Metalloprotein-Inspired UiO-66 Traps for Heavy Metal Removal: An Exploration of Interfacial Dynamics

Metalloproteins coordinate with various metals through their amino acids, performing functions such as catalysis, electron transfer, and sensing. Herein, we functionalized the porous UiO-66 MOF with amino acids, inspired by the metalloproteins' metal-binding site for the capture of heavy metals by Pearson's hard-soft acid-base principle. The defect sites of the MOF were postsynthetically modified with amino acids through solvent-assisted linker exchange, and characterization techniques such as NMR, CHNS, XPS, FTIR, and potentiometric titration were performed to support the confinement of amino acids within the porous network of UiO-66. The functionalized MOFs were successfully applied in capturing heavy metals from water, and the effect of different process parameters in adsorbing heavy metals was studied. Different mathematical models for analyzing the adsorption isotherm, kinetics, and thermodynamics were fit to the experimental data to elucidate the adsorption mechanism. The Langmuir maximum adsorption capacities for Cr6+, As5+, and Hg2+ were 43.47, 99.96, and 195.62 mg/g, respectively. In this work, the interfacial dynamics of adsorption comprising the double- and triple-layer formation and the dissolution of the hydration shell of heavy metals are presented to know the interaction of metals with the MOF system. Specifically, the proton exchange with thiols, followed by the triple layer formation during Hg2+ adsorption, is elaborated. Finally, the competitive binding of metals, the specificity of MOFs toward different heavy metals, and their potential in environmental applications were discussed.