Redox behavior and solubility of plutonium under alkaline, reducing conditions

Abstract

Abstract The solubility and redox behavior of hydrous Pu(IV) oxide was comprehensively investigated by an experimental multi-method approach as a function of different redox conditions in 0.1 M NaCl solutions, allowing a detailed characterization of Pu(IV) and Pu(III) solubility and solid phase stability in these systems. Samples were prepared at ~3≤pHm≤~6 (pHm=–log m H + ) ${{\text{m}}_{{{\text{H}}^{\text{ + }}}}})$ and ~8≤pHm≤~13 at T=(22±2)°C under Ar atmosphere. No redox buffer was used in one set of samples, whereas mildly and strongly reducing redox conditions were buffered in two series with hydroquinone or SnCl2, respectively, resulting in (pe+pHm)=(9.5±1) and (2±1). XRD, XANES and EXAFS confirmed the predominance of Pu(IV) and the nanocrystalline character of the original, aged PuO2(ncr,hyd) solid phase used as a starting material. Rietveld analysis of the XRD data indicated an average crystal (domain) size of (4±1) nm with a mean cell parameter of (5.405±0.005) Å. The solubility constant of this solid phase was determined as log ∗ ​ K ° s , 0 $^ * K{^\circ _{{\text{s}},0}}$ =–(58.1±0.3) combining solubility data in acidic conditions and redox speciation by solvent extraction and CE–SF–ICP–MS. This value is in excellent agreement with the current thermodynamic selection in the NEA-TDB. Synchrotron-based in-situ XRD, XANES and EXAFS indicate that PuO2(ncr,hyd) is the solid phase controlling the solubility of Pu in hydroquinone buffered samples. Under these redox conditions and ~8≤pHm≤~13, the solubility of Pu is very low (~10−10.5 m) and pH-independent. This is consistent with the solubility equilibrium PuO2(am,hyd)+2H2O(l)⇔ Pu(OH)4(aq). Although in-situ XRD unequivocally shows the predominance of PuO2 in Sn(II)-buffered systems, XANES analyses indicate a significant contribution of Pu(III) (30±5%) in the solid phases controlling the solubility of Pu at (pe+pHm)=(2±1). For this system, EXAFS shows a systematic shortening of Pu–O and Pu–Pu distances compared to the starting Pu material and hydroquinone-buffered systems. The solubility of Pu remains very low (~10−10.5 m) at pHm>9, but shows a very large scattering (~10−9–10−10.5 m) at pHm=8. Experimental observations collected in Sn(II) buffered systems can be explained by the co-existence of both PuO2(ncr,hyd) and Pu(OH)3(am) solid phases, but also by assuming the formation of a sub-stoichiometric PuO2−x (s) phase. This extensive study provides robust upper limits for Pu solubility in alkaline, mildly to strongly reducing conditions relevant in the context of nuclear waste disposal. The potential role of Pu(III) in the solid phases controlling the solubility of Pu under these conditions is analysed and discussed in view of the current NEA-TDB thermodynamic selection, which supports the predominance of PuO2(am,hyd) and constrains the formation of Pu(OH)3(am) at pHm>8 outside the stability field of water.