| Microfiltration is one of the main membrane separation technologies based on sieving principle to remove micron-level pollutants(such as particles,bacteria,etc.)in liquid and gas to achieve fluid purification.Microfiltration membrane is the core of microfiltration technology,and there is various microfiltration membrane at present.While the cellulose-based microfiltration membranes(Cellulose acetate,cellulose nitrate,mixed cellulose,etc.)take higher market share,because it is high hydrophilic,excellent permeability,good pollutants resistance,material safety and eco-friendly.However,most of the cellulose-based microfiltration membranes are spongy porosity structure which result in poor compressive strength.What's more,it is mostly made from natural plants so that it is easy to be attached by microorganisms and decrease the permeability Therefore,in order to broaden the application of cellulose-based microfiltration membrane,it ought to solve the problems of low mechanical strength and easy to be polluted by the microorganisms.This thesis takes CA-CN microfiltration membranes as the research object.The nanomaterial is easily doping with polymers to make the defect-free separation membranes.The silica nanoparticles(SiO2 NPs)easily form network structure to enhance the mechanical properties of organic materials,while the silver nanoparticles(Ag NPs)have antibacterial properties.It takes advantage of these properties to prepare the CA-CN microfiltration membranes with better pressure resistance and anti-microbial pollution properties correspondingly by solvent evaporation phase inversion.The research details are as follows:(1)Study on the influence of membrane preparation conditions on membrane structure and performance.In the process of solvent evaporative phase conversion,the ambient temperature and humidity conditions have a great influence on the morphology and structure of the membrane.By investigating the matching relationship between the pore structure and permeability of the membrane under different temperature and humidity conditions,the optimum temperature and humidity conditions were determined to prepare the CA-CN microfiltration membrane with better performance.The results show that when the ambient temperature increases,the channels of CA-CN filter become smaller(the bubble point increases but the water flux decreases),while when the humidity increases,the channels become larger,and the effect on the pore structure is weak.Through comparative study,the optimal phase conversion temperature and humidity conditions were determined:the temperature was 20-25?,and the humidity was 50-60%RH,which were taken as the ambient conditions for subsequent membrane preparation.In addition,growth promotion,microbial recovery and antibacterial circle were used to test the microbial inhibition ability of CA-CN microfiltration membrane,and it was found that CA-CN microfiltration membrane showed no microbial inhibition.(2)Study on pressure resistant CA-CN microfiltration membrane blended with SiO2 NPs.In view of the low mechanical strength of cellulose membrane,the pressure resistance of CA-CN microfiltration membrane is improved by blending SiO2 NPs,because SiO2 NPs can form a three-dimensional network structure to enhance the strength of organic materials.The results showed-that SiO2 NPs could be dispersed in acetone-based casting solution.The compressive strength of the filter membrane decreased slightly at the beginning with the addition of SiO2 NPs,and increased significantly after reaching 0.5%wt.In addition,the hydrophilicity of the membrane increased with the increase of SiO2 NPs content The flux drops first,then rises,and then falls below the pure CA-CN membrane.The content was highest at 0.1%wt.The bubble point was opposite to the water flux,and was the lowest when the SiO2 NPs content was 0.1%wt.The porosity firstly increased and then decreased,and reached the maximum when the content was 0.1%wt.The SEM figures of the membrane surface showed when 0.05-1%wt SiO2 NPs was added,the SiO2 NPs was closely bound to the membrane fiber.The pore diameter of the membrane surface increased,but the membrane surface structure remained uniform.Combined with the above experimental results,it is believed that the compression resistance of CA-CN microfiltration membrane with no less than 0.5%wt SiO2 NPs is significantly improved.(3)Study on CA-CN microfiltration membrane with Ag NPs doping.In order to solve the problem of poor resistance to biological pollution,CA-CN microfiltration membrane was prepared by doping Ag NPs with broad spectrum sterilization function.The results show that the CA-CN microfiltration membrane doped with Ag NPs has a certain resistance to biological contamination.a)Growth promotion studies showed that the filter membrane containing Ag NPs 0.2%wt showed mild inhibition on one of the five experimental microorganisms,while the filter membrane containing Ag NPs 0.4%WT showed inhibition effect on two of them;b)Microbial recovery studies showed that when Ag NPs was more than 0.1%wt,the microfiltration membrane showed inhibitory effects on four of the five experimental microorganisms;c)Antibacterial circle study found that the filter membrane containing Ag NPs showed no inhibitory ability on microorganisms outside the filter membrane range;d)On the surface antibacterial ability,it was found that although the addition of Ag NPs could not inhibit the microorganisms outside the filter membrane,the filter membrane itself obtained antibacterial effects,which increased with the increase of Ag NPs content.In addition,the dispersion of Ag NPs in the casting solution did not significantly affect the pore structure on the microfiltration membrane surface.Therefore,doping Ag NPs can effectively improve the membrane's resistance to microbial contamination on the premise of ensuring better permeability.To sum up,this paper solves the problems of weak compression strength and poor biological contamination resistance of CA-CN cellulose membrane by doping functional nanoparticles,and provides feasible ideas and methods for the long-term operation of CA-CN membrane in the actual separation process. |
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