Studies On Preparation And Separation Performance Of Superhydrophilic-Underwater Superoleophobic Mineralized Membarnes

At present,the pollution of water resources is a very serious global problem.Realizing the efficient separation of oil-water mixture is an useful strategy to alleviate the global problem of water scarcity and reach the healthy development of ecological environment.Membrane separation technology stands out among various separation technologies due to its high efficiency and simplicity of operation,which has attracted the attention of many researchers.However,traditional separation membranes have some drawbacks,such as the use of chemical reagents that may further pollute the environment,the specific reaction conditions such as high temperature and pressure and specialized equipment,and higher costs,and the membranes are easily to be polluted.The preparation of super-impregnated oil-water separation membranes by biomimetic mineralzation under mild reaction conditions at ambient temperature and pressure has received increasing attention.However,the adjustment of the structure,improvement of the stability,antipollution and antibacterial of the biomimetic mineralzed membranes are necessary to be solved.Accordingly,in this paper,calcium carbonate（CaCO₃）mineralized membranes with specific surface morphology were firstly prepared on stainless steel mesh（SSM）by biomimetic mineralization technique.To impart functionality to the oil-water separation membrane,a titanium dioxide（TiO₂）mineralized membrane with photocatalytic properties was prepared on polytetrafluoroethylene（PTFE）membrane,while Ag nanoparticles were introduced to further impart antibacterial properties.The factors influencing the microscopic morphology and surface wettability of the two prepared mineralized membranes were investigated,and their oil-water separation properties,photocatalytic properties and antibacterial properties were systematically studied.The main studies are as follows.（1）To increase the adhesion and stability of the mineralized layer on SSM,SSM-CS membrane was prepared by using positively charged chitosan（CS）cross-linked with glutaraldehyde and attached to SSM.Then,the nucleation and growth of CaCO₃ on the surface of SSM-CS were regulated by the"carbon dioxide diffusion method"through the charge-negative macromolecule polyacrylic acid（PAA）to produce a shape-controlled SSM-CS-CaCO₃ membrane.The effect of glutaraldehyde addition on the membrane-forming property of CS on SSM was investigated,and the results showed that smooth and tight CS membrane was attached on SSM when the glutaraldehyde addition was 0.3 m L.The effects of PAA addition and mineralization time on the surface morphology and properties of CaCO₃mineralized layer were also investigated in detail.The results showed that the morphology of the mineralized membrane could be adjusted by additional PAA content,membrane-like CaCO₃ and hemispherical CaCO₃ membranes could be obtained by change the PAA from 0.1to 0.3 m L and mineralization time for 16 and 24 h.Both of the membranes had super hydrophilic-submerged super oleophobic performance,with separation efficiencies of more than 98%for the oil-water mixtures and separation fluxes of above 3.18×10⁵ L·m^-2·h^-1.The membranes ccould maintain stable separation efficiencies and fluxes even repeated 20 cycles.The mechanical performance of the membrane with the membrane-like CaCO₃ morphology was found to be better than others through wear cycles and water impact experiments.The submerged oil contact angle of that membrane was maintained above 152°within 20 wear cycles and under continuous 48 h water impact.In addition,this method is also adapted to SSM with different pore sizes,such as the membrane CaCO₃ mineralized membrane prepared on 1800 mesh SSM,which has more than 97%separation efficiency for stable emulsions formed by various organic solvents,with separation fluxes above 2.5×10³ L·m^-2·h^-1.Therefore,the CaCO₃ mineralized membrane made on SSM could be adjusted by additional of a cation polymer and change the concentration of an anion polymer.The obtained membrane is able to separate various oil-water mixtures and emulsions with high efficiency and high mechanical stability,which has some prospects for application in the field of oil-water separation.（2）To address the situation where separation membranes are adhered by oil after repeated use,and where the pore blockage flux is reduced due to the need to separate oily sewage containing bacteria in some applications.PTFE-TiO₂-Ag membranes were prepared by the"alternate immersion method"with photocatalytic properties on PTFE membranes,while silver（Ag）nanoparticles were grown on TiO₂ mineralized membranes by the"silver nitrate reduction method"to give photocatalytic and antibacterial properties to oil-water separation membranes.The relationship between the number of alternate immersions and the surface morphology of separation membranes,oil-water separation performance,photocatalytic performance,and antibacterial performance were investigated.The results showed that the underwater oil contact angle of the mineralized membrane reached 154°when the number of alternate immersions was 3,and the highest water flux at this time was 6238 L·m^-2·h^-1.The PTFE-TiO₂-Ag membranes were prepared with 98%separation efficiency for both oil-water mixture and oil-water emulsion;the separation flux was 6238 L·m^-2·h^-1 for oil-water mixture and 663 L·m^-2·h^-1 for water-in-hexane emulsion.After contaminating the PTFE-TiO₂-Ag membrane with stearic acid as a contaminant model,the superhydrophilc property of the membrane surface was lost and the water flux was drastically reduced,while after 30 min of photocatalysis,its water contact angle was restored to 0°and the water flux was restored by more than 80%.In addition to degrading the contaminants on the membrane,it was immersed in 5 mg/L rhodamine B solution,and rhodamine B was almost completely degraded after 1 h of photocatalysis.E.coli and S.aureus were used as bacterial models to test their antimicrobial properties.The presence of Ag nanoparticles gave the PTFE-TiO₂-Ag membrane strong antimicrobial properties and almost no colonies were observed on the petri dishes made by mixed incubation.By cycling the separation of oil-water mixture containing bacteria,it was demonstrated that the mineralized membrane with antimicrobial property has a durable and stable high throughput even when used for the separation of bacteria-containing oil-water mixture,indicating that the PTFE-TiO₂-Ag membrane with photocatalytic and antimicrobial properties is promising for the application in the field of oil-water separation.