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Copper, lead and zinc sorption on Wyoming montmorillonite Cooper, Roger Brian 1976

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COPPER, LEAD AND ZINC SORPTION ON WYOMING MONTMORILLONITE by ROGER BRIAN COOPER B. S c , U n i v e r s i t y of B r i t i s h Columbia, 1973 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF " MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES DEPARTMENT OF GEOLOGICAL SCIENCES We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA J u l y , 1976 (g) Roger B r i a n Cooper, 1976 In p resent ing t h i s t he s i s in p a r t i a l f u l f i l m e n t o f the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that 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 reference and study. I f u r t h e r agree tha t permiss ion fo r ex ten s i ve copying of t h i s t he s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r ep re sen ta t i ve s . It i s understood that copying or p u b l i c a t i o n of t h i s t he s i s f o r f i n a n c i a l gain s h a l l not be a l lowed without my w r i t t e n permi s s ion . Department The U n i v e r s i t y of B r i t i s h Columbia 2075 Wesbrook P l a c e Vancouver, Canada V6T 1W5 4 ABSTRACT Exchange a d s o r p t i o n o f C u 2 , P b 2 and Z n 2 f o r Na on Wyoming mont-m o r i l l o n i t e , i n c h l o r i d e and n i t r a t e e l e c t r o l y t e s o l u t i o n s l e s s t h a n 10 3 M t o t a l heavy m e t a l c o n c e n t r a t i o n , can be m o d e l l e d w i t h a s i m p l e e q u i l i b r i u m i o n - e x c h a n g e r e a c t i o n : Me2+ + 2 Na{Mon£} - Me{Mo*tt}2 + 2 N a + where Me. and Mottt r e p r e s e n t a g i v e n heavy m e t a l and m o n t m o r i l l o n i t e . E q u i -2 + 2 + l i b r i u m c o n s t a n t s f o r t h i s r e a c t i o n a r e e q u a l f o r Cu and Zn a t 3.0 ± 1, ? + s l i g h t l y l e s s t h a n t h a t f o r Pb a t 5.0 ± 1, and comparable t o c o n s t a n t s + + + + c a l c u l a t e d f o r m o n o p o s i t i v e exchange of K f o r Na (3.0 ± 1) and H f o r Na (2.5 ± . 5 ) . Above 10 3 M t o t a l m e t a l c o n c e n t r a t i o n , heavy m e t a l exchange f o r Na i s c o m p l i c a t e d by v a r i a b l e t o t a l Na exchange c a p a c i t y and by a p p a r e n t a n i o n i c i n t e r f e r e n c e s . CuCl2 and Z n C l 2 e l e c t r o l y t e s o l u t i o n s a r e more e f f e c -t i v e Na e x c h a n g e r s a t 0.1 M t h a n C u ( N 0 3 ) 2 , P b ( N 0 3 ) 2 , KC1 or HC1 s o l u t i o n s a t the same c o n c e n t r a t i o n . R e t a r d a t i o n f a c t o r s o f Cu, Pb and Zn s p o t s e l u t e d a c r o s s t h i n l a y e r s o f N a - m o n t m o r i l l o n i t e ( s u p p o r t e d by s i l i c a g e l ) by aqueous N a C l and NaN0 3 s o l u t i o n s of 0.05 t o 3.0 M c o n c e n t r a t i o n s u g g e s t , when i n t e r p r e t e d w i t h a r u d i m e n t a r y i o n - e x c h a n g e mass t r a n s f e r model, t h a t m e t a l s m i g r a t e m a i n l y as m o n o p o s i t i v e s p e c i e s — p e r h a p s as monohydroxo-complexes w i t h i n t h e c l a y m i c e l l e . P r e c i p i t a t i o n o f Pb c h l o r i d e o r h y d r o x y - c h l o r i d e i s i n d i c a t e d by m u l t i p l e Pb s p o t s on N a C l - e l u t e d chromatograms. TABLE OF CONTENTS Page INTRODUCTION 1 Sources of Wyoming Bentonite. 2 Wyoming M o n t m o r i l l o n i t e C r y s t a l Chemistry 3 E a r l y I n v e s t i g a t i o n of Clay M i n e r a l Ion Exchange 8 Stereochemistry of Cation Adsorption.. 13 Previous Study of Heavy Metal Adsorption on M o n t m o r i l l o n i t e . . . . 17 HEAVY METAL SORPTION ON SUSPENDED Na-MONTMORILLONITE I n t r o d u c t i o n 25 Thermodynamics of Exchange Adsorption , 26 Experimental Method , 29 Re s u l t s of Na Exchange Capacity Determinations 31 Re s u l t s of KC1 and HC1 T i t r a t i o n s 35 R e s u l t s of Heavy Metal T i t r a t i o n s 42 I n t e r p r e t a t i o n of R e s u l t s 55 Summary and Conclusions ,. 94 MONTMORILLONITE-SILICA GEL THIN—LAYER CHROMATOGRAPHY In t r o d u c t i o n , 9 7 T h e o r e t i c a l Development., , 9 7 Experimental Procedure. , , . . . 102 R e s u l t s 103 I n t e r p r e t a t i o n and Conclusions 121 LIST OF REFERENCES APPENDICES Methods of S t a n d a r d i z a t i o n . 141 Pr e p a r a t i o n of Na-saturated M o n t m o r i l l o n i t e 142 P a r t i c l e S i z e C h a r a c t e r i z a t i o n of M o n t m o r i l l l o n i t e Suspensions. 1 4 3 Membrane F i l t r a t i o n of Clay Suspensions 145 Test f o r Ca t i o n E x c l u s i o n During F i l t r a t i o n . . . . . 146 Na Exchange Capacity Determination 1 4 7 E f f e c t of Ambient Humidity on Weight of A i r - D r i e d M o n t m o r i l l o n i t e , 149 Page APPENDICES ( c o n t . ) P r e p a r a t i o n o f N a - M o n t m o r i l l o n i t e - S i l i c a G e l T h i n - L a y e r P l a t e s . 151 S o u r c e s of E x p e r i m e n t a l M a t e r i a l s 152 M o d e l l i n g C a t i o n Exchange E q u i l i b r i a 154 i v LIST OF TABLES Table I S t r u c t u r a l formulas f o r s e l e c t e d Wyoming m o n t m o r i l l o n i t e s I I C a tion exchange c a p a c i t i e s determined by displacement of Na from Na-montmorillonite w i t h v a r i o u s 0.1 M e l e c t r o l y t e s o l u -t i o n s . I I I F i l t r a t e e q u i l i b r i u m concentrations of Na and K r e s u l t i n g from a d d i t i o n of 0.100 M KC1 t i t r e to an aqueous suspension of Na-montmorillonite. IV F i l t r a t e e q u i l i b r i u m pH and Na c o n c e n t r a t i o n r e s u l t i n g from a d d i t i o n of 0.100 M HC1 t i t r e to an aqueous suspension of Na-montmorillonite. V F i l t r a t e e q u i l i b r i u m concentrations of Na and Cu r e s u l t i n g from a d d i t i o n of 0.103 M CuCl2 t i t r e to an aqueous suspen-s i o n of Na-montmorillonite. VI F i l t r a t e e q u i l i b r i u m c oncentrations of Na and Cu r e s u l t i n g from a d d i t i o n of 0.1013 M C u ( N 0 3 ) 2 t i t r e to an aqueous sus-pension of Na-montmorillonite. V I I F i l t r a t e e q u i l i b r i u m c oncentrations of Na and Pb r e s u l t i n g from a d d i t i o n of 0.0970 M P b ( N 0 3 ) 2 t i t r e to an aqueous sus-pension of Na-montmorillonite. * V I I I F i l t r a t e e q u i l i b r i u m c o n c e n t r a t i o n s of Na and Zn r e s u l t i n g from a d d i t i o n of 0.0970 M Z n C l 2 t i t r e to an aqueous suspen-s i o n of Na-montmorillonite. IX S t a r t i n g parameters f o r c a l c u l a t i o n of t h e o r e t i c a l concen-t r a t i o n curves. X Corrected volumes and concentrations f o r KC1:Na-montmoril-l o n i t e t i t r a t i o n f i l t r a t e s . XI Summary of mass balance c a l c u l a t i o n s f o r ZnCi2:Na-montmor-i l l o n i t e t i t r a t i o n . X I I Summary of mass balance c a l c u l a t i o n s f o r CuCl2:Na-montmor-i l l o n i t e t i t r a t i o n . X I I I Summary of mass balance c a l c u l a t i o n s f o r Cu ( N O 3) 2 .•Na-mont-m o r i l l o n i t e t i t r a t i o n . XIV Summary of mass balance c a l c u l a t i o n s f o r Pb(N0 3) 2:Na-mont-m o r i l l o n i t e t i t r a t i o n . XV R e t a r d a t i o n f a c t o r s f o r Cu spot centres a f t e r e l u t i o n across N a - m o n t m o r i l l o n i t e - s i l i c a g e l t h i n l a y e r s by aque-ous NaCl and NaN0 3 s o l u t i o n s of v a r y i n g c o n c e n t r a t i o n . XVI R e t a r d a t i o n f a c t o r s f o r Zn spot centres a f t e r e l u t i o n across N a - m o n t m o r i l l o n i t e - s i l i c a g e l t h i n l a y e r s by aque-ous NaCl and NaN0 3 s o l u t i o n s of v a r y i n g c o n c e n t r a t i o n . Page 7 32 36 37 43 44 45 46 59 63 84 85 86 107 108 4 V Table XVII XVIII XIX XX Re t a r d a t i o n f a c t o r s f o r Pb spot centres a f t e r e l u t i o n across N a - m o n t m o r i l l o n i t e - s i l i c a g e l t h i n l a y e r s by aque-ous NaCI s o l u t i o n of v a r y i n g c o n c e n t r a t i o n . R e t a r d a t i o n f a c t o r s f o r Pb spot centres a f t e r e l u t i o n across N a - m o n t m o r i l l o n i t e - s i l i c a g e l t h i n l a y e r s by aque-ous N a ( N n 3 ) 2 s o l u t i o n of v a r y i n g c o n c e n t r a t i o n . Table of symbols used i n FOCAL 69 t h e o r e t i c a l t i t r a t i o n programme. FOCAL 69 programme to c a l c u l a t e t h e o r e t i c a l c a t i o n ex-change t i t r a t i o n curves. Page 109 110 161 162 LIST OF FIGURES Figure Page 1 C r y s t a l s t r u c t u r e of m o n t m o r i l l o n i t e p r o j e c t e d on (001) 4 showing s u p e r p o s i t i o n of octahedral and t e t r a h e d r a l sheets. 2 Diagrammatic c r y s t a l s t r u c t u r e of m o n t m o r i l l o n i t e viewed 5 approximately normal to the e-axis. 3 P l o t of Na exchange c a p a c i t y of e l e c t r o l y t e s against ambient 33 pH. 4' P l o t of f i l t r a t e Na c o n c e n t r a t i o n r e s u l t i n g from a d d i t i o n of 38 0.100 M KC1 t i t r e to 20 ml suspensions of 1% Na-montmoril-l o n i t e . 5 P l o t of f i l t r a t e K c o n c e n t r a t i o n r e s u l t i n g from a d d i t i o n of 39 0.100 M KC1 t i t r e to 20 ml suspensions of 1% Na-montmoril-l o n i t e . 6 P l o t of f i l t r a t e Na co n c e n t r a t i o n r e s u l t i n g from a d d i t i o n of 40 0.100 M HC1 t i t r e to 20 ml suspensions of 1% Na-montmoril-l o n i t e . 7 P l o t of f i l t r a t e pH r e s u l t i n g from a d d i t i o n of 0.100 M HC1 41 t i t r e to 20 ml suspensions of 1% Na-montmorillonite. 8 P l o t of f i l t r a t e Na co n c e n t r a t i o n r e s u l t i n g from a d d i t i o n of 47 0.1030 M C u C l 2 t i t r e to 20 ml suspensions of 1% Na-montmor-i l l o n i t e . 9 P l o t of f i l t r a t e Cu co n c e n t r a t i o n r e s u l t i n g from a d d i t i o n of 48 0.1030 M CuCl2 t i t r e to 20 ml suspensions of 1% Na-montmor-i l l o n i t e . 10 P l o t of f i l t r a t e Na co n c e n t r a t i o n r e s u l t i n g from a d d i t i o n of 4 9 0.1013 M C u ( N 0 3 ) 2 t i t r e to 20 ml suspensions of 1% Na-mont-m o r i l l o n i t e . 11 P l o t of f i l t r a t e Cu c o n c e n t r a t i o n r e s u l t i n g from a d d i t i o n of 50 0.1013 M Cu(N0 3) 2 t i t r e to 20 ml suspensions of 1% Na-mont-m o r i l l o n i t e . 12 P l o t of f i l t r a t e Na co n c e n t r a t i o n r e s u l t i n g from a d d i t i o n of 51 0.0970 M P b ( N 0 3 ) 2 t i t r e to 20 ml suspensions of 1% Na-mont-m o r i l l o n i t e . 13 P l o t of f i l t r a t e Pb c o n c e n t r a t i o n r e s u l t i n g from a d d i t i o n of 52 0.0970 M P b ( N 0 3 ) 2 t i t r e to 20 ml suspensions of 1% Na-mont-m o r i l l o n i t e . 14 P l o t of f i l t r a t e Na c o n c e n t r a t i o n r e s u l t i n g from a d d i t i o n of 53 0.0970 M Z n C l 2 t i t r e to 20 ml suspensions of 1% Na-montmor-i l l o n i t e . 15 P l o t of f i l t r a t e Zn co n c e n t r a t i o n r e s u l t i n g from a d d i t i o n of 54 0,0970 M Z n C l 2 t i t r e to 20 ml suspensions of 1% Na-montmor-i l l o n i t e . V l l Figure Page 16 Sup e r p o s i t i o n of t h e o r e t i c a l Na c o n c e n t r a t i o n curves on data 56 po i n t s of the HC1:Na-montmorillonite t i t r a t i o n . 17 Superposition of t h e o r e t i c a l pH curves on data p o i n t s of the 57 HC1:Na-montmorillonite t i t r a t i o n . 18 Superposition of t h e o r e t i c a l Na co n c e n t r a t i o n curves on data 61 p o i n t s of the KC1:Na-montmorillonite t i t r a t i o n , 19 Su p e r p o s i t i o n of t h e o r e t i c a l K co n c e n t r a t i o n curves on data 62 po i n t s of the KC1:Na-montmorillonite t i t r a t i o n . 20 Sup e r p o s i t i o n of t h e o r e t i c a l Na concentration- curves on data 66 po i n t s of the ZnCl 2:Na-montmorillonite t i t r a t i o n . 21 Su p e r p o s i t i o n of t h e o r e t i c a l Zn co n c e n t r a t i o n curves on data 67 po i n t s of the ZnCl 2:Na-montmorillonite t i t r a t i o n . 22 Sup e r p o s i t i o n of t h e o r e t i c a l Na co n c e n t r a t i o n curves on data 68 po i n t s of the Pb(NO3)2:Na-montmorillonite t i t r a t i o n . 23 Su p e r p o s i t i o n of t h e o r e t i c a l Pb co n c e n t r a t i o n curves on data 69 po i n t s of the Pb ( N O 3) 2:Na-montmorillonite t i t r a t i o n . 24 Sup e r p o s i t i o n of t h e o r e t i c a l Na conc e n t r a t i o n curves on data 71 po i n t s of the CuCl 2:Na-montmorillonite t i t r a t i o n . 25 Su p e r p o s i t i o n of t h e o r e t i c a l Cu co n c e n t r a t i o n curves on data 72 po i n t s of the CuCl 2:Na-montmorillonite t i t r a t i o n . 26 Su p e r p o s i t i o n of t h e o r e t i c a l Na c o n c e n t r a t i o n curves on data 73 po i n t s of the Cu(N0 3) 2:Na-montmorillonite t i t r a t i o n . 27 Sup e r p o s i t i o n of t h e o r e t i c a l Cu co n c e n t r a t i o n curves on data 74 po i n t s of the Cu ( N O 3) 2.Na-montmorillonite t i t r a t i o n . 28 P l o t of observed and t h e o r e t i c a l f i l t r a t e pH f o r the Z n C l 2 : 76 Na-montmorillonite t i t r a t i o n . 29 P l o t of observed and t h e o r e t i c a l f i l t r a t e pH f o r the 77 Pb (NO3) 2 ".Na-montmorillonite t i t r a t i o n , 30 P l o t of observed and t h e o r e t i c a l f i l t r a t e pH f o r the C u C l 2 : 78 Na-montmorillonite t i t r a t i o n . 31 P l o t of observed and t h e o r e t i c a l f i l t r a t e pH f o r the 79 Cu(N03)2:Na-montmorillonite t i t r a t i o n . 32 P l o t of Zn-Na s t o i c h i o m e t r i c exchange constants against 89 t i t r e 0.0970 M Z n C l 2 . 33 P l o t of Cu-Na s t o i c h i o m e t r i c exchange constants against 90 t i t r e 0.1030 M C u C l 2 . 34 P l o t of Cu-Na s t o i c h i o m e t r i c exchange constants against 91 t i t r e 0.1013 M C u ( N 0 3 ) 2 . 35 P l o t of Pb-Na s t o i c h i o m e t r i c exchange constants against 92 t i t r e 0,0970 M P b ( N 0 3 ) 2 . v i i i F i g u r e Page 36 Thin l a y e r chromatograms showing p o s i t i o n s of Cu 5 Zn and Pb 111 spots a f t e r e l u t i o n w i t h 0.5 M NaCI and NaNO s o l u t i o n s . 37 P l o t of Cu spot r e t a r d a t i o n f a c t o r s against l o g a r i t h m of 113 NaNO eluent c o n c e n t r a t i o n . 38 P l o t of Zn spot r e t a r d a t i o n f a c t o r s against l o g a r i t h m of 114 NaNO eluent c o n c e n t r a t i o n . 39 P l o t of Pb spot r e t a r d a t i o n f a c t o r s against l o g a r i t h m of 115 NaNO eluent c o n c e n t r a t i o n . 40 P l o t of Cu spot r e t a r d a t i o n f a c t o r s against logarithm of 116 NaCI eluent c o n c e n t r a t i o n . 41 P l o t of Zn spot r e t a r d a t i o n f a c t o r s against l o g a r i t h m of 117 NaCI eluent c o n c e n t r a t i o n . 42 P l o t of Pb spot r e t a r d a t i o n f a c t o r s against logarithm of 118 NaCI eluent c o n c e n t r a t i o n . 43 Diagrammatic r e p r e s e n t a t i o n of " m i c e l l a r s o l u t i o n " formed 130 between a m o n t m o r i l l o n i t e surface and f r e e s o l u t i o n . 44 C o v a r i a t i o n of m o n t m o r i l l o n i t e weight and ambient r e l a t i v e 150 humidity w i t h time. 45 Flow diagram f o r FOCAL 69 t h e o r e t i c a l t i t r a t i o n programme. 160 LIST OF PLATES P l a t e Page I Photograph of a N a - m o n t m o r i l l o n i t e - s i l i c a g e l chromatogram 105 eluted w i t h 0.3 M NaCI s o l u t i o n and v i s u a l i z e d w i t h 0.5% s-diphenylcarbazone ( i n e t h a n o l ) , ACKNOWLEDGMENTS The author s i n c e r e l y thanks Dr. W.C. Barnes and Dr. M.A. Barnes f o r t h e i r support and patience throughout t h i s study. Their advice and encouragement was i n v a l u a b l e , p a r t i c u l a r l y when events d i d not t r a n s p i r e e x a c t l y as the author thought they should. G r a t e f u l acknowledgment i s a l s o extended to Dr. W.K. F l e t c h e r and Mr. R. L e t t f o r t h e i r suggestions concerning chemical a n a l y s i s , to Mrs. S. F i n o r a and Dr. A. Lewis f o r p a r t i c l e s i z e a n a l y s i s , and to Dr. T.H. Brown and Mr. I . Duncan f o r d i s c u s s i o n s of theory. INTRODUCTION This laboratory investigation attempts to characterize exchange reactions of Cu, Zn and Pb with Na-saturated Wyoming montmorillonite. Two experimental methods were employed: the f i r s t involved cation analysis of electrolyte solutions equilibrated with suspended montmorillonite; and the second made use of thin-layer ion-exchange chromatography. It was hoped that equilibrium constants could be determined for each heavy metal-Na ex-change reaction, thereby establishing a replaceability hierarch that would enable interpretation of chromatographic data, and which might be valuable to the understanding of heavy metal chemistry in natural suspensions, soils and sediments. During the course of study i t was found necessary or expedient to develop new or modified experimental techniques. These included a method for removing clay from suspension by membrane f i l t r a t i o n and a procedure for preparing c l a y - s i l i c a gel thin-layer plates suitable for ion-exchange chromatography. Theoretical models, which relied on general mass action theory and which involved numerical calculation by small computer, were constructed and used to generate equilibrium concentration curves for cat-ions released to (or adsorbed from) solution during t i t r a t i o n of Na-mont-morillonite suspensions with heavy metal electrolytes. Graphical evalua-tion of exchange constants was attempted by comparing theoretical t i t r a t i o n curves with those observed. Stoichiometric constants were also calculated. Retention factors for heavy metal spots, translocated across Na-montmoril-l o n i t e - s i l i c a gel thin-layers by elution with sodium electrolyte solutions of varying ionic strength, were predicted by a rudimentary mass transfer model. These values were compared with chromatographic data. 4 2 Sources of Wyoming Bentonite » M o n t m o r i l l o n i t e used i n t h i s study was e x t r a c t e d from Wyoming bentonite. Bentonite r e f e r s to a rock formed by a l t e r a t i o n of v o l c a n i c ash and c o n s i s t i n g of the c l a y m i n e r a l m o n t m o r i l l o n i t e ( o c c a s i o n a l l y b e i d e l l i t e ) and r e l i c t c r y s t a l fragments. These i n c l u d e quartz, f e l d -spar, b i o t i t e , z i r c o n , amphibole and oxides. Two s u p p l i e r s of bent o n i t e were r e q u i r e d , as a q u a n t i t y of Clay Spur bentonite i n i t i a l l y obtained from Ward's N a t u r a l Science Establishment (Clay M i n e r a l Standard Mont-m o r i l l o n i t e #26) could not be r e p l e n i s h e d . Bentonite from the Newcastle Formation of Crook County, Wyoming was obtained from Source Clay M i n e r a l s Repository, Columbia M i s s o u r i and found a s a t i s f a c t o r y s u b s t i t u t e . Both bentonites are commercially important as s l u r r y i n g agents i n d r i l l i n g muds and p e l l e t i z i n g media i n i r o n ores. Wyoming bentonite i s a l s o used by the ceramics, food and wine i n d u s t r i e s . Clay Spur bentonite i s named a f t e r the extensive bed near the top of the Cretaceous Mowry Formation of northern Black H i l l s D i s t r i c t Wyoming. According to Knechtel and P a t t e r s o n (1962), b e n t o n i t e from t h i s unusually high-grade bed contains about 80% m o n t m o r i l l o n i t e and 20% non-clay m i n e r a l s . X ray and thermal a n a l y s i s reported by these authors revealed no c l a y min-e r a l s other than m o n t m o r i l l o n i t e . Newcastle bentonite comes from a bed i n Newcastle Formation about 80 m s t r a t i g r a p h i c a l l y below the Clay Spur bed. I t i s p h y s i c a l l y and chemi c a l l y s i m i l a r to Clay Spur bentonite except that i t contains a higher percentage of non-clay m a t e r i a l (10-40%) and i s found l o c a l l y w i t h Ca dominating Na as p r i n c i p l e exchange c a t i o n . Newcastle bent o n i t e used i n t h i s study was i d e n t i f i e d as the Na form by s u p p l i e r . 3 Wyoming M o n t m o r i l l o n i t e C r y s t a l Chemistry M o n t m o r i l l o n i t e c r y s t a l chemistry has been summarized by Grim (1968) and Weaver and P o l l a r d (1973). Wyoming m o n t m o r i l l o n i t e i s an expandable, d i o c t a h e d r a l , 2:1 p h y l l o s i l i c a t e w i t h a c a t i o n exchange c a p a c i t y of between 60 and 100 meq/100 g. I t s s t r u c t u r e i s s i m i l a r to muscovite and c o n s i s t s of an oct a h e d r a l sheet sandwiched between two t e t r a h e d r a l sheets (Figure 1). S i l i c a t e t r a h e d r a are l i n k e d together by three shared oxygens to form a sheet w i t h near-hexagonal symmetry (Figure 2 ). A p i c a l oxygens of a l l s i l i c a t etrahedra p o i n t inward and are shared by p a i r s of c a t i o n s of the octahedral sheet. In Wyoming mo n t m o r i l l o n i t e t h i s o c t a h e d r a l sheet i s made up of t r i v a l e n t c a t i o n s i n s i x c o o r d i n a t i o n w i t h four shared oxygens and two shared hydroxyls. Each of the four oxygens i s shared by two octahedral c a t i o n s and one s i l i c o n i n the o v e r l y i n g sheet. Hydroxyls are shared only by p a i r s of o c t a h e d r a l c a t i o n s . Only two t h i r d s , or s l i g h t l y more than two t h i r d s , of a v a i l a b l e octahedral s i t e s are f i l l e d i n m o n t m o r i l l o n i t e , as opposed to t r i o c t a h e d r a l c l a y s i n which a l l are occupied. Weaver and P o l l a r d p o i n t out that analyses i n d i c a t i n g more than 2.10 octahe-d r a l c a t i o n s per Oio(OH)2 are r a r e and probably i n e r r o r . Exact determination of m o n t m o r i l l o n i t e c r y s t a l s t r u c t u r e and sym-metry has been impeded by i t s poor c r y s t a l l i n i t y and v a r i a b l e s t a c k i n g i n the c - d i r e c t i o n (Figure 2). Studies reviewed by Grim (1968) i n d i c a t e d that some m o n t m o r i l l o n i t e s possess s t a c k i n g r e g u l a r i t y which generates a monoclinic c e l l w i t h space group C 2 or C^/m. X ray d i f f r a c t o g r a m s f o r most randomly o r i e n t a t e d m o n t m o r i l l o n i t e powders r e v e a l only hkO and 00Z r e f l e c t i o n s ; hilt r e f l e c t i o n s r e q u i r e s t a c k i n g r e g u l a r i t y i n the sense that Figure 1 . C r y s t a l structure of montmorillonite projected on (001) showing superposition of octahedral and tetrahedral sheets. Unshaded tetrahedra point downward, dark-shaded point up-ward. Upper and lower tetrahedral layers are not shown superimposed to avoid confusion. Hydroxyls are indicated by "H". i 5 A l 3+ © { F e 3 + , F e 2 \ Mg^} Q .2+ 2+1 S i " + { A l 3 + } Oxygen Hydroxyl Figure 2 Diagrammatic c r y s t a l structure of montmorillonite, modified from Grim (1968). Cations i n parentheses substitute for the main cation. 6 successively stacked layers are symmetry-related. Substitution of Mg2 and Fe 2 for A l 3 + in the octahedral sheet and Al for Si in the tetrahedral sheet of Wyoming montmorillonite results in a net negative charge of about 0.66 per unit c e l l . This is balanced largely by adsorption of exchangeable cations on the sheet surface and edge. Air-dried montmorillonite absorbs water between 2:1 units in single, double or multiple layers, depending on relative humidity and exchange cation (Kerns and Mankin, 1968) . Water is thought to be bound to basal oxygens by hydrogen bonds and by electrostatic attraction to exchange cations (i.e., hydration shells). Interlayer water is considerably more ordered than liquid water, possibly having a structure similar to ice (Hendricks and Jefferson, 1938). Structural formulas for oxidized and unoxidized Wyoming montmoril-lonite are presented in Table I, using data of Knechtel and Patterson (1962) and Grim and Kulbicki (1961) . It is noteworthy that formulas are consistent, but that oxidation of ferrous (to ferric) iron in the octahe-dral layer can account for a 30% reduction in layer charge, and hence in total exchange capacity. Exchange cations of natural Clay Spur and Newcastle montmorillonites are, in order of decreasing abundance, Na, Ca and Mg. K is rare, seldom exceeding 3 meq/100 g. Total exchange capaci-ties are about the same in both clays, ranging from 64 to 91 meq/100 g and averaging 78 meq/100 g (Knechtel and Patterson, 1962). Exchangeable anions reported for Clay Spur montmorillonite (Osthaus, 1955) were S0 4 2 (13,0 meq/ 100 g), C l " (3.6 meq/100 g) and C0 3 2~ (10.5 meq/100 g). Minor elements in Clay Spur bentonite were listed (among others) by Knechtel and Patterson as L i 2 0 (0.01%), Zr0 2 (0.03%), Ti0 2 (0.05%), Y 20 3 (0,006%), BaO (0.003%), PbO (0.003%), B 20 3 (0.002%), MnO (0.001%) and Mo03 (0.001%); many of these Table I . Structural formulas for selected Wyoming montmorillonites. Sources are 1Knechtel and Patterson (1962) and 2Grim and Kulbicki (1961). Formulas are for ^  unit c e l l s . Sample Structural Formula Locality Ref. 1 (Al 3g 0 Fe 0 3o 6 F e 2 t 8 Mg 2 2 2) ( S i ^ A1^J 9)0 1 0 (0H)2 Clay Spur bentonite bed, 1 Crook County 1 f (Al[ti Fe03.t3 F e 2 t 6 M g 2 ^ 0 ) ( S i ^ 2 A l 3 ^ ^ „ (OH) 2 2 (Al 3e 2 Fe3.;o Fe 2.t 3 M g ^ 0 ) ( S l ^ 2 A l 0 3 * 8 ) 0 1 0 (OH)2 2 f (A13.J2 F e 3 t 3 F e 2 t 5 Mg 2.t 9)(Si^ 0 Al 3.t 0)O 1 0 (OH) 2 3 f ( A l 3 ^ F e 3 t 5 F e 2 ^ M g 2 t 9 ) ( S i ^ 0 Al 0 3.t 0)O 1 0(OH) 2 Crook County 2 t Oxidized sample 1 8 elements may derive from non-clay components such as zircon and feldspar. Elements not detected by spectrographs analysis were notably Zn, Cd, Cu, Cr, Ni and Co. Knechtel and Patterson noted that almost a l l Wyoming ben-tonites were weakly radioactive, one analysis indicating 0.001% U 20 3 and 0.002% equivalent U 2 O 3 (excess radioactivity presumably from thorium). Early Investigation of Clay Mineral Ion-Exchange The ion exchange properties of clay minerals were f i r s t appreciated by pedologists who found that the a b i l i t y of soils to adsorb cations was in large part due to s o i l clay minerals (Thompson, 1850; Way, 1850) . A p r o l i f i c literature can be found in the s o i l sciences dealing with clay mineral ion-exchange and i t s significance to s o i l f e r t i l i t y . Carroll (1959) has reviewed much of this work. In early chemical literature Jenny (1932) provided an excellent ex-amination of the cation-exchange properties of natural and synthetic alu-minosilicate minerals, including Wyoming montmorillonite, and presented data indicating relative a f f i n i t i e s of these materials for a l k a l i metal cations, ammonium ions and protons. Vanselox^ (1932) u t i l i z e d competitive Cd 2 +-Ba 2 + exchange on montmorillonite (bentonite clay) to estimate a c t i -vity coefficients of these cations in aqueous solution. Agreement of his • results with independent estimates indicated support for his assumption of a simple mass action model and ideal mixing of adsorbed metals. The latter assumption required act i v i t i e s of sorbed cations to be identical with their mole fractions; in other words, unit activity coefficients were assumed. In later publications (Jenny and Reitemeier, .1935; Jenny, 1936; Jenny and Overstreet, 1939) i t was demonstrated that adsorbed cations had « 9 significant mobility. For example, suspended particles of homoionic Putnam montmorillonite were found to carry a net negative charge and move across an electric potential at velocities which depended on (and charac-terized) the adsorbed cation. It was theorized that adsorbed cations could oscillate to some extent between the adsorption surface and free solution, thereby bestowing the clay with limited anionic properties. Comparing particle migration velocities with exchange a f f i n i t y data, Jenny and Reitemeier (1935) found that clays with high velocities contained easily exchangeable cations. More important, a regular exchangeability sequence was established for cations of Groups I and II of the periodic table. It was recognized that the exchanging power of cations was related + + to their ionic radii—smaller, highly hydrated cations such as L i and Na being less strongly adsorbed than larger monovalent cations of Group I. Divalent cations of Group II exhibited a similar trend and were more strongly adsorbed than monovalent cations of the same period. Data indi -cated the following sequence, with adsorption strength increasing l e f t to right: + + + + + + + L i < Na < K < NH^  < Rb < Cs < H Mg 2 + < Ca 2 + < Sr 2 + < Ba 2 + < L a 2 + Mobility of cations sorbed on clay surfaces was also demonstrated by observations of cation interchange across interfaces of various cationic forms of montmorillonite, this in absence of a solution phase or free anions (Jenny and Overstreet, 1939), Significant Fe was found to migrate from (Fe, H)-montmorillonite gel into H, K, Na and Ca-montmorillonite gels, provided these were in direct contact and not separated by water « gaps or thick (non-clay) membranes. Elgabaly and Jenny (1943) reported unusual cation and anion ex-change properties of Zn forms of Wyoming montmorillonite. I t was d i s -covered that zinc was adsorbed i n excess of t h i s clay's normal exchange capacity of 92 meq/100 g, and that t h i s adsorption was accompanied by anion uptake and zinc f i x a t i o n . Of three anions examined (OH , C l and N0 3) hydroxyl was found most strongly adsorbed. Conclusions were drawn 2 + that Zn was sorbed both as Zn and as s i n g l y charged anion complexes, and that i r r e v e r s i b l y adsorbed Zn was incorporated into octahedral va-cancies i n the c l a y structure. Somewhat dubious electrochemical properties of a c i d i f i e d and pre-heated Wyoming bentonite clay were reported by Marshall (1948). So-called b i - i o n i c p o t e n t i a l s , measured across clay membranes separating two d i l u t e e l e c t r o l y t e s o l u t i o n s , were interpreted as measures of the d i f f e r e n c e of adsorption enthalpy and intra-membrane mo b i l i t y of competing cations. Coleman (1952) deduced adsorption enthalpies of a l k a l i cations reacted with protonated forms of synthetic r e s i n s and montmorillonite (Volclay bentonite clay) by measuring heats of n e u t r a l i z a t i o n i n solutions of t h e i r hydroxy-salts, then calculated equilibrium constants and free energy changes for the exchange reactions by measuring concentrations of cations at equilibrium with suspended clay, Coleman also measured b i - i o n i c poten-t i a l s across exchanger membranes and pointed out that these were measures of adsorption free energy d i f f e r e n c e s , not enthalpies as had been proposed by Marshall. Good agreement was found between free energies calculated from equilibrium constants and those calculated from membrane p o t e n t i a l s . For exchange of Na by K at 30°C, an enthalpy change of l e s s than -100 c a l (numerically l e s s than) and a free energy change of -950 c a l indicated an exchange reaction entropy change of between 2,8 and 3.1 e.u. Thus i t was evident that a principle driving force behind the reaction was an entropy increase. Slabaugh (1952) also measured heats of neutralization of mont-morillonite (Wyoming bentonite clay) acidified by resin-exchange and elec-trodialysis. When reacted with NaOH solution, freshly prepared and aged (60 days) H-clays gave markedly different potentiometric and thermometric ti t r a t i o n curves. The aged samples showed decreased buffering capacity and multiple endpoints, one prematurely at low pH. Freshly prepared elec-trodialyzed clay gave equally anomalous results, but freshly prepared resin-exchanged montmorillonite yielded a near "normal" curve with an endpoint near pH 7. In a similar study Coleman and Harward (1953) found that their results and Slabaugh's (op. cit.) were due to spontaneous structural degener-ation of the H-clays, resulting in release of ionic Al to exchange positions. These results clearly demonstrated importance of montmorillonite pretreat-ment to i t s subsequent chemical behavior, and showed the importance of time as an experimental variable. Slabaugh (1954) reported heats of exchange of aliphatic amine ions with Na-montmorillonite (Wyoming bentonite clay) and deduced free energy and entropy changes for the reaction. Entropy changes were less than 1 e.u. for a l l reactions and were found to decrease with increasing chain length, while free energy changes were found to increase (become more negative). Faucher e_t a l . (1952, 1954) applied chromatographic theory and method to analysis of cesium, potassium and sodium exchange on montmorillonite (Chambers, Arizona). Equilibrium cation distribution on clay (supported by asbestos) in chromatographic columns was effectively measured using radio-+ + isotopic tracers of adsorbed cations, For exchange of K by Cs at room temperature, a thermodynamic, equilibrium constant was calculated as 13,1, 1 2 corresponding to a free energy change of about 1520 cal/e. Gaines and Thomas (1953) formulated a rigorous theory for the thermodynamics of exchange adsorption on clays and, in a subsequent paper (1955), presented chromatographic data for strontium-cesium exchange on Chambers montmoril-lonite at temperatures ranging from 5° to 75°C. Stoichiometric e q u i l i b r i -um constants calculated for the reaction S r 2 + + Cs2M ^ SrM + 2Cs + (where M represented montmorillonite) indicated strong preference for Cs adsorption over Sr; this preference decreased with increasing temperature. A thermodynamic equilibrium constant for the reaction at 25°C was calcu-lated as 10 2 - 5 7 , corresponding to a free energy change of +3500 cal. Though certainly incomplete, early investigations reviewed here serve to i l l u s t r a t e some basic exchange properties of montmorillonite. A variety of experimental and theoretical techniques has been shown to lead to a common conclusion that strength of adsorption increases with cationic charge and (anhydrous) radius, at least for cations of Groups I and II of the periodic table. Free energy and enthalpy change measurements show that a favourable positive entropy change is an important motivation for exchange reactions. Perhaps this explains qualitatively why doubly charged cations are favoured for adsorption over singly charged species, since two-for-one exchange would achieve greatest disorder by putting a maximum number of cations in solution. Irregularities of zinc and proton sorption suggest greater complexity of the exchange process than simply mass action consi-derations would encompass. These anomalies, combined with the importance of cationic radius, directs curiousity toward the stereochemistry of sorption. Stereochemistry of Cation Adsorption It i s generally accepted that cation adsorption takes place on (001) surfaces and at broken bonds at c r y s t a l edges of montmorillonite; about 20% of t o t a l cation exchange capacity i s due to edge exchange (Grim, 1968). Geometry of cation adsorption i s not yet established, although X ray d i f f r a c t i o n , i n f r a r e d spectrometry and electron spin resonance studies have shed some l i g h t on the problem. Kerns and Mankin (1968) demonstrated that, at given f i x e d humi-d i t y , the thickness of i n t e r l a y e r water depended on exchange cation. For homoionic clays water thickness increased i n a sequence, K < Na < L i Ba < Sr < Ca < Mg which i s i d e n t i c a l to the sequence given by Jenny (1935) f or migration v e l o c i t y . Generally a l k a l i n e earth metal c a t i o n i c forms had greater i n t e r l a y e r water thicknesses than a l k a l i metal forms, except f o r Ba which l a y between Na and L i i n the above sequence. It i s c l e a r that cations of both groups are adsorbed more strongly and produce more com-pact water i n t e r l a y e r s as t h e i r i o n i c r a d i i increase and t h e i r hydration tendencies decrease. I t may be concluded that there i s competition f o r cations between water, which attempts to organize into hydration s h e l l s about the cations, and negative charge s i t e s on the clay surface. Cations which are les s e f f e c t i v e l y hydrated are more e f f e c t i v e l y adsorbed, and pro-mote i n t e r l a y e r contraction. I t i s important to note the r e l a t i o n s h i p be-tween "exchangeability" (as defined by Jenny) and a commonly measured montmorillonite parameter, that is (001) d-spacing. Electron spin resonance studies of homoionic montmorillonites and hectorites have revealed something of the orientation of cations in clay-water systems. Clementz et_ a l . (1973) were able to demonstrate that hec-torite and montmorillonite (Upton, Wyoming) saturated with Cu gave ani-sotropic electron spin resonance which indicated that Cu ions were orien-tated with their elongated tetragonal axes perpendicular to (001). Basal d-spacing measurements indicated a single water interlayer. It was conclud-2 + ' " -ed that Cu was set in planar four coordination with interlayer water mole-cules and was linked to the clay surface on either end of it s tetragonal axis. In contrast to montmorillonite, air-dried Cu-vermiculites were found to contain double water interlayers and Cu electron spin resonance was non-2 + directional. It was suggested that in vermiculite Cu was six-coordinated O to water with i t s tetragonal axis inclined to (001) at about 45, in this way bringing three water molecules into contact with adjacent basal sheets. Nondirectional ESR spectra were also recorded for water-soaked Cu-hectorite o which was measured to have a d-spacing (001) of approximately 10 A, Under these conditions Cu interlayer cations were thought to tumble rapidly, thereby averaging perpendicular and parallel components of the signal much as free aqueous ions. In a more recent publication (McBride e_t al_., 1975a) evidence was presented which indicated that Mg-saturated hectorite and montmorillonite (Upton, Wyoming) formed well order interlayers at (001) d-spacing of 15.0 o A (at about 40% relative humidity), ESR signals from small quantities of Cu 2 and Mn2 doped into lig-dominated interlayers indicated that cations were orientated with octahedral axes (tetragonal for Cu) perpendicular to (001). The authors suggested that hexa-aquo complexes of interlayer cations 4 were l o c a t e d over, and penetrated s l i g h t l y i n t o , the pseudo-hexagonal c a v i t i e s of the m o n t m o r i l l o n i t e and h e c t o r i t e surface. ESR s i g n a l s from 2 + 2 + Cu and Mn doped i n t o H, L i and Na forms of h e c t o r i t e and m o n t m o r i l l o n i t e f a i l e d to show a x i a l o r i e n t a t i o n . X ray d i f f r a c t i o n revealed one to three water i n t e r l a y e r s i n these c l a y s . McBride et_ a l . (1975b, 1975c) observed an i n v a r i a n t l o w - f i e l d and a v a r i a b l e h i g h - f i e l d ESR s i g n a l from Na, L i , K, Cs and Ca forms of montmor-,+ i l i o n i t e (Upton, Wyoming). Both resonances were c r e d i t e d to Fe adjacent 2 + o to and perturbed by o c t a h e d r a l l y s u b s t i t u t e d Mg . On dehydration at 200 C, h i g h - f i e l d ESR s i g n a l s disappeared f o r a l l c a t i o n i c forms of m o n t m o r i l l o n i t e . Concurrently, s t r u c t u r a l OH i n f r a r e d s t r e t c h i n g v i b r a t i o n was s h i f t e d to higher wavenumbers from a normal 847 cm 1; maximum s h i f t was found f o r Ca-saturated c l a y (855-860 cm"1) followed by Na (855 cm" 1), K (852 cm"1) and Cs (848 cm l ) . Disappearance of h i g h - f i e l d ESR s i g n a l s and i n f r a r e d spec-t r a l s h i f t s were i n t e r p r e t e d as r e s u l t i n g from entrance of exchange cations' i n t o pseudo-hexagonal c a v i t i e s on the m o n t m o r i l l o n i t e s u r f a c e , therby s a t i s -f y i n g chafge imbalance of the octahedral l a y e r and p e r t u r b i n g the nearby OH group r e s i d e n t w i t h i n the c a v i t y . Exchange c a t i o n s were thought, to pene-2 + t r a t e i n t o t h i s c a v i t y according to t h e i r r a d i i , s m a ll c a t i o n s such as Ca o +. o (radius 0.99 A) and Na (radius 0.95 A) en t e r i n g w i t h more f a c i l i t y than + o l a r g e r c a t i o n s . Since Cs (ra d i u s 1.67 A) i s i n f a c t too l a r g e to f i t i n t o O the c a v i t y w i t h i t s r a d i u s of about 1.3 A (Helsen e_t al_., 1975), and i t was as . e f f e c t i v e i n annealing e l e c t r o n s p i n resonance as other c a t i o n s , i t xtfas proposed that s t r u c t u r a l oxygens e f f i c i e n t l y d e l o c a l i z e d o c t a h e d r a l charge. 2 + A second s p e c u l a t i o n was that Ca , apparently having to s a t i s f y negative charges on over- and und e r l y i n g c l a y u n i t s , formed a monohydrate which d i s -+ s o c i a t e d to form a CaOH complex, f r e e to migrate i n t o one c a v i t y , and a proton free to migrate to the other. It i s not known how doubly charged cations such as Ca are able to balance charge deficiency i n montmorillonite, whether they s a t i s f y s i n -gle charges of two adjacent layers or whether they s a t i s f y two adjacent charges of the same sheet. L i t t l e i s known of d i s t r i b u t i o n of charge on the clay sheet or, by c o r o l l a r y , d i s t r i b u t i o n of substituted divalent cat-ions i n the s t r u c t u r a l octahedral l a y e r . If t h i s layer can be pictured as that of g i b b s i t e , i n which octahedra are linked by shared edges to form six-membered rings (each with a c e n t r a l vacancy) and rings are linked by shared sides, i t becomes apparent that a maximum of one s i x t h of t o t a l octahedra may be substituted i f one constrains each A..nd.2.p<2,ndtYVt r i n g to only one divalent member. Since one s i x t h s u b s t i t u t i o n i s c o n s i s t e n t l y found f o r montmorillonite (Weaver and P o l l a r d , 1973), t h i s may be a p r i n -c i p l e l i m i t a t i o n f o r charge d i s t r i b u t i o n i n the clay structure. One can conjecture that, i f two sub s t i t u t i o n s were made i n the same octahedral r i n g , i t would be highly favourable f o r the r e s u l t i n g double negative charge imbalance to be s a t i s f i e d by occupancy of a t h i r d doubly charged cation i n the c e n t r a l vacancy of the r i n g . In such s i t u a t i o n s the struc-t u r a l octahedral layer would take on l o c a l t r i o c t a h e d r a l character. The idea of si n g l e s u b s t i t u t i o n per rin g i s perhaps most a t t r a c t i v e because i t assures even charge d i s t r i b u t i o n within the octahedral layer and through the c r y s t a l as a whole, thereby maximizing separation of l i k e negative char-ges and t h e i r a c c e s s i b i l i t y to exchange cations. Such a model would predict that d i v a l e n t exchange cations s a t i s f y s i n g l e negative charges of adjacent layers , not adjacent charges on the same sheet. 1 7 Previous Study of Heavy Metal Adsorption on Montmorillonite Publications dealing specifically with heavy metal adsorption on montmorillonite have appeared periodically in the s o i l and clay chemistry literature since early work oh zinc sorption by Jenny and Elgabaly (1943). Most of these have focused on adsorption stoichiometry, cation hydrolysis and irreversible exchange. Menzel and Jackson (1950) studied exchange of cupric ions with K~ montmorillonite and kaolinite. They found that, when K-montmorillonite was equilibrated with cupric acetate or cupric chloride solution, copper was adsorbed in excess of potassium released. By back-titrating these equilibrated clays with KC1 solution they found considerable H was re-leased, so much in fact that, i f original residual K was considered, few exchange sites were l e f t for copper. Even i f a l l copper was CuOH there were s t i l l insufficient sites available. These authors attempted to cor-rect their back-titration data to reconcile this discrepancy, but had no real j u s t i f i c a t i o n for doing so. They concluded that hydroxy cupric ions were dominant as the adsorbed copper species, but that these were formed on the clay surface after i n i t i a l sorption of Cu DeMumbrum and Jackson (1956a) reported selected results of an at-tempt to adsorb copper and zinc on Ca-montmorillonite (Otay, California) by dialysis across acetate-buffered solution (pH 7). Their method con-sisted of suspending two dialysis bags in 0.5 N calcium acetate solution, one containing Ca-montmorillonite and the second containing a sparingly soluble salt of copper or zinc. Hydroxide and phosphate salts of both copper and zinc were used, as well as cupric hydroxide. Systems were allowed to- react for periods up to 98 days, some with occasional heating on a steam plate, but most at room temperature. Clay was then analysed for exchange cations to determine quantities of Cu or Zn which had migrated through buffer solution to exchange positions. Although these authors did not control their experiment as carefully as the data i t generated, i t was clear that variable quantities of Cu and Zn were sorbed on montmorillonite (magnitudes depending on parent salt solubility, time and temperature) but, more significant, that in some cases they were adsorbed without concomitant release of Ca . It was shown that Cu could be sorbed 22 meq/100 g in ex-cess of the Ca exchange capacity (110 meq/100 g), while Zn could only be sorbed about 4 meq/100 g in excess. A subsequent paper (DeMumbrum and Jackson, 1956b) described attempts to establish the copper and zinc sorption mechanism using infrared spectro-metry. An absorption peak at 2.8 ym, thought to be from structural OH v i -bration, was found to be less intense in Cu and Zn forms of montmorillonite than in Ca, K or H forms. Two unique absorption peaks at 6.4 and 7.0 ym appreared in spectra of Cu-montmorillonite; these were attributed to vibra-tions of Cu-O-Al and Cu-O-Al groups. Comparable peaks were not found in Zn-montmorillonite spectra. DeMumbrum and Jackson concluded that Zn and Cu were adsorbed by displacing protons from structural hydroxyl groups. Studies by Bingham et_ a l . (1964) tended to contradict earlier find-ings. Adsorption of Cu and Zn on H~montmorillonite (Clay Spur, Wyoming) was measured in chloride, nitrate, sulphate and acetate solutions at pH 2 2 + to 8, In solutions without acetate, total and fractional exchange of Cu and Zn 2 for H was found to be stoichiometric, provided pH was kept below 4.5 for Cu and 6.0 for Zn. Acetate solutions promoted excess cation sorp-tion, but acetate was not i t s e l f adsorbed (as shown by J1*C tracer). Below pH 6,0, Cu and Zn acetate solutions consistently caused exchange saturation 10% to 15% i n excess of that effected by NH^Cl treatment. Above t h i s pH both Cu and Zn were removed from s o l u t i o n i n great excess of that expec-ted for normal exchange capacity (above 300% of t h i s value). The authors 9+ 2+ + concluded normal, stoichiometric exchange of Cu and Zn for H took place on montmorillonite, provided pH and concentration were belcxtf solu-b i l i t y product constants of t h e i r hydroxide s a l t s . Above t h i s concentra-t i o n i n c i p i e n t p r e c i p a t i o n occurred. Acetate-enhancement of apparent me-t a l adsorption was a t t r i b u t e d i n part to i t s pH b u f f e r i n g capacity. Hodgson e t ' a l . (1964) attempted to t e s t the alternate hypotheses of ( s p e c i f i c ) heavy metal sorption on c l a y minerals, namely that heavy metals were adsorbed as mono-charged hydroxy complexes, or that they were exchanged for protons at s t r u c t u r a l OH groups. Unfortunately t h e i r equa-tions , which were hybridized from mass ac t i o n theory for cation hydroly-s i s and from the Langmuir adsorption r e l a t i o n (Langmuir, 1918), l e f t much to the imagination. They did not d i s t i n g u i s h between the two mechanisms as they proposed, nor was data presented for Co sorption on montmorillonite i n 0.1 N C a C l 2 s o l u t i o n c l e a r l y i n support of one adsorption mechanism over the other. A mechanistic approach to Cu adsorption by Ca-montmorillonite (ben-toni t e clay, Fisher S c i e n t i f i c Co.) was also adopted by Steger (1973). Ad-^  sorption of trace copper from calcium acetate s o l u t i o n was measured over a pH range of 4 to 6, then•equilibrium d i s t r i b u t i o n of both dissolved and ad-sorbed species were calculated using modified Langmuir and mass action equa-tion s . This was achieved by computer, using a numerical-model of succes-sive approximation. These c a l c u l a t i o n s , and conclusions a r i s i n g from them, are d i f f i c u l t to s c r u t i n i z e because of ambiguity i n the computer programme. Steger concluded that main adsorbed species were protons, Cu and CuOH ; acetate complexes were not considered adsorbed. ® In a second study Steger (1974) evaluated bentonite clay (Aquagel , Baroid Corporation Canada) as a potential sorber of trace concentrations of heavy metals from lime-treated mine waste waters. These were dominant-ly thiosalt solutions of Ca and Na (10 2 N), and i t was reasonably assumed that normal clay exchange sites would be saturated with the major cations. The fraction of i n i t i a l metal in solution adsorbed on suspended montmoril-lonite vras found to increase with pH (4 to 8), and was thought to be loca-ted at dissociated structural hydroxyls; these were assumed unattractive to major cations. Steger noted that some adsorption might also occur at ® sorbed organic groups (Aquagel carried 0.49% C as determined by combus-tion). In a r t i f i c i a l thiosalt solutions i t was found that Fe(III) was most effectively (and somewhat irreversibly) sorbed, followed by Fe(II), Pb, Cu and Zn. In a sample of lime-treated mine water (pH 5.11) the ad-sorption efficiency sequence was found to be Mn « Fe(II) < Zn < Cu Pb was not included within this ranking because i t s concentration was be-low detection limits. Steger observed that this sequence was very similar to the sta b i l i t y order generalized for true chemical complexes (Irving-Williams Stability Series). An additional important observation was that adsorption of Zn was markedly slower than Cu; for example, complete Cu ad-sorption from a solution of 100 Ug/1 i n i t i a l concentration occurred within ten minutes, whereas maximum Zn adsorption (60% of total metal i n i t i a l l y present in the 100 yg/1 solution) was reached only after about six hours. Maximum adsorption of Cu and Zn corresponded to about 2.7 and 1.4 meq/100 g clay respectively. Recent publications by Reddy and Perkins (1974) and Maes and Cremers (1975) have directed attention to irreversible sorption of Zn by montmorillonite and other clay minerals. Data from Reddy and Perkins was interpreted by this writer to show that up to 18% of total exchange capa-city of bentonite clay (Oklahoma) could be occupied irreversibly by Zn when the clay was treated with 100 meq Zn per 100 g clay at pH 8.8, then alternately wetted and dried three times. Decreased fixation was found at lower pH and Zn concentration. Maes and Cremers reported similar re-sults for Camp Berteau (Morocco) montmorillonite. These authors found about 2% fixed at pH 3.8 to 4.5 (solution pH) when Zn occupancy of exchange capacity was about 60%. Increased fixation of up to 8% was found at 90% Zn occupancy. It was therefore concluded that assumptions of thermodynamic equilibria were not jus t i f i e d at high Zn occupancies, since exchange was neither stoichiometric nor reversible. Similar conclusions were drawn by Jenny (1943), some years earlier. It might be summarized from previous experimental work that there is considerable inconsistency in results and conclusions between authors. Clearly, heavy metals can be adsorbed in excess of a l k a l i and alkaline earth metal cations, but the magnitude of this excess in not well known, nor i s i t s mechanism. Early estimates of excess Cu sorption (DeMumbrum and Jackson, 1956a) are an order of magnitude higher than Steger's (1973) (22 meq versus 2.7 meq/100 g). If excess heavy metal adsorption were loca-lized at exposed structural hydroxyls, as seems most lik e l y , the magnitude of adsorption would be expected to change with particle size and crystal damage. Approximating particle geometry as that of a disk of radius K. and unit thickness, i t can be seen that the ratio of exposed perimeter exchange « 22 s i t e s would vary as the r a t i o of p a r t i c l e perimeter to area, that i s 2-rr/t 2, -P a r t i c l e thickness w i l l have no e f f e c t on t h i s r a t i o provided (001) sur-faces of stacked discs have equal access to exchange cations. This seems a reasonable assumption i n view of e a r l y work by Jenny and Overstreet (1939) which showed that adsorbed cations were quite mobile across the clay sorp-t i o n surface. Thus, v a r i a b i l i t y i n estimates of excess sorption might be explained by v a r i a t i o n i n p a r t i c l e s i z e . Pretreatment e f f e c t s , such as clay a c i d i f i c a t i o n , might cause l a t t i c e damage which could further increase the number of exposed s t r u c t u r a l hydroxyl groups. Steger's data (1974) also indicates the importance of time i n heavy metal sorption, and i t might be expected that some v a r i a t i o n between r e s u l t s of Zn adsorption i n v e s t i g a t i o n s might be a t t r i b u t e d to i t s slow re a c t i o n rate. Investigations endeavoring to deduce heavy metal sorption mechanisms from stoichiometric measurements have r e l i e d heavily on presumption, and often the conclusions drawn are only i n t u i t i v e extrapolations of permissive data. Observed increase i n adsorption with increased pH might be explained by increased h y d r o l y s i s and s e l e c t i v e sorption of complex ions, or by i n -creased d i s s o c i a t i o n of clay s t r u c t u r a l hydroxyls. It has not been shown convincingly that mono-hydroxy complexes are sorbed i n deference to doubly charged cations, nor has i t been demonstrated that protons generated by hydrolysis are not s i g n i f i c a n t l y adsorbed. Indeed, the data of Menzel and Jackson (1950) suggest that, protons are major sorrbed species i n Cu-K ex-change reactions on montmorillonite. It can reasonably be. conjectured that complex ions might be adsorbed by montmorillonite, as Jenny's (1943) data 4 for Zn exchange indicated, but as yet definitive results have not been pro-duced (to this writer's knowledge) which prove this point. It might be also mentioned that organic species, which are strongly sorbed by montmorillonite and other clay minerals, might also be important sorbers of heavy metals. ® Steger (1974) found Aquagel , with i t s relatively high carbon content of 0.49%, to be more effective in sorbing metals than other bentonite clays tested. Zinc fixation by montmorillonite has been well established, but l i t -t le i s known of the fixation mechanism(s). Two possible sites for irrever-sible sorption might be within the structural octahedral layer, perhaps accessible at particle edges or at l a t t i c e irregularities, or within the pseudo-hexagonal cavities of the tetrahedral layer. It is not clear why Cu'1 is not fixed as readily as Zn , since i t s ionic radius (0.72 A) is nearly identical with that of Zn (0.74 A). Indeed, this may suggest that stereochemical factors are involved in the fixation mechanism. Accor-ding to Sucha and Kotrly (1972) Zn 2 , with i t s f i l l e d outer shell elec-tronic configuration (i.e. d 1 0 ) , is unique among metals of the f i r s t transi-tion series because i t tends to behave like alkaline metal cations and ter-3 + 3 + valent cations such as B and Al ; that i s , i t forms complexes with sig-nificant electrostatic character and, unlike Co2 , N i 2 and Cu 2 , tends to form stronger oxo-complexes than halo, n i t r i l o or cyano-complexes. Hence Zn might be more lik e l y to coordinate directly to oxygens of the clay la t t i c e than Cu 2 + ? possibly proxying for Mg 2 +, F e 2 + or A l 3 + at exposed octahedral sites. Since transition metal cations are moderately strong Lewis acids (base-acceptors), i t might be supposed that they enhance proton activity by hydrolysis reactions near the clay surface. It is well established 4 from spectroscopic observations (Bailey and Karickhoff, 1973) that water near the montmorillonite surface is dissociated more strongly than in free liquid. Introduction of heavy metal cations into the clay-water system might result in proton activity in the micellar region much greater than that observed for hydrolysis in free solution, resulting in structural degeneration of the montmorillonite lattice by a process analogous to that observed for HC1 or resin-acidified clays. (Even Na-montmorillonites have limited stability, as demonstrated by conductivity measurements of clay suspensions (Shainberg et a l . , 1974).) Oxidation of ferrous iron in the octahedral layer might also be favoured by high proton activity, as sug-gested by a reaction 2 {Fe 2 +} + h 0 2 + 2 H + ^ 2 {Fe 3 +} + H20 where lat t i c e species are in braces. This reaction depends on ava i l a b i l i t y of free oxygen (or any other effective oxidizing agent) at the clay surface, a contention supported by work of Lahav and Lavee (1973) who observed that the chemiluminescent oxidation of luminol (5-amino-2,3 dihydro-1,4 phthala-zinedione) was highly accelerated in the presence of suspended bentonite clay. These authors attributed this effect to sorbed oxygen or other oxi-dants. Oxidation of ferrous iron would result in reduced cation exchange capacity, and this might be mistakenly attributed to cation fixation in some experimental settings. HEAVY METAL EXCHANGE SORPTION ON SUSPENDED Na-MONTMORILLONITE Introduction A laboratory investigation was undertaken to determine equilibrium constants for the exchange adsorption of Cu, Zn and Pb on Clay Spur mont-morillonite. This was attempted by measuring equilibrium concentrations of a given metal and sodium which resulted from addition of small volumes of 0.1 M heavy metal electrolyte solution (CuCl2, Cu(N0s)2, ZnCl2, and Pb(N03)2) to 1% suspensions of Na-montmorillonite. Equilibrium concentra-tions of both competing cations were plotted against t i t r e of exchange elec-trolyte solution, then compared with theoretical t i t r a t i o n curves calculated for several equilibrium constants. It was hoped that comparison of observed with predicted curves would lead to graphical estimates of equilibrium con-stants for the appropriate exchange reactions at room temperature and pres-sure (23 + 2°C; 760 ± 10 mm Hg). Na-montmorillonite suspensions were also titrated with 0.1 M solutions of KC1 and HC1 in interests of comparing ex-change reactions of singly and doubly charged cations, and also to evaluate pH effects in aqueous exchange systems. No attempt was made to buffer pro-ton activity because of possible interaction of buffering agents, metals and montmorillonite. Exchange reaction stoichiometry was investigated through simple mass balance calculations. By correcting for proton ex-change in heavy metal systems, i t was possible to calculate stoichiometric exchange constants for these metals. These constants were plotted against t i t r e of exchange electrolyte and evaluated from the viewpoint of metal hydrolysis and complex-formation. 1 Thermodynamics of Exchange Adsorption A variety of empirical and semi-empirical equations have been developed to describe quantitatively the absorption and adsorption of molecules and ions on surfaces (or more generally at phase boundaries). Many of these have been reviewed by Bailey.and White (1970), with particu-lar attention to interrelationships between models. Since i t s conception (Langmuir, 1918) as a device to predict sorption of molecular mono-layers on liquid and solid surfaces, the Langmuir equation has found remarkably successful application in describing a wide spectrum of sorption phenomena, including heavy sorption on clay minerals (Steger, 1973, 1974; Jackson, 1975; Bhoojedhur, 1975). In the present study an alternative method has been adopted, based on classical mass action theory. This approach was chosen not s t r i c t l y in preference to other models, but because of familia-r i t y . There is some precedent for mass action modelling of clay ion-exchange reactions (Vanselow, 1932; Body eit a l . , 1947; Slabaugh, 1950; Krishnamoorth et_ a l . , 1948, 1950; Coleman, 1952; Gaines and Thomas, 1953, 1955; Faucher and Thomas, 1953; Brown, 1963; Vansant and Uytterhoeven, 1971). A general equilibrium mass balance equation may be constructed to account for cation-exchange reactions between species of any charge, but in the interests of clarity and economy of notation a more specific example involving exchange of a dipositive cation with Na-montmorillonite is pre-sented here; i.e., Me2 + 2 Na{Mottt} ^ m{Mont}2 + 2 Na (1) Me2 generalizes for any dipositive cation and {Mont} represents that mass of montmorillonite corresponding to one equivalent negative charge. En-compassed within t h i s equation are the constraints that charge balance must be maintained i n both s o l u t i o n and clay, a presumption common to a l l ion-exchange reactions and precluding existence cf species such as {Mont,} From equation (1) a thermodynamic mass action r e l a t i o n may be f o r -mulated as ( Y i " h ) 2 ( / 2 N 2 ) K = (2) ^ (yzm2) ( / 1 W 1 ) 2 where subscripts 1 and 2 r e f e r to Na and Me r e s p e c t i v e l y ; m and y a r e standard molal concentration and a c t i v i t y c o e f f i c i e n t for s o l u t i o n species while W and j are defined here as f r a c t i o n a l equivalent exchange s i t e oc-cupancy and r a t i o n a l a c t i v i t y c o e f f i c i e n t . Similar notation i s found i n the l i t e r a t u r e (Boyd e_t a l . , 1947; Gaines and Thomas, 1953). A c t i v i t y scales and standard states are well established f o r aque-ous species (Robinson and Stokes, 1959; Harned and Owen, 1958) and need not be discussed here. However, no such standard for adsorbed species has found wide acceptance. In t h i s treatment a r a t i o n a l concentration scale i s defined on the basis of f r a c t i o n of exchange equivalents occupied by a given sorbed c a t i o n . An a l t e r n a t i v e scale, based on mole f r a c t i o n occupancy, might at f i r s t sight seem incompatible with the former scale i n charge-heterogenous systems, since i t would bestow doubly charged cat-ions with one hal f the e f f e c t i v e concentration of the equivalent scale. Moreover, at saturation with d i p o s i t i v e cations the mole f r a c t i o n scale would culminate at 0.5, while an equivalent scale would achieve unity. These discrepancies are not troublesome i f only equally charged cations are considered i n exchange reactions, but where such i s not the case ap-2 e parent confusion p e r s i s t s . In f a c t , the two scales are thermodynamically equivalent provided the same standard state i s adopted for both. This point can be i l l u s -trated with the common r e l a t i o n between free energy change for a reac-ti o n and i t s equilibrium constant, here referenced to an a r b i t r a r y stan-dard state —- = In K - In K0 (3) RT m where R i s the ga,s constant; AG 0 i s Gibb's free energy change for the reac t i o n at temperature T r e l a t i v e to a standard state defined by the quotient K$. This quotient has i d e n t i c a l form to the equilibrium quo-t i e n t (constant) K. and represents a c t i v i t i e s of reacting species at a defined standard state. For equation (2) a quotient may be written ( c X i ) 2 a 2 K 0 = —2 (4) a2 ( a i r where subscripts r e t a i n t h e i r o r i g i n a l meaning and standard state a c t i -v i t i e s of sorbed and aqueous species are represented by a and <X respec-t i v e l y . One can quickly confirm that, i f normal standard state unit a c t i v i t i e s of aqueous species are substituted and a standard state f o r sorbed species i s defined out A<vtu/LOtyLon, the quotient of equation (4) becomes 0.5 i f a mole f r a c t i o n concentration scale i s used and one i f the equivalent f r a c t i o n scale i s chosen. It may be surmised that K0 compensates for apparent thermodynamic asymmetry between concentration scales. By combining quotients of equation (3) into one term, i t may be appreciated that the thermodynamic equilibrium constant i s a quo-tient of activity ratios, where denominators are act i v i t i e s at standard state and numerators are activities at equilibrium. Choice of an equi-valent fraction concentration scale is convenient because a l l standard state ac t i v i t i e s of equation (4) become unity and Zn Kn becomes zero. By chosing an equivalent fraction concentration scale and defin-ing a standard state at saturation, a boundary condition i s imposed on an activity coefficient scale—namely, that coefficients tend to unity as saturation is approached. This follows from the relationship a. = / .N . (5) 1 ' I I where subscript " i " generalizes for any sorbed cation. In a rigorous definition i t would be emphasized that saturation is thermodynamically undefined, and that unit activity coefficients, are idealized limits at a standard state which is never practically achieved. Values for a c t i -vity coefficients at equivalent fractional concentrations less than one are essentially unconstrained; unit values may be achieved, but these obviously have no association with the definition of standard state. Experimental Method Experimental methods presented here are brief summaries of com-plete descriptions appearing as appendices. Reagents were obtained from various suppliers (p. .152) and were used without special purification. A l l water was twice-distilled. Electrolyte solutions of Cu, Zn and Pb were standardized by EDTA chelo-metric titrimetry according tc methods of Suk and Malat (1957) and 1 Flaschka and Abdine (1957) (p. 141). Standard NaCl and KCl s o l u t i o n s were made up by weight; standard HC1 s o l u t i o n was obtained commercially. Clay Spur bentonite was dissaggregated and suspended i n d i s t i l l e d water w i t h a blender, then separated i n t o m o n t m o r i l l o n i t e and non-clay components by c e n t r i f u g a t i o n at 6000 RPM f o r two hours. Sodium s a t u r a -t i o n (p. 142) was accomplished by suspending n a t u r a l c l a y i n 1.0 M NaCl s o l u t i o n twice, followed by s i x d i s t i l l e d water washes. Suspensions were separated a f t e r each NaCl treatment or wash by c e n t r i f u g a t i o n f o r two hours at 6000 RPM. T o t a l Na exchange c a p a c i t y was determined by a method developed by t h i s author (p. 147). A i r - d r i e d (at approximately 50% r e l a t i v e humidity) Na-montmorillonite was weighed out a c c u r a t e l y and then suspended i n about 20 ml of 0.1 M s o l u t i o n of app r o p r i a t e exchange e l e c t r o l y t e . Suspensions were s t i r r e d f o r s e v e r a l hours, then f i l t e r e d through S a r t o r i u s 0.01 ym c e l l u l o s e n i t r a t e membranes mounted i n polycarbonate pressure f i l t r a t i o n apparatus; 2.1 kg/cm 2 (30 p s i ) n i t r o g e n pressure was maintained during f i l -t r a t i o n . F i l t r a t e from the o r i g i n a l suspension, and s e v e r a l f l u s h e s of f r e s h e l e c t r o l y t e s o l u t i o n , were c o l l e c t e d i n pre-weighed b o t t l e s . Weights and d e n s i t i e s of f i l t r a t e s were measured, then Na co n c e n t r a t i o n determined by atomic a b s o r p t i o n spectrometry. Na standards f o r atomic a b s o r p t i o n ana-l y s i s were made up w i t h 0.1 M e l e c t r o l y t e s o l u t i o n i d e n t i c a l to that of the f i l t r a t e s . F i l t r a t e pH was determined p o t e h t i o m e t r i c a l l y by a combined glass-Ag:AgCl e l e c t r o d e . Exchange c a p a c i t i e s were determined f o r a l l ex-change e l e c t r o l y t e s by t h i s method. f Suspensions of 0.200 g Na-montmorillonite (Clay Spur) i n 20.0 ml d i s t i l l e d water were reacted w i t h 0.10 to 10.8 ml volumes of 0.1 M ex-change e l e c t r o l y t e s o l u t i o n . D u p l i c a t e samples and blanks (no t i t r e added) were u s u a l l y included w i t h i n t i t r a t i o n s e r i e s f o r each e l e c t r o -l y t e . Suspensions were sealed i n beakers w i t h polyethylene f i l m and s t i r r e d m a g n e t i c a l l y f o r at l e a s t one hour, then f i l t e r e d through 0.01 ym c e l l u l o s e n i t r a t e membranes under 2.1 to 2.5'kg/cm2 (30 to 35 p s i ) n i t r o g e n pressure. F i l t r a t e s were analysed f o r exchange c a t i o n s and sodium by atomic abs o r p t i o n spectrometry, u s i n g s p e c t r a l a b s o r p t i o n l i n e s and standards a p p r o p r i a t e to c o n c e n t r a t i o n range (10 7 to 10 1 M). Sim-p l e , mono-cation standards were used f o r a l l t i t r a t i o n s e r i e s except K-Na. In t h i s case, f i l t r a t e s were i n i t i a l l y analysed w i t h simple standards, then w i t h mixed standards of n e a r l y equal Na/K r a t i o s to c o r r e c t f o r mutual enhancement e f f e c t s . pH of s e l e c t e d f i l t r a t e s was determined as the maximum reading of a p o t e n t i a l that was found to decrease (toward pH 7) during measurement. This behavior i n d i c a t e d i n t e r f e r e n c e at the g l a s s membrane of the combined glass-Ag:AgCl e l e c t r o d e , but i t was con-cluded (with some expediency) that the i n i t i a l , maximum p o t e n t i a l was a good estimate of proton a c t i v i t y (Rechnitz, 1971). R e s u l t s of Na Exchange Capacity Determinations Table I I l i s t s r e s u l t s of Na exchange c a p a c i t y determinations f o r a l l e l e c t r o l y t e s . I t should be s t r e s s e d that these are not orthodox ex-change c a p a c i t i e s s i n c e they do not represent a p a r t i c u l a r c a t i o n ' s a b i l i -t y to be adsorbed, but r a t h e r i t s a b i l i t y to d i s p l a c e Na. A l s o pH of CEC measurement i s the ambient pH of the system, not a b u f f e r e d pH 7 as i s customary i n standard exchange c a p a c i t y determinations. U t i l i t y of t h i s type of CEC measurement w i l l become apparent i n l a t e r d i s c u s s i o n . Table II. Cation exchange capacities determined by displacement of Na from Na-montmorillonite with various 0.1 M electrolyte solutions. CEC and O are average and standard devia-tion for each electrolyte weighted by the reciprocal of analytical error. Electrolyte pH CEC Analytical CEC c meq/100 g Error meq/100 g HC1 1.33 91.0 ±0.8 HC1 1.29 93.1 1.3 91.61 ±0.95 HC1 1.29 91.0 1.6 KC1 5.15 86.6 1.0 KC1 5.10 89.7 2.3 86.75 1.49 KC1 5.43 85.5 1.1 ZnCl 2 3.30 94.3 1.2 ZnCl 2 3.28 95.9 1.5 95.04 0.65 ZnCl 2 3.28 95.1 1.3 CuCl 2 4.05 92.5 0.7 CuCl 2 3.90 93.8 1.5 92.74 0.63 CuCl 2 3.93 92.1 1.7 Cu(N0 3) 2 4.15 84.5 0.2 Cu(N0 3) 2 4.16 86.9 1.0 85.50 1.02 Cu(N0 3) 2 4.18 86.6 0.5 Cu(N0 3) 2 4.24 86.2 0.5 Pb(N0 3) 2 4.38 88.7 0.8 Pb(N0 3) 2 4.42 88.3 0.6 88.54 0.20 Pb(N0 3) 2 4.38 88.7 0.8 3 3 96 • o 92 cs •a: > « ra 9 ( H 86 CuGS S2.74 KG! 86.75 cu[na ZnCI2 95.04 85.50 T " 5 pH HOI 1 91.61 « F i g u r e 3. P l o t of Na exchange c a p a c i t y of e l e c t r o l y t e s a g a i n s t ambient pH. Average va l u e s are i n d i c a t e d by bo l d h o r i z o n t a l b a r s . « 34 Analytical error was estimated by propagating observational errors through the course of calculation. Average Na exchange capacity and stan-dard deviation for each electrolyte was calculated by weighting each deter-mination by the reciprocal of analytical error, thereby giving more credence to better estimates. It can be seen that average CEC varies between elec-trolytes from a minimum of 85.50 meq/100 g for Cu(N03)2 to a maximum of 95.04 meq/100 g for ZnCl 2. It i s also evident that CuCl 2, at 92.74 meq/100 g, is significantly more effective at displacing Na than Cu(N03)2. One might question whether these data indicate true chemical differences between the Na-exchanging power of electrolytes, or whether they reflect analytical im-precision and batch variation. Since determinations were often made using different batch preparations of Na-montmorillonite (and at different times) for the same exchange electrolyte, i t i s thought that data reveal authen-ti c chemical differences between electrolytes. Generalizations are not easily constructed from Na exchange capacity data, but a crude tendency for higher values to be associated with lower ambient pH might be pointed to. Figure 3 shows that this trend occurs both within and between electrolyte data groups. Individual CEC determinations are shown with analytical error bars; average values for each electrolyte are plotted as bold horizontal lines. In most cases plots for a given elec-trolyte group together closely, but an unusually low point (outside analy-t i c a l error bars of other group members) can be seen for Cu(N0 3) 2. Results of KC1 and HC1 Titrations Tables III and IV present data from KC1 and HC1 titr a t i o n experi-ments. Equilibrium molar concentrations of Na and K, and equilibrium pH, are tabulated against t i t r e of 0.100 M exchange electrolyte solution. Errors in concentration represent estimated analytical imprecision from atomic absorption spectrometry; errors in HC1 tit r a t i o n f i l t r a t e pH are readout errors. pH of KC1 titr a t i o n f i l t r a t e s represents maximum values (minimum pH) of a transient glass-Ag:AgCl electrode potential. A total of twelve samples were analysed for the KC1:Na-montmoril-lonite t i t r a t i o n series, including one blank solution which showed trace concentrations of both Na and K. Na concentration can be seen to increase steadily with t i t r e to a maximum of 0.641 x 10 2 M at 5.00 ml, then slowly decrease with further addition. K concentration increases continuously with t i t r e , slowly up to 0.235 ml and rapidly thereafter. Several decades of concentration are spanned by the tit r a t i o n (10 5 to 10 2 M). pH de-creases with increasing t i t r e except for high values at 7.50 and 10.8 ml titre s , perhaps caused by contamination with some buffering agent. Eighteen f i l t r a t e s represent the HC1:Na-montmorillonite tit r a t i o n series, including one duplicate at 1.20 ml. No blank solution was included, within this set. Na concentration increases with t i t r e to a maximum of 0.616 x 10 2 M at 5.00 ml, then decreases gradually as in KC1 titr a t i o n f i l t r a t e s . pH values decrease continuously from 6.16 and 0.100 ml to 1.76 at 7.50 ml; these were non-transient potentials. KC1 and HC1 titration data are presented graphically in Figures 4 to 7. Plots of equilibrium Na concentration against KC1 and HC1 t i t r e (Figures 4 and 6) show common features—steady and almost linear increase Table I I I . F i l t r a t e equilibrium concentrations o f Na and K resulting f rom addition of 0. 100 M KC1 t i t r e to an aqueous suspension of 0.200 g Na-montmorillonite in 20 .0 ml at 23*C. Titre Na Concentration Error K Concentration Error pH 0.000 ml 0.0174 x 10 2 M ±0.0028 x 10~2 M 0.100 x 10~" M ±0.002 X 10 4 M 6.40 0.165 0.0879 0.0014 0.260 0.002 5.85 0.235 0.127 0.003 0.592 0.015 5.90 0.500 0.226 0.008 0.280 x 10~3 0.009 X 10" 3 5.35 0.800 0.331 0.006 0.729 0.012 5.30 1.100 0.422 0.006 0.138 x 10"2 0.002 X io" 2 5.25 1.400 0.475 0.010 0.215 0.005 5.20 1.800 0.527 0.007 0.352 0.004 5.20 2.700 0.600 0.020 0.614 0.008 4.90 5.000 0.641 0.012 0.143 x 10 _ 1 0.002 X 10"1 4.95 7.500 0.624 0.006 0.218 0.005 6.55 10.800 0.585 0.012 0.281 0.005 6.50 to CD Table IV. Filt r a t e equilibrium pH and Na concentration resulting from addition of 0.100 M HC1 t i t r e to an aqueous suspension of 0.200 g Na-montmorillonite in 20.0 ml at 23*C. Titre Na Concentration Error pH 0.100 ml 0.0394 x 10~2 M ±0.0070 x 10"2 M 6.16 0.200 0.0880 0.0069 5.55 0.300 0.135 0.0068 4.86 0.400 0.176 0.0067 3.97 0.500 0.216 0.0068 3.73 0.700 0.286 0.0067 3.36 0.900 0.345 0.0067 3.10 1.000 0.375 0.0080 3.05 1.200 0.420 0.0080 2.88 1.200 0.424 0.0080 2.87 1.500 0.462 0.0067 2.69 1.700 0.496 0.0080 2.58 2.000 0.537 0.0072 2.47 2.500 0.567 0.0072 2.33 3.000 0.590 0.0072 2.18 4.000 0.608 0.0073 2.07 5.000 0.616 0.0081 1.94 7.500 0.600 0.0081 1.76 to >0 1.0 TITRE KGL, ML Figure 4. Plot of f i l t r a t e Na concentration r e s u l t i n g from a d d i t i o n of 0.100 M KC1 t i t r e to 20 ml suspensions of 1% Na-montmorillonite. CO CD 39 « 3.6 3.2 2.8 2.4 7 2.0 1.2 St* Inset 0 1 2 2.0 16 1.2 .1 .2 .3 A H I R E K C I . ML 3 4 5 6 7 TITRE KCI, ML i • • i 9 10 II Figure 5. Plot of f i l t r a t e K concentration r e s u l t i n g from addition of 0 . 1 0 0 M KCI t i t r e to 2 0 . 0 ml suspensions of 1% Na-montmorillonite. 1.0 i C3 .VJ 3 TITRE HCI. 1L Figure 6. Plot of f i l t r a t e Na concentration r e s u l t i n g from a d d i t i o n of 0.100 M HCI t i t r e to 20.0 ml suspensions of 1% Na-montmorillonite. ex. 5 • • _l L. 4 1 9 -0 1 2 3 4 5 6 7 8 9 10-11 TITRE liCI, ML F i g u r e 7 . P l o t o f f i l t r a t e pH r e s u l t i n g f r o m a d d i t i o n o f 0 . 1 0 0 M HC1 t i t r e t o 2 0 , 0 m l s u s p e n s i o n s o f 1% N a - m o n t m o r i l l o n i t e . 1*2 i n c o n c e n t r a t i o n at low t i t r e s ( l e s s than about 1.0 ml) fo l l o w e d by a broad shoulder and a gradual decrease at higher t i t r e s . K c o n c e n t r a t i o n increases continuously w i t h t i t r e i n an S-shaped curve, approaching zero at the s t a r t of t i t r a t i o n (see i n s e t graph, F i g u r e 5 ) and 0.100 M at high t i t r e s . E q u i l i b r i u m pH of HC1 t i t r a t i o n f i l t r a t e s decreases q u i c k l y at low t i t r e s , then approaches an asymptote at pH 1.0 at high t i t r e s . R e s u l t s of Heavy Metal T i t r a t i o n s Data from C u C l 2 , C u ( N 0 3 ) 2 , P b ( N 0 3 ) 2 and Z n C l 2 t i t r a t i o n s of Na-mont-m o r i l l o n i t e are presented r e s p e c t i v e l y i n Tables V to V I I I , P a i r s of graphs showing e q u i l i b r i u m c o n c e n t r a t i o n of Na and heavy metal are shown f o r each t i t r a t i o n i n Figures 8 to 15, i n c l u d i n g i n s e t graphs showing heavy metal co n c e n t r a t i o n at low t i t r e s . Na c o n c e n t r a t i o n of blank samples (no added e l e c t r o l y t e ) v a r i e s from l e s s than 0.02 x 1()""4 M ( P b ( N 0 3 ) 2 t i t r a t i o n ) to 0.935 x 10 _ 1* M ( Z n C l 2 t i t r a -t i o n ) , w i t h an average of 0.48 x 10 ** M. Nine d u p l i c a t e samples show a range of d e v i a t i o n of Na c o n c e n t r a t i o n about p a i r means of zero to 2.1%; i n d i v i -dual p a i r d e v i a t i o n s averaged f o r C u C l 2 , C u ( N 0 3 ) 2 and P b ( N 0 3 ) 2 t i t r a t i o n s are r e s p e c t i v e l y 0.67% (four d u p l i c a t e s ) , 0.38% (three d u p l i c a t e s ) and 1.05% (two d u p l i c a t e s ) . No d u p l i c a t e samples were run f o r the ZnCl2 t i t r a t i o n . Average percentage d e v i a t i o n of Cu co n c e n t r a t i o n of d u p l i c a t e samples i s 6.3% (C u C l 2 t i t r a t i o n ) and 3.5% (Cu(N0 3) 2 t i t r a t i o n ) , and f o r Pb i s 1.2% (P b ( N 0 3 ) 2 t i t r a t i o n ) . . Percentage v a r i a t i o n of d u p l i c a t e (minimum) pH i s 2.2%, 2.3% and 2.9% f o r C u C l 2 , C u ( N 0 3 ) 2 and P b ( N 0 3 ) 2 t i t r a t i o n s . E r r o r s quoted f o r Na and heavy metal c o n c e n t r a t i o n are estimated atomic absorption a n a l y t i c a l p r e c i s o n . E r r o r i n pH i s probably greater than readout p r e c i s i o n Table V. Filt r a t e equilibrium concentrations of Na and Cu resulting from addition of 0.103 M CuCl2 t i t r e to an aqueous suspension of 0.200 g Na-montmorillonite i n 20.0 ml at 23*C. Titre Na Concentration Error Cu Concentration Error pH 0.000 ml 0.0067 x 10"2 M ±0.0044 x 10~2 M -Below detection limit of ~ 10"7 M- 7.73 0.100 0.0997 0.0043 0.0565 x 10"5 M ±0.0180 x 10~5 M 6.67 0.200 0.207 0.0042 0.130 0.0195 6.43 0.300 0.319 0.0040 0.307 0.0255 5.83 0.300 0.320 0.0040 0.257 0.0255 5.63 0.420 0.449 0.0050 0.911 0.0500 5.60 0.420 0.437 0.0044 0.117 x 10~k 0.0180 x 10_l+ 5.80 0.520 0.548 • 0.0050 0.262 0.0057 5.54 0.600 0.614 0.0063 0.501 0.0032 5.48 0.600 0.616 0.0050 0.488 0.0037 5.95 0.720 0.705 0.0077 0.141 x 10~3 0.0021 x 10~3 5.40 0.850 0.789 0.0051 0.444 0.0061 4.48 0.990 0.834 0.0079 0.764 0.0071 4.78 1.300 0.866 0.0064 0.216 x 10 - 2 0.0032 x 10~2 4.83 1.600 0.850 0.0073 0.340 0.0078 5.05 1.600 0.833 0.0052 0.359 0.0143 4.93 2.390 0.834 0.0073 0.658 0.0120 4.80 3.500 0.786 0.0051 0.110 x 10 _ 1 0.0013 x 10 - 1 4.72 4.980 0.749 0.0072 0.164 0.0016 4.65 7.000 0.690 0.0050 0.226 0.0016 4.42 Table VI. Filt r a t e equilibrium concentrations of Na and Cu resulting from addition of 0.1013 M Cu(N03)2 t i t r e to an aqueous suspension of 0.200 g Na-montmorillonite in 20.0 ml at 23'C. Titre Na Concentration Error Cu Concentration Error PH 0.000 ml 0.0030 x 10 - 2 M ±0.0010 x 10~2 M 0.410 x 10 - 6 M ±0.173 x 10 - 6 M 6.70 0.100 0.0945 0.0062 0.775 0.173 5.20 0.200 0.195 0.0060 0.104 x 10"5 0.011 x 10 - 5 5.00 0.300 0.302 0.0063 0.290 0.025 5.45 0.500 0.500 0.0063 0.197 x I0~h 0.0053 x 10_" 4.70 0.500 0.500 0.0063 0.204 0.0087 5.15 0.700 0.671 0.0064 0.122 x 10 - 3 0.0112 x 10 - 3 5.50 0.900 0.764 0.0065 0.532 0.0116 5.20 1.000 0.774 0.0066 0.925 0.0127 5.10 1.000 0.786 0.0066 0.105 x 10 - 2 0.0015 x 10 - 2 5.20 1.200 0.809 0.0066 0.176 0.0120 5.10 1.200 0.815 0.0066 0.185 0.0120 4.95 1.500 0.799 0.0066 0.342 0.0121 4.90 2.000 0.792 0.0066 0.595 0.0123 4.90 3.000 0.767 0.0065 0.100 x 10 _ 1 0.0015 x 10 - 1 4.70 5.000 0.715 0.0076 0.186 0.0044 4.70 7.500 0.656 0.0064 0.254 0.0048 4.40 tr Table VII. Filt r a t e equilibrium concentrations of Na and Pb resulting from addition of 0.0970 M Pb(N0 3) 2 t i t r e to an aqueous suspension of 0.200 g Na-montmorillonite in 20.0 ml at 23'C. T i t r e Na Concentration Error Pb Concentration E r r o r pH 0.000 ml 0.100 0.200 0.200 0.300 0.400 0.500 0.600 0.600 0.800 1.000 1.200 1.500 2.000 3.000 5.000 7.500 <0.0002 x 10"2 M 0.0870 0.186 * 0.194 0.291 0.388 0.494 0.583 0.583 0.721 0.803 0.820 0.822 0.828 0.785 0.715 0.661 -6 -Below detection limit of ~ 10 M-±0.0098 x 10 0.0096 0.0096 0.0101 0.0100 0.0094 0.0112 0.0112 0.0114 0.0128 0.0128 0.0128 0.0128 0.0127 0.0113 0.0113 M 0.185 0.325 0.101 0.293 0.286 0.172 0.715 0.148 0.269 0.481 0.888 0.162 0.236 x 10 5 M x 10 - 1 4 x 10 -3 x 10 -2 X 10 -1 ±0.0022 x 10"3 M 0.0024 0.0022 x 10 0.0049 0.0049 0.0035 x 10 0.0160 0.0028 x 10 0.0038 0.0049 0.0039 0.0039 x 10 0.0040 ^ 3 ^-2 ft ft ft 5.45 5.35 5.05 4.95 4.60 4.60 4.50 4.50 4.30 4.40 4.30 * Hot MeaouAed Table V I I I . F i l t r a t e equilibrium concentrations of Na and Zn r e s u l t i n g f rom a d d i t i o n of 0.0970 M ZnCl 2 t i t r e to an aqueous suspension of 0.200 g Na-montmorillonite i n 20.0 ml at 23'C. T i t r e Na Concentration Error Zn Concentration E r r o r pH 0.000 ml 0.0.935 x 10 3 M ±0.0172 x 10 3 M -Below detection l i m i t of ~ '. 10 7 M- 5.30 0.100 0.0981 x 1 0 - 2 0.0043 x K f 2 0.905 x 1 0 - 6 M ±0.0810 X 1 0 - 6 M ft 0.200 0.198 0.0042 0.126 x 10" 5 0.0085 X 10" 5 * 0.340 0.339 0.0075 0.271 0.0096 5.30 0.400 0.389 0.0044 0.567 0.0089 * 0.470 0.446 0.0054 0.119 x lO""* 0.0023 X 5.54 0.500 0.474 0.0100 0.165 0.0092 ft 0.600 0.583 0.0055 0.385 0.0170 * 0.700 0.681 0.0100 0.105 x 10" 3 0.0011 X 10" 3 * 0.720 0.670 0.0068 0.115 0.0018 5.27 0.900 0.774 0.010 0.366 0.002 1.200 0.825 0.007 0.146 x 1 0 - 2 0.006 X io" 2 5.06 1.500 0.832 0.010 0.276 0.004 5.30 2.000 0.798 0.007 0.490 0.002 4.83 3.500 0.790 0.006 0.108 x 10 _ 1 0.002 X 10" 1 4.80 5.000 0.727 0.007 0.157 0.002 4.52 7.500 0.662 0.010 0.243 0.004 4.26 8.000 0.654 0.007 0.267 0.004 ft * Not Uojua/ind •tr 1.0 .8 *-« .6 a a O O C_3 ..2 T1TRE €uCi 2 , ML Figure 8. Plot of f i l t r a t e Na concentration resulting from addition of 0.1030 M CuCl2 t i t r e to 20.0 ml suspensions of 1% Na-montmorillonite. 3.6 3.2 H 2.Si 2.4 i 2.0 H ^ 1.6 o 1.2 I 4 _J I I I u Sec Inset 2.8 2H 20 L6 12 .2 .3 .4 TITRE C U C l 2 . ML 0 1 2 3 4 5 6 7 TITRE CuCI2 , ML 9 10 11 F i g u r e 9. P l o t o f f i l t r a t e Cu c o n c e n t r a t i o n r e s u l t i n g f r o m a d d i t i o n o f 0 . 1 0 3 0 M C u C l 2 t i t r e t o 2 0 . 0 m l s u s p e n s i o n s o f 1% N a - m o n t -m o r i l l o n i t e . 1.0 .8 l 1 -6 .4 .2J .0 -r-6 3 4 TITRE Cu[^3]2, ML Figure 10. Plot of f i l t r a t e Na concentration r e s u l t i n g from a d d i t i o n of 0.1013 M Cu(N0 3) 2 t i t r e to 20.0 ml suspensions of 1% Na-montmorillonite. 1 50 3.6 3.2 2.8 2.4-m U S3 § 1.6 1.2-1 .0 • i See Intel 2.8 2.4 20H 7 16 <j L2 .1 .2 .3 .4 TURF. CoflOjj. Ml 0 1 2 3 4 5 6 7 8 9 10 11 TITRE Cu [N0312 , ML F i g u r e 11. P l o t o f f i l t r a t e C u c o n c e n t r a t i o n r e s u l t i n g f r o m a d d i t i o n o f 0 . 1 0 1 3 M C u ( N 0 3 ) 2 t i t r e t o 2 0 . 0 m l s u s p e n s i o n s o f 1% N a - m o n t -m o r i l l o n i t e . ess HUE P b ( N 0 3 ] 2 ? ML Figure 1 2 . Plot of f i l t r a t e Na concentration r e s u l t i n g from a d d i t i o n of 0 . 0970 M Pb(N03 )2 t i t r e to 20.0 ml suspensions of 1% Na-montmorillonite. 5 2 3.6 3.2 2.8 •2.4H 2.0 | 1.6 1.2 0 _ l I ' I I I 1 L . Sef lostt 2.4 2.0 o r .1.6 a-d 12 .1 .2 .3 .4 .5 TITRE n f l O j l j . M l 0 1 2 3 4 5 6 7 TITRE P b j i 0 3 l 2 , ML 9 10 11 F i g u r e 13. P l o t of f i l t r a t e Pb c o n c e n t r a t i o n r e s u l t i n g from a d d i t i o n of 0.0970 M Pb(N03>2 t i t r e to 20.0 ml sus p e n s i o n s of 1% Na-mont-m o r i l l o n i t e . CM I 0 5 : cc CO o TITRE Zn GS 2 , ML Figure 14. Plot of f i l t r a t e Na concentration r e s u l t i n g from a d d i t i o n of 0.0970 M Z n C l 2 t i t r e to 20.0 ml suspensions of 1% Na-montmorillonite. 3.6 CM 3.2 2.8 2.4 2.0 J CO = IB o 1.2 .8 Sec Inset TITRE Z N C U , Ml 0 1 2 3 4 5 6 7 8 9 10 11 TITRE ZnC! 2 , ML F i g u r e 15 . P l o t o f f i l t r a t e Z n c o n c e n t r a t i o n r e s u l t i n g f r o m a d d i t i o n o f 0 . 0 9 7 0 M Z n C l 2 t i t r e t o 2 0 . 0 m l s u s p e n s i o n s o f 1% N a - m o n t -m o r i l l o n i t e . 55 of ± 0.02 pH u n i t due to the unstable p o t e n t i a l of the g l a s s e l e c t r o d e . O v e r a l l shape of Na c o n c e n t r a t i o n curves i s s i m i l a r f o r a l l heavy metal t i t r a t i o n s , but f e a t u r e s are more exaggerated than those of KCI and HCI t i t r a t i o n s . Na c o n c e n t r a t i o n increases almost l i n e a r l y w i t h t i t r e up to about 1.0 ml, then i n f l e c t s sharply across a shoulder (1.0 to 2.0 ml) and g r a d u a l l y decreases at higher t i t r e s . Heavy metal c o n c e n t r a t i o n curves are S-shaped w i t h asymptotes at zero and 0.1 M. I n f l e c t i o n s at zero asymp-tot e s are sharper than those of KCI and HCI t i t r a t i o n curves. I n t e r p r e t a t i o n of R e s u l t s Observed d i f f e r e n c e s i n t o t a l Na exchange c a p a c i t y (Table I I ) might be considered s i g n i f i c a n t and due to a u t h e n t i c chemical d i f f e r e n c e s between e l e c t r o l y t e s , or they might be a s c r i b e d to random pre-treatment e f f e c t s a r i s i n g during N a - s a t u r a t i o n and washing of c l a y . Spontaneous, slow degen-e r a t i o n of Na-montmorillonite observed by Shainberg et_ a l . , (1974) could conceivably account f o r 3% l o s s of t o t a l exchangeable Na, but s i n c e t h i s l o s s i s accompanied by d i s s o l u t i o n of c l a y framework, i t would not l i k e l y r e s u l t i n l o s s of net Na exchange c a p a c i t y . A l t e r n a t i v e l y , protons genera-ted by a u t o h y d r o l y s i s of water and formation of t r a c e bicarbonate might exchange f o r Na. Taking an e q u i l i b r i u m constant f o r proton-Na exchange of 2.5 (shown l a t e r ) , i t can be c a l c u l a t e d that approximately 0.4% of t o t a l exchange s i t e s would be proton-occupied at pH 6,5 i n a 5% suspension of o r i g i n a l l y Na-saturated m o n t m o r i l l o n i t e . A l l o w i n g f o r gross i n c o n s i s t e n c i e s i n washing of NaCl-treated c l a y , t h i s might account f o r 1% d i f f e r e n c e i n Na-occupancy between p r e p a r a t i o n s . A more p a u s i b l e c o n s i d e r a t i o n might be o x i d a t i o n of m o n t m o r i l l o n i t e s t r u c t u r a l f e r r o u s i r o n . Data from Knechtel TITRE HC!, ML Figure 16. Superposition of t h e o r e t i c a l Na concentration curves, c a l c u l a t e d for exchange constants of 0.1, 1, 2.5, 3 and 5, on data points of the HCl:Na-montmorillonite t i t r a t i o n . A n a l y t i c a l error bars are indicated. tn <7> 6-^ 8 r~~****)" "• " i '• i 1 '" i i • > • i»» | H• t - v — j 0 1 2 3 4 5 -6 7 8 9 10 11 . TITRE l!C!, ML F i g u r e 17. S u p e r p o s i t i o n o f t h e o r e t i c a l pH c u r v e s , c a l c u l a t e d f o r e x c h a n g e c o n s t a n t s o f 1, 2 . 5 a n d 10 o n d a t a p o i n t s o f t h e H C l : N a - m o n t -m o r i l l o n i t e t i t r a t i o n . « 58 and Patterson (1962) indicates this process under natural weathering conditions, and i t seems reasonable under laboratory conditions when oxidizing agents such as dissolved 0 2 are considered in wash water and electrolyte solution. An oxidation-reduction reaction in aqueous sus-pension might be postulated as 2{Na+:Fe2+} + h 0 2 + 2 H + - 2{Fe3 + } + 2 Na+ + H20 ( 6 ) where species in braces are clay-associated. This reaction i s energeti-cally favoured provided free energies of formation of octahedral iron species are about the same as those of their aqueous ions. Increased oxidation would be expected at lower pH. Since the magnitude of this effect has not been considered in the present study, nor has i t been esta-blished that the reaction occurs at a l l , individual Na exchange capacities for electrolytes are considered accurate and measures of the Na-exchanging power of each electrolyte solution. These values have been used in ca l -culation of theoretical concentration curves. Theoretical concentration curves calculated for the exchange reaction H + + Na{Mon£} - Na+ + H{MOR£} are shown superimposed over data points for the HCI:Na-montmorillonite tit r a t i o n in Figures 16 and 17. Both pH and Na concentration curves sug-gest an equilibrium constant for the exchange reaction of 2.5, with an uncertainty of about ± 0.5. Starting parameters for theoretical calcula-tions (see Appendix for description of method) are listed in Table IX. Theoretical t i t r a t i o n curves calculated for the K-Na exchange reaction Table IX. Starting parameters for calculation of theoretical concentration curves. See appendices for description of method and parameters. Electrolyte Concentration Ionic a -parameter Na exchange capacity KCI 0.100 M 0.30 0.30 nm (K +) (Cl ) 86.75 meq/100 g HCI 0.100 0.90 0.30 (H +) (Cl") 91.61 CuCl 2 0.1030 0.60 0.30 (Cu 2 +) (Cl~) 92.74 Cu(N0 3) 2 0.1013 0.60 0.30 (Cu 2 +) (NO 3 ) 85.50 Pb(N0 3) 2 0.0970 0.45 0.30 (Pb 2 +) (NO 3 ) 88.54 ZnCl 2 0.0970 0.60 0.30 (Zn 2 +) ' (N03 ) 95.04 K + + Na{MoRt} = Na + + KiMont] are shown superimposed over data p o i n t s f o r the KCl:Na-montmorillonite t i t r a t i o n i n Figur e s 18 and 19. The Na co n c e n t r a t i o n curve (Figure 18) i n d i c a t e s an e q u i l i b r i u m constant of about 3 to 4 (at intermediate t i t r e s ) , w h i l e the K c o n c e n t r a t i o n curve (Figure 19) suggests a value between 2 and 3. A median value might be taken as 3, w i t h an u n c e r t a i n t y of about ± 1. Disagreement of Na and K c o n c e n t r a t i o n curves might be explained by con c e n t r a t i o n of f i l t r a t e s o l u t i o n s by evaporation, or by s e l e c t i v e i n t e r -l a y e r s o r p t i o n of water over e l e c t r o l y t e s o l u t i o n during f i l t r a t i o n . This would lea d to an overestimate of exchange constant from the Na co n c e n t r a t i o n curve and an underestimate from the K c o n c e n t r a t i o n curve, much as i s pre-s e n t l y observed. I f mass balance i s assumed, i t i s p o s s i b l e to c a l c u l a t e c o r r e c t e d f i l t r a t e volumes by the f o l l o w i n g method: number of moles Na r e l e a s e d n 2 C-2 V Here s u b s c r i p t s 1 and 2 i d e n t i f y K and Na parameters, n i s number of moles, c i s molar c o n c e n t r a t i o n , v i s volume of 0.1 M t i t r e s o l u t i o n and "V" the cor r e c t e d t o t a l volume. Corrected f i l t r a t e volumes are presented i n Table X, together w i t h r e c a l c u l a t e d Na and K molar c o n c e n t r a t i o n s . S t o i c h i o m e t r i c exchange constants c a l c u l a t e d f o r i n d i v i d u a l data p a i r s are a l s o t a b u l a t e d , and may be compared d i r e c t l y w i t h g r a p h i c a l l y estimated constants s i n c e = number of moles K adsorbed = v(0.1) - nl _ v(o.i) ~ V " C l u(0.1) ( C i + c 2 ) TITRE KCI, Ml Figure 18. Superposition of t h e o r e t i c a l Na concentration curves, c a l c u l a t e d f o r exchange constants of 0.1, 1, 3, 5 and 10, on data points of the KCI:Na-montmorillonite t i t r a t i o n . A n a l y t i c a l error bars are indicated. 62 F i g u r e 19. S u p e r p o s i t i o n o f t h e o r e t i c a l K c o n c e n t r a t i o n c u r v e s , c a l c u l a t e d f o r e x c h a n g e c o n s t a n t s o f 1, 3, 5 a n d 10, o n d a t a p o i n t s o f t h e K C l : N a - m o n t m o r i l l o n i t e t i t r a t i o n . X. Corrected volumes and concentrations f o r KCl.Na-montmorillonite t i t r a t i o n f i l t r a t e s . Stoichiometric exchange constants are ca l c u l a t e d f o r each f i l t r a t e . V/V 0 i s the r a t i o of calculated to o r i g i n a l (assumed) volume expressed as per cent. T i t r e V v/v 0 Na Concentration K Concentration 0.165 18.23 90.41 0.0795 x 1 0 - 2 M 0.235 X 10 4 M 3.44 0.235 17.68 87.37 0.111 0.517 3.19 0.500 19.69 96.02 0.217 0.269 X i o - 3 2.78 0.800 19.81 95.23 0.315 0.694 2.76 1.100 19.64 93.09 0.393 0.128 X 10" 2 2.81 1.400 20.29 94.81 0.450 0.204 2.76 1.800 20.48 93.93 0.495 0.331 2.46 2.700 22.24 97.98 0.588 0.602 3.26 5.000 24.14 96.57 0.619 0.138 X 1 0 _ 1 3.71 7.500 26.75 97.26 0.607 0.212 7.25 10.80 31.81 103.3 — -< 61* Debye-Hlickel activity coefficients for Na and K were essentially identical and self-cancelling at a l l concentrations considered. An average stoichio-metric exchange constant of 3.01 ± 0.40 was calculated from individual data, excluding those for 7.5 and 10.8 ml t i t r e s . This is in agreement with the median graphical estimate of 3. Theoretical concentrations of heavy metals and sodium are plotted against t i t r e in Figures 20 to 27, each curve corresponding to a particular equilibrium exchange constant and a l l constants referring to a reaction writ-ten in the form Me 2 + + 2 Na{Mott£} - 2 Na+ + Me{MoJt£}2 where Me generalizes for Cu, Zn and Pb, A l l theoretical curves were genera-ted according to the method previously described (see also Appendix, p. 154), using starting parameters listed in Table IX and Na exchange capacities aver-aged for each electrolyte (Table II). Na concentration curves are plotted for exchange constants of 1, 10 and 100; curves corresponding to larger con-stants are not significantly different from that for 100 (at present scale). Theoretical heavy metal concentration curves are displayed at two scales, one for high concentration (viz. 10 2 M) and a second (inset graphs) for low con-centration (viz. 10.5 M). Only one theoretical curve, representing an ex-change constant of 10, is plotted in the high concentration range since curves for other constants are not resolvable at this scale. Three theore-t i c a l curves are shown for low-range heavy metal concentration, corresponding to constants of 1, 10 and 100, with additional dashed segments to serve as scale marks indicating positions of curves for constants of 3, 5 and 7. A l l data points are plotted with analytical error bars—provided these are suf-f i c i e n t l y large to be distinguished from plot squares. Figures 20 and 21 show superposition of theoretical concentration curves for Na and Zn on data points of the ZnCl2:Na-montmorillonite t i t r a -tion. From the Na concentration graph (Figure 20) i t can be seen that data are reasonably compatible with a curve for an exchange constant of 10 at titres less than 1,5 ml. Above this volume data points are more scattered and tend toward lower Na concentrations, Zn concentration curves (Figure 21) show agreement of low-scale data (inset graph) xvith a curve for an ex-change constant of 3. High-scale data points l i e close to the theoretical curve (for a constant of 10), excepting extreme values at 7.5 and 8.0 ml which indicate anomalous concentration of Zn—that i s , more than can be ac-counted for by present theory (regardless of exchange constant chosen). Deviation from assumed stoichiometry may be indicated. Figures 22 and 23 show theoretical curves and data points of the Pb(N0a)2:Na-montmorillonite tit r a t i o n . Na concentration data points are seen to l i e close to theoretical concentration curves only at low t i t r e (< 1.0 ml). Near 1.0 ml data depart from theoretical curves and swings across a relatively broad shoulder to anomalously high Na concentration, s i g n i f i -cantly above that allowed by theory. This trend persists to higher titr e s and indicates a deviation from assumed stoichiometry, since theoretical con-centration curves cannot be generated in this range without increasing the estimate of total Na exchange capacity (i.e. 88.54 meq/100 g). Pb concen-tration plots (Figure 23 ) show, at low-range concentration (inset graph), that data points l i e approximately along a theoretical curve corresponding to an exchange constant of 5; at high concentration data are in accord with the curve for an exchange constant of 10. There can be seen a tendency, which i s significant in view of analytical error but not obvious at present F i g u r e 20. S u p e r p o s i t i o n of t h e o r e t i c a l Na c o n c e n t r a t i o n curves, c a l c u l a t e d f o r exchange constants of 1, 10 and 100, on data p o i n t s of the ZnCl 2:Na-montmorillonite t i t r a t i o n . A n a l y t i c a l e r r o r bars are i n d i c a t e d . 6 7 TITRE ZnCI2, ML Figure 21. Superposition of t h e o r e t i c a l Zn concentration curves, c a l c u l a t e d for exchange constants of 1, 10 and 100, on data points of the ZnCl2:Na-montmorillonite t i t r a t i o n . Dashed segments i n d i c a t e positions of curves for exchange constants of 3, 5 and 7. F i g u r e 22. Superposition of t h e o r e t i c a l Na c o n c e n t r a t i o n curves, c a l c u l a t e d f o r exchange constants of 1, 10 and 100, on data p o i n t s of the Pb(N0 3)2:Na-montmorillonite t i t r a t i o n . A n a l y t i c a l e r r o r bars are i n d i c a t e d . CD Figure 23. S u p e r p o s i t i o n o f t h e o r e t i c a l Pb c o n c e n t r a t i o n c u r v e s , c a l c u l a t e d f o r e x c h a n g e c o n s t a n t s o f 1, 10 a n d 1 0 0 , o n d a t a p o i n t s o f t h e P M N O a ) 2 : N a - m o n t m o r i l l o n i t e t i t r a t i o n . D a s h e d s e g m e n t s i n d i c a t e p o s i t i o n s o f c u r v e s f o r e x c h a n g e c o n s t a n t s o f 3 , 5 a n d 7 . i 70 scale, for data points to cross under this curve near 1,0 ml t i t r e then cross back over near 2,0 ml. A trend toward anomalous Pb concentration at 5.0 and 7.5 ml tit r e s may also be inferred, but this may be accounted for by analy-t i c a l error. Figures 24 and 25 show Na and Cu concentration curves and data points for the CuCl2:Na-montmorillonite titr a t i o n , as do Figures 26 and 27 for the Cu(N03)2•Na-montmorillonite ti t r a t i o n . Na concentration data (Figures 24 and 26) are in poor agreement with theory, particularly for Cu(N03)2 data which extend over a broad shoulder at 1.0 ml to anomalously high concentration. As for Pb(N03)2:Na-montmorillonite tit r a t i o n data, these deviations cannot be duplicated by theory, regardless of choice of exchange constant, without increasing estimates of total Na exchange capacity (i.e. 85.50 meq/100 g for Cu(N0 3) 2 and 92.74 meq/100 g for CuCl 2). It may be inferred that better agreement of CuCl 2 Na concentration data with theory (Figure 24) reflects use of a higher estimate of Na exchange capacity. Cu concentration plots (Figures 25 and 27) show that low-range data points (inset graphs) for both CuCl 2 and Cu(N03)2 titrations are near curves for an exchange constant of 3. At high concentration CuCl2 data tend toward anomalously low concentration, while Cu(N0 3) 2 data follow an opposing trend toward high concentration. These deviations cannot be attributed to analytical error. Plots of f i l t r a t e pH versus t i t r e are presented in Figures 28 to 31. Also shown are theoretical pH curves expected for heavy metal hydrolysis in aqueous solution according to a reaction Me 2 + + H20 Me(0H)+ + H + To account for variation between data sources (as tabulated by Sillen and Figure 24. Superposition of t h e o r e t i c a l Na concentration curves, c a l c u l a t e d f o r exchange constants of 1, 10 and 100, on data points of the CuCl 2:Na-montmorillonite t i t r a t i o n . A n a l y t i c a l error bars are indicated. 72 F i g u r e 25. S u p e r p o s i t i o n o f t h e o r e t i c a l Cu c o n c e n t r a t i o n c u r v e s , c a l c u l a t e d f o r e x c h a n g e c o n s t a n t s o f 1, 10 a n d 1 0 0 , o n d a t a p o i n t s o f t h e C u C l 2 : N a - m o n t m o r i l l o n i t e t i t r a t i o n . D a s h e d s e g m e n t s i n d i c a t e p o s i t i o n s o f c u r v e s f o r e x c h a n g e c o n s t a n t s o f 3 , 5 a n d 7 . 1.0 F i g u r e 26. S u p e r p o s i t i o n of t h e o r e t i c a l Na c o n c e n t r a t i o n curves, c a l c u l a t e d f o r exchange constants of 1, 10 and 100, on data p o i n t s of the Cu(N0 3)2:Na-montmorillonite t i t r a t i o n . A n a l y t i c a l e r r o r bars are i n d i c a t e d . F i g u r e 27. S u p e r p o s i t i o n o f t h e o r e t i c a l C u c o n c e n t r a t i o n c u r v e s , c a l c u l a t e d f o r e x c h a n g e c o n s t a n t s o f 1, 10 a n d 1 0 0 , o n d a t a p o i n t s o f t h e C u ( N 0 3 ) 2 : N a - m o n t m o r i l l o n i t e t i t r a t i o n . D a s h e d s e g m e n t s i n d i c a t e p o s i t i o n s o f c u r v e s f o r e x c h a n g e c o n s t a n t s o f 3, 5 a n d 7. M a r t e l l , 1964) two bracketing hydrolysis constants 0C,„) have been chosen UH 1 2+ 2 + ? + + + for each of Cu , Zn and Pb . In addition, formation of CuCl , ZnCl + and Pb(N0 3) complexes has been taken into account by considering reactions Cu 2 + C l " - C u C l + (K - 10°-°) Zn 2 + C l " - Z n C l + (K = io0-8) Pb 2 + Pb(N0 3) + (K - 10 1 0) i n conjunction with hydrolysis reactions. Approximate equilibrium concentra-tions of a l l species were calculated i t e r a t i v e l y by a PDP-8/L computor ( D i g i t a l Equipment Corporation) using t o t a l heavy metal, sodium and anion *~™ 7 """" o concentrations. Hydrolysis constant p a i r s of 10 and 10 were chosen for Cu and Pb , while smaller bracketing values of 10 and 10 ' were chosen 2+ for Zn . I t should be noted that c a l c u l a t i o n s are approximate because of neglect of poly-ligand species, including mixed ligand complexes (e.g. PbCl0.5(0^.5); pH c a l c u l a t i o n s would be depressed by i n c l u s i o n of these equi-l i b r i a . The e f f e c t of dissolved carbonate and s i l i c i c a c i d species has not been considered, though c e r t a i n l y these species existed i n f i l t r a t e solutions and had some influence on proton and possibly free metal a c t i v i t y . For very d i l u t e solutions pH was approximated as that of pure water, that i s 7.0; actual pH was probably nearer 6.8 due to bicarbonate formation. Examination of pH pl o t s reveals great i r r e g u l a r i t y i n data. Most data points for CuCl 2 and Cu(N0 3) 2 f i l t r a t e s (Figures 30 and 31) are seen to f a l l w i thin bracketing curves. However, ZnC l 2 and Pb(N0 3) 2 data generally l i e at lower pH values than theory would predict.. This apparent excess of protons might be a t t r i b u t e d to contributions a r i s i n g from formation of poly-hydroxyl complex species. The large spread of data may be due to u n c e r t a i n t i e s of 1 1 3 4 ~T~6 7 8 9 10 11 TITRE Zn Cl 2 , ML Minimum f i l t r a t e pH p l o t t e d against t i t r e ZnCl2. T h e o r e t i c a l pH curves expected from Zn 2 hydrolysis are plotted f o r hydro-l y s i s constants of 10" 7 5 and 10-9'5. o i i r~~T~~~T' 6 T~ 8 9 13 • ii TITRE Pb [ i !0 3 l 2 , ML 2 9 . M i n i m u m f i l t r a t e pH p l o t t e d a g a i n s t t i t r e P b ( N 0 3 ) 2 . T h e o r e t i c a l pH c u r v e s e x p e c t e d f r o m P b 2 1 - h y d r o l y s i s a r e p l o t t e d f o r h y d r o -l y s i s c o n s t a n t s o f 1 0 - 7 a n d 1 0 ~ 8 . 0 1 T T 10 n TITRE CuCI 2 , ML F i g u r e 3 0 . M i n i m u m f i l t r a t e pH p l o t t e d a g a i n s t t i t r e CuCl2. T h e o r e t i c a l pH c u r v e s e x p e c t e d f r o m C u z + h y d r o l y s i s a r e p l o t t e d f o r h y d r o -l y s i s c o n s t a n t s o f 10" 7 a n d 10~ 8 . 1 2 3 4 5 T 7 T 9~~~10~ 11 TITRE Cu[fJ0 3] 2, ML M i n i m u m f i l t r a t e pH p l o t t e d + a g a i n s t t i t r e C u ( N 0 3 ) 2 - T h e o r e t i c a l pH c u r v e s e x p e c t e d f r o m Cu2 h y d r o l y s i s a r e p l o t t e d f o r h y d r o -l y s i s c o n s t a n t s o f 1 0 ~ 7 a n d 1 0 ~ 8 . measurement, recalling that pH was recorded as minimum values of a tran-sient glass electrode potential. It is not known why potentials drifted from this minimum toward neutrality, but i t is known that interferences of this order were not caused by heavy metal species, sodium or ligands. The electrode response is very similar to that described by Rechnitz (1971) for interference between two species capable of migrating across an ion-selec-tive membrane, one of which migrates much slower than the other. According to Rechnitz the i n i t i a l membrane potential of such a system is very close to the true potential that would be recorded for the fast-migrating ion in the absence of the interfering ion. Since protons are undoubtedly the most mobile species across (pH) glass electrodes, i t seems probable that the minimum potentials recorded were in fact close to true solution pH. Iden-t i t y of the interfering species remains unknown, but trace alumina or col-loidal s i l i c a might be considered; these might also interfere by sorption onto the glass membrane surface. Contamination of f i l t r a t e solutions may also have occurred during atomic absorption spectrometric analysis which preceded pH measurement. Several conclusions may be drawn from comparison of theoretical and observed concentration curves. Na concentration data points for a l l t i t r a -tions show agreement with theoretical curves only (except perhaps ZnCl 2 data) at low titres (< 1.0 ml). Unfortunately, in this region theoretical curves converge and data are inconsequential as far as the elucidation of exchange constants is concerned. At titres greater than about 1.0 ml Na concentra-tion data points for C u C l 2 , Cu(N0 3) 2 and Pb(N0 3) 2 titrations deflect into more rounded shoulders, and carry on to higher concentrations, than can be accounted by theory; data for the ZnCl 2 titration are, however, f a i r l y con-sistent with a curve for an exchange constant of about 10. It might be advanced that disagreement between observation and theory for CuCl 2, Cu(N0 3) 2 and Pb(N0 3) 2 titrations i s caused, at least in part, by underestimation of total Na exchange capacity for these electrolytes. Since Na concentration data points for a l l titrations follow similar curves, and since reasonable agreement is found between ZnCl 2 data and theoretical curves generated using a higher Na exchange capacity of 95.04 meq/100 g, i t might be concluded that this capacity is also appropriate for the other electrolytes—though this contradicts results of Na exchange capacity measurement in 0.1 M solutions. Heavy metal concentration data for a l l titrations are in accord with theory at low t i t r e s , but unlike Na concentration plots, theoretical curves for various exchange constants are well separated in this region and permit graphical estimation for each electrolyte. Data points of ZnCl 2, CuCl 2 and Cu(N03)2 titrations are compatible with theoretical curves for an exchange constant of approximately 3 (uncertainty of about ± 1), while data for Pb(N0 3) 2 suggests a somewhat greater constant of 5 (± 1). At high titres heavy metal concentration data is generally concordant with theory, but sig-nificant deviation is found for ZnCl 2 and Cu(N0 3) 2 f i l t r a t e s above 5.0 ml t i t r e which must indicate change from assumed exchange reaction stoichio-metry, this favouring retention of Cu and Zn in nitrate and chloride solu-tions above 10 2 M concentration (alternatively release of these metals from clay). Data points of CuCl 2 and Pb(N0 3) 2 titrations are very close to theory at high t i t r e (approximately within analytical error), though sys-tematic trends are apparent toward lower (CuCl 2) and higher (Pb(N0 3) 2) than expected metal concentration. To examine the possibility of deviation from assumed exchange reac-tion stoichiometry, as represented by reaction (1), mass balance calculations were made for each tit r a t i o n series. These are summarized in Tables XI to 82 XIV. Total heavy metal adsorbed was calculated for each f i l t r a t e sample (where possible) by subtracting residual metal in solution from that i n i t i -a l l y added; that i s , from the product of t i t r e solution concentration and t i t r e volume. This value (n Me), expressed in equivalents, was subtracted from total equivalents Na released {n Na) to give the net excess or de f i c i t of Na released (6 Na). Fractional equivalent exchange site occupancy for Na (X Na) was then calculated by assuming that a l l Na not released remained at exchange positions. Where excess Na was found to be released, as was the usual situation, i t was assumed that this was due to proton-exchange for Na. Fractional proton-occupied exchange sites (x H) were then calculated as equal to excess Na released divided by total exchange capacity. Fractional equi-2 + valent occupancies of Me (x Me) were calculated as one minus the sum of Na and H fractions. A l l calculations were based on total Na exchange capa-city of 95.0 meq/100 g, rather than individual capacities calculated for each electrolyte (v-tz. Table II). In cases where less Na was released than 2 + Me equivalents adsorbed a l l vacated sites were assigned to Me , with the assumption that additional Me was sorbed by some mechanism other than Na-exchange. From the previous estimate of H:Na-montmorillonite exchange con-stant of 2.5 (Figure 17), from fractional exchange site occupancies of H and Na, and from total Na concentration, an attempt was made to calculate expec-ted equilibrium pH (pH-exch) required to produce the apparent proton occu-pancies. A relation pH = -log C i (X H) 2.5 (X Na) was used, where C i represents Na molar concentration and other symbols are as previously defined. These values were compared with theoretical hydroly-Table XI. Summary of mass balance c a l c u l a t i o n s f o r ZnCl 2:Na-montmorillonite t i t r a t i o n . Number of equiva-l e n t s Zn adsorbed (ft Zn) are subtracted from e q u i v a l e n t s Na r e l e a s e d (ft Na) to g i v e excess Na re l e a s e d (6 Na). Equivalent f r a c t i o n a l exchange s i t e occupancies are c a l c u l a t e d f o r Na (x Na), Zn (x Zn) and protons (X H), assuming excess Na r e l e a s e d i s due to proton exchange. Expected t h e o r e t i c a l pH f o r H-Na exchange e q u i l i b r i u m (pH-exch) i s compared w i t h t h e o r e t i c a l Z n 2 + hydro-l y s i s pH (pH-hydr) and observed pH (pH-obs). S t o i c h i o m e t r i c constants are c a l c u l a t e d f o r Zn-Na exchange e q u i l i b r i u m (K$). T i t r e ft Na ft Zn 6 Na X Na X Zn X H pH-exch pH-hydr pH-obs 0.000 ml 1.870xl(T 6e .9902 9.838x10" '3 6.43 5.30 0.100 1.972x10 5 1.936xl0 _ 5e 0.036xl0~ 5e .8963 .1019 1.865 6.09 'v 7 t * 0.14 0.200 4.000 3.875 0.125 .7896 .2039 6.560 5.18 6.97 * 1.02 0.340 6.895 6.585 0.310 .6372 .3465 1.632x10" 2 4.46 6.80 5.30 3.62 0.400 7.936 7.737 0.199 .5825 .4070 1.046 4.55 6.63 * 3.20 0.470 9.130 9.069 0.061 .5197 .4771 3.174x10" 3 4.96 6.47 5.54 2.95 0.500 9.717 9.632 0.085 .4888 .5067 4.453 4.76 6.40 * 2.89 0.600 1.201x10""* . 1.148X10"4 0.053xl0 - l t .3682 .6040 2.780x10" '2 3.75 6.38 * 3.93 0.700 1.410 1.315 0.095 .2584 .6916 5.005 3.28 5.99 * 4.57 0.720 1.388 1.349 0.039 .2697 .7097 2.057 3.69 5.97 5.27 3.81 0.900 1.618 1.593 0.025 .1490 .8380 1.297 3.57 5.72 * 6.18 1.200 1.749 1.709 0.040 .0799 .8990 2.106 3.06 5.42 5.06 6.56 1.500 1.789 1.723 0.066 .0589 .9066 3.451 2.71 5.28 5.30 6.55 2.000 1.756 1.724 0.032 .0764 .9070 1.662 3.16 5.16 4.83 2.02 3.500 1.857 1.714 0.143 .0233 .9017 7.497 1.99 4.99 4.80 9.60 5.000 1.818 1.850 -0.033 .0267 .9732 4.91 4.52 4.60 7.500 1.821 1.185 0.636 .0422 .6238 3.343x10" '1 1.68 4.82 4.26 0.63 8.000 1.831 0.568 1.263 .0366 .2988 6.646 2.32 4.66 * 0.36 t apptioxAmatzA nejut/iaJUXy {on. pane. wcuteA * not me/uu/Lzd Table XII. Summary of mass balance calculations for CuCl 2:Na-montmorillonite t i t r a t i o n . Number of equiva-lents Cu adsorbed (n Cu) are subtracted from equivalents Na released (n Na) to give excess Na released (6 Na). Equivalent fractional exchange site occupancies are calculated for Na (x Na), Cu (x Cu) and protons (x H), assuming excess Na released i s due to proton exchange. Expected theoretical pH for H-Na exchange equilibrium (pH-exch) i s compared with theoretical Cu 2 + hydro-l y s i s pH (pH-hydr) and observed pH (pH-obs). Stoichiometric constants are calculated for Cu-Na exchange equilibrium (K^). Titre n Na n Cu 6 Na X Na X Cu X H pH-exch pH-hydr pH-obs h 0.000 ml 1.346xl0"6e .9929 7.084xl0"3 6.72 — 7.73 0.100 2.004x10 5 2.058xl0~5e -0.054xl0"5e .8945 .1055 6.71 6.67 0.23 0.200 4.181 4.115 0.067 .7799 .2166 3.508X10"2 5.43 6.50 6.43 1.17 0.300 6.476 6.168 0.308 .6592 .3246 1.622 4.50 6.30 5.83 2.48 0.300 6.496 6.169 0.326 .6581 .3247 1.718 4.48 6.34 5.63 2.99 0.420 9.169 8.615 0.554 .5174 .4534 2.915 3.99 6.04 5.60 3.75 0.420 8.924 8.604 0.319 .5303 .4529 1.681 4.26 5.99 5.80 2.63 0.520 1.124xl0~4 1.060xl0"4 0.640 .4082 .5581 3.371 3.74 5.81 5.54 3.84 0.600 1.265 1.215 0.495 .3343 .6397 2.604 3.72 5.66 5.48 4.31 0.600 1.269 1.216 0.531 .3321 .6399 2.793 3.68 5.67 5.95 4.51 0.720 1.461 1.425 0.360 .2312 .7499 1.894 3.64 5.43 5.40 4.95 0.850 1.645 1.566 0.792 .1342 .8241 4.169 3.01 5.18 4.48 6.42 0.990 1.751 1.719 0.319 .0786 .9046 1.679 3.15 5.06 4.78 13.3 1.300 1.845 1.758 0.867 .0292 .9252 4.565 2.27 4.84 4.83 37.8 1.600 1.836 1.827 0.088 .0337 .9617 4.632X10"3 3.33 4.74 5.05 18.0 1.600 1.799 1.745 0.542 .0530 .9185 2.850X10"2 2.75 4.73 4.93 6.32 2.390 1.867 1.977 -0.110X10"" .0172 .9828 4.60 4.80 35.1 3.500 1.847 2.040 -0.193 .0278 .9722 4.49 4.72 7.04 4.980 1.871 2.065 -0.194 .0153 .9847 4.40 4.65 14.5 7.000 1.863 2.216 0.353 .0195 .9805 4.33 4.40 5.45 CO Table XIII. Summary of mass balance c a l c u l a t i o n s f o r Cu(N0 3) 2:Na-montmorillonite t i t r a t i o n . Number of e q u i -v a l e n t s Cu adsorbed (n Cu) are subtracted from e q u i v a l e n t s Na r e l e a s e d (ci Na) to g i v e excess Na re l e a s e d (<5 Na). Equivalent f r a c t i o n a l exchange s i t e occupancies are c a l c u l a t e d f o r Na • (x Na), Cu (X Cu) and protons (x H), assuming excess Na r e l e a s e d i s due to proton exchange. Expected t h e o r e t i c a l pH f o r H-Na exchange e q u i l i b r i u m (pH-exch) i s compared w i t h t h e o r e t i c a l Cu 2* hydro-l y s i s pH (pH-hydr) and observed pH (pH-obs). S t o i c h i o m e t r i c constants are c a l c u l a t e d f o r Cu-Na exchange e q u i l i b r i u m (K^). T i t r e Yi Na n Cu 6 Na X Na X Cu X H pH-exch pH-hydr pH-obs h 0.100 ml 1.899>a0"5e 2.023xl0~ 5e -0.123xl0" 5e .9000 .1000 > . 6.63 5.20 0.14 0.200 3.939 4.048 -0.109 .7927 .2073 6.56 5.00 1.21 0.300 6.131 6.066 0.064 .6773 .3193 3.388X10"3 5.22 6.31 5.45 2.19 0.500 1.025x10"" 1.005x10 " 0.020x10 * .4605 .5289 1.057x10 2 4.34 5.87 4.70 3.16 0.500 1.025 1.005 0.020 .4605 .5287 1.072 4.33 5.86 5.15 3.06 0.700 1.389 1.368 0.021 .2690 .7198 1.120 3.95 5.46 5.50 3.67 0.900 1.597 1.601 -0.004 .1596 .8404 5.14 5.20 3.62 1.000 1.625 1.638 -0.012 .1445 .8555 * 5.02 5.10 2.65 1.000 1.651 1.585 0.066 .1313 .8342 3.453xl0~ 2 3.08 4.99 5.20 2.85 1.200 1.715 1.685 0.030 .0973 .8868 1.585 3.28 4.88 5.10 3.48 1.200 1.728 1.647 0.081 .0906 .8667 4.263 2.81 4.87 4.95 3.79 1.500 1.718 1.568 0.149 .0959 .8255 7.866 2.58 4.73 4.90 1.68 2.000 1.742 1.434 0.308 .0829 .7547 1.623xl0 - 1 2.21 4.61 4.90 1.16 3.000 1.764 1.478 0.286 .0715 .7779 1.506 2.19 4.50 4.70 0.89 5.000 1.788 0.830 0.958 .0592 .4368 5.039 1.61 4.37 4.70 0.34 7.500 1.804 1.225 0.579 :'0505 .6447 3.047 1.80 4.30 4.40 0.43 CO cn Table XIV. Summary of mass balance calculations for Pb(NO3) 2:Na-montmorillonite t i t r a t i o n . Number of equi-valents Pb adsorbed (ft Pb) are subtracted from equivalents Na released (ft Na) to give excess Na released (6 Na). Equivalent fractional exchange site occupancies are calculated for Na (x Na), Pb (X Pb) and protons (x H), assuming excess Na released i s due to proton exchange. Expected theoretical pH for H-Na exchange equilibrium (pH-exch) i s compared with theoretica 1 Pb 2 + hydro-l y s i s pH (pH-hydr) and observed pH (pH-obs). Stoichiometric constants are calculated for Pb-Na exchange equilibrium (K4). Titre ft Na ft Pb 6 Na X Na X Pb X H pH-exch pH-hydr pH-obs 0.300 5.907xl(f 5e 5.812xl0"5e 0.095xl0"5e .6891 .3059 4.990xl0"3 5.07 6.42 * 3.00 0.400 7.915 7.747 0.168 .5834 .4077 8.866 4.63 6.29 * 5.55 0.500 1.013X10"1* 9.286 0.841 .4670 .4887 4.427xl0"2 3.73 6.03 5.45 5.41 0.600 1.201 1.152X10"1* 0.049x10"'' .3679 .6063 2.582 3.79 5.79 5.35 5.20 0.600 1.201 1.152 0.049 .3679 .6064 2.566 3.79 5.80 5.05 5.32 0.800 1.500 1.480 0.019 .2107 .7792 1.012 3.86 5.40 4.95 5.30 1.000 1.686 1.640 0.047 .1125 .8630 2.453 3.15 5.09 4.60 6.15 1.200 1.738 1.700 0.038 .0851 .8950 1.996 3.11 4.94 4.60 5.62 1.500 1.767 1.753 0.014 .0698 .9228 7.368xl0~3 3.46 4.81 4.50 4.75 2.000 1.822 1.764 0.058 .0413 .9282 3.053X10"2 2.61 4.69 4.50 7.77 3.000 1.806 1.735 0.070 .0497 .9133 3.700 2.63 4.57 4.30 2.56 5.000 1.788 1.600 0.188 .0592 .8421 9.868 2.32 4.46 4.40 0.76 7.500 1.818 1.570 0.248 .0433 .8263 1.304X10"1 2.10 4.40 4.30 0.82 * not mm&vAoA CO 1 87 s i s pH (pH-hydr), as c a l c u l a t e d f o r F i g u r e s 27 to 30, and observed pH (pH-obs) . S t o i c h i o m e t r i c exchange constants were c a l c u l a t e d according to an equation ( d ) 2 ( x Me) K* = 7 ( c 2 ) (X N a ) 2 (where C2 i s Me molar concentration) which i s e s s e n t i a l l y equivalent to the thermodynamic e q u i l i b r i u m constant, K (see equation ( 2 ) ) , d i v i d e d by an e q u i l i b r i u m quotient of a c t i v i t y c o e f f i c i e n t s (making a s l i g h t c o r r e c t i o n f o r molar as opposed to molal s c a l e c o e f f i c i e n t s ) . Thus can be considered as f\ uncorrected f o r e f f e c t s of n o n - i d e a l behavior. R e s u l t s of these c a l -c u l a t i o n s are summarized i n Tables XI to XIV and i n F i g u r e s 31 to 34 where K, values are p l o t t e d against t i t r e of Z n C l 2 , C u C l 2 , C u ( N 0 3 ) 2 and P b ( N 0 3 ) 2 s o l u t i o n s . I t should be borne i n mind that e r r o r propagation becomes very s i g n i f i c a n t at small values of Na f r a c t i o n a l s i t e occupancy. Formal c a l c u -l a t i o n of e r r o r has not been attempted (some i n d i c a t i o n of p r e c i s i o n can be estimated from d u p l i c a t e samples), but values f o r f r a c t i o n a l occupancies (X Na, X Me and X H) and are judged to have two-figure s i g n i f i c a n c e at best, w h i l e other c a l c u l a t e d parameters have t h r e e - f i g u r e s i g n i f i c a n c e . Tabulated values are l i s t e d w i t h extended "accuracy" only f o r purposes of c a l c u l a t i o n . Examination of mass balance c a l c u l a t i o n s r e v e a l s exchange s t o i c h i o -metry much more complex than i n i t i a l l y assumed. In general i t i s seen that excess Na i s u s u a l l y r e l e a s e d r e l a t i v e to heavy metal adsorbed, and that the magnitude of t h i s excess increases (or decreases) s y s t e m a t i c a l l y w i t h t i t r e . S t o i c h i o m e t r i c constants a l s o are seen to vary w i t h t i t r e , g e n e r a l l y i n -4 88 creasing rapidly at the beginning of tit r a t i o n (0 to 1.0 ml) and gradually decreasing thereafter. A plot of K versus t i t r e for ZnCl 2 data (Figure 32) •6 demonstrates the i n i t i a l rapid increase of the stoichiometric quotient, star-ting approximately at the origin and rising to about 6.5 at 1.0 ml, and also a curious "N"-shaped inflection at 0.5 ml. Technically this feature might be considered insignificant, but the contiguity of plots seems to indicate otherwise. While a similar inflection is not seen for CuCl2 data (Figure 33), plots for Cu(N03)2 and Pb(N0s)2 (Figures 34 and 35) do display perturbations at about 1.0 ml and 0.7 ml titre s which, because of closely grouped duplicate plots, are probably authentic. At high titres (> 1#0 ml) stoichiometric con-stants for ZnCl2 and CuCl 2 are scattered, but generally decrease toward the end of titr a t i o n . Plots for Cu(N03)2 and Pb(N03)2, on the other hand, are much more regular and show gradual decrease of stoichiometric quotients from maxima of about 4.0 and 6.0 (near 1.0 ml) to values less than 1.0 at the end of ti t r a t i o n (7.5 ml). Comparison of and 6 Na values (see Tables XI to XIV) shows that, at titres less than about 1.0 ml, stoichiometric quotients increase sympatheti-cally with excess Na. This i s particularly evident for Pb(N03)2 data (Table XIV) which show coincident maxima of and 6 Na at both peaks flanking the inflection at 0.7 ml. Similar trends are found at low titre s for ZnCl 2 and CuCl 2 data (Tables XI and XII), but for Cu(N0 3) 2 (Table XIII) negative 6 Na values are encountered at starting titres (0.10 and 0.20 ml) and again at 0.9 and 1.0 ml near maximum K^. These values indicate more equivalent Cu sorbed than Na released. At titres above about 1.0 ml 6 Na values tend to increase steadily (except for several negative values for CuCl2 and one for ZnCl2 data) while stoichiometric constants decrease. Perhaps the most surprising result of mass balance calculations is the F i g u r e 33. P l o t of Cu-Na s t o i c h i o m e t r i c exchange constants (KA,) a g a i n s t t i t r e 0.1030 M CuCl2. TITRE CuW 2 , ML F i g u r e 34. P l o t of Cu-Na s t o i c h i o m e t r i c exchange constants (K^) against t i t r e 0.1013 M Cu(NC>3)2. v a r i a b i l i t y of apparent heavy metal occupancies of montmorillonite exchange sites. It is apparent that maximum metal occupancies vary between electro-lytes and do not (except for CuCl 2) occur at the end of tit r a t i o n . For ZnCl 2 the maximum Zn occupancy i s about 91% near 2,0 ml t i t r e (excluding an extra-neous value of 97% at 5.0 ml), decreasing to about 30% at the f i n a l t i t r e (8.0 ml). Accounting for about 4% residual Na, this means that 66% of total clay exchange capacity must be proton-occupied, at least according to present assumptions; a theoretical solution pH of 2.32 would be required to support this occupancy, a value far below that observed at about 4.3 (7.5 ml t i t r e ) . CuCl 2 calculations indicate maximum Cu occupancy of about 98% between 2.39 and 7.0 ml ti t r e s , with accompanying excess sorption of Cu (except at 7.0 ml) and no apparent proton occupancy—more concisely, no need to infer proton occupancy. Maximum Cu occupancy from Cu(N03)2 calculations, conversely, seems to occur at intermediate stages of titr a t i o n (v-cz. 1.2 ml) at about 87 to 89% and decreases thereafter to about 40 to 60% at the end of tit r a t i o n . High proton occupancies (30 to 50%) and very low theoretical solution pH are called for by calculations at the f i n a l two tit r e s , these entirely in-consistent with observed pH values of 4.4 and 4.7. Pb(N03)2 calculations are similar to those for Cu(N03)2 , but show a slightly higher maximum Pb occupancy of 93% at about 2.0 ml, this decreasing steadily to about 83% at 7.5 ml where i t i s compensated by increased proton occupancy at 13% and r e s i -dual Na occupancy at about 4%. Again, theoretical solution pH (2.1) is much lower than observed (4.3). 94 Summary and Conclusions It i s evident from Na exchange capacity measurements, from t i t r a t i o n curves and from mass balance calculations that two types of behavior charac-terize the interaction of heavy metal electrolyte solutions with Wyoming montmorillonite. Below titre s of about 1.0 ml (approximately 10 3 M heavy metal and 7 x 10 3 M sodium concentration) heavy metal electrolyte solutions interact with Na-montmorillonite principally by cationic mechanisms— that i s , cation-exchange adsorption. Above these titr e s influences of the anionic component of exchange electrolytes become increasingly more important, sig-nificantly modifying exchange reaction stoichiometry and the a b i l i t y of metal cations to displace sorbed Na. Titration curves enable graphical estimation of exchange constants (reaction (1)) at titr e s less than 1.0 ml, and allow construction of an ex-changeability sequence, V-iz. Pb 2 > Cu 2 , Zn , K > H (5.0 ± 1.0) (3.0 ± 1.0) (2.5 ± 0.5) where a l l cations w i l l replace Na in accordance with their exchange constants listed in parentheses. This sequence is in agreement with that proposed by Brown (1963) and follows the cationic size principle (Jenny et a l . , 1936), at least for the heavy metals. It is interesting to note that the ratios of Pb 2 ionic radius (1.20 A) to those of Cu2 (0.72 A) and Zn 2 (0.74 A) are close to the ratio of exchange constants (i.e. oT5" - f$ - 1.7). If gra-phical estimates of exchange constants can be approximated as thermodynamic constants, then free energy changes for exchange reactions at 23 C are -950 ± 150 cal for Pb, -650 ± 150 cal for Cu, Zn and K, and -540 ± 100 cal for H. Above 1.0 ml t i t r e and i n 0.1 M exchange e l e c t r o l y t e solutions used for Na exchange capacity determination i t must be concluded that e l e c t r o -s t a t i c i n t e r a c t i o n between heavy metals and montmorillonite i s s i g n i f i c a n t l y modified by the anionic component of the e l e c t r o l y t e . Generally, N0 3 seems to i n h i b i t exchange of Na while C l seems to f a c i l i t a t e i t s release. This i s demonstrated by higher Na exchange ca p a c i t i e s for ZnCl2 and CuCl2 com-pared to those of Cu(N03) 2 and Pb(N03)2, and also by larger stoichiometric constants for the chloride e l e c t r o l y t e t i t r a t i o n s (Figures 32 and 33). The reasons for these differences are unknown, but perhaps may be found i n s p e c i -f i c r e action mechanism involving preferred exchange adsorption of p o s i t i v e l y charged metal-chloride complexes rather than n i t r a t e interference ( n i t r a t e being chemically innocuous). Since Na exchange c a p a c i t i e s for n i t r a t e e l e c -t r o l y t e s are s i m i l a r to that for KCI and are i n reasonable agreement with exchange c a p a c i t i e s quoted by other i n v e s t i g a t o r s (Weaver and P o l l a r d , 1973), i t might be proposed that the a d d i t i o n a l capacity measured for CuCl2 and ZnCl2 of about 7 meq/100 g i s due to exchange at s i t e s which are only con-d i t i o n a l l y " a v a i l a b l e " — t h e s e s e n s i t i v e to the anionic component of s o l u t i o n . This value i s comparable to the "edge-exchange" f r a c t i o n of t o t a l exchange capacity often quoted for montmorillonites, about 10-20% (Grim, 1968), and may suggest chloride i n t e r a c t i o n with broken bonds near the clay edge. It might be suggested that cation hydrolysis i s an important considera-t i o n i n exchange reactions involving heavy metal cations i n aqueous systems and that t h i s might explain anomalous behavior evident i n present data at metal concentrations greater than about 10 3 M (Menzel and Jackson, 1950; DeMumbrum and Jackson, 1956a, 1956b; Hodgson et_ al_., 1964). However, t h i s conclusion i s only v a l i d .in systems whose pH i s buffered by a base such as acetate; i n present n i t r a t e and c h l o r i d e e l e c t r o l y t e solutions pH i s set by cation hydrolysis e q u i l i b r i a (Figures 28 to 31) and only a small percentage of t o t a l heavy metal (y 1%) occurs as hydroxo-complexes (Baes and Mesmer, 1976) . It i s not s u r p r i s i n g that stoichiometric c a l c u l a t i o n s do not show evidence for excess metal sorption (excepting a small number of samples) while previous authors (including Steger, 1973), working with acetate s o l u -tions, have found large excesses. Bingham et a l . (1964) concluded that Cu and Zn exchange as divalent cations i n solutions below about pH 5. Mass balance c a l c u l a t i o n s show that, rather than being sorbed i n excess of Na released, heavy metals ( e s p e c i a l l y as Cu(N0 3) 2 and Pb(N03) 2) are sorbed i n equivalent q u a n t i t i e s l e s s than Na released. These data cannot be explained by an H-Na exchange mechanism whereby protons generated by cation hydrolysis exchange for Na, leaving metals i n s o l u t i o n , A possible explanation may be the s t r u c t u r a l degeneration of montmorillonite or the oxidation of octahe-d r a l ferrous i r o n by reaction (6 ), both processes being favoured by low pH. This would lead to release of o r i g i n a l l y adsorbed metal (and Na). E r r a t i c glass electrode p o t e n t i a l s i n f i l t r a t e solutions and recent work of Shainberg et a l . (1974) suggest that s t r u c t u r a l degeneration of montmorillonite may occur more e a s i l y than previously thought (Coleman and Harward, 1953), By whatever mechanism, i t must be concluded that exchange adsorption phenomena involving Cu, Zn and Pb at concentrations above 10 3 M are complicated by stoichiometric i r r e g u l a r i t i e s which preclude simple thermodynamic an a l y s i s . This conclusion follows that of Maes and Cremers (1975) who stated, "...there i s no thermodynamic j u s t i f i c a t i o n f o r expressing the d i s t r i b u t i o n of mono-and divalent cations at high divalent ion occupancy i n terms of an equilibrium s e l e c t i v i t y c o e f f i c i e n t since the reaction i s neither stoichiometric nor r e -v e r s i b l e . . .". MONTMORILLONITE-SILICA GEL THIN-LAYER CHROMATOGRAPHY Introduction As an adjunct to s t a t i c measures of exchange adsorption a f f i n i t y described i n the previous chapter, laboratory i n v e s t i g a t i o n s were dire c t e d toward measurement of heavy metal adsorption on montmorillonite by t h i n -layer chromatography. Chromatography has found wide usage i n the ph y s i c a l sciences as a means of chemical separation, buts i t s a p p l i c a t i o n to geo-chemical problems has been l i m i t e d . Methods and r e s u l t s presented here therefore lack much precedent i n previous geochemical l i t e r a t u r e , but draw instead from a large t h e o r e t i c a l and methodological base founded i n c l a s s i -c a l chemistry. It was hoped that estimates of exchange constants could be obtained by measuring r e t a r d a t i o n factors for Cu, Zn and Pb spots a f t e r e l u t i o n across Na-montmorillonite-silica gel t h i n layers by NaCl and NaN03 solutions of varying concentration. Th e o r e t i c a l Development Basic chromatographic theory has been summarized by Cassidy (1957). In general, chromatography can be considered as a dynamic, mass transf e r process involving displacement of species i n a mobile phase across a reac-t i v e stationary phase. 1 The rate of movement of these species i s p r i n c i p a l l y a function of t h e i r r e l a t i v e a f f i n i t i e s for the two phases, those species not strongly attracted or sorbed by the stationary phase tending to move more r a p i d l y than those having greater a f f i n i t y f o r the stationary phase. A measure of a p a r t i c u l a r species' r e l a t i v e a f f i n i t y f o r the two phases i s 98 given by the partition ratio K = P (7 ) defined as the ratio of solute concentration in the stationary phase (C\) to i t s concentration in the mobile phase (CV), subscript " i " here general-izing for any solute species. In present context the mobile phase is an electrolyte solution which displaces heavy metal species (solute) across the stationary phase represented by montmorillonite-silica gel. If s i l i c a gel can be considered as an inert support medium (cation sorption by s i l i c a gel in aqueous systems is negligible compared to montmorillonite), the dis-tribution of heavy metals and Na betx^een mobile and stationary phases might also be described by mass balance and action equations for a generalized exchange reaction n+ + Me + n Na{M0Ji£} - KeWont} + n Na ( 8 ) which has similar form and notation to reactions described in the previous chapter, but which leaves ambiguity as to the effective charge of the sorbed heavy metal species. Here "n" is the average or apparent charge of a given heavy metal which is sorbed as a. variety of complexed species (e.g., CuCl , CuCl2, CuCl 4 etc.) in addition to doubly charged cations. It may be defined pragmatically as the ratio of moles Na released into solution to moles Me sorbed. Equation ( 8 ) implicitly relies on equi-librium reactions for the formation of complex species and assumes that anion exchange is relatively minor compared to exchange adsorption of positively'charged species. Hence "n" is dependent on the character and 9 9 a c t i v i t y of the anionic component of the s o l u t i o n phase. For purposes of inchoation, and c e r t a i n l y at the expense of rigorous thermodynamic develop-ment, a mass ac t i o n equation corresponding to reaction (8 ) might be w r i t -ten as ( C O 1 1 x K = ( 9 ) * C2 (l-x)n where subscripts 1 and 2 refer to Na and Me r e s p e c t i v e l y , C i s molar con-centration of s o l u t i o n species and X i s the equivalent f r a c t i o n of t o t a l clay exchange capacity occupied by Me. K may be regarded as a s t o i c h i o -•6 metric quotient. Equation (9 ) may be re-arranged into a form s i m i l a r to that of the p a r t i t i o n r a t i o p' fe X p' fe K. (10) ( d ) n * where factors p' (montmorillonite density) and fe have been added to bring concentration of sorbed Me into "molar" u n i t s , e f f e c t i v e l y moles Me per l i t e r montmorillonite. The usefulness of t h i s somewhat awkward trans-p o s i t i o n w i l l become apparent l a t e r . The parameter fe i s given by fe = c ^ CEC (10~ 2) . (11) r for t o t a l clay c a t i o n exchange capacity of CEC meq/100 g. An approximation that the term (1-x) equals one i s required i n transforming equation (10) to (11). This i s j u s t i f i e d i n systems where Na concentration greatly exceeds that of Me. From early t h e o r e t i c a l work of Martin and Synge (1941) a r e l a t i o n 1 0 0 between re t a r d a t i o n factor and p a r t i t i o n r a t i o may be expressed (Skoog and West, 1971) as 1 R o : (12) £ 1 + Kp (A'/A") where i s the retardation factor for the point of maximum concentration of a solute species (with p a r t i t i o n r a t i o K ) eluted across a s e r i e s of t h e o r e t i c a l plates ( c e l l s of f i n i t e thickness i n which l o c a l equilibrium i s attained) whose c r o s s - s e c t i o n a l areas are equal to the sum of p a r t i a l areas of the stationary phase (A 1) and the mobile phase (A"). A summary of so-called "plate theory" leading to equation (12) i s given by Skoog and West (1971). It i s assumed that l o c a l equilibrium i s established i n each t h e o r e t i c a l plate and that no mass transfer occurs between plates except by movement of the mobile phase (solute d i f f u s i o n i s therefore not accounted f o r ) . In addition, i t must be assumed that solute phases react with both phases independently and that t h e i r p a r t i t i o n r a t i o s remain con-stant at a l l concentrations. Since weights of stationary and mobile phases are more amenable to experimental measurement than c r o s s - s e c t i o n a l areas, equation (12) can be p r o f i t a b l y modified to R = (13) * 1 + K P where T' and i " are weights and p' and p" are the d e n s i t i e s of stationary and mobile phases. By s u b s t i t u t i n g the right-hand term of equation (10) for i n equation (13), a new expression for R^ may be written as 1 0 ] 1 + k K* p' Ci n r' o" r" p>. which reduces to n Ci + k r k rwj (14) where T w i s the weight of water (density = 1) which would occupy each t h e o r e t i c a l p late i f i t alone was the mobile phase ( i . e . e l u t i o n with pure water). Equation (14) provides an admittedly informal l i n k between mass action v a r i a b l e s "n" and and measurable chromatographic parameters, and i t must be borne i n mind that much assumption i s contained within the present development. In p a r t i c u l a r , no allowance i s given for v a r i a t i o n of "n" with ligand concentration. The mean ligand coordination number of a metal i n so l u t i o n was de-fined by Bjerrum (1957) as ft = Kn[Z] + 2 K i 2 [ z ] 2 + 3 K i 3 [ Z ] 3 + •»• + NKi* [z]? 1 + Kn[Z] + K 1 2 [ Z ] 2 + K 1 3 [ Z ] 3 + ... + Ku [l]' (15) where ft i s the mean coordination number of ligand Z (at concentration [ Z ] ) which forms consecutive mononuclear complexes with a metal (e.g. M c Z , Me .Z 2 , Me.Z3, M&lt). Corresponding formation constants for metal-ligand com-plexes are K n , K i 2 , K 1 3 , -Ku ; here i n i t i a l subscripts i n d i c a t e that only mononuclear complexes are considered, and second subscripts denote the number of ligands coordinated to each metal (up to " i " ligands per Me.), In a c l a y - e l e c t r o l y t e system where free ligands were s i n g l y charged anions (e.g. - - 2 + 2 + C l or N0 3 ) and free metals were doubly charged cations (e.g. Cu , Zn or « 10 2 Pb 2 ), a r e l a t i o n between Bjerrum's mean coordination number and "n" of the present treatment might be proposed as n = 2 - n (16) Hence, with knowledge of free ligand concentration ([Z]) and metal-ligand complex formation constants, the e f f e c t of changing ligand concentration might be included i n equation (14). Unfortunately, t h i s approach would only be v a l i d i n s i t u a t i o n s where p o s i t i v e l y charged complexes were the dominant species ( i . e . 0 < n < 2) and where a l l species were sorbed by clay with equal f a c i l i t y . Sorption of anionic complexes (anion-exchange adsorption) cannot be accomodated within present development. Although equations (16) and (15) are useful i n r e l a t i n g present and past attempts at describing "average" e f f e c t s of metal complexation, no at-tempt w i l l be made to f u n c t i o n a l i z e "n" against ligand concentration and thereby append equation (14). Instead, and as only an approximation, "n" w i l l be considered constant f o r any given metal-ligand system. Experimental Procedure Experimental technique i n thi n - l a y e r chromatography i s described by Stahl (1965), Randerath (1968) and Peereboom (1971). Sodium-saturated Wyoming and Newcastle Formation montmorillonites were prepared by the pre-v i o u s l y described method (Appendix, p.142) and l e f t i n gel form containing ® the equivalent of 18% a i r - d r y clay. Iron-leached s i l i c a g e l ( S i l i c a Gel H , E. Merck AG), Na-montmorillonite gel and d i s t i l l e d water were mixed i n the proportions (by weight) 40:40:40, homogenized by continuous s t i r r i n g , then spread across standard 20 by 20 cm glass plates with a Desaga^31 apparatus. This spreader was adjusted to produce 0.25 mm thick coatings (Appendix, p. 151). A i r - d r i e d plates were spotted with 1.0 y l volumes of 0.1 M sol u -tions of Cu, Zn and Pb (chloride s a l t s of Cu and Zn; n i t r a t e s a l t s of Cu and Pb) along a baseline about 1 cm above the plate edge. E l u t i o n channels were scored at r i g h t angles to t h i s baseline to separate spots (see Plate I ) . Ascending e l u t i o n with NaCI and NaNO3 solutions, ranging i n concentration from 0.05 to 3.0 M, was c a r r i e d out i n rectangular glass chambers (approxi-mately 25 by 25 by 10 cm) provided with ground glass l i d s . Vapour satura-t i o n during e l u t i o n was aided by f i l t e r paper l i n i n g s along chamber walls. Plates were eluted f o r f i v e to f i f t e e n hours u n t i l the solvent front had progressed about 15 cm past baseline, then'removed from the chamber, marked along the solvent (eluent) front and allowed to dry. Heavy metal spots were v i s u a l i z e d x^ith d i l u t e ethanolic solutions of several commonly used i n d i c a t o r dyes (Ritchie, 1964), including s-diphenylcarbazone, dithizone (diphenylthiocarbazone) and 8-hydroxyquinoline (oxime). A 0.5% so l u t i o n of s-diphenylcarbazone was found s u i t a b l e f o r routine a n a l y s i s . A l t e r n a t e l y spraying plates with v i s u a l i z i n g reagent and exposing them to ammonia fumes several times produced brighter and lo n g e r - l a s t i n g spot colours. V i s u a l i z e d plates were p h o t o s t a t i c a l l y copied and metal retardation values (R^ values) calculated as the r a t i o of distance t r a v e l l e d ( r e l a t i v e to the baseline) by spot centres (points of maximum colour i n t e n s i t y ) to that t r a v e l l e d by the solvent f r o n t . Results Preliminary i n v e s t i g a t i o n s indicated that s i l i c a g el alone was an « 10 4 i n e f f e c t i v e sorber of heavy metals i n aqueous e l e c t r o l y t e s o l u t i o n media; Cu, Zn and Pb spots migrated at the solvent front across t h i n layers of pure s i l i c a g e l , with poorly defined comas attached to Pb spots i n c h l o r i d e -eluted chromatograms. Comparitive chromatograms of Clay Spur and Newcastle Formation Na-montmorillonite-silica gel preparations showed no s i g n i f i c a n t differences from one another. These two clays were therefore used i n t e r -changeably i n the preparation of t h i n layer p l a t e s . T y p i c a l s-diphenylcarbazone spot colours were brown for Cu, red for Zn and purple for Pb (see Plate I ) . These colours varied i n i n t e n s i t y and hue with time and according to spraying procedure. Zinc spots faded r a p i d l y from bright to pale red, while Cu spots tended to r e t a i n t h e i r brownish colour and fade only gradually to a d u l l purple a f t e r several hours. Lead spots were d i f f i c u l t to v i s u a l i z e with s-diphenylcarbazone when plates were thoroughly dried, but became conspicuous when plates were sprayed with water a f t e r a p p l i c a t i o n of dye. Background c o l o r a t i o n of Na-montmorillonite-silica gel was pink immediately a f t e r spraying with s-diphenylcarbazone (Plate I ) , but became b l u i s h a f t e r several minutes. Exposure of plates to ammonia fumes restored the pink background and i n t e n s i f i e d heavy metal spot colours. Spraying of plates with d i l u t e aqueous ammonium hydroxide s o l u t i o n resulted i n an intense red complex which suffused the e n t i r e plate and obscured metal spots. Although t h i s red complex faded and f i n a l l y disappeared over several hours, an equally intense and o b l i t e r a t i n g purple complex r e s u l t i n g from spraying with d i l u t e aqueous HCI s o l u t i o n was found to be permanent. A 0.05% ethanolic s o l u t i o n of dithizone v i s u a l i z e d metals about as e f f e c t i v e l y as s-diphenylcarbazone, giving blue-green Cu spots and l i g h t red Zn and Pb spots. A 1% ethanolic s o l u t i o n of 8-hydroxyquinoline gave yellow-fluorescent Zn and Pb spots under u l t r a v i o l e t l i g h t (366-254 nm), but did not v i s u a l i z e Cu. B I 1 Pb Zn Cu Pb Zn Cu Pb Zn Cu 2 2 0 cm PLATE I Photograph of a N a - m o n t m o r i l l o n i t e - s i l i c a g e l chroraatogram eluted w i t h 0.3 M NaCl s o l u t i o n and v i s u a l i z e d w i t h 0.5% s-diphenylcarbazone ( i n e t h a n o l ) . T y p i c a l p u r p le, red and brown spot colours are seen f o r Pb, Zn and Cu, those i n the right-hand e l u t i o n channels having faded some on d r y i n g . S i n g l e spots are observed f o r Cu and Zn, but f o r Pb two spots (A and B) are vaguely seen. Ret a r d a t i o n f a c t o r s f o r Cu and Zn are approximately equal, about 0.6, and f o r Pb-A and Pb-S spots about 0 . 7 and 0.4 r e s p e c t i v e l y . Eluent f r o n t s and base-l i n e are retouched. 1 Blank 2 Cu spot was i n a d v e r t e n t l y ''flooded" during a p p l i c a t i o n . Eluent f r o n t reached the p l a t e top i n t h i s channel because of t h i c k e n i n g of t h i n l a y e r near the p l a t e edge. f 1 0 6 Retardation factors for Cu, Zn and Pb spots are l i s t e d i n Tables XV to XVIII ( i n c l u s i v e ) under appropriate NaCI and NaN03 eluent concentrations. Conspicuous differences i n R^ values between the two sodium e l e c t r o l y t e s at equal concentrations are not seen for Cu (Table XV) or Zn (Table XVI). For Pb, however, several spot groups ( l a b e l l e d i n order of decreasing R^: A , B and C ) are recognized i n NaCl-eluted chromatograms (Table XVII), while only one group i s seen for NaN03 data (Table XVIII), These differences are i l l u s -trated i n Figure 36, drawings of s-diphenylcarbazone-visualized chromatograms which show A , B and C spots on a 0,5 M NaCl-eluted plate (top figure) and sin g l e Pb spots on a 0.5 M NaN0 3-eluted plate (bottom f i g u r e ) . At l e a s t two spot groups were observed for Pb on chromatograms eluted with NaCI solutions between 0.10 and 2.0 M concentration. D i s t i n c t i o n between Pb spots on these chromatograms was highly subjective because of poor r e s o l u t i o n and c a p r i c i o u s -ness of spot colours, as i s evident i n Plate I where A and B spots vaguely appear as l o c a l colour maxima at retardation factors of about 0.7 and 0.4 r e s p e c t i v e l y . The existence of multiple Pb spots must be questioned i n view of present u n c e r t a i n t i e s i n data. Indeed, attempts to duplicate r e s u l t s at . 0.5 M NaCI eluent concentration f a i l e d to y i e l d chromatograms with unambi-guous spot separations, and i n some instances led to chromatograms which appeared to have only one Pb spot group (si m i l a r to NaN0 3-eluted p l a t e s ) . P lots of retardation factors for Cu, Zn and Pb spots are shown against logarithm of NaCI or NaN03 eluent concentration i n Figures 37 to 42. Also p l o t t e d are t h e o r e t i c a l curves generated by equation (14), using a cation exchange capacity of 95 meq/100 g and a r a t i o f ~ = 0,067 1 w Table XV. Retardation f a c t o r s f o r Cu spot centres a f t e r e l u t i o n across Na-montmorillonite-silica gel t h i n layers by aqueous NaCI and NaN03 solutions of varying concentration. NaCI Concentration 0.05 M 0.10 M 0.50 M 1.00 M 2.00 M 3.00 M r0.170 r0.349 r0.627 L0.705 r0.891 r0.875 0.186 LO.349 0,629 r0.736 0.887 0.874 u0.158 r0.347 J3.640 0.747 t0.892 L0.873 0.315 LO.758 •0.302 r0.685 r0.408 0.685 0.385 10.680 LO.408 NaNO3 Concentration 0.05 M 0.10 M 0.50 M 1.00 M 3.00 M r0.202 r0.325 r0.497 r0.724 TJ.756 0.175 0.263 0.599 0.775 0.831 L0.164 LO.290 L0.630 L0.756 ,0.828 r0.658 r0.720 0.597 0.729 L0.570 L0.725 r evicZo&e. data {nom the i>ame plate. o -J Table XVI. Retardation f a c t o r s for Zn spot centres a f t e r e l u t i o n across N a - m o n t m o r i l l o n i t e - s i l i c a gel t h i n layers by aqueous NaCl and NaN03 solutions of varying concentration. NaCl Concentration 0.05 M 0.10 M 0.50 M 1.00 M 2.00 M 3.00 M r0.186 r0.318 r0.613 L0.707 r0.897 r0.917 0.179 L0.314 0.642 r0.744 0.907 0.903 iD. 171 r0.359 L0.652 0.725 u0.905 L0.908 0.393 L0.723 L0.383 r0.679 0.664 L0.689 NaN03 Concentration 0.05 M 0.10 M 0.50 M 1.00 M 3.00 M r0.171 r0.322 r0.593 r0.760 TJ.815 0.165 0.274 0.595 0.789 0.835 L0.173 L0.390 L0.640 .0.750 L0.835 r0.634 r0.771 0.567 0.796 L0.585 0.714 [ mcJLo&e. data, ^nom the. i>ame. plate. o CO Table XVII. Retardation f a c t o r s f o r Pb spot centres a f t e r e l u t i o n across Na-montmorillonite-silica gel t h i n layers by aqueous NaCI s o l u t i o n of varying concentration. Inferred spot groups are indicated by l e t t e r s A , 8 and C. NaCI Concentration 0 . 0 5 M 0 . 1 0 M 0 . 5 0 M 1 . 0 0 M 2 . 0 0 M 3 . 0 0 M r 0 . 1 7 0 1 0 . 1 7 1 A L0.159 J <L0.484 1 i r 0 . 4 4 6 A 0 . 4 3 8 L0.376 3 f 0 . 3 6 8 0 . 3 7 3 LO . 403 " 0 . 1 6 4 I L 0 . 1 9 4 f r 0 . 8 4 4 ] 0 . 8 4 4 A 0 . 8 9 6 J 0 . 4 7 9 ] 0 . 5 2 4 8 0 . 4 8 1 J 0 . 1 5 0 1 0 . 2 0 1 C L0.182 J , L 0 .726 ] I R 0 . 7 2 9 8 0 . 7 6 5 iD. 737 " 0 . 6 8 2 0 . 6 5 1 L0.713 " b . 6 8 1 ] 0 . 6 5 9 1 .0 .539 J ' [0.436 ' " 0 . 3 6 4 0 . 5 0 6 •0.461 3 r 0 . 3 8 8 0 . 3 3 3 p . 270 '"0.852 ) 0 . 8 8 7 B 0 . 8 6 0 J L0 .487 C R 0 . 9 1 0 ] 0 . 8 9 6 B L O . 9 3 2 J L enclose data, {/torn the home plate Table XVIII. Retardation f a c t o r s f o r Pb spot centres a f t e r e l u t i o n across Na-montmorillonite-silica gel t h i n layers by aqueous NaNC-3 solutions of varying concentration. NaNn3 Concentration 0.05 M 0,10 M 0.50 M 1.00 M 3.00 M r0.185 r0.283 r0.642 r0.775 r0.849 0.175 0.290 0.672 0.783 0.864 10.150 l_0.319 i0.625 L0.762 L0.854 r0.637 r0.793 0.616 0.799 L0.621 L0.758 [ enclose data, {n.om the, £>ame plate 111 r . i -0 a o c -j • • • • • • • • • + s o o CN Cu Zn Pb Cu Zn Pb Cu Zn Pb Figure 36. Thin layer chromatograms showing posit i o n s of Cu, Zn and Pb spots a f t e r e l u t i o n with 0.5 M NaCl (top) and 0.5 M NaN03 (bottom) so l u t i o n s . Plates were v i s u a l i z e d with 0.5% ethanolic solutions of s-diphenylcarbazone. 1 1 2 which was found by weighing wet (water-eluted) and dry Na-montmorillonite ( - s i l i c a gel) t h i n l a y e r s . (Note that T ' i s the weight of Na-montmoril-l o n i t e , not Na-montmorillonite + s i l i c a gel.) A computer programme was written to c a l c u l a t e the sum of squared di f f e r e n c e s between observed and t h e o r e t i c a l R^ values over a seri e s of eluent concentrations (0.05 to 3.0 M). Optimum values of "n" and were found which produced minimum r e s i -dual squares, and these used to generate the "best f i t " curves shown i n Figures 37 to 42. These values are approximate: ±0.1 for "n" and ±1 for K.^. In NaN03 eluents Cu, Zn and Pb retardation factors are not s i g n i f i -cantly d i f f e r e n t , as i s indicated by s i m i l a r "n" and values: 0.8 and 5 for Cu; 0.8 and 4 for Zn; and, 0.9 and 5 for Pb. In NaCI eluents Cu and Zn r e t a r d a t i o n f a c t o r s are not. s i g n i f i c a n t l y d i f f e r e n t from corresponding NaNO3 data, and hence "n" and values are the same for each metal. Pb data are markedly d i f f e r e n t than those for NaN03, however, and when plotted against NaCI eluent concentration (Figure 42) show continuity between A , 8 and C spot groups. Values of "n" and for group A and 8 are r e s p e c t i v e l y 1.6 and 1, and 1.2 and 7.5; i n s u f f i c i e n t data are a v a i l a b l e for group C for c a l c u l a t i o n of a t h e o r e t i c a l curve. In view of the considerable v a r i a t i o n of duplicate R^ values (average standard error = 6%), l i n e a r r e l a t i o n s h i p s between retardation factors and logarithms of eluent concentration might be proposed as a l t e r n a t i v e s to equation (14). NaCI data i n p a r t i c u l a r seem to be approximately c o l l i n e a r and might benefit by a s t r i c t l y empirical evaluation, at l e a s t i n the pre-sent concentration range. Interpretation of these data using equation ' (14) has the advantage of being generally applicable over an i n d e f i n i t e concen-t r a t i o n range, allowing for the asymptotic d e f l e c t i o n s toward R^ values of zero and unity which cannot be accommodated by l i n e a r theory. -2 Log10[NaH03f M] Figure 37. Plot of Cu spot retardation f a c t o r s against logarithm of NaN03 eluent (molar) concentration. A "best f i t " curve, generated by equation (14) using indicated K and "n" values, i s also shown. 4 Figure 38. Plot of Zn spot retardation factors against logarithm of NaN03 eluent (molar) concentration. A "best f i t " curve, generated by equation (14) using i n d i c a t e d and "n" values, i s also shown. • . ,-. •e-Log IO[NaN03, i ] Figure 39. Plot of Pb spot r e t a r d a t i o n factors against logarithm of NaN03 eluent (molar) concentration, A "best f i t " curve, generated by equation (14) using indicated and "n" values, i s also shown. Figure 41. Plot of Zn spot retardation factors against logarithm of NaCl eluent (molar) concentration. A "best f i t " curve, generated by equation (14) using i n d i c a t e d and "n" values, i s also shown. L c g i o [NaCl, i ' Figure 42. Plot of Pb spot retardation factors against logarithm of NaCl eluent (molar) concentration. "Best f i t " curves, generated by equation (14) using indicated and "n" values, are shown fo r spot groups A and B; an i n f e r r e d curve i s drawn through group C points. 4 1 1 9 S i g n i f i c a n t d i f f e r e n c e was found between variance i n duplicate values of the same chromatogram and duplicates between chromatograms. Stan-dard error (standard deviation expressed as percentage of mean) for the former averaged ± 3% for the three metals, compared to ±10% for the l a t t e r . P r e c i s i o n of intra-chromatogram duplicates r e f l e c t e d inhomogeneities within t h i n layers combined with errors i n measurement, the f i r s t r e s u l t i n g from random and systematic changes i n layer thickness ( p a r t i c u l a r l y near plate edges) and the second from s u b j e c t i v i t y i n estimating spot l o c a t i o n . The shapes and areas of metal spots were found to vary within chromatograms (see Figure 36), but were not considered major contributors to intra-chroma-togram PY^  v a r i a n c e — e x c e p t for NaCl-eluted Pb spots which were highly i n t e r -p r e t i v e . Poorer p r e c i s i o n of iriter-chromatogram duplicates resulted from batch v a r i a t i o n i n Na-montmorillonite-silica gel preparations, changes i n layer thickness and texture from plate to p l a t e , and v a r i a t i o n i n conditions of e l u t i o n . An approximate error of ± 10% was estimated i n the preparation of c l a y - s i l i c a s l u r r i e s , due p r i n c i p a l l y to v a r i a t i o n i n i n t i a l water content of Na-montmorillonite and s i l i c a gels, and to a l e s s e r extent readout errors i n weighing. Inconsistency i n spreading even, uniformly thick layers over glass plates was the most conspicuous evidence of preparative v a r i a t i o n , but i t s consequences i n R^ imprecision were impossible to estimate. (Since R^ i s a function, i n theory, of sorbant/eluent mass r a t i o s , not absolute mass, i t might be supposed that layer thickness v a r i a t i o n was inconsequential.) Conditions of e l u t i o n were found to be c r i t i c a l i n minimizing e l u t i o n periods and preventing stagnation or s t a l l i n g of the front during the l a s t stages of e l u t i o n . Insofar as i t was noted that long e l u t i o n periods produced more d i f f u s e arid ambiguous metal spots ( p a r t i c u l a r l y Pb), the e f f e c t s of incom-plete chamber saturation x^ith water vapour may be a s i g n i f i c a n t source of < 12 0 inter-chromatogram variance. Changes i n ambient temperature during e l u t i o n may have caused retro g r e s s i v e motion of the eluent front at the f i n a l stages of e l u t i o n (which were retarded by an opposing hydrostatic gradient down the p l a t e ) . Contamination of t h i n layer plates and eluents l i k e l y occured over the period of experimentation, though i t s possible e f f e c t s were not formally estimated. Absorption of carbon dioxide, ammonia or organic gases by a i r -dried plates was l i k e l y , since they were exposed to the atmosphere of an organic geochemistry laboratory for several days (occasionally weeks). Contamination of eluents was l e s s l i k e l y , but the e f f e c t s of traces of a c i d or base might have been s i g n i f i c a n t i n modifying the near-neutral pH of eluent solutions (pH 5.7 to 6.7). 1 2 1 Interpretation and Conclusions Retardation factors for Cu and Zn i n both NaNC>3 and NaCI eluent sys-tems indi c a t e sorption of metals as mono-charged species, with exchange constants s i m i l a r to those estimated by s t a t i c measurement of exchange equi-l i b r i a (Chapter I I ) . E s s e n t i a l l y i d e n t i c a l "n" and values for both metals i n both eluent systems suggest an adsorption mechanism common to both Cu and Zn, and r e l a t i v e l y i n s e n s i t i v e to ligand type. + In c h l o r i d e systems i t would be natural to suppose that CuCl and ZnCl were the dominant sorbed metal species, a supposition which might be supported by c a l c u l a t i o n of Bjerrum's mean ligand coordination number for Cu and Zn i n c h l o r i d e solutions i n the concentration range under considera-t i o n (0,05 to 3.0 M). Using formation ( s t a b i l i t y ) constants from S i l l e n and M a r t e l l (1964), the average ch l o r i d e ligand coordination number for Cu i n a s o l u t i o n of free c h l o r i d e concentration [Cl ] i s , according to equation (15) ( 1 0 0 9 8 ) [ c r ] + ( 2 ) ( 1 0 ° - 6 9 ) [ c r ] 2 + ( 3 ) ( 1 0 ° - 5 5 ) [ c r ] 3 + (4)UO°-00)[C1T n = = — j  i + a o ° - 9 8 ) [ c r ] + ( i o ° - 6 9 ) [ c r j 2 + d o 0 - 5 5 ) [ c r ] 3 + ( i o o o ° ) [ c r ] 4 Here consecutive (over-all) metal-ligand complex formation constants are represented as base-lO exponents and r e f e r to reactions of the form C u 2 + + i C l " = CuCl2""' [CuCl 2"'] Ku - 7 i [Cu2+][C1-] (Note that Ku i s equivalent to 3^  of S i l l e n and Martell.) Higher order complexes with " i " greater than four are not considered, neither are pos-12 2 s i b l e polynuclear complexes such as CU2CI4, CU2CI 2 etc. Calculations for free chloride concentrations of 0.05, 0.10, 0.50, 1.0, 2.0 and 3.0 M (note the approximation: Cj. - [ci ]) y i e l d corresponding n values of 0.34, 0.53, 1.17, 1.70, 2.47 and 2.91. By equation (16), these lead to "n" values of 1.66, 1.47, 0.83, 0.30, -0.47 and -0.91, i n d i c a t i n g that the chromatograph-i c a l l y derived value for "n" of 0.8 would be expected i n a NaCl eluent sys-tem of about 0.5 M concentration. Negatively charged species would be ex-pected to dominate above 1.0 M concentration according to Bjerrum's method. 2 + Similar c a l c u l a t i o n s for Zn i n ch l o r i d e solutions, using again data from S i l l e n and M a r t e l l (1964; Ka 1 = 1 0 0 7 2 , K12 = l O 0 - 4 9 , /<j 3 = 10 0 1 9 and Klk = l O 0 " 1 8 ) , produced n values of 0.22, 0.38, 1.05, 1.69, 2.77 and 3.33 for the same free c h l o r i d e concentrations used i n Cu c a l c u l a t i o n s . Corresponding "n" values f o r Z n 2 + of 1.78, 1.62, 0.95, 0.31, -0.77 and -1.33 show again agreement of chromatographically derived "n" (0.9) and equation (15)-derived "n" (0.95) at about 0.5 M NaCl concentration. It i s i n t e r e s t i n g to note that Zn-chloride mean coordination numbers are i n i t i a l l y l e s s than those for Cu at low concentration of [ci ], but increase more r a p i d l y with con-centration and eventually surpass Cu-chloride mean coordination numbers at high [Cl~] (> 1.0 M). Two problems a r i s e i n i n t e r p r e t i n g chromatographic "n" values as a re s u l t of sorption of Cu and Zn chloride complexes: the f i r s t i s that one would expect R^ values to vary greatly with NaCl eluent concentration be-cause of changes i n n, e s p e c i a l l y for Zn; and the second i s that a si m i l a r i n t e r p r e t a t i o n cannot be applied to NaN03 eluent systems, since n i t r a t e -metal complexes are not formed with Cu and Zn i n aqueous s o l u t i o n (exclud-ing ion p a i r s ) . An a l t e r n a t i v e approach, equally applicable to both NaCl and NaN03 systems, would be the sorption of metals as hydroxo-complexes. 12 3 By using formation constants from Sucha and Kotrly (1972), the average OH ligand coordination number f or Zn at free hydroxide concentration [OH ] can be calculated as (10 4 4) [OH"] + ( 2 ) ( 1 0 1 ( U ) [ 0 H ~ ] 2 + (3) (lO 1"' 2) [0H~] 3 + (4) (10 1 5 5 ) [0H~] 4 n = • 1 + (10 4 4) [0H _] + ( 1 0 1 I U ) [ 0 H ] 2 + Q0 1 4- 2)[OH~] 3 + ( 1 0 1 5 5 ) [0H~] 4 At [OH"] of 10" 8, 10~ 7, 10~ 6, 10~ 5 and 10~ 4 M the corresponding values of n are thus calculated as 0.0003, 0.003, 0.05, 1.22 and 2.54. If one i s pre-pared to accept the concept of highly di s s o c i a t e d water near the montmoril-l o n i t e surface with K w - 10 1 0 instead of 10 1 4 ( i . e . pH and pfoil] about two decades lower than free l i q u i d water n e u t r a l i t y at 7), then the mean ligand coordination number f or 10 5 M free hydroxide concentration (1.22) might be expected f o r Zn sorbed onto (or c l o s e l y associated with) the clay surface. This i s compatible with chromatographic "n" values of about 0.8 or 0.9. Concentration of free metal i n s o l u t i o n i s important i n considerations of hydroxo-complex formation because of p r e c i p i t a t i o n of metal hydroxides; i t 2 + i s also important i n cupric solutions because Cu tends to form polynuclear 2 + complexes, I U . Z . Cu2(0H )2 , instead of simple step-wise mononuclear complexes c h a r a c t e r i s t i c of Z n 2 + (Sucha and Kotrly, 1972). P r e c i p i t a t i o n of Zn(0H) 2 does not diminish the usefulness of Bjerrum's mean ligand coordination number concept, since H i s concerned only with s o l u t i o n species and i s a function of free ligand concentration only. In t h i s respect i t should be noted that K\2 of equation (15) i s not i d e n t i c a l with K , the s o l u b i l i t y constant, since s o the former involves a term for dissolved Zn(0H)2 concentration not the s o l i d phase. Thus p r e c i p i t a t i o n w i l l a f f e c t t o t a l Zn concentration, but i t w i l l not a l t e r the d i s t r i b u t i o n of^omplexed species remaining i n so l u t i o n at a given value of [OH ]. For Cu, however, i t i s impossible to make t h i s s i m p l i -1 2 4 f i c a t i o n because, for mixed polynuclear complexes, terms for Cu concen-t r a t i o n must appear i n the Bjerrum equation; these cannot be factored out of the summation as they are for homonuclear s e r i e s . Hence e q u i l i b r i a i n cupric solutions are s e n s i t i v e to p r e c i p i t a t i o n of Cu(0H) 2, insofar as free Cu concentration i s changed. Sorption of hydroxo-complexes of Zn and (more co n j e c t u r a l l y ) Cu might seem a reasonable a l t e r n a t i v e to chloride complex sorption, and may explain low "n" values for n i t r a t e eluent systems. The question n a t u r a l l y a r i s e s whether OH can be expected to dominate C l as the p r i n c i p a l metal-coordina-ted ligand at the clay surface. This may be answered, i n part, by consider-ing the r a t i o of t o t a l Zn-coordinated OH to t o t a l Zn-coordinated C l ZOH (lO1'-'*) [0H~] + (2) ( 1 0 1 0 1 ) [0H~] 2 + (3) (10 1 4 2 ) [0H~] 3 4- (4) (10 1 5 5 ) [0H~] 4 ZC1 (10 0- 7 2) [C1"J + (2)(10°- 4 9)[C1~] 2 + (3) CIO 0 1 9) [ C l ~ ] 3 + ( 4 ) ( 1 0 0 1 8 ) [ C 1 ~ ] 4 Substituting a value of 10 5 M for [OH ], r a t i o s corresponding to free chlo-r i d e concentrations of 0.05, 0.10, 0.50, 1.0, 2.0 and 3.0 M can be calculated as 11.65, 5.48, 0.63, 0.15, 0.02 and 0.005. Thus i t becomes evident that 0H~ i s the dominant Zn-coordinated ligand at free chloride concentrations le s s than about 0.50 M, obviously a far more e f f e c t i v e complexing agent than C l considering i t s much lower concentration. Data of Elgabaly and Jenny (1943) i l l u s t r a t e t h i s e f f i c a c y of OH . These authors found that Wyoming H-montmor-i l l o n i t e , leached with 1 N Z n C l 2 s o l u t i o n and washed with 95% methanol, took 2 + up cations and anions i n the proportions 22.2 mmole Zn ,7.2 mmole C l , + 27.1 mmole OH and 1.4 mmole H . If i t can be assumed that a l l ch l o r i d e and hydroxide was sorbed complexed with Zn 2 , the r a t i o E0H/EC1 may be calculated as 3.76. According to present c a l c u l a t i o n s then, t h i s corresponds to a free 1 2 5 chloride concentration at the montmorillonite surface of between 0.10 and 0.50 M (at free hydroxide concentration 10 5 M). This does not ne c e s s a r i l y i n d i c a t e exclusion of C l from the clay m i c e l l e , since free chloride i n 1 N ZnCl2 can be expected to be much lower than t o t a l C l because of complexation with Zn 2 i n free s o l u t i o n . From Elgabaly and Jenny's data (op. c i t . ) i t i s also possible to i n f e r t o t a l , c h l o r i d e and hydroxide mean coordination num-bers for adsorbed Zn. That i s , 7.2 + 27.1 n {Cl+OH} - =1.55 22.2 _ 7 - 2 n {Cl} = = 0.32 22.2 n {OH} = =1.22 22.2 2 + where terms i n braces i d e n t i f y ligands coordinated to Zn . The net mean coordination number of 1.55 i s greater than the values 1.1 to 1.2 indicated by present chromatographic data; the chl o r i d e mean coordination number i s compatible with previous Zn~chloride complex mean coordination number ca l c u -l a t i o n s at approximately 0.10 M [ c l ] (cf. 0.38). Most s i g n i f i c a n t l y , how-ever, the hydroxide mean coordination number of 1.22 i s i d e n t i c a l with that c a l c u l a t e d f o r Zn-hydroxide complexes at 10 5 M (see p.123). This corres-pondence i s l i k e l y c o i n c i d e n t a l , but i t does lend support to an i n t e r p r e t a -t i o n of exclusive hydroxo-complex sorption at the clay surface, t h i s provided one can assign sorbed C l to non-metal associated mechanisms. Observations of Cu adsorption on montmorillonite by Menzel and Jackson (1950) and Steger (1973) i n d i c a t e s i g n i f i c a n t f r a c t i o n s of sorbed + Cu(OH) (at s p e c i f i c , "excess sorption" s i t e s i n Steger's case), but t h e i r « 126 data are presented i n modified forms and cannot be used f o r c a l c u l a t i o n of Cu mean coordination numbers. Retardation f a c t o r s f o r Pb i n NaN03 eluents are not s i g n i f i c a n t l y d i f f e r e n t from those of Cu and Zn, and hence lead to K and "n" values (4 -6 and 0.9) which are s u b s t a n t i a l l y the same. It i s possible to ca l c u l a t e ex-2 + pected mean coordination numbers f o r Pb at any given f r e e n i t r a t e concen-t r a t i o n using formation constant data from S i l l e n and M a r t e l l (op. c i t . ) , i . e . (10 O A 8 ) [ N 0 7 ] + (2)(10 0- 5 2) [NO!] 2 + (3) (lO 0' 0 8) [NO!] 3 + (4)(10"°-61t)[NO;]'( i + do 0 4 8) [NO;] + (IO°- 5 2)[NO7] 2 + (io 0  8 ) [NO!]3 + do"0-6") [NO;]" At [NOl] values of 0.05, 0.10, 0.50, 1.0, 2.0 and 3.0 M, values f o r n can be calculated r e s p e c t i v e l y as 0.14, 0.28, 1.05, 1.62, 2.27 and 2.64. These are si m i l a r to chloride-complex mean coordination numbers for Cu and (especially) Zn. Similar c a l c u l a t i o n s for hydroxo-complex mean coordination numbers of Pb 2 (data from S i l l e n and M a r t e l l , 1964) according to the r e l a t i o n (10 7- 8 2)[OR~] + (2)(10 1 0- 8 8)[OH"] 2 + (3)(10 1 3- 9 , t) [0H~] 3  H 1 + CIO 7" 8 2) [OH"] + (10 1 0- 8 8)[OH"] 2 + (10 1 3- 9 I t)[OH"] 3 y i e l d values of 0.40, 0.87, 0.99, 1,01 and 1.13 at free hydroxide concentra-tions of 10~ 8, 10~ 7, 10*~6, 10~ 5 and 10~ 4 M. These show that P b 2 + complexes 2+ — with hydroxyl with greater equipollency than does Zn . The n value for [OH ] = 10 5 M (1.01) i s i n reasonable agreement with that expected from chromatographic "n" ( i . e . 1.1). Moreover, the noticeably greater r e g u l a r i t y of Pb R^ data (Figure 39) as compared to Cu and Zn data (Figures 37 and 38) might be a t t r i b u t e d to the l e s s e r dependence of Pb-OH coordination numbers on [OH ] concentration. Thus external influences on m i c e l l a r hydroxyl would not have as great an influence on e f f e c t i v e Pb charge as they might for Zn. 1 2 7 Interpretation of Pb retardation f a c t o r s i n NaCI eluent systems i s d i f f i c u l t because of the probable presence of several spot groups. Calcula-t i o n of chloride-complex mean coordination numbers for Pb by the r e l a t i o n ( l O 1 - 6 0 ) ^ " ] + (2) (10 1' 7 8) [ C l ~ ] 2 + (3) (lO 1' 6 8) [Cl~] 3 + (4) (10 1' 3 8) [Cl~]k n i + (ioL 6 0)[cr] + d o L 7 8 ) [ c r ] 2 + cio1-68) [ci~j3 + d o L 3 8 ) [ c r p (data from S i l l e n and M a r t e l l , 1964) y i e l d numbers for free c h l o r i d e concen-t r a t i o n s of 0.05, 0.10, 0.50, 1.0, 2.0 and 3.0 M of, r e s p e c t i v e l y , 0.73, 0.95, 1.70, 2.31, 2.98 and 3.30. These are s i m i l a r to those for Zn-chloride com-plexes. The r a t i o of Pb-coordinated OH to Pb-coordinated C l can be found from EOH (10 7- 8 2)[OH"] + (2)(10 1 0- 8 8) [OH"] 2 + (3) (10 1 3' 9 4) [0H~] 3 EC1 ( l O 1 - 6 0 ) ^ " ] + (2) (10 1- 7 8) [ci~]2 + ( 3 ) ( 1 0 L 6 8 ) [ci~]3 + ( 4 ) ( 1 0 L 3 8 ) [ c r ] ' t which, s u b s t i t u t i n g 10~ 5 M for [0H _] and 0.05, 0.10, 0.50, 1.0, 2.0 and 3.0 M for [ C l ~ ] , gives values of 293, 127, 9.12, 1.69, 0.21 and 0.053 r e s p e c t i v e l y . These r a t i o s i l l u s t r a t e gross predominance of OH as the Pb-coordinated l i -gand at free c h l o r i d e concentrations less than about 0,5 M. It would be impossible, i n a chromatographic system at equilibrium, to separate Pb-complex species (e.g. P b C l + , Pb0H +, PbCl", PbOHj., etc.) and thereby produce i n d i v i d u a l spots on the same chromatogram; t h i s follows from a consideration of equilibrium, which states that a l l species must be present i n eluent at equilibrium with the sorbant. A more probable explanation for multiple spots might be found i n the consideration of p r e c i p i t a t i o n of Pb chloride and hydroxy-chloride. S o l u b i l i t y products ( S i l l e n and M a r t e l l , 1964) for P b C l 2 , PbClOH and PbClo.slOHh.s are re s p e c t i v e l y l O - 4 7 6 , 10"13'7 and 10" 1 7. « 1 2 8 At free hydroxide concentration of 10 7 5 M (corresponding to the free eluent s o l u t i o n pH of ^ 6.5) and free chloride concentration of 0.05 M, the l i m i t i n g concentrations of Pb with respect to the three s o l i d phases would be 7.0 x 10~ 3, 1.3 x 10~ 5 and 8.0 x 10~ 6 M. I n t e r e s t i n g l y , these are much lower than 2 + the l i m i t i n g Pb concentration of 0.50 M expected at equilibrium with s o l i d Pb(0H) 2 (K = l O - 1 5 ' 3 ; S i l l e n and M a r t e l l , 1964). F i r s t - o r d e r c a l c u l a t i o n s show that about 90% of t o t a l Pb spotted onto plates as 0.1 M Pb(N0 3) 2 solu-t i o n would remain i n a desorbed state (calculated on a basis of a l l exchange s i t e s f i l l e d with PbOH ). Hence, at beginning of e l u t i o n with 0,05 M NaCl 2 + solu t i o n , Pb would l i k e l y have been p r e c i p i t a t e d as a hydroxy-chloride. Q u a l i t a t i v e l y , one might expect PbCl0.sfc)H)i.5-type p r e c i p i t a t e s at r e l a t i v e l y low free chloride concentration and PbClOH or PbCl 2 at higher concentration. It i s therefore proposed that multiple Pb spots are the r e s u l t of two " r e t a r -dation" processes: the f i r s t , responsible for t r a i l i n g spots, r e s u l t i n g from p r e c i p i t a t i o n of a Pb hydroxy-chloride; and the second, responsible f o r lead-ing spots, r e s u l t i n g from migration of r e s i d u a l , dissolved Pb ( i n i t i a l l y at equilibrium with the p r e c i p i t a t e ) . Two Pb spots, or possibly a highly d i s -tended s i n g l e spot, might be predicted with t h i s mechanism. The three spot groups A, 8 and C might be explained i n part by assigning B to " f r e e " Pb, moving predominantly as mono-charged complexes and being retarded by cation exchange reactions, and C to a precipitate-retarded form of Pb. This model cannot explain the presence of A spots. A somewhat more complex model might be constructed on the assumption that d i s c r e t e Pb hydroxy-chloride p r e c i p i t a t e s are formed i n eluent systems possessing pH or [ci ] gradients. One might envisage a step-wise p r e c i p i t a -t i o n mechanism whereby, i n eluents of moderate ch l o r i d e concentration (cf. 0.50 M), an i n i t i a l p r e c i p i t a t e such as PbClOH would be formed, r e l e a s i n g 129 Pb remaining i n s o l u t i o n to migrate across the plate with eluent s o l u t i o n . If one assumes a p o s i t i v e pH gradient (or negative [Cl ] gradient) toward the eluent f r o n t , then i t i s conceivable that a second p r e c i p i t a t e such as PbClo.sCOHji.s could form and produce an intermediate spot, Pb remaining i n solu-t i o n at equilibrium with t h i s second p r e c i p i t a t e moving on and forming the leading spot. A somewhat modified p r e c i p i t a t i o n mechanism might be incorporated into a general model for the sorption of Cu, Zn and " f r e e " Pb i n NaCI and NaN03 eluent systems. The c e n t r a l premise of t h i s model would be the con-cept of mobile "micellar s o l u t i o n " (Figure 43), a d i f f u s e zone (greater than three water layers thick) surrounding montmorillonite p a r t i c l e s and charac-t e r i z e d by anomalously disso c i a t e d water molecules (i.e., K > 10 A gradient i n the hydrolysis constant of water would e x i s t across the m i c e l l a r s o l u t i o n , s t a r t i n g at about 10 8 at the clay surface and decreasing to the normal free s o l u t i o n value at the m i c e l l e boundary. Three zones within the m i c e l l a r s o l u t i o n might be recognized (see Figure 43): the f i r s t ( I ) , im-mediately adjacent to the clay surface, would consist of a discontinuous and ephemeral sheet polymer of metal hydroxide interspersed with clay-coordinated Na ions; the second ( I I ) , l y i n g above the hydroxide sheet, would be made up + of mobile cations Na and mono-hydroxo complexes of metal cations, plus protons, hydroxyl and water; the t h i r d ( I I I ) , t r a n s i t i o n a l between II and + free s o l u t i o n , would consist dominantly of Na , anions and occasional metal-ligand complexes (u-tz. MeZ+) . The hydroxide layer would be formed i n i t i a l l y (for example during the spotting of Na-montmorillonite-silica g e l plates) by an exchange reaction involving two steps, i . e . , M e 2 + + 2 H2O -> Me (OH) 2 + 2 H+ (17) I FREE SOLUTION III Na + Z" Na + Z" Na + Z~ Na + Z" Na + Z v -o» H o H v H * y A v v x "°H Na + H + MeOH Na + MeOH+ 5 H H H H \ / \ / \ / „ „ \ / Me2+ Me2+ Me2+ «0 0? Me2* *!o o!! t+ / \ / \ / \ H * H 7 \ H H H H__ _ H H Na Figure 43. Diagrammatic representation of "micellar s o l u t i o n " developed between a montmorillonite surface (shaded) and free s o l u t i o n . Three zones are indic a t e d (I, II and I I I ) , the f i r s t characterized by a discontinuous metal_hydroxid^ sheet, the second by mobile metal hydroxo-complexes,.and the t h i r d by ligands (Z ) and Na derived from f r e e s o l u t i o n . 4 1 3 1 and, 2 H + + 2 Na{Mottt} ^ 2 H{Mon£} + 2 Na + (18 ) Hence protons generated by the nucleation of metal hydroxide would be the ac t i v e agents i n the Na-exchange reaction, migrating d i r e c t l y or by proton transfer mechanisms to negative charges on the montmorillonite surface (or possibly into the l a t t i c e i t s e l f ) . The metal hydroxide sheet might be f o r -mally compared with the b r u c i t e layer of c h l o r i t e or, more properly, with br u c i t e " i s l a n d s " i n montmorillonite-chlorite intergraded clays (MacEwan et a l . , 1961), but i t must be more l a b i l e than i t s Mg(OH)2 analogue and able to " d i s s o l v e " by a r e v e r s a l of r e a c t i o n (18) and by a s l i g h t l y altered reaction (17); i . e . , Me(0H)2 + H + ^ Me.0H+ + H2O (19) Species on the l e f t side of t h i s r e a c t i o n would occur i n zone I of the m i c e l l a r s o l u t i o n , while those on the r i g h t side would migrate into zone I I . By t h i s model the e l u t i o n process would e n t a i l exchange of protons + at the clay surface for Na from zone I I , followed by d i s s o l u t i o n of metal hydroxide and release of M2.0H to zone I I . Early observations of Jenny and Overstreet (1939) on the surface migration of ferrous i r o n across K, Na and H-montmorillonites i n absence of a free s o l u t i o n phase, and more recent e l e c -tron spin resonance data for Cu-montmorillonite (McBride, Pinnavaia and Mortland, 1973), i n d i c a t e that migration of metal hydroxo-complexes within zone II might be a v a l i d transport mechanism. Incorporation of t h i s mech-anism into' the model permits movement of metal across c l a y - s i l i c a gel t h i n 13 2 layers e f f e c t i v e l y as mono-charged species, and without interference of ' ligands present i n free s o l u t i o n (e.g., C l ). A m i c e l l a r s o l u t i o n would therefore have r e l a t i v e l y constant properties for any given solvent, sen-s i t i v e p r i m a r i l y to the a c t i v i t y of exchange cations i n free s o l u t i o n (e.g., Na ) — t h e s e capable of migrating through zone III to I I . Metal species must be postulated to migrate i n m i c e l l a r s o l u t i o n and not free s o l u t i o n , except during i n i t i a l stages of e l u t i o n when excess metal present i n s t a r t -ing spots might move with e l e c t r o l y t e u n t i l i t had access to m i c e l l a r zones (cf. spot enlargement phenomena i n t h i n layer and paper chromatography). Present chromatographic data do not unequivocally support a m i c e l l a r s o l u t i o n model, and considering the approximations leading to equation (14) and the neglect of other mass transfer mechanisms such as d i f f u s i o n one can only regard "micellar s o l u t i o n " as a working hypothesis. It must be con-ceded, however, that there are advantages i n s e t t i n g f o r t h a model which gives more c r e d i t to the i n t e r a c t i o n of solvent and metal cations than do simple ion-exchange mechanisms (which e s s e n t i a l l y ignore metal-solvent i n -t e r a c t i o n and view cation sorption as an e l e c t r o s t a t i c process). One might f i n d some p a r a l l e l i n the very early investigations of e l e c t r o l y t e s o l u t i o n chemistry ( c i r c a 1850-1900) wherein simple e l e c t r o s t a t i c models found ade-quate i n describing the behavior of s a l t solutions of a l k a l i and a l k a l i n e earth metals were found unsatisfactory when applied to those of t r a n s i t i o n metals. In view of the (then) remarkable conclusion that these metals inter-, act strongly with solvent molecules and ligands, often i n proportions d i f -ferent than predicted by t h e i r primary valence (an observation opening the doors of coordination chemistry), i t i s not s u r p r i s i n g that s i m i l a r behavior and conclusions might be encountered i n dealing with t r a n s i t i o n metal reac-tions i n the highly d i s s o c i a t e d aqueous environment near the montmorillonite 4 13 3 surface. The m i c e l l a r s o l u t i o n model predicts that t r a n s i t i o n metal cations near the clay surface are coordinated mainly to hydroxyl (at l e a s t i n aqueous systems), but some pr o v i s i o n must be made for l i m i t e d entry of free s o l u t i o n ligands into the m i c e l l e , p a r t i c u l a r l y at high e l e c t r o l y t e concentrations. This provision might explain the more e r r a t i c r e t a rdation factor data for Cu and Zn i n NaCl eluents (compared to NaN03 eluents) and might also be useful i n p a r t i a l l y r e c o n c i l i n g the anomalous behavior of Pb. I t should be stressed that formation of mixed hydroxo-chloride Pb mono-layers i n zone I of the mi-c e l l e i s not i d e n t i c a l with p r e c i p i t a t i o n of Pb hydroxy-chloride phases, these being separate from m i c e l l a r s o l u t i o n and far l e s s r e a c t i v e . M u l t i p l e Pb spots cannot be explained by the m i c e l l a r concept alone. Conclusions that might best be drawn from present work should not draw heavily from i n t e r p r e t a t i o n of r e s u l t s , since obviously t h i s has been developed around rather l i m i t e d data i n very simple systems. Rather they should emphasize the usefulness of chromatographic methods as tools for i n v e s t i g a t i n g i n t e r a c t i o n s of g e o l o g i c a l l y important solutions, solutes and sorbants. Data presented here ind i c a t e behavior that might not have been foreseen by considering these phases separately; i n p a r t i c u l a r , they point to the s i g n i f i c a n c e of water-clay i n t e r a c t i o n and i t s e f f e c t on sorbed met-a l s . S i m i l a r i t y of R^ values for.Cu and Zn i n both NaCl and NaN0 3 eluent systems suggests the r e l a t i v e unimportance of free s o l u t i o n ligands compared to hydroxyl near the montmorillonite surface. Expected hydroxo-complex mean coordination numbers calculated for [OH ] = 10 5 M and from e x i s t i n g forma-t i o n constant data are i n good agreement with mean coordination numbers indicated by apparent metal c a t i o n i c charges. A m i c e l l a r s o l u t i o n model i s an a t t r a c t i v e means for explaining pre-sent chromatographic data and may o f f e r some insig h t into natural reaction 4 1 34 mechanisms, both inorganic and organic. For example, formation of f e r r i c oxide coatings on argillaceous sediment and s o i l p a r t i c l e s might be explained by dehydration of m i c e l l a r " f e r r i c hydroxide" mono-layers, i r o n being f i r s t adsorbed as Fe (or as Fe and subsequently oxidized). F e r r i c hydroxide gels are well known for t h e i r propensity for evolving into mixed hydrous oxides on aging (Sucha and Kotrly, 1972; Krauskopf, 1967). Dehydration pro-cesses may also play a r o l e i n f i x a t i o n of other i n i t i a l l y adsorbed cations, e s p e c i a l l y tervalent metals and divalent metals such as Zn and Mn which tend to form strong oxygen bonds. Montmorillonite and other clay minerals are l i k e l y natural c a t a l y s t s i n a wide v a r i e t y of organic reactions, e s p e c i a l l y those involving hydrolysis or protonation. Montmorillonite has been shown to be e f f e c t i v e i n degrading s - t r i a z i n e s (commonly used i n herbicides) be-cause of a surface-catalyzed protonation r e a c t i o n . ( R u s s e l l et a l . , 1968). Hence clays may be important i n natural systems ( s o i l s , sediments and sus-pensions) not only because they sorb and immobilize a wide spectrum of or-ganic compounds, but also because they o f f e r a surface environment which i s highly r e a c t i v e and capable of promoting reactions which otherwise might be i n h i b i t e d . As an approach toward modelling natural systems and i n v e s t i g a t i n g processes which take place near the int e r f a c e s of natural s o l i d and l i q u i d materials, chromatography o f f e r s an a l t e r n a t i v e to t r a d i t i o n a l methods of a n a l y s i s . Mass transfer and chemical separation might be modelled p h y s i c a l l y with a chromatographic analogue, thereby circumventing d i f f i c u l t i e s encoun-tered with thermodynamic models—or l e a s t providing an experimental corrobo-rant i n s i t u a t i o n s where data are uncertain, An i l l u s t r a t i o n of the e f f i -cacy of chromatographic analogues i s found i n a recent p u b l i c a t i o n (Brown, 1974) which described Cd, Pb, Zn and Cu sulphide zoning i n s i l i c a gel ana-13 5 lagous to that found i n "White Pine" type base-metal ore bodies. The c l a y - s i l i c a gel thi n layer plate might therefore serve as a means of i n -ve s t i g a t i n g separation processes leading to rather p e c u l i a r metal assem-blages and zoning patterns observed i n sedimentary (low temperature) ore deposits. In p a r t i c u l a r , chromatographic analysis might shed some l i g h t on the exclusive a s s o c i a t i o n of Pb and Zn i n " M i s s i s s i p p i V a l l e y " type m i n e r a l i z a t i o n , perhaps using carbonate as a s o l i d phase. From the view-point of the organic geochemist the chromatographic analogue might be use f u l i n explaining migration and separation of petroleum and natural gases. And for those geochemists venturing across the inorganic-organic b a r r i e r some attempt might be made to model metal separation processes i n clay-organo systems. Perhaps t h i s leads to the most important conclusion: that chromatography o f f e r s a r e l a t i v e l y quick, dynamic and purely empirical approach toward understanding natural processes of mass transfer and chemi-c a l separation. Geologists and geochemists who occasionally f i n d themselves faced with puzzling assemblages, s t a t i c l e f t - o v e r s of a long since past chemical event, might best look to a c t i v e and e a s i l y modifiable chromato-graphic analogues than to the d e l i c a t e web of equilibrium c a l c u l a t i o n s — a t l e a s t i n multi-component systems where thermodynamic models have been un-successful or have not yet dared to tread. LIST OF REFERENCES Baes, C.F., J r . , and R.E. Mesmer, 1976, The Hydrolysis of Cations. John Wiley & Sons, New York, 489 p. Bailey, G.W., and S.W. Karickhoff, 1973, An u l t r a v i o l e t spectroscopic method for monitoring surface a c i d i t y of clay minerals under varying water content. Clays and Clay Minerals 21, 471-477. Bailey, G.W., and J.L. White, 1970, Factors inf l u e n c i n g the adsorption, desorption, and movement of p e s t i c i d e s i n s o i l . Residue Reviews 32, 29-92. Berner, R.A., 1971, P r i n c i p l e s of Chemical Sedimentology. McGraw-Hill Book Company, New York, 240 p. Bhoojedhur, S., 1975, Adsorption and heavy metal p a r t i t i o n i n g i n s o i l s and sediments of the Salmon River area. Ph.D. t h e s i s , The U n i v e r s i t y of B r i t i s h Columbia, 157 p. Bingham, F.T., Page, A.L., and J.R. Sims, 1964, Retention of Cu and Zn by H-montmorillonite. S o i l S c i . Soc. Amer. Proc. 28, 351-354. Bjerrum, J . , 1957, Metal Ammine Formation i n Aqueous Solution. P. Haase & Son, Copenhagen. Boyd, G.E., Schubert, J . , and A.W. Adamson, 1947, The exchange adsorption of ions from aqueous solutions by organic z e o l i t e s . I. Ion-exchange e q u i l i b r i a . Amer. Chem. Soc. J . 69, 2818-2829. Brown, A.C., 1974, An epigenetic o r i g i n for s t r a t i f o r m Cd-Pb-Zn s u l f i d e s i n the lower Nonesuch Shale, White Pine, Michigan. Economic Geology 69, 271-274. Brown, T.E., 1963, Cation exchange reactions on s i z e - f r a c t i o n e d montmoril-l o n i t e s . A d i s s e r t a t i o n . Ph.D. t h e s i s , The U n i v e r s i t y of Texas, 118 p. C a r r o l l , D., 1959, Ion exchange i n clays and other minerals. Geol. Soc. Amer. B u l l . 60, 27-99. i Cassidy, H.G., 1957, Fundamentals of Chromatography. Volume X of Technique of Organic Chemistry. Interscience Publishers, Inc., New York, 447 p. Clementz, D.M., Pinnavaia, T.J., and M.M. Mortland, 1973, Stereochemistry of hydrated copper (II) ions of the i n t e r l a m e l l a r surfaces of layer s i l i c a t e s . An electron spin study. J. Phys. Chem. 77, 196-200. Coleman, N.T., 1952, A thermochemical approach to the study of ion exchange. S o i l Science 74, 115-125. 1 3 7 » Coleman, N.T., and M.E. Harward, 1953, The heats of n e u t r a l i z a t i o n of acid clays and cation-exchange r e s i n s . Amer. Chem. Soc. J . 75, 6045-6046, DeMumbrum, L.E., and M.L. Jackson, 1956a, Copper and zinc exchange from d i l u t e n e u tral solutions by s o i l c o l l o i d a l e l e c t r o l y t e s . S o i l Science 8 1 , 353-357. DeMumbrum, L.E., and M.L. Jackson, 1956b, Infrared absorption evidence on exchange reaction mechanism of copper and zinc with layer s i l i c a t e clays and peat. S o i l S c i . Soc. Amer. Proc. 20, 334-337. Elgabaly, M.M., and H. Jenny, 1943, Cation and anion interchange with zinc montmorillonite clays. J . Phys. Chem. 47, 399-408. Faucher, J.A., Southworth, R.W., and H.C. Thomas, 1952, Adsorption studies on clay minerals. I. Chromatography on cla y s . J. Chem. Phys. 20, 157-160. Faucher, J.A., and H.C. Thomas, 1954, Adsorption studies on clay minerals, IV. The system montmorillonite-cesium-potassium. J. Chem. Phys. 22, 258-261. Flaschka, H., and H. Abdine, 1957, EDTA t i t r a t i o n s using Copper-PAN complex as i n d i c a t o r , i n The EDTA T i t r a t i o n . J.T. Baker Chemical Co., P h i l l i p s b u r g , New Jersey, 25-27. Gaines, G.L., J r . , and H.C. Thomas, 1953, Adsorption studies on clay minerals. I I . A formulation of the. thermodynamics of exchange adsorption. J . Chem, Phys. 21, 714-718. Gaines, G.L., J r . , and H.C. Thomas, 1955, Adsorption studies on clay minerals. V, Montmorillonite-cesium-strontium at several temperatures. J . Chem. Phys. 23, 2322-2326. Garrels, R.M., and C L . Ch r i s t , 1965, Solutions, Minerals and E q u i l i b r i a , Harper and Row, New York, 450 p. Grim, R.E., 1968, Clay Mineralogy. McGraw-Hill Book Company, New York, 596 p. Grim, R.E., and G. K u l b i c k i , 1961, Montmorillonite. High-temperature reac-tions and c l a s s i f i c a t i o n . American Mineralogist 46, 1329-1369. Hanshaw, B.B., and T.B. Coplen, 1973, U l t r a f i l t r a t i o n by a compacted clay membrane - I I , Sodium ion exclusion at various i o n i c strengths. Geochimica et Cosmochimica Acta 37, 2377-2327. Harned, H.S., and B.B. Owen, 1958, The P h y s i c a l Chemistry of E l e c t r o l y t e Solutions. Reinhold Publishing Corporation, New York, 803 p. Helsen, J.A., Drieskens, R.,' and J . Chaussidon, 1975, P o s i t i o n of exchange-able cations i n montmorillonite. Clays and Clay Minerals 23, 334-335. 4 13 8 Hendricks, S.B., and M.E. J e f f e r s o n , 1938, Structure of k a o l i n and t a l c -p y r o p h y l l i t e hydrates and t h e i r bearing on water sorption of c l a y s . American Mineralogist 23, 863-875. Hodgson, J.F., T i l l e r , K.G., and M. Fellows, 1964, The r o l e of hydrolysis i n the r e a c t i o n of heavy metals with soil-forming materials, S o i l S c i , Soc. Amer. Proc. 28, 42-46. Jackson, K.S., 1975, Geochemical d i s p e r s i o n of elements v i a organic com-plexing. Ph.D. t h e s i s , Carleton U n i v e r s i t y , 319 p. Jenny, H., 1932, Studies on the mechanism of i o n i c exchange i n c o l l o i d a l aluminum s i l i c a t e s . J . Phys. Chem. 36, 2217-2258. Jenny, H., 1936, Simple k i n e t i c theory of i o n i c exchange. I. Ions of equal valency. J. Phys. Chem. 40, 501-517. Jenny, H., and R.F. Reitemeier, 1935, Ionic exchange i n r e l a t i o n to the s t a b i l i t y of c o l l o i d a l systems. J. Phys. Chem. 39, 593-609, Jenny, H., and R. Overstreet, 1939, Surface migration of ions and contact exchange. J . Phys. Chem. 43, 1185-1195. Kerns, R.L., J r . , and C.J. Mankin, 1968, S t r u c t u r a l charge s i t e influence on the i n t e r l a y e r hydration of expandable three-sheet clay minerals. Clays and Clay Minerals 16, 73-81. Knechtel, M.M., and S.H. Patterson, 1962, Bentonite Deposits of the Northern Black H i l l s D i s t r i c t Wyoming, Montana, and South Dakota, United States Geological Survey B u l l e t i n 1082-M. Krauskopf, K.B., 1967, Introduction to Geochemistry. McGraw-Hill Book Company, New York, 721 p. Krishnamoorthy, C , Davis, L.E,, and R. Overstreet, 1948, Ionic exchange equations derived from s t a t i s t i c a l thermodynamics. Science 108, p. 439. Krishnamoorthy, C , and R. Overstreet, 1950, An experimental evaluation of ion-exchange r e l a t i o n s h i p s . S o i l Science 69, 41-53. Lahav, N. and S. Lavee, 1973, Chemiluminescence of luminol i n the presence of bentonite and other c l a y s . Clays and Clay Minerals 21, 257-259. Langmuir, I., 1918, The adsorption of gases on plane surfaces of glass, mica, and platinum. J, Amer. Chem. Soc. 40, 1361-1403. MacEwan, D.M.C., Ruiz, A.A., and G. Brown, 1961, I n t e r s t r a t i f i e d clay min-e r a l s . _in The X-ray I d e n t i f i c a t i o n and C r y s t a l Structure of Clay Min-e r a l s . G. Brown ed., M i n e r a l o g i c a l Society, London, 393-445. Maes, A., and A. Cremers, 1975, Cation-exchange hysteresis i n montmorillonite: A pH-dependent e f f e c t . S o i l Science 119, 198-202. 1 13 9 Marshall, C.E., 1948, The electrochemical properties of mineral membranes. VIII. The theory of s e l e c t i v e membrane behavior. J . Phys. & C o l l o i d Chem. 52, 1284-1295. Martin, A.J.P., and R.L.M. Synge, 1941, A new form of chromatogram employing two l i q u i d phases. I. A theory of chromatography. I I . A p p l i c a t i o n to the microdetermination of the higher mono-amino acids i n proteins. Biochem. J. 35, 1358-1368. McBride, M.B., Pinnavaia, T.J., and M.M. Mortland, 1975a, Electron spin resonance study of cation o r i e n t a t i o n i n r e s t r i c t e d water layers on p h y l l o s i l i c a t e s (smectite) surfaces. J . Phys. Chem. 79, 2430-2435. McBride, M.B., Pinnavaia, T.J., and M.M. Mortland, 1975b, Perturbation of s t r u c t u r a l Fe 3 i n smectites by exchange ions. Clays and Clay Minerals 23, 103-107. McBride, M.B., Pinnavaia, T.J., and M.M. Mortland, 1975c, Exchange ion p o s i -tions i n smectite: E f f e c t s on electron spin resonance of s t r u c t u r a l i r o n . Clays and Clay Minerals 23, 162-163, Menzel, R.G., and M.L. Jackson, 1950, Mechanism of sorption of hydroxy cupric ion by clays. S o i l S c i . Soc. Amer. Proc. 15, 122-124. Osthaus, B.B., 1955, I n t e r p r e t a t i o n cf chemical analyses of montmorillonites. Conf. Clays Clay Tech., 1st, 95-100. Peereboom, J.W. Copius, 1971, Paper Chromatography and Thin-Layer Chroma-tography. in_ Comprehensive A n a l y t i c a l Chemistry. Volume II C. C L . Wilson and D.W. Wilson eds., E l s e v i e r Publishing Company, Amsterdam, 1-129. Randerath, K. , 1968, Thin-Layer Chromatography. Academic Press, London, 285 p. Rechnitz, G.A., 1971, Ion-Selective Membrane Electrodes, an Audio Course of the American Chemical Society, Washington, D.C. Reddy, M.R., and H.F. Perkins, 1974, F i x a t i o n of zinc by clay minerals. S o i l S c i . Soc. Amer. Proc. 38, 229-230. R i t c h i e , A.S., 1964, Chromatography and Geology. E l s e v i e r Publishing Company, Amsterdam, 185 p. Robinson, R.A., and R.H. Stokes, 1959, E l e c t r o l y t e Solutions. Academic Press, London, 559 p. R u s s e l l , J.D., Cruz, M. , White, J.L., Bailey, G.W., Payne, W.R., J r . , Pope, J.D., J r . , and J . I . Teasley, 1968, Mode of chemical degradation of s-t r i a z i n e s by montmorillonite. Science 160, 1340-1342. H O Shainberg, I., Low, P.F., and U. K a f k a f i , 1974, Electrochemistry of sodium-montmorillonite suspensions: I ; Chemical s t a b i l i t y of montmorillonite. S o i l S c i . Soc. Amer. Proc. 38, 751-755. Sheldon, R.W., and T.R. Parsons, 1967, A p r a c t i c a l manual on the use of the Coulter Counter i n marine research. Coulter E l e c t r o n i c s Sales Company-Canada, Toronto, 66 p. S i l l e n , L.G., and A.E. M a r t e l l , 1964, S t a b i l i t y Constants of Metal-Ion Complexes. Special P u b l i c a t i o n No. 17, The Chemical Society, London, 754 p. Skoog, D.A., and D.M. West, 1971, P r i n c i p l e s of Instrumental Analysis, Holt, Rinehart and Winston, Inc., New York, 710 p. Slabaugh, W.H., 1952, The heat of n e u t r a l i z a t i o n of hydrogen bentonite, Amer, Chem. Soc. J . 74, 4462-4464. Slabaugh, W.H., 1954, Cation exchange properties of bentonite. J . Phys. Chem. 58, 162-165. Stahl, E., et a l . , 1965, Thin Layer Chromatography. A Laboratory Handbook. E. Stahl ed., Springer-Verlag, B e r l i n , 553 p. Steger, H.F., 1973, On the mechanism of the adsorption of trace copper by bentonite. Clays and Clay Minerals 21, 429-436. Steger, H.F., 1974, Sorption of trace q u a n t i t i e s of metal ions by bentonite clay from a t h i o s a l t medium. Can. Mining M e t a l l . B u l l . 67, 90-95. Sucha, L., and S. Kotrly, 1972, Solution E q u i l i b r i a i n A n a l y t i c a l Chemistry, Van Nostrand Reinhold Company, London, 371 p. Suk, V., and M. Malat, 1957, Pyrocatechol V i o l e t : Indicator for the EDTA t i t r a t i o n , i n The EDTA T i t r a t i o n . J.T. .Baker Chemical Co., P h i l l i p s b u r g , New Jersey, 21-25. Thompson, H.S., 1850, On the absorbent power of s o i l s . J . Roy, Agr. Soc. Engl. 11, 69-74. Vansant, E.F,, and J.B. Uytterhoeven, 1972, Thermodynamics of the exchange of n-alkylammonium ions on Na-montmorillonite. Clays and Clay Minerals 20, 47-54. Vanselow, A.P., 1932, The u t i l i z a t i o n of the base-exchange reaction for the determination of a c t i v i t y c o e f f i c i e n t s i n mixed e l e c t r o l y t e s . Amer. Chem, Soc. J. 54, 1307-1311. Way, J.T., 1850, On the power of s o i l s to absorb manure. J. Roy. Agr. Soc. Engl. 11, 313-379. Weaver, C.E., and L.D. P o l l a r d , 1973, The Chemistry of Clay Minerals. E l s e v i e r Publishing Company, Amsterdam, 213 p. 14 1 APPENDICES Methods of Standardization A l l aqueous solutions and suspensions were prepared with twice-d i s t i l l e d water. No attempt was made to remove carbon dioxide or other dissolved gases. Aqueous solutions were stored u s u a l l y i n polyethylene or polypro-pylene b o t t l e s , but glass containers were also occassionally used. No measurable contamination was found from storage of atomic absorption spec-trometry standards i n glass b o t t l e s . One molar s o l u t i o n standards of NaCI and KC1 were prepared g r a v i -m e t r i c a l l y , weighing out s a l t which had been dried to constant weigh at 110° C. A one molar Na(N0 3) 2 s o l u t i o n was standardized against NaCI s o l u -t i o n by atomic absorption spectrometry. Heavy metal solutions were standardized by ,EDTA (ethylenediamino-t e t r a c e t i c acid) t i t r i m e t r y . A s o l u t i o n of di-sodium EDTA was f i r s t stan-dardized against a primary Bi(N03 )2 s o l u t i o n standard; t h i s was prepared by d i s s o l v i n g an exact weight of a n a l y t i c a l grade bismuth metal i n concen-trated n i t r i c a c i d . Pyrocatechol V i o l e t (3,3',4'-trihydroxyfuchsone-2"-s u l f o n i c acid) provided a sharp and reproducible endpoint i n d i c a t i o n at pH 2 to 3, following the chelometric t i t r a t i o n method of Suk and Malat (1957). Pyrocatechol V i o l e t was also used as an i n d i c a t o r for EDTA t i t r a t i o n of Cu(N0 3) 2, CuCl 2 and Zn C l 2 solutions (Suk and Malat, 1957), while copper-PAN (l-(2-pyridyl-azo)-2 napthol) was used as an i n d i c a t o r f o r Pb(N0 3) 2 t i t r a t i o n s (after the method of Flaschka and Abdine, 1957). Metal concen-t r a t i o n s were determined as averages of four or more i n d i v i d u a l t i t r a t i o n s . 14 2 P r e c i s i o n was approximately 1,5%. Standards f o r atomic absorption spectrometry were made by d i l u t i o n of primary or secondary standard s o l u t i o n s . NaCI and Na(N03>2 s o l u t i o n standards were prepared to span a concentration range of 10 4 to 10 2 M. Sets of heavy metal and KC1 standards were prepared to span concentrations from 10~ 7 to 10~ 2 M. Mixed NaCI and KC1 atomic absorption spectrometry standards were prepared so that mutual de- i o n i z a t i o n enhancement of the two cations could be corrected. Approximate K and Na concentration of f i l t r a t e s was deter-mined using unmixed standards. Then, sets of mixed standards were made up with Na/K r a t i o s equal to those of t h i s determination. Two sets of standards were prepared for each f i l t r a t e , keeping K concentration constant and Na v a r i a b l e i n one set, and Na concentration constant and K v a r i a b l e i n another. Refined Na and K concentrations were then made using the mixed standards. Preparation of Na-saturated Montmorillonite Bentonite from Clay Spur, Wyoming was received i n i t s natural aggre-gated form. About 30 g of t h i s material was mixed with 300 ml d i s t i l l e d water i n a blender. Over a period of three hours a thick, homogenous slur- 4 ry was produced. This was immediately transferred to a 2.5 1 beaker and di l u t e d to 2.0 1 with d i s t i l l e d water. A uniform suspension was produced a f t e r magnetic s t i r r i n g for about two hours, at which point s t i r r i n g was discontinued f o r several minutes to allow coarse grained impurities to set-t l e out. Remaining suspension was transferred to twelve 250 ml polyethy-lene centrifuge b o t t l e s , then centrifuged two hours at 6000 RPM i n a So r v a l l SS-3 centrifuge. A zoned sediment of clay g e l and m i c r o c r y s t a l -l i n e a-quartz was found at the base of each b o t t l e and, a f t e r superna-tant f l u i d had been poured o f f , the opalescent clay g e l was removed from i t s s i l i c a core with a spatula. Clay g e l from s i x b o t t l e s was resuspen-ded i n 1.0 1 of 1.0 M NaCl s o l u t i o n by continuous s t i r r i n g for at l e a s t three hours. This suspension was then transferred to clean centrifuge b o t t l e s and again centrifuged f or two hours at 6000 RPM. Centrifugate brine was removed and replaced with an equal volume of fresh 1.0 M NaCl s o l u t i o n . Resuspension was achieved by w r i s t - a c t i o n shaking of b o t t l e s over a period of about 24 hours. When a l l clay was suspended, b o t t l e s were re-centrifuged and brine drawn o f f , t h i s time being replaced with, approximately 100 ml of d i s t i l l e d water. Bottles were shaken again u n t i l clay was suspended, then centrifuged two hours at 6000 RPM. This process was repeated through a t o t a l of s i x d i s t i l l e d water washes. Following f i n a l c e n t r i f u g a t i o n , clay gel was removed from b o t t l e s and spread across glass plates to a i r - d r y ; t h i s was aided with a warm a i r blower. Dry clay f i l m s were stripped from plates and stored i n loo s e l y covered b o t t l e s . Bentonite from the Newcastle Formation,. Wyoming was received i n a f i n e l y powdered form. This was more e a s i l y suspended than aggregated Clay Spur bentonite, but otherwise behaved s i m i l a r l y . Somewhat more coarse grained, non-clay material was found i n t h i s bentonite. Batch y i e l d s of Na-montmorillonite were therefore proportionately reduced. P a r t i c l e Size Characterization of Montmorillonite Suspensions P a r t i c l e s i z e a nalysis of a very d i l u t e suspension of natural Newcastle Formation montmorillonite was c a r r i e d out by S. Finora using a Coulter Counter (Electrozone Celloscope, P a r t i c l e Data, Inc.). This device 14 4 measures the e f f e c t i v e volume of a p a r t i c l e suspended i n e l e c t r o l y t e solu-t i o n (Calgon solution) by monitoring current f l u c t u a t i o n between a pair of electrodes, these separated i n so l u t i o n by a 20 to 100 ym diameter aperature. Passage of a suspended p a r t i c l e through the aperature causes a momentary de-crease i n current which i s proportional to the volume of e l e c t r o l y t e s o l u -t i o n d i s p l a c e d — t h a t i s , to e f f e c t i v e p a r t i c l e volume. Pulses are counted and classed e l e c t r o n i c a l l y , then data blended, smoothed and normalized by computer. A d e s c r i p t i o n of Coulter Counter operation and a p p l i c a t i o n i s provided by Sheldon and Parsons (1967). The instrument was f i r s t c a l i b r a t e d with standard latex p a r t i c l e s . Two aperatures, 60 and 19 ym, were used to c o l l e c t d a t a — t h e s e segregated into the 128 channels spanning the operation range of each aperature. Count-ing was stopped when a maximum of 4095 entries was reached i n any one channel. Data from both aperatures was blended, smoothed and normalized by a standard numerical method, then corrected to give d i s t r i b u t i o n s based on per cent t o t a l volume instead of number of counts (this analogous to using weight per cent i n sieve analysis instead of p a r t i c l e counts). In broad feature the montmorillonite p a r t i c l e s i z e d i s t r i b u t i o n described a broad, bell-shaped curve extending from about 2.5 ym do\m to the detection l i m i t of 0.7 ym, and centering over a mode at 1.2 ym. About 65% of t o t a l p a r t i c l e volume was associated with t h i s d i s t r i b u t i o n . Two a d d i t i o n a l peaks were superimposed on t h i s broad curve: a prominent sharp peak r i s i n g from the shoulder of the main d i s t r i b u t i o n at 2.0 ym; and a minor peak r i s i n g from the t a i l at 3,3 ym. In d e t a i l , then, the s i z e d i s t r i b u t i o n was tri-modal with an estimated 30% of t o t a l p a r t i c l e volume associated with the f i r s t ac-cessory peak and 5% with the second. An o v e r a l l mean for a l l data was 1.5 ym. Extrapolation indicated that 0.5 ym was a lower p a r t i c l e s i z e l i m i t . 1 145 Membrane F i l t r a t i o n of Clay Suspensions Montmorillonite was removed from suspension by pressure f i l t r a t i o n across 0.01 ym c e l l u l o s e n i t r a t e membranes mounted i n polycarbonate f i l -t r a t i o n apparatus (Sartorius ). Membrane f i l t r a t i o n had advantages of speed (in most cases) and s i m p l i c i t y over c e n t r i f u g a t i o n , and also avoided possible disturbance of exchange equilibrium by pressure and temperature changes incurred by the l a t t e r method. Time required f or f i l t r a t i o n under 2.1 kg/cm2 (30 psi) nitrogen gas pressure was oc c a s i o n a l l y quite long, up to ten hours for suspensions i n d i l u t e e l e c t r o l y t e s o l u t i o n . C o l l e c t i n g b o t t l e s were chosen to f i t snugly against f i l t e r spout to prevent evapora-t i o n . F i r s t attempts to f i l t e r 1% montmorillonite suspensions f a i l e d be-cause a leak-proof seal between membrane and f i l t e r - h o l d e r gasket was very d i f f i c u l t to achieve. This problem was eventually overcome by generously l u b r i c a t i n g gaskets with Dow-Corning s i l i c o n e grease and tightening holder assemblies c a r e f u l l y to avoid d i s t o r t i n g or rupturing d e l i c a t e f i l t e r mem-branes. During f i l t r a t i o n of clay suspensions, a montmorillonite gel ac-cumulated above the c e l l u l o s e membrane and retarded further f i l t r a t i o n . Residual, transluscent g e l plugs were most conspicuous at the end of d i -lu t e KCI:Na-montmorillonite suspension f i l t r a t i o n s , considerably thicker (at about 3 mm) than r e s i d u a l clay f i l m s l e f t a f t e r f i l t r a t i o n of mont-m o r i l l o n i t e : heavy metal s o l u t i o n suspensions (about .5 mm t h i c k ) . F i l t r a -t i o n times for a l l e l e c t r o l y t e solution:montmorillonite suspensions were found to decrease r a p i d l y with increasing i o n i c strength. D i l u t e solutions required hours for complete f i l t r a t i o n , while about ten minutes was required 146 at 10 2 M and l e s s than three minutes at 10 1 M. Thin r e s i d u a l montmoril-l o n i t e f i l m s were associated with rapid f i l t r a t i o n , i n d i c a t i n g that f i l -t r a t i o n rates were c o n t r o l l e d by clay f l o c c u l a t i o n . No s i g n i f i c a n t sorption or desorption of e l e c t r o l y t e of other con-taminants by f i l t r a t i o n apparatus was encountered. Polycarbonate, of which the e n t i r e apparatus (excepting gaskets) i s constructed, i s claimed by the manufacturer to be i n e r t to aqueous solutions at neutral or acid pH. Simi-l a r l y , no s i g n i f i c a n t ion-exchange capacity of c e l l u l o s e n i t r a t e membranes i s reported. Dow. s i l i c o n e grease i s e s s e n t i a l l y i n e r t i n most solvents and had l i m i t e d contact with suspensions and f i l t r a t e s . F i l t e r f a i l u r e was i n -frequent a f t e r a method for mounting and sealing membranes was established, but was immediately apparent by f i l t r a t e t u r b i d i t y . Test for Cation Exclusion During F i l t r a t i o n Since a montmorillonite g e l membrane was created above the f i l t e r membrane during f i l t r a t i o n , i t was considered possible that cation exclu-sion might occur such as has been reported i n compacted clay membranes at high pressures (Hanshaw and Coplen, 1973). A suspension of 0.500 g Na-montmorillonite i n 50 ml d i s t i l l e d water . was f i l t e r e d through a membrane at 2.5 kg/cm2 (35 psi) nitrogen gas pres-sure, producing a g e l disk (about 3 mm thick) above the f i l t e r . A 1.60 x 10~ 2 M NaCI s o l u t i o n was prepared and 250 ml transferred to the f i l t e r r e s e r v o i r . The ensuing f i l t r a t i o n of t h i s e l e c t r o l y t e s o l u t i o n at 2.5 kg/cm pressure was very slow, amounting to about one drop every three minutes at the beginning of f i l t r a t i o n and increasing s l i g h t l y thereafter. F i l t r a t e was c o l l e c t e d i n preweighed containers. After 5 to 20 ml of f i l t r a t e had 1 k 7 c o l l e c t e d , f i l t r a t i o n was temporarily stopped, the f i l t r a t e and container removed and weighed, and an empty container substituted. A 5.00 ml volume of e l e c t r o l y t e s o l u t i o n was also sampled from the f i l t e r r e s e r v o i r at t h i s time. This process was repeated s i x times u n t i l a cumulative volume of 72 ml had passed through the membrane. F i l t r a t e s and r e s e r v o i r samples were analysed for Na by atomic absorption spectrometry. Volumes and cumulate volumes were calculated from i n d i v i d u a l f i l t r a t e weights, and cumulate Na concentration calculated to give the expected Na concentration of a f i l t r a t e had i t been allowed to c o l l e c t i n one container. ". Results indicated that Na concentration increased i n f i l t r a t e s along a curve that would be expected i f o r i g i n a l water present i n clay g e l were displaced gradually by e l e c t r o l y t e s o l u t i o n . That i s , depletion of Na i n f i l t r a t e s was r e a d i l y explained by d i l u t i o n e f f e c t alone. Resevoir concen-t r a t i o n of Na was e s s e n t i a l l y constant within a n a l y t i c a l e r r o r . Hence ion exclusion was concluded to be i n s i g n i f i c a n t under experimental conditions. Na Exchange Capacity Determination Sodium exchange c a p a c i t i e s for 0.1 M solutions of HC1, KC1, ZnCl2, CuCl2, Cu(N03)2 and Pb(N03)2 were found by measuring t o t a l Na released from Na-montmorillonite treated with these e l e c t r o l y t e s . Between 0,1 and 0.2 gram of a i r - d r i e d Na-montmorillonite was weighed ® out into 200 ml Pyrex glass beakers. These beakers, and a l l other g l a s s -ware or plasticware used i n procedure, had been thoroughly been cleaned and rinsed several times with d i s t i l l e d water, being p a r t i c u l a r l y c a r e f u l to avoid contamination with body p e r s p i r a t i o n . Following ad d i t i o n of approx-imately 20'ml of 0.1 M e l e c t r o l y t e s o l u t i o n , beakers were covered with i 1 it 8 polyethylene f i l m and the clay suspended completely by continuous magnetic ® s t i r r i n g f o r three hours. Suspensions were then transferred to a Sartorius pressure f i l t r a t i o n apparatus and f i l t e r e d through a 0.01 ym c e l l u l o s e n i -tr a t e membrane under 2.1 kg/cm2 (30 psi) nitrogen gas pressure. F i l t r a t e was c o l l e c t e d i n dry, pre-weighed polyethylene b o t t l e s . A minimum of three separate 5 to 10 ml volumes of fresh e l e c t r o l y t e s o l u t i o n were added to o r i -g i n a l beakers to f l u s h out r e s i d u a l suspension and s o l u t i o n , then each ri n s e transferred to the f i l t r a t i o n apparatus and f i l t e r e d i n turn. A f t e r f i l -t r a t i o n of the f i n a l r i n s e c o l l e c t i n g b o t t l e s were t i g h t l y capped and weighed. A Techtron model AA4 single-beam atomic absorption spectrophoto-meter was used to determine molar sodium concentration against Na standards (NaCl or Na(N03)2) made up i n 0.1 M solutions of the replacing e l e c t r o l y t e . Good l i n e a r i t y between absorbance and Na concentration of standards was ob-tained using the 330.23-330.30 nm s p e c t r a l doublet of sodium. Density of o f i l t r a t e solutions was determined at 20.0 C with a 10.0 ml pycnometer; pH was found potentiometrically with a combined glass-Ag:AgCl electrode. Na exchange capacity was determined from the following r e l a t i o n : Wi C 10 2 C E C = - ^ r ~ ~ ( 2 0 ) Where CEC i s Na exchange capacity i n meq/100 g; Wj i s f i l t r a t e weight i n grams; W2 i s weight of a i r - d r i e d Na-montmorillonite; C i s molar Na concentration; and, p i s f i l t r a t e density i n g/ml. Error i n exchange capacity determination was estimated by using extreme estimates of measured quantities and calculating maximum and minimum va-lues of CEC by equation (20). A sample calculation for Na exchange capa-city of 0.1 M ZnCl 2 solution i s given below. Wi = 38.65384 ± 0.00010 g W2 = 0.21941 ± 0.00010 g C = (0.5509 ± 0.0066) x i o " 2 M p = 1.02901 ± 0.00022 g/ml 38.65384 (0.5509xl0~2) 102 CEC = — ( 1 < 0 2 9 0 1 ) (o.2l941) = 9 4 ' 3 * U 2 m e q / 1 ° ° g (Error limits for measured quantities represent estimated readout error.) It must be noted that an implicit equivalence betv^een Na molar concentra-tion and Na equivalent concentration has been assumed. Of course, an ap-propriate factor must be applied to equation (20) i f displacement of multi-ply charged cations is being considered. Effect of Ambient Humidity on Weight of Air-Dried Montmorillonite Since exchange capacity measurements, and a l l other calculations involving Na-montmorillonite weight, are referenced to air-dried clay, an attempt was made to evaluate the effect of changes in ambient humidity on clay weight. Figure 43 shows covariation of the weight of a sample of natural Clay Spur montmorillonite with ambient relative humidity. This sample (approximately 0.10 g) was stored in a loosely covered container and weighed repetitively over a period of nine days (July 9 to 18, 1975). 70 Clay weight Relative humidity JULY 1974 Figure 44. Covariation of montmorillonite weight and ambient r e l a t i v e humidity with time. 1 51 At each weighing ambient r e l a t i v e humidity . (laboratory humidity) was found with a rotatable wet and dry bulb thermometer. Relative humidity was found to deviate r e l a t i v e l y l i t t l e , ranging from 46 to 60% r.h., even though outside weather va r i e d through wet and dry s p e l l s . Clay weight increased sympathetically with humidity from 0.09999 to 0.10190 g. A p l o t of clay weight versus r e l a t i v e humidity (not shown) was roughly l i n e a r with a slope of about 12 mg per 10% r.h. change. If a reasonable v a r i a -t i o n of laboratory humidity i s taken to be within l i m i t s of 30 and 70% r.h. and a reference humidity i s taken as 50% r.h., then the maximum v a r i a t i o n i n clay weight would be ± 1.9 per cent. Assuming Na-montmorillonite behaves s i m i l a r l y , t h i s would increase uncertainty i n Na exchange capacity deter-minations by a corresponding amount. Preparation of Na-Montmorillonite-Silica Gel Thin Layer Plates An emulsion s u i t a b l e f o r spreading on conventional glass plates was prepared by homogenizing a mixture of 40 g s i l i c a g e l , 40 g Na-rnontmoril-l o n i t e g e l and 40 g d i s t i l l e d water. These proportions were found by t r i a l and error to produce t h i n layers of good cohesion and permeability for e l u -t i o n with e l e c t r o l y t e solutions by the ascending method. ® S i l i c a Gel H (without calcium sulphate binder) was obtained from E. Merck Company. Iron contaminant was removed with concentrated hydro-c h l o r i c acid:water (1:1) by the method of S e l l e r (Randerath, 1968), dele-tin g a f i n a l step of drying with benzene. Approximately 250 g of dry g e l was treated with acid s o l u t i o n and allowed to stand; supernatant s o l u t i o n was drawn off and replaced with d i s t i l l e d water, t h i s process repeated seven times u n t i l gel was assumed adequately washed. F i n a l r inses were t 1 52 removed by centrifugation and the gel allowed to air-dry almost to com-pletion; gel was kept slightly moist to prevent hardening and s i l i c a dust. Na-montmorillonite (Clay Spur and Newcastle) was prepared by the method previously described, but was kept as a gel containing an equivalent of 18.1 g air-dried clay in 100 g total. Emulsions were up in 250 ml polyethylene bottles and homogenized by vigorous magnetic stirring over thirty minutes. Homogenized mixtures were allowed to rest briefly to free air bubbles, then applied across standard 20 by 20 cm glass plates with a Desaga spreader set to produce 250 ym thick layers. Plates were air-dried. Montmorillonite-silica gel emulsions were found to spread about as easily and evenly as standard s i l i c a gel. Air bubbles lodged along the spreading channel occasionally caused longitu-dinal streaks. Dried thin layers were found to have good cohesion, perhaps due to a binding action of the montmorillonite. Sources of Experimental Materials Suppliers of non-routine materials are lis t e d here. Standard chemi-cal supply houses such as Fisher Scientific Company, J.T. Baker Chemical Company and Mallinckrodt Chemical Works provided common reagents such as electrolytes. Indicator Dyes l-(2-pyridylazo)-2-napthol (PAN) - Aldrich Chemical Company, 940 West St. Paul Street, Milwaukee, Wisconsin 53233 s-Diphenylcarbazone J.T. Baker Chemical Company, 222 Red School Lane, Phillipsburg, N.J. 08865 Pyrocatechol V i o l e t 8-Hydroquinoline (OXIME) Bentonite and S i l i c a Gel Bentonite (Clay Spur) Bentonite (Newcastle Formation) S i l i c a Gel H F i l t r a t i o n Apparatus Sartorius Polycarbonate F i l t r a t i o n Membranes J.T. Baker Chemical Company Mallinckrodt Chemical Works, 2nd and Mallinckrodt Streets, St. Louis, Missouri 63147 Wards Natural Science Establishment, Rochester, New York (no longer a v a i l a b l e ) Clay Minerals Repository, c/o Prof. W.D. Johns, Department of Geology, U n i v e r s i t y of Missouri, Columbia, Missouri 65201 E. Merck Ag (Distributed by Brinkmann Instruments (Canada) Ltd., 50 Galaxy Blvd., Rexdale Toronto, Ontario Unit and Sartorius C e l l u l o s e N i t r a t e • D i s t r i b u t e d by BDH Chemicals Ltd., Vancouver, B r i t i s h Columbia 1 15k Modelling Cation Exchange E q u i l i b r i a A simple model was constructed to predict equilibrium concentrations of two cations competing for clay exchange p o s i t i o n s . This model i s s t r u c -tured on a r e v e r s i b l e r e a c t i o n between a doubly charged exchange cation (re-presented here by Me ) and Na-montmorillonite which may be written as Me 2 + + 2 Na{Mon£} - Ke{Mont}2 + 2 Na + (2l) where {Mont} represents that mass of montmorillonite carrying one equiva-lent negative charge. A mass ac t i o n equation corresponding to re a c t i o n (21) may be written as (Yimi)2(f2M2) = (Y2m2) (f.MO2 ' K° ( 2 2 > where subscripts 1 and 2 r e f e r to Na and Me r e s p e c t i v e l y , y and m are molal scale a c t i v i t y c o e f f i c i e n t and concentration f o r s o l u t i o n species (cations), j and N are r a t i o n a l scale a c t i v i t y c o e f f i c i e n t and equivalent f r a c t i o n f o r sorbed species (exchange ca t i o n s ) , and K i s a thermodynamic equilibrium constant referenced to a standard state defined by the quotient KQ. Choice of concentration scales and standard states has previously been dealt with and w i l l not be repeated here (see pages 27 to 29). I t may be r e c a l l e d that the quotient K0 becomes one as a r e s u l t of these choices. In r e a l i t y , a rigorous formulation such as i s represented by equation (22) i s not r e a d i l y incorporated into t h e o r e t i c a l equilibrium models because i t contains a c t i v i t y c o e f f i c i e n t s , y and j, which are found e m p i r i c a l l y and cannot generally be predicted <X p>vio>vi. Some exception to t h i s statement i s 15 5 the Debye-Ruckel r e l a t i o n , an em p i r i c a l l y supported equation xvhich pre-d i c t s i n d i v i d u a l ion a c t i v i t y c o e f f i c i e n t s as a function of i o n i c strength. This equation takes a general form Az 2 I s -log 10 Y = , h ( 2 3 ) 1 + Ba I where A and B are constants c h a r a c t e r i s t i c of a given solvent at a given temperature. A c t i v i t y c o e f f i c i e n t y i s predicted f o r an i n d i v i d u a l ion of charge z and s i z e parameter "a" at t o t a l i o n i c strength " I " . The s i z e parameter "a" i s c h a r a c t e r i s t i c of each ion and may be regarded as a mea-sure of cl o s e s t approach to other ions i n sol u t i o n ; tabulations of "a" values for various anions and cations are common i n the l i t e r a t u r e (Sucha and Kotrly, 1972; Garrels and C h r i s t , 1965). Values for A and B for water at 23°C are extrapolatable from l i s t i n g s of Berner (1971). Ionic strength may be determined from the r e l a t i o n I = h I m.z2 (24) i where subscript " i " denotes a l l i o n i c species; other parameters are as previously defined, but now generalized to represent a l l ions present i n so l u t i o n . In the present model i t was desired to predict equilibrium concen-t r a t i o n of Na and Me r e s u l t i n g from addition of increments of 0.1 M sol u t i o n of Me e l e c t r o l y t e to a 1% suspension of i n i t i a l l y Na-saturated montmorillonite. Computations were to be arranged to solve for the t i t r e of e l e c t r o l y t e required to displace a prescribed f r a c t i o n of t o t a l exchange capacity. - By incrementing t h i s f r a c t i o n between zero and one, a t i t r a t i o n 156 curve (concentration curve) might be generated for both Na'*' and Me2^ at any given value of the equilibrium constant for the exchange reaction (21). Such a model was developed, but was based on a s i m p l i f i e d mass ac-t i o n equation of the form (YilTli) 2 X Km = (Y 2m 2) (1-x) 2 ( 2 5 ) where terms f o r sorbed cations are replaced by X and 1-X, here defined r e s p e c t i v e l y as the equivalent f r a c t i o n of clay exchange capacity vacated and retained by Na. This modification i s obviously equivalent to an as-sumption of i d e a l mixing of sorbed cations; that i s , unit r a t i o n a l a c t i -v i t y c o e f f i c i e n t s . A c t i v i t y c o e f f i c i e n t s of s o l u t i o n species (Yi and Y 2 ) were estimated from the Debye-Riickel equation (23) . The numerical model r e l i e d on i t e r a t i v e convergence (by successive approximation) of solutions mutually s a t i s f a c t o r y to a set of inhomogenous equations describing equilibrium mass balance and action. Calculations were made by a D i g i t a l Equipment Corporation PDP-8/L computer under d i r e c -t i o n of a small programme written i n FOCAL 69, a highly abbreviated and i n t e r a c t i v e i n t e r p r e t e r language designed for small computers. A copy of t h i s programme, with accompanying symbol table and flow diagram, i s included with t h i s appendix (Table XX ; Figure 44). A b r i e f o u t l i n e of i t s opera-t i o n follows. Cation a c t i v i t i e s , a c t i v i t y c o e f f i c i e n t s , concentrations and t i t r e s of exchange e l e c t r o l y t e s o l u t i o n were ca l c u l a t e d by simultaneous conver-gence over a number of i t e r a t i o n s . To s t a r t , values of K and X were set. Knowing i n i t i a l weight and exchange capacity of Na-montmorillonite, plus t o t a l suspension volume, i t was possible to c a l c u l a t e a f i r s t approximation 1 5 7 + of Na concentration as XH>(CEC)10 5 „ ( 2 6) (Vo+AV) + where: C i i s molar Na concentration; X i s f r a c t i o n of t o t a l exchange capacity replaced; W i s weight of montmorillonite i n grams; CEC i s Na exchange capacity i n meq/100 g; Vo i s s t a r t i n g volume of suspension i n l i t e r s ; and, AV i s t i t r e volume i n l i t e r s . At t h i s point AV was not known and was set to zero, thereby causing a s l i g h t overestimation of Na + concentration. Molar concentration was changed to molal units by an approximation C i mi - — (27) P + where IT?I i s molal Na concentration and p i s s o l u t i o n density (approxi-mated as that of pure water at 23°C). Next an approximation of Me a c t i v i t y was calculated by rea r -ranging equation (25) to the form ( Y i f " i ) 2 X * 2 * % ( 1 " X ) 2 ( 2 8 ) where tXz i s Me a c t i v i t y and other v a r i a b l e s are as previously defined. + An a c t i v i t y c o e f f i c i e n t for Na was not known at t h i s stage and was ap-2 + proximated' as unity. Molal Me concentration was now set as < 1 5 8 m2 * — (29) Y2 and molar concentration C2 calculated from equation (27). Again at t h i s point an a c t i v i t y c o e f f i c i e n t for Me had not been estimated and was set i n i t i a l l y to one. An approximate i o n i c strength was now calculated by equation (24), assuming no anion adsorption, and t h i s value substituted into the Debye-Hiickel equation (23) to give f i r s t estimates of a c t i v i t y c o e f f i c i e n t s Yi and Y2• A f i r s t estimate of AV was then calculated as m 2(V 0+AV) + h x(CEC)w) 10" 5 Av = (30) C t where C i s t i t r e molar concentration. This estimate of t i t r e volume was t then u t i l i z e d i n equation (26) and the e n t i r e sequence of operations r e -peated to a r r i v e at a second estimate of AV. Cycling through t h i s loop was continued u n t i l successive estimates of t i t r e volume d i f f e r e d by l e s s than one part i n one thousand; convergence was usually achieved to t h i s tolerance within ten i t e r a t i o n s . Calculated cation concentrations, plus t h e i r a c t i v i t i e s and a c t i v i t y c o e f f i c i e n t s , were printed out along with t i t r e volume. X was then incremented and a new cycle begun; by i n c r e -menting X from about .05 to .995, a s e r i e s of equilibrium concentrations was generated from which t i t r a t i o n curves could be p l o t t e d . In the development thusfar, the s p e c i f i c case of exchange with doubly charged cations has been considered. The computer programme was generalized to allow for exchange of cations of any charge (n) with Na-montmorillonite. It should be stated that t h i s programme has not been 1 159 executed for c a t i o n charges greater than two, and i t may be necessary to make minor modifications i n convergence c r i t e r i a to allow for greater error propagation i n higher order terms i n equation (25). A second note regarding the programme i s the change of numerical subscripts to 0 and 1 + n+ for Na and Me r e s p e c t i v e l y . Subscript 0 was not used i n general nota-t i o n to avoid confusion with i t s general as s o c i a t i o n with s t a r t i n g condi-ti o n or standard state. START 3-^ STOP INPUT: V 0, p, C , W, CEC, n, a, A, B, K I OUTPUT: Table H e a d i n g s 1 OUTPUT: X, A V , I, c-i, a i , Y i , C2, ffl2, CX2, Y 2 j - * i Increment X I ^ 1 Calculate: Ci mi c t - i " | ' * ~ C a l c u late: a 2 m2 C 2 Calculate: I Calculate: Yi» Y 2 | Calculate: A V Yes A V .converged? No Figure 45. Flox^ diagram for FOCAL 69 t h e o r e t i c a l t i t r a t i o n programme. Table XIX. Table of symbols used i n FOCAL 69 t h e o r e t i c a l t i t r a t i o n programme. FOCAL 69 General Units Meaning Symbol Symbol VW V 0 l i t e r s Starting volume of suspension. VT AV l i t e r s Exchange e l e c t r o l y t e t i t r e . V l i t e r s Dummy v a r i a b l e used to store o l d VT f o r convergence t e s t . D P g/ml Density of s o l u t i o n . MT C t moles/1 Concentration of exchange e l e c t r o l y t e s o l u t i o n . W W grams Weight of Na-montmorillonite. EC CEC meq/100 g Na exchange capacity of cl a y . N n Charge of exchange cat i o n . DR a nm Ionic s i z e parameter f o r Debye-Hiickel equation. A A Debye-Hiickel equation constant. B B Debye-Huckel equation constant. EQ K zq Equilibrium constant f o r exchange r e a c t i o n . X X Equivalent f r a c t i o n of exchange capacity replaced. A0 a.i moles/kg A c t i v i t y of Na +. A l CL2 moles/kg A c t i v i t y of Me n +. M0 m1 moles/kg Molal concentration of Na +, Ml m2 moles/kg Molal concentration of Me n +. C0 Ci moles/1 Molar concentration of Na +. Cl c 2 moles/1 Molar concentration of Me n +. G0- Yi A c t i v i t y c o e f f i c i e n t of Na +. Gl Y 2 A c t i v i t y c o e f f i c i e n t of Me n +. Rl /I (moles/kg) 2 Square root of i o n i c strength. Table XX . FOCAL 69 programme to ca l c u l a t e t h e o r e t i c a l c a t i o n exchange t i t r a t i o n curves. C-FOCAL*1969 01 « 0 1 E . 0 1 . 0 3 A "VOLUME HOH-L "*W*W DENSITY -GM/ML "* D* S 01.05 A "MOLARITY TITRE "*MT*? 01.07 A "UT. CLAY-GM ' "*W*" ' CEC-MEQ/100GM "*EC»S 01.09 A "CATION CHARGE "*N*" K lELAND PARAMETER "*DR*I 01V11 A "A-PARAMETER "^A," B-PARAMETER "*B*S 01.13 A "ECU CONSTANT "*EQ*8f " 02.02 T " X TITRE IONIC STRENGTH"*I 02'; 0 4 T " NA-MQLAR N A-MOLAL NA-ACTIVITY NA-ACT. COEFF.*** ? 02.06 T " M -MOLAR M -MOLAL M -ACTIVITY M "ACT. COEFF."*f 02.08 F K=1*58*T " 02 . 0 9 T U 02.10 F X=.05*.05*.95JS V=0JS G0=1.JS G l = l o l D 3 03.06 S C0=(X*W*EC*lE-5)/CVW+V)*S M0=C0/D1S A0 ° M 0 * G 0 03.08 S Al= ( ( A 0 t N ) / E Q ) * ( X / C l - X ) t t 3 ) ; S Ml<=Al/GWS C 1 = M 1 * D 0 3 . 1 0 S RI = FSQT<.5*(M0+M1#(N'T2> + (M0+M1*N))) 03.12 S G0=1/CFEXP(2.30258*CA*RI)/<1+B*.4*RI))) 03 ' . '14 S Gl = I/<FEXP(2.30258*<A*<Nt2>*RI)y>( 1+B*DR*RI>>> 03.18 S VT=«««(CAl/G13*DT*(W+V>) + <y*EC*lE-5 * C X/N))>/MT 03.20 I <<FABS(VT-V))-CVT * .0001)> 3.23*3.23;S V=VT;G 3 . 0 6 03.23 D 4 03'. 2 4 R 04.05 T %4.04*X*%6.06*V*lE3*%*RIt2*S 04.10 T C0*'K0*A0*Gi3* | 0 4 . ' 1 2 T C 1 J> M1 * A1 * G1 * ! IJR 04.15 G 3.24 

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