AUTHOR=Kiefer Christian , Neill Thomas , Cevirim-Papaioannou Nese , Schild Dieter , Gaona Xavier , Vitova Tonya , Dardenne Kathy , Rothe Jörg , Altmaier Marcus , Geckeis Horst 

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

JOURNAL=Frontiers in Chemistry

VOLUME=10

YEAR=2022

URL=https://www.frontiersin.org/journals/chemistry/articles/10.3389/fchem.2022.1042709

DOI=10.3389/fchem.2022.1042709

ISSN=2296-2646

ABSTRACT=<p>The impact of temperature on a freshly precipitated ThO<sub>2</sub>(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 ThO<sub>2</sub>(am, hyd) at <italic>T</italic> = 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 ThO<sub>2</sub>(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 <italic>T</italic> = 22°C with fresh and aged Th(IV) solid phases show a systematic decrease in the solubility of the solid phases aged at <italic>T</italic> = 80°C. In contrast to the observations gained by solid phase characterization, the ageing time and ageing pH significantly affect the solubility measured at <italic>T</italic> = 22°C. These observations can be consistently explained considering a solubility control by the outermost surface of the ThO<sub>2</sub>(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 *<italic>K</italic>°<sub>s,0</sub>, Δ<sub>f</sub><italic>G</italic>°<sub>m</sub>) 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 ThO<sub>2</sub>(s, hyd)–H<sub>2</sub>O(l) system, with special emphasis on the transformation of the amorphous hydrous/hydroxide solid phases into the thermodynamically stable crystalline oxides.</p>