Fabrication Of Superhydrophilic/Underwater Superoleophobic Membrane And Oil-water Separation Performance

With the rapid development of economy,chemical activities and frequent oil spills have produced a huge amount of oily wastewater,causing serious environmental pollution and ecological damage,endangering human health and safety.Therefore,how to effectively separate oil/water mixtures obtaining clean water to alleviate environmental and ecological problems still faces enormous challenges.At present,the traditional method for oil-water separation is inefficient and easily causes secondary pollution.Although common membrane materials are widely used for oil-water separation,they are time-consuming,Contaminated,lack of selectivity and require pretreatment to limit their application.Therefore,it is necessary to develop new processes and materials with high separation efficiency,selectivity and stability for oil-water separation.Recently,inspired by biological phenomena such as “self-cleaning” of lotus leaves,a series of membrane materials with special wetting properties have been widely used for oil-water separation because of their different wetting properties for oil and water.Comparing with superhydrophobic/superlipophilic membrane materials,superhydrophilic/underwater superoleophobic membrane materials can effectively avoid oil pollution due to the under water superoleophobic properties.In this thesis,TiO₂ nanoparticles?TiO₂ NPs?coated superhydrophilic/underwater superoleophobic stainless steel membranes were prepared by one-step dip-coating and in-situ solidification method.The effects of the viscosity of precursir and concentration on contact angle and oil-water separation efficiency were investigated.According to the above results,TiO₂ nanowires?TiO₂ NWs?modified membrane materials were fabricated to improve the wettability.The effects of TiO₂ NWs concentration on surface roughness,wettability and oil-water separation efficiency were investigated.The results focused on the pore size and the synergistic effect of wettability affects the efficiency of oil-water separation.The main findings are as follows:?1?TiO₂ NPs/PVP-SS membrane with hydrophilic/underwater superoleophobic properties was successfully prepared by coating a mixed solution of TiO₂ NPs and polyvinylpyrrolidone?PVP?onto stainless steel mesh?SS?by one-step in-situ solidification method.SEM and EDS analysis confirmed that TiO₂ NPs and PVP were successfully applied to the stainless steel.The water contact angle?WCA?of the membrane is 0°,and the water completely wetting time is more than 100 s,the underwater oil contact angle is more than 160° and the oil has extremely low adhesion to the membrane,the oil-water separation efficiency is greater than 99.5%.When the concentration of TiO₂ NPs/PVP solution is 3%,which has the best wettability of membrane,and the least oilwater separation time,and the membrane flux is up to 8422.5 L·m^-2·h^-1.After 30 cycles of oil/water separation,the oil-water separation efficiency still more than 99.5%,indicating that the membrane has good stability and reusability.?2?TiO₂ NWs are synthesized by hydrothermal method from TiO₂ NPs.The phase structure changes from rutile to anatase,and Ti-O-Ti tensile vibration bond and Ti-O bond appear.The TiO₂ NWs/PVP mixture was successfully coated onto stainless steel membranes via dip-coating method.Then the membrane surface roughness was increased and a few of TiO₂ NWs aggregations are observed on the membrane.Continuously increasing the loading of TiO₂ NWs,the roughness of membranes increased and TiO₂ NWs interacts networks are formed on the single wire and formed nanoscale structure,which greatly improves the hydrophilicity of the membrane.After coating TiO₂ NWs,a superhydrophilic surface can be formed,the WCA is 0°,and the complete wetting time is at least 8 s.The water flux increases first and then decreases with the increment of loading TiO₂ NWs.The oil-water separation efficiency is more than 99.5%,and the flux is around 11000 L·m^-2·h^-1 at the loading of 1.5%.In addition,the separation efficiency of the membrane for 5 different oil-water mixtures was more than 99.5%.?3?A series of membrane materials with different wettability?superhydrophilic,hydrophilic,hydrophobic and superhydrophobic?were prepared by changing the amount and ratio of TiO₂ NWs,PVP and polydimethylsiloxane?PDMS?in the coating solution.Three stainless steel meshes with different pore sizes of 8,10 and 15 ?m were selected to study the synergistic effects of wettability and membrane pore size on oil-water separation performance.When the membrane pore size is different,the wettability is also different,and the membrane separation efficiency and membrane flux are also different.When the membrane is super-hydrophilic?WCA=0°?,the efficiency of separating water is close to 100%;when the membrane is hydrophilic?0°<WCA<60°?,the water separation efficiency decreases first with the increase of contact angle;when the membrane is hydrophobic?close to 90°-100°?,the oil-water separation effect is lost,and the oil-water mixture passes through;while the membrane is hydrophobic/superhydrophobic?WCA>120°?,the oil separation efficiency increases to 99.5%.For the same contact angle,the membrane flux decreases first and then increases with the increase of the pore size.By calculating the capillary force,the wettability of the membrane and the membrane pore size have synergistic effects on the membrane separation performance,and the calculated results are in agreement with the experimental results.