Study on Mos2 Nanosheets for Desalination and Heavy Metals Removal from Water

Abstract

MoS2-based nanosheets hold great potential in some flexible and low-cost applications for the removal of heavy metal ions and desalination from water due to their excellent electrochemical activity, mechanical and thermal stability, high surface area and abundant negatively charged surfaces, etc. This work studies the properties and mechanisms of MoS2 nanosheets as solar absorbers for solar desalination, as electrode materials for capacitive deionization (CDI) and as adsorbents for heavy metals removal. Meanwhile, some progress has been made to overcome the specified difficulties facing MoS2 nanosheets in these applications. The details of the experimental and results are displayed below. Three elaborate and promising designs of MoS2 nanosheets as the solar absorbers for solar desalination were provided in this work: (I) Magnetic MoS2 nanosheets as recyclable photo-absorbers for high-performance solar steam generation. The magnetic MoS2 nanosheets showed a superhigh solar absorption of over 96% within the whole wavelength range of 200-2500 nm. As a result, a high evaporation efficiency of up to 79.2% under the low illumination of 2.5 kW m-2 was achieved. In addition, the magnetic MoS2 nanosheets not only showed the long-term well dispersion in aqueous solution due to the introduction of hydrophilic PDA but also exhibited a fast and effective separation from aqueous solution with the help of the decorating nano Fe3O4, which much benefited to the continuously efficient solar steam generation and its good recyclability, respectively. (II) Nanoscale MoS2 embed in 3D double-layer structure for high-efficiency solar desalination. With low cost, facile construction pathway and good mechanical flexibility, the MPU-PPU evaporator simultaneously exhibited high evaporation efficiency of 85% under a low illumination of 1.0 kW m−2 and superhigh evaporation efficiencies over 90% under higher solar illumination (1.5–2.5 kW m−2), which outperformed many other reported advanced materials for solar desalination. (III) Facile preparation of high-quality 3D MoS2 aerogel for highly efficient solar desalination. This 3D MoS2 aerogel has an excellent light-absorbing efficiency of over 95% within the whole solar spectrum range, enabling the high evaporation efficiency of 88.0% under low solar irradiation of 1.0 kW m-2 and superhigh evaporation efficiencies of over 90% under the slightly enhanced solar irradiation of 1.5-3.0 kW m-2 as well as the remarkable desalination performance. In addition, the excellent mechanical stability of this MoS2 aerogel renders it to be reused for at least 10 cycles with stable water productivity. Despite MoS2 nanosheets has already been used in CDI, their poor water wettability remained as one of the primary weaknesses for MoS2 nanosheets being used in CDI. In this case, the hydrophilic MoS2/polydopamine (PDA) nanocomposites have been used as the electrode to enhance the capacitive deionization performance. Consequently, MoS2 modified by PDA for 4 h (MoS2/PDA-4) could exhibit not only lower inner resistances due to its better wettability but also much higher specific capacitance because of the incorporation of electroactive PDA compared to original MoS2, resulting in the enhanced electrosorption rate (less than 9 min) and desalination capacity (14.80 mg g-1). MoS2 nanosheets were used as adsorbents to remove Cd (II) from water. The strong complexation between Cd2+ and the intrinsic S planes was proposed for the high adsorption of Cd2+ on MoS2 nanosheets. Aiming at overcoming the difficulty of solid-liquid separation which severely restricted the practical applications of nanoscale MoS2, the mussel-inspired Fe3O4@polydopamine (PDA)-MoS2 core−shell nanospheres were prepared and used as the adsorbent for the removal of Pb2+ from water. As a result, the as-prepared nanocomposites not only exhibited a high adsorption capacity of Pb2+ but also showed an easy and fast separation from aqueous solutions within 120 s by an external magnet as well as a facile regeneration by general acid treatment.