Selective Extraction of Thorium Using Nanoporous Sulfate-Functionalized Fluoro-Bridged Holmium Metal–Organic Frameworks

Abstract

The increasing global demand for sustainable and efficient energy sources has driven the need for alternative nuclear fuels. Thorium has emerged as a promising source due to its abundance, energy efficiency, and lower radiotoxic waste generation compared to uranium. However, the selective extraction of thorium from rare-earth elements and uranium remains a challenge. In this study, a pyrosulfate-functionalized fluoro-bridged holmium metal–organic framework (PS-MOF) was synthesized and characterized for the selective exchange of thorium. The hydrolysis of the pyrosulfate results in metal-bound sulfate ions. The coordination and hydrolysis of the pyrosulfate groups to sulfate were followed by Raman spectroscopy. PS-MOF exhibited selectivity for Th(IV), with a distribution coefficient (K_d) of 1.2 × 10⁵ mL g^–1 and a cation exchange capacity of 145.6 mg g^–1. Kinetic studies revealed that Th(IV) adsorption reached equilibrium within 20 min following a pseudo-second-order model which indicates a chemisorption-controlled process. The PS-MOF retained its crystallinity and adsorption performance over multiple cycles. The Th(IV) can be completely recovered using nitric acid.