Matrix effects on the performance and mechanism of Hg removal from groundwater by MoS₂ nanosheets

Mercury (Hg) contamination in groundwater has been recognized as a serious threat to human health and ecological systems all over the world. This study demonstrated that two-dimensional (2D) molybdenum disulfide (MoS₂) nanosheets can efficiently remove Hg in groundwater, with high Hg uptake capacity, ultrafast removal kinetics, and excellent selectivity. Interestingly, we found that the groundwater matrix has profound implications on the Hg removal efficiency and mechanisms by MoS₂ nanosheets. Specifically, surface adsorption is the dominant removal mechanism for Hg in DI water owing to the high affinity between Hg(II) and MoS₂ via strong Lewis acid/base soft–soft interactions. In groundwater, however, the presence of Cl^- renders HgClOH the dominant species, which can undergo adsorption onto MoS₂ and homolytic cleavage to form thecHgCl radical. As an intermediate radical, cHgCl could either dimerize to form Hg₂Cl₂ or further reduce to Hg⁰. This reduction-based mechanism enhanced the overall removal capacity of Hg to 6288 mg g^-1, which is among the highest values reported to date. Additionally, our desorption tests revealed the high stability of immobilized Hg on MoS₂ nanosheets over conventional adsorbents in various extractant fluids. These impressive features render MoS₂ nanosheets a promising candidate for remediation of Hg-contaminated groundwater.