Investigation On The Mechanism Of Asymmetric Wetting Polymer Microporous Membrane In Membrane Distillation Process

Membrane distillation（MD）process has attracted more and more attention due to its unique advantages in seawater desalination and wastewater recovery in recent years.The process is a membrane technology that uses hydrophobic microporous membranes to separate aqueous solutions containing non-volatile solutes,with the water vapor on the feed side passing through the membrane pores and condensing on the permeate side driven by vapor pressure differential.However,when treating saline wastewater containing non-polar（oil,etc.）and amphipathic（surfactants,etc.）contaminants,conventional hydrophobic membranes face membrane wetting and membrane pollution problems,which severely shorten the life of membranes.Therefore,it is crucial to develop a novel anti-pollution and anti-wetting MD membrane.In this paper,to address the problems of membrane contamination,membrane wetting and temperature polarization in MD process,asymmetric modification of hydrophobic membranes by limited domain crosslinking,interfacial spraying and interfacial deposition,and the asymmetric infiltrating membrane（Janus membrane）with anti-pollution,anti-wetting and good mechanical stability was prepared.The details of the study and the results are as follows:The preparation of polymer-based Janus membranes and the study of membrane distillation performance.Polyvinylidene fluoride（PVDF）membranes were produced by immersed precipitation phase transformation method,and the above PVDF membranes were subsequently modified by 1H,1H,2H,2H-perfluorooctyltrimethoxysilane to further reduce surface energy of the membranes.With the synergistic effect of the micro-nano structure and low surface energy,the bottom surface of the modified PVDF membrane showed superhydrophobic state with water contact angle of 155.64±1.2°.Further,the superhydrophilic layer with water contact angle of 0^o was obtained by controlled domain-limited crosslinking of hydrophilic modified molecules（poly（vinylpyrrolidone-co-vinyltriethoxysilane,P（NVP-VTES）and poly[N,N-dimethyl-[2-（2-methylprop-2-enoyloxy）ethyl]ammonium]propane-1-sulfonic acid inner salt-co-vinyltriethoxysilane,P（SBMA-VTES）））on top surface of the membrane.The structure of Janus membrane showed superstability after strong stirring and ultrasonic treatment.The groups including C=O,Si-O-Si and S=O groups were measured on top surface of membrane by fourier transform infrared spectroscopy（FTIR）,which initially proved that the hydrophilic component was successfully introduced to the top surface of the membrane.The results of X-ray photoelectron spectroscopy（XPS）and energy dispersive spectrometer（EDS）indicated that the top surface of the modified membrane containing Si,N,O and S elements,further proved that the hydrophilic components are successfully loaded by limited domain crosslinking.During direct contact membrane distillation（DCMD）process,oily saline water containing different surfactants（sodium dodecyl sulfate（SDS）,cetyl trimethyl ammonium bromide（CTAB）,polyoxyethylene sorbitan monolaurate（Tween-20）,and a mixture of three surfactants mentioned above）was treated using PVDF,Janus membrane only loaded with P（NVP-VTES）（named as Janus-0）and Janus membrane simultaneously loaded with P（NVP-VTES）and P（SBMA-VTES）（named as Janus-1）,respectively.Compared with PVDF and Janus-0,Janus-1 showed high salt rejection and excellent anti-fouling performance with pure water flux of 23.33 L·m^-2·h^-1 and salt rejection rate of about100%for 13 h.In addition,the membrane also showed excellent performance when treating oily brine containing mixed surfactants.The preparation of composite Janus membranes and the investigation of membrane distillation performance.Based on the bottom surface of PVDF membrane rich in micro and nano structures,different amounts of carboxylated carbon nanotubes（CNT-COOH）with hydrophilicity and high thermal conductivity were sprayed on the surface to improve anti-fouling performance and reduce temperature polarization of the membrane.The carbon nanotubes were preliminarily fixed under the limited domain action of micro-nano structures.Then,in order to further fix CNT-COOH,0.6 wt%Tara gum polysaccharide solution（Tara）was sprayed on its surface,and glutaraldehyde was used for crosslinking to form a cross-network structure interspersed with CNT-COOH.Under the synergic action of micro/nano structure and interfacial crosslinking,the mechanical strength of the composite membrane was improved compared with that directly loaded on the smooth surface.When the surface density of CNT-COOH was 0.04 g/cm²,the tensile strength,elongation at break and Young’s modulus of the membrane were 0.98 MPa,9.93%and 28.47 MPa,respectively.Compared with pure PVDF membrane and direct loading on smooth surface,both mechanical performances were improved.Due to the abundant carboxyl and hydroxyl groups in carbon nanotubes and Tara gum,the surface exhibited superhydrophobicity with a water contact angle of 0°.The anti-fouling ability of the composite Janus membrane to different oil emulsion in MD process was investigated.The results show that the stable running time of the composite Janus membrane can reach 30 h when the oily brine was processed.Compared with pure PVDF membrane(10.50 L·m^-2·h^-1),the average flux was up to 20.89 L·m^-2·h^-1.The preparation of Janus membrane with sandwich structure and the investigation of membranedistillationperformance.Janusmembraneswith hydrophilic/hydrophobic/hydrophilic sandwich structure（PDA-P-CNT）were prepared using PVDF as intermediate/hydrophobic layer,depositing polydopamine（PDA）layer on top surface of PVDF membrane（PDA-P）,and spraying CNT-COOH using PDA as adhesive layer on bottom surface of PVDF membrane（P-CNT）.The thickness of hydrophilic layer can be controlled by adjusting the deposition time of PDA and the spraying surface density of CNT-COOH,thus realizing the regulation of the comprehensive performance of the membrane.Two obvious absorption peaks were detected at 1480 cm^-1 and 1590 cm^-1 on top surface of PDA-P membrane using Fourier transform infrared spectroscopy,which are attributed to the primary amine N-H bending vibration peak and the C=C aromatic stretching vibration peak in PDA structure,respectively.The stretching vibrational peak of hydroxyl in PDA structure was detected at 3000-3600 cm^-1.The peak also appeared on bottom surface of P-CNT membrane at 3000-3500 cm^-1 ascribed to carboxyl.Above results demonstrate that the Janus membrane with sandwich structure was successfully prepared by interfacial deposition and interfacial spraying.The composite Janus membrane showed excellent vapor permeability and anti-fouling properties when treating oily saline water containing surfactants.The pure water flux of PDA-P membrane reached its highest value(20 L·m^-2·h^-1)when the deposition time of the PDA was 5 h.After further spraying of CNT-COOH on bottom surface,the MD performance of the sandwich Janus membrane was significantly further improved,and the pure water flux of the P（5）-P-CNT（0.04）membrane was up to 22.67 L·m^-2·h^-1.It was mainly attributed to two aspects:on the one hand,the hydrophilic layer on the upper surface（toward the feed side）gave the composite membranes better antipollution properties;and on the other hand,the hydrophilic CNT-COOH layer on the lower surface（toward the permeate side）promoted the condensation of vapor and improved the permeate flux.