Precisely modulating the branching functional groups of MIL-53(Al) for highly efficient sequestration of uranium

To fully utilize the customizable characteristics of metal-organic framework (MOF) materials in environmental applications, it is important to precisely modulate the branching groups of MOFs for improved performance. In this work, MIL-53(Al) was modulated by different numbers of nitrogen atoms and carboxyl groups/phosphite groups (MOF-N2-(COOH)3, MOF-N3-(COOH)4, and MOF-N2-(PO(OH)2)3) via a post-synthesis modification method, and the influence of functional groups and chain length on adsorption and the mechanism of modified groups on the adsorption of uranium (U) are demonstrated. Batch experiments suggest the removal of U(VI) reaches 37%, 27%, and 90% for MOF-N2-(COOH)3, MOF-N3-(COOH)4 and MOF-N2-(PO(OH)2)3, respectively (pH = 4.0, T = 25 celcius, I = 0.01 M NaNO3). The surface complexation between UO22+ and the surface of functional groups on the modulated MOF dominates the adsorption. Nevertheless, the steric hindrance in MOF porous structure can explain the inferiority of U(VI) adsorption on MOF-N3-(COOH)4 (27%) to MOF-N2-(COOH)3 (37%). MOF-N2-(PO(OH)2)3 is more conducive to uranyl adsorption than MOF-N2-(COOH)3 or MOF-N3-(COOH)4, which is due to better affinity of U(VI) to phosphate group than the carboxyl group by density functional theory (DFT) calculation. Moreover, the excellent regeneration performance and U(VI) adsorption of MOF-N2-(PO(OH)2)3 in natural water or wastewater further confirmed the practical application of modulated MOFs in practical application.