Selective Th(IV) separation and immobilization by one-dimensional lepidocrocite titanate

Abstract

Titanate nanomaterials have been widely explored for the effective removal of heavy metals and radionuclide ions from environmental wastewater, leveraging their high specific surface areas (SSAs) and ion exchange capacities. In this study, one-dimensional lepidocrocite titanate hierarchical structures (THSs) incorporating exchangeable Li⁺ or tetramethylammonium (TMA⁺), cations were prepared using an organic alkali conversion strategy suitable for large-scale production. These unique adsorbents were tested for the efficient capture and separation of Th(IV). Batch adsorption experiments revealed that the maximum adsorption capacity of THSs for Th(IV) reached 292 mg/g at pH 2.5. THSs demonstrated superior ion selectivity for Th(IV) over uranyl ions and rare earth ions, achieving a Th(IV)/U(VI) separation factor of up to 722. In the context of treating simulated rare earth ore leachates, the corresponding Th(IV) adsorption rate and K_d values were recorded at 98.4 % and 1.52 × 10⁵ mL/g, respectively. Spectroscopic analyses, including XPS and EXAFS, complemented by DFT calculations, substantiated that Th(IV) interacts stably with the oxygen-containing terminations on the surface of THSs, primarily through the formation of inner-sphere complexations. Sintering of Th-loaded THSs led to the formation of stable ThTi₂O₆ phases, facilitating the complete immobilization of Th(IV) within the lattice, with a leaching rate of less than 0.05 % under highly acidic conditions. Overall, THSs demonstrate significant potential for the efficient purification of Th-containing wastewater from rare earth mines, as well as for the secure geological disposal of the resultant waste.