Hydrophilic Modification And Biofouling Behavior Of Organic Membranes Grafted By Polydopamine

Membrane fouling is the main obstacle to the further development of membrane technology in the wastewater treatment. Improving the membrane hydrophilicity is considered as an effective way to reduce the membrane fouling. With the characteristics of dopamine such as self-assemble and chemical activity, polydopamine(PDOPA) deposition was used to improve the surface hydrophilicity of the PVDF microfilitration membrane and the active layer for CTA forward osmosis membrane. In further, mPEG-NH2 was grafted on the PDOPA layer through the adhesion of PDOPA to functionalize the membrane surface. Our work focused on characterization of the modified membranes and the fouling behavior which used in the membrane bioreactor. Firstly, the microfiltration or forward osmosis membranes modified with PDOPA and mPEG-NH2 were characterized with some detecting methods, including composition, morphology analysis, hydrophilicity, roughness and permeate flux. Secondly, for the PVDF microfiltration membrane which operated in MF-MBR under the conditions of short-term and long-term, and the CTA forward osmosis membrane which operated in MF-MBR under the conditions of short-term and long-term, the membrane parameters such as permeate flux, EPS on the membrane surface, flux recovery rate were investigated.Some conclusions were made as follows:(1) The results of FESEM and AFM showed that pore size was reduced and porosity decreased by the adhesion of PDOPA or grafting modification for PVDF membrane. And either the PVDF membrane or CTA membrane, the roughness was increased. The surface hydrophilicity improved after modification, which was indicated by fact that the contact angle of the PVDF membrane of 76.3 °, decreased to 56.2 ° by dopamine coating, 50.3 ° by mPEG-NH2 grafting, and the CTA membrane of 91.4 °, decreased to 71.2 ° by dopamine coating, 64.2 ° by mPEG-NH2 grafting. Compared to the unmodified membrane, the water flux of the PVDF membrane decreased to 87% by dopamine coating, a lower value by mPEG-NH2 grafting, and the CTA membrane decreased to 93% by dopamine coating, a lower value by mPEG-NH2 grafting. Compared to the unmodified membrane, the salt flux of the CTA membrane decreased to 90% by dopamine coating, 86% by mPEG-NH2 grafting, the water permeability(A) of the CTA membrane decreased to 98% by dopamine coating, 93% by mPEG-NH2 grafting, the salt permeability(B) of the CTA membrane decreased to 83% by dopamine coating, 51% by mPEG-NH2 grafting, the support structural parameter(S) of the CTA membrane increased to 118% by dopamine coating, 133% by mPEG-NH2 grafting.(2) The results showed that the modificated PVDF membranes exhibited a considerable anti-fouling ability, which were operating in the MF-MBR during a short-time filtration. The decline rate of water flux increased with the decrease of duration time for modification. When the PVDF membranes operated in the MF-MBR during a long-time filtration, the special flux for all membranes declined severely after membrane cleaning and enabled again. But the membrane by mPEG-NH2 grafting exhibited a bit of anti-fouling ability. The results of membrane resistance and FESEM showed that the accumulation and absorption of pollutants in the pores of modified membranes may be restrained effectively. The protein’s accumulation on the membrane’s surface was also inhibited meanwhile. After the contaminated PVDF membranes cleaned by 1% surfactant solution at 50 °C, the flux recovery rate of unmodified membrane, coated membrane and grafted membrane were 57.2%, 71.6% and 81.1%, respectively.(3) The results showed that the modificated CTA membranes exhibited a considerable anti-fouling ability, which were operating in the OMBR during a short-time or long-time filtration. The decline trend of salt flux was similar to the water flux, which increased with the decrease of duration time for modification. The rate of salt/water flux was a stable value almost, and it did not change with the reduction of water flux or salt flux. For the membrane by dopamine coating, the content of protein and polysaccharide or the proportion of protein for the cake sludge were lower than that of the original film. The proportion of protein for membrane by mPEG-NH2 grafting was lower than the membrane’s by dopamine coating. After hydraulic cleaning for the contaminated CTA membranes, the flux recovery rate of original film, coated film and grafted film reached to 90%, almost. After osmotic backwash, the flux recovery rate of these CTA membranes had a further advance.After enabled the cleaned CTA membranes, comparing these contaminated CTA membranes from the results of FESEM, CLSM and FTIR, we found that all the CTA membranes’ active layer were covered by the massive pollutants. For all CTA membrane, there were least protein and β-polysaccharide accumulating and absorpting on the surface of original film. In a word, the less of anti-fouling and the high level of organic pollution in OMBR greatly weakened the profitable effect of modification on the performance of anti-fouling, there was no significant improvement in anti-fouling for dopamine coating or mPEG-NH2 grafting membranes.