Discrepancies in thorium oxide solubility values: study of attachment/detachment processes at the solid/solution interface

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

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.

Assessment of surface reactivity of thorium oxide in conditions close to chemical equilibrium by isotope exchange 229Th/232Th method

This work aims to assess the solubility and the surface reactivity of crystallized thorium at pH 3.0 in presence of three types of solids: synthesized powder at 1300°C, crushed kernel, and intact kernel. In this study, the kernel is composed by the core solid from high temperature reactors (HTR) sphere particles. The originality of this work consisted in following in a sequential order the kinetic of dissolution, the surface reactivity in presence of isotope tracer 229Th, and its desorption process. Long time experiments (634 days) allowed to get deeper understanding on the behavior of the surface reactivity in contact with the solution. Solubility values are ranging from 0.3×10−7 mol·L−1 to 3×10−7 mol·L−1 with a dissolution rate of 10−6–10−4 g·m−2 day−1. PHREEQC modeling showed that crystallized ThO2(cr, 20 nm) phase controls the equilibrium in solution. Isotope exchange between 229Th and 232Th indicated that well-crystallized phase exist as an inert surface regarding to the absence of exchange between surface solid and solution.

Adsorption of tetravalent thorium by geomedia

We measured the pH-dependent adsorption of Th(IV), an analogue for Pu(IV) and other tetravalent actinides, to two geomedia: goethite (α-FeOOH(s)) and a heterogeneous Fe-containing sand from the southeastern USA. The goal was to examine whether or not the Th(IV)-goethite adsorption data could be used to predict the adsorption of Th(IV) by the heterogeneous sand. In the absence of either geomedia, after forty-eight hours the measured pH-dependent “adsorption” was consistent with the solubility of solid amorphous ThO2(am, aged), despite the fact that ThO2(am, aged) is generally not formed until approximately seventy days. We concluded that ThO2(am, aged) was stabilized by precipitating on the walls of the reaction vessels. Ignoring this phenomenon could lead to experimental artifacts in Th(IV) adsorption studies. Thorium adsorption by both goethite and the sand was strongly pH dependent, with adsorption increasing sharply from pH ∼ 2 to pH ∼ 4. Two methods were utilized to predict the pH-dependent adsorption of Th(IV) by the sand using the Th(IV)-goethite adsorption data. Using the Fe content of the sand and the Th(IV)-goethite adsorption data, we were able to predict the maximum amount of Th(IV) adsorption by the sand within 78% of the actual value (i.e., an error of 22%). In contrast, on a surface-area-normalized basis, we were only able to predict the maximum adsorption of Th(IV) by the sand within a factor of two. These results have important implications to scaling and extrapolating the results of batch-scale tetravalent actinide adsorption studies with pure minerals to predict their field-scale adsorption by and transport in heterogeneous subsurface media.