Effect of membrane pretreatment on performance of solvent resistant nanofiltration membranes in methanol solutions

This paper reports the effect of membrane pretreatment using different organic solvents on the performance of polyamide, polyimide and polydimethylsiloxane (PDMS) membranes in methanol solutions. Membrane pretreatment using acetone, methanol and toluene results in significant changes of membrane flux and rejection for polyamide- and polyimide-based membranes (Desal-DK and STARMEM 228) due to membrane swelling. The Performance of a polydimethylsiloxane (PDMS)-based membrane (MPF-50) in methanol solutions was not significantly affected by membrane pretreatment.     

Study on preparation and performances of cellulose acetate forward osmosis membrane

Cellulose acetate (CA) forward osmosis (FO) membranes were prepared via a phase inversion process. CA was used as membrane material for FO. Acetone and 1,4-dioxane were employed as solvent. Polyvinylpyrrolidone (PVP), maleic acid, and methanol were applied as additives. An orthogonal experiment was performed to optimize the ratio of every component in the casting solution. The membrane with best performance was selected to concentrate an anthocyanin solution. Saturated sucrose solution (about 60°Brix) was fit for using as draw solution in the concentration experiment. Water flux, porosity, and rejection rate were measured to evaluate the membrane properties. Reverse water rinsing was used in cleaning membrane that was fouled by anthocyanin solution. Results showed that under membrane thickness of 100 μm, coagulation temperature at room temperature, and evaporation time of 30 s, the optimum components in casting solution were 13% CA, 45% 1,4-dioxane, 31% acetone, 2% maleic acid, 3% PVP, and 6% methanol. In the concentration experiment, the prepared FO membrane showed water flux of 2.04 L m^?2 h^?1 and rejection rate of 98.61%. In the membrane cleaning experiment, the water flux of the FO membrane recovered 87.51% after rinsing for 1 h. The prepared membranes and previously published membranes were compared which showed the prepared membrane could significantly improve the rejection rate for anthocyanin solution.     

Flux decline during nanofiltration of naturally-occurring dissolved organic matter: effects of osmotic pressure,membrane permeability,and cake formation

Nanofiltration of naturally-occurring dissolved organic matter (NOM) by an aromatic polyamide membrane was measured in a crossflow bench-scale test cell and modeled using a semi-empirical osmotic pressure/cake formation model. Our objective was to examine flux decline due to NOM fouling while explicitly accounting for flux decline due to osmotic effects and changes in membrane permeability. This approach allowed quantification of the effect of ionic composition on specific NOM cake resistance, and yielded insight into flux decline due to enhanced NaCl rejection by the NOM deposit. In the absence of NOM, increasing NaCl concentration reduced salt rejection and decreased membrane permeability. Flux decline was modeled by accounting for changes in osmotic pressure with time, and by employing an effective permeability. The addition of calcium significantly reduced rejection of sodium and feed conductivity, and thus mitigated flux decline. Increasing pH from 4 (near membrane pI) to 10 increased the effective permeability but also increased NaCl rejection, which resulted in greater flux decline. The presence of NOM caused greater flux decline resulting from a combination of NOM cake resistance and increased rejection of NaCl by negatively charged NOM functional groups. Increasing NaCl concentration had little effect on the mass of NOM deposited, but significantly increased the specific resistance of the NOM cake. The effect of ionic strength on specific resistance correlated with a reduction in NOM size, estimated by separate UF permeation experiments and size exclusion chromatography analysis of UF permeate. Therefore, increased specific cake resistance is consistent with a more compact, less porous cake. Flux decline by NOM solutions showed a maximum at pH 7, where salt rejection was also a maximum. Binding of calcium reduced the ability of NOM to enhance NaCl rejection, and likely increased NOM cake resistance. Flux decline caused by NOM fouling in the presence of calcium was only significantly different than that caused by NOM in a solution of NaCl at the same ionic strength when the calcium concentration corresponded to saturation of NOM binding sites.     

Structure-property relationships for novel wholly aromatic polyamide-hydrazides containing various proportions of para-phenylene and meta-phenylene units III. Preparation and properties of semi-permeable membranes for water desalination by reverse osmosis separation performance

Flat sheet asymmetric reverse osmosis membranes were successfully prepared from N,N-dimethylacetamide (DMAc) solutions of a series of novel wholly aromatic polyamide-hydrazides that contained different amounts of para- and meta-phenylene rings. These polyamide-hydrazides were synthesized by a low temperature solution polycondensation reactions of either 4-amino-3-hydroxybenzhydrazide or 3-amino-4-hydroxybenzhydrazide with an equimolar amount of either terephthaloyl dichloride [TCl], isophthaloyl dichloride [ICl] or mixtures of various molar ratios of TCl and ICl in anhydrous DMAc as a solvent. All the polymers have the same structural formula except of the way of linking phenylene units inside the polymer chains. The content of para- to meta-phenylene moieties was varied within these polymers so that the changes in the latter were 10 mol% from polymer to polymer, starting from an overall content of 0-100 mol%. All the membranes were characterized for their salt rejection (%) and water permeability (cm³ cm⁻² day⁻¹) of 0.5 N aqueous sodium chloride feed solution at 3924 kPa operating pressure. The effects of polymers structural variations together with several processing parameters to achieve the best combination of high selectivity and permeability were studied. Effects of various processing parameters of the membranes on their transport properties were investigated by varying the temperature and period of the solvent evaporation of the cast membranes, coagulation temperature of the thermally treated membranes, annealing of the coagulated membranes, casting solution composition, membrane thickness and the operating pressure. During the thermal treatment step, the asymmetric structure of the membranes with a thin dense skin surface layer supported on a more porous layer was established. The former layer seems to be responsible for the separation performance. The results obtained showed that membrane performance was very much influenced by all of the examined processing variables and that membranes with considerably different properties could be obtained from the same polymer sample by using different processing parameters. Thus, the use of higher temperatures and longer exposure times in the protomembrane forming thermal treatment step would result in a membrane of lower solvent content and with a thicker skin layer and consequently led to higher salt rejection at lower water permeability. Most significantly, the membrane properties clearly depended on the polymer structure. Under identical processing condition, substitution para-phenylene rings for meta-phenylene ones within the polymer series resulted in an increase in salt rejection capability of the membranes. This may be attributed to an increase in their chain symmetry associated with increased molecular packing and rigidity through enhanced intermolecular hydrogen bonding. This produces a barrier with much smaller pores that would efficiently prevent the solute particles from penetration. Coagulation temperature controls the structure (porosity) of the membrane particularly its supported layer and consequently its water permeability. Moreover, annealing of the prepared membranes in deionized water at 100 °C was found essential for useful properties in the single-stage separation applications, which required optimum membrane selectivity. Upon annealing, the membrane shrinks resulting in reducing its pore size particularly in the skin layer and consequently improving the salt rejection. Addition of lithium chloride to the casting solution produced a membrane with increased porosity and improved water permeability. Salt rejection capability of the membranes is clearly affected by the applied pressure, reaching its maximum at nearly 4000 kPa. Furthermore, the water permeability is inversely proportional to the membrane thickness, while the salt rejection is not substantially influenced.     

Preparation,structure characteristics and separation properties of thin-film composite polyamide-urethane seawater reverse osmosis membrane

A novel thin-film composite (TFC) seawater reverse osmosis membrane was developed by the interfacial polymerization of 5-chloroformyloxyisophthaloyl chloride (CFIC) and metaphenylenediamine (MPD) on the polysulphone supporting membrane. The performance of the TFC membrane was optimized by studying the preparation parameters, which included the reaction time, pH of the aqueous-MPD solution, monomer CFIC concentration, additive isopropyl alcohol content in aqueous solution, curing temperature and time. The reverse osmosis performance of the resulting membrane was evaluated through permeation experiment with synthetic seawater, and the structure of the novel membrane was characterized by using SEM, AFM and XPS. Furthermore, the separation properties of the TFC membrane were tested by examining the reverse osmosis performances of various conditions, the boron rejection performance and the long-term stability. The results show that the desired TFC seawater reverse osmosis membrane has a typical salt rejection of 99.4% and a flux of about 35 L/m² h for a feed aqueous solution containing 3.5 wt.% NaCl at 5.5 MPa, and an attractive boron rejection of more than 92% at natural pH of 7–8; that the novel seawater reverse osmosis membrane appears to comprise a thicker, smoother and less cross-linking film structure. Additionally, the TFC membrane exhibits good long-term stability.     

Development of a new styrene copolymer membrane for recycling of polyester fibre dyeing effluent

A new ultra-filtration membrane has been developed using indigenously available polymer, low cost solvents and a simple casting technique. The performance evaluation of the developed membrane in terms of pure water permeability (PWP), flux and rejection as compared to commercially available national and international membranes was carried out. It is observed that the newly developed membranes show acceptable performance both in terms of flux and rejection. The compressibility characteristic of the new membrane shows an improvement after suitable chemical modification through cross-linking reactions. The application perspectives of the membranes developed in our laboratory have been evaluated for the selective separation of dyes from typical textile waste stream of polyester fibre dyeing units with an aim to recover and recirculate the auxiliary chemicals and water in the process house. The rejection of dyes >98% and the permeate flux (0.8–1.0 m³/m² per day) values obtained for a specific type of textile effluent is of acceptable standards.The membranes were characterised for pore size and pore size distribution using molecular weight cut-off, combined bubble pressure and solvent permeability method. Membrane morphology has been studied using scanning electron microscopy (SEM). The other features of the developed membrane are its resistance to temperature and adverse chemical environment.     

Bench-scale membrane degumming of crude vegetable oil: Process optimization

Crude vegetable oils contain various minor substances such as phospholipids, coloring pigments, and free fatty acids (FFA) that may affect the quality of the finished oil. Reduction of energy costs and waste disposal are major concerns for many oil refiners who are looking for alternative methods to improve conventional refining methods, and during the last decade, energy-efficient membrane separation technology has evolved dramatically. This paper reports the cross-flow bench-scale crude vegetable oil membrane degumming test results using newly available, modified, hexane-resistant, high flux, and high selectivity non-aqueous membranes. Two membranes (DS-7 and AN03) were evaluated for their flux and rejection properties. Process parameters including pressure, temperature, feed velocity and volumetric concentration factor were examined. A 99.6% rejection of phospholipids and a flux of 26.8 l/m² h were achieved at pressure 300 psi, temperature 40°C, and feed velocity 220 l/h using DS-7 membrane, and significant reduction of coloring pigments was observed as well. Membrane fouling presented no big problem.     

Temperature-sensitive membranes prepared from blends of poly(vinylidene fluoride) and poly(N-isopropylacrylamides) microgels

In this study, temperature-sensitive membranes were prepared by phase transition of the mixture of the temperature-sensitive poly(N-isopropylacrylamides) (PNIPAAM) microgels and poly(vinylidene fluoride). The results of Fourier transformed infrared spectrometer, X-ray photoelectron spectroscopy, elemental analysis, and scanning electron microscope photographs indicate that the PNIPAAM microgels are distributed more in the inner membrane than on the surface. The scanning electron microscope photographs reveal the blend membranes having porous surfaces with nanometer sizes and porous cross-sections with micrometer sizes. The addition of the PNIPAAM microgels is found to improve the porosity, the pore size, water flux, as well as to enhance the hydrophilicity and anti-fouling property of the blend membranes. The blend membrane shows temperature-sensitive permeability and protein rejection with the most dramatic change at around 32 °C which is the lower critical solution temperature of PNIPAAM, when water or bovine serum albumin solution flow through. Specifically, below 32 °C, the blend membrane shows a high protein rejection ratio which decreases with increasing temperature and a low water flux which increases with increasing temperature; above 32 °C, the blend membrane shows a low protein rejection ratio which decreases with increasing temperature and a high water flux which increases with increasing temperature.     

Enhanced transport properties and thermal stability of agro-based RO-membrane for desalination of brackish water

This present work focused on preparation of economic and high performance reverse osmosis membranes, characterized by high transport properties (salt rejection and flux) towards desalination of brackish water. In this respect cellulose acetate from sugar-cane bagasse (BCA) and polymethyl methacrylate (PMMA) wastes were used as the substrates of membrane. The function of PMMA for enhancing the performance of bagasse-based cellulose acetate RO-membranes was investigated at operating pressure 35.85 bar and feed temperature 25 °C. The effects of casting solution, percentage of polymer and treatment of polymer by alkali (HPMMA) on the performance of RO-membrane were discussed. The preferable composition (wt.%) of the 90% BCA and 10% HPMMA was achieved salt rejection 92.18% and flux 325.9 l h⁻¹ m⁻². High water purity was obtained by pre-passing the salted water through membrane made from dissolved bagasse (methylol cellulose) together with PMMA, instead of ion exchanger, followed by passing the accepted water through BCA–HPMMA membrane, whereas the salt rejection increased to 98%. Also, by this approach we obtained high thermal stability membrane compared to CA-RO-membrane. This data gives highlight on possibility of application such type of membrane with high temperature operation conditions.     

Effect of electron beam irradiation on morphology and sieving characteristics of nylon-66 membranes

Nylon-66 is a typical semicrystalline polymer that can be crosslinked through electron beam (EB) irradiation. Crosslinking can dramatically change polymer properties. The objective of this research was to observe how EB irradiation affects morphology and sieving characteristics of nylon-66 membranes. EB irradiation was carried out in air at 60, 70 and 80 kGy doses. Scanning electron microscopy (SEM), swelling and gel content studies evidenced the morphological changes and crosslinking of nylon-66 membranes. Sieving characteristics were also measured using pure water permeation, and rejection of raffinose, vitamin B12, and mono- and divalent salts. These results show that nylon-66 membrane surface and permeation characteristics changed with different irradiation doses. The nylon-66 surface became denser and the gel content increased with increasing irradiation dose. Furthermore, pure water permeation decreased and small molecules were increasingly rejected with greater irradiation doses. The amount of rejection was between 33% and 88.4% for vitamin B12 and between 16% and 83% for raffinose. The highest vitamin B12 and raffinose rejection was seen with a N-80 membrane, and no rejection was measured with N-0 or N-60 membranes. Salt rejection, however, was very low, especially for NaCl, with only 10.51-46% rejected. Based on flux, permeability and uncharged solute (vitamin B12 and raffinose) rejection data, nylon-66 type N-70 and N-80 membranes were estimated to be in the nanofiltration (NF) range.     

Preparation of ceramic γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-TiO<Subscript>2</Subscript> nanofiltration membranes for desalination

As one of the most recently developed membrane separation processes, nanofiltration (NF) has found a number of industrial applications. Ceramic NF membranes are also regarded as the appropriate choice in many applications, due to their higher chemical and physical stability. In this study, the rejection of the chloride ion is investigated using bi-layered γ-Al₂O₃-TiO₂ NF membranes based on α-alumina supports. Compression is used in preparation of the supports and sol-gel dip-coating for the top-layer formation. SEM micrographs, XRD, and nitrogen adsorption/desorption isotherms are used for membrane characterisation. The results show that the calcination temperature (600°C) results in different crystal structures including the brookite phase of TiO₂, the γ phase of Al₂O₃, and a combined phase of aluminium-titanium oxides. The average pore size of the membrane was identified as 1.6 nm using an adsorption/desorption isotherm. The rejection was also studied for the chloride ion, using a cross-flow filtration module. Filtration tests were carried out under different pressures, pH values, and salt concentrations; these showed a smoother behaviour particularly around the isoelectric points (IEPs) due to the dual-layer structure, with the best rejection at pH of approximately 5.     

Characterization of ultrafiltration membranes. : Part IV. Influence of the deformation of macromolecular solutes on the transport through ultrafiltration membranes.

A theory was developed to point out the important parameters involved in the deformation of a flexible polymer in ultrafiltration through membranes. It appears that the deformation, and thus the easier transport of the polymer solute, occurs when the permeate flux reaches (or exceeds) a critical value which depends on the solution characteristics (solvent viscosity, concentration, temperature) as well as the membrane surface characteristics (porosity and pore radius on the surface). The experimental study was carried out with two flexible polymers: polyethylene glycol (PEG) and dextran. In ultrafiltration under constant pressure through a IRIS 3042 membrane, the increase of the concentration of PEG (of molecular weights 15,000, 20,000, 35,000) beyond a certain value caused a steady drop of rejection from a constant value. On the other hand, the increase of the applied pressure in the ultrafiltration of PEG 35,000 and dextran 70,000 afforded a sharp drop of rejections to a zero value as the fluxes increased steadily. Conversely, the changes in the concentration or the applied pressure did not affect the rejection when membranes of low permeability (Nuclepore 150 A, IRIS 3069) or of low pore size (PTGC) were used. These behaviours are consistent with the established theory.     

Fabrication of magnesium bentonite hollow fibre ceramic membrane for oil-water separation

In this study, low-cost magnesium bentonite (MB) was used for the fabrication of bentonite hollow fibre (BHF) membrane with high pure water flux. MB powder was initially characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF), particle size distribution (PSD) analyser, Brunnauer -Emmett- Teller (BET) method, and field emission scanning electron microscope (FESEM). The BHF membrane obtained was then fabricated through dope suspension mixing, phase inversion and sintering process. The dope suspension was prepared by mixing MB, dispersant, polymer binder, and solvent using a planetary ball mill. While the spinning process was carried out at the extrusion rate of 8 mL/min, a fluid bore rate of 10 mL/min and air gap of 5 cm, and this was followed by sintering operation at 950 °C, 1000 °C, 1050 °C, and 1100 °C. The resulting BHF membrane was characterized by scanning electron microscopy (SEM) and XRD; the porosity test, water flux and oil rejection were also examined. The SEM surface morphology of BHF at sintering temperature of 950 °C showed spongy-like and nested macrovoids structure; the porosity was 49.09% with a mean pore size of 3.9 µm. The performance test on the bentonite-based hollow fibre membrane showed that the membrane prepared at 20 wt% and sintering temperature of 1000 °C, which induced high and stable permeate water flux and oil rejection of BHFC membrane were 544 L/m² h and 97%, respectively. The results have shown that the presence of magnesium in bentonite can enhance and promote the needed support material for the fabrication of hollow fibre ceramic membrane.     

Treatment of metal-plating waste water by modified direct contact membrane distillation

In this study, the treatability of metal-plating waste water by modified direct contact membrane distillation (DCMD) at different temperature differences (ΔT = 30°C, 40°C, 50°C, and 55°C was investigated. Two different hydrophobic membranes made of poly(tetrafluoroethylene) (PTFE) and poly(vinylidene fluoride) (PVDF) having different pore sizes (0.22 μm and 0.45 μm) were used. The results indicated that conductivity, COD, sulphate, copper, and nickel could be successfully removed by modified DCMD. The rejection efficiencies for conductivity, COD, and sulphate were 99%, 86%, and 99%, respectively. Copper rejection was effective with both membranes while nickel concentration was below the limit of detection in the effluent. It was found that the pollutant rejection efficiency was affected by the raw water characteristics, membrane properties, and influent heating temperatures. In addition to the water quality parameters, the flux was measured to evaluate membrane performance. A high flux was obtained at 65°C (ΔT = 55°C) with 0.45 μm pore size PTFE membrane (24.1 L m^?2 h^?1) and with PVDF membrane (17.1 L m^?2 h^?1). The flux was mainly affected by temperature and membrane properties. As a result, modified DCMD and all the membranes used in this study were effective for the treatment of metal-plating waste water.     

Thermostable ultrafiltration and nanofiltration membranes from sulfonated poly(phthalazinone ether sulfone ketone)

Modification of poly(phthalazinone ether sulfone ketone) (PPESK) by sulfonation with concentrated or fuming sulfuric acid was carried out in order to prepare thermally stable polymers as membrane materials having increased hydrophilicity and potentially improved fouling-resistance. The sulfonated poly(phthalazinone ether sulfone ketone)s (SPPESK) were fabricated into ultrafiltration (UF) and nanofiltration (NF) asymmetric membranes. The effects of SPPESK concentration and the type and concentration of additives in the casting solution on membrane permeation flux and rejection were evaluated by using an orthogonal array experimental design in the separation of polyethyleneglycol (PEG12000 and PEG2000) and Clayton Yellow (CY, MW 695). One UF membrane formulation type had a 98% rejection rate for PEG12000 and a high pure water flux of 867 kg m⁻² h⁻¹. All the NF membranes made in the present study had rejections of ≥96%, and one had a high water flux of 160 kg m⁻² h⁻¹. Several of the NF membrane formulation types had ∼90% rejection for CY. When the membranes were operated at higher temperatures (80°C), the rejection rates declined slightly and pure water flux was increased more than two-fold. Rejection and flux values returned to previous values when the membranes were operated at room temperature again. Mono- and divalent salt rejections and fluxes were studied on an additional NF membrane set.     

New type of nanofiltration membrane based on crosslinked hyperbranched polymers

Novel nanofiltration (NF) membrane was developed from hydroxyl-ended hyperbranched polyester (HPE) and trimesoyl chloride (TMC) by in situ interfacial polymerization process using ultrafiltration polysulfone membrane as porous support. Fourier transform infrared spectroscopy (FTIR-ATR), scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and water contact angle (CA) measurements were employed to characterize the resulting membranes. The results indicated that the crosslinked hyperbranched polyester produced a uniform, ultra-thin active layer atop polysulfone (PSf) membrane support. FTIR-ATR spectra indicated that TMC reacted sufficiently with HPE. Water permeability and salts rejection of the prepared NF membrane were measured under low trans-membrane pressures. The resulting NF membranes exhibited significantly enhanced water permeability while maintaining high rejection of salts. The salts rejection increase was accompanied with the flux decrease when TMC dosage was increased. The flux and rejection of NF 1 for Na₂SO₄ (1 g/L) reached to 79.1 l/m² h and 85.4% under 0.3 MPa. The results encourage further exploration of NF membrane preparation using hyperbranched polymers (HBPs) as the selective ultra-thin layer.     

A modification of the surface of the thin membranes of EVAL (ethylene vinylalcohol copolymer) by Langmuir-Blodgett films

The surface modification of the membrane as an artificial kidney made of two kinds of ethylene-vinylalcohol copolymers with 32 and 44 mol % contents of ethylene group (EVAL) have been studied by depositing the polymer monolayer on the membrane surface using the Langmuir-Blodgett technique.The permeability to PSS (physiological salt solution) and albumin rejection of the membranes were measured against the number of multilayers. The UFR (ultrafiltration rate) of PSS containing albumin decreased with the increase of the built up layers up to 2 layers and showed constant values from 3 to 10 layers of LB films.On the other hand, albumin rejection increased with the increase of built up layers. Albumin rejection for these deposited membranes of multilayer of EVAL-32 showed 21 times effectiveness more than that of ordinary membrane and 16 times for EVAL-44.Modification of the membrane has been performed by such a network structure formed by the built-up films.     

Synthesis and modeling of composite poly (styrene-co-acrylonitrile) membrane for the separation of chromic acid

In this work, we have prepared a composite styrene acrylonitrile (SAN) membrane on a ceramic clay plate by coating a prepolymer solution prepared using a dual initiator system. This membrane is chemically modified by gas phase nitration followed by amination and quaternization to make it charged and has been characterized by FTIR, SEM, contact angle measurements, AFM, water content, water permeability measurements and molecular weight cut-off experiments. The membrane has been further characterized using chromic acid rejection (real and observed) at different pressures, feed concentrations and pH. The modified membrane is found to possess a real rejection of above 90% with high water flux at low pressure drop.     

Effects of membrane degradation on the removal of pharmaceutically active compounds (PhACs) by NF/RO filtration processes

The impacts of membrane degradation due to chlorine attack on the rejection of pharmaceutically active compounds (PhACs) by nanofiltration and reverse osmosis membranes were investigated in this study. Membrane degradation was simulated by soaking the membranes in a sodium hypochlorite solution of various concentrations over 18 h. Changes in membrane surface properties were characterised by contact angle measurement, atomic force microscopy analysis, and streaming potential measurement. The impacts of hypochlorite exposure to the membrane separation processes were ascertained by comparing the rejection of PhACs by virgin and chlorine-exposed membranes. Overall, the reverse osmosis BW30 membrane and the tight nanofiltration NF90 membrane were much more resilient to chlorine exposure than the larger pore size TFC-SR2 and NF270 nanofiltration membranes. In fact, rejection of all three PhACs selected in this study by the BW30 remained largely unchanged after hypochlorite exposure and further characterisation did not reveal any evidence of compromised separation capability. In contrast, the effects of chlorine exposure to the two loose nanofiltration membranes were quite profound. While chlorine exposure generally resulted in reduced rejection of PhACs, a small increase in rejection was observed when a more dilute hypochlorite solution was used. Changes in the membrane surface morphology as well as observed rejection of inorganic salts and PhACs were found to be consistent with mechanisms of chlorine oxidation of polyamide membranes reported in the literature. Chlorine oxidation consistently resulted in a more negative zeta potential of all four membranes investigated in this study. Conformational alterations of the membrane polyamide active skin layer were also evident as reflected by changes in surface roughness before and after chlorine exposure. Such alterations can either loosen or tighten the effective membrane pore size, leading to either a decrease or an increase in rejection. Both of these phenomena were observed in this study, although the decrease in the rejection of PhACs was overwhelming from exposure to highly concentrated hypochlorite solution.     

Reverse osmosis membrane treatment of acidic etchant wastewater: Effect of neutralization and polyelectrolyte coating on nitrate removal

The removal of nitrate from mixed acid etchant (MAE) wastewater was investigated by neutralization, followed by reverse osmosis (RO) membrane filtration. The coating of a RO membrane was conducted using polyacrylic acid (PAA) in order to enhance the removal of nitrate from the MAE wastewater. The addition of KOH, for the neutralization of the MAE wastewater, was most effective in terms of solid–liquid separation. Double RO filtrations, with crossflow and stirred-flow units, were examined in terms of nitrate rejection and membrane permeability. The Donnan exclusion, due to change in the solution pH, played an important role in nitrate rejection. As a result, RO filtration, at a moderate acidic pH level (e.g., pH 4), provided greater nitrate rejection than that at neutral or alkaline pH levels. The Donnan effect was associated with acetic acid present in MAE wastewater, since it could deprotonate to acetate with a negative charge. Improvement in nitrate rejection occurred with the PAA coating of the original RO membrane. This is because of the enhanced electrostatic repulsion of the nitrate by the carboxyl groups on the coated membrane surface, although the flux declined with the PAA coatings. The effect of charge repulsion was more obvious in the second pass of RO filtration where the ionic strength was relatively low. The increase in nitrate rejection leveled off with a PAA dosage of 0.262 mg/cm² of the membrane, so further coating beyond this level should be prevented.