| Traditional methods to treat oily wastewater(such as centrifugation,solvent extraction and high-temperature decomposition)have the disadvantages of high cost,high energy consumption and easy to cause secondary pollution.Membrane separation technology is widely favored due to its advantages such as environmental friendliness,easy control of the separation process and low equipment cost.Polyvinylidene fluoride(PVDF),as a commonly used membrane separation material,has been widely used in scientific research and industrial production due to its excellent thermal stability,chemical stability,mechanical properties and membrane-forming properties.However,the inherent hydrophobicity of traditional polyvinylidene fluoride(PVDF)membrane makes it easy to be contaminated by oil during the separation process,resulting in the degradation of separation performance,increase of energy consumption and shortening of service life.Studies have shown that hydrophilic chemicals modification of PVDF membrane can effectively improve its separation performance and anti-fouling performance.In this paper,polyvinylidene fluoride graft polyacrylic acid(PVDF-g-PAA)copolymer was synthesized through free radical polymerization,and then optimized the membrane preparation process to study the effect of N,N-dimethyl acetamide(DMAc)content on the structure and performance of PVDF/PVDF-g-PAA blend membrane in the coagulation bath.The hydrophilicity of the membrane was improved by modifying hyperbranched polyamino polymer on the membrane surface,and the membrane had excellent separation and anti-fouling properties for acidic oily wastewater.Furthermore,hyperbranched polyamino polymers were modified on the membrane surface to explore their applications in oil-water separation and treatment of acidic oily wastewater.The design and the specific research contents of the separation membrane are as follows:The membrane-forming thermodynamics of PVDF/PVDF-g-PAA membrane was studied by the cloud point method.Compared with the water/ethanol coagulation bath,water/DMAc coagulation bath would inhibit the phase separation rate of PVDF/PVDF-g-PAA,making the system more stable.The delayed phase conversion technology was adopted to add different amounts of DMAc to the coagulation bath to regulate the crystallization nucleation rate of PVDF,reduce the membrane formation rate of PVDF,and optimize the membrane structure.The results showed that with the increase of DMAc content in the coagulation bath,the dense skin layer of the membrane gradually thinned,the finger-like macrovoids in the membrane cross-section gradually changed to the spongy structure.The membrane surface became rougher,the average pore diameter increased,and the pure water flux of the blending membrane increased.When the DMAc content reached 50%,the pure water flux of the membrane was the highest,reaching 1084±74 L·m-2·h-1.Meanwhile,the membrane had a high fracture strength(2.6±0.1MPa).The surface roughness and permeability of the membrane were greatly improved by adding epoxy silica nanoparticles into the membrane.Furthermore,hyperbranched polyethyleneimine(PEI)was fixed on the membrane surface by surface chemical grafting technology,which aims to realize the synergistic effect of micro-nano rough structure and high surface energy.The pure water flux of PEI grafted membrane was as high as 2358±36 L·m-2·h-1,and the separation efficiency for different kinds of acidic oil-in-water emulsion and oil/water mixture were all higher than 95%.After simple washing with ethanol,the membrane flux recovery rate was 90%,showing excellent oil/water separation performance.This work showed a promising prospect for the treatment of acidic oily wastewater. |
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