Commercial Nanofiltration Membranes Research Articles

Influences of solution chemistry and polymeric natural organic matter on the removal of aquatic pharmaceutical residuals by nanofiltration

This study demonstrates the removal efficiency and the permeate flux behavior during cross-flow nanofiltration (NF) of aqueous solutions of five pharmaceutically active compounds (PhACs). Cephalexin, tetracycline, acetaminophen, indomethacin and amoxicillin were used as models of PhACs, and alginate was selected as model of natural organic matter (NOM). Two commercial composite NF membranes (SR2 and SR3) with different characteristics were used. The highest rejection was observed for tetracycline, i.e., 75–95% for membrane SR 2 and 95–100% for membrane SR 3, while the rejection was least for acetaminophen (32–36% for SR2 and 52–59% for SR3). As the pH of acetaminophen solution was increased (from 6 to 9) the rejection would increase. Changes of ionic content (from 10 to 20 mM) lead to increase (from 89 to 93% for SR 3) or decrease (from 100 to 91% for SR2) of cephalexin rejection depending on the membrane used. The permeate flux would decrease with decreasing the pH solution and increasing ionic strength. The addition of alginate in the feed stream decreased the permeate flux, with lower reduction for SR3, and increased the PhAC rejection except for acetaminophen and amoxicillin. Both size and Donnan exclusions seemed to occur, and the effect of Donnan exclusion was more pronounced for the NF membrane having larger effective pore size (SR2).

Rejection of salt mixtures from high saline by nanofiltration membranes

Nanofiltration (NF) membranes have recently been employed as pretreatment unit operations in seawater desalination processes and as partial demineralization to seawater. The present paper investigates the performance of selected commercial NF membranes to reject salts of high concentrations at salinity levels representative of brackish and sea water. Two commercial nanofiltration membranes (NF90 and NF270) have been investigated in detail to study their performance in filtering aqueous solutions containing different salt mixtures in a cross-flow NF membrane process within the pressure range from 4 to 9 bar. Spiegler-Kedem model (SKM) was used to fit the experimental data of rejection with the permeate flux. The results showed that NF90 membrane was shown to have a distinct ability to reject both monovalent and divalent ions of all investigated mixtures with very reasonable values but with relatively low flux. This will make NF90 more suitable for the application in the pretreatment of desalination processes. On the other hand, NF270 can reject monovalent ions at relatively low values and divalent ions at reasonable values, but at very high permeate flux. The SKM model only fitted well the experimental data of divalent ions in salt mixture. Based on the evaluation of the overall performance of NF90 and NF270 membranes, their distinct ability to reject salts at high salinity from seawater is considered an advantage in the field of pretreatment of seawater feed to desalination units.

Effect of membrane chemistry and coating layer on physiochemical properties of thin film composite polyamide RO and NF membranes: I. FTIR and XPS characterization of polyamide and coating layer chemistry

The physiochemical properties of reserve osmosis (RO) and nanofiltration (NF) polyamide (PA) membranes are largely determined by their PA chemistry and coatings, if any. Knowledge on such inherent relationship is critically needed in advancing membrane technology. This paper presents a consistent and in-depth characterization on diagnosing the chemistry of polyamide and the presence of any coating or modifying agent. Fourier-transform infrared (FTIR) and x-ray photoelectron spectra (XPS) of 17 commonly used commercial thin film composite polyamide RO and NF membranes are presented. The FTIR spectra for fully aromatic trimesoyl chloride and 1,3-benzenediamine based membranes had an amide II band (1541 cm −1) and an aromatic amide band (1609 cm −1) that were absent for the semi-aromatic membranes. Consistent with that, the XPS binding energy shift for carbon atoms in fully aromatic amide groups was higher than that for semi-aromatic ones likely due to the more electron withdrawing environment. An additional intermediate peak with a binding energy shift of 1.1–1.6 eV was present in the XPS spectra of C(1s) for some commercial RO and NF membranes. The additional peak, coupled with FITR analysis over the high wave number region and XPS elemental analysis, provided consistent evidence that these membranes were either coated with an additional coating layer or had a modified PA chemistry.

Integrated nanofiltration cascades with low salt rejection for complete removal of pesticides in drinking water production

In this study, the aim is to remove micropollutants (pesticides) from water completely while removing only a small fraction of salts. To achieve the latter, loose commercial nanofiltration (NF) membranes were selected (Desal51HL, N30F and NF270). To realize a high removal of organic compounds, combined with a low salt rejection, cascades of NF stages are proposed. Experiments with pesticides (aldrin, atrazine, bentazone, dieldrin, and propazine) and NaCl confirmed that a nearly complete rejection of pesticides is possible, depending on specific properties of the solutes such as molecular size and chemical structure (e.g. hydrophobicity). Salt rejection was highly dependent on the specifications of the membrane and on the ion charge, and ranged from low to high values. Through modeling based on transport equations and mass balances, it was shown that the separation was significantly improved by adopting the principle of an integrated cascade of NF membranes.

Fouling resistant, high flux nanofiltration membranes from polyacrylonitrile- graft-poly(ethylene oxide)

Membranes possessing both fouling resistance and permeate size cut-offs in the nanometer range would open membrane processes to numerous molecular separations in the biochemical, pharmaceutical and food industries. This study describes the preparation of novel thin film composite (TFC) nanofiltration (NF) membranes with the above properties, where the selective layer is a dense coating of the amphiphilic comb copolymer polyacrylonitrile- graft-poly(ethylene oxide) (PAN- g-PEO). PAN- g-PEO is synthesized by free-radical polymerization using a macromonomer method, and solution coated onto a PAN ultrafiltration (UF) membrane support. Upon coagulation of the coating, microphase separation occurs between the hydrophobic PAN backbone and hydrophilic PEO side chains, as determined by differential scanning calorimetry (DSC). Transmission electron microscopy (TEM) reveals the formation of an interconnected network of PEO domains that act as water-permeable nanochannels and provide the size-based separation capability of the membrane. In filtration studies, the pure water permeability of PAN- g-PEO TFC NF membranes was found to be 85 ± 25 L/m 2 h MPa, over 4 times that of a commercial NF membrane control. The permeate size cut-off was determined by the filtration of a series of rigid dye molecules to be less than 1 nm, and the fractionation of a mixture of two anionic dyes, Congo Red and Ethyl Orange, was demonstrated. Further, the membrane showed complete resistance to fouling, defined as flux loss that cannot be recovered by a water rinse, by 1 g/L bovine serum albumin (BSA) in a 24-hour dead-end filtration experiment, demonstrating the promise of these membranes for biomolecule separations.

Model Analysis of Separation Performance of Commercial Nanofiltration Membranes Improved by Tannic Acid

Model Analysis of Separation Performance of Commercial Nanofiltration Membranes Improved by Tannic Acid

Separation of binary heavy metals from aqueous solutions by nanofiltration and characterization of the membrane using Spiegler–Kedem model

Heavy metals are important sources of environmental pollution and are non-degradable, and therefore, continue to exist in water. Membrane technology is one option for the separation of heavy metals from wastewater without generating any pollution load. This paper presents the binary heavy metals (cadmium and nickel) separation capability of a commercial nanofiltration membrane from aqueous solutions. The influence of applied pressure, feed solute concentration, feed flowrate, feed pH and nature of anion on the retention of cadmium and nickel ions is studied. It is observed that the rejection increases with increase in feed pressure and decreases with increase in feed concentration at constant feed flowrate. The maximum observed solute rejection of nickel and cadmium ions are 98.94% and 82.69%, respectively, for an initial feed concentration of 5 ppm. The NF membrane is characterized by using the Spiegler–Kedem model based on irreversible thermodynamics coupled with film theory. Boundary layer thicknesses, as well as the membrane transport parameters are estimated by using the Levenberg–Marquardt method. The estimated parameters are used to predict the membrane performance and found that the predicted values are in good agreement with the experimental results.

Modeling on electrokinetic phenomena of charged porous membrane

Studies on streaming potential (SP), an important electrokinetic phenomena, is chiefly within the scope of microfiltration (MF) and ultrafiltration (UF) membrane processes based on the Helmholtz-Smoluchowski (HS) equation. However, the HS equation has some limitations, which is an adequate cause of the less study of SP for nanofiltration (NF) membranes. In this paper, experiments of SP for two commercial NF membranes (NTR7250 and ESNA 1-K) have been carried in four electrolytes (KCl, MgCl 2, Na 2SO 4 and MgSO 4) solutions. We proposed a new modified HS equation to analyze the experimental data and predict the electrical properties of NF membranes. Results show that the modified HS equation is more applicable than the classical HS equation in the evaluation of surface electrical properties of NF membranes. The established strategy to evaluate the SP of two typical NF membranes in the paper has important significance for other membranes processes.

High flux nanofiltration membranes based on interfacially polymerized polyamide barrier layer on polyacrylonitrile nanofibrous scaffolds

Electrospun polyacrylonitrile (PAN) nanofibrous scaffold was used as a mid-layer support in a new kind of high flux thin film nanofibrous composite (TFNC) membranes for nanofiltration (NF) applications. The top barrier layer was produced by interfacial polymerization of polyamides containing different ratios of piperazine and bipiperidine. The filtration performance (i.e., permeate flux and rejection) of TFNC membranes based on electrospun PAN nanofibrous scaffold was compared with those of conventional thin film composite (TFC) membranes consisting of (1) a commercial PAN ultrafiltration (UF) support with the same barrier layer coating and (2) two kinds of commercial NF membranes (i.e., NF90 and NF270 from Dow Filmtec). The nanofiltration test was carried out by using a divalent salt solution (MgSO 4, 2000 ppm) and a cross-flow filtration cell. The results indicated that TFNC membranes exhibited over 2.4 times more permeate flux than TFC membranes with the same chemical compositions, while maintaining the same rejection rate (ca. 98%). In addition, the permeate flux of hand-cast TFNC membranes was about 38% higher than commercial NF270 membrane with the similar rejection rate.

Ultra- and nanofiltration of aqueous extracts from distilled fermented grape pomace

Aqueous extracts from pressed distilled grape pomace were processed using ultrafiltration (UF) and nanofiltration (NF) membranes to obtain fractions enriched in compounds with antioxidant activity. The properties of concentrates (composition, phenolic content and antioxidant activity) were determined for different commercial UF and NF membranes as a function of the volume reduction factor. Membrane concentrates obtained under selected conditions were extracted with ethyl acetate to yield fractions with enhanced antioxidant activity. The radical scavenging capacities of ethyl acetate extracts were in the range reported for synthetic antioxidants. Cleaning resulted in the recovery of 92–100% of the initial permeability.

Polymeric membranes modified via plasma for nanofiltration of aqueous solution containing organic compounds

Polymeric nanofiltration membranes were prepared modifying via low temperature plasma treatment flat membranes based on a modified poly(ether ether ketone) (PEEKWC). PEEKWC membranes, prepared using wet phase inversion using methanol (M2m) and 1-butanol (M2b) as internal non-solvent, were further plasma surface modified (RF 13.56 MHz) introducing amino groups on the membrane using two different pre-treatment gases, Ar (M2bpsm/Ar/NH 3 and M2mpsm/Ar/NH 3) and H 2 (M2bpsm/H 2/NH 3 and M2mpsm/H 2/NH 3). The performances of unmodified and plasma modified membranes M2b and M2m and of a commercial nanofiltration membrane (NFTFC50) were tested in treatment of aqueous solution containing two dyes, characterized by similar molecular weight (≈320 g/mol) and opposite charge, methylene blue and methyl orange, positively and negatively charged, respectively. The observed rejections resulted optimized in terms of fouling for the plasma modified membranes with respect to unmodified and commercial NFTFC50 membranes. In particular, methylene blue was retained for 100% by M2mpsm/Ar/NH 3 with a relative flux of 100% compared to 100% and 95% rejections and 85% and 90% relative fluxes observed for unmodified PEEKWC M2b membrane and commercial NFTFC50, respectively.

Removal of metal ions from an acidic leachate solution by nanofiltration membranes

This paper presents the feasibility of the application of two commercial nanofiltration (NF) membranes (Desal5 DK and NF-270) in the removal of metal ions from an acidic leachate solution generated from a contaminated soil using H 2SO 4 as a soil washing agent. The experimental results of soil washing indicated that H 2SO 4 is highly effective in removing metal ions from contaminated soil. Following this process, the treatment of this acidic solution by nanofiltration membranes showed good metal ion rejection (between 62% to 100%) where divalent ions were better rejected than monovalent ions. For characterization purposes, the membrane experiments were conducted using K 2SO 4 solutions at different pHs. Membrane performance criteria were evaluated according to membrane permeability and ionic retention in the tank and permeate, taking into account different operating conditions such as pressures, flow rate and pH. These results demonstrated the effectiveness and feasibility of the application of nanofiltration treatments in the cleaning-up of contaminated water residues generated during soil washing processes.

Potentials of using nanofiltration to recover phosphorus from sewage sludge

Due to the depletion of mineral phosphorus resources there is an increasing demand for efficient phosphorus recovery technologies. In this study the potential of nanofiltration to recover phosphorus from pre-treated sewage sludge is investigated. The efficiency of three commercial nanofiltration membranes (Desal 5DK, NP030; MPF34) was tested using model solutions. Desal 5DK showed the best selectivity for phosphorus. A pH of lower than 1.5 was found to be most suitable. Desal 5DK was used on four different sewage sludge ash eluates and on one sewage sludge. In these experiments it was shown that a separation of phosphorus from undesired components such as heavy metals was possible with significant variations in the efficiency for the different ash and sludge types. Additionally the achievable product recovery was investigated with model solutions. A product recovery of 57.1% was attained for pH 1 and 41.4% for pH 1.5.

Characteristics and performance of new nanoporous PEEKWC films

This rapid communication reports a summary of the key findings of preparation and characterization of new polymeric membranes for nanofiltration of organic compounds. A series of nanoporous asymmetric membranes of PEEKWC, a modified poly(etheretherketone) was prepared by means of the dry–wet phase inversion method. In particular, the type and concentration of internal non-solvent were varied in order to obtain membranes for nanofiltration. The optimization of these factors led to reproducible membranes, which were characterized for hydrophobicity, roughness, morphology, surface charge. The performance was studied by doing dead-end filtration experiments with aqueous solutions of uncharged and charged organic compounds. The new polymeric films exhibited interesting performance compared to commercial nanofiltration membranes in terms of retention and relative flux for positively charged organic compounds.

Novel approach combining physico-chemical characterizations and mass transfer modelling of nanofiltration and low pressure reverse osmosis membranes for brackish water desalination intensification

The aim of the present work is to establish a systematic approach in the range of characterization of commercial nanofiltration (NF) and low pressure reverse osmosis (LPRO) membranes materials for a better help to the users. We developed two sorts of characterizations: (i) first physico-chemicals in terms of hydrophobicity/hydrophilicity, morphology and topography and (ii) secondly by mass transfer in terms of pure water and saline solution permeabilities, monovalent (NaCl) and divalent (Na 2SO 4) solutes rejections and molecular weight cut-off determination. A model inspired by the phenomenological approach proposed by Kedem and Katchalsky (KK) and completed by Spiegler (S) will help us to quantify separately both parts of mass transfer occurring, the pure convection or advection and the pure diffusion for three membranes in polyamides, 2 NF and 1 LPRO, denoted NF270, NF90 and BW30, respectively, purchased from Dow. We have limited our study to low concentration polarization by using diluted solutions (10 −3 to 10 −1 M) and high flow rate (4 m s −1) under low conversion (5%) operational conditions to develop a new and original approach to classify and better understand the behavior of the commercialized NF and LPRO membranes under brackish waters in order to guide the users to find a suitable membrane.

New generation of low fouling nanofiltration membranes

The nanofiltration (NF) membranes and their properties are quite diverse, but can generally be described as having rejection characteristics that range from “loose” RO to “tight” ultrafiltration. The uniqueness of these membranes is highlighted by their ability to selectively reject different dissolved salts, and have high rejection of low molecular weight, dissolved components. Nanofiltration membranes are mainly used to partially soften potable water, allowing some minerals to pass into the product water and thus increase the stability of the water and prevent if from being aggressive to distribution piping material. Additionally, NF membranes are finding increasing use for purifying industrial effluents, and minimizing waste discharge. The key to using NF membranes for particular application is the selection of a membrane with appropriate rejection characteristic and the design of a suitable process. NF membranes are generally characterized by a high charge density and pore sizes in the range of nanometers. The surface charge is most often negative and has the greatest effect on the selective passage nature of these membranes. The design of a NF system can often be more complex than a RO system. Because the required transmembrane pressure is so low, a system design that will result in balanced flux throughout the array is difficult to achieve. One common approach is to design NF systems with multiple element types. Such so called “hybrid” designs can help more finely control flow in the NF system. Also, membrane fouling can cause a change of rejection selectivity and/or reduction of membrane permeability that could affect both the water quality and operating cost due to increased energy requirement and higher consumption of chemicals needed for stabilization of permeate. The rejection characteristics of NF membranes depend on chemistry and surface charges of the separating barrier layer, ionic composition and concentration of feedwater and operating parameters of the NF system: recovery rate and permeate flux. Although, theoretical models have been developed to calculate selective ion rejection of NF membranes, they are mostly adequate only for simple water systems in limited range of water compositions and concentrations. In algorithms applied for projection of permeate composition in commercial NF systems, the theoretical models are augmented by empirical correction coefficients developed from field results. Selection of proper NF membrane type and prediction of performance is further complicated by membrane fouling. In NF projects involving treatment of feedwater with high concentration of dissolved organics the membrane selection process usually include pilot unit operation to evaluate stability of salt rejection and water permeability of candidate membrane elements. The paper will discuss present offering of commercial NF membranes, case studies and models applied to calculate their performance. This will also include information on recently developed NF membranes with low fouling characteristic that are successfully applied in the world largest (150,000 m 3/d, 40 mgd) NF plant in Boca Raton, FL. The process of extensive field testing and parallel membrane performance optimization that led to selection of these nanofiltration elements for the Boca Raton plant will be described as well.

Mechanism of Ion Exclusion by Sub-2nm Carbon Nanotube Membranes

AbstractCarbon nanotubes offer an outstanding platform for studying molecular transport at nanoscale, and have become promising materials for nanofluidics and membrane technology due to their unique combination of physical, chemical, mechanical, and electronic properties. In particular, both simulations and experiments have proved that fluid flow through carbon nanotubes of nanometer size diameter is exceptionally fast compared to what continuum hydrodynamic theories would predict when applied on this length scale, and also, compared to conventional membranes with pores of similar size, such as zeolites. For a variety of applications such as separation technology, molecular sensing, drug delivery, and biomimetics, selectivity is required together with fast flow. In particular, for water desalination, coupling the enhancement of the water flux with selective ion transport could drastically reduce the cost of brackish and seawater desalting. In this work, we study the ion selectivity of membranes made of aligned double-walled carbon nanotubes with sub-2 nm diameter. Negatively charged groups are introduced at the opening of the carbon nanotubes by oxygen plasma treatment. Reverse osmosis experiments coupled with capillary electrophoresis analysis of permeate and feed show significant anion and cation rejection. Ion exclusion declines by increasing ionic strength (concentration) of the feed and by lowering solution pH; also, the highest rejection is observed for the salts (A=anion, C=cation, z= valence) with the greatest zA/zC ratio. Our results strongly support a Donnan-type rejection mechanism, dominated by electrostatic interactions between fixed membrane charges and mobile ions, while steric and hydrodynamic effects appear to be less important. Comparison with commercial nanofiltration membranes for water softening reveals that our carbon nanotube membranes provides far superior water fluxes for similar ion rejection capabilities.

Prediction of physical properties of nanofiltration membranes using experiment and theoretical models

Two commercial nanofiltration (NF) membranes, viz., Desal-HL and NF 700 MWCO were investigated experimentally using neutral and charged solutes, viz., glucose, sodium chloride and magnesium chloride. Effect of pH was studied for sodium chloride rejection and isoelectric point of the membrane was deduced. Experimental results were analyzed using Donnan steric pore and dielectric exclusion models. Dielectric exclusion arises due to the difference in dielectric constant between the bulk and the nano-pore. Born dielectric effect was used as dielectric exclusion phenomena in the present investigation. Stokes–Einstein, Born effective and Pauling radii were used for theoretical simulation, which accurately predicted different charge densities. Empirical correlations were proposed between charge density, concentration and pH for each radius. Charge density decreased drastically when dielectric exclusion term was included in the theoretical model, which showed the real physical characteristics of the membranes employed. Charge density and radius of pore was found to be an important surface parameter in predicting the separation effects in NF membranes.

A Comprehensive Characterization of Commercial Nanofiltration Membranes

This paper presents a comprehensive characterization study of NF 90 (Filmtec), NF 270 (Filmtec), NF 2 (Sepro), and NF PES 10 (Microdyn‐Nadir) membranes by their morphology, charge, and separation performance parameters. Extensive investigations of the relations between membrane performance and membrane charge/morphological properties were performed by various measurement methods and active layer mass transport modelling. For this purpose, a broad experimental program was exerted with saccharides and polyethylene glycol at various pressures and with single salt solutions of NaCl, CaCl2, Na2SO4, and MgSO4 at various concentrations. pH effect on membrane performances was investigated as well. A Fortran program code of a hydrodynamic model was developed in order to determine the pore sizes of the membranes using the experimental data with organics. In addition, an equation‐based software, COMSOL Multiphysics, was utilized for the modelling with salt solutions. This eliminated the effort demanding program code writing for concentration and potential differential equations derived from extended Nernst‐Planck (ENP) equation, which is a common method in many publications. By this model, transport phenomena inside the membrane, relation between the membrane performance and its charge, as well as performance prediction of the membranes could be quested in detail.

Investigation of nanopores in nanofiltration membranes using slow positron beam techniques

AbstractThe pore distribution of the commercial nanofiltration membranes Desal 5DL, Desal 51HL, N30F, NTR 7450 and NF‐PES‐10 were analyzed by positron annihilation spectroscopy using slow positron beam analysis. From the depth selective 3γ/2γ analysis a dense nanoporous filtration layer could be determined. From the depth selective lifetime analysis the size and distribution of the nanopores could be extracted. The measurements indicate the presence of 2 pore sizes in the samples with a radius of 1.25‐1.55 Å and 3.2‐3.95 Å. These experimental values are compared with the molecular weight cut‐off values of these membranes. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)