Retention measurements of nanofiltration membranes with electrolyte solutions

Retention measurements with single salt solutions of CaCl₂, NaCl and Na₂SO₄ revealed that the rejection mechanism of commercial polymeric nanofiltration membranes investigated in this study may be divided into two categories:

• 1.Membranes for which Donnan exclusion seems to play an important role.
• 2.Membranes for which retention is determined by both Donnan exclusion and size effects.

In category 1 both positively and negatively charged membranes were found.Ceramic γ-Al₂O₃ ultrafiltration membranes with a pore size of 3 nm showed a same type of salt retention behavior as the positively charged polymeric membranes.The extended Nernst–Planck equation in combination with the Donnan equilibrium has been used to model the flux-retention experiments for the salt solutions. The numerical calculations resulted in a good agreement with experimental data and acceptable values for the fixed charge densities have been determined. The effective membrane thicknesses calculated were higher than those observed by scanning electron microscopy.     

Modeling the separation performance of nanofiltration membranes for the mixed salts solution

A new model is proposed to evaluate the separation performance of nanofiltration (NF) membranes for the mixed salts solution. In the model, the observed transmission of an ion through a NF membrane is applied to express the separation performance of the membrane for the ion in the mixed salts solution, which has a relationship with the total concentration, the equivalent fraction and the species of each ion in the mixed salts solution. The verification of the model was carried out in the permeation experiments of some mixed salts solutions ((1) Na⁺, Cl⁻ and F⁻; (2) Na⁺, K⁺ and Cl⁻; (3) Na⁺, F⁻, Cl⁻ and NO₃⁻; (4) Na⁺, Cl⁻, NO₃⁻ and SO₄²⁻) through three commercial NF membranes (ESNA 1-LF, ESNA 1 and LES 90). According to the permeation experiments of three NF membranes for some binary salts solutions, the competition coefficients of ions were obtained. The model evaluation results agreed quite well with the experimental data. Finally, the model was applied to evaluate the observed transmission of each ion in the mixed salts solution (Na⁺, F⁻, Cl⁻, NO₃⁻ and SO₄²⁻) through three NF membranes. The agreement between the model evaluation results and the experimental data indicated that the model is suitable for evaluating the separation performance of three NF membranes for the mixed salts solution.     

Effect of organic ion on the separation of salts by nanofiltration membranes

The aim of this study is to investigate the transport mechanism of NaCl through nanofiltration membranes in mixed solutions. A model based on the Spiegler–Kedem and Perry–Linder models has been developed. Concentration polarization effects of an organic ion and NaCl were not included in the previous studies. The effects of concentration polarization on salt removal were added to the model. Reactive Black 5 (RB5) was used as an organic ion in the rejection of NaCl by DS5 membrane in all experimental runs. Different concentrations of RB5 and NaCl have been studied and a coefficient of the gel polarization effect of dye on salt removal (α) was defined depending on molar concentration ratios of dye and NaCl. Experimental and calculated results were shown to be in good agreement.     

Mass diffusion-based separation of sugars in a microfluidic contactor with nanofiltration membranes

Processes such as chromatographic separation and nanofiltration can remove low molecular weight sugars from liquid mixtures of oligosaccharides. As an alternative for the separation of such liquid mixtures, we studied mass diffusion separation of such sugars in a microfluidic device with incorporated nanofiltration membranes. This separation method is based on differences between diffusivities of components and does not require high transmembrane pressures. The effects of channel depth and flow rate were studied in experiments. The key parameters selectivity and rejection increased with increasing channel depth due to increased external mass transfer limitations. Among the studied membranes, the obtained selectivities and rejections correlated to the specified retention values by the manufacturers. Compared to more conventional nanofiltration where high pressure forces solutes through membranes, we obtained corresponding selectivities and fluxes of only an order of magnitude smaller. Simulated results indicated that with optimized microchannel and membrane dimensions, the presented separation process can compete with currently available separation technologies.     

Nanocomposite membranes for organic solvent nanofiltration

Organic solvent nanofiltration (OSN) is a molecular separation method which offers a sustainable and reliable solution compared to the conventional energy-intensive separation processes. OSN can be successfully applied to several applications, such as food, pharmaceutical, petrochemical and fine-chemical industries. Current research on OSN membranes mainly focuses on polymeric materials due to the ease of processing, controlled formation of pores, lower fabrication costs and higher flexibility as compared with inorganic materials. However, there are some limitations for the polymeric membranes which can be partially surmounted by adding nanoscale fillers into the polymeric matrix to make nanocomposite membranes. This review aims to comprehensively evaluate and report the advances in nanocomposite membranes prepared by using either different nanoscale fillers or various fabrication methods for OSN applications. Nanoparticles that will be discussed include metal-organic framework, graphene oxide, carbon nanotubes, silica, titanium, gold, zeolite and other fillers. The incorporation of these nanoscale fillers into the polymeric membranes can positively influence the mechanical strength, chemical and thermal stability, hydrophilicity, solute selectivity and solvent permeance. This study may provide helpful insights to develop next-generation of OSN membranes for years to come.     

Fouling of reverse osmosis and nanofiltration membranes by humic acid—Effects of solution composition and hydrodynamic conditions

Fouling of reverse osmosis (RO) and nanofiltration (NF) membranes by humic acid, a recalcitrant natural organic matter (NOM), was systematically investigated. The membrane flux performance depended on both hydrodynamic conditions (flux and cross-flow velocity) and solution composition (humic acid concentration, pH, ionic strength, and calcium concentration), and was largely independent of virgin membrane properties. While increasing humic acid concentration and ionic strength, and lowering cross-flow velocity affected flux performance moderately, severe flux reduction occurred at high initial flux, low pH, and high calcium concentration. At a calcium concentration of 1 mM, all the membranes exhibited an identical stable flux, independent of their respective intrinsic membrane permeabilities. The effect of solution composition was more significant at higher fluxes. Improved salt rejection was observed as a result of humic acid fouling, which was likely due to Donnan exclusion by humic material close to membrane surfaces. Greater rejection improvement was observed for membranes with rougher surfaces.     

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 transmembrane electrical potential of nanofiltration membranes in KCl and MgCl2 solutions

The transmembrane electrical potential (TMEP) across two commercial nanofiltration membranes (ESNA1-K and Filmtec NF) was investigated in KCl and MgCl(2) solutions. TMEP was measured in a wide range of salt concentrations (1-60 mol·m(-3)) and pH values (3-10) at the feed side, with pressure differences in the range of 0.1-0.6 MPa. A two-layer model based on the Nernst-Planck equation was proposed to describe the relation between TMEP and permeation flux. From the pattern of these curves, the information of membrane structure could be deduced. In the concentration range investigated, TMEP in KCl solutions was always positive and decreased as the salt concentration increased. The contribution of the membrane potential to the TMEP decreased. TMEP was greatly affected by the feed pH. When the feed pH increased, the mobility of cations increased, which indicated that the charges of NF membranes were more negative. The zero point of TMEP and the minimum of rejection in KCl solution were consistent and occurred at the isoelectric point of NF membranes, while in MgCl(2) solution the zero point of TMEP located at a higher pH value. The TMEP in MgCl(2) solutions changed its sign at a given concentration, and by calculating the transport number the location of the minimum rejection could be determined.     

On the prediction of permeate flux for nanofiltration of concentrated aqueous solutions with thin-film composite polyamide membranes

The nanofiltration of binary aqueous solutions of glucose, sucrose and sodium sulfate was investigated using thin-film composite polyamide membranes with different molecular weight cut-off's. The NF experiments, in total recycle mode, were performed in a plate-and-frame module Lab 20 (AlfaLaval), at 22 °C and with a flowrate of 8.2 L/min, using the membranes NF90, NF200 and NF270 from FilmTec (Dow Chemical), for transmembrane pressures between 1 and 6 MPa and with aqueous solutions with osmotic pressures of between 0.5 and 3.0 MPa. The permeate flux was predicted by the osmotic pressure model, using the membrane hydraulic resistance and the solution viscosity inside the membrane pores, and computing the concentration polarization with recourse to a mass-transfer correlation specific for the plate-and-frame module used. The flux predictions, using the pure water viscosity, agree reasonably with the experimental data only for low transmembrane pressures and with the most diluted solutions. For higher transmembrane pressures and for higher solute concentration the predicted fluxes can be as far as 2.5, 4.1 and 9.6 times higher than the experimental one, for the aqueous solutions of Na₂SO₄, glucose and sucrose, respectively. These deviations are strongly reduced when the pure water viscosity is replaced by the solution viscosity adjacent to the membrane. In this case, the maximum deviation between predictions and experiments occurs also for higher transmembrane pressures and for higher solute concentration, but the maximum ratio between predicted values and the experiments were reduced now to 1.8, 2.1 and 2.9, for the aqueous solutions of Na₂SO₄, glucose and sucrose, respectively. Even using the solution viscosity adjacent to the membrane, and for the systems investigated, the osmotic pressure model must used with caution for design purposes because it may over predict the permeate flux by a factor of about 2 when the solute concentration is high.     

Evaluating the charge of nanofiltration membranes

In this study, several methods were used to determine the charge of commercially available nanofiltration membranes, and were compared. First the ion-exchange capacity was determined by titration, this method is able to distinguish between positively and negatively charged functional groups on the membrane. Secondly, measurements of the streaming potential gave a value for the charge density at the exterior membrane surface; the effect of the pH of the solution on the membrane charge was studied. Finally, measurements of the membrane potential allowed to evaluate the total membrane charge density.The results of the three methods were used to compare the membrane charge of nanofiltration membranes mainly in a qualitative way. It is shown that measurements of the membrane potential are preferred for the evaluation of the membrane charge.     

Physico-chemical characterization of nanofiltration membranes.

This study presents a methodology for an in-depth characterization of six representative commercial nanofiltration membranes. Laboratory-made polyethersulfone membranes are included for reference. Besides the physical characterization [molecular weight cut-off (MWCO), surface charge, roughness and hydrophobicity], the membranes are also studied for their chemical composition [attenuated total reflectance Fourier spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS)] and porosity [positron annihilation spectroscopy (PAS)]. The chemical characterization indicates that all membranes are composed of at least two different layers. The presence of an additional third layer is proved and studied for membranes with a polyamide top layer. PAS experiments, in combination with FIB (focused ion beam) images, show that these membranes also have a thinner and a less porous skin layer (upper part of the top layer). In the skin layer, two different pore sizes are observed for all commercial membranes: a pore size of 1.25-1.55 angstroms as well as a pore size of 3.20-3.95 angstroms (both depending on the membrane type). Thus, the pore size distribution in nanofiltration membranes is bimodal, in contrast to the generally accepted log-normal distribution. Although the pore sizes are rather similar for all commercial membranes, their pore volume fraction and hence their porosity differ significantly.     

纳滤膜分离机理及其应用研究进展

综述了纳滤膜的分离机理及其应用研究现状和进展,纳滤膜分离过程是一个不可逆过程,其分离机理可以运用电荷模型（空间电荷模型和固定电荷模型）和细孔模型,以及近年才提出的静电排斥和立体阻碍模型等来描述。纳滤膜应用研究现状的介绍包括低聚糖分离和精制、果汁的高浓度浓缩、多肽和氨基酸的分离、抗生素的浓缩与纯化、牛奶及乳清蛋白的浓缩、农产品的综合利用以及纳滤膜生化反应器的开发等。     

Computer program for simulation of mass transport in nanofiltration membranes

A computer program, NanoFiltran, was developed to simulate the mass transport of multi-ionic aqueous solutions in charged nanofiltration (NF) membranes, based on the Donnan steric partitioning pore and dielectric exclusion (DSPM&DE) model, with incorporation of the non-ideality of electrolyte solutions and concentration polarization effects in the membrane/feed-solution interface. With this computer program, the extended Nernst–Planck (ENP) equations are discretized inside the membrane, using the finite-difference scheme. The discretized ENP equations together with the other model equations are linearized in order to obtain a system of equations that are solved simultaneously. The linearized system of equations is based on an initial guess for the electrical potential and ions concentrations profiles, which are updated iteratively. A robust method of under-relaxation of the electrical potential and ions concentrations ensures that the convergence is achieved even for NF systems that exhibit a very stiff numerical behaviour.     

Streaming potential measurements as a characterization method for nanofiltration membranes

The streaming potentials of two different nanofiltration membranes were studied with several electrolyte solutions to investigate the influence of salt type and concentration on the zeta potential and kinetic surface charge density of the membranes. The zeta potentials decreased with increasing salt concentration, whereas the kinetic surface charge densities increased. The kinetic surface charge densities could be described by Freundlich isotherms, except in one case, indicating that the membranes had a negligible surface charge. The kinetic surface charge density observed was caused by adsorbed anions. Salt retention measurements showed different mechanisms for salt separation for the two investigated membranes. One membrane showed a salt retention that could be explained by a Donnan exclusion type of separation mechanism, whereas for the other membrane the salt rejection seemed to be a combination of size and Donnan excluion. Comparing the results obtained by the streaming potential measurements with those of the retention measurements, it could be concluded that the membrane with the highest kinetic surface charge density showed the Donnan exclusion type of separation, whereas the membrane with the lower surface charge density showed a separation mechanism that was not totally determined by Donnan exclusion, size effects seemed to play a role as well.     

Modeling of the adsorption of organic compounds on polymeric nanofiltration membranes in solutions containing two compounds.

The adsorption of organic compounds in aqueous solution on polymeric nanofiltration membranes is studied; this process is one of the most important fouling mechanisms influencing the flux and retention behavior of nanofiltration membranes. It is shown that the adsorption of dissolved organic compounds on polymeric nanofiltration membranes is comparable to that on activated carbon. Freundlich and Langmuir isotherms are used to describe the relation between the adsorbed mass on the membrane and the equilibrium concentration of the organic compound in a single-compound solution. Based on these results, three models for the adsorption of solutions containing several compounds [i.e., the simple competitive adsorption model (SCAM), the model of Jain-Snoeyinck, and the model of Butler-Ockrent] were used to predict the adsorption behavior of an organic compound in an aqueous solution containing two compounds. The results of the three models were compared to experimental observations. It is shown that the SCAM allows a good prediction of the adsorption behavior.     

Cross-linked polyimide membranes for solvent resistant nanofiltration in aprotic solvents

Solvent stable nanofiltration membranes were prepared through the chemical cross-linking of asymmetric Matrimid^®-based polyimide membranes with p-xylylenediamine. The influence of this straightforward post-treatment on membrane stability, morphology and performance in dimethylformamide (DMF), N-methylpyrrolidinone (NMP), dimethylacetamide (DMAc) and dimethylsulfoxide (DMSO) was thoroughly investigated. With permeabilities up to 5.4 l/m² bar h and rejections up to 98% for low molecular weight dyes in these demanding solvents, optimally performing, truly solvent resistant nanofiltration membranes were obtained. Nanozeolite-filled membranes were prepared in parallel to study the effect of an inorganic filler on the cross-linking reaction and performance in aprotic solvents. The outstanding stability and performance of these membranes and their easy preparation clearly offer vast potential for applications in harsh solvent environments.     

Cobalt-doped silica membranes for gas separation

Cobalt-doped silica membranes were synthesized using tetraethyl orthosilicate-derived sol mixed with cobalt nitrate hexahydrate. The cobalt-doped silica structural characterization showed the formation of crystalline Co₃O₄ and silanol groups upon calcination. The metal oxide phase was sequentially reduced at high temperature in rich hydrogen atmosphere resulting in the production of high quality membranes. The cobalt concentration was almost constant throughout the film depth, though the silica to cobalt ratio changed from 33:1 at the surface to 7:1 at the interface with the alumina layer. It is possible that cobalt has more affinity to alumina, thus forming CoOAl₂O₃. The He/N₂ selectivities reached 350 and 570 at 160 °C for dry and 100 °C wet gas testing, respectively. Subsequent exposure to water vapour, the membranes was regenerated under dry gas condition and He/N₂ selectivities significantly improved to 1100. The permeation of gases generally followed a temperature dependency flux or activated transport, with best helium permeation and activation energy results of 9.5 × 10⁻⁸ mol m⁻² s⁻¹ Pa⁻¹ and 15 kJ mol⁻¹. Exposure of the membranes to water vapour led to a reduction in the permeation of nitrogen, attributed to water adsorption and structural changes of the silica matrix. However, the overall integrity of the cobalt-doped silica membrane was retained, given an indication that cobalt was able to counteract to some extent the effect of water on the silica matrix. These results show the potential for metal doping to create membranes suited for industrial gas separation.     

Magnetically activated micromixers for separation membranes

Presented here is a radically novel approach to reduce concentration polarization and, potentially, also fouling by colloids present in aqueous feeds: magnetically responsive micromixing membranes. Hydrophilic polymer chains, poly(2-hydroxyethyl methacrylate) (PHEMA), were grafted via controlled surface-initiated atom transfer radical polymerization (SI-ATRP) on the surface of polyamide composite nanofiltration (NF) membranes and then end-capped with superparamagnetic iron oxide magnetite (Fe(3)O(4)) nanoparticles. The results of all functionalization steps, that is, bromide ATRP initiator immobilization, SI-ATRP, conversion of PHEMA end groups from bromide to amine, and carboxyl-functional Fe(3)O(4) nanoparticle immobilization via peptide coupling, have been confirmed by X-ray photoelectron spectroscopy (XPS) and field emission scanning electron microscopy (FESEM). These nanoparticles experience a magnetic force as well as a torque under an oscillating external magnetic field. It has been shown, using particle image velocimetry (PIV), that the resulting movement of the polymer brushes at certain magnetic field frequencies induces mixing directly above the membrane surface. Furthermore, it was demonstrated that with such membranes the NF performance could significantly be improved (increase of flux and salt rejection) by an oscillating magnetic field, which can be explained by a reduced concentration polarization in the boundary layer. However, the proof-of-concept presented here for the active alteration of macroscopic flow via surface-anchored micromixers based on polymer-nanoparticle conjugates has much broader implications.     

Characterization of multilayer nanofiltration membranes using positron annihilation spectroscopy

Positron annihilation spectroscopy (PAS) coupled with a slow positron beam was used to characterize in situ the layer structure and depth profile of the cavity size in thin film composite (TFC) polyamide nanofiltration (NF) membranes prepared by the interfacial polymerization method. Two techniques, using PAS coupled with a slow positron beam of Doppler broadening energy spectra (DBES) and positron annihilation lifetime spectroscopy (PALS) designed to reveal the layer structure and the cavity sizes contained in a multilayer thin film composite NF membrane, were assessed. To the best knowledge of the authors, a characterization of the depth profile of cavities in NF membranes using PAS coupled with a slow positron beam has never been reported. The membranes selected have a composite structure containing three layers: a selective polyamide layer, a transition layer, and a porous support prepared by the phase inversion technique. Furthermore, the cavity size distribution in the selective top layer plays an important role in determining the performance of the NF membranes.     

High-flux nanofiltration membranes prepared by adsorption of multilayer polyelectrolyte membranes on polymeric supports

Layer-by-layer deposition of anionic and cationic polyelectrolytes readily converts polymeric ultrafiltration membranes into materials capable of nanofiltration. ATR-FTIR spectra confirm that layer-by-layer deposition occurs on the ultrafiltration substrates, and adsorption of as few as 2.5 bilayers of poly(styrenesulfonate) (PSS)/protonated poly(allylamine) (PAH) or 3.5 bilayers of PSS/poly(diallyldimethylammonium chloride) (PDADMAC) reduces the molecular weight cutoff of polyethersulfone ultrafiltration supports from 50 kDa to <500 Da. Deposition of multilayer polyelectrolyte films on 300 and 500 kDa membranes also decreases molecular weight cutoffs, but solute rejections are significantly lower when using these supports, suggesting that the polyelectrolyte films do not completely cover large (0.2-0.4 microm in diameter) pores. On the 50 kDa substrates, PSS/PDADMAC films containing 3.5 bilayers exhibit a 95% rejection of SO(4)(2-) and a chloride/sulfate selectivity of 27, whereas 4.5-bilayer PSS/PAH coatings show a glucose/raffinose selectivity of 100. Pure water flux for [PSS/PAH](3)PSS-coated membranes at 4.8 bar is 1.6 m(3)/(m(2)day), which is more than 2-fold higher than that through a commercial 500 Da membrane.