Liquid Membranes - American Chemical Society

Chapter 13

Actinide Removal from Aqueous Waste Using Solid Supported Liquid Membranes Anthony C. Muscatello, James D. Navratil , and Marlene Y. Price 1

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 3, 2015 | http://pubs.acs.org Publication Date: July 24, 1987 | doi: 10.1021/bk-1987-0347.ch013

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

184

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 .


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


13.

MUSCATELLO ET AL.

Actiniae Removal from Aqueous Waste

185


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


LIQUID MEMBRANES: THEORY AND APPLICATIONS

186


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

Liquid Membranes - American Chemical Society

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