Laminar MoS2 membrane for high-efficient rejection of methyl orange from aqueous solution
Introduction
Water pollution has been a global problem with the rapid outbreak of industrialization, and it is of critical importance to remove the contaminants from water [1], [2]. Membrane technology has become one of the emerging technologies in the past decades by virtue of its low energy consumption, environmental benignancy, high stability, easy operation and convenient maintenance [3], [4] compared with other technologies including adsorption, solvent extraction, electrochemical process, etc. [5], [6], [7], [8]. The good performance of membrane in the water treatment greatly depends on the membrane materials. According to the chemical composition, the membrane could be made by organic materials mainly composed of polymeric materials and inorganic materials, such as mesoporous ceramic and zeolite-based materials [9], [10]. Polymeric membranes suffer from instability at high temperature, strongly acidic/alkaline solutions and organic solvents, while the mesoporous membranes achieve a relatively low rejection of contaminants.
Two-dimensional (2D) materials have drawn widespread attention as membrane materials [11], [12]. Different from traditional materials, they present another way to make of the membrane through piling up themselves and forming nanocapillary channels [13]. The membranes prevent solute and allow transport of solvent through nanopores. The nano-size of interlayer space enables 2D materials-based membranes to achieve highly specific separation, minimal transport resistance and extraordinary flux rate at the same time [14]. The nanopores could be controlled through modifying the size or surface functionality of 2D materials and then the contaminants can be removed selectively through the accurate regulation of nanopores [14]. Graphene oxide (GO) is widely investigated as membrane material [3], [9], [15], while the membranes are prone to swell after long-term exposure to water and present low rejection of the contaminants.
Molybdenum disulfide (MoS2), as one of the hottest 2D materials [16], [17], [18], exhibits potential applications in hydrogen production, light detectors, photovoltaic power and tumortherapy, etc. due to its remarkable optical and electronic properties [19], [20], [21]. Recent researches have suggested that MoS2 nanosheets might be an excellent material for membranes [22]. Comparing with GO membranes, MoS2 membranes had a wider pH range, higher rigidity, higher antibacterial activity, no toxicity which were beneficial to water treatment [14]. It was reported that MoS2 membranes can achieve 100% ion rejection for arsenic removal from contaminated aqueous solution due to its fish-bone nanostructure and nanopores [23], [24]. The molecular dynamics simulation suggested that the rejection of mercury ions was largely related to the type and size of MoS2 pores [5]. The laminar MoS2 membranes exhibit extraordinary ionic sieving for the majority of cations existed in sea water after special functionalization. In addition, MoS2 membrane also showed good rejection for macromolecular proteins such as macromolecular bovine serum albumin (BSA) [25], and organic contaminants, Evans blue molecule as an example [26]. Most of the current treatment of waste water with MoS2 membranes is based primarily on pressure filtration, however this treatment sets a high requirement for the mechanical strength of the membranes and needs a high cost because of the use of pressure. The electroosmosis is another commonly used water treatment method and has got the practical application stage [27], [28]. It induces the harmless ions to pass through the membrane and rejects the contaminants through applying a low level of electrical current rather than pressure. It is therefore, waste water treatment by using electroosmosis is more cost effective.
In this work, an attempt was made to study the rejection performance of molybdenum disulfide membrane for the removal of methyl orange (MO) from water through electroosmosis. The synthesized MoS2 was qualitatively analyzed by X-ray diffraction (XRD) and Raman spectra. The membranes before and after filtration were studied by scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) in order to characterize the physicochemical properties and separation mechanism. Furthermore, MO rejection rate was calculated through the concentration variation before and after the electroosmosis, which was measured by UV spectrophotometer. The aim was to obtain a clear understanding in the performance and mechanism of MoS2 membranes in the rejection of MO through electroosmosis, as well as to give a precise guidance for the research about MoS2 membranes, and the exploration in other filtration materials for water treatment.
Section snippets
Materials
Ammonium molybdate tetrahydrate ((NH4)6Mo7O24·4H2O), thiourea (CH4N2S), isopropanol (C3H8O), ethanol absolute (C2H6O), methyl orange (C14H14N3NaO3S), sucrose (C12H22O11) and sodium hydroxide (NaOH) used in this work were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). And hydrochloric acid (HCl) was from Xinyang chemical reagent (China). All the reagents were of analytical grade. Porous nickel and titanium wire that used for making electrode were bought from Yongsheng company
Characterization of MoS2
The synthesized MoS2 was detected by XRD (Fig. 2a). The result showed that there were diffraction peaks appeared at 13.24°, 32.23°, 35.10°, and 57.55°, which corresponded to (0 0 2), (1 0 0), (1 0 3) and (1 1 0) planes of MoS2, respectively. No impurities were found, indicating a high purity of MoS2. The low intensity of characteristic peaks demonstrated a low crystallinity of synthesized MoS2 compared with natural MoS2 [36]. Then the measurement of Raman was carried out. As showed in Fig. 2b,
Conclusion
In conclusion, homogeneous membranes were produced from MoS2 nanosheets via vacuum filtration. MoS2 membranes exhibited a high and stable rejection performance of MO under the acceleration of electroosomosis. And the rejection performance increased with the decrease of initial concentration of MO and applied potential, and increase of membrane thickness, respectively. The excellent rejection performance should be ascribed to the Donnan theory and sieve theory, and the electrostatic repulsion
Acknowledgements
The financial support for this work from the Excellent Dissertation Cultivation Funds of Wuhan University of Technology under the project no. 2018-YS-048 is gratefully acknowledged.
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