Preparation And Characterization Of MoS₂ Composite Nanofiltration Membranes

In order to relieve the severe situation of water contamination and shortage,membrane separation technology has received increasing attention.As a typical pressure-driven membrane technique,nanofiltration(NF)can replace reverse osmosis in some applications due to its many merits such as high water permeability and low operation pressure.Common NF membranes are based on a composite design,wherein a relatively dense skin layer is formed on a porous support layer(microfiltration/ultrafiltration membrane).It is worth mentioning that the design,fabrication,and optimization of the skin layer is of vital importance.To further improve the performance of NF membranes,various nanomaterials have been used to develop novel composite membranes.MoS2 nanosheets,as a sort of two-dimensional nanomaterial,have been applied in many water filtration techniques including reverse osmosis,NF,pervaporation,etc.Compared with graphene oxide(GO),MoS2 nanosheets have many advantages such as smooth surface,high rigidity,and easy functionality.Moreover,the lamellar MoS2 membrane has smaller swelling degree than the pure GO membrane.In this work,we fabricated novel composite NF membranes containing MoS2 nanosheets,which provides some useful guidelines for the assembling of MoS2 nanosheets on the surface of ceramic hollow fiber(CHF)to form selective separation layer,and the seamless incorporation of MoS2 nanosheets into the polyamide layer by interfacial polymerization.The detailed content of this paper is as follows:(1)Preparation of MoS2 nanosheets through liquid phase exfoliation:MoS2 nanosheets were obtained by liquid phase exfoliation.In order to gain MoS2 nanosheets with high concentration and large lateral size,we optimized the initial concentration of bulk MoS2,followed by size selection through controlled centrifugation.According to experimental results,we find that the concentration of MoS2 nanosheets increases linearly with the increase of the initial MoS2 concentration to 5 mg mL-1.Further increasing the initial MoS2 concentration would cause a sharp decline of the concentration of MoS2 nanosheets,which corresponds to a decrease of the yield of MoS2 nanosheets.Therefore,5 mg mL-1 was selected as the optimal initial MoS2 concentration.After size selection by controlled centrifugation with final centrifugation rate of 500 rpm,it is found that the lateral size of MoS2 nanosheets has been obviously augmented.Finally,MoS2 nanosheets with relatively large lateral size are systematically characterized in terms of surface morphology,crystallinity,hydrophilicity,and surface charge.(2)Construction of MoS2/polyethyleneimine(PEI)composite NF membranes on the CHF:Firstly,Tio2 interlayer was prepared on the outer surface of the original CHF with large pores via dip-coating and sintering.Then,we employed the filtration-assisted method to assemble MoS2 nanosheets and PEI on the surface of the TiO2-CHF substrate and thus obtained the MoS2/PEI composite membrane.According to experimental results,it is clear that the Tio2 interlayer substantially decreases the pore size as well as the surface roughness.It is found that PEI plays an important role in both the morphology and physicochemical properties of the membrane.That is,PEI enhances the interfacial adhesion between adjoining MoS2 nanosheets,as well as the MoS2/PEI layer and the substrate.Moreover,PEI endows the composite membrane with positively charged and hydrophilic surface.The obtained MoS2/PEI composite membrane displays an outstanding NF performance with pure water permeability of 4.6 L m-2 h-1 bar-1 and MgCl2 rejection of 95.5%,which can be used for efficient removal of heavy metal ions commonly found in water bodies(e.g.,Cu2+、Zn2+、Mn2+、Ni2+).(3)Construction of thin-film nanocomposite(TFN)membranes containing tannic acid(TA)-Fe3+modified MoS2 nanosheets:MoS2 nanosheets were modified with TA-Fe3+coordination complexes and then embedded within the polyamide layer to construct TFN NF membranes.It is observed that the TFN membrane with 0.01 wt%addition of TA-MoS2 nanosheets demonstrates increased surface roughness with the appearance of scattered protuberances.On the other hand,TA-MoS2 nanosheets introduce interfacial crosslinking,which can compete with the hydrolysis of acid chloride groups as well as the reaction between amine groups and acid chloride.Thus,the TFN membrane exhibits reduced surface hydrophilicity and enhanced crosslinking degree.Compared with the control membrane without the addition of TA-MoS2 nanosheets,the TFN membrane displays approximately 60%improvement of water permeability accompanied by slightly enhanced rejection properties.