Functionalization of Defective Zr Metal-Organic Frameworks for Water Decontamination: Mechanistic Insight into the Competitive Roles of -NH2 and -SH Sites in Removal of As(III) Species.

Abstract

Direct removal of trivalent arsenic, As(III), arsenite, or H3AsO3, is a great challenge in accessing clean sources of water. Different methodologies and materials were applied in this regard, but among them, direct removal of As(III) species using a metal-organic framework (MOF)-based adsorbent shows a great deal of potential. Although some studies were conducted on As(III) removal using MOFs, studies of functional groups are still quite rare. For this purpose, three novel functionalized defective Zr-MOFs, using UiO-66 [Zr6(OH)4O4(BDC)6, where BDC2- = benzene-1,4-dicarboxylate], were fabricated to investigate the competitive or cooperative roles of the free -NH2 and/or -SH site in the removal of As(III). UiO-66 was functionalized with monocarboxylate linkers, including glycine (Gly, NH2-CH2-COOH), cysteine [Cys, SH(CH2)-NH2(CH)-COOH], and mercaptopropionic acid [Mer, SH-(CH2)2-COOH]. Gly@UiO-66, Cys@UiO-66, and Mer@UiO-66 were applied for the direct removal of As(III) species. Although Cys@UiO-66 is functionalized with both amine and thiol functional groups, Gly@UiO-66 has a higher adsorption capacity (301.4 mg g-1) with respect to As(III), which is among the best reported values. This is due to the fact that (1) the affinity of amine sites in Gly@UiO-66 for As(III) is higher than that of thiol sites in Mer@UiO-66 and (2) Cys@UiO-66 has a very small surface area compared to that of Gly@UiO-66. Mechanistic studies using X-ray photoelectron spectroscopy and vibrational spectroscopy reveal that not only the functionalization and chemical nature of the function but also other parameters such as the protonation-deprotonation mechanisms and chemical state of the function are other critical factors for designing a functional MOF-based adsorbent with high affinity for and maximum capacity with respect to the target analyte.