Geochemical Processes at Mineral Surfaces: An Overview - ACS

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Geochemical Processes at M i n e r a l Surfaces: An Overview 1

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James A. Davis and Kim F. Hayes 1

Water Resources Division, U.S. Geological Survey, Menlo Park, CA 94025 Environmental Engineering and Science Group, Department of Civil Engineering, Stanford University, Stanford, CA 94305-4020

Downloaded by UNIV EDINBURGH on June 15, 2012 | http://pubs.acs.org Publication Date: November 13, 1987 | doi: 10.1021/bk-1987-0323.ch001

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The phase discontinuity that occurs at the mineral-water interface greatly influences the geochemical cycles of many elements. The composition of natural waters and the flux of material through the hydrosphere are largely controlled by the weathering of minerals and the precipitation of new phases - - processes in which the mineral water interface plays a fundamental role. In addition, mineral surfaces may act as catalysts for chemical or biological transformations that occur within the hydrosphere. Reactions at the mineral-water interface are of interest in the study of ore genesis, geochemical exploration, mineral separation processes such as flotation and sedimentation, transport of adsorbed nutrients or pollutants in rivers and lakes, scavenging of trace elements in the oceans, and the transport of nuclear or other hazardous waste materials in groundwaters. Because these processes are so complex, we rely to a great extent on models to understand our observations of the geochemical behavior of solutes in water. The models typically contain several components; for example, solute transport models may be composed of a hydrologie model coupled with a chemical or biological submodel. The chemical submodel can be as simple as a distribution coefficient to represent the partitioning of an element between solid and aqueous phases, or i f the aqueous chemical composition is expected to be controlled by the dissolution of a mineral, a solubility product or rate constant for dissolution would be included in the model. Ideally, the chemical model would describe geochemical behavior in terms of a series or combination of elementary processes, e.g. adsorption, ion exchange, precipitation, dissolution, or electron transfer reactions. In reality, the behavior of geochemical systems is often so complex that the actual mechanisms of the processes observed are not well understood. In this overview we discuss recent advances in the study of chemical reactions at the mineral-water interface as we introduce the 0097-6156/ 86/ 0323-0002S06.00/ 0 © 1986 American Chemical Society

In Geochemical Processes at Mineral Surfaces; Davis, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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DAVIS A N D HAYES

Overview

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c h a p t e r s c o n t a i n e d i n t h i s v o l u m e . Our o b j e c t i v e i s t o i n d i c a t e i m p o r t a n t f e a t u r e s of each c h a p t e r to the f i e l d of aqueous g e o c h e m i s t r y and ways i n which the c h a p t e r s r e l a t e t o each o t h e r . The p a p e r i s d i v i d e d i n t o s e c t i o n s on p h y s i c a l p r o p e r t i e s o f t h e interface; a d s o r p t i o n and i o n e x c h a n g e ; surface spectroscopy; d i s s o l u t i o n , p r e c i p i t a t i o n , and s o l i d s o l u t i o n f o r m a t i o n ; and t r a n s f o r m a t i o n r e a c t i o n s a t the m i n e r a l - w a t e r i n t e r f a c e . Each s e c t i o n touches on the importance o f t h a t t o p i c i n geochemical p r o c e s s e s and i n t e r d e p e n d e n t r e l a t i o n s h i p s among the t o p i c s c o v e r e d .


Physical Properties of the Mineral-Water Interface The c h e m i c a l r e a c t i v i t y o f t h e m i n e r a l - w a t e r i n t e r f a c e i s i n f l u e n c e d by some i m p o r t a n t p r o p e r t i e s w h i c h d i s t i n g u i s h t h e i n t e r f a c i a l e n v i r o n m e n t from t h a t o f b u l k w a t e r , e.g. 1) water i s more s t r u c t u r e d i n t h e i n t e r f a c i a l r e g i o n , 2) i o n s and w a t e r m o l e c u l e s a r e l e s s m o b i l e , 3) t h e d i e l e c t r i c c o n s t a n t o f w a t e r i s d e c r e a s e d , and 4) e l e c t r i c a l charge and p o t e n t i a l may d e v e l o p a t t h e s u r f a c e , l e a d i n g t o t h e f o r m a t i o n o f an e l e c t r i c a l d o u b l e l a y e r . A l t h o u g h e x p e r i m e n t a l s t u d i e s o f m i n e r a l - w a t e r i n t e r f a c e have been e x t e n s i v e , the e f f e c t s o f t h e p e r t u r b e d l a y e r o f water and t h e e l e c t r i c a l d o u b l e l a y e r on c h e m i c a l r e a c t i o n s at the i n t e r f a c e are s t i l l u n r e s o l v e d i s s u e s (l11). M u l l a ( C h a p t e r 2) c o m p a r e s s i m u l a t i o n s o f i n t e r f a c i a l w a t e r s t r u c t u r e from v a r i o u s s t a t i s t i c a l - m e c h a n i c a l m o d e l s , i n c l u d i n g Monte C a r l o and M o l e c u l a r D y n a m i c s m o d e l s . The r e s u l t s p r e d i c t t h e e x i s t e n c e o f m o l e c u l a r l a y e r i n g w i t h o r d e r e d d i p o l e o r i e n t a t i o n s at the i n t e r f a c e , d e n s i t y o s c i l l a t i o n s which extend many Angstroms away from t h e s u r f a c e , and fewer hydrogen bonds between w a t e r m o l e c u l e s i n the i n t e r f a c i a l r e g i o n . The e x i s t e n c e o f d e n s i t y o s c i l l a t i o n s at the i n t e r f a c e has r e c e n t l y been c o n f i r m e d e x p e r i m e n t a l l y (12). Reduced d i p o l e r e l a x a t i o n times are a l s o p r e d i c t e d , which suggests t h a t i n t e r f a c i a l w a t e r e x p e r i e n c e s h i n d e r e d r o t a t i o n . U n f o r t u n a t e l y the d i e l e c t r i c p r o p e r t i e s cannot be e f f e c t i v e l y modeled, but the r e s u l t s do s u g g e s t t h a t i n t e r f a c i a l w a t e r i s n o t a u n i f o r m d i e l e c t r i c continuum. The development o f improved models i n t h e f u t u r e appears p r o m i s i n g , and t h e s e m o d e l s s h o u l d i n c r e a s e o u r u n d e r s t a n d i n g o f p r o p e r t i e s w h i c h are not e a s i l y q u a n t i f i e d at p r e s e n t , e.g. h y d r a t i o n f o r c e s , hydrophobic e f f e c t s , and d o u b l e l a y e r f o r c e s . G i e s e and C o n s t a n z o ( C h a p t e r 3) p r e s e n t t h e r e s u l t s o f an infrared study of the bonding of i n t e r c a l a t e d water i n s y n t h e t i c h y d r a t e d k a o l i n i t e s . Two t y p e s o f w a t e r were i d e n t i f i e d : 1) w a t e r m o l e c u l e s which were bonded t o the d i t r i g o n a l h o l e s o f the s i l i c a t e l a y e r , and 2) a s s o c i a t e d water m o l e c u l e s which were hydrogen bonded t o t h o s e o f the f i r s t group. The m o b i l i t y o f water m o l e c u l e s bound t o t h e d i t r i g o n a l h o l e s was g r e a t l y r e d u c e d i n c o m p a r i s o n t o t h e a s s o c i a t e d water. A l t h o u g h h y d r a t e d k a o l i n i t e s are not found i n n a t u r e , G i e s e and C o n s t a n z o s u g g e s t t h a t t h e surface-water i n t e r a c t i o n s observed in t h i s study are r e p r e s e n t a t i v e of i n t e r a c t i o n s w i t h the s u r f a c e s o f o t h e r s i l i c a t e m i n e r a l s . The p r o p e r t i e s o f t h e e l e c t r i c a l d o u b l e l a y e r (EDL) h a v e been the s u b j e c t o f c o n s i d e r a b l e r e s e a r c h (1,3,5,8,10). U n l i k e r e v e r s i b l e e l e c t r o d e s , where s u r f a c e p o t e n t i a l i s c o n t r o l l e d and charge d e v e l o p s i n response to changes i n e l e c t r o d e p o t e n t i a l , m i n e r a l s u r f a c e s d e v e l o p p o t e n t i a l i n response t o the f o r m a t i o n o f s u r f a c e charge (8). On t h e s u r f a c e o f h y d r o u s o x i d e s , f o r e x a m p l e , h y d r o x y l g r o u p s



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G E O C H E M I C A L P R O C E S S E S AT M I N E R A L S U R F A C E S

( B r o n s t e d a c i d s i t e s ) o r m e t a l atoms w i t h u n s a t i s f i e d c o o r d i n a t i o n ( L e w i s a c i d s i t e s ) r e a c t w i t h w a t e r t o f o r m s u r f a c e c h a r g e (1.3). Isomorphic s u b s t i t u t i o n i n the i n t e r l a y e r r e g i o n o f l a y e r e d s i l i c a t e s r e s u l t s in a n e g a t i v e surface charge. In e a c h c a s e c h e m i c a l "exchange" o f i o n s between phases r e s u l t s i n the f o r m a t i o n o f s u r f a c e charge and the development o f an e l e c t r i c a l p o t e n t i a l . I t i s i m p o r t a n t t o e s t a b l i s h t h e o r i g i n and m a g n i t u d e o f t h e a c i d i t y (and h e n c e , t h e c h a r g e ) o f m i n e r a l s u r f a c e s , b e c a u s e t h e r e a c t i v i t y of the surface i s d i r e c t l y r e l a t e d to i t s a c i d i t y . Several m i c r o s c o p i c - m e c h a n i s t i c m o d e l s h a v e been p r o p o s e d t o d e s c r i b e t h e a c i d i t y o f h y d r o x y l g r o u p s on o x i d e s u r f a c e s ; most d e s c r i b e t h e s u r f a c e i n terms o f amphoteric weak a c i d groups (14-17), but r e c e n t l y a m o n o p r o t i c weak a c i d model f o r t h e s u r f a c e was p r o p o s e d ( ! 8 ) . The m o d e l s d i f f e r p r i m a r i l y i n t h e i r d e s c r i p t i o n o f t h e EDL and t h e a s s u m p t i o n s used to d e s c r i b e i n t e r f a c i a l s t r u c t u r e . " I n t r i n s i c " a c i d i t y c o n s t a n t s t h a t are d e r i v e d from t h e s e models can have s u b s t a n t i a l l y d i f f e r e n t v a l u e s because o f the d i f f e r e n t assumptions e m p l o y e d i n e a c h model f o r t h e s t r u c t u r e o f t h e EDL (5). W e s t a l l ( C h a p t e r 4) r e v i e w s s e v e r a l d i f f e r e n t a m p h o t e r i c m o d e l s w h i c h d e s c r i b e the a c i d i t y o f o x i d e s u r f a c e s and compares t h e a p p l i c a b i l i t y o f t h e s e models w i t h the m o n o p r o t i c weak a c i d m o d e l . The assumptions employed by each o f the models t o e s t i m a t e v a l u e s o f thermodynamic c o n s t a n t s are c r i t i c a l l y examined. The d i f f i c u l t y i n c h a r a c t e r i z i n g the i n t e r f a c e a r i s e s from the f a c t t h a t t h e e l e c t r o s t a t i c i n t e r a c t i o n s are c l o s e l y c o u p l e d t o the c h e m i c a l i n t e r a c t i o n s . An i n d e p e n d e n t measurement o f e l e c t r o s t a t i c energy w o u l d be u s e f u l f o r p r o b i n g the s e p a r a t i o n o f c o u l o m b i c and c h e m i c a l components i n the EDL models. Bousse and M e i n d l (Chapter 5) d e s c r i b e a t e c h n i q u e f o r measuring the e l e c t r i c a l p o t e n t i a l a t o x i d e s u r f a c e s u s i n g i o n - s e n s i t i v e f i e l d e f f e c t t r a n s i t o r s ( I S F E T s ) . In t h i s method one may r e g u l a t e t o t a l e l e c t r i c a l p o t e n t i a l o f t h e i n t e r f a c e , and t h i s a l l o w s e s t i m a t e s o f i n t r i n s i c a c i d i t y c o n s t a n t s t h a t a r e i n d e p e n d e n t o f p r o t o n a d s o r p t i o n d a t a . The m e a s u r e m e n t o f the t o t a l e l e c t r i c a l p o t e n t i a l i s p r e f e r a b l e to t h a t of zeta p o t e n t i a l , s i n c e t h e e x a c t l o c a t i o n o f t h e l a t t e r measurement i s i n d e t e r m i n a t e . F u r t h e r m o r e , i t has been shown t h a t t h e dependence o f p r o t o n a d s o r p t i o n as a f u n c t i o n o f pH may depend t o a g r e a t degree on the extent of complex formation w i t h adsorbed c o u n t e r i o n s (19-21), w h e r e a s t h e t o t a l p o t e n t i a l as a f u n c t i o n o f pH i s r e l a t i v e l y i n s e n s i t i v e t o c o m p l e x a t i o n (21). Chan ( C h a p t e r 6) p r e s e n t s a s i m p l e g r a p h i c a l method f o r e s t i m a t i n g t h e f r e e e n e r g y o f EDL f o r m a t i o n a t t h e o x i d e - w a t e r i n t e r f a c e w i t h an amphoteric model f o r the a c i d i t y o f s u r f a c e groups. S u b j e c t t o t h e a s s u m p t i o n s o f t h e EDL m o d e l , t h e g r a p h i c a l method a l l o w s a comparison o f the magnitudes o f the c h e m i c a l and c o u l o m b i c components o f s u r f a c e r e a c t i o n s . The a n a l y s i s a l s o i l l u s t r a t e s the r e l a t i o n s h i p b e t w e e n model p a r a m e t e r v a l u e s and t h e d e v i a t i o n o f s u r f a c e p o t e n t i a l from the N e r n s t e q u a t i o n . The r e l a t i v e i m p o r t a n c e o f t h e EDL f o r r e a c t i o n s o t h e r t h a n a d s o r p t i o n i s not w e l l u n d e r s t o o d . S u r f a c e c o m p l e x a t i o n models have r e c e n t l y been a p p l i e d t o p r o c e s s e s i n which a d s o r p t i o n r e p r e s e n t s the f i r s t s t e p i n a sequence o f r e a c t i o n s . For example, Stumm e t a l . (22) h a v e a p p l i e d a model w i t h an EDL component i n t h e i r s t u d i e s o f t h e r o l e o f a d s o r p t i o n i n d i s s o l u t i o n and p r e c i p i t a t i o n r e a c t i o n s . The e f f e c t o f s u r f a c e c h a r g e and p o t e n t i a l on p r e c i p i t a t i o n and t h e


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DAVIS A N D HAYES

Overview

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f o r m a t i o n o f m e t a s t a b l e s o l i d phases i s d i s c u s s e d by Zawacki et a l . ( C h a p t e r 3 2 ) . W a i t e ( C h a p t e r 20) r e v i e w s t h e i m p o r t a n t r o l e o f t h e i n t e r f a c i a l environment i n c a t a l y z i n g l i g h t - i n d u c e d redox r e a c t i o n s . V o u d r i a s and R e i n h a r d ( C h a p t e r 22) d i s c u s s t h e e f f e c t s o f s u r f a c e a c i d i t y on t r a n s f o r m a t i o n s o f o r g a n i c compounds. The r a t e s o f c h e m i c a l r e a c t i o n s a r e a l s o i n f l u e n c e d by t h e E D L . A s w i t h e q u i l i b r i u m models, the r e l a t i v e c o n t r i b u t i o n o f t h e EDL i n d e t e r m i n i n g the overall r e a c t i o n r a t e i s d e p e n d e n t on t h e i n t e r f a c i a l model chosen (see Chapters 7 and 12).


Adsorption S o r p t i o n p r o c e s s e s h a v e r e c e i v e d much s t u d y b e c a u s e o f t h e i r fundamental importance i n g e o c h e m i s t r y , a n a l y t i c a l c h e m i s t r y , and i n i n d u s t r i a l a p p l i c a t i o n s . The t h r e e p r i n c i p a l s o r p t i o n p r o c e s s e s are a d s o r p t i o n , a b s o r p t i o n , and s u r f a c e p r e c i p i t a t i o n ; t h e d i f f e r e n c e s among t h e s e p r o c e s s e s a r e d i s c u s s e d by S p o s i t o ( C h a p t e r 1 1 ) . I f t h e s p e c i f i c p r o c e s s l e a d i n g t o t h e l o s s o f a s o l u t e f r o m aqueous s o l u t i o n i s not known, then the g e n e r a l term, s o r p t i o n , may be used. The abundance o f l i t e r a t u r e on e x p e r i m e n t a l s t u d i e s o f i o n a d s o r p t i o n has been r e v i e w e d by K i n n i b u r g h and J a c k s o n (23) and H i n g s t o n ( 2 4 ) . R e c e n t l a b o r a t o r y s t u d i e s o f s o r p t i o n p r o c e s s e s have been d i r e c t e d more toward a b e t t e r u n d e r s t a n d i n g o f the mechanisms o f r e a c t i o n s , a c h a r a c t e r i z a t i o n o f the bonding o f adsorbed s p e c i e s , and improvements in a d s o r p t i o n models. It i s i n c r e a s i n g l y c l e a r , however, t h a t the m a c r o s c o p i c approaches t h a t have been commonly used t o s t u d y s o r p t i o n p r o c e s s e s , e.g. a d s o r p t i o n i s o t h e r m s , s o l u b i 1 i t y c a l c u l a t i o n s , and k i n e t i c methods, cannot u n e q u i v o c a b l y d i s t i n g u i s h between p r o c e s s e s such as a d s o r p t i o n and s u r f a c e p r e c i p i t a t i o n (see C h a p t e r 11). I t i s l i k e l y t h a t f u t u r e developments i n t h i s f i e l d w i l l come from s t u d i e s u t i l i z i n g m o l e c u l a r t e c h n i q u e s such as i n - s i t u s u r f a c e s p e c t r o s c o p y . Mechanisms of Sorption Processes. K i n e t i c s t u d i e s are v a l u a b l e f o r h y p o t h e s i z i n g m e c h a n i s m s o f r e a c t i o n s i n homogeneous s o l u t i o n , but the i n t e r p r e t a t i o n o f k i n e t i c d a t a f o r s o r p t i o n p r o c e s s e s i s more d i f f i c u l t . R e c e n t l y i t has been shown t h a t t h e m e c h a n i s m s o f v e r y f a s t a d s o r p t i o n r e a c t i o n s may be i n t e r p r e t e d f r o m t h e r e s u l t s o f c h e m i c a l r e l a x a t i o n s t u d i e s (25-27). Yasunaga and Ikeda (Chapter 12) summarize r e c e n t s t u d i e s t h a t have u t i l i z e d r e l a x a t i o n t e c h n i q u e s t o e x a m i n e t h e a d s o r p t i o n o f c a t i o n s and a n i o n s on h y d r o u s o x i d e and a l u m i n o s i l i c a t e s u r f a c e s . Hayes and L e c k i e ( C h a p t e r 7) p r e s e n t new i n t e r p r e t a t i o n s f o r the mechanism o f l e a d i o n a d s o r p t i o n by g o e t h i t e . In b o t h p a p e r s i t i s c o n c l u d e d t h a t t h e k i n e t i c and e q u i l i b r i u m a d s o r p t i o n d a t a a r e c o n s i s t e n t w i t h the r a t e r e l a t i o n s h i p s d e r i v e d from an i n t e r f a c i a l model i n which metal i o n s are l o c a t e d n e a r e r t o the s u r f a c e than adsorbed c o u n t e r i o n s . The s u r f a c e s o f m i n e r a l s are g e n e r a l l y not homogeneous; k i n k s , s t e p s , e d g e s , d i s l o c a t i o n s , o r p o i n t d e f e c t s may p r o v i d e r e a c t i v e zones. M i c r o c r y s t a l 1 i n e p r e p a r a t i o n s o f s o l i d s , which are f r e q u e n t l y used i n l a b o r a t o r y s o r p t i o n e x p e r i m e n t s , may have s e v e r a l c l e a v a g e p l a n e s w i t h d i f f e r e n t s i t e e n e r g i e s . The i m p o r t a n c e o f t h e h i g h energy s i t e s t h a t r e s u l t from t h e s e i m p e r f e c t i o n s i s w e l l r e c o g n i z e d f o r t h e p r o c e s s e s o f d i s s o l u t i o n and c r y s t a l g r o w t h (28). S e v e r a l e q u i l i b r i u m s t u d i e s of i o n a d s o r p t i o n have suggested t h a t hydrous o x i d e s u r f a c e s a r e composed o f h e t e r o g e n e o u s s i t e s ( 2 9 - 3 2 ) . Some p r e l i m i n a r y r e s u l t s from a k i n e t i c study t h a t suggest the e x i s t e n c e



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o f heterogeneous s i t e s on the s u r f a c e o f g o e t h i t e are g i v e n i n Chapter 7. D e s p i t e the growing e v i d e n c e o f s u r f a c e s i t e h e t e r o g e n e i t y , most s u r f a c e c o m p l e x a t i o n models are based on the concept o f a homogeneous s u r f a c e w i t h a v e r a g e d EDL p r o p e r t i e s ( 5 , 8 , 1 8 , 3 3 - 3 8 ) . In a d d i t i o n to h e t e r o g e n e o u s s i t e s , m i n e r a l s u r f a c e s may c o n t a i n e i t h e r p o o r l y c r y s t a l 1 i z e d or wel 1 h y d r a t e d m a t e r i a l . Thus, m u l t i p l e s o r p t i o n mechanisms may o p e r a t e at the b e g i n n i n g o f many l a b o r a t o r y s t u d i e s o f s o r p t i o n k i n e t i c s . S p o s i t o (Chapter 11) argues t h a t t h i s m u l t i p l i c i t y prevents a simple i n t e r p r e t a t i o n of k i n e t i c or e q u i l i b r i u m data in terms o f a s i m p l e f i r s t o r d e r r a t e l a w o r an a d s o r p t i o n mechanism. R e l a x a t i o n s t u d i e s have shown t h a t the attachment o f an i o n t o a s u r f a c e i s v e r y f a s t , but the e s t a b l i s h m e n t o f e q u i l i b r i u m i n w e l l d i s p e r s e d s u s p e n s i o n s o f c o l l o i d a l p a r t i c l e s i s much s l o w e r . A d s o r p t i o n o f c a t i o n s by h y d r o u s o x i d e s may a p p r o a c h e q u i l i b r i u m w i t h i n a m a t t e r o f m i n u t e s i n some s y s t e m s ( 3 9 - 4 0 ) . H o w e v e r , c a t i o n and a n i o n s o r p t i o n p r o c e s s e s o f t e n e x h i b i t a r a p i d i n i t i a l stage of a d s o r p t i o n t h a t i s f o l l o w e d by a much s l o w e r r a t e o f uptake (24,4143). S e v e r a l s t u d i e s of s h o r t - t e r m i s o t o p i c exchange of phosphate i o n s between aqueous s o l u t i o n s and o x i d e s u r f a c e s have demonstrated t h a t the k i n e t i c s of phosphate d e s o r p t i o n are very slow (43-45). Numerous hypotheses have been suggested f o r t h i s slow attainment of e q u i l i b r i u m i n c l u d i n g 1) the f o r m a t i o n o f b i n u c l e a r complexes on the s u r f a c e (44); 2) dynamic p a r t i c l e - p a r t i c l e i n t e r a c t i o n s i n which an a d s o r b i n g i o n enhances c o n t a c t adhesion between p a r t i c l e s (43,45-46); 3) d i f f u s i o n o f i o n s i n t o a d s o r b e n t s ( 4 7 ) ; a n d 4) surface p r e c i p i t a t i o n (48-50). Bleam and M c B r i d e (51-52) r e c e n t l y p r e s e n t e d e v i d e n c e t h a t the a r r a n g e m e n t o f g r o u p s o f s i t e s on m i n e r a l s u r f a c e s may i n f l u e n c e a d s o r p t i o n . These a u t h o r s argued t h a t , under c e r t a i n c o n d i t i o n s , the f o r m a t i o n o f a monolayer o f a d s o r b i n g i o n s may be l e s s f a v o r a b l e than the f o r m a t i o n o f a m u l t i l a y e r c l u s t e r o f p o l y m e r i z e d o r p r e c i p i t a t e d m a t e r i a l . S e v e r a l s t u d i e s h a v e i n d i c a t e d t h a t a d s o r p t i o n may be d e s c r i b e d by c o m p l e x a t i o n r e a c t i o n s a t d i s c r e t e s u r f a c e s i t e s a t low s u r f a c e c o v e r a g e , but t h a t p o l y m e r i z a t i o n and h y d r o x i d e p r e c i p i t a t i o n may o c c u r a t h i g h s u r f a c e c o v e r a g e ( 5 3 - 5 5 ) . F a r l e y e t a l . (56) r e c e n t l y proposed a model f o r s o r p t i o n o f c a t i o n s on hydrous o x i d e s t h a t a l l o w s f o r a c o n t i n u u m b e t w e e n a d s o r p t i o n and s u r f a c e precipitation as t h e s o r p t i o n d e n s i t y i n c r e a s e s . Surface c o p r e c i p i t a t e s ( s o l i d s o l u t i o n s ) may form when an a d s o r b i n g c a t i o n i s capable of occupying s t r u c t u r a l s i t e s i n the adsorbent l a t t i c e . Experimental evidence o f t h i s t y p e o f p r o c e s s has been g i v e n by M c B r i d e (57) f o r a l u m i n a and by D a v i s e t a l . (49) f o r c a l c i t e . S p e c t r o s c o p i c t e c h n i q u e s may p r o v i d e t h e l e a s t ambiguous methods f o r v e r i f i c a t i o n of a c t u a l s o r p t i o n mechanisms. Z e l t n e r et a l . ( C h a p t e r 8) h a v e a p p l i e d F T I R ( F o u r i e r T r a n s f o r m Infrared) s p e c t r o s c o p y and m i c r o c a l o r i m e t r i c t i t r a t i o n s i n a s t u d y o f t h e a d s o r p t i o n o f s a l i c y l i c a c i d by g o e t h i t e ; t h e s e t e c h n i q u e s p r o v i d e new i n f o r m a t i o n on the s t r u c t u r e o f o r g a n i c a c i d complexes formed at t h e g o e t h i t e - w a t e r i n t e r f a c e . Ambe e t a l . ( C h a p t e r 19) p r e s e n t t h e r e s u l t s o f an e m i s s i o n Mossbauer s p e c t r o s c o p i c s t u d y o f sorbed Co(II) and Sb(V). A l t h o u g h Mossbauer s p e c t r o s c o p y can o n l y be used f o r a few c h e m i c a l e l e m e n t s , the t e c h n i q u e p r o v i d e s d e t a i l e d i n f o r m a t i o n about t h e m o l e c u l a r b o n d i n g o f s o r b e d s p e c i e s a n d may be u s e d t o d i f f e r e n t i a t e between a d s o r p t i o n and s u r f a c e p r e c i p i t a t i o n .



1.

DAVIS A N D HAYES

Overview

1

E m p i r i c a l models. N a t u r a l s y s t e m s c o n t a i n a w i d e v a r i e t y o f mineral s u r f a c e s t h a t may be i n v o l v e d i n s o r p t i o n processes. A p p l i c a t i o n of surface complexation models r e q u i r e s a d e t a i l e d c h a r a c t e r i z a t i o n of each a d s o r b e n t p r e s e n t , and t h e amount o f i n f o r m a t i o n r e q u i r e d g e n e r a l l y exceeds our knowledge of the p r o p e r t i e s o f n a t u r a l m a t e r i a l s (58). W h i l e t h e m o d e l s h a v e been m o d e r a t e l y s u c c e s s f u l i n d e s c r i b i n g the r e s u l t s from l a b o r a t o r y s t u d i e s w i t h p u r e m i n e r a l p h a s e s (e.g. 5 9 ) , t h e y h a v e n o t y e t been a p p l i e d i n f i e l d s t u d i e s . Some a u t h o r s (58,60) have made c a l c u l a t i o n s for hypothetical s e d i m e n t s f o r p r e d i c t i v e p u r p o s e s . In t h e s e c a l c u l a t i o n s the c o n v e n t i o n a l approach has been t o assume t h a t the overall a d s o r p t i o n o f an i o n f o r a m i x t u r e o f m i n e r a l s c a n be d e s c r i b e d as the sum o f the a d s o r p t i v e c o n t r i b u t i o n o f each m i n e r a l . However, Honeyman (61.) has d e m o n s t r a t e d t h a t t h i s c o n c e p t of "adsorptive a d d i t i v i t y " does not h o l d , even i n s i m p l e experiments w i t h b i n a r y m i x t u r e s o f o x i d e p h a s e s . In t h e a b s e n c e o f a u n i f i e d t h e o r e t i c a l m o d e l , g e o c h e m i s t s have o f t e n f o r m u l a t e d e m p i r i c a l approaches t h a t u t i l i z e macroscopic parameters to d e s c r i b e the a d s o r p t i o n p r o c e s s ( 6 0 , 6 2 - 6 3 ) . Honeyman and L e c k i e ( C h a p t e r 9) show t h a t t h e s e m a c r o s c o p i c p a r a m e t e r s a r e t h e n e t r e s u l t o f numerous m i c r o s c o p i c s u b r e a c t i o n s o c c u r r i n g i n t h e s y s t e m . In p a r t i c u l a r , Honeyman and L e c k i e r e v i e w the use o f m a c r o s c o p i c p r o t o n c o e f f i c i e n t s (e.g. Kurbatov c o e f f i c i e n t s ) i n p r a c t i c a l s o r p t i o n m o d e l s . The a u t h o r s show t h a t m a c r o s c o p i c p r o t o n c o e f f i c i e n t s are r a r e l y o b s e r v e d t o have i n t e g r a l v a l u e s , d e s p i t e the f a c t t h a t p r o t o n c o e f f i c i e n t s o f t h e m i c r o s c o p i c a d s o r p t i o n r e a c t i o n s o f i n t e r e s t may have i n t e g r a l v a l u e s . A mathematical d e r i v a t i o n supporting t h i s c o n c l u s i o n is p r e s e n t e d i n Chapter 7. S o r p t i o n of o r g a n i c compounds. A d s o r p t i o n may p l a y an i m p o r t a n t r o l e i n the t r a n s f o r m a t i o n s o f o r g a n i c compounds i n t h e e n v i r o n m e n t . For example, a d s o r p t i o n o f o r g a n i c p o l l u t a n t s a t t h e m i n e r a l - w a t e r i n t e r f a c e may c a t a l y z e t h e c o n v e r s i o n o f t h e s e compounds t o l e s s harmful p r o d u c t s . The c h e m i c a l f a c t o r s c o n t r o l l i n g t h e s o r p t i o n o f h y d r o p h o b i c compounds i n s e d i m e n t s w i t h m o d e r a t e t o h i g h o r g a n i c c o n t e n t have r e c e i v e d c o n s i d e r a b l e s t u d y (64), but s o r p t i o n p r o c e s s e s f o r sediments c o n t a i n i n g low o r g a n i c c o n t e n t are p o o r l y u n d e r s t o o d . C u r t i s e t a l . (Chapter 10) d i s c u s s c u r r e n t problems i n u n d e r s t a n d i n g the s o r p t i o n b e h a v i o r o f h y d r o p h o b i c o r g a n i c compounds, i n c l u d i n g r e a c t i o n k i n e t i c s , h y s t e r e s i s e f f e c t s , and the i n f l u e n c e o f d i s s o l v e d macromolecular organic m a t e r i a l . Ion Exchange The d i s t r i b u t i o n o f major e l e m e n t s between s o i l s and s o i l s o l u t i o n s i s known t o be g o v e r n e d p r i m a r i l y by i o n e x c h a n g e p r o c e s s e s ( 6 5 ) . These p r o c e s s e s are i m p o r t a n t because t h e y g r e a t l y i n f l u e n c e the uptake o f n u t r i e n t s by p l a n t s and o t h e r l i v i n g o r g a n i s m s . Even though an i o n exchange r e a c t i o n c o u l d be c l a s s i f i e d as a t y p e o f a d s o r p t i o n r e a c t i o n , i t i s u s u a l l y t r e a t e d as a s e p a r a t e s o r p t i o n p r o c e s s as a m a t t e r o f c o n v e n i e n c e and t r a d i t i o n . D e s c r i b i n g i o n exchange r e a c t i o n s as t h o s e t a k i n g p l a c e a t " c o n s t a n t c h a r g e " s u r f a c e s , e.g. i n the i n t e r l a y e r r e g i o n s o f c l a y m i n e r a l s , d i s t i n g u i s h e s them from a d s o r p t i o n r e a c t i o n s t h a t occur at " c o n s t a n t p o t e n t i a l " s u r f a c e s , such as those o f hydrous o x i d e s (8).


8


W h i l e the t h e o r y and e x p e r i m e n t a l measurements o f the e q u i l i b r i a o f i o n e x c h a n g e a r e w e l l e s t a b l i s h e d ( 6 6 , 6 7 ) , some m o d e l s e m p l o y u n v e r i f i e d a s s u m p t i o n s c o n c e r n i n g t h e s t r u c t u r e o f exchanged i o n s . For example, t h e e x t e n t t o which c a t i o n s are d e h y d r a t e d upon e n t e r i n g an i o n e x c h a n g e r i s n o t known u n e q u i v o c a b l y ( 6 8 ) . S p o s i t o (10) d i s c u s s e s t h e e x p e r i m e n t a l e v i d e n c e f o r f o r m a t i o n o f both i n n e r - and outer-sphere complexes for various exchanging ions. Camontmori 1 1 o n i t e s u s p e n s i o n s e x h i b i t a d(001) s p a c i n g t h a t is c o n s i s t e n t w i t h t h e f o r m a t i o n o f an o u t e r - s p h e r e s u r f a c e c o m p l e x between e x c h a n g e a b l e C a i o n s and a p a i r o f o p p o s i n g s i l o x a n e d i t r i g o n a l c a v i t i e s . A d d i t i o n a l evidence for t h i s s t r u c t u r e includes q u a s i e l a s t i c neutron s c a t t e r i n g experiments which suggest the e x i s t e n c e of a r i g i d o c t a h e d r a l s o l v a t i o n s h e l l for C a ^ i n Camontmori11onitg (69). E l e c t r o n spin resonance spectra for exchangeable C u and M n i n montmori 1 1 o n i t e a l s o suggest the f o r m a t i o n o f o u t e r - s p h e r e c o m p l e x e s ( 7 0 , and s e e M c B r i d e , C h a p t e r 17). Goodman ( C h a p t e r 16) r e v i e w s t h e c u r r e n t s t a t e o f k n o w l e d g e c o n c e r n i n g t h e s o r p t i o n o f metal i o n s by a l u m i n o s i l i c a t e m i n e r a l s and t h e v a r i o u s s p e c t r o s c o p i c t e c h n i q u e s w h i c h h a v e been e m p l o y e d t o c h a r a c t e r i z e the bonding e n v i r o n m e n t . When i s o m o r p h i c s u b s t i t u t i o n o f A l for S i o c c u r s i n the t e t r a h e d r a l s h e e t o f a p h y l 1 o s i 1 i c a t e , t h e e x c e s s n e g a t i ve c h a r g e w i l l d i s t r i b u t e i t s e l f p r i m a r i l y o v e r t h e t h r e e s u r f a c e oxygens o f one t e t r a h e d r o n . T h i s a l l o w s f o r the f o r m a t i o n o f s t r o n g complexes w i t h c a t i o n s ( Π ) ) . In p a r t i c u l a r , t h e f o r m a t i o n o f i n n e r - s p h e r e complexes w i t h K i s l i k e l y , because the i o n i c diameter of K i s a l m o s t e q u a l t o t h e s i z e o f the d i t r i g o n a l c a v i t y i n the b a s a l p l a n e s o f v e r m i c u l i t e and i l l i t i c m i c a s . The i n f l u e n c e o f such s t r u c t u r e s and bonding f o r c e s on i o n exchange p r o c e s s e s i n v o l v i n g K and C a are d i s c u s s e d i n d e t a i l by G o u l d i n g (Chapter 15) and more g e n e r a l l y by Maes and C r e m e r s ( C h a p t e r 13). In b o t h p a p e r s , e v i d e n c e f o r t h e e x i s t e n c e o f h i g h l y s e l e c t i v e and s p e c i f i c s u r f a c e s i t e s i n l a y e r e d m i n e r a l s i s reviewed. Goulding presents a c h a r a c t e r i z a t i o n of several s e l e c t e d l a y e r e d s i l i c a t e m i n e r a l s i n terms o f s p e c i f i c s i t e t y p e s which can be i d e n t i f i e d by t h e e n t h a l p i e s o f i o n exchange. The a u t h o r a l s o d i s c u s s e s reasons f o r t h e s l o w exchange o r " f i x a t i o n " o f K i n s o i l s . In Chapter 13, Maes and Cremers p r e s e n t a comprehensive r e v i e w o f t h e i n f l u e n c e i o n e x c h a n g e r c h a r g e d e n s i t y and t h e r e l a t i v e p o l a r i z a b i 1 i t y o f e x c h a n g i n g i o n s on i o n e x c h a n g e p r o c e s s e s . The f a c t o r s t h a t l e a d t o h i g h l y s e l e c t i v e exchange b e h a v i o r i n montmori 1 l o n i t e s and z e o l i t e s are emphasized. The paper by Yasunaga and Ikeda (Chapter 12) examines the mechanisms o f i n t e r c a l a t i o n and d e i n t e r c a l a t i o n i n l a y e r e d , c h a n n e l e d , and c a g e - s t r u c t u r e d m i n e r a l s . E b e r l e t a l . (Chapter 14) p r e s e n t e v i d e n c e f o r t h e dynamic r o l e o f w e t t i n g and d r y i n g c y c l e s on t h e w e a t h e r i n g o f s m e c t i t e s and Kf e l d s p a r s . The e x c h a n g e o f K w i t h o t h e r c a t i o n s l e a d s t o t h e f o r m a t i o n o f i l l i t e - l i k e l a y e r s i n s m e c t i t e s , but t h i s r e a c t i o n i s c o m p l e t e l y r e v e r s i b l e when e x c h a n g e d a g a i n w i t h c a t i o n s o f h i g h h y d r a t i o n e n e r g y s u c h as C a (71.). H o w e v e r , E b e r l e t a l . show t h a t K - s m e c t i t e may f i x K i r r e v e r s i b l y when s u b j e c t e d t o w e t t i n g and d r y i n g c y c l e s . The m o i s t u r e c o n t e n t o f s o i l s can d r a m a t i c a l l y a f f e c t the r e a c t i v i t y o f s o i l m i n e r a l s . Under v e r y low m o i s t u r e c o n d i t i o n s b o t h t h e s u r f a c e and i n t e r l a y e r a c i d i t y c a n be g r e a t l y i n c r e a s e d , l e a d i n g t o an i n c r e a s e i n t h e r a t e s o f a c i d - c a t a l y z e d r e a c t i o n s . Thus, c h e m i c a l t r a n s f o r m a t i o n s may o c c u r a t q u i t e d i f f e r e n t r a t e s i n 2 +

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1.

DAVIS A N D HAYES

9

Overview

u n s a t u r a t e d v s . s a t u r a t e d s o i l z o n e s . In a d d i t i o n t o t h e p a p e r by E b e r l e t a l . , t h i s t o p i c i s a d d r e s s e d i n t h e p a p e r s by V o u d r i a s and R e i n h a r d (Chapter 22) and V e l b e l (Chapter 30).


Surface Spectroscopy Surface spectroscopy o f f e r s the best o p p o r t u n i t y to e l u c i d a t e the s t r u c t u r e s o f c h e m i c a l s p e c i e s a t t h e m i n e r a l - w a t e r i n t e r f a c e (see S p o s i t o , C h a p t e r 11). The a p p l i c a t i o n o f s p e c t r o s c o p i c m e t h o d s t o probe the m o l e c u l a r environment of the i n t e r f a c e i s s t i l l a r e l a t i v e l y new f i e l d . C h a p t e r s 16-19 p r e s e n t r e v i e w s and some r e c e n t a d v a n c e s i n i n v e s t i g a t i o n s o f m o l e c u l a r s t r u c t u r e a t the m i n e r a l w a t e r i n t e r f a c e . A r e c e n t r e v i e w o f s p e c t r o s c o p i c methods a p p l i e d t o s o i l and c l a y m i n e r a l systems i s g i v e n i n S t u c k i and Banwart (72). S p e c t r o s c o p i c t e c h n i q u e s can be c l a s s i f i e d a c c o r d i n g t o t h e type of i n t e r f a c i a l environment being i n v e s t i g a t e d . W h i l e the terms " s u r f a c e " and " i n t e r f a c e " are o f t e n i n t e r c h a n g e d , each has a d i s t i n c t meaning: S u r f a c e r e f e r s t o the f a c e o f a s o l i d which i s exposed t o a gas o r l i q u i d phase; an i n t e r f a c e i s a narrow r e g i o n o f f i n i t e w i d t h between the s u r f a c e and a l i q u i d or gas phase. S u r f a c e s p e c t r o s c o p i c t e c h n i q u e s o f t e n r e q u i r e vacuum o r u l t r a h i g h vacuum c o n d i t i o n s and a r e s o m e t i m e s r e f e r r e d t o as e x - s i t u t e c h n i q u e s . S p e c t r o s c o p i c t e c h n i q u e s a p p l i e d t o m i n e r a l - a q u e o u s systems are r e f e r r e d t o as i n s i t u t e c h n i q u e s i n t h a t d i r e c t i n v e s t i g a t i o n o f aqueous s u s p e n s i o n s is possible. S p e c t r o s c o p i c t e c h n i q u e s w h i c h a n a l y z e t h e c o m p o s i t i o n and s t r u c t u r e o f m i n e r a l s u r f a c e s i n c l u d e Auger E l e c t r o n Spectroscopy ( A E S ) , X - r a y P h o t o e l e c t r o n S p e c t r o s c o p y ( X P S , o r t h e o l d e r name, ESCA), and S e c o n d a r y Ion Mass S p e c t r o s c o p y ( S I M S ) . P e r r y ( C h a p t e r 18) d i s c u s s e s t h e a p p l i c a t i o n o f X P S , A E S , and SIMS t o s t u d i e s o f n a t u r a l m a t e r i a l s . Each o f t h e s e t e c h n i q u e s c a n y i e l d d e t a i l e d i n f o r m a t i o n a b o u t t h e s t r u c t u r e and b o n d i n g o f m i n e r a l s and o f c h e m i c a l s p e c i e s p r e s e n t on t h e s u r f a c e s o f m i n e r a l s , b u t P e r r y i l l u s t r a t e s t h e f a c t t h a t a much g r e a t e r knowledge can be g a i n e d by c o m b i n i n g d a t a f r o m two o r more m e t h o d s . The a u t h o r a l s o d i s c u s s e s the i m p o r t a n t s p e c t r a l parameters t h a t y i e l d s t r u c t u r a l i n f o r m a t i o n , t h e a p p l i c a t i o n o f depth p r o f i l i n g methods, and problems c r e a t e d by the h i g h vacuum n e c e s s a r y f o r a n a l y s i s . The use o f XPS and SIMS i n an i n v e s t i g a t i o n o f the o x i d a t i o n s t a t e o f c o b a l t sorbed by b i r n e s s i t e i s r e p o r t e d i n D i l l a r d and Schenck (Chapter 24). A v a r i e t y o f i n - s i t u s p e c t r o s c o p i c t e c h n i q u e s have been used t o i n v e s t i g a t e the m i n e r a l - w a t e r i n t e r f a c e , i n c l u d i n g Raman ( 7 3 ) , F o u r i e r Transform I n f r a r e d (FTIR)(74-75), N u c l e a r M a g n e t i c Resonance (NMR)(76), E l e c t r o n P a r a m a g n e t i c R e s o n a n c e (EPR)(77-80), E l e c t r o n N u c l e a r Double Resonance (END0R)(81), Mossbauer (82), and Extended Xr a y A b s o r p t i o n F i n e S t r u c t u r e (EXAFS)(83) s p e c t r o s c o p i e s . I n ^ s r t u s p e c t r o s c o p i c i n v e s t i g a t i o n s o f t h e m i n e r a l - w a t e r i n t e r f a c e can be f o u n d i n t h e p a p e r s by Z e l t n e r e t a l . ( F T I R , C h a p t e r 8 ) , M c B r i d e (EPR, Chapter 17) and Ambe e t a l . (Mossbauer, C h a p t e r 19). M c B r i d e (Chapter 17) s t u d i e d the o r i e n t a t i o n and m o b i l i t y o f C u i o n s sorbed at exchange s i t e s o f l a y e r s i l i c a t e m i n e r a l s . The EPR s p e c t r a l d a t a r e v e a l e d that the r o t a t i o n a l motion of C u i n t h e s e m i n e r a l s was h i g h l y dependent on the s i z e o f t h e i n t e r l a y e r r e g i o n , and hence, the degree o f i n t e r l a y e r e x p a n s i o n . Ambe e t a l . (Chapter 19) d i s c u s s the s t r u c t u r e s o f s o r b e d C o ( I I ) and Sb(V) i o n s on t h e s u r f a c e o f z +

z +



10


h e m a t i t e . An i n - s i t u e m i s s i o n Mossbauer s p e c t r o s c o p i c s t u d y o f t h e s e s u r f a c e s r e v e a l e d two c h e m i c a l f o r m s o f a d s o r b e d C o ( I I ) : one a t t r i b u t a b l e t o c o o r d i n a t i v e bonds t o s u r f a c e s i t e s and t h e o t h e r to w e a k l y b o u n d ( p o s s i b l y h y d r o g e n b o n d e d ) i o n s . The relative p r o p o r t i o n s o f the two forms v a r i e d as a f u n c t i o n o f pH. Two forms o f a d s o r b e d Sb(V) were a l s o f o u n d i n t h i s s t u d y . Goodman ( C h a p t e r 16) d i s c u s s e s t h e use o f EPR and o t h e r s p e c t r o s c o p i c t e c h n i q u e s i n s t u d i e s of the a d s o r p t i o n of metal i o n s a n d c o m p l e x e s by aluminosilicate minerals. Many o t h e r methods c a n be u s e d t o o b t a i n i n f o r m a t i o n a b o u t m i n e r a l s u r f a c e s i n c l u d i n g Low E n e r g y E l e c t r o n D i f f r a c t i o n (LEED), S c a n n i n g E l e c t r o n M i c r o s c o p y (SEM) and T r a n s m i s s i o n E l e c t r o n M i c r o s c o p y (TEM) (72). The d i f f r a c t i o n methods are w e l l e s t a b l i s h e d f o r s t u d y i n g s t r u c t u r e and the m i c r o s c o p i c t e c h n i q u e s can r e v e a l the morphology o f s u r f a c e s . G i e s e and Constanzo (Chapter 3) s t u d i e d the bonding of i n t e r c a l a t e d water m o l e c u l e s i n s y n t h e t i c hydrated k a o l i n i t e by i n f r a r e d (IR) a b s o r p t i o n . V o u d r i a s and R e i n h a r d (Chapter 22) r e v i e w the a p p l i c a t i o n of a v a r i e t y of s p e c t r o s c o p i c techniques f o r i d e n t i f y i n g s o r b e d r e a c t a n t s , p r o d u c t s , and i n t e r m e d i a t e s i n o r g a n i c r e a c t i o n s at the m i n e r a l - w a t e r i n t e r f a c e . D i s s o l u t i o n , P r e c i p i t a t i o n , and S o l i d S o l u t i o n F o r m a t i o n D i s s o l u t i o n . C h e m i c a l w e a t h e r i n g i s c e r t a i n l y among t h e most i m p o r t a n t geochemical p r o c e s s e s t h a t o c c u r on t h e e a r t h ' s s u r f a c e . In t h e l a s t decade, r e s e a r c h i n t h i s f i e l d has focused on t h e k i n e t i c s and m e c h a n i s m s o f m i n e r a l d i s s o l u t i o n r e a c t i o n s , s i n c e a d e p a r t u r e from e q u i l i b r i u m i n n a t u r a l systems i s apparent (84-85). Hypotheses f o r d i s s o l u t i o n m e c h a n i s m s and r a t e - l i m i t i n g s t e p s d u r i n g t h e w e a t h e r i n g p r o c e s s c a n be g r o u p e d i n t o two s c h o o l s o f t h o u g h t (86)· One s c h o o l proposes t h a t the d i s s o l u t i o n r a t e i s c o n t r o l l e d by the f o r m a t i o n o f a r e s i d u a l l a y e r at the s u r f a c e o f t h e r e a c t i n g m i n e r a l , t h r o u g h which t h e r e a c t a n t s and p r o d u c t s o f w e a t h e r i n g must d i f f u s e ( 8 7 , 8 8 ) . The s e c o n d g r o u p p r o p o s e s t h a t the d i s s o l u t i o n r a t e i s c o n t r o l l e d by the r a t e o f a s u r f a c e r e a c t i o n (89,90). The proponents o f t h e d i f f u s i o n - c o n t r o l m e c h a n i s m b a s e t h e i r c o n c l u s i o n s on t h e temporal e v o l u t i o n o f aqueous s o l u t i o n c o m p o s i t i o n , which suggests t h a t t h e k i n e t i c s o f m i n e r a l d i s s o l u t i o n u s u a l l y obeys a p a r a b o l i c r a t e law. E v i d e n c e f o r the l a t t e r h y p o t h e s i s i n c l u d e s the r e s u l t s o f s u r f a c e s p e c t r o s c o p i c s t u d i e s which have f a i l e d t o d e t e c t any l e a c h e d l a y e r (89-91). D i b b l e and T i l l e r (28) noted t h a t the c o n s i s t e n c y o f the k i n e t i c d a t a w i t h p a r a b o l i c r a t e l a w may i n d i c a t e t h a t t h e r a t e determining step i n v o l v e s d i f f u s i o n of a r e a c t i o n product or impurity i o n t o o r from the i n t e r f a c e . W h i l e mass t r a n s f e r through t h e l i q u i d phase at the i n t e r f a c e i s a r e l a t i v e l y f a s t s t e p , d i f f u s i o n c o u l d become r a t e - d e t e r m i n i n g i f a d s o r p t i o n r e t a r d s m o l e c u l a r detachments a t k i n k s o r l a y e r e d g e s (92). V e l b e l ( C h a p t e r 30) r e v i e w s t h i s a r e a o f r e s e a r c h and d i s c u s s e s c u r r e n t a p p l i c a t i o n s o f t h e c o n c l u s i o n s to n a t u r a l systems. A common phenomenon i n the d i s s o l u t i o n o f s i l i c a t e m i n e r a l s i s t h e f o r m a t i o n o f e t c h p i t s a t t h e s u r f a c e ( 9 0 - 9 3 . , 9 3 - 9 4 ) . When t h i s o c c u r s , the o v e r a l l r a t e o f m i n e r a l d i s s o l u t i o n i s n o n - u n i f o r m , and d i s s o l u t i o n o c c u r s p r e f e r e n t i a l l y at d i s l o c a t i o n s or defects that i n t e r c e p t the c r y s t a l s u r f a c e . P r e f e r e n t i a l d i s s o l u t i o n of the m i n e r a l c o u l d e x p l a i n why s u r f a c e s p e c t r o s c o p i c s t u d i e s have f a i l e d



1.

DAVIS A N D H A Y E S

Overview

11

t o d e t e c t a l e a c h e d l a y e r a t t h e s u r f a c e (93). B r a n t l e y e t a l . ( C h a p t e r 31) e x a m i n e t h e f o r m a t i o n o f e t c h p i t s on q u a r t z i n l a b o r a t o r y e x p e r i m e n t s a t 300°C and on q u a r t z p a r t i c l e s f r o m a n a t u r a l s o i l p r o f i l e . T h e i r r e s u l t s suggest t h a t t h e t h e o r y o f e t c h p i t f o r m a t i o n a t d i s l o c a t i o n s may be u s e f u l i n d e s c r i b i n g m i n e r a l w e a t h e r i n g b e h a v i o r a t l o w t e m p e r a t u r e s as w e l l as d u r i n g hydrothermal a l t e r a t i o n . Numerous g e o c h e m i c a l s t u d i e s h a v e a t t e m p t e d t o i n t e r p r e t t h e c o m p o s i t i o n o f waters i n terms o f c h e m i c a l r e a c t i o n s between parent m i n e r a l s and w e a t h e r i n g p r o d u c t s i n n e a r - s u r f a c e w e a t h e r i n g e n v i r o n m e n t s . The s t u d i e s s u g g e s t , or i n some cases assume, t h a t the water i s i n e q u i l i b r i u m w i t h e i t h e r o b s e r v e d o r i n f e r r e d w e a t h e r i n g p r o d u c t s o r h y p o t h e t i c a l m e t a s t a b l e phases (95). However, c h e m i c a l e q u i l i b r i u m i s n o t n e c e s s a r i l y e x p e c t e d i n an open s y s t e m t h r o u g h w h i c h w a t e r i s f l u x i n g r a p i d l y . A more r e a l i s t i c a p p r o a c h i s t o r e l a t e t h e w a t e r c h e m i s t r y t o t h e k i n e t i c s o f d i s s o l u t i o n and p r e c i p i t a t i o n o f p r i m a r y and s e c o n d a r y phases (87,96). Weathering r a t e s i n n a t u r e are u s u a l l y e s t i m a t e d from geochemical mass b a l a n c e e q u a t i o n s ( 9 7 ) . V e l b e l ( C h a p t e r 30) c o m p a r e s t h e few e s t i m a t e s o f m i n e r a l w e a t h e r i n g r a t e s measured i n n a t u r e which can be n o r m a l i z e d on t h e b a s i s o f s u r f a c e a r e a w i t h t h o s e m e a s u r e d i n t h e l a b o r a t o r y (28,98). The r e s u l t s i n d i c a t e t h a t the r a t e s o f d i s s o l u t i o n i n n a t u r e are much s l o w e r than p r e d i c t e d from l a b o r a t o r y e x p e r i m e n t s . V e l b e l o u t l i n e s p o s s i b l e reasons f o r t h i s d i s c r e p a n c y and p r e s e n t s areas i n which f u r t h e r r e s e a r c h are needed. P r e c i p i t a t i o n . An i m p o r t a n t element o f any geochemical a n a l y s i s o f n a t u r a l waters i s an e v a l u a t i o n o f which m i n e r a l s a r e p r e s e n t and t h e e x t e n t t o w h i c h t h e s y s t e m c a n be r e p r e s e n t e d by e q u i l i b r i u m m o d e l s . T y p i c a l q u e s t i o n s t h a t need t o be a n s w e r e d a r e : 1) I s t h e water s u p e r s a t u r a t e d , u n d e r s a t u r a t e d , o r a t e q u i l i b r i u m w i t h a g i v e n m i n e r a l ? and 2) I f more t h a n one s o l i d p h a s e c a n f o r m f o r a g i v e n e l e m e n t , which phase i s more s t a b l e i n t h a t p a r t i c u l a r e n v i r o n m e n t ? P r e c i p i t a t i o n can o c c u r i f a w a t e r i s s u p e r s a t u r a t e d w i t h respect to a s o l i d phase; however, if the growth of a t h e r m o d y n a m i c a l l y s t a b l e phase i s s l o w , a m e t a s t a b l e phase may form. D i s o r d e r e d , amorphous p h a s e s s u c h as f e r r i c h y d r o x i d e , a l u m i n u m h y d r o x i d e , and a l l o p h a n e are thermodynamical 1 y u n s t a b l e w i t h r e s p e c t to c r y s t a l l i n e phases; n o n e t h e l e s s , these d i s o r d e r e d phases are f r e q u e n t l y f o u n d i n n a t u r e . The r a t e s o f c r y s t a l l i z a t i o n o f t h e s e phases are s t r o n g l y c o n t r o l l e d by t h e presence o f adsorbed i o n s on t h e s u r f a c e s o f p r e c i p i t a t e s ( 9 9 ) . Z a w a c k i e t a l . ( C h a p t e r 32) present evidence that adsorption of a l k a l i n e earth ions g r e a t l y i n f l u e n c e s t h e f o r m a t i o n and g r o w t h o f c a l c i u m p h o s p h a t e s . W h i l e h y d r o x y a p a t i t e was t h e t h e r m o d y n a m i c a l l y s t a b l e p h a s e u n d e r t h e c o n d i t i o n s s t u d i e d by t h e s e a u t h o r s , i t i s shown t h a t s e v e r a l d i f f e r e n t m e t a s t a b l e p h a s e s may f o r m , d e p e n d i n g upon t h e d e g r e e o f s u p e r s a t u r a t i o n and the i n i t i a t i n g s u r f a c e phase. P r e c i p i t a t i o n must b e g i n w i t h t h e f o r m a t i o n o f n u c l e i ; n u c l e a t i o n can be homogeneous (formed i n t h e aqueous s o l u t i o n by the spontaneous a s s o c i a t i o n o f i o n s ) o r heterogeneous ( o r i g i n a t i n g on the s u r f a c e o f an i m p u r i t y o r v i a s e e d p a r t i c l e s w h i c h a c t as c r y s t a l l i z a t i o n c a t a l y s t s ) . In n a t u r e , i t i s t h o u g h t that heterogeneous n u c l e a t i o n i s the predominant process which begins p r e c i p i t a t i o n ( 1 0 0 ) . The f a c t o r s d e t e r m i n i n g growth k i n e t i c s may be d i v i d e d i n t o two main groups: 1) t r a n s p o r t p r o c e s s e s the t r a n s p o r t



12


o f r e a c t a n t s up t o a c r y s t a l s u r f a c e and t h e t r a n s p o r t o f r e a c t i o n p r o d u c t s away f r o m t h e s u r f a c e , and 2) s u r f a c e p r o c e s s e s , w h i c h may i n c l u d e a d s o r p t i o n o f m o l e c u l e s and i o n s on a c r y s t a l surface, m i g r a t i o n o f i o n s a l o n g the s u r f a c e , a change o f t h e degree o f h y d r a t i o n o f i o n s , f o r m a t i o n o f two d i m e n s i o n a l s u r f a c e n u c l e i , and t h e f i t t i n g o f i o n s i n t o the c r y s t a l l a t t i c e (101). N e i l sen (Chapter 29) r e v i e w s some o f the i m p o r t a n t a s p e c t s o f c r y s t a l growth p r o c e s s e s and t h e f a c t o r s w h i c h i n f l u e n c e t h e r a t e - d e t e r m i n i n g mechanism f o r c r y s t a l growth. A d s o r p t i o n may i n f l u e n c e p r e c i p i t a t i o n by means o t h e r than the p r o c e s s e s mentioned above. D a v i e s (Chapter 23) d i s c u s s e s t h e r o l e o f t h e s u r f a c e as a c a t a l y s t f o r o x i d a t i o n o f a d s o r b e d Mn . Redox r e a c t i o n s may c o n t r i b u t e s u b s t a n t i a l l y t o the f o r m a t i o n o f manganese o x i d e c o a t i n g s on m i n e r a l s u r f a c e s i n s o i l s and sediments. Sol i d Sol u t i o n s . The aqueous c o n c e n t r a t i o n s o f t r a c e e l e m e n t s i n n a t u r a l waters are f r e q u e n t l y much l o w e r than w o u l d be expected on the b a s i s o f e q u i l i b r i u m s o l u b i l i t y c a l c u l a t i o n s o r o f s u p p l y t o the w a t e r f r o m v a r i o u s s o u r c e s . I t i s o f t e n assumed t h a t a d s o r p t i o n o f t h e element on m i n e r a l s u r f a c e s i s the cause f o r t h e d e p l e t e d aqueous c o n c e n t r a t i o n o f t h e t r a c e e l e m e n t (97). However, S p o s i t o ( C h a p t e r 11) shows t h a t t h e methods commonly u s e d t o d i s t i n g u i s h between s o l u b i l i t y o r a d s o r p t i o n c o n t r o l s are c o n c e p t u a l l y f l a w e d . One o f the i m p o r t a n t problems i l l u s t r a t e d i n Chapter 11 i s t h e e v a l u a t i o n o f the s t a t e o f s a t u r a t i o n o f n a t u r a l waters w i t h r e s p e c t t o s o l i d phases. G e n e r a l l y , t h e c o n c l u s i o n t h a t a t r a c e element i s u n d e r s a t u r a t e d i s based on a comparison o f i o n a c t i v i t y p r o d u c t s w i t h known pure s o l i d phases t h a t c o n t a i n the t r a c e element. I f a s o l i d phase i s p u r e , then i t s a c t i v i t y i s equal t o one by thermodynamic c o n v e n t i o n . However, when a t r a c e c a t i o n i s c o p r e c i p i t a t e d w i t h a n o t h e r c a t i o n , t h e a c t i v i t y o f t h e s o l i d phase end member c o n t a i n i n g t h e t r a c e c a t i o n i n t h e c o p r e c i p i t a t e w i l l be l e s s t h a n one. I f t h e a q u e o u s p h a s e i s a t equi 1 ibriurn w i t h the c o p r e c i p i t a t e , then the ion a c t i v i t y product wi 1 1 be 1 e s s t h a n t h e s o l u b i 1 i t y c o n s t a n t o f t h e p u r e s o l i d p h a s e c o n t a i n i n g t h e t r a c e element. T h i s c o n d i t i o n c o u l d then l e a d t o the c o n c l u s i o n t h a t a n a t u r a l water was u n d e r s a t u r a t e d w i t h r e s p e c t to t h e pure s o l i d phase and t h a t the aqueous c o n c e n t r a t i o n o f the t r a c e c a t i o n was c o n t r o l l e d by a d s o r p t i o n on m i n e r a l s u r f a c e s . W h i l e t h i s m i g h t be t r u e , S p o s i t o p o i n t s o u t t h a t t h e i o n a c t i v i t y p r o d u c t comparison w i t h the s o l u b i l i t y p r o d u c t does not p r o v i d e any c o n c l u s i v e e v i d e n c e as t o whether an a d s o r p t i o n o r c o p r e c i p i t a t i o n p r o c e s s c o n t r o l s the aqueous c o n c e n t r a t i o n . T h e r e i s c o n s i d e r a b l e e v i d e n c e t h a t c o p r e c i p i t a t i o n and t h e f o r m a t i o n o f s o l i d s o l u t i o n s i s s i g n i f i c a n t i n s o i l s and sediments ( 1 0 ) . W h i l e i d e a l s o l u t i o n m o d e l s h a v e been w i d e l y p r o p o s e d f o r v a r i o u s m i n e r a l s o l i d s o l u t i o n s , e x p e r i m e n t a l i n v e s t i g a t i o n s and s t u d i e s o f n a t u r a l m i n e r a l assemblages have shown t h a t m i s c i b i l i t y g a p s a r e common i n a l m o s t e v e r y m a j o r m i n e r a l g r o u p ( 1 0 2 ) . The e x i s t e n c e o f such gaps r e q u i r e s n o n i d e a l s o l u t i o n models t o d e s c r i b e t h e d i s t r i b u t i o n o f c o m p o n e n t s i n t h e s o l i d and a q u e o u s p h a s e s . D r i e s s e n s ( C h a p t e r 25) r e v i e w s t h e i m p o r t a n t l i t e r a t u r e c o n c e r n i n g l a b o r a t o r y i n v e s t i g a t i o n s o f s o l i d s o l u t i o n s and p r e s e n t s t h e t h e o r y o f i d e a l s o l i d s o l u t i o n s and n o n i d e a l s o l i d s o l u t i o n s , i n c l u d i n g the more g e n e r a l m o d e l s o f r e g u l a r s o l i d s o l u t i o n s w i t h and w i t h o u t o r d e r i n g . Many o f the c a l c u l a t e d a c t i v i t y c o e f f i c i e n t s f o r end-member s o l i d p h a s e s i n s o l i d s o l u t i o n s a r e b a s e d on an a s s u m p t i o n t h a t a


1.

DAVIS A N D H A Y E S

13

Overview

system has reached e q u i l i b r i u m when the s o l i d phase c o m p o s i t i o n i s m e a s u r e d . Plummer ( C h a p t e r 26) e x a m i n e s t h e c r i t e r i a f o r d e c i d i n g when a s y s t e m has r e a c h e d e q u i l i b r i u m ; h i s a n a l y s i s shows t h a t p r e v i o u s c a l c u l a t i o n s f o r the K C l - K B r system at 25°C may be i n e r r o r because the system a n a l y z e d was o n l y near e q u i l i b r i u m . N o n - l a t t i c e s i t e s may p l a y an i m p o r t a n t r o l e i n the i n c o r p o r a t i o n of l a r g e foreign ions in c r y s t a l s t r u c t u r e s during c o p r e c i p i t a t i o n ; P i n g i t o r e (Chapter 27) d i s c u s s e s the importance o f t h e s e s i t e s i n the formation of c o p r e c i p i t a t e s of c a l c i u m carbonate c o n t a i n i n g S r or Ba . W h i t e and Yee ( C h a p t e r 28) d i s c u s s the d i f f u s i o n of a l k a l i i o n s i n t o d e f e c t s t r u c t u r e s i n t h e s u r f a c e s o f g l a s s e s and c r y s t a l l i n e feldspars. z +


Transformation Reactions at the Mineral/Water Interface C e r t a i n p r o p e r t i e s of the m i n e r a l - w a t e r i n t e r f a c e or of r e a c t i v e s i t e s on m i n e r a l s u r f a c e s may l o w e r the a c t i v a t i o n energy o f v a r i o u s transformation reactions, e.g. e l e c t r o n t r a n s f e r r e a c t i o n s or h y d r o l y s i s r e a c t i o n s o f o r g a n i c compounds. The presence o f B r o n s t e d o r L e w i s a c i d s i t e s on m i n e r a l s u r f a c e s i s a p r i m a r y f a c t o r i n c a t a l y z i n g such r e a c t i o n s at the s u r f a c e . Other f a c t o r s are the s t r u c t u r e and charge o f the m i n e r a l s u r f a c e ( i n c l u d i n g any i n t e r l a y e r s p a c i n g ) and t h e s i z e and charge o f r e a c t i n g s o l u t e s . I t i s g e n e r a l l y b e l i e v e d t h a t t h e m e c h a n i s m s o f t h e s e t r a n s f o r m a t i o n r e a c t i o n s are s i m i l a r t o those t h a t o c c u r i n homogeneous aqueous s o l u t i o n s , but the c o n d i t i o n s at t h e m i n e r a l - w a t e r i n t e r f a c e may a c c e l e r a t e the r a t e s o f c e r t a i n r e a c t i o n s . A l t h o u g h many i m p o r t a n t r e a c t i o n s o f o r g a n i c compounds a r e c a t a l y z e d by m i n e r a l s u r f a c e s u n d e r d e s s i c a t e d c o n d i t i o n s at e l e v a t e d temperatures (103), l i t t l e i n f o r m a t i o n i s a v a i l a b l e on c a t a l y s i s by s u r f a c e s i n aqueous e n v i r o n m e n t s . Electron Transfer Reactions. Important redox r e a c t i o n s i n v o l v i n g aqueous s o l u t e s and m i n e r a l s u r f a c e s i n c l u d e o x i d a t i v e o r r e d u c t i v e d i s s o l u t i o n , o x i d a t i o n o r r e d u c t i o n o f s o l u t e s by r e a c t i o n w i t h s u r f a c e s i t e s , and p o l y m e r i z a t i o n r e a c t i o n s o f o r g a n i c compounds. The theory of e l e c t r o n t r a n s f e r r e a c t i o n s i s w e l l e s t a b l i s h e d i n homogeneous s o l u t i o n ; however, the mechanisms o f e l e c t r o n t r a n s f e r r e a c t i o n s w h i c h o c c u r a t t h e m i n e r a l - w a t e r i n t e r f a c e a r e more d i f f i c u l t t o e s t a b l i s h because o f the d i f f i c u l t y i n i d e n t i f y i n g the reacting species. Two t y p e s o f e l e c t r o n t r a n s f e r m e c h a n i s m s h a v e been f o u n d f r o m k i n e t i c s t u d i e s i n homogeneous s o l u t i o n : 1) i n n e r sphere and 2) o u t e r - s p h e r e . S t o n e ( C h a p t e r 21) d i s c u s s e s the a n a l o g y b e t w e e n e l e c t r o n t r a n s f e r r e a c t i o n s i n homogeneous and heterogeneous systems. Waite (Chapter 20) r e v i e w s t h e l i t e r a t u r e on the a b i l i t y o f l i g h t to i n i t i a t e or enhance t h e r a t e s o f redox r e a c t i o n s w h i c h o c c u r on m i n e r a l s u r f a c e s . A s t u d y o f a c c e l e r a t e d oxidation of M n i n the presence o f hydrous i r o n o x i d e i s p r e s e n t e d by D a v i e s ( C h a p t e r 2 3 ) . D i l l a r d and S c h e n c k ( C h a p t e r 24) r e p o r t on r e d o x r e a c t i o n s o f C o ( I I ) and C o ( I I I ) - c o m p l e x e s on t h e s u r f a c e o f birnessite. The s u r f a c e s o f c l a y m i n e r a l s can c a t a l y z e t h e p o l y m e r i z a t i o n o f o r g a n i c compounds through a f r e e r a d i c a l - c a t i o n i c i n i t i a t i o n p r o c e s s . T h i s t y p e o f r e a c t i o n i s b e l i e v e d t o be i n i t i a t e d by t h e a b s t r a c t i o n o f an e l e c t r o n by L e w i s a c i d s i t e s on m i n e r a l s u r f a c e s ; h o w e v e r , B r o n s t e d a c i d i t y has a l s o been shown t o be i m p o r t a n t i n c e r t a i n cases (see C h a p t e r 2 2 ) . z +



14

GEOCHEMICAL PROCESSES ATMINERAL SURFACES

A d d i t i o n a l Transformation Reactions. Other r e a c t i o n s t h a t can be c a t a l y z e d by m i n e r a l s u r f a c e s a r e s u b s t i t u t i o n , e l i m i n a t i o n , and a d d i t i o n r e a c t i o n s o f o r g a n i c m o l e c u l e s . S u b s t i t u t i o n and e l i m i n a t i o n a r e two g e n e r a l t y p e s o f r e a c t i o n s t h a t o c c u r a t s a t u r a t e d c a r b o n atoms o f o r g a n i c m o l e c u l e s . Both types a r e i n i t i a t e d by n u c l e o p h i l i c a t t a c k ; however, i n e l i m i n a t i o n r e a c t i o n s i t i s t h e b a s i c i t y o f t h e n u c l e o p h i l e that determine i t s r e a c t i v i t y r a t h e r than i t s nucleophilicity. S i n c e m i n e r a l s u r f a c e s a r e expected t o have both n u c l e o p h i l i c and b a s i c p r o p e r t i e s , t h e s e t y p e s o f r e a c t i o n s s h o u l d a l s o o c c u r a t m i n e r a l - w a t e r i n t e r f a c e s (see C h a p t e r 22). I t remains t o be shown w h e t h e r o r n o t t h e s e r e a c t i o n s a r e c a t a l y z e d u n d e r environmental conditions. H y d r o l y s i s r e a c t i o n s o c c u r by n u c l e o p h i l i c a t t a c k a t a carbon s i n g l e bond, i n v o l v i n g e i t h e r t h e water m o l e c u l e d i r e c t l y o r t h e h y d r o n i u m o r h y d r o x y l i o n . The most f a v o r a b l e c o n d i t i o n s f o r h y d r o l y s i s , e.g. a c i d i c o r a l k a l i n e s o l u t i o n s , depend on t h e n a t u r e o f t h e bond w h i c h i s t o be c l e a v e d . M i n e r a l s u r f a c e s t h a t have B r o n s t e d a c i d i t y have been shown t o c a t a l y z e h y d r o l y s i s r e a c t i o n s . E x a m p l e s o f h y d r o l y s i s r e a c t i o n s w h i c h may be c a t a l y z e d by t h e s u r f a c e s o f m i n e r a l s i n s o i l s i n c l u d e p e p t i d e bond f o r m a t i o n by amino a c i d s which a r e adsorbed on c l a y m i n e r a l s u r f a c e s and t h e d e g r a d a t i o n of p e s t i c i d e s (see Chapter 22). C o n c l u d i n g Remarks Our k n o w l e d g e o f t h e p h y s i c a l and c h e m i c a l n a t u r e o f t h e m i n e r a l water i n t e r f a c e i s s t i l l a d v a n c i n g . The s i g n i f i c a n c e o f t h e i n t e r f a c e i n p r o c e s s e s s u c h as s o r p t i o n , i o n e x c h a n g e , p r e c i p i t a t i o n , and d i s s o l u t i o n h a s been r e c o g n i z e d f o r some t i m e , b u t new s t u d i e s a r e d e m o n s t r a t i n g t h a t t h e s e p r o c e s s e s a r e i n t e r r e l a t e d i n complex and i n t e r e s t i n g ways. A f u l l a p p r e c i a t i o n o f t h e fundamental importance o f i n t e r f a c i a l r e a c t i o n s i n geochemical p r o c e s s e s i s s t i l l emerging, and i t i s i n c r e a s i n g l y c l e a r t h a t t h e i n t e r f a c e may p l a y a c r i t i c a l r o l e i n a c c e l e r a t i n g t h e r a t e s o f redox r e a c t i o n s , p o l y m e r i z a t i o n , h y d r o l y s i s , and o t h e r t r a n s f o r m a t i o n s . T h i s v o l u m e p r e s e n t s a compilation of state-of-the-art theoretical and e x p e r i m e n t a l approaches which a r e b e i n g a p p l i e d i n s t u d i e s o f t h e m i n e r a l - w a t e r i n t e r f a c e . T h e s e new c o n c e p t s must be i n t e g r a t e d i n t o g e o c h e m i c a l models i f a comprehensive c h e m i c a l d e s c r i p t i o n o f n a t u r a l systems i s t o be a c h i e v e d . Acknowledgments The a u t h o r s w o u l d l i k e t o thank A. Maest, C. C h i s h o l m , and C. F u l l e r for t h e i r c r i t i c i a l reviews o f the manuscript.

Literature Cited 1. Sci. 2. 3.

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Arnold, P.W. In "The Surface Chemistry of Soil Constituents"; Greenland, D.J.; Hayes, M.H.B., Eds.; John Wiley and Sons, New York, 1978. Westall, J . ; Hohl, H. Adv. Colloid Interf. Sci. 1980, 12, 265294. Derjaguin, B.V.; Churaev, N.V. In "Progress in Surface and Membrane Science"; Cadenhead, D.A.; Danielli, J.F., Eds.; Academic Press: New York, 1981; Vol. 14, pp. 69-130. Sposito, G.; Prost, R. Chem. Rev. 1982, 82, 553-573. James, R.O.; Parks, G.A. Surface and Colloid Sci. 1982, 12, 119217. Void, R.D.; Void, M.J. "Colloid and Interface Chemistry"; Addison-Wesley: Reading, Mass., 1983; Chaps. 7,9. Sposito, G. "The Surface Chemistry of Soils"; Oxford: New York, 1984. Ruckenstein, E.; Schiby, D. Lajiajnuijr 1985, 1, 612-615. Pashley, R.M.; Israelachvi1i, J.N. J. Colloid Interface Sci. 1984, 101, 511-523. Schindler, P.W. In "Adsorption of Inorganics at the Solid Liquid Interface"; Anderson, M.A.; Rubin, A . J . , Eds.; Ann Arbor Science, Ann Arbor, 1981, Chap. 1. Huang, C.P., Stumm, W.J. Colloid Interf Sci. 1973, 43, 409. Stumm, W.; Hohl, H.; Dalang, F. Croatica Chem. Acta 1976, 48, 491. Bowden, J.W.; Posner, A.M.; Quirk, J.P.Aust. J. Soil Res. 1977, 15, 121. Davis, J.Α.; James, R.O.; Leckie, J.O.J. Colloid Interf. Sci. 1978, 63, 480-499. van Riemsdijk, W.H.; Bolt, G.H.; Koopal, L.K.; Blaakmeer, J. J. Colloid Interf. Sci. 1986, 109, 219-228. Sprycha, R. J. Colloid Interf. Sci. 1984, 102, 173-185. Smit, W.; Holten, C.L.M. J. C o l l o i d Internf. Sci. 1980, 78, 1. Bousse, L.; de Rooij, N.F.; Bergveld, P. Surface Sci. 1983, 135, 479. Stumm, W.; Furrer, G.; Kunz, B. Croatica Chem. Acta 1983, 56, 593-611. Kinniburgh, D.G.; Jackson, M.L. In "Adsorption of Inorganics at Solid-Liquid Interfaces"; Anderson, M.A.; Rubin, A.J., Eds.; Ann Arbor Science: Ann Arbor, MI, 1981; Chap. 3. Hingston, F.J. In "Adsorption of Inorganics at Solid-Liquid Interfaces"; Anderson, M.A.; Rubin, A.J., Eds.; Ann Arbor Science: Ann Arbor, MI, 1981; Chap. 2. Mikami, N.; Sasaki, M.; Hachiya, K.; Astumian, R.D.; Ikeda, T.; Yasunaga, T. J. Phys. Chem. 1983, 87, 1454-1458. Hachiya, K.; Sasaki, M.; Saruta, Y.; Mikama, N.; Yasunaga, T. J. Phys. Chem. 1984, 88, 23-27. Hachiya, K.; Sasaki, M.; Ikeda, T.; Mikami, N.; Yasunaga, T. J. Phys. Chem. 1984, 88, 27-31. Dibble, W.E. Jr.; T i l l e r , W.A. Geochem. Cosmochim. Acta 1981, 45, 79-92. Benjamin, M.M.; Leckie, J.O. J. Colloid interf. Sci. 1981, 79, 209-221. Kinniburgh, D.G.; Barker, J.A.; Whitfield, M. J. Colloid Interf. Sci. 1983, 95, 370.


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1.

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61. Honeyman, B.D. Ph.D. Thesis, Stanford University, Stanford, Calif., 1984. 62. Tessier, Α.; Rapin, F.; Carignan, R. Geochim. Cosmochim. Acta 1985, 49, 183-194. 63. Balistrieri, L.S.; Murray, J.W. Geochim. Cosmochim. Acta 1983, 47, 1091-1098. 64. Karickhoff, S.W. J. Hydraulic Eng. 1984, 110, 707-735. 65. Sposito, G.; Mattigod, S.V. Son Sci. Soc. Am. J. 1977, 41, 323329. 66. Sposito, G. "Thermodynamics of Soil Solutions"; Oxford Clarendon Press: Oxford, 1981. 67. Bruggenwert, M.G.M.; Kamphorst, A. In "Soil Chemistry, B. Physico-Chemical Models" Bolt, G.H. Ed.; Elsevier Sci. Publ.: Amsterdam, 1979, Chap. 5. 68. Maes, Α.; Cremers, A. In "Soil Chemistry B. Physico-Chemical Models" Bolt, G.H. Ed.; Elsevier Sci. Publ.: Amsterdam, 1979, Chap. 6. 69. Ross, D.K.; Hall, P.L. In "Advanced Chemical Methods for Soil and Clay Mineralogy Research"; Stucki, J.W.; Banwart, W.L., Eds.; Reidel: Boston, 1980, p. 93. 70. McBride, M.B.; Pinnavaia, T.J.; Mortland, M.M. J. Phys. Chem. 1973, 77, 196-200. 71. Goulding, K.W.T.; Talibudeen, O. J. Colloid Interface Sci. 1980, 78, 15-24. 72. Stucki, J.W.; Banwart, W.L., Eds. "Advanced Chemical Methods for Soil and Clay Minerals Research"; Reidel: Boston, 1980. 73. Johnston, C.T.; Sposito, G.; Birge, R.R. Clays andC l a yMin. 1985, 33, 483-489. 74. Tejedor-Tejedor, M.I.; Anderson, M.A. Langmuir 1986, 2, 203-210. 75. Foley, J.K.; Pons, S. Anal. Chem. 1985, 57, 945A-956A. 76. Young, J.R. PhD. Thesis, California Institute of Technology, Pasadena, Calif., 1981. 77. Motschi, H. Colloids and Surfaces 1984, 9, 337-347. 78. Fransesca, M.O.; Ceresa, E.M.; Visca, M. J. Colloid Interface Sci. 1985, 108, 114-122. 79. Bassetti, V.; Burlamacchi, L.; Martini, G. J. Amer. Chem. Soc. 1979, 101, 5471-5477. 80. Clementz, D.M.; Pinnavaia, T.J.; Mortland, M.M. J. Phys. Chem. 1973, 77, 196-200. 81. Rudin, M.; Motschi, H. J. Colloid Interface Sci. 1984, 98, 385393. 82. Ambe, F.; Okada, T.; Ambe, S.; Sekizawa, H. J. Phys. Chem. 1984, 88, 3015. 83. Waychunas, G.A.; Brown, G.E. In EXAFS and Near Edge Structure III; Hodgson, K.O.; Hedman, B.; Penner-Hahn, J . E . , Eds.; Springer-Verlag, New York, pp. 336-342. 84. Berner, R.A. Aim J. Sci. 1978, 278, 1235-1252. 85. Lerman, Α., "Geochemical Processes"; Wiley: New York, 1979. 86. Wollast, R.; Chou, L. In "The Chemistry of Weathering"; Drever, J.I., Ed.; Reidel: Boston, 1985, pp. 75-96. 87. Paces, T. Geochim. Cosmochim. Acta 1973, 37, 2641-2663. 88. Wollast, R. Geochim. Cosmochim. Acta 1967, 31, 635-648. 89. Holdren, G.R.; Berner, R.A. Geochim. Cosmochim. Acta 1979, 43, 1161-1171.



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90. Berner, R.A.; Holdren, G.R. Geochim. Cosmochim. Acta 1979, 43, 1173-1186. 91. Schott, J . ; Berner, R.A.; Sjoberg, E.L. Geochim. Cosmochim. Acta 1981, 45, 2123-2135. 92. Zutic, V.; Stumm, W. Geochim. Cosmochim. Acta 1984, 48, 14931504. 93. Berner, R.A.; Holdren, G.R.; Schott, J. Geochim. Cosmochim. Acta 1985, 49, 1657-1658. 94. Berner, R.A.; Schott, J. Am. J. Sci. 1982, 282, 1214-1231. 95. Velbel, M.A. In "The Chemistry of Weathering"; Drever, J.I., Ed.; Reidel: Boston, 1985, pp. 231-247. 96. Paces, T. In "Interpretation of Environmental Isotope and Hydrochemical Data in Groundwater Hydrology", International Atomic Energy Agency: Vienna, pp. 85-108. 97. Drever, J.I. "The Geochemistry of Natural Waters"; Prentice-Hall: Englewood Cliffs, NJ, 1982. 98. Paces, T. Geochim. Cosmochim. Acta 1983, 47, 1855-1863. 99. Schwertmann, U. In "The Chemistry of Weathering"; Drever, J.I., Ed.; Reidel: Boston, 1985, pp. 119-120. 100. Stumm, W.; Furrer, G.; Wieland, E.; Zinder, B. In "The Chemistry of Weathering"; Drever, J.I., Ed.; Reidel: Boston, 1985, pp. 5574. 101. Neil sen, A.E. In "Treatise on Analytical Chemistry", 2nd Ed.; Part I; Vol. 3; Kolthoff, I.M.; Elving, P.J., Eds.; John Wiley: New York, 1983, Chap. 27. 102. Nordstrom, D.K.; Munoz, J.L. "Geochemical Thermodynamics"; Benjamin-Cummings Publ.: Menlo Park, Calif., 1985, pp.152-162. 103. Solomon, D.H.; Hawthorne, D.G. "Chemistry of Pigments and Fillers" Wiley-Interscience: New York, 1983. RECEIVED August 4, 1986


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