Chapter 13
Actinide Removal from Aqueous Waste Using Solid Supported Liquid Membranes Anthony C. Muscatello, James D. Navratil , and Marlene Y. Price 1
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Rockwell International, Rocky Flats Plant, Golden, CO 80402-0464
The actinides americium and plutonium can be removed and recovered from nitrate-nitric acid waste solutions using solid supported liquid membranes. The bifunctional organophosphorus extractants dihexyl-N,N-diethylcarbamoylmethylphosphonate (DHDECMP) and octylphenyl-N,N-diisobutylcarbamoylmethylphosphine oxide (0ØD(iB)CMPO) effectively remove actinides from low acid-high nitrate conditions when supported on Accurel polypropylene hollow fibers. Studies of the sorption of americium on inorganic ion exchangers from 0.25M oxalic acid and water are included to demonstrate the feasibility of actinide concentration from the strip solution after membrane transfer.
Our r e s e a r c h program a t Rocky F l a t s has been aimed a t r e d u c i n g the l e v e l s of americium and p l u t o n i u m i n the n i t r i c a c i d waste w h i c h a r i s e s from the a n i o n exchange p u r i f i c a t i o n of p l u t o n i u m . These wastes c u r r e n t l y c o n t a i n TRU o r t r a n s u r a n i c elements t o such a c o n c e n t r a t i o n t h a t the waste must be s t o r e d i n a g e o l o g i c a l r e p o s i t o r y a f t e r n e u t r a l i z a t i o n and f l o c c u l a n t p r e c i p i t a t i o n . T a b l e I shows the average c o m p o s i t i o n of the w^ste. _7 Reducing the p l u t o n i u m l e v e l t o 10 g/1 and americium t o 10 g/1 would a l l o w p r o d u c t i o n of a s l u d g e w h i c h can be s t o r e d as l o w - l e v e l waste. I n a d d i t i o n , economic c o n s i d e r a t i o n s and r a d i a t i o n exposure l i m i t s a^so f a v o r r e d u c t i o n of p l u t o n i u m t o 10 g/1 and americium t o 17 hours o f c o n t i n u o u s o b s e r v a t i o n . The d e t e c t o r was c a l i b r a t e d by t h e use o f s o l u t i o n s c o n t a i n i n g known amounts o f americium.
In Liquid Membranes; Noble, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
LIQUID MEMBRANES: THEORY AND APPLICATIONS
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The feed and s t r i p s o l u t i o n s were c i r c u l a t e d t h r o u g h t h e module u s i n g Cole-Parmer M a s t e r - F l e x t u b i n g pumps. Unless o t h e r w i s e n o t e d , Table I I .
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Exchanger Z i r c o n i u m phosphate Zirconium tungstate Sodium t i t a n a t e
I n o r g a n i c I o n Exchangers
Grade "for research technical
only"
-
Ferrite Peatmoss
magnetite acid treated
Alumina Bone char
150 mesh a c t i v a t e d c a l c i u m phosphate
Supplier P f a l t z & Bauer Alfa R. G. Dosch, Sandia N a t ' l Lab. Pfizer (from Glenamoy, Co· Mayo, Ireland) Aldrich Stauffer C h e m i c a l Co.
f l o w r a t e s were m a i n t a i n e d a t 10 ml/min. The i n i t i a l and f i n a l americium c o n c e n t r a t i o n s were a l s o determined radiometrically· U n c e r t a i n t i e s i n the d a t a a r e e s t i m a t e d t o be 5-10%. D i s t r i b u t i o n r a t i o experiments w i t h i n o r g a n i c c a t i o n exchangers were performed by e q u i l i b r a t i n g 1.0 g o f the exchanger w i t h 10 ml o f e i t h e r 0.25M o x a l i c a c i d o r water i n a 14 ml v i a l on a r o t a t i n g wheel f o r up t o 72 h o u r s . The aqueous pH was a d j u s t e d w i t h 50% NaOH o r 7M HNO^. The i n i t i a l and f i n a l americium and p l u t o n i u m concen t r a t i o n s were determined r a d i o m e t r i c a l l y . The volume d i s t r i b u t i o n r a t i o , D, i s d e f i n e d as [Am] ( g ^ g f l j d * [Am]/mj^ajueous P^fgli*™* * c a l c u l a t e d from 10 χ ([Am]aq - [Am]aq /[Am]aq ). s
P e r m e a b i l i t y C a l c u l a t i o n s . D a n e s i e t a l . (5) d e r i v e d t h e f o l l o w i n g e q u a t i o n t o c a l c u l a t e the p e r m e a b i l i t y (P ) o f t h e m e t a l i o n through the membrane: Ln [M]_ = P_At
(1)
where:
[M]
q
= Feed c o n c e n t r a t i o n a t time z e r o ,
[M]
T
= Feed c o n c e n t r a t i o n a t time t ,
A
= S u r f a c e a r e a o f the membrane,
t
= Time, and
V
= Volume o f c e l l .
A p l o t of t h e l e f t hand s i d e o f E q u a t i o n s t r a i g h t l i n e whose s l o p e i s Ρ A/V.
1 versus
time g i v e s a
In Liquid Membranes; Noble, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
13.
MUSCATELLO ET AL.
Actiniae Removal from Aqueous Waste
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R e s u l t s and D i s c u s s i o n E f f e c t o f N i t r i c A c i d . P r e l i m i n a r y experiments showed i n c o m p l e t e t r a n s f e r o f americium from 7.0M n i t r i c a c i d t h r o u g h a membrane o f u n d i l u t e d DHDECMP t o 0.25M o x a l i c a c i d . D a n e s i e t a l . ( 2 ) have shown t h a t such membranes a l s o t r a n s p o r t n i t r i c a c i d . C o n s e q u e n t l y , the n i t r i c a c i d c o n c e n t r a t i o n o f the s t r i p s o l u t i o n i n c r e a s e s w i t h time and the d r i v i n g f o r c e o f the t r a n s f e r , the n i t r a t e c o n c e n t r a t i o n g r a d i e n t , i s n e u t r a l i z e d . When t h i s o c c u r s , e q u i l i b r i u m i s reached and no f u r t h e r net changes i n americium c o n c e n t r a t i o n a r e o b s e r v e d . See H o r w i t z et_ a l . ( 6 ) f o r the e q u a t i o n s d e s c r i b i n g t h e c h e m i s t r y o f the e x t r a c t i o n . To c o u n t e r a c t the e f f e c t , experiments were performed w i t h d e c r e a s i n g amounts o f n i t r i c a c i d mixed w i t h sodium n i t r a t e t o s i m u l a t e p a r t i a l n e u t r a l i z a t i o n o f the a c i d waste. T a b l e I I I summarizes the r e s u l t s o f t h i s s t u d y .
T a b l e I I I . T r a n s f e r o f A m e r i c i u m ( I I I ) Through a DHDECMP Membrane on A c c u r e l H o l l o w F i b e r s . ( I n i t i a l [Am]=l(f g/1) P m
-4 -1 [HNOJ ,M % Transfer 10 cm s e c [ N a N O j ,M •j _ 7.0 0.0 61 5.0 2.0 64 3.0 4.0 76 1.0 6.0 90 7.2 0.1 6.9 94 Source: Reproduced w i t h p e r m i s s i o n from Ref. 9. C o p y r i g h t 1987, M a r c e l Dekker. The feed volume was 50 ml and the s t r i p volume was 15 m l . The net membrane a r e a was 8.0 cm . Smooth curves were observed f o r p l o t s a c c o r d i n g t o E q u a t i o n 1 w i t h i n c r e a s i n g americium t r a n s f e r w i t h d e c r e a s i n g n i t r i c a c i d c o n c e n t r a t i o n and a c o n s t a n t t o t a l n i t r a t e c o n c e n t r a t i o n o f 7.0M. Only a t 0.1M n i t r i c a c i d i s t h e r e s u f f i c i e n t t r a n s f e r t o a l l o w the f i t t i n g o f a s t r a i g h t l i n e t o t h e d a t a . The c a l c u l a t e d p e r m e a b i l i t y i s l i s t e d i n T a b l e I I . T h i s v a l u e f o r americium, 7.2 χ 10 cm/sec, i s about t w i c e t h a t measured by D a n e s i e t a l . ( 2 ) from 1.0M n i t r i c a c i d , u s i n g a 430 m i c r o n t h i c k A c c u r e l h o l l o w - f i b e r loaded w i t h 0.25M O0D(iB)CMPO p l u s 0.75M TBP i n decalin. Thus, the best c o n d i t i o n o f those t e s t e d f o r americium t r a n s f e r i s 0.1M n i t r i c a c i d p l u s 6.9M sodium n i t r a t e . A c t u a l Rocky F l a t s waste streams may be brought t o these c o n d i t i o n s by t i t r a t i o n w i t h the a p p r o p r i a t e amount o f sodium h y d r o x i d e . A s i m i l a r study o f t h e t r a n s f e r o f p l u t o n i u m (IV) , u s i n g the same c o n d i t i o n s shows a s i m i l a r p e r m e a b i l i t y , but t h e f r a c t i o n t r a n s f e r e d i s lower ( 7 0 % ) . A p o s s i b l e e x p l a n a t i o n f o r lower t r a n s f e r o f p l u t o n i u m may be the t r a n s p o r t o f o x a l i c a c i d i n t o t h e feed s o l u t i o n from the s t r i p s o l u t i o n by DHDECMP. The o x a l i c a c i d then i n t h e feed would s t r o n g l y complex the p l u t o n i u m ( I V ) , p r e v e n t i n g i t s complete
In Liquid Membranes; Noble, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
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e x t r a c t i o n by t h e c a r r i e r , Americium ( I I I ) would n o t be n o t i c e a b l y a f f e c t e d by t h e o x a l i c a c i d . E f f e c t o f Feed t o S t r i p R a t i o . F u r t h e r experiments were performed to i n v e s t i g a t e t h e e f f e c t o f f e e d - t o - s t r i p (F:S) r a t i o on americium t r a n s f e r and t o e v a l u a t e t h e p o s s i b i l i t y of c o n c e n t r a t i n g t h e m e t a l i o n s s i m u l t a n e o u s l y w i t h r e m o v a l . A l i n e a r r e l a t i o n s h i p would be expected between t h e F:S r a t i o and t h e c o n c e n t r a t i o n f a c t o r , t h e r a t i o o f the f i n a l s t r i p c o n c e n t r a t i o n d i v i d e d by t h e i n i t i a l f e e d c o n c e n t r a t i o n assuming no c o m p l i c a t i o n s a r i s e . However, t h e r e s u l t s shown i n T a b l e IV i n d i c a t e t h e a c t u a l c o n c e n t r a t i o n f a c t o r i s much l e s s than the F:S r a t i o , p o s s i b l y because a l a r g e f r a c t i o n of t h e americium i s h e l d i n t h e membrane a t t h e c o n c l u s i o n o f t h e r u n . I n a d d i t i o n , t h e r e l a t i o n s h i p between t h e r a t i o and t h e p e r c e n t t r a n s f e r r e d i s r o u g h l y i n v e r s e because o f t h e I n c r e a s e d amount o f n i t r i c a c i d t r a n s f e r r e d i n t o the s t r i p s o l u t i o n w i t h l a r g e r feed volumes.
T a b l e I V . E f f e c t o f F e e d : S t r i p Volume R a t i o on Americium T r a n s f e r from 7.0M HNO~ t o 0.25M H C O Through a DHDECMP SLM ( I n i t i a l [Am] =10 g/1) Cone. Feed Volume, ml S t r i p Volume, ml Volume R a t i o % T r a n s f e r F a c t o r 0.90 80 50 3.33:1 15 0.62 52 50 10 5:1 0.58 45 150 15 10:1 Source: Reproduced w i t h p e r m i s s i o n from Ref. 9. C o p y r i g h t 1987, M a r c e l Dekker. C a t i o n Exchange o f t h e S t r i p S o l u t i o n . I n o r d e r t o improve t h e somewhat d i s a p p o i n t i n g r e s u l t s g i v e n above f o r t h e c o n c e n t r a t i o n f a c t o r , and t o c o n v e r t t h e americium i n t o a d i s c a r d a b l e form, t h e s t r i p s o l u t i o n was c i r c u l a t e d through a column o f c a t i o n exchange r e s i n , Dowex 50WX8, p l a c e d i n s e r i e s w i t h t h e membrane module. T a b l e V shows t h e r e s u l t s o f t h i s s t u d y .
T a b l e V.
Americium T r a n s f e r and S o r p t i o n w i t h t h e Combined DHDECMP Membrane/Cation Exchange System
Solution
Run
Volume, I n i t i a l ml [Am],g/1
Transfer, %
Final [Am],g/l
Equilibrium Time, m i n .
342 82 1 8.0xl0" 1.5xl0~* Feed 75 1