Photochemistry of Aqueous Plutonium Solutions - ACS Publications

18 Photochemistry of Aqueous Plutonium Solutions J. T. BELL, L. M. TOTH, and H. A. FRIEDMAN

Downloaded by NANYANG TECHNOLOGICAL UNIV on June 3, 2016 | http://pubs.acs.org Publication Date: May 19, 1983 | doi: 10.1021/bk-1983-0216.ch018

Oak Ridge National Laboratory, Chemical Technology Division, Oak Ridge, TN 37830

Aqueous plutonium photochemistry is briefly reviewed. Photochemical reactions of plutonium in several acid media have been indicated, and detailed information for such reactions has been reported for perchlorate systems. Photochemical reductions of Pu(VI) to Pu(V) and Pu(IV) to Pu(III) are discussed and are compared to the U(VI)/(V) and Ce(IV)/(III) systems respectively. The reversible photoshift in the Pu(IV) disproportionation reaction is highlighted, and the unique features of this reaction are stressed. The results for photoenhancement of Pu(IV) polymer degradation are presented and an explanation of the post-irradiation effect is offered.

The a u t h o r i t a t i v e documents o n p l u t o n i u m (J_,2_) do n o t i n c l u d e p h o t o - c h e m i c a l r e a c t i o n s o f p l u t o n i u m i n aqueous systems. The f i r s t p a p e r s i n W e s t e r n w o r l d l i t e r a t u r e o n s t u d i e s t h a t were d e d i c a t e d t o aqueous p l u t o n i u m p h o t o c h e m i s t r y a p p e a r e d i n 1976 (3_,4_), e v e n t h o u g h p h o t o c h e m i c a l changes i n o x i d a t i o n s t a t e s were i n d i c a t e d a s e a r l y a s 1952 (_5,6_>7_)* Other reasons f o r i n v e s t i g a t i n g plutonium photochemistry i n the m i d - s e v e n t i e s i n c l u d e d t h e w i d e l y known u r a n y l p h o t o c h e m i s t r y and t h e s i m i l a r i t i e s o f t h e a c t i n y l s p e c i e s , t h e e x c i t i n g p o s s i b i l i t i e s o f isotope s e p a r a t i o n o r enrichment, the poten t i a l f o rchemical separation o r interference i n separation pro cesses f o r n u c l e a r f u e l r e p r o c e s s i n g , the p o s s i b l e photoredox e f f e c t s on p l u t o n i u m i n t h e environment, and the d e s i r e t o e x p a n d t h e f u n d a m e n t a l knowledge o f p l u t o n i u m c h e m i s t r y .

0097-6156/ 83/0216-0263$06.00/0 © 1983 American Chemical Society

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

264

PLUTONIUM CHEMISTRY

B u r g e r a n d c o w o r k e r s (5_) i n 1952 r e p o r t e d t h a t some distribution coefficients forPu02 i n organic-aqueous systems a t l i g h t e d c o n d i t i o n s were d i f f e r e n t f r o m t h o s e o b s e r v e d f o r d a r k c o n d i t i o n s , a n d t h o s e a u t h o r s b e l i e v e d t h a t some P u 0 2 had been p h o t o c h e m i c a l l y r e d u c e d . That r e d u c t i o n was c o n f i r m e d b y o t h e r s ( 6 ) i n 1965, a n d i n 1969 a r e p o r t s u g g e s t e d t h a t most aqueous p l u t o n i u m r e a c t i o n s were a f f e c t e d b y l i g h t (7_). S t u d i e s o f a c t i n i d e p h o t o c h e m i s t r y a r e always dominated by the r e a c t i o n s that photochemically reduce the u r a n y l , U(VI), species. Almost any U V - v i s i b l e l i g h t w i l l e x c i t e the u r a n y l s p e c i e s s u c h t h a t t h e l o n g - l i v e d , ^lO"" * s e c o n d s , e x c i t e d - s t a t e s p e c i e s w i l l r e a c t w i t h most r e d u c t a n t s , a n d t h e quantum y i e l d f o r t h i s r e d u c t i o n o f UO2 "*" t o U 0 2 i s v e r y n e a r u n i t y ( 8 ) . Because o f t h e c o n t i n u e d h i g h l e v e l o f i n t e r e s t i n u r a n y l photo c h e m i s t r y a n d t h e s i m i l a r i t i e s i n t h e a c t i n y l s p e c i e s , one w o n d e r s why aqueous p l u t o n i u m p h o t o c h e m i s t r y was n o t i n v e s t i gated e a r l i e r . Isotope photoseparation techniques f o r a c t i n i d e s probably w i l l i n c l u d e o n l y g a s e o u s s y s t e m s , h e x a f l u o r i d e s and m e t a l v a p o r s . Hence, aqueous a c t i n i d e p h o t o c h e m i s t r y i s n o t l i k e l y to influence isotope separations. However, t h e i n t e n s e i n t e r e s t i n l a s e r s e p a r a t i o n t e c h n i q u e s f o r t h e gaseous s y s t e m s p r o m o t e s i n t e r e s t i n t h e aqueous s y s t e m s . The p o s s i b l e a p p l i c a t i o n o f aqueous p l u t o n i u m photochem i s t r y t o n u c l e a r f u e l r e p r o c e s s i n g p r o b a b l y h a s been t h e b e s t r e c e i v e d j u s t i f i c a t i o n f o r i n v e s t i g a t i n g t h i s s u b j e c t . The n e c e s s a r y c o n t r o l s o f a n d changes i n Pu o x i d a t i o n s t a t e s c o u l d p o s s i b l y be i m p r o v e d b y p l u t o n i u m p h o t o c h e m i c a l r e a c t i o n s t h a t were c o m p a r a b l e t o t h e u r a n y l p h o t o c h e m i s t r y . The p r i m a r y r e a s o n f o r s t u d y i n g aqueous p l u t o n i u m p h o t o c h e m i s t r y h a s been t h e s c i e n t i f i c v a l u e . No o t h e r aqueous m e t a l system has such a wide range o f c h e m i s t r y ; f o u r o x i d a t i o n s t a t e s c a n c o - e x i s t ( I I I , I V , V, a n d V I ) , a n d t h e P u ( I V ) s t a t e c a n f o r m p o l y m e r m a t e r i a l . C a t i o n c h a r g e s on t h e s e s p e c i e s r a n g e f r o m 1 t o 4, a n d t h e r e a r e m o l e c u l a r a s w e l l a s m e t a l l i c ions. A wide v a r i e t y o f a n i o n a n d c h e l a t i n g c o m p l e x c h e m i s t r y a p p l i e s t o the r e s p e c t i v e o x i d a t i o n s t a t e s . F i n a l l y , a l l o f t h i s aqueous p l u t o n i u m c h e m i s t r y c o u l d be a f f e c t e d by t h e a b s o r p t i o n o f l i g h t , a n d p e r h a p s new p l u t o n i u m s p e c i e s c o u l d be d i s c o v e r e d by photon e x c i t a t i o n . V i s i b l e and UV s p e c t r o p h o t o m e t r y t e c h n i q u e s a r e most c o n v e n i e n t f o r s t u d y i n g t h e polymer and v a r i o u s o x i d a t i o n s t a t e s of plutonium. The s p e c t r a o f t h e p l u t o n i u m s t a t e s and t h e p r o c e d u r e f o r r e s o l u t i o n o f t h e c o n c e n t r a t i o n s were p r e v i o u s l y d e s c r i b e d ( 9 ) . Changes i n t h e r e l a t i v e c o n c e n t r a t i o n s o f t h e o x i d a t i o n s t a t e s and o f t h e polymer g e n e r a l l y a r e determined from c o r r e s p o n d i n g changes i n t h e s p e c t r a and a comparison o f t h e changes t o s t a n d a r d s p e c t r a o f t h e v a r i o u s s t a t e s . These t e c h n i q u e s have been u s e d e x c l u s i v e l y f o r s t u d y i n g t h e p h o t o c h e m i s t r y o f aqueous p l u t o n i u m . 2 +


2 +

1

2

+



18.

BELL ET AL.

Photochemistry of Aqueous Pu Solutions

265

O n l y t h e o b v i o u s s t u d i e s o f aqueous p l u t o n i u m photochem i s t r y have been c o m p l e t e d , a n d t h e r e s u l t s a r e summarized b e l o w . The c o u r s e o f d i s c u s s i o n w i l l f o l l o w t h e p a r t i c u l a r p h o t o c h e m i c a l r e a c t i o n s t h a t have been o b s e r v e d , b e g i n n i n g w i t h the higher o x i d a t i o n s t a t e s . This d i s c u s s i o n w i l l consider p r i m a r i l y t h o s e s t u d i e s o f aqueous p l u t o n i u m i n p e r c h l o r i c a c i d m e d i a b u t w i l l i n c l u d e one r e a c t i o n i n n i t r i c a c i d m e d i a . Aqueous s y s t e m s o t h e r t h a n p e r c h l o r a t e may a f f e c t p a r t i c u l a r p l u t o n i u m s t a t e s by redox r e a c t i o n s and complex f o r m a t i o n and c o u l d obscure photochemical changes. D e t a i l e d experimental s t u d i e s o f p l u t o n i u m p h o t o c h e m i s t r y i n o t h e r aqueous s y s t e m s s h o u l d a l s o be c o n d u c t e d . P h o t o c h e m i c a l R e d u c t i o n o f Aqueous P l u t o n i u m ( V I ) Plutonium VI e x i s t s i n s o l u t i o n as the molecular p l u t o n y l ion, Pu0 . The s t r u c t u r e o f t h i s i o n i s e s s e n t i a l l y t h e same a s t h a t o f t h e u r a n y l i o n , b u t t h e p l u t o n y l i o n i s much l e s s s t a b l e t h a n t h e u r a n y l i o n . The g r e a t e r s t a b i l i t y o f t h e u r a n y l s p e c i e s r e s u l t s f r o m i t s h a v i n g o n l y 12 b o n d i n g e l e c t r o n s which e x a c t l y f i l l the bonding o r b i t a l s o f the a c t i n y l molecu l a r o r b i t a l m o d e l , w h i l e t h e p l u t o n y l i o n h a s t h e 12 b o n d i n g e l e c t r o n s p l u s two n o n - b o n d i n g e l e c t r o n s ( 1 0 ) . The two e x t r a e l e c t r o n s i n the p l u t o n y l system a r e e n e r g e t i c a l l y favored as u n p a i r e d i n t h e two l o w e s t n o n b o n d i n g o r b i t a l s , and t h e g r o u n d s t a t e f o r t h e p l u t o n y l i o n i s a t r i p l e t s t a t e . From t h e o r e t i c a l c o n s i d e r a t i o n s , one w o u l d p r e d i c t t h a t P u ( V I ) w o u l d be r e d u c e d t o P u ( V ) when e x c i t e d b y l i g h t and r e a c t e d w i t h a m i l d r e d u c i n g agent such as e t h a n o l . Stronger reducing agents such a s n i t r o u s a c i d w o u l d d i r e c t l y r e d u c e P u ( V I ) , and t h e s t r o n g e r a g e n t s s h o u l d be a v o i d e d i n t h e p h o t o c h e m i c a l e x p e r i m e n t s . The reaction 2 +

2

Pu0

2 +

+ R = Pu0

2

+ 2

+ R

+

(1)

where R i s a n a g e n t w i t h r e d u c i n g p o t e n t i a l s i m i l a r t o e t h a n o l , does n o t proceed i n d a r k c o n d i t i o n s . However, t h e r e a c t i o n was o b s e r v e d when t h e aqueous p e r c h l o r a t e s y s t e m was e x p o s e d t o UV l i g h t [ 3 ] . Both P u 0 and Pu *" " were o b s e r v e d a f t e r t h e UV i r r a d i a t i o n , ( F i g . 1 ) , a n d t h e s p e c t r a showed t h a t P u 0 was i n i t i a l l y formed a n d t h e n d i s p r o p o r t i o n a t e d t o f o r m Pu*"*". A quantum y i e l d o f 0.02 was d e t e r m i n e d f o r r e a c t i o n 1 a n d was r e p o r t e d t o be a l o w v a l u e , p e r h a p s by a s much a s a f a c t o r o f 10, b e c a u s e o f i n t e r f e r i n g r e a c t i o n s . The l o w quantum y i e l d r e l a t i v e t o that f o r the s i m i l a r U 0 reaction i s likely r e l a t e d t o the short l i f e - t i m e o f the e x c i t e d - s t a t e P u 0 . The l o w e s t - l e v e l , e x c i t e d t r i p l e t s t a t e o f UC^ "*" has a n unu s u a l l y l o n g l i f e t i m e o f - 1 0 " , w h i c h i s ample t i m e f o r r e a c t i o n w i t h a reductant that i s e i t h e r a s s o c i a t e d with the U 0 ion o r t h a t i s a n u n a s s o c i a t e d s p e c i e s i n t h e same s o l u t i o n . One +

1

1

2

+

2

2 +

2

2 +

2

2

2 +

2



PLUTONIUM CHEMISTRY

WAVE

LENGTH.millimicrons

F i g u r e 1. P l u t o n i u m s p e c t r a showing tonyl. ( I ) 0.385 M P u 0 e t h a n o l i n 1.32 M. HC10/+. f i r s t UV i r r a d i a t i o n . ( I t h e f i r s t UV i r r a d i a t i o n . t h e s e c o n d a n d 25 h a f t e r

2 +

2

the photoreduction of p l u , some P u a n d 1.5 11 ( I I ) Immediately a f t e r the l l ) Twenty-four hours a f t e r (IV) Immediately a f t e r t h e f i r s t UV i r r a d i a t i o n s (_3). 4 +


18.


BELL ET AL.

267

can s p e c u l a t e on t h e b a s i s o f c o n t i n u i t i e s i n t h e a c t i n y l s p e c t r a (10) t h a t t h e UV a b s o r p t i o n by P u 0 involves the e x c i t a t i o n of a bonding e l e c t r o n t o a non-bonding l e v e l , pro d u c i n g a n e x c i t e d - s t a t e s p e c i e s w i t h two o r f o u r u n p a i r e d e l e c t r o n s t h a t w o u l d have a v e r y s h o r t l i f e t i m e . Thus o n l y a low f r a c t i o n o f t h e e x c i t e d - s t a t e s p e c i e s would r e a c t w i t h o t h e r s o l u t i o n components. T h i s t h e o r y i s s u p p o r t e d o n l y by t h e s i m i l a r i t i e s t o t h e a c t i n y l i o n s and i s s u b j e c t t o c o r r e c t i o n by e x p e r i m e n t a l e f f o r t s . 2 +


2

Photochemical Reduction

o f Aqueous

Plutonium(IV)

Plutonium(IV), i n p e r c h l o r i c a c i d with ethanol present, i s s t a b l e under d a r k c o n d i t i o n s b u t p r o d u c e s P u ( I I I ) when t h e s o l u t i o n i s e x p o s e d t o UV l i g h t . A quantum y i e l d o f 0.03 was r e p o r t e d (3_) f o r t h e r e a c t i o n , 4

3

Pu* * + R = P u + + R

+

(2)

+

where R was a n i n t e r m e d i a t e o x i d i z e d e t h a n o l s p e c i e s . When t h e r e d u c t a n t i s e t h a n o l , f o r m i c a c i d , o r f o r m a t e , t h e r e i s no d a r k - s t a t e r e a c t i o n . When a s t r o n g e r r e d u c t a n t i s u s e d , s u c h that the dark s t a t e r e a c t i o n progresses s l o w l y , the a d d i t i o n of UV l i g h t i n c r e a s e s t h e r e a c t i o n r a t e . F o r e x a m p l e , h y d r a z i n e w i l l s l o w l y r e d u c e P u ( I V ) , a n d t h e a d d i t i o n o f UV p h o t o n s w i l l appreciably increase the r e a c t i o n rate. Some u n p u b l i s h e d and more r e c e n t work by t h e s e authçrs examined t h e p h o t o r e d u c t i o n o f Pu(IV) i n n i t r i c a c i d s o l u t i o n that contained hydrazine. T h a t r e d u c t i o n was shown t o o c c u r w i t h o r a f t e r t h e f o r m a t i o n o f a P u ( I V ) p e r o x y complex. This c o m p l e x i s r e s p o n s i b l e f o r t h e enhanced r a t e s o f P u ( I V ) r e d u c t i o n i n d a r k p e r i o d s f o l l o w i n g t h e p h o t o l y s i s . The p h o t o l y s i s o f HNO3 i n t h e p r e s e n c e o f Ν Η< p r o d u c e s H 02, w h i c h t h e n c o m p l e x e s w i t h P u ( I V ) ; i t i s known t h a t H 0 does n o t a p p e a r i n t h e p h o t o l y s i s o f p u r e HN0 s o l u t i o n s , and i t s f o r m a t i o n i n t h e presence of d e m o n s t r a t e s some o f t h e c o m p l e x c h e m i s t r y a s s o c i a t e d w i t h Î^H^ s o l u t i o n s . T h i s may have u s e f u l a p p l i c a t i o n , because the peroxy complex o f Pu(IV) i s reduced a t a f a s t e r r a t e w i t h Î^H^ t h a n i s t h e u n c o m p l e x e d Pu i o n . The P u ( I V ) t o ( I I I ) aqueous p h o t o l y s i s c a n be compared t o the Ce(IV) t o ( I I I ) l i g h t - s e n s i t i v e r e a c t i o n s , although Pu(IV) i s a much weaker o x i d i z i n g a g e n t t h a n C e ( I V ) . The C e ( I V ) / C e ( I I I ) c o u p l e h a s a s t a n d a r d r e d u c t i o n p o t e n t i a l o f 1.7 V i n HCIO^, w h i l e t h e comparable p o t e n t i a l f o r the P u ( I V ) / P u ( I I I ) couple i s 0.98 V. I n HCIO4 s o l u t i o n s , UV l i g h t and C e ( I V ) w i l l o x i d i z e H2O w i t h a quantum y i e l d o f 0.14, b u t t h e s i m i l a r r e a c t i o n w i t h P u ( I V ) h a s n o t been r e p o r t e d . With the low r e d u c t i o n p o t e n t i a l , i t i s not l i k e l y t o occur. An i n t e r e s t i n g f e a t u r e o f t h e p h o t o r e d u c t i o n of Ce(IV) t o C e ( I I I ) i s the s t a b i l i t y of C e ( I I I ) , which 2

+

2

2

2

3


268

PLUTONIUM

CHEMISTRY


e n a b l e s t h e C e ( I V ) - ( I I I ) e q u i l i b r i u m t o be e a s i l y s t u d i e d . Con v e r s e l y , P u ( I I I ) i s r e l a t i v e l y u n s t a b l e and g e n e r a l l y r e q u i r e s a h o l d i n g r e d u c t a n t , m a k i n g t h e P u ( I V ) - ( I I I ) e q u i l i b r i u m more d i f f i c u l t t o study, even w i t h o u t c o n s i d e r i n g t h e e x p e r i m e n t a l problems a s s o c i a t e d w i t h r a d i o a c t i v i t y . Therefore, even though P u ( I V ) h a s a more i n t e r e s t i n g s o l u t i o n c h e m i s t r y b e c a u s e o f more i n t e r a c t i o n s w i t h l i g a n d s a n d aqueous m e d i a , i t i s u n l i k e l y t h a t the photochemical r e d u c t i o n o f Pu(IV) t o P u ( I I I ) w i l l ever r e c e i v e the a t t e n t i o n a l r e a d y g i v e n t o the cerium system. Photo S h i f t i n the Pu(IV)

Disproportionation

The d i s p r o p o r t i o n a t i o n r e a c t i o n o f P u ( I V ) i s g e n e r a l l y w r i t t e n as 3Pu*

+

3

+ H 0 = 2Pu + + P u 0 2

2 + 2

+ 4H+

(3)

T h i s o v e r a l l r e a c t i o n was shown more t h a n t h r e e d e c a d e s ago ( 1 1 ) t o i n c l u d e a t l e a s t two s e p a r a t e r e a c t i o n s : the r e l a t i v e l y slow r e a c t i o n , 2Pu* and

+

+ H 0 = Pu0 + + P u 2

2

3 +

+ 4H+

(4)

the f a s t electron-exchange r e a c t i o n , Pu** + P u 0

+ 2

= Pu0

2 + 2

+ Pu

3 +

(5)

A f t e r o b s e r v i n g t h e photochemical r e d u c t i o n o f Pu(VI) and P u ( I V ) , i t seems o b v i o u s t h a t r e a c t i o n (3) s h o u l d be l i g h t sensitive. However, i t i s n o t o b v i o u s how p h o t o n s w o u l d a f f e c t the e q u i l i b r i u m concentrations of the plutonium s p e c i e s . The e x p e r i m e n t a l r e s u l t s [ 3 , 4 ] a r e v e r y i n t e r e s t i n g and a r e d e s c r i b e d b e l o w , b u t a c o m p l e t e e x p l a n a t i o n i s y e t t o be d e v e l o p e d . The UV i l l u m i n a t i o n o f c a r e f u l l y s e l e c t e d s o l u t i o n s o f 1 0 " M_Pu(IV) i n 0.5 M_ HC10i+ s i g n i f i c a n t l y i n c r e a s e d t h e d i s p r o p o r t i o n a t i o n r a t e and s h i f t e d t h e e q u i l i b r i u m i n f a v o r o f Pu , Pu0 , a n d a c i d . Those e f f e c t s a r e shown i n F i g . 2. The r e a c t i o n p r o c e e d e d a t 22°C f o r 27 h i n d a r k c o n d i t i o n s a n d was n o t n e a r e q u i l i b r i u m . The sample was e x p o s e d s e v e r a l t i m e s for 1 h periods. A f t e r e a c h e x p o s u r e , t h e r e was a s i g n i f i c a n t decrease i n t h e Pu(IV) c o n c e n t r a t i o n and s t o i c h i o m e t r i c a l l y e q u a l i n c r e a s e s i n t h e P u ( I I I ) and Pu(VI) c o n c e n t r a t i o n s . A f t e r d a r k - c o n d i t i o n e q u i l i b r i u m was e s t a b l i s h e d , a s i n d i c a t e d b y t h e v i s i b l e s p e c t r a , t h e p h o t o - s h i f t i n e q u i l i b r i u m was o b s e r v e d t o be c o m p l e t e l y r e v e r s e d when t h e i l l u m i n a t i o n ceased. T h i s p h o t o g a l v a n i c e f f e c t m a i n t a i n e d a mass b a l a n c e i n t h e s y s t e m , w i t h no r e a g e n t consumed o r g e n e r a t e d d u r i n g t h e d a r k - l i g h t - d a r k c y c l e . This observation suggested that the p l u t o n i u m system i n t h e proper network o f a c o n c e n t r a t i o n c e l l 2

2 +

3

2



1

F i g u r e 2. The e f f e c t s o f UV l i g h t on t h e d i s p r o p o r t i o n a t i o n o f P u ( I V ) The i n c r e a s e d r a t e i e d e m o n s t r a t e d by t h e r a p i d d e c r e a s e i n P u ( I V ) , and c o r r e s p o n d i n g i n c r e a s e i n P u ( I I I ) and P u ( I V ) . A r e v e r s i b l e s h i f t i n the e q u i l i b r i u m i s obvious from the r e t u r n to dark c o n d i t i o n c o n c e n t r a t i o n s a f t e r each exposure. The e f f e c t s o f i n c r e a s i n g t e m p e r a t u r e a r e compared ( 3 ) .

ι

Γ


PLUTONIUM CHEMISTRY


270

c o u l d c o n v e r t l i g h t i n t o an e.ni.f. w i t h o u t t h e e x p e n d i t u r e o f chemical reactants or the generation of chemical products. L a t e r e x p e r i m e n t s ( 4 ) were d e s i g n e d t o d e t e r m i n e a c e l l e.m.f. f o r t h e p l u t o n i u m d i s p r o p o r t i o n a t i o n s y s t e m w i t h a p a r t i c u l a r l i g h t source. Concentration quotients f o r the l i g h t and d a r k c o n d i t i o n s , Qe a n d Q^, were d e t e r m i n e d , and a n e n e r g y d i f f e r e n c e o f 1.65 k c a l ( 3 2 mV) was c a l c u l a t e d by t h e r e l a t i o n - R T l n Qe/QdT h i s r e v e r s i b l e p h o t o c h e m i c a l s h i f t may be t h e o n l y s i n g l e - e l e m e n t s y s t e m known a t t h i s t i m e a n d c e r t a i n l y i s the s i m p l e s t such system. Even t h o u g h t h e r a d i o a c t i v e p r o p e r t i e s c o u l d p r e v e n t d e v e l o p m e n t and u t i l i z a t i o n o f a p l u t o n i u m p h o t o c o n v e r s i o n system, these s t u d i e s c e r t a i n l y suggest t h a t s i m i l a r n o n r a d i o a c t i v e a n d more a c c e p t a b l e s y s t e m s c o u l d be d i s c o v e r e d and developed. C o n s i d e r a t i o n o f where t h e l i g h t e f f e c t o c c u r s i n t h e d i s p r o p o r t i o n a t i o n mechanism i s a n i n t e r e s t i n g e x e r c i s e . The e f f e c t i s t w o - f o l d i n i n c r e a s i n g t h e r e a c t i o n r a t e and i n r e v e r s i b l y s h i f t i n g the r e a c t i o n . Since r e a c t i o n (4) i s r a t e c o n t r o l l i n g , and assuming t h a t t h e r a t e o f r e a c t i o n (5) i s so f a s t t h a t a p h o t o e f f e c t t o i n c r e a s e t h a t r a t e w o u l d n o t be o b s e r v e d by t h e m i l d i l l u m i n a t i o n i n t h e e x p e r i m e n t s , we suggest that the i n i t i a l photoeffect i s e x c i t a t i o n of the Pu(IV) species. Then t h e r a t e a n d t h e d e g r e e o f t h e r e a c t i o n for the e x c i t e d - s t a t e species are greater than that f o r the non-excited state species. P h o t o E f f e c t s on D e g r a d a t i o n o f P u ( I V ) P o l y m e r Plutonium(IV) polymer i s a product o f Pu(IV) h y d r o l y s i s and i s formed i n aqueous s o l u t i o n s a t l o w a c i d c o n c e n t r a t i o n s . D e p o l y m e r i z a t i o n g e n e r a l l y i s a c c o m p l i s h e d by a c i d r e a c t i o n t o form i o n i c Pu(IV), but a c i d degradation o f polymer i s s t r o n g l y d e p e n d e n t on t h e age o f t h e p o l y m e r a n d t h e c o n d i t i o n s u n d e r w h i c h t h e p o l y m e r was f o r m e d ( 1 2 ) . Photoenhancement o f P u ( I V ) d e p o l y m e r i z a t i o n was f i r s t o b s e r v e d w i t h a f r e s h l y p r e p a r e d p o l y m e r m a t e r i a l i n 0.5 _M HC10t+, F i g . 3 (3_). Depolymerization p r o c e e d e d i n d a r k c o n d i t i o n s u n t i l a f t e r 140 h , 18% o f t h e p o l y m e r r e m a i n e d . F o u r r a t h e r m i l d 1-h i l l u m i n a t i o n s o f i d e n t i c a l samples a t 5, 25, 52, a n d 76 h enhanced t h e d e p o l y m e r i z a t i o n r a t e s so t h a t o n l y 1% p o l y m e r r e m a i n e d a f t e r t h e f o u r t h l i g h t exposure ( F i g . 3 ) . An enhancement i n d e p o l y m e r i z a t i o n r a t e a f t e r t h e l i g h t was t u r n e d o f f was a l s o o b s e r v e d . T h i s c o n t i n u e d enhancement was v e r y o b v i o u s a f t e r t h e e x p o s u r e s a t 25 and 52 h, and i s believed to represent r e l a t i v e l y f a s t depolymerization of small p o l y m e r p a r t i c l e s w h i c h were f r a g m e n t e d f r o m l a r g e r p a r t i c l e s during i l l u m i n a t i o n . Enhancement o f d e p o l y m e r i z a t i o n r a t e s f o r aged p o l y m e r ( p o l y m e r w i t h h i g h d e g r e e o f P u 0 c h a r a c t e r ( 1 3 ) were o b s e r v e d 2



BELL ET AL.


TIME,

hrs

F i g u r e 3. The e f f e c t s o f f o u r 1 h , l o w i n t e n s i t y , UV l i g h t e x p o s u r e s t o enhance t h e d e g r a d a t i o n o f f r e s h l y prepared Pu(IV) polymer. The d a r k e n e d l i n e r e p r e s e n t s d e p o l y m e r i z a t i o n under d a r k c o n d i t i o n s . The s o l u t i o n c o n t a i n e d 0.0093 M_ t o t a l Pu and 0.47 _M HClOi^ a t 22°C ( 3 ) .


272

PLUTONIUM

CHEMISTRY


a t a l a t e r t i m e (4_)). P o l y m e r m a t e r i a l s w h i c h h a d been a g e s t a b i l i z e d by t i m e a n d t e m p e r a t u r e were p a r t i a l l y d e p o l y m e r i z e d i n three a c i d s o l u t i o n s and w i t h almost continuous i l l u m i n a tion. P l o t s o f p e r c e n t p o l y m e r v e r s u s e x p o s u r e t i m e a r e shown i n F i g . 4, a n d a r e t h e r e compared w i t h t h e c o r r e s p o n d i n g amounts o f p o l y m e r r e m a i n i n g u n d e r d a r k c o n d i t i o n s . The p h o t o enhancement o f P u ( I V ) d e p o l y m e r i z a t i o n i s c l e a r l y e s t a b l i s h e d f o r p o l y m e r aged b o t h b y t i m e a n d t e m p e r a t u r e .

1

Γ

Time Aged Polymer in A 0.13/V HCIO4 B 0.45/V HCIO4 C 2.O/VHCIO4

Temperature Aged Polymers in

90h

D

20

1.95/VHCI0

F

4.0/V HCIO4 ( 5 0 ° C )

4

I 40

30

UV EXPOSURE T I M E

O.I6/VHCIO4

Ε

I 50

(hours)

F i g u r e 4: The e f f e c t s o f UV i r r a d i a t i o n t o enhance t h e d e g r a d a t i o n o f aged P u ( I V ) p o l y m e r i n HCIO^. D e p o l y m e r i z a t i o n under dark c o n d i t i o n s f o r each e x p e r i m e n t i s shown b y a d a t a p o i n t d i r e c t l y above t h e l a s t l i g h t sample p o i n t ( 4 ) .


18.

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273


Concluding Remarks Several notable photochemical reactions have been observed for aqueous plutonium systems. Only the perchlorate system has been described i n detail, and the effects of complexing ligands and various reducing agents are not known. The effects of self radiolysis have not been compared to direct photochemistry of plutonium chemistry. However, these effects generally change the aqueous medium, and the radiolysis products of the medium then react with the plutonium. In studies considered herein, the direct absorption of UV light by plutonium in aqueous media and the subsequent chemistry of this excited state plutonium have overwhelmed the effects of self radiolysis. Application of aqueous plutonium photochemistry probably w i l l remain limited to nuclear fuel processing, as discussed i n a recent publication (14). However, the most important consideration of plutonium photochemistry may be with respect to plutonium i n the environment, where the transport rates would be dependent on i t s oxidation states and complexing reactions. Acknowledgments Research sponsored by the Division of Material Sciences, Office of Basic Energy Sciences, U.S. Department of Energy under contract W-7405-eng-26 with the Union Carbide Corporation. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

Cleveland, J. M. "The Chemistry of Plutonium"; Gordon and Breach: New York; 1970. Wick, O. J. "The Plutonium Handbook"; Vol. I. Gordon and Breach: New York; 1967. B e l l , J. T . ; Friedman, H. A. J. Inorg. Nucl. Chem. 1976, 38, 83. Friedman, Η. Α.; Toth, L. M.; Bell, J. T. J. Inorg. Nucl. Chem. 1977, 39. Burger, L. L . ; Rehn, I. M.; Slansky, C. M. USAEC Report, HW-19949; 1952. Mazumar, A. S. Ghosh; Sivarmakrishnan, C. K. J. Inorg. Nucl. Chem. 1965, 27, 2423. Palei, P. N . ; Nemodruk, Α. Α.; Berzogova, Ε. V. Radiochimk 1967, 11, 30. Bell, J. T . ; B i l l i n g s , M. R. J. Inorg. Nucl. Chem. 1975, 37, 2529. Costanzo, D. Α.; Biggers, R. E . ; B e l l , J. T. J. Inorg. Nucl. Chem. 1973, 35, 609. B e l l , J. T. J. Inorg. Nucl. Chem. 1969, 31, 703. Connick, R. E. J. Am. Chem. Soc. 1949, 71, 1528.


274

12.

13. 14.

PLUTONIUM CHEMISTRY

Toth, L. M.; Osborne, M. M. "Further Aspects of Pu(IV) Hydrous Polymer Chemistry," Symposium on the Chemistry of Plutonium, American Chemical Society Meeting, Kansas City, Missouri, September 1982. Johnson, G. L . ; Toth, L. M. "Plutonium(IV) and Thorium(IV) Hydrous Polymer Chemistry," ORNL/TM-6365, 1978. B e l l , J. T . ; Toth. L. M. Radiochimica Acta 1978, 25, 225.


RECEIVED December 21, 1982


Photochemistry of Aqueous Plutonium Solutions - ACS Publications

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