Selective Capture Mechanism of Radioactive Thorium from Highly Acidic Solution by a Layered Metal Sulfide

Thorium as a potential nuclear fuel for the next-generation thorium-based molten salt reactors holds significant environmental and economic promise over the current uranium-based nuclear reactors. However, because thorium (Th⁴⁺) usually coexists with other rare earth elements, alkali or alkaline earth metals in minerals, or highly acidic radioactive waste, seeking acid-resistant sorbents with excellent selectivity, high capacity, and fast removal rate for Th⁴⁺ is still a challenging task. In this work, we investigated a robust layered metal sulfide (KInSn₂S₆, KMS-5) for Th⁴⁺ removal from strong acidic solutions. We report that KMS-5 could capture Th⁴⁺ from a 0.1 M HNO₃ solution with extremely high efficiency (∼99.9%), fast sorption kinetics (equilibrium time < 10 min), and large distribution coefficient (up to 1.5 × 10⁶ mL/g). Furthermore, KMS-5 exhibited excellent sorption selectivity towards Th⁴⁺ in the presence of large amounts of competitive metal ions like Eu³⁺, Na⁺, and Ca²⁺. This extraordinary capture property for Th⁴⁺ is attributed to the facile ion exchange of Th⁴⁺ with K⁺ in the interlayers and subsequent formation of a stable coordination complex via Th-S bonds. These results indicate that KMS-5 is a promising functional sorbent for the effective capture of Th⁴⁺ from highly acidic solutions.