Behavior of Plutonium in Natural Waters - ACS Publications

19 Behavior of Plutonium in Natural Waters BERT ALLARD Chalmers University of Technology, Department of Nuclear Chemistry, S-41296 Göteborg, Sweden

Downloaded by UNIV OF MARYLAND COLL PARK on October 17, 2014 | http://pubs.acs.org Publication Date: May 19, 1983 | doi: 10.1021/bk-1983-0216.ch019

JAN RYDBERG

1

Lawrence Livermore National Laboratory, Livermore, CA 94550

The use of nuclear power leads to waste streams containing plutonium. The large scale of plutonium production combined with the high radiotoxicity of this element makes it mandatory to understand how plutonium behaves in nature, particularly how it will migrate in water and be taken up by living species. The paper reviews the plutonium sources, the relevant chemistry of natural waters, and what plutonium species are expected to be formed. Measurements of interaction between natural solid materials (clay, rock, etc) and plutonium dissolved in natural waters are used to show the low environmental risk associated with storage of large amounts of plutonium in repositories located in deep granite bedrock. The large and widespread production of plutonium i n nuclear power s t a t i o n s combined with i t s high r a d i o t o x i c i t y has caused great p u b l i c concern about "plutonium poisoning". Consequently, the spread of plutonium i n the environment has been e x t e n s i v e l y studied f o r s e v e r a l decades. The r e s u l t s have been reported i n j o u r n a l s and more r e c e n t l y i n a number of monographs and conference proceedings (e.g. J_-7). Except f o r large scale a c c i d e n t a l r e l e a s e s (e.g. nuclear explosions or c a t a s t r o p h i c accidents a t nuclear p l a n t s ) , water w i l l be the main transport medium of plutonium to man. Therefore the s i z e and l o c a t i o n of plutonium sources, i t s pathways to man and i t s behaviour i n n a t u r a l waters are e s s e n t i a l knowledge r e quired f o r the e v a l u a t i o n of i t s e c o l o g i c a l impact. That informat i o n , combined with r a d i o l o g i c a l h e a l t h standards, allows an assessment of the o v e r a l l r i s k to the p u b l i c from plutonium e.g. from a waste r e p o s i t o r y f o r spent unreprocessed r e a c t o r f u e l e l e ments i n deep g r a n i t e bedrock (8, 9 ) . 1

Permanent address: Chalmers University of Technology, Department of Nuclear Chemistry, S-41296 Gôteborg, Sweden.

0097-6156/83/0216-0275$06.25/0 © 1983 American Chemical Society

In Plutonium Chemistry; Carnall, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

276

PLUTONIUM CHEMISTRY

P l u t o n i u n Sources About 300 tonnes of plutonium (95% P u ) are p r e s e n t l y stored i n nuclear weapons (6). Production f i g u r e s f o r weapons grade p l u t o nium are unknown, but they may be estimated to be Am>U>Np ( 1 5 ) . B e c a u s e n e p t u n i u m i s u s u a l l y p e n t a v a l e n t , u r a n i u m h e x a v a l e n t and a m e r i c i u m t r i v a l e n t , w h i l e p l u tonium i n n a t u r a l systems i s m a i n l y t e t r a v a l e n t , i t i s c l e a r from t h e a c t i n i d e homologue p r o p e r t i e s t h a t t h e o x i d a t i o n s t a t e o f p l u tonium w i l l a f f e c t t h e o b s e r v e d K d - v a l u e . The o x i d a t i o n s t a t e o f p l u t o n i u m depends on t h e r e d o x p o t e n t i a l ( E h - v a l u e ) o f t h e g r o u n d w a t e r and i t s c o n t e n t o f o x i d a n t s o r r e d u c t a n t s . I t i s a l s o f o u n d t h a t n a t u r a l l i g a n d s l i k e CO^ " and f u l v i c a c i d s , w h i c h complex p l u t o n i u m (see next s e c t i o n ) , a l s o i n f l u e n c e the Kd-value. 5

3

k

2

C o m p o s i t i o n of N a t u r a l Waters W a t e r c o n t a i n s d i s s o l v e d and p a r t i c u l a t e i n o r g a n i c and o r g a n i c m a t t e r . To d i s t i n g u i s h b e t w e e n d i s s o l v e d and p a r t i c u l a t e m a t t e r a 0.45 pm p o r e s i z e f i l t e r i s o f t e n u s e d t o s e p a r a t e t h e f r a c t i o n s .


19.

ALLARD AND RYDBERG

Behavior of Pu in Natural Waters

T a b l e I . D i s t r i b u t i o n (Kd) o f p l u t o n i u m b e t w e e n d i f f e r e n t com partments o f t h e ecosystem. E x p e r i m e n t a l range i s i n d i c a t e d by


T r a n s p o r t from > t o Water > Sediment P a c i f i c Ocean (Enewetak) I r i s h Sea (Windscale) Lake M i c h i g a n W h i t e Oak L a k e ( T e n n e s s e e ) Hudson R i v e r Water > O r g a n i s m s W h i t e Oak L . (Tenn.) > seaweed > sediment s p e c i e s Lake M i c h i g a n > phytoplankton > zooplankton " > plant eating fish > fish eating fish W h i t e Oak L a k e (Tenn.) > f i s h I r i s h Sea > f i s h P a c i f i c Ocean (Enewetak) > f i s h Lake M i c h i g a n > c o a s t a l p l a n t s > c o a s t a l animals Sediments > water organisms (a) > algea > phytoplankton > zooplankton > plant eating fish Sediment, s o i l , sludge > p l a n t s Sediment (L. M i c h i g a n ) > c o a s t a l p l a n t s S o i l (b) > t r e e s > soybean > tomato S o i l ( C a l i f o r n i a ) > wheat g r a i n Sludge (c) > corn " > broccoli " > tomato Food > a n i m a l s Sediment ( L . M i c h . ) > c o a s t a l a n i m a l s V e g e t a t i o n (d) > c o t t o n r a t s L i c h e n (Lappland) > r e i n d e e r Food ( P u a s o x i d e o r h y d r o x i d e ) > man " ( P u i n o t h e r common f o r m s ) > man ( P u - c i t r a t e ) > man 11

11

11

11

11

11

11

l o g Kd

Ref.

5.0±1.5 4.8±0.6 5.5 4.7±0.8 4.8

29 29 16 15 16

4.3 3 4.0 2.5 1.5 0.4±0.6 0.1+0.5 0.1 3.7±2.3 3.3±0.4 3.110.2

15 15 15,30 15,30 15,30 15,30 15 29 29 15,30 15,30

212 -1.5 -3.0 -2.310.7

15 15 15 15

-1.110.3 -4.2 -2.7 -4.0 -6.210.8 -5.5 -3.8 -4.1

15 14 14 14 31 32 32 32

-1.210.3 -0.9 -5.210.3 -5 -4 -3

15 33 34 35 35 36

(a) R e f e r s t o L a k e M i c h i g a n , where l o g Kd f o r w a t e r > s e d i m e n t 5.5. ( b ) W e s t e r n U.S. f l o o d p l a i n . ( c ) Waste s l u d g e f r o m sewage t r e a t m e n t p l a n t , (d) Savannah R i v e r P l a n t ( S . C a r o l i n a ) , where l o g Kd f o r s o i l > r a t i s - 1 . 9 .


PLUTONIUM CHEMISTRY


280

The amount o f p a r t i c u l a t e m a t t e r v a r i e s f r o m v e r y h i g h v a l u e s i n s i l t c a r r y i n g r i v e r s ( t h e M i s s i s s i p p i R.iver c a r r i e s an a v e r a g e o f 2600 m g / l i t e r a t f l o o d t i m e ) t o p r a c t i c a l l y z e r o (0.05 m g / l i t e r ; r e f . 17) i n t h e o c e a n . A t y p i c a l v a l u e may be 1-10 m g / l i t e r . The m i n e r a l p a r t i c l e s o f t e n c o n s i s t o f c l a y w i t h i o n exchange p r o p e r ties . C o n s i d e r i n g the v a r i e t y of sources f o r o r g a n i c m a t e r i a l , a great d i v e r s i t y i n the c o n c e n t r a t i o n of p a r t i c u l a t e o r g a n i c matter o c c u r s ; h o w e v e r , i t i s u s u a l l y l e s s t h a n a few p e r c e n t o f t h e t o t a l p a r t i c u l a t e m a t t e r , e x c e p t i n a r e a s o f e x c e s s i v e human c o n t a m i n a t i o n . A t y p i c a l v a l u e f o r e s t u a r i n e w a t e r s w o u l d be 5 m g / l i t e r o f p a r t i c u l a t e o r g a n i c m a t t e r ( 1 8 ) . The o r g a n i c m a t e r i a l has t h e f u n c t i o n a l g r o u p s -C00H, -OH and - N H 2 , w h i c h may c o m p l e x m e t a l s a t i t s surface. When t h e w a t e r becomes s t a g n a n t , t h e p a r t i c u l a t e m a t t e r s e t t l e s as b o t t o m s e d i m e n t s . I n t h i s p r o c e s s i t c a r r i e s w i t h i t c o n s i d e r a b l e amounts o f p l u t o n i u m , i f any has b e e n d i s s o l v e d i n t h e water (see T a b l e I ) . T a b l e I I summarizes a n a l y t i c a l d a t a f o r d i s s o l v e d i n o r g a n i c m a t t e r i n a number o f n a t u r a l w a t e r s o u r c e s ( 8 , 9^, 19_ 20_ 21) . B e c a u s e o f t h e i n t e r a c t i o n o f r a i n w a t e r w i t h s o i l and s u r f a c e m i n e r a l s , w a t e r s i n l a k e s , r i v e r s and s h a l l o w w e l l s (100 m d e p t h . R a i n i n e q u i l i b r i u m w i t h a t m o s p h e r i c C0 » b u t u n c o n t a m i n a t e d by i n d u s t r i a l e m i s s i o n s , s h o u l d have a pH o f 5.7. However, atmos p h e r i c p o l l u t i o n f r o m b u r n i n g f o s s i l f u e l s has r e s u l t e d i n a c i d r a i n o f pH as l o w as 3.5 (24) . I f t h i s c o n d i t i o n c o n t i n u e s f o r a l o n g t i m e , i t may l e a d t o a change i n g r o u n d w a t e r c o m p o s i t i o n , w h i c h may c o n s i d e r a b l y change t h e m i g r a t i o n o f p l u t o n i u m i n n a t u r e . A g r e a t d i v e r s i t y i n the c o n c e n t r a t i o n of d i s s o l v e d o r g a n i c m a t t e r a l s o o c c u r s i n n a t u r a l w a t e r . Commonly, t h e c o n c e n t r a t i o n r a n g e s f r o m 0.5 t o 50 m g / l i t e r . F r e s h w a t e r and s e a w a t e r t y p i c a l l y h a v e v a l u e s o f 0.5-1.5 m g / l i t e r ( 1 8 ) . About h a l f o f the d i s s o l v e d o r g a n i c c a r b o n may a p p e a r i n humic o r f u l v i c a c i d s . These a r e h i g h - m o l e c u l a r w e i g h t o r g a n i c compounds o f a c o m p o s i t i o n w h i c h i s somewhat u n c e r t a i n . They c o n t a i n a r o m a t i c h y d r o x y l and c a r b o x y l g r o u p s w h i c h h a v e t h e a b i l i t y t o b i n d t o met a l i o n s . R i v e r s and e s t u a r i e s t y p i c a l l y c o n t a i n 10 m g / l i t e r o f a c i d w i t h an exchange c a p a c i t y of 5-10 mmoi/g, m a i n l y due t o c a r b o x y l i c 9

9

2

2



2 +

2

2

volts mg/liter mg/liter mg/liter mg/liter mg/liter mg/liter mg/liter mg/liter mg/liter mg/liter mg/liter mg/liter mg/liter mg/liter mg/liter mg/liter mg/liter

m years

Units

0.1-4 0 $20(1-5) 0 0 $0.5

$1

0.1-20

0 0 4-6 0.9 10 0.3-20 0.1-4 0.5-5 0.1-0.5

Rainwater

60-200 $10 $0.1 3-300(20) 3-15 0 0.05 M a n d pH Ρ υ θ 2 £ Ρ π > Ρ υ 0 . S e v e r a l r e c e n t c o m p i l a t i o n s and c r i t i c a l e v a l u a t i o n s o f d a t a on p l u t o n i u m r e d o x a n d complex c h e m i s t r y a r e a v a i l a b l e ( e . g . 3 8 - 4 2 ) . However, e x p e r i m e n t a l d a t a o n p l u t o n i u m c o m p l e x e s w i t h e.g. C 0 3 ~ , ΗΡ0ι+ , OH , e t c . a r e s c a r c e a n d many o f t h e s u g g e s t e d f o r m a t i o n c o n s t a n t s a r e m e r e l y e x t r a p o l a t e d o r even e s t i m a t e d d a t a , e.g. f r o m comparisons w i t h the s i m i l a r elements thorium, uranium, neptunium and a m e r i c i u m . The u n c e r t a i n t y e v e n i n m e a s u r e d f o r m a t i o n c o n s t a n t s i s u s u a l l y a f a c t o r o f a t l e a s t 2 t o 3, and o f t e n up t o o r d e r s o f m a g n i t u d e f o r t h e h i g h e r c o m p l e x e s . M o r e o v e r , t h e e x i s t e n c e o f many of t h e suggested p l u t o n i u m s p e c i e s h a s never been e x p e r i m e n t a l l y v e r i f i e d . S t i l l , i n any d i s c u s s i o n o f s p e c i a t i o n and s o l u b i l i t y i n n a t u r a l systems i t i s i m p e r a t i v e t h a t a l l r e l e v a n t s p e c i e s a r e con s i d e r e d . T h u s , i t i s j u s t i f i e d , and n e c e s s a r y , t o i n c l u d e e s t i m a t e d o r e x t r a p o l a t e d d a t a f o r p o o r l y s t u d i e d s p e c i e s and even d a t a f o r n o n - i d e n t i f i e d p l u t o n i u m s p e c i e s t h a t a r e l i k e l y t o e x i s t . Some s e l e c t e d c o m p l e x f o r m a t i o n c o n s t a n t s w h i c h c o u l d be v a l i d f o r p l u t o n i u m a r e g i v e n i n T a b l e I I I (41) , c o n s i d e r i n g t h e m a j o r c o m p l e x ing anions i n n a t u r a l waters (Table I I ) . Selected standard poten t i a l s are given i n Table IV (41). 2

2

2

+

2 +

3 +

+

2

2

2

Table IV.

Pu

Selected standard p o t e n t i a l s 4+

λ-τ + e =Pu D

PuO * + 4H"" + e" = P u 94ι + 4H

2

Pu0

2 + 2

4 +

+ 2H 0 / 4_

+ 2 e ~ = Pu

+ e~ = P u 0

(41)

l . o i

1

Pu0

(V)

+ 2

1.10

+ 2H 0 2

1.03 0.96


284

PLUTONIUM

CHEMISTRY

L i t t l e i s known a b o u t a c t i n i d e c o m p l e x a t i o n by humic o r f u l v i c acids although l o g ^ - v a l u e s f o r Am , T h and U 0 with h u m i c a c i d a t pH 4.0-4.5 as 6.8, 11.0 and 5.8, r e s p e c t i v e l y , a r e reported (43). 3 +

4 +

2 +

2


P l u t o n i u m S o l u b i l i t y and S p e c i a t i o n i n N a t u r a l W a t e r s S e v e r a l a t t e m p t s t o c a l c u l a t e p l u t o n i u m s o l u b i l i t y and s p e c i a t i o n i n environmental systems a r e g i v e n i n t h e l i t e r a t u r e ( e . g . 39, 4 1 , 4 4 - 4 7 ) . C o n s i d e r i n g t h e a n i o n c o n c e n t r a t i o n r a n g e s i n n a t u r a l w a t e r s ( T a b l e I I ) and t h e m a g n i t u d e o f t h e c o r r e s p o n d i n g p l u t o nium s t a b i l i t y c o n s t a n t s (Table I I I ) , the c h e m i s t r y o f p l u t o n i u m , as w e l l a s t h a t o f u r a n i u m and n e p t u n i u m , i s a l m o s t e n t i r e l y d o m i n a t e d by h y d r o x i d e and c a r b o n a t e c o m p l e x a t i o n , c o n s i d e r i n g i n o r g a n i c complexes o n l y ( 4 1 , 48, 4 9 ) . I n F i g u r e 2 t h e s o l u b i l i t y and s p e c i a t i o n o f p l u t o n i u m h a v e b e e n c a l c u l a t e d , u s i n g s t a b i l i t y d a t a f o r t h e h y d r o x y and c a r b o n a t e c o m p l e x e s i n T a b l e I I I and s t a n d a r d p o t e n t i a l s f r o m T a b l e I V , f o r t h e w a t e r s i n d i c t e d i n F i g u r e 2. H e r e , t h e v a r i o u s c a r b o n a t e c o n c e n t r a t i o n s w o u l d c o r r e s p o n d t o a n open s y s t e m i n e q u i l i b r i u m w i t h a i r ( b ) and c l o s e d s y s t e m s w i t h a t o t a l c a r b o n a t e c o n c e n t r a t i o n o f 30 m g / l i t e r ( c , e ) and 485 m g / l i t e r ( d , f ) , r e s p e c t i v e l y . The two r e dox p o t e n t i a l s w o u l d r o u g h l y c o r r e s p o n d t o w a t e r i n e q u i l i b r i u m w i t a i r ( a - d ; c f 50) and s y s t e m s b u f f e r e d by an F e ( I I I ) ( s ) / F e ( I I ) ( s ) e q u i l i b r i u m ( e , f ) , r e s p e c t i v e l y . T h u s , t h e n a t u r a l span o f c a r b o n a t e c o n c e n t r a t i o n s and r e d o x c o n d i t i o n s i s i l l u s t r a t e d . Under o x i c c o n d i t i o n s , t h e s o l u b i l i t y o f p l u t o n i u m i s l i m i t e d by P u 0 ( s ) w i t h P u i O H ) ^ a s t h e d o m i n a t i n g s p e c i e s i n s o l u t i o n i n t h e i n t e r m e d i a t e pH-range 6-9. A t a pH b e l o w 5-6 t h e p e n t a v a l e n t Pu0 w o u l d d o m i n a t e and b e l o w pH 3-4, P u w o u l d be t h e m a j o r s p e c i e s . A t h i g h carbonate c o n c e n t r a t i o n s p e n t a v a l e n t carbonate spe c i e s l i k e P u 0 ( C 0 ) ~ w o u l d d o m i n a t e i n t h e h i g h pH-range (above 8-9). Hexavalent s p e c i e s would not c o n t r i b u t e s i g n i f i c a n t l y . Under r e d u c i n g c o n d i t i o n s t h e s o l u b i l i t y l i m i t i n g s p e c i e s w o u l d s t i l l be P u 0 ( s ) a t h i g h pH (above 7-8) b u t P u ( C 0 3 ) ( s ) i n t h e l o w p H - r a n g e . T r i v a l e n t s p e c i e s w o u l d d o m i n a t e i n t h e pH-range o f e n v i r o n m e n t a l i n t e r e s t (up t o pH 9 - 1 0 ) . The o v e r a l l s o l u b i l i t y o f p l u t o n i u m w o u l d be c o n s i d e r a b l y h i g h e r u n d e r r e d u c i n g c o n d i t i o n s t h a n i n o x i c s y s t e m s , i n t h e p r e s e n c e o f c a r b o n a t e . Any o f t h e o x i d a t i o n s t a t e s I I I , I V o r V c o u l d be o b t a i n e d . P r e v i o u s l y , i t h a s b e e n s u g g e s t e d (39) t h a t P u 0 2 ( s ) w o u l d be c o m p a r a t i v e l y s t a b l e i n t e r r e s t r i a l w a t e r s i n most pH and r e d o x p o t e n t i a l ranges. S o l u t i o n s i n e q u i l i b r i u m w i t h Pu02(s) o r Pu(0H^(s are c l a i m e d t o c o n t a i n P u 0 2 as t h e dominating s p e c i e s under o x i c c o n d i t i o n s and a t pH b e l o w 6-7, a c c o r d i n g t o r e c e n t r e s u l t s ( 5 0 ) . These e x p e r i m e n t a l l y d e t e r m i n e d s o l u b i l i t i e s a r e i n g e n e r a l a g r e e ment w i t h t h e c a l c u l a t e d c u r v e s a-d i n F i g u r e 2. A l s o t h e p o s s i b l e r e d u c t i o n t o t h e t r i v a l e n t s t a t e i n n a t u r e by d i s s o l v e d o r g a n i c s u b s t a n c e s ( 1 4 , 15, 51) a t l o w o r i n t e r m e d i a t e p H - v a l u e s h a s b e e n suggested. 2

3 +

2

5

2

3

3

2

2

+


3


19.

I

e

I

I

I

1

I

-

1

3

2

\ \ — . \\

1

\\

~ PuC0 * 3

285

1

Pu^CO^isi Pu0is>

Pu *\

ο

Behavior of Pu in Natural Waters

ALLARD AND RYDBERG

ι1

-

N^-^k

P u O H ^ ^ . . V^v 2

| \ V \

— PutOH)/ -10 -

^ν^Λ^Χ

^

Pu(0H)

4

_

Ρι40Η) --^ >Χ >

5

I

1

1

1

1

1

1

8 PH F i g u r e 2. P l u t o n i u m s o l u b i l i t i e s (M) a n d s p e c i e s i n s o l u t i o n ( 4 1 ) . ( a ) C a r b o n a t e f r e e , (b) E q u i l i b r i u m w i t h a t m o s p h e r i c c a r b o n d i o x i d e ; p ( C 0 ) =10 -3.5 atm. ( c , e ) T o t a l c a r b o n a t e 0.5 mM (30 m g / l i t e r ) , ( d , f ) T o t a l c a r b o n a t e 8 mM (485 m g / l i t e r ) . ( a , b , c,d) A e r a t e d s y s t e m s ; Eh = 0.8 - 0.06pH. ( e , f ) Water c o n t a i n i n g F e ( I I ) ; Eh = 0.2 - 0.06pH. 2


PLUTONIUM CHEMISTRY

286


The e x i s t e n c e o f p l u t o n i u m w i t h an o x i d a t i o n s t a t e o f V ( o r V I ) i n n e u t r a l s o l u t i o n s o r a t h i g h pH and i n t h e p r e s e n c e o f c a r b o n a t e was p r e v i o u s l y o b s e r v e d ( 5 1 ) . I t has a l s o b e e n s u g g e s t e d t h a t Pu(V) i s t h e d o m i n a n t o x i d a t i o n s t a t e i n s e a - w a t e r and t h a t P u ( V I ) i s r a p i d l y r e d u c e d t o Pu(V) i n t h e s e w a t e r s ( 5 2 ) . However, i t has b e e n c o n c l u d e d f r o m s o r p t i o n and d i f f u s i o n experiments t h a t p l u t o n i u m e x i s t s l a r g e l y i n the t e t r a v a l e n t s t a t e (53) and c l e a r l y n o t as P u ( V ) , i n t h e i n t e r m e d i a t e pH-range u n d e r o x i c c o n d i t i o n s and a t l o w c a r b o n a t e c o n c e n t r a t i o n . T h i s w o u l d be r e p r e s e n t a t i v e o f many g r o u n d w a t e r s and a l s o i n agreement w i t h t h e c a l c u l a t e d c u r v e s o f F i g u r e 2. The s t r o n g p l u t o n i u m c o m p l e x i n g a b i l i t y o f humic a c i d s i s i l l u s t r a t e d by t h e u p t a k e o f p l u t o n i u m on Oak R i d g e s o i l , f r o m w h i c h i t c o u l d n o t be e l i m i n a t e d e i t h e r by l e a c h i n g w i t h m i l d a c i d s o r s t r o n g c o m p l e x f o r m e r s l i k e c i t r a t e o r EDTA ( 1 4 ) . However, c o n s i d e r i n g t h e r e p o r t e d c o n c e n t r a t i o n r a n g e s o f humic a c i d i n n a t u r a l w a t e r s and t h e i r p k - v a l u e s ( s e e above) and a s s u m i n g a l o g 3 i ~ v a l u e o f

Behavior of Plutonium in Natural Waters - ACS Publications

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