| Poly(vinylidene fluoride)(PVDF)is a membrane material with the advantages of high mechanical strength,good chemical resistance and thermal stability,excellent aging resistance and membrane-forming property,which was extensively employed in the preparation of ultrafiltration membrane.Nevertheless,the intrinsic hydrophobic native of PVDF materials results in severe membrane fouling and low water permeability when treating an aqueous solution containing protein or organic matters.Therefore,it is of great significance to prepare modified PVDF membrane with high permeate flux and excellent antifouling property by hydrophilic modification tecgniques,and it has a potential application in treatment of oily wastewater.In this study,the monomer glycerol dimethacrylate(GDMA)was grafted on PVDF backbone via atom transfer radical polymerization(ATRP)to synthesize an amphiphilic copolymer PVDF-g-GDMA.The resulting copolymer as a hydrophilic modifier was incorporated in PVDF matrix to fabricate PVDF/PVDF-g-GDMA modified membranes through non-solvent induce phase separation method(NIPS).The existence of grafted GDMA in PVDF powder was confirmed by fourier transform infrared spectroscopy(FTIR)spectroscopy and nuclear magnetic resonance hydrogen spectroscopy(1H NMR)spectra.The X-ray photoelectron spectroscopy(XPS)analysis verified also the presence of GDMA on the PVDF backbone.The surface chemical structure,morphology,element composition,charge and hydrophilicity of PVDF/PVDF-g-GDMA modified membranes were characterized by attenuated total reflectance fourier transform infrared spectroscopy(ATR-FTIR),scanning electron microscopy(SEM),XPS,Zeta potential measuring instrument and water contact angle tester,respectively.The influence of the copolymer additive PVDF-g-GDMA on the permeate flux,retention ratio and antifouling properties of the modified membrane was investigated by a cross-flow experimental setup.The modified membrane was applied to filtrate oily wastewater,and its separation efficiency of iol/water separation was discussed.The experimental results were as follows:(1)Chemical structure analysis of PVDF-g-GDMA: The peak at 1738 cm-1 ofIV FTIR spectra was attributed to the characteristic absorption O-C=O.In XPS analysis,the content of O and C increased obviously,while the content of F decreased,and binding energy at 289.0 e V was ascribed to C 1s core-level spectrum of O-C=O.All indicates that the monomeric GDMA had been successfully grafted to the PVDF backbone.(2)Chemical structure discussion of PVDF/PVDF-g-GDMA modified membrane:The peak at 1732 cm-1 of ATR-FTIR spectra was assigned to the characteristic absorption O-C=O.In the wide-scan XPS spectra of the modified membranes,as compared with the PVDF membrane two additional peaks with binding energies of287.5 e V and 289 e V are attributable to O-C and O-C=O signal can be observed,respectively.A homogeneous,dense and smooth surface was presented for all modified membranes on the top morphology,and an asymmetric porous structure,consisting of a compact skin layer,a much thicker finger-like porous sub-layer and a spongy-like bottom support was observed in cross-section SEM images.Moreover,the water contact angle of the modified membrane decreased from 79.7° to 60.8° with the increasing of the PVDF-g-GDMA content,suggesting that the hydrophilicity of the modified membrane has improved obviously.The zeta potential value declined with the increasing of copolymer PVDF-g-GDMA content at each testing p H,indicating that the formation of a more negatively charged membrane surface within p H the range of 5.0 – 9.0.(3)The permeability and antifouling test of PVDF/PVDF-g-GDMA modified membranes showed that the mean pore size,porosity and pure water flux of the modified membrane gradually increased as the increasing amount of copolymer PVDF-g-GDMA,while the retention ratio of BSA was above 90% with no significant change.It was found that the modified membrane with 4wt% PVDF-g-GDMA exhibited a noticeable pure water flux(144.62 L·m-2·h-1)and a remarkable flux recovery ratio(FRR)of 97.07% in comparison with the pristine PVDF membrane(57.71 L·m-2·h-1and 50.06%,respectively).The enhanced flux and FRR of modified membrane(M4)was attributed to an increase in hydrophilicity.All mean that the modified membranes possess improved water permeability and outstandingantifouling properties.(4)The experimental of iol/water separation of membrane M0 and M4: The experimental results showed that the modified membrane exhibited high-efficiency oil/water separation efficiency and outstanding antifouling properties in treatment of soybean synthetic-oily wastewater compared with PVDF membrane.Alone with the increasing of the concentration of soybean oil,the membrane fouling worsened continually,and the FRR gradually decreased.In addition,we have also investigated the flux recovery method of fouled membrane.The rinse with 0.4% Na OH solution obtained 99.87% of the FRR than the rinse with deionized water or acidic aqueous solution.The long-term stability of the treatment of oily wastewater has been discussed.The FRR of the modified membrane was 70% after 6 h filtration,while the PVDF membrane was only about 50%.Using 0.4% Na OH solution to wash the membrane surface about 30 min,the FRR of modified membrane was reach to 80%,which indicated that the modified membrane had better separation stability.(5)The membrane resistance analysis of PVDF/PVDF-g-GDMA modified membrane in oil/water separation illustrated that the total resistance(3.86×1012 m-1)of the modified membrane has declined by 62.38% than that(10.26×1012 m-1)of PVDF membrane.The filter cake resistance of pristine and modified membrane occupy20.86% and 15.80% of the total resistance,respectively.The above result clearly demonstrated that the antifouling properties of PVDF membranes had been effectively improved through incorporating the amphiphilic copolymer PVDF-g-GDMA. |
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