Glass transition behaviors of interfacially polymerized polyamide barrier layers on thin film composite membranes via nano-thermal analysis

Thin film composite (TFC) reverse osmosis (RO) membranes enjoy widespread use in desalination, but their sensitivity to oxidizing agents such as chlorine remains a continuing challenge. In contrast to many reports on the chemical aspects associated with decreased membrane performance after chlorine exposure, studies on the fundamental physical properties of the polyamide barrier layer (PBL) of TFC membranes are scarce. This omission is mostly due to the lack of techniques capable of characterizing such interfacially polymerized PBLs, which are ultrathin and insoluble. The focus of this study is the development of an AFM-based nano-thermal analysis technique that provides the first-ever result for the direct measurement of the glass transition temperature (T_g) of the PBL on several commercial TFC RO membranes. Moreover, the technique is utilized to study the changes in T_g of the PBL after exposure to chlorine solutions as a function of concentration and duration at constant pH. Results indicate significant and systematic reduction in T_g of the PBL with increasing chlorine concentration and exposure time.     

Flux enhancement of thin film composite RO membrane by controlled chlorine treatment

A major stumbling block in polyamide thin film composite RO membrane performance is its incompatibility with chlorine and oxidizing agents. The amide bond of the membrane is highly vulnerable to chlorine attack. Two reactions are possible with chlorine exposure i.e. N-H bond chlorination and/or aromatic ring chlorination. In this way, chlorine may cause degradation/modification in the membrane leading to deterioration in performance.However, low concentration of chlorine up to a certain time may give synergistic effect on membrane and improve its performance. Chlorine solution, if exposed to membrane for certain time gives enhancement in trans-membrane flux of the membrane.The same solution if exposed for more time deteriorates ultra-thin polyamide layer of TFC membrane. Conspicuously, the membrane with poor salt rejection and flux benefited more as compared to the membrane with better performance. In the present study, membranes with different salt rejection and flux were taken and exposed to the inorganic chlorine solution. The inorganic chlorine solutions were made by dissolving sodium hypochlorite in pH buffer. The different solutions were made by varying pH to investigate the pH dependence. The membrane samples were kept in solution for different time durations. The exposure time was monitored and the exposure level was taken in terms of ppm h (ppm chlorine solution exposed to membrane for a fixed time in h). With the same chlorine concentration, effect of varying pH was studied. Spiral wound TFC membrane modules were also subjected to chlorine solution to study its effect.     

Preparation and characterization of thermally crosslinked chlorine resistant thin film composite polyamide membranes for reverse osmosis

Currently, polyamide reverse osmosis membranes are highly effective for desalination, industrial process water, and home drinking water. However, they have poor resistance to strong oxidants especially chlorine due to chain cleavage of aromatic polyamide. In general, aromatic polyamide RO membranes are essentially random copolymers consisting of the linear and crosslinked structures. The amide ring is sensitive to attack by chlorine because it is an electron-rich region. Therefore, the activated carbon or sulfite addition processes are essential to remove the chlorine in the separation processes. Many research groups have studied to improve the chlorine-resistance RO membrane having hydrophilic groups (− SO₃H and − COOH) or nitro groups (− NO₂) such as electron acceptors. In this study, thin film composite polyamide RO membranes were prepared by interfacial polymerization method including cross-linking agents having hydroxyl groups to improve the chlorine-resistance. The chlorine-resistance of polyamide RO membrane was influenced by the thermal cross-linking conditions (temperature and time) and cross-linking density of polyamide membranes.     

New chlorine‐resistant polyamide reverse osmosis membrane with hollow fiber configuration

New asymmetric hollow fiber reverse osmosis (RO) membrane was developed from a new chlorine‐resistant copolyamide [4T‐PIP(30)] with a piperazine moiety by a conventional phase‐separation method. The new 4T‐PIP(30) hollow fiber membrane has the same low‐pressure RO performance as cellulose triacetate hollow fiber membrane (FR = 205 L/m² day, Rj = 99.6%) and superior chlorine resistance as well as pH resistance to conventional aramid RO membranes. Structural analysis and viscoelastic study revealed that the new hollow fiber consisted of a top skin, dense layer, and microporous layer, and that it began to decrease its elasticity at 80°C in water, which is possibly related to its good and stable RO performance around room temperature. Several kinds of RO modules were made from the new hollow fiber membranes, for which RO performances were stable for 2 years in chlorinated feed water desalination (the free residual chlorine ranged from 0.l to 1.1 mg/L). © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 517–527, 2001     

Change of membrane performance due to chlorination of crosslinked polyamide membranes

A systematic investigation of the relationship between chlorine exposure of a thin film composite crosslinked polyamide membrane (LE membrane, FilmTec©) and changes in membrane performance (water flux and salt rejection) is discussed here. Performance change of crosslinked polyamide membranes due to chlorination depended on pH and concentration of chlorine in the soaking bath. Membranes chlorinated at low pH and high chlorine concentration showed flux decreases at an early stage of filtration and then increases with filtration time. On the other hand, membranes chlorinated at high pH and low chlorine concentration showed flux increases at an early stage and then decreases with filtration time. Performance of chlorinated polyamide membranes was determined by the balance between rearrangement of polymer chains and the distortion of the chains due to chlorination. A conceptual model of performance change was proposed consistent with the chlorination of polyamide membranes. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5895–5902, 2006     

Impact of halogen based disinfectants in seawater on polyamide RO membranes

The effect of halogen based disinfectants including monochloramines (NH₂Cl), free chlorine (HOCl/OCl⁻), and free oxidants (mixture of HOCl/OCl⁻ and HOBr/OBr) on polyamide membrane was studied in synthetic Ocean seawater. Formation and stability of these oxidants were also examined. Permeability and salt rejection of flat sheet polyamide RO membranes following exposure to the halogen based oxidants were compared to the baseline performance of unexposed membranes. The ratio between free chlorine and free bromine was found to depend on the ratio between the bromides, naturally found in seawater, and the added chlorine. Bromide enhanced the degradation of monochloramines but did not affect the stability of free chlorine. All the oxidants damaged the polyamide membranes studied while the free oxidants appeared to be the most aggressive.     

The chlorination and chlorine resistance modification of composite polyamide membrane

Membrane‐based separation technology is one of the most active separation technologies being employed in water treatment. Polyamides (PA) are widely used membrane materials because they exhibit excellent performance, such as high flux with high salt rejection, and enhanced stability against wide range of pH and temperature. Unfortunately, PA membranes exhibit extremely poor resistance to chlorine leading to increased operation costs and decreased membrane lifetime. In this study, we find new ways for prolonging membrane lifetime and reducing the operating costs by investigating the chlorination and modification of PA membranes. Varying concentrations of hypochlorite were used to chlorinate a commercial reverse osmosis membrane (BW‐30, DOW). The results showed that short‐time exposure to high concentrations of hypochlorite could cause more serious problems to membranes than long‐time exposure to low concentrations under the similar total exposure. The performance of the chlorinated membranes was recovered to some extent after treatment with NaOH solution (pH 10), indicating that the alkali treatment could initiate the reversible regeneration of chlorinated membranes. Furthermore, an industrial grade epoxy resin was used to modify the membranes to enhance the chlorine resistance via the reaction between the amide nitrogen and epoxy bond. The successful modifications were confirmed by attenuated total reflectance Fourier transform infrared spectroscopy, X‐ray photoelectron spectroscopy, scanning electron microscopy, and atomic force microscopy. Moreover, the chlorination tests showed that the modifications performed in these experiments enhanced the chlorine resistance of the membranes, especially for the membranes exposed to low concentration of chlorine. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41584.     

Self-doped sulfonated polyaniline ultrafiltration membranes with enhanced chlorine resistance and antifouling properties

Membranes heavily rely on chlorination to diminish (bio)fouling, but chlorination can also lead to membrane degradation. We developed sulfonated polyaniline (S-PANI) ultrafiltration (UF) membranes with improved chlorine resistance and intrinsic antifouling properties. The S-PANI membranes were synthesized through Non-solvent Induced Phase Separation (NIPS). Membrane performance was evaluated under harsh chlorine conditions (250 ppm sodium hypochlorite for 3 days under different pH conditions). The S-PANI membranes showed improved chlorine resistance including a stable performance without changes in model foulant BSA rejection. In contrast, PANI membranes suffered critical structural damage with complete leakage and commercial PES membranes showed a 76% increase in pure water flux and a noticeable change in BSA rejection. Small changes in S-PANI membrane performance could be linked to membrane structural changes with pH, as confirmed by SEM, IR spectroscopy, and contact angle measurements. Additionally, the S-PANI membranes showed better antifouling properties with a high flux recovery ratio in comparison to PANI membranes using alginic acid, humic acid, and BSA model foulants. Chemical cleaning by sodium hypochlorite re-instated the transport properties to its initial condition. Overall, the developed S-PANI membranes have a high chlorine tolerance and enhanced antifouling properties making them promising for a range of UF membrane applications.     

耐氧化芳香聚酰胺纳滤膜的研究进展

芳香聚酰胺纳滤膜不耐氧化,易被活性氯氧化降解,导致膜性能急剧下降,缩短膜的使用寿命,目前已成为制约芳香聚酰胺纳滤膜应用和发展的关键问题之一。本文综述了芳香聚酰胺纳滤膜的材料和结构,重点概述了芳香聚酰胺氯化和膜性能下降的机制,并进一步介绍了近年来耐氧化芳香聚酰胺纳滤膜的研究进展。     

Recent advances in nanofiltration,reverse osmosis membranes and their applications in biomedical separation field

In the face of human society's great requirements for health industry, and the much stricter safety and quality standards in the biomedical industry, the demand for advanced membrane separation technologies continues to rapidly grow in the world. Nanofiltration (NF) and reverse osmosis (RO) as the high-efficient, low energy consumption, and environmental friendly membrane separation techniques, show great promise in the application of biomedical separation field. The chemical compositions, microstructures and surface properties of NF/RO membranes determine the separation accuracy, efficiency and operation cost in their applications. Accordingly, recent studies have focused on tuning the structures and tailoring the performance of NF/RO membranes via the design and synthesis of various advanced membrane materials, and exploring universal and convenient membrane preparation strategies, with the objective of promoting the better and faster development of NF/RO membrane separation technology in the biomedical separation field. This paper reviews the recent studies on the NF/RO membranes constructed with various materials, including the polymeric materials, different dimensional inorganic/organic nanomaterials, porous polymeric materials and metal coordination polymers, etc. Moreover, the influence of membrane chemical compositions, interior microstructures, and surface characteristics on the separation performance of NF/RO membranes, are comprehensively discussed. Subsequently, the applications of NF/RO membranes in biomedical separation field are systematically reported. Finally, the perspective for future challenges of NF/RO membrane separation techniques in this field is discussed.     

Improvement of performance of polyamide reverse osmosis membranes using dielectric barrier discharge plasma treatment as a novel surface modification method

In this research, surface modification of aromatic polyamide thin film composite (TFC) reverse osmosis (RO) membranes was carried out using dielectric barrier discharge (DBD) plasma treatment to improve the performance and fouling resistance of prepared RO membranes. First, polyamide TFC RO membranes were synthesized via interfacial polymerization of m‐phenylenediamine and trimesoyl chloride monomers over microporous polysulfone support membrane. Next, the DBD plasma treatment with 15 s, 30 s, 60 s, and 90 s duration was used for surface modification. The surface properties of RO membranes were characterized by attenuated total reflectance Fourier transform infrared spectroscopy (ATR‐FTIR), SEM, AFM, and contact angle measurements. The ATR‐FTIR results indicated that DBD plasma treatment caused hydrogen bonding on the surface of RO membranes. Also, the contact angle measurement showed that the hydrophilicity of the membranes was increased due to DBD plasma treatment. The changes in the membranes surface morphology indicated that the surface roughness of the membranes was increased after surface modification. In addition, it was found that the DBD plasma treatment increased the water permeation flux significantly and enhanced sodium chloride (NaCl) salt rejection slightly. Moreover, the filtration of bovine serum albumin revealed that the antifouling properties of the modified membranes had been improved. POLYM. ENG. SCI., 59:E468–E475, 2019. © 2018 Society of Plastics Engineers     

Characterization of methyl‐substituted polyamides used for reverse osmosis membranes by positron annihilation lifetime spectroscopy and MD simulation

Three kinds of polyamides were synthesized from three diamines and 1,3,5‐benzenetricarbonyl trichloride (TMC). The diamines used were m‐phenylene diamine, N‐methyl‐m‐phenylenediamine, and N,N′‐dimethyl‐m‐phenylenediamine. The average free volume sizes of the polyamides were measured by positron annihilation lifetime spectroscopy (PALS), and the free volume fractions were evaluated by molecular dynamics (MD) simulations. The methyl substitution on amino groups of diamines brought about an increase in interstitial space of molecular chains of the polyamides. In addition, reverse osmosis (RO) membranes were prepared by interfacial polymerization from the three diamines and TMC. The increase in the degree of methyl‐substitution of diamines led to increased chlorine resistance and decreased salt rejections of the polyamide RO membranes. Thus, the methyl‐substitution of diamines significantly influenced membrane performance. The vacancy sizes and fractional volumes in polyamides evaluated by PALS measurement and MD simulation were well correlated with salt rejection of polyamide RO membranes. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Reverse osmosis membrane sensitivity to ozone and halogen disinfectants

The sensitivity of commercial reverse osmosis (RO) membranes toward halogen and ozone disinfectants has been measured at carefully controlled concentration and pH levels. Membrane sensitivity varies with polymer type, disinfectant chemical, and solution pH. Aromatic polyamide membranes are damaged by halogen addition to aromatic rings within the polymer. This process follows predictable reaction kinetics. Polymer viscosity changes with increasing membrane damage have also been followed. Results of this study will be useful in planning disinfection strategies for RO units in the field.     

Poly(2-vinylimidazoline) composite reverse osmosis membranes

The preparation of poly(2-vinylimidazoline) (PVI) composite reverse osmosis membranes is described. The membranes were prepared from PVI interfacially crosslinked by the use of 1,3-benzenedicarbonyl chloride, 3-(chlorosulphonyl)benzoyl chloride (A) and 3,5-(dichlorosulphonyl)benzoyl chloride (B), the latter two crosslinking agents being synthesized for the purpose of the study. Initial screening of the reverse osmosis (RO) performance of the membranes are reported on as well as the more detailed testing in the presence of chlorine and at various pH values. Modifications to the PVI/A composite membranes by the addition of hydrophilic fillers is also described.     

Permselectivities of 2,2′‐dimethyl‐4,4′‐bis(aminophenoxyl)biphenyl diphenyl methane–based aromatic polyamide membranes for aqueous alcohol mixtures in pervaporation and evapomeation

The separation of aqueous alcohol mixtures was carried out by use of a series of novel aromatic polyamide membranes. The aromatic polyamides were prepared by the direct polycondensation of 2,2′‐dimethyl‐4,4′‐bis(aminophenoxyl)biphenyl (DBAPB) with various aromatic diacids, such as terephthalic acid (TPAc), 5‐tert‐butylisophthalic acid (TBPAc), and 4,4′‐hexafluoroisopropylidenedibenzoic acid (FDAc). The pervaporation and evapomeation performance of these novel aromatic polyamide membranes for dehydrating aqueous alcohol solution were investigated. The solubility of ethanol in the aromatic polyamide membranes is higher than that of water, but the diffusivity of water through the membrane is higher than that of ethanol. The effect of diffusion selectivity on the membrane separation performances plays an important role in the evapomeation process. Compared with pervaporation, evapomeation effectively increases the permselectivity of water. Moreover, the effect of aromatic diacids on the polymer chain packing density, pervaporation, and evapomeation performance were investigated. It was found that the permeation rate could be increased by introduction of a bulky group into the polymer backbone. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2688–2697, 2003     

Study of the structure and transport of asymmetric polyamide membranes for reverse osmosis using the electron spin resonance (ESR) method

The electron spin resonance technique (ESR) was used to study the structure and transport of asymmetric aromatic polyamide membranes. TEMPO (2,2,6,6-tetramethyl-1-piperridinyloxy free radical) was used as a spin probe that was brought into the membrane either by (a) immersion ofthe membranes in aqueous TEMPO solutions, (b) reverse osmosis (RO) experiments with feed solutions involving TEMPO or (c) blending TEMPO in casting solutions. The membranes were further tested for the separation of sodium chloride and TEMPO from water by RO. It was concluded that aromatic polyamide membranes contain water channels in the polymer matrix like cellulose acetate membranes. The presence of such water channels allows aromatic polyamide membranes to be used as RO membranes. The diffusion of organic solutes through the water channels seems much slower in aromatic polyamide membranes than in cellular acetate membranes, which probably causes a higher separation of organic solutes by aromatic polyamide membranes than cellulose acetate membranes. A comparison was made with other RO membranes (cellulose acetate, CA) and ultrafiltration membranes (polyethersulphone, PES). It was observed that the ESR technique can be used to study the structure of OF and RO membranes. The presence of water channels in the polymer matrix seems indispensable for the RO membrane.     

Study of reverse osmosis membranes fouling by inorganic salts and colloidal particles during seawater desalination

Fouling phenomenon is considered among the major reasons that cause significant increase of operating cost of desalination plants equipped with reverse osmosis (RO) membranes. This phenomenon is studied in the present work in the case of RO polyamide aromatic membranes using model seawater containing inorganic salts and colloidal compounds. Different solubility conditions of CaCO₃ and CaSO₄ were applied to study RO performances with and without colloid presence. During experiments, the membrane permeate fluxes were continuously monitored. Moreover, studies of chemical composition, structure, and morphology of the materials deposited on the membrane surface were conducted using energy dispersive microanalysis (EDS) X-ray diffraction and scanning electronic microscopy (SEM). Results show that in conditions of calcium carbonate oversaturation there is a reduction in the permeate flow of 11.2% due to fouling of the membrane by the precipitation of this compound. While in the same conditions of calcium sulphate oversaturation the reduction of the flow is 5%, so we can conclude that in conditions of oversaturation of both salts, calcium carbonate produces a greater fouling of the membrane that in its view causes greater decrease in the flow of permeate. All this based on the results of the test with both salts in oversaturated conditions. Resulting in the formation of calcite and gypsum crystals onto the membranes as XRD analyses stated. Additional presence of colloidal silica in those conditions intensifies strongly the fouling, leading until to 24.1% of permeate flux decrease.     

Reverse osmosis performance and chlorine resistance of new ternary aromatic copolyamides comprising 3,3′‐diaminodiphenylsulfone and a comonomer with a carboxyl group

The preparation of asymmetric flat membranes from a series of novel aromatic polyamides comprising 3,3′‐diaminodiphenylsulfone and a comonomer with a carboxyl group(&BOND;COOH) were studied and the measurements of reverse osmosis (RO) performance and chlorine resistance were carried out. It was confirmed that the introduction of comonomer with a carboxyl group (MC or T_m) to the aromatic polyamides (3I or 3T) comprising 3,3′‐diaminodiphenylsulfone was very effective for the improvement of its RO performance. In particular, 3T‐MC(30), which was prepared from terephthaloyl dichloride and mixed diamine components of 3,3′‐diaminodiphenylsulfone and 3,5‐diaminobenzoic acid, exhibited not only some potential for sea water desalination (FR = 159 L/m² day, Rj = 98.3%) but also higher chlorine resistance than conventional Nomex‐type aramid [MI‐MC(0)]. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 505–513, 2001     

Fouling of reverse osmosis membranes by hydrophilic organic matter: implications for water reuse

Effluent organic matter (EfOM) is suspected as a major cause of fouling of reverse osmosis (RO) membranes in advanced wastewater reclamation. Among the main constituents in EfOM, polysaccharides are the most ubiquitous. The influence of solution chemistry and hydrodynamics on RO membrane fouling with alginate — a model for polysaccharides in secondary wastewater effluent — was systematically investigated. Results of fouling runs with alginate demonstrate that RO membrane fouling increases with decreasing pH, increasing ionic strength, and addition of calcium ions. At fixed solution ionic strength and pH, the presence of divalent calcium ions, at concentrations typical of those found in secondary wastewater effluent, had a dramatic effect on membrane fouling. However, for similar concentrations of divalent magnesium ions, fouling was negligible. The severe fouling in the presence of calcium is attributed to the formation of a thick, dense alginate gel layer on the membrane surface via calcium-alginate complexation and crosslinking (bridging) of alginate macromolecules by calcium. In addition to solution chemistry, hydrodynamic operating conditions — initial permeate flux and crossflow velocity — were also shown to influence RO membrane fouling with alginate.     

Elimination of biological fouling in seawater reverse osmosis desalination plants

The main trouble in the RO method of seawater desalination is biological fouling (bio-fouling) from microorganism growth. In this work, the growth rate of microorganisms in the Red Sea and sterilization by chlorine injection were measured and quantified. Furthermore, actual demonstration of RO pilot tests using cellulose tri-acetate (CTA) RO membranes with chlorine resistance was performed in RO plants where bio-fouling actually occurred. By carrying out direct chlorine sterilization of the RO membrane with an intermittent chlorine injection method, bio-fouling was eliminated. The combination of the CTA membrane and chlorine injection successfully prevented increases of differential pressure and stabilized product water quantity and quality.