Enhanced mercury removal from wastewater using thiol-functionalized UiO-66-NH2 MOF: experimental and theoretical insights†

Abstract

Disposal of mercury ions and Hg(II) in industrial wastewater poses serious risks to humans and aquatic organisms owing to their bio-accumulative nature. Despite several reports on the adsorptive removal of Hg(II) ions from water, designing effective, selective, and highly stable adsorbents remains a great challenge. In this study, we reported the systematic design of thiol-functionalized UiO-66-NH₂ MOF, namely, UiO-66-NH₂-(MAA)₂, using a modulator synthetic strategy. The synthesized MOF exhibited a high Hg(II) uptake of 890 mg g⁻¹, remarkable selectivity with a distribution coefficient K_d of 6.0 × 10⁶ mL g⁻¹, one of the highest reported for Hg(II) ions, and fast adsorption kinetics, removing 99.9% of Hg(II) ions (C_o = 10 mg L⁻¹) in just 10 min. The synthesized MOF also conformed to the Freundlich adsorption isotherm and followed pseudo-second-order adsorption kinetics, with a rate constant k₂, of 9.611 g mg⁻¹ min⁻¹ at an extremely low dosage of 2 mg. Meanwhile, excellent recyclability with negligible loss in efficiency was achieved during 5 consecutive cycles of Hg(II) removal on the UiO-66-NH₂-(MAA)₂ MOF. Using the BET adsorption isotherms, it was confirmed that the structural integrity of the MOF was retained after several cycles of adsorption and desorption. First-principles DFT simulations revealed that thiol functionalization resulted in the significant enhancement of the charge transfer characteristics of the UiO-66-NH₂ MOF, forming stable complexes with Hg(II) ions. This prevented leaching and the constitution of secondary pollution during the adsorption process. This study demonstrated the rational design of highly selective adsorbents for Hg(II) removal, which could be employed in the practical remediation of Hg(II) from real wastewater.