Plutonium Chemistry - American Chemical Society

20 Aquatic Chemistry of Plutonium ROBERT L. WATTERS

Downloaded by NANYANG TECHNOLOGICAL UNIV on June 3, 2016 | http://pubs.acs.org Publication Date: May 19, 1983 | doi: 10.1021/bk-1983-0216.ch020

U.S. Department of Energy, Ecological Research Division, Washington, DC 20036

Research into the aquatic chemistry of plutonium has produced information showing how this radioelement is mobilized and transported in the environment. Field studies revealed that the sorption of plutonium onto sediments is an equilibrium process which influences the concentration in natural waters. This equilibrium process is modified by the oxidation state of the soluble plutonium and by the presence of dissolved organic carbon (DOC). Higher concentrations of fallout plutonium in natural waters are associated with higher DOC. Laboratory experiments confirm the correlation. In waters low in DOC oxidized plutonium, Pu(V), is the dominant oxidation state while reduced plutonium, Pu(III+IV), is more prevalent where high concentrations of DOC exist. Laboratory and field experiments have provided some information on the possible chemical processes which lead to changes in the oxidation state of plutonium and to its complexation by natural ligands. Early i n o u r r e s e a r c h p r o g r a m , we r e a l i z e d t h a t a n u n d e r s t a n d i n g o f how p l u t o n i u m may move t h r o u g h t h e e n v i r o n m e n t t o t h e human p o p u l a t i o n h i n g e d u p o n a k n o w l e d g e o f i t s s o l u t i o n c h e m i s try. T h i s was i m p o r t a n t k n o w l e d g e f o r t h e d e s c r i p t i o n o f b o t h contemporaneous movement and l o n g - t e r m m o b i l i z a t i o n , s i n c e p l u t o n i u m s d i s t r i b u t i o n between s o l i d phases ( s o i l s and sediments) and aqueous p h a s e s ( s o i l s o l u t i o n s a n d n a t u r a l w a t e r b o d i e s ) l a r g e l y d e t e r m i n e d i t s a c c e s s t o f o o d c h a i n s CO. E a r l i e r obser v a t i o n s h a d shown t h a t s o i l a n d s e d i m e n t i n m o s t e n v i r o n m e n t s h e l d the b u l k o f plutonium, but t h a t a s m a l l m o b i l e f r a c t i o n a l w a y s seemed t o b e p r e s e n t . We e n c o u r a g e d t h e s t u d y o f t h e chem i s t r y o f t h i s s o l u b l e f r a c t i o n i n terms o f t h e i d e n t i f i c a t i o n and 1

This chapter not subject to U.S. copyright. Published 1983, American Chemical Society

Carnall and Choppin; Plutonium Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


298

PLUTONIUM CHEMISTRY

q u a n t i f i c a t i o n of i t s c h e m i c a l s p e c i e s , the p o s s i b l e r e a c t i o n s t h a t l e d t o s p e c i a t i o n , and t h e s o r p t i o n - d e s o r p t i o n phenomena on s o l i d phases. The s t u d y o f t h e c h e m i s t r y o f s o l u b l e p l u t o n i u m i n n a t u r a l w a t e r s seemed a l m o s t i m p o s s i b l e when f i r s t c o n s i d e r e d . We needed to d e v e l o p the i n f o r m a t i o n i n a q u a t i c systems where the r a n g e of pH i s 4 t o 10, Eh i s 0.6v t o -0.2v(_2) and t h e i o n i c s t r e n g t h v a r i e s from t h a t of the oceans to l a k e s w i t h c o n d u c t i v i t i e s l e s s t h a n 20yS/cm. I n a d d i t i o n , t h e r a n g e o f d i s s o l v e d o r g a n i c c a r b o n v a r i e s f r o m a b o u t lppm to g r e a t e r t h a n lOOppm. To compound t h e d i f f i c u l t i e s , the r e p o r t e d c o n c e n t r a t i o n s of s o l u b l e plutonium i n n a t u r a l waters ranged from 1 0 " to 1 0 " m o l e s / l i t e r depending upon t h e i r s o u r c e . These l e v e l s w e r e beyond t h e s e n s i t i v i t y l i m i t s of chemical i n s t r u m e n t a t i o n . However, by t h e u s e o f p l u t o n i u m s r a d i o a c t i v i t y and s e v e r a l n o n i s o t o p i c c a r r i e r t e c h n i q u e s , a c o n s i d e r a b l e amount o f i n f o r m a t i o n a b o u t i t s e n v i r o n m e n t a l chem i s t r y has b e e n a c c u m u l a t e d . 1 3

1 7

1

Sources of Plutonium

f o r Environmental

Research

The m a j o r s o u r c e o f p l u t o n i u m i n n a t u r a l w a t e r s i s t h e a t m o s p h e r i c f a l l o u t f r o m n u c l e a r weapons t e s t s . F a l l o u t plutonium i s u b i q u i t o u s i n m a r i n e and f r e s h w a t e r e n v i r o n m e n t s o f t h e w o r l d w i t h h i g h e r c o n c e n t r a t i o n s i n the n o r t h e r n hemisphere where the b u l k of n u c l e a r weapons t e s t i n g o c c u r r e d ( 3 ) . Much o f t h e r e s e a r c h on t h e a q u a t i c c h e m i s t r y o f p l u t o n i u m t a k e s p l a c e i n m a r i n e and f r e s h w a t e r s y s t e m s where o n l y f a l l o u t i s p r e s e n t . F a l l o u t p l u t o n i u m a r r i v e s i n n a t u r a l w a t e r s e i t h e r by d i r e c t a t m o s p h e r i c d e p o s i t i o n o r by e r o s i o n a n d / o r d i s s o l u t i o n f r o m t h e l a n d . A l t h o u g h i n t h e p a s t , t h i s p l u t o n i u m was c o n s i d e r e d t o be i n a r e f r a c t o r y f o r m due t o f o r m a t i o n w i t h i n t h e f i r e b a l l , i t seems more l i k e l y t h a t m o s t o f t h e p l u t o n i u m o r i g i n a t e d i n t h e s t r a t o s p h e r e by t h e d e c a y o f N p (from U formed d u r i n g the d e t o n a t i o n ) ( 4 ) . D e p o s i t i o n o c c u r s p r e d o m i n a n t l y w i t h one o r a few atoms i n c o r p o r a t e d i n a r a i n d r o p . I n v e s t i g a t i o n s by F u k a i i n d i c a t e t h a t c o l l e c t e d r a i n c o n t a i n s s o l u b l e p l u t o n i u m w h i c h has o x i d a t i o n s t a t e s that are almost t o t a l l y Pu(V+VI)(5). A s e c o n d s o u r c e o f p l u t o n i u m , d i s p e r s e d more l o c a l l y , i s l i q u i d e f f l u e n t from f u e l r e p r o c e s s i n g f a c i l i t i e s . One s u c h i s the f u e l 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 , Cumbria i n the U n i t e d Kingdom w h e r e l i q u i d w a s t e i s r e l e a s e d t o t h e I r i s h S e a ( 6 ) . Chem i c a l a n a l y s i s o f t h i s e f f l u e n t shows t h a t a b o u t one p e r c e n t o r l e s s o f t h e p l u t o n i u m i s i n an o x i d i z e d f o r m b e f o r e i t c o n t a c t s t h e m a r i n e w a t e r (7^). A p p r o x i m a t e l y 95 p e r c e n t o f t h e p l u t o n i u m r a p i d l y a d s o r b s t o p a r t i c u l a t e m a t t e r a f t e r d i s c h a r g e and d e p o s i t s on t h e seabed w h i l e 5 p e r c e n t i s removed f r o m t h e a r e a as a s o l u b l e component(8)· B e c a u s e t h i s s o u r c e p r o v i d e d c o n c e n t r a t i o n s t h a t were r e a d i l y d e t e c t e d , p i o n e e r i n g f i e l d r e s e a r c h i n t o p l u t o n i u m o x i d a t i o n s t a t e s i n t h e m a r i n e e n v i r o n m e n t was c o n d u c t e d at this location. 2 3 9

2 3 9


20.

Aquatic Chemistry of

WATTERS

Pu

299


A t h i r d source of aquatic plutonium i s l i q u i d e f f l u e n t d i s c h a r g e d f r o m l a b o r a t o r y o p e r a t i o n s i n t o ponds and s t r e a m s . An example o f t h i s i s a f o r m e r w a s t e pond a t Oak R i d g e N a t i o n a l L a b o r a t o r y , Pond 3513, t h a t r e c e i v e d l i q u i d w a s t e s w i t h l o w c o n c e n t r a t i o n s o f t r a n s u r a n i c e l e m e n t s b e f o r e i t was r e t i r e d . T h i s im poundment has w a t e r q u a l i t y s i m i l a r t o h i g h pH n a t u r a l p o n d s . S i n c e t h e pond i s e a s i l y a c c e s s i b l e and s h a l l o w , s c i e n t i s t s a t ORNL p e r f o r m e d s e v e r a l m a n i p u l a t i v e e x p e r i m e n t s w i t h c a i s s o n s t o d e t e r m i n e t h e e f f e c t s o f a e r o b i c and a n a e r o b i c e n v i r o n m e n t s u p o n the s p e c i a t i o n of a c t i n i d e e l e m e n t s ( 9 ) . Correlative

Observations

E n v i r o n m e n t a l c h e m i s t s f u n d e d by t h e D e p a r t m e n t o f E n e r g y h a v e s t u d i e d t h e s e s o u r c e s t o l e a r n a s much as t h e y c a n a b o u t t h e c h e m i s t r y o f p l u t o n i u m d i s p e r s e d i n f r e s h w a t e r and m a r i n e e c o s y s tems. Much o f t h e e a r l y w o r k d e t e r m i n e d t h e c o n c e n t r a t i o n s i n v a r i o u s w a t e r b o d i e s and t h e d i s t r i b u t i o n b e t w e e n w a t e r and s e d i ment. T a b l e I shows r e s u l t s o f v a r i o u s f r e s h w a t e r and m a r i n e surveys(10).

Table I . D i s t r i b u t i o n C o e f f i c i e n t s , Κ , For Plutonium i n D i f f e r e n t N a t u r a l Waters(10) Concentration on p a r t i c l e s (pCi-g- )

Concentration i n solution (pCi-ltr- )

Κ (ml'g ) X 10*

Bombay H a r b o u r

0.4-2.9

4.0-20

4.8-13

Enewetak L a g o o n

0.1-75

2.0-75

5.0-60 4.0-30

Hudson R i v e r

0.02

0.3

6.7

Irish

36 23-80 9.0-29 0.22 0.37

500 370-650 310-460 35 50

7.5 4.6-21 1.9-9.3 0.6 0.7

Lake Michigan

0.2

0.6

33

Lake Washington

-

-

1.0-8.5 6.9-27

Mediterranean

-

-

1.6-9.4

0.014-0.1

0.24-2.4

4.2-41

Sampling

area

1

Sea

Savannah R i v e r

Sea

1


- 1

300

PLUTONIUM CHEMISTRY

A number o f f r e s h w a t e r l a k e s w e r e s u r v e y e d f o r c o n c e n t r a t i o n s of p l u t o n i u m , the r a t i o of i t s upper to lower o x i d a t i o n s t a t e s , pH, and t h e c o n c e n t r a t i o n o f d i s s o l v e d o r g a n i c c a r b o n (DOC), w h i c h a r e shown i n T a b l e I I ( 1 1 ) .


T a b l e I I . C o n c e n t r a t i o n s o f A c t i n i d e s and D i s s o l v e d O r g a n i c C a r b o n and Pu O x i d a t i o n S t a t e R a t i o s i n F i l t e r e d L a k e W a t e r ( 1 1 )

Lake

Concentrations i n f i l t e r e d water pH Pu(III+IV) Pu(V+VI) ( f C i L- )

Pu(V+VI) Pu(III+IV)

1

Clear Lake 8 .0 L a k e M i c h i g a n 8 .0 ELA 302S 6 .3 ELA 224 6 .4 ELA 223 5 .6 L a s t M o u n t a i n 8 .4 Lake GSL, McLeod 7 .5 Bay ELA 161 6 .8 GSL, M a i n 8 .2 Basin GSL, C h r i s t i e 8 .5 Bay ELA 305 6 .9 ELA 227 6 .6 Lake of the 7 .1 Woods L a k e K a t h e r i n e 8 .5 AELA-885 8 .4 ELA-239 6 .3 ELA-661-77 4 .8 ELA-661-78 5 .6 V o l o Bog 5 .5 Okeef enokee 3 .9 ELA 241 5 .9 L i t t l e M a n i t o >8 .7

Dissolved Organic c a r b o n DOC (ppm)

0.09 0.06 0.13 0.18 0.19 0.28

0.57 0.39 0.21 0.30 0.30 0.46

6.5 6.5 1.6 1.6 1.6 1.6

0.34

0.45

1.3

2.5

0.28 0.13

0.35

Plutonium Chemistry - American Chemical Society

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