Development Of Robust And Superhydrophobic Membranes To Mitigate Membrane Scaling And Fouling In Membrane Distillation

Membrane distillation is a thermally membrane separation process driven by vapor pressure difference between the two sides of the membrane,which is separated by a microporous hydrophobic membrane.Membrane distillation?MD?has attracted comprehensive attention for seawater and brackish water desalination,wastewater treatment,and concentration of juices and pharmaceutical solutions due to its mild operation conditions,high effluent water quality and treatment for high-concentration feed solutions.Hydrophobic microporous membrane is the key component of membrane distillation,and practical application has been restricted by lack of a sort of qualified membrane with a property of excellent flux,anti-wetting and anti-fouling.Therefore,it is very important for the development of membrane distillation to fabricate superhydrophobic membranes with properties such as anti-wetting,anti-fouling,and high flux.In view of the above problems,this study fabricated a robust and superhydrophobic membrane via electrospinning followed by electrospray.The robust superhydrophobic membrane comprised a superhydrophobic surface layer fabricated by electrospraying a polyvinylidene fluorid?PVDF?/Polydimethylsiloxane?PDMS?/silica fume blended solution and an electrospun nanofibrous PVDF support.And the optimal composite membrane,PDMS-3,exhibited outstanding superhydrophobicity with a water contact angle of 170?±1?and sliding abgle of 4.3°±0.1°by controlling the addition of Si O₂nanoparticles.The mechanism of the formation of the superhydrophobic surface of the composite membrane was demonstrated:during the electrospray process,PDMS molecules were brought onto the microspheres by the volatilization of the solvent to solidify and crosslink,and were entangled by the large molecular mass PVDF forming microsphere structure.Then the phase separation process on the surface of the microspheres facilitated a rough surface as the first level,and the Si O₂nanoparticles on the surface constituted the second level of the multilayer structure.After 40 abrasions cycles,the surface of PDMS-3 still possessed a slippery superhydrophobic property and had a contact angle of 152°±1°,which repelled water droplets for bouncing on the membrane surface.The phenomena referred above proved PDMS-3 characterized with a unique hydrophobic properties and abrasion resistance due to the multilayer structure through water droplet bouncing experiments and abrasion cycles experiments.PDMS-3 also achieved a superior MD water flux of28 L m^-2h^-1and a stable salt rejection of 99.99%during a continuous 160-h DCMD operation when the feed and permeation solutions were 3.5 wt%Na Cl solution at 333K and distillate water at 293 K,respectively.This study compared the DCMD performance of PDMS-3 with nanofiber membrane PVDF and commercial membrane PVDF in the treatment of different fouling feeds,and illustrated the mechanisms of antifouling property of superhydrophobic membranes:1)When treating Ca SO₄ solution,the representative of scaling,PDMS-3 maintained a flux of 28.1±0.7 Lm^-2 h^-1 in the 28-h DCMD test and a conductivity of 4.1±0.9?s/cm,while the nanofibrous PVDF membrane exhibited a significant increased conductivity caused by wetting.The unique surface characters of PDMS-3 reduced liquid-solid contact area between membrane surface and feed solutions,and declined the contact possibility of membrane and crystal and lowered nucleation propensity impeding scaling fouling to adhere on the membrane surface;2)When treating the representative fouling of organic pollution,humic acid,PDMS-3 achieved a flux of 21.4±0.5 Lm^-2h^-1 and conductivity under 5.0?s/cm after18-h operation of DCMD,whereas the other two membranes were fouled more severely.It is easier for PDMS-3 to flush the loose fouling layer due to poor interactions between organic fouling and membrane surface and self-cleaning character;3)Herein,the interactions between different membrane surface and surfactant were also studied with cationic surfactant dodecylammonium bromide?DTAB?and anionic surfactant sodium dodecylsulfonate?SLS?in feed solutions.When DTAB was added to the feed solution,the water flux and conductivity of all three membranes gradually changed;whereas the feed solution containing the anionic surfactant SLS,all membranes showed stable flux and rejection.The contact angle of the membrane after DCMD test of surfactant was higher than the other two membranes.The reason why PDMS-3 retained good hydrophobicity was that its low surface tension and turbulence flow above surface made surfactant molecules difficult to adsorb on the membrane surface.This study fabricated outstanding anti-wetting superhydrophobic membranes and explained the mechanism of fabricating superhydrophobic membrane by electrospinning and electrospray.Mechanisms of interactions was also illustrated between different foulings and hydrophobic membrane surfaces subsequently which could offer a solution to improve anti-fouling and anti-wetting property of MD membrane and facilitate the practical application of MD.