Aromatic Polyamide Reverse Osmosis Membranes Research Articles

Supercritical CO2 permeation through dense commercial polymeric membranes: Permeability measurements and influence of pressure.

In the food industry, supercritical CO2 (sc-CO2) is widely used as an extraction solvent, particularly for the extraction of lipids from seeds. After the extraction, the decompression is usually used to recover the products. Afterwards, the recompression for CO2 recycling is highly energy consuming and this handicaps industrial development when medium to low added-value post-extraction products are considered. Recovery of solutes using membrane technology is an interesting alternative for the regeneration of CO2 loaded with oil. The suitable membrane must have a high permeability towards CO2 as reasonable filtration area is needed. In this work, we have studied the CO2 membrane permeability of two different dense polymeric membranes, AG aromatic polyamide reverse osmosis membrane and PuraMem™ silicone nanofiltration membrane. It was considered that the pure sc-CO2 permeation is governed by the solution-diffusion model, which explain the observed permeability changes as a function of CO2 viscosity and fugacity (i.e. pressure) and the polymer properties as chemical composition and structure.Indeed, it was observed that the permeability increased with pressure to reach a maximum value at a permeate pressure of 180 bar (17.3 ± 0.9 kg/(h.m2.bar)), and then decreased in the case of a crosslinked polymer (AG membrane). While, for a slightly crosslinked polymer (PuraMem™ membrane), the permeability was found much higher and it is about 751.6 ± 26.4 kg/(h.m2.bar) at a permeate pressure of 160 bar.

Design and construct alkali-responsive nanocontainers for self-healing thin-film composite reverse osmosis membranes

Poor chlorine resistance is a major factor affecting service life of aromatic polyamide reverse osmosis membranes. Herein, alkali-responsive polymer nanocontainers are designed and constructed via emulsified cross-linking and electrostatic self-assembly of sodium alginate, chitosan, and polyaspartic acid around chitosan nanoparticles, and the nanocontainer is integrated into polyamide layer of thin film composite membranes. The nanocontainer does not only provide more channels for water molecules to improve membrane permeability, but also during routine alkaline washing process, polyaspartic acid at the outer layer reacts with alkaline agent to release healing agents: sodium alginate and chitosan to heal damaged sites to restore membrane performances. For example, with the addition of 0.005 wt% nanocontainers, water flux was increased by 32.8% compared to pristine membranes without sacrificing NaCl rejection. After soaking in alkaline cleaning solution, NaCl rejection of the membrane degraded by 2000 ppm active chlorine can be recovered by 97.3%. Therefore, self-healing nanocontainers can greatly improve chlorine resistance, water flux, and stability of polyamide reverse osmosis membranes.

Radioactive wastewater treatment with modified aromatic polyamide reverse osmosis membranes via quaternary ammonium cation grafting

With the growing application of nuclear power, development of methods for the treatment of radioactive wastewater has become increasingly important. In this study, a positively charged aromatic polyamide reverse osmosis (RO) membrane was prepared using a quaternary ammonium salt (2, 3-epoxypropyltrimethylammonium chloride, EPTAC) grafting to improve nuclide removal efficiency. After modification, the zeta potential of the RO membrane surface increased from −22.89 mV to 21.20 mV in neutral conditions. The positively charged RO membrane promoted more efficient removal of nuclides when compared with the original RO membrane. The decontamination factor (DF) of cesium (I) increased from 11.69 to 151.12 and the DF of cobalt (II) increased from 28.62 to 1670.84. In addition, the surface properties of the modified RO membrane were measured, including hydrophilicity, chemical elements, and morphology. The results of these analyses demonstrated that the hydrophilicity of the membrane was significantly improved as the water contact angle decreased from 65° to 10°. The analysis also demonstrated that the EPTAC-modified RO membrane had a stable nuclide DF under different pH conditions. A brief comparison of the DF of nuclides between commercial RO membranes and modified membranes demonstrated a potential application of EPTAC-modified RO membranes for the treatment of radioactive wastewater from nuclear power plants.

1-methylimidazole as a novel additive for reverse osmosis membrane with high flux-rejection combinations and good stability

A novel aqueous solution additive, 1-methylimidazole (1-MI), was applied to prepare aromatic polyamide (PA) reverse osmosis (RO) membranes with excellent performance by interfacial polymerization of trimesoyl chloride (TMC) and m-phenylenediamine. The RO membrane structure and performance were characterized in detail. The results indicated that the addition of 1-MI was beneficial to forming the membrane with a dense and thin PA layer. This is because 1-MI could react with TMC, which is in favor of the formation of back surface structure with more pores. The product generated by 1-MI and TMC was hydrolyzed easily, and then endowed the membrane with more carboxylic acid groups. The membrane desalination performance for feeds with different NaCl concentration was evaluated. The membrane prepared with 1-MI possessed a higher flux and rejection because of the thinner and denser PA layer and more carboxylic acid groups. Considering that the membranes inevitably face chemical cleaning with acid or alkaline solutions in the real processes, the RO membranes were exposed to acidic-alkaline solution, and the results showed that the membrane prepared with 1-MI had good pH stability. This study offers a new route for preparing an excellent-performance RO membrane via using 1-MI as an additive.

Iron catalyzed degradation of an aromatic polyamide reverse osmosis membrane by free chlorine

The catalytic effect of iron on the degradation of a polyamide (PA) reverse osmosis (RO) membrane by free chlorine was evaluated by measuring the permeability, P, and salt rejection, R, at circumneutral pH, cumulative exposures (CT) of 6000 and 3000 ppm-h, varying free chlorine concentrations, and exposure times. The chemical transformations of the polyamide layer were characterized by X-ray photoelectron spectroscopy (XPS) and attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR). The role of free radicals was probed by studying the effect of the short-chained alcohols of increasing size (methanol (MeOH), ethanol (EtOH) and tertiary-butyl alcohol (tBuOH) on membrane performance and transformation. In the absence of iron, CT up to 6000 ppm-h caused N-chlorination (up to 4.9%), disrupted intra-molecular hydrogen bonding and caused membrane tightening. A one-time-addition of iron nitrate (Fe(NO3)3) (1 mg/L) to the recirculating feed water significantly accelerated oxidation and increased P and decreased R. The proposed membrane degradation mechanism involves Fe precipitates lodged on the membrane surface catalyzing the formation of hydroxyl radical by Fenton-like reactions. MeOH accelerated membrane degradation, apparently because the secondarily formed hydroxymethyl radical diffused into the separation layer. By contrast, tBuOH inhibited membrane degradation likely because tBuOH quenched hydroxyl radical and secondary radicals of tBuOH were prevented from diffusing into the separation layer due to their large size. In the initial loss of membrane permeability and the rejection after CT of 3000 ppm-h were proportional to the free chlorine concentration. Results highlight the potential significance of radical formation and quenching processes when evaluating membrane degradation by oxidants to under field conditions.

High-performance polyamide membrane with tailored water channel prepared via bionic neural networks for textile wastewater treatment

A novel aromatic polyamide (PA) reverse osmosis membrane with nanoscale water channels and hydrophilic molecular skin surface, similar to those of biomimetic neural networks, was realized for textile wastewater treatment via the facile strategy of interfacial polymerization with the addition of free radicals.

Density, Elastic Constants, and Thermal Conductivity of Interfacially Polymerized Polyamide Films for Reverse Osmosis Membranes

The thickness, volumetric mass density, longitudinal elastic constant, and thermal conductivity of fully aromatic polyamide reverse osmosis membrane active layers synthesized by interfacial polymerization were measured using a combination of Rutherford backscattering spectrometry, quartz crystal microbalance measurements of areal mass density, optical ellipsometry, atomic force microscopy, picosecond acoustics, and time-domain thermoreflectance (TDTR). A support-free synthesis approach was used to produce smooth three-dimensionally cross-linked polyamide films from m-phenylenediamine (MPD) and trimesoyl chloride (TMC) at a fixed molar ratio of 99:1 MPD:TMC with 0.1–2 wt % MPC and 0.005–0.1 wt % TMC. A novel method to measure the areal mass density of nanoscale polymer films by Rutherford backscattering spectrometry was developed to aid in characterization of the volumetric mass density of the membranes. The volumetric mass density and longitudinal elastic constant of free-standing polyamide films increase...

Zwitterion-like, Charge-Balanced Ultrathin Layers on Polymeric Membranes for Antifouling Property.

Zwitterions of charge-balanced units have super-low fouling properties induced by ionic solvation, but their extensive applications in polymeric substrates are strictly constrained by current constructing strategies. A zwitterion-like, charge-balanced ultrathin layer with high antifouling capacity was covalently constructed on delicate aromatic polyamide (PA) reverse osmosis (RO) membranes via a mild and solvent-free grafting-to strategy. Two oppositely charged commercial short-chain carbonyl alkenes, 2-acrylamido-2-methylpropanesulfonic acid (AMPS) and methacryloxyethyltrimethylammonium chloride (DMC), were directly mixed-grafted with amino groups on PA RO membrane surface via Michael addition. Under ambient temperature and pressure, these oppositely charged compounds were assembled into a zwitterion-like, charge-balanced ultrathin layer. The dynamic fouling experiments indicated that the modified membrane exhibited strong antifouling properties and excellent permeation recovery abilities. Surface characterization revealed that the selective layer thickness and surface roughness were not measurably changed. More meaningful is that the typical ridge-and-valley surface structure and the excellent separation performance were both well preserved after modification. This opens a universal avenue to construct a zwitterion-like, ultrathin antifouling layer on the delicate polymer substrate without compromising its original matrix structure and performance, which has promising application in areas of biosensors, tissue engineering, and biomaterials.

Chlorine-Resistant Polyamide Reverse Osmosis Membrane with Monitorable and Regenerative Sacrificial Layers.

Improving chlorine stability is a high priority for aromatic polyamide (PA) reverse osmosis (RO) membranes especially in long-term desalination. In this Research Article, PA RO membranes of sustainable chlorine resistance was synthesized. Glycylglycine (Gly) was grafted onto the membrane surface as a regenerative chlorine sacrificial layer, and the zeta-potential was used to monitor the membrane performance and to conduct timely regeneration operations for chlorinated Gly. The Gly-grafted PA membrane exhibited ameliorative chlorine resistance in which the N-H moiety of glycylglycine served as sacrificial pendants against chlorine attacks. Cyclic chlorination experiments, combined with FT-IR and XPS analysis, were carried out to characterize the membrane. Results indicated that the resulting N-halamines could be fast regenerated by a simple alkaline reduction step (pH 10). A synchronous relationship between the zeta-potential and the chlorination extent of the sacrificial layer was observed. This indicated that the zeta-potential can be used as an on-site sensor to conduct a timely regeneration operation. The intrinsic mechanism of the surface sacrificial process was also studied.

Experimental Study and Mathematical Modelling of Zinc Removal by Reverse Osmosis Membranes

In this study, aromatic polyamide reverse osmosis membranes were used to remove zinc ions from electroplating wastewater. Influence of different operating conditions such as time, zinc concentration and pressure on reverse osmosis process efficiency was studied. The experimental results showed, concentration of zinc in permeate increase with increases of time from 0 to 70 min, and flux of water through membrane decline with time. While, the concentrations of zinc in permeate increase with the increase in feed zinc concentration (10–300 mg/l), flux decrease with the increment of feed concentration. The raise of pressure from 1 to 4 bar, the zinc concentration decreases and the flux increase. The highest recovery percentage was found is 54.56% for reverse osmosis element, and the highest rejection of zinc was found is 99.49%. Experimental results showed that the concentrations of zinc ion in permeate was lower than the permissible limits (i.e. ˂ 10 ppm). A mathematical model describing the process was investigated and solved by using MATLAB PROGRAM. Theoretical results were consistent with the experimental results approximately 90%.

Modeling organic fouling of reverse osmosis membrane: From adsorption to fouling layer formation

A combined model is developed for flux decline of reverse osmosis (RO) membranes, which accounts for initial fouling due to adsorption and subsequent fouling due to the growth of a fouling layer. The predicted data are in excellent agreement with the experimental ones obtained over crossflow filtration of organic wastewaters using aromatic polyamide RO membranes over the entire course of the filtration. The model also provides a smooth transition from the adsorption regime to the fouling layer filtration regime. Based on the flux profiles and the analysis of the membrane surface, a two-step fouling mechanism is proposed to describe the evolution of fouling during continuous crossflow operation of RO membranes.

Potential applications of abandoned aromatic polyamide reverse osmosis membrane by hypochlorite degradation

Reverse osmosis (RO) membranes might experience significant changes in surface structure and performance after disinfection has been applied, or after membrane cleaning, because of hydrolysis and oxidation processes.

Enhancing the performance of aromatic polyamide reverse osmosis membrane by surface modification via covalent attachment of polyvinyl alcohol (PVA)

Surface modification is a promising way to improve membrane performance. In this study, a commercial aromatic polyamide thin-film composite reverse osmosis membrane was modified through sequential surface treatment with glutaraldehyde aqueous solution followed by polyvinyl alcohol (PVA) aqueous solution. ATR-FTIR spectroscopy, SEM, AFM, measurements of streaming potential and contact angle and cross-flow permeation tests were employed to investigate the influence of modification on membrane performance. It was illustrated that PVA molecules were covalently attached on the surface of the pristine membrane and both membrane surface physico-chemical and permeation properties could be tuned through changing the PVA content. The covalent attachment of PVA resulted in an improved surface hydrophilicity, a declined surface negative charge and a slightly increased surface roughness, and could enhance membrane salt rejection and water flux simultaneously. The modification was proved to be effective in improving membrane antifouling property to the foulants of bovine serum albumin, sodium dodecy sulfate and dodecyltrimethyl ammonium bromide through increasing anti-adsorption capability and in enhancing membrane chlorine stability through reducing chlorination sites and preventing the underlying polyamide backbones from chlorine attack. Furthermore, the modification could effectively enhance the membrane rejection performance and antifouling property in tertiary treatment of industrial effluent.

Improving the water flux and bio-fouling resistance of reverse osmosis (RO) membrane through surface modification by zwitterionic polymer

Biofouling is a critical problem to the development of reverse osmosis (RO) membrane technology. In this study, a commercial available aromatic polyamide RO membrane (TE membrane) was surface modified to improve its permselectivity and anti-biofouling property. The TE membrane was modified by the redox initiated graft polymerization of N,N′-dimethylaminoethyl methacrylater (DMAEMA), followed by the surface quaternization reaction with 3-bromopropionic acid (3-BPA) to obtain the zwitterionic carboxybetaine methacrylate (CBMA) polymer chains on the membrane surface and the modified membrane was named TE–PCBMA membrane. The grafting of DMAEMA and the quaternization reaction were analyzed by ATR–FTIR and XPS. The zeta analysis (at pH 7.0) indicated that the TE–PCBMA membrane surface was near neutrality, while the unmodified TE membrane surface was negative charged. The water flux of the TE–PCBMA membrane increased 22.55% than that of the unmodified membrane, while the salt rejections of the membranes were nearly the same. The proteins fouling tests (contaminations were positively charged lysozyme and negatively charged bovine serum albumin, pH 6.8), indicated that the water fluxes of the TE–PCBMA membrane all decreased less than those of the unmodified TE membrane, and after the membranes were cleaned, the water flux recovery rates of the TE–PCBMA membrane were higher than 92%, which were far higher than those of the unmodified TE membrane. Moreover, the mortality of Escherichia coli and Bacillus subtilis, after contacted with the TE–PCBMA membrane, could reach to 99%. The results indicated that the TE–PCBMA membrane had improved water flux, and was endowed easy-cleaning, anti-adhesive and anti-microbial properties to prevent bacterial deposition and growth on the membrane surface.

Positively charged aromatic polyamide reverse osmosis membrane with high anti-fouling property prepared by polyethylenimine grafting

In this work, Polyethylenimine (PEI) with different molecular weights was used to alter the surface charge of the aromatic polyamide RO membrane, and the positively charged RO membrane was fabricated by carbodiimide-induced grafting with PEI. The effect of PEI grafting on the surface property (charge, hydrophilicity, chemical elements and morphology) and permselectivity of membrane was analyzed. The results show that the surface charge of resulting membrane was successfully reversed (isoelectric point changed from pH3.1 to pH8.0) by the PEI grafting. The membrane surface hydrophilicity was obviously improved as well, which could not only compensate the effect of the increased trans-membrane resistance resulted from the grafted PEI on water flux, but also benefit the anti-fouling property. The morphology of membrane was not obviously affected by the PEI modification. Furthermore, two kinds of representative pollutants, basic protein (lysozyme) and cationic surfactant (DTAC and CTAC) were used as the model pollutant to evaluate membrane anti-fouling property. The PEI-modified membranes show high anti-fouling property to the positively charged pollutants. The novel PEI-modified membrane complements the existing short supply of positively charged RO membranes.

Thinning of reverse osmosis membranes by ionic liquids

In this study, ionic liquids (ILs) were used to thin out the dense layer and, in turn, tune the surface properties and separation performance of commercial aromatic polyamide reverse osmosis membranes. It was observed that the structure of the ILs and dipping time had a strong impact on the dense layer thickness and morphology. This can be understood in terms of the dissolubility and interaction force between ILs and the organic membrane surface, such as hydrogen bonding and π–π interactions. Among the ILs synthesized, 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) showed the most promising thinning effects. It was observed that the thickness of the dense layer on the surface decreased from 127 to 67nm after dipping treatment with [BMIM]Cl for 24h. The water flux was enhanced by 20% at the expense of a slight decline of salt rejection. AFM, contact angle and zeta potential analyses suggest that the surface hydrophilicity and electronegativity increased, while the roughness decreased, which improved the anti-fouling properties.

Surface modification of seawater desalination reverse osmosis membranes: Characterization studies & performance evaluation

In this work we report surface modification of commercial reverse osmosis membranes by depositing ultrathin copolymer coatings, which could potentially enhance the biofouling resistance of RO membranes. Hydrophilic monomer hydroxyethyl methacrylate (HEMA) and a hydrophobic monomer, perfluorodecyl acrylate (PFDA) were copolymerized directly on the active layer of commercial aromatic polyamide reverse osmosis (RO) membranes using an initiated Chemical Vapor Deposition (iCVD) technique. Attenuated total reflective Fourier transform infrared spectra (ATR-FTIR) verified the successful modification of the membrane surfaces as a new FTIR adsorption band around 1730cm−1 corresponding to carbonyl groups in the copolymer film appeared after the deposition. X-ray Photoelectron spectroscopy (XPS) analysis also confirmed the presence of the copolymer film on the membrane surface by showing strong fluorine peaks emanating from the fluorinated alkyl side chains of the PFA molecules. Contact angle measurements with deionized water showed the modified membrane surfaces to be initially very hydrophobic but quickly assumed a hydrophilic character within few minutes. Atomic Force Microscopy (AFM) revealed that the deposited films were smooth and conformal as the surface topology of the underlying membrane surface remained virtually unchanged after the deposition. FESEM images of the top surface also showed that the typical ridge-and-valley structure associated with polyamide remained intact after the deposition. Short-term permeation tests using DI water and 2000ppm NaCl water showed that the deposited copolymer coatings had negligible effect on permeate water flux and salt rejection.

Improving fouling resistance of thin-film composite polyamide reverse osmosis membrane by coating natural hydrophilic polymer sericin

In this study, a commercial thin-film composite aromatic polyamide reverse osmosis membrane was modified through coating a surface layer of natural polymer sericin for improved antifouling property. The deposition of sericin on the membrane was carried out through dip-coating followed by in situ cross-linking and was confirmed by ATR-FTIR spectroscopy. The changes of surface charge, hydrophilicity and roughness that resulted from the sericin application were analyzed using zeta-potential analysis, contact angle measurement and atomic force microscopy, respectively. The separation performance was evaluated through cross-flow permeation tests. It was found that the sericin-coated membrane showed improved surface hydrophilicity, enhanced surface negative charge, smoothed surface morphology, and decreased pure water permeability and salt permeability coefficient. The results of fouling experiments with bovine serum albumin aqueous solution revealed that the fouling resistance of the reveres osmosis membrane could be effectively improved by coating sericin surface layer. Although the initial flux of the modified membrane was lower than that of the unmodified membrane under the same operating pressure due to the additional hydraulic resistance, the rate of flux decline slowed after modification due to the mitigation of foulant deposition on the membrane surface and compensated for the initial flux decline within 40h.

Surface modification of a commercial thin-film composite polyamide reverse osmosis membrane through graft polymerization of N-isopropylacrylamide followed by acrylic acid

Surface modifications including physical and chemical treatments are effective approaches to improve the fouling and chlorine resistances of the commercial thin-film composite (TFC) aromatic polyamide reverse osmosis (RO) membranes. However, the major problem facing the modification is the influence on membrane performance. In this study, surface modification of a commercial TFC RO membrane was performed through redox initiated graft polymerization of N-isopropylacrylamide (NIPAm) followed by acrylic acid (AA) and the modified membrane showed improved membrane properties. Membrane characterization was conducted through ATR-FTIR analysis, zeta-potential analysis, contact angle measurement and cross-flow permeation test. Changes in surface characteristics confirmed the graft polymerizations of NIPAm and AA. The surface of the membrane became more hydrophilic and negatively charged after modification. The membrane modified under certain conditions showed both increased water flux and salt rejection. Fouling experiments with bovine serum albumin (BSA) aqueous solution demonstrated that the modification could mitigate the deposition of foulants on the membrane surface through enhancing electrostatic repulsion and lowering hydrophobic interaction between BSA molecules and membrane surface and thereby improving fouling resistance. Furthermore, chlorination tests also showed that surface modification through graft polymerization of NIPAm followed by AA could effectively enhance the chlorine stability of the polyamide membrane.

Fouling resistant zwitterionic surface modification of reverse osmosis membranes using amino acid l-cysteine

The suppression of membrane fouling has been a severe challenge faced by membrane industry. In this study, zwitterionic amino acid l-cysteine were grafted on to the commercial thin-film composite (TFC) aromatic polyamide reverse osmosis membranes in order to improve membrane fouling resistance. The modification was carried out through bonding the thiol group of the l-cysteine to the allyl functionalized membrane. Attenuated total reflection spectroscopy (ATR-FT-IR) and X-ray photoelectron spectroscopy (XPS) tests confirmed the success of grafting reaction. Contact angle measurement and atomic force microscopy (AFM) images indicated higher hydrophilicity and smoother surface of the modified membrane. Water permeability and salt rejection of the membranes were measured using a stirred dead end filtration cell. Although membrane modification tended to enhance membrane salt rejection, membrane permeability declined. Cross flow filtration of bovine serum albumin (BSA) solution and dodecyltrimethylammonium bromide (DTAB) as model foulants displayed a lower fouling propensity of the modified membrane. Improved fouling resistance of the modified membrane was attributed to its enhanced hydrophilicity and lower surface roughness.