Synthesis, characterization and solubility of M-Pu(VI)-O(H) solid phases

Compared to U(VI) and Np(VI), the hydrolysis behavior of Pu(VI) in aqueous solution shows some unique features. One of them is the fact that alkalization of acidic solutions of UO22+ and NpO22+ with alkali metal bases leads to the spontaneous formation of sparingly soluble An(VI)-(hydr)oxide solids, e.g., AnO2(OH)2xH2O(cr) and Na2An2O7xH2O(cr) with An = U and Np, whereas Pu(VI) forms polynuclear hydrolysis products, (PuO2)y(OH)x2y−x(aq).1,2,3 Some of these species, especially the ones forming at greater [Pu(VI)]tot are metastable, with lifetimes going up to several months, making the spontaneous formation of analogous Pu(VI)-(hydr)oxides kinetically slow.3 This behavior may explain why available information on Pu(VI)-(hydr)oxides including thermodynamic data are overall sparse. In this contribution we report on the synthesis, characterization and solubility behavior of Pu(VI) in MOH-MCl (M = K, Rb) solutions.

Experiments were performed with 242Pu in glove boxes under inert anoxic Ar atmosphere. Oxidation state pure 242PuO22+ stock solutions were alkalized with MOH-MCl (M = K, Rb), and mildly tempered for several weeks resulting in the formation of brownish to blackish precipitates. ... mehrThe latter were routinely characterized by powder XRD, SEM-EDX, wet chemical analysis by combined LSC+OES or ICPMS, Pu oxidation state analysis by Vis/NIR, and Pu L3-edge XANES and EXAFS at the INE Beamline for Radionuclide Research at the KIT Light Source. If applicable, additional characterization methods including single crystal (sc) XRD, solid state Vis/NIR, and Raman were used.

In total, three new crystalline M-Pu(VI)-O(H) compounds were obtained and identified under the experimental conditions. Compound (I) and (II) are K-Pu(VI)-O(H) solid phases with structural K:Pu ratios of 1:2 (platelet-like morphology), and 2:3 (rod-like), respectively. Compound (III) is a Rb-Pu(VI)-O(H) solid phase with Rb:Pu ratio of 1:2 (rod-like, obtained at pHm ≈ 12.7). The crystal structure of (I) was revealed by sc-XRD and is very similar to a class of reported U(VI)-O(H) minerals to which, e.g., Becquerelite and Metaschoepite belongs to, but holds overall unique structural features.4 While sc-XRD analysis is ongoing for (II) and (III), principal structural parameters were evaluated by Pu L3-edge XANES, EXAFS and powder XRD, confirming a very pronounced structural similarity to (I). To get information about the thermodynamic stability in solution, the solubility behavior of (II) and (III) were studied in 2 M M-Cl-OH with pHm = 12-14. The systematic pHm dependence in this pHm region reveal that the Pu(VI) solubility is controlled by equilibria between the solid M-Pu(VI)-O(H) phases and the (1,4) hydroxo complex of Pu(VI), PuO2(OH)42−. The corresponding thermodynamic constants were derived. In the presentation, the results obtained for Pu(VI) are compared to analogous experiments performed with Np(VI).