Plutonium Chemistry - American Chemical Society

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16 Reactions of Plutonium Ions with the Products of Water Radiolysis J. C. SULLIVAN Downloaded by NANYANG TECHNOLOGICAL UNIV on June 3, 2016 | http://pubs.acs.org Publication Date: May 19, 1983 | doi: 10.1021/bk-1983-0216.ch016

Argonne National Laboratory, Chemistry Division, Argonne, IL 60439

The free radicals produced in the radiolysis of HO range from the powerful reductant e (aq) to the oxidant OH. The reactions of the hydrated electron with Pu(VI) are reviewed. The effect of absorption on a SiO colloid markedly reduces the rate k(e (aq) + Pu(VI)). The reactions of OH with CO and subsequent reaction with Pu(V) are discussed. Salient problems in the process and waste disposal chemistry of Pu are outlined. 2

-

2 -

=

3

The g i f t e d c h e m i s t s who worked on t h e " M a n h a t t a n P r o j e c t " r e c o g n i z e d and a t t e m p t e d t o q u a n t i t a t i v e l y d e s c r i b e t h e e f f e c t s o f α s e l f - r a d i o l y s i s soon a f t e r t h e p r e p a r a t i o n o f m a c r o s c o p i c quantities of Pu. The p r e s e n t symposium provides an a p p r o p r i a t e t i m e and p l a c e t o c i t e a number o f t h e s e i n d i v i d u a l s f o r t h e i r c o n t r i b u t i o n s t o a n i m p o r t a n t a s p e c t o f Pu s o l u t i o n chemistry. The a u t h o r s o f r e p o r t s r e f e r r e d t o i n V o l . 14-B ( 2 ) who p a r t i c i p a t e d i n t h e s e e a r l y i n v e s t i g a t i o n s under t h e d i r e c t i o n o f P r o f e s s o r R. E. C o n n i c k a r e ( i n a l p h a b e t i c a l o r d e r ) S. C. C a r n i g l i a , M. K a s h a , W. H. McVey, G. E. Moore, G. E. S h e l i n e and W. K. W i l m a r t h . These e a r l y s t u d i e s were c a r r i e d o u t i n HClOi+ and HC1 s o l u t i o n s and t h e n e t e f f e c t was a r e d u c t i o n t o P u ( I I I ) . To p r o v i d e a n a p p r o p r i a t e b a c k g r o u n d f o r a discussion of the t i t l e r e a c t i o n s note t h a t ~ 1 0 ~ sees a f t e r i r r a d i a t i o n t h e w a t e r d e c o m p o s i t i o n ( p r o d u c t s ) c a n be e x p r e s s e d a s : 2 3 9

9

H 0 ~ ~ • H ^ , OH, e ^ , H, H ^ , H , 0H~ 2

q

q

2

(1)

The amounts o f t h e s e p r i m a r y p r o d u c t s p e r 100 eV a d s o r b e d (G v a l u e s ) a r e r e s p e c t i v e l y ; 2.9, 2.75, 2.65, 0.65, 0.70., 0.45 and 0.25. These v a l u e s a r e f o r n e u t r a l w a t e r and a C o - y s o u r c e . The G v a l u e s may v a r y somewhat w i t h e n e r g y and t y p e o f p a r t i c l e 6 0

0097-6156/83/0216-0241$06.00/0 © 1983 American Chemical Society

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

242

PLUTONIUM CHEMISTRY

u s e d f o r t h e i r r a d i a t i o n , but n o t m a r k e d l y o v e r a w i d e r a n g e o f p a r t i c l e s and e n e r g i e s . The thermodynamically powerful oxidant(OH) and reductant e~(aq) are produced. E s t i m a t e s have been made o f t h e v a l u e s f o r t h e p o t e n t i a l s o f some o f t h e c h a r a c t e r i s t i c r e a c t i o n s o f t h e so-called primary products of r a d i o l y s i s . These v a l u e s a r e presented i n Table I . I t i s of importance to note t h a t the H atom can a c t as e i t h e r an o x i d a n t o r r e d u c t a n t

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Table

I.

Half-Cell

Potentials

Reactions

2.65 2.30 1.83 1.73 1.47 1.44 -0.33 -2.30 -2.87

+

OH + H + e" = H 2 O H + H + e~ = H OH + e = OH +

2

02

+

2H

+

H + H2O H

+

+

+

e~

=

H2O2

+ e~" = H 2 +

H02

+

e"

=

OH

H202

O 2 + e" = O2 H+ + e" = H e~ = e~aq

(a)

H 0 ( l i g ) , H 2 ( g , 1 atm 25°), ( 0 2 g , 1 atm 25°) states. S c h w a r t z Η. Α., J . Chem. Ed. 1981, J58,

as s t a n d a r d 101.

2

The d e v e l o p m e n t o f p u l s e r a d i o l y s i s t e c h n i q u e s have l e d t o t h e d e t e r m i n a t i o n o f a number o f t h e i m p o r t a n t k i n e t i c p r o c e s s e s o f t h e s p e c i e s p r o d u c e d by t h e i r r a d i a t i o n o f H 2 O . The r e s u l t s t h a t have been o b t a i n e d f o r a number o f t h e most i m p o r t a n t r e a c t i o n s a r e p r e s e n t e d i n T a b l e I I . These r e s u l t s d e m o n s t r a t e t h a t t h e n e t e f f e c t o f r a d i a t i o n i s H2O d e c o m p o s i t i o n i n t h e a b s e n c e o f any r e a c t i v e s u b s t r a t e . Table

II.

R e a c t i o n s o f the R a d i c a l s 9 Rate Constant M s l

Reaction

e"(aq) + e~(aq) + 2H2O2 — • e""(aq) + H 3 0 — • Η + H20 e " ( a q ) + OH — • 0H~ OH + OH — • H 02 OH + Η — • H20 H -f Η — • H2 H+ + OH" — • H20 +

2

H 2 + 20H

1

6.0 23.5 3.0 5.0 20.0 10.0 100.0

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

χ

10

16.

SULLIVAN

Pu Ions and the Products of Water Radiolysis

243

The s o l u t i o n c h e m i s t r y o f Pu i s u n i q u e i n t h a t i t i s p o s s i b l e f o r a l l f o u r o f t h e common o x i d a t i o n s t a t e s t o be p r e s e n t i n t h e same s o l u t i o n . The w e l l - k n o w n r e l a t i o n s a r e p r e s e n t e d i n e q u a t i o n s 2, 3 and 4. P u ( I I I ) + Pu(V) P u ( I I I ) + Pu(VI)

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2Pu(V)

^

^

^

2Pu(IV)

Pu(IV) + Pu(V)

Pu(IV) + Pu(VI)

(2) (3) (4)

The r e a c t i o n s t o be e x p e c t e d between t h e r a d i c a l s p r o d u c e d by t h e r a d i a t i o n o f H20 w i t h t h e p l u t o n i u m i o n s i n aqueous solution are: P u ( V I I ) + e" aq

— •

P u ( V I ) + OH — •

Pu(VI)

(5)

Pu(VII)

(6)

P u ( V I ) + ( e ~ , H) — • aq P u ( V ) + OH

— •

Pu(VI)

P u ( V ) + ( e ~ , H) — • aq P u ( I V ) + OH — •

Pu(V)

Pu(IV) Pu(V)

(7) (8) (9) (10)

P u ( I V ) + ( e ~ , H) — • aq

Pu(III)

(11)

P u ( I I I ) + (OH, H) — •

Pu(IV)

(12)

There i s i n a d d i t i o n t o t h e p u l s e r a d i o l y s i s t e c h n i q u e t h e c l a s s i c a l c o n t i n u o u s r a d i a t i o n method. This l a t e r procedure u s e s e i t h e r t h e C o - y s o u r c e s o r i n t h e c a s e o f Pu t h e s e l f α i r r a d i a t i o n due t o t h e r a d i o a c t i v e d e c a y . The v a s t m a j o r i t y o f t h e s t u d i e s on t h e r e a c t i o n s s u c h as 5-12 have been the c l a s s i c a l type i n acidic solutions, predominantly i n nitric acid solutions f o r obvious process related interests. The r e s u l t s o f s u c h s t u d i e s have been summarized by Newton ( 4 ) s o o n l y a v e r y b r i e f r e c a p i t u l a t i o n i s warranted. 6 0

1.

The p r e d o m i n a n t r e a c t i o n s i n p e r c h l o r i c a c i d t e n d t o r e d u c e Pu t o a " s t e a d y s t a t e " m i x t u r e o f P u ( I I I ) and P u ( I V ) .

2.

I n s u l f u r i c a c i d s o l u t i o n s Pu(VI) i s reduced o n l y t o P u ( I V ) .

3.

I n HC1 whether

solutions there are c o n f l i c t i n g observations r e d u c t i o n o f P u ( V I ) and P u ( I V ) o c c u r s .

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

as t o

244

PLUTONIUM CHEMISTRY

4.

The v a s t amount o f work c a r r i e d o u t i n n i t r i c a c i d s o l u t i o n s can n o t be a d e q u a t e l y s u m m a r i z e d . S u f f i c e i t to say r e s u l t s i n t h e s e s o l u t i o n s a r e p l a g u e d w i t h i r r e p r o d u c i b i l i y and induction periods, but the general pattern i s reduction of Pu(VI). T h e r e a r e a l i m i t e d number o f s u c h " s t e a d y radiolysis" s t u d i e s o f Pu i n n e u t r a l o r b a s i c s o l u t i o n s t h a t have been reported. Here t o o , t h e r e s u l t s a r e n o t u n a m b i g u o u s . For e x a m p l e , i n a s e r i e s o f s t u d i e s ( 5 ) w i t h ~20 mM Pu and added Cm (1.8 x 1 0 D/mM/ml) t h e Pu(lfï) d e c r e a s e d o v e r p e r i o d s o f days i n s o l u t i o n s c o n t a i n i n g C I " ( i . e . , a r t i f i c i a l s e a w a t e r and WIPP B r i n e ) a s w e l l a s i n t r i p l y d i s t i l l e d w a t e r . The most s t r i k i n g r e s u l t was t h e g r o w t h and d i s a p p e a r a n c e o f P u ( V I ) i n d i s t i l l e d H2O o v e r some 300 d a y s . In a d i f f e r e n t s e t of experiments s o l u t i o n s o f P u ( I I I ) o r P u ( V I ) i n H2O ( A r s a t u r a t e d , pH a d j u s t e d t o 7, 2 χ Ι Ο " M i n Pu) were i r r a d i a t e d i n t h e ANL ^ C o - y s o u r c e a t a p o s i t i o n where t h e dose was 1 megarad/hr (6) ~1 mM H2O2 p r o d u c e d / h r . Cation e x c h a n g e column b e h a v i o r was u s e d i n a n a t t e m p t t o i d e n t i f y Pu o x i d a t i o n s t a t e s , s e e T a b l e I I I . The r e s u l t s o b t a i n e d a f t e r a n i r r a d i a t i o n o f 1 h r . were i n d i s t i n g u i s h a b l e f r o m t h e " b l a n k " , i.e. a s o l u t i o n not subjected to i r r a d i a t i o n . The i r r a d i a t i o n f o r a 24 h r . p e r i o d f a i l e d t o d e m o n s t r a t e a marked i n c r e a s e i n t h e amount o f P u ( I V ) p r o d u c e d t h a t c o u l d be a s c r i b e d t o t h e effects of r a d i o l y s i s . When a s o l u t i o n o f P u ( V I ) i s s u b j e c t e d t o t h e 24 h r . i r r a d i a t i o n t h e b u l k o f t h e Pu h a s been r e d u c e d t o t h e P u ( I I I ) oxidation state. T h e r e e x i s t s t h e a m b i g u i t y t h a t r e s i d e n c e on t h e column may c a u s e some r e d u c t i o n o f P u ( V I ) P u ( V ) and Pu(IV) t o P u ( I I I ) . The g e n e r a l c o n c l u s i o n i s s t i l l v a l i d t h a t under these c o n d i t i o n s t h e e f f e c t o f r a d i a t i o n i s t o reduce t h e Pu. I t i s apparent that steady r a d i o l y s i s experiments i n t h e n e a r n e u t r a l pH r e g i m e a r e s u b j e c t t o t h e same t y p e o f p r o b l e m s of i n t e r p r e t a t i o n as a r e those s t u d i e s c a r r i e d o u t i n s t r o n g acid. T h e r e i s an a d d i t i o n a l p r o b l e m t h a t a r i s e s i n c o n t i n u o u s radiolysis studies. I f t h e s o l u t i o n s c o n t a i n , C I " , SO^, NO3 CO3 i o n s t h e p r o d u c t s o f H2O r a d i o l y s i s c a n r e a c t t o p r o d u c e t h e corresponding radicals. These r a d i c a l s , C I 2 , SO^, o r CO3 c a n t h e n undergo r e d o x r e a c t i o n s w i t h t h e Pu i o n s s u c h as

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2i+t+

9

5

6

o

Pu(III) + C l ~

— •

P u ( I V ) + 2C1"

r

(13)

The a p p l i c a t i o n o f techniques of pulse r a d i o l y s i s o f f e r s the p o t e n t i a l t o determine r a t e s o f p r i m a r y r a d i o l y s i s i n d u c e d reaction processes. T h i s k n o w l e d g e c a n be o f g r e a t v a l u e i n t h e d e t e r m i n a t i o n o f r e d o x p r o c e s s e s o f Pu i o n s o c c u r r i n g i n a w i d e v a r i e t y o f aqueous s o l u t i o n s . As a m a t t e r o f f a c t , such i n f o r m a t i o n i s e s s e n t i a l t o a p r e d i c t i o n o f t h e Pu o x i d a t i o n s t a t e s t o be e x p e c t e d i n b r e a c h e d r e p o s i t o r y s c e n a r i o s . F o r an

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

Carnall and Choppin; Plutonium Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983. 0.04

P u ( I I I ) 1 h o u r b l a n k a c i d wash

P u ( I I I ) 1 hour

0.2

0.3 0.2 0.4

0.06 0.18 1.3

P u ( I I I ) 24 h o u r b l a n k a c i d wash

P u ( I I I ) 24 h o u r

31.5 51.4 0.1 0.2

0.1 7.5 1.1 0.00

P u ( V I ) 24 h o u r b l a n k

P u ( V I ) 24 h o u r b l a n k a c i d wash

P u ( V I ) 24 h o u r

Note:

HC1

17.4

62.2

1.0 0.2

0.02

0.3

18.5

60.6

6.8 5.6 86.1

1.3

0.00 0.00

22.6

0.4

1.0 6.1

98.0

0.6

0.9 0.6

97.5

17.6

0.7

79.3

0.3

0.2

98.3

0.6 9.8

9.5 5.1

0.00

12.4

12 M

22.5

4.2

25.9

HC1

Pu(IV) 6 M

10.3

93.7

58.2

95.4

60.0

HC1

The f i n a l pH o f a l l s o l u t i o n s showed l i t t l e c h a n g e . However, t h e s o l u t i o n s i r r a d i a t e d f o r 24 h o u r s changed t h e m o s t , pH ^ 6 . 2 , as compared t o i n i t i a l pH = 7.

0/ ·j . r.^ ^ (cmp/ml) Σ v o l u m e o f f r a c t i o n % Oxidation State = - — ^ - s τ—-,-r ν r-: Σ cmp o f a l l f r a c t i o n s

P u ( V I ) 24 h o u r i r r a d i a t e d a c i d wash

irradiated

0.4

0.3

P u ( I I I ) 24 h o u r i r r a d i a t e d a c i d wash

irradiated

blank

P u ( I I I ) 24 h o u r

0.01

1.7 0.6

0.7

0.01

0.3

3 M

Pu(III)

States*

HC10. 4

Pu(VI) 2.0 M

7.5

HC10, 4

0.00

0.3

0.6

0.5 M

Pu(V)

D i s t r i b u t i o n of Oxidation

P u ( I I I ) 1 h o u r i r r a d i a t e d a c i d wash

irradiated

0.7

P u ( I I I ) 1 hour blank

3xDW

Table I I I .

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246

PLUTONIUM

CHEMISTRY

u n d e r s t a n d i n g o f the e f f e c t s o f r a d i o l y s i s i n process streams the E d i s o n i a n ( s t e a d y r a d i o l y s i s ) approach can p r o v i d e adequate information. In pulse r a d i o l y s i s studies i t i s f e a s i b l e to e l i m i n a t e u n d e s i r e d r a d i c a l r e a c t i o n s by t h e u s e o f " s c a v e n g e r s " . For example, t o produce a t o t a l l y r e d u c i n g system s i m p l e a l i p h a t i c a l c o h o l s a r e added and t h e e n s u i n g r e a c t i o n OH + CH OH — •

CH OH + H ^

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3

(14)

2

produces a radical which i s slow to react on t h e p u l s e r a d i o l y s i s time s c a l e . To e l i m i n a t e e " ( a q ) t h e s o l u t i o n i s s a t u r a t e d w i t h N2O and t h e r e a c t i o n N 0 + e" — • 2 aq o

N

0

2

+ OH + 0H~

(15)

i n s u r e s t h a t t h e s y s t e m i s p r e d o m i n a n t l y an o x i d i z i n g s y s t e m . The rate parameters f o r the r e a c t i o n s of e~(aq) w i t h substrates are generally determined by monitoring the d i s a p p e a r a n c e o f t h e h y d r a t e d e l e c t r o n a t 600-700 nm. The f i r s t o r d e r r a t e parameters a r e g e n e r a l l y determined over a range of s u b s t r a t e c o n c e n t r a t i o n s and t h e second o r d e r r a t e parameter calculated from the r e s u l t i n g linear relation. The data available f o r such s t u d i e s w i t h Pu i o n s a r e p r e s e n t e d i n Table IV. Table IV. [Pu]

VI VI VI VI VII

(a) (b) (c) (d)

1

[Μ] χ ΙΟ""*

0.12-1.0 0.09-1.08 0.2-0.8 0.26 0.2

R e a c t i o n R a t e s o f e ~ ( a q ) w i t h Pu Solution

pH 5.6 ( H C 1 0 O pH 6.1 ( H C I O O 0.05 M Na2C03 2 M NaOH 2 M NaOH

k(10

6.4 5.8 2.3 1.9 4.2

1 0

± ± ± ± ±

1

1

M" s" )

0.4 0.3 0.3 0.3 0.8

(b) (b) (c) (d) (d)

Ambient t e m p e r a t u r e , 25°C f o r ( b ) and ( c ) , 20-22°C f o r (d). S u l l i v a n , J . C. ; G o r d o n , S.; Cohen, D.; M u l a c , W. and S c h m i d t , Κ. H., J . P h y s . Chem. 1976, _80, 1684. M u l a c , W. and S u l l i v a n , J . C., u n p u b l i s h e d r e s u l t s . P a k a e v , A. K.; M e f a d ' e v a , M. P.; K n o t , Ν. N. and S p i t s y n , V. I . , H i g h E n e r g y Chem. 1973, J7_, 448.

The a m a z i n g f e a t u r e o f t h e s e r e s u l t s i s t h e f a c t t h a t t h e r e a c t i o n occurs at e s s e n t i a l l y the d i f f u s i o n c o n t r o l l e d limit o v e r a w i d e range o f s o l u t i o n c o m p o s i t i o n . In p a r t i c u l a r i n 0.05 M Na2C03 s o l u t i o n s t h e P u ( V I ) e x i s t s p r e d o m i n a n t l y as t h e

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

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16.

SULLIVAN

Pu

Ions and

the

Products of

247

Water Radiolysis

t r i s - c a r b o n a t o complex. Y e t t h e r e i s no marked e f f e c t i n t h e r a t e of the r e a c t i o n d e s p i t e the f a c t t h a t s i m p l e e l e c t r o s t a t i c c o n s i d e r a t i o n s w o u l d p r e d i c t _a p r i o r i t h a t t h e f o r m a l n e g a t i v e c h a r g e on the P u ( V I ) w o u l d be a k i n e t i c b a r r i e r f o r the r e a c t i o n with e~(aq). The d a t a i n t h i s t a b l e a r e s u f f i c i e n t t o p r o v i d e a b a s i s f o r the p r e d i c t i o n t h a t i n homogeneous s o l u t i o n s the r a t e of t h e r e a c t i o n o f e ~ ( a q ) w i t h P u ( V I ) w i l l n o t be influenced m a r k e d l y by any c o m p l e x i n g of t h e P u ( V I ) . T h e r e a r e two a d d i t i o n a l p u l s e r a d i o l y s i s i n v e s t i g a t i o n s o f P u ( V I ) r e a c t i o n s t h a t a r e of i m p o r t a n c e . These a r e t h e r a t e o f r e a c t i o n o f e~~(aq) w i t h P u ( V I ) a d s o r b e d on S1O2 c o l l o i d s and t h e r a t e of t h e r e a c t i o n between Pu(V) and the c a r b o n a t e i o n r a d i c a l Pu(V)

+ C0~

— •

P u ( V I ) + CO^

(16)

The f o r m e r i n v e s t i g a t i o n was m o t i v a t e d , i n p a r t by the f a c t t h a t i n a p r e v i o u s s t u d y (7) t h e r e had been a marked d i f f e r e n c e on the rates of reactions of e~(aq) and U(VI) between homogeneous s o l u t i o n s and those c o n t a i n i n g m i c e l l a r m a t e r i a l . When t h e r a t e of d i s a p p e a r a n c e of the h y d r a t e d e l e c t r o n i s measured o v e r a r a n g e of c o n c e n t r a t i o n s f r o m 2 χ ΙΟ"" M t o 8 χ 10 M a t pH = 9.7 i n s o l u t i o n s f o r m a l l y 0.003 M S 1 O 2 , the calculated s e c o n d o r d e r r a t e p a r a m e t e r i s 1.4 χ 1 0 M'-^s" . T h i s i s a marked d e c r e a s e f r o m any o f t h e p r e v i o u s measurements and e m p h a s i z e s the p o i n t t h a t the p r e d i c t i o n o f Pu c h e m i s t r y i n a n a t u r a l w a t e r s y s t e m must t a k e c o g n i z e n c e o f f a c t o r s t h a t a r e not u s u a l l y deemed s i g n i f i c a n t . The p o i n t has p r e v i o u s l y been made t h a t common a n i o n s can r e a c t w i t h the p r o d u c t s o f w a t e r r a d i o l y s i s t o p r o d u c e r e a c t i v e s p e c i e s · One s u c h w e l l documented (8_) r e a c t i o n i s 5

_1+

9

C ; 0

+ OH

k

=

4

X

M

"

l

s

-

1

>

1

C0-+0H-

(17)

The p r o d u c t CO3 r a d i c a l can r e a c t w i t h a v a r i e t y o f i n o r g a n i c i o n s i n c l u d i n g the c a r b o n a t o complex of P u ( V ) . Thus, i n a p u l s e r a d i o l y s i s e x p e r i m e n t , P u ( V I ) i n 0.05 M Na2C03, He saturated, t h e r e a r e t h e p a r a l l e l r e a c t i o n s (17) and (18)

P u ( V I ) + e" aq followed

k = 2 χ 10

l

M s

1 Q

1

^

(

P

u

(

v

)

)

( 1 8 )

by

Pu(V)

+ C0~

k

Thus P u ( V I ) i n c a r b o n a t e due t o r a d i o l y s i s .

=

3

X

1 ( ) 7

M

l g

media w i l l

1

»

not

P u ( V I ) + CO3 suffer

a net

(19) reduction

American Chemical Society Library 1155 16th St., N.W. Washington, U.C. 20036 Carnall and Choppin; Plutonium Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

PLUTONIUM CHEMISTRY

248

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Future

Trends

It h a s been n o t e d that r e s u l t s of steady radiolysis experiments provide adequate data f o r separations related problems. The d i f f i c u l t y i s t h a t i n t h e a b s e n c e o f k i n e t i c d a t a f o r t h e p r i m a r y p r o c e s s i t becomes n e c e s s a r y t o r e p e a t t h i s t y p e o f e x p e r i m e n t f o r e a c h p a r t i c u l a r s e t o f c o n c e n t r a t i o n s and times· One problem t h a t should be o f p a r t i c u l a r i n t e r e s t f o r separation processes i s the identification and kinetic c h a r a c t e r i z a t i o n o f t h e r e a c t i v e r a d i c a l s t h a t o c c u r when s t r o n g n i t r i c acid solutions are subject to i o n i z i n g r a d i a t i o n . The important reducing r a d i c a l i n s u c h s o l u t i o n s i s t h e H atom. T h e r e a r e p r e s e n t l y no d i r e c t measurements o f t h e r a t e o f r e d u c t i o n o f H atoms w i t h any Pu o x i d a t i o n s t a t e . I n marked c o n t r a s t t h e r e e x i s t s a volumnous l i t e r a t u r e o n pulse r a d i o l y s i s studies of n i t r i c acid solutions (9). The NO3 r a d i c a l h a s been i d e n t i f i e d as a m a j o r p r o d u c t a l t h o u g h t h e mechanism o f f o r m a t i o n i s s t i l l a m a t t e r o f d e b a t e . While a number o f r e a c t i o n s o f t h i s r a d i c a l have been r e p o r t e d ( 10) t h e r e h a s e v i d e n t l y n o t been a n y p u l s e r a d i o l y s i s s t u d i e s o f r e a c t i o n s w i t h Pu i o n s . F o r r e a c t i o n s i n t h e n e a r n e u t r a l o r s l i g h t l y b a s i c pH r e g i m e - w h i c h a r e o f i n t e r e s t i n e n v i r o n m e n t a l c h e m i s t r y and w a s t e d i s p o s a l p r o b l e m s - t h e p r e s e n c e o f t h e common a n i o n s C l ~ , S O i and CO3 c a n i n t r o d u c e a d d i t i o n a l c o m p l i c a t i o n s . We have a l r e a d y discussed the reactions of Pu(VI) i n c a r b o n a t e s o l u t i o n s b u t t h e r e i s no i n f o r m a t i o n a v a i l a b l e on r e a c t i o n s o f C I 2 o r S0Î r a d i c a l s w i t h any o f t h e Pu s p e c i e s w h i c h may be p o s t u l a t e d t o be p r e s e n t . The p u b l i s h e d d a t a on the r e a c t i v i t y o f these r a d i c a l s i n d i c a t e s t h a t they a r e both o x i d a n t s and c o u l d t h e r e f o r e be e x p e c t e d t o p r o v i d e important p a t h s f o r t h e d i s t r i b u t i o n o f Pu o x i d a t i o n s t a t e s . W h i l e t h e r e have been o n l y a l i m i t e d number o f r e a c t i o n s o f e ~ ( a q ) and Pu i o n s i n near neutral or basic solutions, apparently systematic reactivity patterns provide some additional insight. F o r e x a m p l e , i t h a s been n o t e d t h a t t h e second order r a t e p a r a m e t e r s (MT^-s*" ) f o r t h e r e a c t i o n s o f N p ( V I ) and Np(V) ( 1 1 ) (pH=6) a r e r e s p e c t i v e l y ( 5 . 4 ± 0.2) χ 1 0 and (2.13 ± 0.03) χ 1 0 . The v a l u e s o f t h e s e c o n d o r d e r r a t e p a r a m e t e r s when Am(VI) and Am(V) a r e t h e o x i d a n t s (12) are (3.9 ± 0.9) χ 1 0 and (3.18 ± 0.08) χ 1810. So one c a n p r e d i c t t h a t t h e v a l u e f o r k ( e " ( a q ) + P u ( V ) ) a t pH=6 w o u l d be 3 ± 1 χ 10 H-le-1. The c o n c l u s i o n t o be drawn i s t h a t t h e d a t a s e t o f k i n e t i c i n f o r m a t i o n on 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 w i t h p r i m a r y products of H2O r a d i o l y s i s needs to be s u b s t a n t i a l l y augmented. The s t e a d y r a d i o l y s i s s t u d i e s t h a t have been and a r e currently o u t a r e o f immediate pragmatic value. It is imperative t h a t s e l e c t e d r a t e d a t a f o r r e a c t i o n s i n aqueous 1

1 0

1 0

1 0

1 0

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

16. SULLIVAN systems that obtained.

Pu Ions and the Products of Water Radiolysis249 can be used to model the aquatic reservoir be

Acknowledgment

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The skillful and dedicated collaborative efforts of members of the ANL Chemistry Division Radiation Chemistry Group, S. Gordon, W. Mulac and Κ. H. Schmidt have provided not only the technical expertise but also continuing intellectual stimulation which has made the pulse radiolysis studies possible. Literature Cited 1.

2. 3.

This work was performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Sciences, U. S. Department of Energy, under contract number W-31-109-ENG-38. Transuranium Elements, NNES, IV-14-B, 1944, p. 334. Draganic, I. G. and Draganic, Z. D. "The Radiation Chemistry of H O"; Academic Press: New York, 1941. (a) Newton, T. W. "The Kinetics of the Oxidation­ -Reduction Reactions of Uranium, Neptunium, Plutonium and Americium in Aqueous Solutions"; National Technical Information Service, U. S. Department of Commerce: Springfield, Virginia, 1975. (b) Buxton, G. V. and Sellers, R. M. "The Radiation Chemistry of Metal Ions in Aqueous Solutions" in Coordination Chemistry Reviews, A. B. P. Lever, Ed., 1977, Vol. 22, pp. 195-274. (c) Miner, F. J . and Seed, J . R. Chem. Rev. 1967, 67, 299. Fried, S.; Friedman, Α.; Sullivan, J . C.; Nash, K.; Cohen, D. and Sjoblom, K, "Scientific Basis for Waste Management", Ed. Northrup, Jr, C. J . Μ., Plenum Publ. Co.: New York, 1980. Soldano, J . Ε . , Report of ANL Student Research Participation Program, Fall, 1981. Meisel, D.; Mulac, W. and Sullivan, J . C., Inorg. Chem. 1981, 20, 4247. Dorfman, L. M. and Adams, G. E . , "Reactivity of the Hydroxyl Radical in Aqueous Solutions", NSRDS-NBS46, U. S. Government Printing Office: Washington, D.C., 1973. See for example, Brosckiewicz, R. K.; Kozlaraska-Milner, E. and Blum, Α., J . Phys. Chem. 1981, 85, 2258 and references therein. Ross, A. B. and Neta, P. "Rate Constants for Reactions of Inorganic Radicals in Aqueous Solutions", NSRDS-NBS65, U. S. Government Printing Office, Washington, D.C., 1979. Sullivan, J . C.; Gordon, S.; Cohen, D.; Mulac, W. and Schmidt, Κ., J . Phys. Chem. 1976, 1684. Gordon, S.; Mulac W.; Schmidt, Κ. H.; Sjoblom R. K. and Sullivan, J . C., Inorg. Chem. 1978, 17, 294. 2

4.

5.

6. 7. 8. 9. 10. 11. 12.

RECEIVED December 27, 1982

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