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

Abstract

We investigated the removal of heavy metals from water by two-dimensional MoS 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 MoS nanosheets. Ag was used as a model species and MoS nanosheets were prepared via chemical exfoliation of bulk powder. We found that the Ag removal capacity of suspended MoS 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 MoS membranes were able to retain a similar high removal capacity, and attribute this capability to the formation of a conductive, permeable multilayer MoS structure, which enables a corrosion-type reaction involving electron transfer from a MoS 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, MoS membranes can be used effectively to remove and recover precious heavy metals from wastewater.

Colloidal Template Synthesis of Nanomaterials by Using Microporous Organic Nanoparticles: The Case of C@MoS2 Nanoadsorbents

“…We suggest that the excellent adsorption performance of C@MoS 2 ‐900 compared with C‐MON‐2, MoS 2 ‐NS, and S‐C@MoS 2 was owing to their size (80 nm), negative zeta potential (−39.5 mV), high surface area (508 m 2 g −1 ), and core–shell structure (Figure b, c and Table ). Notably, the diameters and surface areas of other recently reported MoS 2 ‐based adsorbents were within the range 100 nm–2 μm and 15.09–76.85 m 2 g −1 , respectively (Table ) –…”

“…X‐ray photoelectron spectroscopy (XPS) of the C@MoS 2 materials revealed Mo 3d peaks at 232.33 and 229.18 eV, and S 2p peaks at 163.23 and 162.03 eV (Figure b), which matched well with those of previously reported MoS 2 materials –. Raman spectroscopy revealed the presence of D and G bands for the carbon components in C@MoS 2 ‐700 at 1357 and 1587 cm −1 , respectively, with an I D / I G ratio of 0.82.…”

“…According to the N 2 ‐adsorption/‐desorption isotherms, the surface areas of C@MoS 2 ‐700, C@MoS 2 ‐800, and C@MoS 2 ‐900 were 418, 423, and 508 m 2 g −1 , respectively (see the Supporting Information, Figure S6). PXRD studies of C@MoS 2 ‐900 showed diffraction peaks at 14.3, 33.1, 38.3, and 58.3°, which corresponded to the (002), (100), (103), and (110) diffraction peaks of MoS 2 (JCPDS#37‐1492), respectively (Figure c) –. As the temperature decreased from 900 °C to 800 and 700 °C, the (002) peaks broadened and gradually shifted from 14.3° to 14.1° (C@MoS 2 ‐800) and 13.8° (C@MoS 2 ‐700), thus indicating the elongation of the interlayer distance in MoS 2 , and a decrease in crystallinity.…”

“…Whilst C‐MON‐2 exhibited a positive zeta potential of 14.8 mV, C‐NP, S‐C@MoS 2 , C@MoS 2 ‐700, C@MoS 2 ‐800, C@MoS 2 ‐900, and MoS 2 ‐NS all exhibited negative zeta potentials of −13.3, −29.8, −38.1, −38.4, −39.5, and −40.7 mV, respectively (Figure c, Table , and the Supporting Information, Figure S8). The zeta potentials of previously reported MoS 2 ‐based adsorbents were all within the range −20.6–−36.0 mV (Table ) –…”

“…C@MoS 2 ‐900 showed relatively poor adsorption performance for an anionic dye, methyl orange, thus indicating that coulombic interactions between the negatively charged surface of MoS 2 and a cationic dye was critical for efficient adsorption (see the Supporting Information, Figure S9). It has previously been reported that the surface of MoS 2 consists of negatively charged sulfide groups and countercations, such as H + , and that the adsorption/desorption of cationic adsorbates onto/from the surface of MoS 2 results from ion exchange between the adsorbates and the adsorbents –. According to our XPS studies, the Mo 3d and S 2p peaks of C@MoS 2 ‐900 were red‐shifted by 0.25 and 0.28 eV, respectively, following the adsorption of MB, thus indicating the existence of interactions between Mo‐2S dipoles and the adsorbates (Figure a) –…”