Phosphorus-Modified Metal-Organic Frameworks for the Effective Removal of Thorium from Aqueous Solutions

Thorium (Th), as a promising nuclear fuel, can cause serious radioactive contamination even in a small amount in nuclear wastewater. Therefore, it is urgent to develop effective adsorbents for the removal of thorium. Herein, phytic acid modification of carboxylated zirconium based on metal-organic framework (UIO-66-COOH-PA) was obtained by rapid high-temperature carbonization, which exhibited excellent performance in removing Th (IV) in acidic solutions. The maximum adsorption capacity of UIO-COOH-PA-2 (with phytic acid mass ratio of 40%) reached 315.5 mg/g at pH of 4, which was more than twice that of the unmodified Zr-based MOF material (151.2 mg/g). The thorium adsorption process reached dynamic equilibrium quickly at different temperatures. Freundlich model fitted the thorium adsorption kinetics of UIO-COOH-PA better and its adsorption dynamics followed the pseudo-second-order kinetic model. The excellent adsorption performance of UIO-COOH-PA-2 at low pH values indicated that the adsorption materials containing phosphorus were suitable for the removal of thorium especially in acidic systems. It also exhibited excellent selectivity (up to 90.2%) for Th (IV) adsorption in the presence of many coexisting rare-earth ions. These results will provide a new strategy for the design of adsorbents for efficient removal of thorium from radioactive acidic wastewater environments.