Inspired by the hydrophilicity effect of arginine (Arg) in water channel aquaporins (AQPs), Arg was incorporated into the polyamide layer during interfacial polymerization to enhance the permeation and antifouling performance of the nanofiltration (NF) membranes. Due to the presence of active amine groups, Arg became another aqueous phase monomer along with piperazine (PIP) to react with trimesoyl chloride (TMC) during interfacial polymerization, which was incorporated into the polyamide network. The resulting polyamide NF membranes were characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), atomic force microscopy (AFM), static water contact angle, zeta potential, and positron annihilation spectroscopy (PAS) measurement. The effects of incorporating Arg in aqueous phase on water permeability and the rejection of dyes and inorganic salts of the NF membranes were studied, respectively. Similar to its function in AQPs, Arg apparently increased the hydrophilicity and the negative charges of the membrane surface and, consequently, the permeation performance. When the addition of Arg reached 40% to PIP, the water flux was doubled and the rejection ratios of Congo red and Orange GII were still >90%. Meanwhile, the antifouling experiments verified that the modified polyamide NF membranes possessed excellent fouling-resistant performance for negatively charged foulants of BSA, emulsified oil droplet, and humic acid. The flux was decreased below 15%, and recovery even rose to 89%.Keywords: antifouling; arginine; hydrophilicity; nanofiltration membrane; negative charge;

•A in-situ generated TiO2 approach was used to fabricate loose nanofiltration membrane.•The membrane contained small channels owing to the interaction between TiO2 and the polyamide.•The membranes exhibited high water fluxes and separation performance for dye/salt solutions.In this study, a high flux nanofiltration (NF) membrane with hybrid polymer-nanoparticle active layer was fabricated by chemical crosslinking of piperazine (PIP) and 1, 3, 5-benzene tricarbonyl trichloride (TMC). An in-situ generated method was applied to deposit titanium dioxide (TiO2) nanoparticles uniformly on the membrane surface, leading to the enhancement of the surface hydrophilicity, roughness and relative surface area of the polyamide (PA) layer. The morphology of the modified membrane was investigated by scanning electron microscopy (SEM) and Atomic force microscopy (AFM), also energy dispersive X-ray microanalysis (EDX) was used to analyze the distribution of Ti element. Chemical structure was observed by Fourier transmission infrared attenuated total reflectance (FTIR-ATR) spectroscopy. Remarkably, the optimal water flux of the loose NF membrane was 65.0 Lm−2 h−1 bar−1 nearly 5 times as much as the pure PA membrane flux. The rejections of the loose NF membranes for dyes were almost all greater than 95.0%, while the rejection for sodium sulfate (Na2SO4) was only about 17.0%, which indicated that the modified membrane had an impressive potential application for dye desalination and purification.

In this study, carbon nanotubes coated with poly(sulfobetaine methacrylate) (SBMA@CNT) particles were synthesized via a precipitation polymerization method. The SBMA@CNT particles were used as a novel kind of modifier to fabricate polyethersulfone (PES) ultrafiltration membranes by a non-solvent induced phase separation method (NIPS). During the membrane formation process, the surface segregation phenomenon of the SBMA@CNT particles was found, which was demonstrated by an energy dispersive spectrometer (EDS) mapping and X-ray photoelectron spectroscopy (XPS). This phenomenon was ascribed to the self-organization of hydrophilic SBMA@CNT particles spontaneously on the interface of membrane/water in the formation of membranes. In the ultrafiltration of a bovine serum albumin (BSA) feed solution, the best-performing membrane was found to effectively reduce protein adhesion. Its irreversible and reversible flux declines were remarkably decreased and the flux recovery was as high as 98.9%.

A novel green coating method was proposed to prepare composite nanofiltration (NF) membranes without using organic solutions or toxic reagents in the formation of the active layer compared with traditional interfacial polymerization. Tannic acid (TA) and iron(III) chloride (FeCl3) were chosen as the two reactive monomers dissolved in the aqueous phase. The stable metal–polyphenol complex coating was formed via the coordination reaction between TA and iron ions (FeIII) upon porous support. Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and water contact angle were used to characterize the chemical features of the prepared TA–FeIII/polyethersulfone (PES) composite NF membranes. Scanning electron microscope (SEM) and atomic force microscopy (AFM) were utilized to observe the surface morphologies. The effects of reactive monomer concentration and reaction time on the permeability of water and rejection of dyes and inorganic salts were investigated, respectively. The TA–FeIII/PES composite NF membranes possessed good structural stability and oxidation resistance ability.

Zwitterions bearing balanced charge groups have attracted increasing attention to fabricate antifouling membranes due to their highly hydrated structure. The simple and efficient method of covalent connection to enhance the stability of the zwitterions on membrane surfaces is still challenging. Herein, s branched polyethyleneimine (PEI) was employed to synthesize the zwitterion by a quaternary amination reaction with sodium chloroacetate. A novel zwitterionic surface with neutral surface charge was constructed by grafting the PEI-based zwitterion (Z-PEI) onto the hydrolyzed PAN (H-PAN) membrane surface. Covalent bonds were formed between the Z-PEI and H-PAN membrane while the electrostatic attraction would promote this reaction. The zwitterionic membranes exhibited superior antifouling ability (flux recovery ratio of about 99.8% and total flux decline of about 31.4%) due to the formation of a tight hydration layer by zwitterions on the membrane surface. Furthermore, the flux recovery ratio was not changed obviously during the long term experiment and could be maintained at as high as 96.3 and 98.4% after immersion in acid and alkali solution, respectively. These results demonstrated that the long term and chemical stability of zwitterionic functional surfaces were significantly enhanced via multisite covalent anchorage.

Poly(vinyl chloride) (PVC) ultrafiltration membranes with improved antifouling properties were prepared by using a non-solvent-induced phase inversion process with in situ amination and subsequent surface zwitterionicalization. PVC was directly reacted with triethylenetetramine (TETA) in casting solutions. The introduction of amino groups not only enhanced the hydrophilic property of PVC membranes but also provided the active chemical sites on the membrane surfaces. The aminated PVC membranes were then immersed into the sodium chloroacetate solution to carry out quaternary amination reaction. The zwitterionic groups were formed on the PVC membrane surfaces and pore walls. The surface chemical compositions of modified PVC membranes were confirmed by energy dispersive X-ray spectroscopy (EDX), Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS). Water contact angle and zeta-potential measurement were employed to explore surface property. In the ultrafiltration experiment of bovine serum albumin (BSA) solution, the zwitterionic PVC membranes exhibited better permeability and fouling resistance ability than the initial PVC membrane. The corresponding resistances of membrane, cake formation, adsorption, and pore blocking were calculated to explore the membrane fouling mechanism.

Polyvinyl chloride (PVC) and chlorinated polyvinyl chloride (CPVC) were used as membrane materials to fabricate blend ultrafiltration membranes. Polyethylene glycol (PEG2000) and polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer (Pluronic F127) were used as both pore forming agent and surface modifier to improve the permeability. The advantage of amphiphilic Pluronic F127 is that it enables higher −CH2–CH2–O– segment coverage on the membrane surfaces. The increase of CPVC proportion in membrane materials could improve the water fluxes of PVC/CPVC blend membranes with the slight change of protein rejection ratios. All the PVC/CPVC blend membranes with an additive of Pluronic F127 have excellent antifouling property. The blend method is an appropriate way to prepare new antifouling PVC/CPVC ultrafiltration membranes which have lower cost and better performance.

Cellulose acetate (CA) was used as a hydrophilic additive to blend with poly(vinylidene fluoride) (PVDF) for preparing casting solutions. A novel freeze and immersion precipitation coupling method was developed to improve the performance of PVDF microporous membranes. X-ray photoelectron spectroscopy (XPS) analysis confirmed the enrichment of CA segments on the PVDF membrane surfaces. The level of CA surface segregation on PVDF membrane surfaces was dramatically influenced by the freezing time. The hypothesis of solid (frozen DMSO)–liquid–solid (precipitated polymer) phase separation was proposed to explain the CA surface enrichment phenomenon. The mechanical strength and antifouling property of the PVDF microfiltration membranes were simultaneously improved by the synergistic action of freeze and immersion precipitation coupling technique. The PVDF microfiltration membranes exhibited higher permeability and better antifouling property in bioseparation process than that fabricated by the individual freeze technique.

Pluronic F127/polyethersulfone (PES) blend nanofiltration membranes were fabricated by varying polymer weight ratios of WF127/WPES in casting solutions from 0.0 to 100.0 wt %. The initial wet membranes prepared via the phase inversion method were post-treated by drying at room temperature and then immersion into water for rewetting to obtain Pluronic F127/PES nanofiltration membranes. Membrane shrinkage and swelling, cross section and surface morphologies, water uptake, surface hydrophilicity, X-ray diffraction pattern, water flux, and rejection of dyes for Pluronic F127/PES nanofiltration membranes were investigated. The introduction of Pluronic F127 into PES membranes can substantially alleviate the membrane shrinkage during drying treatment. All of the prepared Pluronic F127/PES nanofiltration membranes can completely reject Alcian blue with a molecular weight of 1299. The water permeance of Pluronic F127/PES nanofiltration membrane at WF127/WPES of 80% is as high as 176.2 L/(m2 h MPa). After long-term immersion into sodium hypochlorite solution, Pluronic F127/PES nanofiltration membranes still keep over 95.7% rejection of Alcian blue.

Preparation of poly(vinylidene fluoride) (PVDF) microporous membranes with high water fluxes and excellent mechanical properties by a simple and effective freeze method was carried out in the present study. Dimethylsulfoxide (DMSO) was selected as a solvent, the solidification of casting solutions was achieved at a temperature of −10 °C with a refrigerator. The frozen solvent was exchanged with water and PVDF microporous membranes were formed at the same time. The structures and properties of the fabricated PVDF microporous membranes were investigated with scanning electron microscopy (SEM), X-ray diffraction (XRD), porosity, and water flux. It was observed that the formation of the spherulites throughout the PVDF membranes. The mechanical properties of PVDF membranes are dramatically enhanced, porosity and water fluxes of PVDF microporous membranes are gradually decreased with an increase of PVDF concentration in DMSO casting solutions. The freeze method was further developed as a general approach for fabricating cellulose acetate (CA), polyethersulfone (PES), and polyacrylonitrile (PAN) microporous membranes.Graphical abstractResearch highlights▶ Casting solution. ▶ Frozen solvent and solidified polymer. ▶ Microporous membrane.

To improve the surface coverage of polyvinylpyrrolidone (PVP) on membrane surfaces and further enhance the antifouling property, a silica−PVP nanocomposite was synthesized and used as a novel hydrophilic additive to modify polyethersulfone (PES) membranes. Transmission electron microscopy (TEM) observation showed that PES membranes, using additives of PVP and silica−PVP nanocomposites, have similar asymmetric structures. X-ray photoelectron spectroscopy (XPS) measurement indicated that the near-surface coverage of PVP for PES membrane with a silica−PVP nanocomposite additive is greater than that with a PVP additive. Protein ultrafiltration experiment also showed that the antifouling ability of PES membrane with a silica-PVP nanocomposite additive is stronger than that with a PVP additive. The hydrophilic modification with a silica−PVP nanocomposite is an appropriate method for improved antifouling property of PES ultrafiltration membranes.

Preparation of antifouling ultrafiltration membranes by self-organization with amphiphilic materials is a simple and effective method. A series of amphiphilic poly(ethylene glycol)-graft-polyacrylonitrile (PEG-g-PAN) copolymers with various molecular weights of PEGs were synthesized by water-phase precipitation copolymerization using ceric(IV) ammonium nitrate as an initiator. PEG-g-PAN ultrafiltration membranes were fabricated by phase inversion in a wet process. Scanning electron microscopy (SEM) revealed that PEG-g-PAN membranes have the typical asymmetric structure. X-ray photoelectron spectroscopy (XPS) analysis confirmed the enrichment of PEG segments on PEG-g-PAN membrane surfaces. The surface segregation enhances the hydrophilic property and antifouling ability of PEG-g-PAN membranes. All prepared PEG-g-PAN ultrafiltration membranes have lower BSA adsorption, higher flux for protein solution, higher flux recovery ratio, and lower membrane fouling during protein ultrafiltration in comparison with the control PAN membrane. The excellent antifouling property endows PEG-g-PAN membranes with potential applications in protein separation and purification.

Adsorption and recovery of methylene blue (MB) from aqueous solution through ultrafiltration technique with polyethersulfone (PES) membrane was developed. Cationic dye MB can be adsorbed by PES membrane from aqueous solution. The amount of adsorbed MB on PES membrane was higher at the condition of basic solution and lower ionic strength. The electrostatic interaction may be the important driving force of MB adsorption. During ultrafiltration, the rejection ratio of MB can reach 100% in the initial operation stage due to convection mass transport through membrane pores and adsorption of MB on the surface and pores of PES membrane. The final recovery of MB in the permeate solution was easily achieved by decreasing the solution pH to desorb MB. The adsorption and recovery of dyes from aqueous solution through ultrafiltration technique have potential application in wastewater treatment.

The permeability of a weak polyelectrolyte ultrafiltration membrane based on poly(acrylonitrile and 2-dimethylaminoethyl methacrylate) (PAN-DMAEMA) copolymer was measured at various conditions of solution pH and ionic strength. Bovine serum albumin (BSA) and lysozyme were used as model proteins to investigate the antifouling property of PAN-DMAEMA ultrafiltration membrane. Electrostatic interactions, the conformation of PDMAEMA chains, and the nature of protein play important roles in enhancing the antifouling property of PAN-DMAEAM membrane. During BSA ultrafiltration, BSA rejection ratio is decreased, flux recovery ratio is remarkably increased, and the total membrane fouling and irreversible membrane fouling are decreased at higher pH value and ionic strength. During lysozyme ultrafiltration, the PAN-DMAEAM membrane displays excellent antifouling property in a broad range of pH and ionic strength.

Poly(2-dimethylamino ethyl methacrylate) (PDMAEMA) is a weak cationic polyelectrolyte in aqueous solution which was incorporated in the porous poly(acrylonitrile and 2-dimethylamino ethyl methacrylate) (PAN−DMAEMA) membrane. The signals of the formation of ion-pair complexes between protonated DMAEMA groups and anions of the added salts were clearly observed in the X-ray photoelectron spectroscopy (XPS) spectra of the PAN−DMAEMA membrane. Since the electrostatic interactions between propotated DMAEMA groups and anions are highly dependent on the valency of anions, which influences the conformation of PDMAEMA and pore sizes of membrane, ion-specific flux was found for the PAN−DMAEMA membrane. At the same salt concentration, water fluxes of the PAN−DMAEMA membrane are enhanced in the order of trivalent > divalent > monovalent anions.

Poly(acrylonitrile) (PAN)-based zwitterionic membranes, composed of PAN and poly(N,N-dimethyl-N-methacryloxyethyl-N-(3-sulfopropyl) copolymer, are electrolyte-sensitive smart membranes. The hydrophilicity was increased and protein adsorption was remarkably decreased for the membranes in response to environmental stimuli. FTIR spectroscopic analysis directly provided molecular-level observation of the enhanced dissociation and hydration of zwitterionic sulfobetaine dipoles at higher electrolyte concentrations. The smart PAN-based zwitterionic membranes can close or open channels for protein transport under different NaCl concentrations. The electrolyte-sensitive switch of on/off behavior for protein transport is reversible.

A weak polyelectrolyte ultrafiltration membrane based on poly(acrylonitrile and 2-dimethylamino ethyl methacrylate) (PAN-DMAEMA) copolymer was prepared by phase inversion in a wet process. PAN-DMAEMA membrane has the typical asymmetric structure. The results of X-ray photoelectron spectroscopy (XPS) analysis have shown enrichment of PDMAEMA on the membrane surface. Water flux of PAN-DMAEMA ultrafiltration membrane is tunable due to the switch of the stretched and collapsed states of PDMAEMA chains at different pH and NaCl concentration in the feed solutions. The water flux of PAN-DMAEMA membrane cannot reversibly recover its original flux after environmental stimuli; the reason may be that the residual exchanged-ions and ionic pairs disturb the switch of polymer conformation of PDMAEMA in the skin layer of ultrafiltration membrane.