Preparation And Characterizations Of Anti-fouling Polyethyleneimine Hollow Fiber Composite Nanofiltration Membranes

Positively charged nanofiltration（NF）membranes show excellent rejection performance for di-/multi-valent cations and positively charged particles.Hence,they have been widely applied to recoveries of heavy metal ions,separation of antibiotics,treatment of electroplating wastewater,etc.However,organic fouling occurring on membrane surfaces would result in the decrease of the permeate flux and the increase of the operating pressure.Most of the current studies in the field of NF membranes are concentrated on preparation and characterizations of flat-sheet composite NF membranes.Compared to the flat-sheet membranes,hollow fiber membranes have some advantages such as higher packing density,higher specific surface area,and self-supporting capability.Therefore,it is of great significance to develop positively charged NF membranes with excellent anti-fouling propertiesIn view of the problem that polyamide（PA）NF membranes are easily to be fouled,they were modified with polyvinyl alcohol（PVA）andβ-cyclodextrin（β-CD）,respectively.Fourier transform attenuated total reflection infrared spectroscopy（ATR-FTIR）,scanning electron microscopy（SEM）,contact angle（CA）measurement,and surface zeta potential analyzer,were employed to characterize the chemical structures of the membrane surfaces,membrane morphologies,hydrophilicities,and eletrokinetic characteristics of the membrane surfaces,respectively.Bovine serum albumin（BSA）being the model organic foulant,the anti-fouling properties of the resultant composite NF membranes were investigated through observing the variances in the permeate flux.Firstly,Polyethyleneimine（PEI）,isophthaloyl chloride（IPC）,and polyvinylidene fluoride（PVDF）ultrafiltration（UF）membrane were employed as the monomer of aqueous phase,the monomer of the oil phase,and the base membrane,respectively.The hollow fiber composite NF membranes were prepared via the method of interfacial polymerization（IP）.The influences of molecular weight（Mw）of PEI and the preparation conditions on the rejection performance of the resultant composite NF membranes were investigated.The optimum preparation conditions for the development of PEI hollow fiber composite NF membranes with the strongest positively charge and the stable rejection performances were as the following:M_wof PEI 70,000 Da,PEI concentration in aqueous phase 4 wt.%,immersion time in the aqueous phase min,IPC concentration in organic phase 0.25 w/v.%,time period for IP 90 s,no post-heat treatment required.At ambient temperature and 0.2 MPa,the rejections of the resultant PEI composite NF membrane to the aqueous solutions of 1.0 g·L^-1inorganic electrolytes decreased in the following order:ZnCl₂（96.0%）>MgCl₂（95.1%）>MgSO₄（60.4%）>NaCl（25.4%）>Na₂SO₄（21.4%）,in accordance with the rejection performances of positively charged NF membranes to inorganic electrolytes.The permeate flux to 1.0g·L^-1Mg Cl₂aqueous solution was 12.7 L·m^-2·h^-1.The hollow fiber composite NF membranes were prepared via the method of IP,using polyvinyl alcohol（PVA）and PEI as the monomers of the aqueous phase,and IPC as the monomer of the oil phase.The active layer of the composite NF membrane was composed of polyamide and polyester.The hydrophilicity of the resultant composite NF membrane was improved due to the introduction of numerous hydroxyl groups（-OH）in PVA polymer chains.As the PVA ratio increased,the static water contact of the NF membrane surface decreased gradually from 61.3°to 25.4°.In the anti-fouling experiment using BSA as the foulant,the increase of PVA ratio could significantly decrease the sedimentation of BSA on the membrane surface,alleviating the decline of the permeate flux.Additionally,the recovery rate of the permeate flux was significantly improved as well after the fouled membrane surface being washed with deionized（DI）water.Based on the studies regarding to PEI composite NF membranes mentioned above,surface modification was employed to improve the anti-fouling property of the resultant composite NF membranes.After the reaction betweenβ-cyclodextrin（β-CD）and the residual acyl chlorides,a polyester layer was formed on the PA layer.It suggested that the introduction ofβ-CD into the PA active layer could improve the hydrophilicity of the resultant composite NF membrane significantly.As the concentration ofβ-CD being increased from 0 to 2.0 wt.%,the static water contact angle decreased from 61.3°to 23.0°,and the resultant hollow fiber composite NF membranes showed excellent anti-fouling properties.In the anti-fouling experiment using BSA as the model organic foulant,the sedimentation of BSA on the surface of the resultant NF membrane decreased remarkably due to the increase in the hydrophilicity of the membrane surface and the decrease in the roughness of membrane surface.β-CD concentration being 2.0wt.%,the resultant composite NF membrane showed the best anti-fouling performance.The permeate flux remained at 93.3% of the original value,and the recovery rate of permeate flux was 97.6% after being washed with deionized（DI）water.