Recent advances in effective capture of inorganic mercury from aqueous solutions through sulfurized 2D-material-based adsorbents

Abstract

The inorganic form of mercury is considerably dangerous because of its highly volatile elemental form, exposure to which causes many serious health challenges including nervous system and lung damage as well as potential birth defects in expectant mothers. Compared to the reported technologies for removing toxic heavy metal ions from water, adsorption is widely employed because of its simplicity and practical applicability. Recently, many two-dimensional (2D)-based materials were reported to capture different toxic heavy metal ions from water. This review critically summarizes the ultrahigh adsorption densities of recently developed sulfurized 2D materials (S-2D), such as metal–organic frameworks (MOFs), MoS₂/MoS₂-based composites, graphene materials, and layered double hydroxide (LDH)-based materials, for the efficient removal of mercury/mercury ions from water. The chemistry of the interaction between the S-2D materials and mercury and the insights into their mechanisms were comprehensively discussed. Possible mechanisms based on the coordination covalent bond, Lewis acid–base, and electrostatic and cation–pi interactions as well as the ligand-exchange mechanism (depending on the nature of the active sites in 2D materials) based on the hard and soft acids and bases (HSAB) principle were well-examined by characterization techniques, such as X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), Fourier-transform infrared (FTIR) spectroscopy, powder X-ray diffraction (PXRD) analysis, the extended X-ray absorption fine structure (EXAFS) technique, and scanning electron microscopy (SEM). The reported mercury adsorption densities and the selectivity test results were much higher than those of other sulfurized nanomaterials and inorganic nanocomposites. The advantages and necessity of functionalizing the adsorption technique for the effective capture of mercury ions from water were thoroughly described. This review also critically discussed the regeneration methods, real-water analysis, and cost-effectiveness of the reported materials. In particular, the clean-up and disposal methods for mercury-adsorbed materials and other areas requiring further research were addressed.