Functionalized metal organic frameworks for thorium (IV) adsorption: Mechanisms, speciation, and practical challenges

Functionalized metal-organic frameworks (MOFs) have emerged as a promising material for Th(IV) removal from nuclear wastewater and rare earth processing effluents because of their tunable pore structures and binding environments. In these systems, the adsorption behaviour of Th(IV) is strongly influenced by hydrolysis, colloidal speciation, and competing ions, which complicate mechanistic interpretation and often limit practical separation performance. This review critically evaluates recent advances in the functionalized MOFs for Th(IV) capture by correlating aqueous thorium speciation with reported adsorption mechanisms, framework functionalization strategies, and operational performance. Pre-synthetic functionalization, post-synthetic functionalization, and defect engineering are analysed with respect to their influence on adsorption capacity, selectivity, regeneration, and framework stability. The reliability of proposed adsorption mechanisms is critically evaluated using spectroscopic and computational approaches, including EXAFS, XPS, and DFT analyses. The review highlights that adsorption capacities alone cannot explain Th(IV) uptake without simultaneous consideration of aqueous speciation, coordination environment, and competing adsorption pathways. Current limitations in the field include the absence of standardized testing protocols, limited evaluation under realistic wastewater conditions, insufficient continuous-flow and column studies, and the poor understanding of radiation-induced framework degradation and self-irradiation effects. In addition, techno-economic challenges associated with solvent-intensive synthesis, material shaping, regeneration chemistry, and scale-up feasibility are critically discussed. Future research should prioritize the integration of thorium speciation analysis, rigorous mechanistic validation, radiation stability assessment, and scalable process design to improve the practical applicability of MOF-based separation systems.