| The water scarcity and pollution of aquatic environments have become a common problem throughout the world.At the same time,sea water and brackish water accounts for about 97.5%of global water resources,which could not be used for human production and life due to their high salinity.Membrane distillation(MD),as a new type of membrane separation technology,can make full use of low-grade thermal energy such as solar energy,geothermal energy and waste heat,indicating a broad application prospect in seawater desalination and wastewater treatment.MD is the thermally driven and non-isothermal membrane separation process.In the MD process,only vapor molecules can transport across the hydrophobic membrane under a driving force of partial vapor pressure difference and other nonvolatile species are rejected to produce high-quality water.Compared with the traditional thermal desalination technology or other pressure driven membrane separation processes,MD has lower operation temperature and pressure.The separation membrane is the core of MD technology.Therefore,developing a high-performance MD membrane is the key factor to realize the large-scale commercial applications of MD.The ideal MD membrane should exhibit sufficient hydrophobicity,high porosity,good mechanical strength,and chemical and thermal stability to ensure high permeability and anti-wetting during long-term operation.However,the traditional hydrophobic microporous membranes are limited by the drawbacks of low porosity,closed cell pore structure and insufficient hydrophobicity,resulting in their poor permeation flux and salt rejection.In contrast,the electrospun nanofibrous membranes have several advantages of high porosity,interconnected pore structure,and controllable membrane thickness,which are beneficial to high permeability and improve the drawback of low water flux in traditional MD process.Inspired by the lotus leaf with superhydrophobic property resulting from the cooperation of hydrophobic wax-like material and hierarchical surface structures,superhydrophobic nanofibrous membranes are fabricated based on electrospinning technique to avoid membrane wetting,which consist of functional coating and porous nanofibrous support.Compared with the pristine nanofibrous membrane,the superhydrophobic coating could ensure narrow pore size distribution and high wetting resistance to achieve high selectivity,and the interconnected nanofibrous support could provide direct paths for vapor transport to achieve high permeability.Herein,based on the poly(vinylidene fluoride)(PVDF)nanofiber membrane as the support,the hydrophobic agent perfluorododecyltrichlorosilane(FTCS)plus the benign and inexpensive polydimethylsiloxane(PDMS),amorphous polypropylene(aPP),and isotactic polypropylene(iPP)were selected as low surface energy materials to construct superhydrophobic nanofibrous membranes with hierarchically structured coatings by various surface/interface modification methods.We successively constructed polymerized FTCS coating on nanofiber surface and CNTs/PVDF functional coating,porous aPP skin layer,and iPP microsphere coating with interlocked structure on nanofibrous membrane surface for higher-performance MD membranes.The effects of structural and material design on surface hydrophobicity and membrane structural properties including pore size,porosity,and liquid entry pressure of water(LEPw)were comprehensively investigated.We tried to tune the morphology and structure of the functional layer to cooperatively regulate the membrane structural properties and surface superhydrophobicity,thereby improving permeation flux,durability and anti-fouling performance.1.In consideration of the current complicated hydrophobic modification processes,the omniphobic nanofibrous membranes were fabricated by directly functionalizing the fiber surface with FTCS consisting of a highly water-sensitive trichlorosilane headgroup and a long fluoroalkyl chain via one-step solution immersion without any assistance of roughening treatments.With the increase of FTCS concentration,the morphologies of polymerized FTCS(PFTCS)coatings evolved from the newly budding willow twigs with tiny bumps,to vigorous buds and then to intertwined fillets,which could serve as a robust barrier to low-surface-tension liquid penetration.By constructing hierarchically re-entrant structures of low-surface-energy PFTCS coating layer onto the PVDF nanofibers,the modified nanofibrous membranes were endowed with omniphobic property,low water-adhesion property,and remarkable mechanical property.At the optimal concentration(2 wt%),the resultant membranes with increased membrane thickness presented the reduction in pore size,porosity and gas permeability,and the promotion in LEPw as well as more stable permeate conductivity.Significantly,the modified nanofibrous membrane exhibited a competitive permeate vapor flux of36.9 kg/m~2h in highly salty solution(3.5 wt%NaCl salt feed;?T=40°C)over 24 h DCMD operation and also demonstrated robust DCMD performance even in the presence of surfactant.2.We demonstrated a novel strategy to deposit high-aspect ratio CNTs on nanofibrous support via vacuum filtration method in order to precisely control the maximum pore size by using inorganic nanomaterial with ultrafine diameters.According to the proportional relationship between the fibrous material diameter and the pore size,the large scale surface gaps between adjacent nanofibers could be replaced by smaller pores from the CNTs layer on the nanofibrous substrate.The PDMS was used as a crosslinker to in-situ solidify the loose CNTs layer to prepare a robust dual-layer nanofiber composite membrane with superhydrophobicity,high porosity,controllable pore size and high LEPw.By controlling the concentration of PDMS,the structural stability of the CNTs/PDMS composite membranes could be greatly improved,leading to higher LEPw(2.45bar)than the commercial PVDF membrane.Benefitting from the superhydrophobicity and high LEPw,the composite membrane showed excellent anti-wetting performance.The optimized composite membrane(the CNTs deposition amount:6.4 g/m~2;the PDMS concentration:0.8wt%)presented a competitive permeate vapor flux of 50.8 kg/m~2h with a high salt rejection of>99.99%over 50 h DCMD operation(3.5 wt%NaCl salt feed;?T=40°C).3.In order to avoid the environmental and cost problems caused by the hydrophobic agents and inorganic nanomaterials,we successfully engineered an intact and porous thin film onto the surface of PVDF nanofibrous support via a simple vacuum assisted filtration method.For the first time,this nanofibrous composite membrane was elaborately designed by transforming aPP solution into thermodynamically unstable state to deposit aPP-rich phase onto nanofibrous support and drain away the aPP-poor phase,forming a porous polymeric skin layer.With the cooperation of the hierarchical structures and low surface free energy of the aPP matrix,the resultant composite membrane exhibited excellent superhydrophobicity and low water-adhesion property,indicating by the increased water contact angle from 131.8°to156.3°.In this strategy,the robust dual-layer aPP/PVDF membranes presented controllable pore size and LEPw without severely compromising the porosity.The three-dimensional network separation layer acted as additional superhydrophobic barrier to avoid pore wetting without severe mass transfer resistance and combined the interconnected nanofibrous support layer to guarantee direct paths for vapor transport.The optimized composite membrane(the aPP solution temperature:60?;the aPP deposition amount:24 g/m~2)presented an excellent permeate vapor flux of 53.1kg/m~2h with a high salt rejection of>99.99%over a long-term DCMD test duration of 6 days(3.5 wt%NaCl salt feed;?T=40°C),which was more than twice that of C-PVDF membrane.4.To achieve more comprehensive performances especially higher mechanical strength and antifouling property for complicated feed solution,a robust superhydrophobic two-tier composite membrane with interlocked structure was elaborately designed for DCMD by constructing iPP functional coating reinforced onto electrospun nanofibrous membrane.The middle transitional interlocking zone between the crystalline iPP microsphere coating and PVDF nanofbers endowed the resultant composite membrane with excellent structural integrity including the remarkable enhancement in mechanical performance compared with PVDF flat sheet or aPP/PVDF composite membranes,and also resulted in the iPP/PVDF composite membranes with robust durability against ultrasonication in isopropanol and strong acid/base attacks.By controlling the solution temperatures and amounts,the morphological architecture of the interlocking microsphere coating on PVDF nanofibrous substrate made the membrane superhydrophobic and could optimize the membrane structural properties.The unique surface structure with air entrapped underneath served as a cushion to avoid fouling and pore wetting.For the simulated methylene blue(MB)or high salinity sunset yellow(SY)wastewater,the optimized superhydrophobic composite membrane(the iPP solution temperature:58?;the iPP deposition amount:32 g/m~2)exhibited a competitive permeate flux of 53.9 kg/m~2h,complete rejection,and firmly sustainable liquid repellency over 50 h DCMD operation(?T=40°C).Consequently,series of superhydrophobic nanofibrous membranes with proper hierarchically structured coatings were specially designed by taking full advantage of various functional materials with novel material process strategies.By cooperatively regulating the membrane structural properties and surface superhydrophobicity,these composite membranes exhibited quite high separation efficiency for simulated seawater or high salinity wastewater. |
PDF Full Text Request