Aspects of Plutonium Solution Chemistry - ACS Publications

14 Aspects of Plutonium Solution Chemistry GREGORY R. CHOPPIN

Downloaded by NANYANG TECHNOLOGICAL UNIV on June 3, 2016 | http://pubs.acs.org Publication Date: May 19, 1983 | doi: 10.1021/bk-1983-0216.ch014

The Florida State University, Department of Chemistry, Tallahassee, FL 32306

The effect of pH and complexation on the relative stabilities of the oxidation states of Pu is discussed. A set of ionic radii are presented for Pu in different oxidation states and different coordination numbers. A model for Pu hydration is presented and the relation between hydrolysis and oxidation state evaluated, including the problem of hydrous polymerization. Complexation of Pu is discussed in terms of the relative stabilities of different oxidation states and the "effective" ionic charge of PuO and PuO . An equation is proposed for calculating stability constants of Pu complexes and its correlation with experimental values demonstrated. The competition between inner vs outer sphere complexation as affected by the oxidation state of Pu and the pKa of the ligand is reviewed. Two examples of uses of specific complexing agents for Pu indicate a useful direction for future studies. + 2

+2

2

The i n v e s t i g a t i o n o f p l u t o n i u m c h e m i s t r y i n aqueous s o l u t i o n s p r o v i d e s u n i q u e c h a l l e n g e s due i n l a r g e p a r t t o t h e f a c t t h a t p l u t o n i u m e x h i b i t s an u n u s u a l l y b r o a d r a n g e o f o x i d a t i o n s t a t e s from 3 t o 7-and i n many s y s t e m s s e v e r a l o f t h e s e o x i d a t i o n s t a t e s can c o e x i s t i n e q u i l i b r i u m . Following the normal pattern f o r polyvalent c a t i o n s , lower o x i d a t i o n s t a t e s o f plutonium a r e s t a b i l i z e d by more a c i d i c c o n d i t i o n s w h i l e h i g h e r o x i d a t i o n s t a t e s become more s t a b l e a s t h e b a s i c i t y i n c r e a s e s . As w i t h a l l s u c h g e n e r a l i z a t i o n s , t h i s o n e c a n be n e g a t e d by o t h e r f a c t o r s s u c h a s c o m p l e x i n g w h i c h c a n even r e v e r s e t h e trends and t h e r e l a t i v e s t a b i l i t y o f t h e d i f f e r e n t o x i d a t i o n states. F o r example, t h e g r e a t e r s t r e n g t h o f c o m p l e x a t i o n o f

0097-6156/83/0216-0213$06.00/0 © 1983 American Chemical Society

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


214

PLUTONIUM CHEMISTRY

Pu(IV) c a t i o n s r e l a t i v e to that of P u ( I I I ) i n c r e a s e s the s t a b i l i t y o f t h e ( I V ) s p e c i e s compared t o ( I I I ) . While P u ( I I I ) i s s t a b l e to a i r and warm w a t e r i n a c i d i c s o l u t i o n s , i n t h e p r e s e n c e o f a c e t a t e o r EDTA (pH 3.5), i t p a r t i a l l y o x i d i z e s t o P u ( I V ) ( 1 ) . This i n c r e a s e d s t a b i l i z a t i o n o f the IV s t a t e i s r e f l e c t e d i n the v a r i a t i o n i n t h e s t a n d a r d p o t e n t i a l (E°) f o r t h e ( I I I ) / ( I V ) c o u p l e w h i c h i s -0.98v i n 1M H C H ^ compared t o +0. 63v a t pH 7 (_2). The change i n t h e Ε ° v a l u e i s due t o t h e much more e x t e n s i v e h y d r o l y s i s a t pH 7 o f t h e P u ( I V ) compared t o P u ( I I I ) . Disproportionation r e a c t i o n s , which lead to several o x i d a t i o n states simultaneously i n s o l u t i o n , are also a s i g n i f i c a n t aspect o f p l u t o n i u m c h e m i s t r y , p a r t i c u l a r l y f o r t h e I V and V s p e c i e s . P l u t o n i u m c a t i o n s i n w h a t e v e r o x i d a t i o n s t a t e can be d e s c r i b e d a s h a r d a c i d s and i n t e r a c t w i t h a n i o n i c s p e c i e s by i o n i c bonding. As a r e s u l t c e r t a i n g e n e r a l i z a t i o n s can be made about the r e l a t i v e complexing t e n d e n c i e s of the d i f f e r e n t oxidation states. In t h i s r e v i e w the n a t u r e of the v a r i o u s o x i d a t i o n s p e c i e s i n s o l u t i o n a r e r e v i e w e d and g e n e r a l i z a t i o n s drawn a b o u t t h e i r com p l e x i n g behavior. Oxidation

States

P u ( I I I ) can be r a t h e r e a s i l y formed and m a i n t a i n e d i n a c i d s o l u t i o n s u s i n g a r e d u c i n g a g e n t s u c h as Ν Ηι* and NH 0H. Pu(IV) i s a l s o s t a b l e i n c o n c e n t r a t e d a c i d s o l u t i o n s but undergoes s t r o n g h y d r o l y s i s even a t r e l a t i v e l y l o w pH's and i s removed f r o m t h e s o l u t i o n p h a s e by a d s o r p t i o n t o s u s p e n d e d p a r t i c l e s and t o t h e w a l l s of the r e a c t i o n v e s s e l s . Pu0 i n c r e a s e s i n s t a b i l i t y as t h e pH o f an aqueous s o l u t i o n i s i n c r e a s e d t o a b o u t 7 a f t e r w h i c h t h e s t a b i l i t y again decreases. T h i s o x i d a t i o n s t a t e has a s t r o n g t e n d e n c y t o d i s p r o p o r t i o n a t e b u t t h e r e i s some e v i d e n c e t h a t i t may be t h e p r e d o m i n a n t p l u t o n i u m s p e c i e s a t u l t r a t r a c e c o n c e n t r a t i o n s i n s o l u t i o n and some n a t u r a l w a t e r s where t h e p r o b a b i l i t y o f two P u 0 i o n s i n t e r a c t i n g i s v e r y s m a l l ( 3 ) . Pu02 ^% ^ aqueous s o l u t i o n have a s t a b i l i t y between t h a t o f t h e UO2 and NpÛ2 species. The V I o x i d a t i o n s t a t e can be a c h i e v e d i n s o l u t i o n by t h e use o f o x i d a n t s s u c h as b r o m a t e and p e r s u l f a t e . P u ( V I I ) i s r e d u c e d r a p i d l y by w a t e r i n a c i d s o l u t i o n s b u t t h e r e d u c t i o n i s c o n s i d e r a b l y slower i n b a s i c s o l u t i o n . In such b a s i c s o l u t i o n s t h e P u ( V I I ) s p e c i e s seems t o be t r i n e g a t i v e , c o r r e s p o n d i n g most s i m p l y t o P u O s " , a l t h o u g h i t has been p r o p o s e d t h a t a more l i k e l y f o r m u l a t i o n i s P u 0 ( 0 H ) " ( 8 9) b a s e d on a model o f a s m a l l number o f t i g h t l y bound w a t e r s . A more c o m p l e t e d e s c r i p t i o n o f t h e h y d r a t i o n i s s u g g e s t e d by t h e c o r r e l a t i o n o f t h e e n t r o p y o f h y d r a t e d c a t i o n s w i t h t h e c a t i o n i c c h a r g e d e n s i t y Z / r , as shown i n F i g u r e 2 (10). The i n c r e a s i n g c u r v e o f S q f r o m N a ^ q ) t o P u ( q ) i n d i c a t e s t h a t the o r i e n t i n g e f f e c t o f t h e c a t i o n s on t h e h y d r a t i o n w a t e r s s t e a d i l y i n c r e a s e s w i t h the charge d e n s i t y . Perhaps a b e t t e r h y d r a t i o n m o d e l , t h e n , i s one o f a p r i m a r y l a y e r o f w a t e r m o l e c u l e s a b o u t t h e p l u t o n i u m c a t i o n s w i t h a weaker o r i e n t i n g e f f e c t e x t e n d i n g some d i s t a n c e i n t o t h e s o l v e n t beyond t h a t f i r s t l a y e r . F i g u r e 2 i n d i c a t e s t h a t the t o t a l h y d r a t i o n a l o r d e r i n g of P u i s about twice that of P u w h i l e t h a t of P u O j i s comparable to t h a t of Ca . An e s t i m a t e o f t h e e x t e n t o f t h e h y d r a t i o n a l o r d e r i n g by Pu can be o b t a i n e d f r o m a r e c e n t s t u d y o f G d which i n d i c a t e d t h a t t h e h y d r a t i o n e f f e c t e x t e n d s a b o u t 50A f r o m t h e G d c a t i o n (12). 1

2

2

2

a

a

a

+ i f

+ 3

2

+ 2

+ 3

+ 3

Q

+ 3



14.

CHOPPiN

217

Pu Solution Chemistry

Estimated

Table I Effective Ionic Radii

Coordination Number

+

f o r Plutonium

Oxidation III

IV

State V

VI

4

0.79

0.65

0.53

0.50

6

1.00

f

0.86

0.74

0.71

7

1.06

0.92

0.80

0.77

8

1.10

0.96

0.84

0.81

9

1.15

1.01

0.89

0.86

10

1.21

1.01

0.95

0.92

12

1.26

1.12

1.00

0.97

The

r a d i i a r e i n A ( = 10" cm) u n i t s

The

underlined values

8

a r e from r e f e r e n c e

6.



218

Figure

PLUTONIUM CHEMISTRY

2.

The c o r r e l a t i o n between -S° and Ζ / r ; f o r t h e r a d i u s , aq r , t h e C.N. = 6 v a l u e s Pu0

+ 2 2

a r e u s e d f o r N a ( l ) , C a ( l l ) and

, t h a t o f C.N. = 8 f o r P u ( I V ) ,

f o r P u ( l l l ) and t h a t o f C.N. = 4 values

t h a t o f C.N.

f o r PuO„ ζ

.

The

a r e f r o m r e f e r e n c e 11.


=_9 S aq

14.

219


CHOPPIN

A s i g n i f i c a n t aspect of plutonium h y d r a t i o n i s the i n c r e a s e w i t h pH o f h y d r o l y s i s r e a c t i o n s s u c h a s : + n

Pu


mH 0

=

>

Pu0

>

+ ( n

m )

+

Pu(OH) ~ + mH (1) m The o r d e r o f h y d r o l y s i s ( i . e . , o r d e r o f i n c r e a s i n g pH f o r o n s e t o f h y d r o l y s i s ) f o l l o w s the sequence Pu

+

2

2

Pu

3

>

Pu0

(2)

2

which d i f f e r s from t h a t i n F i g u r e 2 f o r the h y d r a t i o n e n t r o p i e s which i s , Pu

+ i +

>

Pu

+ 3

>

Pu02

2

>

Pu0

(3)

2

The d i f f e r e n c e i n t h e s e p a t t e r n s p r o b a b l y r e f l e c t s t h a t t h e hydrate e n t r o p i e s are r e l a t e d simply to the net p o s i t i v e charge on t h e c a t i o n i c s p e c i e s ( i . e . , +2 f o r P u 0 ) w h i l e t h e h y d r o l y s i s r e a c t i o n i s the r e s u l t o f i n t e r a c t i o n o f a water m o l e c u l e w i t h t h e m e t a l atom i t s e l f — i . e . , Pu i n P u 0 . If this i s a valid e x p l a n a t i o n , the h y d r o l y s i s o r d e r i n d i c a t e s t h a t t h e c h a r g e on Pu in Pu0 i s a c t u a l l y between +3 and +4 and p r o b a b l y a b o u t +3.3. The v a r i a t i o n o f t h e c o n c e n t r a t i o n o f t h e f r e e ( n o n - h y d r o l y z e d ) c a t i o n s w i t h pH i s shown f o r t h e o x i d a t i o n s t a t e s o f I I I t o V I i n F i g u r e 3. These c u r v e s a r e b a s e d on e s t i m a t e d v a l u e s o f t h e h y d r o l y s i s c o n s t a n t s but are o f s u f f i c i e n t accuracy to i n d i c a t e t h e pH v a l u e s a t w h i c h h y d r o l y s i s becomes s i g n i f i c a n t ( e . g . . -6-8 f o r P u , < 0 f o r P u \ 9-10 f o r P u 0 and 4-5 f o r P u O j * . Values o f the h y d r o l y s i s c o n s t a n t s are l i s t e d i n Table H a n d r e p r e s e n t the "best v a l u e s ( e x p e r i m e n t a l or estimated) from various laboratories. The s t u d y o f p l u t o n i u m h y d r o l y s i s i s c o m p l i c a t e d by t h e f o r m a t i o n o f o l i g o m e r s and p o l y m e r s once t h e s i m p l e m o n o n u c l e a r h y d r o l y s i s species s t a r t forming. The r e l a t i v e m o n o - o l i g o m e r c o n c e n t r a t i o n s a r e d e p e n d e n t on t h e p l u t o n i u m c o n c e n t r a t i o n - e.g. t h e r a t i o o f Pu p r e s e n t as ( P u 0 ) ( 0 H ) t o t h a t as P u 0 ( 0 H ) i s 200 f o r [PU3T = 0.1 M, 5.6 f o r ΙΟ"** M and 0.05 f o r ΙΟ" M. The h y d r o l y s i s o f P u can r e s u l t i n t h e f o r m a t i o n o f polymers which are r a t h e r i n t r a c t a b l e to r e v e r s a l to s i m p l e r species. G e n e r a l l y such p o l y m e r i z a t i o n r e q u i r e s [ P u ] ^ > 10~ M b u t , due t o t h e i r r e v e r s i b i l i t y , d i l u t i o n o f more c o n c e n t r a t e d h y d r o l y s i s s o l u t i o n s below t h i s v a l u e would not d e s t r o y the polymers. The r a t e o f p o l y m e r i z a t i o n has been found t o be t h i r d o r d e r i n Pu c o n c e n t r a t i o n s and has a v a l u e o f 5.4 X 1 0 ~ moles/hr a t 50°C and [ P u ] ζ 0.006 M, CHN0 ] 0.25 M ( 1 3 ) . Soon a f t e r f o r m a t i o n , such p o l y m e r s can be decomposed r e a d i l y t o s i m p l e s p e c i e s i n s o l u t i o n by a c i d i f i c a t i o n o r by o x i d a t i o n t o P u ( V l ) . However, as the p o l y m e r s age, t h e d e c o m p o s i t i o n p r o c e s s r e q u i r e s i n c r e a s i n g l y r i g o r o u s treatment. The r a t e o f such i r r e v e r s i b l e aging v a r i e s w i t h temperature, P u ( l V ) c o n c e n t r a t i o n , the nature o f 2

2

2

2

2

2

+ 3

+

2

1 1

2

2

2

+

2

2

-

8

+ i f

6

5

+ l f

2

T

3



220

PLUTONIUM CHEMISTRY

PH F i g u r e 3.

The e f f e c t o f h y d r o l y s i s ,

as a f u n c t i o n

o f pH, o n t h e

c o n c e n t r a t i o n o f h y d r a t e d Pu c a t i o n i c s p e c i e s .

The

i n i t i a l ( l o g [ P u ] a t pH = 0) c o n c e n t r a t i o n s a r e t h o s e a t pH > 8 w h i c h c o r r e s p o n d t o t h e k v a l u e s o f the Τ . . sp h y d r o x i d e p r e c i p i t a t e o f each s p e c i e s .



14.

221


CHOPPiN

Table I I H y d r o l y s i s Constants f o r Plutonium l o g *3.. = [ M . ( O H ) J ij

1

[HD-VM

J

Τ = 25°C, I = 1.0 H i C l o O , l o g *3n

M +

Pu 3

log *β

ί 2

1

l o g *El3

-7.8 (-7.84) *

-16.6

-25.9

-0.3 (-1.5)

-2.3 (-2.7)

-5.4 (-6.6)

P*

log -36.2

w

= 13.8 l o g *&22 -12.8

l o g *&35 -32

1

Pu

+ 4

Pu0 2 J

Pu0 1

+ o

+ 2 9

-9.7 (-10.8)

-9.2 (-9.9) -5.5 (-6.0)

-11.6

-5.1 (-8.59)

-14.5 (-22.1)

V a l u e i n ( ) a r e f r o m r e f e r e n c e _28, e x c e p t ( ) ^ w h i c h i s f r o m r e f e r e n c e 10 ( f o r Am 3 ) . The o t h e r v a l u e s a r e f r o m r e f e r e n c e 29.


222

PLUTONIUM CHEMISTRY

anions present i n s o l u t i o n , etc. A reasonable model of the aging involves i n i t i a l formation of aggregates with hydroxy b r i d g i n g which convert with time to s t r u c t u r e s with oxygen b r i d g i n g . The r e l a t i v e percentage of oxygen bridges presumably determines the r e l a t i v e inertness of the polymer. The polymers apparently increase i n aggregate s i z e as the pH increases. At pH 4, 99% i s s t i l l suspended i n s o l u t i o n a f t e r 6 days but at pH 5 t h i s has f a l l e n to 7.4% and at pH 6, to 0.1% (14).


Complexation In studies where d i f f e r e n t o x i d a t i o n states of plutonium have been complexed by the same l i g a n d , the sequence of complex ing strength most commonly observed i s that described f o r the hydrolysis reactions: i . e . : Pu

+if

>

Pu0

2 2

>

Pu

+ 3

>

PuO^

1

0

The sequence Pu > Pu i s expected from the i o n i c nature of the P u - - X i n t e r a c t i o n which should increase with η f o r constant m. The observation that the complexing strength of the PuOj cations i s greater than the P u cations can be understood by assuming that the c e n t r a l metal atom, Pu, i n the l i n e a r [0-Pu-0] c a t i o n has an e f f e c t i v e charge that i s greater than +3. I t i s t h i s e f f e c t i v e charge that determines the strength of complexing since the ligands bind i n the e q u a t o r i a l p o s i t i o n s about the l i n e a r Puo£ . Wadt (15) r e c e n t l y analyzed U 0 £ by the r e l a t i v i s t i c core p o t e n t i a l method and obtained a value of +2.4 for the atomic charge on the uranium. However, the e f f e c t i v e charge i n the e q u a t o r i a l plane would be expected to exceed t h i s value and an estimate of an e f f e c t i v e charge_ of +3.3(i 0,1) was obtained from a n a l y s i s of complexation by F anions {16). S i m i l a r e v a l u a t i o n of the s t a b i l i t y constants f o r N p 0 2 F and N P O 2 S O Î lead to an estimated e f f e c t i v e charge on the Np atom i n the l i n e a r species of 2.3 t 0.2 (17). E f f e c t i v e charges f o r Pu of +3.3 i n PuOj? and +2.3 i n P u O j ^ a r e consistent with the complexation s t a b i l i t y constants of these species. Measurement of the s t a b i l i t y constants of plutonium complexes i s hampered by d i f f i c u l t i e s of maintaining a p a r t i c u l a r o x i d a t i o n s t a t e . Formation of complexes of P u , except i n very a c i d s o l u t i o n s , i s accompanied and o f t e n obscured by complexation catalyzed o x i d a t i o n to Pu . Study of complexation of Pu is o f t e n confused by competition with h y d r o l y s i s above pH 1-2. Except i n t r a c e r l e v e l s o l u t i o n s where i t s concentration i s o f t e n d i f f i c u l t to a s c e r t a i n , PuOj i s normally present i n a mixture of Pu o x i d a t i o n states i n which that of PuOj i s much lower than PuOj and P u \ PuOj can be formed i n good y i e l d and maintained with a h o l d i n g oxidant (which may be a complexor i t s e l f ) . Conversely, PuOj may be reduced by many complexing agents, (18), leading to c a l c u l a t i o n of erroneous s t a b i l i t y constants i f the extent of r e d u c t i o n i s not taken i n t o account. Photochemical + n

m

2

+ 3

+2

2

2

1

+ 3

+i+

2

+

+I+

2

2


14.


CHOPPiN

223

r e d o x r e a c t i o n s c a n be a f u r t h e r c o m p l i c a t i o n i n p l u t o n i u m s y s t e m s (19). A c o m p i l a t i o n o f t h e r e p o r t e d s t a b i l i t y c o n s t a n t s i s a v a i l a b l e ( 2 0 ) and t h e p a u c i t y o f d a t a r e f l e c t s t h e d i f f i c u l t i e s i n studying plutonium complexation. D e s p i t e t h e problems o f d i r e c t e x p e r i m e n t a l e v a l u a t i o n o f plutonium s t a b i l i t y c o n s t a n t s , t h e y a r e needed i n m o d e l i n g o f the b e h a v i o r o f p l u t o n i u m i n r e p r o c e s s i n g systems i n waste r e p o s i t o r i e s and i n g e o l o g i c a l and e n v i r o n m e n t a l m e d i a . A c t i n i d e a n a l o g s such a s A m , T h , N p 0 and IK)!" c a n be u s e d w i t h c a u t i o n f o r p l u t o n i u m i n t h e c o r r e s p o n d i n g o x i d a t i o n s t a t e s and v a l u e s f o r s t a b i l i t y c o n s t a n t s o f t h e s e a n a l o g u e s a r e t o be f o u n d a l s o i n r e f e r e n c e 20. I t i s p o s s i b l e t o o b t a i n f a i r l y r e l i a b l e v a l u e s f o r many c o m p l e x e s o f 1:1 ( m e t a l : l i g a n d ) s t o i c h i o m e t r y by u s i n g a n e x t e n d e d e m p i r i c a l e q u a t i o n i n w h i c h the d i e l e c t r i c c o n s t a n t i s dependent on t h e c a t i o n i c c h a r g e ( 2 1 ) . F o r t h e c o m p l e x a t i o n r e a c t i o n : +3

+

2


2

Pu at

+ Z l

+ Χ"

2 2

= PuX

+ ( Z l

"

Z

2

>

(4)

1

25°C, t h e e q u a t i o n (AG i n k j - m ) i s : AG = -

3

(1.387 χ 1 0 ) Ζχ » Z

?

+

^

^

+

^

n

f

(

5

)

where D i s t h e d i e l e c t r i c c o n s t a n t , d i t h e i n t e r n u c l e a r c a t i o n a n i o n d i s t a n c e i n %( = π + r ) , Ζχ, Z , t h e i o n i c c h a r g e s . Also e

2

2

llnf

= -(0.30 Δ Ζ

2

2

+ 0.75) 1^ + 0.015 I

(6)

and ΔΖ

= Ζ

2z

2

2

- (Ζ Pu

PuX

2

- Ζ ) x

(7)

T h i s e q u a t i o n p r o v i d e s v a l u e s i n good agreement w i t h e x p e r i ment f o r i n n e r s p h e r e c o m p l e x a t i o n by b o t h i n o r g a n i c and o r g a n i c ligands. I n o r g a n i c l i g a n d s , t h e v a l u e o f Z i s assumed t o be p r o p o r t i o n a l t o the a c i d c o n s t a n t , p K , o f the bonding group (or to £ K f o r polydentate ligands). Figure 4 i s the r e l a t i o n between Z and E p K w h i c h was o b t a i n e d by u s i n g e q u a t i o n ( 5 ) with experimental free energies of protonation, holding a l l values constant e x c e p t Z ( 1 0 ) . These v a l u e s o f Z , i n t u r n , were u s e d w i t h e q u a t i o n ( 5 ) t o e s t i m a t e t h e AG o f c o m p l e x a t i o n o f A m ( l l l ) and N p ( l V ) by a m i n o c a r b o x y l a t e l i g a n d s , o f A m ( l l l ) and T h ( l V ) by a c e t a t e and t h e T h ( l V ) by m a l o n a t e . The c o m p a r i s o n o f e x p e r i m e n t a l and c a l c u l a t e d v a l u e s i s shown i n F i g u r e 5. The v a l u e u s e d f o r D was 57 f o r A m ( l l l ) and 50 f o r T h ( l V ) and N p ( l V ) ; f o r a l l t h e l i g a n d s , r = 1.55 A w h i l e r i was o b t a i n e d f r o m T a b l e I (CN = 8 ) . The c a l c u l a t e d and e x p e r i m e n t a l v a l u e s f o r U 0 with a c e t a t e and m a l o n a t e show s i m i l a r agreement when Ζ χ ( ϋ 0 ) = 3.2 and D = 55. F o r N p 0 M a l " when Ζχ(Νρθ£) = 2 . 3 and D = 65, t h e c a l c u l a t e d AG a g r e e d w i t h t h e e x p e r i m e n t a l v a l u e . 2

a

a n

2

a n

2

2

e

2

2

2

2

2

1

e

2

e



224 PLUTONIUM CHEMISTRY



14.

CHOPPiN

225


I50h

EDTA

HEDTA DCTA EDTA. • NTA HEDTA / ' t

100

*HIDA

3

NTA %

/

50| IDA /

x

e

/

Malonate

^x^cetate r ^-Acetate 50

A6 F i g u r e 5.

100 C0|C

150 1

(kJm- )

C o m p a r i s o n o f t h e AG c a l c u l a t e d by e q u a t i o n ( 5 ) a n d t h e AG f r o m e x p e r i m e n t : · = A m ( l l l ) ; χ = Th ( I V ) ; Q = Np(lV). The s o l i d l i n e r e p r e s e n t s A G ( c a l c ) AG ( e x p e r . ) .


226

PLUTONIUM CHEMISTRY

A l l o f t h e p o l y d e n t a t e l i g a n d s y s t e m s w h i c h showed agreement between e x p e r i m e n t a l and c a l c u l a t e d AG v a l u e s i n v o l v e d 5 o r 6 membered c h e l a t e r i n g s . The e x p e r i m e n t a l v a l u e s f o r c o m p l e x e s o f c h e l a t e r i n g s o f 7 o r more members were p r o g r e s s i v e l y l o w e r t h a n t h e c a l c u l a t e d v a l u e s as t h e r i n g s i z e i n c r e a s e d . By t h a t c r i t e r i o n , p o o r agreement between t h e c a l c u l a t e d and e x p e r i m e n t a l v a l u e f o r U0 NTA (5-membered r i n g s ) i n d i c a t e s t h a t t h e e x p e r i mental value i s probably i n c o r r e c t . U n f o r t u n a t e l y , f o r l i g a n d s o f s t r o n g a c i d s , t h i s e q u a t i o n may u n d e r e s t i m a t e t h e s t a b i l i t y c o n s t a n t as i t c a l c u l a t e s v a l u e s f o r i n n e r s p h e r e f o r m a t i o n o n l y . E i g e n ( 2 2 ) has p r o p o s e d t h a t t h e f o r m a t i o n o f c o m p l e x e s p r o c e e d s s e q u e n t i a l l y as f o l l o w s :


2

M

(aq)

+

X

(aq) ^

M

(

H

*

0

)

n

X

(aq) ^

Μ

(

Η

>°

)

Χ

(aq)

( 8 )

^(aq)

The f i r s t s t e p i s d i f f u s i o n c o n t r o l l e d w h i l e t h e s e c o n d r e p r e s e n t s t h e f o r m a t i o n o f an o u t e r s p h e r e c o m p l e x i n w h i c h the m e t a l i o n and t h e l i g a n d a r e s e p a r a t e d by a t l e a s t one m o l e c u l e t o w a t e r . I n t h e f i n a l s t e p , t h i s o u t e r s p h e r e c o m p l e x e j e c t s t h e w a t e r and forms an i n n e r s p h e r e c o m p l e x i n w h i c h t h e m e t a l and l i g a n d a r e d i r e c t l y a s s o c i a t e d . Some l i g a n d s c a n n o t d i s p l a c e t h e w a t e r and complexation a p p a r e n t l y terminates w i t h the f o r m a t i o n o f the o u t e r s p h e r e c o m p l e x . P l u t o n i u m c a t i o n s f o r m b o t h i n n e r and o u t e r s p h e r e c o m p l e x e s , d e p e n d i n g on t h e l i g a n d p K . For t r i v a l e n t plutonium, we can a s s i g n a p r e d o m i n a n t o u t e r s p h e r e c h a r a c t e r t o t h e h a l i d e , n i t r a t e , s u l f o n a t e and t r i c h l o r o a c e t a t e c o m p l e x e s and an i n n e r s p h e r e c h a r a c t e r t o t h e f l u o r i d e , i o d a t e , s u l f a t e and a c e t a t e c o m p l e x e s (.23). A s t u d y o f A m , T h and U 0 complexation by c h l o r o a c e t a t e l i g a n d s ( C l H 3 _ C C 0 , η = 0-3) ( 2 4 ) l e a d s to a s s i g n m e n t o f c o m p l e x a t i o n c h a r a c t e r f o r 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 p l u t o n i u m as g i v e n i n T a b l e I I I . a

+3

+lf

2

2

n

n

2

Some a t t e n t i o n has b e e n g i v e n t o t h e c o m p l e x a t i o n o f a c t i n i d e s by m a c r o c y l i c s and s i m i l a r m u l t i d e n t a t e l i g a n d s . G e n e r a l l y , t h e s t r e n g t h of the i n t e r a c t i o n of the m e t a l - f u n c t i o n a l group i s too weak t o d i s p l a c e t h e m e t a l - w a t e r bonds i f t h e f u n c t i o n a l g r o u p i s an e t h e r o r a p h e n o l . Amine g r o u p s a r e p r o t o n a t e d b e l o w pH 6 .to 8. p r e v e n t i n g Pu-N i n t e r a c t i o n t h r o u g h t h e r e p u l s i o n o f t h e p o s i t i v e l y c h a r g e d p l u t o n i u m and amine g r o u p s . As a r e s u l t , h y d r o l y s i s u s u a l l y c o m p l e t e s s u c c e s s f u l l y a t l o w e r pH's w i t h a c t i n i d e - a m i n e i n t e r a c t i o n . By c o n t r a s t , i n a m i n o p o l y c a r b o x y l a t e s , t h e amine g r o u p does b i n d t o p l u t o n i u m , i n c r e a s i n g t h e s t a b i l i t y o f t h e c o m p l e x a t i o n CtOi Raymond's g r o u p has s t u d i e d t h e c o m p l e x a t i o n o f Pu""** by l i n e a r and c y c l i c c a t e c h o y l a m i d e l i g a n d s i n w h i c h t h e p l u t o n i u m has CN = 8 t h r o u g h i n t e r a c t i o n w i t h p h e n o l a t e g r o u p s ( F i g u r e 6 ) . The l i g a n d s were d e s i g n e d t o be s p e c i f i c c o m p l e x i n g a g e n t s f o r Pu f r o m b i o l o g i c a l systems (25). Another s p e c i f i c complexing 1

+ t f


14.

227


CHOPPiN


Table I I I R e l a t i o n s h i p of Complexation o f P l u t o n i u m and L i g a n d Complexation

Cation Pu Pu

+3 +4

PuO

••+.2 (

F i g u r e 6.

Character pKa

Character

i/o < 1

2.0

< pKa > 2.0

i/o > 1

i/o < 1

1.0

< pKa > 1.0

i/o > 1

i/o < 1

1.7

< pKa > 1.7

i/o > 1

The

Pu

+4

t e t r a c a t e c h o y l a m i d e complex.



228

PLUTONIUM CHEMISTRY

F i g u r e 7.

f

f

N,N - d i h e p t y l - N,N , 6 , 6 - t e t r a m e t h y l - 4 , 8 d i o x a u n d e c a n e u s e d i n t h e PuO^ CWE.

••vss 150

pH

An Op U Np Pu U Np Pu U

log [An

F i g u r e 8.

θ| ] +

Response o f t h e a c t i n y l ( V l ) CWE t o c o n c e n t r a t i o n s + 2

+

2

of U 0 ( ) , NpO (- - ) and P u 0 pH 2 ( x ) , 3 ( ο , · , θ ) and 4 (π, Α, Δ ) . 2

+ 2 2

(···) a t



14. CHOPPIN


229

agent, N,Nf - diheptyl-N,N' 6,6-tetramethy1- 4,8-dioxaundecane diamide (Figure 7), for Uoj w a s shown to provide the basis for a uranyl specific ion electrode (26). We have used this ligand to develop a simple coated wire type electrode for PuOj2 which reacts rapidly and reproducibly (27). It has very good specificity for PuOj2 against a greater than 10 fold excess of +2 and +3 cations and a 100 fold excess of NpOj but does show interference by Th+l+as its concentration exceeds that of the PuOj2. The electrode operates well between pH 2 and 5 and for PuOj2 concentra7 tions from 10~5to 10*~2 M (Figure 8) and has been found useful in redox and complexation studies as it responds to the concentration of "free" plutonyl, not the total. Conclusion Studies of ligands which might provide specificity in binding to various oxidation states of plutonium seems a particularly promising area for futher research. If specific ion electrodes could be developed for the other oxidation states, study of redox reactions would be much facilitated. Fast separation schemes which do not change the redox equilibria and function at neutral pH values would be helpful in studies of behavior of tracer levels of plutonium in environmental conditions. A particularly important question in this area is the role of PuOj which has been reported to be the dominant soluble form of plutonium in some studies of natural waters (3,14). The research from F.S.U. described in this review was con ducted under contracts with the Offices of Basic Energy Science and of Health and Environmental Research of the U.S.D.O.E. Literature Cited (1). (2).

Foreman, J.K. and Smith, T.D., J. Chem. Soc., 1957, 1752. Connick, R.E., The Actinide Elements, Ch. 8, ed. G.T. Sea borg and J.J. Katz, McGraw H i l l , New York, 1954. (3). "Transuranics in the Environment", ed. H.D. Hanson, DOE/ TIC-22800, U.S.D.O.E., 1980. (4). Ahrland, S., private communication. (5). Allard, Β., Kipatsi, Η., and Liljensin, J.O., J. Inorg. Nucl. Chem., 1980, 42, 1015. (6). Shannon, R.D., Acta Cryst., 1976, A32, 751. (7). Choppin, G.R., Pure Appl. Chem., 1971, 27, 23. (8). Choppin, G.R. and Strazik, W.F., Inorg, Chem., 1965, 4, 1250. (9). Lundquist, R., Hulet, Ε. K. and Baisden, P. Α., Acta Chem. Scand., 1981, A35, 653. (10). Choppin, G. R. Radiochim. Acta, in press. (11). Fuger, J., and Oetting, F. L., The Chemical Thermodynamics of Actinide Elements and Compounds; Part 2. The Actinide Aqueous Ions., Inter. At. Ener. Agen., Vienna, 1976. (12). Svoronos, D. R., Antic-Fidancev, Ε., Lamaitre-Blaise, Μ., and Caro, P., Nouveau J. Chim., 1981, 5, 547.


230

(13). (14). (15). (16).


(17). (18). (19). (20). (21). (22). (23). (24). (25).

(26). (27). (28). (29).

PLUTONIUM CHEMISTRY

Toth, L. Μ., Friedman, Η. Α., and Osborne, M. M . , J. Inorg. Nucl. Chem., 1981, 43, 2929. Rai, D. and Swanson, J. L . , Nucl. Tech., 1981, 54, 107. Wadt, W. R., J. Am. Chem. Soc., 1981, 103, 6053. Choppin,G.R.,and Unrein,P.J.,Transplutonium Elements ,W.Mul ler and R.Lindner,eds., North-Holland, Amsterdam,1976, p.97 Rao, L. F . , and Choppin, G. R., unpublished data. Bertrand, P. A. and Choppin, G. R., to be published. Toth, Τ. Μ., B e l l , J. T . , and Friedman, Η. Α . , Actinide Separations, J. D. Navratil and W. W. Schultz, eds., ACS Symposium Series 117, Washington, D. C . , 1980, pp.253-266. Martell, Α. Ε., and Smith R. Μ., C r i t i c a l Stability Constants, Vol. 1-5., Plenum Press, New York, 1974-81. Munze, R., J. Inorg. Nucl. Chem., 1972, 34, 661. Eigen, Μ., and Wilkins, R., Adv. Chem. Ser. No. 49, 1965, p. 55. Choppin, G. R., and Bertha, S. L . , J. Inorg. Nucl. Chem., 1973, 35, 1309. K h a l i l i , F . I . , and Choppin, G. R., to be published. Raymond, Κ. Ν., Kappel, N. J., Pecoraro, V . L . , Harris, W.R., Carrano, C. J . , Weith, F. L . , and Durbin, P. W., Actinides in Perspective, Edelstein, Ν. M . , ed., Pergamon, New York, 1982, p. 491. Senkyr, J . , Ammann, D., Meier, P. C . , Morf, W. E., Pretsch, E . , and Simon, W., Anal. Chem., 1979 , 51, 786. Bertrand, P. Α . , Choppin, G. R., Rao, L. F . , and Bunzli, J. C . , submitted for publication. Baes, C. F . , and Mesmer, R. Ε . , The Hydrolysis of Cations, Wiley-Interscience, New York, 1976. Allard, B., Actinides in Perspective, N. Edelstein, ed., Pergamon Press, 1982, p. 553.

RECEIVED December 21, 1982


Aspects of Plutonium Solution Chemistry - ACS Publications

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