Initial Assessment of Hydrous Thorium(IV) Solubility and Speciation in Geological Environments: An Experimental Approach in Presence of Organic Ligands

Abstract

Data on the hydrolysis of Thorium(IV) and solubility of ThO₂ in presence of organic ligands are of critical importance to the safe disposal and treatment of high-level nuclear waste. Such data are required to predict reliably thorium behaviour in high-level waste repositories where organic anions may be present. In addition, Th(IV) is considered to be a chemical analogue of tetravalent actinides such as Pu(IV). To establish the effect of pH and organic ligands on the hydrolysis of Th(IV), experiments on the solubility of hydrous ThO₂ in aqueous solution at room temperature were performed, covering a wide range of pH (pH = 3-12) in 0.1 molal NaNO₃ solutions. Experiments also were conducted in the presence of citrate, oxalate and EDTA with a concentration range of 100-500 μmol. The solubility of ThO₂ decreases with increase of pH (pH 3 to 6) and remains near the detection limit of ICP-MS (10^-9-10^-10 M) in the range pH 6-12. These values are in accordance with data in the literature. The solubility of ThO₂ does not appear to be a sensitive function of the concentration of the investigated organic ligands at any given pH providing the fact that bulk of organic ligands is potentially absorbed by ThO₂ solid phase. Thorium concentrations obtained from unfiltered and filtered samples showed almost an order of magnitude variation in molal concentration indicating a significant amount of colloid formation.