Study On Interface Modification Of Ultrafiltration Membrane And Its Anti-Oil Pollution Characteristics

Treatment of oily wastewater has always been a challenge in the field of water treatment,and ultrafiltration technology is widely used in the treatment of oily wastewater because of its simplicity,efficiency and low cost.Polyvinylidene fluoride（PVDF）is widely used in drinking water and wastewater treatment because of its high mechanical strength,high stability and oxidation resistance.However,the membrane contamination caused by oily substances limits the engineering application of ultrafiltration.In this paper,we first introduced different contents of Fe₃O₄ nanoparticles into PVDF ultrafiltration membranes through co-blending modification,and then immersed the prepared modified membranes in aqueous solution of P（VP-VTES）copolymer obtained by the reaction of N-vinyl-2-pyrrolidone（NVP）and vinyltriethoxysilane（VTES）to further cross-link hydrophilic polymers,and prepared a series of organic-inorganic ultrafiltration membranes by submerged precipitation phase transformation method.Then a polydopamine/polyethyleneimine（PDA/PEI）coating was deposited on the surface of PVDF ultrafiltration membranes soaked with aqueous P（VP-VTES）copolymer solution,followed by the construction of an array ofβ-Fe OOH mineralized layers on this coating,and PVDF ultrafiltration membranes with different modification degrees were obtained according to the modification steps.Finally,the modified ultrafiltration membranes were tested and analyzed by the characterization and evaluation methods of ultrafiltration membranes,and finally,the mechanism of photocatalytic removal of oil and grease contaminants by theβ-Fe OOH mineralized layer constructed on the surface of ultrafiltration membranes was investigated.It was found that:the best performance of co-blended Fe₃O₄ nanoparticles was achieved at 5%,and higher content of nanoparticles would form agglomerates causing the degradation of ultrafiltration membrane performance;PPS-PVDF@5%Fe₃O₄ ultrafiltration membrane would preferentially adsorb hydrophilic component N-vinyl-2-pyrrolidone（NVP）when immersed in P（VP-VTES）copolymer aqueous solution,and the thickness of the formed hydrated layer was the largest and the best stability;PPS-PVDF@5%Fe₃O₄ ultrafiltration membrane has the highest pure water flux of 235.12L/（m²·h）and the retention rate of 98%when separating bovine serum protein（BSA）and soybean oil contaminant solution,which has better ultrafiltration performance compared with other modified ultrafiltration membranes;When separating simulated pollutants,it was found that the co-blended modified ultrafiltration membrane had the best separation performance for humic acid pollutants,followed by oil and grease pollutants,and poorer separation efficiency for protein pollutants.The PVDF@PDA/PEI-β-Fe OOH ultrafiltration membrane obtained by surface modification has an underwater oil adhesion force of only 1.3μN,which is better than that of the unmodified PPS-PVDF base membrane and the preliminary modified PVDF@PDA/PEI ultrafiltration membrane,it possesses better resistance to oil adhesion when separating grease-based contaminant solutions.After being polluted by oils and fats,the pure water flux of PVDF@PDA/PEI-β-Fe OOH ultrafiltration membrane can be restored to 215.32L/（m²·h）in the presence of light and trace H₂O₂,and the flux attenuation is within 10%during long-term circulating filtration,which can keep the permeation flux relatively stable.After the photocatalytic cleaning,the surface pore size of the PVDF@PDA/PEI-β-Fe OOH ultrafiltration membrane contaminated by oils and fats can be restored from 0.31μm to 0.35μm,and the water contact angle can be restored from 135°to 124°.The oil adhesion was restored from 6μN to4μN,indicating that the interface characteristics of the ultrafiltration membrane can be effectively restored after exposure to light.PVDF@PDA/PEI-β-Fe OOH ultrafiltration membrane can effectively remove oil and grease pollutants adsorbed on the surface of ultrafiltration membrane through the active hydroxyl radicals generated by photocatalysis,and has good photocatalytic self-cleaning properties;The polydopamine coating under the photocatalytic coating can also act as a free radical scavenger to protect the ultrafiltration membrane substrate from damage caused by the active hydroxyl radicals generated by photocatalysis,while acting as an adhesion intermediate layer.The flux recovery rate of PPS-PVDF@Fe₃O₄ ultrafiltration membrane was only about60%during the long-term cycling of the oil and grease contaminant solution and would continue to decrease as the number of cycles increased,but the flux recovery rate of PVDF@PDA/PEI-β-Fe OOH ultrafiltration membrane was up to 75%and increased to about80%after light exposure,indicating that PVDF@PDA/PEI-β-Fe OOH ultrafiltration membranes have better resistance to oil and grease contamination,and the resistance to oil and grease contamination of ultrafiltration membranes can be further enhanced by photocatalytic cleaning.