Study On The Preparation And Performance Of PVDF-based FO Membrane

In recent years,the forward osmosis membrane with high efficiency,energy saving and low pollution has been widely welcomed by researchers.At present,the researches on the forward osmotic membrane are mainly carried out from three aspects:supporting layer,selective layer and draw solution.This thesis focuses on the two aspects: supporting layer and selective layer.The hydrophilic nanomaterials(O-MWCNTs,AT and M-PHMB)were added to the casting solution to improve the inherent hydrophobic problem of PVDF to improve the hydrophilicity and permeability of the substrates and promote the interfacial polymerization to improve the permeability and rejection of NaCl of TFC membrane.On the other hand,in order to form free-defect TFC membranes with excellent performance,surfactants(Tween 80,SDS,CTAC and CTAT)was added to amine solution to reduce the interfacial tension of aqueous solution to promote the degree of interfacial polymerization to obtain a TFC membrane with excellent performance.First of all,oxidized-multiwalled carbon nanotube(O-MWCNTs)was prepared by mixed acid oxidation of multiwalled carbon nanotube(MWCNTs).Much hydrophilic carboxyl(-COOH)groups was modified on the surface of MWCNTs.Hydrophilic nanomaterizals of attapulgite(AT)and modified oxidized-multi-walled carbon nanotubes(O-MWCNTs)were introduced to the hydrophobic polyvinylidene fluoride(PVDF)substrates to prepare free-defect polyamide(PA)layer of thin-film composite(TFC)membranes with improved performance for forward osmosis(FO)process.The effects of substrates prepared by the single or composite addition of dope solution were investigated.And the hydrophilicity,membrane morphology,porosity and mechanical performance of as-prepared substrates were also investigated in detail.The corresponding TFC membranes also had better performance and improved selectivity of NaCl.All in all,TFC membrane FO-MAC received improved water flux(of 20.37 LMH and 28.23 LMH and lower salt flux()of 6.5 gMH and 9.5 gMH under FO mode(active layer facing feed solution,AL-FS)and pressure-retarded osmosis(PRO)mode(active layer facing draw solution,AL-DS),respectively.The draw solution and feed solution were deionized water and 2M NaCl solution,respectively.Secondly,on the basis of the previous chapter,M-PHMB was synthesized by grafting hydrophilic polyhexylene biguanide hydrochloride(PHMB)onto the surface of O-MWCNTs.The prepared M-PHMB was added to the casting solution as an additive to prepare the substrates with excellent properties and good hydrophilicity.The effects of the addition of M-PHMB on the pore size,porosity,thickness and hydrophilicity of the substrate were investigated,as well as the water flux,reverse salt flux and salt rejection of the corresponding TFC membrane were also studied systematically.To sum up,with the increase of improved addtion content of M-PHMB,the permeability,hydrophilicity and porosity of the substrates were improved.In addition,hydrophilic and porous bottom films are good for preparing TFC membranes with excellent properties.In this chapter,TFC membrane with an addition of 0.5wt% of M-PHMB had the optimal performance.In PRO mode and FO mode,the water flux of the modified TFC membrane was 19.69 LMH and 13.56 LMH,respectively(draw solution and feed solution were 1M NaCl solution and deionized water,respectively).Compared with the pure membrane M0,the reverse salt flux was lower.Finally,surfactant-modified TFC membranes were successfully prepared by adding different surfactants(Tween 80,SDS,CTAC and CTAT)to MPD amine solution to optimize the degree of interfacial polymerization.The performance of TFC membranes prepared by interfacial polymerization of different surfactants and different concentrations of CTAT into amine solutions were studied.In summary,the prepared TFC membrane had improved surface roughness,free-defect PA layer,optimized membrane surface,and increased free volume of the membrane surface and all of these were conducive for TFC membrane to achieve higher permeability.The water flux(of the improved TFC membrane was increased to 31.75 LMH and 21.68 LMH(PRO mode and FO mode,respective).In addition,the lower surface tension of amine solution helps to form uniform,free-defect and rough PA layer.The structure of TFC membrane obtain lower reverse salt flux()of 1.68 and 0.91 gMH in PRO mode and FO mode with 1M NaCl solution and deionized water working as draw solution and feed solution,respectively.