I
ARTHUR BRUNSTAD' Hanford Atomic Products Operation, General Electric Co., Richland, Wash.
Polymerization and Precipitation of PIutonium(1V) in Nitric Acid The formation of colloidal plutonium(1V) hydroxide from plutonium(1V)nitrate solutions has been determined as a function of acid and plutonium concentration at several temperatures. Hydrolysis, polymerization, and precipitation take place in unstable solutions with a resulting increase in acidity to a concentration sufficient for a stable true solution. Except for colloidal sols formed from solutions of a few grams per liter, the acidities become too high for the plutonium to remain as a stable colloid. T H E incipient precipitation of colloidal plutonium(1V) hydroxide in processing low acid solutions may result in formation of critically unsafe deposits and plugging of transfer lines. The dispersed colloid and precipitated plutonium hydroxide are very difficult to redissolve. The presence of the colloid also causes foaming in concentration by evaporation, and emulsification in solvent extraction operations. I t seemed desirable to establish the conditions necessary to avoid hydrolytic polymerization of quadrivalent plutonium.
Experimental A 2.5M nitric acid stock solution containing 400 grams of plutonium(1V) per liter was used. Dilution with distilled water or standard nitric acid solutions was the only chemical alteration of the 1 Present address, U. S. Atomic Energy Commission, Hanford Operations Office, Richland, Wash.
plutonium solution. Several series of solutions were prepared with constant acid-plutonium ratios and with various plutonium concentrations within each series. The solutions were mixed at room temperature. A part of each solution was used for spectrophotometric studies and for stability observations a t room temperature. The remainder was divided, placed in water baths for 2 hours a t 80",90", and 100" C., and allowed to cool and stand overnight before being centrifuged. The absorption spectrum between 1200 and 400 mp was obtained about 10 minutes after mixing of the solutions, except for rate determinations where times are specifically noted. Plutonium solutions greater than 3 grams per liter were diluted to 3 grams per liter with nitric acid solution of equal acidity. Absorbance measurements were made with a Beckman Model DK-2 recording spectrophotometer. The spectra of low acid (0.024 to 1. OM) solutions were studied for criteria of polymer formation. Solutions stored a t room temperature were examined after 4 days for precipitation. The solutions were centrifuged and the supernatant was sampled and analyzed for plutonium concentration by alpha counting. Acidity was determined by titration, using a n oxalate complexing method. The supernatant acidity found was considered to be the acidity necessary to stabilize the plutonium found in the supernatant. Stability tests near boiling temperature were made with increasingly higher acid
and plutonium concentrations until precipitation no longer occurred. Depolymerization rates in 2, 4, and 6M nitric acid were obtained from the optical densities a t 415 m+ Precipitation formed a t room temperature and at 80" C. was dispersed in nitric acid solution. A dispersion suitable for spectrophotometric measurement was formed when one volume of precipitate was dispersed in 10 volumes of acid solution. A second dilution in the range of 1 to 50 resulted in readable optical densities.
Discussion Pure polymeric plutonium(1V) hydroxide sols have a beautiful emeraldgreen color. Nitrate solutions 0.5 to 5.0M are brown, but those of higher acidity are green and resemble the hydroxide sols. The emerald-green colloidal polymer in sols or precipitates can be easily recognized when this species is the principal form, but incipient polymerization is not so easily recognized. Formation of the colloidal sol is accompanied by increasing absorbance in the low wave length end of the visible spectrum, due to light scattering by the colloidal particles. Absorbance at 415 mp was chosen as a measure of polymerization, values greater than 20 were taken to indicate that polymerization occurred. Absorbance here is molar absorbance and is synonymous with molar extinction coefficient, usually denoted by E, where
100I
1
0
z a
LEGEND: 0
cnlo
Background Literature Ref. Early work on plutonium chemistry showed formation of colloidal plutonium hydroxide in low acid chloride and nitrate solutions Colloidal plutonium hydroxide sols free of monomeric ions obey Beer's and Lambert's laws. X-ray diffraction data indicate t h e colloid and precipitated plutonium "hydroxide'' to be hydrated dioxides Precipitation temperature related to plutonium and acid concentration in sulfuric acid solutions
38
m,
(4)
I 0.02 (g)
(1)
INDUSTRIAL AND ENGINEERING CHEMISTRY
0.05
0.I
0.2
0.6
J
fy "03 Figure 1. Polymerization begins at higher acidity and is more extensive a t higher plutonium concentration
I"",
I 0.02 Figure 2. ratios
E D I
= = =
I
I 0.05 0.I -M "03
I 0.2
I
I
0.4
Polymerization is greatest in solutions of low acid-plutonium
D/CL
optical density = loglo 1OO/I per cent transmittance C = molar concentration of plutonium L = optical path length of cell, cm. The effects of plutonium concentration and the nitric acid-plutonium ratio on polymer formation are shown in Figures 1 and 2. In Figure 1, points of equal plutonium concentration are connected, and in Figure 2, for the same data, equal nitric acid-plutonium ratios are connected. Figure 1 indicates that polymerization begins a t higher acidity and is more extensive in solutions of higher plutonium concentration. Figure 2 shows that polymerization is greatest in solutions of low acid-plutonium ratios. The tendencies noted ' appear to be violations of Beer's law, which states that absorbance is independent of concentration. Polymerization has been recognized as a cause of anomalous behavior in spectrophotometry. The polymer is, however, a different species than the various ions which contribute to the plutonium (IV) nitrate spectrum. Polymeric plutonium sols, free from monomeric ions, have been shown to obey Beer's and Lambert's laws (2). In Figure 3, the course of the hydrolysis is represented by the line from the square point to the circular point. A decrease in solution plutonium concentration and an acidity increase are indicated. The final compositions (circular points) describe a stability line or the line of minimum acidity necessary for maintenance of a true solution. Solutions represented by points on the left of this line are unstable and will hydrolyze to form a solid phase of plutonium hydroxide. The green solid phase settled or was centrifuged from the brown solutions, but no precipitate was formed in the green solutions, which were the most dilute. These stable colloids are of compositions represented by the shaded area in the lower left corner of
Figure 3. Solutions should be kept at acidities greater than indicated by the stability line
Figure 3. Only the very low acid solutions of a few grams per liter plutonium formed stable colloids when prepared in this manner. The dilution line represents concentrations resulting from the dilution of the 400 grams per liter plutonium, 2.5M nitric acid stock solution. Dilution of this stock solution to below 42 grams of plutonium per liter resulted in the unstable solutions represented by the area between the dilution line and the stability line. Dilutions to concentrations greater than that represented by point B should remain stable. I t is difficult, however, to make such dilutions without some part of the solution, a t any time, being more dilute than the composition represented by B. Although the final acidity of such a dilution is sufficient for stability, the rate of depolymerization is so small a t this acidity as to be negligible. Precipitates may, therefore be formed as the result of dilution technique rather than final acid deficiency. Figure 3 should, therefore, be taken as a general guide only, and solutions should be kept a t acidities greater than indicated by the stability line. I t is practically impossible to dilute plutonium(1V) solutions with water without formation of colloid, because of regions of momentary high pH. Plutonium(1V) solutions should therefore be diluted with acid and never consciously with water. The points above 40 grams per liter were obtained with stock solutions of lower acid-plutonium ratio. These solutions were stored G months, centrifuged, and again set aside for observation. No further precipitation took place. The experimental average increase in acidity per mole of plutonium precipitated was 3.1M . The stoichiometric increase would be 4.OM according to @e
0
01
0.2 0.3 0.4 0.5
M "03
following equation :
Pu+*+- 4HzO +Pu(0H)a f 4H+ A comparison of Figure 1 and Figure 3 shows good agreement between spectrophotometric and precipitation data. The spectrophotometric data indicate polymer formation a t slightly higher acidity. As the spectrophotometric data were obtained within 10 minutes after mixing, it may be assumed that hydrolysis, and polymerization to the colloidal state, are very rapid. A recent investigation (7) with sulfuric acid solution of plutonium(1V) related precipitation temperatures to plutonium and acid concentrations. The temperature of precipitation increased with acidity and decreased with increasing plutonium concentration. The precipitation of plutonium hydroxide from boiling plutonium nitrate solution has been investigated in this laboratory (3). The plutonium concentration a t which precipitation occurred was shown to be a function of hydrogen ion concentration below 1.23M H+. A part of the 25' C. stability curve from Figure 3 is included in Figure 4. Examples of the course of the changes in concentrations of solution plutonium and acidity which occurred upon heating the starting solution to the respective temperatures are shown by dotted lines. The precipitation may be partial or complete, depending on concentrations of acid and plutonium. Where precipitation is partial, hydrolysis proceeds until there is an increase in acidity sufficient to prevent further hydrolysis. Precipitation a t these higher temperatures was well defined and appeared to be complete after 2 hours. Disproportionation was visually evident in the more concentrated plutonium VOL. 51, NO. 1
JANUARY 1959
39
solution at high temperatures. Reproportionation in these solutions on cooling to room temperature was also evident. I t is probable that the persistence of the very low plutonium concentrations (Figure 4) was the result of valence states other than plutonium(IV). S o corrections were made for the effects of the disproportionation reaction: 3 Pu(1V) q 2 Pu(II1j
+ Pu(V1j
The observed formation of stable colloids (or precipitates) may be explained on the basis of the pioneering work of \’on Wiemarn. The formation of a stable colloid is directly related to the rate of particle formation-the higher rate of formation yields smaller particles of greater stability.
I
40 ~
c
3
25 “C.
f
I
0 I
3
a
80°C.
.
’\.
L o . ,
/
20 L 4
0°C
‘%!/
10
0
_ Q -_ S
S
S Q
= =
T =
solubility of polymer 01- colloid concentration of monomer or polymerizing species-Pu(0H j4.xH?O initial rate of condensation
Figure 4. Precipitation may be partial or complete, depending on acid and plutonium concentration
The polymerizing plutonium hydroxide is formed by the hydrolysis of plutonium nitrate:
+ 4H:O %
Pu(NO.I)J
Pu(OH)?+ 4HNOs
The solubility of the polymer increases with nitric acid concentration and the concentration of plutonium hydroxide monomer decreases with an increase in nitric acid concentration. The initial rate of particle formation will, therefore, be high if acidity is low, and particles of colloidal dimensions will be formed. Higher acid concentrations will result in a lower rate of initial particle formation and the particles will have an opportunity to grow in size. This favors the formation of a precipitate rather than a colloid. The more difficult solubility of precipitates formed a t higher temperatures suggests that the polymer may be partly a n oxide or suboxide, as suggested for other metallic sols. The decreased solubility of the polymer formed a t high temperature would indicate higher oxide content. Evidence indicates that precipitated colloidal plutonium and quadrivalent plutonium “hydroxide” are just hydrated plutonium dioxides (2). Precipitated poIymer was washed ivith water, centrifuged, and redispersed in nitric acid solutions of several concentrations. Some of these dispersions were sufficiently stable to obtain absorption spectra series from which the depolymerization rate could be calculated. Optical density readings a t 41 5 mp were corrected for plutonium nitrate concentration, as indicated by the 476 m,u peak. The resulting data yielded straight-line semilog plots indicating first-order dependence on polymer concentration. Half-times for depolymerization can therefore be calculated as in the following example :
40
INDUSTRIAL AND ENGINEERING CHEMISTRY
Depolymeriaatiori
Time Interval.
Coriected
Depolymerization
Half-Time,
Min.
Absorbance
Rate Constant. k
TI,?. M i l l .
60 300
2.303 0.60 300-60 min. log 0 7 1 =
-
0.60‘,< 0.llj
100
0.0069
Some depolymerization half-times found are : Formatioil Temp., C . 25
80
_
trirh c_ i d C on( _ STi~ _entIation
-
ZJI
4.11
6T f
20 days 1 year
400 min.
100 min. 15 days
The rapid rate of polymer formation and the sloiv rate of depolymerization make possible the existence of the polymer in solutions subjected only to transient unstable conditions. The polymer may form in overheated pumps or in places where steam or water leaks might cause dilution. I t may be precipitated a t the place of formation or carried by the solution as a colloid. Precipitation of the colloid plutonium(1V) may subsequently take place where the acidity is sufficient for a stable true solution, but unfavorable for the existence of the sol. Conclusions
Serious problems in handling IOLV acid plutonium(1V) solutions may result from increased solution temperatures and water dilution. Steam or water leaks into piping or tanks carrying plutonium(IV), localized high temperatures resulting from overheating in pumps, and excessive heat of reaction should be avoided. Temperatures employed in concentration by evaporation must not exceed the precipitation temperature for the solution. These conditions could
*..
result in serious criticality conditions. plugged transfer lines, and solutions requiring drastic treatment before becoming suitable for further processing. Plutonium(I1’) solutions should be diluted xvith acid; never purposely with water. Depolymerization should be effected at the highest practical acidit\ and temperature. literature Cited (1) Grant, D. W.: Glanville, D. E., ‘.Hydrolytic Behavior of Plutonium Ions in
Sulfuric Acid,” Atomic Energy Research Establishment (Gt. Britain), AERE-C-R2155 (unclassified) (May 1957). (2) Ockenden, D. W., WeIch, G. .\., J . Chem. SOC. 1956, pp. 3358-63. (3) Pugh, R. A,, U. S. Atomic Energy Comm., HW-32100 (April 28, 1954) (classified). (4) Seaborg, G. T., Katz, J. J., rds., “ T h e Actinide Elements,” Natl. Nuclear Ene r g Series, Div. IV, vol. 14A, pp. 225, 31 7. McGraw-Hill, New York, 1954. RECEIVED for review hpril 7, 1958 ACCEPTED October 27, 1958
Division of Industrial and Engineering Chemistry, Symposium on Chemistry of Heavy Elements, 133rd Meeting, ACS, San Francisco, Calif., April 1958.
