Ion-Exchange Equilibria between Montmorillonite and Solutions of

17

Ion-Exchange Equilibria between Montmorillonite and Solutions of Moderate-to-High Ionic Strength

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on June 30, 2016 | http://pubs.acs.org Publication Date: April 6, 1979 | doi: 10.1021/bk-1979-0100.ch017

S.-Y. SHIAO, P. RAFFERTY, and R. E. MEYER Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830 W. J. ROGERS Department of Chemistry, The University of Tennessee, Knoxville, TN 37916 Prediction of the migration rates of ions in geologic formations is of extreme importance in the fields of nuclear waste d i s posal and enhanced oil recovery. Demonstration of the safety of nuclear waste repositories requires that the repository be so designed and situated that migration rates from the repository be acceptably low in the event of water flow through the repository. On the other hand, the principal criterion for selection of t r a cers to monitor fluid flow in the flooding techniques of enhanced oil recovery is that the tracers be not retained on the formation, i.e., migration rates must be high. Migration rates are dependent on the distribution coefficient of the species between fluid and the geologic media such that the greater the distribution coefficient the slower the rate of migration. Therefore, for reliable prediction of migration rates, accurate knowledge of the adsorption behavior of the nuclides and tracers must be known as a function of all of the pertinent variables. Obtaining the necessary data for this complex situation is complicated by both the large number of different minerals which may be present and the wide range of solution compositions. In order to accumulate a systematic body of information pertinent to this complex situation, we are measuring adsorption of ions of interest on minerals typical of classes, from a wide range of aqueous solution compositions. I n i t i a l l y , montmorillonite was selected for attention because its high adsorptive capacity for ions may cause i t to dominate the adsorptive proportion of the formation in which i t is present. Reported here are distribution coefficients of a l k a l i metal and alkaline earth ions, and of one rare earth, between montmorillonite predominantly in the sodium and calcium forms and aqueous solutions of controlled pH, at moderate to high s a l i n i t i e s (>0.01MNaCl). The ions were selected for initial study, not only because of their i n t r i n s i c interest for these applications but also because they allow evaluation of the extent to which equilibria can be described by conventional ion-exchange equations with minimum d i f f i c u l t i e s from hydrolysis, precipitation, and complexing. Correlation of results as a 0-8412-0498-5/79/47-100-297$07.00/0 © 1979 American Chemical Society

Fried; Radioactive Waste in Geologic Storage ACS Symposium Series; American Chemical Society: Washington, DC, 1979.


298

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f u n c t i o n of i o n i c strength at c o n t r o l l e d pH, temperature, and l o a d i n g , so f a r as i t i s f e a s i b l e , reduces g r e a t l y the number of measurements necessary to p r e d i c t behavior. The exchange of sodium and calcium on c l a y minerals i s of s p e c i a l importance, because i t l a r g e l y determines the i o n i c form of the c l a y , which i n t u r n a f f e c t s the performance of s u r f a c t a n t and polymer f l o o d s and the d i s t r i b u t i o n of other ions of i n t e r e s t . In concentrated NaCl environments, c l a y e x i s t s mainly i n the sodium form; however, at low i o n i c strength and moderate hardness, c l a y may be e s s e n t i a l l y i n the calcium form. Measurements with the sodium, calcium, and mixed forms are t h e r e f o r e of i n t e r e s t . Previous s t u d i e s of the adsorption of ions on montmorilloni t e have emphasized low s a l t concentration regions (see, e.g. r e f . 1-11). Our main i n t e r e s t i s i n higher s a l t concentrations, because many of the b r i n e s i n o i l r e s e r v o i r s are i n t h i s range, and because some favored l o c a t i o n s f o r nuclear waste d i s p o s a l , bedded s a l t deposits and s a l t domes, may r e s u l t i n high i o n i c strength environments. Experimental C l a y s . Two c l a y s , SWy-a Na-montmorillonite (Crook County, Wyoming) and STx-1 Ca-montmorillonite (Gonzales County, Texas) were obtained from the Department of Geology, U n i v e r s i t y of Miss o u r i . Montmorillonite #27 ( B e l l Fourche, South Dakota) and montmorillonite #31 (Cameron, Arizona) were purchased from Ward's N a t u r a l Science Establishment. T r a c e r s . Radioactive t r a c e r s were employed i n ments' C s " , N a , K^ , Ca , and B a were made bombardment i n the High Flux Isotope Reactor at Oak Laboratory. Sr and E u were purchased from New Nuclear (Boston, Massachusetts). 1

37

2 2

8 5

2

kl

1 5 5

1 3 3

the e x p e r i by neutron Ridge N a t i o n a l England

P u r i f i c a t i o n and Preparation of Clay Samples. The sand f r a c t i o n of the c l a y samples was separated by slow-speed c e n t r i f u g a t i o n of a suspension of the c l a y before the p u r i f i c a t i o n steps. The c l a y was p u r i f i e d f o l l o w i n g Jackson's p u r i f i c a t i o n procedure (12) (unless otherwise mentioned), that i s , removing i n s o l u b l e carbonates by using 1 M acetate b u f f e r (pH 5), removing organic matter by using 30% hydrogen peroxide, and removing i r o n not i n corporated i n the c l a y s t r u c t u r e by u s i n g sodium c i t r a t e and sodium d i t h i o n i t e . F i n a l l y , the c l a y was converted to a monoionic form ( e i t h e r sodium or calcium form) by c o n t a c t i n g the c l a y with concentrated sodium or calcium c h l o r i d e s o l u t i o n s s e v e r a l times. The excess calcium or sodium s a l t s were washed out with 60% methanol-water mixtures. Removal of c h l o r i d e by the washes was checked with a chloridometer (Buchler Instruments, Inc., F o r t Lee, N. J . ) . The c l a y was then d r i e d i n a vacuum d e s i c c a t o r and ground i n a j a r m i l l . The f i n e f r a c t i o n (] ι A>

„ J

V_

(1)

(n

a+ i s the concentration of A in solution, moles/liter. (A ) i s the c o n c e n t r a t i o n on the s o l i d , i n moles/ k g dry s o l i d , (n^) ^ and (n^) are the counts of r a d i o a c t i v e t r a c e r on the s o l i d and i n the s o l u t i o n , r e s p e c t i v e l y , at e q u i l i b r i u m (or the amounts of A, i f analy s i s i s done by other methods). ^ A^i "** * ^ I t l a l t o t a l counts i n the aqueous phase, w i s the weight of dry s o l i d i n monoionic form i n kg, V i s the volume of s o l u t i o n , i n l i t e r s . A Nal w e l l - t y p e s c i n t i l l a t i o n was used to count the γ emission of the t r a c e r s i n the s o l u t i o n before and a f t e r e q u i l i b r a t i o n . Most of the c l a y samples were p r e - e q u i l i b r a t e d with the aqueous media to be used i n the adsorption measurements to insure a t t a i n ing the s p e c i f i e d pH, u s u a l l y 5, maintained with acetate b u f f e r . M o n t m o r i l l o n i t e , e s p e c i a l l y the sodium form, swells i n lowi o n i c - s t r e n g t h s o l u t i o n s ; consequently, c e n t r i f u g a t i o n i n moder a t e l y high f i e l d s i s needed to separate the c l a y p a r t i c l e s and the s o l u t i o n s . Clay suspensions were c e n t r i f u g e d w i t h a Du Pont S o r v a l l RC-5 r e f r i g e r a t e d super-speed c e n t r i f u g e at 15,000 rpm (28,000 g) f o r 15 minutes, and the supernatant was then withdrawn f o r a n a l y s i s . No c o r r e c t i o n was made f o r p o s s i b l e ion e x c l u s i o n where

(A

a+

( M

I W Λ

clay A

)

a+

y

n

s

t

i e


300

RADIOACTIVE

WASTE

IN GEOLOGIC

STORAGE


i n the water i n the c l a y pack at the bottom of the c e n t r i f u g e tube; i t was assumed t o have the same c o n c e n t r a t i o n as that of the supernatant. For s o l u t i o n - c l a y r a t i o s used here, there would probably be no s i g n i f i c a n t e r r o r i f there were some i o n e x c l u s i o n i n the c l a y pack. Polypropylene tubes were used f o r e q u i l i b r a t i o n and c e n t r i f u g a t i o n . Under the c o n d i t i o n s of these exper iments, the a d s o r p t i o n of r a d i o a c t i v e t r a c e r s on the tubes was found not to have a s i g n i f i c a n t e f f e c t on r e s u l t s . Cation Exchange Capacity. Various techniques have been used to measure the c a t i o n exchange capacity of the c l a y samples. Unless otherwise noted, i n computation of e q u i l i b r i u m q u o t i e n t s , we s h a l l use a value of 0.78 equivalents/kg c l a y , determined by a column method (14) on the calcium form of Wyoming montmoril l o n i t e at pH 5. a +

Equations. The e q u i l i b r i u m between A and (in a solu t i o n with a common monovalent anion, X~) i n aqueous (no sub s c r i p t ) and s o l i d ( s u b s c r i p t " c l a y " ) phases, a

b

bA

a +

+

aB t

- b c l a y -«-

At

+ aB

b +

(2)

clay

can be expressed by

(A )!L„ m;a , b

a + va+

!

*

« W

O" ' e l . ,

where the a c t i v i t y c o e f f i c i e n t quotient i s d e f i n e d and can be evaluated by / \κ

/ \

/

xMa+l)/

( O Heu, M

r

x

a(b+l)

clay a(b+l) Γ , Γ = Γ c l a y aq clay /

\b(a+l)

(4)

The symbol, γ , denotes the mean i o n i c a c t i v i t y c o e f f i c i e n t . Customarily, i n the s o l u t i o n phase, the symbol γ i s used i n con j u n c t i o n with c o n c e n t r a t i o n s i n m o l a l i t y u n i t s , or moles per kg +



17.

SHiAO ET AL.

Ion-Exchange

301

Equilibria

of s o l v e n t . We have expressed solution-phase concentrations i n Equation 3 i n m o l a l i t y , m, f o r consistency; D i s the d i s t r i b u t i o n c o e f f i c i e n t corresponding to t h i s convention. For present pur poses, there i s l i t t l e d i f f e r e n c e between m o l a l i t y and m o l a r i t y s c a l e s up to one molar NaCl; d i f f e r e n c e s i n the range of 10% are i n c u r r e d near saturated NaCl. The s e p a r a t i o n of T^g i n t o r a t i o s f o r the s o l u t i o n and s o l i d phases i s u s e f u l when s o l u t i o n phase a c t i v i t y c o e f f i c i e n t s are a v a i l a b l e (see, e.g., 15) because i t allows e v a l u a t i o n of v a r i a t i o n s from i d e a l i t y of the c l a y phase over a range of experimental c o n d i t i o n s . When measurements f o r the aqueous mixed e l e c t r o l y t e systems i n question are not a v a i l able, adequate estimates can f r e q u e n t l y be made by Debye-Huckel equations or by v a r i o u s methods from measurements on two-component systems (see, e.g., 16). C e r t a i n s p e c i a l cases are of i n t e r e s t here. When one i o n i s present at concentrations low enough f o r l o a d i n g of the s o l i d phase to be only a small f r a c t i o n of i o n exchange c a p a c i t y , C, expressed i n equivalents/kg of s o l i d , Equation 3 may be w r i t t e n

a — a

£ /r _ " Κ — AB AB - A A R

n

A

b

~ — ,

( c / h )

( (

/ 1 3

b+v

/ b )

a

a

(5)

If i s constant over the range of c o n d i t i o n s , and i f the e x c l u s i o n of coion X" from the c l a y phase i s e s s e n t i a l l y complete, Equation 5 can be d i f f e r e n t i a t e d to give d log

D

d log

(B )

A

-a/b

(6)

b +

+

i . e . , a p l o t of l o g vs l o g (B^ ) w i l l be l i n e a r , with slope - ( a / b ) , or, e.g., -2 i f A is Ca and B i s Na . The r e s t r i c t i o n of coion e x c l u s i o n , although i m p l i c i t l y i n cluded i n " i d e a l " behavior here, i s a somewhat d i f f e r e n t a p p r o x i mation than constancy of or r ^ . Equations which take i n vasion of coions i n t o the c l a y phase i n t o account can r e a d i l y be developed, f o r example by a s s i g n i n g the same r e f e r e n c e s t a t e to both phases and expressing concentration of ions i n the c l a y phase i n terms of moles/kg water, equivalent to Κ = 1 (17). The behavior of l o g ^ v s l o g (B^ ) given by Equation 6 i s approached as i o n i c strength i s decreased, and coion concentration i n the c l a y presumably becomes n e g l i g i b l e i n comparison with c a p a c i t y . We have not measured coion i n v a s i o n i n t h i s work and t h e r e f o r e w i l l d i s c u s s r e s u l t s i n terms of the equations given. I t i s perhaps worthwhile to d i s t i n g u i s h between the j u s t discussed coion e x c l u s i o n i n the l a y e r s of m o n t m o r i l l o n i t e , where presumably most of the ion-exchange c a p a c i t y i s l o c a t e d , from the e a r l i e r reference to p o s s i b l e e f f e c t s of e x c l u s i o n i n the water a +

2 +

c

b +

+

a v

+


RADIOACTIVE W A S T E IN GEOLOGIC STORAGE

302


of the c l a y pack on i n t e r p r e t a t i o n of experimental r e s u l t s . The l a t t e r , which we have assumed t o be n e g l i g i b l e , would a r i s e from the i n f l u e n c e on i n t e r s t i t i a l s o l u t i o n between p a r t i c l e s of charges on the surface of the p a r t i c l e s , the charge d e n s i t y of which, averaged over pack-water volume, would be f a r l e s s than the fixecj-charge d e n s i t y i n the l a y e r s , i f the l a y e r s are only 10 - 20 A apart. Results reported here do not c l a r i f y the degree of coion e x c l u s i o n i n e i t h e r environment. Results We s h a l l f i r s t d i s c u s s e f f e c t s of loading of the c l a y on d i s t r i b u t i o n c o e f f i c i e n t s . With these r e s u l t s , we hope to i d e n t i f y c o n d i t i o n s under which d i s t r i b u t i o n c o e f f i c i e n t s are independent of l o a d i n g ( " l i n e a r isotherm" r e g i o n s ) . Measurements made i n these regions can then be used to evaluate e f f e c t s o f other v a r i a b l e s , p r i m a r i l y of i o n i c strength. I t i s expected, of course, from Equation 3 that Ό w i l l decrease s u b s t a n t i a l l y at high l o a d i n g ; what we are concerned with here are s i g n i f i c a n t e f f e c t s at lower l o a d i n g than p r e d i c t e d f o r constant r c i a v . Because d i s t r i b u t i o n c o e f f i c i e n t s are o f t e n dependent on pH, b u f f e r s were used. The pH o f 5 was s e l e c t e d . f o r most of the ex periments because montmorillonite i s s t a b l e at t h i s a c i d i t y , i n t e r f e r e n c e of observations from h y d r o l y s i s and from p r e c i p i t a t i o n of a l k a l i n e earth carbonates i s precluded, and adsorption on p o s s i b l e hydrous oxide i m p u r i t i e s i s minimized. An acetate b u f f e r was u s u a l l y used t o maintain pH because acetate does not have a strong tendency to complex many i o n s . Loading Sodium Form of Montmorillonite (Wyoming). C s ( I ) : F i g u r e 1 summarizes the d i s t r i b u t i o n c o e f f i c i e n t s of C s from 0.5 M NaCl, pH 5, s o l u t i o n s , as a f u n c t i o n of l o a d i n g of C s from t r a c e l e v e l to w e l l over 10% o f c a p a c i t y . There i s an appreciable d i f f e r e n c e between t r a c e and the lowest macro concentration, and a s i g n i f i cant, though not p r e c i p i t o u s , d e c l i n e up to about 10% l o a d i n g . Wahlberg and Fishman (_3) reported l o a d i n g curves f o r C s on two samples of montmorillonite a t s e v e r a l concentrations of NaCl up to 0.2 M. They a l s o found a gentle decrease of DQ with i n c r e a s i n g loading. A l k a l i n e earths: The e f f e c t o f S r ( I I ) l o a d i n g on D$ f o r s e v e r a l d i f f e r e n t concentrations of Na+ i s shown i n Figure 2. Up to 0.01 moles S r ( I I ) / k g c l a y , e f f e c t s are very s m a l l , and values at t r a c e r l e v e l s agree with those at macro concentrations. Wahl berg, et a l . (7) report l o a d i n g curves f o r a sample of montmoril l o n i t e a t N a concentrations up to 0.2 M. T h e i r values o f D$ . appeared constant with l o a d i n g up t o about the same S r ( I I ) loading as i n Figure 2, and t h e i r values of D i n the l i n e a r isotherm r e gion were about 50% higher than those i n Figure 2 a t overlapping +

+

+

S

R

+

T


17.

SHIAO

Ion-Exchange

E T A L .

Equilibria

t o o

ζ ο

I

| | | | | | | — I

I

l|ll|||

I I l|llfl|

I

I

M

b r

Ul

ο ο

10

m

10 -5

TRACE

3

i o ~

10" L0A0IN6, mole Cs(I)/kg 4

10" 10"

Figure 1. Ε feet of loading on distribution coefficients of Cs(I) on the sodium form of Wyoming montmorillonite (0.5M NaCl + 0.01M NaOAc, pH 5, equili bration for 74 hr)

1 1 1

...i t I Γ

Sr /Na 2 +

:

V

1

1 1

M

EXCHANGE ON MONTMORILLONITE

+

1

I

:

!1 Il 1 1 1 ι 1 1

1 ! J

1

1 1 1

R>

1


1000^1 ι ι ι m u — ι ιι1—I

303

1

1 1 1 1 1 1

I I

η°

Ο ,

,

;

"

1

-«*·

-i*

M*

1

1 i ti 1 1 i 1 1 11 1

TRACE

IM

:

0.0001

1

1 1

0.001

1

0.01

1 1 11 11 11 11 0.1

LOADING, moles/kilogram 1 1 1 1 1

Figure 2. Effect of loading on distribution coefficients of Sr(H) on the sodium form of Wyoming montmorillonite. NaCl and pH 5 acetate buffer: (O), 0.1 M N a ; (Φ), 0.2M Na+; (+), 0.6M Na+; (χ), LIU Na\

A


+

RADIOACTIVE W A S T E IN GEOLOGIC STORAGE


304

i o n i c s t r e n g t h . The agreement i s considered good, i n view of d i f ferences i n c l a y samples and p u r i f i c a t i o n s ( i n r e f . _7, c l a y s were exposed to hot 1 M HC1-1 M NaCl s o l u t i o n s ) . F i g u r e 3 summarizes e f f e c t s of l o a d i n g on Ό f o r C a ( I I ) , S r ( I I ) , and B a ( I I ) , f o r the sodium form of montmorillonite. The S r ( I I ) r e s u l t s are from a d i f f e r e n t set of measurements than those i n F i g u r e 2, but are i n good agreement with them. E f f e c t s of l o a d i n g are small up to the s e v e r a l percent of ion-exchange c a p a c i t y covered. 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 f o r these three ions f a l l i n a narrow range. E u ( I I I ) : The l o a d i n g e f f e c t on the d i s t r i b u t i o n c o e f f i c i e n t of E u ( I I I ) on the Na form of montmorillonite f o r both a c e t a t e b u f f e r e d and unbuffered s o l u t i o n s i s shown i n F i g u r e 4. The Ds are f a i r l y constant i n the low l o a d i n g range. We s h a l l comment l a t e r on the d i f f e r e n c e s i n presence and absence of b u f f e r . %

Calcium Form of M o n t m o r i l l o n i t e (Wyoming). A l k a l i metal i o n s : We present these r e s u l t s i n terms of p l o t s of K/Tclay * a f u n c t i o n of l o a d i n g (Figures 5 and 6 ) . The solution-phase a c t i v i t y c o e f f i c i e n t r a t i o f o r sodium was obtained from a compilation of l i t e r a t u r e data on the N a C l - C a C l 2 - H 2 0 systems (15), and the same values were used f o r cesium. E q u i l i b r i u m quotients f o r the d i f f e r e n t Ca(II) concentrations agree w e l l . In comparison w i t h the r e s u l t s on the sodium form of m o n t m o r i l l o n i t e , the most s t r i k ing d i f f e r e n c e i s the low f r a c t i o n a l l o a d i n g (about 0.1% of i o n exchange c a p a c i t y ) at which v a r i a t i o n s i n D, r e f l e c t e d as changes i n K/T ± y, become apparent, f o r both sodium and cesium. Wahlberg and Fishman (3) a l s o r e p o r t strong decreases i n DQ s t a r t i n g at low l o a d i n g from s o l u t i o n s up to 0.1 M C a C l 2 . The values of t h e i r D's at t r a c e loadings imply much higher values of e q u i l i b r i u m quotients than those at lowest l o a d i n g i n F i g u r e 6, two orders of magnitude f o r one c l a y sample they used and a f a c t o r of about 600 f o r the other. T h e i r K/T values imply that at the same Ca(II) c o n c e n t r a t i o n , t h e i r DQ would be a f a c t o r of 10 to 25 lower than ours. Van B l a d e l , et a l . , (9), report e q u i l i b r i u m quotients f o r Na(I)/Ca(II) exchange from aqueous N a C l - C a C l 2 s o l u t i o n s of t o t a l n o r m a l i t y up to 0.025. I t i s not c l e a r from t h e i r g r a p h i c a l pre s e n t a t i o n whether t h e i r measurements i n the low-sodium-fraction r e g i o n i n c l u d e p o i n t s i n the range of r a p i d l y i n c r e a s i n g % (de c r e a s i n g K/T i y), although at l e a s t at t h e i r highest Ca(II) con c e n t r a t i o n , t h e i r values of the e q u i l i b r i u m quotient appear to l e v e l o f f , at values corresponding to those i n F i g u r e 6 at l o a d ings of N a of about 1 to 10% of ion-exchange c a p a c i t y . In t h i s regard, we wish to comment on some p r e l i m i n a r y r e s u l t s reviewed at a recent meeting (18). At that time, we had not com p l e t e d the l o a d i n g s t u d i e s of D and of K/T. The values f o r the calcium form i n Figures 15 and 16 of Ref. are i n the l o a d i n g r e g i o n i n which they are r a p i d l y changing. This accounts f o r d i f ferences with the r e s u l t s i n Figure 6 of t h i s paper, and probably a n c

c

a

s

a

S9

S

a

c

a

+



17.

SHiAO ET AL.

100 b cr

~

Ion-Exchange

I

I

l

305

Equilibria

jMllj

1

I I

llllll

1

I I

I

lltt

10

ο υ ζ ο t3 CD

ο

0.1 10"

ι ι Ιιιιιΐ

I ι ι Ιιιιιΐ

10 -2 10" LOADING,mole /kg

1 ιι Im 10"

Figure 3. Effect of loading on distribution coefficients of Ca(ll), Sr(II), and Ba(H) on the sodium form of Wyoming montmorillonite (0.5M NaCl O.J M NaOAc, pH 5, equilibration for more than 90 hr): (·), Ca(II); (Αλ Ba(II); β λ Sr(II).



306

RADIOACTIVE

ι Γ Α Ι 111 mil TRACE

1 0

~

I ι ilinil 5

I ι ilinil

WASTE

IN

I i ilinil

GEOLOGIC

I I 11iml I I

*0~ 10~ 10~ LOADING, mole Eu (IH)/kg 4

3

STORAGE

2

10"

1

Figure 4. Effect of loading on distribution coefficients of Eu(HI) on the sodium form of Wyoming montmorillonite (pH 5, equilibration for 44 hr): f | ) , O.J M NaCl; (Φ), O.J M NaCl + 0.0JM NaOAc.



17.

SHiAO ET AL.

Ion-Exchange

307

Equilibria

Ξ 1 ll|lilll 1 m u m

l|llll|

11

1

1 M |HN|

M|l

1

/

7

J

/ ~ l 10"

Mill

ll ^ΗΊΤΙΙΙΙ -5

10

I lllllll 1

iliiiil

1

10" mole

ΙΟ" LOADING, 4

—£

11

1 11 Hull

i l l

10-1

10" Cs (I)/kg

Figure 5. Effect of loading on equilibrium quotients lm Vaq/(Ca *) i yD c ] of Wyoming montmorillonite predominately in the calcium form (pH 5, equilibra tion for 77 hr): (φ), 0.5M CaCl + 0.01M Ca(OAc) ; « λ 0.05M CaCl + 0.01M Ca(OAc) . 2

Ca

2

2

2

c a

8

2

2


10

u

RADIOACTIVE W A S T E IN

308

GEOLOGIC STORAGE

as w e l l f o r the d i s c r e p a n c i e s noted i n Ref. 18^ from those i n Ref. 9. S r ( I I ) : The e f f e c t of S r ( I I ) l o a d i n g on ϋ$ on the Ca(II) form of montmorillonite i s shown i n Figure 7. Up to 10" moles S r ( I I ) / k g c l a y , values of Ός are e s s e n t i a l l y constant, and do not vary g r e a t l y up to 10~ moles S r ( I I ) / k g c l a y . As we have j u s t d i s cussed, e f f e c t s were apparent at a f a c t o r of ten lower l o a d i n g f o r Cs+ and Na . Wahlberg, et a l . , (7) report very s i m i l a r l o a d i n g e f f e c t s on a calcium montmorillonite. The value of P g they report f o r t h e i r c l a y at t r a c e S r ( I I ) , from 0.01 M C a ( I I ) , i s about 40% higher than the value i n F i g u r e 7. τ

3

τ

2


+

r

D at Low Ion

Loading as a Function of Concentration of E q u i l i b r a t i n g

Sodium Form of M o n t m o r i l l o n i t e : A l k a l i metal i o n s : Distribution c o e f f i c i e n t s of C s as a f u n c t i o n of sodium concentration are summarized i n Figure 8 f o r c l a y s from four d i f f e r e n t sources. As we discussed i n connection with F i g u r e 1, there i s a m i l d de pendence on cesium loading f o r t h i s exchange i n the range of loading of these experiments, 0.02 to 5% of ion-exchange c a p a c i t y . The slopes of l o g DQ VS l o g (Na ) from 0.5 to 4 molar sodium are however c l o s e to the i d e a l value, -1 (Equation 6), f o r i n d i v i d u a l c l a y samples. As l o a d i n g i s higher at low (Na ) and DQ therefore r e l a t i v e l y lower, the f a c t that the slopes appear s l i g h t l y l e s s than u n i t y i s i n the r i g h t d i r e c t i o n f o r a l o a d i n g e f f e c t . These c l a y s had been subjected to a l l steps i n the Jackson p u r i f i c a t i o n procedure except removal of f r e e i r o n oxide. With one sample, measurements were c a r r i e d out at pH 5 (0.1 M sodium acetate p l u s a c e t i c a c i d b u f f e r ) as w e l l as 8 (0.1 M NaHC03 b u f f e r ) , and there appeared to be l i t t l e e f f e c t on D. Values of D at a f i x e d sodium concentration v a r i e d by about a f a c t o r of three between the montm o r i l l o n i t e s of highest DQ ( c l a y #31) and of the lowest (Wyoming and #27). E x t r a p o l a t i o n of the r e s u l t s i n Figure 8 to 0.2 molar Na , the highest concentration of measurements i n Ref. 3^, gives a range of DQ of about 60 to 180. Wahlberg and Fishman r e s u l t s f o r t r a c e Cs at t h i s sodium concentration were about 250 f o r one c l a y and 550 f o r the other. Some of the d i f f e r e n c e may a r i s e from the higher loading i n our experiments, but i n any case t h i s much v a r i a t i o n between c l a y samples i s probably not s u r p r i s i n g . T h e i r slopes of l o g D vs l o g sodium concentration at t r a c e C s were some what l e s s (absolute value) than minus one. Tamura and Jacobs (19) report adsorption on a montmorillonite sample from 6 M NaN03, from which we compute DQ - 5, i n the range of extrapolated values from Figure 8. Lewis and Thomas (4) report K/T of about 40 f o r low cesium loading and 0.04 M s a l t concentra t i o n , and Gast (10) about 35 f o r 0.001 M. These values are i n the range implied by r e s u l t s i n Figure 8 (K/T 15 to 50). +

+

S

+

S

S

+

S

+

S


17.

SHiAO ET AL.


10

2Γ

Equilibria

309

Ξ ι ιιμιιιι ι ιιμιιιι | ιΐ|ΐιιΐ| | ιιμιιιι | ι 2

tz.

ο

·

HT

•Λ

k/Γ IC/rclay • • 0.01 M Ca(0Ac)

1

oc ο t-, 0.1

Ion-Exchange

0.05 M CaCIg + O.OI Μ Co (0Ac)

2

b=

0.01 10"

10i - 6

10" 10" LOADING, mole

10"

10"

Να (I)/kg

10"

Figure 6. Effect of loading on equilibrium quotients [mcaV q/(Ca *) i yO ] of Wyoming montmorillonite predominately in the calcium form (pH 5, equilibra tion for 14 hr) 2

a

LOADING, mole

2

c

a

Na

Sr(II)/kg

Figure 7. Effect of loading on distribution coefficients of Sr(II) on the calcium form of Wyoming montmorillonite (0.01M Ca(OAc) , pH 5, equilibration for 65 hr) 2


RADIOACTIVE W A S T E IN GEOLOGIC


310

_

0.3

Ο Να

BICARBONATE

8

Δ Co

BICARBONATE

8

Ο NO. 27

BICARBONATE

8

0 NO. 34

BICARBONATE

8

0.5

1.0

2.0

STORAGE

4.0

Ν α (I) (moles/1)

Distribution of Cs (I) Between Aqueous NaCi Solutions and Montmorillonite from Several S o u r c e s Γ~10~ M C s ( I ) ] 3

Figure 8. Adsorption of Cs(I) on the sodium form of montmorillonites from sev eral sources (Loading: 2 X 10~ — 4 X 10 mol Cs(I)/kg, equilibration for 24 hr.). 4

2


17.

SHiAO ET AL.

Ion-Exchange

311

Equilibria

D i s t r i b u t i o n c o e f f i c i e n t s f o r potassium on one c l a y are sum marized i n Figure 9. The i d e a l slope of -1 f o r l o g D^vs l o g sodium concentration was c l o s e l y approximated. Values of Ό were about 1/3 those f o r cesium on the same c l a y . A value o f K/T [(K+) lay(Na+)/(K+) (Na+) c l a y ! °f about 3.75 can be estimated from Figure 9. T h i s agrees s a t i s f a c t o r i l y with values o f about 2.5 r e ported both by Gast (10) f o r 0.001 M Na+ and by Shainberg and Kemper (20) f o r 0.05 M Na+. A l k a l i n e earth i o n s : The d i s t r i b u t i o n c o e f f i c i e n t s of C a ( I I ) , S r ( I I ) , and Ba(II) on the Na form of Wyoming montmorillonite are shown i n Figures 10a, b, and c. Values o f #ca> ^Sr> * % a s i m i l a r f o r equal sodium concentration. The slope of l o g D vs log sodium concentration i s c l o s e t o -2, the slope f o r i d e a l divalent-monovalent i o n exchange. Measurements were c a r r i e d out i n the presence of sodium acetate b u f f e r o f 0.1 M and 0.01 Af, and the r e s u l t s were not s i g n i f i c a n t l y d i f f e r e n t at the same sodium concentration. T h i s i n d i c a t e s that there i s no i n t e r f e r e n c e from formation of complexes between the a l k a l i n e earths and acetate ions. One sample of Wyoming montmorillonite was p u r i f i e d only by passage through a Dowex 50 ion-exchange column i n the sodium form. The # c i s sample agreed w i t h i n s c a t t e r with the same c l a y which had been subjected to the complete Jackson procedure. From the r e s u l t s i n Figures 10a, b, c, the computed values o f K/T [ ^ ) c l a y ( ) / ( X N a * " ^ ] were approximately 3. A value of 1.4 i s reported f o r Ca(II) i n 0.025 M NaCl (9) and of about 1 f o r Ba(II) i n 0.04 M NaCl (4). Tamura (5) reported l i n e a r de pendence o f l o g D vs l o g sodium concentration from 0.01-0.6 molar sodium with a slope o f -2; h i s values of Z)g were about h a l f of those of Figure 10b a t the same sodium concentrations. From r e s u l t s reported by S p i t s y n and Gromov (1), we c a l c u l a t e a D$ of about 70 a t 0.1 M NaCl, t h e i r highest concentration, i n comparison with about 100 i n Figure 10b. E u ( I I I ) : Figure 11 summarizes the adsorption of Eu(III) on the Na form of Wyoming montmorillonite a t pH 5, c o n t r o l l e d with 0.01 M acetate b u f f e r , and adjusted to the same a c i d i t y without b u f f e r by HC1. The values o f Z? i n the presence of acetate are about a t h i r d of those without, a d i f f e r e n c e s i m i l a r to that seen i n the l o a d i n g curve, Figure 4. Formation of Eu(III) acetate com plexes, presumably the source of the d i f f e r e n c e s , has been r e ported elsewhere (21). For the unbuffered system, the p l o t of l o g D

Ion-Exchange Equilibria between Montmorillonite and Solutions of

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