Computational Investigation of Adsorptive Removal of Pb2+ from Water by the UiO-66 Metal–Organic Framework: Comparison of Adsorption Sites on Defects and Functionalised Linkers

Abstract

Adsorption using metal–organic frameworks (MOFs) such as UiO-66 has shown great promise in remediating water sources contaminated with toxic heavy metals such as Pb2+, but detailed information about the adsorption process remains limited. In this article, we gained mechanistic insights into Pb2+ adsorption using both functionalised and defective UiO-66 by performing density functional theory calculations using cluster models. Our benchmarked approach led to a computational model of solvated Pb2+ (a hemidirected Pb(H2O)62+ complex) fully consistent with experimental reports. The analysis of Pb2+ adsorption using functionalised UiO-66 determined that factors such as electrostatic attraction, chelation, and limited constraints on the Pb2+ coordination geometry lead to enhanced binding affinity. For these reasons, UiO-66-COO– was identified as the most promising functionalised MOF, consistent with experimental literature. We additionally explored a novel aspect of Pb2+ adsorption by UiO-66: the role of missing linker defects that often characterise this MOF. We found that the defects expected to form in an aqueous environment can act as excellent adsorption sites for Pb2+ and the preferred adsorption geometry is again determined by electrostatic attraction, chelation, and constraints on the Pb2+ coordination geometry. Overall, we conclude that functional groups and defect sites can both contribute to Pb2+ adsorption and our study provides crucial design principles for improving the UiO-66 MOF performance in toxic Pb2+ removal from water.