Plutonium Chemistry - ACS Publications - American Chemical Society

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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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232

PLUTONIUM CHEMISTRY

H

4

[Pu(OH) ] ~ x

x

2

PuCOH)^] ^-*)

4

+ [ ( H O ^ P u ] - * = [(ΗΟ)χ-ιΡιι

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H

(2)

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

(1)

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

<

2

I

ο -J

ι

1 0.25

0.20

M H

0.12

+

_L 0

1

2 -LOG

3

4

[ Pu ( I V ) ]

Figure 2. Determination of r e a c t i o n order f o r Pu(IV) polymer growth stage at 50° C. S l o p e d 3 f o r a l l HN0~ concentrations indicated. (Reprinted with permission from Ref. 2.)

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

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τ ο τ Η ET AL.

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Pu(IV) Hydrous Polymer Chemistry

w h i c h s h i f t s t h e above e q u i l i b r i u m t o t h e l e f t . (The same s h i f t i n t h e e q u i l i b r i u m i s o b t a i n e d b y a d d i n g NaN03 o r Ca(N03)2«) T h i s s h i f t c a u s e s a l o w e r a c i ­ d i t y and hence a n i n c r e a s e d r a t e o f p o l y m e r i z a t i o n .

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(2)

+

The p r e s e n c e o f U U 2 ^ c a u s e s a n e t d e c r e a s e i n t h e r a t e o f p o l y m e r i z a t i o n w h i c h means t h a t i t must o v e r ­ come t h e n i t r a t e i o n e f f e c t t h a t a c c o m p a n i e s i t .

C h e m i c a l a n a l y s e s r e v e a l t h a t m e a s u r a b l e amounts o f u r a n y l i o n a r e a c t u a l l y p r e s e n t i n P u ( I V ) p o l y m e r s grown i n m i x t u r e s o f P u ( I V ) and u r a n y l n i t r a t e s u g g e s t i n g t h a t u r a n y l i o n i s b e i n g t a k e n up i n t h e p o l y m e r n e t w o r k and c o n s e q u e n t l y hampers the growth through a c h a i n t e r m i n a t i o n process as suggested i n F i g . 3. The u r a n y l s e r v e s t o t e r m i n a t e a c t i v e s i t e s b e c a u s e i t does n o t t y p i c a l l y f o r m e x t e n s i v e p o l y m e r i c a g g r e g a t e s a s does P u ( I V ) ; i n s t e a d i t t e n d s o n l y t o d i m e r i z e a n d , a t most, t r i m e r i z e (4). The p r o o f o f t h e c h a i n t e r m i n a t i n g f u n c t i o n o f U 0 2 could be o b t a i n e d by t h e i d e n t i f i c a t i o n o f a u r a n y l - t o - p o l y m e r bond t h r o u g h t h e usage o f Raman s p e c t r o s c o p y . However, Raman s p e c t r a o f p l u t o n i u m s o l u t i o n s a r e d i f f i c u l t t o measure s i n c e t h e s o l u t i o n s must be k e p t i n a l p h a - c o n t a i n m e n t b o x e s ; we, t h e r e f o r e , have t u r n e d t o p o l y m e r s o l u t i o n s o f T h ( I V ) i n p l a c e of Pu(IV) f o r the i n i t i a l s t u d i e s . I t i s through the decrease i n the frequency of the U 0 symmetric s t r e t c h i n g v i b r a t i o n t h a t t h e a t t a c h m e n t o f UO2 t o the Th(IV) polymer network i s d e m o n s t r a t e d , f o r when u r a n y l d i m e r i z e s i n p u r e s o l u t i o n s a s i m i l a r s h i f t occurs. (One o b s e r v e s s e p a r a t e s y m m e t r i c s t r e t c h i n g v i b r a t i o n s due t o monomeric u n h y d r o l y z e d u r a n y l , d i m e r i c and t r i m e r i c h y d r o x y l - b r i d g e d u r a n y l e a c h a t p r o g r e s ­ s i v e l y l o w e r v i b r a t i o n a l f r e q u e n c i e s (5_).) I n u r a n y l - t h o r i u m ( I V ) m i x t u r e s t h a t a r e a d j u s t e d t o pH = 3.1, t h e a p p e a r a n c e o f t h e u r a n y l symmetric s t r e t c h i n g v i b r a t i o n a t t r i b u t a b l e t o a d i m e r i c s p e c i e s a p p e a r s q u i t e r e a d i l y ( i n a d d i t i o n t o t h a t o f t h e mono­ m e r i c s p e c i e s ) e v e n t h o u g h t h e pH i s n o t h i g h enough f o r u r a n y l t o h y d r o l y z e a n d a g g r e g a t e on i t s own. I t i s f o r t h i s r e a s o n t h a t we b e l i e v e U 0 2 ^ c h e m i c a l l y a t t a c h e s t o t h e p o l y m e r i c n e t ­ work t h r o u g h h y d r o x y l b r i d g e s . 2 +

2 +

2

+

Carbon D i o x i d e A d s o r p t i o n on D r i e d P o l y m e r . Other un­ e x p e c t e d i n t e r a c t i o n s o f t h e s e h y d r o l y t i c p o l y m e r s have b e e n n o t e d p r e v i o u s l y d u r i n g t h e measurement o f i n f r a r e d s p e c t r a o f d r i e d Pu(IV) polymers ( l i k e those used f o r d i f f r a c t i o n studies). V i b r a t i o n a l bands f i r s t a t t r i b u t e d t o n i t r a t e i o n were o b s e r v e d i n s a m p l e s e x p o s e d t o room a i r ; h o w e v e r , t h e s e bands were n o t p r e s e n t i n s a m p l e s p r e p a r e d u n d e r n i t r o g e n a t m o s p h e r e s ( s e e F i g . 4) (6). Chemical analyses e s t a b l i s h e d enough c a r b o n i n t h e e x p o s e d s a m p l e s t o c o n f i r m t h e a s s i g n m e n t o f t h e e x t r a n e o u s bands t o t h e c a r b o n a t e f u n c t i o n a l g r o u p

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

PLUTONIUM CHEMISTRY

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236

F i g u r e 4, I n f r a r e d s p e c t r a o f K B r p e l l e t s c o n t a i n i n g P u ( I V ) p o l y m e r p r e c i p i t a t e s , (A) p r e p a r e d i n ^ - p u r g e d g l o v e b a g f r e e o f CO2; (B) p r e p a r e d i n l a b o r a t o r y a t m o s p h e r e . (Reprinted with p e r m i s s i o n f r o m R e f . 6.)

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

15.

Pu(IV) Hydrous Polymer Chemistry

TOTH ET AL.

formed by t h e r a p i d a d s o r p t i o n o f a t m o s p h e r i c CO2. Although t h e a f f i n i t y o f t h e p o l y m e r f o r CO2 i s n o t u n u s u a l , i t has n o t o f t e n been c o n s i d e r e d i n p r e v i o u s d i f f r a c t i o n s t u d i e s and c o u l d h a v e compromised some o f t h e e a r l i e r r e s u l t s . The i m p l i c a t i o n o f t h e s e two e x a m p l e s i s t h a t t h e medium i n w h i c h t h e P u ( I V ) h y d r o l y s i s c h e m i s t r y i s s t u d i e d has a s t r o n g b e a r i n g on t h e outcome o f t h e r e s u l t s . I n t h e p a s t , we were c o n t e n t t o t r e a t t h e p u r e s y s t e m s and e i t h e r i g n o r e e x t e r ­ n a l i n t e r f e r e n c e s ( s u c h as t h e a t m o s p h e r e ) o r i n f e r t h e b e h a v ­ i o r of m i x t u r e s (such as P u and U 0 2 ) b a s e d on t h e known c h e m i s t r i e s o f t h e i n d i v i d u a l s p e c i e s . The example o f U02 i n t e r a c t i o n s w i t h Pu(IV) polymer demonstrates t h a t n e i t h e r of these approaches i s a c c u r a t e . Therefore, future r e s e a r c h e f f o r t s w i l l n e c e s s a r i l y have t o c o n s i d e r p l u t o n i u m h y d r o l y s i s r e a c t i o n s i n more d e t a i l t h a n has p r e v i o u s l y been done. 4 +

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237

2 +

2 +

Parametric

Studies

R e f l u x Experiments. More r e c e n t e f f o r t s have been d i r e c t e d a t a q u a n t i t a t i v e e v a l u a t i o n of those parameters t h a t a f f e c t polymer growth, namely a c i d i t y , p l u t o n i u m c o n c e n t r a t i o n , t e m p e r a t u r e , and r e f l u x a c t i o n . The l a s t i s an i n t e r e s t i n g e x a m p l e t o i l l u s t r a t e s i n c e t h e a d m i s s i o n o f low a c i d c o n d e n ­ s a t e s o r d i l u e n t s t o a P u ( I V ) s o l u t i o n c a u s e s some p o l y m e r f o r ­ m a t i o n e v e n when t h e b u l k s o l u t i o n i s o t h e r w i s e a c i d i c enough t o p r e v e n t any m e a s u r a b l e d e g r e e o f h y d r o l y s i s . T h i s u n t i m e l y p o l y m e r f o r m a t i o n i s u n d e r s t o o d t o be c a u s e d b y t h e v e r y r a p i d h y d r o l y s i s and a g g r e g a t i o n o f monomeric Pu(IV) s p e c i e s ( a t the r e g i o n of condensate r e e n t r y i n t o the hot plutonium s o l u t i o n ) t o produce hydrous polymers t h a t are not r e a d i l y depolymerized. At h i g h t e m p e r a t u r e s s u c h as f o u n d under r e f l u x c o n d i t i o n s , the polymer r a p i d l y ages through the c o n v e r s i o n of h y d r o x y l - to oxo-bridges: H n+ Pu

Pu'

Pu -

n+ +

H0 2

(4)

0 and

t h u s becomes h i g h l y r e s i s t a n t t o r e d i s s o l u t i o n . A l t h o u g h t h e e f f e c t o f r e f l u x on p o l y m e r f o r m a t i o n has b e e n r e c o g n i z e d f o r many y e a r s , l i t t l e d e t a i l e d i n f o r m a t i o n i s a v a i l a b l e c o n c e r n i n g t h e e x t e n t t o w h i c h c h a n g e s i n tem­ p e r a t u r e , a c i d i t y , and p l u t o n i u m c o n c e n t r a t i o n a f f e c t i t . R e c e n t work i n t h i s l a b o r a t o r y has s o u g h t t o p r o v i d e some o f t h i s i n f o r m a t i o n , u n f o r t u n a t e l y , absorption spectrophotometry i s not s u i t a b l e f o r m o n i t o r i n g the f o r m a t i o n of Pu(IV) polymer

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

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CHEMISTRY

u n d e r r e f l u x c o n d i t i o n s , b e c a u s e t h e aged p o l y m e r formed a t a b o u t 100°C does n o t r e m a i n s u s p e n d e d i n t h e s o l u t i o n . Instead i t p r e c i p i t a t e s and p r o d u c e s e x t e n s i v e l i g h t s c a t t e r . More q u a l i t a t i v e techniques which i n v o l v e simply r e f l u x i n g the s o l u ­ t i o n s u n t i l p r e c i p i t a t e s a r e v i s u a l l y o b s e r v e d have t h e r e f o r e been used. R e f l u x r a t e s o f a p p r o x i m a t e l y 0.3 m l / m i n i n t h e s e e x p e r i m e n t s p r o d u c e a t u r n o v e r o f t h e 10 m l samples i n 0.5 h . R e f l u x e d s a m p l e s were compared w i t h n o n r e f l u x e d samples h e l d i s o t h e r m a l l y i n a n o i l b a t h . The l e n g t h o f t i m e u n t i l p o l y m e r was o b s e r v e d i n 0.05 M_ Pu s o l u t i o n s i s shown i n T a b l e 1. I t i s s e e n t h a t above 2 H_ HNO3 no p o l y m e r f o r m e d i n t h e n o n r e f l u x c o n t r o l s a m p l e s , whereas p o l y m e r f o r m e d u n d e r a l l c o n d i t i o n s tested at reflux. F u r t h e r m o r e , t h e p o l y m e r t h a t formed was r e s i s t a n t t o a l l attempts a t r e d i s s o l u t i o n i n n i t r i c a c i d , thus s u g g e s t i n g a v e r y aged s t a t e o f t h e p o l y m e r . T a b l e 1. F o r m a t i o n o f aged p o l y m e r under r e f l u x and n o n r e f l u x c o n d i t i o n s w i t h 0.05 M Pu a t 105°C a

HNO3 concentration (M)

Hours t o f o r m o b s e r v a b l e p o l y m e r Reflux Nonreflux

1 2 3 4 6

1.5 2.0 4.0 22.0 48.0

[0.013] [0.029] [0.050] [0.013] [0.007]

c

d

18 191 b b b

[0.049] [0.049] [0.050] [0.050] [0.045]

a

the

V a l u e s i n brackets a r e the Pu(IV) molar c o n c e n t r a t i o n s a t end o f t h e e x p e r i m e n t .

^No p o l y m e r f o r m a t i o n was o b s e r v e d , e v e n a f t e r a t o t a l o f 596 h . c

Decrease

[HNO3]

i n a c i d i t y n o t e d a t end o f e x p e r i m e n t ; f i n a l

= 3.5M.

^ D e c r e a s e i n a c i d i t y n o t e d a t end o f e x p e r i m e n t ;

final

[ H N O 3 ] = 5.0 M.

T a b l e 2 shows t h e e f f e c t o f r e f l u x o n h i g h c o n c e n t r a t i o n s o f p l u t o n i u m ( u p t o 0.55 M) i n c o m p a r i s o n t o n o n r e f l u x s i t u a t i o n s o f comparable a c i d i t i e s . Again, the nonreflux s o l u ­ t i o n s d i d n o t p o l y m e r i z e above a f i x e d a c i d c o n c e n t r a t i o n ( i n t h i s c a s e 3M) whereas a l l s o l u t i o n s t h a t were r e f l u x e d showed the f o r m a t i o n o f p o l y m e r . I f one t a k e s t h e 4 o r 6 M H N O 3 d a t a under r e f l u x c o n ­ d i t i o n s and e v a l u a t e s t h e i n c r e a s e i n r a t e s w i t h i n c r e a s i n g c o n c e n t r a t i o n o f P u ( I V ) , he w o u l d f i n d t h a t t h e t i m e t o f o r m o b s e r v b l e p o l y m e r i s i n v e r s e l y p r o p o r t i o n a l t o t h e f i r s t power (1.1 ± .4) o f t h e P u ( I V ) c o n c e n t r a t i o n . At f i r s t glance t h i s m i g h t seen t o c o n t r a d i c t e a r l i e r f i n d i n g s (7_) t h a t showed t h e

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

15.

TOTH ET AL.

239

Pu(IV) Hydrous Polymer Chemistry

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p o l y m e r g r o w t h r a t e t o f o l l o w a t h i r d power dependency on P u ( I V ) c o n c e n t r a t i o n ; b u t t h e f o r m e r c a s e was f o r t h e g r o w t h r a t e a f t e r p o l y m e r was o b s e r v e d a n d t h e p r e s e n t c a s e i s f o r t h e appearance o r i n i t i a t i o n o f polymer. The i n i t i a t i o n o f p o l y m e r i n v o l v e s a mechanism t h a t h a s n o t y e t been e l u c i d a t e d a n d , t h e r e f o r e , the f i r s t order k i n e t i c s found here a r e not i n c o n t r a d i c t i o n w i t h the previous study. ( I f t h e time t o form o b s e r v a b l e polymer f o l l o w e d t h i r d o r d e r k i n e t i c s , a 1000-fold i n c r e a s e i n r a t e w o u l d be e x p e c t e d f o r a 1 0 - f o l d i n c r e a s e i n concentration.) Table

2.

F o r m a t i o n o f aged p o l y m e r u n d e r r e f l u x and n o n r e f l u x c o n d i t i o n s w i t h 0.27 a n d 0.55 M Pu a t 105°C

HN0 concentration

Pu concentration

3 4

0.55 0.27 0.55 0.55

3

6

Hours t o perform observable parameters Nonreflux Reflux 97 [0.493] 7.6 [0.093] 1.5 [0.261] 3.0 [0.357]

b b

[0.455] [0.348]

a

V a l u e s i n brackets a r e the Pu(IV) molar c o n c e n t r a t i o n s a t t h e end o f t h e e x p e r i m e n t . ^No p o l y m e r f o r m a t i o n o b s e r v e d , 596 h.

even a f t e r a t o t a l o f

Conclusions Many r e p o r t s on t h e h y d r o l y s i s o f P u ( I V ) a n d p o l y m e r i z a ­ t i o n (aggregation) of the primary h y d r o l y s i s products e x i s t i n one f o r m o r a n o t h e r . The v a l i d i t y o f some o f t h e e a r l i e r d a t a may be s u b j e c t t o q u e s t i o n b e c a u s e t h e e x p e r i m e n t a l c o n d i t i o n s were n o t p r o p e r l y c o n t r o l l e d . T h e r e f o r e , these systems deserve f u r t h e r c o n s i d e r a t i o n f o r t h e sake o f r e f i n e m e n t s . Nevertheless, the major area o f i n t e r e s t f o r the f u t u r e w i l l remain w i t h i n t e r a c t i o n s between P u ( I V ) h y d r o l y s i s p r o d u c t s and o t h e r r e a c ­ t i v e species present i n the s o l u t i o n . There i s n o t o n l y c o n ­ s i d e r a b l e promise o f e l u c i d a t i n g n o v e l chemical i n t e r a c t i o n s , b u t t h e r e i s a l s o a g r e a t p r a c t i c a l need t o f u l l y u n d e r s t a n d t h e e x t e n t o f t h e s e i n t e r a c t i o n s i n o r d e r t o e n s u r e t h e most complete c o n t r o l of plutonium i n r e p r o c e s s i n g o p e r a t i o n s . Acknowledgments R e s e a r c h s p o n s o r e d by t h e O f f i c e o f Spent F u e l Management and R e p r o c e s s i n g S y s t e m s , U.S. Department o f E n e r g y u n d e r c o n t r a c t W-7405-eng-26 w i t h t h e U n i o n C a r b i d e C o r p o r a t i o n .

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3. 4. 5. 6. 7.

Johnson, G. L.; Toth, L. M. "Pu(IV) and Th(IV) Hydrous Polymer Chemistry" ORNL/TM-6365, May 1978. Figure taken from Toth, L.M.; Friedman, Η. Α.; Osborne, M. M. J. Inorg. Nucl. Chem. 1981, 43 (11) 2929. Harvey, W. W.; Turner, M. J.; Slaughter, J.; Makridges, A. C. "Study of Silica Scaling from Geothermal Brines," EIC Corp. Prog. Rep. March-September 1976, COO-2607-3. Baes, C. F.; Mesmer, R. E. "The Hydrolysis of Cations"; Wiley, New York, 1976, pp. 177-182. Toth, L. M.; Begun, G. M. J. Phys. Chem. 1981, 85, 547. Figure taken from Toth, L. M.; Friedman, H. A. J. Inorg. Nucl. Chem. 1978, 40, 807. Toth, L. M.; Friedman, Η. Α.; Osborne, M. M. J. Inorg. Nucl. Chem., 1981 43 (11) 2929.

RECEIVED December

21,1982

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