Amorphous molybdenum selenide intercalated magnetite as a recyclable trap for the effective sequestration of elemental mercury

Abstract

The key to obtaining a cost-effective and environmentally friendly technique for elemental mercury (Hg⁰) immobilization from industrial flue gases lies in the development of a highly effective and immediately recyclable sorbent that is capable of accommodating volatile Hg⁰ into a stable mercury species. Amorphous molybdenum selenide intercalating magnetite [MoSe_x(inter)Fe₃O₄] was thus purposefully designed in this work to give an ideal response to these challenging requirements. With the assistance of hexadecyl trimethyl ammonium bromide, the active MoSe_x nanosheets were well intercalated among the Fe₃O₄ nanoparticles, leading to a structured composite that is beneficial for exerting the Hg⁰ removal ability of MoSe_x while also maintaining the magnetization of Fe₃O₄, allowing it to be easily recycled. Approximately 100% Hg⁰ removal efficiency was achieved under extremely harsh experimental conditions with gas hourly space velocity (GHSV) and Hg⁰ concentration values as high as 90 000 h⁻¹ and 500 μg m⁻³, respectively. The Hg⁰ adsorption capacity and uptake rate reached approximately 135 mg g⁻¹ and 49 μg g⁻¹ min⁻¹, record-high values compared to previously reported magnetic and recyclable Hg⁰ sorbents. The recycling tests demonstrated that MoSe_x(inter)Fe₃O₄ mixed with large amounts of impurities could be effectively retrieved and reused to adequately exploit the capacity of MoSe_x(inter)Fe₃O₄ for Hg⁰ removal. The dominant product, mercury selenide (HgSe), a mercury species known for negligible environmental leachability, was successfully obtained by the reaction between Hg⁰ and diselenide (Se₂²⁻) in MoSe_x. This was ascribed to the fact that Se₂²⁻ played dual roles in the Hg⁰ immobilization process to simultaneously oxidize Hg⁰ to Hg²⁺ and to immobilize Hg²⁺ to HgSe, a property that distinguished Se₂²⁻ from monoselenide (Se²⁻) with no redox capabilities.