Geochemical Behavior of Disposed Radioactive Waste - American

4 Actinide and Technetium Sorption on Iron-Silicate and Dispersed Clay Colloids J. W. SHADE, L. L. AMES, and J. E. MCGARRAH

Downloaded by UCSF LIB CKM RSCS MGMT on October 6, 2014 | http://pubs.acs.org Publication Date: March 8, 1984 | doi: 10.1021/bk-1984-0246.ch004

Pacific Northwest Laboratory, Richland, WA 99352

Two different colloidal suspensions, representative of those found i n waste package interaction tests, were prepared from iron metal and s i l i c a powders or sodium-bentonite at 90°C. Aliquots were spiked with U, Np, Pu, or Tc at pH ranges from 2 to 12, then shaken for 24 hours followed by a 1 5 Å f i l t r a t i o n . Zeta potential measurements were made on unspiked samples. Similar sorptive properties were observed for both colloids. At 25°C both U and Pu exhibit maximum sorption (50-90%) near pH 6. Sorption drops by about a factor of 5 at pH >8. Slight sorption of Np occurs at pH 11 and decreases to zero at lower pH values. Tc does not sorb on Fe-silicates and is only slightly sorbed (10%) on smectites. 233

235

237

95m

233

237

235

95m

As part o f an e f f o r t to evaluate the e f f e c t s o f waste package components ( c a n i s t e r , waste form, b a c k f i l l ) on waste form l e a c h i n g behavior, i n t e r a c t i v e t e s t s have been conducted that i n c l u d e m o n o l i t h i c specimens o f i r o n i n the same container with m o n o l i t h i c glass specimens. These experiments i n d i c a t e that the leach rates of elements i n t o groundwater from the glass are enhanced r e l a t i v e to rates observed i n comparable g l a s s - o n l y t e s t s ÇL) . The enhanced leach rates are suggested to be the r e s u l t o f decreased s o l u b l e s i l i c a a c t i v i t y caused by r e a c t i o n s between i r o n and s i l i c a to form hydrated i r o n s i l i c a t e r e a c t i o n products. Similar experiments (2-3) conducted with sodium-bentonite b a c k f i l l m a t e r i a l s i n place of i r o n a l s o y i e l d enhanced l e a c h rates from g l a s s . This might be a t t r i b u t a b l e to the a l k a l i s o r p t i o n p r o p e r t i e s of the smectite component o f bentonite o r , because glass leach rates appear to be r e l a t e d to d i s s o l v e d s i l i c a concentrations (4), p o s s i b l e s i l i c a - b e n t o n i t e p o l y m e r i z a t i o n o r condensation r e a c t i o n s s i m i l a r to that suggested f o r s i l i c i c

0097-6156/ 84/0246-0067S06.00/0 © 1984 American Chemical Society

In Geochemical Behavior of Disposed Radioactive Waste; Barney, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.


68

GEOCHEMICAL BEHAVIOR OF RADIOACTIVE WASTE

a c i d (5) may be an i n f l u e n c e . Although the i n t e r a c t i o n mechanisms are not understood, ICP a n a l y s i s of s e q u e n t i a l l y f i l t e r e d leachates (0.45 pm to 15Â) and m i c r o - e l e c t r o p h o r e t i c examination during zeta p o t e n t i a l measurements have i n d i c a t e d the existance of s i l i c a - r i c h p a r t i c l e s of c o l l o i d a l s i z e i n both i r o n and bentonite systems (JL) , but no a d d i t i o n a l c h a r a c t e r i z a t i o n was attempted. I f c o l l o i d s generated by waste package component i n t e r a c t i o n s r e a d i l y f l o c c u l a t e or are otherwise removed from s o l u t i o n soon a f t e r formation, they may not represent a waste management problem because c o l l o i d a l transport of r a d i o n u c l i d e s would be l i m i t e d . In the previous i n t e r a c t i o n experiments (1-3), no evidence f o r f l o c c u l a t i o n or p r e c i p i t a t i o n was reported over the pH range 6 to 9.5, which implies that the c o l l o i d s remain as s o l s and could p o t e n t i a l l y be transported. I f transport i s p o s s i b l e , then i t i s d e s i r a b l e to know the extent to which sorbed r a d i o n u c l i d e s could a l s o be transported. Such a mechanism would require m o d i f i c a t i o n s to some concepts of s o l u b i l i t y c o n t r o l l e d r e l e a s e . This report concerns a p r e l i m i n a r y e f f o r t to determine some of the s o r p t i o n p r o p e r t i e s of c o l l o i d a l species r e p r e s e n t a t i v e of those formed during waste form/waste package i n t e r a c t i o n t e s t s . Sorption of a c t i n i d e s and Tc on those c o l l o i d s as a f u n c t i o n of pH at 25°C was s t u d i e d . The g l a s s - i r o n and g l a s s - b e n t o n i t e i n t e r a c t i o n experiments (1-2) that r e s u l t e d i n c o l l o i d formation were conducted at 90°C i n deionized water and i n low i o n i c strength groundwater w i t h f i n a l pH values g e n e r a l l y i n the range 8 to 9.5. On the b a s i s of ICP analyses of f i l t r a t e s (1), i t i s thought that c o l l o i d s i n the g l a s s - i r o n system are s i l i c a - r i c h , i r o n - b e a r i n g m a t e r i a l s ( p o s s i b l y s i m i l a r to those described i n Reference 6) rather than an i r o n hydroxide such as g e o t h i t e . In the absence of f u r t h e r c h a r a c t e r i z a t i o n , these c o l l o i d s are simply considered as i r o n silicates. Thus, c o l l o i d s generated with powdered i r o n and s i l i c a under c o n d i t i o n s s i m i l a r to the experiments i n Reference 1, but at a s l i g h t l y higher pH, are thought to be r e p r e s e n t a t i v e of i r o n s i l i c a t e c o l l o i d s from waste glass i n t e r a c t i o n systems but exclude p o s s i b l e complications from leachable waste g l a s s elements. Moreover, c o l l o i d s from the simple i r o n - s i l i c a systems provide a b a s i s f o r comparison with g l a s s - b e a r i n g systems i n future work. Experimental Procedure Iron s i l i c a t e c o l l o i d s were prepared by p l a c i n g f i f t y grams of i r o n powder and s i l i c i c a c i d or Na s i l i c a t e i n a l i t e r of 0.01 M NaOH s o l u t i o n , then h e a t i n g the s o l u t i o n at 90°C f o r four to f i v e days. A f t e r t h i s d i g e s t i o n p e r i o d , the s o l u t i o n was c e n t r i f u g e d at 3500 rpm f o r one hour and the supernate decanted. A l i q u o t s of the supernate were examined f o r the presence of c o l l o i d s using a m i c r o - e l e c t r o p h o r e s i s u n i t , and the amount of c o l l o i d s i n suspension was estimated by weight l o s s a f t e r evaporation. This same procedure was used with Na-bentonite to generate c o l l o i d a l


4.

s i z e p a r t i c l e s except that deionized water was used i n s t e a d of a hydroxide s o l u t i o n . The Na b e n t o n i t e was the same m a t e r i a l used i n Reference 7. Separate 20 ml a l i q u o t s of supernate were then adjusted to a d e s i r e d pH w i t h i n the range 2 to 12 with HC1 or NaOH and spiked with Pu, U, Np, Tc, C s , o r S r . Only one isotope was used f o r a given a l i q u o t , and the amount of spike was on the order of 1 0 " or 1 0 " M f o r the a c t i n i d e s and 10" M f o r u r a n i u m — w e l l below expected s o l u b i l i t y l i m i t s . The spiked samples were shaken f o r 24 hours at 25°C, then f i l t e r e d through 15* f i l t e r s . Counting the i n i t i a l s o l u t i o n and the 15& f i l t r a t e allowed a determination of the percentage of n u c l i d e sorbed on the c o l l o i d . C o r r e c t i o n s were made f o r s o r p t i o n on container walls. In a d d i t i o n , the s u r f a c e charge (or zeta p o t e n t i a l ) of suspended c o l l o i d s was measured on separate pH-adjusted a l i q u o t s using a l a s e r m i c r o - e l e c t r o p h o r e t i c instrument. These measurements were made between the pH range of 2 to 12. 2 3 7

2 3 3

2 3 5

1 2


69

Actiniae and Technetium Sorption

S H A D E ET AL.

9 5 m

1 3 7

8 5

1 3

7

Results and D i s c u s s i o n The percentages of r a d i o n u c l i d e s sorbed by i r o n s i l i c a t e and sodium b e n t o n i t e are l i s t e d i n Tables I and I I along w i t h i n i t i a l and f i n a l pH v a l u e s . The percentages of U and P u sorbed on i r o n s i l i c a t e and b e n t o n i t e c o l l o i d s are shown g r a p h i c a l l y i n Figure 1. I t i s apparent that s i m i l a r s o r p t i o n behavior occurs for both c o l l o i d s w i t h these i s o t o p e s . Both e x h i b i t maximum s o r p t i o n near pH 6 with l e s s s o r p t i o n at high pH and, to some extent, a l s o at low pH. P u seems to sorb more than 233„ while U e x h i b i t s a somewhat greater pH dependence—at l e a s t under c o n d i t i o n s of r e l a t i v e l y d i l u t e s o l u t i o n s . In an attempt to o f f e r a p a r t i a l e x p l a n a t i o n f o r these observations, the zeta p o t e n t i a l of the c o l l o i d s measured as a f u n c t i o n of pH i s shown i n comparison w i t h p o s s i b l e s p e c i a t i o n of U and Pu (Figure 2 ) . The s o r p t i o n experiments were conducted under o x i d i z i n g c o n d i t i o n s , so only the +200 to +400 mV range of Eh i s considered. The s p e c i a t i o n of U and Pu shown was taken from References 8 and 9, r e s p e c t i v e l y . I t i s recognized that dominance f i e l d s f o r s p e c i f i c species vary as functions of parameters such as i o n i c s t r e n g t h , t o t a l d i s s o l v e d carbonate, e t c . , and a l s o that a c t i n i d e s themselves may form polymers (10-11). For purposes of t h i s d i s c u s s i o n , however, the i n t e n t i s to i l l u s t r a t e that d i s s o l v e d species of U and Pu tend to be n e g a t i v e l y charged a t high pH values and p o s i t i v e l y charged at low pH. When viewed i n these terms, simple e l e c t r o s t a t i c c o n s i d e r a t i o n s suggest that low s o r p t i o n would be expected at high pH because both c o l l o i d and dominant a c t i n i d e species are n e g a t i v e l y charged. I f , at i n t e r mediate pH, the dominant a c t i n i d e species change from negative to p o s i t i v e while the c o l l o i d z e t a p o t e n t i a l i s s t i l l negative ( i . e . , before the i s o e l e c t r i c p o i n t ) , then maximum s o r p t i o n would be 2 3 3

2 3 7

2 3 7

2 3 3


70

GEOCHEMICAL BEHAVIOR OF RADIOACTIVE WASTE TABLE I .

Sorption of Radionuclides

by Iron S i l i c a t e


S o l i d s - t o - s o l u t i o n r a t i o was 108 m g / £ . hours. A l l samples were 15A f i l t e r e d .

Radionuclides

Initial Concentration moles/

233

4.591 χ 10

235

0 0 2.92 35.12 42.72 15.10 6.96 0,0 1 2

5.135 χ 1 θ " P

237

1.230 χ 1 0 "

24-hour Sorption,%

1 3

U

5.82 0 0 0

at 25°C.

Contact time was 24

I n i t i a l pH

F i n a l pH

11.10 10.00 8.00 5.99 5.00 4.00 2.99 2.00

11.20 10.10 8.05 6.09 5.30 4.00 3.00 2.0

11.20 10.00 8.00 6.00

11.25 10.02 8.01 6.10

44.37 11.10 15.12 10.00 23.27 8.00 55.40 6.01 61.19 5.00 57.73 4.05 56.75 50.15 2.98 2.00

11.25 10.10 8.00 6.20 5.40 4.10 2.90 2.60

95m

4.187 χ 1 θ "

1 4

0 0 0 0

11.03 10.00 7.99 6.00

11.15 10.00 7.99 6.15

137

2.593 χ Ι Ο "

1 1

19.77 10.74 11.09 8.19 0

10.00 8.00 6.00 4.00 2.00

9.92 8.18 7.45 3.97 1.99

85

2.370 χ 1 0 "

64.52 48.08 44.72 20.74 7.68

10.00 8.00 6.00 4,00 2.00

10.28 8.24 7.09 3.82 1.90

1 3


4. SHADE ET AL.

Actinide

and

Technetium

Sorption

71

TABLE I I . Sorption of Radionuclides by Na Bentonite at 25°C


S o l i d s - t o - s o l u t i o n r a t i o was 68.2 mg/£. Contact time was 24 hours. A l l sample s were 15A f i l t e r e d .

Radionuclides

Initial Concentration moles/it

233u

4.591 χ 10"

235

N p

237

24-hour Sorption, %

I n i t i a l pH

F i n a l pH

7

21.33 14.95 41.18 71.55 86.94 90.21 76.51

11.20 10.00 8.00 6.00 5.02 4.00 4.00

11.00 9.90 8.00 6.90 6.26 5.66 4.05

5.135 χ 1 0 "

1 2

14.23 4.02 6.94 8.58 3.65 7.43

11.20 10.00 8.05 6.05 5.00 3.95

11.15 9.80 8.05 7.50 6.04 5.37

P u

1.230 χ 1 0 "

1 3

56.79 58.06 82.71 80.62 75.30 77.94

11.20 10.00 8.00 6.00 5.00 4.02

11.00 9.80 8.10 6.85 6.06 5.65

95m

Tc

4.187 χ 1 0 "

1 4

0.89 1.50 3.89 13.40 9.51 9.41

11.20 10.00 8.00 6.00 5.00 4.02

10.10 9.85 8.00 7.00 6.85 5.68

137

C s

2.593 χ 1 0 "

77.05 73.57 68.71 62.69 18.57

10.00 8.05 6.00 4.00 2.00

9.90 8.09 6.69 5.53 2.09

98.04 96.35 95.02 94.43 35.48

10.00 8.00 6.00 4.00 2.00

9.83 7.99 6.48 4.42 1.94

1 1

2.370 χ 10-13





Actinide and Technetium Sorption


SHADE ET AL.


74

GEOCHEMICAL BEHAVIOR O F RADIOACTIVE WASTE

expected. At even lower pH values, both c o l l o i d and a c t i n i d e are l i k e l y to be p o s i t i v e l y charged so that s o r p t i o n i s reduced. A more 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 c o l l o i d - a c t i n i d e systems would permit a q u a n t i t a t i v e i n t e r p r e t a t i o n , but t h i s was not w i t h i n the scope of t h i s e f f o r t . The same type o f experiments were conducted using Tc and N p s p i k e s , and these r e s u l t s are shown i n Figure 3. Very l i t t l e s o r p t i o n was observed on e i t h e r c o l l o i d . The l a r g e r amounts of N p s o r p t i o n a t high pH f o r both c o l l o i d s suggest the presence of a p o s i t i v e l y charged s p e c i e s , but i t apparently i s not present i n l a r g e amounts. The dominance o f the pertechnetate i o n at o x i d i z i n g c o n d i t i o n s r e a d i l y accounts f o r the low s o r p t i o n on n e g a t i v e l y charged c o l l o i d s . The s l i g h t amount o f T c and Np s o r p t i o n on bentonite might be a t t r i b u t e d to l o c a l i z e d s o r p t i o n at s i t e s o f ferrous i r o n i n bentonite or to r e d u c t i o n by small amounts o f o r g a n i c s . The i r o n a s s o c i a t e d with i r o n s i l i c a t e c o l l o i d s i s l i k e l y f e r r i c because they were formed under oxidizing conditions. The r e s u l t s o f a d d i t i o n a l experiments conducted with Sr and C s spikes are shown i n Figure 4. The w e l l known s o r p t i o n c h a r a c t e r i s t i c s o f bentonite f o r Sr and Cs ions i s apparent ( 7 ) . The s o r p t i o n p r o p e r t i e s o f bentonite are reduced a t low pH, which i s c o n s i s t e n t w i t h an e l e c t r o s t a t i c concept. C s sorption on the i r o n s i l i c a t e c o l l o i d s i s considerably l e s s than that observed with b e n t o n i t e , even though the c o l l o i d zeta p o t e n t i a l s are s i m i l a r , which suggests that mechanisms other than simple e l e c t r o s t a t i c concepts may be i n v o l v e d . A l s o , the l i n e a r trend of data f o r Sr i n i r o n s i l i c a t e systems i s considered to represent p r e c i p i t a t i o n r a t h e r than s o r p t i o n . 9 5 m

2 3 5


2 3 5

9 5 m

2 3 5

8 5

1 3 7

1 3 7

Conclusions This p r e l i m i n a r y work suggests that s o r p t i o n p r o p e r t i e s of i r o n s i l i c a t e c o l l o i d a l s i z e p a r t i c l e s generated i n waste package i n t e r a c t i o n experiments are s i m i l a r to those o f bentonite f o r U and Pu as w e l l as f o r Tc and Np. Although no s p e c i f i c e f f o r t was made i n t h i s i n v e s t i g a t i o n to determine c o l l o i d s t a b i l i t y , f l o c c u l a t i o n was not observed i n the short 24-hour experiments w i t h i n the pH range 2 to 12 f o r low i o n i c strength s o l u t i o n s . A s i g n i f i c a n t amount o f s o r p t i o n was observed which j u s t i f i e s a more d e t a i l e d c h a r a c t e r i z a t i o n e f f o r t to evaluate c o l l o i d growth r a t e s , composition and f l o c c u l a t i o n c h a r a c t e r i s t i c s over a range of r e p o s i t o r y - r e l e v a n t temperatures and r a d i a t i o n f i e l d s . This type of e f f o r t w i l l be r e q u i r e d before a f u l l e v a l u a t i o n of the s i g n i f i c a n c e of c o l l o i d s i n waste package systems can be made.




S H A D E ET AL.




1 3 7

8 5

Figure 4. S o r p t i o n of C s and S r by C o l l o i d a l Iron S i l i c a t e s and Na Bentonite at 25°C.


4. SHADE ET AL.


77

Literature Cited 1. 2.

3.


4.

5. 6. 7.

8. 9. 10. 11.

McVay, G. L.; Buckwalter, C. Q. J . Am. Ceramic Soc. 1983, 66:3, 170-4. Shade, J . W.; Chick, L. A. Abs. f o r American Ceramic Society Meeting: C i n c i n n a t i , Ohio, 1982; PNL-SA-10012, P a c i f i c Northwest Laboratory, Richland, Washington Lanza, F.; Ronsecco, C. i n " S c i e n t i f i c Basis f o r Nuclear Waste Management"; Lutze, W., Ed.; E l s e v i e r P u b l i s h i n g Company: New York, 1982; V o l . 5, pp. 125-133. Pederson, L. R.; Buckwalter, C. Q.; McVay, G. L.; Riddle, B. L. i n " S c i e n t i f i c Basis f o r Nuclear Waste Management"; Brookins, D. G., Ed.; E l s e v i e r P u b l i s h i n g Company: New York, 1983; V o l . VI, pp. 47-54. Stober, W. Adv. i n Chem. S e r i e s 1967, 67, 161-182. Yokoyama, T.; Nakazato, T.; T a r u t a n i , T. B u l l . Chem. Soc. Japan 1980, 53, 850-8. Hodges, F. N.; Westsik, J . H.; Bray, L. A. i n " S c i e n t i f i c Basis f o r Nuclear Waste Management"; Lutze, W., Ed.; E l s e v i e r P u b l i s h i n g Company: New York, 1982; V o l . 5, pp. 641-8. Langmuir, D. Geochim et Cosmechim Acta. 1978, 42, 547. R a i , D.; Seme, R. J . ; Swanson, J . L. J . E n v i r o n . Q u a l i t y 1980, 9, 417-20. R a i , D.; Swanson, J . L. Nucl. Tech. 1981, 54, 107. Olofsson, U.; A l l a r d , B.; Anderson, K.; T o r s t e n f e l t , B. i n "The S c i e n t i f i c Basis f o r Nuclear Waste Management"; Topp, S. V., Ed.; E l s e v i e r S c i e n t i f i c P u b l i s h i n g Company: New York, 1982; V o l . 6, p. 191.

RECEIVED December 6, 1983


Geochemical Behavior of Disposed Radioactive Waste - American

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