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Potassium-Calcium Exchange Equilibria i n Aluminosilicate M i n e r a l s a n d Soils Keith W. T. Goulding Rothamsted Experimental Station, Harpenden, Herts AL5 2JQ, England Potassium-calcium exchange equilibria in some selected layer silicate minerals and soils were studied using exchange isotherms and microcalorimetry. Groups of homogeneous exchange sites, identified by their differential enthalpies of Ca->2K exchange, comprised eight distinct types. These, in turn, were tentatively associated with the surfaces of mica (one type of s i t e ) , hydrous mica or illite (three types of s i t e ) , vermiculite (one type of s i t e ) , and montmorillonite (three types of s i t e ) . If the identification is correct, the method can be used to estimate the amount of each mineral in a clay or soil and thus more precisely determine clay composition. Plants take up the potassium (K) they require from the soil solution. Very l i t t l e Κ i s present in this form, however, perhaps 5-10 kg ha" in the surface soil (0-20 cm). Almost a l l of the Κ in soil is adsorbed on the surfaces of soil colloids, chiefly on aluminosilicate minerals; very l i t t l e Κ i s held on organic materials (see below). In temperate s o i l s , about 500 kg Κ ha~l i s present in this exchangeable form in topsoil , and i t i s released into solution by a simple cation exchange reaction. By far the greatest proportion of Κ in soils of temperate regions, 80-99% or 2-50 Mg ha~l, i s held within the interlayers of partially-weathered and unweathered mica minerals, and within the crystal structure of feldspars. Although the term nonexchangeable' is widely used for this K, this i s not s t r i c t l y correct. The Κ i s exchangeable, but only very slowly. It i s released into more available forms - exchangeable or solution Κ by slow weathering, which again involves cation exchange in the case of micas. The release of Κ into solution from the solid phase is important in plant nutrition, and i t has been extensively studied at Rothamsted as at other agricultural research stations throughout the world. We have been interested in the process both from the practical standpoint of determining the availability of Κ to crops, and from the more fundamental view of understanding its 1
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a d s o r p t i o n and r e l e a s e p r o p e r t i e s and the n a t u r e o f the s u r f a c e s and s i t e s on which Κ i s a d s o r b e d . We have thus examined both the e q u i l i b r i a and k i n e t i c s o f Κ exchange. Because c a l c i u m (Ca) i s the dominant c a t i o n i n most B r i t i s h a g r i c u l t u r a l s o i l s , we have c o n c e n t r a t e d on K/Ca exchange. A l s o , s o i l s are mul ticomponent systems, c o n t a i n i n g a l u m i n o s i l i c a t e ( p h y l l o s i l i c a t e s o r l a y e r l a t t i c e c l a y s , e t c . ) and o r g a n i c e x c h a n g e r s . To determine something o f the c o n t r i b u t i o n o f each component to the exchange c h a r a c t e r i s t i c s o f a s o i l we have t h e r e f o r e examined K/Ca exchange r e a c t i o n s on s o i l s and on some o f the i n d i v i d u a l a l u m i n o s i l i c a t e s most commonly found i n the c l a y f r a c t i o n s o f temperate s o i l s . P.W. A r n o l d ( K 3 ) was an e a r l y p i o n e e r o f the work a t Rothamsted. The p r e s e n t r e s e a r c h programme was begun by 0. T a l i b u d e e n ( 4 - 6 ) , w i t h whom I began c o l l a b o r a t i o n i n 1974. T h i s paper summarises our r e s u l t s and my more r e c e n t work on exchange e q u i l i b r i a (4-16). Materials The s o i l s and c l a y s used i n the work were d e s c r i b e d i n f u l l i n the o r i g i n a l r e p o r t s ( 7 , 1 1 - 1 6 ) . The s o i l s used were taken c h i e f l y from l o n g - t e r m experiments on p l a n t n u t r i t i o n a t Rothamsted and a t e x p e r i m e n t a l s t a t i o n s and farms t h r o u g h o u t B r i t a i n . Those o f c o n t r a s t i n g m i n e r a l o g y and c o n s t i t u t i o n were s e l e c t e d where p o s s i b l e and a l s o those t h a t had been s u b j e c t e d to d i f f e r e n t f e r t i l i z e r and c r o p p i n g sequences, and which t h e r e f o r e had very d i f f e r e n t Κ c o n t e n t s and exchange c h a r a c t e r i s t i c s . G e n e r a l l y , the w h o l e , u n t r e a t e d s o i l s were examined but a l s o , i n one c a s e , the v a r i o u s p a r t i c l e s i z e s e p a r a t e s . The a l u m i n o s i l i c a t e s examined were chosen as end members o f those groups o f p h y l l o s i l i c a t e s t h a t commonly o c c u r i n s o i l s : m u s c o v i t e and b i o t i t e m i c a , F i t h i a n and M o r r i s i l l i t e , Montana v e r m i c u l i t e , m o n t m o r i l l o n i t e s from Upton (Wyoming b e n t o n i t e ) , Camp B e r t e a u , R e d h i l l and New M e x i c o , and k a o l i n i t e s from S t . A u s t e l l , E n g l a n d , and G e o r g i a , U . S . A . Methods E q u i l i b r i a . P o t a s s i u m - c a l c i u m exchange e q u i l i b r i a were s t u d i e d u s i n g s t a n d a r d exchange i s o t h e r m t e c h n i q u e s and by the m i c r o c a l o r i m e t r i c measurement o f e n t h a l p i e s o f exchange; the methods were d e s c r i b e d i n f u l l by G o u l d i n g and T a l i b u d e e n (_7). E s s e n t i a l l y the s o i l s and c l a y s were s a t u r a t e d w i t h Ca and the e n t h a l p i e s o f Ca •* 2K exchange measured a t 303K by adding s u c c e s s i v e 5 μΐ a l i q u o t s o f 0 . 5 M KC1 to a s u s p e n s i o n o f about 50 peq o f the s o l i d i n 2-5 ml water i n an LKB B a t c h m i c r o c a l o r i m e t e r . An i n j e c t i o n system f i t t e d to the c a l o r i m e t e r , d e s c r i b e d by Tal ibudeen e t a l . (8) and M i n t e r and T a l i b u d e e n (9), greatly speeded up the e x p e r i m e n t a l work. The e n t h a l p y measured a f t e r each s t e p was summed to g i v e the i n t e g r a l e n t h a l p y o f exchange ( Η ) , and the r a t e o f change o f t h i s w i t h Κ s a t u r a t i o n was the d i f f e r e n t i a l e n t h a l p y o f exchange, d U H ) / d x ( F i g u r e 1 ) . The Δ Η : Κ s a t u r a t i o n r e l a t i o n s h i p appeared to be a s e r i e s o f s t r a i g h t l i n e s r a t h e r than a smooth c u r v e . T h u s , t h e d e r i v e d Δ
Χ
x
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(kJ eq- ) 1
0
0.25
0.50
0.75
1.0
Fractional K saturation (x)
F i g u r e 1. I n t e g r a l ( Δ Η ) and d i f f e r e n t i a l ( d ( A H ) / d x ) e n t h a l p i e s o f Ca -+ 2K (I) and 2K + Ca (0) exchange on Wyoming b e n t o n i t e as a f u n c t i o n o f t h e f r a c t i o n a l Κ s a t u r a t i o n o f t h e exchange c a p a c i t y . χ
x
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d U H ) / d x : K s a t u r a t i o n r e l a t i o n s h i p became a s e r i e s o f sharp steps (Figure 1 ) . To show i f t h i s was t r u l y the c a s e , and to o b t a i n the b e s t f i t f o r the d a t a , the Rothamsted Maximum L i k e l i h o o d Program (17) was used to f i t and compare a l i n e a r s p l i n e ( s p l i t l i n e ) w i t h s e v e r a l smooth c u r v e s ( q u a d r a t i c , e x p o n e n t i a l , exponential plus l i n e a r ) . In a l l the c a s e s r e f e r r e d to h e r e , the l i n e a r s p l i n e gave the s t a t i s t i c a l l y b e t t e r f i t . The e x t e n t o f exchange f o l l o w i n g each i n j e c t i o n was determined s e p a r a t e l y i n a s c a l e d - u p v e r s i o n o f the c a l o r i m e t e r e x p e r i m e n t . The two procedures a l l o w e d a l l the thermodynamic parameters o f c a t i o n exchange to be c a l c u l a t e d f o l l o w i n g the procedure o f Gaines and Thomas ( 1 8 J . The usual working standard s t a t e s were u s e d : the homoionic exchanger i n e q u i l i b r i u m w i t h a s o l u t i o n o f the s a t u r a t i n g c a t i o n o f c o n s t a n t i o n i c s t r e n g t h ( 0 . 1 M ) , and an i d e a l solution at u n i t m o l a r i t y . In t h i s p a p e r , o n l y the d i f f e r e n t i a l e n t h a l p i e s o f exchange, d e f i n e d above, and the s t a n d a r d f r e e energy U G ) , e n t h a l p y ( Δ Η ) , and entropy (àS ) o f exchange are c o n s i d e r e d . The l a t t e r f u n c t i o n s r e p r e s e n t the i n t e g r a t i o n over the whole exchange o f the change i n s e l e c t i v i t y , bonding s t r e n g t h and o r d e r , r e s p e c t i v e l y , when changing from a Ca/water/sol i d to a 2K/water/sol i d s y s t e m . x
0
0
0
R e s u l t s and D i s c u s s i o n Al urn i no s i 1 i c ate m i n e r a l s . Standard f r e e e n e r g i e s o f Ca -*· 2K exchange, AG , were always n e g a t i v e , showing t h a t a l l o f the 2:1 p h y l l o s i l i c a t e m i n e r a l s ( h e r e a f t e r d e s c r i b e d s i m p l y as c l a y s ) s t u d i e d , and the 1:1 m i n e r a l , k a o l i n i t e , were s e l e c t i v e f o r Κ o v e r Ca ( T a b l e I ) . 0
Table
I.
Standard f r e e e n e r g i e s ( Δ 9 ) , e n t h a l p i e s (ΔΗ ) and e n t r o p i e s ( T A S ) o f Ca + 2K exchange i n the s o i l s and p h y l l o s i l i c a t e m i n e r a l s (From r e f e r e n c e s 7,15) 0
0
0
Material
C a t i o n exchange c a p a c i t y (Ca-forni) (^eq
M u s c o v i t e m i c a , i l l i t e = vermiculite > montmorillonite. S e l e c t i v i t y f o r Κ o v e r Ca has been a s c r i b e d to the low h y d r a t i o n number and p o l a r i z a b i l i t y o f K (19), t o wedge s i t e s a t the weathered edge o f c l a y
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c r y s t a l l i t e s ( 2 0 , 21), and to the c h a r a c t e r i s t i c s o f the a d s o r p t i o n s i t e g e n e r a l l y [20_, 2 2 ) . F o r the s e r i e s o f c l a y s examined h e r e , i t must r e f l e c t T T f f e r e n c e s i n the a d s o r p t i o n s i t e s , and t h e r e f o r e , i n the s u r f a c e s a t which exchange o c c u r s . W h i l e AG i s a measure o f s e l e c t i v i t y , Η i s a measure o f bond s t r e n g t h . The change i n e n t h a l p y r e f l e c t s the breakage and f o r m a t i o n o f a l l bonds i n the i o n / w a t e r / c l a y system ( i o n - i o n , ion-water, i o n - c l a y , water-clay, c l a y - c l a y , water-water). A n e g a t i v e ( e x o t h e r m i c ) e n t h a l p y change i n going from Ca-*2K i m p l i e s s t r o n g e r bonds i n the 2K/water/clay system than i n the C a / w a t e r / c l a y s y s t e m . As S p o s i t o (23J s t a t e s , thermodynamic q u a n t i t i e s c a n n o t be i n t e r p r e t e d d i r e c t l y i n terms o f m o l e c u l a r s t r u c t u r e , nor be a s s i g n e d to any one component w i t h o u t f u r t h e r nonthermodynamic e v i d e n c e . However, they can be seen to agree o r d i s a g r e e w i t h a p a r t i c u l a r m o d e l . The d a t a here agree w i t h S p o s i t o ' s (23) model o f i o n exchange i n terms o f s u r f a c e complexes wïïTch i s based on x - r a y ( 2 4 J , i n f r a r e d ( 2 5 J , and neutron s c a t t e r i n g (26J d a t a . A c c o r d i n g to the m o d e l , potassium adsorbed on 2 : 1 l a y e r s i l i c a t e m i n e r a l s i s i n an i n n e r - s p h e r e complex w i t h no s o l v e n t m o l e c u l e s between i t and the s u r f a c e f u n c t i o n a l g r o u p ; the Κ i s h e l d by i o n i c o r c o v a i e n t bonds o r some c o m b i n a t i o n o f the two. Good examples o f t h i s are the v e r y s t a b l e K-mica and K - v e r m i c u l i t e s u r f a c e complexes i n s o i l ( 2 4 ) . By c o m p a r i s o n , adsorbed Ca i s h e l d by e l e c t r o s t a t i c bonïïs i n an o u t e r sphere complex w i t h a t l e a s t one water m o l e c u l e between i t and the s u r f a c e f u n c t i o n a l g r o u p , e . g . the t w o - l a y e r h y d r a t e o f Camontmorillonite. The g r e a t e r bond s t r e n g t h i n the K - c l a y system decreases i n the same o r d e r as the s e l e c t i v i t y f o r K, s u g g e s t i n g a t f i r s t t h a t bond s t r e n g t h c o n t r o l s s e l e c t i v i t y , as Brouwer e t a l . (27) s t a t e d . However, the data show the range o f e n t h a l p i e s to be v e r y much l e s s than t h a t o f f r e e e n e r g i e s , and t h a t e n t r o p i e s e x e r t a s t r o n g i n f l u e n c e , e s p e c i a l l y i n the m u s c o v i t e m i c a . The s t a n d a r d e n t r o p y change accompanying Ca + 2K exchange on the m i c a , expressed as TAS , i s p o s i t i v e , w h i l e t h a t o f a l l the o t h e r clays i s negative (Table I). T h i s i m p l i e s g r e a t e r o r d e r i n the Ca than the Κ system i n m i c a , b u t v i c e v e r s a f o r the o t h e r c l a y s . I t i s not easy i n such a complex system to d e c i d e which component o r components i s i n f l u e n c i n g the entropy changes m o s t . The o v e r a l l o r d e r o f the system i n c l u d e s t h a t o f the c l a y s u r f a c e s , the adsorbed c a t i o n s and the water m o l e c u l e s - both those a s s o c i a t e d w i t h the c a t i o n s and those i n the f r e e s o l u t i o n . F o l l o w i n g work o f PI ancon e t a l . (28) and Eberl and Srodon ( t h i s volume) which showed how t h e ~ T : l l a y e r s o f N a - s m e c t i t e s r o t a t e and r e a l i g n on a d s o r b i n g Κ, I t h i n k t h a t t h i s r e a l i g n m e n t i s the p r i n c i p l e component o f e n t r o p y changes i n i l l i t e s , v e r m i c u l i t e s and s m e c t i t e s , g i v i n g them g r e a t e r o r d e r on a d s o r b i n g K. In m i c a , no rearrangement i s p o s s i b l e , and the p r i n c i p a l component o f AS i s the i n c r e a s e d randomness i n d i s t r i b u t i o n o f adsorbed Κ o v e r adsorbed C a . D i f f e r e n t i a l e n t h a l p i e s o f exchange, p l o t t e d as a f u n c t i o n o f Κ s a t u r a t i o n , e x h i b i t a stepped c h a r a c t e r ( F i g u r e 1 and T a b l e II). T h i s can o n l y r e f l e c t d i f f e r e n t types o f a d s o r p t i o n s i t e s on the c l a y s u r f a c e s , the group o f s i t e s w i t h the most n e g a t i v e e n t h a l p y Δ
0
0
0
0
vm
H /dx) as a f u n c t i o n o f f r a c t i o n a l Κ s a t u r a t i o n f o r the Batcombe s e r i e s s o i l s from v a r i o u s p l o t s o f the Broadbalk C l a s s i c a l Experiment a t Rothamsted. x
appeared to cause a r e v e r s a l o f the normal p a t t e r n o f d i f f e r e n t i a l enthalpy curves (Figure 2 ) . S i t e s t h a t bound Κ w e a k l y appeared to be f i l l e d b e f o r e those t h a t bound Κ more s t r o n g l y . T h i s s h a r p l y c o n t r a s t i n g b e h a v i o u r has n o t y e t been examined f u r t h e r . The d i f f e r e n c e s between the two s o i l s c o u l d be because ( i ) exchange f i r s t t a k e s p l a c e on o r g a n i c exchange s i t e s which b i n d Κ o n l y weakly b u t which are favoured by e n t r o p y c h a n g e s , ( i i ) the o r g a n i c m a t e r i a l , w h i l e n o t d i r e c t l y i n v o l v e d i n the exchange, c o a t s m i n e r a l s u r f a c e s b l o c k i n g some o f the exchange s i t e s . There may a l s o be some e f f e c t o f the d e c o m p o s i t i o n o f o r g a n i c m a t t e r on c l a y weathering. E n t r o p y changes show an i n c r e a s e i n o r d e r o v e r a l l the exchange when Κ i s a d s o r b e d , an i n c r e a s e so l a r g e t h a t the Batcombe s o i l t r e a t e d w i t h FYM p r e f e r s C a . T h i s may r e f l e c t the c h a r a c t e r i s t i c s o f o r g a n i c exchange s i t e s , but i t c o u l d a l s o r e f l e c t the b l o c k i n g a c t i o n o f o r g a n i c m a t e r i a l s . Other workers have suggested t h a t o r g a n i c m a t e r i a l b l o c k s exchange s i t e s ( 3 6 , 3 7 ) , and the d e s t a b i l i z a t i o n o f c l a y - C a - o r g a n i c b r i d g e s by monovalent c a t i o n s has been noted b e f o r e [38). C e r t a i n l y the Κ on the p l o t s r e c e i v i n g FYM on some Rothamsted C l a s s i c a l Experiments has been observed as r e a c t i n g d i f f e r e n t l y , namely b e i n g 'more a v a i l a b l e ' , b e f o r e ( 3 9 ) . G e n e r a l l y , c l a y m i n e r a l c o n t e n t was the most i m p o r t a n t f a c t o r d e t e r m i n i n g Κ s e l e c t i v i t y i n s o i l , w i t h w e a t h e r i n g , f e r t i l i z e r s and o r g a n i c r e s i d u e s a f f e c t i n g s e l e c t i v i t y through t h e i r m o d i f y i n g e f f e c t s on m i n e r a l s u r f a c e s . Conclusions P o t a s s i u m - c a l c i u m exchange e q u i l i b r i u m s t u d i e s , i n v o l v i n g p a r t i c u l a r l y the measurement o f d i f f e r e n t i a l e n t h a l p i e s o f exchange, q u a n t i t a t i v e l y i d e n t i f y t y p e s o f c a t i o n exchange s i t e s i n s o i l s and p h y l l o s i l i c a t e m i n e r a l s which can be i n t e r p r e t e d i n terms o f m i c a , hydrous m i c a , v e r m i c u l i t e and m o n t m o r i l 1 o n i t e s u r f a c e s . Combined w i t h XRD and o t h e r a n a l y t i c a l t e c h n i q u e s , such s u r f a c e measurements p r o v i d e a means o f more p r e c i s e l y d e t e r m i n i n g m i n e r a l c o m p o s i t i o n , and o f examining the e f f e c t s o f w e a t h e r i n g and o f v a r i o u s c o a t i n g s on the s u r f a c e s o f a l l a l u m i n o s i l i c a t e m i n e r a l s w i t h an exchange c a p a c i t y . At t h i s s t a g e i t i s not
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p o s s i b l e to i d e n t i f y c o n c l u s i v e l y the v a r i o u s types o f s i t e s w i t h known p h y s i c a l c h a r a c t e r i s t i c s o f a l u m i n o s i l i c a t e s , p a r t i c u l a r l y because charge d e n s i t y ( c o u l o m b i c ) e f f e c t s w i l l a l s o be a s t r o n g force i n determining s e l e c t i v i t y . However, t h e mass o f e v i d e n c e p o i n t s t o t h e e x i s t e n c e o f ( i ) a few h i g h l y Κ s e l e c t i v e , s p e c i f i c s i t e s which b i n d Κ s t r o n g l y a t p a r t i c l e and step e d g e s , a t c r a c k s , and around i s l a n d s o f 1 0 A c o r e s i n weathered micas ( i l l i t e s ) ; ( i i ) s i t e s on t h e s u r f a c e s o f m i c a s , w h i c h a r e v e r y Κ s e l e c t i v e because o f e n t r o p i e e f f e c t s b u t which do n o t b i n d Κ e s p e c i a l l y s t r o n g l y ; ( i i i ) s i t e s o f i n t e r m e d i a t e Κ s e l e c t i v i t y on i l l i t i c and v e r m i c u l i t i c s u r f a c e s ; ( i v ) s i t e s o f low Κ s e l e c t i v i t y on e x t e r n a l p l a n a r s i t e s on m o n t m o r i l l o n i t i c s u r f a c e s . I t has o f t e n been argued t h a t c l a y m i n e r a l s c a n n o t be used as models f o r s o i l s i n s u r f a c e c h e m i s t r y because s o i l c l a y s a r e t o o heterogeneous and impure. The work r e p o r t e d here shows t h a t , f o r both e q u i l i b r i u m and k i n e t i c s t u d i e s , s t a n d a r d al u m i n o s i l i c a t e s a r e u s e f u l models f o r s o i l s .
Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13 14. 15. 16. 17. 18. 19.
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