Molecular Weight Cut-off Research Articles

Membrane-organic solute interactions in asymmetric flow field flow fractionation: Interplay of hydrodynamic and electrostatic forces

The structure and size characterization of organic matter (OM) using flow field-flow fractionation (FFFF) is interesting due to the numerous interactions of OM in aquatic systems and water treatment processes. The estimation of hydrodynamic and electrostatic forces involved in the fractionation of OM over different molecular weight cut-off (MWCO) membranes is vital for a better understanding of the FFFF process. This work aims to understand the membrane-OM interactive forces with respect to membrane MWCO, solute molecular weight, flow rates, solution pH and ionic strength. Polystyrene sulfonate sodium salt (PSS) of molecular weights 10, 30 and 65 kDa were used as model organic solutes for fractionation over ultrafiltration (UF) membranes of MWCO 1–30 kDa. Maximum fractionation of PSS was achieved by using a tight membrane of 1 kDa MWCO at the conditions of high permeate flow rate (1.5–2.0 mL·min−1), low concentrate flow rate (0.2–0.3 mL·min−1) and low ionic strength (10 mM). The better fractionation corresponds to high permeate drag force and low concentrate drag force. A low membrane-solute DLVO interaction is favourable for the retention of a small solute. This study illustrated that FFFF characteristics can be analyzed based on membrane-solute interactive forces controlled by selected flow, size and charge parameters.

Fouling of polyethersulphone ultrafiltration membranes during the decaffeination of ground coffee brews

The production of reduced caffeine coffee beverages (so called ‘half-decaf’) has recently gained increased commercial attention. Ultrafiltration (UF) can offer a viable alternative to the use of solvents in the decaffeination process, also operating at ambient temperatures, and a low operational cost. This paper reports the development and evaluation of the fouling occurring during the selective reduction of caffeine from coffee brews using a commercially available synthetic tight ultrafiltration (TUF) polyethersulphone (PES) membrane (GP95PP – Alfa Laval) with a 2 kDa molecular weight cut off (MWCO). Performance is reported in terms of fluxes, hydraulic resistances, and component rejection ratios. A cross-flow rig was operated at transmembrane pressures of 2–9 bar and cross-flow velocities of 0.04–0.1 m/s at temperatures in the range 25–50 oC. The process is viable: a retentate is produced with a much reduced caffeine concentration whilst still rich in higher molecular weight bioactives. Commercial PES membranes (GP95PP – Alfa Laval) had a permeate flux of 6.5 L m-2 h-1 and a fouling index (FI) of ca 60%. Rejection ratios were ca 30% for caffeine, ≥ 90% for both polyphenols & proteins, & ≥ 80% for melanoidins, over a 29 hr filtration period. Membrane surface modification due to fouling takes place, altering the hydrophobicity. An effective cleaning protocol is reported comprised of 0.5% w/v NaOH at 500 C. The flux decline and recovery are reported for multiple fouling and cleaning cycles.

Fabrication of Loose Nanofiltration Membranes with High Rejection Selectivity between Natural Organic Matter and Salts for Drinking Water Treatment.

Loose nanofiltration (LNF) membranes with a molecular weight cut-off (MWCO) of about 1000 Da and high surface negative charge density have great application potential for drinking water treatment pursuing high rejection selectivity between natural organic matter (NOM) and mineral salts. This study was conducted to exploit the novel method coupling non-solvent induced phase separation (NIPS) and interfacial polymerization (IP) for the preparation of high-performance LNF membranes. A number of LNF membranes were synthesized by varying the polyethersulfone (PES) and piperazine (PIP) concentrations in the cast solution for the PES support layer preparation. Results showed that these two conditions could greatly affect the membrane water permeance, MWCO and surface charge. One LNF membrane, with a water permeance as high as 23.0 ± 1.8 L/m2/h/bar, when used for the filtration of conventional process-treated natural water, demonstrated a rejection of NOM higher than 70% and a low rejection of mineral salts at about 20%. Both the mineral salts/NOM selectivity and permselectivity were superior to the currently available LNF membranes as far as the authors know. This study demonstrated the great advantage of the NIPS–IP method for the fabrication of LNF membranes, particularly for the advanced treatment of drinking water.

Ultrafiltration-based diafiltration for post-delignification fractionation of lignin from a deep eutectic solvent comprised of lactic acid and choline chloride

Lignocellulosic biomass is a valuable renewable resource that has the potential to be refined into multiple fractions of valuable biomaterials. Extracting and purifying lignin without compromising cellulose quality is a challenging goal. Deep Eutectic Solvents (DESs) have been reported as suitable delignification solvents, and for process development for cellulose extraction, lignin removal is essential. This work describes an investigation on continuous ultrafiltration (UF)-based diafiltration (DF) for recovering and purifying lignin dissolved in a DES-pulping stream comprising lactic acid and choline chloride following lignocellulose delignification. Experiments have been carried out in a stirred dead-end setup. The results indicate that lignin with a molecular weight of 200 g/mol or higher can be rejected by a polyethersulfone UF membrane with a molecular weight cutoff of 5000, reaching rejection values close to 0.85 for the feed solution with lignin concentration ranging from 4.5 to 30 g/L. It was discovered that membrane fouling and flux decline are DES concentration-dependent. A modeling approach was developed to predict the system size and operating conditions necessary to conduct the diafiltration process on a large scale, for which experimental data on lignin fouling at various feed concentrations and hydrodynamic flow characteristics were used. Based on the modeling results, lignin dissolved in DES-dark liquor (lignin-loaded DES) may well be separated and purified by a continuous UF-DF system.

Relationship between the Hansen solubility parameter and changes in membrane mass-transfer channels: A quantitative model

The relationship between swelling, the difference (Δδ) in theoretical Hansen solubility parameters (HSP) between the two substances, and the change of membrane mass-transfer channel has been controversial. In this work, a completely new parameter, Swell-Cavity Ratio (SCR), defined as the ratio of membrane molecular weight cut off (MWCO), was introduced to characterize the changes in mass transfer channels before and after membrane swelling. Models of the relationship between Δδ and SCR were successfully constructed for nanoporous glassy (SCR = 0.19 × Δδ0.40) and rubbery (SCR = 17.7 × Δδ-0.93) membranes, using the swelling as a bridge. Based on this contradiction in models, we speculate that SCR variation is strongly related to the separation mechanism of the membranes. The theoretical model accurately calculates the influence of the solvent on the membrane mass transfer channel, allowing for the selection of the best membrane material and pretreatment process to maximize treatment efficiency.

Polysulfate membrane prepared with a novel porogen for enhanced ultrafiltration performance

A novel deep eutectic solvent (DES) is proposed as a multifunctional porogen to simultaneously improve the permeability and selectivity of the polysulfate (PSE) ultrafiltration (UF) membrane. The effects of DES on the formation process of PSE membrane were explored by the viscosity, cloud point, and phase inversion properties. The molecular weight cutoff (MWCO) of the prepared membranes decreased as the DES content increased to 3.0 wt%, reaching as small as 18.5 kDa, with an improved porosity of 81.9%. In addition, the water flux improved to 212.3 L/m2*h, which was 2.3-fold larger than that of the pristine PSE membrane, and the rejection efficiency for natural organic matter (NOM) was simultaneously enhanced. Considering the easy and cheap preparation of the DESs, the proposed method offers a new possibility for high-performance UF membranes preparation.

Loose nanofiltration membranes for selective rejection of natural organic matter and mineral salts in drinking water treatment

Loose nanofiltration (LNF) is a promising technology for advanced treatment of drinking water, primarily owing to the potentially high rejection of natural organic matter (NOM) and high passage of mineral salts, particularly divalent cations, along with the low operating pressure. This study was devoted to relating the performance with the properties of LNF membranes, and obtaining the membrane properties most suitable for drinking water treatment. Cluster analysis indicated that the commercially available LNF membranes would behave quite differently, mainly depending on the molecular weight cut-off (MWCO). By using a treated natural water as the feed, a best-performing membrane had a mineral salts rejection lower than 30% and an NOM rejection higher than 70%, with the membrane water permeance at ∼17 L/m2/h/bar. The LNF membranes with high mineral salts/NOM selectivity should have an MWCO of around 1000 Da, sufficiently uniform pore sizes, and high surface negative charge density. The rejection characteristics for mineral ions by LNF membranes were similar with the relatively dense conventional NF membranes, but with a lower rejection difference among the ions, manifesting less strong co-ion and counter-ion competition effects. The LNF membranes with an MWCO of ∼1000 Da could satisfactorily remove (>80%) disinfection byproduct precursors, with small-sized protein-like and other substances remaining in the permeate water. This study helps to understand the rejection characteristics and mechanisms of LNF membranes, which will benefit the synthesis of high-performance LNF membranes for drinking water treatment and other applications.

Antibacterial Activity of Silver Nanoflake (SNF)-Blended Polysulfone Ultrafiltration Membrane

The aim of this research was to study the possibility of using silver nanoflakes (SNFs) as an antibacterial agent in polysulfone (PSF) membranes. SNFs at different concentrations (0.1, 0.2, 0.3 and 0.4 wt.%) were added to a PSF membrane dope solution. To investigate the effect of SNFs on membrane performance and properties, the water contact angle, protein separation, average pore size and molecular weight cutoffs were measured, and water flux and antibacterial tests were conducted. The antimicrobial activities of the SNFs were investigated using Escherichia coli taken from river water. The results showed that PSF membranes blended with 0.1 wt.% SNFs have contact angles of 55°, which is less than that of the pristine PSF membrane (81°), exhibiting the highest pure water flux. Molecular weight cutoff values of the blended membranes indicated that the presence of SNFs does not lead to enlargement of the membrane pore size. The rejection of protein (egg albumin) was improved with the addition of 0.1 wt.% SNFs. The SNFs showed antimicrobial activity against Escherichia coli, where the killing rate was dependent on the SNF concentration in the membranes. The identified bacterial colonies that appeared on the membranes decreased with increasing SNF concentration. PSF membranes blended with SNF, to a great degree, possess quality performance across several indicators, showing great potential to be employed as water filtration membranes.

Preparation of Silica/Graphene Oxide Composite Membranes for Nanofiltration

Graphene oxide (GO) membranes have received widespread attention. However, GO membranes exhibit low structural stability in water. We attempted to solve this problem by cross-linking GO with silica. The water permeance of a silica/GO membrane, which was prepared with a tetraethyl silicate/GO mass ratio of 190 and a reaction time of 3 h, was six times that of an unmodified GO membrane. The molecular weight cutoff of the silica/GO membrane was 800, which was almost the same as that of the unmodified GO membrane.

A super-hydrophilic partially reduced graphene oxide membrane with improved stability and antibacterial properties.

In order to improve stability and antibacterial property, a novel super-hydrophilic partially reduced graphene oxide membrane was prepared by interfacial polymerization of piperazine and partially reduced graphene oxide as aqueous solution and trimesoyl chloride as organic solution. Fourier transform infrared spectroscopy, scanning electron microscope, and contact angle measurement were conducted to probe the morphology and properties of the membranes. The modified membrane possessed super-hydrophilicity, improved durability and swelling resistance. The optimized membrane had a molecular weight cut off of about 674 Da and possessed a pure water permeability of 49.86 L·m-2·h-1·MPa-1. The retention order of salts was Na2SO4 > MgSO4 > MgCl2 > Na2CO3 > CaCl2 > NaCl, while the rejection for four kinds of pharmaceuticals followed the order of ibuprofen (92%) > carbamazepine (87%) > amlodipine (80%) > atenolol (76%), indicating that the negatively charged membrane could improve the retention performance by the electrostatic repulsive effect. Moreover, the enhanced antibacterial performance of membrane attributed to the dual effects of the super-hydrophilicity and the tea polyphenols antibacterial material loading, which may alter the charge distribution on and within the membrane, leading to loss of cell viability.

Tailoring pore size of positively-charged nanofiltration membrane via a facile self-assembly method

Positively-charged nanofiltration (NF) membranes have attracted more attention compared with traditional negative NF membranes. In this work, one-step self-assembly strategy was designed to prepare positively-charged NF separation layer by the close packing of poly(styrene-r-quaternized 2-(dimethylamino)ethyl methacrylate) (P(St-r-QDMA)) nanoparticles (NPs) with intramolecular crosslinking structure. Meanwhile, the pore size of NF membrane was tailored by regulating the aggregation of the NPs using different ions and annealing conditions. Herein, water was used as a dispersant to obtain P(St-r-QDMA) NPs dispersion, which is non-toxic and green. The pore size cut-off (PSCO) of the separation layer is well controlled in the range of 0.98–4.04 nm. The optimal membrane showed molecular weight cut-off (MWCO) of 380 Da (i.e., PSCO, 0.98 nm) with the isoelectric point of 9.4, which effectively improved the separation performance for divalent cations (97.2 % for MgCl2 and 96.0 % for CaCl2) and neutral small molecules (97.7 % for VB12 and 92.5 % for PEG400). This work gives a novel and convenient method to construct positively-charged NF membrane with controlled pore sizes, which will be a promising candidate for water softening and molecular separation.

Preparation of microporous organic solvent nanofiltration (OSN) composite membrane from a novel tris-phenol monomer

As an emerging membrane separation technology, organic solvent nanofiltration (OSN) has shown great application potential in the field of pharmaceutical and chemical industries referring to organic media. Herein, we prepared a novel microporous thin-film composite (TFC) OSN membrane with superior perm-selective performances, which was fabricated by applying 4,4′,4″-trihydroxytriphenylmethane (Tris-Phenol) to react with trimesoyl chloride (TMC) by interfacial polymerization (IP) technique. The rigid, non-planar and multifunctional nature of Tris-Phenol endowed the polyarylate nanofilm with enhanced microporosity. Of note, the prepared composite membrane showed good selectivity to small organic solutes with molecular weight cut-off (MWCO) as low as ∼295 Da, meanwhile had medium methanol permeance up to 5.86 LMH/bar, by the help of surfactant dodecyl trimethyl ammonium chloridea (DTAC) during the IP reaction. The experiments showed that the Tris-Phenol membrane exhibited extraordinary long-term operation stability. In addition, the Tris-Phenol membrane could achieve separation of solutes with similar molecular weights but different charges. This research well illustrated the novel IP monomer design strategy to fabricate new TFC OSN membrane with high perm-selectivity.

Membrane Technology for Valorization of Mango Peel Extracts

Mango peel is rich in nutritional and functional compounds, such as carbohydrates, dietary fibers, proteins, and phenolic compounds, with high potential to be applied in the food industry. Most of the investigation about recovery of bioactive compounds from fruit bioproducts involves extraction techniques and further separation of target compounds. There is still a lack of information about the potential of membrane processes to recover the nutritive/functional compounds present in aqueous extracts of those bioproducts. This research is addressed to study the performance of ultrafiltration (UF), followed by nanofiltration (NF) of UF permeates, to fractionate the compounds present in aqueous extracts of mango peel. Both UF and NF concentration processes were carried up to a volume concentration factor of 2.0. Membranes with molecular weight cut-offs of 25 kDa and 130 Da were used in the UF and NF steps, respectively. UF and NF concentrates showed antioxidant activity, attributed to the presence of phenolic compounds, with rejections of about 75% and 98.8%, respectively. UF membranes totally rejected the higher molecular weight compounds, and NF membranes almost totally concentrated the fermentable monosaccharides and disaccharides. Therefore, it is envisaged that NF concentrates can be utilized by the food industry or for bioenergy production.

Nanocomposite Polymeric Membranes for Organic Micropollutant Removal: A Critical Review.

The prevalence of organic micropollutants (OMPs) and their persistence in water supplies have raised serious concerns for drinking water safety and public health. Conventional water treatment technologies, including adsorption and biological treatment, are known to be insufficient in treating OMPs and have demonstrated poor selectivity toward a wide range of OMPs. Pressure-driven membrane filtration has the potential to remove many OMPs detected in water with high selectivity as a membrane's molecular weight cutoff (MWCO), surface charge, and hydrophilicity can be easily tailored to a targeted OMP's size, charge and octanol-water partition coefficient (Kow). Over the past 10 years, polymeric (nano)composite microfiltration (MF), ultrafiltration (UF), and nanofiltration (NF) membranes have been extensively synthesized and studied for their ability to remove OMPs. This review discusses the fate and transport of emerging OMPs in water, an assessment of conventional membrane-based technologies (NF, reverse osmosis (RO), forward osmosis (FO), membrane distillation (MD) and UF membrane-based hybrid processes) for their removal, and a comparison to the state-of-the-art nanoenabled membranes with enhanced selectivity toward specific OMPs in water. Nanoenabled membranes for OMP treatment are further discussed with respect to their permeabilities, enhanced properties, limitations, and future improvements.

Using molecular weight-based fluorescent detector to characterize dissolved effluent organic matter in oxidation ditch with algae.

Implementation of microalgae has been considered for enhancing effluent wastewater quality. However, it can cause environmental issues due to the release of extracellular and algal organic matter in the biological process. This study aimed to investigate the characteristics of dissolved effluent as algae- and bacteria-derived organic matter during the oxidation ditch process. Furthermore, experiments were conducted under three combinations filled by Spirulina platensis, Chlorella vulgaris, and without microalgae. The results showed that dissolved effluent organic matter was more aromatic and hydrophobic than before treatment. Fluorescence spectroscopy identified two components-aromatic protein-like and soluble microbial product-like components-at excitation/emission of 230/345nm and 320/345nm after treatment, instead of fulvic acid-like at 230/420nm and humic acid-like at 320/420nm in raw wastewater. These components were fractionated based on the average of molecular weight cut-offs (MWCOs), and high (MWCOs > 50,000Da), medium (MWCOs 50,000-1650Da), and low molecular weights (MWCOs < 1650Da) were reported. Biological oxidation ditch under symbiosis algal bacteria generated humic and fulvic acid with a higher MWCOs than the process without algal. The quality and quantity of dissolved effluent organic matter in an oxidation ditch reactor were significantly affected by algal-bacteria symbiotic.

Influence of molecular weight cut-off (MWCO) of commercial ultrafiltration substrate on the performance of thin film composite nanofiltration membrane

This study investigated the impacts of polyether sulfone (PES) substrate with four different molecular weight cut-offs (MWCO) on the performance of the thin-film composite (TFC) nanofiltration membranes. When the MWCO of the substrate increased: (i) the effective thickness of the substrate decreased; (ii) the average pore size of the substrate enlarged in general; and (iii) the surface roughness of the substrate increased. When the MWCO of substrates was increased, the TFC membranes synthesized from them showed the following characteristics: (i) the thickness of the polyamide layer expanded; (ii) average size of the nodules on the membrane surface enlarged, (iii) permeate flux enhanced in general, (iv) rejection of NaCl (RNaCl) increased and rejection of MgSO4 (RMgSO4) did not change significantly; (v) flux recovery rates decreased. The study showed that the MWCO of the substrate had a similar influence on the permeate fluxes produced by the substrate and the TFC membrane. Lower MWCO substrates induced higher selectivity to divalent and monovalent ions.

Interafacially grown ultrathin high flux polymeric nanofilm for molecular separation: An improved trade-off between permeance and selectivity

Polymeric membranes with enhance permeability and selectivity are highly anticipated approach for liquid separation. Polymeric membranes for controlled molecular weight cut-off and high liquid permeance are desirable to separate small organic molecules and multivalent ions from a water stream. This demands active polymeric layer to be defect-free, thin, narrow pores and pore size distribution and surface charge to maximize membrane performance. We described an interfacial polymerization reaction between phloroglucinol (PG) and 1,3,5-trimesyol chloride (TMC) at water-hexane interface to form polymeric layer. Controlling the monomer concentration at the interphase resulted into the formation of the defect free polymeric thin film down to 20 nm (nanofilm). Electron and force microscopy suggested the presence crumpled and crater morphology of the polymeric nanofilm. FT-IR analysis clearly suggested the formation of ester bonds. Pressure driven transport data demostrate that polyaryl nanofilm shows high water permeance of ∼108 Lm−2h−1 bar−1 and demonstrates the high rejection (>90 %) of molecules (Molecular weight cutoff > 585 gmol−1) and high permeance of ∼92 Lm−2h−1 bar−1. High rejection (>95) of multivalent ions (K3[Fe(CN)6]) achieved as compared to monovalent (∼2%) and divalent salt (∼14%) with the water permeance of ∼94 Lm−2h−1 bar−1.

Construction of hollow fiber nanofiltration separation layer with bridging network structure by polymer-anchored co-deposition for high-concentration heavy metal ion removal

In this work, a positively charged hollow fiber nanofiltration (NF) membrane with a bridging network structure was prepared by applying the polymer-anchored co-deposition method. The microstructural regularity of the bridging network structure during the formation and evolution process were interpreted. The acquired results indicated that both tannic acid (TA) and ferric ion (Fe3+) can be rapidly deposited and then anchored on the membrane surface by the chemical bridging with polyethyleneimine (PEI), which effectively filled the pores and significantly improved the positive charge of the membrane surface. Interestingly, the proposed membrane showed an excellent removal efficiency of high-concentration heavy metal ion (4000 mg L−1, Fe3+ (95.7%), Cu2+ (94.1%), Mn2+ (92.9%), Zn2+ (91.1%)), while keeping a high pure water permeance (23.18 L m−2 h−1 bar−1). By combining the relationship between the hydrated diameter about the heavy metal ions, the molecular weight cut-off (MWCO) of NF membrane and the surface charge of membrane, the underlying mechanism of high-concentration heavy metal ion removal was expounded. In addition, the membrane exhibited excellent ability to resist acid/base, anti-fouling capacity and long-term stability. Our work provides a new method for the fabrication of hollow fiber NF membrane with high separation performance to high-concentration heavy metal ions.

Fabrication and characterization of inner selective antibiofouling forward osmosis hollow fiber membranes for simultaneous wastewater treatment and desalination

Forward osmosis (FO) hollow-fiber membranes were prepared by in situ thin layer assembly of polyethyleneimine (PEI) on antifouling PES-based porous nanocomposite membranes during membrane formation by spinning and phase inversion, via incorporation of surface-modified silver nanoparticles (NPs) in the dope solution and addition of PEI in the bore liquid; after cross-linking with glutaraldehyde the FO selective layer was obtained at the lumen surface while the shell surface had ultrafilter (UF) properties. This design enabled the utilization of the FO membranes in active layer facing draw solution (DS) orientation to employ maximum effective osmotic pressure, while the UF layer in combination with the antibacterial NPs was supposed to reduce fouling by the wastewater used as feed solution (FS). The quality of the active layer assembly was characterized using atomic force microscopy and scanning electron microscopy analyses, as well as salt rejection, structural parameter (S), and molecular weight cut-off (MWCO) measurements. The separation performance of these membranes was evaluated in an osmotic membrane bioreactor (OMBR) system, showing for the best FO nanocomposite membranes less than 10% fouling and flux reduction over 5 cycles of operation over a total period of 5 days. The best-suited membrane showed promising separation performance in the OMBR system by providing 43.4 LMH water flux and only around 0.2 g/l specific reverse salt flux (SRSF) with 1 M NaCl and DI water serving as DS and feed, respectively. This study focused also on distinctive performance criteria in combined FO and OMBR systems, such as the membrane’s potential in a pretreatment step of brine streams (used as DS) to provide proper feed composition for RO units and introduced a new specific performance index (SPI) for better characterization of FO membranes which summarizes all essential characteristics of the membrane application, instead of using existing structural parameter equations for membrane characterization. The best-suited FO membrane offered an SPI of 778.2 gl−1 m−2 h−1, meaning 778.2 g/l reduction of TDS in DS per unit membrane surface area per hour.

The Effect of Heat Sterilization on Key Filtration Performance Parameters of a Commercial Polymeric (PVDF) Hollow-Fiber Ultrafiltration Membrane.

Membrane processes can be integrated with fermentation for the selective separation of the products from the fermentation broth. Sterilization with saturated steam under pressure is the most widely used method; however, data concerning heat sterilization applicability to polymeric ultrafiltration (UF) membranes are scarcely available. In this study, the effect of the sterilization process on the filtration performance of a commercial polyvinylidene difluoride (PVDF) hollow fiber UF membrane was evaluated. Membrane modules were constructed and sterilized several times in an autoclave. Pure water flux tests were performed, to assess the effect of heat sterilization on the membrane’s pure water permeance. Dextran rejection tests were performed for the characterization of membrane typical pore size and its fouling propensity. Filtration performance was also assessed by conducting filtration tests with real fermentation broth. After repeated sterilization cycles, pure water permeance remained quite constant, varying between approx. 830 and 990 L·m−2·h−1·bar−1, while the molecular weight cut-off (MWCO) was estimated to be in the range of 31.5–98.0 kDa. Regarding fouling behavior, the trans-membrane pressure increase rate was stable and quite low (between 0.5 and 7.0 mbar/min). The results suggest that commercial PVDF UF membranes are a viable alternative to high-cost ceramic UF membranes for fermentation processes that require heat sterilization.