5 Adsorption of Nuclides on Hydrous Oxides Sorption Isotherms on Natural Materials R. E. MEYER, D. A. PALMER, W. D. ARNOLD, and F. I. CASE
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Oak Ridge National Laboratory, Oak Ridge, TN 37830
Hydrous oxides and minerals which have adsorbing groups that behave like hydrous oxides are ubiquitous components of geological formations and may dominate the adsorptive properties of the formations at conditions of natural groundwaters. An understanding of the adsorptive behavior of hydrous oxides i s therefore necessary for reliable prediction of migration of nuclides through the formations. Various isotherms are derived from equilibrium ion-exchange theory for the sorption of non-hydrolyzed ions on hydrous oxides. These isotherms are compared with experimental isotherms for sorption of Cs , Sr , Eu , and TcO - on several hydrous oxides. The experimental isotherms for cations show l i t t l e dependency of sorption on the ionic strength at intermediate pH values but considerable dependence at higher pH values. In the pH range of negligible hydrolysis, sorption increases with pH for cation sorption, and the slope of this dependency on pH increases with the charge of the cation. General features of these isotherms are predictable from ion-exchange equilibrium theory as applied to hydrous oxides. By combining isotherms for hydrous oxides with those for layer-type clay minerals, many unusual features of isotherms found on geological materials can be explained. +
2+
3+
4
Safety a n a l y s i s of nuclear waste r e p o s i t o r i e s r e q u i r e s r e a l i s t i c p r e d i c t i o n of the rates of migration of n u c l i d e s from the reposi t o r y through the host g e o l o g i c a l medium to the a c c e s s i b l e environment. These p r e d i c t i o n s r e q u i r e s o r p t i o n isotherms r a t h e r than s i n g l e values of d i s t r i b u t i o n c o e f f i c i e n t s , and there must be s u b s t a n t i a l confidence that the s o r p t i o n isotherms
0097-6156/ 84/0246-0079S06.00/0 © 1984 American Chemical Society
Barney et al.; Geochemical Behavior of Disposed Radioactive Waste ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
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GEOCHEMICAL BEHAVIOR O F RADIOACTIVE WASTE
are r e a l i s t i c . T h e o r e t i c a l j u s t i f i c a t i o n of the isotherms can provide t h i s confidence, p a r t i c u l a r l y i f the j u s t i f i c a t i o n i s obtained from l o n g - e s t a b l i s h e d p r i n c i p l e s such as chemical e q u i l i b r i u m theory. Hydrous oxides and minerals with adsorbing groups which behave l i k e hydrous oxides are ubiquitous components of geologi c a l formations. These formations may a l s o c o n t a i n other h i g h l y adsorbing m a t e r i a l s , such as the c l a y minerals, and, i n pract i c e , samples of the formations w i l l probably be mixtures of o x i d e s , c l a y s , and other m a t e r i a l s . One approach to modelling these formations i s to consider them to be mixtures of the i n d i v i d u a l minerals of the formations and to c a l c u l a t e s o r p t i o n isotherms from s o r p t i o n isotherms of the components. I f chemic a l e q u i l i b r i u m theory, f o r example as a p p l i e d to i o n exchange, c o u l d be used to p r e d i c t the form of these isotherms, the task of p r e d i c t i o n of the o v e r a l l isotherm of the formations would be considerably s i m p l i f i e d . In t h i s paper, we show that many of the f e a t u r e s of s o r p t i o n isotherms found with n a t u r a l m a t e r i a l s can be p r e d i c t e d with r e l a t i v e l y simple r e l a t i o n s derived from o r d i n a r y e q u i l i b r i u m i o n exchange equations. Further, i n the pH range 6-9 of most n a t u r a l groundwaters, the s o r p t i o n p r o p e r t i e s of many n a t u r a l m a t e r i a l s are dominated by the s o r p t i o n propert i e s of hydrous oxides. Here, we w i l l r e f e r p r i m a r i l y to three types of isotherms: p l o t s of e q u i l i b r i u m d i s t r i b u t i o n coeff i c i e n t , D, vs c o n c e n t r a t i o n of the n u c l i d e on the adsorbent; D vs i o n i c s t r e n g t h ; and D vs pH. The f i r s t of these types may a l s o be p l o t t e d as an e s s e n t i a l l y equivalent graph of concentrat i o n of the n u c l i d e on the adsorbent vs c o n c e n t r a t i o n i n the solution. In d e r i v i n g the shapes of these isotherms, we f i r s t must d e f i n e isotherms f o r i d e a l c l a y s and i d e a l oxides. We then combine these i d e a l f u n c t i o n s i n t o o v e r a l l isotherms and compare the derived f u n c t i o n s to experimental isotherms determined f o r v a r i o u s adsorbents. We w i l l show that observations which have been s a i d to preclude i o n exchange are, i n f a c t , q u i t e cons i s t e n t with ion-exchange behavior. We w i l l not attempt to d e r i v e a c t u a l values of e q u i l i b r i u m d i s t r i b u t i o n c o e f f i c i e n t s , but r a t h e r we seek only to define the shapes of the isotherms. Isotherms f o r I d e a l Clay We w i l l define an i d e a l c l a y as one with a f i x e d negative charge i n the l a t t i c e and i n which the charge i s completely independent of pH. Further, we w i l l s t i p u l a t e that t h i s charge i s the only source of the s o r p t i o n c a p a c i t y of the exchanger. This implies that adsorption i s e n t i r e l y independent of pH except f o r the i n c l u s i o n of the hydrogen i o n as one of the exchangeable c a t i o n s . Using t h i s assumption, we can c a l c u l a t e isotherms from normal i o n exchange e q u i l i b r i u m equations. For exchange of a c a t i o n , A , with a u n i v a l e n t c a t i o n , B , we can write the n +
+
Barney et al.; Geochemical Behavior of Disposed Radioactive Waste ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
5.
Adsorption of Nuclides on Hydrous Oxides
MEYER ET AL.
following
equations: A Κ -
n +
s
[A
a d s
+
n
+ nB , = A + + ads ads
+
nB s
(1)
][B ]»/{[A ](C-n[A s
s
D = [A ds]/[A ] - Κ a
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81
(C-n[A
S
n
a d s
]) }
n
a d s
]) /[B ]
(2) n
s
(3)
where Κ i s the e q u i l i b r i u m constant f o r the ion exchange reac t i o n , [ A ] and [A ] are the concentrations of the nuclide on the c l a y and i n the s o l u t i o n , [ B ] and [B ] are the con c e n t r a t i o n s of the exchanging c a t i o n s , η i s the charge of the n u c l i d e , C i s the c a p a c i t y of the c l a y , and D i s the e q u i l i b r i u m distribution coefficient. Isotherms derived from these equations are quite f a m i l i a r . Values of D are constant as the c o n c e n t r a t i o n of the c a t i o n i s increased u n t i l the amount adsorbed approaches the c a p a c i t y . E q u i v a l e n t l y , the slope of a p l o t of l o g [ A j ] vs l o g [ A ] i s one at low c a p a c i t y and approaches zero as the capacity of the exchanger i s approached. These equations show the s e n s i t i v i t y of t h i s type of s o r p t i o n to competing ions, and t h i s s e n s i t i v i t y i s o f t e n used as a t e s t f o r the presence of ion exchange. If the s o l u t i o n contains ions of only one charge, m, e.g., a l l monovalent or a l l d i v a l e n t , and i f the n u c l i d e , A , i s present at trace concentrations, then p l o t s of l o g D vs logJBg **"] are equal to -n/m. This r e l a t i o n i s i l l u s t r a t e d i n Figure 1 f o r exchange with monovalent c a t i o n s , c a l c u l a t e d f o r a h y p o t h e t i c a l c l a y , from Equation 3. Unfortu n a t e l y , i t i s sometimes assumed that the converse i s true, i . e . that i f these r e l a t i o n s are not found, ion exchange i s not the s o r p t i o n process i n v o l v e d . [In Figure 1 c o r r e c t i o n s are applied f o r the known a c t i v i t y c o e f f i c i e n t s i n mixtures of the c h l o r i d e s of sodium and the adsorbing n u c l i d e s , C s ( I ) , S r ( I I ) , and E u ( I I I ) . This c o r r e c t i o n produces some curvature on these plots.] a d s
s
a d s
a
