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(Gamma minus) radiolytic generation of polysulphide from aqueous sodium sulphide solutions. You, Young-Soo 1973

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d Y- RADIOLYTIC GENERATION OF POLYSULPHIDE FROM AQUEOUS SODIUM SULPHIDE SOLUTIONS by YOUNG - SOO YOU B.S.F. Korea University, Seoul, 1965 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department of Forestry We accept t h i s thesis as conforming to the required standard. THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1973 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the require-ments for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y a v a i l a b l e for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. I t i s under-stood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Faculty of Forestry The University of B r i t i s h Columbia Vancouver 8, Canada Date A p r i l 17, 1973 i i ABSTRACT The mechanism of polysulphide formation by y-radiation of aqueous sodium sulphide (Na2S) solutions was investigated. The roles of solvated electron (e ) and hydroxyl r a d i c a l s aq (•OH), as primary products of water r a d i o l y s i s , were studied i n a systematic manner by using N 20 as eaq a n ^ 2-propanol as •OH scavengers. Polysulphide formation i n Na2S solutions was found to be i n i t i a t e d by the o x i d i z i n g intermediate, the «0H r a d i c a l . The scavenger experiments were supplemented by other studies on e f f e c t s of solute concentration, dose at a single dose rate, pH, oxygen and hydrogen sulphide (H2S) saturation. Rates of polysulphide formation increased according to presence of N 20 and H2S, Na 2S concentration, i r r a d i a t i o n dose and were highest at pH 7.0. In these experiments, highest polysulphide y i e l d , 14 g/1, was obtained i n the 80 g/1 aq Na 2S-H 2S system at pH 12.7 and exposed for 20 Mrad dosage. The e f f e c t of oxygen was to increase r a d i a t i o n y i e l d for lower dose ranges. At higher doses, however, G-values fluctuated markedly. ACKNOWLEDGEMENTS The author would l i k e to express his gratitude to Dr. J.W. Wilson, Professor, and Dr. L. Paszner, Research Associate, Faculty of Forestry, U. B. C. for t h e i r i n t e r e s t , assistance and encouragement throughout the course of t h i s research. He i s also g r a t e f u l to Dr. S-t. Chiu, who was the f i r s t to observe polysulphide formation under conditions employed here. Special thanks are due Dr. D.C. Walker, Department of Chemistry, and Dr. R.W. Wellwood, Faculty of Forestry for t h e i r h e l p f u l suggestions and review of the th e s i s . In addition, the author wishes to thank Messrs. G. Bohnenkamp, G.D. Jensen, and U. Rumma, Technicians, Faculty of Forestry, for technical assistance given. F i n a n c i a l support by the Faculty of Forestry has been greatly appreciated. i v TABLE OF CONTENTS Page TITLE PAGE i ABSTRACT i i ACKNOWLEDGEMENTS i i i TABLE OF CONTENTS i v LIST OF TABLES v i LIST OF FIGURES v i i LIST OF SYMBOLS ix 1.0. INTRODUCTION 1 2.0. LITERATURE SURVEY 4 2.1. Polysulphide 4 2.1.1. Pulping with polysulphide liquors 4 2.1.2. Polysulphide solution character-i s t i c s 6 2.1.3. Polysulphide sol u t i o n preparations 10 2.1.4. Polysulphide solution (fresh/used) analyses 12 2.2. Rad i o l y t i c Processes i n Aqueous Environment 14 2.2.1. Ionized and excited species 14 2.2.2. Solvated electrons (e ) 19 aq 2.2.3. Hydroxyl r a d i c a l (-OH) 21 2.2.4. E f f e c t s of oxygen 25 2.2.5. E f f e c t s of pH 28 2.2.6. Scavengers 32 2.2.7. Other e f f e c t s 35 2.3. Radiolysis of Inorganic Sulphur Compounds i n Aqueous Environment 36 3.0. MATERIALS AND METHODS 41 3.1. Sample Preparation 41 3.2. Sample I r r a d i a t i o n 43 3.3. Analyses 46 V Page 4.0. RESULTS 51 5.0. DISCUSSION 53 5.1. Mechanism of Polysulphide Formation 53 5.2. Reaction Variables 61 6.0. RECOMMENDATIONS 71 7.0. CONCLUSION 72 8.0. LITERATURE CITED 74 v i LIST OF TABLES Table page 1. Reactions of free r a d i c a l s i n i r r a d i a t e d water 82 2. Ra d i o l y t i c generation of sodium polysulphide (g/1) as related to sodium sulphide concentra-t i o n , a d d i t i v e ( s ) , i n i t i a l pH and ra d i a t i o n dose 83 v i i LIST OF FIGURES Figure Page 1. V a r i a t i o n with pH of primary y i e l d s i n y - i r r a d i a t e d water 84 2. Dependence of primary r a d i c a l and molecular y i e l d s of water y - r a d i o l y s i s on pH, derived from measurements on formic acid-oxygen solutions 84 3. Stainless s t e e l high pressure c e l l and glass d i f f u s i o n tube i n cut-off Erlenmeyer f l a s k 85 4. Schematic diagram of the pressurized system 86 3+ 5. Absorbance of Fe at 304 nm (10 mm c e l l ) vs. i r r a d i a t i o n time for c a l c u l a t i o n of ferrous sulphate dosimetry 87 6. Absorbance at 285 nm (2 mm c e l l ) vs. p o l y s u l -phide excess sulphur concentration (g/1) at three sodium sulphide concentrations, a l l containing 1 g/1 elemental sulphur 88 7. Polysulphide y i e l d related to r a d i a t i o n dose for 10 g/1 aq Na~S, N 90 (20 psi) and H,S (20 p s i ) ; i n i t i a l pH 12.7 7 89 8. Polysulphide y i e l d related to r a d i a t i o n dose for 40 g/1 aq Na~S and no additives; i n i t i a l pH 12.7 7 90 9. Polysulphide y i e l d related to r a d i a t i o n dose for 40 g/1 aq Na_S and N ?0 (20 p s i ) ; i n i t i a l pH 1.0 7 7 91 10. Polysulphide y i e l d r e l a t e d to r a d i a t i o n dose for 40 g/1 aq Na_S and N 90 (20 p s i ) ; i n i t i a l pH 7.0 7 92 11. Polysulphide y i e l d r e l a t e d to r a d i a t i o n dose for 40 g/1 aq Na„S and N_0 (20 p s i ) ; i n i t i a l pH 12.7 7 7 93 12. Polysulphide y i e l d r e l a t e d to r a d i a t i o n dose for 40 g/1 aq Na~S and 2-propanol (0.5 M); i n i t i a l pH 1.0 .7 94 13. Polysulphide y i e l d related to r a d i a t i o n dose for 40 g/1 aq Na~S and 2-propanol (0.5 M); i n i t i a l pH 12.7 7 95 v i i i Figure Page 14. Polysulphide y i e l d related to r a d i a t i o n dose for 40 g/1 aq Na„S and H,S (20 p s i ) ; i n i t i a l pH 12.7 7 7 96 15. Polysulphide y i e l d related to ra d i a t i o n dose for 40 g/1 aq Na~S and 0 9 (20 p s i ) ; i n i t i a l pH 12.7 7 7 97 16. Polysulphide y i e l d related to r a d i a t i o n dose for 40 g/1 aq Na„S, N 90 (20 psi) and H~S (20 p s i ) ; i n i t i a l pH 12.7 7 98 17. Polysulphide y i e l d related to radiati o n dose for 80 g/1 aq Na^S, N„0 (20 psi) and H,S (20 p s i ) ; i n i t i a l pH 12.7 7 99 IX LIST OF SYMBOLS A Group IV, V and VI elements i n the periodic table. Br Bromide ion. •Br Bromine r a d i c a l . C l Chloride ion. •Cl Chlorine r a d i c a l . E Redox p o t e n t i a l , e Electron. e Solvated (hydrated) electron, aq e., Thermalized electron, therm F Faraday i n appropriate unit . + jj Radiation y i e l d as sum of primary reducing species. G M Radiation y i e l d of molecules. Gp Radiation y i e l d of f i n a l products. G R Radiation y i e l d of r a d i c a l species. G.Y. Radiation y i e l d of measured t o t a l X (atoms, r a d i c a l s or molecules) produced i n and out of spurs of a given reaction. G Radiation y i e l d of X produced only i n spurs. H + Hydrogen ion. H Hydride ion. HO2 Perhydroxy ion. •HOj Perhydroxy r a d i c a l . H 2 + A c i d i c form of hydrogen molecule ion. X •H Hydrogen r a d i c a l . H 2 Hydrogen molecule. P o s i t i v e parent ion i n water r a d i o l y s i s . H 2 O * Excited water molecule. H 2 ° 2 Hydrogen peroxide. HgO+ Hydrated proton (acidic hydrogen). HS Hydrogen sulphide ion. • H S O 2 Hydroxosulphuroxy r a d i c a l . K.R 1 Reaction rate constant between r a d i c a l species («R) and ^ scavenger concentration (<j>) . LET Linear energy transfer (kev/M). This i s defined as the l i n e a r - r a t e energy loss ( l o c a l l y absorbed) by an i o n i z i n g p a r t i c l e traversing a material medium. M Molecule i n general. Mrad Unit of r a d i a t i o n dose (10^ rad). 0 Monooxygen (-1). OH Hydroxide ion. •OH Hydroxyl r a d i c a l . O2 Oxygen molecule. 0 2 ~ Hyperoxide. 0^" Ozonide ion. 0^ Oxyanion i n general. P Product. x i Q Substance reacted. R Gas constant. R H or a l k y l group. •R Radical species. Spur Small c l u s t e r when r e f e r r i n g to small groups of excited and ionized species. S c Elemental sulphur. *S Sulphur r a d i c a l . S* Labelled sulphur atom. S Sulphide ion. SC>2 Sulphite ion. SO^ Sulphate ion. S2O3 Thiosulphate ion. S nS ~ Polysulphide excess sulphur. Track Path by which protons or p a r t i c l e s i n r a d i a t i o n chemistry can ionize or excite a large number of molecules. X Ratio between polysulphide excess sulphur and sulphide sulphur. In loving memory of my mother, DORRAE YOU (1904 - 1968) and my brother, BONG - SOO YOU (1922 - 1966 1 1.0. INTRODUCTION In recent years, much pulping research has been aimed at modifying e x i s t i n g processes or finding new ones ad d i t i o n a l to those already practiced. The improvements sought are i n the area of higher pulp y i e l d and/or q u a l i t i e s . Further im-provements are desirable with respect to ease and extent of spent chemical recovery and reduced impact on environmental q u a l i t y . One a t t r a c t i v e process appears to reside i n the r e d i s -covered e f f e c t s of polysulphide i n a l k a l i n e pulping l i q u o r s . Polysulphide pulping, as a modification of k r a f t pulping by addition or generation of sodium polysulphide, i s one of the few p o t e n t i a l methods by which increased y i e l d and changes i n k r a f t pulp properties can be obtained economically. During recent years several modifications to the k r a f t or a l k a l i n e cooking processes have been proposed. Some additives, such as d i t h i o n i t e (52 to 55), sodium borohydride (63) and hydrazine (71) are only of t h e o r e t i c a l i n t e r e s t due to higher production costs with t h e i r use, as compared to the conventional k r a f t cook (95). One of the oldest suggestions, cooking with l i q u o r containing polysulphide, seems to be more a t t r a c t i v e . Thus, new attention has been focussed by d i f f e r e n t researchers on r e v i v a l of polysulphide cooking. Changes observable i n pulp q u a l i t y , when pulping with polysulphide l i q u o r s , depend upon the amount of excess sulphur used i n cooking. With 4% excess sulphur, for example, approx-imately 10% reduction i n wood consumption i s experienced and 2 pulp c h a r a c t e r i s t i c s are changed rather d r a s t i c a l l y . F i r s t of a l l , the pulp i s beaten more e a s i l y . The e f f e c t i s believed to r e s u l t from an increase i n retained hemicelluloses. The resultant paper, however, has lower tear resistance, which i s one of the main drawbacks of polysulphide pulps (41). From the point of response to beating, and stock freeness, p o l y s u l -phide pulps seem to be a very a t t r a c t i v e a l t e r n a t i v e to conven-t i o n a l k r a f t pulps, but where tear i s an important strength consideration polysulphide cooking i s less favored. During black liquor r e c i r c u l a t i o n i n the k r a f t recovery process, the viscous black li q u o r contains less than 100% of the o r i g i n a l inorganic elements present i n the i n i t i a l white l i q u o r . This i s p a r t l y due to the f a c t that the pulp cannot be washed completely free of inorganic ions. Therefore, makeup chemical i s added i n an amount equivalent to these losses (59). The decrease of chemical concentration i n r e c i r c u l a t e d p o l y s u l -phide cooking liquors also f a l l s into t h i s category. While cooking li q u o r makeup involves only addition of elemental s u l f u r to the white l i q u o r , polysulphide s t a b i l i z a t i o n , e s p e c i a l l y at high (cooking) temperatures, appears to be exceedingly d i f f i c u l t . Much work has been done i n attempts to prevent chemical losses from polysulphide a l k a l i n e pulping liquors (64). No studies have been reported previously which involve r a d i o l y t i c generation of polysulphide. The general scheme of r a d i a t i o n chemical investigations may be represented as follows (73): chemical system r a d i a t i o n w w w i o n i z a t i o n e x c i t a t i o n 3 what species? ^ how many? F i n a l , stable products Intermediate Species Previous studies i n these laboratories have shown that polysulphide can be formed from aqueous sodium sulphide by Y-radiation (23, 99). The r a d i a t i o n y i e l d (G-value) was about 1.0 i n a system containing 80 g/1 Na 2S under 270 p s i H2S and exposed for 70.8 Mrad. The mechanism of polysulphide forma-t i o n and e f f e c t s of dosage, pH, oxygen, H 2S and X s~values were not c l e a r . ' The objectives of the present work were: (i) to study the r o l e of two r a d i c a l species, e and «OH, formed from water r a d i o l y s i s i n polysulphide generation; and ( i i ) to examine some parameters, such as solute concentration, additives and i n i t i a l pH, which may r e l a t e to polysulphide y i e l d by r a d i o l y s i s . The r o l e of e and «OH on mechanism of polysulphide aq formation was studied by using r a d i c a l scavengers. Nitrous oxide was used as an electron scavenger and 2-propanol as the •OH r a d i c a l scavenger. 4 2.0. LITERATURE SURVEY 2.1. Polysulphide A review of l i t e r a t u r e pertinent to the present study divides as: pulping c h a r a c t e r i s t i c s , solution preparations and associated a n a l y t i c a l techniques. 2.1.1. Pulping with polysulphide liquors In 1946 Hagglund (38) f i r s t noted an increase i n pulp y i e l d a f t e r d i r e c t sulphur addition to caustic pulping l i q u o r s . Subsequently, numerous studies have reported on the nature of y i e l d increase with d i f f e r e n t woods (41, 85), mechanism of polysulphide pulping (10, 66, 67, 85) and characterization of polysulphide pulps (9, 10, 12, 22, 25, 37, 66, 67, 85). In the l a s t few years attention has been given to optimization of y i e l d increase, as exemplified by the work of Teder and co-workers (9, 37, 88, 90, 92, 93) and Clayton and Sakai (24, 25). Commercial adaptation of the polysulphide process has been announced by only one m i l l . The Norwegian k r a f t m i l l , Lovenskiold-Vaekero, Hurum Fabriker, converted to polysulphide pulping i n October 1967 (47). This m i l l reported a 3.5 to 4.0% y i e l d increase by addition of 2.2% elemental sulphur. Assum-ing constant s u l p h i d i t y , t h i s means that sulphur losses must be i n the neighbourhood of 90 lb/ton of pulp (oven-dry b a s i s ) . Herein i s a major reason why the polysulphide process has not replaced conventional k r a f t pulping; without a recovery process for the added sulphur, losses are extremely high and these occur mainly as objectionable p o l l u t a n t s . Another factor which might be a drawback i s increased s u l p h i d i t y r e s u l t i n g from d i r e c t sulphur addition. Dissolution of polymer molecules, physical rearrange-ments r e s u l t i n g i n increased odor, degradation through 'peeling-o f f , a l k a l i n e hydrolysis of g l u c o s i d i c bonds, and redeposition of polymer molecules, are reactions occurring during p o l y s u l -phide cooking (41). Polysulphide can oxidize terminal aldose groups on wood polysacharides to aldonic acids, which then can not rearrange to form ketoses. A l k a l i n e peeling can then begin only at end-groups which have escaped oxidation, or at end-groups created by a l k a l i n e hydrolysis (24). The increase i n y i e l d appears to r e s u l t from oxidation of polysaccharide reduc-ing end-groups to acids which provides resistance to 'peeling-o f f ' degradation i n hot a l k a l i . With polysulphide pulping the y i e l d of a l l wood carbohydrates increases, p a r t i c u l a r l y gluco-mannan i n coniferous woods and glucuronoxylans i n pored woods (10, 85, 95) . Polysulphide also reacts with wood l i g n i n s . Evidence that the polysulphide ion might react with l i g n i n was given by Nakano et a l . (67), who experimented with l i g n i n model com-pounds. They found that when v a n i l l y l alcohol and apocynol were treated with polysulphide l i q u o r the reactions yielded much more v a n i l l i n and acetoguaiacone, respectively, than a f t e r s i m i l a r treatment with k r a f t l i q u o r . The higher y i e l d of v a n i l -l i n might have been due to oxidation v i a v a n i l l y l disulphide, since t h i s intermediate compound was i s o l a t e d a f t e r polysulphide treatment (66), but not a f t e r treatment with k r a f t l i q u o r . 6 The rate of d e l i g n i f i c a t i o n i n polysulphide pulping i s considerably f a s t e r than i n the k r a f t process. Resulting coniferous wood pulps are comparable to k r a f t i n a l l strength properties, with the possible exception of tear (24, 85, 88). The presence of l i g n i n decreases s t a b i l i t y of polysulphide solutions. Although i n i t i a l lignin-polysulphide reactions might lead to consumption of polysulphide i n wood pulping, t h i s e f f e c t i s smaller than thermal degradation of polysulphide solutions. The r e s u l t i n g l i g n i n modification could possibly make i t more e a s i l y removed during subsequent stages of the pulp-ing process (88). The color of polysulphide pulp i s usually darker than that of comparable k r a f t , thus requiring stronger bleaching treatments. In order to optimize pulp y i e l d and q u a l i t y , two stage or multistage polysulphide a l k a l i cooks have been proposed (24, 51). Aqueous calcium polysulphide solutions are known to be more e f f e c t i v e i n s t a b i l i z i n g carbohydrates toward a l k a l i n e de-gradation than corresponding aqueous sodium polysulphide solu-tions (87). The considerable a d d i t i o n a l carbohydrate s t a b i l -i z a t i o n obtained i n the presence of calcium ions originates only to a minor extent from accelerated metasaccharinic acid end-group formation. I t mainly r e s u l t s from extensive and rapid oxidation of carbonyl end-groups to mannoic aci d . 2.1.2. Polysulphide solution c h a r a c t e r i s t i c s The composition of aqueous sodium polysulphide solutions includes various polysulphide ions, S nS , as well as HS , S and 0H~ ions (93). The e f f e c t of S , 0H~ and X s (the r a t i o between polysulphide excess sulphur and sulphide sulphur) i s si m i l a r , i r r e s p e c t i v e of whether calcium or sodium i s used as the cation (87) . Several workers have investigated e q u i l i b r i a i n aqueous solutions at room temperature between S nS of d i f f e r e n t chain lengths and HS , S and OH . Rapid acidimetric t i t r a t i o n and redox p o t e n t i a l measurements have been used (26, 62, 85). Cloke (26) indicated i n 1963 that the mean polysulphide ion i s larger than the stoichiometric composition, i . e . , that consider able amounts of HS or S e x i s t i n so l u t i o n . A l l equilibrium studies indicate that S^S and S^S ions are the dominant species. Single protonated polysulphide ions seem to be un-stable as polysulphide solutions decompose into HS and S Q when they are slowly a c i d i f i e d to pH values below 9 (35). Teder (89) postulated that even i f i t were impossible to prepare polysulphide ions larger than S^S the p o s s i b i l i t y could not be neglected, since, for instance, S^S and larger ions could be i n equilibrium with smaller ions than S^S i n the stoichiometric composition. This contradicts previous studies on polysulphide e q u i l i b r i a . Ames and Wil l a r d (12), Rodziewicz et a l . (82) and Voge and Libby (96) have reported that a l l sulphur atoms, upon forma t i o n of t h i o s a l t s and d i - and polysulphides, are interchangeabl with each other, as: S"~ + S* » S*S~" [2-1] 0 0 0 0 0 S* + S S 0~~ » 0 S*S + S 0 .. [2-2] 0 0 0 0 8 I t i s known that aqueous polysulphide .solutions have a stable redox p o t e n t i a l that varies with composition at room temperature and at comparatively low a l k a l i n i t i e s (26). Accord-ing to Teder (90), the redox p o t e n t i a l of polysulphide solutions at given temperature i s dependent on the X s value, excess s u l f u r concentration (S Q) and OH . The a b i l i t y of polysulphide solu-t i o n to s t a b i l i z e carbohydrates can be predicted from the redox p o t e n t i a l (E), by: E + — p — • ln[OH~] + l n [ S 0 ] J [2-3] where: E = electromotive force (emf), v o l t s ; R = gas constant, 0.08206 1-atm deg ^-mole T = absolute temperature, °K; and F = faraday i n appropriate units, 96,487 coulombs/ eq. = 23,060 c a l / v o l t - eq. If the above sum i s below a c e r t a i n value (-725 mv at 90°C) no s t a b i l i z a t i o n i s obtained. Above t h i s l i m i t the a b i l i t y of polysulphide solutions to s t a b i l i z e hydrocellulose and carbo-hydrates i n wood seems to be an approximately l i n e a r function of the above variables at a given temperature. The k i n e t i c s of polysulphide rearrangement reactions were studied by spectrophotometric methods (92). In aqueous solutions of Na2S, the d i f f e r e n t polysulphide ions are i n e q u i l i brium with HS and OH : S(m+n) S"~ + H S ~ + 0 H " » s m s " " + S n S " + H 2 ° • • t 2 ~ 4 l 9 The e q u i l i b r i a were disturbed by addition of OH or HS . Rate of rearrangement was found to increase with increased S Q, i o n i c strength, and temperature, and decrease with decreased OH and X s l e v e l . Rearrangement reactions occurring i n polysulphide solutions are thought to be considerably f a s t e r than reactions with organic compounds or that of thermal polysulphide decompo-s i t i o n . The thermal decomposition of sodium polysulphide has been studied i n a l k a l i n e aqueous solutions (24, 37, 75). Un-wanted polysulphide losses can occur while polysulphide pulping liquor i s being prepared, c a u s t i c i z e d or stored at elevated temperatures (70 to 130 C). Polysulphide ions decompose into t h i o s u l f a t e and hydrogen sulphide ions, as: S nS"~ + (n-l)0H~ + (1-2-) H 20 > ( I + J J H S " + jS2°" "' [ 2 - 5 ] According to Olsson and Samuelson (75) , and Teder (88) , t h i s decomposition i s accelerated by increased temperature, alka-l i n i t y and X s. Thermal polysulphide decomposition i s known to occur as a r e s u l t of a l k a l i n e degradation of polysulphide ions. This i s independent of both acid decomposition of polysulphide into SQ and HS ions, and oxidation of polysulphide with molecular oxygen to S2°3 ( 2 4' 7 5 ) • The mechanism of polysulphide ion a l k a l i n e degradation can be drawn from the a l k a l i induced s c i s s i o n of the S—S bond i n organic d i s u l f i d e s . When hydrogen i s not s p l i t o f f from the organic groups, the i n i t i a l reaction i s believed to be: R' - S - S - R/+ OH" > RS - OH + RS~ [2-6] Whether a polysulphide solution i s stable or not at a given temperature i s determined by HS~ and OH concentrations, whereas S2°3 concentration has no observable influence (37). Since the s t a b i l i z i n g e f f e c t of HS i s considerably larger than the counteracting e f f e c t of OH , concentrated polysulphide solu-tions are as a rule more stable than d i l u t e ones. Decomposi-t i o n of polysulphide solutions having low s t a b i l i t y seems to s t a r t with a nucleophilic attack of OH on the polysulphide ion. This reaction i s probably rate determining. A c t i v a t i o n energy i s lower for solutions containing larger polysulphide ions than for solutions with smaller mean polysulphide ion siz e (37) . 2.1.3. Polysulphide solution preparations Early investigations on polysulphide seem to have o r i g -inated with Bloxam i n 1895 and 1900 (78). He found that S Q dissolves i n aqueous S ~ solutions forming S nS and S2O3 and that formation of the l a t t e r i s depressed by decreased a l k a l i n i t y . I t had been known that the d i s s o l v i n g rate of S D was fas t e r i n S nS than i n S solution (21), as: [2-7] 11 n = 3, the reaction i s slow, n = 4 or n = 0, the reaction i s slowest. The simplest and most common way to prepare aqueous polysulphide solutions for laboratory or commercial pulping purposes i s to dissolve S Q i n aqueous a l k a l i n e sulphide solutions. For example, white liquor or recycled li q u o r from the f i r s t stage of a m u l t i -stage pulping process may be treated. The S 0 can be produced by any of the procedures proposed for chemical recovery (20, 56), or i t can be a "make-up" chemical. The rate of S D d i s s o l u t i o n i n aqueous Na2S at various a l k a l i n i t i e s has been studied (42) and i s known to be approx-imately proportional to s p e c i f i c surface of the S Q. Mass trans-fer i n the l i q u i d i s of minor importance, as long as a l l p a r t i c l e s are i n contact over t h e i r e n t i r e surface with the l i q u o r . In t h i s , sulphide ions seem to follow a second order reaction at moderate rate. Polysulphide ions, possibly i n combination with S , react at a high rate according to a mixed f i r s t order re-action. Although the former reaction has a f a i r l y low a c t i v a t i o n energy, the l a t t e r e xhibits even lower value. Experimental evidence suggests that small polysulphide ions are more active but less stable than larger ones. The p o s s i b i l i t y of making polysulphide at elevated temperature by processing sodium-sulphur compounds avail a b l e i n the k r a f t recovery system (Na2SO^, Na2S, Na2S20^) has not been considered previously. I t i s known, however, that there are three p o s s i b i l i t i e s for formation of polysulphides from 12 these compounds (8), namely: (i) thermal decomposition of S2O.J ; ( i i ) p a r t i a l reduction of SO^ ; and ( i i i ) p a r t i a l oxidation of S ~ i n the absence of water, at atmospheric pressure and with r e s t r i c t e d a i r contact. 2.1.4. Polysulphide solution (fresh/used) analyses In early stages of polysulphide a n a l y t i c a l i n v e s t i g a -t i o n s , Ahlgren and Hartler (6) determined polysulphide excess sulphur by the copper column method. This was modified by Bilberg and Landmark (18) who employed copper metal to i s o l a t e polysulphide, which was then reduced and analyzed. This method gives polysulphide values higher than those obtained by other a n a l y t i c a l techniques. Olsson and Samuelson (74) developed an ion exchange procedure for S nS , S and S2O3 determination i n black l i q u o r s . Teder (88), however, chose the Bilberg - Landmark method i n preference to ion exchange. The present TAPPI method (T 624 - OS 68) i s intended p r i m a r i l y for k r a f t l i q u o r s i n which SO^ concentration i s low. I t i s not r e l i a b l e for analyzing a l k a l i n e liquors containing large amounts of SO^ , due to p a r t i a l c o p r e c i p i t a t i o n of zinc sulphite with the zinc sulphide during the zinc carbonate metathesis step. Modification of t h i s well-known volumetric standard method for the determination of S , S2°3 a n ^ S 0 3 o r S n S * i a s k e e n investigated (81). In 1967, Ahlgren (5) examined an acidimetric method for determining S sulphur and S Q. The advantage of 13 t h i s procedure, by comparison with other methods known for analyzing polysulphide l i q u o r s , i s the p o s s i b i l i t y of determin-ing i n a single operation a l l sulphur components of i n t e r e s t to pulping. I t should be pointed out, however, that f o r each single component i n the li q u o r other methods may e x i s t which are more precise than acidimetric t i t r a t i o n (6). I t i s known that aqueous polysulphide solutions have a stable redox p o t e n t i a l that varies with composition (26, 62, 78). Methods for determining excess sulphur i n polysulphide solutions, based on redox t i t r a t i o n , have been investigated (7, 77, 90). Redox potentials for polysulphide solutions, within wide ranges of X s, S D and OH~ concentration were deter-mined. The redox p o t e n t i a l of a polysulphide solution at a given temperature i s dependent on a l l of these (90). Recently, spectrophotometry studies on polysulphide solutions have been c a r r i e d out by Teder (89, 91, 94). D i f f i -c u l t i e s i n obtaining stable spectra from polysulphide solutions have been reported (26). The spectroscopic method i s based on measurement of sample absorbance at 285 nm i n t h i n s i l i c a c e l l s a f t e r d i l u t i o n with s l i g h t l y a l k a l i n e NaCl sol u t i o n . At t h i s wave-length, aqueous Na2SnS solutions of d i f f e r e n t types have almost the same ab s o r p t i v i t y based on excess sulphur. The spectroscopic method has been compared with other methods and i s generally found to be equally accurate, but more rapid. For determining differences i n excess sulphur concentration within a series of polysulphide solutions of the same type, the spec-troscopic technique i s considerably more accurate than other known methods (89). 14 2.2. Ra d i o l y t i c Processes i n Aqueous Environment Polysulphide formation by i r r a d i a t i o n of aqueous Na 2S solutions has not been reported i n the l i t e r a t u r e pre-viously. Radiation chemistry of water, however, has been much explored. solutions, i t i s necessary to understand the r a d i o l y s i s of water. Since water i s the main component of environment i n d i l u t e aqueous systems, i t i n i t i a l l y absorbs almost a l l the rad i a t i o n energy. R e l a t i v e l y l i t t l e r a d i a t i o n energy i s absorbed d i r e c t l y by the solute, but the solute may be a f f e c t -ed i n d i r e c t l y . By absorbing most of the r a d i a t i o n energy, the solvent produces reactive species and reaction between these and solutes may then occur. 2.2.1. Ionized and excited species Gamma-radiation of water produces ionized and excited species. The i o n i z a t i o n process i s believed to be of much greater importance to chemical changes than the e x c i t a t i o n processes i n l i q u i d water. The i o n i z a t i o n process may be represented (86), as: In order to probe r a d i o l y s i s mechanisms of aqueous H 20 Y ^ H 20 + + e [2-8] A considerable part of the t o t a l energy absorbed from high-energy r a d i a t i o n (10 to 15%) i s dissipated by sub-excita-t i o n of electrons which ultimately end as thermalized electrons 15 { e - t j i e r m ) . There i s a close r e l a t i o n s h i p between the forma-t i o n of ; primary r a d i c a l products i n water and the fate of sub-excitation and e., (33). therm Samuel and Magee (84) calculated that the time required f o r an electron to return to the parent ion from a distance varying between 5 and 100 R ranges from 10 to 10 sec. -13 The thermalxzmg process takes about 10 sec. Due to ele c -t r o s t a t i c a t t r a c t i o n , the electron would be drawn back to the po s i t i v e parent ion (H20 +), thereby n e u t r a l i z i n g i t and pro-ducing a highly excited water molecule, H^O*, such as: H 20 + + e ^ H 20* [2-9] This excited molecule may further d i s s o c i a t e into a hydrogen r a d i c a l (*H) and a hydroxyl r a d i c a l (-OH), according to: H 2 ° * > * H + - ° H [2-10] The «H and «OH r a d i c a l s are formed with s u f f i c i e n t energy to di f f u s e from the solvent cage. Concerning the fate of H 20 + formed i n the primary i o n i z a t i o n [2-8] , i t i s highly probable that t h i s species reacts with another water molecule before i t i s neutralized by the electron (57): H 20 + + H 20 ^ H 30 + + -OH ... [2-11] 16 When H^O* i s neutralized by recapture of an electron, the unstable, p a r t i a l l y solvated e n t i t y , H^O, i s produced. This can undergo two thermal d i s s o c i a t i o n reactions, as: H 30 + + e > H 30 » H 20 + -H ... [2-12] H,0 ;—» H o0 + + e „ > H_0 + «H ... [2-13] where: 3 <—' 3 aq 2 e represents the solvated electron; i . e . , e a g = H 20" = (H 20)". The net outcome of eithe r [2-10] or [2-11] and [2-12] i s pro-duction of the same products, *H and *OH r a d i c a l s . According to Gray (36) and Lea (58), the,secondary electron would t r a v e l approximately 150 2 from the H 20 + before i t s energy i s thermalized. They postulated that the *OH r a d i c a l i s produced near the track of the secondary electron by [2-11]. With regard to the secondary electron, which i s free of ele c -t r o s t a t i c a t t r a c t i o n from H 20 + and away from the track, they suggested that i t produces -H atoms on reaction with H 20 mol-ecules, as: H 20 + e > OH + -H [2-14] Further, «H reacts with any solute present. Platzman (79) considered energy loss by secondary 17 electrons and estimated that a 10 eV electron would t r a v e l approximately 50 A5 away from i t s parent ion before i t s energy i s thermalized. At such a distance the electron would not be influenced by the e l e c t r o s t a t i c f i e l d of the parent ion. Thermal equilibrium between e_, and water was established ^ therm and the reaction was found to be r e l a t i v e l y slow (approximately 10 ^ sec). From t h i s point of view, it-was concluded that electrons ejected i n the water i o n i z a t i o n process become ther-malized and hydrated: e_, -> e .. [2-15] ' therm aq Both the Samuel-Magee and Lea-Gray-Platzman models are pres-ently accepted as describing processes occurring i n i r r a d i a t e d aqueous systems. In spurs, where i n i t i a l r a d i c a l concentration i s high, the following recombination reactions (86) are possible: •H + -OH > H 20 [2-16] H + -H > H 2 [2-17] 2H-0 i + e - > H, + 20H~ [2-18] aq aq 2 •OH + 'OH > H 20 2 [2-19] The hydrogen gas and hydrogen peroxide are molecular products, whereas the «H, e and «OH formed [2-10 to 2-15] are r a d i c a l aq products. 18 Radiation-induced water decomposition can be summarized as: H 2 ° Y > H3°aq' ' 0 H ' e a q ' * H' H2°2' H2 • • • I 2 " 2 0 ] Of these species, the r o l e of •OH r a d i c a l s and solvated elec-trons, e , are the most important i n r a d i a t i o n chemical reac-aq t i o n s . G-values of water r a d i o l y s i s products have been shown (97) to be: G ( i ) = 2 ' 7 GCOH) " 2 ' 7 5 aq G(.H) " ° - 5 5 G(0H-) = °- 1 G ( H 3 0 + ) ~ 2 * 8 G ( H 2 ° 2 ) " °' 7 G ( H 2 ) = ° ' 4 5 The formation of these products i n i r r a d i a t e d water i s inde-pendent of type and energy of r a d i a t i o n . Radiation y i e l d s (G-values), depend on e f f e c t s of l i n e a r energy transfer (LET) from r a d i a t i o n and upon other parameters, such as scavenger r e a c t i v i t y , pH, oxygen and dose rate (33). The hydroxyl r a d i c a l , «0H, and i t s dimer H202 are the main, and p r a c t i c a l l y only o x i d i z i n g primary products of water r a d i o l y s i s . Reducing species among primary products are the e and «H also present i n i r r a d i a t e d water (33). Since e aq r aq 19 and «0H play important roles i n aqueous systems (33) , only these two are discussed further. 2.2.2. Solvated electrons (e ) aq The symbol, e a g ' ^ s u s e ( ^ t o represent the solvated (hydrated) electron, although I^o" or (1^0) may be preferred as more convenient terms when balancing chemical equations (86). E a r l i e r investigations have shown that most chemical reactions occurring i n i r r a d i a t e d aqueous solutions can be explained, at l e a s t s u p e r f i c i a l l y , by mechanisms involving •H and -OH. In neutral and a l k a l i n e solutions more d e t a i l e d studies on reactions have indicated that e i s to be viewed aq as the major reducing species (16, 39, 98). Hart (39) proposed that e i s produced by the primary i o n i z a t i o n of i r r a d i a t e d water, as: 2H-0 — > H->0+ + e + -OH [2-21] This combines [2-8] and [2-11]. The e may be converted to «H aq under appropriate conditions [2-13], Conversion of e to «H i s completed even i n d i l u t e acids: e + H + } -H [2-22] aq In many reactions both e and «H lead to the same aq f i n a l p r o d u c t s . With a few e x c e p t i o n s , however, e i s a much more r e a c t i v e s p e c i e s than «H,as can be seen from r a t e c o n s t a n t s i n T a b l e 1 ( 3 3 ) . A l l e n and Schwarz ( 1 1 ) r e p r e s e n t e d the r e l a t i o n s h i p between r e d u c i n g s p e c i e s , i . e . , e , «H and an a c i d i c form aq of the hydrogen atom, H 2 , as: -OH" + H + , i „ < > -H C > -H, [ 2 - 2 3 ] Work on the r a d i o l y s i s of monochloroacetic a c i d by Hayon and Weiss ( 4 4 ) supported the concept of a second r e d u c i n g s p e c i e s , ea q « Hydrogen i s a major product from r a d i o l y s i s of monochloro-a c e t i c a c i d s o l u t i o n s but i s r e p l a c e d by c h l o r i d e i o n s i n n e u t r a l s o l u t i o n s . T h i s can be e x p l a i n e d i f the r e d u c i n g s p e c i e s p r e s e n t are *H and e a g , a s : •H + C 1 C H 2 C 0 0 H > H 2 + C 1 C H C 0 0 H [ 2 - 2 4 ] e +' C 1 C H - C 0 0 H > C l + -CH OC00H .. [ 2 - 2 5 ] clC[ *C /L Regarding d i f f e r e n c e s i n chemical behaviour of e a g and *H, i t was observed t h a t e ^ does not a b s t r a c t hydrogen atoms from o r g a n i c compounds, whereas *H does (86). A prominent r o l e i s p l a y e d by c o n c e n t r a t i o n of s o l u t e s i n aqueous s o l u t i o n s on e a ^ decay k i n e t i c s . Decay of e ^ a t low and h i g h c o n c e n t r a t i o n s was found to f o l l o w second o r d e r 21 k i n e t i c s , which has been interpreted as i n d i c a t i v e of one r decay mode (39) . Second order k i n e t i c s r e s u l t when: 2H„0 hq + e a q — H 2 + 20H [2-26] or; e a q + * H * H2 + 0 H [ 2 " 2 7 ] In a l k a l i n e solution and i n solutions containing *0H scavengers, the reaction of e with *H i s i n f e r r e d from decay curves. On aq J scavenging *0H, the remaining reactant for e a ^ i s «H and eac_ decays by [2-26] and [2-27]. Almost a l l e reactions seem to be electron-transfer aq reactions. In a l l these cases the primary product acquires an a d d i t i o n a l electron: e n + A n > A n _ 1 [2-28] where; A i s an atom or a polyatomic molecule and n i s an integer i n d i c a t i n g charge. Often the primary product i s thermodynamically unstable and undergoes futher reactions such as protonation, d i s s o c i a t i o n , disproportionation or charge transfer. P o s i t i v e inorganic ions are much more reactive with e a ^ than are negative ones, and the higher the charge the higher the rate constant (33). 2.2.3. Hydroxyl r a d i c a l (*0H) Both stationary state and pulse r a d i o l y s i s i n v e s t i g a -tions indicate that -OH, which i s an ox i d i z i n g species, i s an important fragment i n i r r a d i a t e d water. The *0H ari s e s mainly from rapid reaction of H 20 +, which i s formed d i r e c t l y during the energy absorption process [2-8] . Diss o c i a t i o n of excited water molecules produced by either d i r e c t electron e x c i t a t i o n or as a r e s u l t of electron recapture by the parent H 2 0 + can also form *0H [2-10 and 2-11]. The .OH ox i d i z i n g species i s produced by photolysis of hydrogen peroxide (H 20 2), as : H 20 2 + hv > 2-OH [2-29] Other reactions also lead to production of 'OH r a d i c a l s i n rad i a t i o n chemistry. Thus, the primary reducing species (•H or e ) reacted with H 00 0 produces «OH r a d i c a l s (33), accord-ing to: H 20 2 + -H ) H 20 + -OH [2-30] H o0 o + I > OH" + -OH • [2-31] 2 2 aq The conversion of e to «OH i s achieved e f f i c i e n t l y aq by nitrous oxide, as: N 20 + e a g > N 2 + 0" [2-32] N 2 + O" + H 20 > N 2 + -OH + OH" .. [2-33] Four main types of reactions occur between *0H r a d i c a l s and stable species or free r a d i c a l s : (i) electron transfer; ( i i ) addition; ( i i i ) hydrogen atom transfer; and possibly (iv) group abstraction. Electron transfer i s the p r i n c i p a l mechanism of«OH-induced oxidation for both inorganic anions and cations. With organic molecules, hydrogen atom abstraction and *0H addition are the most common types of reaction. Addition reactions occur with free r a d i c a l s (33). Electron transfer and addition have been observed as examples of anionic oxidation i n pulsed r a d i o l y s i s . For exampl the ferrocyanide ion i s oxidized to f e r r i c cyanide by -OH,and the l a t t e r ion has a strong o p t i c a l absorption spectrum (3, 30, 40): Fe(CN) 6"" 4 + -OH * Fe(CN) 6~ 3 + OH~ .. [2-34] Dainton and Watt (30) and Rabani and Matheson (80) indicated that the OH anion, 0 , produced at high a l k a l i n i t y , reacted more slowly with the ferrocyanide ion. They measured the i o n i z a t i o n constant of «OH by studying e f f e c t s of pH on these k i n e t i c s . Adams et a l . (4) proposed the formation of O v i a the following reaction: 24 • OH + OH > O + H 20 [2-35] The rate constant f o r [2-35] was found to be 3.6 x 10 M sec ^. Competitive reactions among OH and other *OH scavengers determined the amount of 0 i n solution. The following reaction was found to be very e f f i c i e n t i n a l k a l i n e solutions: 0 + 0 2 > 0 3 [2-36] Adams et a l . (2) have observed also a number of tran-sient absorption sepctra r e s u l t i n g from electron transfer from oxyanions of Group IV, V and VI elements. The spectra are due to electron oxidation products i n which the elements are i n an oxidation state one higher than the reacting solute, as: A 0 " n + -OH » A O ~ ( n 1 J + OH~ ..[2-37] x y x y Other reactions include those of transient species, such as S 2 0 3 from thiosulphate; CO^ and SiO^ from carbonate and s i l i c a t e ; N0 2 from n i t r i t e ; SeO^ from selenite; and TeO^ from t e l l u r i t e . The halide ions C l , Br and I react r a p i d l y with 'OH. Electron transfer here leads to formation of free r a d i c a l s : Br~ + *OH > -Br + OH- [2-38] •Br + Br > B r 2 [2-39] Numerous c a t i o n i c electron-transfer reactions are w e l l -known by which «0H produces stable ions. Thereby, transient spectra for unstable higher oxidation states of many metals have been characterized (33), such as: F e 2 + + «0H > F e 3 + + 0H~ [2-40] 2.2.4. E f f e c t s of oxygen Radiolysis of aqueous solutions i s much affected by dissolved oxygen for two reasons. F i r s t l y , free r a d i c a l s produced from the solute may react with oxygen, leading to formation of peroxy r a d i c a l s . The reaction of oxygen with free r a d i c a l intermediates may change the f i n a l products from those obtained i n the absence of oxygen. In the case of either organic solutes or organic impurities i n aerated aqueous inorganic systems t h i s i s usually more important. For example: • OH + RH * «R + H 20 [2-41] i •R + 0 2 > -R02 [2-42] •R02 + H + + F e 2 + > F e 3 + + R02H .. [2-43] R02H + F e 2 + > F e 3 + + -RO + OH- [2-44] •RO + H + + F e 2 + > F e 3 + + ROH ... [2-45] where; R yrepresents a l k y l or a l l y l groups or hydrogen. 26 +3 Thus, each -OH brings about the oxidation of 3 Fe ions instead +3 of only one. Y i e l d of Fe i s commensurately higher. Secondly, oxygen i s a very e f f i c i e n t scavenger for hydrogen atoms and combines with them to give the perhydroxyl r a d i c a l , *H02. In i r r a d i a t e d aqueous solutions t h i s reaction occurs almost i n v a r i a b l y i f dissolved oxygen i s not completely removed [2-42]. Because of the high p r o b a b i l i t y of t h i s con-version, traces of oxygen modify the course of many r a d i a t i o n -induced reactions. For example, when oxygen i s present i n the Fricke dosimetric sol u t i o n [2-42] takes place, as do (86): F e 2 + + «0H > F e 3 + + 0H~ [2-40] H 20 2 + F e 2 + > F e 3 + + -OH + OH" [2-46] •H02 + F e 2 + > F e 3 + + H0 2" [2-47] H0 2" + H + > H 20 2 [2-48] Therefore, the ra d i a t i o n y i e l d of G^Fe3+^ i s : G(Fe 3 +) - 2 G(H 20 2) + 3G(-H) + G(-OH) " 1 5 ' 6 However, i n the absence of oxygen, [2-42] i s replaced by [2-23] and: •H + H + -> H 2 + [2-23] 27 H 2 + + F e 2 + > F e 3 + + H 2 [2-50] Reactions [2-47 and 2-48] do not occur. For t h i s reason, G^ F e3+j i n the absence of oxygen i s given by: G(Fe + 3) = 2 G(H 20 2) + G(.H) + G(.OH) = 8 ' 2 [ 2 " 5 1 ] The *H or e a (_ i s replaced by 'HO,, or Oj , i . e . , oxygen i s very e f f i c i e n t scavenger [2-42 and 2-52]: °2 + e a q > °2 [ 2 " 5 2 ] In both above cases, 0 2 can convert reactive species of marked reducing character into less reactive species of mainly o x i d i z i n g character. The rate constant for conversion of «H or e n to .H0o or 0 ~ by 0 o i s 2 x 1 0 1 0 M~ 1sec~ 1 (33). aq £ & A Form of *H02 changes with pH. Neutral, *-H02, and basic, 0 2 , forms are generally accepted, and pKa values of 4.5 and 4.8 have been reported (33) for the equilibrium: H0 2 > H + + 0 2" [2-53] Hence, at high pH values 0 2 i s generally the predominant species. Oxidation state of *H02 l i e s between that for 0 2 and H2^2* T ^ e *H<^2 m a ^ t*ehave as either a reducing or ox i d i z i n g 28 agent (33) •H02 + C e 4 + i H + + 0 2 + C e 3 + ... [2-54] In comparison with -H, 'HC>2 i s a consistently weaker reducing agent for abstracting hydrogen atoms from saturated organic compounds. Consequently, hydrogen abstraction by *H02 i s possible only for those hydrogens e a s i l y removed, such as with ascorbic acid: R2 R2 I I H°2 + R x - C - H > H 20 2 + R][ - C ..[2-55] R3 R 3 As an ox i d i z i n g agent *H02 reacts with an -OH r a d i c a l or another -H02 r a d i c a l to produce 0 2 (33, 86): H0 2 + «0H > 0 2 + H 20 [2-56] •H02 + «H0 2 > 0 2 + H 20 2 [2-57] 2.2.5. E f f e c t s of pH Radiochemical reactions i n i r r a d i a t e d aqueous solutions may be greatly influenced by pH. In some solutions pH deter-mines structure of the solute, but i n a l l solutions changes i n pH may a l t e r nature and r e a c t i v i t y of primary r a d i c a l s , formed during r a d i o l y s i s . The po s i t i o n of equilibrium for substances which are f r a c t i o n a l l y dissociated i n water may s h i f t due to a change i n pH. On the other hand, reactive species produced i n i r r a d i a t e d water may react much more re a d i l y with either the ionized or non-ionized form of the solute. A "prominent r o l e i s played by pH on properties of reactive species present i n i r r a d i a t e d water. The following pH dependent e q u i l i b r i a have been established for reducing species (86): (i) i n acid solution the r e l a t i v e l y slow association + of «H or hydrogen ion (H ) [2-23]; and \_ ( i i ) i n a l k a l i n e solution conversion of «H to e aq [2-23]. For o x i d i z i n g species (86): ( i i i ) i n acid medium [2-11] holds (iv) i n neutral or mildly a l k a l i n e solutions d i s s o c i a -t i o n of «H0 2 follows [2-53] •H02 ) H + + 0 2" [2-53]; and (v) i n a l k a l i n e solution d i s s o c i a t i o n of *0H occurs according to [2-35] with pK about 10. a Many e a r l i e r studies have indicated that pH has con-siderable e f f e c t on y i e l d of reducing species and «OH. The increased r a d i a t i o n y i e l d of r a d i c a l s , G R, without correspond-ing decrease i n r a d i a t i o n y i e l d of molecules (molecular hydrogen 30 and hydrogen peroxide), G M, at pH below about 3 has been accepted without argument (Fig. 1, 2 (19)). At high pH (above about 11), however, increase i n G was observed with s l i g h t decrease i n the y i e l d of ^ 2°2 ^ n P r e v ^ o u s studies. Recently Draganic and Draganic (33) proposed that G R and G M values i n i r r a d i a t e d aqueous systems do not change above pH 13. For above pH 13: G „ _ = 4.09 G„ - = 3.18 2 aq G. 0 H = 2.72 = 0.45 The increase i n a c i d i t y induces an increase i n water decompo-s i t i o n . For example, at pH 1.3 the following values were derived: G-H,0 - 4 ' 3 5 GH+e = 3 ' 4 9 z aq G. 0 H = 2.85 = 0.43 In general, primary y i e l d s do not depend much upon pH (Fig. 2), but do r e l a t e to solute concentration. The increase i n G below pH 3 may be attr i b u t e d to the r e l a t i v e abundance of «H compared to e . This may a f f e c t G D values at extreme pH readings, since species produced by conversion have d i f f e r e n t c o e f f i c i e n t s and d i f f e r e n t recombina-t i o n rate constants i n acid media for G- [2-13]. aq + -H Hence, fewer e are combined with *0H to form non-radical aq products by the following reaction (98): e + «0H } H_0 + OH [2 r58] aq z -c-' Dainton and Peterson (29) have interpreted increased G_. i n acid solutions as due to attack upon the acid by an intermedi-ate. The intermediate i s either an excited water molecule, H20*, or an i s o l a t e d r a d i c a l p a i r , *H and «0H, trapped i n a solvent cage, as: H 20* + -H » H 2 + + -OH [2-59] or; (•H + -OH) + H + > H 2 + + -OH [2-60] The above reactions reduce the extent of [2-16] and the follow-ing reverse r e a c t i o n : H20* } H 20 [2-61] Czapski (27) concluded that i n many cases where G-value seemed to depend on pH or on scavenger concentration, e f f e c t s a c t u a l l y resulted from disregarded back reactions. There i s no strong dependence of primary y i e l d on pH, although the s i t u a t i o n at extreme pH i s not yet c l e a r . In t h i s connection i t should be noted that e f f e c t s at extremely high a c i d i t y or a l k a l i n i t y represent s p e c i a l problems which require p a r t i c u l a r study. \ ^ 2.2.6. Scavengers If a substance Q reacts p r e f e r e n t i a l l y with the primary r a d i c a l *R, formed i n i r r a d i a t e d water, to give the product P according to the reaction: •R + Q > P [2-62] then increase of [Q] should be followed by increase i n stable f i n a l product y i e l d G(pj« This increasing process should continue u n t i l a l l «R species escaping recombination: •R + -R > M [2-63] are scavenged. Scavenging i s the term applied to the deliberate addi-t i o n to a r a d i c a l reaction of a compound which reacts prefer-e n t i a l l y with r a d i c a l s , at the expense of the normal r a d i c a l reaction. The objective i s to i d e n t i f y the r a d i c a l species taking part i n the reaction and to determine what part of the o v e r a l l reaction i s due to scavengeable free r a d i c a l s . Using moderate scavenger concentrations, the scavenging action w i l l be l i m i t e d to r a d i c a l s which have diffused into the bulk of the sample. Reactions i n the track zone w i l l be unaffected, C w thereby allowing the reactions i n these two regions to be d i f f e r e n t i a t e d (86). Effectiveness with which the scavenger reacts with the species *R above also depends on reaction rate constant K«R+cj) ( r e a c t i v i t Y i s usually represented as • [<J>] • sec ^) . Consequently, scavenger concentration, <j), i n the i r r a d i a t e d solution i s very important i f G ^ i s to be taken as a measure of G R. Scavengers do not react with a l l r a d i c a l s at very low -5 - 4 concentrations (10 to 10 M). Other reactions between them and molecular products or impurities may also play an important r o l e , not always e a s i l y c o n t r o l l e d . At moderately low concentrations, scavengers remove a l l those r a d i c a l s which are i n the bulk of the sol u t i o n . When r e a c t i v i t y i s s u f f i c i e n t l y high, scavenger Q also competes for •R species from intra-spur reactions. Then an increase i n G p begins to occur because of G^. The increase i n concentration of a substance reacting with -OH leads to decreased y i e l d of H~On. When the substance i s reacted with *H or e the y i e l d 2 2 aq J of i s decreased ( 3 3 ) . In concentrated solutions the d i r e c t action of radia-t i o n on solute might introduce uncertainties as to the reaction scheme. T h i s d i r e c t e f f e c t might a l s o occur by the i n t e r f e r -ence of s o l u t e s i n e a r l i e r stages of r a d i o l y s i s r a t h e r than a l l o w i n g i n t r a - s p u r r e a c t i o n s . F o r the p r e s e n t , such e f f e c t s are not w e l l e s t a b l i s h e d (33). Dependence o f r a d i c a l y i e l d s on s o l u t e c o n c e n t r a t i o n f o r a number o f s o l u t e s i n c l u d i n g NO^", I and T i + , as w e l l as N.,0, has been s t u d i e d by Hayon (43). In every case the data show t h a t G and G _.„ i n c r e a s e w i t h i n c r e a s i n g c o n c e n t r a -• H+e_ 'OH 3 aq t i o n of s o l u t e (43). N i t r o u s o x ide (N 20) i s w i d e l y used as an e l e c t r o n scavenger because of e f f i c i e n c y of r e a c t i o n [2-32] and because the i n e r t product n i t r o g e n (N 2) a f f o r d s a convenient measure of G. In e a r l y i n v e s t i g a t i o n s on r a d i o l y s i s of N 20 s o l u t i o n s , Dainton and Peterson (29) found t h a t as N 20 c o n c e n t r a t i o n was i n c r e a s e d G^ N ^ i n c r e a s e d to a p l a t e a u v a l u e of 3.1 f o r N 20 2 -2 c o n c e n t r a t i o n s l a r g e r than 1.3 x 10 M. R e c e n t l y , Head and Walker (45) s t u d i e d G ( N j as a f u n c t i o n of N^O over the range -2 2 0<[N 0]<1.2 x 10 M. They observed t h a t f o r low c o n c e n t r a t i o n s 2 G^ N j i n c r e a s e d r a p i d l y from 0 to 2.45 as N 20 c o n c e n t r a t i o n 2 . -5 was r a i s e d t o 4 x 10 M. The r e a c t i o n then f o l l o w e d a slower G / v t . i n c r e a s e from 2.45 to 3.1 as N.O c o n c e n t r a t i o n was r a i s e d (N~) 2 -2 t o 1.2 x 10 M. These r e s u l t s were i n t e r p r e t e d as i n d i c a t i n g t h a t e f f i c i e n t scavenging a t low N 20 c o n c e n t r a t i o n r e p r e s e n t s r e a c t i o n [2-32]. Hence, G. . = G,- , = 2.45. The second and slower r i s e was a t t r i b u t e d to scavenging of a second r a d i a -t i o n product e n t i t y , rather than intra-spur scavenging of ^ aq« This second species was not i d e n t i f i e d by the authors. I t seems u n l i k e l y that i t could be *H, since Mahlman (60) has reported that for saturated N 20 solutions G ( N 2 j i s unaffected by the presence of e f f i c i e n t «H scavengers such as 2-propanol and sodium formate. On the other hand, i t has been suggested (19, 28) that the following may be a source of N 2 at high N 20 concentration: H 20* + N 20 » N 2 + 20H [2-64] In N 20 saturated solutions at room temperature, i t s concentra--2 t i o n i s about 2.5 x 10 M. C l e a r l y , under these conditions G< N2) = G ( i a q ) * 2.2.7. Other e f f e c t s I t i s evident that chemical e f f e c t s of i o n i z i n g r a d i a -t i o n depend upon rate at which i o n i z a t i o n occurs i n the aqueous medium. The density of spurs w i l l depend upon i n t e n s i t y of i o n i z i n g r a d i a t i o n . Chemical e f f e c t s w i l l depend i n turn upon t h i s i n t e n s i t y or dose rate. In other words, increased dose rate should have e f f e c t s on r a d i a t i o n y i e l d s i m i l a r to those observed i n the case of LET, since increased density of primary events i s involved. Therefore, y i e l d s of stable re-action products should increase or decrease, depending upon whether these are formed by reactions with molecular or r a d i c a l products (33). 36 Increase of temperature might lead to a broader d i s -t r i b u t i o n of primary species and, thereby, to increased GR and decreased G M. According to Hochanadel and Ghormley (48), changes caused by increased temperature are indeed i n agreement with these expectations. Since changes are small, however, i t i s understandable why usual v a r i a t i o n s of room temperature have no appreciable e f f e c t on measured y i e l d s of primary pro-ducts. S i m i l a r l y , the e f f e c t of temperature on reaction rate constants i s small. These increase by only a few tenths of a per cent per degree. Increased pressure causes considerable s t r u c t u r a l change i n water. However, Hentz et a l . (46) concluded that there i s no pressure e f f e c t on the primary y i e l d s of free r a d i c a l s and molecular products i n acid medium. The rate constants of some of the reactions studied were found to be appreciably affected by changes i n pressure, although no e f f e c t of pressure on the primary y i e l d s from water r a d i o l y s i s could be proven. 2.3. Radiolysis of Inorganic Sulphur Compounds i n Aqueous Environment Radiolysis of aqueous solutions for many inorganic compounds have been studied. Systems discussed here are those which are c h i e f l y anionic. S p e c i f i c a l l y , t h i s short review considers only sulphur containing anions. Concerning the r a d i o l y s i s of anions i n aqueous media, the presence of *0H with high ox i d i z i n g power or «H (or e ) with aq high reducing power probably causes most anions to undergo electron transfer reactions, such as: A" +-0H > A + OH" [2-65] A + e a g » A" . [2-66j^) (A + -H > A~ + H +) The r a d i o l y s i s of aqueous inorganic sulphur solutions has been of some recent i n t e r e s t . In constrast to n i t r i c and perchl o r i c acid, sulphuric acid i s r e l a t i v e l y stable to radia-t i o n (31). In d i l u t e or moderately concentrated solutions of sulphuric acid, peroxysulphuric acids are formed, i . e . , H2SO5 and H2S20g. A product with reducing properties s i m i l a r to hydro gen was i d e n t i f i e d , but there was no evidence for presence of SO2 (31) . However, Hochanadel et a/1. (49) did f i n d evolution of SO2 upon i r r a d i a t i o n of aqueous sulphuric acid solutions of varying concentrations. These investigators found decreased rate of SO2 production with increased dose. Addition of hydro-gen peroxide or oxygen to the soluti o n also diminished SO2 formation. Packer (76) i r r a d i a t e d aqueous H 2S solutions with 6 0 C o y-rays. The products found were S Q, SO4 , H2 and H2O2. Complete scavenging of *H and «0H r a d i c a l s was assumed i n the mechanism proposed: 38 H 2S + -OH > -HS + H 20 [2-67] H 2S + -H > -HS + H2 [2-68] 2»HS y H 2S + -S ..... [2-69] Both presence of 0 2 and increased concentration of H 2S pro-duced a r i s e i n G ( s D ) . Packer (76) suggested a chain mechanism involving the reactions: •HS + 0 2 > «HS0 2 [2-70] •HS02 + H 2S » H 2 S 0 2 + * H S [2-71] (slow step) The «HS0 2 probably react r a p i d l y among themselves and with H 2S and H 20 2 to y i e l d «S and S O 4 . Aqueous potassium thiocyanate solutions have been i r r a d i a t e d with y-rays from 6 0 £ O sources (34). The products were CN , S O 4 and S Q. Since y i e l d of sulphur did not i n -crease with concentration of potassium thiocyanate, formation of S Q was attributed to i n d i r e c t action. Abellan (1) studied the e f f e c t of 6 0 C o y-radiation on Na2S203 a l k a l i n e solutions. The r a d i o l y s i s products report-ed were SO3 and H2S gas. Presence of S Q was noted only at higher doses. The acid was estimated potentiometrically by decrease i n solution a l k a l i n i t y . As a measure of S O 3 and H 2S gas quantities formed, iodometric estimation of the increase 39 i n reducing power was taken. Removal of E^S gas by bubbling N 2 permitted estimation of S0 3 ions alone. The following mechanism was suggested: 2S 20 3~~ + 4-OH + 2-H > HS" +3S03"~ + 3H + + H 20 ...[2-72] Here, G, — . was found to increase with concentration and ^ b2 u3 ' i t s asymptotic l i m i t was reached at 0.06 M. I t has been shown by Ivanitskaya and Nanobashvili (50), Murthy (65) and N a t r o s h v i l i and Nanobashvili (70) that i n r a d i a t i o n of Na_S~0-, solutions SO. , SO-, ~ and S are formed 2 2 3 4 - 3 o with G-values of 0.8 ± 0.1, 0.55 ± 0.1 and 0.16 ± 0.02, respective-ly , while G. „ _ _ » i s 0.65 ± 0.12. The intermediate formed (-Na 2S 20 3) i s SO- and end products are SO. and S . If i r r a d i a t i o n i s 3 4 o done i n the presence of N 20, G ( _ N a s o ) = 2«95. A r a d i c a l mechanism i s involved with «OH formed as a reactive intermediate i n the i r r a d i a t e d water during r a d i o l y s i s of aqueous Na 2S 20 3 solutions. Nanobashvili et a l . (68, 69) investigated r a d i o l y t i c oxidation and reduction of sulphur compounds. Main chemical e f f e c t s of high energy r a d i a t i o n on d i l u t e aqueous solutions were formation of -H and -OH. The oxidative e f f e c t s of x- and y - i r r a d i a t i o n on various classes of sulphur compounds, aqueous solutions of sulphides and thiocyanates of a l k a l i metals have been studied. The HS~, S~~, [Fe(CN) g]~ 3, [Fe(CN) g]~ 4 and CNS~ ions become oxidized by «OH or H 20 2 molecules. The G-value was dependent on concentration and r a d i a t i o n i n t e n s i t y , which indicates that a chain reaction mechanism has taken place. No reference to polysulphide formation i n aqueous sulphide solutions by r a d i a t i o n has been found i n the l i t e r a t u r e . However, several investigations on y - r a d i o l y s i s of sulphur containing organic compounds have shown disulphides to be formed as main conversion products (22, 72). Nishimura et a l . (72) have investigated y - r a d i o l y s i s of c h a r a c t e r i s t i c sulphoxide amino acids i n Allium species, e.g., PCSO (s-n-propyl-L-cystein sulphoxide). Di-n-propyl disulphide, alanine and c y s t e i c acid formed as main products from PCSO i r r a d i a t e d i n oxygen-free aqueous solutions were i d e n t i f i e d by IR and mass spectrometry. In order to elucidate degradation mechanism for the deoxygenated v o l a t i l e products (di-n-propyl sulphide and di-n-propyl d i s u l -phide) obtained from i r r a d i a t e d PCSO, 20 M oxygen-free aqueous solutions of various PCSO concentrations containing N 20 ( s p e c i f i c scavenger for e ) or KBr ( s p e c i f i c scavenger for -OH) were aq i r r a d i a t e d . As a r e s u l t i t was proposed that PCSO reacts with •OH and with both e and «OH to produce di-n-propyl sulphide and di-n-propyl disulphide, respectively. 41 3.0. MATERIALS AND METHODS A series of i r r a d i a t i o n experiments was c a r r i e d out with aqueous sodium sulphide (Na2S) solutions and additives; such as nitrous oxide (N 20), 2-propanol (CH3CHOHCH3), oxygen (0 2) and hydrogen sulphide (H 2S). Nitrogen gas (N2) was bubbled through the system to remove oxygen. Following i r r a d i a t i o n treatments, polysulphide analyses were done by the UV spectrophotometric technique and G ( s n s ) values were calculated. 3.1. Sample Preparation A l l chemicals were of the best grades av a i l a b l e ; N 20, N 2 and 0 2 of medical therapy grade were obtained from Matheson of Canada; a n a l y t i c a l grade Na 2S and CH3CHOHCH3 were purchased from Fisher Chemical Co. Solution pH was adjusted, where desired, with reagent grade H2S04. P u r i f i e d d i s t i l l e d water was obtained from the Chemistry Department, Univ. of B.C. This ultra-high p u r i t y water was further treated by i r r a d i a -t i o n with 1 Mrad to destroy any trace organic impurities. The Na 2S«9H 20 contained 1 to 2% impurities as oxida-t i o n products, among which Na 2S 203, Na 2S03, Na 2S 2 and NaOH are the most common. To obtain pure anhydrous Na 2S, the Na 2S«9H 20 was f i r s t heated slowly under vacuum to 300°C to remove hydration water, then further heated to 600°C to remove the remaining water and impurities (83) . The anhydrous product thus obtained was white and repeat analyses according to Bethe (17) indicated a value of 100.0 ± 0.2% Na 2S. The white color was also an i n d i c a t i o n of high p u r i t y . For instance, the presence of as l i t t l e as 0.1% polysulphide was found to cause noticeable d i s c o l o r a t i o n . Aqueous solutions were made with the treated u l t r a -high purity water and prepared anhydrous Na 2S. To prevent reaction with other impurities, and to obtain a high c a t i o n i c -3 concentration for polysulphide s t a b i l i z a t i o n , 10 M NaCl was added to these solutions. Working concentrations of Na 2S solutions were 10, 40 and 80 g/1. I n i t i a l pH of the corres-ponding solutions was 12.7. Solutions at pH 1.0 and 7.0 were made by adding H2SC"4. A l l pH measurements were made on a Radiometer Type M 26 pH meter. Samples (50 ml) of the above solutions were introduced into Erlenmeyer fl a s k s which were sealed i n a s t a i n l e s s s t e e l (SS) vessel. Gas (N 2, N 20, H2S, 0 2) was admitted to the vessel as required. Sample solutions examined i n t h i s study can be c l a s s i -f i e d according to the following four groups: (i) Na 2S solutions (40 g/1 at i n i t i a l pH 12.7) without additives; ( i i ) Na 2S solutions (10, 40 or 80 g/1 at i n i t i a l pH 12.7, 40 g/1 at i n i t i a l pH 1.0 and 7.0) with 'OH and e aq scavengers; ( i i i ) Na 2S solutions with scavengers and H 2S at i n i t i a l pH 12.7; and (iv) Na 2S solutions with H 2S at i n i t i a l pH 12.7. Certain preparatory experiments were done to t e s t s u i t a b i l i t y of the systems used. A SS high pressure c e l l of in t e r n a l diameter 50 mm and 100 mm depth, and containing a truncated Erlenmeyer f l a s k as container, was used (Fig. 3). The g a s i f i c a t i o n apparatus i s shown i n F i g . 4. Pr i o r to each experimental series the whole glass apparatus was cleaned. F i r s t , i t was cleaned by soaking i n permanganate-acid (KMnC>4 i n 95% H2SC>4) and then rinsed with d i s t i l l e d water. Second, i t was soaked i n a biodegradable deter-gent solution to remove a c i d i c traces, rinsed i n tap water, then d i s t i l l e d water and, f i n a l l y , with the treated u l t r a -high pu r i t y water. Following this/the apparatus was dried i n an oven at 150°C and stored i n dust free atmosphere. 3.2. Sample I r r a d i a t i o n Cobalt-60 y-rays from a Gamma C e l l 220 were used to i r r a d i a t e the solutions, which were at 34°C and kept under a gas pressure of 20 p s i during i r r a d i a t i o n . Solutions were f i r s t deoxygenated by bubbling N 2 through the system for about 20 min before purging for 20 min with experimental gas to completely remove N 2. The solution was then pressurized to 20 p s i . At t h i s pressure v i r t u a l l y a l l gas dissolved i n the solution. The pressurized sample was l e f t to e q u i l i b r a t e for about 1 hr. During i r r a d i a t i o n , a g l a s s - d i f f u s i o n tube was used i n order to provide mixing of the l i q u i d and pressurized gas i n the f l a s k . I r r a d i a t i o n dosages ranged from 0.02 to 20 Mrad. Af t e r i r r a d i a t i o n , the vessel was slowly depressurized through the trap system, the SS vessel cap was removed and the solution was analyzed. In order to determine the absorbed dose, several methods may be employed. For these experiments, Fricke dosimetry was used. Dose rates of the Cobalt-60 source inside and outside the SS vessel were determined by Fricke chemical dosimetry as f i r s t proposed by Fricke i n 1929 and since modified by others (86). Fricke dosimetry u t i l i z e s an a i r saturated solution of ferrous sulphate i n 0.8 N H2SO4 to measure absorbed dose. In the a c i d i c solution the two primary reducing species, e and «H atoms, are converted to o x i d i z i n g species by [2-13, 2-42 and 2-43] or the following reactions: H + + H2C>2 + F e 2 + ^ F e 3 + + -OH + H 20 ..[3-1] H + + .OH + F e 2 + > F e 3 + + H 20 [3-2] Thus, each «H formed from [2-13] leads to oxidation 2+ 2+ of 3 Fe , whereas each «OH oxidizes only 1 Fe . For Cobalt-3+ 60 y r a y s the t o t a l G-value of Fe has been determined as G(pe3+) = 15.5 (86). The f e r r i c ion i s determined spectro-photometrically at 304 nm, where the molar e x t i n c t i o n c o e f f i -c ient (e) i s 2,197 M^cm - 1 at 25°C. e increases by 0.69% for each degree increase i n temperature. Absorbed dose, D (in rad), i s calculated by the follow-ing general formula (33): „ _ N A (CO) 100 , r_ D = —r~T7n r oT 5 T " rad [3-3] e • 10J • G(pe3+) • f • p • 1 where: 23 N = Avogadro's number at 6.022 x 10 molecules per mole; A (OD) = difference between o p t i c a l d ensities of i r r a d i a t e d and control samples; e = molar e x t i n c t i o n c o e f f i c i e n t of 2,174 M~1cm-1 at 23.7°C; f = conversion factor for t r a n s i t i o n -from electron v o l t s per m i l i l i t r e units into rad (6.245 x 1013); p = density of the dosimeter solution at 1.024 for 0.8 N H 2S0 4; and 1 = o p t i c a l path length i n cm. The ferrous sulphate dosimeter i s very s e n s i t i v e to traces of organic impurities. When organic material (R' H) i s present [2-41 to 2-45] may occur. Thus, each *0H brings 3+ about the oxidation of 3 Fe instead of only one, and the 3+ y i e l d of Fe i s commensurately higher, i . e . , G(Fe3+) > 15.5, 4 6 This leads to errors i n determination of dose or dose rate. To suppress [2-41] C l ~ i s normally added to the dosimeter solution: •OH + C l " * 0H~ + -Cl [3-4] •Cl + F e 2 + > F e 3 + + C l " [3-5] Since C l i s i n high concentration r e l a t i v e to organic impuri-t i e s the fate of -OH w i l l be reaction [3-4], rather than [2-41]. 2+ Because of the e f f e c t of -Cl on Fe , i . e . , equivalent e f f e c t to «0H, there w i l l be no change i n G(Fe3+) (86). If oxygen i s not present i n the solution, [2-42] and [2-43] w i l l not occur. The r e s u l t i s that G (pe3+j becomes 8.2 (86). Dosimetry r e s u l t s for i r r a d i a t i o n i n the high pressure SS c e l l are plotted as the absorbance vs. i r r a d i a t i o n time i n F i g . 5. From slope of the l i n e , and taking G(p e3+) = 15.5, dose rate inside/outisde the SS vessel was calculated 3 3 by [3-3] to be 8.876 x 10 / 12.24 x 10 rad per minute on March 10, 1972. Subsequent corrections were made for v a r i a t i o n i n dose rate due to source decay. 3.3. Analyses Standard polysulphide solutions were prepared by heat-ing known amounts of p u r i f i e d anhydrous Na 0S together with 1 g/1 of sublimated sulphur i n ethanol. Polysulphide i n ethanol i s known to have an X value of about 7 (88). After s the ethanol was evaporated under vacuum at 50°C, an appropriate amount of boiled d i s t i l l e d water was added. To avoid oxida-t i o n of S nS to S 2 0 3 , the solution was l e f t standing with the suspended sulphur overnight under nitrogen. Subsequent removal of the suspended sulphur together with the impurities by f i l t r a t i o n i s known (88) to r e s u l t i n a polysulphide solu-t i o n having a high X g value (= 4), low S 2 0 3 content (<0.~005 moles / l ) and low a l k a l i n i t y (0.001 M OH~/l). The solution thus obtained was stored under a layer of p a r a f f i n o i l . The c h a r a c t e r i s t i c absorbance of S S at 285 nm wave n length was measured with a TJnicam SP 800 UV Spectrophotometer using trough-like s i l i c a c e l l s of 0.01, 0.2 and 1.0 cm path lengths. Most of the solutions could be studied without d i l u t i o n throughout concentration ranges encountered i n t h i s work by using these c e l l s of varying path length. Due to the strong a b s o r p t i v i t y of polysulphide, and i n s t a b i l i t y of very d i l u t e polysulphide solutions, the usual 1.0 cm c e l l s are unsuited for measurements at high polysulphide concentrations (15, 37, 89, 91, 94). Since polysulphide concentration i s known to be dependent on the sodium sulphide concentration r (15), three c a l i b r a t i o n curves were prepared corresponding to Na 2S concentrations (10, 40 and 80 g/1) used i n the study. Peak heights i n r e l a t i v e units at 285 nm absorbance were plotted against polysulphide excess sulphur concentration to give c a l -48 i b r a t i o n curves (Fig. 6). These plots of absorbance vs. polysulphide excess sulphur followed Beer's law. Spectrophotometric polysulphide analyses were performed on the various experimental solutions according to the method of Teder (89). Some solutions were d i l u t e d with corresponding o r i g i n a l stock s o l u t i o n before measurements could be done. An al k a l i n e 3 M NaCl (10~ 2 to 1 0 - 3 M 0H~ + 3 M NaCl) solution was used as reference. For wave length c a l i b r a t i o n s i n the u l t r a v i o l e t region an absorption spectrum was obtained by measuring absorbance of a 2 mm thick piece of holmium oxide glass (14). Since with even the greatest of care, small amounts of polysulphide were formed i n stock solutions, experimental data were adjusted accordingly. Cleaning of UV c e l l s between measurements was as follows. F i r s t , a l l traces of sample solution were removed and the c e l l s were washed i n carbon disulphide (CS 2) and d i s t i l l e d water. Second, a washing was done with concentrated n i t r i c acid (HNO^) i n an ul t r a s o n i c bath, followed by r i n s i n g with d i s t i l l e d water. Several changes of hot and cold d i s t i l l e d water were used. Third, the c e l l s were rinsed thoroughly i n cool reagent grade ethanol, and allowed to dry while protected from dust contamina-t i o n . ^ From 3 to 5 r e p l i c a t i o n s were done for each treatment. Although substantial y i e l d s of polysulphide were observed to accompany treatments, the r e s u l t s were not e n t i r e l y reproducible. In addition, i t was not possible to determine exactly how much polysulphide was produced i n the solution alone, since at a l l times t h i s was under an over-gas. The concentration of polysulphide excess sulphur i n the sample was calculated from absorbance as follows (89): [3-6] where: C = concentration of polysulphide excess sulphur i n the sample, g-atom l i t r e ; A = absorbance; E = a b s o r p t i v i t y = 1,360 l i t r e , cm ^ (g-atom excess sulphur)"^"; and b = path length of the c e l l (cm). In order to express the y i e l d of a species or product formed by r a d i o l y s i s the term "G-value" i s used. G-value i s defined as the number of molecules, ions, atoms or r a d i c a l s produced (or consumed) per 100 eV of energy absorbed by the r a d i a t i o n . In t h i s system, G-value was" calculated as the number of sulphur atoms associated per 100 eV r a d i a t i o n energy. Since polysulphide excess sulphur i s a heterogeneous compound, i t s exact molecular weight cannot be e a s i l y determined by normal chemical analysis. Thus, the G-value i s also an expression of X value, s For "primary" species or "molecular" products, the 50 notation "Gx" i s generally used and G x i s normally i n the range of 0 to 10. For secondary processes involving chain reactions i t i s not usual for a G-value to exceed 10. I t i s necessary to know the t o t a l amount of energy absorbed by the system i n order to measure the G-value of any species formed by r a d i a t i o n . This amount of energy i s termed the "absorbed dose" which i s expressed i n a va r i e t y of units, among which "rad" i s commonly used. One rad i s defined as the equivalent of 100 ergs per gram i n energy. G-values were obtained from the i n i t i a l slope on graphs of polysulphide y i e l d vs. ra d i a t i o n dose. 51 4.0. RESULTS The r o l e of -OH and e on formation of polysulphide was studied by using scavengers. E f f e c t s of ra d i a t i o n dose, Na 2S concentration, pH, E^S over-pressure and 0^ on p o l y s u l -phide y i e l d were also studied (Fig. 7 to 17). The mechanism of polysulphide formation from aq Na 2S solutions by y r a d i a t i o n was investigated by using N 20 as an e scavenger and 2-propanol as a «OH scavenger (Fig. 7, 9 to aq 13, 16 and 17). An aq 40 g/1 Na 2S solution without scavengers was found to have G^g g — ^ value of 0.75 ± 0.01. Adding N 20 gave the much higher G,„ c — . value of 5.79 ± 0.6, while a ' n ' G / c c — . value of 0.39 ± 0.01 was obtained for the comparable ( S n } 2-propanol system (Fig. 8, 11 and 13 and Table 2). Dependence of r a d i a t i o n y i e l d on absorbed dose for aq Na 2S solutions i s given i n Table 2. The absorbed dose i s expressed i n Mrad, while y i e l d s are given i n g/1. G.g g — . n values were calculated by p l o t t i n g polysulphide y i e l d s as functions of absorbed dose. I t i s observed from dose-yield r e l a t i o n s h i p s that y i e l d per uni t absorbed dose increased d i r e c t l y with dose, except i n 0 2 addition experiments. The highest y i e l d obtained was 14 g/1 polysulphide from 20 Mrad i r r a d i a t i o n of the 80 g/1 aq Na 2S - N 20 - H 2S system (Table 2). Solute concentrations of Na 2S solutions used were 10, c 40 and 80 g/1. For the 10 g/1 aq Na 2S solution, G.g g — . n was founded to be 1.35 + 0.2. A G-value of 7.84 ± 0.8 was obtained for the 80 g/1 solution, and for the 40 g/1 solution a larger range of G. c — . values were observed depending upon the experimental system used (Fig. 7, 16 and 17, and Table 2). The e f f e c t of pH (1.0, 7.0 and 12.7) on r a d i a t i o n y i e l d s was also studied. The G-value at pH 12.7 was 5.79 ± 0.6 for the 40 g/1 aq Na 2S - N,,0 system. The comparatively lower G-values at pH 1.0 were 0.63 ± 0.01 for Na 2S - N 20, and 0.53 ± 0.01 for N a 2 s " 2-propanol systems. The highest G-value i n the pH series studied was 10.06 ± 1.0, found with the 7.0 pH 40 g/1 aq Na 2S - N 20 (Fig. 9 to 12 and Table 2). The e f f e c t of H 2S on polysulphide y i e l d was also studied. As noted i n the case of the 40 g/1 aq Na 2S - N 20 system, the G-value obtained was 5.79 ± 0.6; for the 40 g/1 aq Na 2S - N20 -H 2S system the G-value was 7.23 ± 0.7. This increase suggests an additive e f f e c t of hydrogen sulphide on formation of po l y s u l -phide (Fig. 11 and 16 and Table 2). This system also gave the second highest concentration of polysulphide among experiments t r i e d (10.5 g/1 at 20 Mrad). Oxygen saturated aq Na 2S solutions showed an increase i n r a d i a t i o n y i e l d with G. c — . i n t h i s case at 108.30 ± 5.0 ( b n b } over the lower dose range. At higher doses, however, the G-value appeared to decrease (Fig. 15 and Table 2). The r e l a t i v e errors of the G-values were considered to be ± 5 to 10% of t h e i r calculated values. t •j '. 53 5.0. DISCUSSION This thesis compares polysulphide G-values determined from r a d i a t i o n y i e l d s obtained under various experimental condi-tions; and attempts to describe some features of mechanism i n polysulphide formation by r a d i a t i o n . F i r s t , mechanism i s d i s -cussed, then factors which a f f e c t these reactions are discussed. 5.1. Mechanism of Polysulphide Formation Radiation y i e l d of polysulphide was not s i g n i f i c a n t l y higher for 80 g/1 aq Na 2S solutions than for 40 g/1 solutions (Fig. 16 and 17 and Table 2). Consequently, the r a d i o l y s i s of aqueous sodium sulphide solutions used did not give any d i r e c t e f f e c t of ra d i a t i o n on the nature of polysulphide formation. Water i s the main component of d i l u t e aqueous systems, and one has to consider mechanisms involving products r e s u l t i n g from y - r a d i o l y s i s Q f water. As discussed, these products are excited water molecules (H,>0*) and various reactive intermedi-ates, such as «H, 'OH, e and HO Thus a mechanism involving aq A 2. i n d i r e c t r a d i a t i o n e f f e c t s may proceed from excited water mol-ecules and from formation of intermediate molecules. In r a d i o l y s i s of aqueous solutions the contributions made by excited water molecules to observed chemical changes are generally regarded as n e g l i g i b l e (33). The reactions involving excited water molecules may proceed as: H 20 - > H 20* [5-1] 54 An aqueous sodium sulphide solution consists of the following components: Na 2S + H 20 > Na + + SH~ + Na + + 0H~ ..[5-2] SH" + 0H~ > S + H 20 [5-3] SH" + H 20 > H 2S + OH~ [5-4] The excited water molecules, i n turn, may transfer energy to SH~ (or S ) ions. H20* + SH~ >• H 20 + (SH -)* [5-5] H 20* + S"" >> H 20 + (S"")* [5-6] The excited (SH )* or (S )* ions may either revert to the ground state by losing e x c i t a t i o n energy to the surroundings or else undergo the following reactions: (SH -)* > -S + H~ [5-7] (SH~)* } S~ + -H [5-8] (S~~)* — y S"" [5-9] The r a d i c a l s and ions, which are formed with low energy i n the above reactions, recombine^-and d i s s i p a t e the energy of recom-bination as heat without evident chemical change. If the above were the only reactions, steps involving excited species would not give r i s e to new products. Most chemical changes observed i n the r a d i o l y s i s of aqueous solutions can be explained by means of reactive i n t e r -mediates formed i n i r r a d i a t e d water. These intermediates are summarized [2-20]. The molecular products H 2 and ^2°2 a r e p r o ~ duced i n small amounts. The nature and amount of intermediates formed i n i r r a d i a t e d water depend on various factors, such as solution pH, presence of oxygen and the nature of the solute. Thus consideration has to be given to reactive intermediates formed i n i r r a d i a t e d water, e s p e c i a l l y at higher pH, such as 12.7. At t h i s pH l e v e l the predominant species are expected to be e and -OH r a d i c a l s . However, a few 'H from primary "SI events i n water r a d i o l y s i s and a small amount of ^2°2 w-"-11 also be present. In order to examine the roles of the e and *OH r a d i c a l aq i n polysulphide formation, N 20 was used as an electron scav-enger and 2-propanol was used as an «0H scavenger. Conversion of e into 'OH i s e f f i c i e n t l y achieved by aq nitrous oxide, since N 20 i n the system enhances the amount of •OH r a d i c a l s i n the H 20 r a d i o l y s i s as [2-32, 2-33, 2-64] and: •H + N 20 > N 2 + «0H [5-10] H I A common ra d i a t i o n - induced reaction of alcohols (R,Ro-C-0H) i s loss of an a-hydrogen atom to give the r a d i c a l R^^C-OH H (86) such that: OH OH CH3-{:-CH3 ^ ? CH3-C-CH3 + -H [5-11] H and the r a d i c a l s formed react as [2-16] and : OH OH CH3-fc-CH3 + -H ) CH3-C-CH3 + H 2 [5-12] H K = 2 x 10 7 x M _ 1 s e c " 1 OH OH CH 3-£-CH 3 + -OH } CH3-C-CH3 + H 20 . [5-13] H K = 1.7 x 10 9 M~ 1sec~ 1 OH OH CH3-C-CH3 + OH > CH3-C=CH2 + H 20 . [5-14] OH H-.C CH-, , 3, | 3 2CH--C-CH- } CH--C-C-CH_ [5-15] HO OH Thus, 2-propanol strongly suppresses the -OH r a d i c a l , whereas N 20 increases -OH y i e l d . f From experimental r e s u l t s (Fig. 8, 11, 13 and Table 2), i t i s strongly suggested that the *OH r a d i c a l i n i t i a t e s the polysulphide reaction. Polysulphide formation was enhanced by adding N 20 and was decreased by adding 2-propanol. Since a l l experiments were conducted i n the absence of a i r , the oxidi z i n g species can be considered to be *0H r a d i c a l s and ^2Q2 f o r m e c ^ ^ n i r r a d i a t e d water. The r o l e of oxi d i z i n g intermediates i n polysulphide formation requires some discussion. In aqueous solution the Na 2S reacts with E^O (59) according to [5-2] or: Na 2S + H 20 > NaOH + NaSH [5-16] The anion S and H 20 then react according to [5-3] and[5-4], Because OH ions enter into these e q u i l i b r i a , the extent of i o n i z a t i o n depends on pH. I t i s known that at pH=7, H 2S i s present mainly as H2S; while at pH = 11 to 13,such as in k r a f t pulping (pH = 12.7 i n the present work), H 2S i s mainly present as SH ion from the 1st and 2nd i o n i z a t i o n constant (59, 76). Since i n the pH range of k r a f t pulping formation of H 2S i s n e g l i g i b l e (59, 76), these anions could react with intermediates formed by water r a d i o l y s i s , to y i e l d polysulphides i n the following ways: SH~ + «0H } -SH + OH" [5-17] •SH + -OH y -S + H 20 [5-18] 58 •S + S 2 ~ ~ > S 3"" •> S n~~ .. [5-20] •S + S > S S"~ [5-21] n ' n S + -OH \> S~ + OH~ [5-22] S" + S" V S ~~ > S .. [5-23] 2 ' n I t i s possible, however, that some *S might be converted to elemental sulphur by the following reaction: •S ^ | S 0 r [5-24] This elemental sulphur, i n turn, might produce: S Q + S~~ y S 2"~ [5-25] nS° + S~~ ^ S S~~ [5-26] f n or; nS° + SH" + 0H~ ^ S n S " ~ + H 2 ° [ 5 - 2 7 ] Due to high concentrations of Na 2S, most -S formed would probably recombine with S or HS to y i e l d S S n HS" + •OH > H 20 + S" [5-28] S" + .HS > HS 2" [5-29] HS 2" + -HS -y H 2S 3" > S n"~ .. [5-30] I^C^ i t s e l f , as an ox i d i z i n g species formed i n i r r a d i a t e d water, can produce addi t i o n a l r a d i c a l s , as well as i n i t i a t e the reaction for polysulphide formation [2-30, 2-31 and 5-17] and: •SH + 2-OH > * H S 02 + H2 1 5 - 3 1 3 •SH + H 20 2 > * H S 02 + H2 **• [ 5 _ 3 2 ] •HS02 + * SH > H 2S0 2 + -S ... [5-33] H 2S0 2 +4-OH •—* S04"~ + 2H20+2H+. [5-34] S~" + 2-OH ) S0 2~~ + H 2 ... [5-35] S + H 20 2 > S0 2~~ + H 2 ... [5-36] •S + S0 2~" > S 2 0 2 ~ [5-37] S 2 0 2 + -OH > H S 2 ° 3 [5-38] As shown i n the scavenger experiments, the r o l e of e & q seems to be n e g l i g i b l e i n polysulphide formation. Its importance i s i n reducing intermediates through the reaction of S with reducing species. In some cases the reducing species (*H atoms and e a g ) react with the same solute to give d i f f e r e n t products (44). The present work at pH = 12.7 includes e ^ as the major reducing species. At t h i s pH the conversion of e a q to •H by [2-13] i s u n l i k e l y . However, a small number of -H might be a v a i l a b l e at t h i s pH from primary events [2-8 to 2-10]. In general, the following reaction of «H with SH i s suggested; SH + 2-H » H 9S + H [5-39] At pH 12.7 t h i s reaction does not occur (59, 76) because of the i o n i z a t i o n constant for ^ S . The S ion can react with • H as: S + «H > SH [5-40] On the other hand, "H i n h i b i t s the chain reaction for p o l y s u l -phide formation: S n~" + -H > SH" + S^_1 [5-41] The solvated electron reacts either with S or with another * — e . The rate constant for e + e i s known to be 4.5 ± aq aq aq 9 -1 -1 0.07 x 10 M sec (13). Even though the rate constant for S + e i s smaller than that for e + e , the concentration aq aq aq of S i s r e l a t i v e l y large compared to e . Thus i t i s quite aq l i k e l y that the solvated electron w i l l react mainly with S The following reactions can be considered as possible: no reaction [5-42] 61 SH + e aq no reaction [5-43] e aq HS or S~ S n [5-44] With regard to [5-44], formation of H 2S from an aq sodium sulphide solution at t h i s pH i s not l i k e l y . ments (Fig. 8, 11 and 13 and Table 2) that the -OH r a d i c a l s formed i n r a d i o l y s i s of water i n i t i a t e d polysulphide formation. 5.2. Reaction Variables A l i k e l y explanation for the e f f e c t of saturated oxygen on polysulphide y i e l d i s given below. The r a d i c a l *H02 (or 0 2) which might be expected to be formed i n oxygenated solutions i s a f a i r l y strong o x i d i z i n g agent [2-42, 2-52]. changes with pH. The neutral ('HO,,) and basic (0 2) forms are generally accepted, and pKa values of 4.5 and 4.8 have been assigned the equilibrium [2-53]. Therefore, at high pH values 0^ i s generally the pre-dominant species. There w i l l now be an excess of the number equivalents of o x i d i z i n g species over the number of reducing species equivalents. I t would appear l i k e l y that t h i s would r e s u l t Therefore, i t can be concluded from scavenger experi-i n an increase i n the y i e l d of polysulphide as was observed i n the lower dose ranges. However, a y i e l d decrease i n the higher dose range i n oxygenated solutions might occur f o r the following reasons. Polysulphide decomposition i s known to occur through oxidation of S nS with oxygen to $2^3 ' F o r e x a m P l e (15), sodium disulphide oxidizes by the process: 2Na 2S 2 + 30 2 > 2Na 2S 20 3 [5-45] Whether a polysulphide solution i s stable or not at a given temperature i s determined by HS~ and 0H~. The s t a b i l i z -ing e f f e c t of HS i s considerably larger than the counteracting OH" e f f e c t (37). On the other hand, during long i r r a d i a t i o n , the HS~ i s decreased as follows: HS" + -H02 ^ S0~ + H 2 [5-46] HS~ + 0~ > HS0 2" [5-47] Since e q u i l i b r i a of [2-5] or [5-27] could be destroyed by decreased SH , the S S y i e l d seems to be decreased. n Since pH may determine structure of the solute and a l t e r properties of the primary reactive species i n i r r a d i a t e d water, changes i n pH may a f f e c t to a great extent the reaction by which polysulphide i s formed. As mentioned, the following 63 pH dependent e q u i l i b r i a have been established (86). 1. For reducing species: (i) i n a c i d i c solution the r e l a t i v e l y slow association of «H and H + [2-23]; and ( i i ) i n a l k a l i n e solution the conversion of •H to e a g [2-23]. 2. For ox i d i z i n g species: (i) i n a c i d i c medium existence of the equilibrium • OH + H 30 + *. > H 20 + + H 20 [5-48] i s possible but needs experimental v e r i f i c a t i o n ; ( i i ) i n neutral medium the oxid i z i n g r a d i c a l i s not charged and most l i k e l y e x i s t s i n the form of «0H; and ( i i i ) i n a l k a l i n e solution the conversion of «0H to 0 becomes important accord-ing to the equilibrium - k f •OH + OH > 0 + H o0 [5-49] k b where; K f = 1.2 x 1 0 1 0 M _ 1 Sec" 1 K. = 9.2 x 10 7 M - 1 Sec" 1; and b (iv) i n a l k a l i n e solution or neutral solu-t i o n , there i s d i s s o c i a t i o n of the H0 2« r a d i c a l [2-53]. Further, the 0 ion r a d i c a l i s an oxid i z i n g species l i k e the «0H r a d i c a l but behaves quite d i f f e r e n t l y i n the following respects (33): 0~ reacts r a p i d l y with 0 2, but «0H i s i n e r t or reacts slowly with 0 2; 0~ reacts more r a p i d l y with H 2 and H 20 2 than does 'OH; the bimolecular combination of 0 i s considerably slower than that of *0H; and i n electron trans-f e r reactions, 0 reacts much more slowly than does 'OH (in •H abstractions, the 0 ion i s almost the same as *0H). It i s known that G^ F e3+j increases with increasing a c i d i t y (33). when pH =5.8, G ^ F e + 3 j = 13.95 G . H + e a g = 3.29, G 0 H = 2.74 when pH =1.3, G ^ F e + 3 j = 15.52 G-H+e = 3.7, G-0H = 2 , 9 aq I t i s also known that the increase i n a c i d i t y induces an increase i n water decomposition. when 3^ pH _< 13, G(-H 20) = 4 , 0 9 G U J _ = 3.18, G H = 0.45, •H+e ' "2 ' aq G-OH = 2 * 7 2 ' G H 2 0 2 ' ° ' 6 8 when pH =1.3, G, =4.36-(-H20) 4 > 5 G-H+S = 3.49- G = 2.85-a q 3.7, 0 H 2.90, G„ _ = 0.76. G„ = 2.85 H2°2 H2 The present experimental r e s u l t s can be explained i n the following way. Since the formation of 'OH r a d i c a l i s highest i n the neutral N 20 system, t h i s system shows largest ^ s g — j values. A chain reaction must occur i n t h i s system G ' S i because G._, _ — . exceeds G. + G._ .. The main o x i d i z i n g (S nS ) (.OH) ( e a q ) species i s 0 instead of *0H as found i n the a l k a l i n e solution. The 0 reacts much more slowly than does >0H i n an electron transfer reaction, which i s an important reaction i n po l y s u l -phide formation. Therefore, i n the a l k a l i n e N^O system the ra d i a t i o n y i e l d shows a smaller value than does that of the neutral N 20 system. In the extremely a c i d i c solution (pH 1.0), the G-value was the smallest of any of the N 20 systems. The «H(is the main reducing species i n a c i d i c solution. The electron scavenger N-0 can transform e into «0H, but i t can not react with *H 2 aq ' atoms [2-32, 2-33] and: •H + N 20 )• no reaction [5-50] Therefore, the e f f e c t of N 20 on polysulphide formation i n a c i d i c solutions i s n e g l i g i b l e . The unreacted *H thus i n h i b i t s polysulphide formation by chain reaction through formation of HS". The smaller G-value obtained i n the a l k a l i n e 2-propanol system has already been discussed. A s l i g h t l y higher G-value shows i n the a c i d i c 2-propanol system than i n the a l k a l i n e 2-propanol system. This i s believed to be due to an increase i n water decomposition by r a d i o l y s i s with increasing a c i d i t y . The e f f e c t s of changes i n solute concentration upon ra d i a t i o n y i e l d are i l l u s t r a t e d i n F i g . 7, 16, 17 and Table 2. As noted, i n aq Na,,S solution the e q u i l i b r i a have been established as [5-2] and [5-16]. The SH - and OH- Ions then re-act with S G to form polysulphide as a general chemical prepara-tory method [5-27]. For low Na 2S concentrations, since there i s less chance for recombination of S to S nS , the po l y s u l -phide y i e l d should be small. Therefore, the degree of polysul-phide formation i s dependent on sulphide concentration. Baker (15) has already mentioned that the degree of oxidation to polysulphide with Mn0o i s dependent on sulphide concentration: Na 2S + Mn02 + H 20 > S Q + 2NaOH + MnO .. [5-51] (n-1) S Q + Na 2S > Na 2S n [5-52] The influence of d i r e c t r a d i a t i o n e f f e c t s on po l y s u l -phide formation i s not suggested because y i e l d of polysulphide was not much higher for 80 g/1 aq solutions than for 40 g/1 solutions (Fig. 16, 17 and Table 2). However, i f a d i r e c t e f f e c t could be considered at these higher concentrations, the chemical transformation i n a solute due to energy deposition d i r e c t l y i n the solute would become important and could be explained as follows. In concentrated solutions the major energy deposition would s t i l l be with water components providing only " i n d i r e c t e f f e c t s " . Assuming, as a working approximation, that these modes of r a d i o l y s i s are independent, then the o v e r a l l decom-pos i t i o n may be described by the l i n e a r r e l a t i o n : G ( s s — ) ~ G ( s s - - ) „ + G ( s s — ) - • • • [ 5 _ 5 3 ] n n " 2 Here G / c c . s i g n i f i e s the c o e f f i c i e n t of net ( n 'H 20 polysulphide formation by the " i n d i r e c t e f f e c t s " and Gtc c-2\ t n e c o e f f i c i e n t for polysulphide formation from ( b n ^SH" S by "d i r e c t e f f e c t s " . The determination of these i n d i r e c t and d i r e c t e f f e c t s at higher concentration i s not easy. At higher concentrations, a d i r e c t action may be s i g n i f i c a n t due to interference with spur and track reactions, and there i s some evidence that H 20* molecules or subexcitation may transfer energy d i r e c t l y to the solute (32, 86). Hydrogen sulphide has an important e f f e c t on y i e l d increase i n polysulphide formation. Packer (76) proposed that H^S disappeared i n the r a d i o l y s i s of aq H2S solutions through i t s i n t e r a c t i o n with reactive species l i k e *0H and -H. Polysulphide was produced as shown i n the series [2-67 to 2-69, 5-19 and 5-23] . The oxidation of hydrogen sulphide to «S may be attributed (76) to [2-67 and 5-18]. The o v e r a l l reaction i s : H 2S + 2-OH * «S + 2H20 [5-54] 68 The a l t e r n a t i v e , as shown i n the preliminary experi-mental report (99), i s that aq Na 2S solution can be a c i d i f i e d by H2S. This can be explained by progressive «0H r a d i c a l oxidation of H2S to SO^ (61). In the present work i t i s d i f f i c u l t to d i s t i n g u i s h between t h i s p a r t i c u l a r series of reactions and other a l t e r n a t i v e s which would r e s u l t i n the same o v e r a l l reaction. For example [2-16] and: • HS + -OH » HSOH [5-55] HSOH + ..OH > -HSO + H 20 [5-56] •HSO + -OH * H 2 S 0 2 [5-57] By a s i m i l a r sequence, the addition of an *0H r a d i c a l i s followed by «H abstraction by another «OH r a d i c a l ; the H 2 S 0 2 might be converted to sulphate. The o v e r a l l reaction i s : H 2S + 8-OH > S0 4~ + 4H20 + 2H+.. [5-57] Chiu (23) suggested that the apparent Na 2S degradation y i e l d G. i n aqueous solution i s pH dependent. His values { — f N a 2 o / of G» .T o X ranged between 5.0 for a soluti o n at pH 12.5 to 7.5 v - N a 2 o j at pH 8.4. The y i e l d of polysulphide excess sulphur G^s g was also shown to be pH dependent. (S S ) n 69 At i n i t i a l pH = 8.38 i G ( s n s " ) - 1 ' 1 G(-Na 2S) = 7 ' 5 At i n i t i a l pH = 12.46 G ( s s ~ ) = n G(-Na 2S) = 5 ' ° The Na 2S - H 2S system gave considerably higher y i e l d s of S nS (Fig. 7, 14, 16 and 17 and Table 2). This may be due to an increase i n S concentration by the H 2S gas treatment. Thus changes i n pH by using H 2S i n the sulphide solution would r e s u l t i n both an increase i n , and a change i n nature of, the i n t e r -mediates. This i s e s p e c i a l l y true with regard to o x i d i z i n g species formed i n water r a d i o l y s i s , as noted i n the previous section. These w i l l have important e f f e c t s i n polysulphide formation. Response to the dose e f f e c t was generally found to increase l i n e a r l y only i n the lower dose range. Experimental r e s u l t s beyond the i n i t i a l dose range with 0 2 addition showed that polysulphide decreased with further i r r a d i a t i o n up to 20 Mrad. This i s not unusual i n r a d i a t i o n chemistry. However, most treatments gave increased polysulphide y i e l d s as dose increased, even beyond the dose range used for G-value c a l c u l a -70 tions (Fig. 1, 8, 11 to 14 and 17). A l i k e l y explanation for t h i s pheomenon i s occurrence of S Q i n the r a d i o l y s i s of sulphur containing compounds at l a t e r i r r a d i a t i o n stages as mentioned. This S Q would appear to i n i t i a t e a polysulphide chain reaction. As suggested by Abellan (1), with regard to r a d i o l y s i s of aq "R&^S.p^, the presence of S o was noticed only at higher doses. On the other hand, organic impurities could also be responsible for these spurious r e s u l t s . 71 6 . 0 . RECOMMENDATIONS j (1) Detailed estimation of r a d i o l y s i s products, i . e . , OH and SH , i n the generated polysulphide solution would be of value i n estab l i s h i n g stoichiometric r e l a t i o n s h i p s for the polysulphide reaction. (2) A study of r a d i o l y t i c generation of polysulphide i n 0 2 saturated aq Na 2S solutions should be attempted i n order to observe e f f e c t s of 0 2 at higher doses on ra d i a t i o n products. The e f f e c t of oxygen on polysulphide formation showed marked f l u c t u a t i o n of G-values at higher doses, suggesting depolyminiza-t i o n . (3) Some other stable e a (_ scavengers should be t r i e d for promotion of polysulphide y i e l d . ( 4 ) E f f e c t of dose rate or i n t e n s i t y on polysulphide formation should be studied. (5) Overall k i n e t i c s should be studied with respect to es p e c i a l l y 'OH propagation and termination steps. ( 6 ) Further studies on v a r i a t i o n of parameters defined at higher dose could help elucidate factors c r i t i c a l to optimum polysulphide y i e l d s . This area of research could have commer-c i a l importance i f suitable systems are found. 72 7.0. CONCLUSION J The following general conclusions may be drawn from t h i s study: (1) Polysulphide can be formed from aq Na 2S solution by y-radiation. (2) Polysulphide i s thought to be formed i n d i r e c t l y through action of o x i d i z i n g intermediates available i n i r r a d i a t e d aqueous solutions. (3) The polysulphide y i e l d varied according to several factors including dosage, presence of N 20, pH of the sol u t i o n , presence of S and H2S, and whether 0 2 was present. Thus the y i e l d a f f e c t i n g factors may be considered as follows: (i) polysulphide y i e l d was almost l i n e a r l y proportional to dosage; ( i i ) use of N 20 as an electron scavenger was found to e f f i c i e n t l y increase polysulphide y i e l d , ( i i i ) pH had a great e f f e c t on polysulphide formation, wherein a neutral solution i s recommended to produce the highest y i e l d ; (iv) concentration of dissolved Na 2S had considerable a f f e c t on y i e l d , (v) H 2S over-pressure considerably increased y i e l d ; and (vi) 0 2 was found to have strong e f f e c t s only during early stages of i r r a d i a t i o n . 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Studies on y - r a d i o l y s i s of sulphur containing amino acids.II. V o l a t i l e products from S-n-propyl-L-cysteine sulphoxide and S-allyl-L-Cysteine sulphoxide i r r a d i a t e d i n oxygen free aqueous solutions. Tetrahedron 27: 307-313. 73. O'Donnell, J.H. and D.F. Sangster. 1970. P r i n c i p l e s of Radiation Chemistry. E l s e v i e r , New York. p. 2. 74. Olsson, J.E. and 0. Samuelson. 1965. Determination of sulphide, thiosulphate and polysulphide i n black l i q u o r s . Svensk Papperstidn. 68: 179-185. 75. . 1966. Inorganic reac-tions during polysulphide cooking. Svensk Papperstidn. 69: 703-710. 76. Packer, J.E. 1962. Radiolysis of aqueous solutions of hydrogen sulphide and on in t e r p r e t a t i o n of r a d i o l y t i c t h i o l oxidation. Nature 194: 81-82. 80 77. Papp, J. 1971. Potentiometric determination of s u l f u r compounds i n white, green and black liquors with sulphide i o n - s e l e c t i v e electrode. 1. Determination of sulphide, thiosulphate, sulphite, polysulphide and mercaptan i n the presence of one another by means of potentiometric t i t r a -t i o n . C e llulose Chem. Tech. 5: 147-152. 78. Peschanski, D. and G. Valensi. 1949. Electrochemistry of aqueous solutions of polysulphide. J . Chim. Phys. 46: 602.(CA 44:4816g). 79. Platzman, R.L. 1953. Basic mechanics i n radiobiology. I I . Physical and chemical aspects. Publ. No. 305. Nat. Res. Council, Washington. 22 pp. 80. Rabani, J . and M.S. Matheson. 1964. Pulse r a d i o l y t i c determination of pk for hydroxyl i o n i c d i s s o c i a t i o n i n water. J. Am. Chem. Soc. 86: 3175-3176. 81. Rice, J.W. and M. Zimmerman. 1967. An improved procedure for analyzing neutral and a l k a l i n e mixtures of sodium sulphide, thiosulphate, sulphite or polysulphide i n aqueous solution. Tappi 50: 72-75. 82. Rodziewicz, W., Bernasik, S. and K. Grzedzicke. 1962. Examination of the exchange of s u l f u r atoms i n t h i o s a l t s and polysulphides by means of sulfur-35. Roczni Chem. 36: 1361. (CA 58: 10958c). 83. Rosen, E. and R. Tegman. 1971. A preparative and X-ray powder d i f f r a c t i o n study of the polysulphides Na 2S 2, Na 2S 4 and Na 2S 5. Acta. Chem. Scand. 25: 3329-3336. 84. Samuel, A.H. and J.L. Magee. 1953. Theory of r a d i a t i o n chemistry. I I . Track e f f e c t s i n r a d i o l y s i s of water. J. Chem. Phys. 21: 1080-1087. 85. Sanyer, N. and J.F. Laundrie. 1964. Factors a f f e c t i n g y i e l d increase and f i b r e q u a l i t y i n polysulphide pulping of l o b l o l l y pine, other softwoods and red oak. Tappi 47: 640-652. 86. Spinks, J.W.T. and R.J. Woods. 1964. An Introduction to Radiation Chemistry. John Wiley and Sons, New York. 477 pp. 87. Szabo, I. and A. Teder. 1970. Calcium polysulphide pre-treatment i n the s t a b i l i z a t i o n of carbohydrates against a l k a l i n e degradation. Svensk Papperstidn. 73: 404-409. 88. Teder, A. 1965. The e f f e c t of l i g n i n on the s t a b i l i t y of polysulphide solutions. Svensk Papperstidn. 68: 825-833. 81 Teder, A. 1967. Spectrophotometeric determination of polysulphide excess sulf u r i n aqueous solutions. Svensk Papperstidn. 70: 197-200. 1968. Redox p o t e n t i a l of polysulphide solu-tions and carbohydrate s t a b i l i z a t i o n . Svensk Papperstidn. 71: 149-160. . 1968. The spectra of aqueous polysulphide solutions. I. The resolution into t r a n s i t i o n energy l e v e l . Arkiv for Kemi 30: 379-391. . 1969. The rate of rearrangement reaction i n aqueous polysulphide solutions. Svensk Papperstidn. 72: 245-248. . 1969. Some aspects of chemistry of po l y s u l -phide pulping. Svensk Papperstidn. 72: 294-303. . 1969. The spectra of aqueous polysulphide solutions. I I . The e f f e c t of a l k a l i n i t y and s t o i c h i o -metric composition at equilibrium. Arkiv for Kemi 31: 173-198. Venmark, E. 1964. Some ideas on polysulphide pulping. Svensk Papperstidn. 67: 157-166. Voge, H.H. and W.F. Libby. 1937. Exchange reactions with r a d i o s u l f u r . J . Am. Chem. Soc. 59: 2474. Wallace, S.C. 1971. Subnanosecond pulse r a d i o l y s i s studies. Unpubl. Ph.D. thesis, Dept. Chem., Univ. B.C., Vancouver, p. 34. Weiss, J . 1960. Primary process i n the action of i o n i z i n g radiations on water: Formation and r e a c t i v i t y of s e l f -trapped electrons ("polarons"). Nature 186: 751-752. You, Y.S. 1972. Y - r a d i o l y t i c generation of polysulphide from sodium sulphide solutions and k r a f t green l i q u o r . Unpubl. Rept.,Fac. For., Univ. B.C., Vancouver. 57 pp. 82 Table 1. Reactions of free r a d i c a l s i n i r r a d i a t e d water ( 3 3 ) . Rate constant, Reaction A / " 1 sec - 1 pH c f , - r e " ' — — > HH - 2 0 H - '5.5 x l O 9 13.3 5 x 10° 10-13 6 x 10° 11 e ~ + H •• ";°-> H j + OH" 2.5 x IO 1 0 10.5 3 x 10'° 10.9 e"+OH — — > OH" 3.0 x l O 1 0 11 cf,-r°" > 20H" 2.2 X l O 1 0 13 e " + K 3 0 + > H + H 2 0 2.06 x 10'° 2.1-4.3 2.36 X 10 1 0 4-5 2.2 x 10'° — 2.26 x IO 1 0 4.1-4.7 e - v H j O j > OH + OH- 1.23 x 10'° 7 1.36 x 10'° 11 1.1 x 10'° — 1.3 x 10'° 11 c^ + K O j " > 0- + OH" 3.5X10° 13 e^ + K j O > H + OH" 16 8.3-9.0 H + H > H 2 1.5 x IO 1 0 0.2-0.S.VH;SO. 1.0 X 10 1 0 2.1 7,75 X 10 9 3 1.3 x 10'° 0.4-3 1.25 x 10'° 2-3 H + OH • H ; 0 3.2 x 10'° 0.4-3 H + OH" > e" 1.8 x 107 11.5 2.2 x 107 11-13 H + H i O j > H 2 0 + OH 5 x 107 acid 1.6 x 10 s 0.4-3 9 x 107 2 4 x 107 — OH + OH > H,0 2 6 X 10 9 0.4-3 4 x 10 9 7 5 x l O 9 — 0- + 0" > O;-H j O > HO," + OH" 1 x 109 13 O H + O H " > 0~ + H20 3.6 x 10s — 0 H + K 2 0 2 > K O , + H 2 0 4.5 x 107 .7 1.2 x 107 0.4-3 2.25 x 107 — 0- + HO," > 0 2- + 0 H " 7 x 10s 13 2.74 x 10s . 13 o i l | I t , ••• - » II ! I I ,O ('. j o ' 7 4.5 x IO' 7 •O-' + Hj -. > 11 + OH" Ii x 107 13 H j O + + OH" > 2H 20 14.3 x IO 1 0 — 15 x 10'° — 4.4 X l O 1 0 " — ° Low value obtained in neutral solution with high-intensity pulses. T a b l e 2. R a d i o l y t i c g e n e r a t i o n o f s o d i u m p o l y s u l p h i d e (g/1) a s r e l a t e d t o s o d i u m s u l p h i d e c o n c e n t r a t i o n , a d d i t i v e ( s ) , i n i t i a l pH and r a d i a t i o n d o s e . Na-S,g/1 10 ' 4 0 80 A d d i t i v e s ( s ) N 2 ° a H 2 S None N 2 0 H 2 S 0 4 N-0 H 2 S 0 4 2-prop. H 2 S 0 4 a 2 - p rop H 2S o 2 a b N_0 K 2 S N 2 ° H 2S I n i t i a l pH 12.7 12.7 1.0 7.0 12.7 1.0 12.7 12.7 12.7 12.7 12.7 D ose, K r a d .96 0.050 0.029 0.023 0.405 0.180 0.017 0.012 0.031 1.120 0.256 0.245 1.28 0.065 0.034 0.029 0.460 0.235 0.023 0.016 0.045 0.790 0.330 0.310 1.60 0.080 0.036 0.035 0.510 0.300 0.023 0.020 0.054 0.720 0.425 0.385 1.92 0.095 0.040 0.040 0.545 0.360 0.024 0.025 0.064 0.610 0.490 0.485 5.12 0.290 0.047 0.110 1.950 1.100 0.034 0.046- 0.823 0.310 2.250 2.460 10.00 0.600 0.063 0.500 3.800 3.000 0.046 0.075 2.725 0.413 7.500 4.850 15.00 0.800 0.120 0.565 4.900 4.850 0.052 0.151 3.704 0.132 10.000 9.000 20.00 2.200 0.340 0.625 6.100 6.150 C.068 0.340 8.324 0.151 10.500 14.000 G ( S n S ~ ) 1.35 ±0.20 0.75 ±0.01 0.63 ±0.01 10.06 ±1.00 5.79 ±0.60 0.53 ±0.01 0.39 ±0.01 1.04 ±0.10 108.30 ± 5.00 7.23 ±0.70 7.84 ±0'.80 F i g . 7 8 9 10 11 12 • 13 14 15 16 17 NjO, H 2S and 0 2 m a i n t a i n e d a t 20 p s i ; 2 - p r o p a n o l a t 0.5 M. A d d i t i o n a l 0 2 e x p e r i m e n t s [Mrad ( y i e l d , g/1)] : 0.02 ( 0 . 0 8 ) ; 0.04 ( 0 . 1 7 ) ; 0.16 ( 0 . 6 5 ) ; 0.32 ( 1 . 1 2 ) ; 0.48 ( 1 . 8 7 ) . 1 I 1 1 1 1 1 1 1 1 1 1 1 -~ • G0H --X 1 1 ! i 1 p i r i 1 I i 1 1 2 3 4 5 6 7 8 9 10 11 12 13 PH F i g . 2. Dependence of primary r a d i c a l and molecular y i e l d s of water y - r a d i o l y s i s on pH, derived from measurements on formic acid-oxygen solutions ( 3 3 ) . 85 Gas Traps Fig. 4. Schematic Diagram of the Pressurized System. CO QK i . i . i . i 0 2 4 6 8 10 I r r a d i a t i o n time, min 3+ g. 5. Absorbance of Fe at 304 nm (10 mm c e l l ) vs. i r r a d i a t i o n time for c a l c u l a t i o n of ferrous sulphate dosimetry. 8 8 Concentration F i g . 6. Absorbance at 285 nm (2 mm c e l l ) vs. polysulphide excess sulphur concentration (g/1) at three sodium sulphide concentrations, a l l containing 1 g/1 elemental sulphur. I 1 1 2 m 1 I 1 0 I—I ,,.1 , • . 1 1 0.10 0.08 0.06 0.04 0.02 10 0.5 1.0 Dose, Mrad 15 1.5 20 - 1 1 -• — • * 1 . . 1 . . t • 1 • • 1 2.0 F i g . 7. Polysulphide y i e l d related to ra d i a t i o n dose for 10 g/1 aq Na 2S, N-0 (20 psi) and H2S (20 p s i ) ; i n i t i a l pH 12.7. 90 fr 10 15 20 0.04 i i i 0.02 . . • * 0 . • : . . • ' • • 0 0.5 1.0 1.5 2 Dose, Mrad F i g . 8. Polysulphide y i e l d related to radiati o n dose for 40 g/1 aq Na 2S and no additives; i n i t i a l pH 12.7. 91 r-i 1 1 1 0.8 -0.6 0.4 -0.2 — . 1 . . . . I . . . 1 5 10 15 20 0.06 0.04 1 i l 0.02 K___ -J _ L ^ . . m , . • • . 1 . . . . 1 . . . . 1 * • t 0.5 1.0 1.5 2.( Dose,'Mrad F i g . 9. Polysulphide y i e l d related to r a d i a t i o n dose for 40 g/1 aq Na-,S and N 20 (20 p s i ) ; i n i t i a l pH 1.0. 92 8 6 4 2 L * • r H • H • ' ~ - • 15 20 0.6 - 1 1 1 0.4 -0.2 0 . 1 . . . . 1 . . 1 C • 0.5 1.0 1.5 2. Dose, Mrad. F i g . 10. Polysulphide y i e l d related to r a d i a t i o n dose for 40 g/1 aq Na 2S and N 20 (20 p s i ) ; i n i t i a l pH 7.0. 93 -1 1 — 1 • 1 . . . . 1 . CD •H 3 0.4 0.3 0.2 0.1 10 15 20 -1 1 1 — . . 1 . . . 1 . . . . 1 . 0 0.5 1.0 1.5 2 Dose, Mrad F i g . 11. Polysulphide y i e l d related to ra d i a t i o n dose for 40 g/1 aq Na 2S and N 20 (20 p s i ) ; i n i t i a l pH 12.7. 94 0 . 0 8 L 10 Dose, Mrad F i g . 12. Polysulphide y i e l d related to ra d i a t i o n dose for 40 g/1 aq Na 2S and 2-propanol (0.5 M); i n i t i a l pH 1.0. 0 . 0 2 4 I 0 . 0 2 0 L F i g . 1 3 . P o l y s u l p h i d e y i e l d r e l a t e d t o r a d i a t i o n d o s e f o r 40 g / 1 a q N, i n i t i a l pH 1 2 . 7 .   /   a - S a n d 2 - p r o p a n o l (0.5M); 96 0.02 0 0.5 1.0 1.5 2.0 Dose, Mrad F i g . 14. P o l y s u l p h i d e y i e l d r e l a t e d to r a d i a t i o n dose f o r 40 g/1 aq Na 2S and H 2S (20 p s i ) ; i n i t i a l pH 12.7. 97 2 I.O . 0.8 _ 0.6 -0.4 -0.2 0.1 0.2 Dose, Mrad 0.3 0.4 F i g . 15. Polysulphide y i e l d related to ra d i a t i o n dose for 40 g/1 aq Na 2S and 0^ (20 p s i ) ; i n i t i a l pH 12.7. 10 I i 1 — 8 -6 — 4 — 2 -0 i i 0 5 10 15 20 0.6 I i 1 0.4 -0.2 - . • -* - -0 . i . 1 . . . . 1 . . C 0.5 1.0 1.5 2. Dose, Mrad F i g . 16. Polysulphide y i e l d related to ra d i a t i o n dose for 40 g/1 aq Na 2S, N-0 (20 psi) and H2S (20 p s i ) ; i n i t i a l pH 12.7. 99 i 1 I 20 - -16 * 12 8 — rH \ 4 t tr> •d rH 0 ... «—-r" i . . . i d) •H S J 0 5 10 15 2 0 . 6 1 i I 0 .4 - — 0 .2 -. • • — 0 . • • * , 1 , . 1 • . . 1 . 0 0.5 1.0 1.5 2.0 Dose, Mrad F i g . 17. Polysulphide y i e l d related to ra d i a t i o n dose for 80 g/1 aq Na-S, N 20 (20 psi) and H-S (20 p s i ) ; i n i t i a l pH 12.7. 

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