23 Effects of Humic Substances on Plutonium Speciation in Marine Systems 1
G. R. Choppin, R. A. Roberts , and J. W. Morse
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Department of Chemistry, Florida State University, Tallahassee, FL 32306-3006
The dominant oxidation state of plutonium in the dissolved phase of seawaters has been shown to be Pu(V). Data are presented that indicate a signif icant role by humic materials which cause rapid re duction of Pu(Vl) to Pu(lV). The latter leaves the solution phase via hydrolysis. The humic material also seems to reduce Pu(V) but at a much slower rate and, in sunlight, this reduction may be negated by an oxidation process of unknown origin at this time. The role of humics sorbed on suspended particulate and sedimentary matter is discussed. Sources and Amounts o f P l u t o n i u m i n the Environment. Since 1945 a p p r o x i m a t e l y 3300 k g o f p l u t o n i u m has been i n j e c t e d i n t o t h e e n vironment, m o s t l y (>90%) from atmospheric e x p l o s i o n s o f n u c l e a r weapons. T h i s corresponds t o about 380 k C i t o t a l a l p h a r a d i o a c t i v i t y . The a d d i t i o n t o t h i s amount by r e l e a s e s from n u c l e a r power o p e r a t i o n s i s much s m a l l e r ; the major c o n t i n u i n g a d d i t i o n i s ca. 0.1 k C i per month r e l e a s e d t o t h e I r i s h Sea from the B r i t i s h n u c l e a r r e p r o c e s s i n g p l a n t a t W i n d s c a l e . About 2/3 o f the p l u t o n i u m from n u c l e a r e x p l o s i o n s would be formed i n t o h i g h f i r e d o x i d e s which would be r a t h e r i n e r t c h e m i c a l l y . However, the remainder, c r e a t e d d u r i n g the e x p l o s i o n as s i n g l e atoms v i a t h e U(n, Ï ) U ( 2 $ " V P u r e a c t i o n sequence, should be more r e a c t i v e and behave s i m i l a r l y t o t h a t r e l e a s e d from r e p r o c e s s i n g p l a n t s o r n u c l e a r waste r e p o s i t o r y sites. Sampling o f f i l t e r e d water samples o f the P a c i f i c Ocean i n d i c a t e s a c o n c e n t r a t i o n o f c a . 2 x l 0 ' M ( i . e . 1 0 " dpm L"*) ( 1 ) . These v a l u e s , however, are open t o q u e s t i o n as the p l u t o n i u m a s s o c i a t e d w i t h suspended p a r t i c u l a t e s may be more than an o r d e r o f magnitude g r e a t e r than t h a t i n t r u e s o l u t i o n . For example, f o r t h e Z J 9
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Current address: Mallinckrodt Medical Products Research and Development, St. Louis, MO 63134. Current address: Department of Oceanography, Texas A&M University, College Station, TX 77843.
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0097-6156/86/0305-0382S06.00/0 © 1986 American Chemical Society
Sohn; Organic Marine Geochemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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C H O P P I N ET AL.
Plutonium Speciation in Marine Systems
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M e d i t e r r a n e a n Sea, the p l u t o n i u m a c t i v i t y i n u n f i l t e r e d water samples was twenty f i v e times g r e a t e r than t h a t o f f i l t e r e d sam p l e s ( 2 ^ . I n experiments i n which p l u t o n i u m was added to seawater, the s o l u b i l i t y o f i o n i c s p e c i e s a f t e r 30 days was found t o be 5χ10~Ί% ( 3 ) . A d d i t i o n o f r e l a t i v e l y l a r g e q u a n t i t i e s o f humic mat e r i a l i n these experiments i n c r e a s e d the p l u t o n i u m s o l u b i l i t y s i x fold. P r o p e r t i e s o f P l u t o n i u m . As n u c l e a r power u t i l i z a t i o n w i t h i t s a s s o c i a t e d r e p r o c e s s i n g and waste d i s p o s a l o p e r a t i o n s expands, the f a t e o f any r e l e a s e d p l u t o n i u m assumes g r e a t e r importance. However, t h e r e i s another r a t i o n a l e f o r s t u d y i n g the e n v i r o n m e n t a l b e h a v i o r o f p l u t o n i u m . Because o f i t s s o l u t i o n c h e m i s t r y , i t i s an element w i t h r a t h e r unique q u a l i t i e s as a probe o f e n v i r o n m e n t a l p r o p e r t i e s . P l u t o n i u m has f o u r o x i d a t i o n s t a t e s , V I , V, IV, and I I I , a l l o f which can e x i s t i n aqueous s o l u t i o n w i t h i n the and pH range found i n n a t u r e . Under the proper c o n d i t i o n s , a l l f o u r s t a t e s can c o e x i s t , a l t h o u g h one o r more o f the s t a t e s i s u s u a l l y f a v o r e d . The I I I and IV o x i d a t i o n s t a t e s g e n e r a l l y e x i s t as h y d r a t e d o r complexed c a t i o n s . The V and VI s t a t e s , on the o t h e r hand, e x i s t as P u 0 and Pu0„ the d i o x o c a t i o n s - a l s o h y d r a t e d o r complexed. N o r m a l l y , a c i d i c c o n d i t i o n s s t a b i l i z e the lower o x i d a t i o n s t a t e s w h i l e more b a s i c c o n d i t i o n s f a v o r the h i g h e r s t a t e s . D i s t r i b u t i o n o f p l u t o n i u m between i t s d i f f e r e n t o x i d a t i o n s t a t e s i s , o f c o u r s e , dependent not o n l y on pH. C o m p l e x a t i o n , f o r example, can change the r e l a t i v e s t a b i l i t i e s o f the d i f f e r e n t s t a t e s . The v a r i o u s o x i d a t i o n s t a t e s d i f f e r i n t h e i r a b i l i t y t o form complexes, w i t h Pu(V) e x h i b i t i n g the weakest c o m p l e x a t i o n . P u ( V l ) and P u ( l l l ) are r a t h e r s i m i l a r i n c o m p l e x a t i o n s t r e n g t h a l t h o u g h P u ( V l ) i s usu a l l y s t r o n g e r than P u ( l l l ) f o r most l i g a n d s . P u ( l V ) forms the s t r o n g e s t complexes. Hydrolysis, including hydrolytic polymeriza t i o n , can a l s o p l a y an i m p o r t a n t r o l e i n s t a b i l i z i n g one o x i d a t i o n s t a t e over another. The e f f e c t s o f redox p o t e n t i a l s , c o m p l e x a t i o n , h y d r o l y s i s , e t c . can combine t o g i v e the p o s s i b i l i t y o f more than one o x i d a t i o n s t a t e c o e x i s t i n g under the same c o n d i t i o n s which c o m p l i c a t e s the study o f the e n v i r o n m e n t a l b e h a v i o r o f p l u t o n i u m but a l s o can serve t o d e f i n e the Ε, , e t c . o f systems. The r e l a t i v e t e n d e n c i e s o f v a r i o u s s p e c i e s o f trie d i f f e r e n t o x i d a t i o n s t a t e s t o form c o l l o i d s o r t o sorb t o p a r t i c u l a t e s add f u r t h e r dimensions to the chemical behavior of plutonium. A t o p i c which r e q u i r e s f u r t h e r study i s the e f f e c t o f o r g a n i c s on the b e h a v i o r o f p l u t o n i u m i n the marine environment. Based on a b i o a s s a y method, F i s h e r , e t a l . (4) have suggested t h a t marine humic and f u l v i c a c i d s produce no s u b s t a n t i a l c o m p l e x a t i o n o f t r a n s u r a n i c elements i n the oceans. However, Dahlman, et a l . (5) support the i d e a o f c o m p l e x a t i o n by humic and f u l v i c a c i d s and p r o v i d e ex p e r i m e n t a l evidence o f r e d u c t i o n o f P u ( V l ) t o P u ( l V ) by f u l v i c a c i d . Complexation i s a l s o supported by the work o f P i l l a i and Mathew ( 3 ) , a l t h o u g h the c o n c e n t r a t i o n s o f o r g a n i c s used i n t h e i r s t u d i e s were u n r e a l i s t i c a l l y high. +
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Sohn; Organic Marine Geochemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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O R G A N I C M A R I N E GEOCHEMISTRY
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Previous Studies. Studies from the I r i s h Sea (6) and the P a c i f i c Ocean (_7) show that the soluble form of plutonium i s predominantly in the oxidized forms (V and VI states) with evidence that V i s the more important state. However, modeling calculations with ex perimental complexation and redox data led Aston (8) to conclude that Pu(VT) i s the dissolved form (no effect of organic material was included). In a recent extensive review of the geochemistry of plutonium i n water environments, Sholkovitz (90 concluded the o x i dation state d i s t r i b u t i o n i n waters requires more investigation. In this paper we report the result of studies designed to c l a r i f y the dominant oxidation state of plutonium in sea water and the role of humic material. Experimental The s t a b i l i t y of Pu(Vl) and Pu(V) was studied i n solutions of stand ard a r t i f i c i a l (same mineral constitution) seawater and of true seawater from the Gulf of Mexico (collected ca. 5 miles from the Florida coastline below Tallahassee). A l l solutions were buffered to pH 8.0 by addition of trishydroxymethylaminomethane ("tris") and f i l t e r e d by vacuum through 0.45 micron M i l l i p o r e f i l t e r s . The glass ware, pipets, etc. used were treated to reduce adsorption losses of Pu at pH 8 by a method developed i n this laboratory (10). The humic acid was recovered from Bahamian marine sediments obtained from sites i n less than 5 m water depth and at least 0.5 miles from shore. The i s o l a t i o n and p u r i f i c a t i o n procedures have been described e a r l i e r ( 1 Ό . 238 239 Pu(Vl) was prepared by oxidation of acidic stock Pu or Pu tracer solutions with KMnO^. The oxidized tracer solution was neu t r a l i z e d with NaOH prior to addition to the experimental solution. Pu(V) was prepared by photolysis of a thenoyltrifluoracetone (TTA) solution as described previously (_12 ) . The Pu(V) was stripped from the organic-TTA solution d i r e c t l y into the experimental aqueous solution. The concentration of ^39p io~^ £ n experi ments while that of ^**Pu was ca. lO'^M. Ten ml of the test solutions were kept in treated screw cap glass v i a l s and 0.500 ml aliquots withdrawn p e r i o d i c a l l y . Plutonium was separated by oxidation state by a solvent extraction method (13) and the alpha a c t i v i t y counted with a l i q u i d s c i n t i l l a t i o n counter using "Handifluor" (Mallinckrodt) as the c o c k t a i l . u w a s
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