Effect of Aging on the Solubility and Crystallinity of Np(IV) Hydrous Oxide

8 Effect of Aging on the Solubility and Crystallinity of Np(IV) Hydrous Oxide

Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 29, 2018 | https://pubs.acs.org Publication Date: March 8, 1984 | doi: 10.1021/bk-1984-0246.ch008

RICHARD G. STRICKERT, DHANPAT RAI, and ROBERT W. FULTON Pacific Northwest Laboratory, Richland, WA 99352

Neptunium(IV) hydrous oxide (ΝpO ·xΗ O) was aged i n aqueous suspensions with pH values ranging from 4 to 12. I n i t i a l l y amorphous ΝpO ·xΗ O developed strong X-ray diffraction peaks within one month that corre sponded to crystalline NpO . The oxidation state analyses of the 1.8 nm f i l t e r e d solutions showed the Np to be primarily present as Np(V). The s o l u b i l i t y of ΝpO ·xΗ O in suspensions at pH values 8.5 were complicated by the p a r t i a l conversion of the ΝpO ·xΗ O to an amorphous, oxidized Np s o l i d , and possibly by Np aqueous speciation other than NpO OH°. 2

2

2

2

2

2

2

+

2

2

2

-

2

2

2

C u r r e n t l y proposed l i c e n s i n g r e g u l a t i o n s f o r g e o l o g i c nuclear waste r e p o s i t o r i e s r e q u i r e a performance assessment involving long-term p r e d i c t i v e c a p a b i l i t i e s . Previous work (1-5) has shown the importance of s o l u b i l i t y controls f o r modeling maximum a c t i n i d e concentrations i n r e p o s i t o r y groundwaters. However, u n t i l r e l i a b l e data are a v a i l a b l e on the a c t i n i d e s o l i d phases that may be present or that may p r e c i p i t a t e i n the environment, the. s o l u b i l i t y of s o l i d phases such as hydrous oxides that have f a s t p r e c i p i t a t i o n k i n e t i c s can be used to i n i t i a l l y set maximum s o l u t i o n concentration l i m i t s . In general, metal hydrous oxides i n aqueous suspensions g r a d u a l l y age to form c r y s t a l l i n e metal oxides. These changes i n c r y s t a l l i n i t y of s o l i d phases are accompanied by decreases i n the standard f r e e energy of formation. However t h i s c r y s t a l l i z a t i o n process i s opposed by r a d i o l y t i c e f f e c t s such that the p r o p e r t i e s of the steady-state r a d i o a c t i v e s o l i d w i l l be between those of the hydrous oxide and the c r y s t a l l i n e oxide ( 6 ) . Foç example, Rai and Ryan (6) have shown that over a 3.5-year p e r i o d 2 P u 0

0097-6156/84/0246-0135S06.00/0 © 1984 American Chemical Society

Barney et al.; Geochemical Behavior of Disposed Radioactive Waste ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

136

GEOCHEMICAL BEHAVIOR O F RADIOACTIVE WASTE

( c r y s t a l l i n e ) was a l t e r e d to a l e s s c r y s t a l l i n e Pu0 phase and that amorphous Pu(IV) hydrous oxide was converted to the same l e s s c r y s t a l l i n e phase with a corresponding decrease i n s o l u bility. They ^ s o showed t h a t , due to i t s greater alpha a c t i v i t y , crystalline Pu0 was converted to a steady-state s o l i d with p r o p e r t i e s , i n c l u d i n g s o l u b i l i t y , s i m i l a r to Pu(IV) polymer. q

2


2

237 239 Because of the longer h a l f - l i f e of Np compared with Pu, the alpha r a d i a t i o n e f f e c t s should be s i g n i f i c a n t l y l e s s . There f o r e , Np(IV) hydrous oxide i s expected to r a p i d l y develop c r y s t a l l i n i t y r e s u l t i n g i n a decrease i n s o l u b i l i t y and thus a decrease i n the maximum p r e d i c t e d s o l u t i o n c o n c e n t r a t i o n . The objective*? of t h i s study were to determine the s o l u b i l i t y of Np(IV) hydrous oxide and to determine the e f f e c t of aging Np(IV) hydrous oxide on i t s s o l u b i l i t y and c r y s t a l l i n i t y . M a t e r i a l s and Methods 237 A Np(IV) stock s o l u t i o n was prepared from an aniQn-exchange p u r i f i e d and f i l t e r e d (1.8 nm) s o l u t i o n of 4.3x10"" "ΤΙ Np ( i n 1M HC1) that was bubbled with H through a black platinum gauze for two days. At the end of bubbling the s o l u t i o n appeared to be l i g h t brown due to some formation of Np(III) but changed to green i n d i c a t i v e of Np(IV) w i t h i n one day. Spectrophotometric measure ments made on the reduced stock s o l u t i o n confirmed that the s o l u t i o n contained Np(IV) with l e s s than 4% Np(V). Another o x i d a t i o n s t a t e a n a l y s i s of the Np s o l u t i o n s used the TTA e x t r a c t i o n t e c h nique (7) which d i s t i n g u i s h e s the (IV) o x i d a t i o n s t a t e from the o x i d i z e d s t a t e s (V and V I ) . The u n c e r t a i n t y of the TTA e x t r a c t i o n procedure was estimated to be about 5%. The TTA e x t r a c t i o n a n a l y s i s of the reduced stock s o l u t i o n a l s o i n d i c a t e d that the Np was i n a reduced s t a t e >90% TTA e x t r a c t a b l e as ( I V ) . 2

A l l experiments were done i n an atmospheric c o n t r o l chamber ( i n i t i a l l y n e a r l y 100% Ar, 8.5

270 DAY EQUILIBRATION

LOG Np = -9.8±0.7 + pe SI

3 -β

F i g u r e 2· Log Np concentrations i n f i l t e r e d s o l u t i o n s con t a c t i n g Np(IV) hydrous oxide f o r v a r i o u s e q u i l i b r a t i o n times as a f u n c t i o n of ρβ· S o l i d l i n e s are f i t t e d to data from pH8.5

ν EQUILIBRATION, DAYS V

• '

-5

\ o

SLOPE =

ν

ο

70

Δ

110

•

206

Ο

270

•

ν

-6

•

-7 11

Ν

V

I

1

I

I

12

13

14

15

16

pe + pH

Figure 4. Log Np concentrations i n f i l t e r e d s o l u t i o n s (pHJ>8.5) c o n t a c t i n g Np(IV) hydrous oxide for various e q u i l i b r a t i o n times as a f u n c t i o n of pe + pH. The s o l i d l i n e i s drawn to i n d i c a t e a slope of -1.


STRICKERT ET A L .

8.

Np(IV) Hydrous Oxide

143


greater than 8.5 the s o l u b i l i t y - c o n t r o l l i n g s o l i d phase i s not Np(IV) hydrous oxide alone. This c o n c l u s i o n i s supported by Np o x i d a t i o n s t a t e analyses of the s o l i d phases from various suspensions (discussed i n the f o l l o w i n g s e c t i o n ) . There i s a l s o the p o s s i b i l i t y that the Np s o l u t i o n species may not e x i s t as the simple mono-hydroxy Np(V) complex i n the pH range 8.5 to 12. S o l i d Phase Analyses. P o r t i o n s of the Np s o l i d s from s e v e r a l suspensions were removed a f t e r 200 days, r i n s e d and d i s s o l v e d i n HC1. The Np o x i d a t i o n s t a t e analyses of the d i s s o l v e d s o l i d s (Table I I ) i n d i c a t e that while the s o l i d s from various low pH suspensions c o n t a i n predominantly Np(IV), high pH suspensions contain Np s o l i d s of mixed-Np o x i d a t i o n s t a t e s . Even though the TTA values are lower, the sum of the extracted and non-extracted neptunium ( r e l a t i v e to the t o t a l Np s o l u t i o n concentration) does not account f o r a l l the Np. This may i n d i c a t e that the Np(IV) values could be h i g h e r . Recoveries i n predominantly Np(V) s o l u t i o n s were n e a r l y 100%.

Table I I .

Np Oxidation State Analyses of D i s s o l v e d Np

Suspension

% Extractable i n TTA Np (IV)

Fresh ppt

88

Low

pH

High pH

%

Solids*

Non-extracted i n TTA Np(V and VI) 8 1 1

73 + 4

±

1

42 _+ 2

43 + 3

* Standard d e v i a t i o n based on m u l t i p l e samples. The r e s u l t s of XRD a n a l y s i s (Table I I I ) show that f r e s h l y prepared Np(IV) hydrous oxide i s amorphous. However, w i t h i n one month s e v e r a l samples from both low and high pH suspensions showed X-ray d i f f r a c t i o n peaks that corresponded to N p 0 ( c ) . No other c r y s t a l l i n e s t r u c t u r e was observed i n the s o l i d s examined. Theref o r e , the Np concentrations i n high pH s o l u t i o n s may be c o n t r o l l e d by an amorphous, o x i d i z e d Np s o l i d phase. A s i m i l a r study (6) of ^ P u ( I V ) hydrous oxide demonstrated that c r y s t a l l i n i t y s i m i l a r to Pu0 d i d not begin to appear u n t i l a f t e r 3 and 1/2 y e a r s . The r a p i d i t y of the c r y s t a l l i z a t i o n of Np(IV) hydrous oxide compared to Pu(IV) hydrous oxide i s no doubt a s s o c i a t e d with the d i f f e r e n c e i n h a l f - l i v e s between the two alpha-emitting i s o t o p e s . 2

2

9

2

The r e s u l t s presented i n t h i s paper along with other work (6) show that alpha r a d i a t i o n a f f e c t s the nature of the e q u i l i b r i u m


G E O C H E M I C A L BEHAVIOR OF RADIOACTIVE WASTE

144

Table I I I .

Summary of X-Ray D i f f r a c t i o n Analyses

Equilibration Time, Days

Solution d Spacing

(A)


Fresh ppt 41

7.7

112

6.55

289

5.75

3.17

—

1.93

1.64

1.36

1.22

9.6

3.17

2.70

1.94

1.66

1.38

1.24

1.92

1.64

1.35

1.25

31 126

12.05

289

11.5

NpO*

* From Reference

3.16

2.69

1.92

1.65

1.36

1.24 1.24

1.64

3.19

2.74

1.92

1.65

1.37

1.23

3.14

2.72

1.92

1.64

1.36

1.24

13.

s o l i d phase and thus the maximum s o l u b i l i t y - c o n t r o l l e d concentration. Such r e s u l t s i n d i c a t e that modeling f o r r e p o s i t o r y p e r f o r mance assessments must i n c l u d e the e f f e c t s of alpha r a d i o l y s i s on the p r o p e r t i e s of a c t i n i d e s o l i d phases. Acknowledgment s This research was performed f o r the U.S. Department of Energy under Contract No. DE-AC06-76RLO 1830. We thank Robert F u l t o n f o r h i s e f f o r t s i n the experimental measurements, Harvey Tenny f o r the X-ray d i f f r a c t i o n analyses, and Frank Hara f o r ICP analyses. The p r e p a r a t i o n and p r e s e n t a t i o n of t h i s paper was supported through the M a t e r i a l s C h a r a c t e r i z a t i o n Center. Literature

Cited

1. Rai, Dhanpat; Serne, R. J. "Solid Phases and Solution Species of Different Elements in Geologic Environments." U.S. Department of Energy Report PNL-2651, Pacific Northwest Laboratory, Richland, Washington, 1978.


8.

STRICKERT ET AL.

2.


3.

4.

5.

Np(IV) Hydrous Oxide

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Effect of Aging on the Solubility and Crystallinity of Np(IV) Hydrous Oxide

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