Interlink between solubility, structure, surface and thermodynamics in the ThO2(s, hyd)–H2O(l) system

Abstract

The impact of temperature on a freshly precipitated ThO2(am, hyd) solid phase was investigated using a combination of undersaturation solubility experiments and a multi-method approach for the characterization of the solid phase. XRD and EXAFS confirm that ageing of ThO2(am, hyd) at T = 80°C promotes a significant increase of the particle size and crystallinity. TG-DTA and XPS support that the ageing process is accompanied by an important decrease in the number of hydration waters/hydroxide groups in the original amorphous Th(IV) hydrous oxide. However, while clear differences between the structure of freshly precipitated ThO2(am, hyd) and aged samples were observed, the characterization methods used in this work are unable to resolve clear differences between solid phases aged for different time periods or at different pH values. Solubility experiments conducted at T = 22°C with fresh and aged Th(IV) solid phases show a systematic decrease in the solubility of the solid phases aged at T = 80°C. In contrast to the observations gained by solid phase characterization, the ageing time and ageing pH significantly affect the solubility measured at T = 22°C. These observations can be consistently explained considering a solubility control by the outermost surface of the ThO2(s, hyd) solid, which cannot be properly probed by any of the techniques considered in this work. Solubility data are used to derive the thermodynamic properties (log *K°s,0, ΔfG°m) of the investigated solid phases, and discussed in terms of particle size using the Schindler equation. These results provide new insights on the interlink between solubility, structure, surface and thermodynamics in the ThO2(s, hyd)–H2O(l) system, with special emphasis on the transformation of the amorphous hydrous/hydroxide solid phases into the thermodynamically stable crystalline oxides.

Overlooked impact of surface hydroxylation on the solubility of less-soluble compounds: a case study of CeO2

“…This hypothesis is in line with previous studies that have revealed that the dissolution process of cerium dioxide and other less soluble substances is not representative of the bulk phase, but is rather controlled by the surface processes of a few monolayers of the corresponding oxide. 28,29,[60][61][62] After chemical precipitation, a significant amount of hydroxyl groups is present on the surface of CeO 2 particles. Some studies have proposed considering particles of metal dioxides obtained from solutions as being particles with a core/shell structure, or even proposed judging the hydroxylated surface layer as a microphase.…”