Study Of Composite Nanofiltration Membrane Based On Dopamine Modified Nanofibrous Scaffolds

Nanofiltration(NF)is a pressure-driving membrane filtration process,mainly used in the separation of multivalent ions or organic and colloidal substances with a molecular weight range from 200 to 1000.Nanofiltration as the alternative to reverse osmosis(RO)in the application of filtration process is often used in water softening,in reduction of chemical oxygen demand,in purification of waste water,in treatment of dye solutions etc.Its advantage lies in its low energy cost and high permeate flux.The thin film composite structure is the most common format in NF and RO membranes,it is always consisted of one microporous support layer and one functional barrier layer.However,because of the two layers are made of different materials,they have different interfacial compatibility and different surface tension which means the structure of the composite membrane may not be stable and tend to be compromised after long time use.In this study,polyethersulfone(PES)nanofibrous scaffold known as the support layer was obtained through electrospinning method and the functional barrier layer was prepared on top of PES nanofibrous scaffold by interfacial polymerization(IP)reaction to form the composite NF membrane.In order to improve composite membrane's stability,dopamine as one derivative functional element of marine mussel proteins was used not only to improve the hydrophilicity of the PES support layer but also to bring in extra functional groups.Dopamine solution with the concentration of 1.Og/L was used to modify PES nanofibrous membrane and the modification time was 3h.Pure water permeate flux rises from 1.1 × 104L/m2h to 2.1 X 104L/m2h(feeding pressure was 0.2MPa)and water contact angle drops from 84° to 24° after dopamine modification process.Positively charged composite NF membrane was obtained by interfacially polymerizing polyethyleneimine(PEI)and trimesoyl chloride(TMC)on top of dopamine modified PES nanofibrous membrane.Conditions such as molecular weight and concentration of water-soluble PEI,concentration of water-insoluble TMC and curing temperature have been discussed in this study.With the average water permeate flux of 30.5L/m2h(feeding pressure was 0.6MPa),dye rejection of the positively charged composite NF membrane follows the order:Alcian Blue>Reactive Orange>Sunset Yellow>99.0%;and the composite NF membrane's rejection towards 1000ppm salt solution follows the order:MgCl2>MgS04>Na2S04>NaCl,which also proves the membrane is positively charged.The barrier layer of the composite NF membrane was prepared under following conditions:PEI(M.W.=10000)solution with concentration of 4.0wt%,TMC solution with concentration of 0.4wt%,reaction time is 60s,curing for 2min under 100?.Amines' property is one of the parameters that can affect barrier layer's performance obtained by interfacial polymerization method.Different amines have different diffusion rates.Some of them have large molecular weight and complicated structure which would slow down the amines diffusion rate,such as the performance of the composite NF membrane prepared by interfacial polymerization(IP)reaction between PEI and TMC on dopamine modified nanofibrous PES membrane is influenced by the long chain structure of PEI.On the contrary,piperazidine(PIP)has a relatively simple chemical structure and low molecular weight which would not be bothered by the diffusion problem.In this part,PIP was chosen as the water-soluble diamine to react with TMC solution to produce the functional barrier layer.By the discussion of the factors which influenced the IP reaction,parameters of the preparation of the barrier layer for the composite NF membrane was as follows:PIP solution with the concentration of 1.Owt%,TMC solution with concentration of 0.6wt%,reaction time is 60s,curing for 30min under 100?.The composite NF membrane's filtration property was tested through cross-flow examination under 0.6MPa feeding pressure.It has a high water permeate flux at 61.OL/m2h toward 1000ppm Na2SO4 solution while remaining a perfect rejection ratio of about 99.1%.The composite NF membrane is negatively charged and is proved by the result of its rejection order towards different salt solutions with a pH value of 6(Na2SO4>MgSO4>MgCl2>NaCl).