The Effect Of The Composition Of The Coagulation Bath On The Membrane Structure And Anti-pollution Properties Of The PVDF-G-PACMO Copolymer

Polyvinylidene fluoride(PVDF)membrane is widely used in the field of membrane separation because of excellent mechanical properties,chemical stability and thermal stability.However,PVDF has low interfacial tension and hydrophobicity,which make the pollutants easily adhere to the membrane surface even membrane pores,thus resulting in membrane fouling.Based on previously our research of ACMO modified PVDF membranes,PVDF-g-PACMO copolymer flat membranes were prepared via immersed phase inversion.In this paper,from the perspective of optimizing the membrane technology,the effects of ethanol and sodium chloride in the coagulation bath on the structure and anti-fouling properties of PVDF-g-PACMO copolymer membranes were investigated.Additionally,the membrane fouling models were introduced to analyze the membrane fouling mechanism of pure PVDF membrane and PVDF-g-PACMO copolymer membranes.On the one hand,the effects of different ethanol content(ethanol:deionized water=0:100,15:85,30:70,45:55,60:40,wt%)on the structure and anti-fouling of copolymer membranes were investiged.The delayed phase separation of copolymer in coagulation bath occurred because of the addition of ethanol,and the membrane formation rate decreased,which made hydrophilic ACMO chains migrated and accumulated to the membrane surface and the membrane pores.The results showed that,with the increase of ethanol content,the pore structure of cross-section changed from finger-like pore to sponge pore,and the average pore diameter and porosity increased,which increased the pure water flux of copolymer membrane.Additionally,the ACMO content of membrane surface increased,and then the hydrophilicity of membrane increased,protein adsorption capacity decreased,flux recovery rate increased,membrane fouling resistance decreased.Thus,the antifouling property of membrane increased.However,when the ethanol content is up to 60%,the ACMO content of membrane surface decreased.The hydrophilicity of membrane decreased and the amount of protein adsorbed onto membrane surface increased,which was not conducive to the improvement of the protein fouling resistance.The fouling blocking models showed that,pure PVDF membrane accorded with the standard blocking model.The protein molecules entered into the membrane pore and formed more irreversible pollution.The copolymer membranes in containing ethanol coagulation baths all accorded with the filter cake blocking model.The protein molecules mainly attached to the surface of the membrane,and the reversible fouling increased.After washing,the membrane had high recovery rate and thus the anti-protein fouling property of membrane improved.On another hand,inorganic salt sodium chloride was used as coagulation bath additive(sodium chloride:deionized water = 0:100,5:100,15:100,25:100,35:100,wt%)to further optimize the coagulation conditions and improve the anti-fouling performance of copolymer membranes.With the increase of sodium chloride content in the coagulation bath,the water activity in coagulation bath decreased and the exchange rate of solvent with water decreased,which could induce the hydrophilic PACMO to accumulate on the surface of the membrane and form microspheres;surface roughness and the average pore size of copolymer membrane increased;the hydrophilicity increased;the surface adsorption amount of protein decreased;the pure water flux increased;the flux recovery rate increased and the pollution resistance decreased.At the same time,the cross-sectional structure of the copolymer membranes changed from the finger-like pore to the sponge-like pore;porosity decreased;which enhanced mechanical properties of copolymer membranes.The pollution blocking model indicated that,the copolymer membranes prepared by sodium chloride coagulation bath accorded with the cake blocking model.The increasing surface roughness also makes the surface of copolymer membranes form loose filter cake layer.Thus,the irreversible pollution is transformed to reversible pollution.Therefore,the copolymer membranes still maintain a high flux recovery rate after washing,and the anti-protein fouling property improved obviously.