Removal and Recovery of Heavy Metal Ions by Two-dimensional MoS2 Nanosheets: Performance and Mechanisms

We investigated the removal of heavy metals from water by two-dimensional MoS₂ nanosheets suspended in aqueous solution, and restacked as thin film membranes, respectively. From these studies we elucidated a new heavy metal ion removal mechanism that involves a reduction-oxidation (redox) reaction between heavy metal ions and MoS₂ nanosheets. Ag⁺ was used as a model species and MoS₂ nanosheets were prepared via chemical exfoliation of bulk powder. We found that the Ag⁺ removal capacity of suspended MoS₂ nanosheets was as high as ∼4000 mg/g and adsorption accounted for less than 20% of removal, suggesting the reduction of Ag⁺ to metallic silver as a dominant removal mechanism. Furthermore, we demonstrated that MoS₂ membranes were able to retain a similar high removal capacity, and attribute this capability to the formation of a conductive, permeable multilayer MoS₂ structure, which enables a corrosion-type reaction involving electron transfer from a MoS₂ site inside the membrane (anode) to another site on membrane surface (cathode) where heavy metal ions are reduced to metallic particles. The membrane surface remains active to efficiently recover metallic particles, because the primary oxidation products are soluble, nontoxic molybdate and sulfur species, which do not form an insulating oxide layer to passivate the membrane surface. Therefore, MoS₂ membranes can be used effectively to remove and recover precious heavy metals from wastewater.