Complexation of the Plutonium(IV) Ion in Carbonate-Bicarbonate

21 Complexation of the Plutonium(IV) Ion in Carbonate-Bicarbonate Solutions J. I. KIM, CH. LIERSE, and F. BAUMGARTNER

Downloaded by UNIV LAVAL on October 23, 2015 | http://pubs.acs.org Publication Date: May 19, 1983 | doi: 10.1021/bk-1983-0216.ch021

Institut für Radiochemie, TU München,8046Garching, Federal Republic of Germany

The complexation of Pu(IV) with carbonate ions is investigated by solubility measurements of PuO in neutral to alkaline solutions containing sodium carbonate and bicarbonate. The total concentration of carbonate ions and pH are varied at the constant ionic strength (I = 1.0), in which the initial pH values are adjusted by altering the ratio of carbonate to bicarbonate ions. The oxidation state of dissolved species in equilibrium solutions are determined by absorption spectrophotometry and different i a l pulse polarography. The most stable oxidation state of Pu in carbonate solutions is found to be Pu(IV), which is present as hydroxocarbonate or carbonate species. The formation constants of these complexes are calculated on the basis of solubility data which are determined to be a function of two variable parameters: the carbonate concentration and pH. The hydrolysis reactions of Pu(IV) in the present experimental system assessed by using the literature data are taken into account for calculation of the carbonate complexation. 238

2

With growing i n t e r e s t i n the c h e m i c a l behaviour o f a c t i n i d e i o n s i n the environment ( 1 ) , the complexation o f these i o n s with c a r b o n a t e a n i o n s h a s been r e c e n t l y a t t r a c t i n g p a r t i c u l a r a t t e n t i o n ( 2 - 1 0 ) due t o t h e u b i q u i t o u s p r e s e n c e o f c a r b o n a t e i o n s i n n a t u r e ( 1 1 , 12) a n d t h e i r p r o n o u n c e d t e n d e n c y t o f o r m c o m p l e x e s w i t h heavy m e t a l i o n s (7^, 1 0 - 1 4 ) . In s p i t e o f t h e c a r b o n a t e c o m p l e x a t i o n of a c t i n i d e s being considered important chemical reactions for understanding the chemistry of a c t i n i d e s i n n a t u r a l f l u i d s , n o t many e x p e r i m e n t s have been d e v o t e d up t o now t o t h e q u a n t i t a t i v e s t u d y o f t h e s u b j e c t , t h o u g h numerous q u a l i t a t i v e o b s e r v a t i o n s a r e d i s c u s s e d i n t h e l i t e r a t u r e . A l t h o u g h t h e r e a r e a few papers r e p o r t i n g the formation c o n s t a n t s o f carbonate complexes 0097-6156/ 83/0216-0317$06.00/0

© 1983 American Chemical Society

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

318

PLUTONIUM CHEMISTRY

o f P u ( I I I ) and P u ( V I ) ( 1 5 , 1 6 ) , i n a c r i t i c a l c o m p i l a t i o n o f s t a b i l i t y c o n s t a n t s ( 1 3 ) none o f t h e s e d a t a h a s b e e n c o n s i d e r e d w o r t h y o f i n c o r p o r a t i o n . To o u r p r e s e n t k n o w l e d g e no r e c o r d e d d a t a have been known f o r t h e c a r b o n a t e c o m p l e x a t i o n o f P u ( I V ) i n t h e l i t e r a t u r e e x c e p t one f o r t h e f o r m a t i o n o f PuCO* species (17). The s t u d y o f c a r b o n a t e c o m p l e x e s o f Pu i s c o m p l i c a t e d by v a r i o u s e x p e r i m e n t a l d i f f i c u l t i e s . The low s o l u b i l i t y o f many c a r b o n a t e s (7), l e a v i n g a v e r y d i l u t e Pu c o n c e n t r a t i o n i n s o l u t i o n , r e s u l t s i n d i f f i c u l t i e s to the experiments with e l e c t r o c h e m i c a l o r s p e c t r o p h o t o m e t r y methods. However, t h e r a d i o m e t r i c method w i t h s o l v e n t e x t r a c t i o n o r s o l u b i l i t y measurement i s e a s i l y a p p l i c a b l e f o r the purpose. U n l i k e the s o l u t i o n with anions, l i k e CI"*, N 0 " e t c . , t h e c o n c e n t r a t i o n o f w h i c h c a n b e v a r i e d at a constant pH, t h e p r e p a r a t i o n o f s o l u t i o n s w i t h v a r y i n g c a r b o n a t e c o n c e n t r a t i o n a c c o m p a n i e s i n d i s p e n s a b l y t h e c h a n g e o f pH o f t h e s o l u t i o n . As a r e s u l t , t h e f o r m a t i o n o f c a r b o n a t e c o m p l e x e s i n v o l v e s a c c o r d i n g l y t h e h y d r o l y s i s r e a c t i o n s o f Pu i o n s i n s o l u t i o n s u n d e r i n v e s t i g a t i o n . I t i s t h e r e f o r e p r e r e q u i s i t e t o know the s t a b i l i t y constants o f Pu(lV) hydroxides p r i o r to the study of i t s carbonate complexation. The p r e s e n t s t u d y i s c o n d u c t e d u n d e r c o n s i d e r a t i o n o f t h u s m e n t i o n e d d i f f i c u l t i e s . The s o l u b i l i t y measurement i s a p p l i e d t o t h e p r e s e n t i n v e s t i g a t i o n , s e l e c t i n g t h e pH r a n g e 6 ^ 12 i n w h i c h t h e c a r b o n a t e c o n c e n t r a t i o n c a n be m a i n t a i n e d g r e a t e r t h a n 5xl0~* M / l . The c a r b o n a t e c o n c e n t r a t i o n a n d pH o f e x p e r i m e n t a l s o l u t i o n s , both b e i n g m u t u a l l y dependent i n a g i v e n s o l u t i o n , a r e taken i n t o a c c o u n t a s two v a r i a b l e p a r a m e t e r s i n t h e p r e s e n t e x p e r i m e n t a n d h e n c e t h e f i n a l e v a l u a t i o n o f f o r m a t i o n c o n s t a n t s i s b a s e d on three dimensional f u n c t i o n s . For c a l c u l a t i o n purpose, the hydrol y s i s c o n s t a n t s o f Pu(IV) a r e t a k e n from t h e l i t e r a t u r e ( 1 8 ) . In o r d e r t o d i f f e r e n t i a t e t h e i n f l u e n c e o f h y d r o l y s i s r e a c t i o n s on the c a r b o n a t e complexation so f a r as p o s s i b l e , t h e c a l c u l a t i o n i s b a s e d on t h e s o l u b i l i t i e s f r o m s o l u t i o n s o f c a r b o n a t e c o n c e n tration > lCf M / l a n d pH > 8.


3

6

Experimental The Pu0 i s p r e p a r e d by p r e c i p i t a t i o n o f P u ( I V ) h y d r o x i d e f r o m n i t r i c a c i d s o l u t i o n w i t h a n a d d i t i o n o f c o n e . NhÔH, w h i c h i s f o l l o w e d by w a s h i n g , f i l t r a t i o n , d r y i n g and c a l c i n a t i o n o f t h e h y d r o x i d e f o r 6 h r s a t 700 ° C . A t t h e t i m e o f p r e p a r a t i o n , the Pu0 i s f o u n d t o be i n c r y s t a l l i n e s t a t e w i t h t h e l a t t i c e c o n s t a n t o f 5.399 A a n d t h e a v e r a g e c r y s t a l s i z e o f 225 A . The i s o t o p i c c o m p o s i t i o n o f p l u t o n i u m i s known t o be 9 4 . 8 4 % Pu, 4.89 % Pu,.0.24 % P u , 0 . 0 2 4 % * P u , 0 . 0 0 4 % ?u and 6 . 2 ppm Pu. 2 3 8

2

2 3 8

2

0

2 3 8

2 3 9

2 I f 0

2 l

x

lhl

2 3 6

A b o u t 5 mg o f t h u s p r e p a r e d Pu0 i s i n t r o d u c e d i n each e x p e r i m e n t a l s o l u t i o n o f 10 m l , w h i c h i s p r e p a r e d w i t h v a r y i n g t h e c o n c e n t r a t i o n o f c a r b o n a t e i o n a n d pH by c h a n g i n g t h e N a H C 0 / Na C0 r a t i o a t t h e c o n s t a n t i o n i c s t r e n g t h (I = 1.0) a d j u s t e d 2 3 8

2

3

2

3


21.

Pu(IV) Ion in Carbonate-Bicarbonate Solutions 319

KIM ET A L .


by a d d i t i o n o f N a C l û \ . A fevi/ s o l u t i o n s p r e p a r e d w i t h h i g h c a r b o n a t e c o n c e n t r a t i o n , up t o 1 M / l , a r e a l s o i n c l u d e d . A l l s a m p l e s a r e p l a c e d i n g l a s s v i a l s vi/ith t i g h t l y c l o s e d c o v e r and a l l o w e d t o stand f o r a long p e r i o d o f time with o c c a s i o n a l s h a k i n g . Measu r e m e n t s a r e made a f t e r 4 , 9 and 12 months c o n t a c t t i m e . The s a m p l i n g o f s o l u t i o n f o r a c t i v i t y measurement i s c a r r i e d o u t by f i l t r a t i o n w i t h 0 . 2 2 μπι M i l l e x f i l t e r ( M i l l i p o r e C o . ) w h i c h i s e n c a p s u l e d a n d a t t a c h e d t o a s y r i n g e f o r handy o p e r a t i o n . The randomly s e l e c t e d f i l t r a t e s a r e f u r t h e r p a s s e d t h r o u g h Amicon C e n t r i f l o membrane f i l t e r ( C F - 2 5 ) o f 2 nm p o r e s i z e . The a c t i v i t i e s m e a s u r e d f o r t h e f i l t r a t e s f r o m t h e two d i f f e r e n t p o r e s i z e s a r e o b s e r v e d t o be i d e n t i c a l w i t h i n e x p e r i m e n t a l e r r o r . A c t i v i t i e s a r e m e a s u r e d by a l i q u i d s c i n t i l l a t i o n c o u n t e r . F o r e a c h s a m p l e s o l u t i o n , t r i p l i c a t e s a m p l i n g s and a c t i v i t y measurements a r e u n d e r t a k e n and t h e a v e r a g e o f t h r e e v a l u e s i s u s e d f o r c a l c u l a t i o n . A b s o r p t i o n s p e c t r a o f e x p e r i m e n t a l s o l u t i o n s a r e measured u s i n g a Beckman UV 5260 s p e c t r o p h o t o m e t e r f o r t h e a n a l y s i s o f o x i d a t i o n s t a t e s o f d i s s o l v e d Pu i o n s . T h i s a n a l y s i s i s f u r t h e r s u p p o r t e d by t h e measurements w i t h d i f f e r e n t i a l p u l s e p o l a r o g r a p h y ( P o l a r e c o r d Ε 5 0 6 , Metrohm C o . ) u s i n g a d r o p p i n g Hg e l e c t r o d e . To e x a m i n e t h e most s t a b l e o x i d a t i o n s t a t e o f Pu i o n s i n c a r b o n a t e s o l u t i o n s , t h e e x p e r i m e n t i s c o n d u c t e d by c o m b i n i n g an e l e c t r o c h e m i c a l r e a c t i o n c e l l t o t h e a b s o r p t i o n s p e c t r o p h o t o m e t r y . The Pu s o l u t i o n o f d i f f e r e n t o x i dation states prepared electrochemically i s continuously c i r c u l a t e d b e t w e e n t h e r e a c t i o n c e l l and t h e c u v e t t e i n t h e s p e c t r o p h o t o m e t e r . The o r i g i n a l s o l u t i o n o f 0.1 M HClOi, c o n t a i n i n g 10"~ M / l Pu i s t i t r a t e d w i t h N a C 0 s o l u t i o n and t h e c h a n g e i n o x i d a t i o n s t a t e s i s o b s e r v e d w i t h g r a d u a l l y v a r y i n g t h e pH and c a r bonate c o n c e n t r a t i o n . 3

2

3

At the time o f sampling, the t o t a l c o n c e n t r a t i o n o f carbonate i o n s i n e a c h s o l u t i o n and i t s pH a r e d e t e r m i n e d u s i n g a C 0 - e l e c trode (Orion Research C o - T y p e 9 5 - 0 2 ) and a g l a s s e l e c t r o d e , r e s p e c t i v e l y . B a s e d on t h e s e r e s u l t s , t h e c o n c e n t r a t i o n s o f f r e e b i c a r b o n a t e and c a r b o n a t e i o n s i n e a c h s o l u t i o n a r e c a l c u l a t e d by t a k i n g a c c o u n t o f d e g r e e s o f d i s s o c i a t i o n o f N a H C 0 , N a C 0 , H C0 and H C 0 , w h i c h a r e t a k e n f r o m t h e l i t e r a t u r e ( 1 5 , 1 8 , 19) or assessed through i n t e r p o l a t i o n or e x t r a p o l a t i o n o f r e l e v a n t l i t e r a t u r e data (20, 21). 2

3

2

3

Results

2

3

3

and

discussion

Hydrolysis reactions. As t h e s y s t e m u n d e r i n v e s t i g a t i o n c o n t a i n s not o n l y c a r b o n a t e i o n s but a l s o h y d r o x i d e i o n s o f c o n s i d e r a b l e c o n c e n t r a t i o n , i t i s q u i t e p l a u s i b l e that the r e a c t i o n s o f h y d r o l y s i s a n d c a r b o n a t e c o m p l e x f o r m a t i o n compete w i t h e a c h o t h e r . S i n c e the h y d r o l y s i s r e a c t i o n i s not i n v e s t i g a t e d s e p a r a t e l y i n t h i s e x p e r i m e n t , t h e magnitude o f t h i s r e a c t i o n as a f u n c t i o n o f pH i s e v a l u a t e d on t h e b a s i s o f t h e f o r m a t i o n c o n s t a n t s a v a i l a b l e i n the l i t e r a t u r e ( 1 8 ) , which are reproduced


320

PLUTONIUM

CHEMISTRY

i n T a b l e I, t o g e t h e r w i t h t h e s o l u b i l i t y p r o d u c t o f P u 0 taken f r o m t h e same l i t e r a t u r e . The d i s s o l v e d c o n c e n t r a t i o n o f P l u t o n i u m , [ P u ] s , due t o h y d r o l y s i s r e a c t i o n i s t h e sum o f a l l h y d r o xide species: 2

[Pu]s

= [Ρυ* ] +

+ [Pu(0H)

3 +

]

+

+ [PuCOH). *- ] 1

(1)

1

wjîch c a n be w r i t t e n i n t e r m s o f t h e s o l u b i l i t y p r o d u c t o f P u 0 K and f o r m a t i o n c o n s t a n t o f e a c h h y d r o x i d e s p e c i e s β ^ , s u c h a s : 2

s d

Pu0

+ 2H 0

2

1

t Pu ** +

2

P u * * + OH"

t Pu(0H)

Pu

t PuiOH) ?"


1

+ iOH"

Î +

40H"

K sp 3

3 +

1

H

U

3u*

1

1

[Pu^HOrT]

(2)

[PuCOHi^/CPu^lCOH"]

(3)

[Pu(0H)^ ]/[Pu i

l t +

][0H-]

i

Hi

and h e n c e K [

P

u

]

8

H

=

l_un J

5 ( Χ Ι β

[OH"] ) 1

Η

(4)

lrU

B a s e d on E q u a t i o n 4 , i t i s p o s s i b l e t o e v a l u a t e t h e d i s s o l v e d c o n c e n t r a t i o n o f Pu a s a f u n c t i o n o f pH, p r o v i d e d p o l y m e r and o t h e r complex s p e c i e s a r e not p r e s e n t . However, t h e p o l y m e r i z a t i o n o f h y d r o l y s e d s p e c i e s e n h a n c e s t h e s o l u b i l i t y o f P u 0 and h e n c e t h e d i s s o l v e d Pu c o n c e n t r a t i o n i s e x p e c t e d t o be g r e a t e r t h a n t h e q u a n t i t y c a l c u l a t e d by E q u a t i o n 4 . On t h e o t h e r h a n d , t h e p r e s e n c e o f a s t r o n g c o m p l e x i n g a n i o n , e . g . c a r b o n a t e i o n , may 2

Table

I.

Species:

Pu(0H)

The f o r m a t i o n c o n s t a n t s

Pu(0H)

of

Pu(IV)

i

l

0

3

24.6

+

34.7

Pu(0H)g

44.3

Pu(0H)I 2

52.7 -

log y

a) b) c)

a

HiCo

Pu(0H)f

Pu0 (s)

(18)

12,2

3 +

Pu(0H)!

9

hydroxides

c a l c u l a t e d b a s e d on t h e v a l u e s symbol used i n t h i s paper ( c f . the v a l u e taken as the average

K

H

= 52.0

c

)

sp

i n T a b l e s 4 . 4 and 9 . 9 E q u a t i o n 14) o f t h o s e g i v e n i n (7)

in and


(18) (18)

21.

K i M ET AL.

Pu(IV)

Ion

in Carbonate-Bicarbonate

321

Solutions

s u p p r e s s t h e p o l y m e r i z a t i o n and h i g h e r d e g r e e o f h y d r o l y s i s r e a c t i o n s through formation o f carbonate or hydroxocarbonate s p e c i e s o f the Pu(IV) i o n . By t h e a i d o f t h e l i t e r a t u r e d a t a o f h y d r o l y s i s c o n s t a n t s , t h e a b s o l u t e and r e l a t i v e amounts o f e a c h h y d r o x i d e s p e c i e s o f P u ( I V ) a s a f u n c t i o n o f pH a r e c a l c u l a t e d and g i v e n i n F i g u r e s l a and l b . I t i s shown i n t h i s f i g u r e t h a t i n t h e r e g i o n pH > 8 o n l y t h e P u ( 0 H ) i a p p e a r s i n t h e s o l u t i o n . By a d d i n g t h e c a r b o n a t e i o n o f i n c r e a s i n g c o n c e n t r a t i o n and t h u s i n c r e a s i n g pH, i t i s e x p e c t e d t h a t the Pu(0H)s i o n i s t r a n s f o r m e d t o d i f f e r e n t c a r b o n a t e com p l e x e s and a t t h e same t i m e t h e s o l u b i l i t y p a t t e r n o f P u 0 changes.


2

Complexation o f Pu(IV) i o n with carbonate a n i o n . The s o l u b i l i t i e s o f P u 0 a r e d e t e r m i n e d t o be v a r y i n g w i t h r e s p e c t t o t h e c a r b o n a t e c o n c e n t r a t i o n a s w e l l a s pH s i m u l t a n e o u s l y . The d i s s o l v e d c o n c e n t r a t i o n s o f Pu d e t e r m i n e d a f t e r 4 and 12 months c o n t a c t t i m e a r e g i v e n i n F i g u r e 2 a s a f u n c t i o n o f pH, w h e r e a s t h e c o n c e n t r a t i o n s o f f r e e c a r b o n a t e and b i c a r b o n a t e i o n s d e t e r m i n e d i n e a c h e x p e r i m e n t a l s o l u t i o n a r e p r e s e n t e d i n F i g u r e 3 . The s o l u b i l i t i e s m e a s u r e d a f t e r 9 and 12 months a r e f o u n d t o be a l m o s t i d e n t i c a l . In F i g u r e 2, i t i s o b s e r v e d t h a t t h e s o l u b i l i t y o f P u 0 a t f i r s t d e c r e a s e s s l o w l y w i t h i n c r e a s i n g pH u n t i l pH = 8 . 5 , t h e n d e c r e a s e s r a p i d l y between pH = 8.5 ^ 1 0 . 0 , and f i n a l l y i n c r e a s e s s t e e p l y above pH = 1 0 . 0 . T h i s d e v e l o p m e n t o f s o l u b i l i t y w i t h i n c r e a s i n g pH does n o t r e f l e c t a monotonous f u n c t i o n w i t h s i n g l e v a r i a b l e as shown by E q u a t i o n 4 b u t i n f e r s t o a f u n c t i o n o f two v a r i a b l e p a r a m e t e r s , i . e . pH and c a r b o n a t e c o n c e n t r a t i o n . The minimum s o l u b i l i t y o b s e r v e d a t pH = 10 and [ C 0 " " ] = 3 χ 10*" M/l implies that there i s a transformation of s o l i d species (s) t a k i n g p l a c e , i . e . from P u 0 ( s ) t o P u ( 0 H ) C 0 ( s ) . The e x i s t e n c e o f s u c h a p r o c e s s c a n be i n d i r e c t l y e v i d e n c e d by t h e i r r e g u l a r change o f d e c r e m e n t o b s e r v e d i n f r e e c a r b o n a t e ( a l s o b i c a r b o n a t e ) c o n c e n t r a t i o n s n e a r pH = 10 a s i l l u s t r a t e d i n F i g u r e 3 . T h i s may be a s c r i b e d t o t h e c o n s u m p t i o n o f f r e e c a r bonate ions f o r the formation o f the plutonium hydroxocarbonate p r e c i p i t a t e . The r e a c t i o n s i n v o l v e d i n t h i s p r o c e s s a r e i n t e r p r e t e d as f o l l o w s : 2

2

3

3

2

Pu0 (s) 2

+ 2H 0 2

Pu *"*" + 20H~ + C O ? " 1

h+

t Pu

2

+ 40H~

+ Pu(0H) C0 (s) 2

. Pu0 (s) 2

log -log

.

+ 2H 0 + C O P 2

where l o g

3

3

Κ c a n be

+ Pu(0H) C0 2

X-52

3

K^ K

p

= -52

(5)

= X

(6)

H C

SE! + 20H"

log

.

Κ = X-52

< 0

F o r c o n v e n i e n c e o f i l l u s t r a t i o n and f u r t h e r e x p l a n a t i o n s t o be followed l a t e r , the f r e e P u i o n i s t a k e n a s an i n t e r m e d i a t e r e a c t a n t i n t h e f o r m u l a t i o n o f E q u a t i o n s 5 and 6, a l t h o u g h t h i s 4


(7) (8)

322

PLUTONIUM CHEMISTRY

3.0 (1): P u 1.8 "h2 \ \ ^

(2): P u ( 0 H )

-ι.


-10 H -5.0

(5)

-9.0 -14.0

(a) 4 +

3 +

(3):

Pu(0H)2

(4):

Pu(0H)3

(5):

Pu(0H)°

(6):

Pu(OH)-

(7):

T o t a l Pu d i s s o l v e > r

+

\

ι -12.0

Λ \ ι ν ί \ -10.0

ι

.

-8.0 log

[0H"]

ι

-6.0

,

ι

,

-4.0

ι

,

-2.0

0.0

-2.0

0.0

(M/L)

f p e e

(b)

The r e g i o n o f i n te r e s t

-L -14.0

-12.0

-10.0

-8.0 log

[0H"]

-6.0 f r e e

-4.0

(M/L)

Figure 1. The a b s o l u t e ( a ) a n d r e l a t i v e ( b ) amounts o f P u ( I V ) h y d r o x i d e i o n c o n c e n t r a t i o n ( o r p H ) , c a l c u l a t e d by E q u a t i o n 4 b a s e d on t h e d a t a g i v e n i n T a b l e I. The r e g i o n o f i n t e r e s t f o r t h e p r e s e n t i n v e s t i g a t i o n i s marked by d o t t e d l i n e s .




KIM ET AL.

j

-4

h

-5

h

-6

h

-7

h

(·):

after

4 months c o n t a c t

time

(χ):

a f t e r 12 months c o n t a c t

time

*

-8

I

-9 -

9

-

8

-

7

-

I

I

I

6

I

I ι ι ι -5

l 0

9

H

[0 "]

ι

1 -4

-3

-2

M/L

free

< )

Figure 2. The s o l u b i l i t i e s o f P u 0 ( c ) a s a f u n c t i o n o f t h e pH o f e q u i l i b r i u m s o l u t i o n s . Up t o pH = 10 t h e s o l u b i l i t i e s a f t e r 4 and 12 months a r e a l m o s t i d e n t i c a l , w h i l e a t pH > 10 t h e v a l u e s a f t e r 12 months d i f f e r f r o m t h o s e a f t e r 4 months w h i c h a r e n o t i n e q u i l i b r i u m . The v a l u e s a f t e r 12 months and 9 months ( n o t p l o t t e d h e r e ) a r e f o u n d t o be i d e n t i c a l . 2 3 8

2


324

PLUTONIUM CHEMISTRY


_

* * · •

free HCO, ion -

·

··

"«

•

2- s o r p t i o n

"

•·

CO,

» «

. .ν·· °

·

"

-"*· m * •

. \

. «1. 1 · «. •·

. •. .·

* * · ·*

··

·

J

Λ

;••

-6 "

" - f r e e CO3" ion

-8

*

"

-1

-10 5

6

1

1

1

1

1

7

8

9

10

11

12

PH

Figure 3. The c o n c e n t r a t i o n s determined at

of

free carbonate

solubility

and b i c a r b o n a t e

e q u i l i b r i u m as

D e c r e m e n t s o f t h e c o n c e n t r a t i o n n e a r pH = 10 formation

of

solubilities

t h e Pu(0H) C0 2

of

Pu0

2

(cf.

3

suggest

p r e c i p i t a t e and h e n c e

Figure

ions

a function of

pH. the

lowering

2).


21.


κ ί Μ ET AL.

i o n i s n o t v i a b l e s p e c i e s i n t h e s o l u t i o n o f pH > 8. The r e a c t i o n g i v e n by E q u a t i o n 7 c a n be f o r m u l a t e d a l t e r n a t i v e l y vi/ith more or l e s s r e a l i t y r e l e v a n t r e a c t i o n s : Pu0 (s) 2

t

2

Pu(0H)I

(9)

P u ( 0 H ) i + CO?"

+ Pu(0H) C0 (s)

+ 30H"

(10)

Pu0 (s)

*

+ 20H"

(11)

2


+ 2 H 0 + OH"

2

+ 2 H 0 + C01" 2

3

Pu(0H) C0 (s) 2

3

As i s a p p a r e n t i n F i g u r e s l a and l b , t h e P u ( 0 H ) i s p e c i e s i n p r e d o m i n a n t a t pH > 8 i n t h e a b s e n c e o f c a r b o n a t e i o n s . I t appears reasonable to expect that i n the presence of carbonate ions i n s o l u t i o n the Pu(0H)i i o n undergoes the f o r m a t i o n of the P u ( 0 H ) C 0 p r e c i p i t a t e , o f w h i c h t h e s o l u b i l i t y d e p e n d s on t h e c a r b o n a t e c o n c e n t r a t i o n as w e l l as pH. C o n s i d e r a t i o n o f P u ( 0 H ) C 0 as a l e a s t s o l u b l e s p e c i e s under the p r e s e n t e x p e r i m e n t a l system i s a t t r i b u t e d t o t h e s i m i l a r compound, i . e . T h O C 0 * n H 0 , w h i c h a p p e a r s s p a r ingly soluble i n a l k a l i n e carbonate s o l u t i o n s (22). Equation 8 may r e s u l t i n a p o s i t i v e o r n e g a t i v e v a l u e ; t h e f o r m e r i n d i c a t e s Pu(0H) C0 l e s s s o l u b l e than Pu0 while the l a t t e r s u g g e s t i n g the r e v e r s e . 2

2

3

2

3

3

3

2

2

The d i s s o l v e d s p e c i e s a r e c o n s i d e r e d t o be a c o m p o s i t e o f numerous h y d r o x i d e s , c a r b o n a t e s , and h y d r o x o c a r b o n a t e s o f P u ( I V ) , w h i c h c a n be e x p r e s s e d by [Pu]s

=

[Pu^ ]-r

[Pu(0H)

+

+[Pu(C0 ) 3

2 +

]+

3 +

+[Pu(0H) *~ ]+ 1

+[Pu(0H).(C0 ) " ]+

+

2

3

C0 )

2 j

3

3

i

i

[Pu(0H)(C0 ) ]+

+[Pu(C0 ) t" ]+[Pu(0H) J l

]+

2J

i

3

] + . . .+[Pu(0H) ( C O s ) ; - - ^ ] 1

1

(12) and i n a g e n e r a l

[Pu]s

, = [pu"*]

Γ J i-

form: _n ;>Z i=0

m

. β

j=0

Η

"

ί

Γ

ί

1

L

J

[OH"]

1

[C01"]

J

-1 ! :

/ -t -τ \ {

U

)

where

ο 3

HiCj

[Pu(0H).(C0,).] =

— - i

[Pu- ] +

J

r

[0H-] [C0i ] 1

-

(14)

J

The f r e e c o n c e n t r a t i o n o f P l u t o n i u m , [Pu *"*"], AS c o n t r o l l e d by either s o l u b i l i t y products, Κ f o r Pu0 or for Pu(0H) C0 , sp 1

2


2

3

326

PLUTONIUM CHEMISTRY

namely

K

[Pu" ]

=

+

[

P

u

i

+

]

=

HC S£

(15)

[0H"] [C0 "] 2

3

Which o f t h e two i s a p p l i c a b l e t o t h e p r e s e n t e x p e r i m e n t d e p e n d s m a i n l y on w h e t h e r E q u a t i o n 8 r e s u l t s i n a p o s i t i v e o r n e g a t i v e v a l u e . The s o l u b i l i t y c h a n g e s q u i t e d i f f e r e n t l y i n pH > 8 a n d r e a c h e s t h e minimum ( [ P u ] s = 2 χ l C f M / l ) a t pH = 10 and [ C 0 l ~ ] = 3 χ 1 0 " M / l . S i n c e t h e minimum i s much l o w e r t h a n t h e v a l u e a s s e s s e d by E q u a t i o n 4 ( c f . F i g u r e l a ) , i t i s p l a u s i b l e t h a t E q u a t i o n 8 becomes a p o s i t i v e v a l u e . As d e s c r i b e d a b o v e , i n pH < 8 t h e c a r b o n a t e c o n c e n t r a t i o n s a r e r e l a t i v e l y s m a l l , < 10 M / l , and i t i s t h e r e f o r e e x p e c t e d t h a t i n t h a t c o n d i t i o n d i f f e r e n t hydroxide s p e c i e s are predominant ( c f . F i g u r e l b ) . In pH > 8 where [ C 0 | ~ ] > 10" * M / l , t h e f o r m a t i o n o f h y d r o x o c a r b o n a t e and c a r b o n a t e c o m p l e x e s may t a k e p l a c e . F o r t h i s r e a s o n , e v a l u a t i o n o f t h e f i n a l r e s u l t s i s m a i n l y b a s e d on t h e e x p e r i m e n t a l d a t a f r o m t h e s o l u t i o n s o f pH > 8 and [ C O ^ ] > 10" * M / l . F o r t h e d e t e r m i n a t i o n o f 3^-Q - t h e c o m p u t a t i o n a l . s o l u t i o n o f e x p e r i m e n t a l r e s u l t s i s c a r r i e c h O u t by E q u a t i o n 1 3 . The BîCo v a l u e s f o r h y d r o l y s i s r e a c t i o n s a r e taken i n t o a c c o u n t as g i v e n i n T a b l e 1. The i and j v a l u e s a r e v a r i e d f r o m 0 t o 5 f o r t h e f o r m a t i o n o f h y d r o x o c a r b o n a t e c o m p l e x e s . The i n p u t p a r a m e t e r s f o r E q u a t i o n 1 3 : [ P u ] s , [ O H " ] ( o r p H ) , and [ C O " ] a r e a l l e x p e r i m e n t a l v a l u e s a f t e r 12 months c o n t a c t t i m e . 9


3

- l f

1

2

1

2

V e r i f i c a t i o n of Pu(IV) i o n i n s o l u t i o n . Whether t h e d i s s o l v e d Pu i o n s a r e t e t r a v a l e n t i s v e r i f i e d by t h e f o l l o w i n g e x p e r i m e n t s . The a b s o r p t i o n s p e c t r o p h o t o m e t r y o f t h e s o l u t i o n s w i t h h i g h e r c a r b o n a t e c o n c e n t r a t i o n (> 0.1 M / l ) , i n w h i c h t h e [ P u ] s v a l u e s a r e g r e a t e r t h a n 1 0 " M / l , shows t h a t t h e d i s s o l v e d s p e c i e s a r e P u ( I V ) i o n s . The i n d i r e c t c o n f i r m a t i o n o f a s t a b l e s p e c i e s o f Pu i o n s i n c a r b o n a t e s o l u t i o n s i s made by a g r a d u a l a d d i t i o n of Na C0 s o l u t i o n t o 0.1 M HClOif s o l u t i o n c o n t a i n i n g Pu i o n s o f d i f f e r e n t o x i d a t i o n s t a t e s . With i n c r e a s i n g the carbonate i o n c o n c e n t r a t i o n i n t h e s o l u t i o n , where pH b e i n g a l s o i n c r e a s e d a c c o r d i n g l y , i t i s o b s e r v e d t h a t the d i s t i n c t i v e a b s o r p t i o n peaks o f P u ( I I I ) , P u ( I V ) , Pu(VI) i n HCIO4 s o l u t i o n d i s a p p e a r g r a d u a l l y and the a b s o r p t i o n spectrum o f Pu(IV) i n c a r b o n a t e s o l u t i o n i s f i n a l l y observed. F i g u r e 4 manifests the t r a n s f e r process o f d i f f e r e n t o x i d a t i o n s t a t e s o f Pu i o n s f r o m t h e HClOt* s o l u t i o n t o P u ( I V ) i n t h e c a r b o n a t e s o l u t i o n . The s p e c t r o p h o t o m e t r y r e s u l t s a r e f u r t h e r s u p p o r t e d by t h e measurements w i t h d i f f e r e n t i a l p u l s e p o l a r o g r a p h y , w h i c h d e m o n s t r a t e s t h a t t h e most s t a b l e Pu o x i d a t i o n state i n carbonate solutions i s Pu(IV). 5

2

3

The l o w e s t s o l u b i l i t y o b s e r v e d i n o u r e x p e r i m e n t s i s 2 x l 0 " a t pH = 10 and [ C 0 ~ ] = 3 χ 1 0 ~ M / l . When t h e d i s s o l v e d s p e 9

M/l

3

3


Pu(IV) Ion in Carbonate-Bicarbonate Solutions


21. κίΜ ET AL.

Wavelength / nm

Figure 4. The a b s o r p t i o n tions.

Curve

Pu(IV), after

of

Pu(V)

and P u ( V I ) of Na C0

1

lution is

2

c u r v e 3:

(pH = 1 0 . 4 ; (1-5)

t h e Pu i o n s

The s t a r t i n g

addition

10" * M / l ) ; tion

spectra

1:

χ 10~

3

after [COi""] 5

ions

solution in

0.1

solution

in different containing M HClOt,;

(pH = 6 . 7 ;

further addition = 0.3

M/l).

of

The Pu

solu

Pu(III),

curve

2:

[C0|~] Na C0 2

= 3

so

concentra

M/l.


327

328

PLUTONIUM CHEMISTRY

c i e s may be o t h e r t h a n P u ( I V ) i o n , i t i s t o e x p e c t t h a t t h e s o l u b i l i t y u n d e r t h i s c o n d i t i o n must be h i g h e r t h a n t h e o b s e r v e d v a l u e , s i n c e the o x i d a t i o n o r r e d u c t i o n enhances the s o l u b i l i t y o f t h e l e a s t s o l u b l e P u ( I V ) compound. I t i s q u i t e w e l l known t h a t t h e t e t r a v a l e n t a c t i n i d e compounds a r e l e s s s o l u b l e t h a n t h e same c o m p l e x compounds o f o t h e r o x i d a t i o n s t a t e s . In t h e l i t e r a t u r e ( 3 ) , t h e p o s s i b l e f o r m a t i o n o f p l u t o n y l b i c a r b o n a t e complex i s d i s c u s s e d . In o r d e r t o v e r i f y w h e t h e r we a r e d e a l i n g w i t h b i c a r b o n a t e o r c a r b o n a t e c o m p l e x e s , t h e Pu(IW) s o l u t i o n s p r e p a r e d i n NaHC0 a n d N a C 0 s o l u t i o n s a r e examined by s p e c t r o p h o t o m e t r y . The a b s o r p t i o n s p e c t r a m e a s u r e d up t o 900 nm show no v i s i b l e d i f f e r e n c e f o r b o t h s o l u t i o n s . F o r t h i s r e a s o n i t i s b e l i e v e d t h a t t h e Pu(IV) i o n forms c a r b o n a t e complexes i r r e s p e c t i v e o f carbonate or bicarbonate ions present i n s o l u t i o n .


3

2

3

Formation constants o f carbonate complexes. The c o m p u t a t i o n a l t r e a t m e n t o f e x p e r i m e n t a l r e s u l t s by E q u a t i o n 13 i s made by an i t e r a t i o n p r o c e s s . F i g u r e s 5 a n d 6 d e m o n s t r a t e c o m p a r i s o n s o f computed v a l u e s w i t h e x p e r i m e n t a l d a t a a s a f u n c t i o n o f two d i f f e r e n t v a r i a b l e p a r a m e t e r s , [OH"] and [ C 0 § ~ ] . S i n c e t h e e x p e r i m e n t a l e r r o r i n t h e measurement o f c a r b o n a t e c o n c e n t r a t i o n i s r e l a t i v e l y l a r g e due t o s m a l l volume o f s o l u t i o n a v a i l a b l e t o u s e , a c e r t a i n e x t e n t o f d e v i a t i o n i s e x p e c t e d from t h e comparison o f computed v a l u e s w i t h e x p e r i m e n t a l d a t a . H o w e v e r , F i g u r e s 5 and 6 show t h a t r e a s o n a b l e a g r e e m e n t s between t h e two v a l u e s a r e a t t a i n e d a s a f u n c t i o n o f pH a s w e l l a s o f t h e c a r b o n a t e c o n c e n t r a t i o n . S i n c e e a c h s o l u b i l i t y p o i n t i s r e l a t e d w i t h t h e two p a r a m e t e r s s i m u l t a n e o u s l y , t h e computed p o i n t s do n o t f o l l o w smooth c u r v e s b u t s e n s i t i v e l y f l u c t u a t e i n a c c o r d a n c e w i t h t h e v a r i a t i o n o f the conjugate parameter. Because t h e carbonate c o n c e n t r a t i o n a n d pH a r e m u t u a l l y d e p e n d e n t i n a g i v e n s o l u t i o n and b e c a u s e t h e OH" a n d C 0 | " i o n s a r e c o m p e t i t i v e l i g a n d s f o r c o m p l e x a t i o n o f the Pu(IV) i o n , i t i s found necessary t o i n v e s t i g a t e t h e c a r b o n a t e c o m p l e x a t i o n o f P u ( I V ) by v a r y i n g t h e two p a r a m e ters simultaneously. The c a l c u l a t e d f o r m a t i o n c o n s t a n t s o f B u - p presented i n T a b l e 2. Among c a l c u l a t e d v a l u e s f o r p r o b a D i e complex s p e c i e s , t h e r e l a t i v e amounts o f d o m i n a n t a n d l e s s d o m i n a n t c o m p l e x s p e c i e s a r e d i s t i n g u i s h e d by a

'HaCb

r°H-] [C0!-] a

η 1 I

r

e

b

(a,b

m III

=

const.)

(16)

The r e l a t i v e amount o f d o m i n a n t s p e c i e s u n d e r t h e e x p e r i m e n t a l c o n d i t i o n a r e shown i n F i g u r e 7 . F i v e s p e c i e s a r e p r e d o m i n a n t i n the s o l u t i o n : Pu(0H)C0 , P u ( C 0 ) § , P u ( C 0 ) r , Pu(C0 )iT, and P u ( C 0 ) " . The s p e c i e s o t h e r t h a n t h o s e g i v e n h e r e a r e n o t p r e s e n t i n s i g n i f i c a n t amount. +

3

3

3

3

3

5


21. KiM ET AL.



M

H

(χ): experimental value ( · ) : calculated value

J -

6

-

I

5

I

I

-

fm

I

I

-

4

log [0H"]

-J

free

I

L_

_l

I

3

(M/L)

Fiqure 5. The c o m p a r i s o n o f c a l c u l a t e d s o l u b i l i t i e s by E q u a t i o n 13 vi/ith t h e v a l u e s f r o m e x p e r i m e n t (12 months c o n t a c t t i m e ) . The v a l u e s a r e p l o t t e d as a f u n c t i o n o f t h e c o n c e n t r a t i o n of free hydroxide ion at s o l u b i l i t y e q u i l i b r i a (cf. Figure 6).


l_

-2

330

PLUTONIUM CHEMISTRY


_

Μ

Κ

*• • • •

(χ): experimental value ( · ) : calculated value

Κ

-

•

··

:

-

: -

χ l

l

-5

l

l

i

l

· l

.

·

l

l

-4

l

J

1

-3 l

Figure 6.

l

o

g

1 L

-2 [ 3 ]free C0

_

(

M/L

i .-I

L

l i i ]

-1

>

The c o m p a r i s o n o f c a l c u l a t e d s o l u b i l i t i e s by E q u a t i o n 13 w i t h t h e v a l u e s f r o m e x p e r i m e n t (12 months c o n t a c t t i m e ) . The v a l u e s a r e p l o t t e d a s a f u n c t i o n o f t h e c o n c e n t r a t i o n o f free carbonate i o n at s o l u b i l i t y e q u i l i b r i a (cf. Figure 5 ) .


ι

0


KIM ET A L .


1.0

-

^ \

P

U

(

C

0

/»u(C0 )|-

) 2

3

3

Pu(C0 )^

/

3

-

\

XPU(C0 )\ 3

.

Pu(OH)(C0 )

+

3

1

0.0 -4.0

1

1

l.Tfc

1

log [ C 0 ^ ]

1 1

-1.1

-2.0

-10

1. 1

1 ^ — \

f r e e

0.0

(M/L)

Figure 7. The

relative

carbonate

amounts o f

species

of

predominant

Pu(IV)

in

the

hydroxocarbonate investigated

and

solutions.


332

PLUTONIUM CHEMISTRY

The complex formation constants obtained from t h i s experiment are very l a r g e i n comparison w i t h the complexation o f the Pu(IV) i o n w i t h other l i g a n d s , l i k e OH", F" e t c . In view o f s o l u b i l i t i e s observed i n higher carbonate c o n c e n t r a t i o n ([Pu]s ^ 10" M/l) i t i s considered p o s s i b l e t h a t the formation constants o f carbon ate complexes o f the Pu(IV) i o n must be r e l a t i v e l y l a r g e . The p o s t u l a t i o n made i n t h i s work as f o r the formation o f the very i n s o l u b l e Pu(0H) C0 under the present experimental c o n d i t i o n i s p r i m a r i l y based on the e a r l i e r works (22) on ThOC0 «nH 0, which i s found t o be not much s o l u b l e i n a l k a l i n e s o l u t i o n . However, the experimental v e r i f i c a t i o n o f the p o s t u l a t i o n i s s t i l l i n pro gress f o r the moment. The r a d i o l y s i s e f f e c t due t o the P u nu c l i d e used i n the present experiment i s not i n v e s t i g a t e d sepa r a t e l y . The recent paper (23) d e a l i n g w i t h the s o l u b i l i t y o f P u 0 i n pure water d i s c u s s e s the change o f pH due t o the r a d i o l y s i s e f f e c t , i . e . decrease o f pH on account o f the formation of HN0 i n the s o l u t i o n open t o a i r and hence i n c r e a s i n g the so l u b i l i t y o f P u 0 . In our separate experiment made i n a system c l o s e d t o a i r c o n t a c t , the s o l u b i l i t i e s o f P u 0 a f t e r 200 days i n water w i t h v a r y i n g pH = 5 'v* 9 are observed t o be i d e n t i c a l w i t h the values reported i n the l i t e r a t u r e f o r Pu0 •nHzOiam) (24) which were measured a f t e r 250 days. From experience, i t i s b e l i e v e d t h a t the r a d i o l y s i s e f f e c t can be o f two k i n d s : the d i r e c t e f f e c t which changes the c r y s t a l l i n e s t r u c t u r e o f the oxide and thus a l t e r s the k i n e t i c s o f d i s s o l u t i o n ; the i n d i r e c t e f f e c t which changes the s o l u t i o n and components t h e r e i n , e.g. producing HN0 i n case o f the s o l u t i o n being i n contact with a i r , and hence l e a d i n g t o a d i f f e r e n t s t a t e o f e q u i l i b r i u m , namely higher s o l u b i l i t y . However, the thus i n c r e a s e d s o l u b i l i t y f o l l o w s the e q u i 3

2

3

3

2


2 3 8

2 3 8

2

3

2 3 8

2

2 3 8

2

239

2

3

Table I I . The formation constants o f Pu(IV) carbonates and hydroxocarbonates log 3

Species: P u ( 0 H ) ( C 0 ) . i

PuC0

(

3

2+

47.1

+

3.0

Pu(C0 )°

55.0 ± 2.5

Pu(C0 )I"

57.9

+

2.7

59.6

+

2.9

62.4

± 2.4

3

3

Pu(C0 )ÎT 3

Pu(C0 )|" 3

55.2 ± 2.3

Pu(0H)C0Î

- log K r 68.8 ± 2.4 sp aT symbol used i n t h i s paper ( c f . Equation 13) H C

Pu(0H) C0 (s) 2

3

y


21. KIM ET AL.



librium function with respect to pH, unless other complexing agents are present in solution. In a carbonate solution, the ra diation effect changes pH and therewith the carbonate concentra tion, and the Pu02 solubility varies accordingly. The solubility function shown in Figure 2 is not altered after 9 months, not after 4 months at :pH < 10, although individual experimental points are shifted to other positions in accordance with new val ues of pH and the carbonate concentration determined. The primary mechanisms of radiolysis effect on the solubility of Pu02 are, however, an important subject for separate investigation. Similar experiments with 2 3 9 Pu0 2 are in progress in our la boratory and will be used to verify or to improve the preliminary results presented in this paper. Acknowledgments The authors are much indebted to Dr. F. X. Koppold for the computer calculation. Thanks are due to Mr. Y. X. Xia for his help in measuring the absorption spectra and the technical assist ance of Miss Ch. Rahner throughout this work is acknowledged. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

Hanson, W. C., Ed.; "Transuranic Elements in the Environ ment"; DOE/TIC-22800, 1980. Billon, Α., Ed.; "Techniques for Identifying Transuranic Speciation in Aquatic Environments"; STI/PUB/613, IAEA: Vienna, 1981; p 65. Sullivan, J. C.; Bertrand, P. Α.; Choppin, G. R. Radiochim. Acta, in press. Wood, M.; Mitchell, M. L.; Sullivan, J. C. Inorg. Nucl. Chem. Lett. 1978, 14, 465. Rai, D.; Seme, R. J. J. Environ. Qual. 1977, 6, 89. Langmuir, D. Geochim. Cosmochim. Acta 1978, 42, 547. Edelstein, W. M., Ed.; "Actinides in Perspective"; Pergamon Press: New York, 1981; p 553. Bidoglio, G. Radiochem. Radioanal. Lett. 1982, 53, 45. Brückl, N.; L i , G. H.; Kim, J. I. Paper presented in the meeting of German Chemical Society, Nuclear Chemistry: Karlsruhe, Sep. 21-25, 1982. Jensen, B. S. RIS-Report on Critical review of available information on migration phenomena of radionuclides into the geosphere, 1981. Stumm, W.; Morgan, J. J., Eds.; "Aquatic Chemistry"; John Wiley & Sons: New York, 1981. Lindsay, W. L., Ed.; "Chemical Equilibria in Soils"; John Wiley & Sons: New York, 1979. Smith, R. M.; Martell, A. E . , Eds.; "Critical Stability Constants"; Plenum Press: New York, 1976.


334 14. 15. 16. 17.


18. 19. 20. 21. 22. 23. 24.

PLUTONIUM CHEMISTRY

Saltelli, Α.; Avogadro, Α.; Bertozzi, G. "Proc. Workshop on the Migration of Long-lived Radionuclides in the Geo sphere"; OECD/NEA: Paris, 1979; p 147. Sillen, L. G.; Martell, A. E . , Eds.; "Stability Constants of Metal Ion Complexes"; The Chemical Society: London, 1964. Cleveland, J. M., Ed.; "The Chemistry of Plutonium"; Gordon & Breach Science, 1970. Moskvin, A. I.; Gel!man, A. D. Russ. J. Inorq. Chem. 1958, 3, 962. Baes, C. F.; Mesmer, R. G., Eds.; "The Hydrolysis of Cations"; John Wiley & Sons: New York, 1976. Walker, J.; Cormack, W. J. Chem. Soc. 1900, 77, 11. Frary, F. C.; Nietz, A. H. J. Am. Soc. 1915, 37, 2271. Seyler, C. A.; Lloyd, P. V. J. Chem. Soc. 1917, 111, 138. Gmelin Handbuch der Anorg, Chem. "Thorium und Isotope"; Sys. Nr. 44, Verlag Chemie: Weinheim, 1955; p 300. Rai, D.; Ryan, L. Radiochim. Acta in press. Rai, D.; Serne, R.; Moore, D. A. Soil Sci. Soc. Am. J. 1980, 44, 490.

RECEIVED December 21,1982


Complexation of the Plutonium(IV) Ion in Carbonate-Bicarbonate

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