A novel approach to Th immobilization via coprecipitation with Pb-apatite on a mixture of Pb-modified clinoptilolite and hydroxylapatite

Radioactive storage is becoming a critical component of industrial waste management. Thorium and Uranium are the primary waste of nuclear power plants and increasingly present in waste from other sectors. While bentonite is often suggested as the primary backfill in repositories, zeolites (e.g. clinoptilolite) are frequently proposed and used as specialized reactive fillers and barrier materials within storage containers and concrete matrices.

In this research we test a novel reactive barrier material for immobilization of Th via coprecipitation with low-solubility lead phosphates (pyromorphite, Pb₅(PO₄)Cl). The material consists of a mixture of Pb-modified zeolite clinoptiloite and hydroxylapatite. Appropriate methodology of zeolite activation allows us to obtain a Pb-modified clinoptilolite which is safe for the environment (no Pb release in water) but still is reactive enough that in contaminated waters it acts as a Pb source for the precipitation of Pb phosphates [1]. In this system, hydroxylapatite acts as a source of PO₄^3- anions and as a source of Ca ions which can replace Pb on the surface of zeolite through ion exchange. Upon contact with a solution containing radioactive contaminants and Cl, a reaction is expected to produce pyromorphite with incorporated Th as the dominant phase.

To test this model an experiment was conducted in which a mixture of Pb-modified zeolite and hydroxylapatite reacted with solutions containing Th and Cl in pH = 5. Analyses of both the solutions and solid phases were carried out to ascertain the efficiency and mechanisms of the processes.

The reaction with a mixture of Pb-zeolite and hydroxylapatite results in formation of Th-bearing pyromorphite. Powder X-ray diffraction and SEM analyses of the precipitates has shown that a reaction has occurred in all the experiments, with pyromorphite crystals up to 2 μm formed on the surfaces of both zeolite and hydroxylapatite. The reaction is very effective, with Th concentrations lowering from 5 ppm to less than 10 ppb. Precipitation of pyromorphite in the presence of Th (1g/L) results in complete removal of Th from the solution (control experiment).

The proposed mechanism is coprecipitation of Th with Pb-phosphates, principally pyromorphite (potentially accompanied with Th sorption on zeolite). The coprecipitation of Th with pyromorphite occurs according to the following reaction:

Pb-zeolite   +   Ca₅(PO₄)₃OH   +   Th⁴⁺   +   Cl^-   =>    (Pb,Th)₅(PO₄)₃Cl ↓    +   Ca-zeolite

It is likely that minor amounts of other Pb phosphates precipitate together with pyromorphite and may contribute to the removal of Th from solution. This will be addressed in future studies. The method described might lead to a new technology that allows for selective, permanent and effective way to remove radioactive elements. A mixture of Pb-modified clinoptilolite and hydroxylapatite may serve as a reactive barrier and as a supplementary backfill material in storage containers and underground repositories for radioactive waste. This research was partly funded by NCN research grant no. 2024/55/B/ST10/01958.

[1] Stępień, E., Manecki, M., Bajda, T. 2025. Mimetite precipitation on Pb-clinoptilolite: an effective approach for arsenate removal from water. Mineralogia, vol. 56, 44-51. DOI: 10.2478/mipo-2025-0006