Design Of Photothermal Membranes With High Performance For Water Purificaion

Water scarcity is becoming a threat to sustainable development of human society and has become one of the most severe global challenges.Solar-driven water purification technologies can extract freshwater from seawater or sewage,by utilizing abundant and green solar energy.Solar-driven water purification technologies have shown great potential for alleviating water shortages,especially in remote and underdeveloped regions,due to their advantages of environmental friendliness,low cost and energy consumption.Solar-driven water purification technologies mainly include interfacial evaporation and solar-driven membrane distillation(SDMD).The interfacial evaporation,mainly depending on an interfacial evaporator,has a high rate of water vapor generation.However,the interfacial evaporation suffers from a low rate of water vapor condensation.In combination with interfacial evaporation and traditional membrane distillation process,solar-driven membrane distillation(SDMD)has been designed.Photothermal membranes play a key role in SDMD systems.The stability,anti-wetting property and salt resistance of photothermal membranes have been significantly improved,by introducing various photothermal materials and improving the preparation process of the membrane.In view of the fact the density of solar irradiation in most parts of earth is no more than 1 sun(1 kW m-2),the existing photothermal membranes exhibit low energy efficiency and distillate flux under this solar irradiation,which seriously impedes its practical application.Thus,it is urgent to improve the energy efficiency of photothermal membranes in SDMD under low solar irradiation.In addition,there are a large amount of volatile organic compounds(VOCs)in natural water and industrial wastewater.In the process of solar-driven water purification,VOCs and water evaporate simultaneously,and the evaporation rate of VOCs is usually higher than that of water.As a result,VOCs are enriched in the distillate,leading to secondary pollution.Therefore,it is of great significance to highly efficient removal of VOCs in water during solar-driven water purification.In recent years,through optimizing categories and structure of photothermal materials,interfacial evaporators have achieved huge progress in light-to-heat conversion property,solar energy efficiency and water evaporation rate,etc.Inspired by this,polyaniline(PANI)nanofibers are used as photothermal materials in this thesis.By controlling the morphology of PANI nanofibers on the membrane surface,the structure of the photothermal layer on the membrane surface was regulated,thus improving the energy utilization rate of the photothermal membrane in SDMD.Based on this,PANI nanofibers are combined with zeolitic imidazolate metal-organic framework(ZIF-8)to effectively remove VOCs in water.The details are as follows:The hydrophilic PANI nanofiber dispersion was synthesized by chemical oxidation polymerization in a dilute solution,and then filtered onto the surface of a hydrophobic polyvinylidene fluoride(PVDF)microfiltration membrane,to construct a photothermal layer consisting of PANI nanofiber net.The influence of PANI photothermal layer on the SDMD performance of the photothermal membrane has been investigated.It is found that there is an optimum value for the thickness of PANI photothermal layer on the surface of the membrane.When the thickness of PANI photothermal layer is lower than the optimum thickness,the increase in the thickness of PANI photothermal layer can enhance the light absorption of the membrane,its distillate flux and energy efficiency thus increase.When the thickness of PANI photothermal layer is higher than the optimum thickness,the increase of PANI photothermal layer thickness will lead to an increase in heat loss of PANI photothermal layer to the feed,instead of improving the light absorption of the membrane.Accordingly,the distillate flux and energy efficiency of the photothermal membrane are reduced.Apart from aforementioned photothermal layer composed of PANI nanofiber net,inspired by the anti-reflective nanostructure on the surface of moth’s eyes,vertically arranged PANI nanofiber layer was directly grown on the surface of a hydrophobic PVDF microfiltration membrane by chemical oxidation polymerization.Based on experiments and theoretical simulation,the vertical PANI nanofiber layer has a strong light-harvest ability,which can cause multiple reflection of incident light inside the PANI nanofiber layer until it is absorbed,thus offering the photothermal membrane a very high light absorption(95%).When the photothermal membrane is used for direct contact SDMD without pre-heating feed under one sun irradiation,it shows a distillate flux as high as 1.09 kg m-2 h-1 and corresponding energy efficiency up to 74.15%,which is higher than the photothermal membrane used in the same SDMD reported before.In order to achieve the efficient removal of VOCs in water,a polyaniline(PANI)nanofiber layer was coated on the surface of polyethersulfone microfiltration membrane,and then a thin layer of ZIF-8 was grown on the surface of PANI nanofibers layer,which was modified by a hydrophobic substance.As a result,a photothermal membrane with PANI/ZIF-8 bilayer structure was obtained.The minimum pore size of ZIF-8 is between that of water molecules and VOCs(such as aniline).Based on the pore size seiving effect,ZIF-8 allows water molecules to permeate and repel VOCs molecules.The experimental results show that the rejection of common VOCs in water by photothermal membrane is 99%,which is obviously superior to the existing interfacial evaporators reported in the literature.Moreover,the photothermal membrane can effectively treat the wastewater with high concentration(50-400 mg L-1)of VOCs,and shows excellent long-term stability.This study provides a new avenue for the efficient treatment of VOCs wastewater in the process of solar-driven water purification.