Fabrication Of Composite Membranes With High Flux And Fouling-resistance Property For Membrane Distillation

Membrane distillation（MD）technology possesses broad application prospects in the desalination field due to its high salt rejection rate and the effective use of low-carbon clean energy such as solar energy.However,various factors hinder the development and industrial applications of MD technology,such as membrane fouling,membrane wetting,and lack of suitable MD membranes.In this paper,aiming at membrane fouling and membrane wetting problems in the high salinity wastewater treatment,MD membranes with excellent performances were fabricated via structural design innovation and surface chemical modification of hydrophobic membranes,and the mechanisms of anti-wetting and anti-fouling for MD membranes were explained.In addition,the resistance to different characteristic pollutants and the separation efficiency for MD membranes in high salinity solutions were researched systematically.To improve the anti-fouling property of MD membrane,a biomimetic omniphobic membrane was prepared by the solution immersion precipitation method and fluorosilane treatment,with the biological cohesiveness of polydopamine.Meanwhile,the multiple re-entrant morphology structure composed of silica nanoparticles and polyvinylidene fluoride（PVDF）microspheres was constructed on the membrane surface.The characterization results showed that the contact angles of water and oil on the omniphobic membrane surface were 169.0°and 112.1°,respectively,and the fabricated membrane maintained good chemical stability in acid and alkali environments.Concentrating the high-concentration mixed salt solution containing organics and surfactants for 90 h,the initial hydrophobic membrane flux gradually decreased to 10.8 kg/m²·h,while the omniphobic membrane flux remained above 35.0kg/m²·h and displayed high salt rejection rate（nearly 100%）,presenting excellent anti-fouling performance.To enhance the anti-wetting property of MD membrane,the synthesized dual-scale composite micro/nanoparticles Ti O₂@PDA@Cu_xO were immobilized on the hydrophobic membrane via chemical bonding,and then the membrane surface was dealt with fluorosilane to fabricate an omniphobic membrane with multi-level re-entrant structure.The omniphobic membrane displayed over 150°contact angles to various droplets with different properties.The MD test results showed that the omniphobic membrane maintained a stable membrane flux of 34.4 kg/m²·h in 26 h concentration with hypersaline solution as the feed.The surfactant system was utilized to test the anti-wetting performance of the omniphobic membrane,it is found that the pristine hydrophobic membrane flux decreased by 72.13%,while the omniphobic membrane flux kept nearly at a constant,exhibiting good anti-wetting and anti-fouling performance.To solve the problem of membrane flux decline caused by surface modification,the composite membrane with omniphobicity and high permeate flux was fabricated via electrospraying synthesized heat-conducting nanofibers Ti-CNFs on the hydrophobic membrane and fluorosilane treatment simultaneously,presenting nest-like morphology structure.Under the temperature difference of 50°C,the omniphobic membrane exhibited a membrane flux of 36.3 kg/m²·h and increased by 21.4%,compared to the pristine hydrophobic membrane.The omniphobic membrane demonstrated the lyophobic feature to the droplets with different properties,and displayed high membrane flux as well as outstanding wetting resistance and anti-fouling ability in the membrane distillation fouling test.Aiming at the problems of oil pollution and surfactant wetting,a Janus membrane with the hydrophilic protective layer was prepared by two-step strategy,namely the dilute solvent welding method combined with electrospraying method was used to construct the omniphobic layer on the hydrophobic membrane firstly,and then the polymer hydrogel was introduced on it by vacuum filtration technology.The modified surface of the Janus membrane demonstrated hydrophilic feature and underwater superoleophobic characteristic.In the membrane distillation fouling test,the Janus membrane showed approximately 100%salt rejection rate,favorable anti-fouling and anti-wetting performances for the salt solution containing surfactant-oil emulsion.It is verified by the above experiments that the omniphobic membrane and the Janus membrane have the potential to be applied for high salinity wastewater treatment in MD,fabricated via constructing rough morphologies and changing chemical compositions.