Immobilization of radioactive Th by coprecipitation with lead apatites

There is a constant search for materials that could be used as geochemical barriers and fillers for radioactive waste storage containers. One of effective methods of immobilizing toxic elements is their precipitation in the form of sparingly soluble crystalline phases. The purpose of this study was to test the effectiveness of co-precipitation of Th from aqueous solutions in the form of crystalline lead phosphates. Such phases have high stability and low solubility and the presence of Pb has an additional effect on reducing the radiation.

Three sets of experiments were carried out by adding reagents into a solution containing non-radioactive Th at a concentration of 400 ppm:

(A) a solution containing PO₄^3- ions (at a concentration of 1 g/L);

(B) a solution containing Pb²⁺ and a solution containing PO₄^3- ions (at concentrations of 3.5 g/L and 1 g/L, respectively) added dropwise simultaneously;

(C) solutions containing Pb²⁺, PO₄^3-, and Cl^- ions (at concentrations of 3.5 g/L, 1.0 g/L, and 0.12 g/L, respectively) added dropwise simultaneously.

All experiments were repeated at pH = 3, 5 and 7. No reaction occurred in experiment (A), while in experiments (B) and (C) the solid phases crystallized, and the Th concentration dropped from 400 to about 0.05 ppm. In the absence of Cl (experiment B), the reaction at pH=3 led to the formation of "phosphoschultenite" PbHPO₄ while at pH = 5 and 7 Th-bearing hydroxylpyromorphite Pb₅(PO₄)₃OH was formed. In the presence of Cl ions (experiments C), Th-bearing pyromorphite Pb₅(PO₄)₃Cl was formed over the entire pH range. Th was removed from solution by coprecipitation with Pb and incorporation of Th into lead phosphate structure.

The results showed that Pb must be present for Th to be effectively immobilized from solution in the form of phosphate phases. The presence of Cl is not as important as the presence of Pb in terms of removal efficiency, but it may be crucial for the long-term stability of the precipitated phases, since Pb₅(PO₄)₃Cl has higher stability and lower solubility than Pb₅(PO₄)₃OH. These findings pave the way for development of potential innovative techniques for immobilizing Th from waste and contaminated solutions, which could have implications for the safety and long-term viability of various nuclear waste disposal programs. This research was funded by National Science Center research grant no. 2021/43/O/ST10/01282.