Structural Designs And Performance Studies Of Membranes For Solar-driven Membrane Distillation

The issue of fresh water shortage urgently requires a solution.As a new and efficient membrane separation technology,membrane distillation is rapidly developing in the fields of seawater desalination and wastewater treatment,using the vapor pressure gradient generated by the temperature difference between the liquid on both sides of the separation membrane as the driving force,with moderate operating conditions,simple operation,and the ability to use a variety of low-quality heat sources.The ideal membrane for membrane distillation should be a hydrophobic microporous membrane with sufficient permeability as well as stability to ensure long-term operation of membrane distillation.Traditional hydrophobic microporous membranes are not sufficient to meet the requirements of membrane distillation,but the nanofiber membranes prepared by electrospinning technology have interpenetrating porous structure,high specific surface area,high porosity and adjustable membrane thickness,providing a large number of channels for volatile components,improving the shortcomings of the traditional membrane distillation with closed pore structure and increasing the flux.Additionally,making full use of low-quality heat sources(e.g.solar energy)improves the problem of low energy utilization efficiency due to the centralized heating of the feed liquid by conventional membrane distillation,and the direct heating of the feed liquid on the membrane surface also eliminates the effect of temperature polarization.This article combines photothermal conversion materials and electrospinning technology to prepare nanofibrous composite membranes with photothermal conversion effect,and replaces the traditional membrane distillation with solar-driven membrane distillation,which makes full use of renewable energy and provides a new idea for seawater desalination.Herein,two different types of membranes(hydrophilic/hydrophobic bilayer nanofibrous membrane and superhydrophobic nanofibrous composite membrane)with photothermal conversion effect were designed and prepared based on polyvinylidene fluoride(PVDF)nanofibrous porous membrane,with silver nanoparticles and polypyrrole nanotubes(PPy NTs)as photothermal conversion materials,respectively,as follows:(1)In order to increase the heated liquid volume on the membrane surface,hydrophilic electrospun polyacrylonitrile(PAN)nanofibrous membrane was selected as the carrier of silver nitrate(Ag NO₃),and nanofibers loaded with silver nanoparticles were obtained under the reduction of sodium borohydride(Na BH₄),and then the hydrophilic/hydrophobic bilayer nanofibrous membrane was prepared by continuous electrospinning technology,meanwhile,to improve the hydrophobicity of the support,a polydimethylsiloxane(PDMS)coating with low surface energy was formed on the PVDF nanofibers surface by one-step immersion method.The PDMS concentration was regulated to ensure the maximum anti-wetting property of the substrate membrane without clogging the PVDF nanofiber membrane pores,and in addition,the maximum addition of Ag NO₃ was explored to improve the photothermal conversion performance of the composite membrane.The optimized composite membrane can reach a surface temperature of95.3°C under simulated solar irradiation of 1 kw/m~2,while exhibiting excellent solar-driven membrane distillation(SD-DCMD)performance.The SD-DCMD desalination performance was tested by using 20?,3.5 wt%Na Cl as the feed solution and 20?,deionized water as the condensate for 10 h.The highest water flux of 1.2 kg/m~2·h and stable condensate-side conductivity were demonstrated,and the photothermal conversion efficiency reached 78%.(2)To overcome the problem of easy wetting of nanofiber membranes with large pores,we prepared nanotubular polypyrrole by the template method.According to the proportional relationship between fiber materials diameter and pore size,the large pore size between adjacent nanofibers can be replaced by the small pore size formed by PPy NTs with finer diameters.The PPy NTs were loaded onto the PVDF nanofibrous membrane by vacuum filtration,while low surface energy PDMS was selected as in-situ bonding between the PPy layer and the PVDF substrate membrane,resulting in a complete and stable dual-layer composite membrane with superhydrophobicity,high porosity and adjustable pore size.By optimizing the PPy NTs loading and PDMS concentration to maximize the structural stability and SD-DCMD performance of the composite membrane,the optimized composite membrane reached a water contact angle of156±1.9°,and its surface temperature reached 96°C under simulated sunlight irradiation of 1 kw/m~2,under SD-DCMD desalination performance testing at 20?with 3.5 wt%Na Cl as feed solution and20?deionized water as condensate for 10 h,the highest water flux of 1.3 kg/m~2·h and stable condensate-side conductivity were demonstrated,and the photothermal conversion efficiency reached 81.6%.