Preparation And Application Research Of TiO₂+Al₂O₃/PVDF Ultrafiltration Membrane

Polyvinylidene fluoride （PVDF） is an excellent organic polymer which has beenfavoured for some performances, such as heat resistance, radiation resistance, chemicalstability, fouling resistance and easy membrane forming characteristics. However, thePVDF membrane has a strong hydrophobic due to the lower surface energy of PVDFmembrane, which makes it easy to be polluted by contaminants in the membraneseparation process, and which cause water flux of the membrane greatly reduced.Modification of membrane materials is the fundamental way to solve the membranefouling problem.In order to improve the hydrophilic and anti-pollution capacity of PVDF ultrafiltrationmembranes in the waste water treatment process, TiO₂+Al₂O₃/PVDFultrafiltrationmembranes modified by inorganic nano-sized particles were prepared by phase inversionprocess. Nanoparticles modified ultrafiltration membrane’s pure water flux and rejectionrate were investigated using cup ultrafiltration apparatus; the contact angles of themembrane with pure water were measured by a contact angle meter for characterizing thehydrophilicity changes of the membrane; the membranes strength were measured by thestrength tester; filtration rate method and liquid-liquid displacement method were usedto investigate modified membrane’s pore size and pore size distribution. Theexperimental results show that the modified membrane’s （PVDF1311） pure water flux hasbeen increased79.5%, from93.28L/（m²h） increase to167.43L/（m²h）; mechanicalstrength of the modified membranes have increase, the largest increase is41.6%; purewater contact angle of modified membrane PVDF1311dropped from78.68°to50.54°, themodified membranes hydrophilic have been significantly improved. Thereby themembrane fouling resistance has been increased. The average pore diameter of themodified membrane is greater than that of organic membrane; pore size distributionshows certain regularity. Thus, the membranes water flux, and mechanical strength andanti-pollution performance have been improved by nanoparticles, which makemembrane’s average pore increase and have a more uniform pore size distribution.Scanning electron microscope （SEM） and atomic force microscopy （AFM） have beenused to analyze the membranes surface cross-section morphology and membrane internalpore structure. Fourier transform infrared spectroscopy （FT-IR） has been used to test theeffect of nano-particles on PVDF crystal during the process of membrane formation. Theresults show that the modified membrane’s average surface roughness has been increased,which result in the ultrafiltration membrane’s filtration area increase, and therebyincrease the modified membrane pure water flux. The modified membrane has the samestructure as the organic membrane, that consistued by parallel dense cortex and finger-like pore structure; But the modified membrane has a relatively smooth the bore wallsurface, which reduces the contaminant clogging the membrane module during operation, thereby improves the stain resistance of the modified membrane; The nano-particlesadded can induce the casting solution stress changes in the type of the modified filmPVDF crystallization, increased the number of α and β polymorph. No new absorptionpeaks generated in the IR spectrum of the modified membrane, confirming that theorganic polymer membrane modified by the inorganic nanoparticles is physical blend.The turbidimetric titration has been used to determine the cloud point of differentcasting solution system, and draw ternary phase diagrams for the study of thethermodynamic effects of nanoparticles on PVDF ultrafiltration membrane formingprocess; UV spectrophotometer has been used to measure light transmittance of thesystem of the membrane casting solution in the gel process, and draw transmittancedecline curve for the study of the dynamics of nanoparticles on the PVDF membraneforming process. The results show that the ability of non-solvent accommodates in thecasting solution is decreased during the nano-particles addition and makes membraneformation in smaller non-solvent concentration. The nano-particles in the casting solutioncan increase two-way transfer resistance of solvent and non-solvent and shorten themembrane curing time that casting solution is immersed in the the gel bath moment tofilm curing time.The tubular membrane modules of organic-inorganic modified membrane and organicmembrane were used to treat the oil filed waste water. The conditions of membranefouling were observed by SEM and AFM. Contaminants were analyzed with the unitingof gas chromatography and mass spectrum （GC/MS）. In order to know the flux recoveryconditions of fouled membranes after washing, fouled membranes were cleanedchemically using different cleaning agents. The results show that the pollutant removalrate of modified membrane is better, the major pollutants in treated water, oil content isless than0.7mg/L, turbidity and suspended solids are less than2.0NTU and0.5mg/Lrespectively, the COD and TOC removal rates are80-90%and95%respectively. Themain materials which make membranes fouled are crude oil and inorganic minerals. Andthe kinds and amounts of contaminants adsorbed on modified membrane are less than thatof unmodified one. The favorable flux recovery ratio can be obtained through washingwith alkaline medicaments and flux recovery ratio of modified membrane can reach96.3%after chemical washing with1%content of OP-10surfactant solution （pH=10）.Field model is a math model which is used widely in membrane fouling ofultrafiltration process. Ideal Field model includes cake filtration model, completeblocking model and intermediate blocking model. For different test running conditions,membranes fouling adapts to different models. Three kinds of fouling mechanisms ofField model were chose and analyzed with the test data of oily waste water treatment inspot of modified membrane. It was found that complete blocking model was the mostsuitable membrane fouling model for the test conditions. And we established the mathequations based on the model, which adapted to the flux changing with time for theultrafiltration process of oily waste water treatment. Flux values calculated by the equations were coincident with the values measured by test. The trend of the fluxchanging with time could be forecasted well with the established equations. And it wouldprovide a reference value for the ultrafiltration process of oily waste water treatment.