Fine tuning of pore size in metal–organic frameworks for superior removal of U(vi) from aqueous solution

Abstract

The pore structure of metal–organic frameworks (MOFs) is crucial to their adsorption performance, and it is still a challenge to precisely control the pore size to realize superior removal of uranium. The pore size of MOF-A, MOF-B and MOF-C was fine-tuned by using three organic chains with different lengths (fumaric acid, terephthalic acid and naphthalene dicarboxylic acid, respectively). The synthesized MOF-A, MOF-B and MOF-C had ordered increase in pore sizes with 3.8 Å, 5.7 Å and 7.6 Å, respectively. Pore size of 7.6 Å in MOF-C was a little larger than the geometric dimension of the UO₂²⁺ ion (maximum length 6.04–6.84 Å), which facilitated diffusion of UO₂²⁺ ions and achieved highly efficient adsorption. The maximum adsorption capacity (584 mg g⁻¹) and removal rate (99.75%) of uranium for MOF-C were higher than those for MOF-A and MOF-B. The excellent adsorption performance of MOF-C was attributed to the specific pore structure of MOF-C and effective complexation of oxygen-containing groups with U(VI). Our work proposes a new strategy for the outstanding removal of uranium by using different organic chains to precisely regulate the pore size of MOF.