Coprecipitation of Ce(III) oxide with UO2

Abstract

The neutralization of acidic solutions containing U (IV) and Ce (III) at room temperature in glove box atmosphere and in the presence of dithionite results in coprecipitation of these elements as amorphous solid solutions Ce_xU_1-xO_2±y. The solubilities of the precipitates with different mole fractions (x) of Ce(OH)₃ (x = 0.01 or 0.1) were determined in 1 M NaClO₄ solutions between pH 2.2 and 12.8 under reducing conditions. The solids were investigated by a variety of methods (chemical analysis, SEM-EDX, XRD, XPS, XAS) to determine the nature of the solid solutions formed, their composition and the valence state of Ce and U. X-ray photoelectron spectroscopy confirmed the oxidation states of the solids both before and after the equilibration as Ce (III) and U (IV). The amorphous coprecipitates reached equilibrium relatively fast (∼1 week). The release of Ce from the coprecipitates was totally dominated by the release of uranium over the whole pH range. The Ce concentrations decrease slightly with the decrease of Ce content in the solid, suggesting that Ce_xU_1-xO_2±y solids behave thermodynamically as solid solutions. The concentrations of U in equilibrium with the coprecipitate were in excellent agreement with the solubility of UO₂(s) under reducing conditions reported in the literature. The conditional solubility product of Ce(OH)₃ from the coprecipitate was several orders of magnitude (∼4 in the near neutral pH range and ∼18 in the acidic range) lower than that of pure Ce(OH)₃(s). The activities and activity coefficients of Ce(OH)₃(s) in the coprecipitate were also estimated. Activity coefficients are much less than 1, indicating that the mixing of Ce(OH)₃ with UO₂ is highly favorable.

Keywords: Ce (III); UO2; actinides; amorphous; coprecipitation; solid solution; solubility.

Figure 1

UV-visible spectrophotometric analysis of U stock solution.

Figure 2

(a) Evolution of U and Ce total concentrations during 30 days equilibration for all pH values investigated for the solid with 1% Ce. (b) Evolution of U and Ce concentrations during 30 days equilibration for two randomly selected samples at −log[H⁺] values of 2.99 and 8.18. The dotted lines are horizontal and centered at the value of the last point.

Figure 3

SEM micrograph of (Ce_0.1U_0.9)O_2±x solid (center) and SEM-EDX U mapping (left) and Ce mapping (right) of the same area. Analysis made before equilibration.

Figure 4

SEM-EDX spectrum of the Ce_0.1U_0.9 solid. Solid after equilibration; S originates from di­thio­nite.

Figure 5

SEM micrograph of (Ce_0.1U_0.9)O_2±x solid (center) and SEM-EDX U mapping (left) and Ce mapping (right) of the same area. Analysis made after equilibration.

Figure 6

XRD spectra of the Ce_0.10U_0.90 solid before (upper) and after (lower) equilibration.

Figure 7

U–Ce equilibrated solid heated at 900°C under reducing conditions. The upper spectrum is for Ce_0.10U_0.90 while the lower spectrum is for Ce_0.01U_0.99.

Figure 8

Survey XPS spectrum of the as-received Ce_0.01U_0.99 sample. (b) High resolution XPS scans in the Ce 3d region for the Ce_0.01U_0.99 solid. (c) High resolution XPS scans in the U 4f region for the Ce_0.01U_0.99 solid.

Figure 9

(a). Survey XPS spectrum of the as-received Ce_0.10U_0.90 sample. (b) High resolution XPS scans in the Ce 3d for the Ce_0.10U_0.90 solid. (c). High resolution XPS scans in the U 4f region for the Ce_0.10U_0.90 solid.

Figure 10

Normalized raw XANES spectra of cerium (III) oxide, Ce₂O₃ (light blue line) and cerium (IV) oxide, CeO₂ (brown line). Raw data, not normalized, of U_0.90Ce_0.10 at two different spots (yellow and gray lines) show that cerium is present as cerium (III) in the studied samples.

Figure 11

Normalized raw XANES spectra of uranium (IV) oxide, UO₂ (light blue line), and the studied uranium samples, ‘sample UO₂_pure_1’ (red line), ‘sample UO₂_pure_2’ (green line), ‘sample U_0.99Ce_0.01’ (purple line) and ‘sample U_0.90Ce_0.10’ (yellow line). Note that samples ‘UO₂_pure_1’, ‘UO₂_pure_2’, ‘U_0.99Ce _0.01’ and ‘U_0.90Ce_0.10’ all consist of uranium (VI) oxide.

Figure 12

Close up of Fig. 11 ▸ at the white line peak region

Figure 13

UV-vis analysis of the MAX IV laboratory samples after dissolution in HClO₄ (top; left Ce_0.01U_0.99, right Ce_0.10U_0.90) compared with spectra of pure U (VI) and U (IV) in the same solvent (bottom).

Figure 14

Concentrations of U in equilibrium with the Ce_0.01U_0.99 solid solution at 30 days. The dotted line with slope −3 is from Fig. 5 of Rai et al. (1997 ▸), while the horizontal dotted lines indicate log[U] = −8.5 ± 1. The data of Rai et al. (1997 ▸) are for equilibration times 8–420 days.

Figure 15

Concentrations of Ce and U in equilibrium with Ce_0.01U_0.99 oxide coprecipitate compared with concentrations of Ce in equilibrium with Ce(OH)₃(s) (Kragten & Decnop-Weever, 1978 ▸) at 1 M NaClO₄.

Figure 16

Concentrations of Ce and U in equilibrium with Ce_0.10U_0.90 oxide coprecipitate. The dotted lines in the basic range indicate the solubility of UO₂(s), log[U] = −8.5, and the horizontal part of Ce concentrations for the pure oxide and the coprecipitate.