15 Aspects of Plutonium(IV) Hydrous Polymer Chemistry
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L. M. TOTH, H. A. FRIEDMAN, and M. M. OSBORNE Oak Ridge National Laboratory, Chemical Technology Division, Oak Ridge, TN 37830
The polymerization of Pu(IV) hydrolysis products in aqueous nitric acid solutions containing uranyl nitrate has been examined as a function of pH, temperature, and concentration. Even in the absence of the uranyl solute, an induction period usually follows the polymer growth stage during which time formation of primary hydrolysis products occurs. Uranyl nitrate retards the polymerization rate by approximately 35% in spite of the counteracting influence of the nitrate ions associated with this solute; evidence is given to demonstrate that the uranyl ion attaches through hydroxyl bridges to active sites in the polymer network and functions as a chain-terminating unit. The rate of polymer growth has been shown to be third order with respect to Pu(IV). The reflux of aqueous Pu(IV) solutions containing 3 M. 3
2
3
The h y d r o l y s i s o f P u ( I V ) Pu
4 +
4
+ xli 0 = [Pu(0H) ] ~ 2
x
x
+ xli+
and subsequent a g g r e g a t i o n o f h y d r o l y s i s
products
0097-6156/83/0216-0231$06.00/0 © 1983 American Chemical Society
Carnall and Choppin; Plutonium Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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PLUTONIUM CHEMISTRY
H
4
[Pu(OH) ] ~ x
x
2
PuCOH)^] ^-*)
4
+ [ ( H O ^ P u ] - * = [(ΗΟ)χ-ιΡιι
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H
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h a s been a s u b j e c t o f c o n s i d e r a b l e s t u d y o v e r t h e p a s t f o u r decades. S i n c e t h i s s t r o n g l y a c i d dependent c h e m i s t r y i s n o t u n u s u a l when compared w i t h t h a t o f o t h e r m e t a l c a t i o n s , t h e e x p e r i m e n t a l techniques a r e t y p i c a l o f those used i n s t u d i e s o f o t h e r h y d r o l y s i s and a g g r e g a t i o n r e a c t i o n s . These i n c l u d e ( 1 ) t h e e l e c t r o c h e m i s t r y and s p e c t r o s c o p y o f t h e elementary hydro l y s i s r e a c t i o n s , l i g h t s c a t t e r i n g and u l t r a c e n t r i f u g e examina t i o n of the s i z e of the hydrous polymeric aggregates, d i f f r a c t i o n and m i c r o s c o p y e x p e r i m e n t s on t h e n a t u r e o f t h e a g g r e g a t e s , and g e n e r a l c h e m i c a l s t u d i e s on t h e r e a c t i v i t y o f P u ( I V ) and i t s p o l y m e r i c p r o d u c t s . Although the r e s u l t s from t h e s e p r e v i o u s p l u t o n i u m i n v e s t i g a t i o n s may a p p e a r t o be s u f f i c i e n t f o r an understanding o f the c h e m i s t r y i n v o l v e d i n the h y d r o l y s i s r e a c t i o n s , there s t i l l remain u n c e r t a i n t i e s that must be worked o u t t o e n s u r e r e l i a b l e p e r f o r m a n c e i n t h e aqueous r e p r o c e s s i n g o f n u c l e a r f u e l s . The need f o r a d d i t i o n a l work stems i n p a r t f r o m s e v e r a l deficiencies. One o f t h e s e , w h i c h i s c h a r a c t e r i s t i c o f a l l a c t i n i d e s , i s t h a t much o f t h e work h a s o n l y been documented, at b e s t , i n unrefereed l a b o r a t o r y p u b l i c a t i o n s and, a t worst, i n the notebooks or r e c o l l e c t i o n s o f that g e n e r a t i o n o f a c t i n ide researchers. I n a d d i t i o n , t h e e a r l i e r r e s e a r c h was f o c u s e d m a i n l y on p r o v i d i n g b a s i c i n f o r m a t i o n a b o u t i s o l a t e d P u ( I V ) systems; and w h i l e t h i s has been n e c e s s a r y as a f i r s t s t e p , t h e r e a l i z a t i o n t h a t most r e a l s i t u a t i o n s i n v o l v e p l u t o n i u m i n t h e p r e s e n c e o f many i n t e r a c t i v e i o n s d i c t a t e s t h e d e s i g n o f more c o m p l i c a t e d a n d e x t e n s i v e e x p e r i m e n t s . The r e c e n t i n t e r e s t s i n P u ( I V ) h y d r o l y s i s c h e m i s t r y have a r i s e n from a d e s i r e t o d e f i n e i t s behavior i n these r e a l i s t i c s i t u a t i o n s , e . g . , i n t h e p r e s e n c e o f l a r g e amounts o f accom p a n y i n g s o l u t e s , namely u r a n y l n i t r a t e . T h i s work h a s l e d t o t h e r e v e l a t i o n o f some v e r y i n t e r e s t i n g a s p e c t s o f i n t e r a c t i o n between u r a n y l i o n s and p l u t o n i u m polymer. S t r u c t u r a l aspects o f t h e p o l y m e r n e t w o r k g l e a n e d t h r o u g h t h e usage o f i n f r a r e d and Raman s p e c t r o s c o p y have r e v e a l e d t h e e x a c t n a t u r e o f t h e s e i n t e r a c t i o n s w i t h o t h e r i o n s ; t h e s e t e c h n i q u e s have g r e a t promise of p r o v i d i n g f u r t h e r i n s i g h t s i n t o the i n t e r a c t i o n s b e t w e e n p l u t o n i u m a n d t h e medium i n w h i c h i t r e s i d e s . The g e n e r a l g o a l i n t h e r e c e n t e f f o r t s h a s been v e r y p r a c t i c a l — t o ensure b e t t e r c o n t r o l o f Pu(IV) i n low a c i d r e p r o c e s s i n g steams t h r o u g h q u a n t i t a t i v e measurements o f t h e parameters that determine polymer f o r m a t i o n . Admittedly, simi l a r s t u d i e s have been c o n d u c t e d t o some d e g r e e a l r e a d y , b u t
Carnall and Choppin; Plutonium Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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15.
τ ο τ Η ET AL.
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Pu(IV) Hydrous Polymer Chemistry
w i t h b e t t e r t e c h n i q u e s we c a n o f f e r r e f i n e m e n t s t h a t more a c c u r a t e l y g u i d e t h e d e s i g n e n g i n e e r away f r o m s i t u a t i o n s where plutonium(IV) hydrolysis w i l l p r e v a i l . An example o f t h i s p a r a m e t r i c s t u d y w i l l be g i v e n t o w a r d t h e end o f t h i s p r e s e n tation. The t e c h n i q u e s u s e d i n t h e work h a v e g e n e r a l l y b e e n spectroscopic; visible-uv f o r quantitative determinations of s p e c i e s c o n c e n t r a t i o n s and infrared-Raman f o r s t r u c t u r a l a s p e c t s o f t h e polymer. A l t h o u g h t h e f o r m e r h a s o f t e n been u s e d i n t h e s t u d y o f p l u t o n i u m s y s t e m s , t h e r e h a s been c o n s i d e r a b l y l e s s usage made o f t h e l a t t e r i n t h e a c t i n i d e h y d r o l y s i s mechanisms. N a t u r e o f t h e Hydrous
Polymer R e a c t i o n
I f t h e growth o f Pu(IV) hydrous polymer i s monitored s p e c t r o p h o t o m e t r i c a l l y a t 400 nm a s a f u n c t i o n o f t i m e , i t i s o b s e r v e d under many c i r c u m s t a n c e s ( c f . , F i g . 1) ( 2 ) t h a t t h e polymer growth i n n i t r i c a c i d s o l u t i o n o f t e n proceeds a f t e r a n i n d u c t i o n period which i s determined by the p u r i t y o f the system. The s u b s e q u e n t g r o w t h s t a g e o f t h e p o l y m e r i n n i t r i c a c i d s o l u t i o n occurs through a r e a c t i o n that i s t h i r d order w i t h r e s p e c t t o Pu(IV) c o n c e n t r a t i o n a s demonstrated by t h e p l o t o f r a t e c o n s t a n t s as a f u n c t i o n o f Pu(IV) c o n c e n t r a t i o n shown i n F i g . 2 ( 2 ) . Shown h e r e a r e s e v e r a l s e t s o f d a t a t h a t a l l have a s l o p e o f 3. I n c l u d e d o n t h e p l o t i s a s i n g l e p o i n t f o r a HNO3 c o n c e n t r a t i o n o f 0.12_M t o i n d i c a t e where o t h e r d a t a a t t h i s a c i d i t y would f a l l under t h e assumption t h a t t h e t h i r d o r d e r r e l a t i o n s h i p was s t i l l v a l i d . These c h a r a c t e r i s t i c s o f h y d r o u s p o l y m e r f o r m a t i o n a r e t y p i c a l o f many h y d r o l y t i c systems, e.g., S i ( 0 H ) 4 , and l e n d c o n f i d e n c e i n e x t r a p o l a t i n g t h e s e d a t a t o more e x t r e m e a n d d i f f i c u l t t o measure c o n d i t i o n s . U r a n y l N i t r a t e I n f l u e n c e o n P o l y m e r Growth. The e f f e c t o f a s o l u t e s u c h a s u r a n y l n i t r a t e on t h i s p o l y m e r f o r m a t i o n i s s o complex t h a t t h e n e t e f f e c t on t h e polymer growth r a t e cannot be p r e d i c t e d . Experimentally, i t i s observed that the rates o f g r o w t h a t g i v e n i n i t i a l HNO3 c o n c e n t r a t i o n a r e a l w a y s s l o w e r i n the p r e s e n c e o f U 0 ( N 0 3 ) a s i n d i c a t e d b y t h e s o l i d c u r v e i n F i g . 1. T h i s o c c u r s i n s p i t e o f a n o b s e r v e d b a c k - s h i f t i n t h e Pu(IV) d i s p r o p o r t i o n a t i o n e q u i l i b r i u m , 2
2
4
3
2
3Pu + + 2H 0 = 2Pu + + P u 0 + + 2
(3)
2
and t h e a c c o m p a n y i n g d e c r e a s e i n t h e a c i d i t y o f t h e s o l u t i o n when u r a n y l n i t r a t e i s added. Thus t h e p r e s e n c e o f υ θ ( Ν θ 3 ) i n t h e s o l u t i o n c a u s e s two phenomena t o o c c u r . 2
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2
the i n c r e a s e i n n i t r a t e i o n c o n c e n t r a t i o n causes t h e s t a b i l i z a t i o n o f Pu(IV) through n i t r a t e complexation
Carnall and Choppin; Plutonium Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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PLUTONIUM CHEMISTRY
28
0.18 100
150
200
M 300
MINUTES
Figure 1. Percent Pu(IV) polymer vs. time f o r 0.05 M Pu s o l u t i o n s at 50°C. Solid/dashed l i n e s — s o l u t i o n s with/without 0.05 M U0 (NOo)2 added. Makeup HN0 concentrations f o r s o l u t i o n s are i n d i c a t e d . (Reprinted with permission from Ref. 2.) 2
3
^
3
_i Ο Ο ι ι
