Thiol Functionalized Metal-Organic Framework for Efficient Silver Adsorption and Removal from Aqueous Solution.

Abstract

MOF-808, a zirconium-based metal–organic framework (Zr-MOF, allows desired functionalization using six monocarboxylate ligands. In this study, a thiol (-SH) group was incorporated into MOF-808 using thiolactic acid (ThA) as a modulator. MOF-808 with 25 molar equivalents of ThA (M-ThA-25) achieved a -SH loading of 5.62 per cluster, twice that of thioglycolic acid as a modulator. Due to strong Ag–S interactions predicted by the hard–soft acid–base (HSAB) theory, M-ThA-25 demonstrated an excellent Ag⁺ adsorption capacity, with a maximum uptake of 365.47 mg•g-1, which is well described by the Langmuir isotherm. The kinetics were impressive, achieving a 100% removal of 500 ppm of Ag+ in under 10 minutes. Post-adsorption powder X-ray diffraction and scanning electron microscopy images showed that chloride impurities can lead to the precipitation of AgCl as observed in MOF-808 with 50 equivalents of acetic acid (M-AA-50). In M-ThA-25, the adsorbed Ag is mainly in the form of Ag+. X-ray photoelectron spectroscopy analysis of M-ThA-25-Ag confirmed that the adsorption occurred via an ion-exchange mechanism, where incoming Ag ions replaced H⁺ in the -SH group. To evaluate selectivity, M-ThA-25 was exposed to a mixed-ion solution containing Al3+, Pb2+, and Ag+ at concentrations representative of those found in solar panel leachates. It showed that Ag⁺ uptake wasn't affected by the presence of competing Al3+. This highlights M-ThA-25's potential as an efficient silver adsorbent. However, regenerating silver is challenging due to the strong Ag–S bonds, so future work should focus on developing better desorption methods for practical recovery.