Computational Insights into the Hydrolysis and Aqueous Stability of Thorianite (ThO₂) and its Environmental Implications for Groundwater Systems.

Thorianite (ThO₂) is a naturally occurring mineral form of thorium dioxide found in diverse geological settings such as pegmatites, carbonatites, and heavy mineral sands. The environmental importance of the material lies in the fact that the mobility of thorium species in the mineral weathering and hydrolysis processes is likely to reduce the quality of groundwater and radiochemical stability. This study aims to computationally investigate the hydrolysis and hydrolysis and aqueous stability behavior of Th⁴⁺ and hydrated ThO₂ complexes to understand their stability and potential environmental implications. Density functional theory (DFT) calculations were performed using Gaussian 09W, Revision D.01 to simulate the hydrolysis of Th⁴⁺ and ThO₂(H₂O)ₙ species (n = 1 to 10). The relative stabilities and electronic characteristics of the hydrated thorium species, optimised geometries, total electronic energies, and thermodynamic parameters, such as entropy and Gibbs free energy, were calculated. The coordination number of Th⁴⁺ was found to be decreasing progressively during stepwise hydrolysis. Among the hydrated species, ThO₂(H₂O)₉ exhibited the highest stability with a total energy of −29764.9950 eV and total free energy of −20701.0604 eV, indicating strong hydration stabilization. ThO₂(H₂O)₆, possessing C₂v symmetry, shows a total energy of −23539.8782 eV and entropy of −865.8066 cal mol⁻¹K⁻¹. The results suggest that hydration plays a critical role in stabilizing thorium oxide clusters, which can influence thorium mobility under aqueous conditions. The computational insights highlight that the hydrolysis of ThO₂ can yield stable hydrated complexes, which may enhance thorium solubility and potential leaching into groundwater systems. These findings contribute to understanding the environmental geochemistry of thorium-bearing minerals and their long-term behavior in natural waters. The study underscores the importance of quantum chemical modeling in predicting mineral water interactions relevant to radioactive element dispersion and environmental safety.