| Oily wastewater is a serious environmental pollutant. Every year, large amount of oily wastewater is produced around the world. Oily wastewater has high COD value, and if released to the environment without proper treatment, it will seriously pollute the environment and damage the ecosystem. Compared with conventional methods, membrane technology has many advantages, such as no addition of reagents is needed, concentrated solution is easy to recycle, the composition of oil has little effect on the separation process, and low cost. Treating oily wastewater by membrane technology requires the membrane to be either highly hydrophilic or highly hydrophobic, but conventional hydrophobic membrane materials have some disadvantages such as low water flux and easy fouling, and conventional hydrophilic membrane materials also have some problems like poor membrane-forming properties or instability in water. In this study, in order to get membranes with high mechanical strength and good membrane-forming properties as well as good oil/water separation performances,polyvinylidene fluoride (PVDF) material was hydrophilic modified. The contents of this study can be divided into four parts.(1) PVDF material was modified by ozone-induced graft polymerization of acrylic acid (AAc) side chains. PVDF solution in N-methyl pyrrolidone (NMP) was first treated by ozone to introduce active peroxide groups, and then grafted with PAAc side chains via free radical polymerization of AAc monomers. The effects of modification conditions on the grafting degree were studied. The results showed that, with longer ozone treatment time, the grafting degree increased; the initiating agent concentration had an optimal value, and lower grafting degrees were get either below or above this value; the graft polymerization proceeded well under0℃, and with higher temperature, grafting degree decreased because of side reaction; with higher monomer amount, grafting degree increased, but when the monomer amount was too high, the by-product interrupted the following treatments. The best modification conditions were as follow:45min ozone treatment time,4:1mass ratio of AAc and PVDF,0℃polymerization temperature,5h reaction time. PVDF-g-AAc prepared under these conditions had a grafting degree of65.3%(2) For oily wastewater produced in petroleum industry, low concentration suspended and dissolved hydrocarbon oil is left after initial treatment by conventional methods such as gravity or flocculence treatments, so further treatment is required. Flat membranes for this purpose were prepared using PVDF-g-AAc material, and the effects of membrane casting conditions on the surface hydrophilicity and the pore size of the membranes were studied. The results showed that, NMP presence in the coagulation bath lowered surface hydrophilicity as well as pore size, but when the NMP content reached40%, pore size increased due to the structure change of the membrane; changing casting solution concentration or air exposure time lead to the change of pore size without affecting the surface hydrophilicity; with increased casting solution concentration, pore size decreased, and with increased air exposure time, pore size decreased. Then the effect of pore size and surface hydrophilicity on the oil/water separation performances was studied. The results showed that, for membranes with the same pore size, higher hydrophilicity lead to much higher flux; for the same retention value, membrane with higher hydrophilicity reached this value at a larger pore size. These phenomena can be explained by the membrane phase separation mechanism.(3) Oil/water emulsion with high oil concentration is often produced in industries, and treating this kind of oily wastewater requires the concentration of oil in order to recycle it. PVDF-g-AAc flat membranes were used for this kind of oil emulsion separation. The effects of membrane parameters were studied. The results showed that, at the initial stage of the separation process, the main mechanism was demulsification caused the adsorption of surfactant on the membrane. Membrane with higher hydrophilicity had better adsorption capability, so more floated oil was produced by demulsification, and the saturation adsorption was reached faster; on the other hand, pore size had no apparent effect on this process. When the separation process was carried out for1-2h, saturation adsorption of the surfactant was reached, and the main separation mechanism shifted to screening mechanism, where larger pores lead to lower retention and higher flux. The membrane with the best performances concentrated kerosene emulsion with an original concentration of108g·L-1into a concentration of390.5g·L-1with a flux of114L·m-2·h-1under the transmembrane pressure of0.1MPa, and the kerosene loss was15%.(4) Hollow fiber is an important type of membrane in industrial applications because its high membrane area per unit volume.Surface modification is relatively difficult for hollow fiber membranes, while bulk modification used in this study is suitable for the preparation of modified hollow fiber membranes. Using PVDF-g-AAc material, hollow fiber membranes were prepared by dry-wet spinning method. The effects of membrane casting conditions were studied. The hollow fiber membranes showed good performances, with a pure water flux of over400L·m-2·h-1, a retention against50mg·L-1oil suspension of over95%, high retention against all oil concentrations within5～100mg·L-1, good anti-fouling properties, and mechanical stength enough for use. |
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