Chemical Modeling in Aqueous Systems - American Chemical Society

16 Critical Review of Plutonium Equilibria of Environmental Concern JESS M. C L E V E L A N D

Downloaded by UNIV OF ARIZONA on November 29, 2012 | http://pubs.acs.org Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch016

U.S. Geological Survey, Water Resources Division, Lakewood, CO 80225

The complex i n t e r r e l a t i o n s h i p s of three types of chemical e q u i l i b r i a , namely o x i d a t i o n - r e d u c t i o n , h y d r o l y s i s , and complexat i o n , as w e l l as p o l y m e r i z a t i o n , a nonequilibrium process, d e t e r mine the nature and s p e c i a t i o n of plutonium i n aqueous environmental systems. T h i s paper presents a s e l e c t i v e , c r i t i c a l review of the l i t e r a t u r e d e s c r i b i n g these processes. Although most research has been conducted under non-environmental c o n d i t i o n s — that i s , macro concentrations of plutonium and high a c i d i t i e s — t h e r e s u l t s i n some cases are a p p l i c a b l e to environmental c o n d i t i o n s . In other cases the behavior i s d i f f e r e n t , however, and care should always be e x e r c i s e d i n e x t r a p o l a t i n g macro data to environmental conditions. Oxidation-Reduction P o t e n t i a l s The o x i d a t i o n - r e d u c t i o n behavior of plutonium i s described by the redox p o t e n t i a l s shown i n Table I. (For the purposes of t h i s paper, the unstable and environmentally unimportant heptavalent o x i d a t i o n s t a t e w i l l be ignored.) These values are of a high degree of accuracy, but are v a l i d only f o r the media i n which they are measured. In more s t r o n g l y complexing media, the potent i a l s w i l l change. In weakly complexing media such as 1 M HCIO4, a l l of the couples have p o t e n t i a l s very n e a r l y the same; as a r e s u l t , i o n i c plutonium i n such s o l u t i o n s tends to d i s p r o p o r t i o n ate. Plutonium i s unique i n i t s a b i l i t y to e x i s t i n a l l four o x i d a t i o n s t a t e s simultaneously i n the same s o l u t i o n . I t s behavi o r i s i n c o n t r a s t to that of uranium, which i s commonly present i n aqueous media as the uranyl(VI) i o n , and the transplutonium a c t i n i d e elements, which normally occur i n s o l u t i o n as t r i v a l e n t ions. Hydrolysis I t i s important to emphasize the often-overlooked f a c t that r e a c t i o n s between a metal i o n and water molecules (hydration) or

0-8412-0479-9/79/47-093-321$05.00/0 This chapter not subject to U.S. copyright Published 1979 American Chemical Society In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL MODELING IN AQUEOUS SYSTEMS


322

Table I

Plutonium Formal P o t e n t i a l s ( i n V o l t s ) at 25° (1)

In 1 M HC10: P u

0_J^3_

P u

3 + -0.9819

^+-1.1702 I

^

-0,9164

^

-1.0433

-1.0228 In n e u t r a l s o l u t i o n (pH = 7 ) :

Pu

3 + 0.63

p u ( 0 H )

^.

y H 2

VxiOt

Q( ) s

"°*

7 7

Pu0 (OH) (aq) 2

2

-0.94 In 1 M OH":

Q

Pu(OH) «xH 0 ' 3

2

9 5

Pu(OH) ·yH 0 ~ ° ' h

!

2

7 6

Pu0 (OH)(aq)"°' 2

* -0.4

26

Pu0 (OH)3(aq)

I 2

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

16.

CLEVEbAND

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hydroxide ions ( h y d r o l y s i s ) are a c t u a l l y complexation r e a c t i o n s that do not d i f f e r i n kind from those with a c i d anions and organic l i g a n d s . H y d r o l y s i s w i l l be considered as a separate process, however, because i t can lead to p r e c i p i t a t i o n and/or formation of polymeric species. Each plutonium o x i d a t i o n s t a t e hydrolyzes by the successive a d d i t i o n of hydroxide ions as the pH i s increased, the f i n a l product i n each case being an i n s o l u b l e hydroxide p r e c i p i t a t e . The various o x i d a t i o n states decrease i n t h e i r tendency to hydrolyze i n the order


Pu

4 +

»

PuO§

+

> Pu

> PuOt .

3 +

The f i r s t h y d r o l y s i s constant f o r p l u t o n i u m ( I I I ) , the e q u i l i b r i u m constant f o r the r e a c t i o n

Pu

3 +

t

+ H0 2

Pu(OH)

2+

that i s ,

+

+

H,

has been determined from acid-base t i t r a t i o n curves at an i o n i c strength (I) of 0.069 M to be 7.5 X 10~ (2). The s o l u b i l i t y product of the n e u t r a l hydroxide p r e c i p i t a t e , Pu(0H) , i s reported to be 2 X 1 0 (3, p. 299). H y d r o l y s i s of p l u t o n y l (VI) has been i n v e s t i g a t e d e x t e n s i v e l y , but not a l l the data are relevant to environmental concentrations. Potentiometric data at I = 1 M and 25° have been i n t e r p r e t e d to i n d i c a t e the formation of three species, Pu02(0H) , ( P u 0 ) 2 2 5 and (Pu0 )3(OH)5, with r e s p e c t i v e o v e r a l l h y d r o l y s i s constants of 1.1 X 10~ , 3.1 X 10" , and 6.9 X 1 0 ~ ( 4 ) . Another potentiometric study (5), at I = 3 M and 25°, y i e l d e d the f o l l o w i n g o v e r a l l h y d r o l y s i s constants: P u 0 ( 0 H ) , Κ = 5.0 Χ 10" ; ( P u 0 ) ( O H ) , Κ = 5.74 Χ 10" ; 8

3

- 2 0

+

+

2

2

6

9

2 3

+

7

2 +

2

2

( P u 0 ) ( 0 H ) t , Κ = 2.0 2

3

9

2

22

Χ 1 0 " ; (Pu0 )4(OH)^, Κ = 7.68 2

3 0

Χ 10" .

The polynuclear species would not be expected at environmental plutonium concentrations, at which the most common h y d r o l y s i s product i s Pu02(0H) . The c i t e d h y d r o l y s i s constants f o r t h i s species are i n s a t i s f a c t o r y agreement. Least hydrolyzed of the four o x i d a t i o n states i s p l u t o n y l ( V ) , which has a reported f i r s t h y d r o l y s i s constant of 2 Χ 1 0 ~ at I = 3 Χ ΙΟ M and 25° (2, pp. 478-499). Plutonium (IV) i s the most r e a d i l y hydrolyzed of the four o x i d a t i o n s t a t e s , but only the f i r s t h y d r o l y s i s constant i s known with any confidence. For the r e a c t i o n +

1υ

- 3

Pu

4 +

+ H0 2

t

Pu(0H)

3+

+

H

+

the best value f o r the f i r s t h y d r o l y s i s constant, K j , i s 0.031, as determined p o t e n t i o m e t r i c a l l y at I = 1 M and 25° (6). Step wise values f o r to have been c a l c u l a t e d from TTA e x t r a c t i o n


324

C H E M I C A L M O D E L I N G IN

AQUEOUS

SYSTEMS

4

y

data at I = 1 M to be 0.35, 0.18, 5.0 Χ 10" , and 5.0 X 10" , r e s p e c t i v e l y (_7), but r e s u l t s are q u e s t i o n a b l e . Successive h y d r o l y s i s constants s u f f e r from such an accumulation of e r r o r s that they become v i r t u a l l y meaningless. I n p a r t i c u l a r , the value f o r Kj^, i f i t r e f e r s to the e q u i l i b r i u m between Pu(0H)"3 and s o l i d Pu(OH) 1+, y i e l d s values f o r the e q u i l i b r i u m c o n c e n t r a t i o n of p l u t o nium (IV) i n n e a r - n e u t r a l s o l u t i o n s that are too high by many orders of magnitude. On the assumption of a r e g u l a r p r o g r e s s i o n of the s t a b i l i t i e s of s u c c e s s i v e h y d r o l y s i s products, o v e r a l l values f o r K i to Ki+ (that i s , f o r the formation of P u ( 0 H ) from Pu(OH)n-i) were estimated to be 0.32, 5.0 Χ 10" , 5.0 Χ 10" , and 3.2 Χ 1 0 " ( 8 ) , but these r e s u l t s cannot be taken s e r i o u s l y i n view of the u n v e r i f i e d assumptions made i n t h e i r c a l c u l a t i o n . The reported s o l u b i l i t y product of Pu(0H)i+, 7 X 10 as measured by pH t i t r a t i o n Ç3, p. 300), i s an exceedingly s m a l l number. I f i t were a t r u e r e p r e s e n t a t i o n of the c o n c e n t r a t i o n of plutonium i n s o l u t i o n , at pH 7 there would be only 7 X 1 0 ~ mole of plutonium per l i t e r ; thus the e q u i l i b r i u m c o n c e n t r a t i o n of p l u t onium i n n e u t r a l water would be about one atom per 2400 l i t e r and there would be no problem w i t h plutonium-contaminated ground water. The s o l u b i l i t y product does not a c c u r a t e l y d e f i n e the concentrat i o n of plutonium i n aqueous s o l u t i o n s because i t merely s t a t e s the c o n c e n t r a t i o n of the P u i o n . At pH values of environmental i n t e r e s t , plutonium w i l l be present not p r i m a r i l y as P u , but as species such as Pu(0H)£ , Pu(OH)"^, u n i o n i z e d PuiOH)^, c o l l o i d a l polymeric forms to be discussed l a t e r , as w e l l as other o x i d a t i o n s t a t e s formed by d i s p r o p o r t i o n a t i o n at low a c i d i t i e s . Thus, the t o t a l plutonium c o n c e n t r a t i o n w i l l be much higher than that d e s c r i b e d by the s o l u b i l i t y product of PuiOH)^. n

3

6


10

6

28

4 +

4 +

+

Complexes A b r i e f review of methodology i s i n order before embarking on a d i s c u s s i o n of plutonium complexes. Determination of s t a b i l i t y constants of metal complexes depends on the measurement of the e f f e c t produced by the p a r t i c u l a r l i g a n d on an e q u i l i b r i u m i n v o l v ing the uncomplexed metal i o n . The most common methods are i o n exchange, s o l v e n t e x t r a c t i o n , spectrophotometry, s o l u b i l i t y , potentiometry, and polarography, of which only the l a s t two g i v e values based on a c t i v i t i e s r a t h e r than c o n c e n t r a t i o n s . For t h i s and other reasons, these two methods g e n e r a l l y y i e l d more thermodynamically s i g n i f i c a n t constants. Unfortunately,, they have been r a t h e r i n f r e q u e n t l y used i n studying plutonium complexes. The l e a s t r e l i a b l e method, s o l u b i l i t y , s u f f e r s from the d i f f i c u l t y of a c h i e v i n g e q u i l i b r i u m s o l u b i l i t i e s and from problems i n p r o p e r l y i n t e r p r e t i n g the data. In cases where the s t a b i l i t y constants of plutonium complexes are not known, i t i s p o s s i b l e to o b t a i n a rough estimate by comp a r i s o n w i t h analogous metal ions (that i s , L a for P u , T h f o r Pu " ", NpOj f o r PuO"J, υ θ f o r PuO^ ) . With one e x c e p t i o n , 3 +

4

1

2 +

3 +

+


4 +

16.

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t h i s approach i s not a p p l i e d i n t h i s paper because of space l i m i t a t i o n s . Moreover, attempting to e x t r a p o l a t e s t a b i l i t y constants from one element to another i s a s h o r t c u t that can e a s i l y be abused. The analogous ions d i f f e r s i g n i f i c a n t l y from the respec t i v e plutonium ions i n such p r o p e r t i e s as i o n i c p o t e n t i a l and o x i d a t i o n - r e d u c t i o n behavior, which can r e s u l t i n d i f f e r e n c e s i n complexation. Two types of s t a b i l i t y constants should be d e f i n e d . For the r e a c t i o n of one or more l i g a n d s , L, w i t h a metal i o n , M, the stepwise s t a b i l i t y constant i s expressed by the r e l a t i o n


[MLJ K

n

=

[ML

n - 1

][L] '

whereas the o v e r a l l s t a b i l i t y constant i s d e f i n e d as [ML ] β 11

6

I t follows that Κ

. β

[M][L]

n

n

= — n

s_ .

=

Moreover, s t a b i l i t y constant

equations

η-1

i n v o l v i n g protonated s p e c i e s , such as u n d i s s o c i a t e d or p a r t i a l l y d i s s o c i a t e d a c i d , may be converted to the above expressions by a p p l i c a t i o n of the a p p r o p r i a t e a c i d d i s s o c i a t i o n constant. Standard d e v i a t i o n s are not g i v e n , s i n c e they merely e s t a b l i s h the r e p r o d u c i b i l i t y of a given method and do not n e c e s s a r i l y r e f l e c t the accuracy of the data. Carbonate Complexes. Of the many l i g a n d s which are known to complex plutonium, o n l y those of primary environmental concern, that i s , carbonate, s u l f a t e , f l u o r i d e , c h l o r i d e , n i t r a t e , phos phate, c i t r a t e , t r i b u t y l phosphate (TBP), and ethylenediaminetetr a a c e t i c a c i d (EDTA), w i l l be d i s c u s s e d . Of these, none i s more important i n n a t u r a l systems than carbonate, but data on i t s r e a c t i o n s w i t h plutonium are meager, p r i m a r i l y because of competi t i v e h y d r o l y s i s at the low a c i d i t i e s that must be used. No s t a b i l i t y constants have been p u b l i s h e d on the carbonate complexes of p l u t o n i u m ( I I I ) and p l u t o n y l ( V ) , and the data f o r t h e ' p l u t o n i um (IV) species are not c r e d i b l e . R e s u l t s from s t u d i e s on the s o l u b i l i t y of plutonium(IV) o x a l a t e i n K2CO3 s o l u t i o n s of v a r i o u s c o n c e n t r a t i o n s have been i n t e r p r e t e d (9) to i n d i c a t e the e x i s t e n c e of complexes as h i g h as Pu(C0 3 ) ^ , a species that i s most u n l i k e l y from both e l e c t r o s t a t i c and s t e r i c c o n s i d e r a t i o n s . From the i n f l u e n c e of K2CO3 c o n c e n t r a t i o n on the s o l u b i l i t y of PuiOH)^ at an i o n i c s t r e n g t h of 10 M, the s t a b i l i t y constant of the complex P u ( C 0 ) " was c a l c u l a t e d (10) to be 9.1 Χ 1 0 at 2 0 ° . This value 2 _

3

2 4

4 6


326


i s based on poor experimental methodology and i n c o r r e c t i n t e r p r e t a t i o n of the data, and i s best ignored. From comparisons w i t h other plutonium complexes and w i t h carbonate complexes of analogous metal i o n s , i t appears to be h i g h by about 30 orders of magnitude. In the case of p l u t o n y l ( V I ) , somewhat more p l a u s i b l e r e s u l t s have been obtained. Data on the s o l u b i l i t y of p l u t o n y l ( V I ) hydrox ide i n (NH4)2C03 s o l u t i o n s of v a r y i n g concentrations have been i n t e r p r e t e d (11) to r e v e a l the presence of three complexes, P u 0 ( C 0 ) | " , Pu0 (C0 )(OH)", and Pu0 (C0 )(0H)£~, w i t h s t a b i l i t y constants of 6.7 Χ 1 0 , 4.5 Χ 1 0 , and 2.3 Χ 1 0 , at I = 1 M and 20°. The v a l u e f o r the dicarbonato species i s reasonably c o n s i s t e n t w i t h published constants f o r the analogous u r a n y l ( V I ) s p e c i e s , but the constants f o r the hydroxy species need f u r t h e r verification. 2

3

2

3

2


1 3

3

2 2

2 2

S u l f a t e Complexes. Knowledge of the s u l f a t e complexes of p l u t o n i u m ( I I I ) and (IV) has advanced somewhat i n recent years. Ion exchange data i n d i c a t e d the presence of two complexes of the t r i v a l e n t i o n , Pu(S0i+) and P u ( S 0 ) , w i t h r e s p e c t i v e stepwise s t a b i l i t y constants of 44.7 and 43.7 at I - 2 M and 25° (12). I t is unlikely and K would have such s i m i l a r v a l u e s , and hence K must be regarded w i t h s u s p i c i o n . Because these values were the same i n both 1 M and 2 M HCIO^, i t was concluded that no protonated species were formed. This value of Kj takes precedence over e a r l i e r data (13) i n d i c a t i n g the e x i s t e n c e of two complexes, Pu(S0i ) , w i t h K - 18.1 and PuiHSO^)^, w i t h Κχ = 9.9, at I = 1 M and 28°. In the l a t t e r case a l l determinations were at a constant a c i d i t y of 1 M, and t h e r e f o r e the presence of the protonated complex PuiHSO^)* was not demonstrated by s t u d i e s at v a r y i n g acidity. Numerous determinations have been made of the s t a b i l i t y con s t a n t s of s u l f a t e complexes of plutonium(IV), and the r e s u l t s vary by three orders of magnitude. The most p l a u s i b l e values (14) have come from c a r e f u l i o n exchange s t u d i e s at I = 2 M and 25°, which i n d i c a t e the presence of two s p e c i e s , Pu(S0[+) and P u ( S 0 ^ ) , w i t h stepwise s t a b i l i t y constants of 6.6 X 1 0 and 5.8 Χ 1 0 , respec t i v e l y . This K i i s i n s a t i s f a c t o r y agreement w i t h the value 4.6 Χ 1 0 obtained p o t e n t i o m e t r i c a l l y at I = 1 M and 25° ( 6 ) . Spectrophotometric and e l e c t r o p h o r e t i c s t u d i e s i n d i c a t e that p l u t o n y l ( V I ) forms complexes c o n t a i n i n g as many as four s u l f a t e groups, w i t h a n i o n i c species predominating at s u l f a t e concentra t i o n s above 1 M (15). The monosulfato complex has a reported s t a b i l i t y constant of 14.4 as determined by e x t r a c t i o n s t u d i e s at I = 2 M and 25° (16). +

4

2

2

2

+

+

x

2+

2

3

2

3

F l u o r i d e Complexes. F l u o r i d e i s known to complex plutonium s t r o n g l y , but q u a n t i t a t i v e data on these environmentally important complexes are l i m i t e d . C a t i o n exchange s t u d i e s (17) y i e l d e d values of 4.5 Χ 1 0 a t I = 1 M and 7.9 Χ 1 0 at I = 2 M f o r -the s t a b i l i t y constant of the monofluoro complex of plutonium(IV), which are i n s a t i s f a c t o r y agreement w i t h the value 1.2 Χ 1 0 obtained from 7

7

8


16.

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e x t r a c t i o n data a t I = 2 M and 25° (18). The l a t t e r study a l s o gave 3.4 Χ 1 0 as the o v e r a l l s t a b i l i t y constant, $2> of PuF^ under the same c o n d i t i o n s . (The values c i t e d i n references 19 and 20 have been c o r r e c t e d f o r the i o n i z a t i o n constant of HF to convert them to s t a b i l i t y constants as d e f i n e d i n t h i s d i s c u s s i o n . ) Four f l u o r o complexes of p l u t o n y l ( V I ) have been i d e n t i f i e d from c a t i o n exchange s t u d i e s (19) a t I = 1 M, w i t h o v e r a l l s t a b i l i t y constants as f o l l o w s : Pu0 î ", 3i = 130; P u 0 F , 3 = 1-4 u 0 F 3 , Β3 +

1 4

H

x

2

2

2

p

2

2

= 1.2 Χ 1 0 ; Pu0 F^"", 3i+ = 2 . 0 Χ 1 0 . These r e s u l t s are somewhat clouded by the a u t h o r s f a i l u r e to demonstrate the absence of p l u t o n i u m ( I V ) , a p o t e n t i a l source of e r r o r i n t h i s system. A lower v a l u e , 3i = 12, has been reported from e x t r a c t i o n data a t I = 2 M and 25° (16). 6

6

2


1

C h l o r i d e Complexes. Because of marine d i s p o s a l or storage of a c t i n i d e wastes i n s a l t formations, the r e l a t i v e l y weak c h l o r o complexes of plutonium could become important over extended time p e r i o d s . Two p l u t o n i u m ( I I I ) complexes, P u C l and P u C l J , have been reported (20), the former o c c u r r i n g a t c h l o r i d e c o n c e n t r a t i o n s above 2 M and the l a t t e r i n the presence of 8 M or greater c h l o r i d e i o n c o n c e n t r a t i o n . No s t a b i l i t y data were given. Plutonium(IV) forms complexes c o n t a i n i n g from one to s i x c h l o r i d e l i g a n d s . Q u a n t i t a t i v e values e x i s t f o r the three lower complexes i n 4 M HCIO^ a t 20° as determined by c a t i o n exchange: P u C l , 2 +

3 +

= 1.4; PuCl£ , K = 1.2; PuCl^, K = 0 . 1 (21). Other values f o r K i agree s a t i s f a c t o r i l y w i t h the above: 1.38 by potentiometry (22) , and 1.42 by an e x t r a c t i o n technique a t I = 2 M and 25° (23) ; thus the v a l u e f o r K]_ seems w e l l e s t a b l i s h e d . Values f o r K show more v a r i a t i o n : 0.49 by potentiometry (22) and 0.16 by e x t r a c t i o n (23). Agreement between the i o n exchange and p o t e n t i o m e t r i c values i s s u f f i c i e n t l y good to suggest that K i s somewhere i n the range 0,5 t o 1.2. P o t e n t i o m e t r i c s t u d i e s Ç3, pp. 312-13) a t u n s p e c i f i e d tempera t u r e and i o n i c s t r e n g t h have i n d i c a t e d K j f o r the p l u t o n y l ( V ) complex PUO2CI t o be 0.67, a v a l u e that should be accepted w i t h c a u t i o n i n view of the experimental vagueness. P l u t o n y l ( V I ) forms complexes c o n t a i n i n g up to three or f o u r c h l o r i d e i o n s , but q u a n t i t a t i v e data have been reported f o r only the mono and d i c h l o r o s p e c i e s . The f o l l o w i n g values have been found, a l l by spectrophotometry, f o r the s t a b i l i t y constant of P u 0 2 C l : 1.25 at I = 2 M and 25° (24); 0.73 a t I = 1 M and 23° (3, pp. 312-13); 0.56 a t I = 1 M and 20° (25). Again, agreement i s reasonably good, although i t i s r e g r e t t a b l e t h a t a l l values were obtained by the same technique. The s i n g l e reported s t a b i l i t y constant, K 2 , f o r P u 0 C l , a l s o by spectrophotometry, i s 0.35 at I = 2 M and 25° ( 2 4 ) . +

2

3

2

2

+

2

2

N i t r a t e Complexes. Although there i s spectrophotometric evidence f o r the e x i s t e n c e of n i t r a t e complexes of p l u t o n i u m ( I I I ) ,


328


they are unstable because of o x i d a t i o n to plutonium(IV). The l a t t e r forms complexes c o n t a i n i n g as many as s i x n i t r a t e groups, and they are important i n the chemical p r o c e s s i n g of plutonium by i o n exchange and s o l v e n t e x t r a c t i o n . Values f o r Κχ are i n good agreement: 5.5 by c a t i o n exchange at I = 4 M and 20° (21); 5.3 by e x t r a c t i o n at I = 6 M (26); 4.4 by e x t r a c t i o n at I = 2 M and 25° (23). Strong disagreement e x i s t s among the values f o r K : 23.5 by i o n exchange at I = 4 M and 20° (21); 3.0 by e x t r a c t i o n at I = 2 M and 25° (23); 0.96 at I = 6 M, a l s o by e x t r a c t i o n (26). The i o n exchange v a l u e i s probably the l e a s t r e l i a b l e because of compounding of e r r o r s ; on t h i s b a s i s the most p l a u s i b l e K i s i n the range 1 t o 3. Values f o r K 3 become even l e s s r e l i a b l e ; of the two r e p o r t e d , 15 by i o n exchange at I = 4 M and 20° (21) and 0.33 by e x t r a c t i o n at I • 6 M (26), n e i t h e r can be accepted w i t h assurance. Mono-, d i - , and t r i n i t r a t o complexes of p l u t o n y l ( V I ) have been i d e n t i f i e d , but they are weak. Of the two p l a u s i b l e v a l u e s r e p o r t e d f o r K i , t h a t i s , 0.93 by e x t r a c t i o n at I = 4.1 M (27) and 0.25, a l s o by e x t r a c t i o n , at I = 4.6 M and 25° (28), the l a t t e r i s more r e l i a b l e because of more c a r e f u l experimental c o n t r o l s and the more v a l i d assumptions made i n i t s calculation. 2


2

Phosphate Complexes. Of a l l the systems d e s c r i b e d i n t h i s paper, the most d i f f i c u l t to evaluate i s t h a t of the phosphate complexes of plutonium. Many of the s t a b i l i t y constants were c a l c u l a t e d from s o l u b i l i t y measurements, which o f t e n y i e l d data of dubious accuracy. The values t h a t have been obtained are reported i n Table I I , and many appear to be too h i g h , a common e r r o r i n s o l u b i l i t y determinations. The i o n exchange values f o r the a c i d phosphate complexes of p l u t o n i u m ( I I I ) are the most p l a u s i b l e . The data f o r plutonium(IV) and p l u t o n y l ( V I ) should be used w i t h c a u t i o n . C i t r a t e Complexes. Because of t h e i r a b i l i t y to form s t e r i c a l l y favored c h e l a t e s t r u c t u r e s , many organic l i g a n d s complex plutonium more s t r o n g l y than i n o r g a n i c anions. One of the s t r o n g est n a t u r a l l y o c c u r r i n g c h e l a t i n g agents i s c i t r a t e , and i t forms a number of very s t a b l e complexes w i t h plutonium. Three p l u t o nium(III) complexes have been reported (32), P u ^ B ^ O y ) , Pu(H C H 0 )"£, and P u ( H C H 0 ) 3 , w i t h r e s p e c t i v e s t a b i l i t y 2

6

5

7

2

8

6

5

7

6

1 0

constants of 7.3 X 1 0 , 4.0 X 1 0 , and 1.0 X 1 0 , but these v a l u e s are suspect because they d i f f e r by s e v e r a l orders of magnitude from constants f o r c i t r a t e complexes of other a c t i n i d e s and l a n t h a n i d e s . The apparent e r r o r i s p r i m a r i l y a r e s u l t of f a i l u r e t o a l l o w f o r p l u t o n i u m ( I I I ) h y d r o l y s i s and complexing by s u l f i t e formed from the sodium formaldehyde s u l f o x y l a t e used as a reducing agent, and by improper i n t e r p r e t a t i o n of the data. More r e l i a b l e data are a v a i l a b l e f o r the plutonium(IV) complexes, which are present at c i t r a t e c o n c e n t r a t i o n s as low as 1 0 ~ M (33). S t a b i l i t y constants of the unprotonated species ΡυζΟβΗ^Ογ)" " 15

1


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Table I I Phosphate Complexes of Plutonium Complex

S t a b i l i t y Constant

Method

Reference

Pu ( I I I ) 1 9

Pu(POiJ 2 +

Pu(H P0 ) 2

Ki

4

Ρη(Η Ρ0 )$

K

2

Pu(H P0 )

K

3

2

4

2

4

3

Pu(H P0 )^ 2

Ki+

4

Pu(IV) PuCHPOiO Pu(HP0 )2_ PuCHPO^)^ Pu(HP0 )Î_ PuCHPO^)^ 2

Ki K K Kit K

4

2

3

4

5

Pu(VI) Pu0 (HP0O

Pu0 (H P0 ) 2

3

= = = = =

8.3 6.7 4.8 6.3 6.3

Χ Χ Χ Χ Χ

10 10 10 10 10

Κχ = 1 . 5 X 1 0

2

2

1.5 Χ 1 0 at I = 0.5 30.2 at I = 1 M 5.25 at I = 1 M 5.01 at I = 1 M 3.98 at I = 1 M

Ki

+

Solubility

(29)

C a t i o n exchange

(29)


(29)


(29)


(29)

Solubility Solubility Solubility Solubility Solubility

(30) (30) (30) (30) (30)

Unspecified; presumably solubility Solubility

(31)

M and 20° and 20° and 20° and 20° and 20°

1 2

1 0

9

9

8

at 1 = 0

Κχ = 200 a t I = 0


(31)


330

and PuCCgHsOy)!" have been determined by three separate methods w i t h g e n e r a l l y good agreement, as shown i n Table I I I . Table I I I

S t a b i l i t y Constants of C i t r a t e Complexes

of Plutonium(IV) at I = 0.5 M and


Method Spectrophotometric Potentiometric pH t i t r a t i o n

3χ

β lk

5.3 X 10 3.5 Χ 1 0 6.9 Χ 1 0

Reference

2

1.6 Χ 1 0 1.0 Χ 1 0 1.0 Χ 1 0

1 5

1 5

25°

3 0

3 0

2 9

(33) (34) (34)

ΤBP Complexes. T r i b u t y l phosphate (TBP) does not normally occur i n the environment, but i t s use i n nuclear f u e l r e p r o c e s s i n g makes i t a probable c o n s t i t u e n t of r a d i o a c t i v e wastes and hence a p o s s i b l e ground water contaminant. TBP r e a c t s w i t h n i t r a t e com plexes of plutonium(IV) and (VI) to form the species Pu(NO3)4·2TBP and Pu02(N03)2*2TBP, which although not e x c e p t i o n a l l y s t a b l e , are u s e f u l i n separations chemistry because of t h e i r s o l u b i l i t y i n organic s o l v e n t s and i n s o l u b i l i t y i n aqueous media. By c o n t r a s t , the p l u t o n i u m ( I I I ) complex, Pu(NO3)3·3TBP, i s p o o r l y e x t r a c t e d by organic s o l v e n t s . The s t a b i l i t y constant of the neptunium(IV) complex, d e f i n e d as [Np(Ν0 ) ·2ΤΒΡ] 3

4

Κ =

, 4

[Νρ][Ν0 ] [ΤΒΡ]

2

3

i s reported to be 130 at I = 2 M (35), and the v a l u e f o r the p l u tonium(IV) complex should be s i m i l a r . Although these complexes and t h e i r s o l v e n t are w a t e r - i n s o l u b l e , they could e x i s t i n ground water i n an e m u l s i f i e d form. EDTA Complexes. E t h y l e n e d i a m i n e t e t r a a c e t i c a c i d (EDTA) and i t s homologues form the most s t a b l e known complexes of plutonium. This d i s c u s s i o n w i l l be l i m i t e d to EDTA, which i s most l i k e l y to be found i n the environment as a r e s u l t of i t s use as a medium f o r the a d d i t i o n of s o l u b l e i r o n to s o i l s . The e q u i l i b r i u m constant f o r formation of the 1:1 c h e l a t e of p l u t o n i u m ( I I I ) , as given by the expression [PuY-J[H+]

2

Κ = 3+

2

[Pu ][H Y ""] 2

(where Ηι+Y would represent the u n i o n i z e d EDTA molecule) , was found i n 0.1 Ν KC1 s o l u t i o n to be 3.9 Χ 1 0 at pH 1.5 by spec trophotometry (36) and 1.32 Χ 1 0 at pH 3.3 and 20° by i o n 1 8

1 8


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exchange (37). Another i o n exchange study, at I = 1 M and pH values from 1.2 to 3.4, y i e l d e d a s t a b i l i t y constant f o r PuY~ of 2.28 Χ 1 0 and i n d i c a t e d the presence at pH 1.5 to 2.0 of another s p e c i e s , PuHY, w i t h a s t a b i l i t y constant of 1.61 Χ 1 0 (38). A s t a b i l i t y constant of 1.6 X 10* f o r PuY~ formation from Pu * and Υ^~ at I = 0.1 M and 20° has been c a l c u l a t e d from spectrophoto m e t r y and p o l a r o g r a p h i c data (39). Determination of the s t a b i T T t y constants f o r EDTA c h e l a t e s of plutonium(IV) i s rendered d i f f i c u l t by h y d r o l y s i s of the metal i o n at f a i r l y low pH values and by p r o t o n a t i o n of the EDTA molecule i n more s t r o n g l y a c i d s o l u t i o n . Thus attempts to d e t e r mine the e q u i l i b r i u m constant i n the expression 1 7

9


2

34

[PuY][H+]

2

Κ = +

2

[Ρ^ ][Η Υ -] 2

1 7

at pH 3.3 i n 0.1 Ν KC1 s o l u t i o n gave values of 4.57 Χ 1 0 by i o n exchange (37) and 1.26 Χ 1 0 by spectrophotometry (36), but these numbers d e s c r i b e the complexing of a p a r t i a l l y hydrolyzed p l u t o nium s p e c i e s , r a t h e r than the Pu " i o n . In 1 M HNO3 a s p e c t r o 1 7

44

2Lf

p h o t o m e t r i c a l l y determined v a l u e of 1.59 X 1 0 was reported (36), but t h i s , too, i s suspect because i t f a i l s to a l l o w f o r the apparent p r o t o n a t i o n of EDTA(Hi+Y) to form species such as H5Y " and H g Y (40). The d i s s o c i a t i o n constants of these species were determined and combined w i t h other i o n i z a t i o n constants of EDTA to c a l c u l a t e a more accurate v a l u e f o r the c o n c e n t r a t i o n of Y ~" ions i n 1 M HNO3 and from t h i s the s t a b i l i t y constant of PuY at I = 1 M and 25° was c a l c u l a t e d to be 5.7 X 1 0 (41). A more recent v a l u e of 4.0 X 1 0 at I = 0.1 M determined by s p e c t r o photometry and polarography (39) i s i n e x c e l l e n t agreement. Even p l u t o n y l ( V ) i s s t r o n g l y complexed by EDTA. The s t a b i l i t y constant of P u 0 Y has been determined by three methods: 7.7 X 1 0 by spectrophotometry at 20° (42); 8 X 1 0 by p o t e n t i o metry i n 0.1 M KC1 at room temperature (43); 1.5 X 1 0 by i o n exchange at I = 0.05 M (44). The f i r s t two v a l u e s , by v i r t u e of t h e i r c l o s e agreement, take precedence. The s t a b i l i t y constant of the p l u t o n y l ( V I ) c h e l a t e , P u 0 Y ~ , i s somewhat higher than that f o r p l u t o n y l ( V ) . Two d i f f e r e n t spectrophotometric s t u d i e s y i e l d e d values of 1.07 Χ 1 0 at pH 4.0 (36) and 4.0 X 1 0 C42), both a t 20°, whereas an i o n exchange-derived value of 2.46 Χ 1 0 at pH 3.3 and 20° has been reported (37). Although these values i n d i c a t e a high degree of s t a b i l i t y f o r the complex, the metal i o n i s unstable toward r e d u c t i o n (36, 42). EDTA reduces p l u t o n y l ( V I ) to p l u t o n y l ( V ) , or plutonium(IV) i f an excess of l i g a n d i s present (45). However, because a l l o x i d a t i o n s t a t e s are s t r o n g l y complexed by EDTA, the r e d u c t i o n does not r e s u l t i n the r e l e a s e of uncomplexed plutonium i o n s . 4

2+

4

2 5

2 5

3

2

1 2

1 2

1 0

2

2

1 6

1 6


1 4

332



Polymerization The e q u i l i b r i a discussed above are u s e f u l i n c h a r a c t e r i z i n g plutonium and p r e d i c t i n g i t s behavior i n aqueous systems, but o n l y i f i t i s i n t r u e s o l u t i o n . This i s f r e q u e n t l y the case f o r p l u t o n y l ( V ) and ( V I ) , but much l e s s common w i t h p l u t o n i u m ( I I I ) and ( I V ) . I n p a r t i c u l a r , plutonium(IV) i s very s u s c e p t i b l e t o h y d r o l y s i s and p o l y m e r i z a t i o n a t pH values above 1 (46). I n 0.009 M plutonium(IV) s o l u t i o n s a t 25°, p o l y m e r i z a t i o n increased from e s s e n t i a l l y zero i n 0.1 M HNO3 (pH 1) t o 98 percent i n 0.04 M HNO3 (pH 1.4). Although p o l y m e r i z a t i o n would not occur a t such low pH v a l u e s w i t h the c o n c e n t r a t i o n s of plutonium found i n n a t u r a l waters, i t would be expected a t the higher pH values most common i n the environment. Whether the r e s u l t i n g polymer remains d i s p e r s e d i n the aqueous medium or p r e c i p i t a t e s depends on a number o f f a c t o r s , i n c l u d i n g i t s molecular weight, pH, temperat u r e , and the type and c o n c e n t r a t i o n of anions i n s o l u t i o n ; i n g e n e r a l , t h e presence of polymer w i l l r e s u l t i n a higher concent r a t i o n o f plutonium i n the aqueous phase than would otherwise be the case. I t i s not i n t r u e s o l u t i o n , however, but i s present i n c o l l o i d a l form. Values f o r i t s molecular weight range from 4000 to 1 0 (47), depending on c o n d i t i o n s of formation. Formation of the polymer i s r e l a t i v e l y r a p i d , g e n e r a l l y o c c u r r i n g w i t h i n a matter o f minutes (47); d e p o l y m e r i z a t i o n , on the other hand, r e q u i r e s hours t o days, depending on a c i d i t y and temperature (48), and i s even slower f o r aged polymers (47). Because of t h i s , polymer formation i s o f t e n considered t o be i r r e v e r s i b l e . The presence o f a l a r g e molar excess of c i t r a t e r e t a r d s polymerizat i o n and enhances d e p o l y m e r i z a t i o n (49), doubtless because of complex formation. The s i z e s of the c o l l o i d a l p a r t i c l e s v a r y , but g e n e r a l l y i n c r e a s e w i t h aging, increased i o n i c s t r e n g t h , and the presence o f bicarbonate a t environmental c o n c e n t r a t i o n s (50). 1 0

The nature of the polymer has not been f i r m l y e s t a b l i s h e d . I t has g e n e r a l l y been considered an i n t e r m e d i a t e h y d r o l y s i s product i n which p a r t i a l l y hydrolyzed plutonium species a r e l i n k e d by hydroxide o r oxide b r i d g e s i n t o long c h a i n s . I n t h i s view, the e f f e c t of aging i s t o i n c r e a s e the s i z e of the polymer u n i t s . A more recent c o n c l u s i o n (51), supported by spectrophotom e t r y , x - r a y d i f f r a c t i o n , e l e c t r o n d i f f r a c t i o n , and e l e c t r o n microscopy data, i s t h a t the polymer c o n s i s t s of s m a l l , d i s c r e t e primary p a r t i c l e s ( s i z e 0.5 to 2.0 nm), which may be amorphous o r c r y s t a l l i n e , and secondary p a r t i c l e s which a r e aggregates of the primary p a r t i c l e s . C o l l o i d a l s o l s would thus c o n s i s t of d i s p e r sions of the primary p a r t i c l e s i n aqueous media. I n t h i s concept the slower d e p o l y m e r i z a t i o n of aged polymer i s the r e s u l t of the conversion o f amorphous p a r t i c l e s t o c r y s t a l l i n e p a r t i c l e s on standing. X-ray d i f f r a c t i o n p a t t e r n s of the c r y s t a l l i n e p a r t i c l e s correspond t o that f o r Pu02« A s i m i l a r p a t t e r n i s found f o r p r e c i p i t a t e d PuiOH)^ (47), l e a d i n g t o the suggestion that the primary p a r t i c l e s , which a r e presumably hydrated Pu02> a r e a l s o the f i r s t p a r t i c l e s t o form during p r e c i p i t a t i o n (51).


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Dispersed polymer can be adsorbed from aqueous media onto surfaces w i t h which i t comes i n contact. Among s o l i d s whose surfaces can adsorb plutonium are numerous m i n e r a l s , g l a s s , and s t a i n l e s s s t e e l . Glass r e p o r t e d l y can adsorb 1.6 Ug c o l l o i d / c m of s u r f a c e , and the a d s o r p t i o n by s t e e l i s much higher (47). A d s o r p t i o n i s h i g h l y v a r i a b l e , depending on c o l l o i d c o n c e n t r a t i o n , pH, age of polymer, nature of presence of anions and complexing l i g a n d s , and nature of the s u r f a c e . I t i s important to bear i n mind t h a t t h i s a d s o r p t i o n i s s t r i c t l y a surface phenomenon and i s g e n e r a l l y i r r e v e r s i b l e ; the r a t e of d e s o r p t i o n i s dependent on depolymerization and s o l u b i l i t y c o n s i d e r a t i o n s . Because of the nature of the process and i t s i r r e v e r s i b i l i t y , i t cannot be d e s c r i b e d by e q u i l i b r i u m i o n exchange parameters, such as d i s t r i b u t i o n c o e f f i c i e n t s [ K ^ ] . The two processes are completely


2

u n r e l a t e d ; i n t e r e s t i n g l y enough, the polymer i s r e p o r t e d l y not adsorbed by i o n exchange r e s i n s (46) . A c c o r d i n g l y , i t i s important f o r researchers to e s t a b l i s h the absence of polymeric species i n a given s o l u t i o n before u s i n g i t to determine e q u i l i b r i u m d i s t r i b u t i o n values. Areas Needing Further Research With the exception of a few areas such as p o l y m e r i z a t i o n , the aqueous chemistry of plutonium has r e c e i v e d much study. However, most of the current knowledge a p p l i e s to macro concent r a t i o n s of the element i n strong a c i d s o l u t i o n s , and i t s a p p l i c a t i o n to the picogram l e v e l s and n e u t r a l pH values of environmental systems r e q u i r e s utmost c a u t i o n . Since h y d r o l y s i s i s r e l a t i v e l y more important, i t i s p o s s i b l e that the complex species formed are not simple m e t a l - l i g a n d m o i e t i e s , but r a t h e r i n v o l v e hydroxylated metal ions i n complex species not p r e v i o u s l y i d e n t i f i e d . The present s t a t e of knowledge of plutonium chemistry under environmental c o n d i t i o n s does not appear adequate to permit chemical modeling w i t h any degree of confidence. To reach t h i s d e s i r e d o b j e c t i v e , a d d i t i o n a l data, determined under simulated environmental c o n d i t i o n s — n o t e x t r a p o l a t e d from g r o s s l y d i f f e r e n t c o n d i t i o n s — a r e needed. Several areas of research seem to merit top p r i o r i t y : 1. attempt to v e r i f y published s t a b i l i t y constants of e n v i r o n mental i n t e r e s t at lower metal concentrations and higher pH; 2. determine s t a b i l i t y constants that are not c u r r e n t l y known, the prime example being the plutonium-carbonate system; 3. assess the i n t e r p l a y of complexation, h y d r o l y s i s , and polyme r i z a t i o n a t environmental pH v a l u e s , as these f a c t o r s are important but not w e l l understood under n e u t r a l c o n d i t i o n s ; 4. study the complex chemistry of p l u t o n y l ( V ) , which some workers b e l i e v e to be an important species i n ground waters; 5. attempt to e l u c i d a t e the nature and behavior of polymeric species w i t h the u l t i m a t e o b j e c t i v e of developing q u a n t i t a t i v e , r e p r o d u c i b l e expressions f o r d i s p e r s i o n , p r e c i p i t a t i o n ,


334


a d s o r p t i o n , and d e s o r p t i o n of the polymer. This i s a b i g order, and may appear u n r e a l i s t i c to some. I n at l e a s t one man's opinion, however, progress i n these f i v e areas i s d e s i r a b l e , not only f o r meaningful modeling, but more i m p o r t a n t l y , f o r a true under standing of the behavior of plutonium i n the environment.


Abstract The behavior of plutonium in aqueous environmental systems i s governed by the complex interrelationships of three types of chemical equilibria - oxidation state, hydrolysis, and complexa tion - and by polymerization, which i s a non-equilibrium process; and our a b i l i t y to understand i t s environmental behavior w i l l de pend on our knowledge of these fundamental chemical reactions. Oxidation potentials are known with satisfactory precision. Hydrolysis constants and hydroxide solubility products are of varying but generally dubious r e l i a b i l i t y . The situation with regard to complex s t a b i l i t y constants i s somewhat variable. Com plexes with strong chelating legands such as c i t r i c acid, EDTA, and DTPA have been quantitatively described with reasonable accuracy, but data on plutonium complexes with such environmentally-important ions as carbonate and phosphate are meager and questionable. Least understood of all are the irreversible polymerization reac tions of plutonium which play so large a part in i t s aqueous chemistry. Data for all of these reactions w i l l be reviewed and evaluated, accompanied when necessary by a critique of the methodo logy. The environmental consequences of this information w i l l be assessed; for example the fallacies of attempting to use solubility products to calculate plutonium concentrations, or of employing equilibrium ion exchange concepts to describe non-equilibrium sur face adsorption of polymer w i l l be emphasized. Moreover, since most of the literature refers to macro concentrations of plutonium and frequently to relatively strong acid solution, great caution is necessary in extrapolating to environmental conditions. In conclusion, suggestions w i l l be made of areas of plutonium chemis try that most urgently require further study. Literature Cited 1. 2.

3. 4.

Cleveland, J. M. "The Chemistry of Plutonium," 653 p. Gordon and Breach Science Publishers, Inc., New York, 1970. Kraus, Κ. Α., and Dam, J. R., p. 466-477, in Seaborg, G. T., Katz, J. J., and Manning, W. Μ., eds., "The Transuranium Elements," McGraw-Hill Book Co., New York, 1949. Katz, J. J., and Seaborg, G. T. "The Chemistry of the Actinide Elements," John Wiley and Sons, Inc., New York, 1957. Cassol, Α., Magon, L., Portanova, R., and Tondello, E. Hydrolysis of plutonium(VI): acidity measurements in perchlorate solutions, Radiochim. Acta, 17, 28 (1972).


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1978.


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Chemical Modeling in Aqueous Systems - American Chemical Society

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