Incorporation of hierarchical porous MIL-101@MXene into double-network hydrogel for an enhanced strategy for selective and effective removal of hazardous Hg2+ from water

Hg 2+ contamination in aquatic environments represents a significant threat to human health and ecological systems due to its pronounced toxicity and enduring nature. It is imperative to develop a high-performance, recyclable adsorbent for Hg 2+ removal to enable effective treatment of wastewater. Herein, MIL-101@MXene was synthesized through in-situ hydrothermal growth of MIL-101 on MXene substrates, and was subsequently incorporated into a double-network hydrogel matrix comprising polyvinyl alcohol (PVA) and chitosan (CS), resulting in the formation of the MIL-101@MXene/hydrogel adsorbent. A comprehensive characterization of MIL-101@MXene/hydrogel was conducted utilizing a series of analytical techniques, including X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) analysis, and X-ray photoelectron spectroscopy (XPS). The porous hydrogel network of MIL-101@MXene/hydrogel, along with the micro-mesoporous structure of MIL-101@MXene, creates a hierarchical pore architecture. In this configuration, the macropores of the hydrogel facilitate efficient diffusion of Hg 2+ into the material's interior while promoting physical adsorption. Meanwhile, the micro-mesopores not only provide a substantial specific surface area but also reveal numerous active adsorption sites, thereby enhancing the overall adsorption capacity. Batch adsorption experiments demonstrated that MIL-101@MXene/hydrogel exhibited optimal performance for the removal of Hg 2+ at a pH of 5, achieving a maximum adsorption capacity. In addition, MIL-101@MXene/hydrogel demonstrated competitive selectivity and reusability, with only about a 25 % reduction in Hg 2+ adsorption capacity in the presence of various competing ions, and retaining over 55 % of its initial capacity after six successive adsorption-desorption cycles. The adsorption mechanism was further elucidated through FT-IR and XPS analyses conducted both before and after the adsorption process. The results highlighted the coordination of Hg 2+ with unsaturated Fe 3+ sites and carboxyl oxygen atoms, as well as surface complexation between Hg 2+ and the functional groups on MXene. Physisorption also contributed to the overall adsorption process. Additionally, there were interactions characterized by hydrogen bonding and coordination between the hydrogel matrix and Hg 2+ . This work indicates that MIL-101@MXene/hydrogel is an effective adsorbent with practical potential for Hg 2+ -contaminated wastewater remediation.