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 MoS2via 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 the˙HgCl radical. As an intermediate radical, ˙HgCl 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.