Synergistic Optimization Of The Performance Of GO-Janus Composite Membranes Based On Dual Polymer Cross-linking In Membrane Distillation

Membrane distillation is a combined technology that couples the characteristics of both distillation and membrane separation processes.A hydrophobic membrane served as the medium and the vapor pressure difference between the two sides of the membrane is the driving force to achieve the separation.Membrane distillation technology is widely used in seawater desalination,high salt industrial wastewater treatment and so on.However,membrane wetting and fouling in the membrane distillation process largely limit its industrial development.Therefore,the development of membrane distillation membranes with simultaneous anti-wetting/fouling properties is of great significance.At present,graphene oxide(GO),as a two-dimensional(2D)nanomaterial,is widely used for the modification of membrane,and the research on GO in membrane distillation process is mostly focused on its effect on permeate flux,but further exploration is still needed to enhance the performance of membrane distillation with simultaneous anti-wetting and anti-fouling.In this paper,the surface modification of polyvinylidene fluoride(PVDF)commercial hydrophobic membranes by GO is aimed at improving the anti-wetting and anti-scaling ability of PVDF hydrophobic membranes in the membrane distillation.In this paper,four commonly used cross-linking agents,polyethyleneimine(PEI),polyethylene glycol(PEG),thiourea(TU)and copper(Cu),were selected as the crosslinkers and GO as the membrane surface modification material.After physically blending GO and the cross-linking agents separately,P-GO/PEI membrane,P-GO/PEG membrane,P-GO/TU membrane and P-GO/Cu membrane were prepared by vacuum filtration method.Sodium dodecyl sulfate(SDS)was used as a were selected for anti-wetting experiments.Based on the anti-wetting experiments,PAA was further introduced in GO 2D nanochannels for synergistic performance optimization,and a mixture of sodium sulphate and calcium chloride was used as the feed solution to test the anti-scaling performance of the GO-Janus composite membrane.The anti-wetting and anti-scaling mechanisms of the GO-Janus composite membrane was analyzed.The main experimental results are as following:(1)In the performance preference experiments of the anti-wetting GO-Janus composite membrane,the P-GO/PEI membrane was characterized by Fourier transform infrared spectroscopy,etc.It is known that PEI crosslinks GO by reacting with the functional groups on the GO nanosheets to generate amide bonds.Meanwhile,the layer spacing of GO layer was calculated by the Bragg equation,and from the results it is known that the swelling rate of the pure GO membrane is 47.2%,and the P-GO /PEI membrane surface layer of GO/PEI layer swelling rate is only 1.4%,indicating that the cross-linking of PEI is good at inhibiting GO swelling under water.In the anti-wetting membrane distillation experiments,PVDF,P-GO,P-GO/PEG,P-GO/TU and P-GO/Cu membranes all showed a decrease in flux and an increase in conductivity,with poor anti-wetting effects,the P-GO/PEI membrane maintained a constant flux during the 16 h of continuous operation,showing excellent antiwetting performance.The P-GO/PEI membranes were investigated by SDS diffusion experiments,retention experiments and membrane penetration pressure measurements,which confirmed that the P-GO/PEI membranes prevented the penetration of SDS and retained 84.5% of SDS at a pressure of 0.5 MPa,At the same time,the GO/PEI layer has a high capillary force,which not only retains SDS,but also prevents SDS from crossing the GO layer and contacting the PVDF base membrane,thus avoiding the appearance of membrane wetting.(2)Based on the above GO-Janus composite membrane anti-wetting preference experiments,the P-GO/PEI/PAA composite membrane was further prepared by introducing anionic polyelectrolyte(PAA)for synergistic cross-linking.By calculating the layer spacing,the difference in layer spacing between P-GO/PEI/PAA membranes in dry and wet conditions was only 0.04 nm,indicating that the co-crosslinking of PAA did not reduce the stability of the 2D nanochannels.The P-GO/PEI/PAA composite membrane has good resistance to wetting and the flux is stable during long-term operation.In the anti-scaling membrane distillation experiments,PVDF,P-GO and P-GO/PEI membranes all showed a rapid decrease in flux,losing flux after 5 h,4 h and10 h respectively,thus indicating that the three membranes were not effective in retarding scaling as a whole.The introduction of PAA synergistic cross-linking slowed a down the rate of membrane flux decline,but the slowing down effect was related to the amount of PAAadded.The best effect of retarding membrane surface crystallization was achieved when PAA was added at 0.2 mg,and the flux was still about 10% of the initial flux after 42 h of operation.The membrane contamination characterization showed that the calcium sulphate crystallization on the membrane surface after P-GO/PEI/0.2PAA contamination showed an amorphous morphology,which may be due to the chemical combination of the carboxyl group in the PAA molecule with the calcium ion,making the calcium sulphate precursors in solution stable,thus limiting the conversion of sub-stable amorphous calcium sulphate to calcium sulphate dihydrate.While PAA may affect the uniform and non-uniform crystallization of calcium sulphate,thus increase the crystallization induction time to inhibit fouling.The experimental results in this paper show that GO-Janus composite membranes prepared by cross-linking PEI and PAA polymers have excellent anti-wetting and anti-scaling properties.The stable 2D structure of GO-Janus composite membranes and the ultra-high capillary force inside the GO/PEI layer improve the anti-wetting performance of GO-Janus membranes,while the synergistic cross-linking of PAA within GO nanochannels suppresses the growth of scaling,thus promoting the mineral scaling resistance of membrane.The results of this study are expected to provide a reference to improve membrane wetting and membrane mineral fouling of Janus membrane during membrane distillation,thus promoting the industrial application of membrane distillation technology.