Thiol-Functionalized Zr-Based Metal–Organic Framework for Capture of Hg(II) through a Proton Exchange Reaction

Rational design and facile synthesis of thiol-modified metal–organic frameworks (MOFs) for the efficient capture of highly toxic mercuric ions from water has attracted great attention. However, the corresponding adsorption mechanism is not well understood. In this paper, a thiol-modified Zr-based MOF (Zr-DMBD) with free-standing and accessible thiol groups was prepared. It exhibited remarkable performance in the capture of Hg(II), and its maximum adsorption capacity was 171.5 mg·g–1, approximately 9 times that of the pristine UiO-66. Impressively, the maximum value of the selective coefficient was as high as 28899.6. Additionally, 99.64% of Hg(II) could be eliminated by Zr-DMBD from the actual wastewater, rendering the concentration of Hg (II) below 0.05 ppm (Emission Standard of Mercury (GB30770-2014)). The excellent adsorption capacity and outstanding selectivity were ascribed to the remarkable coordination between S2– and Hg(II), as supported by the results of FT-IR and XPS. Unexpectedly, a good correlation (R2 = 0.982) between the increased H+ concentration after adsorption and its corresponding adsorption capacity was obtained. This result suggested that the thiol groups' sulfur atoms coordinated with Hg(II) while the hydrogen atoms in thiol groups were replaced and released as hydrogen ions in the solution, thus extending a proton exchange reaction mechanism for Hg(II) adsorption.