Investigation of 137Cs+, 85Sr2+ and 241Am3+ ion exchange on thorium phosphate hydrogenphosphate and their immobilization in the thorium phosphate diphosphate

Abstract

Thorium phosphate diphosphate (TPD) has been proposed as a host matrix for the long-term storage of tetravalent actinides such as U(IV), Np(IV) and Pu(IV), which form solid solutions with TPD. Small amounts of di- or trivalent cations can also be immobilized in this matrix. Another method of loading of TPD, using the ion-exchange properties of thorium phosphate hydrogenphosphate (TPHP), a precursor of TPD, was applied to ¹³⁷Cs⁺, ⁸⁵Sr²⁺ and ²⁴¹Am³⁺. The adsorption of these cations on TPHP was studied in terms of pH in NaClO₄ and NaNO₃ media. The ion-exchange process on TPHP was found to be reversible for ⁸⁵Sr²⁺ and ²⁴¹Am³⁺. Doped TPHP obtained in this way was heated at 1050 °C until TPD was formed. Several leaching tests were performed in acidic media on TPD doped with radionuclides with ionic strength equal to 0.1 M. The average partial order related to the proton was found to be equal to 0.24 ± 0.10 and the normalized leaching rate constant was in the range of 3 × 10⁻⁶ to 2 × 10⁻⁵ g m⁻² d⁻¹. These results are similar to those obtained for thorium plutonium(IV) phosphate diphosphate solid solutions. On the basis of the results obtained from the adsorption and leaching studies, this adsorption-immobilization process, based on the TPHP matrix, can be proposed for the decontamination of low level radioactive liquid wastes.