Commercial Nanofiltration Membranes Research Articles

Poly(vinylidene fluoride) Substrate-Supported Polyamide Membrane for High-Temperature Water Nanofiltration

Conventional polysulfone and polyethersulfone membrane-supported polyamide (PA) nanofiltration (NF) membranes suffer an obvious performance decline due to the drastic pore shrinkage of their membranes during high-temperature (HT) desalination processes. Herein, a thermally stable PA NF membrane (PVDF-IP-2) was successfully constructed over a relatively hydrophobic polyvinylidene fluoride (PVDF) membrane using a continuous interfacial polymerization (CIP) method. The PVDF-IP-2 membrane had a moderate water permeance of 8.3 L m–2 h–1 bar–1 and a 99.7% Na2SO4 rejection at room temperature, which were comparable to those of commercial NF membranes. Introducing calcium ions (Ca2+) into the aqueous solutions further confined the transport rate of the piperazine monomer across the reaction interface through the coordination functions between Ca2+ and piperazine molecules, forming a high-flux (15.9 L m–2 h–1 bar–1 at room temperature) NF membrane (PVDF-IP-Ca) with an excellent Na2SO4 rejection (∼98.6%). Impressively, the PVDF-IP-Ca membrane presented an excellent antifouling behavior and performance stability during both fouling and high-temperature filtration tests. The Ca2+-assisted IP (Ca–CIP) strategy is expected to be a facile and universal approach to preparing TFC desalination membranes for HT environmental applications on relatively hydrophobic supports.

Highly efficient separation of organic substances from high salinity brine by nanofiltration

Efficient separation of organic compounds from the high salinity brines during the treatment of reverse osmosis concentrate (ROC) is highly desirable and also very challengeable for the purpose of zero liquid discharge (ZLD). For the general treatment process, the efficient removal of organic compounds from the NaCl rich solution before the evaporation-crystallization stage is one of the key challenges. In this work, separation of N-(2-hydroxypropyl)morpholine (NMH), 4-morpholineacetate (MHA), the major organic components of ROC in refinery, and monovalent/divalent ions from the high salinity brines by three commercial nanofiltration (NF) membranes, i.e. Desal-5DK, Desal-5DL and NF270 were systematically investigated. The effects of various parameters on the rejection performance of these three NF membranes were experimentally investigated. The results showed that the retentions of NMH and MHA were dominated by flux and pH. Single stage filtration experiments showed that the rejection to NMH and MHA with various operating parameters was high and stable for DK. In comparison, DL and NF270 had higher separation efficiency for monovalent and divalent ions. The experiments determined that the rejections of NMH and MHA were 99.16% and 98.42% under the optimized conditions, respectively, indicating that NF membranes were highly efficient for separating organic compounds from the high salinity brine.

Highly permeable polyamide nanofiltration membrane incorporated with phosphorylated nanocellulose for enhanced desalination

Nanofiltration (NF) membranes with excellent permeability and selectivity are highly desirable for the efficient separation but still remain a great challenge. Herein, a novel thin-film nanocomposite (TFN) membrane with excellent permeability and enhanced desalination was prepared by in-situ incorporating phosphorylated cellulose nanofibers (P–CNF) during interfacial polymerization. The unique attributes of P–CNF, especially the well dispersibility in water, excellent hydrophilicity and highly negative charges, were propitious to the formation of defect-free separation layer with hydrophilic nanochannels and high surface charge density. The composite membrane obtained thereby exhibited not only enhanced permeance of up to 17.0 L m −2 h −1 bar −1 but a high rejection of 95.4% for Na 2 SO 4 , successfully overcoming the trade-off effect between permeability and selectivity. Importantly, a low rejection of down to 5.3% for NaCl was achieved for the membrane due to a slight increase in effective pore size. Therefore, this composite membrane demonstrated elevated separation factor of up to 18.0 towards mono/divalent salts, 3–6 times higher than those of the reported TFN membranes and commercial NF membranes. Additionally, this membrane displayed highly efficient antibiotic desalination, suggesting its promising potential in pharmaceutical fields. As a result, the incorporation of P–CNF yielded a great enhancement to the water permeance and desalination ability of the polyamide membranes by tuning the microstructure. This work provided a novel biobased green nanomaterial for the fabrication of advanced NF membrane with outstanding perm-selectivity. The developed highly selective and permeable NF membrane has huge potentials in the efficient separation of divalent salt/antibiotic and monovalent salt. • P–CNF possesses excellent water dispersibility, compatibility with PA layer and highly negative charge. • TFN membrane exhibits not only increased permeance of up to 17.0 L m −2 h −1 bar −1 but enhanced divalent salt rejection. • A state-of-the-art separation factor of up to 18.0 towards mono/divalent salts is observed. • This membrane also displays highly efficient antibiotic desalination. • Above superior performances are attributed to the synergism of nanocellulose and phosphorylated groups.

Membrane-based Operations for the Fractionation of Polyphenols and Polysaccharides From Winery Sludges

The present work investigated the impact of ultrafiltration (UF) and nanofiltration (NF) membranes on the recovery and fractionation of polyphenolic compounds and polysaccharides from Sangiovese and Cabernet Sauvignon wine lees. A laboratory-made flat-sheet membrane in cellulose acetate (CA400-38) was used in the UF treatment of Sangiovese wine lees; three laboratory-made flat-sheet membranes in cellulose acetate (CA316, CA316-70, CA400-22) and a polyamide commercial membrane (NF90) were used in the NF treatment of Cabernet Sauvignon wine lees. All membranes were characterized in terms of hydraulic permeability and rejection toward references solutes; the performances of the membranes were measured in terms of productivity, fouling index, cleaning efficiency and retention toward target compounds.Experimental results indicated that all UF and NF membranes were effective in separating target compounds rejecting more than 92% of polysaccharides with polyphenols preferentially permeating through the membrane. The UF membrane rejected more than 40% of total polyphenols; rejections toward non-flavonoids and flavonoids were less than 25% and 12.5%, respectively.The laboratory-made NF membranes exhibited higher permeate flux values (of the order of 11–12 L/m2h) in comparison with the commercial NF membrane, despite the observed differences in the retention of specific solutes. Among the prepared membranes the CA316 showed a total rejection toward most part of non-flavonoids and flavonoids.The experimental results support the use of UF and NF processes in a sequential design to fractionate and refine phenolic compounds from winery sludge for the production of concentrated fractions with high antioxidant activities.

Purification of Underground Water From Heavy Metal Ions with a Composite Nanofiltration Membrane

A method for obtaining a composite membrane with a surface layer for water treatment processes is proposed. Nylon-AC composite membranes were obtained by immersion of a polymer substrate made of nylon-66 in a solution of cellulose acetate (AC) in acetone. The properties of membranes have been studied, the parameters of nanofiltration of composite membranes have been established. The content of heavy metal ions in underground water has been studied. The specific performance of membranes for distilled and underground water is determined depending on the number of applied layers of AC. The selectivity of the composite membrane with respect to metal ions was established, the retention capacity for which averaged 86 % with a specific productivity of 137 dm3/(m2·h), which exceeds the performance of a commercial nanofiltration membrane.

Two-dimensional nanoporous and lamellar membranes for water purification: Reality or a myth?

• Progress in 2D nanoporous and lamellar membranes is critically reviewed. • Sub-nanoporous membranes can achieve high water permeance and rejection. • Lamellar membranes provide effective rejection of dye molecules at high water flux. • Enhanced ion rejection by lamellar membranes requires 2D material functionalisation. • Research is required on scalability, stability, fouling and pore size engineering. Wastewater contains only ∼ 1% impurities that are required to be separated precisely for water recycling and reuse applications, particularly in water scare regions. Membrane-based separation has been recognised as an environmentally-friendly and energy-efficient process for effective pollutant removal and clean water production. To further improve the selectivity and water flux of membranes, two-dimensional (2D) materials, such as graphene and those exhibiting graphene-like properties, have been intensively studied for nanoporous and lamellar membrane fabrication in the last decade. Herein, we critically review the progress in 2D membranes for water purification and uniquely discuss the performance governing factors and research gaps from the standpoint of environmental and material chemists, which are critical to realize 2D membrane commercialization. The major points of this review are: (1) among the six types of reviewed 2D membranes, graphene, graphene oxide and reduced graphene oxide membrane have been predominantly studied for water treatment. (2) atomically thin nanoporous membranes with a defined pore size can achieve excellent water permeance and selectivity but large-scale fabrication of nanoporous membranes with uniform shape and size of nanopores remains a challenge. (3) salt separation performance of the pressurized lamellar membranes may not be comparable to commercial nanofiltration membranes. (4) functionalization or crosslinking of 2D materials is inevitable to stabilize 2D membrane pore structure and/or improve molecule separation. (5) membranes fabricated using emerging 2D materials such as born nitride and MXene appear to show better stability than graphene oxide membranes. (6) several aspects, particularly membrane stability, long-term performance, and membrane fouling propensity need attention to realize commercial application of 2D membranes in water treatment.

Exploring ions selectivity of nanofiltration membranes for rare earth wastewater treatment

The separation of metal ions with different valences is an intractable challenge in the treatment of rare earth (RE) ore leaching solutions, and nanofiltration (NF) is one of the most promising approaches to separate ions with different valences. Therefore, the separation behaviors of three commercial NF membranes (NF5, NF270 and NF90) were investigated in this study. The results showed that NF technology could be used for the separation of cations with different valences from nitrate and chloride; the NF270 membrane could be used for the separation of Na+/Ca2+ due to its appropriate pore size and negative charge. The NF5 membrane could be used for Ca2+/Nd3+ separation due to its large pore size and suitable zeta potential. Increasing the operating pressure and cross-flow velocity was beneficial to increase the separation factor of Ca2+/Nd3+. Increasing the solution pH and salt concentration facilitated the adsorption of metal cations on the membrane surface, resulting in the separation factor of Ca2+/Nd3+ increasing first and then decreasing. This work may provide new strategies to recycle valuable metals from RE wastewater or other complex mediums.

Water recovery from reverse osmosis concentrate by commercial nanofiltration membranes: A comparative study

In this work, the potential of three commercial nanofiltration membranes for the treatment of the RO concentrate of Fajr Petrochemical Complex, Iran, was examined. The effect of membrane type (TW30, NE70, and NE90), feed flow rate, and feed pressure on the permeation flux, flux decline, TDS removal, and ion rejections were investigated. The experimental results showed that by increasing the feed pressure and feed flow rate, the rejection and permeation flux were enhanced. Among the studied membranes, the TW30 membrane resulted in the highest TDS removal of 93.4% and water permeability of 2.84 L·m−2·h−1·bar−1 at 10 bar but showed more flux decline than NE90 and NE70. The rejection of calcium, magnesium, and chloride by TW30 membrane was 96.1%, 98.7%, and 90.3%, respectively, and the total hardness was reduced by 96.2%. The NE90 membrane showed also a good TDS removal (88.0%) but it had lower water permeability. The NE70 membrane exhibited the highest water permeability among the studied membranes but the permeate quality was not good and a TDS removal of 24.9% was observed which is not as good as the two other membranes.

Open and dense hollow fiber nanofiltration membranes through a streamlined polyelectrolyte-based spinning process

Polyelectrolyte composite hollow fiber membranes show a high potential to become applied in industrial nanofiltration applications such as water purification or downstream processing in several industries. This material platform allows multiple routes to achieve tailored selectivities and high permeances. Hollow fiber membranes promise a higher process efficiency due to higher packing densities and their backwash stability. However, the fabrication of composite polyelectrolyte hollow fiber nanofiltration membranes is more challenging compared to state of the art spiral wound modules. While the Layer-by-Layer deposition has proven versatile opportunities and is recently entering the commercial market, the multi-step post-treatment is elaborate. In this work, we streamline the fabrication of polyelectrolyte composite hollow fiber membranes. We extend our recently presented ”chemistry-in-a-spinneret” approach with a membrane modification in a subsequent coating bath. The additional modification can be incorporated in the composite hollow fiber membrane manufacturing process without additional process steps. Sulfonated polyethersulfone as a polyanionic additive is used in the polymer solution which complexates with the polycations polyethylenimine (PEI) or poly(diallyldimethylammonium chloride (PDADMAC) in the bore fluid during fiber spinning. The extruded fibers have a positively charged lumen surface which allows a subsequent modification with the polyanion poly(sodium 4-styrenesulfonate) (PSS) in the coating bath. The spun fibers reveal with 16-4 LMH/bar pure water permeance (PWP) and 2000-1100 Dalton (Da) molecular weight cut-off (MWCO) open nanofiltration characteristics, while the PEI+PSS modified fibers show dense nanofiltration characteristics (0.6 LMH/bar PWP and 360 Da MWCO). The fibers are backwash stable and the PDADMAC and PDADMAC+PSS fibers show a significantly higher sodium hypochlorite (NaOCl) stability compared to the PEI and PEI+PSS fibers. This new technique creates composite hollow fiber nanofiltration membranes with a polyelectrolyte bilayer without multi-step post-treatment. Therefore, it provides a promising alternative to the Layer-by-Layer post-modification, where two coating steps and one water washing post-treatment step are required to create a polyelectrolyte bilayer on a porous hollow fiber membrane support. • Composite hollow fiber nanofiltration membranes without multi-step post-treatment. • The chemistry in a spinneret technique is extended with a polyanion coating step. • Backwash stable hollow fibers feature open or dense NF characteristics. • PDADMAC and PDADMAC+PSS show higher NaOCl stability compared to PEI and PEI+PSS. • Fibers with open NF characteristics are competitive with commercial nanofiltration membranes.

Understanding Selectivity in Solute-Solute Separation: Definitions, Measurements, and Comparability.

The development of membranes capable of precise solute-solute separation is still in its burgeoning stage without a standardized protocol for evaluating selectivity. Three types of membrane processes with different driving forces, including pressure-driven filtration, concentration difference-driven diffusion, and electric field-driven ion migration, have been applied in this study to characterize solute-solute selectivity of a commercial nanofiltration membrane. Our results demonstrated that selectivity values measured using different methods, or even different conditions with the same method, are generally not comparable. The cross-method incomparability is true for both apparent selectivity, defined as the ratio between concentration-normalized fluxes, and the more intrinsic selectivity, defined as the ratio between the permeabilities of solutes through the active separation layer. The difference in selectivity measured using different methods possibly stems from the fundamental differences in the driving force of ion transport, the effect of water transport, and the interaction between cations and anions. We further demonstrated the difference in selectivity measured using feed solutions containing single-salt species and that containing mixed salts. A consistent protocol with standardized testing conditions to facilitate fair performance comparison between studies is proposed.

Intrinsic Dependence of Groundwater Cation Hydraulic and Concentration Features on Negatively Charged Thin Composite Nanofiltration Membrane Rejection and Permeation Behavior.

Commercial nanofiltration membranes of different molecular weight cut-offs were tested on a pilot plant for the exploration of permeation nature of Ca, Mg, Mn, Fe, Na and ammonium ions. Correlation of transmembrane pressure and rejection quotient versus volumetric flux efficiency on nanofiltration membrane rejection and permeability behavior toward hydrated divalent and monovalent ions separation from the natural groundwater was observed. Membrane ion rejection affinity (MIRA) dimension was established as normalized TMP with regard to permeate solute moiety representing pressure value necessary for solute rejection change of 1%. Ion rejection coefficient (IRC) was introduced to evaluate the membrane rejection capability, and to indicate the prevailed nanofiltration partitioning mechanism near the membrane surface. Positive values of the IRC indicated satisfactory rejection efficiency of the membrane process and its negative values ensigned very low rejection affinity and high permeability of the membranes for the individual solutes. The TMP quotient and the efficiency of rejection for individual cations showed upward and downward trends along with flux utilization increase. Nanofiltration process was observed as an equilibrium. The higher the Gibbs free energy was, cation rejection was more exothermic and valuably enlarged. Low Gibbs free energy values circumferentially closer to endothermic zone indicated expressed ions permeation.

Separation of succinic acid from fermentation broth: Dielectric exclusion, Donnan effect and diffusion as the most influential mass transfer mechanisms

Nanofiltration can be used to separate succinic acid (SA) from other organic acids in a fermentation broth. However, commercial nanofiltration membranes can be insufficiently selective since some produced organic acids have similar molecular weights. SA production from biomass fermentation entails separation of succinic acid from the other concomitantly produced organic acids such as acetic acid, formic acid, pyruvic acid etc. In this study, five commercial nanofiltration membranes were tested for SA rejection and selectivity under dead-end and crossflow modes of SA recovery from selected synthetic solutions and a real fermentation broth. SA rejections up to 94.1 ± 1.5% and 95.5 ± 3.5% were observed for the synthetic broth and the fermentation broth, respectively. The SA fractions obtained in the retentate were nearly 70% and 60% for synthetic and fermentation broth, respectively. The highest rejections and SA fraction values were achieved with DK and NF270 membranes, which also showed the highest permeate flux. Model evaluation using the Donnan-steric pore model with dielectric exclusion (DSPM-DE) showed that 99% of the SA rejection occurs at the membrane interface and not inside the membrane. Based on the modelling, the most important mechanism affecting separation was dielectric exclusion at both sides of the membrane surfaces. Moreover, identified sensitive model parameters, i.e. pore dielectric constant (εpore), charge density (Xd) and pore radius (rp), were estimated through model fitting and were found to be highly correlated with significant variations. Finally, for the first time, a calibrated model with synthetic broth was successfully used to predict organic acids rejection for a real fermentation broth (with 12% and 3% prediction error for succinate rejection by NF270 and DK, respectively). Such prediction was achieved only by measuring organic acid concentrations in the fermentation broth and using the three sensitive parameters. Furthermore, the potential contribution of different transport mechanisms of SA was quantified for the first time.

Stability of Polymeric Membranes to UV Exposure before and after Coating with TiO2 Nanoparticles.

The combination of photocatalysis and membrane filtration in a single reactor has been proposed, since the photocatalytic treatment may degrade the pollutants retained by the membrane and reduce fouling. However, polymeric membranes can be susceptible to degradation by UV radiation and free radicals. In the present study, five commercial polymeric membranes were exposed to ultraviolet (UV) radiation before and after applying a sol–gel coating with TiO2 nanoparticles. Membrane stability was characterized by changes in hydrophilicity as well as analysis of soluble substances and nanoparticles detached into the aqueous medium, and by Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), and energy-dispersive X-ray spectrometry (EDS) for structural, morphological, and elemental distribution analysis, respectively. The TiO2 coating conferred photocatalytic properties to the membranes and protected them during 6 h of UV radiation exposures, reducing or eliminating chemical and morphological changes, and in some cases, improving their mechanical resistance. A selected commercial nanofiltration membrane was coated with TiO2 and used in a hybrid reactor with a low-pressure UV lamp, promoting photocatalysis coupled with cross-flow filtration in order to remove 17α-ethinylestradiol spiked into an aqueous matrix, achieving an efficiency close to 100% after 180 min of combined filtration and photocatalysis, and almost 80% after 90 min.

Recent developments in the preparation of improved nanofiltration membranes for extreme pH conditions

• Substantial R&D efforts have been devoted to preparing improved pH stable membranes. • Polyamine and polyurea membranes are more acid stable than polyamide membranes. • Specific polyelectrolyte multilayer membranes (PEM) are acid and alkaline stable. • PEM show better permeance than commercial NF membranes at extreme pH conditions. • Further R&D is needed before developed membranes can be widely applied in industry. Conventional commercially available nanofiltration (NF) membranes show limited stability at extreme pH conditions, whereas commercially available NF membranes that can cope with these conditions often show (too) low permeance or are relatively open. Since NF applications in many industrial sectors require pH stable NF membranes with a range of molecular weight cut-offs (MWCO), including tight NF membranes, substantial research and development efforts are being devoted to the research and development of these membranes. This review discusses these developments as reported in open literature, with a focus on the period between 2016 and 2021. Most developments relate to the use of interfacial polymerization to produce thin film composite membranes. Polyamine and polyurea membranes with good chemical stability for extreme pH conditions have been prepared. For polysulfonamide membranes indications for good chemical stability at low pH have been shown. These membrane types show improved stability compared to developed poly(aryl cyanurate), polyesteramide, poly(amide-sulfonamide), and polyamide membranes, which are more susceptible to hydrolysis and therefore less chemically stable at extreme pH. Furthermore, layer-by-layer coating using strong cation - and anion polyelectrolytes has led to new pH stable NF membranes. Despite the extension of the pH stable NF membrane portfolio, most of the developed membranes still do not meet the performance characteristics of the best commercially available NF membranes applicable for the common pH range between 2 and 11. Only a few layer-by-layer coated polyelectrolyte membranes combine high permeance, relatively low MWCO and stability at extreme pH. However, these membranes are not yet commercially available for full-scale applications requiring high pressure operation. Consequently, there is still room for research and development to further improve pH stable NF membranes.

Assessment on Performance-properties of Asymmetric Nanofiltration Membranes from Polyethersulfone/n-Methyl-2-pyrrolidone/water Blends with Poly(vinyl pyrrolidone) as Additive

In this study, the effect of poly(vinyl pyrrolidone) (PVP) additive on the fabrication of asymmetric nanofiltration (NF) membranes was investigated in terms of performance, structural details and key properties. On addition of PVP ranging from 2 to 10 wt% into the dope solution, the fabricated NF membranes exhibited significantly different in properties and improved performance. In particular, the membranes made from 2 wt% PVP had the highest water flux and salt rejection of about 3.61 × 10–6 m3/m2s and 44.49 %, respectively. Modeling results revealed that small amount of PVP (2–4 wt%) produced finer structural properties. Moreover, the key properties (rp, ∆x/Ak and ζ) of the fabricated NF membranes were found to be within the range of that of commercial NF membranes.

Mitigating Silica Fouling and Improving PPCP Removal by Modified NF90 Using In Situ Radical Graft Polymerization.

This study in-situ modified a commercial nanofiltration membrane, NF90, through the concentration-polymerization-enhanced radical graft polarization method by applying two agents of 3-sulfopropyl methacrylate potassium salt (SPM) and 2-hydroxyethyl methacrylate (HEMA) with different dosages. Surface characterization revealed that the modified membranes became rougher and more hydrophilic compared with the pristine membrane. The modified membranes exhibited considerably enhanced separation performance with 5.8–19.6% higher NaCl rejection and 17.2–19.9% higher pharmaceuticals and personal care products (PPCPs) rejection than the pristine membrane. When treating the feedwater with high silica concentration, the modified membranes exhibited relatively less flux decline with high percentage of reversible fouling, especially the ones modified using a lower monomer concentration (0.01 M SPM and 0.01 M HEMA). Moreover, membrane modification enhanced the PPCP rejection (1.3–5.4%) after silica fouling by mitigating foulant deposition on the membrane surface. The fouling mechanism was confirmed to be intermediate blocking of membrane pores. Therefore, the in-situ modification technique with a low monomer concentration proved to be effective for mitigating silica fouling and improving PPCP rejection, which can be easily performed and cost-effective in practical application.

Reconstruction of the polyamide film in nanofiltration membranes via the post-treatment with a ternary mixture of ethanol-water-NaOH: Mechanism and effect

Flexible regulation of the membrane performance via the reconstruction of structural and physicochemical features is a promising technical route for the modification of nanofiltration (NF) membranes. Here we present a facile but effective post-treatment method with a ternary mixture of Ethanol-Water-NaOH (EWN). It was demonstrated the polyamide (PA) film in the nascent NF membrane could be effectively reconstructed by the EWN which consisted of 1 mol/L NaOH, 70% (v/v) ethanol, and 30% (v/v) ultrapure water. The EWN post-treatment greatly improved the water permeability (almost 100%) while maintaining high Na2SO4 rejection (~95.9%), which made membrane separation performance closer to the “upper bound” related to the “trade-off” between water permeability and Na2SO4 rejection. This improvement was attributed to the “etching effect” brought by the synergistic interaction between NaOH and ethanol, which partially broke the polyamide chains, thus reducing the PA film thickness from 43.1 nm to 18.5 nm, decreasing the water contact angle from 38° to 28°, and decreasing the zeta potential from −34.4 mV to −69.1 mV. In addition, the long-term stability testing up to 60 h verified the good durability of the post-treated PA film. The performance improvement of different commercial NF membranes confirmed the general applicability of EWN post-treatment.

Comparison of Commercial Organic Solvent Nanofiltration (OSN) Membrane Performance

Comparison of Commercial Organic Solvent Nanofiltration (OSN) Membrane Performance

Rejection Capacity of Nanofiltration Membranes for Nickel, Copper, Silver and Palladium at Various Oxidation States.

Electroplating and metalworking industries produce enormous amounts of waste containing heavy metals in their effluents, leading to potential threats to biotic and abiotic life. According to regulation, heavy metal contamination must be kept within the regulated standard of a few parts per million, which has led to a recent pique in interest in the utilization of nanofiltration technology for metal recovery. The effect of feed pH, pressure, metal concentration, and oxidation of metal on the rejection of heavy metal ions using three commercial nanofiltration membranes (NF, NF90, and NF270) were explored. To begin, studies of electrolyte salts, contact angle, and water permeability were employed to characterize the nanofiltration membranes. A dead-end module was used to test the permeation and retention capacities of the nanofiltration membranes. The results showed an increase in salt rejection for all metals examined irrespective of the membrane, at a pH below the isoelectric point. For divalent cations, the NF90 membrane achieved recovery capacities of 97% and 85% at 200 ppm and 20 ppm respectively, as compared to the recovery observed for Ni2+, Cu2+, and Pd2+ ions by NF and NF270. At a pH 2, 20 ppm and 5 bar, the NF90 membrane had the highest percent recovery, but at a pH 3, the recovery was at 95%. Mono and divalent stable Ag+ and Ni2+ ions showed a comparatively high percent recovery as compared to Pd2+ and Cu2+, which have high molecular weight and charge effect. In the presence of chelating agents, the membrane surface area is increased, resulting in high divalent ion recovery capacities due to favourable interaction with the polyamide functional group of the membranes. This study establishes the significance of oxidation in high removal efficiency cation in varying experimental conditions.

Facile preparation of antifouling nanofiltration membrane by grafting zwitterions for reuse of shale gas wastewater

Complex organic matter causes severe fouling when membranes are applied for shale gas wastewater (SGW) treatment. This study reports the grafting of a zwitterionic polymer brush consisting of poly (sulfobetaine methacrylate) (PSBMA) onto the surface of a commercial nanofiltration (NF) membrane via electron transfer-atom transfer radical polymerization (ARGET-ATRP) to achieve anti-fouling property, especially against organic foulants. Compared to the pristine NF membranes, the PSBMA-grafted NF membrane showed high performance when challenged by SGW as a feed stream: (1) The flux stability was significantly improved during long-term operation, with a 64.28% increase in flux normalization at 50% recovery rate of SGW, while maintaining a suitable initial flux and near constant ion removal rate; (2) Based on excitation-emission-matrix spectra integrated in the fluorescence region, the removal of protein-like organic matters and humus-like organic matters increased by 34% and 16.5%, respectively; (3) The XDLVO theory supports the hypothesis that the hydrophobic interactions between the membrane surface and organic foulants were reduced by enhancing the Lewis acid-base interaction energy. The proposed anti-fouling zwitterionic membranes has potential in industrial application for the on-site reuse of SGW.