) . I f t h e s i d e r e a c t i o n s o f o x y g e n e v o l u t i o n a r e r e s p o n s i b l e f o r t h e o b s e r v e d pH b e h a v i o u r , i t f o l l o w s t h a t t he pH r e s p o n s e must be r e l a t e d t o t h e c u r r e n t b e c a u s e the r a t e a t w h i c h t h e s i d e r e a c t i o n s o c c u r w i l l u l t i m a t e l y be d e t e r m i n e d by t h e c u r r e n t a p p l i e d . The pH r e s p o n s e was t h e n compared a t d i f f e r e n t c u r r e n t s ( 0 , 3 , 6 , and 10 A) w i t h a l l t h e o t h e r e x p e r i m e n t a l c o n d i t i o n s h e l d c o n s t a n t . The pH v s t i m e c u r v e s a r e a l s o r e p r e s e n t e d i n F i g . 1 3 . When w o r k i n g a t 6 A (Run 2 -5 ) t h e pH d r o p p e d f rom 9 . 4 t o 3 .7 i n 15 m i n , s h o w i n g t h e same t e n d e n c y d e s c r i b e d p r e v i o u s l y f o r Run 2 - 2 . A l o w e r c u r r e n t o f 3 A (Run 2 -4 ) p r o d u c e d an u n e x p e c t e d r e s u l t . I n 15 m i n t h e pH o f t h e sample a t t h e o u t l e t o f t h e c e l l had i n c r e a s e d f rom 9 .4 t o 1 1 . 7 . The same b e h a v i o u r was o b s e r v e d when t h e s o l u t i o n was r e c i r c u l a t e d w i t h o u t any c u r r e n t a p p l i e d (Run 2 - 3 ) . Two p o s s i b l e e x p l a n a t i o n s a r e p r o p o s e d f o r t h i s e f f e c t . S i n c e t h e membrane u sed i n t h e s e e x p e r i m e n t s was t h e c a t i o n i c M C - 3 4 7 0 , t h e o r e t i c - a l l y i t o n l y a l l o w s t h e t r a n s p o r t o f p o s i t i v e i o n s b u t as t h e i o n i c 58 s e l e c t i v i t y i s n o t 100%, i t i s p o s s i b l e t h a t a c e r t a i n amount o f h y d r o x y l i o n s p a s s e d t o t h e a n o l y t e f r o m t h e a l k a l i n e c a t h o l y t e , t h e r e f o r e r a i s i n g t h e p H . A n o t h e r p o s s i b i l i t y i s t h a t t h e pH i n c r e a s e i s a s s o c i a t e d w i t h a l e a d d i o x i d e r e a c t i o n . T h i s w o u l d e x p l a i n why a p o t e n t i a l d i f f e r e n c e was d e t e c t e d w i t h o u t an a p p l i e d c u r r e n t (Run T a b l e 2 -3 ) and t h e l e a d c o n - c e n t r a t i o n was b u i l d i n g up d u r i n g the r u n . I n o t h e r w o r d s , t h e e l e c t r o d e may have behaved as i n t h e l e a d b a t t e r y where the Pb02 r e d u c e s i n t h e p r e s e n c e o f s u l p h a t e i o n s g e n e r a t i n g a f l o w o f c u r r e n t ( R e a c t i o n R 1 7 ) . I n o r d e r t o e n s u r e t h a t t h e d i f f e r e n t pH changes o b s e r v e d were a s s o c i a t e d w i t h t h e a p p l i e d c u r r e n t , d u r i n g Run 2-4 the c u r r e n t was s u d d e n l y changed f rom 3 t o 10 A , a f t e r 90 m i n and a g a i n the pH d r o p p e d f r o m 1 1 . 8 t o 3 . 8 d u r i n g t h e n e x t 15 m i n , f o l l o w i n g t h e same b e h a v i o u r o b s e r v e d i n Runs 2 -2 and 2 -5 ( F i g . 1 3 ) . The e f f e c t o f p H - c u r r e n t changes on t h e r a t e o f p h e n o l o x i d a t i o n i s shown i n F i g . 1 4 . When c o m p a r i n g the % T . O . C . removed i n Runs 2 - 2 , 2 - 4 , and 2 - 5 , i t i s o b s e r v e d t h a t t h e t h r e e c u r v e s t ended t o a 17% T . O . C . removed i n 90 m i n , b u t when t h e c u r r e n t was changed i n Run 2 -4 f r o m 3 t o 10 A , a h i g h e r p e r c e n t o f t h e c a r b o n was o x i d i z e d i n t h e i n t e r v a l f rom 90 t o 120 m i n ( F i g . 1 3 ) . The most p r o b a b l e r e a s o n f o r t h i s d i f f e r e n c e was t h e h i g h e r pH o b s e r v e d a t 90 m i n i n Run 2 - 4 . T h i s was t h e f i r s t i n d i c a t i o n o f an enhancement o f T . O . C . r e m o v a l a t h i g h p H . To i n v e s t i g a t e i f t h e r e was any d i f f e r e n c e i n t h e pH r e s p o n s e when a d i f f e r e n t c a t i o n membrane was u s e d , e x p e r i m e n t 2-8 was c a r r i e d o u t . U s i n g t h e NAFION-127 membrane, s t a r t i n g a t pH = 12 and 10 A t h e pH d r o p p e d even f a s t e r t h a n i n Run 2-2 ( F i g . 1 5 ) . A f t e r 15 m i n t h e pH had d r o p p e d f rom 12 t o 2 . I n Run 2 -10 t h e pH was k e p t i n t h e b a s i c r a n g e f o r a l o n g e r p e r i o d 59 F i g . 14 . E f f e c t o f c u r r e n t on % p h e n o l o x i d a t i o n a t i n i t i a l pH = w i t h IONAC MC-3470 membrane. 9 .4 60 61 o f t i m e , by s t a r t i n g a t a pH o f 1 2 . 8 . As shown i n F i g . 16 t h e pH s t a r t e d t o d rop a f t e r 45 m i n , and went f rom 1 2 . 2 t o a v a l u e o f 2 . 2 i n t h e n e x t 15 m i n u t e s . I n t h e same f i g u r e , i t c a n a l s o be o b s e r v e d t h a t t h e o x i d a t i o n o f p h e n o l i s s e v e r e l y l i m i t e d by a h i g h p H . A t 15 m i n and pH = 1 2 . 7 (Run 2 - 1 0 ) o n l y a 10% o f t h e p h e n o l had been o x i d i z e d , whereas a t pH - 2, 65% o f t h e p h e n o l had a l r e a d y been o x i d i z e d a f t e r t he same t i m e (Run 2 - 9 ) . On t h e o t h e r h a n d , t h e c u r v e f o r % T . O . C . removed v s t i m e c o r r e s p o n d i n g t o t h e h i g h pH r u n i s above t h e % T . O . C . c u r v e c o r r e s p o n d i n g t o t h e l o w pH r u n . I t s h o u l d be n o t e d t h a t t h e pH i s measured a t t h e o u t l e t o f t h e c e l l , and when t h e pH d r o p i s r e c o r d e d t h e r e must be a pH p r o f i l e w i t h i n t h e c e l l ( h i g h e r pH a t t h e e n t r a n c e ) , and a v a r i a b l e pH i n t h e r e c i r c u - l a t i o n t a n k . T h i s makes t h e i n t e r p r e t a t i o n o f t he r e s u l t s more d i f f i - c u l t , b u t s t i l l t h e r e i s a d e f i n i t e enhancement i n T . O . C . r e m o v a l a t h i g h p H . These e x p e r i m e n t s i n d i c a t e d t h a t an opt imum pH sequence w o u l d be a l o w pH' a t t h e b e g i n n i n g o f t h e r u n , f a v o u r i n g p h e n o l o x i d a t i o n , and a h i g h pH a t t h e e n d , f a v o u r i n g T . O . C . r e m o v a l . T h i s i d e a s u g g e s t e d t h a t an a n i o n s e l e c t i v e membrane c o u l d p r o v i d e p r e c i s e l y s u c h an opt imum pH s e q u e n c e . I f t he t r a n s p o r t o f h y d r o x y l i o n s f rom t h e NaOH c a t h o l y t e , and t h r o u g h t h e membrane were s u f f i c i e n t t o p r o d u c e a pH i n c r e a s e f rom abou t 2 t o 1 2 , d u r i n g t h e c o u r s e o f a r u n , a t a g i v e n c u r r e n t , an opt imum t y p e o f r u n c o u l d be p o s s i b l e . W i t h t h i s o b j e c t i v e , s e v e r a l p r e l i m i n a r y t e s t s were p e r f o r m e d w i t h t h e a n i o n i c membrane IONAC M A - 3 4 7 5 . I n F i g . 1 7 , Run 2 - 1 1 , i t i s o b s e r v e d t h a t t h e d e s i r e d i n c r e a s e i n pH i n d e e d o c c u r s , and a t 30 m i n u t e s 90% o f 62 63 64 the phenol had been oxidized, before the pH started to increase. When the pH increase was produced, the T.O.C. started to be oxidized at a higher rate than in Run 2-10. I t should be noted that in the high pH range the oxidized organic carbon remains in solution in the form of inorganic carbon (e.g., Run 2-11) probably carbonates, due to the higher solubility of carbon dioxide in alkaline solutions. Particularly interesting colour changes were observed during these experiments. The electrolyte at the outlet of the c e l l showed a brown- reddish colour when the pH was alkaline (higher than 9.4), and in those runs where the pH dropped spontaneously, the colour changed to light yellow. For example, in Pom 2-10 the electrolyte took a deep brown- reddish colour u n t i l 60 min and when the pH drop was produced, the outlet showed the light yellow colour while tank was s t i l l brown. These colour reactions, when the pH was changed, were also observed in a pure benzo- quinone solution, prepared for comparison. 5.3 Effect of current using the divided c e l l In experiments 2-3, 2-4, and 2-5, current and pH were dependent variables and therefore the unique effect of the current can not be analyzed separately. However, in Fig. 13 i t can be seen that the runs at 6 and 10 A showed approximately similar pH drops and therefore the current effect can be compared. The % T.O.C. removed vs^ time curves for those two runs are practically coincidental. From Fig. 14 i t i s observed that at 10 A and 15 min, 12% more of the phenol had been oxidized than at 6 A. At 10 A, total phenol conversion was achieved in 90 min, but at 6 A, 120 min were necessary to oxidize the 65 p h e n o l c o m p l e t e l y . Two r u n s were c a r r i e d ou t w i t h t h e c a t i o n i c membrane MC-3470 t o s t u d y t h e e f f e c t o f t h e c u r r e n t w i t h o u t s i g n i f i c a n t pH changes (Runs 2 - 6 , 2 - 7 ) . The r e s u l t s a r e p l o t t e d i n F i g . 1 8 , where i t i s o b s e r v e d t h a t t h e % p h e n o l o x i d i z e d v s t i m e c u r v e s a r e v e r y c l o s e and t o t a l p h e n o l c o n v e r - s i o n i s a c h i e v e d a t 75 m i n f o r b o t h c u r r e n t s . Howeve r , t h e T . O . C . a n a l y s e s r e v e a l e d t h a t a t 20 A , 47% o f t h e c a r b o n had been o x i d i z e d t o c a r b o n d i o x i d e i n 120 m i n , whereas a t 10 A o n l y a 12% had been o x i d i z e d d u r i n g t h e same t i m e . 5 .4 C o m p a r i s o n s o f membrane p e r f o r m a n c e s The p e r f o r m a n c e s o f c a t i o n i c membranes IONAC MC-3470 and NAFION 127 c a n be compared f rom Runs 2-6 and 2 - 9 , b o t h a t 20 A and pH = 2 . The r e s u l t s a r e p l o t t e d i n F i g . 1 9 . The % p h e n o l o x i d i z e d v s t i m e c u r v e s showed t h a t when u s i n g t h e IONAC M C - 3 4 7 0 , abou t 77% o f t h e p h e n o l had been o x i d i z e d i n 15 m i n v e r s u s 65% when u s i n g N A F I 0 N - 1 2 7 . But a t 75 m i n t o t a l p h e n o l c o n v e r s i o n had been a c h i e v e d w i t h b o t h membranes. The % T . O . C . removed v s t i m e c u r v e s a r e v e r y c l o s e b u t n o t e x a c t l y c o i n c i d e n t a l . A t 120 m i n , 10% more c a r b o n was o x i d i z e d i n t h e r u n w i t h IONAC M C - 3 4 7 0 . The a n i o n i c membrane MA-3475 gave t h e b e s t p e r f o r m a n c e as f a r as T . O . C . r e m o v a l i s c o n c e r n e d , because i t p r o v i d e s t h e f a v o u r a b l e pH i n c r e a s e as d e s c r i b e d p r e v i o u s l y ( F i g . 1 7 ) . I n 15 m i n , p h e n o l o x i d a t i o n was 70%, and t o t a l p h e n o l o x i d a t i o n was a l s o a c h i e v e d i n 75 m i n . How- e v e r , t h e membrane was found t o be changed i n c o l o u r f r o m y e l l o w t o b r o w n , when t h e c e l l was opened a f t e r Run 2 - 1 1 . The a c t u a l r e a s o n f o r t h i s i s unknown, bu t i t i s p o s s i b l e t h a t t h e p o l y m e r s t r u c t u r e o f t h e membrane e x p e r i e n c e d o x i d a t i o n , o r t h a t i t i n t e r a c t e d i n some way w i t h t h e p h e n o l 66 F i g . 1 8 . C u r r e n t e f f e c t on % T . O . C . and % p h e n o l o x i d a t i o n a t pH = 2 . 5 w i t h IONAC M C - 3 4 7 0 . 67 F i g . 1 9 . Type o f c a t i o n i c membrane e f f e c t on % T . O . C . and % p h e n o l o x i d a t i o n a t 20 A and pH = 2 . 5 . o x i d a t i o n p r o d u c t s . These changes may a f f e c t t h e l i f e t i m e o f t h e mem- b r a n e . The c a t i o n i c membrane MC-3142 was o n l y t e s t e d d u r i n g t h e p r e l i m i n a r y e x p e r i m e n t s w i t h a n o d i z e d l e a d . On s e v e r a l o c c a s i o n s the membrane was p e r f o r a t e d by p r o t r u d i n g p a r t i c l e s , w h i c h r e s u l t e d i n s h o r t c i r c u i t s . F o r t h i s r e a s o n , a p l a s t i c s c r e e n was i n t r o d u c e d be tween t h e membrane and t h e p a r t i c l e s . A n o t h e r d i s a d v a n t a g e o f t h e IONAC MC-3142 membrane i s t h a t i t s n y l o n s u p p o r t w i l l d e t e r i o r a t e w i t h u se i n NaOH s o l u t i o n s . I n t e rms o f m e c h a n i c a l r e s i s t a n c e , t h e IONAC membranes MC3470 and MA3475 were s t r o n g e r t h a n t h e NAFION-127 and IONAC MC3142. P r o b l e m s o f b r e a k i n g n e v e r o c c u r r e d w i t h t h e f i r s t two o w i n g t o t h e i r d i f f e r e n t s u p - p o r t m a t e r i a l s and t h i c k n e s s e s ( A p p e n d i x 1 ) . The p o l y p r o p y l e n e s c r e e n , u sed i n Run 2 - 1 , p r o d u c e s much h i g h e r p o t e n t i a l d r o p s t h a n t h e s a r a n c l o t h u sed i n Run 2 - 2 , p r o b a b l y due t o i t s c l o s e l y p a c k e d s c r e e n s t r u c t u r e . These e x p e r i m e n t s show t h a t t h e use o f t h e d i f f e r e n t p l a s t i c s c r e e n s does n o t a f f e c t t h e r e s u l t s as f a r as p h e n o l and T . O . C . r e m o v a l i s c o n c e r n e d . 5 . 5 E f f e c t o f pH u s i n g t h e u n d i v i d e d c e l l From t h e pH v s t i m e c u r v e i n F i g . 2 0 , i t c a n be seen t h a t when w o r k i n g w i t h o n l y one chamber t h e pH does n o t d r o p as f a s t as i n t h e d i v i d e d c e l l . I n Run 3-2 t h e pH o n l y d r o p p e d f rom 12 t o 1 1 . 5 i n two h o u r s whereas i n c o r r e s p o n d i n g Run 2 -8 ( F i g . 15 ) t h e pH d r o p p e d f r o m 12 t o 2 i n 15 m i n . The r e a s o n f o r t h i s d i f f e r e n t pH b e h a v i o u r i s t h a t i n t h e c a s e o f t h e u n d i v i d e d c e l l , t h e r e a c t i o n o f h y d r o g e n e v o l u t i o n t a k i n g p l a c e a t t h e c a t h o d e changes t h e h y d r o x y l i o n b a l a n c e i n t h e e l e c t r o l y t e . When s t a r t i n g a t pH = 9 .5 t h e pH d r o p p e d t o 3 .8 w h i c h a l s o r e p r e - s e n t s a s m a l l e r d r o p t h a n i n t h e c a s e o f t h e d i v i d e d c e l l , and when s t a r t i n g a t pH = 2 . 5 , t h e pH was h e l d p r a c t i c a l l y c o n s t a n t . As was found i n e x p e r i m e n t s w i t h t h e d i v i d e d c e l l , t h e % • p h e n o l o x i d i z e d v s t i m e c u r v e s a t 10 A f o r t h e u n d i v i d e d c e l l ( F i g . 2 0 ) , show t h a t p h e n o l i s o x i d i z e d much f a s t e r a t a l o w pH t h a n a t a h i g h p H . I n 15 m i n , 70% o f t h e p h e n o l was o x i d i z e d when t h e pH was 2 . 5 , compared w i t h o n l y a 33% when t h e pH was 1 2 . On t h e o t h e r h a n d , more T . O . C . was o x i d i z e d a t t h e h i g h p H . A t 120 m i n , 19% o f t h e c a r b o n was o x i d i z e d a t pH = 2 . 5 , whereas a t pH = 1 2 , a 32% o f t h e c a r b o n had been o x i d i z e d b u t r e m a i n e d i n s o l u t i o n as i n o r g a n i c c a r b o n ( e . g . , Run 3 - 2 ) . The pH e f f e c t was a l s o s t u d i e d when w o r k i n g a t 20 and 30 A ( F i g s . 21 and 22) and t h e r e s u l t s showed t h e same t e n d e n c y . P h e n o l o x i d a t i o n i s f a v o u r e d by a l o w pH whereas t h e f u r t h e r o x i d a t i o n o f i n t e r m e d i a t e s i s i m p r o v e d a t a h i g h p H . When c o m p a r i n g F i g s . 2 0 , 21 and 22 i t i s a l s o o b s e r v e d t h a t as t h e c u r r e n t i n c r e a s e s , t h e pH has a g r e a t e r e f f e c t on T . O . C . o x i d a t i o n t h a n on p h e n o l o x i d a t i o n . 5 .6 E f f e c t o f c u r r e n t u s i n g t h e u n d i v i d e d c e l l F i g u r e 23 d e s c r i b e s t h e e f f e c t o f 1 0 , 2 0 , and 30 A c u r r e n t s a t c o n - s t a n t l o w pH ( 2 . 5 ) , and F i g . 24 a t a h i g h pH ( 1 2 ) . I t s h o u l d be n o t e d t h a t d u r i n g t h e r u n s a t 10 A , t h e t e m p e r a t u r e o f t h e e l e c t r o l y t e r e m a i n s c o n s t a n t a t room t e m p e r a t u r e . A t 20 A s l i g h t h e a t i n g o c c u r s , and a 4°C t e m p e r a t u r e i n c r e a s e c a n be d e t e c t e d i n t h e r e c i r c u l a t i o n t a n k a f t e r 120 m i n o p e r a t i o n . When w o r k i n g a t 30 A , a p p r o x i m a t e l y a 1 2 ° C t e m p e r a - t u r e i n c r e a s e i s r e g i s t e r e d ( e . g . , Runs 3-6 and 3 - 7 ) . These t e m p e r a t u r e i n c r e a s e s a r e s m a l l and t h e s i d e e f f e c t s due t o t e m p e r a t u r e v a r i a t i o n s 70 F i g . 2 0 . pH e f f e c t on % T . O . C . and % p h e n o l o x i d a t i o n a t 10 A i n an u n d i v i d e d c e l l . F i g 2 1 . pH e f f e c t on % T . O . C . and % p h e n o l o x i d a t i o n a t 20 A i n an u n d i v i d e d c e l l . I JL 100 r- 20 KEY RUN N 2 PH O X 3 - 6 3 - 7 2.5 12.0 MASS T R A N S F E R - CONTROLLED REGION 1 1 1 i 30 60 90 120 TIME (min) F i g . 22. Effect of pH on % T.O.C. and % phenol oxidation at 30 A in an undivided c e l l .  74 100 100 O 80 UJ N Q X O 60 UJ JO. CL 40 20 0 0 120 —a KEY RUN N 2 I (A) • 3 - 2 10 3 - 5 20 O 3 - 7 30 _L JL 30 24. 60 9 0 TIME (min) E f f e c t o f c u r r e n t on % T . O . C . and % p h e n o l o x i d a t i o n a t i n i t i a l pH = 1 2 , i n an u n d i v i d e d c e l l . 120 75 can probably be neglected. In Fig. 23 and Fig. 24 the effect of increasing current i s to sub- stantially raise the i n i t i a l rate of phenol oxidation, thus decreasing the time to complete oxidation and to increase the % of T.O.C. removed. These effects are not proportional to the current since there i s a bigger increase in T.O.C, removal and in the rate of phenol oxidation i n going from 10 A to 20 A than in going from 20 A to 30 A (i.e., Fig. 24). It can also be observed that in Fig. 23 the % phenol oxidized vs time curves are closer together than i n Fig. 24, which indicates that at high pH the current has a greater effect on phenol oxidation than at low pH. The maximum % T.O.C* oxidation i s encountered i n Run 3-7, for the highest current and alkaline pH, where 92% of the carbon was oxidized i n 120 minutes, 5.7 Comparisons of divided and undivided cells It i s possible to evaluate the performances of divided and undivided cells under similar conditions. Table 5 shows the operating condition and results of three comparisons. Cationic membranes IONAC MC-3470 and NAFION 127 gave similar per- formances to the undivided cells i n terms of phenol oxidation. Con- sidering that 2% error arises i n the determinations of.phenol and T.O.C. .... i t can be said that the results seem to favour the undivided c e l l slightly in terms of T.O.C. removal. Run 2-11, with anionic membrane MA-3475, can be either compared with Run 3-4 or with Run 3-5. Because in Run 2-11 the pH increased from 2.4 to 12, higher % T.O.C. oxidation was achieved than i n Run 3-4, but 76 TABLE 5 COMPARISONS OF DIVIDED AND UNDIVIDED CELLS Run N o . % P h e n o l o x . % T . O . C . o x . C o n d i t i o n s ( c e l l c o n f i g u r a t i o n ) a t 15 m i n a t 120 m i n 1 (A) pH Run 2-7 70 11 10 2 . 5 (IONAC MC-3470) Run 3-3 70 19 10 2 . 5 ( u n d i v i d e d ) Run 2 -9 77 47 20 2 . 5 (NAFION 127) Run 3-4 75 53 20 2 . 5 ( u n d i v i d e d ) Run 2 -11 72 67 20 2 . 4 (IONAC MA 3475) t o 12 Run 3-5 53 73 20 12 ( u n d i v i d e d ) s i m i l a r % p h e n o l o x i d a t i o n . When c o m p a r i n g Run 2 -11 w i t h Run 3-5 ( a t a c o n s t a n t pH o f 12) i t i s o b s e r v e d t h a t h i g h e r T . O . C . o x i d a t i o n was a c h i e v e d i n Run 3 - 5 , even i f t h e % p h e n o l o x i d i z e d a t 15 m i n was l o w e r due t o t h e h i g h e r p H . On t h e b a s i s o f t h e s e r e s u l t s and b e c a u s e t h e u n d i v i d e d c e l l i s s i m p l e r t o o p e r a t e , i t was d e c i d e d t o p e r f o r m t h e r e s t o f t h e e x p e r i - ments w i t h t h e u n d i v i d e d c e l l . 5 . 8 E f f e c t o f c o n d u c t i v i t y o f t h e e l e c t r o l y t e F i g u r e 25 shows t h e e f f e c t o f e l e c t r o l y t e c o n d u c t i v i t y when w o r k i n g a t 20 A . A t a pH o f 1 2 , i n Runs 3-5 and 3 - 1 1 , t h e e l e c t r o l y t e c o n d u c - - 3 - 3 - 1 t i v i t y was changed f rom 8 * 10 t o 32 x 10 (ft.cm) , by a d d i n g 5 g / 1 and 30 g / 1 o f N a 2 S 0 i + , r e s p e c t i v e l y . The r e s u l t s showed t h a t t h e i n c r e a s e  i n c o n d u c t i v i t y d i d n o t a f f e c t t h e p h e n o l o x i d a t i o n , n o r t h e T . O . C . r e m o v a l . B o t h e x p e r i m e n t s gave a l m o s t p e r f e c t l y c o i n c i d e n t a l c u r v e s o f % p h e n o l o x i d i z e d v s t i m e and o f % T . O . C . v s t i m e . The same r e s u l t was o b t a i n e d when t h e c o n d u c t i v i t y was c h a n g e d , a t pH = 2 . 5 i n Runs 3-4 and 3 - 1 0 . However , when w o r k i n g a t 10 A , and pH = 1 2 , t h e same i n c r e a s e i n c o n d u c t i v i t y p r o d u c e d an e x t r a 20% c a r b o n o x i d i z e d a f t e r 120 m i n , bu t t h e % p h e n o l o x i d i z e d v s t i m e c u r v e r e m a i n e d p r a c t i c a l l y unchanged ( F i g . 2 6 ) . S i m i l a r l y , a t 10 A , and pH = 2 . 5 ( F i g . 27) abou t 8% more c a r b o n was - 3 o x i d i z e d a f t e r 120 m i n when c o n d u c t i v i t y was changed f rom 8 .4 x 10 t o - 3 - 1 30 x 10 (fi .cm) , bu t a g a i n , t h e % p h e n o l o x i d i z e d v s t i m e c u r v e showed no v a r i a t i o n . An e x p l a n a t i o n c a n be p r o p o s e d f o r t h e s e o b s e r v a t i o n s . An i n c r e a s e i n c o n d u c t i v i t y o f t h e e l e c t r o l y t e p r o d u c e s a l o w e r s o l u t i o n p o t e n t i a l , and t h e r e f o r e a h i g h e r e l e c t r o d e p o t e n t i a l . A t 10 A , s u c h an i n c r e a s e i n e l e c t r o d e p o t e n t i a l r e s u l t s i n an a p p r e c i a b l e change i n t h e r a t e o f f u r t h e r o x i d a t i o n o f i n t e r m e d i a t e s t o CO2. B u t a t 20 A , t h e m e t a l p o t e n t i a l i s much h i g h e r so t h a t t h e d e c r e a s e i n s o l u t i o n p o t e n t i a l d i d n o t change t h e n e t e l e c t r o d e p o t e n t i a l a p p r e c i a b l y , and t h e r e f o r e no change i s d e t e c t e d i n t h e % o f T . O . C . r e m o v e d . 5 .9 E f f e c t o f i n i t i a l p h e n o l c o n c e n t r a t i o n F i g u r e 28 r e p r e s e n t s t h e % p h e n o l o x i d i z e d v s t i m e f o r t h r e e d i f f e r - en t i n i t i a l c o n c e n t r a t i o n s o f p h e n o l (Runs 3 - 3 , 3 - 1 2 , and 3 - 1 3 ) . As t h e i n i t i a l p h e n o l c o n c e n t r a t i o n was i n c r e a s e d f rom 93 mg/1 t o 525 mg/1 t o 1100 m g / 1 , t h e % p h e n o l o x i d i z e d a f t e r 15 m i n d e c r e a s e d f rom 70% t o 38% and t o 27%. H o w e v e r , i n F i g . 29 i t i s p o s s i b l e t o see t h a t a g i v e n 79 F i g . 2 6 . E f f e c t o f e l e c t r o l y t e c o n d u c t i v i t y a t 10 A and i n i t i a l pH = 1 2 . 80 F i g . 2 7 . E f f e c t o f e l e c t r o l y t e c o n d u c t i v i t y a t 10 A and i n i t i a l pH - 2 . 5 81 100 Q UJ N X o UJ X 0_ 60 T I M E ( m i n ) F i g . 2 8 . % p h e n o l o x i d i z e d v s t i m e f o r v a r i o u s i n i t i a l p h e n o l c o n c e n t r a t i o n a t 10 A and pH = 2 . 5 . N5 VO PHENOL CONCENTRATION (mg/L ) o — "2. m 3 CO ho t i m e t h e n e t amount o f p h e n o l o x i d i z e d i s h i g h e r as t h e c o n c e n t r a t i o n i n c r e a s e s . T h u s , 350 mg/1 o f p h e n o l were o x i d i z e d i n 15 m i n i n Run 3-13 compared t o 200 mg/1 i n Run 3 -12 and o n l y 75 mg/1 i n Run 3 - 3 . I n t h i s f i g u r e i t i s e a s i e r t o o b s e r v e t h e p a r a l l e l i s m be tween t h e t h r e e c u r v e s . F o r e x a m p l e , i f t h e c u r v e f o r Run 3-12 was d i s p l a c e d a l o n g t h e t i m e a x i s u n t i l i t ma tched t h e c u r v e c o r r e s p o n d i n g t o Run 3 - 1 3 , b o t h c u r v e s w o u l d be p r a c t i c a l l y c o i n c i d e n t a l . I n o t h e r w o r d s , a f t e r ' 30 m i n o f Run 3 - 1 3 , t h e same r e s u l t s w o u l d be o b s e r v e d as i n Run 3-12 f r o m t i m e z e r o . As t h e s e r u n s were p e r f o r m e d a t 10 A and pH = 2 . 5 , t h e T . O . C . r e m o v a l i s r e l a t i v e l y l o w . When t h e i n i t i a l c o n c e n t r a t i o n was 93 m g / 1 , 25% o f t h e c a r b o n was removed i n 2 h v e r s u s 11% when t h e i n i t i a l c o n c e n - t r a t i o n was 525 m g / 1 . B u t f o r t h e i n i t i a l c o n c e n t r a t i o n o f 1100 mg/1 o f p h e n o l , t h e n e t change i n T . O . C . was p r a c t i c a l l y u n d e t e c t a b l e due t o t h e h i g h e r c o n c e n t r a t i o n o f c a r b o n p r e s e n t i n s o l u t i o n . 5 . 1 0 E f f e c t o f e l e c t r o l y t e f l o w r a t e Run 3-15 was p e r f o r m e d a t a f l o w r a t e o f 0 . 5 5 £ / m i n . A c o m p a r i s o n o f t h e r e s u l t s w i t h t h o s e f r o m Run 3-4 where t h e f l o w , was 1.12 £ / m i n , u n d e r o t h e r w i s e e q u a l o p e r a t i n g c o n d i t i o n s , shows t h a t a t b o t h f l o w r a t e s p r a c t i c a l l y t h e same % p h e n o l v s t i m e and % T . O . C . o x i d i z e d v s t i m e were o b t a i n e d . T h u s , p h e n o l was 75% o x i d i z e d a f t e r 15 m i n i n Run 3-4 and abou t 73% i n Run 3 - 1 5 . The % T . O . C . o x i d i z e d a f t e r 120 m i n were 53% and 54% r e s p e c t i v e l y . ( R e f e r t o Run t a b l e s i n A p p e n d i x 2 . ) The e f f e c t o f t h e f l o w r a t e i s masked by t h e p r e s e n c e o f t h e r e l - a t i v e l y l a r g e r e c i r c u l a t i o n t a n k . I n a s i n g l e p a s s e x p e r i m e n t , a h i g h e r c o n v e r s i o n w o u l d be e x p e c t e d a t a l o w e r f l o w r a t e . W i t h r e - c i r c u l a t i o n , a l o w e r f l o w r a t e means i t t a k e s more t i m e f o r a g i v e n 84 effect to be detected i n the recirculation tank analysis. The effect of both inlet concentration and flow rate, is more easily examined in single pass experiments where the c e l l operates in steady state. Thus some single pass experiments were carried out. Figure 30 represents the % phenol oxidized in a single pass through the c e l l vs flow rate, at different i n i t i a l concentrations, when operat- ing at 10 A and pH = 2.5. Runs 4-1, 4-2, and 4-3 were carried out at essentially the same phenol concentration of 100 ± 5 mg/1 to check reproducibility. They produced practically coincidental % phenol oxi- dized vs flow rate curves. About 90% of the phenol was oxidized at a flow rate of 0.11 £/min, and as the flow rate was increased the % of phenol oxidized dropped, reaching a 20% at a flow rate of 1.1 2,/.mini Run 4-4 was performed under the same conditions (pH = 2.5, I = 10 A) but starting at the higher phenol concentration of 580 mg/1. The phenol analysis showed that about 70% was oxidized when the liquid flow rate was 0.11 Ji/min and that the % phenol oxidized decreased with increasing flow rate, to about 8% when the flow rate reached 1.1 2,/min. Similar effects were observed when operating at a current of 20 A, for i n i t i a l concentrations of 110 and 510 mg/1. The table for Run 4-1 shows that when the flow rate was 0.11 £/min at 10 A, the temperature of the electrolyte was raised from 24°C at the inlet to 32°C at the outlet of the c e l l . But when the flow was increased to 0.25 £/min the outlet temperature increased only 4°C above the inlet temperature. The operation can be considered practically isothermal for a l l flows equal or higher than 0.25 il/min. At 20 A (i.e., Run 4-5) the temperature at the outlet increased more than at 10 A, as could be expected, but the operation can be 100 80 Q UJ U O 60 X o o Z LU X 0_ 40 20 0 J. (a) K E Y R U N N 2 P H E N O L C 0 ( m g / l ) O 4 - 1 105 + 4-2 95 4-3 100 • 4-4 580 0.2 0.4 0.6 0.8 1.0 FLOWRATE (L/min) 1.2 1.4 100 80 Q LU N 9 60 X o LU 40 X 0_ 20 0 T 02 1 K E Y R U N N2 P H E N O L C 0 (mg/L) 4-5 110 • 4-6 510 0.4 3 0 . 06 0.8 1.0 FLOWRATE (L/min) E f f e c t o f f l o w r a t e on t h e s i n g l e p a s s % p h e n o l o x i d a t i o n a t (a) 10 A , (b) 20 A . 1.2 1.4 86 considered p r a c t i c a l l y isothermal f o r flows above or equal to 0.55 £/min. In retrospect, i t would have been better to report a l l the r e s u l t s under constant room temperature to eliminate any possible side e f f e c t due to temperature v a r i a t i o n s . However, t h i s would require the design of a t o t a l l y d i f f e r e n t c e l l with a b u i l t - i n heat exchanger to remove the heat released by the current. 5.11 E f f e c t of p a r t i c l e s i z e Figure 31 shows the % of phenol oxidized vs flow using three d i f f e r - ent anode surface areas. The lower curve corresponds to experiment 4-8, where no p a r t i c l e s were present and the anode was j u s t the lead dioxide on graphite p l a t e . The intermediate curve corresponds to the average % phenol oxidized from Runs 4-1, 4-2, and 4-3 where a p a r t i c l e s i z e between 1.7 and 2.0 mm was used and the upper curve was obtained when working with p a r t i c l e s i z e s between 0.7 and 1.1 mm. Taking as a reference, the flow of 0.4 A/min, where the operation i s p r a c t i c a l l y isothermal, i t i s observed that when working without p a r t i c l e s the s p e c i f i c area was 3.3 cm ^ and only 20% of the phenol was oxidized, whereas f or a s p e c i f i c area of 20.6 cm ^ using the la r g e r p a r t i c l e s , about a 59% oxidation was achieved. With the smaller p a r t i c l e s (sizes = 0.7-1.1 mm) about a 67% of the phenol was oxidized and the s p e c i f i c area was 41.5 cm ^. (Calculations of the s p e c i f i c electrode areas are found i n Appendix 4.) These e f f e c t s w i l l be discussed i n the next section based on mathe- matical models f or the process. 87 0 0.2 0.4 0 6 0.8 1.0 1.2 1.4 F L O W R A T E ( I / m i n ) F i g . 31. E f f e c t o f anode s u r f a c e a r e a - p a r t i c l e s i z e on t h e % p h e n o l o x i d i z e d i n a s i n g l e - p a s s v s f l o w r a t e . 88 5 .12 C o m p a r i s o n s o f e x p e r i m e n t a l r e s u l t s w i t h m a t h e m a t i c a l m o d e l s 5 . 1 2 . 1 B a t c h e x p e r i m e n t s I f t h e r a t e o f p h e n o l d i s a p p e a r a n c e i s c o n t r o l l e d by mass t r a n s f e r o f p h e n o l f r o m t h e b u l k e l e c t r o l y t e t o t h e s u r f a c e o f t h e e l e c t r o d e , t h e n f o r a b a t c h r e c i r c u l a t i o n s y s t e m , t h e f o l l o w i n g e q u a t i o n r e l a t e s t h e p h e n o l f r a c t i o n a l c o n v e r s i o n w i t h d i m e n s i o n l e s s t i m e and mass t r a n s f e r g r o u p s ( A p p e n d i x 3 ) . X = 1 - exp (exp - K a L m K a L - 1 ) — - m t u m H e r e , t i s t h e t i m e t h e e l e c t r o l y t e spends i n t h e m i x i n g t a n k , a t h e s p e c i f i c s u r f a c e a r e a o f t h e e l e c t r o d e , L t h e e l e c t r o d e h e i g h t , and u t h e s u p e r f i c i a l e l e c t r o l y t e v e l o c i t y . The m o d e l c a l c u l a t i o n s a r e r e p r e s e n t e d as a band o r r e g i o n o f c o n - v e r s i o n v s t i m e , t a k i n g i n t o a c c o u n t t h a t a 10% e r r o r c a n be e x p e c t e d f r o m t h e e m p i r i c a l e q u a t i o n f o r t h e mass t r a n s f e r c o e f f i c i e n t . As w e l l , t h e c o r r e l a t i o n c h o s e n may u n d e r e s t i m a t e t h e mass t r a n s f e r c o e f f i c i e n t f o r t h i s p a r t i c u l a r c a s e , as d i s c u s s e d i n A p p e n d i x 3 , page 1 6 0 . C a l c u - l a t i o n s o f X v s t a r e p r e s e n t e d i n A p p e n d i x 4 , T a b l e A - 2 . F i g u r e 28 shows t h e d e v i a t i o n s o f t h e e x p e r i m e n t a l r e s u l t s f rom t h e mass t r a n s f e r mode l when o p e r a t i n g a t 10 A . As e x p e c t e d , t h e c u r v e c o r - r e s p o n d i n g t o t h e l o w e s t i n i t i a l p h e n o l c o n c e n t r a t i o n (93 mg/1) i s c l o s e s t t o t h e mass t r a n s f e r - c o n t r o l l e d r e g i o n w i t h a 15 m i n c o n v e r s i o n a p p r o x i m a t e l y 15% l o w e r t h a n p r e d i c t e d by t h e mass t r a n s f e r m o d e l . Where t h e i n i t i a l c o n c e n t r a t i o n o f p h e n o l was 1100 mg/1 (Run 3 -13) t h e e x p e r i m e n t a l % p h e n o l o x i d i z e d a f t e r 15 m i n i s abou t 55% b e l o w t h a t p r e d i c t e d by t h e mass t r a n s f e r m o d e l . T h i s i m p l i e s t h a t t h e e l e c t r o - c h e m i c a l r e a c t i o n k i n e t i c s c o n t r o l s t h e r a t e o f p h e n o l o x i d a t i o n u n d e r 89 such conditions. However, as the phenol concentration drops during the course of the experiments, the curves approach that of the mass transf e r model. In other words, the process i s c o n t r o l l e d by r e a c t i o n k i n e t i c s at the beginning and as phenol depletes, mass tr a n s f e r becomes the con- t r o l l i n g mechanism. Figures 21 and 22 show that as the current i s increased at an i n i t i a l phenol concentration of the order of 100 mg/1 (Runs 3-4, 3-6) the experimental % phenol oxidized vs time curve moves cl o s e r to the mass tr a n s f e r region as might be expected. At 20 A, the experimental curve i s s t i l l below the mass t r a n s f e r region u n t i l about 75 min when the con- v e r s i o n was complete. But at 30 A the mass tr a n s f e r band encloses the experimental curve from Run 3-6, at 30 min time. For both 20 and 30 A currents, the curve at low pH i s cl o s e r to the mass t r a n s f e r model than the curve corresponding to the high pH run. It should be noted that the experimental curves obtained with e i t h e r the divided or the undivided c e l l are never above the mass t r a n s f e r region. The c l o s e s t experimental curve to the mass t r a n s f e r model i s that of Run 3-6 at 30 A and pH = 2.5. 5.12.2 Continuous experiments For a.single pass through the packed bed reactor where phenol d i s - appears, by a rate process f i r s t order i n phenol concentration, the assumption of plug flow y i e l d s (Appendix 3) - Jtn(l - X) = K a L/u where X i s the f r a c t i o n of phenol oxidized and K i s the o v e r a l l r a t e constant, which can be re l a t e d to the mass t r a n s f e r and electrochemical rate constants by using the concept of a d d i t i v e mass tr a n s f e r and e l e c t r o - chemical resistances as, 90 Using the data from those experiments performed i n the continuous mode, i t i s possible to evaluate an experimental rate constant. When - &n(l - X) i s plotted vs -jj a straight line i s obtained (Fig. 32) and the experimental rate constant K can be determined from the slope. Thus, using available correlations for K , K can be determined. m r The nature of the electrochemical rate constant K was alluded to in r Chapter 2. Unlike a chemical reaction rate constant, i t i s dependent on electrode potential. As discussed in Appendix 3, when operating at a fixed current rather than at a fixed potential, changes i n phenol concen- tration, flow rate, surface area, etc. w i l l result.in changes in potential and hence in K . Therefore, the analysis of results in terms of K to be r 3 r presented here, i s of limited usefulness except to indicate what resistance (mass transfer or electrochemical kinetics) i s more important under deter- mined conditions. However, considerations of the effects of the process variables on K does show qualitative agreement with what i s expected from the simple model, as w i l l be shown with the following examples. Taking the average fractional conversion from experiments 4-1, 4-2, 4-3, performed at 10 A and i n i t i a l concentrations of phenol of 100 ± 5 mg/1, with particle sizes between 1.7 and 2.00 mm, the experimental:rate con- stant i s obtained from the corresponding straight line represented in Fig. _3 32. The resulting value of K is 3.1 x 10 cm/s. Using the correlation by Pickett and Stanmore (45) for mass transfer coefficient in electro- chemical packed bed reactors K and K are calculated at various flows m r (Table A-3). 91 92 F o r e x a m p l e , a t 0 . 2 5 V m i n : K = 3 . 9 x 1 0 ~ 3 c m / s , K = 1 5 . 7 x i o " 3 c m / s , and m ' r ' a t 1 .10 A / m i n : K = 8 .8 x i o " 3 c m / s , K = 4 . 8 x i o " 3 c m / s . m r T h i s i m p l i e s t h a t a t t h e l o w f l o w r a t e , mass t r a n s f e r c o n t r o l s t h e p h e n o l o x i d a t i o n b e c a u s e t h e mass t r a n s f e r r e s i s t a n c e (~~) r e p r e s e n t s abou t an m 80% o f t h e o v e r a l l r e s i s t a n c e . On t h e o t h e r h a n d , a t t h e h i g h f l o w r a t e , t h e r e s i s t a n c e t o r e a c t i o n ( ~ ) r e p r e s e n t s a 65% o f t h e o v e r a l l r e s i s - t a n c e ( t r ) . The same c a l c u l a t i o n p r o c e d u r e was a p p l i e d t o t h e d a t a f r o m Run 4-4 when t h e i n i t i a l c o n c e n t r a t i o n was 580 m g / 1 , w o r k i n g u n d e r o t h e r w i s e e q u a l c o n d i t i o n s as i n t h e p r e v i o u s e x a m p l e . The r e s u l t i n g e x p e r i m e n t a l r a t e _3 c o n s t a n t was 1.4 x 10 c m / s . I n t h i s c a s e t h e e l e c t r o c h e m i c a l r e a c t i o n r e s i s t a n c e was h i g h e r t h a n t h e mass t r a n s f e r r e s i s t a n c e a t a l l f l o w s ( T a b l e A - 4 ) - 3 - 3 i . e . , a t 0 . 2 5 A / m i n : K = 3 . 8 x 10 c m / s , K = 2 . 2 1 x 10 cm/s m r a t 1 .10 l/mln : K = 8 .8 x i o " 3 c m / s , K = 1.67 x 1 0 ~ 3 c m / s . m r The r e s u l t s f r o m b o t h examples a r e i n q u a l i t a t i v e agreement w i t h t h e p r e - d i c t i o n s o f t h e m o d e l , s i n c e a t a c o n s t a n t i n i t i a l c o n c e n t r a t i o n , K ' r s h o u l d d e c r e a s e when t h e f l o w r a t e i n c r e a s e s , and a t a c o n s t a n t f l o w r a t e , K s h o u l d a l s o d e c r e a s e when t h e i n i t i a l c o n c e n t r a t i o n i n c r e a s e s as r d e s c r i b e d i n A p p e n d i x 3 . The e x p e r i m e n t a l r a t e c o n s t a n t was a l s o e v a l u a t e d f o r t h e s m a l l e r p a r t i c l e s i z e ( 0 . 7 - 1 . 1 mm). U s i n g t h e r e s u l t s f rom Run 4 - 8 , t h e e x p e r i - - 3 m e n t a l r a t e c o n s t a n t i s K = 1.8 x 10 cm/ sec ( T a b l e A - 5 ) , and t h e mass t r a n s f e r and r e a c t i o n c o e f f i c i e n t s a t t h e e x t r e m e f l o w s a r e , a t 0 . 2 5 £ / m i n : K = 5 . 3 x 1 0 ~ 3 c m / s , K = 2 . 6 8 x 1 0 ~ 3 cm/s m ' r 1 .10 A / m i n : K = 1 2 . 1 x i o " 3 c m / s , K = 2 . 0 9 x i o " 3 c m / s . m ' r 93 These d a t a i n d i c a t e t h a t t h e r e a c t i o n o f f e r e d a h i g h e r r e s i s t a n c e t h a n mass t r a n s f e r . I n t h i s c a s e , t h e i n i t i a l c o n c e n t r a t i o n o f p h e n o l was 100 mg/1 and a l l t h e e x p e r i m e n t a l c o n d i t i o n s e x c e p t f o r p a r t i c l e s i z e , we re e q u a l t o t h o s e i n t h e f i r s t e x a m p l e . The mass t r a n s f e r c o e f f i c i e n t s K a r e h i g h e r a t a g i v e n f l o w r a t e t h a n i n t h e f i r s t m e x a m p l e , due t o t h e dependance o f t h e e m p i r i c a l mass t r a n s f e r c o e f f i c i e n t on p a r t i c l e s i z e , b u t l o w e r r e a c t i o n c o e f f i c i e n t s K r a r e o b t a i n e d w i t h t h e s m a l l e r p a r t i c l e s . T h i s r e s u l t i s a l s o i n q u a l i t a t i v e agreement w i t h t h e t h e o r e t i c a l m o d e l , s i n c e f o r a h i g h e r s p e c i f i c s u r f a c e a r e a o f t h e bed ( s m a l l e r p a r t i c l e s ) , t h e e l e c t r o d e p o t e n t i a l w i l l be l o w e r f o r t he same i n p u t c u r r e n t a p p l i e d t o t h e c e l l , and t h u s l o w e r v a l u e s o f K r a r e e x p e c t e d . Even t h o u g h a l o w e r v a l u e o f t h e o v e r a l l r a t e c o n s t a n t K i s o b t a i n e d w i t h t h e s m a l l e r p a r t i c l e s , t h e n e t e f f e c t i s t h a t h i g h e r c o n v e r s i o n s a r e a c h i e v e d a t a g i v e n f l o w r a t e t h a n w i t h t h e l a r g e r p a r t i c l e s ( F i g . 3 1 ) , due t o t h e i n c r e a s e o f t h e s p e c i f i c s u r f a c e a r e a o f t h e b e d , f r o m 2 0 . 6 t o 4 1 . 5 cm \ T h i s i s e x p e c t e d f rom t h e p l u g f l o w e q u a t i o n : K a X = 1 - exp r u I t s h o u l d be e m p h a s i z e d t h a t t h e v a l u e s r e p o r t e d h e r e f o r t h e r e a c - t i o n r a t e c o n s t a n t s a r e p a r t i c u l a r f o r t h e s e t o f c o n d i t i o n s u sed i n e a c h e x p e r i m e n t , and t h e r e f o r e s h o u l d n o t be u s e d i n a s c a l e - u p , s i n c e f o r a l a r g e r c e l l even w i t h t h e same p a r t i c l e s i z e s t h a n u sed i n t h i s s t u d y , t h e a v e r a g e e l e c t r o d e p o t e n t i a l ( o r t h e a v e r a g e v a l u e ) w i l l be d i f f e r e n t f o r t h e same n e t c u r r e n t i n p u t , a c c o r d i n g t o t h e m o d e l . U s i n g t h e d a t a f rom Run 4 -7 where t h e anode was t h e e l e c t r o d e p o s i t e d Pb02 on g r a p h i t e f e e d e r p l a t e ( w i t h o u t p a r t i c l e s ) , t h e e x p e r i m e n t a l r a t e c o n s t a n t o b t a i n e d f rom t h e s l o p e o f t h e s t r a i g h t l i n e on F i g . 32 i s _3 K = 5 x 10 c m / s e c . F o r t h e r a n g e s o f f l o w s used i n t h e e x p e r i m e n t s t h e Re numbers r e f e r r e d t o t h e h y d r a u l i c d i a m e t e r , de = 2SW/(S + W) a r e l o w e r t h a n 7 0 0 . F o r R e ^ e < 2 0 0 0 , an a p p l i c a b l e c o r r e l a t i o n f o r t h e mass t r a n s f e r c o e f f i c i e n t i n a p a r a l l e l p l a t e r e a c t o r i s (page 1 3 3 , R e f . 10) m = 1.47 ( R e , S c ^ % ) 1 / 3 D v de L S F o r l i q u i d f l o w r a t e s be tween 0 . 2 5 and 0 . 1 1 J l / m i n , t h i s c o r r e l a t i o n - 4 r e s u l t s i n mass t r a n s f e r c o e f f i c i e n t v a l u e s be tween 4 x 10 and - 4 7 x 10 cm/ sec w h i c h a r e abou t t e n t i m e s l o w e r t h a n t h e c a l c u l a t e d e x p e r i m e n t a l c o n s t a n t . S i n c e t h e o v e r a l l r a t e c o n s t a n t c a n n e v e r be h i g h e r t h a n t h e mass t r a n s f e r c o e f f i c i e n t , i t means t h a t t h e v a l u e s o f t h e mass t r a n s f e r c o e f f i c i e n t o b t a i n e d by t h e c o r r e l a t i o n a r e an u n d e r - e s t i m a t i o n o f t h e r e a l s i t u a t i o n . I t c o u l d be s u g g e s t e d t h a t i n p r a c - t i c e t h e r e i s an enhancement i n t h e mass t r a n s f e r c o e f f i c i e n t due t o gas e v o l u t i o n , w h i c h i s n o t t a k e n i n t o a c c o u n t by t h e c o r r e l a t i o n . Mos t o f t h e c o r r e l a t i o n s f o r mass t r a n s f e r c o e f f i c i e n t s i n gas e v o l v i n g p a r a l l e l p l a t e e l e c t r o d e s have been d e v e l o p e d f o r t h e c a s e o f s t a t i o n a r y s o l u t i o n where m i x i n g o f t h e e l e c t r o l y t e i s o n l y p r o v i d e d by t h e gas b u b b l e s . I t i s known t h a t mass t r a n s f e r c o e f f i c i e n t s unde r gas e v o l u t i o n a r e be tween f o u r o r f i v e t i m e s g r e a t e r t h a n t h e mass t r a n s f e r c o e f f i c i e n t s f rom c o n v e n t i o n a l f r e e - c o n v e c t i o n c o r r e l a t i o n s i n f l a t p l a t e s ( 1 0 ) . B u t f o r t h e c a s e o f f o r c e d c o n v e c t i o n and gas e v o l v i n g e l e c t r o d e s , an e m p i r i c a l c o r r e l a t i o n c o u l d n o t be found i n t h e l i t e r a t u r e . H o w e v e r , t h e r e s u l t s seem t o i n d i c a t e t h a t mass t r a n s f e r c o n t r o l s t h e r a t e o f p h e n o l o x i d a t i o n on t h e f l a t p l a t e , s i n c e even i f t h e mass t r a n s f e r c o e f - f i c i e n t s were i n c r e a s e d by a f a c t o r o f f i v e , t h e r e s u l t i n g v a l u e s 95 would s t i l l be lower than the experimental rate constant. 5.13 Current efficiencies, energy requirements and energy costs for phenol oxidation 5.13.1 Batch experiments Typical current efficiencies, energy requirements, and costs are estimated for batch experiments nos. 3-3, 3-12, 3-13, after recirculation times of 15 and 90 min. A sample calculation is given in Appendix 4, page 178. For the estimation of the % C.E. i t was assumed that four electrons are transferred from the phenol molecule as proposed by Covitz (Reaction R9). Energy per g mol phenol oxidized i s calculated taking $0.02/Kw-h as a basis. Table 6 shows that for lower i n i t i a l concentrations of phenol or higher recirculation times, the % C.E. are relatively lower and the energy costs are higher. TABLE 6 TYPICAL CURRENT EFFICIENCIES, ENERGY REQUIREMENTS AND ENERGY COSTS IN BATCH EXPERIMENTS WITH UNDIVIDED CELL (Q=1.12 £/min) Run No. \ (mg/1) AV (volts) t (min) % (mg/1) X (%) C.E. (%) Energy rKw-h s ĝ moi ; E l e c t r i c a l costs ($/g mol) 3-3 93 8.6 15 90 28 0 70 100 11 3 6.7 22.6 0.14 0.45 3-12 525 8.3 15 90 320 5 39 99 36 19 2.5 4.8 0.05 0.10 3-13 1100 8.5 15 90 792 22 28 98 54 35 1.7 2.4 0.03 0.05 96 Run 3-3 a t C . = 9 3 m g / 1 , y i e l d s t h e maximum e n e r g y c o s t o f A 0 $ 0 . 4 5 / g m o l o x i d i z e d f o r t h e 90 m i n o p e r a t i o n , where p r a c t i c a l l y no p h e n o l was p r e s e n t a f t e r t r e a t m e n t . 5 . 1 3 . 2 C o n t i n u o u s e x p e r i m e n t s U s i n g t h e d a t a f rom Runs 4 -3 and 4 - 4 , i t i s p o s s i b l e t o e s t i m a t e % C . E . and e n e r g y c o s t s f o r v a r i o u s f l o w s i n a s i n g l e p a s s t h r o u g h t h e c e l l . A sample c a l c u l a t i o n i s g i v e n i n A p p e n d i x 4 , page 1 8 0 , and t h e r e s u l t s a r e p r e s e n t e d i n T a b l e 7. TABLE 7 T Y P I C A L CURRENT E F F I C I E N C I E S , ENERGY REQUIREMENTS AND ENERGY COSTS I N CONTINUOUS EXPERIMENTS WITH UNDIVIDED CELL C C E n e r g y E l e c t r i c a l Run 1 Q AV 2 X C . E . r K w - h . c o s t s N o . (mg/1) ( A / m i n ) (V) (mg/1) (%) (%) ^g m o l ' ' ( $ / g m o l ) 4 -3 95 0 . 2 5 8 .7 39 59 9 . 6 9 .7 0 . 1 9 5 0 . 5 5 8 . 3 63 34 1 2 . 2 7 .4 0 . 1 4 8 1.1 7 .4 76 20 1 4 . 3 5 .6 0 . 1 1 0 4-4 580 0 . 2 5 8 .6 365 37 3 6 . 7 2 . 5 0 . 0 5 0 0 . 5 5 7 . 9 470 19 4 1 . 5 2 . 0 0 . 0 4 1 1 .10 7 . 0 526 9 3 9 . 3 1.9 0 . 0 3 7 The e l e c t r i c a l c o s t o f t r e a t m e n t d e c r e a s e s as t h e i n i t i a l p h e n o l c o n c e n t r a t i o n and e l e c t r o l y t e f l o w s a r e i n c r e a s e d . C o m p a r i n g t h e r e s u l t s f rom Run 3-12 a f t e r 15 m i n ( T a b l e 6) w i t h t h o s e f r o m Run 4-4 a t 0 . 2 5 5 , /min , i t c an be s e e n t h a t i n t e rms o f c u r r e n t e f f i c i e n c e s and e n e r g y r e q u i r e m e n t s , b o t h s i t u a t i o n s a r e p r a c t i c a l l y e q u i v a l e n t . 5 . 1 3 . 3 C o s t c o m p a r i s o n s I t i s o f i n t e r e s t t o compare t h e e s t i m a t e d e l e c t r i c a l c o s t s w i t h t h e o p e r a t i n g c o s t s f o r o t h e r t r e a t m e n t s g i v e n by K a t z e r ( 8 ) . T a b l e 8 97 TABLE 8 OPERATING COSTS OF VARIOUS TREATMENT METHODS, ESTIMATED FOR 1974 FOR A CATALYTIC CRACKER EFFLUENT CONTAINING 700 mg/1 PHENOL (8) T r e a t m e n t c o s t s P r o c e s s ( $ / 1 0 0 0 g a l ) ( $ / g m o l ) ( 5 ) O x i d a t i o n p o n d ^ 0 . 1 4 - 0 . 5 1 0 . 0 0 5 - 0 . 0 1 8 A c t i v a t e d s l u d g e ^ 0 . 2 4 0 . 0 0 9 A c t i v a t e d s l u d g e , w i t h d i l u t i o n { } 2 . 4 0 . 0 9 0 (3) C a r b o n a d s o r p t i o n 0 . 8 6 0 . 0 3 1 (4) C a t a l y t i c o x i d a t i o n ' 0 . 57 0 . 0 2 0 E l e c t r o c h e m i c a l (JJ) o x i d a t i o n 2 . 0 0 U / ) 0.08^ ' (1) $ 0 . 3 8 / 1 0 3 g a l i n 1967 ; o p e r a t i n g c o s t s o f b i o l o g i c a l o x i d a t i o n ponds a t B i l l i n g M o n t a n a O i l R e f i n e r y ; e x t r a p o l a t i o n t o 1974 g i v e s $ 0 . 5 1 / 1 0 3 g a l . C a p i t a l i n v e s t m e n t s n o t i n c l u d e d . (2) E x t r a p o l a t e d f rom 1968 t o 1974 u s i n g a f a c t o r o f 1 . 3 4 ; i f d i l u t i o n o f e f f l u e n t i s r e q u i r e d c o s t o f t r e a t m e n t w i l l i n c r e a s e ; 1 0 : 1 d i l u t i o n f a c t o r r e s u l t s i n $ 2 . 4 / 1 0 3 g a l . 1 0 6 g a l / d a y d e s i g n f o r t r e a t i n g u n f i l t e r e d a c t i v a t e d s l u d g e p l a n t e f f l u e n t t o p r o d u c e w a t e r w i t h 8 mg/1 C . O . D . (4) C a l c u l a t e d f o r a 99% p h e n o l r e m o v a l by c a t a l y t i c o x i d a t i o n . ^ " ^ C a l c u l a t e d f r o m t h e o r i g i n a l p a p e r ( R e f . 8) a s s u m i n g t h a t t o t a l p h e n o l c o n v e r s i o n was a c h i e v e d i n a l l t h e e x a m p l e s . ^ C a l c u l a t e d i n t h i s s t u d y f o r t r e a t i n g a 51 v o l u m e w i t h 99% r e m o v a l ( A p p e n d i x 4 ) . C a l c u l a t e d i n t h i s s t u d y by i n t e r p o l a t i n g f r o m T a b l e 6 f o r 700 mg/1 p h e n o l i n i t i a l c o n e , u s i n g a f a c t o r o f $ 0 . 0 2 / K w - h f o r e l e c t r i c a l e n e r g y c o s t . 98 shows t h e 1974 c o s t s f o r t r e a t i n g a c a t a l y t i c c r a c k e r e f f l u e n t c o n t a i n i n g 700 mg/1 p h e n o l . F o r t h e e l e c t r o c h e m i c a l p r o c e s s , e l e c t r i c a l e n e r g y c o s t s a r e i n t e r p o l a t e d f rom T a b l e 6 u s i n g t h e d a t a f rom Run 3-12 (525 mg/1 p h e n o l ) and Run 3-13 (1100 mg/1 p h e n o l ) a t 90 m i n , when p h e n o l c o n - v e r s i o n s a r e 98-99%. To t r e a t an e f f l u e n t w i t h 700 mg/1 p h e n o l , a p p r o x i m a t e l y $ 0 . 0 8 / g m o l p h e n o l w o u l d be n e c e s s a r y . I n A p p e n d i x 4 i s e s t i m a t e d t h e e l e c t r i c a l c o s t p e r v o l u m e o f w a s t e i n about $ 2 . 0 0 / 1 0 3 g a l , t o compare w i t h t h e d a t a by K a t z e r . E l e c t r i c a l e n e r g y c o s t i s abou t t e n t i m e s h i g h e r t h a n t h e a c t i v a t e d s l u d g e c o s t i n n o r m a l c o n d i t i o n s , b u t o f t h e o r d e r o f t h e a c t i v a t e d s l u d g e c o s t i f d i l u t i o n i s r e q u i r e d . The c o s t o f o t h e r p r o c e s s e s a p p e a r t o be l o w e r t h a n t h e e s t i m a t e d e l e c t r i c a l c o s t ; f o r e x a m p l e , c a r b o n a d s o r p t i o n o p e r a t i n g c o s t s w o u l d be l e s s t h a n a h a l f ( $ 0 . 0 3 / g m o l ) . I t s h o u l d be n o t e d t h a t t h e e l e c t r i c a l c o s t ( $ 0 . 0 8 / g m o l ) has been e s t i m a t e d f o r t h e a r b i t r a r y c e l l c h a r a c t e r i s t i c s and o p e r a t i n g c o n d i t i o n s u sed i n t h e e x p e r i m e n t s . To draw f i r m c o n c l u s i o n s abou t f e a s i b i l i t y o f t h e p r o c e s s w o u l d r e q u i r e an o p t i m i z a t i o n o f o p e r a t i n g c o s t s and a c a p i t a l c o s t e s t i m a t e o f t h i s and t h e o t h e r p r o c e s s e s . Some recommen- d a t i o n s f o r t h e f o r m e r a r e g i v e n i n C h a p t e r 7 . CHAPTER 6 CONCLUSIONS An i n v e s t i g a t i o n was made o f t h e e l e c t r o c h e m i c a l o x i d a t i o n o f p h e n o l on l e a d d i o x i d e p a c k e d bed a n o d e s . I n a l l t h e e x p e r i m e n t s p e r f o r m e d , p h e n o l o x i d a t i o n o c c u r r e d more e a s i l y t h a n t h e f u r t h e r o x i d a t i o n o f i n t e r - m e d i a t e o r g a n i c s t o c a r b o n d i o x i d e o r c a r b o n a t e s . The c o n c l u s i o n s o f t h i s s t u d y a r e summar ized b e l o w . 1. E l e c t r o d e p o s i t e d l e a d d i o x i d e was f o u n d t o be a b e t t e r anode t h a n t h e a n o d i z e d l e a d s h o t i n t e rms o f p h e n o l o x i d a t i o n and c o r r o s i o n r e s i s t a n c e . 2 . The o x i d a t i o n o f p h e n o l was more r a p i d u n d e r a c i d i c c o n d i t i o n s b u t t h e r e m o v a l o f o x i d i z e d p r o d u c t s (measured by t h e t o t a l o r g a n i c c a r b o n ) was f a v o u r e d by a l k a l i n e c o n d i t i o n s . I n d i v i d e d c e l l s , o p e r a t e d w i t h r e c i r c u l a t i o n o f s o l u t i o n , t h e e l e c t r o l y t e pH depended on t h e t y p e o f i o n s e l e c t i v e membrane u s e d . An a n i o n i c membrane w h i c h p r o v i d e d a pH i n c r e a s e f r o m a c i d i c t o a l k a l i n e p r o v e d t o be s u p e r i o r t o a c a t i o n i c membrane i n t e r m s o f T . O . C . r e m o v a l . 3 . The r a t e s o f p h e n o l o x i d a t i o n i n d i v i d e d and u n d i v i d e d c e l l s were s i m i l a r . I n t e rms o f T . O . C . r e m o v a l no improvement was o b t a i n e d w i t h t h e d i v i d e d c e l l even u n d e r opt imum pH c o n t r o l l e d c o n d i t i o n s p r o v i d e d by t h e a n i o n i c membrane. 4 . The e x t e n t s o f p h e n o l and T . O . C . o x i d a t i o n i n c r e a s e d w i t h a p p l i e d c u r r e n t d e n s i t y a t h i g h o r l o w p H . B u t a t h i g h p H , c u r r e n t d e n s i t y 99 changes a f f e c t e d the rate of phenol oxidation more strongly than at low pH. -3 -3 5. An increase i n e l e c t r o l y t e conductivity from 8 x 10 to 32 x 10 (ft.cm) ^ had no e f f e c t on the rates of phenol or T.O.C. oxidation at high or low pH at c.d. = 1052.6 A/m2, but at 526.3 A/m2 c.d., the same increase i n conductivity produced higher T.O.C. oxidation rates, even though the phenol oxidation rates remained constant. 6. The e f f e c t of increasing the i n i t i a l phenol concentration i n a s i n g l e pass was to reduce the % phenol oxidized at a given time or i n a s i n g l e pass. However, the current e f f i c i e n c y f o r phenol oxidation increased. 7. Increasing e l e c t r o l y t e flow r a t e reduced the s i n g l e pass phenol conversion at a given i n l e t phenol concentration i n continuous experi- ments. 8. Increasing the s p e c i f i c surface area of the anode i n the range of 3.3 to 41.5 cm2/cm3 produced higher single-pass phenol conversions at a given flow rate and i n l e t phenol concentration of the e l e c t r o l y t e . 9. Comparisons of the experimental r e s u l t s from batch experiments with the mass t r a n s f e r model indicated that the oxidation of phenol i s . ..-• co n t r o l l e d by the electrochemical reaction at the beginning and as phenol depletes, the experimental % phenol oxidized vs time approach that of the mass transf e r model. Continuous experiments showed that the electrochemical r e a c t i o n r e s i s t a n c e becomes more important at higher e l e c t r o l y t e flow r a t e s , higher i n l e t phenol concentrations and smaller p a r t i c l e s i z e s . 10. E l e c t r i c energy costs estimated were generally higher than operating costs of other processes. However, power costs can l i k e l y be reduced with further work. CHAPTER 7 RECOMMENDATIONS I n v e s t i g a t i o n o f o t h e r f a c t o r s r e g a r d i n g t h e e l e c t r o c h e m i c a l o x i d a - t i o n o f p h e n o l c o u l d p o s s i b l y r a i s e t h e p r o s p e c t i v e f e a s i b i l i t y o f t h e p r o c e s s . Improvements t o t h e m o d e l w o u l d l e n d c o n f i d e n c e t o t h e f e a s - i b i l i t y c a l c u l a t i o n s . On t h e b a s i s o f t h i s s t u d y , t h e f o l l o w i n g r ecommenda t i ons a r e made, t o y i e l d a more c o m p l e t e k n o w l e d g e o f t h e p r o c e s s . 1. A n a l y s i s o f t h e p h e n o l o x i d a t i o n p r o d u c t s : R o u t i n e a n a l y s i s o f t h e p h e n o l o x i d a t i o n p r o d u c t s , ( b e n z o q u i n o n e , h y d r o q u i n o n e and p o s s i b l y m a l e i c a c i d ) s h o u l d be a t t e m p t e d , t o draw d e f i n i t e c o n c l u s i o n s abou t e f f l u e n t q u a l i t y u n d e r d i f f e r e n t o p e r a t i n g c o n d i t i o n s . The T . O . C . a n a l y s i s w o u l d p e r m i t one t o d e t e r m i n e what f r a c t i o n o f t h e c a r b o n i s i n t h e f o r m o f e a c h compound. 2 . Measurements o f e l e c t r o d e p o t e n t i a l : R e f e r e n c e e l e c t r o d e s s h o u l d be l o c a t e d a t c e r t a i n p o i n t s on t h e b e d , t o d e t e r m i n e e l e c t r o d e p o t e n t i a l v a r i a t i o n s . I t w o u l d be o f i n t e r - e s t t o d e t e r m i n e t h e e f f e c t o f e l e c t r o d e p o t e n t i a l on t h e k i n d o f o x i d a t i o n p r o d u c t s unde r d i f f e r e n t o p e r a t i n g c o n d i t i o n s . A l s o e l e c t r o d e p o t e n t i a l measurements a r e o f i n t e r e s t f o r t h e p r o c e s s m o d e l l i n g . 3 . C o n t r o l o f t h e e l e c t r o l y t e t e m p e r a t u r e : A h e a t e x c h a n g e r b u i l t i n t h e e l e c t r o l y t i c c e l l w o u l d p e r m i t 101 102 t e m p e r a t u r e c o n t r o l w i t h i n t h e c e l l i n s i n g l e p a s s e x p e r i m e n t s . H o w e v e r , t h e c o n s t r u c t i o n o f a c e l l e q u i p p e d w i t h s u c h a h e a t e x c h a n g e r m i g h t be d i f f i c u l t . 4 . A n a l y s i s o f t h e gases p r o d u c e d d u r i n g e l e c t r o l y s i s : A gas a n a l y s i s t e c h n i q u e c o u l d be a s s e s s e d f o r c o m p l e t e m o n i t o r i n g o f t h e p r o c e s s . I t w o u l d p e r m i t a t o t a l c a r b o n b a l a n c e . A l s o a measurement o f t h e r a t e o f gas p r o d u c t i o n w o u l d be o f i n t e r e s t t o i n c l u d e t h e e f f e c t o f gas e v o l u t i o n i n t h e p r o c e s s m o d e l i n g . A f t e r t h e s e e x p e r i m e n t a l improvemen t s were a s s e s s e d , t h e e f f e c t o f some o f t h e o t h e r i m p o r t a n t f a c t o r s on t h e e l e c t r o c h e m i c a l o x i d a t i o n o f p h e n o l m i g h t be s t u d i e d , e . g . , t h e e f f e c t o f d i f f e r e n t anode m a t e r i a l s on t h e k i n d o f o x i d a t i o n p r o d u c t s , and t h e e f f e c t o f t e m p e r a t u r e and e l e c - t r o d e p o t e n t i a l on t h e r a t e s o f o x i d a t i o n o f p h e n o l and i n t e r m e d i a t e s . The f o l l o w i n g r ecommenda t ions w o u l d a l l o w improvemen t s t o t h e m a t h e m a t i c a l m o d e l i n g o f t h e p r o c e s s i n a d d i t i o n t o f u r t h e r i n g g e n e r a l u n d e r s t and i n g . 5 . The e f f e c t o f gas e v o l u t i o n on t h e mass t r a n s f e r c o e f f i c i e n t s h o u l d be s t u d i e d . A n o t h e r e l e c t r o c h e m i c a l r e a c t i o n t h a t i s p u r e l y mass t r a n s f e r c o n t r o l l e d c o u l d be u s e d f o r t h i s p u r p o s e ( 4 5 ) . 6 . C e l l l e n g t h , w i d t h , and t h i c k n e s s o f t h e bed s h o u l d be v a r i e d . Measurements o f e l e c t r o d e p o t e n t i a l a t d i f f e r e n t p o i n t s on t h e bed w o u l d p e r m i t one t o d e t e r m i n e u n d e r w h i c h c o n d i t i o n s t h e a s s u m p t i o n o f a u n i f o r m o r a v e r a g e e l e c t r o d e p o t e n t i a l i s r e a s o n a b l e . 7. E x p e r i m e n t s c o u l d be s e t up t o d e t e r m i n e t h e r e l a t i o n s h i p be tween e l e c t r o d e p o t e n t i a l and e l e c t r o c h e m i c a l r a t e c o n s t a n t u n d e r d i f f e r e n t o p e r a t i n g c o n d i t i o n s . 8 . The e f f e c t o f gas e v o l u t i o n and e l e c t r o l y t e c o n d u c t i v i t y on t h e 103 e l e c t r o d e p o t e n t i a l c o u l d be s t u d i e d t o i n c l u d e t h e s e e f f e c t s i n t h e m a t h e m a t i c a l m o d e l s . 9 . Computer methods c o u l d be u s e d t o c o r r e l a t e a l l t h e v a r i a b l e s a f f e c t i n g t h e p r o c e s s , and d e t e r m i n e c o s t s o f e n e r g y unde r d i f f e r e n t o p e r a t i n g c o n d i t i o n s t o o p t i m i z e o p e r a t i n g c o s t s . 104 NOMENCLATURE T y p i c a l u n i t s a s p e c i f i c s u r f a c e a r e a o f t h e bed c m 2 / c m 2 a . , b . c o n s t a n t s o f t h e T a f e l e q u a t i o n f o r t h e J ~* r e a c t i o n j V c d . c u r r e n t d e n s i t y r e f e r r e d t o t h e s u r f a c e a r e a o f t h e f e e d e r p l a t e A / m 2 C . E . c u r r e n t e f f i c i e n c y C . i n i t i a l p h e n o l c o n c e n t r a t i o n i n b a t c h e x p e r i m e n t s mg/1 C ^ i n l e t p h e n o l c o n c e n t r a t i o n mg/1 C . o u t l e t p h e n o l c o n c e n t r a t i o n mg/1 A 2 C. p h e n o l c o n c e n t r a t i o n i n t h e b u l k o f s o l u t i o n mg/1 % C ^ p h e n o l c o n c e n t r a t i o n a t t h e s u r f a c e o f t h e s e l e c t r o d e mg/1 C c o n c e n t r a t i o n a s s o c i a t e d w i t h a w a t e r w / - i e l e c t r o l y s i s r e a c t i o n mg/1 dp a v e r a g e p a r t i c l e d i a m e t e r mm D d i f f u s i v i t y o f p h e n o l i n w a t e r c m 2 / s F F a r a d a y ' s c o n s t a n t c o u l / g e q u i v . i ^ l o c a l c u r r e n t d e n s i t y f o r p h e n o l o x i d a t i o n A / m i g l o c a l c u r r e n t d e n s i t y c a r r i e d by t h e s o l u t i o n A / m i l o c a l c u r r e n t d e n s i t y c a r r i e d by t h e m e t a l A / m m 2 i ^ a v e r a g e c u r r e n t d e n s i t y f o r p h e n o l o x i d a t i o n A / m i w a v e r a g e c u r r e n t d e n s i t y f o r w a t e r e l e c t r o l y s i s A / m 2 i t o t a l a v e r a g e c u r r e n t d e n s i t y A / m 2 I a p p l i e d c u r r e n t A e l e c t r i c a l c o n d u c t i v i t y o f t h e a n o l y t e e l e c t r i c a l c o n d u c t i v i t y o f t h e c a t h o l y t e e l e c t r i c a l c o n d u c t i v i t y o f t h e m e t a l e l e c t r i c a l c o n d u c t i v i t y o f t he s o l u t i o n e l e c t r o c h e m i c a l r e a c t i o n r a t e c o n s t a n t mass t r a n s f e r c o e f f i c i e n t o v e r a l l o r e x p e r i m e n t a l r a t e c o n s t a n t l e n g t h o f t h e c e l l p r e s s u r e e l e c t r o l y t e f l o w r a t e u n i v e r s a l gas c o n s t a n t R e y n o l d s number S c h m i d t number t h i c k n e s s o f t h e bed ( i n the d i r e c t i o n o f c u r r e n t ) t e m p e r a t u r e t i m e o f e l e c t r o l y s i s d i m e n s i o n l e s s t i m e r e s i d e n c e t i m e i n t h e m i x i n g t a n k s u p e r f i c i a l v e l o c i t y anode p o t e n t i a l c a t h o d e p o t e n t i a l r e v e r s i b l e e q u i l i b r i u m p o t e n t i a l f o r t h e r e a c t i o n j s t a n d a r d r e d u c t i o n p o t e n t i a l f o r t h e r e a c t i o n j t o t a l v o l t a g e d r o p t h r o u g h t h e c e l l vo lume o f t h e m i x i n g t a n k 106 W w i d t h o f t h e bed cm X p h e n o l f r a c t i o n a l c o n v e r s i o n y v a r i a b l e l e n g t h o f t h e bed cm z number o f e l e c t r o n s a s s o c i a t e d w i t h p h e n o l o x i d a t i o n G r e e k l e t t e r s a t r a n s f e r c o e f f i c i e n t E v o i d a g e o f t h e bed rij o v e r p o t e n t i a l f o r t h e r e a c t i o n j E, shape f a c t o r f o r t h e p a r t i c l e s 9 K a L / u d i m e n s i o n l e s s mass t r a n s f e r g roup m v s a mc s c k i n e m a t i c v i s c o s i t y o f w a t e r ( c m 2 / s ) cf> m e t a l p o t e n t i a l o f t h e anode V ma s o l u t i o n p o t e n t i a l o f t h e a n o l y t e V m e t a l p o t e n t i a l o f t h e c a t h o d e V s o l u t i o n p o t e n t i a l o f t h e c a t h o l y t e V 107 BIBLIOGRAPHY 1. 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S o c . 1 0 3 , 1 ( 1 9 5 6 ) . 3 9 . R u e t s c h i , P . , and C a h a n , B . D . " A n o d i c c o r r o s i o n and h y d r o g e n and o x y g e n o v e r v o l t a g e on l e a d and l e a d a n t i m o n y a l l o y s . " J . o f E l e c t r o c h e m . S o c . 1 0 4 , N o . 7 , 406 ( 1 9 5 7 ) . 4 0 . G r i g g e r , J . C , M i l l e r , H . C , and L o o m i s , F . D . " L e a d d i o x i d e anode f o r c o m m e r c i a l u s e . " J . E l e c t r o c h e m . S o c . 1 0 5 , 100 ( 1 9 5 8 ) . 4 1 . G i b s o n , F . D . " I n e r t l e a d d i o x i d e anode and p r o c e s s o f p r o d u c t i o n . " U . S . P a t . 8 9 3 , 8 2 3 . ( A p r i l 1 1 , 1 9 6 2 ) . 4 2 . S t a n d a r d methods f o r t h e e x a m i n a t i o n o f w a t e r and w a s t e w a t e r . A m e r i c a n P u b l i c H e a l t h A s s o c i a t i o n , 1 2 t h e d . (1955) p p . 5 1 6 , 5 1 1 . 4 3 . S h i e l d s , J . R . , and C o u l l , J . " R a t e s t u d i e s i n t h e e l e c t r o c h e m i c a l o x i d a t i o n o f p h e n o l . " T r a n s . Am. E l e c t r o c h e m . S o c . 8Q, 113 ( 1 9 4 1 ) . 4 4 . W e s l e y , W. Wate r q u a l i t y e n g i n e e r i n g f o r p r a c t i c i n g e n g i n e e r s . B a r n e s & N o b l e , New Y o r k ( 1 9 7 0 ) , p . 2 5 . 110 4 5 . P i c k e t t , D . J . , and S t a n m o r e , B . R . " A n e x p e r i m e n t a l s t u d y o f a s i n g l e l a y e r p a c k e d bed c a t h o d e i n an e l e c t r o c h e m i c a l f l o w r e a c - t o r . " J . A p p . E l e c t r o c h e m . _5, 95 ( 1 9 5 7 ) . 4 6 . G r o t , W. G . F . , Munn , G . E . , and W a l m s l e y , P . N . " P e r f l u o r i n a t e d i o n exchange membranes . " 1 4 1 s t m e e t i n g o f t h e E l e c t r o c h e m i c a l S o c . H o u s t o n , T e x a s , May 7 , 1 9 7 2 . 4 7 . Sedahmed, G . H . "Mass t r a n s f e r b e h a v i o u r o f gas e v o l v i n g p a r t i c u - l a t e - b e d e l e c t r o d e . " J . o f A p p l . E l e c t r o c h e m . % 37 ( 1 9 7 9 ) . 4 8 . A l k i r e , R . "Two d i m e n s i o n a l c u r r e n t d i s t r i b u t i o n w i t h i n a p a c k e d bed e l e c t r o c h e m i c a l f l o w r e a c t o r . " J . E l e c t r o c h e m . S o c . 1 2 1 , N o . 1, 95 ( 1 9 7 4 ) . 4 9 . P e r r y , R . H . , and C h i l t o n , C . H . C h e m i c a l e n g i n e e r s handbook . F i f t h e d i t i o n , McGraw H i l l , New Y o r k ( 1 9 7 3 ) . 5 0 . B i r d , R. B . , S t e w a r t , W. E . , and L i g h t f o o t , E . N . T r a n s p o r t phenomena. J o h n W i l e y & S o n s , I n c . , New Y o r k (1960) p . 5 1 5 . 5 1 . W e a s t , R. Handbook o f c h e m i s t r y and p h y s i c s . The C h e m i c a l Rubber C o . 5 3 r d e d i t i o n ( 1 9 7 2 / 7 3 ) . I l l APPENDIX 1 S p e c i f i c a t i o n o f A u x i l i a r y Equ ipmen t and M a t e r i a l s Power s u p p l y S o r e n s o n DCR 4 0 - 2 5 B V o l t m e t e r r a n g e : 0 -40V Am m e t e r r a n g e : 0 - 2 5 A V o l t m e t e r C e n t r a l S c i e n t i f i c C o . , D . C . V o l t m e t e r S c a l e s : 0 - 1 . 5 v o l t s 0 -15 v o l t s R o t a m e t e r s a) A n o l y t e : B r o o k s , f u l l v i e w i n d i c a t i n g r o t a m e t e r T y p e : 7 -1110 Tube N o . : R - 7 M - 2 5 - 1 F l o a t : 316 s t a i n l e s s s t e e l M a x . f l o w : 1400 c c / m i n ( s . g . = 1) S c a l e : 0-100% l i n e a r b) C a t h o l y t e : B r o o k s , f u l l v i e w i n d i c a t i n g r o t a m e t e r T y p e : 8 -1110 Tube N o . : R - 8 M - 2 5 - 2 F l o a t t y p e : 8 - R S - 8 M a x . f l o w : 1 U . S . G . P . M . ( s . g . = 1) S c a l e : 0 -250 mm l i n e a r Gas l i q u i d s e p a r a t o r s 2 . 5 cm I . D . and 60 cm l o n g g l a s s t u b e . L i q u i d o u t l e t l o c a t e d a t 40 cm f rom t h e b o t t o m . P a c k e d t o a p p r o x i m a t e l y 20 cm d e p t h w i t h 2 mm d i a m e t e r g l a s s b e a d s . F i l t e r s 3 . 0 cm I . D . and 15 cm l o n g g l a s s t u b e f i l l e d w i t h g l a s s w o o l ( M e r c k ) . P r e s s u r e gauges M a r s h - t y p e 3 - 1 0 0 - S S w i t h 316 s t a i n l e s s s t e e l t u b e S c a l e : 0 -30 p s i ( 1 / 4 p s i / d i v ) . 112 Pumps B a r r i s h Pumps C o . , N . Y . M o d e l t y p e : 1 2 A - 6 0 - 3 1 6 F l o w d a t a : 21 6 . P . H . a t 40 p s i d , 29 6 . P . H . a t 0 p s i d . ( p s i d i n d i - c a t e s o u t l e t p r e s s u r e minus i n l e t p r e s s u r e . ) Pumps a r e p r e s e t a t 45 p s i d b u t a r e a d j u s t a b l e t o 65 p s i d max. pH me te r C o r n i n g , M o d e l 101 ( a c c u r a c y ± 0 . 0 0 1 pH) E l e c t r o d e : F i s h e r , p l a s t i c b o d y - p r o t e c t e d b u l b t y p e M o d e l N o . 1 3 - 6 3 9 - 9 7 C o n d u c t i v i t y me te r S e i b o l d , M o d e l L T A . P r o v i d e d w i t h a d j u s t a b l e maximum r a n g e s ( f rom 1 mmho cm --'- t o 100 mmho cm --'- maximum) C o n d u c t i v i t y c e l l c o n s t a n t = 0 . 8 8 cm --'- T u b i n g I m p e r i a l Eas tman " P o l y F l o " 6 6 - P - 3 / 8 " V a l v e s W h i t e y , f o r g e d body r e g u l a t i n g 316 s t a i n l e s s s t e e l . 3 / 8 " c o n n e c t i o n s F i t t i n g s Swage lok c o m p r e s s i o n t u b e f i t t i n g s 316 s t a i n l e s s s t e e l 3 / 8 " Membranes a) IONAC membranes The IONAC membranes a r e s u p p l i e d by I o n a c C h e m i c a l S y b r o n C o r p o r a t i o n , B i r m i n g h a m , N . J . T a b l e A - l i n c l u d e s some o f t h e s p e c i f i c a t i o n s e n t by t h e m a n u f a c t u r e r . b) NAFI0N 127 membrane S u p p l i e d by duPon t de Nemours & C o . , D e l a w a r e . NAFION 127 c o n s i s t s o f an homogeneous f i l m o f 1200 e q u i v a l e n t w e i g h t p o l y m e r , 7 m i l s t h i c k ( p e r f l u o r o s u l f o n i c a c i d p o l y m e r ) . S u p p o r t s a r e made f r o m t e f l o n . D e t a i l s o f membrane p r o p e r t i e s s u c h as s t r e n g t h , i o n i c t r a n s p o r t , w a t e r p e r m e a b i l i t y e t c . , a r e a v a i l a b l e i n a p u b l i c a t i o n s u p p l i e d by t h e m a n u f a c t u r e r ( 4 6 ) . 113 TABLE A - l SUMMARY OF T Y P I C A L PROPERTIES OF IONAC MEMBRANES A n i o n Exchange C a t i o n Exchange Membranes Membranes P r o p e r t y MC-3142 MC-3470 MA-3475 E l e c t r i c a l R e s i s t a n c e ( o h m - c m 2 , A . C . m e a s u r e - ment) O . I N N a C l l . O N N a C l 14 5 12 6 17 8 % P e r s e l e c t i v i t y ( 0 . 5 N N a C l / l . O N N a C l ) ( 0 . 2 N N a C l / O . l N N a C l ) 9 4 . 1 9 9 . 0 9 6 . 2 9 9 . 0 Wa te r P e r m e a b i l i t y ( m l / h r / f t . 2 / 5 p s i ) n e g l i g i b l e ( l e s s t h a n 30) ( l e s s t h a n 30) M u l l e n B u r s t S t r e n g t h (minimum p s i . ) 175 200 200 Membrane T h i c k n e s s ( m i l s ) 7 15 15 A p p r o x . D e n s i t y Ne t as s h i p p e d o z . / y d 2 g / m 2 6 202 12 405 12 405 C a p a c i t y meq/g 1.08 1.22 0 . 7 0 D i m e n s i o n a l S t a b i l i t y ( a b i l i t y t o r e w e t a f t e r d r y i n g ) good good good C h e m i c a l S t a b i l i t y H2S01+ H C l NaOH S a l t up t o 6 0 ° C up t o 5% up t o 4% NR* OK a t a l l c o n e . up t o 1 2 5 ° C up t o 35% c o n e . H C l 50% NaOH OK a t a l l c o n e . up t o 1 2 5 ° C S u p e r i o r t o MC-3470 i n l o w and h i g h pH m e d i a S i z e A v a i l a b l e n o m i n a l s h e e t s i z e s ( i n c h e s 40 x 120 40 x 30 x 120 96 40 x 120 *Not recommended 114 P l a s t i c s c r e e n s S u p p l i e d by C h i c o p e e M a n u f a c t u r i n g C o . , G e o r g i a S a r a n t y p e Max . o p e r a t i n g t e m p e r a t u r e = 1 2 5 ° F C h e m i c a l r e s i s t a n c e : good r e s i s t a n c e t o a c i d s and most a l k a l i s S t y l e : 6100900 W e i g h t / s q . y d . = 7 o z . P o l y p r o p y l e n e t y p e M a x . o p e r a t i n g t e m p e r a t u r e : 1 8 0 ° F C h e m i c a l r e s i s t a n c e : e x c e l l e n t r e s i s t a n c e t o most a c i d s and a l k a l i s w i t h e x c e p t i o n o f c h l o r o s u l f o n i c a c i d s and o x i d i z i n g a g e n t s S t y l e : 60070XX W e i g h t / s q . y d . = 8 .7 o z . Ca thode chamber p a c k i n g m a t e r i a l S t a i n l e s s s t e e l 304 - 20 x 20 mesh A n a l y t i c equ ipment and o p e r a t i n g c o n d i t i o n s s p e c i f i c a t i o n s a) Gas c h r o m a t o g r a p h y s p e c i f i c a t i o n s Gas c h r o m a t o g r a p h M a n u f a c t u r e r : V a r i a n A e r o g r a p h M o d e l : 1440 s e r i e s , s i n g l e co lumn m o d e l D e t e c t o r : H2 f l a m e i o n i z a t i o n d e t e c t o r C h r o m a t o g r a p h i c c o l u m n S u p p l i e r : W e s t e r n C h r o m a t o g r a p h y S u p p l i e s , New W e s t m i n s t e r , B . C . M a t e r i a l : g l a s s D i m e n s i o n s : 2mm I . D . , 6 . 4 m m 0 . D . , 6 f t l o n g P a c k i n g : 10% S P - 2 1 0 0 ON 1 0 0 / 1 2 0 S u p e l c o p o r t ( d e t a i l s o f t h e p a c k i n g a r e g i v e n i n B u l l e t i n 742D by S u p e l c o , I n c . ) O p e r a t i n g c o n d i t i o n s I n j e c t o r p o r t t e m p e r a t u r e 1 5 0 ° C Column t e m p e r a t u r e 1 2 0 ° C D e t e c t o r t e m p e r a t u r e 1 7 5 ° C C a r r i e r gas N 2 C a r r i e r gas f l o w 30 m l / m i n A i r f l o w 300 m l / m i n H 2 f l o w 30 m l / m i n A t t e n u a t i o n s e t t i n g 4 x 10 R e c o r d e r M o d e l : C o r n i n g 840 s e r i e s R e s p o n s e 1 mv f u l l s c a l e C h a r t s p e e d : 1 cm/min S y r i n g e S u p p l i e r : U n i m e t r i c s Sample s i z e : 1 uil 115 b) T . O . C . a n a l y s i s s p e c i f i c a t i o n s M o d e l : Beckman 915 t o t a l o r g a n i c c a r r o n A n a l y z e r : Beckman 865 i n f r a r e d a n a l y z e r O p e r a t i n g c o n d i t i o n s T e m p e r a t u r e o f t h e t o t a l c a r b o n c h a n n e l 1 0 0 0 ° C T e m p e r a t u r e o f t h e i n o r g a n i c c a r b o n c h a n n e l 1 5 0 ° C Oxygen f l o w i n e a c h c h a n n e l 250 m l / m i n S y r i n g e H a m i l t o n w i t h a u t o m a t i c p l u n g e r Sample s i z e 50 y£ R e c o r d e r M o d e l : H e w l e t t P a c k a r d 7127A R e s p o n s e : 1 mv f u l l s c a l e C h a r t s p e e d : 1 cm/min c ) A t o m i c a b s o r p t i o n s p e c i f i c a t i o n s M a n u f a c t u r e r : J a r r e l A s h , D i v i s i o n o f F i s h e r S c i . C o . M o d e l : 810 A t o m i c a b s o r p t i o n s p e c t r o p h o t o m e t e r L e a d l a m p : W e s t i n g h o u s e h o l l o w l e a d c a t h o d e Wave l e n g t h : 2170 A F l a m e : r i c h ( a i r - a c e t y l e n e ) R e c o r d e r H e w l e t t P a c k a r d 7127A S p e e d : v a r i a b l e Range : 1 o r 2 mv f u l l s c a l e R e a g e n t s P h e n o l — l i q u i f i e d r e a g e n t M a t h e s o n Coleman & B e l l M a n u f . Chem. C o d e : PX 5 1 1 - C B - 1 0 4 0 A s s a y : m i n 88% p h e n o l , max 12% H2O N a O H — p e l l e t s , r e a g e n t A m e r i c a n S c i e n t i f i c C h e m i c a l C o d e : SS 350 A s s a y : NaOH m i n 98% H 2 S 0 4 — r e a g e n t A . C . S . A l l i e d C h e m i c a l Code : 001180-009580 A s s a y : 9 5 . 5 - 9 6 . 5 % N a 2 S 0 i t — a n h y d r o u s r e a g e n t g r ade A m e r i c a n S c i e n t i f i c and C h e m i c a l SS 530 B u f f e r s : F i s h e r S c i e n t i f i c (2 ± 0 . 0 2 ) A m e r i c a n S c i e n t i f i c and C h e m i c a l (10 ± 0 . 0 1 ) L e a d s t a n d a r d s o l u t i o n f o r a t o m i c a b s o r p t i o n : F i s h e r S c i e n t i f i c , 1000 ppm Wate r f o r s o l u t i o n s : s i n g l e d i s t i l l e d w a t e r 116 APPENDIX 2 E x p e r i m e n t a l D a t a 1. C h a r a c t e r i s t i c s o f e a c h g roup o f e x p e r i m e n t s Group N o . 1 a) Mode o f o p e r a t i o n : B a t c h e x p e r i m e n t s u s i n g t h e d i v i d e d c e l l and t h e a n o d i z e d l e a d e l e c t r o d e b) C e l l d e s c r i p t i o n : C a t h o d e : S t a i n l e s s s t e e l 316 p l a t e and mesh A n o d e : F r e s h l e a d f e e d e r p l a t e and p a r t i c l e s P a r t i c l e s : l e a d s p h e r e s S i z e : dp = 2 mm W e i g h t : 250 g V o l u m e : " ° f — = 22 cm 3 ( r e f 49) 1 1 . 3 3 7 g / c c V o i d f r a c t i o n : e = (57 - 2 2 ) / 5 7 = 0 . 6 1 Membranes t e s t e d : IONAC MC-3142 o r IONAC MC-3470 p r o t e c t e d by s a r a n s c r e e n Group N o . 2 a) Mode o f o p e r a t i o n : B a t c h e x p e r i m e n t s u s i n g t h e d i v i d e d c e l l and t h e e l e c t r o d e p o s i t e d Pb02 e l e c t r o d e b ) C e l l d e s c r i p t i o n : C a t h o d e : S t a i n l e s s s t e e l 316 f e e d e r p l a t e and mesh A n o d e : Pb02 e l e c t r o d e p o s i t e d on g r a p h i t e f e e d e r p l a t e P a r t i c l e s : e l e c t r o d e p o s i t e d Pb02 c r u s h e d and s i z e d S i z e : 1.7 < dp < 2 . 0 mm W e i g h t : 215 g V o l u m e : 23 c m 3 V o i d f r a c t i o n (e) = 0 . 6 Membranes t e s t e d : C a t i o n i c membranes: IONAC MC-3470 NAFION 127 A n i o n i c membrane: IONAC MA 3475 I n a l l r u n s , a s a r a n s c r e e n was l o c a t e d be tween p a r t i c l e s and membrane t o p r e v e n t membrane b r e a k i n g and s h o r t c i r c u i t . The o n l y e x c e p t i o n was Run 2 -1 where a p o l y p r o p y l e n e s c r e e n was u s e d . Group N o . 3 a) Mode o f o p e r a t i o n : B a t c h e x p e r i m e n t s u s i n g t h e u n d i v i d e d c e l l ( o n l y one chamber) and t h e e l e c t r o d e p o s i t e d Pb02 e l e c t r o d e 117 b ) C e l l d e s c r i p t i o n : C a t h o d e : s t a i n l e s s s t e e l 316 p l a t e (no p a c k i n g mesh u s e d t o a v o i d b y - p a s s i n g o f t h e l i q u i d ) A n o d e : f e e d e r and p a r t i c l e s a r e t h e same as d e s c r i b e d i n g roup N o . 2 . S e p a r a t o r : two p i e c e s o f s a r a n s c r e e n be tween c a t h o d i c p l a t e and Pb02 p a r t i c l e s Group N o . 4 a) Mode o f o p e r a t i o n : C o n t i n u o u s e x p e r i m e n t s ( s i n g l e p a s s ) u s i n g t h e u n d i v i d e d c e l l and e l e c t r o d e p o s i t e d Pb02 anode b) C e l l d e s c r i p t i o n : The same as i n g roup N o . 3 w i t h e x c e p t i o n s o f Runs 4-7 and 4 - 8 . I n Run 4 - 7 : t h e anode was t h e f e e d e r p l a t e o n l y ( w i t h o u t p a r t i c l e s ) I n Run 4 - 8 : t h e p a r t i c l e s had t h e f o l l o w i n g c h a r a c t e r i s t i c s : S i z e : 0 . 7 < dp < 1.1 mm W e i g h t : 230 g V o l u m e : 2 4 . 5 cm V o i d f r a c t i o n (e) : ( 5 7 - 24.5)7 ,57 = 0 .57 2 . V o l u m e , f l o w p r e s s u r e and t e m p e r a t u r e o f t h e e l e c t r o l y t e s F o r g r o u p s N o . 1 and N o . 2 ( u n l e s s o t h e r w i s e s t a t e d ) E l e c t r o l y t e Volume ( £ ) F l o w s U / m i n ) P + ( kPa ) A n o l y t e 5 1.12 2 . 4 0 C a t h o l y t e 5 1 .54 2 . 4 0 + P r e s s u r e s may v a r y ± 20 k P a due t o gas e v o l u t i o n . N o t e : The s y m b o l (*) w h i c h a p p e a r s i n e a c h Run t a b l e (Groups 1 and 2) i n d i c a t e s t h a t t h e c a t h o l y t e pH and e l e c t r i c a l c o n d u c t i v i t y r e p o r t e d were measured on sample d i l u t e d by a f a c t o r o f t e n . F o r g roup N o . 3 t h e e l e c t r o l y t e v o l u m e f l o w and p r e s s u r e a r e t h e same g i v e n f o r t h e a n o l y t e i n g r o u p s N o . 1 and 2 w i t h e x c e p t i o n o f Run 3-15 where d a t a a r e r e c o r d e d . I n e x p e r i m e n t s group N o . 4 t h e e l e c t r o l y t e f l o w was v a r i e d and t h e r e - f o r e t h e p r e s s u r e , t o o . D a t a a r e t h e n r e p o r t e d i n e a c h p a r t i c u l a r Run t a b l e . T e m p e r a t u r e s A l l t h e e x p e r i m e n t s were s e t up a t room t e m p e r a t u r e (22 t o 2 4 ° C ) . D u r i n g t h e b a t c h e x p e r i m e n t s (g roups 1 , 2 , 3 ) a t c u r r e n t s b e l o w o r e q u a l t o 10 A t h e e l e c t r o l y t e t e m p e r a t u r e s r e m a i n e d c o n s t a n t . Bu t f o r a l l t h o s e e x p e r i m e n t s p e r f o r m e d a t 20 A ( i . e . , Runs 3 - 4 , 3 -5 ) a t e m p e r a t u r e i n c r e a s e o f 4°C was a l w a y s d e t e c t e d i n t h e r e c i r c u l a t i o n t a n k ( a f t e r t h e 120 m i n r u n ) . A l s o , when w o r k i n g a t 30 A t h e e l e c t r o l y t e t e m p e r - a t u r e r o s e a b o u t 1 2 ° C ( i . e . , Runs 3-6 and 3 - 7 ) . 118 F o r t h e c o n t i n u o u s e x p e r i m e n t s (g roup N o . 4) t h e t e m p e r a t u r e v a r i a t i o n s a t a g i v e n c u r r e n t a r e a f u n c t i o n o f t h e f l o w and w i l l be r e p o r t e d i n e a c h e x p e r i m e n t t a b l e . 3 . A n o d i z a t i o n B e f o r e each e x p e r i m e n t t h e anode was t r e a t e d w i t h 20% H2SO4 a t 10 A f o r 1 h o u r . ( R e f e r t o e x p e r i m e n t a l method s e c t i o n . ) Any e x c e p t i o n t o t h e s t a n d a r d p r o c e d u r e i s g i v e n i n t h e p a r t i c u l a r r u n t a b l e . 119 RUN 1-1 Membrane: IONAC MC-3142 A n o d i z a t i o n : S t a r t i n g f r o m t h e f r e s h l e a d p a r t i c l e s f o r 1 h ( a t t h e s p e c i f i e d c o n d i t i o n s ) E l e c t r o l y t e s A p p r o x . Cone . P H A n o l y t e 1% H 2S0i+ • 1.5 C a t h o l y t e 10% H 2S0i+ 1 .0* I = 20 A c . d . = 1 0 5 2 . 6 A / m 2 t (min) AV (V) T . O . C . (mg/1) % T . O . C . 0 7 . 3 72 0 15 6 . 8 68 6 30 6 . 5 62 14 , 60 5 .7 59 17 90 5 .7 57 21 120 5 . 6 52 28 Comments: P a r t i c l e s showed an homogeneous brown c o l o u r a f t e r t h e 1 h a n o d i z a t i o n . * I n d i c a t e s t h a t t h e pH was measured on a s ample d i l u t e d by a f a c t o r o f 1 0 . 120 RUN 1-2 Membrane: MC-3142 A n o d i z a t i o n : S t a r t i n g f r o m f r e s h l e a d p a r t i c l e s f o r 12 h ( a t t h e s p e c i f i e d c o n d i t i o n s ) E l e c t r o l y t e s A p p r o x . Cone . pH A n o l y t e 1% H2SO4 1 . 5 C a t h o l y t e 10% H 2 S0tj 1.1* I = 20 A c d . = 1 0 5 2 . 6 A/m t AV (min) (V) . T . O . C . (mg/1) % T . O . C . (Pb) (mg/1) 0 7 . 3 80 0 0 15 6 .9 75 6 30 6 . 4 72 11 0 . 8 60 5 . 8 66 18 0 . 7 90 5 .6 61 24 0 . 6 120 5 . 5 5 30 0 . 4 Comments: P a r t i c l e s showed homogeneous b rown c o a t i n g ( w i t h t h e same a p p e a r a n c e t h a n a f t e r Run 2 -1 ) 121 RUN 1-3 Membrane: IONAC MC-3142 A n o d i z a t i o n : The bed was o r i g i n a l l y a n o d i z e d f o r 12 h and t h e n was r e a n o d i z e d f o r 1 h b e f o r e t h e r u n E l e c t r o l y t e C o n e . pH A n o l y t e 5 g / 1 N a C l 5 .4 C a t h o l y t e 10% H 2S0i+ 1 .0* I = 10 A c . d . = 5 2 6 . 3 A / m 2 t AV T . O . C . (min) (V) mg/1 (%) 0 10 73 0 15 5 55 24 30 3 52 28 Comments: I t was o b s e r v e d t h a t t h e g l a s s w o o l f i l t e r c o l l e c t e d s m a l l f r a g m e n t s t h a t had f l a k e d o f f t h e e l e c t r o d e , and the s o l u t i o n t o o k a d a r k g r e y c o l o u r , i n d i c a t i n g t h a t t h e e l e c t r o d e was r a p i d l y d i s s o l v i n g . When t h e c e l l was opened t h e e l e c t r o d e had l o s t t h e brown o x i d e ^ c o a t i n g s h o w i n g t h e u n d e r l a y i n g g r e y l e a d . The membrane was f o u l e d w i t h d e p o s i t s . RUNS 1-4 t o 1-8 ( C o r r o s i o n s t u d i e s on a n o d i z e d l e a d ) Membrane: IONAC MC3142 A n o d i z a t i o n t i m e : 12 h s t a r t i n g f rom f r e s h l e a d , and 1 h p r i o r t o e a c h r u n I = 10 A c . d . = 5 2 6 . 3 A / m 2 C a t h o l y t e : 25 g / 1 NaOH (pH* = 1 2 . 7 K * = 14 x 10 (ft c m ) " 1 ) c Run N o . 1-4 1--5 1--6 1--7 1--8 A n o l y t e 5 g / 1 NaOH 5 g / 1 Na 2 S0i t 30 g / 1 NaaSOtt 30 g /1 Na 2 S0i t 30 g / 1 Na 2 S0i t NaOH t o a d j . pH NaOH t o a d j . pH NaOH t o a d j . pH NaOH t o a d j . pH t ( P b ) 1 ( P b ) 2 (Pb) (Pb) (Pb) (Pb) (min) pH (mg/1) (mg/1) pH (mg/1) pH (mg/1) PH (mg/1) pH (mg/1) 0 1 2 . 7 140 0 . 0 9 . 8 0 . 0 9 . 8 0 . 0 1 2 . 0 0 . 0 7 . 0 0 . 0 15 1 2 . 6 36 2 . 7 2 . 9 1.7 3 . 2 4 . 2 3 . 4 5 . 0 2 . 5 3 . 4 30 1 2 . 5 24 1.7 2 . 7 1.6 2 . 9 3 .5 2 . 9 5 . 0 2.2- 3 . 0 45 1 2 . 4 13 1.7 2 . 6 1.4 2 . 8 2 . 7 2 . 6 4 . 3 2 . 0 2 . 3 60 1 1 . 8 3 0 .7 2 . 5 1.3 2 . 7 2 . 0 2 . 5 3 .3 1.9 2 . 2 75 2 . 4 1.1 2 . 6 1.7 2 . 4 3 . 0 1.8 2 . 0 90 2 . 3 0 . 8 2 . 5 1.4 2 . 3 2 . 3 : T h e e l e c t r o l y t e s were r e c i r c u l a t e d w i t h o u t a p o t e n t i a l a p p l i e d . 2 A p o t e n t i a l was a p p l i e d b e f o r e t h e e l e c t r o l y t e s e n t e r e d the c e l l . *Measured on a sample d i l u t e d by a f a c t o r o f 1 0 . RUN 1-9 Membrane: IONAC MC-3470 I = 10 A c d . = 5 2 6 . 3 A / m 2 E l e c t r o l y t e s C o n c e n t r a t i o n K x 1 0 3 e _ i (n cm) pH A n o l y t e 5 g / 1 N32S01+ NaOH t o a d j u s t pH 6 .4 9 . 4 1 C a t h o l y t e 25 g / 1 NaOH 16* 1 2 . 6 * K e  X l ° 3 P h e n o l T . O . C . (Pb) t AV a (min) (V) (fi cm) 1 P a (mg/1) (%) (mg/1) (%) (mg/1) 0 6 . 0 6 .4 15 6 . 0 7 . 9 30 5 .7 9 . 0 45 5 . 4 1 0 . 5 60 5 . 2 1 1 . 5 75 5 . 0 1 2 . 0 90 5 . 0 1 2 . 7 9 . 4 1 104 3 . 0 0 55 47 2 . 8 0 42 60 2 . 7 0 33 68 2 . 6 3 27 74 2 . 5 8 19 82 2 . 5 4 11 89 80 74 8 2 . 2 72 11 1.7 71 13 1.5 70 14 0 . 9 68 15 0 . 3 67 16 0 . 3 RUN 2-1 Membrane: IONAC MC-3470 a g a i n s t p o l y p r o p y l e n e s c r e e n I = 10 A c . d . = 5 2 6 . 3 A / m 2 K x 1 0 3 e E l e c t r o l y t e C o n c e n t r a t i o n (ft cm) P H A n o l y t e 5 g / 1 Na2S0i+ 6 .4 9 .44 NaOH t o a d j u s t pH C a t h o l y t e 25 g / 1 NaOH 14* 1 2 . 7 * K x l O 3 P h e n o l T . O . C . (Pb) t AV e a _ 1 pH r a (min) (V) (ft cm) (mg/1) (%) (mg/1) (%) (mg/1) 0 1 2 . 9 6 .4 9 .44 1 0 2 . 0 78 0 0 . 0 15 1 5 . 0 8 . 1 3 .04 5 1 . 0 50 0 . 2 30 1 6 . 0 9 .4 2 . 8 0 2 9 . 0 71 72 8 0 . 1 45 1 6 . 0 1 1 . 0 2 .67 9 . 0 91 60 1 5 . 0 1 2 . 0 2 . 5 8 1.0 99 67 14 0 . 1 75 1 4 . 0 1 2 . 5 2 . 5 2 0 . 0 100 90 1 4 . 0 1 3 . 0 65 16 0 . 1 105 1 3 . 5 1 3 . 5 2 . 4 8 120 1 3 . 5 1 4 . 0 2 . 4 2 62 21 0 . 0 Comments: The p o l y p r o p y l e n e s c r e e n p r o d u c e s a h i g h e r p o t e n t i a l d rop compared w i t h t h e s a r a n s c r e e n . RUN 2 -2 Membrane: IONAC MC-3470 a g a i n s t s a r a n s c r e e n I = 10 A c d . = 5 2 6 . 3 A / m 2 K x 1 0 3 E l e c t r o l y t e C o n c e n t r a t i o n e - i (fi cm) pH A n o l y t e 5 g / 1 N a 2 S 0 i t 6 . 5 9 .42 NaOH t o a d j u s t pH C a t h o l y t e 25 g / 1 NaOH 14* 1 2 . 6 * K x l O 3 P h e n o l T . O . C . (Pb) t AV e a (min) (V) ($2 cm) 1 P a (mg/1) (%) (mg/1) (%) (mg/1) 0 8 . 0 6 . 5 9 .42 100 - 77 0 . 0 15 8 . 0 7 . 8 3 . 1 2 45 55 72 6 0 . 2 30 8 . 0 8 . 8 2 . 9 25 75 70 9 0 . 1 45 7 . 9 9 . 5 2 . 8 2 11 89 68 13 0 . 1 60 7 . 5 1 0 . 5 2 . 7 5 5 95 66 14 0 . 0 75 7 .3 1 1 . 0 2 . 6 8 2 98 65 16 0 . 0 90 7 . 1 1 1 . 5 2 . 6 0 0 100 64 17 0 . 0 105 7 . 0 1 2 . 0 2 . 5 8 63 18 120 7 . 0 1 2 . 0 2 . 5 6 61 21 RUN 2-3 Membrane: IONAC MC-3470 1 = 0 (no c u r r e n t a p p l i e d ) - K x 1 0 3 E l e c t r o l y t e C o n c e n t r a t i o n (ft cm) pH A n o l y t e 5 g / 1 N32S01+ 6 .5 9 .47 NaOH t o a d j u s t pH C a t h o l y t e 25 g / 1 NaOH 14* 1 2 . 7 * K x l O 3 P h e n o l T . O . C . (Pb) t AV e a P H a (min) (V) (ft cm) (mg/1) (mg/1) (mg/1) 0 0 . 8 6 . 5 9 .47 95 73 0 . 0 15 0 . 7 5 5 .7 10 .74 95 73 0 .5 30 0 . 7 6 . 1 1 1 . 2 2 95 73 0 .7 45 0 . 0 1 6 . 3 11 .47 95 73 1.0 60 6 . 6 11 .64 95 73 1.2 75 6 . 6 1 1 . 8 95 73 1.5 90 6 . 6 1 1 . 9 2 . 0 Comments: No a p p r e c i a b l e change i n p h e n o l o r T . O . C . c o n c e n t r a t i o n was d e t e c t e d , b u t w h i l e t h e f i r s t 31 were w i t h d r a w n ( t o p u r g e t h e s y s t e m ) , t h e s o l u t i o n showed t h e b r o w n i s h c o l o u r c h a r a c t e r i s t i c o f t h e o x i d a t i o n o f p h e n o l . RUN 2-4 Membrance: IONAC MC-3470 I = 3 A c d . = 1 5 7 . 9 A / m 2 E l e c t r o l y t e C o n c e n t r a t i o n K x 1 0 3 e - i (ft cm) pH A n o l y t e 5 g / 1 Na 2S0i+ NaOH t o a d j u s t pH 6 . 2 9 .44 C a t h o l y t e 25 g / 1 NaOH 15* 1 2 . 6 * t [min) AV (V) K x i o 3 e 3 - 1 (ft cm) p H a P h e n o l (mg/1) (%) (mg/1) T . O . C . (%) (Pb) (mg/i; 0 4 . 7 6 . 2 9 .44 100 _ 77 0 . 1 15 - 4 . 7 1 1 . 7 0 83 17 76 1 0 . 4 30 4 . 7 7 . 0 1 1 . 7 7 68 32 73 5 0 . 4 45 4 . 5 1 1 . 8 2 58 42 70 9 0 . 4 60 4 . 5 7 . 2 1 1 . 8 3 53 47 66 14 0 . 4 75 4 . 5 1 1 . 8 4 49 51 64 17 0 . 4 90 4 . 5 7 . 2 1 1 . 8 8 43 57 63 18 0 . 4 105 7 . 9 3 . 3 8 18 82 58 25 . 0 . 4 120 7 . 9 8 . 0 3 . 0 0 6 94 44 43 0 . 3 135 7 .7 2 . 8 0 2 98 40 48 0 . 2 150 7 . 5 9 . 0 2 . 7 3 0 100 37 52 0 . 2 N o t e : C u r r e n t was changed t o 10 A a t 90 m i n t o o b s e r v e pH r e s p o n s e . D u r i n g t h e h i g h pH i n t e r v a l t h e s o l u t i o n had a b r o w n - r e d d i s h c o l o u r and a f t e r t h e pH d r o p t h e c o l o u r changed t o l i g h t y e l l o w . RUN 2-5 Membrane: IONAC MC-3470 I = 6 A c . d . = 3 1 5 . 8 A / m 2 K x 1 0 3 , 6 N - l E l e c t r o l y t e C o n c e n t r a t i o n (ft cm) pH A n o l y t e 5 g / 1 Na 2SOi+ 6 .2 9 .42 NaOH t o a d j u s t pH C a t h o l y t e 25 g / 1 NaOH 15* 1 2 . 6 * K x l O 3 P h e n o l T . O . C . t AV e a (min) (V) (ft cm) L p H a (mg/1) (%) (mg/1) 0 5 .8 6 . 2 9 . 4 2 85 15 3 .74 49 42 65 4 30 5 .7 6 . 5 3 .50 28 67 62 7 45 3 . 2 1 19 78 59 10 60 5 .6 7 . 1 3 .05 12 86 56 14 75 2 . 8 8 7 92 54 17 90 5 . 5 8 . 3 2 . 8 0 5 94 53 18 105 2 . 7 8 2 98 53 18 120 5 . 5 8 . 5 2 . 7 6 0 100 52 20 KJ OO RUN 2-6 Membrane: IONAC MC-3470 I = 20 A c d . = 1 0 5 2 . 6 A / m 2 K x 1 0 3 E l e c t r o l y t e C o n c e n t r a t i o n (ft cm) PH A n o l y t e 5 g / 1 N a 2 S 0 i t 8.5 2 . 4 5 0 . 4 4 g / 1 H 2 S O 4 C a t h o l y t e 25 g / 1 NaOH 14* 1 2 . 7 * K x l O 3 . . . e P h e n o l T . O . C . AV a 1 (min) (V) (ft c m ) _ 1 P a . (mg/1) (%) (mg/1) 0 1 2 . 7 8 . 5 2 . 4 5 105 81 15 1 2 . 4 24 77 81 0 30 9 .7 1 2 . 5 1.80 5 95 80 1 45 8 .9 2 98 78 4 60 8 .7 1 5 . 5 1.67 1 99 72 11 75 3 . 4 0 100 66 18 90 8 . 2 1 7 . 0 1.58 59 27 105 7 . 9 52 36 120 7 .8 1 7 . 5 1.50 42 48 RUN 2-7 Membrane: IONAC MC-3470 I = 10 A c . d . = 5 2 6 . 3 A / m 2 E l e c t r o l y t e C o n c e n t r a t i o n K x 1 0 3 e - i (ft cm) pH A n o l y t e 5 g / 1 Na 2S0i+ 0 . 4 4 g / 1 H 2 S0 i t 8 .3 2 .46 C a t h o l y t e 25 g / 1 NaOH 14* 1 2 . 7 * t (min ) AV (V) K x l O 3 e a - 1 (ft cm) P R a (mg/1) P h e n o l (%) (mg/1) T . O . C . <%: 0 7 .5 8 . 3 2 .46 100 0 75 0 15 30 70 30 6 . 7 1 0 . 5 1.96 8 92 74 1 45 4 96 60 6 . 3 1 2 . 5 1 .81 2 98 74 1 75 0 100 90 6 . 0 1 4 . 0 1.72 72 4 105 120 5 . 9 1 5 . 0 1.66 67 11 RUN 2-8 Membrane: NAFION 127 I = 10 A c d . = 5 2 6 . 3 A / m 2 K x 1 0 3 e E l e c t r o l y t e C o n c e n t r a t i o n (ft cm) PH A n o l y t e 5 g / 1 Na 2 S0ix 8 .0 1 2 . 0 3 0 . 4 4 g / 1 NaOH C a t h o l y t e 25 g / 1 NaOH 14* 1 2 . 6 * t (min) AV (V) K x l O 3 e a - 1 (ft cm) p H a P h e n o l (mg/1) (%) T . O . C . (mg/1) (%) 0 5 . 3 8 .0 1 2 . 0 3 95 76 15 2 . 1 53 44 75 1 30 5 . 5 9 . 0 1.8 20 79 74 3 45 7 93 60 5 .2 1 1 . 0 1.54 3 97 73 4 75 1 99 90 5 . 1 1 3 . 0 1.38 0 100 70 8 120 5 . 0 1 4 . 5 1.29 67 12 150 4 . 9 1 5 . 0 1.26 66 13 N o t e : C o l o u r change o b s e r v e d f rom brown r e d d i s h t o y e l l o w when t h e pH d r o p p e d . RUN 2-9 Membrane: NAFION 127 I = 20 A c . d . = 1 0 5 2 . 6 A / m 2 K x 10 3 E l e c t r o l y t e C o n c e n t r a t i o n (ft cm) PH A n o l y t e 5 g/1 Na2S0LL 8.2 2 . 4 8 0 . 44 g/1 H 2 S 0 t t C a t h o l y t e 25 g/1 NaOH 15* 1 2 . 7 * K x i o ^ P h e n o l T . O . C . t AV e a _ (min) (V) (ft cm) 1 P a (mg/1) (%) (mg/1) (%) 0 8 .5 8 .2 2 . 4 8 15 7 . 5 30 6 . 8 1 2 . 5 1.7 45 6 .4 60 5 .9 1 5 . 0 1.67 75 5 .5 90 5 . 5 1 6 . 5 1.64 105 5 .5 120 5 .5 1 7 . 5 1.62 102 85 37 64 84 1 4 91 82 3 1 98 77 9 0 100 72 15 0 100 67 21 63 26 58 32 53 38 to RUN 2-10 Membrane: NAFION 127 I = 20 A c . d . = 1 0 5 2 . 6 A / m 2 K x I O 3 , e - l E l e c t r o l y t e C o n c e n t r a t i o n (ft cm) pH A n o l y t e 5 g / 1 N a 2 S 0 i t 30 1 2 . 8 4 5 g / 1 NaOH C a t h o l y t e 25 g / 1 NaOH 1 5 * 1 2 . 7 K x l O 3 T o t a l I n o r g . t AV 6 a _ 1 P h e n o l Ca rbon Carbon T . O . C . (min) (V) (ft cm) 1 p H a (mg/1) (%) (mg/1) (mg/1) (mg/1) (%) 0 3 . 9 3 0 . 0 1 2 . 8 4 102 0 82 0 82 15 3 . 9 2 3 . 5 1 2 . 6 6 92 10 82 2 80 2 30 4 . 9 1 7 . 0 1 2 . 5 4 69 32 82 4 78 5 45 5 . 0 1 1 . 0 1 2 . 2 4 36 65 82 8 74 10 60 7 . 0 2 . 1 5 15 85 72 5 67 18 75 5 . 0 9 . 5 1.56 3 97 62 2 60 26 90 5 . 0 1 1 . 0 1 .40 0 100 55 3 52 37 105 5 . 0 120 5 . 0 1 3 . 5 1 .28 43 0 43 48 135 5 . 0 150 5 . 0 1 4 . 0 1 .26 39 0 39 52 Comments: The c o l o u r o f t h e e l e c t r o l y t e changed f rom brown r e d d i s h t o y e l l o w when the pH d r o p was p r o d u c e d . RUN 2-11 Membrane: IONAC MA-3475 ( a n i o n i c ) I = 20 A c . d . = 1052.6 A / m 2 K x 1 0 3 e E l e c t r o l y t e C o n c e n t r a t i o n (ft .cm) pH A n o l y t e 5 g / 1 N a 2 S 0 i i 0 .44 g / 1 H^SOtt 8 2 .4 C a t h o l y t e 25 g / 1 NaOH 15* 1 2 . 7 * K x 1 0 ^ t AV e a . _ P h e n o l Carbon (mg/1) (min) (V) (ft cm) P a (mg/1) (%) T o t a l I n o r g . T . O . C . 0 7 .7 8 .0 2 .40 94 15 2 .80 26 30 7 .6 8 .1 3 .15 9 45 8 .33 4 60 7.4 8 .2 1 1 . 2 0 2 75 11 .75 0 90 7.1 8 .3 12 .08 120 7 .0 9 . 5 12 .38 150 6 .6 1 0 . 0 12 .53 0 78 0 78 0 72 2 76 3 90 78 3 75 4 96 7 71 9 98 78 18 60 23 100 33 45 42 78 45 33 58 78 52 26 67 78 60 18 77 Comment: The a n i o n i c membrane showed a change i n c o l o u r ( f rom y e l l o w t o brown) a f t e r t h e r u n . RUN 3-1 E l e c t r o l y t e 1 ( A ) c . d . (A/m 2 ) 5 g / 1 Na2S0it 10 5 2 6 . 3 NaOH t o a d j u s t pH t (min) AV (V) ' K e (ft x 1 0 3 cm) P H ( m g / D P h e n o l (%) (mg/1) T . O . C . (%: 0 9 .7 6 . 2 9 .46 110 84 15 1 0 . 5 6 .5 3 .78 60 45 80 5 30 9 . 7 6 .7 3 .62 40 64 78 7 45 9 . 1 6 . 8 3 .52 19 83 77 8 60 8 .9 6 .9 3 .46 9 92 75 11 75 8 .7 6 . 9 3 .42 3 97 71 15 90 8 . 5 7 . 0 3 .39 1 99 67 20 120 8 .5 7 . 0 3 .36 0 100 60 29 RUN 3-2 E l e c t r o l y t e 1 ( A ) c d . (A/m 2 ) 5 g / 1 Na 2SOtx 0 . 4 4 g / 1 NaOH 10 5 2 6 . 3 t AV K x 1 0 3 6 - 1 P h e n o l C a r b o n (mg/1) (min) (V) (ft cm) pH (mg/1) (%) T o t a l I n o r g . T . O . C . % T . O . C . 0 6 . 5 8 . 0 1 1 . 9 8 106 81 0 81 15 6 .4 7 . 5 1 1 . 8 8 71 33 81 5 76 7 30 6 . 3 7 .2 1 1 . 8 1 52 51 81 7 74 9 45 6 . 3 7 . 0 1 1 . 7 2 34 68 81 8 73 10 60 6 . 3 6 . 9 1 1 . 6 7 18 83 81 10 70 13 75 6 .4 6 . 8 1 1 . 5 8 12 89 81 13 68 16 90 6 . 5 6 . 8 1 1 . 5 7 9 92 81 18 63 22 120 6 . 6 6 . 7 1 1 . 5 4 5 95 81 26 55 32 RUN 3-3 E l e c t r o l y t e 1 ( A ) c d . (A/m ) 5 g / 1 Na2S0i+ 10 5 2 6 . 3 0 . 4 4 g / 1 H 2 S 0 L L A T 7 K x 1 0 3 P h e n o l T . O . C . t AV e (min) (V) (ft c m ) " 1 pH (mg/1) (%) (mg/1) (%) 0 7 . 8 8 .4 2 . 5 0 93 75 15 7 . 3 8 . 6 28 70 75 0 30 7 . 2 8 . 8 2 . 4 5 12 86 74 1 45 6 . 9 8 . 9 4 95 71 5 60 6 .7 8 .9 2 . 4 3 2 98 69 7 75 6 .5 8 .9 1 99 68 9 90 6 . 4 8 .9 2 . 4 1 0 100 65 13 120 6 .4 8 .9 2 .39 61 19 150 6 .4 8 . 9 2 . 3 8 56 25 RUN 3-4 E l e c t r o l y t e 1 ( A ) c d . (A/m ) 5 g / 1 Na2S0ix 20 1 0 5 2 . 6 0 . 4 4 g / 1 H2SO11 t AV K x 1 0 3 T P h e n o l T . O . C . (min) (V) (ft c m ) - 1 pH ( ° c ) ( m g / D (%) (mg/1) (%: 0 9 . 6 8 . 5 2 . 4 7 23 95 73 15 9 . 7 9 . 1 2 . 2 8 23 75 71 3 30 9 . 2 9 . 2 2 . 2 8 25 10 89 68 7 45 8 .4 9 . 2 2 . 2 7 3 97 64 12 60 8 .2 9 . 2 2 . 2 5 26 2 98 59 19 75 8 .2 9 . 3 2 . 2 4 1 99 54 26 90 8 .2 9 . 4 2 . 2 2 27 0 100 47 36 120 9 .3 9 . 5 2 . 2 0 28 34 53 150 1 0 . 7 9 . 6 2 . 1 9 28 23 68 (Pb)" = u n d e t e c t a b l e . RUN 3-5 E l e c t r o l y t e 1 ( A ) c d . (A/m ) 5 g / 1 Na 2S0i+ 20 1 0 5 2 . 3 0 . 4 4 g / 1 NaOH t (min) AV (V) K e (a x io3 cm) pH T ( ° C ) (mg/1) P h e n o l (%) Ca rbon (mg/1) T o t a l I n o r g . T, . O . C . (%) (Pb) (mg/1] 0 9 . 2 8 .2 1 2 . 0 4 24 102 86 0 86 0 0 . 0 15 9 . 1 7 . 9 1 1 . 9 1 48 53 84 6 78 9 0 . 2 30 8 .9 7 .7 1 1 . 7 6 25 26 75 84 13 71 17 0 . 2 45 8 .9 7 . 5 1 1 . 5 6 12 88 84 22 62 28 0 . 0 60 9 . 0 7 . 2 1 1 . 2 6 26 3 97 84 30 54 37 0 . 0 75 9 . 4 7 . 1 1 0 . 7 6 0 100 84 37 46 46 0 . 0 90 1 0 . 4 7 . 1 1 0 . 2 6 27 84 46 38 56 0 . 0 120 1 1 . 4 7 . 1 9 . 5 6 28 83 60 23 73 0 . 0 150 1 2 . 0 7 . 0 8 .84 28 81 62 19 78 0 . 0 RUN 3-6 _2 _ E l e c t r o l y t e 1 ( A ) c . d . (A/m ) 5 g / 1 NaaSOit 30 0 . 4 4 g / 1 H 2 S O 4 1 5 7 8 . 9 t (min) AV (V) K e (ft x 1 0 3 cm) pH T ( ° C ) (mg/1) P h e n o l (%) (mg/1) T . O . C . (%) 0 1 4 . 0 8 .6 2 . 4 8 24 96 76 15 1 1 . 9 8 .9 2 . 4 0 26 20 79 71 7 30 1 1 . 6 9 . 1 2 . 3 6 28 4 96 68 11 45 1 1 . 4 9 . 3 2 . 3 2 29 1 99 61 20 60 1 3 . 0 9 . 5 2 . 3 0 31 0 100 47 38 75 1 4 . 0 9 . 6 2 . 2 8 33 0 100 37 51 90 1 5 . 0 9 . 7 2 . 2 6 34 28 63 120 1 5 . 5 9 . 8 2 . 2 5 36 24 69 ( P b ) : u n d e t e c t a b l e RUN 3-7 E l e c t r o l y t e 1 ( A ) c . d . (A /m ) 5 g / 1 Na 2 SOL f 30 1 5 7 8 . 9 0 . 4 4 g / 1 NaOH t AV K e x 1 0 3 T P h e n o l C a r b o n (mg/1) (min) (V) (ft cm) pH ( ° C ) (mg/1) (%) T o t a l I n o r g . T . O . C . (%: 0 1 1 . 9 8 . 1 1 2 . 0 8 23 110 0 86 0 86 0 15 1 0 . 7 ' 1 1 . 9 7 25 36 67 74 10' 74 14 30 1 0 . 4 7 .6 1 1 . 8 0 26 12 89 61 22 61 29 45 1 0 . 5 1 1 . 4 7 28 1 99 48 35 48 44 60 1 2 . 9 7 . 2 1 0 . 8 0 29 0 100 32 51 32 63 75 1 3 . 4 1 0 . 4 3 31 25 58 23 73 90 1 3 . 5 7 . 3 1 0 . 1 8 32 16 64 16 81 120 1 3 . 6 7 . 0 9 .5 35 7 73 7 92 [ P b ] : u n d e t e c t a b l e RUN 3-8 E l e c t r o l y t e 1 ( A ) c d . (A/m ) 5 g / 1 N a 2 S O i + 10 5 2 6 . 3 0 . 4 4 g / 1 H 2S0LL t (min) AV (V) K x 1 0 3 - 1 (ft cm) pH (mg/1) P h e n o l (%) (mg/1) T . O . C . (%: 0 4 . 5 3 0 . 5 2 . 5 108 0 83 0 15 33 70 80 4 30 4 . 2 3 1 . 0 2 .44 13 88 79 5 45 5 95 77 7 60 4 . 1 3 1 . 5 2 . 4 2 1 99 72 13 75 0 100 68 18 90 4 . 1 3 1 . 5 2 . 4 0 67 19 120 4 . 1 3 1 . 5 2 . 3 8 61 27 RUN 3-9 E l e c t r o l y t e 1 ( A ) c . d . (A/m ) 5 g / 1 Na 2S0i+ 10 5 2 6 . 3 0 . 4 4 g / 1 NaOH AV K e X 1 0 3 P h e n o l Ca rbon (mg/1) (min) (V) (ft c m ) " 1 pH (mg/1) (%) T o t a l I n o r g . T . O . C . (%) 0 4 . 6 32 1 2 . 0 6 105 80 0 80 0 15 4 . 6 32 1 1 . 8 8 72 31 80 2 78 3 30 4 . 6 32 1 1 . 8 1 48 54 80 5 75 6 45 4 . 6 32 1 1 . 6 4 38 64 80 8 72 10 60 4 . 6 32 1 1 . 4 1 18 83 80 14 66 18 75 4 . 6 32 1 0 . 9 6 7 93 80 20 60 25 90 4 . 6 32 1 0 . 3 6 1 99 80 27 53 34 120 4 . 6 32 9 .24 0 100 80 40 40 50 150 4 . 6 32 7 . 7 72 45 27 66 RUN 3-10 E l e c t r o l y t e 1 ( A ) c . d . (A/m 30 g / 1 N32S01+ 20 1 0 5 2 . 6 0.44 g / 1 t (min) AV (V) K x 1 0 3 e - i (ft cm) PH T ( ° C ) (mg/1) P h e n o l (%) T . O . C . (mg/1) (%: o' 6 . 6 32 2 . 4 6 25 98 0 80 0 15 20 80 77 4 30 6 . 1 32 2 . 4 2 25 7 93 75 6 45 3 97 70 13 60 6 . 0 32 2 . 4 0 26 1 99 65 19 75 0 100 58 28 90 6 .2 32 2 . 3 8 28 54 33 120 6 .4 32 40 50 150 6 .7 32 2 . 3 6 28 28 65 RUN 3-11 E l e c t r o l y t e 1(A) c d . (A/m ) 30 g / 1 N32S01, 20 1 0 5 2 . 6 0 . 4 4 g / 1 NaOH K x 1 0 3 Phenol Carbon (mg/1) t AV e s (min) (V) (ft cm) pH (mg/1) (%) Total Inorg. T.O.C. (%) 0 4 . 4 3 2 . 0 1 2 . 0 3 113 0 87 0 87 15 54 52 87 7 80 8 30 4 . 4 3 1 . 5 1 1 . 9 3 29 74 87 13 74 15 45 14 88 87 22 65 25 60 4 . 5 3 1 . 0 1 1 . 7 0 3 98 87 32 55 37 75 0 100 87 40 47 45 90 4 . 6 3 0 . 5 1 1 . 1 6 87 51 36 59 120 4 . 7 3 0 . 0 1 0 . 4 2 87 65 22 75 150 87 70 17 80 RUN 3-12 - 2 . E l e c t r o l y t e K A ) c . d . (A/m ) 5 g / 1 N a 2 S 0 4 10 5 2 6 . 3 0 . 4 4 g / 1 H2S0I+ t AV K e x 1 0 3 s-1 cm) P h e n o l T . O . C . (min) (V) (ft pH (mg/1) (%) (mg/1) (%: 0 8 .7 8 . 0 2 . 4 3 525 0 395 15 320 39 30 8 .6 8 .7 2 . 2 5 175 67 380 4 45 75 86 60 8 . 5 9 . 3 2 .18 25 95 375 5 75 15 97 90 8 .2 9 . 3 2 .16 5 99 370 8 120 7 . 8 9 . 3 2 .14 0 100 350 11 RUN 3-13 E l e c t r o l y t e K A ) _2 c . d . (A/m ) 5 g/1 Na2S0Lj 10 5 2 6 . 3 0 . 4 4 g/1 H2S01+ K x 1 0 3 P h e n o l t AV e (min) (V) (ft cm) 1 pH (mg/1) (%) 0 9 . 3 8 .7 2 . 4 2 1100 0 15 792 28 30 8 .3 8 . 9 2 . 3 2 506 54 45 341 69 60 8 . 0 8 .9 2 . 2 8 187 83 75 88 92 90 7 . 9 9 . 1 2 .27 22 98 120 7 .8 9 . 2 2 .26 5 100 Comment: The n e t change i n T . O . C . was p r a c t i c a l l y u n d e t e c t a b l e due t o t h e h i g h amount o f c a r b o n p r e s e n t i n s o l u t i o n . RUN 3-14 E l e c t r o l y t e KA) _2 c . d . (A/m ) 5 g / 1 Na 2SOi+ 10 5 2 6 . 3 2 . 2 g / 1 H2S01+ t (min) AV (V) K x 1 0 3 e - i (ft cm) pH (mg/1) P h e n o l <%) (mg/1) T . O . C . (%: 0 5 . 4 1 3 . 5 1.8 90 77 15 5 .2 1 3 . 7 30 67 74 4 30 5 . 1 1 4 . 0 1.78 9 90 73 5 45 5 . 0 1 4 . 2 3 97 72 6 60 4 . 9 1 4 . 3 1.75 1 99 69 10 75 4 . 9 1 4 . 5 0 100 68 12 90 4 . 9 1 4 . 5 1.75 66 14 120 4 . 9 1 4 . 5 1.75 64 17 RUN 3-15 E l e c t r o l y t e 1(A) c . d . (A/m ) Q ( £ / m i n ) P ( K p a ) 5 g / 1 NaaSOi, 20 1 0 5 2 . 6 0 . 5 5 145 0 . 4 4 g / 1 H 2S0i+ . „ K x 1 0 3 P h e n o l T . O . C . t AV e (min) (V) (ft cm) 1 pH (mg/1) (%) (mg/1) (%) 0 1 2 . 1 8 . 3 2 . 5 105 0 79 0 15 28 73 74 6 30 1 0 . 5 8 . 5 2 .43 9 91 71 10 45 2 98 69 13 60 1 0 . 9 8 . 7 2 . 4 0 1 99 62 22 75 0 100 55 30 90 1 1 . 1 8 .8 2 .38 48 39 120 1 2 . 0 8 . 9 2 .37 36 54 RUN 4 - 1 K x 1 0 3 T . P h e n o l 6 i n E l e c t r o l y t e 1 ( A ) c . d . ( A / m ~ 2 ) (ft c m ) " 1 pH ( ° C ) C i n ( r a g / : L ) 5 g / 1 Na 2 S0i4 10 5 2 6 . 3 7 . 9 2 . 5 24 100 0 . 4 4 g / 1 H2SO4 Q AV P T P h e n o l (Jo/min) (V) (kPa) ° ™ C „ (mg/1) (%) 0 . 1 1 8 .7 108 32 9 91 0 . 2 5 8 . 6 120 28 42 58 0 . 4 0 8 . 3 129 26 55 45 0 . 5 5 7 . 9 143 25 67 33 0 . 8 5 7 .4 184 24 75 25 1 .10 7 . 1 232 24 79 21 RUN 4 -2 K x I O 3 T . P h e n o l 6 i n E l e c t r o l y t e 1(A) c d . ( A / m ~ 2 ) (ft. c m ) " 1 pH ( ° C ) C i n ( m g / 5 g / 1 N32S04 10 5 2 6 . 3 8 . 1 2 . 5 24 105 0 .44 g / 1 H 2 S 0 i t Q AV P T P h e n o l ( A / m i n ) (V) (kPa) C (mg/1) (%)' 0 . 2 5 8 .6 119 27 43 60 0 . 4 0 8 . 5 129 26 60 44 0 . 5 5 8 .3 143 25 71 34 0 . 8 5 7 .8 184 25 80 26 1 .10 7 . 3 232 24 85 21 1 .30 7 . 0 280 24 90 17 RUN 4-3 K g x I O 3 T P h e n o l E l e c t r o l y t e 1 ( A ) c d . ( A / m " 2 ) (ft c m ) " 1 ) pH ( ° C ) C i n ( n i g / 1 ) 5 g / 1 UazSOn 10 5 2 6 . 3 8 . 0 2 . 5 23 95 0 . 4 4 g / 1 H 2S0i+ Q AV P U/min) (V) (kPa ) 0 . 2 5 0 8 .7 108 0 . 4 0 0 8 . 6 120 0 . 5 5 0 8 .3 129 0 . 8 5 0 7 . 9 143 1 .100 7 .4 184 1 .300 7 . 1 232 T P h e n o l ( ° C ) C o u t ( m g / 1 ) ( % ) 27 39 59 25 53 44 24 63 34 24 72 24 23 76 20 23 81 17 RUN 4-4 K g x 1 0 3 T P h e n o l E l e c t r o l y t e 1 (A) c . d . ( A / m " 2 ) (ft c m ) " 1 pH ( ° C ) c±a^lAs/^) 5 g / 1 N a 2 S 0 4 10 5 2 6 . 3 8 2 . 4 5 23 580 0 . 4 4 g / 1 H 2S0i+ F l o w AV P U/min) (V) (kPa) 0 . 1 1 8 .7 108 0 . 2 5 8 . 6 120 0 . 4 0 8 . 3 129 0 . 5 5 7 . 9 143 0 . 8 5 7 .4 184 1 .10 7 . 0 234 T P h e n o l ( ° C ) C o u t ( m8 / 1> ( % ) 31 175 70 27 365 37 26 435 25 25 470 19 23 510 12 23 526 9 RUN 4-5 K g x 1 0 3 T P h e n o l E l e c t r o l y t e 1 (A) c d . ( A / m " 2 ) (ft c m ) " 1 pH ( ° C ) C i n ( m g ^ 5 g / 1 Na2S0k 20 1052.6 7 .9 2 .43 24 110 0 .44 g / 1 H 2 S0it Q AV P T P h e n o l (4 / m i n ) (V) (kPa) ™ \ C Amg/1) (%) .11 13 .4 110 48 7 94 .25 1 3 . 4 122 38 40 63 .40 1 3 . 3 129 32 62 44 .55 1 2 . 6 145 27 73 34 .85 1 1 . 5 185 26 80 27 1.10 1 0 . 8 234 25 90 19 RUN 4-6 K x 1 0 3 T . P h e n o l E l e c t r o l y t e K A ) _2 c . d . (A/m ) e - i (ft cm) i n pH ( °C) C (mg/1) i n 5 g / 1 Na 2S0i+ 20 1 0 5 2 . 6 7 .9 2 . 4 5 24 515 0 . 4 4 g / 1 NaOH Q AV P T P h e n o l ( £ / m i n ) (V) (kPa) ou t ( ° C ) C (mg/1) ou t (%) 0 . 1 1 1 2 . 2 110 48 56 89 0 . 2 5 1 1 . 8 121 38 260 49 0 . 4 0 1 1 . 3 129 30 335 35 0 . 5 5 1 0 . 8 145 27 390 24 0 . 8 5 1 0 . 1 185 25 430 16 1 .10 9 . 7 234 24 450 12 H1 RUN 4-7 K g x I O 3 T i n P h e n o l E l e c t r o l y t e 1 (A) c d . ( A / m ~ 2 ) (ft c m ) " 1 pH ( ° C ) C i n ( m g / 5 g / 1 N32S04 10 5 2 3 . 6 8 .1 2 .43 25 110 0 . 44 g / 1 H2S01+ Q AV P T P h e n o l ( £ / m i n ) (V) (kPa) C (mg/1) (%) 0 . 1 1 5 . 9 110 30 68 38 0 . 2 5 5 . 9 115 28 87 21 0 . 4 0 5 . 9 115 27 98 11 0 . 5 5 5 . 9 115 26 100 9 0 . 8 5 5 . 9 115 25 103 6 1 .10 5 . 9 115 25 106 4 N o t e : The anode u sed was t h e f e e d e r p l a t e o n l y ( r e f e r t o g e n e r a l s p e c i f i c a t i o n s ) . T h e r e f o r e , t h e p r e s s u r e was p r a c t i c a l l y c o n s t a n t a t 115 K p a . RUN 4-8 E l e c t r o l y t e KA) - 2 c d . (A/m ) K x 1 0 3 e - i (ft cm) T . P h e n o l P H ( ° C ) C i n ( m g / 1 ) 5 g / 1 Na 2SOi+ 0 . 4 4 g / 1 H2SOi+ 10 5 2 6 . 3 8 .0 2 . 4 3 24 100 Q (Jo/min) AV (V) P (kPa) T «. out (°c) P h e n o l C (mg/1) (%) ou t 0 . 1 1 6 . 3 112 31 5 95 0 . 2 5 6 . 0 128 28 33 67 0 . 4 0 5 . 9 143 27 47 53 0 . 5 5 5 .8 162 26 55 45 0 . 8 5 5 . 7 204 25 66 34 1.10 5 .6 271 25 71 29 N o t e : T h i s r u n c o r r e s p o n d s t o t h e s m a l l e r p a r t i c l e s i z e ( 0 . 7 < dp < 1.1 mm) F o r bed d a t a r e f e r t o g e n e r a l s p e c i f i c a t i o n s . 158 APPENDIX 3 Mathematical Models The electrochemical oxidation of phenol is a heterogeneous process that takes place on the surface of the anode. A simplified picture of the process is that the disappearance of phenol is the result of two steps which occur in series: a) the transfer of phenol molecules or phenoxonium ions from the bulk of the solution to the surface cf the electrode b) an electrochemical reaction by which the phenol is converted into some oxidation products as discussed in Chapter 2. Two limiting cases are considered in which either of the two steps are so slow that they control the overall rate of the process. Such models assume that adsorption phenomena and transfer of oxidation products from the electrode surface are not rate-limiting. A third case is also presented where both the resistance to mass transfer and to electro- chemical reactions are comparable in magnitude. 1. Mass transfer controlled model The model presented here deals with a packed bed electrochemical reactor operating continuously i n plug flow. Assumptions - The resistance to the electrochemical reaction i s negligible compared with the resistance to mass transfer. In other words, the concen- tration of phenol at the surface of the electrode is negligible 159 compared w i t h t h e c o n c e n t r a t i o n o f p h e n o l i n t h e b u l k o f t h e s o l u t i o n . V a r i a t i o n s i n p h e n o l c o n c e n t r a t i o n i n t h e d i r e c t i o n s p e r p e n d i c u l a r t o t h e f l o w a r e n e g l e c t e d compared t o t h e v a r i a t i o n s o f c o n c e n t r a t i o n i n t h e d i r e c t i o n o f f l o w . A l l t h e bed i s a c t i v e f o r p h e n o l o x i d a t i o n . Q , C Mass b a l a n c e f o r a d i f f e r e n t i a l h e i g h t . Q d C A = - K a ( S W d y ) ( C - ) [ A . l ] b 11 D " s The s u p e r f i c i a l v e l o c i t y r e f e r r e d t o t h e c r o s s s e c t i o n a l a r e a o f t h e bed i s g i v e r . b y : u = Q/W S Q , C N e g l e c t i n g , ( E q . A . l ) c a n be e x p r e s s e d s F i g . A - 1 P l u g - f l o w p a c k e d b e d r e a c t o r K a dy d c . = _ . 3 C i u A. i n t e g r a t i n g , cA(y) dC A l K a dv m u In C . ( y ) K a v A ' m u [ A . 2] w h i c h c a n a l s o be e x p r e s s e d i n t e rms o f t h e f r a c t i o n a l c o n v e r s i o n X , f o r and A j A 2 A 2 _ _ _ _ - 1 - — [ A . 3] 160 t h e n , K a L Jon(l - X) = - - 5 L [ A . 4 ] The s p e c i f i c s u r f a c e a r e a o f t h e bed i s g i v e n by t h e sum o f t h e s p e c i f i c s u r f a c e a r e a o f t h e f e e d e r p l a t e p l u s t h e s p e c i f i c s u r f a c e a r e a o f t h e p a r t i c l e s , as where 5 i s a shape f a c t o r f o r t h e p a r t i c l e s , and e i s t h e f r a c t i o n o f v o i d s ( 4 9 ) . The a v e r a g e mass t r a n s f e r c o e f f i c i e n t c a n be e s t i m a t e d u s i n g t h e c o r r e l a t i o n by P i c k e t t and Stanmore (45) as f o l l o w s , K dp - 2 = — = 0 . 8 3 (u dp 0 . 56 V 1/3 I v J [ A . 6 ] f o r 23 < Re < 520 T h i s e q u a t i o n was d e v e l o p e d u s i n g a s i n g l e l a y e r o f s p h e r e s and c o r r e l a t e d t h e e x p e r i m e n t a l d a t a w i t h i n ± 10%. I t s h o u l d be n o t e d t h a t t h e e f f e c t o f gas e v o l u t i o n i s n o t i n c l u d e d i n e q u a t i o n A . 6 . S t u d i e s o f mass t r a n s f e r on g a s - e v o l v i n g p a c k e d bed e l e c t r o d e s have b e e n made r e c e n t l y ( 4 7 ) , b u t f o r t h e c a s e o f a s t a t i o n a r y s o l u t i o n where t h e m o t i o n o f t h e e l e c t r o l y t e i n t h e c e l l i s o n l y p r o v i d e d by gas b u b b l e s . I t was found t h a t t h e r a t e o f mass t r a n s - f e r was i n c r e a s e d by gas e v o l u t i o n . F o r t h e p r e s e n t c a s e o f f o r c e d c o n v e c t i o n , a c o r r e l a t i o n f o r mass t r a n s f e r i n gas e v o l v i n g p a r t i c u l a t e e l e c t r o d e s c o u l d n o t be f o u n d i n t h e l i t e r a t u r e . H o w e v e r , i t i s r e a s o n a b l e t o e x p e c t t h a t enhancement o f t h e mass t r a n s f e r c o e f f i c i e n t due t o gas b u b b l i n g i s l e s s s i g n i f i c a n t t h a n i n f r e e c o n v e c t i o n , s i n c e t h e e l e c t r o l y t e i s m a i n l y moved by mechan - i c a l c i r c u l a t i o n . 161 2 . E l e c t r o c h e m i c a l r e a c t i o n c o n t r o l l e d m o d e l A s s u m p t i o n s - The r e s i s t a n c e t o e l e c t r o c h e m i c a l r e a c t i o n i s so h i g h t h a t t h e c o n c e n - t r a t i o n o f p h e n o l a t t h e s u r f a c e o f t h e p a r t i c l e s i s e q u a l t o t h e c o n - c e n t r a t i o n a t t h e b u l k o f t h e s o l u t i o n . - The o x i d a t i o n r e a c t i o n i s assumed t o be f i r s t o r d e r i n p h e n o l c o n c e n - t r a t i o n . - The e l e c t r o d e p o t e n t i a l i s assumed t o be u n i f o r m a l l o v e r t h e c e l l . T h i s a s s u m p t i o n r e p r e s e n t s a s i g n i f i c a n t s i m p l i f i c a t i o n b e c a u s e i n r e a l i t y an e l e c t r o d e p o t e n t i a l d i s t r i b u t i o n e x i s t s w i t h i n and a l o n g t h e e l e c t r o d e ( 1 0 ) . I n a d i f f e r e n t i a l l e n g t h o f t h e a n o d e , t h e p o t e n t i a l d i s t r i b u t i o n w o u l d be as i n d i c a t e d i n F i g . A -2 . C U R R E N T PACKED F E E D E R BED.ANODE ' k 1 d<f> o i =-K - r ^ s e dx s 7? U J O 0. -• OO ) m a j 0 L K — m e dx m i = i + i . s m x = 0 The p o t e n t i a l d i s t r i b u t i o n i n t h e x d i r e c t i o n o r i g i n - a t e s b e c a u s e t h e t o t a l c h a r g e a t a c e n t r a l x i s c a r r i e d by X=SQ b o t h the m e t a l and t h e s o l u t i o n . F i g . A - 2 . P o t e n t i a l d i s t r i b u t i o n i n a p a r t i c u l a t e e l e c t r o d e . 162 The t o t a l c h a r g e a t x = 0 i s e n t i r e l y c a r r i e d by t h e m e t a l o f t h e f e e d e r p l a t e b u t a t x = S t h e c u r r e n t i s e n t i r e l y c a r r i e d by t h e 3. s o l u t i o n . T h i s i m p l i e s t h a t t h e m e t a l p o t e n t i a l p r e s e n t s i t s maximum g r a d i e n t a t x = 0 and t h e s o l u t i o n p o t e n t i a l a t x = S , so t h a t t h e shape a o f t h e c u r v e s <j> v s x and cj> v s x a r e as i n d i c a t e d i n t h e f i g u r e , m s I n t h e i d e a l c a s e o f a m e t a l o f i n f i n i t e c o n d u c t i v i t y , t h e p o t e n - t i a l d r o p t h r o u g h t h e m e t a l i s z e r o and i s r e p r e s e n t e d by t h e h o r i z o n t a l d o t t e d l i n e . G e n e r a l l y , t h e c o n d u c t i v i t y o f t h e m e t a l i s much g r e a t e r t h a n t h a t o f t h e e l e c t r o l y t e , t h e r e f o r e t h e s o l u t i o n p o t e n t i a l d r o p w o u l d be g r e a t e r t h a n t h e m e t a l p o t e n t i a l d r o p . T h u s , t h e e l e c t r o d e p o t e n - t i a l V * = <j> - cb t e n d s t o i n c r e a s e t o w a r d s x = S . T h i s means t h a t t h e T m s a s i d e r e a c t i o n s a r e more l i k e l y t o o c c u r a t t h e edge o f t h e p a c k e d bed o p p o s i t e t o t h e f e e d e r e l e c t r o d e . A two d i m e n s i o n a l m o d e l f o r p o t e n t i a l , c u r r e n t , and c o n c e n t r a t i o n d i s t r i b u t i o n s has been d e v e l o p e d (48) f o r a c o n c e n t r i c c y l i n d r i c a l e l e c - t r o d e , b u t t h e m a t h e m a t i c a l s o l u t i o n i n v o l v e d i s e x t r e m e l y c o m p l i c a t e d . The mode l t o be p r e s e n t e d h e r e assumes t h a t some a v e r a g e e l e c - t r o d e p o t e n t i a l e x i s t s w i t h i n t h e c e l l . T h i s p e r m i t s a s i m p l e c o r r e l a - t i o n o f a l l t h e v a r i a b l e s a f f e c t i n g t h e p r o c e s s . F o r t h e c a s e where t h e e l e c t r o c h e m i c a l r e a c t i o n i s t h e r a t e l i m i t i n g s t e p , t h e mass b a l a n c e i n a d i f f e r e n t i a l l e n g t h o f c e l l i s : Q dC = - K a S W dy C [ A . 7] % r A - The e l e c t r o c h e m i c a l r e a c t i o n c o e f f i c i e n t K i s r e l a t e d t o t h e e l e c t r o d e r p o t e n t i a l by d e f i n i t i o n ° A Z A V a F 163 Under t h e a s s u m p t i o n o f u n i f o r m e l e c t r o d e p o t e n t i a l and C = C , s % e q . A . 7 c a n be i n t e g r a t e d ( w i t h u = Q/W S ) , c A(y) dC A l K a dy r A An C A ( y ) K a y r A [ A . 9 ] f o r y = L c A(y) = C A 2 a n d x = 1 - A i t h e n , £ n ( l - X) = - K a L J ^ A u [ A . 1 0 ] a) S i n g l e r e a c t i o n I f t h e o n l y r e a c t i o n o c c u r r i n g a t t h e e l e c t r o d e i s t h e e l e c t r o - c h e m i c a l o x i d a t i o n o f p h e n o l , a l o c a l r e a c t i o n r a t e ( r e f e r r e d t o t h e t r u e e l e c t r o d e a r e a ) w o u l d be g i v e n b y : i A(y) T T = K r C A ( ^ [ A . 1 1 ] S o l v i n g f o r C . ( y ) f rom e q . A . 9 and s u b s t i t u t i n g i n e q u a t i o n A . 1 1 i A(y) ~TY~ " K r C A l e X p K a y r A [ A . 1 2 ] t h e a v e r a g e t r u e c u r r e n t d e n s i t y t h r o u g h t h e c e l l X A = a" i s , i A(y) X A = dy [ A . 1 3 ] dy 164 S u b s t i t u t i n g e q . A . 1 2 i n t o e q . A . 1 3 and i n t e g r a t i n g , an a v e r a g e r e a c t i o n r a t e c a n be o b t a i n e d a s : X A . A * z F a L E q u a t i o n A . 1 4 i m p l i e s t h a t i f t h e i n i t i a l c o n c e n t r a t i o n o f p h e n o l i s i n c r e a s e d h o l d i n g a l l t h e o t h e r p a r a m e t e r s c o n s t a n t , t h e v a l u e o f w i l l be l o w e r t o s u s t a i n t h e same c u r r e n t f l o w i n g . To i l l u s t r a t e t h i s , e q . A . 8 i s combined w i t h e q . A . 1 4 , t o f i n d t h e r e l a t i o n be tween t h e a v e r a g e r e a c t i o n r a t e , and t h e e l e c t r o d e p o t e n t i a l , as r e p r e s e n t e d i n F i g . A - 3 . ( V * ) ' V * F i g A - 3 S c h e m a t i c r e p r e s e n t a t i o n o f e q . A . 1 4 K a u [ A . 1 4 ] I f t h e i n i t i a l c o n c e n t r a t i o n o f p h e n o l i s i n c r e a s e d f rom C. t o A] ( C . ) 1 a t a c o n s t a n t c u r r e n t i n p u t , t h e v a l u e o f t h e e l e c t r o d e p o t e n t i a l A l ( V * ) ' w i l l be l o w e r , and t h e r e f o r e t h e v a l u e o f K w i l l d r o p t o o . A n a l o g o u s r e a s o n i n g shows t h a t K r w i l l a l s o d e c r e a s e i f u i s i n c r e a s e d , o r i f a i s i n c r e a s e d a t a c o n s t a n t a p p l i e d c u r r e n t . E q u a t i o n A . 1 4 a l s o i m p l i e s t h a t when t h e e l e c t r o d e p o t e n t i a l , t e n d s t o i n f i n i t y , t h e 165 r a t e o f r e a c t i o n i ^ / z F w i l l t e n d t o a maximum g i v e n by C A u a L b) More t h a n one e l e c t r o c h e m i c a l r e a c t i o n o c c u r s a t t h e e l e c t r o d e I f , f o r e x a m p l e , a s i d e r e a c t i o n o f w a t e r e l e c t r o l y s i s o c c u r s i n p a r a l l e l t o t h e p h e n o l o x i d a t i o n , t h e a v e r a g e c u r r e n t d e n s i t y w i l l be g i v e n by t h e sum o f t h e a v e r a g e c u r r e n t d e n s i t i e s d r i v i n g e a c h r e a c t i o n . I f w a t e r e l e c t r o l y s i s i s r e p r e s e n t e d w i t h t h e s u b s c r i p t w , t h e c u r r e n t b a l a n c e w o u l d be e x p r e s s e d b y , i = i . + i where i = I / a A w U s i n g e q . A . 1 4 f o r b o t h p a r t i a l a v e r a g e c u r r e n t s , 1 = Z A  F C , u A A i 1 - exp K a L r A u z F C u w w a L' 1 - exp K a L r w u where K = K exp r A r A A A and K = K exp r r w w a . z V * F A A a R T a z V * F w w a [ A . 1 5 ] [ A . 8 ] R T T h e r e f o r e , t h e a v e r a g e c u r r e n t d e n s i t y , r e f e r r e d t o t h e t r u e a r e a o f t h e b e d , can be r e l a t e d t o t h e e l e c t r o d e p o t e n t i a l as r e p r e s e n t e d i n F i g . A-4, where i t i s assumed t h a t b o t h r e a c t i o n s t a k e p l a c e a t a l l p o t e n t i a l s g r e a t e r t h a n z e r o . F i g u r e A-4 shows t h a t t h e r e i s o n l y one e l e c t r o d e p o t e n t i a l a t w h i c h i . + i = i f o r e a c h s e t o f p a r a m e t e r s a , L , u , C . A w A]̂ I f t h e i n i t i a l c o n c e n t r a t i o n o f p h e n o l i s i n c r e a s e d h o l d i n g a l l o t h e r p a r a m e t e r s c o n s t a n t , a l o w e r v a l u e o f V * w i l l s u s t a i n t h e same t o t a l a v e r a g e c u r r e n t d e n s i t y i . v* F i g . A - 4 S c h e m a t i c r e p r e s e n t a t i o n o f e q . A.15 A n a l o g o u s l y » i f u o r a i s i n c r e a s e d V* s h o u l d a l s o d e c r e a s e , w h i c h means t h a t t h e v a l u e s o f t h e r e a c t i o n r a t e c o n s t a n t s K and K w o u l d be A w l o w e r . 3. Mass t r a n s f e r and e l e c t r o c h e m i c a l r e a c t i o n c o n t r o l l e d m o d e l i The most g e n e r a l c a s e t o . c o n s i d e r i s when b o t h r e s i s t a n c e s t o mass t r a n s f e r and t o e l e c t r o c h e m i c a l r e a c t i o n a r e c o m p a r a b l e . U n d e r s t e a d y - s t a t e (when t h e c o n c e n t r a t i o n p r o f i l e s h a v e b e e n d e v e l o p e d ) , t h e r a t e o f e l e c t r o c h e m i c a l r e a c t i o n and t h e r a t e o f mass t r a n s f e r p e r u n i t a r e a o f t h e e l e c t r o d e , w i l l be e q u a l i n a d i f f e r e n t i a l o f c e l l l e n g t h ( d y ) , - Q d C A K a S-W dy ( C . - C A ) [ A . l ] \ m \ A s — Q d C . = K a S W dy C . [A.7] *b r A s T h e r e f o r e , u n d e r s t e a d y - s t a t e , K ( C . - C A ) = K C . m A, A r A b s s w h i c h p e r m i t s t o e x p r e s s t h e c o n c e n t r a t i o n a t t h e s u r f a c e a s a f u n c t i o n o f t h e c o n c e n t r a t i o n i n t h e b u l k o f s o l u t i o n , 167 K C . m *b X (K + K ) s m r [ A . 1 6 ] S u b s t i t u t i n g e q . A . 1 6 i n t o e q . A . l w i t h Q = u/W S, and i n t e g r a t i n g , y CA(y) d C . C A ^ K K a dy m r (K + K ) u 0 m r An c A(y) K K m r a y J A i (K + K ) u r f o r y = L , C . ( y ) = C A and w i t h X = 1 - — J ' A J hz C [ A . 1 7 ] A n ( l - X) = - K K m r a L (K + K ) u m r where K K K m r K + K m r [ A . 1 8 ] [ A . 1 9 ] i s t h e o v e r a l l r a t e c o n s t a n t , w h i c h c o u l d a l s o be f o u n d a p p l y i n g t h e con - c e p t o f r e s i s t a n c e s i n s e r i e s a s : K K K m r [ A . 2 0 ] A n a l o g o u s t o e q . A . 1 5 , i n t h e g e n e r a l c a s e when a s i d e r e a c t i o n o f w a t e r e l e c t r o l y s i s o c c u r s i n p a r a l l e l t o p h e n o l o x i d a t i o n , t h e t o t a l a v e r a g e c u r r e n t d e n s i t y w o u l d be g i v e n b y , z . F C . u l = a L 1 - exp K a L u z F C u w w a L 1 - exp K a L r w u [ A . 2 1 ] 168 Note that K i s used for the water electrolysis reaction, since i t w i t i s an activation controlled process and would not be affected by mass transfer. In this case, an increase in the phenol inlet concentration C. or in the specific surface area of the bed, would result in lower A l values of (or V*) for a same average current density, as discussed previously. However, an increase in the superficial velocity u would result in increased mass transfer coefficients (eq. A .6) but at the same time, lower reaction rate constants K , K should be expected. r ' r A w 4. Mass transfer model for a batch recirculation system If the c e l l i s operated in a batch recirculation system, as shown in Fig. A-5 both inlet and outlet concentrations w i l l be a function of time. + C A (t) F i g . A-5 Schematic repre- s e n t a t i o n of a batch sys- tem It i s possible to find an expression to correlate the inlet and outlet concentrations with time, u t i l i z i n g the approach of Pickett (32, p. 325) From equations A .3 and A .4, JA 2 exp K a L m u [A.22] An instantaneous material balance over the recirculation tank would be written as, dC. Ct - C, = t A 2 A j m dt [A.23] 169 w i t h t = V /Q m m C o m b i n i n g e q u a t i o n A . 22 w i t h A . 2 3 w o u l d g i v e , - cA = t A i m a t C . exp A l K a L m [ A . 2 4 ] i n t e g r a t i n g , y i e l d s , r t f > K a L m exp u - 1 d t t m J A i d C , J A i J A 0 In A i : A 0 exp ( K a L m u - 1 t t m C . = C . exp A l A 0 f K a L] * m - 1 t exp u t m^ [ A . 2 5 ] S u b s t i t u t i n g C . f rom e q . A . 2 5 i n t o e q . A . 2 2 , A l C . = C . exp A 2 A 0 f f K a l l exp m u - i - K a L t m t u m [ A . 2 6 ] d e f i n i n g , t * = = d i m e n s i o n l e s s t i m e m K a L m = d i m e n s i o n l e s s mass t r a n s f e r g roup A 0 A 2 X = = f r a c t i o n a l c o n v e r s i o n E q u a t i o n A . 2 6 c a n be w r i t t e n a s , X = 1 - exp [ (exp (-0) - 1) t * - 0] [ A . 2 7 ] N o t e t h a t i f t h e r e a c t i o n c o n t r o l s t h e p r o c e s s , a n a n a l o g o u s m a t h e m a t i c a l s o l u t i o n f o r a c o n s t a n t a p p l i e d c u r r e n t o p e r a t i o n i s n o t p o s s i b l e , b e c a u s e t h e v a l u e o f w i l l change a s t h e c o n c e n t r a t i o n changes w i t h t i m e , a s d i s c u s s e d f r o m e q . A . 1 4 . 171 APPENDIX 4 Calculations 1. Batch experiments. C a l c u l a t i o n of t h e o r e t i c a l phenol f r a c t i o n a l conversion i f mass tr a n s f e r c o n t r o l s . This estimate w i l l be s u i t a b l e to compare a l l those experiments i n groups No. 2 and No. 3, performed under the following experimental condi- t i o n s , Flow rate 1.12 £./min P a r t i c l e s i z e 1.7 < dp < 2.00 mm P a r t i c l e weight 250 g 250 e o P a r t i c l e volume = n . " & , — r = 23 cm3 *density of Pb02 from 9.375* g/cm3 r e f . (49) Void f r a c t i o n (57-23)/57 = 0.60 S p e c i f i c surface area of the bed (eq. A.5) a = l + 6 * • ( ! - e) S % x dp taking £ = 0.75 f o r A and, from r e f . (49), and dp =0.185 cm. 1 . 6 x ( l - Q.57) o n , -1 a = -T—Z 1— r» "7c ^ n - i c e = /U.o cm 0.3 cm 0.75 x 0.185 Determination of the mass tr a n s f e r c o e f f i c i e n t from eq. A.6,. . . K = 0.83 f ( R e ) ° - 5 6 ( S c ) 1 / 3 m dp Using an equation f o r d i f f u s i v i t y of l i q u i d s given by Wilke (50). (O r i g i n a l nomenclature.) D = = y V A ° - 6 D = 7.4 x IP' 8 ( 2 . 6 x 1 8 ) ^ x 297 = ^ x 1 Q - 6 c m 2 / s 1 x (105)°- 6 S u p e r f i c i a l v e l o c i t y of the l i q u i d , u = 1 , 1 2 £ / m l n x (I0 3cm 3 / J o ) x (1 min/60 s) = 12.4 cm/s 5 x 0.3 cm 2 u dp 12.4 cm/s x 0.185 cm o o r , . Re - c-= = 229.4 V 0.01 cm2/s S c = V 0.01 cm2/s = 1 0 g 6 9.2 x 10 cm2/s Q 9 x m"^ —^ K = 0.83 x * r\ i QC (229.4) (1086) = 8.7 x 10 J cm/s ± 1 0 % m U.lo5 Extreme values of K , taking i n t o account the 10% error i n c o r r e l a t i o n A.6, K~ = 0.078 cm/s K + = 0.0096 cm/s m m The dimensionless mass tr a n s f e r group i s given by K a L K 20.6 cm"1 38 cm e = _m = _ni u 12.4 cm/s Therefore, the extreme values of 0 w i l l be e~ = 0.49 6 + = 0.60 The phenol f r a c t i o n a l conversion f o r mass tr a n s f e r c o n t r o l i s given by, X = 1 - exp[(exp(-6) - 1) t * - 6] [A.27] t 5 I with t * = — and t = , , „ . , — = 4.464 min. t m 1.12 £/mm m 173 U s i n g t h e ex t r eme v a l u e s o f 6, e q u a t i o n A . 2 7 i s u sed t o c a l c u l a t e t h e r a n g e o f t h e o r e t i c a l f r a c t i o n a l c o n v e r s i o n f o r mass t r a n s f e r c o n t r o l a t a g i v e n t i m e , as shown i n T a b l e A - 2 . TABLE A - 2 THEORETICAL PHENOL FRACTIONAL CONVERSION VS TIME FOR A MASS TRANSFER- -CONTROLLED BATCH SYSTEM t (min) + t * X ~ X 0 . 00 0 . 3 9 . 0 . 4 5 15 3 .36 0 . 8 3 0 . 8 8 30 6 .72 0 . 9 5 0 .97 45 1 0 . 0 8 0 . 9 9 0 . 9 9 60 1 3 . 4 4 1 .00 1 .00 A t h e o r e t i c a l mass t r a n s f e r c o n t r o l l e d r e g i o n i s o b t a i n e d i n t h i s manner w h i c h i s r e p r e s e n t e d i n F i g . 28 f o r c o m p a r i s o n w i t h t h e e x p e r i m e n t a l r e s u l t s . 2 . C o n t i n u o u s e x p e r i m e n t s . D e t e r m i n a t i o n o f e x p e r i m e n t a l , mass t r a n s f e r , and r e a c t i o n r a t e s c o n s t a n t s . P r o c e d u r e a) A c c o r d i n g t o t h e p l u g f l o w e q u a t i o n , A n ( l - X) = - [ A . 18] I f - J in/ (1 - X) v s i s p l o t t e d f rom t h e e x p e r i m e n t a l d a t a , a s t r a i g h t l i n e s h o u l d be o b t a i n e d , w i t h a s l o p e e q u a l t o K a L . The e x p e r i m e n t a l r a t e c o n s t a n t c a n be d e t e r m i n e d f rom t h e s l o p e . The s p e c i f i c s u r f a c e a r e a o f t h e e l e c t r o d e i s c a l c u l a t e d f rom e q . A . 5 . 174 b) U s i n g t h e e m p i r i c a l e q u a t i o n f o r t h e mass t r a n s f e r c o e f f i c i e n t , i s e s t i m a t e d : K = 0 . 8 3 ( R e ) ° ' 5 6 ( S c ) 1 / 3 22 < Re < 520 m dp c ) The r e a c t i o n r a t e c o n s t a n t i s t h e n c a l c u l a t e d f rom t h e e q u a t i o n o f r e s i s t a n c e s i n s e r i e s : 1 K _1 K r m [ A . 2 0 ] where K K K m r K - K m TABLE A - 3 CALCULATION OF EXPERIMENTAL, MASS TRANSFER AND REACTION RATE CONSTANTS FROM EXPERIMENTS 4 - 1 , 4 - 2 , 4 - 3 * (USING AVERAGE PHENOL FRACTIONAL CONVERSION) Q , / m i n ) u ( c m / s ) - 1 U - 1 ( cm/s ) X - £ n ( l - X) Re K m ( cm/s ) K r ( cm/s ) 0 . 2 5 2 . 7 8 0 . 3 6 0 0 . 59 0 . 8 9 5 1 . 4 3 . 85 x 1 0 " 3 15 .65 x 10' 0 . 4 0 4 . 4 4 0 . 2 2 5 0 . 44 0 . 5 8 8 2 . 1 5 . 01 x 1 0 ~ 3 8.06 x 10' 0 . 5 5 6 . 1 1 0 . 1 6 3 0 . 34 0 . 4 2 1 1 3 . 0 5 . 98 x 1 0 ~ 3 6 .39 x 10' 0 . 8 5 9 . 4 4 0 . 1 0 6 0 . 25 0 . 2 9 174 .6 7. 64 x 1 0 ~ 3 5.19 x 10' 1 .10 1 2 . 2 2 0 . 0 8 2 0 . 21 0 . 2 4 226 .0 8. 82 x 1 0 " 3 4 .76 x 10' 1 .30 1 4 . 4 4 0 .069 0 . 17 0 . 1 9 2 6 7 . 0 9 . 69 x 1 0 - 3 4 .54 x 10' * F o r t h e s e e x p e r i m e n t s : C = 100 ± 5 mg/1 1.7 < dp < 2 . 0 0 mm a = 2 0 . 6 cm" ( f rom page 171) Ao From F i g . 3 2 : S l o p e = 2 . 4 2 cm/sec = K = — 2 . 4 2 cm/s = 3 > Q 9 x 1 Q - 3 c m / g 2 0 . 6 cm x 38 cm u dp u x 0 . 1 8 5 cm . n r Re = 1 1 = n j-.— = 1 8 . 5 u v 0 . 0 1 c m z / s K = 0 . 8 3 - f ( R e ) 0 ' 5 6 ( S c ) 1 / 3 = 0 . 8 3 x 9 - \ \ l f x R e ° ' 5 6 ( 1 0 8 6 ) 1 / 3 = 4 . 2 4 x 10" m dp U . L o D TABLE A - 4 CALCULATION OF EXPERIMENTAL, MASS TRANSFER AND REACTION RATE CONSTANTS FOR EXPERIMENT ( 4 - 4 ) * u 1 K K Q u m r ( J l /min) ( cm/s ) ( c m / s ) X - £ n ( l - X) Re (cm/s ) (cm/s) 0 . 2 5 2 . 7 8 0 . 3 6 0 . 3 7 0 . 4 6 5 1 . 4 3 .85 X io"3 2 . 2 1 X 10' 0 . 4 0 4 . 4 4 0 . 2 2 5 0 . 2 5 0 . 2 9 8 2 . 1 5 .01 X io"3 1.95 X 10' 0 . 5 5 6 . 1 1 0 . 1 6 3 0 . 1 9 0 . 2 1 1 1 3 . 0 5 .98 X io"3 1.84 X 10" 0 . 8 5 9 .44 0 . 1 0 6 0 . 1 2 0 . 1 3 1 7 4 . 6 7 .64 X I O " 3 1.72 X 10 1 .10 1 2 . 2 2 0 . 0 8 2 0 . 0 9 0 . 0 9 2 2 6 . 0 8 .82 X io"3 1.67 X 10 * I n e x p e r i m e n t ( 4 - 4 ) : C . = 580 mg/1 1.7 < dp < 2 . 0 0 mm a = 2 0 . 6 c m " 1 "(from page 171) A 0 From F i g . 3 2 : S l o p e = 1.1 cm/s = K = ! • ! cm/s = 1 < 4 0 5 x 1 0 ~ 3 c m / s 2 0 . 6 cm x 38 cm w i t h , Re = 1 8 . 5 u and K = 4 . 2 4 x 1 0 ~ 4 R e ° " 5 6 (as i n T a b l e A - 3 ) TABLE A - 5 CALCULATION OF EXPERIMENTAL, MASS TRANSFER AND REACTION RATE CONSTANT FOR EXPERIMENT ( 4 - 8 ) * Q ( i l / m i n ) u ( c m / s ) - 1 u ( c m / s ) 1 X A n ( l - X) Re K m ( cm/s ) K r ( cm/s ) 0 . 2 5 2 . 7 8 0 . 3 6 0 .67 1. 11 25 5 . 3 x 1 0 ~ 3 2 .68 x 1 0 ~ 3 0 . 4 0 4 . 4 4 0 . 2 2 5 0 . 5 3 0 . 76 40 6 .9 x 1 0 " 3 2 .40 x 1 0 " 3 0 . 5 5 6 . 1 1 0 . 1 6 3 0 .45 0 . 60 55 8 .2 x 1 0 ~ 3 2.27 x 1 0 - 3 0 . 8 5 9 . 4 4 0 . 1 0 6 0 .34 0 . 42 85 1 0 . 5 x 1 0 " 3 2.14 x l o " 3 1.10 1 2 . 2 2 0 .082 0 .29 0 . 34 110 1 2 . 1 x 1 0 ~ 3 2 .09 x 1 0 - 3 * I n e x p e r i m e n t ( 4 - 8 ) C . = 100 mg/1 0 .7 < dp < 1.1 e = 0 . 5 7 A 0 S p e c i f i c s u r f a c e a r e a o f t h e bed ( e q . A . 5 ) : 1 (1 - 0 . 5 7 ) x 6 , - 1 a * 0 . 3 cm ( 0 . 7 5 ) ( 0 . 0 9 ) cm " 1 0 c m From F i g . 3 2 : S l o p e - 2 . 8 1 cm/s = K : 2 , 8 1 f m / s - 1 .78 x 1 0 ~ 3 cm/s (41 .5cm ) (38cm) Re =«= u d p / v = u 0 . 0 9 / 0 . 0 1 = 9 u K . 0 . 8 3 x 9 - 2 * 1 0 " 6 c m 2 / s ( R e ° - 5 6 ) ( 1 0 8 6 ) 1 / 3 = 8 .72 x l o " 4 ( R e ) 0 ' 5 6 m U.uy cm 178 3 . E s t i m a t e s o f t y p i c a l % c u r r e n t e f f i c i e n c y a) B a t c h e x p e r i m e n t s % C . E . = F Z ( m ^ S ° * i d i z e d > x 100 [ E q . 7] A s s u m i n g a 4 - e l e c t r o n t r a n s f e r f r o m t h e p h e n o l m o l e c u l e as p r o p o s e d by C o v i t z ( R e a c t i o n R . l l ) , t h e % C . E . c a n be c a l c u l a t e d . Sample c a l c u l a t i o n : Run 3 - 1 3 , r e c i r c u l a t i o n t i m e = 15 m i n . F o r t h e c a l c u l a t i o n , i t i s s u p p o s e d t h a t a f t e r 15 m i n , r e c i r c u l a t i o n i s s t o p p e d and t h e v o l u m e r e m a i n i n g i n t h e t a n k i s t r e a t e d c o n t i n u o u s l y . The s t e a d y s t a t e f i n a l c o n c e n t r a t i o n a t t h e o u t l e t i s assumed t o be e q u a l t o t h e r e c o r d e d a t 15 m i n t i m e . t o t a l e l e c t r o l y s i s t i m e = 15 m i n + 5 A / 1 . 1 2 A / m i n = 1 9 . 5 m i n m o l e s p h e n o l o x i d i z e d p h = ( 1 1 0 0 - 7 9 2 ) m g / l x 5A x 1 g m o l / 9 4 g x 1 g / 1 0 3 mg = 1 6 . 4 x 1 0 g m o l _3 _ 96500 c o u l / e q x 4 e q / m o l x 1 6 . 4 x 1 0 g m o l .. n n n 10 A x 1 9 . 5 m m x 60 s e c / m m T h i s i s a s ample c a l c u l a t i o n f o r T a b l e 6 , where % C . E . i s g i v e n a f t e r 15 and 90 m i n r e c i r c u l a t i o n t i m e s , b) C o n t i n u o u s e x p e r i m e n t s F z Q X C . A l % C . E . = - x 100 T h e r e f o r e , c u r r e n t e f f i c i e n c i e s f o r p h e n o l o x i d a t i o n i n a s i n g l e p a s s , a r e a f u n c t i o n o f f l o w , c o n v e r s i o n , i n i t i a l p h e n o l c o n c e n t r a t i o n , and c u r r e n t a p p l i e d . Sample c a l c u l a t i o n : Run 4 - 4 , a t Q = 0 . 2 5 A / m i n , % = 96500 c o u l / e q x 4 e q / m o l x 0 . 2 5 A / m i n x 0 . 3 7 x 580 mg/1 x 1 0 Q 10 A x 1 0 3 mg/g x 94 g / g m o l x 60 s e c / 1 m i n % C . E . = 3 6 . 7 . 179 4 . E s t i m a t e o f t y p i c a l e l e c t r i c a l e n e r g y r e q u i r e m e n t s and c o s t s a) B a t c h e x p e r i m e n t s E n e r g y r e q u i r e m e n t s _ I AV t M o l e s o f p h e n o l o x i d i z e d V (C . - C . ) K m AQ A 2 Sample c a l c u l a t i o n : Run 3 - 1 3 , r e c i r c u l a t i o n t i m e = 15 m i n . As i t was assumed i n t h e p r e v i o u s s e c t i o n , a f t e r t h e b a t c h o p e r a t i o n t h e t a n k v o l u m e i s t r e a t e d c o n t i n u o u s l y . T o t a l e l e c t r o l y s i s t i m e = 15 m i n + 5 1/1.12 A / m i n = 1 9 . 5 m i n A v e r a g e AV = 8 . 5 V E n e r g y = 10 A * 8 .5 V x 1 k w / 1 0 3 w * 1 9 . 5 m i n x 1 h / 6 0 m i n m o l 5A x (1100-792) mg/1 x i g / l 0 3 m g x 1 g m o l / 9 4 g g m o l Power c o s t s a r e t a k e n as 2 ^ / k w - h f o r i l l u s t r a t i o n , C o s t = 1 . 6 9 k w - h / g m o l x 0 . 0 2 $ / k w - h = 0 . 0 3 $ / g m o l E n e r g y r e q u i r e m e n t s and c o s t s a r e p r e s e n t e d i n T a b l e 6 f o r t h e b a t c h e x p e r i m e n t s N o s . 3 - 3 , 3 - 1 2 , 3-12 a t 10 A , a f t e r 15 and 90 m i n r e c i r c u l a - t i o n t i m e . I n T a b l e 8 t h e e n e r g y r e q u i r e m e n t s p e r vo lume o f w a s t e i s e s t i m a t e d as f o l l o w s : i n Runs 3-12 and 3 - 1 3 , 99% and 98% p h e n o l r e m o v a l i s a c h i e v e d a t 90 m i n r e c i r c u l a t i o n t i m e . T o t a l e l e c t r o l y s i s t i m e = 90 mm + 5 A / 1 . 1 2 A / m i n = 9 4 . 5 m i n 10 A x 8 .5 V x 9 4 . 5 m i n x l p 3 k w / w E n e r g y = g 0 y . / n , = 0 . 0 2 7 k w - h / A O J 5A x 60 m m / 1 h E n e r g y = 0 . 0 2 7 k w - h / A x 3 .785 A / 1 g a l = 0 . 1 0 2 2 k w - h / g a l 180 The e l e c t r i c a l c o s t f o r t r e a t i n g 10 g a l o f w a s t e w o u l d b e : 1 0 2 . 2 k w - h / 1 0 0 0 g a l x 0 . 0 2 $ / k w - h = 2 $ /1000 g a l T h i s w o u l d be t h e a p p r o x i m a t e c o s t f o r t r e a t i n g a 700 mg/1 e f f l u e n t w i t h a 99% p h e n o l r e m o v a l , u n d e r t h e o p e r a t i n g c o n d i t i o n s o f e x p e r i m e n t s 3-12 o r 3 - 1 3 . b) C o n t i n u o u s e x p e r i m e n t s e n e r g y r e q u i r e d I AV m o l p h e n o l o x i d i z e d Q ( C . - C . ) A i A 2 Sample c a l c u l a t i o n : Run 4-4 a t Q - 0 . 2 5 £ / m i n , v 10 A x 8 .5 V x 1 0 3 kw/w x 1 0 3 mg/g x 94 g / g m o l . . . , . E n 6 r g y " 0 . 2 5 £/mln * ( 5 8 0 - 3 6 5 ) m g / l x 60 m i n / 1 h = 2 ' 5 1 k w ~ h / g m o 1 C o s t o f e l e c t r i c a l e n e r g y p e r m o l o f p h e n o l o x i d i z e d : 0 . 0 2 $ / k w - h x 2 . 5 1 k w - h / g m o l = 0 . 0 5 $ / g m o l 181 APPENDIX 5 R e l e v a n t P h y s i c a l D a t a TABLE A - 6 pH OF SOLUTIONS OF NaOH AND H ^ O ^ AT 2 0 ° C (51) N A q . NaOH s o l u t i o n s A q . H ^ O ^ s o l u t i o n s g / 1 pH g / 1 pH .1 4 0 . 0 14 4 9 . 0 0 . 3 0 . 1 4 . 0 13 4 . 9 1.2 0 . 0 1 0 . 4 12 0 . 4 9 2 . 1 TABLE A - 7 CONDUCTIVITIES OF AQUEOUS SOLUTIONS OF NaOH, H 2S0i+ AND N a 2 S O l + AT 20°C (51) NaOH H 2 S 0 i t N a 2 S O i + K x 1 0 3 K x 1 0 3 K x 1 0 3 c o n e . e c o n e . e c o n e . e ( g / 1 ) (ft cm) 1 ( g / 1 ) (ft cm) 1 ( g / 1 ) (ft cm) 5 2 4 . 8 5 . 0 2 4 . 3 5 . 0 5 . 9 1 0 . 1 4 8 . 6 1 0 . 0 4 7 . 8 1 0 . 1 1 1 . 2 1 5 . 2 " 7 1 . 3 1 5 . 1 7 0 . 3 1 5 . 2 1 5 . 7 2 0 . 4 9 3 . 1 2 0 . 2 9 2 . 0 2 0 . 3 1 9 . 8 2 5 . 5 2 3 . 9 3 0 . 8 2 7 . 9 182 % p h e n o l i o n i z e d v s pH a t 2 0 ° C d i s s o c i a t i o n c o n s t a n t " 1 .28 x 10 ^ (51) K d = [if] [ C 6 H 5 0 ~ ] / [ C 6 H 5 O H ] I o n i z e d = r l [ c R ] f r a c t i o n J D 0 I o n i z e d _ d = d = 1 .28 x 10 x u f r a c t i o n " [ R + ] 1 Q - p H = 1 Q - p H % i o n i z e d = f r a c t i o n / ( f r a c t i o n + 1) x ioo TABLE A - 8 % PHENOL IONIZED VS pH pH 2 4 6 8 10 12 i o n i z e d t o u n i o n i z e d _ f i _/- _ / _ o f r a c t i o n 1.28 x 10 1.28 x 10 1.28 x 10 1.28 x 10 1.28 128 % i o n i z e d 1.28 x i o " 6 1.28 x i o " 4 1.28 x i o " 2 1.16 x i f f 1 56 99

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