Hydrophilic/hydrophobic Membranes With High Flux And Anti-oil-fouling Capacity And Its Membrane Distillation Performance

Membrane distillation(MD)is a thermally driven membrane separation technique,in which a hydrophobic porous membrane functions as a barrier and the vapor pressure difference across the membrane provides driving force for mass transfer.However,low flux as well as membrane fouling and wetting are hindering its application and development.Research on the enhancement of MD flux or membrane anti-fouling/wetting capacities has made progress in the last few years,while simultaneous improvement in both features has yet to be achieved,namely,there exists a trade-off between increasing flux and improving anti-fouling capacity.The comprehensive performance and stability of MD membranes still need to be improved.As a result,it is of great significance to fabricate MD membranes with simultaneous high flux and anti-fouling/wetting performance.In this work,we mainly focused on how to break through the aforementioned trade-off effect from the perspective of constructing novel hydrophilic/hydrophobic Janus composite membranes.(1)Three hydrophilic/hydrophobic Janus membranes were prepared in sequence based on a commercial polytetrafluoroethylene(PTFE)membrane.Through a multi-step surface modification strategy,which includes polydopamine(PDA)coating,in-situ immobilization of silver nanoparticles(AgNPs)and PDA ultrathin sealing,we obtained P-PTFE,Ag/P-PTFE and P/Ag/P-PTFE membranes.Membrane surface physicochemical features including elementary composition and morphology were characterized.Although the mean pore size was smaller than the pristine PTFE membrane,Janus membranes still maintained a surface porous feature(72-86nm).Moreover,the surface of Janus membranes was in-air hydrophilic and underwater oleophobic and exhibited stronger electronegativity compared with the pristine PTFE membrane.After the in-situ metallization,a large number of uniformly distributed AgNPs with a diameter 36.8 ± 5.1nm appeared,endowing the Ag/P-PTFE membrane with a multilevel rough surface,so its hydrophilicity and underwater oleophobicity were further enhanced.The final PDA ultrathin sealing significantly improved the stability of AgNPs but did not impact the wettability of the membrane surface.The in-air water contact angle and underwater oil contact angle of the P/Ag/P-PTFE membrane were 55.7±5.6° and 146.4±3.9°,respectively.(2)The desalination performance of the pristine PTFE membrane and Janus membranes were investigated through vacuum membrane distillation(VMD).When dealing with a 3.5%NaCl solution at 70?(vacuum degree 90kPa),all membranes had the salt rejection rate above 99.9%.The VMD flux of the pristine PTFE membrane was 19.8kg m-2h-1.The VMD flux of P-PTFE and P/Ag/P-PTFE membranes soared to 66.9kg m-2h-1 and 84.3kg m-2h-1,respectively,which was 237.0%and 324.7%higher than the pristine PTFE membrane.When treating an oily saline emulsion(500ppm mineral oil,3.5%NaCl),in contrast to the severe fouling on the PTFE membrane and the gradual flux decline of the P-PTFE membrane,the P/Ag/P-PTFE membrane exhibited a quite stable desalination performance.Its initial flux and ultimate flux was 39.14kg m-2h-1 and 37.65kg m-2h-1,respectively,only a decrease in 3.8%,and the salt rejection was always higher than 99.9%.It can be seen that the first PDA coating was the main driver of the flux increase,and AgNPs also contributed to the alleviation of temperature polarization and the improvement of anti-fouling capacity.The outstanding comprehensive performance of the P/Ag/P-PTFE membrane came from their synergetic effect.(3)The mechanism of flux increase from the pristine PTFE membrane to Janus membranes was further discussed via experimental results and theoretical analysis.The results of Raman spectra confirmed that there were masses of intermediate water(IW)confined in the hydrophilic PDA coating.Compared with free water(FW),IW possesses lower evaporation enthalpy,so it was speculated that the hydrophilic layer of Janus membranes might have the function of accelerating water evaporation.However,the structure of the hydrophilic layer has significant influences on the water transfer across the hydrophilic layer(water recharge)and the temperature polarization.The experimental results demonstrated that the effect of accelerating water evaporation in the hydrophilic layer could be readily offset by the insufficiency of liquid water replenishment.Theoretical analysis also showed that there was a competitive relationship between water evaporation acceleration,water supply across the hydrophilic layer,and aggravated temperature polarization.(4)PDA modification of carbon nanotubes(CNTs)and their dispersion behavior in different solvents were preliminarily studied.Compared to AgNPs,CNTs not only has high thermal conductivity but also can accelerate the water transfer in the tube.Therefore,it is expected that CNTs will further enhance the MD desalination performance based on the concept of hydrophilic/hydrophobic Janus membrane.Given that raw CNTs will always agglomerate severely and have poor dispersibility,which will negatively influence the subsequent application in constructing a hydrophilic layer,we modified the surface of CNTs by coating PDA.Results indicated that the PDA layer could significantly improve the dispersibility of CNTs through the synergistic effect of physical shielding and electrostatic repulsion.By comparing the agglomeration process of CNTs in deionized water,N,N-dimethylacetamide and ethanol before and after modification,it could be found that within 170minutes after ultrasonication,more than 95%of the pristine CNTs presented bundles or aggregates,but over 95%of modified CNTs(0.5-PDA@CNTs)were monodisperse.The results showed that the PDA modified CNTs exhibited outstanding dispersion stability.In summary,we demonstrated in this study that the trade-off effect between increasing flux and improving anti-fouling capacity,especially in the surface modification process for the MD membrane,could be broken through by constructing a well-structured hydrophilic layer on a hydrophobic porous membrane.As a result,we could obtain a high-performance hydrophilic/hydrophobic Janus membrane with both enhanced MD flux and improved anti-oil-fouling ability.In order to further optimize the physical structure and chemical properties of the hydrophilic layer,we also gave some preliminary suggestions based on the theoretical analysis.We hope the findings in this study can provide a new platform and methodology for designing high-performance membranes for MD desalination.