Surface properties of two thorium phosphate compounds: Th4(PO4)4P2O7 and Th3(PO4)4; comparison and consequences for the retention of actinides

Two thorium phosphate compounds, Th3(PO4)4 and Th4(PO4)4(P2O7), with different solubilities and structures, are compared in the present work from the point of view of their surface charge and retention capacity. The sorption of Th(IV) and Am(III) onto the phosphate powders or colloids is shown to depend essentially on the parameters characterizing the solution in contact with the solid phase: namely the pH and the nature of the electrolyte which affect the surface charge of the solid phase. Taking into account the speciation of the two actinides under consideration, two mechanisms involving one phosphate site and either one (pH ≤ 4.7 for Th, 4 ≤ pH ≤ 7-8 for Am) or 3-4 exchanged protons (pH > 8 for Am, pH > 4.7 for Th) can be proposed to describe the sorption processes. However, this interpretation of the results needs to be confirmed by a better knowledge of the equilibria which involve Th and Am in the phosphate-containing solutions and also of the acid/base properties of the phosphate groups situated at the solid/solution interface. (orig.)