Carboxyl-functionalized UiO-66-NH₂ for efficient capture of Th(IV) in aqueous solution: Experimental and DFT study

Thorium is an excellent alternative to uranium as a potential nuclear fuel due to the gradual depletion of uranium resources. Therefore, the separation and enrichment of thorium from tailing wastewater is of great significance. In this study, a novel adsorbent, UiO-66-NH-PMA, was prepared by grafting pyromellitic acid (PMA) onto UiO66-NH2 composite via a simple amination reaction for the separation and enrichment of thorium from aqueous solution. The presence of a significant number of -COOH functional groups on PMA provides an abundance of adsorption sites for thorium binding. Under acidic conditions (pH = 3.0), UiO-66-NH-PMA exhibited a theoretical maximum adsorption capacity of 168.38 mg/g. Moreover, the adsorbent reached adsorption equilibrium within 150 min. After five cycles of adsorption-elution experiments, UiO-66-NH-PMA retained 84.6 % of its firsttime thorium adsorption, demonstrating excellent regenerative performance. Additionally, the mechanism of UiO-66-NH-PMA complexed with Th(IV) was analyzed in detail by combining XPS, FT-IR, XRD and DFT calculation. This work demonstrates the high efficiency of carboxyl-functionalized UiO-66-NH2 for disposal of thorium-containing nuclear wastewater, and offers new insights into the mechanism of thorium adsorption on the adsorbents containing carboxyl groups.