Studies On Preparation And Performance Of Super Hydrophilic PVDF Fibrous Composite Membranes

With the increasing development of textile,petroleum,chemical and other industries,the amount of industrial wastewater is increasing,which makes the problem of water pollution increasingly prominent.The discharge of oily wastewater and dyeing wastewater is very large.Through long-term exploration and research,it is found that membrane separation technology is one of the most scientific methods to treat these wastewater.Among many membrane materials,polyvinylidene fluoride（PVDF）stands out with its excellent corrosion resistance,wear resistance,thermal stability and other characteristics.However,its biggest disadvantage is poor anti-pollution performance,which leads to PVDF membrane being easily polluted when treating wastewater.Therefore,it is of practical significance to find effective modification methods to improve the anti-fouling performance of PVDF membrane.In addition,photocatalysis technology can decompose toxic dyes into harmless substances,which is an efficient and environmentally friendly treatment method for printing and dyeing wastewater,but it also has some drawbacks,such as the difficulty in recycling the catalyst.Under this context,the combination of photocatalysis technology and membrane separation technology to prepare a membrane with dual functions of separation and dye degradation has practical application value.（1）For the separation membrane used to treat oil-water mixture wastewater,this study first blended PVDF and ethylene-co-maleic anhydride copolymer（PEMA）,prepared fiber composite membrane by electrospinning,and then acidified to hydrolyze the maleic anhydride bond to carboxyl,and then used the coordination effect of carboxyl and Ca²⁺to grow micro-nano CaCO₃particles on the membrane surface,to prepare super-hydrophilic PVDF/CaCO₃fiber composite membrane.The test results showed that with the increase of PEMA content,the water contact angle of the composite membrane declined firstly and then increased,and the water flux increased firstly and then declined.When the mass ratio of PEMA to PVDF was 1:4,the hydrophilic property of the carboxyl-rich PVDF fiber composite membrane was the best,in which the water contact angle of the membrane decreased from 131.12°to61.1°,and the pure water flux rose from 0 to 87.75 L·m^-2·h^-1under gravity.After the growth of CaCO₃particles,the hydrophilicity of the membrane was improved again,among which the PVDF/CaCO₃fiber composite membrane after three cycles of mineralization had the best hydrophilicity,the water contact angle was 18.55°,and the pure water flux was 391.96 L·m^-2·h^-1.In addition,the rejection rate of the membrane for the five mixtures of n-hexane/water,toluene/water,petroleum ether/water,soybean oil/water,and dichloroethane/water was more than 98.5%,and the separation efficiency and flux of the membrane were basically unchanged after 10 cycles of filtration experiments.At the same time,the composite membrane had good washability.（2）For oil-water emulsion and dyeing wastewater,TiO₂nanoparticles were grown on the surface of carboxyl-rich PVDF fiber composite membrane by hydrothermal method using carboxyl group induction strategy,and PVDF/TiO₂fiber composite membrane with dual functions of oil-water separation and dye degradation was prepared.The influence of hydrothermal time on the surface morphology,hydrophilicity and photocatalytic performance of the membrane was investigated.The experimental results indicated that with the increase of hydrothermal reaction time,the water contact angle of the composite membrane declined firstly and then increased,and the water flux and dye removal increased firstly and then declined.After 2 h hydrothermal reaction,the TiO₂particles on the membrane surface were the most uniform,the membrane had the best hydrophilicity and photocatalytic performance.Its water contact angle decreased from 61.16°to 15.63°,the underwater oil contact angle was up to 156.65°,and the pure water flux at 0.1 bar transmembrane pressure was 5547.57 L·m^-2·h^-1.When separating soybean oil/water emulsion,the oil-water permeation flux and retention rate reached 678.64 L·m^-2·h^-1and 98.95%respectively,and the flux recovery rate was 99.19%,and the separation efficiency could still reach over 98%after 5 cycles;The XRD spectrum showed that the TiO₂particles on the PVDF/TiO₂fiber composite membrane were anatase.The removal rate of the membrane after the degradation of the cationic rhodamine B dye（RhB,p H=7）for 7 h was 97%,and the removal rate of the anionic acid green 25 dye（AG-25,p H=3）for 12h was 90.2%.After 5 cycles of experiments,the membrane could still remove 96%of RhB and 88%of AG-25.