Hollow Fiber Nanofiltration Research Articles

Fabrication of a high-flux thin film composite hollow fiber nanofiltration membrane for wastewater treatment

A high-flux thin film composite (TFC) hollow fiber nanofiltration (NF) membrane was fabricated using a barrier layer of polypiperazine amide synthesized via interfacial polymerization (IP) on a previously prepared dual-layer (PES/PVDF) hollow fiber substrate, which was synthesized via the two-step TIPS/NIPS method. The permeability of the TFC membrane was smaller by an order of magnitude than that of the substrate, which was due to the formation of a thick barrier layer. The structure of the barrier layer was controlled via the addition of cyclic ethers (dioxane, oxolane and trioxane). By increasing the polarity of cyclic ethers, the thickness of the barrier layer and the dense layer was reduced due to the piperazine concentration in the organic phase. Comparing with oxolane and trioxane, the addition of dioxane resulted in a narrower pore size as that of the conventional IP process, which was ascribed to the narrow IP reaction zone via the immiscibility of interface. With the addition of dioxane up to 2wt%, the barrier layer became thinner, the pure water flux of the resultant membrane was improved and the dextran rejection was maintained. The novel hollow fiber NF membrane has a permeability of 16.6Lm−2h−1bar−1, a MWCO of 330Da and a tensile strength of 10.3MPa. This membrane had a higher rejection of total organic carbon and a lower rejection of total dissolved solids of the secondary effluent from a petrochemical industry plant, which proves the potential of this membrane in municipal, agricultural and industrial wastewater treatment.

Determination of surface potential of hollow fiber membranes by streaming potential method

Streaming potential method at tangential flow of solution has been proposed as a means of measuring the surface potential of hollow fiber membranes. Zeta potential and surface charge for polyacrylonitrile hollow fiber nanofiltration membranes have been measured. The measurements have been made on the membranes of different porosities obtained by partial drying of initially wet membranes. Porosity has been evaluted from the rate of pressure-driven filtration of a solution through membrane pores. An equation has been derived that allows for assessment of the effect of solution flow in the porous layer. It has been shown that more correct values of membrane surface potential can be calculated using this equation.

Mitigating the hydraulic compression of nanofiltration hollow fiber membranes through a single-step direct spinning technique.

Most nanofiltration (NF) membranes have been made through complicated multistep or thin-film composite processes. They also suffer the compaction issue that reduces permeate flux in pressure-driven filtration processes. A single-step coextrusion hollow fiber fabrication technique via immiscibility induced phase separation (I(2)PS) process is presented in this study to fabricate NF hollow fiber membranes. A protective layer is concurrently formed on top of the selective layer during the phase inversion process. The fabricated hollow fiber membrane has a narrow pore size distribution with a molecular weight cutoff (MWCO) of 470 Da. The outer layer of the I(2)PS hollow fiber is found to serve as a buffering layer that mitigates hydraulic compression on the compaction of dense-selective layer and sublayer and helps to retain membrane performance during nanofiltration operations. The newly fabricated NF hollow fiber membrane exhibits an average pure water permeability of 3.2 L m(-2) h(-1) bar(-1) and shows good rejections toward the testing dyes. This study may offer a simple, direct, and cost-effective approach to fabricate NF hollow fiber membranes.

Comparative study on the treatment of raw and biologically treated textile effluents through submerged nanofiltration

Raw and biologically treated textile effluents were submerged filtrated using lab-fabricated hollow fiber nanofiltration membrane with a molecular weight cut-off of about 650g/mol. Permeate flux, chemical oxygen demand (COD) reduction, color removal, membrane fouling, and cleaning were investigated and compared by varying the trans-membrane pressure (TMP) and volume concentrating factor (VCF). It was found that both raw and biologically treated textile effluents could be efficiently treated through submerged nanofiltration. The increase of TMP resulted in a decline in water permeability, COD reduction, color removal, and flux recovery ratio, while the increase of VCF resulted in both increased COD reduction and color removal. Under the TMP of 0.4bar and VCF of 5.0, fluxes of 1.96 and 2.59l/m2h, COD reductions of 95.7 and 94.2%, color removals of 99.0, and 97.3% and flux recovery ratios of 91.1 and 92.9% could be obtained in filtration of raw and biologically treated effluents, respectively. After filtration, the COD and color contents of the raw effluent declined sharply from 1780 to 325mg/l and 1.200 to 0.060Abs/cm, respectively, while for the biologically treated effluent, they decreased from 780 to 180mg/l and 0.370 to 0.045Abs/cm, respectively.

Stabilization of composite hollow fiber nanofiltration membranes with a sulfonated poly(ether ether ketone) coating

Composite hollow fiber nanofiltration membrane coated with a thin layer of sulfonated poly(ether ether ketone) (SPEEK) can separate monovalent ions from multivalent ions with a low energy demand. It has found applications in desalination of glyphosate wastewater and removal of arsenic from drinking water. Before large scale application of such membranes, one of the main issues is the long time performance stability. This paper reports, for the first time, systematic investigations on the origins of the instability of the SPEEK composite membrane in aqueous phase. Membrane preparation parameters, the coating polymer concentration, the coating time, and the post-treatment temperature were correlated to the membrane stability. A reptation mechanism was proposed to explain the rejection decline and flux increase with time, based on swelling and dissolution of SPEEK. To reduce the swelling and loss of SPEEK, a PVA/SPEEK blend was used as the coating layer, which after cross-linking by glutaraldehyde, yielded a NF membrane with constant rejection during 2000h of operation.

Charged micropollutant removal with hollow fiber nanofiltration membranes based on polycation/polyzwitterion/polyanion multilayers.

Hollow fiber nanofiltration membranes can withstand much higher foulant concentrations than their spiral wound counterparts and can be used in water purification without pretreatment. Still, the preparation of hollow fiber nanofiltration membranes is much less established. In this work, we demonstrate the design of a hollow fiber nanofiltration membrane with excellent rejection properties by alternatively coating a porous ultrafiltration membrane with a polycation, a polyzwitterion, and a polyanion. On model surfaces, we show, for the first time, that the polyzwitterion poly N-(3-sulfopropyl)-N-(methacryloxyethyl)-N,N-dimethylammonium betaine (PSBMA) can be incorporated into traditional polyelectrolyte multilayers based on poly(styrenesulfonate) (PSS) and poly(diallyldimethylammonium chloride) (PDADMAC). Furthermore, work on model surfaces allows a good characterization of, and insight into, the layer build-up and helps to establish the optimal membrane coating conditions. Membranes coated with these multilayers have high salt rejection of up to 42% NaCl, 72% CaCl2, and 98% Na2SO4 with permeabilities of 3.7-4.5 l·m(-2)·h(-1)·bar(-1). In addition to the salt rejections, the rejection of six distinctively different micropollutants, with molecular weights between 215 and 362 g·mol(-1), was investigated. Depending on the terminating layer, the incorporation of the polyzwitterion in the multilayer results in nanofiltration membranes that show excellent retentions for both positively and negatively charged micropollutants, a behavior that is attributed to dielectric exclusion of the solutes. Our approach of combining model surfaces with membrane performance measurements provides unique insights into the properties of polyzwitterion-containing multilayers and their applications.

Treatment of highly concentrated wastewater containing multiple synthetic dyes by a combined process of coagulation/flocculation and nanofiltration

The treatments of dyes (acid, basic and reactive dyes) wastewater were studied by applying individual coagulation/flocculation (CF) and nanofiltration (NF) processes as well as their combination (referred as CF–NF). For the treatment of highly concentrated multiple dyes wastewater (MDW, 1000ppm), polyaluminum chloride (PAC) and polydiallyldimethyl ammonium chloride (PDDA) were found to be the most effective coagulant and flocculant, respectively. The CF process can achieve about 90% of dye removal at the optimal dosage of PAC/PDDA=400/200ppm, and the MDW with pH>3 is favorable for the CF treatment. A positively charged NF hollow fiber membrane was fabricated and used for NF treatment. It is able to remove almost 100% dyes with a permeate flux of about 1.0Lm−2h−1 under an operating pressure of 1bar. The combination of CF and NF can complement each other׳s strengths and overcome their individual limitations. The NF treatment can completely remove the strong color left in CF treated dye solutions, while the efficiency of coagulant/flocculant is improved by treating NF concentrated streams and subsequently results in much less sludge. In addition, membrane fouling is abated and NF permeate flux is increased by applying the CF process as a pretreatment. Thus, the combination of CF–NF improves the overall performance for the dyes wastewater treatment.

Nanofiltration hollow fiber membranes for textile wastewater treatment: Lab-scale and pilot-scale studies

The textile industry is a water intensive industry that generates a vast amount of wastewater. The wastewater generated from the textile industry is generally loaded with pollutants comprising spent textile dyes, suspended solids, mineral oils, electrolytes, surfactants, etc. Therefore, it must be properly treated before disposal or reuse. A systematic study was conducted to evaluate the performance of newly developed polyamide-imide hollow fiber nanofiltration (NF) membrane in various operating conditions such as feed temperature (i.e., 25, 40, 50, 70°C), solute concentration (i.e., 100, 500, 1000ppm) and pH (i.e., 3, 7, 10). The results indicate that the NF membrane has satisfactory rejections (average: >90%) against various dyes at most testing conditions. In addition, more than 80% of NaCl and 90% of Na2SO4 permeate through the membrane. As a result, these salts have the potential to be be recovered and reused for the next dyeing process. The robustness of the membrane was proven by showing satisfactory and stable performance under cycles of chemical cleaning during the lab-scale and pilot-scale evaluations.

Dual layer composite nanofiltration hollow fiber membranes for low-pressure water softening

Nanofiltration (NF) membrane process has become increasingly attractive due to their unique characteristics to selectively remove specific compounds or ions. The most commonly NF membranes are negatively charged which is unsuitable for hardness removal. Therefore, the development of novel NF membranes with a positively charged skin has become a key issue for low pressure water softening.In this study, dual layer microporous hollow fiber membranes were fabricated using Torlon poly(amide-imide) (PAI) as the material of the selective outer layer and polyethersulfone (PES) as the material of the porous support inner layer. A positively charged NF-like selective layer was developed by a simple polyelectrolyte cross-linking using polyallylamine (PAAm). The newly developed (PAI–PAAm)–PES dual layer hollow fiber membranes show a salt water permeability of 15.8 L/m2 h bar and high Mg2+ and Ca2+ rejections of 94.2% and 92.3%, respectively, at operating pressure of 2 bar using a 3000 ppm TDS feed solution. This study provides a simple and effective approach to produce positively charged NF hollow fiber membranes for water softening applications with low energy consumption.

Positively charged thin-film composite hollow fiber nanofiltration membrane for the removal of cationic dyes through submerged filtration

Positively charged thin-film composite hollow fiber nanofiltration membranes were developed by the dip-coating method using polypropylene hollow fiber microfiltration membrane as support, polyvinyl alcohol (PVA) and polyquaternium-10 (the reaction product of hydroxyethyl cellulose with a trimethylammonium substituted epoxide, PQ-10) as coating materials, and glutaraldehyde as cross-linking agent. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, field emission scanning electron microscopy (FESEM), measurements of streaming potential and contact angle were employed to characterize the obtained membranes. The permeation properties were estimated through pressurized cross-flow permeation tests and the dye removal performance were evaluated through submerged filtration tests using cationic dye aqueous solutions. It was found that the salt rejection order of the membrane was CaCl2>MgCl2>NaCl>MgSO4>Na2SO4 at neutral pH. The desired membrane with a molecular weight cut-off of about 650Da had a pure water flux of 25.7l/m2h and a rejection of 92.8% to 500mg/l CaCl2 aqueous solution at 3.0bar. The membrane could effectively remove cationic dyes from aqueous solution with good long-term performance stability through submerged filtration. The observed rejections to Brilliant green, Victoria blue B and Crystal violet were 99.8, 99.8 and 99.2%, respectively, under the trans-membrane pressure of 0.7bar.

Characterization and application of a thin-film composite nanofiltration hollow fiber membrane for dye desalination and concentration

This study focused on the application of a thin-film composite nanofiltration (NF) hollow fiber membrane formed by interfacial polymerization on a polysulfone/polyethersulfone supporting membrane for dye desalination and concentration. The fabricated NF hollow fiber membrane had a hydrophilic skin layer with an isoelectric point at a pH value of approximately 6.6. Permeation tests revealed that the membrane possessed a molecular weight cut-off (MWCO) of approximately 520Da and a pure water flux of 47.5L/m2h at 0.4MPa. Rejections of different salts followed in the order of MgSO4>Na2SO4>MgCl2>NaCl at a pH value of 6.8, with up to 96.20% rejection for MgSO4. Additionally, due to the negative charge on the membrane surface, rejections of the anionic dyes reactive brilliant blue X-BR and acid red B were 99.99% and 99.90%, respectively. The dye rejection and permeate flux were affected by the dye concentration as well as the presence of NaCl in the X-BR aqueous solutions. Both the dye rejection and permeate flux would decline with the increase of feed dye and salt concentration. The fabricated NF hollow fiber membrane was applied to the process of dye desalination and concentration using an X-BR and NaCl mixed aqueous solution as the pollutant. After finishing the process, the volume of retentate was concentrated 6.25 times, and the dye concentration in the retentate reached 2854.8mg/L. Moreover, the recovery rate of dye was up to 91.4%, and over 95.3% NaCl was removed from the dye/salt mixtures, which indicated that the fabricated composite NF hollow fiber membrane could be successfully applied to the dye desalination and concentration process.

Color removal and COD reduction of biologically treated textile effluent through submerged filtration using hollow fiber nanofiltration membrane

Color removal and chemical oxygen demand (COD) reduction of biologically treated textile effluent through submerged filtration using nanofiltration membrane were investigated in this study. Submerged filtration tests of biologically treated textile effluent were conducted under different trans-membrane pressures (TMP) and various volume concentrating factors (VCF) employing lab-made hollow fiber membrane modules. It was found that the color and COD presented in the biologically treated textile wastewater could be effectively removed through submerged filtration using hollow fiber nanofiltration membranes with molecular weight cut-off of about 620Da, while the electrolytes presented in the effluent were retained by the membrane to a very small extent, especially under VCF higher than 2.0, so that the nanofiltration membrane module could be operated under a low suction pressure. The increase of TMP and/or VCF resulted in a decline in water permeability and an increase in both color removal and COD reduction. Under the TMP of 0.8bar and VCF of 4.0, the submerged nanofiltration system exhibited a steady permeate flux of 5.15l/m2h, a color removal rate of 99.3% and a COD reduction rate of 91.5%. Furthermore, the submerged nanofiltration system possessed high water cleaning efficiency of more than 93.0%.

Cellulose acetate hollow fiber nanofiltration membrane with improved permselectivity prepared through hydrolysis followed by carboxymethylation

Cellulose acetate (CA) hollow fiber membrane with improved permselectivity for nanofiltration was developed by hydrolysis and carboxymethylation of an original cellulose triacetate semi-permeable membrane. The effects of these modifications on the structure and surface properties were characterized by ATR-FTIR, XRD, SEM and measurements of streaming potential and contact angle. The permeation attributes of the obtained membrane were evaluated through cross-flow permeation tests and the improved permselectivity was also demonstrated through dye removal tests. The results showed that hydrolysis greatly increased the membrane pore size and surface hydrophilicity, but decreased the membrane surface charge, while carboxymethylation had little effect on the membrane pore size, but increased the membrane surface hydrophilicity and negative charge. CA hollow fiber nanofiltration membrane with a pure water permeability of 5.2l/m2hbar, NaCl rejection of 66.4% and Na2SO4 rejection of 95.4% could be obtained through modification an original CA semi-permeable membrane having a pure water permeability of 2.3l/m2 h bar, NaCl rejection of 90.8% and Na2SO4 rejection of 91.5%. Additionally, in treating Congo red and Methyl blue aqueous solutions, the modified CA membrane exhibited improved dye removal rates and higher water fluxes of more than two times compared with the original membrane.

Composite hollow fiber nanofiltration membranes for recovery of glyphosate from saline wastewater

A high performance versatile composite hollow fiber nanofiltration (NF) membrane is reported for the separation of glyphosate from saline waste streams. Preparation of SPEEK based on an amorphous poly (ether ether ketone, PEEK) was investigated. The membrane was prepared by coating sulfonated polyether ether ketone (SPEEK) onto a polyethersulfone (PES) ultrafiltration (UF) hollow fiber membrane. The composite membrane was characterized by water permeability, scanning electron microscopy, and rejection toward sodium sulfate (Na2SO4), sodium chloride (NaCl), and calcium chloride (CaCl2). About 90% rejection toward sulfate anions and only 10% rejection for calcium cations were obtained. A water permeability around 10–13 LMHBar and 90% rejection for polyethylene glycol (PEG) with a molecular weight of 4000–6000 Da were observed. In the separation of glyphosate from saline wastewater, the membrane rejected less than 20% of NaCl and higher than 90% of glyphosate at an operating pressure of 5 bars and pH = 11.0. An economic analysis indicated that the cost for recovery of glyphosate was comparably low to the value gained by an increase in the productivity. The results may lead to a new promising low energy solution for the environmental problem faced by the herbicide industry.

Fabrication of poly(amide-imide)-polyethersulfone dual layer hollow fiber membranes applied in forward osmosis by combined polyelectrolyte cross-linking and depositions

Delamination-free ultrafiltration (UF) dual layer hollow fibers have been successfully fabricated by dry jet-wet spinning technique. Two different polymers of poly(amide-imide) (PAI) and polyethersulfone (PES) were utilized as the selective layer on the outer side and the porous support layer on the inner side, respectively. The dual layer substrates were subsequently modified to develop a nanofiltration (NF)-like skin on the outer layer by polyethyleneimine (PEI) cross-linking followed by multilayer polyelectrolyte depositions. These membranes were characterized by a series of standard protocols in terms of membrane structure, permeability and salt rejection, and were applied in FO process.Experiments reveal that the dual-layer NF hollow fiber modified by polyelectrolyte cross-linking and depositions can achieve pure water permeability of 4.1l/m2.h.bar and a high rejection to MgCl2 up to 97%. In FO process, the dual-layer hollow fibers exhibited a water flux of 39 and 21lm−2h−1 in the orientations of active layer facing draw solution and feed water, respectively, by using 0.5M MgCl2 as draw solution and de-ionized (DI) water as feed at room temperature. It suggests that the combined treatment of polyelectrolyte cross-linking and deposition is an effective way to form a NF-like selective layer for FO dual-layer hollow fiber membranes.

Preparation of polysulfone–polyamide thin film composite hollow fiber nanofiltration membranes and their performance in the treatment of aqueous dye solutions

Thin film composite hollow fiber nanofiltration membranes containing poly(m-phenylene-trimesamide) active layer on polysulfone ultrafiltration membrane were prepared by in situ interfacial polymerization between m-phenylenediamine and trimesoyl chloride under different conditions. The nanofiltration membranes exhibited molecular weight cut-off values in the range of 490 to 730g/mole. They showed the rejections in the range of 60–97% for reactive black-5 and rhodamine-B with water flux of 10–35mL/m2h at 25psi for feed solutions containing 400–2000ppm dye. The flux was decreased slightly while the rejection was remained almost the same upon increasing the dye concentration from 400ppm to 2500ppm in feed. The dye rejection and flux of the membranes were found to be reduced by 2–5% when tested with feed solutions containing a mixture of dye and NaCl (up to 10,000ppm). This may be attributed to either the decrease in Donnan's exclusion effect or change in membrane pore structure by salting out or by pore swelling effect. The membranes were characterized by SEM and AFM for surface morphology. The surface roughness was found to be in the range of 20 to 40nm, as observed by AFM, depending upon polyamide preparation conditions.

Novel dual-layer hollow fiber membranes applied for forward osmosis process

Abstract A novel dual-layer forward osmosis (FO) hollow fiber membrane has been designed and successfully fabricated by using a triple orifice spinneret. The fiber consists of two layers made from polyamide-imide (PAI) polymer for the outer layer and polyethersulfone (PES) polymer for the porous inner layer. Specifically, after obtaining asymmetric microporous PAI/PES dual-layer hollow fibers via non-solvent induced phase inversion, polyethyleneimine (PEI) polyelectrolyte modification on the outer PAI layer was applied to produce a nanofiltration (NF)-like thin layer, while the PES porous inner layer remained intact as PES is inert to PEI. The membrane morphology, structure and surface property were carefully tailored by adjusting polymer dope composition and spinning conditions. These membranes were subsequently characterized by a series of standard protocols in terms of membrane structure, permeability and salt rejection, and were utilized in FO process. It was found that the resultant dual-layer NF hollow fiber membrane can achieve pure water permeability of 15.9 l m −2 h −1 bar −1 and a high rejection to divalent cations up to 89%. In FO process, the dual-layer hollow fiber exhibited a water flux of 27.5 l m −2 h −1 in the orientation of active layer facing feed water by using 0.5 M MgCl 2 as draw solution and de-ionized (DI) water as feed at room temperature. The newly developed dual layer hollow fibers outperform all the single layer and dual-layer NF hollow fibers reported in the literature for FO applications.

Preparation and characterization of negatively charged hollow fiber nanofiltration membrane by plasma-induced graft polymerization

A new negatively charged nanofiltration (NF) membrane has been prepared by grafting of 2-acrylamido-2-methylpropanesulfonic acid (AMPS) on the polysulfone (PSf) hollow fiber ultrafiltration membrane using two-step plasma method. First, PSf substrate was pretreated by plasma. Then, the pretreated membrane was soaked in AMPS solution and the loaded monomer was grafted on the membrane by plasma-induced. ATR-FTIR, FESEM, water contact angle (WCA) and breaking strength were employed to characterize the structural and morphological changes of membranes. The effects of pretreatment time, AMPS concentration and grafting reaction time on the membrane performances were studied. In addition, the rejections to single salt solutions and mixed salt solutions were examined. The results indicated that the pretreatment of PSf substrate played an important role in the preparation of this NF membrane. The NF membrane, pretreated for 30 s and grafted for 80 s with 5% AMPS solution, showed a rejection to 1 g/L Na 2SO 4 of 80.8 ± 1.2% with a permeate flux of 6.4 ± 0.2 L m − 2 h − 1 at 0.2 MPa and at 23 ± 1 °C. The rejection sequence of single salts was Na 2SO 4 > MgSO 4 > NaCl > MgCl 2. And the negatively charged NF membrane showed a better selectivity to SO 4 2 − than Cl − in a mixed salt solution.

Preparation of solvent stable polyphenylsulfone hollow fiber nanofiltration membranes

In this study, the preparation and extensive characterization of polyphenylsulfone (PPSU) hollow fiber membranes (HFM) was carried out. PPSU is a polysulfone with better solvent resistance than Polysulfone (PSf) and poly(ether sulfone) (PES), which makes investigation of its potential use in solvent resistant nanofiltration (SRNF) membranes interesting. The membranes were prepared via the dry wet spinning method with three different concentrations of PPSU (22.5%, 25% and 27.5% W/W) in N-methyl-2-pyrrolidone (NMP) as the dope solution. They were characterized on the basis of their morphology, porosity and permeo-selective properties. In addition, their resistance in different organic solvents was investigated. Scanning electron microscopy (SEM) was used to explore the morphological characteristics and the structure of the cross section. It was found that the fiber has a typical asymmetric structure with a nearly dense skin layer and a porous substructure. An increasing amount of macro-voids in the porous substructure was observed when the polymer concentration decreased. The maximum pore size was estimated by bubble point measurements. Iso-propanol permeability and the dye rejection tests were performed in order to study the permeo-selective properties. An increase in the polymer concentration in the dope produces a decrease in the iso-propanol permeability and an increase in the rejection of dyes. In terms of rejection the membranes offer good potential for successful use in nanofiltration applications, although the permeability is low and needs further optimization. The effect of the membrane exposure to ethyl acetate, n-hexane, toluene, diethyl ether, iso-propanol and acetone was investigated by measuring the change in the length of the fiber before and after solvent treatment for ten days. The membranes were visually stable in most of the solvents except for methyl ethyl ketone. The methanol, ethanol, iso-propanol, n-hexane, n-heptane, acetone and toluene permeability has been tested. Except for the last two solvents, the membranes were stable during the permeation test. Mechanical tests show the loss of mechanical stability of fibers in contact with toluene, while their mechanical strength remains the same in contact with n-hexane and iso-propanol.

Hyperbranched Polyethyleneimine Induced Cross-Linking of Polyamide−imide Nanofiltration Hollow Fiber Membranes for Effective Removal of Ciprofloxacin

This study aims to develop a positively charged nanofiltration (NF) hollow fiber membrane for effective removal of ciprofloxacin from water. A novel NF membrane was fabricated by hyperbranched polyethyleneimine (PEI) induced cross-linking on a polyamide-imide hollow fiber support. The spongy-like, fully porous membrane support provides minimal transport resistance and sufficient mechanical strengths for water permeation under high pressures. It is found that the PEI modification significantly influences NF performance through the mechanisms of size exclusion, charge repulsion, and solute-membrane affinity. Specifically, after PEI induced cross-linking, the membrane pore size is significantly reduced. The membrane surface becomes more hydrophilic and positively charged. As a result of these synergic effects, the rejection of ciprofloxacin is substantially enhanced. Furthermore, experimental results show that the molecular weight of PEI has tremendous effect on NF performance of the as-modified membrane. The NF membrane modified by a high molecular weight PEI_60K exhibits the highest rejection, the lowest fouling tendency, and keeps a constant flux over the whole pH range. This study may have great potential for developing high-performance antifouling NF hollow fiber membranes for various industrial applications.