Study On Surface Chemncal Modification Of Poly(Vinylidene Fluoride) UF Membrane And Oily Wasterwater Treatment

In the application involving oily wastewater, the inherent hydrophobic characteristic of PVDF often causes serious membrane fouling and a rapid decline of permeation flux. In this thesis, Fenton reagent was carried out to modify PVDF membranes, while maintaining excellent physical and chemical properties of materials, but also the introduction of hydrophilic groups. In the treatment of oily emulsions, modified membranes exhibited good anti-fouling properties in reducing the membrane fouling, water flux was increased and the system could maintain stable operation for a long time.The experiments were divided into three parts: (1) Different alkali treatment system on the impact of HF removal, the optimal reaction conditions of Fenton treatment and modified mechanism were explored. (2)Oily wastewater was treated by modified membrane through homemade internal pressure operated module and flux testing device. The optimal operating conditions were studied in the plot experments. (3) Hydraulic washing and sodium hydroxide, hydrochloric acid and surfactant as cleaning agents on the effect of membrane flux recovery rate have been studied in the membrane fouling.The following conclusions were obtained.(1)The research of PVDF membrane modification①It was found that 4M NaOH treatment system could obtain higher removal rate of HF and maintain excellent physical and chemical properties of PVDF membrane compared with other alkali treatment systems.②The optimal modification condition was confirmed,which was as follows: the pH was about 3, 20mmol/L of Fe²⁺, 400mmol / L of H₂O₂, 30min of reaction time at 30℃. Contact angle and water absorbency showed that the hydrophilicity of modified membrane was enhanced while water flux was increased by 20%. The thermal stability of modified membrane was less declined according to the result of TG-DTA experiments. The pore structure was observed by scanning electron microscopy (SEM) test,which showed modification only occurred in the membrane surface.③Carbonyl, -C=C- and other unsaturated groups were formed after alkali treatment as well as hydroxyl and carboxyl were introduced on the surface of modified membrane after Fenton electrophilic addition and oxidation reactions on unsaturated groups by reflection of Fourier transform infrared (FT-IR) tests. The pure water flux, hydrophilicity and antifouling properties were improved much after modification.(2) Small scale experiments about treatment of oily wastewater by ultrafiltration①The results indicated that modified PVDF membrane was strong applicable for oil/water emulsions. The flux of modified membrane declined only 9.3% after 1h than that of unmodified membrane, while polysulfoneand polyvinyl chloride membranes were quickly fouled.②It was said that the rejection ratio was above 95% at different concentrations while the oil concentration of efflux was 9.7mg/L, reaching the water letting standard and the influent concentration had no significant effect on removal. The water flux decreased with an increase in the concentration of feed. The results also showed that the optimum operating conditions were a crossflow velocity of 2 m·s^-1, operation pressures at 0.1MPa, a feed temperature between 20²5°C in the 365mg/L of feed concentration, under which the perfect flux could be gained and stabilized after running for 4-6h.(3) Membrane fouling and cleaning①In the study of membrane fouling, it was found that membrane was contaminated while contaminants deposited on the membrane surface and gradually formed a gel layer as well as water flux decreased. According to the result of hydraulic washing for 1h on the contaminated module running for 4-6h, the flux recovery was only 75% and the amount of water for cleaning was far greater than permeate. This demonstrated that only loose layer of pollution was removed through hydraulic washing. In order to completely recover water flux, further chemical cleaning was required.②When HCl was used as cleaning agent, the flux recovery rate was 80.6%, far less than 95.3% with NaOH as cleaning agent. When adopted 0.1M SDS as cleaning agent, surfactant pass through membrane, which indicated that the effluent quality was deteriorated and oil removal was reduced. The main pollutants were roughly determined to be oily substance and pollutants were basically removed after chemical cleaning by FT-IR test. The results of SEM study confirmed that there was contamination existed on the membrane surface instead of pore. The membrane surface was smooth after chemical cleaning. Comparing the flux recovery ratio of different cleaning agents and the results of SEM tests, it was found that the mixture of SDS and NaOH as cleaning agent was effective.