Influence of colloidal fouling and feed water recovery on salt rejection of RO and NF membranes

The influence of colloidal fouling and feed water recovery (or concentration factor, CF) on salt rejection of thin-film composite reverse osmosis (RO) and nanofiltration (NF) membranes was investigated. Fouling experiments were carried out using a laboratory-scale crossflow test unit with continuous permeate disposal to simulate the CF and recovery as commonly observed in full-scale RO/NF systems. For feed waters containing only salt (NaCl), permeate flux declined linearly as CF was increased and salt rejection was nearly constant for both RO and NF membranes. On the other hand, a sharp decrease in permeate flux and significant decline in salt rejection with increasing CF were observed under conditions where colloidal fouling takes place. For both RO and NF membranes, the marked permeate flux decline was attributed to the so-called “cake-enhanced osmotic pressure”. The decline in salt rejection when colloidal fouling predominated was much more substantial for NF than for RO membranes. In all cases, the decline in salt rejection was higher under conditions of more severe colloidal fouling, namely at higher ionic strength and initial permeate flux.     

Polyethersulfone-polyvinylpyrrolidone composite membranes:Effects of polyvinylpyrrolidone content and polydopamine coating on membrane morphology,structure and performances

Hydrophilic modification is a promising method to inhibit fouling formation on ultrafiltration membrane.In this work,different mass concentrations (1％-16％) of hydrophilic polyvinylpyrrolidone were incorpo-rated into polyethersulfone (PES) membranes fabricated by none-solvent induced phase separation.Then,polydopamine (PDA) coating on the surface of prepared membrane was carried out at pH 8.5.The mor-phology and structure,surface hydrophilicity,permeation flux,BSA rejection,antifouling and stability performances of PES and PDA/PES modified membranes were investigated in detail.The results indicated that PDA was successfully attached onto the membranes.Membrane hydrophilicity was evaluated by water contact angle measurement.The contact angles of modified membranes reduced remarkably,sug-gesting that the membrane hydrophilicities were significantly increased.The results of filtration tests,which were done by dead-end filtration of bovine serum albumin solution,showed that the properties of permeability and fouling resistance were obviously improved by PDA modification.When polyvinylpyrrolidone mass content reached 10％,flux recovery ratio of modified membrane was up to 91.23％,and its BSA rejection were over 70％.The results of stability tests showed that the modified mem-branes had good mechanical stability and chemical stability.This facile fabrication procedure and out-standing performances suggested that the modified membranes had a potential in treating fouling.     

Flux,antifouling and separation characteristics enhancement of nanocomposite polyethersulfone mixed-matrix membrane by embedding synthesized hydrophilic adipate ferroxane nanoparticles

Polyethersulfone (PES) nanofiltration (NF) membranes were prepared by blending of synthesized hydrophilic adipate ferroxane nanoparticles (AFNPs) as a novel multifunctional nanofiller via the phase inversion method. The water contact angle measurement indicated the higher hydrophilicity of the NF membranes. The water flux of the membranes improved significantly after the addition of AFNPs, from 10.4 to 32.2 kg/m²h. Antifouling characteristics of AFNPs/PES membranes were improved by increased hydrophilicity and decreased membrane surface roughness. The 0.6 wt% AFNPs/PES membrane exhibited the highest FRR (96%) and the lowest irreversible fouling resistance (6%). The nanofiltration performance of the prepared membranes was evaluated by dye removal and salt retention. The results proved the high dye removal capability of modified membranes (98% rejection) compared with the unfilled PES membrane (89% rejection). The salt retention sequence for membrane with 0.2 wt% of nanoparticles was Na₂SO₄ (70%)>MgSO₄ (60%)>NaCl (18%).     

Enhanced forward osmosis from chemically modified polybenzimidazole (PBI) nanofiltration hollow fiber membranes with a thin wall

To develop high-flux and high-rejection forward osmosis (FO) membranes for water reuses and seawater desalination, we have fabricated polybenzimidazole (PBI) nanofiltration (NF) hollow fiber membranes with a thin wall and a desired pore size via non-solvent induced phase inversion and chemically cross-linking modification. The cross-linking by p-xylylene dichloride can finely tune the mean pore size and enhance the salt selectivity. High water permeation flux and improved salt selectivity for water reuses were achieved by using the 2-h modified PBI NF membrane which has a narrow pore size distribution. Cross-linking at a longer time produces even a lower salt permeation flux potentially suitable for desalination but at the expense of permeation flux due to tightened pore sizes. It is found that draw solution concentration and membrane orientations are main factors determining the water permeation flux. In addition, effects of membrane morphology and operation conditions on water and salt transport through membrane have been investigated.     

Preparation and characterization of PVA/SA composite nanofiltration membranes

Nanofiltration (NF) composite membranes based on poly(vinyl alcohol) (PVA) and sodium alginate (SA) were prepared by coating PVA/SA (95/5 in wt %) mixture solutions on microporous polysulfone (PSF) supports. For the formation of a defect free thin active layer on a support, the PSF support was multi‐coated with a dilute PVA/SA blend solution. The PVA/SA active layer formed was crosslinked at room temperature by using an acetone solution containing glutaraldehyde as a crosslinking agent. The prepared composite membranes were characterized with a scanning electron microscopy (SEM), a Fourier transform infrared spectroscopy (FTIR), an electrokinetic analyzer (EKA) and permeation tests: The thicknesses of the active layers were about 0.25 μm and 0.01 μm depending on the preparation conditions. The crosslinking reaction of the active layers were completed in less than three minutes via the formation of acetal linkage. The surface of the PVA/SA composite membrane was found to be anionic. The permeation properties of the composite membrane were as follows: 1.3 m³/m² day of flux and > 95% of rejection at 200 psi for 1000 ppm PEG600 solution. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 347–354, 2000     

Separation of aqueous salt solution by pervaporation through hybrid organic–inorganic membrane: Effect of operating conditions

Hybrid polymer-inorganic membranes were prepared by crosslinking poly(vinyl alcohol) (PVA), maleic acid (MA) and silica via an aqueous sol–gel route. Membrane characterisation results revealed silica nanoparticles (< 10 nm) were well dispersed in the polymer matrix and significantly reduced swelling of the membrane. The membranes were tested for pervaporation separation of aqueous salt solution with NaCl concentrations of 0.2–5.0 wt% at temperatures 20–65 °C, feed flowrates 30–150 mL/min and permeate pressures 2–40 Torr. The salt rejection remained high (up to 99.9%) under all operating conditions. A high water flux of 11.7 kg/m² h could be achieved at a feed temperature of 65 °C and a vacuum of 6 Torr. The effect of operating conditions on water flux is discussed in relation to diffusion coefficients of water and fundamental transport mechanism through the membrane. The activation energy for water permeation was found to vary from 23.8 to 20.1 kJ/kmol when the salt concentration in the feed was increased from 0.2 to 5.0 wt%.     

Facile modification of thin-film composite nanofiltration membrane with silver nanoparticles for anti-biofouling

This study focuses on the modification of a commercial nanofiltration (NF) membrane by an in-situ reaction to load silver nanoparticles (AgNPs) for anti-biofouling. Poly (vinyl alcohol) (PVA) was coated onto the NF membrane firstly, and silver salt was then deposited on the surface of PVA layer. Through thermal reduction, AgNPs with 10–20 nm in diameter were formed and immobilized onto the membrane surface by the interaction between AgNPs and PVA, as confirmed by UV–vis absorption spectrum, SEM and XPS analysis. Compared to the pristine NF90 membrane, the PVA composite membranes (NF90-PVA) and AgNPs (NF90-PVA-AgNPs) modified membranes exhibit lower water flux and slightly higher salt rejection. Release of silver ion experiments were assessed via batch method, and the results indicate silver ion can be released from the AgNPs modified membrane continuously and steadily, which may be responsible for the improved and long-time antibacterial ability of the membrane. Due to the simplicity of the method, the ability to immobilize the AgNPs to avoid leaching out, and the strong antibacterial activity, this NF90-PVA-AgNPs composite membrane displays potential applications in industrial water-treatment.     

Removal of dyes by poly(p-phenylene terephthamide)/polyvinylidene fluoride hollow fiber in-situ blend membranes

In order to obtain the compatible poly(p-phenylene terephthalamide) (PPTA)/polyvinylidene fluoride (PVDF) blend membranes, the casting solution was synthesized via the in situ polycondensation process and flat sheet blend membranes were successfully prepared through the immersion precipitation phase inversion method in our previous study. In this study, the polycondensation solution was directly used as the spinning dope to fabricate PPTA/PVDF hollow fiber in-situ blend membrane by the dry-wet spinning technique. Hollow fiber membranes were employed to remove the dyes including Congo red (CR) and methylene blue (MB) from the dyeing liquor. Effects of operation conditions on dye rejection and membrane water flux were investigated. With the increase of operation pressure, feed concentration and feed temperature, dye rejection rates gradually decreased, but the rejection value of CR and MB still remained above 99.5%. On the contrary, the permeation water flux basically increased. During the continuous dye filtration of 300 min, hollow fiber membrane can maintain high dye rejection rates and stable water flux. The combination method of physical backwashing and chemical cleaning can effectively alleviate membrane fouling and recover membrane water flux. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48569.     

Conferring pH-sensitivity on poly (vinylidene fluoride) membrane by poly (acrylic acid-co-butyl acrylate) microgels

In this paper, cross-linked poly (acrylic acid-co-butyl acrylate) microgels were utilized to impart pH-sensitivity to poly (vinylidene fluoride) membranes by phase separation of a casting solution of poly (vinylidene fluoride)/poly (acrylic acid-co-butyl acrylate)/DMF in aqueous solution. The effect of microgels content on morphologies, surface composition, and chemistry of the as-prepared membranes was studied by varieties of spectroscopic and microscopic characterization techniques. By using the filtration of water and protein aqueous solution, the performance of the membrane was evaluated. Results indicated that the as-prepared membrane was pH-sensitive to water flux, bovine serum albumin rejection and antifouling property. Besides, the as-prepared membrane showed an obvious improvement of water flux and proper bovine serum albumin rejection ratio, compared to the pristine PVDF membrane. Meanwhile, dynamic bovine serum albumin fouling resistance and flux recovery property were also greatly enhanced due to the improvement of surface hydrophilicity. Hopefully, the hydrophilic microgels additive would be favorable to fabricate other polymer membranes for water treatment.     

Improved separation performance of polyamide based reverse osmosis membrane incorporated with poly(dopamine) coated carbon nanotubes

A series of polyamide thin-film nanocomposite (PA TFN) membranes have been fabricated by incorporating hydrophilic poly(dopamine) (PDA) coated carbon nanotubes (CNTs@PDA) into the PA selective layer via interfacial polymerization. The effects of PDA coating thickness on surface characteristics and separation performances of membranes are studied in detail. The PDA coating makes the surface of PA TFN membrane more hydrophilic, smoother and less electronegative. The desalination performance is obviously influenced by the coating thickness of PDA and the loading concentration of PDA@CNTs. The water fluxes of PDA@CNTs incorporated PA TFN membranes have been improved without sacrificing NaCl rejections. When the loading concentration is 0.0010%, the maximum water flux is 48.1 L m⁻² h increasing by 45% compared with that of pristine PA membrane. Meanwhile, the NaCl rejection is up to 99.8%. The CNTs@PDA incorporated PA TFN membranes exhibit better anti-fouling property and separation performance durability. This work proves that CNTs@PDA has great potential application in PA TFN membranes.     

Synthesis of polydopamine‐mediated PP hollow fibrous membranes with good hydrophilicity and antifouling properties

Membrane fouling remains a major barrier to membrane separation, particularly obvious in polymer membranes. Dopamine (DA) is of great value as a precursor for conjugation hydrophilic molecules. In this study, PP hollow fibrous membranes were first modified by DA to form a layer of polydopamine (PDA) coating. Then taurine and glycidol were introduced respectively with assisted by PDA reactive layer, the prepared membranes corresponding to PP‐T and PP‐G membranes, respectively. PP and the modified PP membranes were confirmed by a thorough membrane characterization of ATR‐FTIR, XPS, and FESEM measurements. The hydrophilic properties and permeability were measured by water contact angle (WCA) and permeation flux test. BSA was used to as model protein to evaluate the antifouling properties of the membranes. The results showed that taurine and glycidol were successfully introduced onto the membrane surface. The WCA of PP‐T and PP‐G membranes can be reduced to 32° and 26°, and the flux recovery ratio increased around 90.6% and 89.8%, respectively. Based on the experimental results, taurine and glycidol effectively improved the hydrophilic and anti‐fouling performance of PP membrane. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44430.     

Nanofiltration and reverse osmosis thin film composite membrane module for the removal of dye and salts from the simulated mixtures

Nanofiltration (NF) and reverse osmosis (RO) thin film composite polyamide membrane modules were used to remove the color from the contaminated solution mixture. The feasibility of membrane processes for treating simulated mixture by varying the feed pressures (100-400 psi) and feed concentrations was studied to assess the separation performance of both NF and RO membranes. It was found that the efficiency of NF and RO membranes used in the treatment of colored water effluents was greatly affected by the presence of salts and dyes in the mixture. Color removal by NF with a high rejection of 99.80% and total dissolved solids (TDS) of 99.99% was achieved from RO by retaining significant flux rate compared to pure water flux, which suggested that membranes were not affected by fouling during the simulated wastewater process operation. The effect of varying concentrations of Na₂SO₄ salt and methyl orange (MO) dye on the performance of spiral wound membranes was determined. Increasing the dye concentration from 500 to 1000 mg/L resulted in a decrease of salt rejection at all operating pressures and for both concentrations of 5000 and 10,000 mg/L as the feed TDS. Increasing the salt concentration from 5000 to 10,000 mg/L resulted in a slight decrease in dye removal.     

Preparation,structure, and transport properties of ultrafiltration membranes of poly(vinyl chloride) and poly(vinyl pyrrolidone) blends

Ultrafiltration (UF) membranes based on poly(vinyl chloride) and poly(vinyl pyrrolidone) blends were prepared by the phase inversion method, and the factors governing membrane properties were investigated. The membranes were characterized by scanning electron microscopy and atomic force microscopy. The fouling characteristics of the membranes were determined by UF of aqueous solutions of bovine serum albumin (BSA) over a pH range of 2–9 and varying salt concentrations. The maximum adsorption of the protein on the membrane surface occurred near the isoelectric point (pI 4.8) of BSA, and the presence of the salts increased the fouling of the membrane. The results can be explained in terms of the nature of the membrane polymer and the effect of different ionic environments on the permeability of the deposited protein layer. The net charge on the BSA molecules appears to be a dominant factor in determining the flux of water through the blend membranes. The UF flux is correlated by a model based on the membrane resistance, adsorbed protein resistance, and time dependent resistance of the concentration polarization layer near the membrane surface. The ζ potentials of the membranes were also determined before and after UF to characterize the surface potential of the membrane. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2606–2620, 2000     

天然有机物对国产纳滤膜的污染及清洗效果研究

为考察水体中天然有机物(NOM)对纳滤膜性能产生的影响,以腐殖酸(HA)、牛血清蛋白(BSA)和海藻酸钠(SA)分别模拟水中常见NOM,腐殖质、蛋白质和多糖,对国产NF-1812纳滤膜进行单组分及其混合物定性定量有机污染及清洗实验。结果表明,有机污染造成膜通量下降,膜污染程度为SA>HA>BSA;NOM截留率可稳定在99.2%~99.6%;膜污染阻力主要为浓差极化阻力,其次是凝胶层阻力和内部污染阻力,有机污染液综合黏度和综合含量越大,浓差极化阻力的比例越高;对多组分有机污染膜进行错流速度9 cm/s的物理水力清洗和pH=10.0的质量分数分别为0.1%的NaOH+0.025%Na-SDS化学药剂清洗,膜通量、NOM截留率、苦咸水截留率、SEM成像均恢复至原膜状态,纳滤膜清洗效果良好,适用于中国西北苦咸水地区。     

Effect of ethylene vinyl acetate content on the performance of VMD using HDPE co-blending membrane

Membranes were fabricated with high-density polyethylene(HDPE) and ethylene vinyl acetate(EVA) blend through thermally induced phase separation and were then used for vacuum membrane distillation(VMD).The membranes were supported by nonwoven polyester fabric with a special cellular structure. Different membrane samples were obtained by adjusting the polymer concentration, HDPE/EVA weight ratio, and coagulation bath temperature. The membranes were characterized by scanning electron microscopy(SEM) analysis, contact angle test, and evaluation of porosity and pore size distribution. A series of VMD tests were conducted using aqueous NaCl solution(0.5 mol·L~(-1)) at a feed temperature of 65 ℃ and permeate side absolute pressure of 3 kPa. The membranes showed excellent performance in water permeation flux, salt rejection, and long-term stability. The HDPE/EVA co-blending membranes exhibited the largest permeation flux of 23.87 kg·m~(-2)·h~(-1) and benign salt rejection of ≥99.9%.     

Dehydration of glycerin/water mixtures by pervaporation using homo and copolymer membranes

Separation characteristics of glycerol/water mixtures were studied using hydrophilic poly(acrylonitrile-comethacrylic acid) (PANMAC), poly(acrylonitrile-co-hydroxyethyl methacrylate) (PANHEMA), Poly(vinyl alcohol) GFT-1001, and poly(vinyl alcohol) (PVA) crosslinked with maleic anhydride (PVAManh) membranes. All membranes were found to be highly water selective. PVAManh membrane yielded the highest permeation flux for water over the entire range of water concentration studied. Homopolymers (PVAManh and GFT-1001) gave better permeation rates than copolymer membranes (PANHEMA and PANMAC). But the swelling of homopolymers is nuch greater than that of copolymers, which is why PVA membranes have poor longevity. No effect on selectivity of the membrane was observed with a change in operating parameters. No decomposition/polymerization of glycerin was observed, as there was no involvement of high temperatures as there is with distillation. A comparison of pervaporation with vapor-liquid equilibrium data showed that pervaporation of glycerin/water mixtures yielded better selectivity than vapor-liquid equilibrium, particularly for glycerol concentrations above 90 wt%.     

纳滤和反渗透膜形貌结构与膜性能关系探索

纳滤和反渗透膜表面形貌结构、亲疏水性的性质与膜脱盐率、水通量等性能存在一定关系。对几款商用纳滤、反渗透膜进行表面形貌结构、表面粗糙度、亲水性表征。结果表明,纳滤膜表面平整粗糙度低、亲水性强、脱盐率较低,但水通量高。反渗透膜表面存在大量疏松的峰谷结构,比纳滤膜粗糙度更大、亲水性强。对比两款海水反渗透膜,推测调整反渗透膜"叶片"大小和数量可调节反渗透膜的脱盐率和水通量性能。     

Hydrophilic modification of P(VDF-co-CTFE) porous membranes

This paper describes fabrication of a poly(vinylidene difluoride-co-chlorotrifluoroethylene) (P(VDF-co-CTFE)) porous membrane via non-solvent induced phase inversion and subsequent hydrophilic modification using high efficient surface initiated atom transfer radical polymerization (ATRP). The effect of viscosities of casting solutions on microstructures of the P(VDF-co-CTFE) membrane was investigated. The surface chemistry, thermal stability, morphological structure, and hydrophilicity of the modified membranes were evaluated by Fourier Transform Infrared Attenuated Total Reflection (FTIR-ATR), Differential Scanning Calorimeter (DSC), Scanning Electron Microscope (SEM), and contact angle measurements, respectively. The degree of grafting and the degree of swelling were measured to analyze the effect of polymerization time on the wettability. The mechanical strength of the membranes after modification was also investigated. The permeability and fouling resistance were evaluated according to pure water flux and protein solution filtration measurements. The results demonstrate that the hydrophobic P(VDF-co-CTFE) membrane can be feasibly modified by immobilization of hydrophilic poly(ethylene glycol) methyl ether methacrylate (PEGMA) brushes via surface initiated ATRP.     

Effect of Poly(vinyl pyrrolidone) on Antifouling Properties of Asymmetric Poly(ethylene‐co‐vinyl alcohol) Membranes

Poly(ethylene‐co‐vinyl alcohol)/poly(vinyl pyrrolidone) (EVAL/PVP) blend membranes with antifouling properties were prepared by nonsolvent induced phase separation. Residual PVP in the sample was calculated by infrared spectroscopic data and confirmed by thermogravimetric analysis. The effect of residual PVP on hydrophilicity and permeation characteristics of the membranes was evaluated. Porosity and equilibrium water content of the membranes were influenced by the addition of PVP. The effect of protein fouling on flux using bovine serum albumin as a model system was studied in detail. The residual PVP content could enhance the antifouling property of the membrane. All membranes proved to have sufficient mechanical strength to withstand pressure‐driven filtrations.     

Poly(vinyl alcohol) and poly(vinyl amine) blend membranes for isopropanol dehydration

Blended membranes of hydrophilic polymers poly(vinyl alcohol) (PVA) and poly(vinyl amine) (PVAm) were prepared and crosslinked with glutaraldehyde. The prepared membranes were characterized using infrared (attenuated total reflection mode) spectroscopy, differential scanning calorimetry, X‐ray diffractometry, and scanning electron microscopy measurements. Pervaporation performances of the membranes were evaluated for the separation of water‐isopropanol (IPA) mixtures. As the PVAm content increased from PVAm0 to PVAm1.5, the flux through a 70 μm film increased from 0.023 to 0.10 kg/m² h at an IPA/water feed ratio of 85/15 at 30 °C. The driving force for permeation of water increased due to the temperature but it has no effect on IPA permeation. Activation energies for the permeation of IPA and water were calculated to be 17.11 and 12.46 kJ/mol, respectively. Controlling the thickness of the blend membrane could improve the permeation flux with only a marginal reduction in the separation factor. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45572.