Study On Structure And Properties Of Hydrophilic CPVC Composite Loose Nanofiltration Membrane

To create a high flux nanofiltration membrane for separating salts and dyes from dye wastewater and recycling dyes,researchers used a chlorinated polyvinyl chloride(CPVC)ultrafiltration membrane as the base.The resulting CPVC composite loose nanofiltration membrane was made by interfacial polymerization of dopamine(DA)and polyethyleneimine(PEI)on the membrane surface,as well as trimesoyl chloride(TMC).On this basis,the SMA/CPVC composite nanofiltration membrane was prepared by hydrophilic modification of CPVC by blending polystyrene-maleic anhydride(SMA).The effects of SMA on the hydrophilicity,anti-fouling performance and mechanical properties of CPVC substrate membrane were discussed.The effects of PEI concentration and molecular weight on the structure and properties of CPVC composite loose nanofiltration membrane and SMA/CPVC composite loose nanofiltration membrane were also discussed.The following conclusions are obtained through research :(1)During the preparation of the CPVC composite loose nanofiltration membrane,interfacial polymerization was carried out after co-depositing DA and PEI.Increasing the concentration of PEI resulted in an initial increase in pure water flux followed by a decrease,while PEG800 rejection first decreased and then gradually increased.This is attributed to the increase in the number of hydrophilic amine monomers participating in the reaction.Michael addition or Schiff base reaction occurs between PEI and DA,which delays the self-polymerization of DA,reduces agglomeration,and gradually reduces the surface roughness of the membrane.The reaction between high concentration PEI and TMC is more intense,and the thickness of the selective layer increases.As the molecular weight of PEI gradually increased,the pure water flux gradually decreased,and the rejection rate of PEG800 increased.This is because the PEI molecular chain becomes longer,the distance between the terminal amino groups of the DA reaction becomes longer,the aggregates generated by the reaction decrease,the membrane surface becomes smooth and dense,and the cross-linking degree of PEI and TMC increases,resulting in a significant reduction in membrane surface permeability.When the PEI concentration was 5g/L and the molecular weight was 1800,the prepared CPVC composite loose nanofiltration membrane achieved the best performance.The pure water flux reached 5.46 LMH,and the rejection rate of PEG800 was 93.5%,reaching the nanofiltration level.(2)SMA and CPVC were blended to prepare the substrate membrane to improve the hydrophilicity of CPVC.Then,SMA/CPVC composite loose nanofiltration membrane was prepared by co-deposition and interfacial polymerization.When the SMA content in the membrane was increased,it led to a rise in pure water flux,a decline in PEG800 rejection rate,and an increase in flux recovery rate.Conversely,elevating the PEI concentration initially caused a growth in pure water flux,which was followed by a decrease,while the PEG800 rejection rate declined at first and then gradually rose with further increase in PEI concentration.Ultimately,the flux of the membrane fabricated under the best conditions was 38% greater than that of the CPVC composite loose nanofiltration membrane.This is attributed to the emergence of a large number of hydrophilic anhydride groups on the membrane surface,which improves the wettability of the membrane surface,increases the surface roughness of the membrane,improves the pore structure of the membrane,and increases the porosity.The high concentration of PEI reacts more thoroughly with TMC,and the density and thickness of the generated PA selective layer increase.When the SMA content is 3%and the PEI concentration is 5g/L,the membrane performance is optimal,the pure water flux is 7.55 LMH,and the rejection rate of PEG800 is 90.2%,reaching the nanofiltration level.(3)In the treatment of RB5 simulated dye wastewater by CPVC composite loose nanofiltration membrane,it was found that with the increase of PEI concentration,the flux of the dye solution showed an initial increase followed by a decrease,and the rejection of dye exhibited a decline followed by an increase.This phenomenon can be attributed to the rise in the quantity of hydrophilic amine monomers,which delays the self-polymerization of DA,reduces agglomeration,and gradually reduces the surface roughness of the membrane.The reaction of high concentration PEI with TMC is more intense,and the thickness of the selective layer increases.With the increase of PEI molecular weight,the flux of dye solution decreased gradually,and the retention rate of RB5 dye increased.This is because the PEI molecular chain becomes longer,the distance between the terminal amino groups of the DA reaction becomes longer,the aggregates generated by the reaction decrease,the membrane surface becomes smooth and dense,and the cross-linking degree of PEI and TMC increases,resulting in a significant reduction in membrane surface permeability.The CPVC composite loose nanofiltration membrane prepared under the optimal conditions has a PEI concentration of 5g/L,a molecular weight of 1800,a dye solution flux of 4.64 LMH,a dye rejection rate of98.5%,a salt rejection rate of 2.1%,and a dye and salt separation factor of 46.9,while maintaining treatment efficiency and stability under long-term operation.In the treatment of RB5 simulated dye wastewater,it was found that with the increase of SMA content,increasing the PEI concentration led to an increase in the flux of dye solution,while the rejection rate of RB5 dye and salt decreased.Moreover,with the increase of PEI concentration,the flux of the dye solution demonstrated an initial growth followed by a decline,and the rejection of dye exhibited a decrease followed by an increase.Ultimately,the membrane prepared under optimal conditions exhibited a 45% higher flux than the CPVC composite loose nanofiltration membrane.This is attributed to the appearance of a large number of hydrophilic anhydride groups on the membrane surface,which improves the wettability of the membrane surface,increases the surface roughness of the membrane,improves the pore structure of the membrane,and increases the porosity.The interfacial polymerization reaction between high concentration PEI and TMC is more intense,and the thickness and density of the generated PA layer are greatly improved.When the SMA content is 3% and the PEI concentration is 5g/L,the prepared composite loose nanofiltration membrane achieves the best effect.The dye flux is 6.75 LMH,the dye rejection rate is 95.9%,the salt rejection rate is 1.6%,the separation factor of dye and salt is 59.9,and it maintains good stability in the long-term operation test.