Biomimetic Construction Of Superhydrophobic Electrospun Nanofibrous Membranes For Membrane Distillation

Recently,a new type of highly efficient membrane separation technology,membrane distillation?MD?,has gained substantial attention.Membrane distillation is a thermally-driven non-isothermal membrane separation process,in which only vapor molecules can transfer through a hydrophobic microporous membrane.The driving force in the MD process is the vapor pressure difference induced by the temperature difference between the feed and permeate side.Compared with the traditional distillation methods and other membrane separation technologies,MD technology possesses several outstanding advantages,including lower operation pressure and temperature,and high rejection factor,which can take full advantage of the solar,geothermal energy,and industrial waste heat.Therefore,seawater desalination by MD technology is expected to become a new method for preparing fresh water with low energy water recovery.With respect to MD membrane,it should possess sufficient hydrophobicity,microporosity,and excellent breathability that will be beneficial to the osmotic transmission of volatile components,while the mechanical property,thermal stability,and corrosion resistance of the membrane are also indispensable prerequisites for the practical long-term MD operation.As traditional MD membranes,flat-sheet membranes prepared by phase inversion method and hollow fiber membranes fabricated by wet spinnng technology are often suffered from the low permeate flux and membrane pore wetting because of its inner disconnected pore structure,low porosity,small pore size and insufficient hydrophobicity,which limit the large-scale industrial application of MD technology.Electrospun nanofibrous membrane?ENM?exhibits a series of attractive characters,such as a fully interconnected porous web structure,high porosity,and uniform fiber morphology with controllable pore size and membrane thickness,etc.,which could remarkably improve the drawback of low permeate flux in the MD process.Furthermore,inspired by the micro/nano-scaled structures of the superhydrophobic biological organisms from nature,a superhydrophobic electrospun nanofibrous membrane?ENM?can be fabricated by constructing hierarchical roughness on the nanofiber surface and the surface modification with low surface energy materials,which can avoid the membrane pores wetting efficiently during the long-term MD operation process.Herein,the low surface energy and hydrophobic materials including polystyrene?PS?,polyvinylidene fluoride?PVDF?,polysulfone?PSU?,polydimethylsiloxane?PDMS?,hydrophobic silica nanoparticles?SiO₂ NPs?and hydrophilic polyacrylonitrile?PAN?were used for the preparation of superhydrophobic electrospun nanofibrous membranes?ENMs?.Through a one-step electrospinning technology,colloid electrospinning technology,eccentric-axial electrospinning technology and the coating modification method,various biomimetic hierarchical structures were fabricated on the nanofiber surfaces.The construction of superhydrophobic surface property and the optimization of membrane structural properties were obtained by comprehensively investigating the effect of different fiber microstructures on the resultant membrane performances.The relationship between the separation performance in the MD process and the membrane structural properties was also studied completely,i.e.,the optimized superhydrophobic ENMs for MD can be achieved through the design of nanofibrous membranes with different structural characteristics.1.A new type of dual-biomimetic hierarchically rough PS superhydrophobic micro/nano-fibrous membrane was fabricated via a one-step electrospinning technique at various polymer concentrations from 15 to 30 wt%.The obtained micro/nano-fibers exhibited a nanopapillose,nanoporous,and microgrooved surface morphology that originated from mimicking the micro/nanoscale hierarchical structures of lotus leaf and silver ragwort leaf,respectively.This kind of fiber surface roughness endowed the PS micro/nano-fibrous membrane with a water contact angle of 150.2°.Superhydrophobicity and high porosity of such resultant electrospun nanofibrous membranes make them attractive candidates for membrane distillation?MD?application with low energy water recovery.The mean flow pore?MFP?size,porosity,liquid entry pressure of water?LEPw?and gas permeability?under the pressure of 0.2 bar?of two optimized PS membranes with thickness of 60 ?m and 120 ?m were 0.76 ?m and 1.15 ?m,69% and 77.5%,0.6 bar and 0.8 bar,249.3 cm³/s and 189.0 cm³/s,respectively,which were obviously better than those of traditional phase inversion membranes.These two optimized PS micro/nano-fibrous membranes with thickness of 60 ?m and 120 ?m were applied to seawater desalination via direct contact MD?DCMD?.The membranes maintained a high and stable permeate flux?104.8 kg/m2·h,20.0 g/L Na Cl salt feed for a thinner PS membrane;51.0 kg/m2·h,35.0 g/L Na Cl salt feed for the thicker sample;?T = 50??and stable fine hierarchical structures on the fiber surface for a test period of 10 h without remarkable membrane pores wetting detected.These results were better than those of typical commercial PVDF MD membranes or related PVDF nanofibrous membranes reported in literature,suggesting excellent competency of PS nanofibrous membranes for MD applications.2.Based on the different solution properties of PVDF/DMF solution blended with hydrophobic silica with different particle sizes?40 nm,167 nm and 210 nm?,electrospun superhydrophobic PVDF-S-x organic/inorganic composite nanofibrous membranes with different micro morphologies and membrane structural properties were fabricated and applied to desalination via DCMD,where x stands for the silica particle size.Benefiting from the utilization of three different SiO₂ NPs,the PVDF-S-x ENMs were endowed with three different delicate nanofiber morphologies and fiber diameter distribution,high porosity,and superhydrophobic property,which resulted in excellent waterproofing and breathability.Wherein,the water contact angles of PVDF-S-x ENMs were increased form 152.3° to 163.1° with the increase of silica particle sizes.Significantly,structural attributes analyses have indicated the major contributing role of fiber diameter distribution on determining the augment of permeate flux through regulating MFP.Meanwhile,the extremely high LEPw?2.4 bar?,robust nanofiber morphology of PVDF immobilized SiO₂ NPs,remarkable mechanical properties,thermal stability,and corrosion resistance endowed the as-prepared membranes with prominent desalination capability and stability for long-term MD process.The resultant choreographed PVDF-S-40 ENMs with optimized MFP presented an outstanding permeate flux of 41.1 kg/m2·h and stable low permeate conductivity??2.45 ?s/cm??3.5 wt % Na Cl salt feed;?T = 40??over a DCMD test period of 24 h without membrane pores wetting detected.Furthermore,the smaller the MFP,the lower the permeate flux.3.Inspired by the profiled structure of polar bear hair that possesses excellent thermal insulation properties,we designed a novel eccentric-axial electrospinning technology for the first time that allows the fabrication of profiled PAN-PS core-shell nanofiber with groove structure to increase the membrane porosity,and then promoting the thermal efficiency and enhancing the permeate flux.The design of the spinneret containing two eccentric-axial capillary,i.e.,the inner capillary deviates from the central axis by a certain degree.Fiber surface morphology analyses indicated the major contributing role of applied voltage and the shell feeding rate in determining the stability of the electrospinning fluid jet,groove length and width,and membrane structural characteristics.A relatively lower applied voltage of 18 k V was beneficial to form a stable electrospinning fluid jet with a compound Taylor cone and uniform groove structure throughout the whole fiber.The groove width and porosity of the PAN-PS core-shell nanofiber decreased from 503.8 nm to 92.8 nm,and from 85.9% to 70.7%,respectively,with the increase of PS shell feeding rate,and the cross-sectional topographies varied from a distinct C-shape structure to the resultant frizzy C-shape morphology.However,when we replaced the soft PAN with crystalline PVDF,the peculiar groove structure would not be formed on the PVDF-PS fiber surface,which exhibited smooth and uniform core-shell nanofiber morphology,resulting in the membrane with lower porosity of 60.8%.The superhydrophobic properties resulting from the surface hierarchical roughness make the free-standing core-shell nanofibrous membranes as promising candidates for MD application.As compared to the lower permeate flux of PVDF-PS core-shell nanofiber membrane without groove structure applied to wastewater treatment by DCMD?20.0 g/L Na Cl and 1000 ppm Sunset Yellow FCF aqueous solution as feed,?T = 40??,the resultant grooved PAN-PS core-shell ENMs presented higher permeate flux of 60.1 kg/m2·h due to the higher thermal efficiency that is derived from the higher porosity with C-shape groove structure,and the larger the groove width the higher the permeate flux.Meanwhile,the superhydrophobicity of the resultant core-shell nanofibrous membranes guaranteed the production of high quality water with stable low permeate conductivity?².45 ?s/cm?and without any detected absorbance of SY FCF at the wavelength of 482 nm over a DCMD test period of 36 h.4.Compared with the high water-adhesion behavior of the superhydrophobic nanofibrous membranes with “petal effect”,the nanofibrous membarnes with “lotus effect” or the self-cleaning surface property,the water droplets will be immediately rolled/slid off the membrane surface during the MD process,which is more beneficial to avoid the membrane pores wetting.In this sense,we describe a novel and facile design strategy for the fabrication of self-cleaning PSU-PDMS ENMs for desalination via DCMD by electrospinning PSU nanofibers and in situ curing of PDMS coating layer,followed by the cold-press post-treatment.The results showed that the PDMS adhesions and/or agglomerations would be formed on the membrane surface with the increase of PDMS concentration.Meanwhile,it exhibited superhydrophobicity,low water-adhesion behavior and self-cleaning property due to the low surface energy of PDMS and the formed surface hierarchical roughness.The PDMS fusions would be gradually generated on the surface nanofibrous layer with the increase of pressure,while the LEPw of the PSU-PDMS ENMs could be impressive improved.The cross-section FE-SEM images showed that the inner nanofibers were also wrapped by the PDMS coating,but the PDMS fusions would not be formed in the internal nanofibrous layers.The resultant optimized PSU-PDMS ENMs presented a competitive permeate flux of about 21.5 kg/m2·h and stable low permeate conductivity?².5 ?s/cm??30.0 g/L Na Cl salt feed,?T = 50??over a DCMD test period of 12 h without detection of membrane pore wetting,which overcoming the major obstacles of traditional MD membrane including low permeate vapor flux and membrane pores wetting,and demonstrating the feasibility of the self-cleaning PSU-PDMS ENMs as a novel membrane material for MD applications.Consequently,biomimetic construction of superhydrophobic electrospun nanofibrous membranes for membrane distillation can be obtained by investigating the formation mechanism and controlling the parameters of electrospinning process.The MD performance?permeability and selectivity?can be further optimized by designing different membrane structural properties,such as the fiber diameter distribution,LEPw,porosity,etc.,and achieving the comprehensive study of the integration of structure and performance.