Preparation And Desalination Application Of Polyvinylidene Fluoride Photothermal Nanofiber Membrane

Membrane distillation technology plays an increasingly important role in the desalination field.However,the traditional membrane distillation technology is affected by the phenomenon of temperature polarization,resulting in a temperature gradient between bulk fluid and membrane surface,which brings a decrease on membrane's permeation flux.In this paper,nano-doped tin dioxide(ATO)with photothermal function was introduced into polyvinylidene fluoride(PVDF)to prepare photothermal membrane,which was used to reduce temperature polarization during the process of vacuum membrane distillation(VMD).On this basis,the membrane with hydrophilic/hydrophobic bilayer structure was prepared and used for the process of solar seawater desalination.Firstly,PVDF/ATO nano-fiber membranes were prepared by electrospinning with polyvinylidene fluoride(PVDF)as the membrane-forming polymer and doped with ATO particles,and then heat-press process was applied to optimize the pore structure.The effect of membrane structure and properties due to the heat-pressing temperature and the loading amount of ATO were investigated.The results showed that the fiber layer was denser,the pore size distribution was more uniform,the mechanical strength was increased by 70%relative to the primary membrane,and the contact angle was maintained above 1300°after the membrane heating at 170 ? for 1 h.Moreover,the fiber membrane had a better overall performance when the ATO loading was 3 wt%.Secondly,the photothermal properties of the membrane were characterized and the VMD process was tested.The results showed that the surface temperature of PVDF/ATO membrane was 42 ? higher than the pure PVDF membrane under the same direct infrared irradiation conditions,the permeation flux of PVDF/ATO membrane was 26 kg-m-2·h-1,which is 2.6 times higher than that of pure PVDF membrane,and the salt rejection maintained above 99%.The PVDF/ATO membrane had a light absorption rate of over 92%in the wavelength range of 300-2500 nm,indicating a good photothermal properties.Then,the hydrophilic/hydrophobic double-layer PVDF/PAN/ATO nanofiber membranes(PPAMs)were prepared by electrospinning.The effect of the PPAMs'solar desalination performance due to the ATO loading,PAN layer and light intensity were investigated.The results showed that the membrane exhibited the best photothermal performance when the ATO loading was 5 wt%,the seawater evaporation rate of the membrane was 0.93 kg·m-2·h-1 and the energy conversion efficiency was 56.7%under the light intensity of 1 kW·m-2·h-1.The evaporation rate of PPAMs is increased by 0.2 kg-m-2·h-1 compared with PVDF/ATO membrane;In seawater evaporation test,the surface temperature of PPAM s was increased from 18.5 ? to 37.1 ?,12 ? higher than pure water,and the evaporation flux increased from 0.27 kg·m-2·h-1 to 0.93 kg·m-2·h-1;and the light conversion efficiency of the membrane increases from 56.7%to 62%when the light intensity is in the range of 1?3 kW·m-2,.Finally,in the cycle test of the membrane,after 14 days of operation or after several cleaning processes,the evaporation rate of the PPAMs remained stable,showing good cycle stability.