Structural Design And Electrochemically Enhanced Membrane Separation Performance Of Inorganic-Substrate Based Carbon Nanotube Composite Membranes For Water Treatment

Water pollution and water resource scarcity affect both human health and ecological environment.In order to mitigate water pollution and augment fresh water supply,membrane-based desalination technologies have been applied to separate and purify traditional wastewater resources such as seawater,brackish water and various wastewater such as high-salinity industrial wastewater.Among them,membrane distillation?MD?offers promising advantages,such as excellent separation efficiency,ultra-high salt enrichment and potential opportunities for resource and energy extraction,particularly for challenging desalination applications such as high salinity water treatment.MD can be highly competitive when using low-grade waste heat.However,existing MD membranes generally involve hydrophobic polymeric membranes and hydrophobically-modified inorganic membranes such as ceramic membrane,which have insufficient long-term stability,leading to undesirable issues such as wetting,fouling,and loss of flux and rejection under harsh and long-term operating environments.Compared with ceramic membrane,metal-based membranes such as stainless steel?SS?membranes have better mechanical strength and flexibility.Nevertheless,membrane fouling and corrosion are the key technical bottlenecks limiting its wide application.Therefore,in order to improve the performance of high salinity water treatment,it is of great academic value and practical significance to design and fabricate novel desalination membranes and study its anti-fouling and anti-corrosion mechanism.The key topics of this thesis are shown as follows:the alumina?Al₂O₃?hollow fiber membrane with long finger-like macro-void structure was obtained by optimizing the preparation conditions.Through in situ growth of carbon nanotubes?CNTs?,the hydrophobically-modified inorganic membranes have the higher water flux in the application of membrane distillation.In order to address the brittleness and poor processability of ceramic membranes,a novel method is proposed to prepare a superhydrophobic SS hollow fiber?SSHF?membrane through in situ growth of carbon nanotubes?CNTs?.The mechanically stronger flexible SS substrate featuring high flux was systematically designed and prepared by adjusting its structure.Then,without any external addition of a catalyst precursor,superhydrophobic electro-conductive CNTs network was in situ functionally constructed on SS substrates to finally form robust superhydrophobic SS-CNT membrane by employing a simple surface activation followed by self-catalysis chemical vapor deposition?CVD?.A micro-electrical field-coupling strategy coupled with the MD process is demonstrated to enhancing anti-fouling and anti-corrosion performance for the treatment of high-salinity water.Through systematic research,anti-fouling and anti-corrosion mechanism under micro-electric field strategy was proposed.The main research contents and research conclusions are listed as follows.Fabrication and performance of inorganic ceramic-carbon nanotube desalination membrane for water treatment.Alumina hollow fiber membrane with highly controllable structure consisting of adjustable sponge-like region and finger-like macro-voids could be prepared by via phase inversion and sintering method by adjusting the external coagulants with different ethanol contents.Effects of dry-wet spinning parameters on membrane performance including pore size,porosity,water flux and mechanical strength were comprehensively investigated.The results showed that solid-state loading of 50 wt.%,bore liquid flow rate of 20 mL·min^-1,ethanol content of 60 vol.%and air-gap distance of 10 cm were defined as the optimized dry-wet spinning parameters.CNTs network was in situ constructed on alumina substrates to finally form Al₂O₃-CNT membrane for high salinity wastewater treatment in direct contact membrane distillate process.The results showed that liquid entry pressure?LEP?of Al₂O₃-CNT membrane was 1.6 bar,which was enough to inhibit pore wetting in the long-term membrane desalination process.When the temperature difference was 55?,the water flux of Al₂O₃-CNT membrane after 7 h MD operation was about 30 L·m^-2·h^-11 with a salt rejection over 99.9%.Microstructure optimization of SS substrate with high strength,high flux and much better flexibility.The stainless steel?commercial 316 stainless steel powder as raw material?hollow fiber membrane with sandwich structure was fabricated by phase inversion and dry-sintering method.The effects of dry-wet spinning parameters and sintering temperature on membrane properties such as pore size,nitrogen permeance,water flux and mechanical strength were comprehensively studied.The results exhibited that solid-state loading of 75wt.%,bore liquid flow rate of 50 mL·min^-1,water as the external coagulant,air-gap distance of 10 cm were defined as the optimized dry-wet spinning parameters.When the sintering temperature was 1050?,the average pore size,nitrogen permeance,water flux,mechanical strength and electro-resistance of SS hollow fiber membrane were 1.6 cm,5056m³·m^-2·h^-1·bar^-1,38799 L·m^-2·h^-1·bar^-1,244.2±9.8 MPa and 35.9?·cm,respectively.The flexible SS hollow fiber membrane exhibited high strength and excellent permeability,outperforming most existing ceramic and SS membranes.In situ growth of superhydrophobic,superporous and electro-conductive carbon nanotube networks.Without external catalyst addition,CNT network was in situ constructed on SS substrates to form superhydrophobic SS-CNT membrane with superior superhydrophobic characteristic?water contact angle as high as¹71°?by taking advantage of a simple and cost-effective surface activation followed by self-catalysis chemical vapor deposition?CVD?.In situ growth mechanism of CNT and superhydrophobic properties of the composite membranes were investigated.The results showed that growth of CNTs to form SS-CNT membrane involved two stages:surface activation of catalytic sites via a simple oxidation and reduction process;in situ nucleation and growth of CNT via self-catalyzed CVD.The results reveal that a tip growth model mechanism of in situ growth of CNTs was dominant.Water treatment performance and anti-fouling and anti-corrosion mechanism of SS-CNT membrane.Desalination performance of SS-CNT membrane was further evaluated under different operating conditions such as operation time,feed concentration and temperature.The treatment performance of high salinity water containing humic acid was investigated under a micro-electrical field-coupling strategy coupled with the MD process.The mechanism of anti-fouling and anti-corrosion was proposed in this study.The results showed that SS-CNT membrane exhibited a high permeate flux 43.2 L·m^-2·h^-11 for simulated seawater at 75°C after 12 h operation,while salt rejection was maintained a high level?above99.9%?,outperforming most existing inorganic membranes.For high salinity water with organic(NaCl 70 g·L^-1,HA 30 mg·L^-1),the water flux of SS-CNT membrane was significantly reduced by 58.4%during the open circuit,due to serious membrane fouling and corrosion.Significantly improved water flux was observed at negative polarization,much higher?².4 times?than that under open circuit.The mechanistic insights show that anti-fouling was realized via enhanced electrostatic repulsion between the membrane surface and humic acid with the same negative charges,while anti-corrosion was improved via an electron supply mechanism.The fabrication method and a micro-electrical field-coupling strategy of SS-CNT membrane can be expected to extend toward fabrication of high-performance membranes derived from other metal substrates,as well as additional applications in water treatment such as antibiotics,pharmaceutical and personal care products,dyes and endocrine disrupting compounds in waters.