Comparative investigation of Ti- and V-based MXenes for adsorption of Th(IV) and U(VI): Mechanistic insights from surface structural evolution and complexation behavior

The removal of radionuclide contaminants from aqueous systems remains critical for mitigating long-term radiological and chemical risks and for improving the practical feasibility of nuclear wastewater treatment. This study systematically investigated the adsorption behavior of Th(IV) and U(VI) ions on Ti- and V-based MXenes (Ti₂CT_x, Ti₃C₂T_x, V₂CT_x, and V₄C₃T_x) in comparison with bentonite as a reference adsorbent. Through batch experiments, the influences of pH, dosage, contact time, initial concentration, and coexisting ions were evaluated. The adsorption mechanism was elucidated using Visual MINTEQ species differentiation modeling and physicochemical characterization analysis. Th(IV) exhibited strong uptake at pH 4.0 due to its highly charged hydrolyzed species, with V-based MXenes showing particularly high affinity driven by their more negative surface potentials and heterogeneous high-energy sites. In contrast, U(VI) adsorption at pH 6.0 was dominated by inner-sphere coordination to oxygenated Ti terminations, resulting in distinctly higher capacities on Ti-MXenes than on V-MXenes. Notably, the maximum adsorption capacities reached 769 mg/g for Th(IV) on V₄C₃T_x and 66.9 mg/g for U(VI) on Ti₂CT_x under the optimal pH conditions. Despite oxidation-induced structural differences, all MXenes retained their layered framework after adsorption, confirming surface complexation as the primary uptake mechanism. This work demonstrates that MXene metal centers and surface terminations critically govern radionuclide selectivity and affinity. These findings provide mechanistic insights and design principles for developing MXene-based sorbents for nuclear wastewater treatment.