Fabrication Of Polyvinylidene Fluoride Membrane With Surface Omniphobicity Via “Full-covalent-bond” For Membrane Distillation Application

With the increasing demand for clean water and the shortage of water resources,the regeneration of clean water has been paid more and more attention.Membrane distillation（MD）technology,as an emerging thermal desalination process,can effectively treat hypersaline wastewater and regenerate fresh water.However,wetting/fouling and functional layer instability are among the great challenges facing MD technology,which greatly hinder the expension of this technology.Therefore,the development of new MD membrane materials has attracted the attention of scientists.Accordingly,this study discards the traditional electrpstatic bonding method and processes an effective strategy of full-covalent bonding to prepare two types of omniphobic/slippery MD membranes with excellent anti-wetting,anti-fouling and mechanical stability,and verifies the feasibility and effectiveness of this strategy through experiments,which provides new ideas and technical guarantees for the robust design of subsequent high-performance membrane.This study mainly attempts to solve two key problems:（1）How to improve the anti-wetting and anti-fouling performance of MD membrane;（2）How to strengthen or solve the insufffient stability of MD membrane.The specific research content is as follows:（1）The traditional MD membrane usually adopts electrostatic deposition to realize the attachment of nanoparticles on the surface of the membrane to construct the required micro/nano hierarchical structure,which leads to instability of the functional layer and attenuation or even loss of membrane performance.In this study,taking into account the excellent adhesion of dopamine,firstly use it as a medium to covalently bind functionalized silica nanoparticles（Si O₂）to build a micro/nano hierarchical structure,and then react to Si O₂ by reacting amino groups and hydroxyl groups on the surface of silica particles with acryloyl chloride,and finally covalently bonded 1H,1H,2H,2H-perfluorodecanethiol to realize the fluorination of the MD membrane surface.The optimal PDA/PVDF-C membrane has a very high water contact angle（>150°）and a very low sliding angle（<10°）,and it can maintain the origin surface properities even after abrasion cycles（40 cycles）and a long time of sonication（270 min）,indicating that the membrane has excellent mechanical stability.In addition,the membrane exhibited high contact angles for various liquids with different surface tensions（e.g.,ethylene glycol,mineral oil,ethanol）and could stably treat salt solutions containing 2.0 m M SDS for 10 h over a long period of time,reflecting its excellent anti-wetting ability.The anti-fouling performance of the membrane was further investigated,and it was found that the membrane could delay the formation of gypsum scale and mitigate the performance degradation caused by membrane scaling to a certain extent.Finally,the membrane showed good desalination performance in the treatment of shale gas oil wastewater,with 60%water recovery in only 1200 min.（2）Although the above-mentioned PDA/PVDF-C membrane can greatly enhance the mechanical stability of MD membranes and achieve a significant improvement of the anti-wetting performance.However,due to the presence of dense polydopamine hydrophilic layer,the membrane still faces more serious inorganic scale contamination in the treatment of multi-component hypersaline wastewater,and the flux attenuation is obvious.Therefore,we considered abandoning the hydrophilic layer of polydopamine,directly treating the surface of the pristine membrane with alkali,and introducing olefins on the surface of the Si O₂ nanoparticles through a multi-step chemical reaction to build a micro/nano hierarchical structure,and finally realized low surface energy the membrane surface based on thiol-ene click chemistry.The results show that the optimal membrane PVDF-C also exhibits ultra-high water contact angle（WCA:166.5±1.4°）and very low sliding angle（SA:5.0±1.1°）,and maintains good surface and MD performance under harsh conditions（abrasion cycles,ultrasonic treatment,and prolonged operation）,further supporting the effectiveness and feasibility of the full-covalent strategy.In addition,the membrane exhibits excellent anti-wetting properties for liquids with different low surface tensions and is able to achieve stable and prolonged treatment of salt solutions up to 2 m M SDS,which is significantly better than commercial membranes.The membrane also exhibits excellent resistance to inorganic fouling due to its special omniphobic/slippery properties on the surface,it can achieve a long-term stable treatment of 20 m M Ca SO₄.In view of these excellent properties,the membranes exhibited a very high desalination capacity for the comprehensive treatment of multi-component hypersaline wastewater.The regeneration of clean water from the wastewater（up to 60%water recovery）was achieved in a relatively short period of time（1050 min）,contributing to the resource utilization of the wastewater.In summary,this study has developed a series of new MD membranes based on full-covalent bonding through the basic research and rational design of new membrane materials.The membranes have both excellent anti-wetting,anti-fouling and mechanical stability,which fully verified the effectiveness and feasibility of the full-covalent strategy.This project provides new insights and ideas for further development and exploitation of high performance MD membranes,and is expected to realize resourceful treatment of complex hypersaline wastewater.