Development of asymmetric BTDA-TDI/MDI (P84) copolyimide flat sheet and hollow fiber membranes for ultrafiltration: Morphology transition and membrane performance

In this paper, the effects of γ-butyrolactone (GBL) weight ratio (w_GBL) and membrane thickness on the formation of asymmetric flat sheet membranes prepared with P84 (BTDA-TDI/MDI co-polyimide)/N-methyl-2-pyrrolidone (NMP)/GBL casting solutions are investigated. With the increase of membrane thickness, the transition of membrane morphology from sponge-like to finger-like structure occurs at critical structure-transition thickness L_c. L_c and the general sponge-like structure thickness (L_gs) increase with w_GBL. For 20 wt.% P84/NMP/GBL casting solution, the membrane morphology changes from finger-like to sponge-like structure at the critical weight ratio of GBL (w^? = 0.69). The membrane morphology and performance of hollow fibers spun with various w_GBL are observed. Compared with the hollow fiber membranes made of 18 wt.% P84/NMP/GBL dope solution with w_GBL = 0.75, the hollow fiber membranes spun with w_GBL = 0.25 present a higher permeation flux and a larger MWCO. As w_GBL increases from 0.25 to 0.75, the membrane morphology transfers from finger-like to sponge-like structure. An increase in shear rate shifts the rejection curves towards left, and lowers the MWCO of hollow fiber membranes. For hollow fiber membranes spun with w_GBL = 0.75, a relatively high permeation flux and a large MWCO are obtained by the wet spinning process.     

Analysis and optimization of the influence of operating conditions in the ultrafiltration of macromolecules using a response surface methodological approach

In this work, the ultrafiltration of macromolecules was analysed using a response surface methodological approach. The behaviour of two different inorganic membranes was investigated. The membranes selected were a Carbosep M2 membrane (Orelis, France) with a molecular weight cut-off (MWCO) of 15 kDa and a Tami MSKT membrane (Tami Industries, France) with a MWCO of 5 kDa. The solute employed was polyethylene glycol of 35 kDa molecular weight. The influence of transmembrane pressure (0.1, 0.2, 0.3, 0.4 and 0.5 MPa), crossflow velocity (1, 2 and 3 m/s) and feed concentration (5, 10 and 15 g/L) on permeate flux and permeate flux decline was investigated. Analysis of variance was proved to be a useful tool to determine the effect of operating variables on both parameters. The method used demonstrated the presence of coupled effects between factors as well as squared effects that are relevant to the ultrafiltration process. The surface contours obtained from fitted models were used for the optimization of the operating conditions. The goal was to simultaneously maximize the average permeate flux and minimize the flux decline. The optimal operating conditions for the Carbosep M2 membrane were a transmembrane pressure of 0.38 MPa and a crossflow velocity of 3 m/s. The optimal operating conditions for the Tami MSKT membrane could not be determined by means of multiple response optimization due to the low accuracy of the regression model obtained for the cumulative permeate flux decline (SFD) response variable.     

Processing of alumina-coated clay–diatomite composite membranes for oily wastewater treatment

Crack-free alumina-coated clay–diatomite composite membranes were successfully prepared by a simple pressing and dip-coating route using inexpensive raw materials at a temperature as low as 1000 °C in air. The changes of porosity, flexural strength, pore size, flux, and oil rejection rate of the membranes were investigated while changing the diatomite content. A simple burn-out process subjected to the used membranes in air completely recovered the specific surface area, steady state flux, and oil rejection rate of the virgin membranes. The recycled membranes showed an exceptionally high oil rejection rate (99.9%) with a feed oil concentration of 600 mg/L at an applied pressure of 101 kPa. The typical porosity, pore size, flexural strength, oil rejection rate, and steady state flux of the recycled alumina-coated clay–diatomite composite membrane were 36.5%, 0.12 μm, 32 MPa, 99.9%, and 6.91×10⁻⁶ m³ m⁻² s⁻¹, respectively, at an applied pressure of 101 kPa.     

Positively charged loose nanofiltration membrane grafted by diallyl dimethyl ammonium chloride (DADMAC) via UV for salt and dye removal

A novel positively charged loose nanofiltration (NF) membrane was fabricated feasibly by UV-induced photografting polymerization of diallyl dimethyl ammonium chloride (DADMAC) on Polysulfone ultrafiltration membrane. A possible reaction mechanism was proposed that a linear chain structure and/or pyrrole like five-membered nitrogen heterocycles structure on the side chain were grafted to form the active barrier layer. NF membrane demonstrated a looser average pore size of 8.6 nm and positive charges surface. Owing to the nanoscale ultrathin nanoscale barrier layer and the combination of Donnan exclusion and steric hindrance, NF membrane exhibited good hydrophilicity, a high pure flux of 60 L/m² h (0.5 MPa), a good salt rejection to Mg²⁺ (90.8%), Al³⁺ (94.0%), Ca²⁺ (91.5%), and a high dye rejection to methylene blue (99.4%) and congo red (100.0%) respectively. The salts rejection of NF membrane to different salts followed the order of AlCl₃ > CaCl₂ > MgCl₂ > NaCl > LiCl > MgSO₄ > Na₂SO_4. NF membrane showed certain fouling resistance to seawater and BSA solution. The grafting polymerization kinetics were comprehensively investigated including irradiation time, monomer concentration and irradiation intensity. X-ray Photoelectron Spectroscopy (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM) and contact angle measurement were employed to investigate membrane chemistry, morphologies, and hydrophilicity.     

Application of membrane distillation technology in the treatment of table olive wastewaters for phenolic compounds concentration and high quality water production

Direct contact membrane distillation process (DCMD) is proposed for the treatment of table olive wastewaters (TOW) for high quality water production and concentration of their phenolic compounds. The main objective was to investigate the effectiveness of DCMD process to concentrate phenolic compounds from TOW that can be reused as a potential source for powerful natural antioxidants. The performance of three commercial membranes, made from polytetrafluoroethylene supported by polypropylene net (TF200, TF450 and TF1000), were tested. The permeate fluxes and polyphenols concentration in both the permeate and retentate have been monitored under different DCMD operating temperatures. It was found that the three membranes exhibit an excellent separation coefficient (greater than 99.5%) even after 4 h of DCMD operation with TOW. High concentration factors were obtained with the membrane TF450 at 70 °C, while the membrane TF200 having the lowest pore size was found to be more resistant to fouling phenomenon compared to the other membranes since the reduction of its water permeate flux after TOW treatment did not exceed 2.9%. High quality of the permeate was obtained with phenolic concentration lower than 16 mg of TYE/L. In addition, the values of electrical conductivity of the permeate were lower than 193 μS/cm for the membranes TF450 and TF200, and lower than 355 μs/cm for the membrane TF1000. Consequently, DCMD proved to be an effective process for the treatment of TOW for high quality water production and a phenolic-rich concentrate.     

Enhanced hollow fiber membrane performance via semi-dynamic layer-by-layer polyelectrolyte inner surface deposition for nanofiltration and forward osmosis applications

The layer-by-layer (LBL) polyelectrolyte deposited membranes have drawn increasing attention in various applications due to the ease of selective layer formation and their stability and versatility. In this study, the LBL deposition was performed at the inner surface of the polyethersulfone (PES) hollow fiber substrate to form composite nanofiltration (NF) membrane. The semi-dynamic deposition procedure was adopted with the aid of syringes. The newly developed inner deposited (id-LBL) membranes were then tested in NF and forward osmosis (FO) applications and the performance were compared with outer surface deposition as well as some literature data. The id-LBL membranes could not only withstand higher operating pressure but also possess superior hardness rejection especially in high concentration mixed salt solutions (more than 95% rejection to Mg²⁺ and Ca²⁺ in a 5000 ppm total dissolved salt (TDS) mixture under 4.8 bar). As for the FO process, with only two layer deposition, the id-LBL membranes also demonstrated significant performance improvement with increased water flux (up to 70 L/m² h using 0.5 M MgCl₂ as draw solution in active layer facing draw solution configuration) and reduced salt leakage (around 0.5 g/m² h using 1 M MgCl₂ draw solution in active layer facing feed water configuration). This study suggests that for hollow fiber substrate, the inner surface is more suitable for the formation of the selective layer via LBL deposition than the outer surface.     

Synthesis and characterization of PVA/PES thin film composite nanofiltration membrane modified with TiO2 nanoparticles for better performance and surface properties

Novel polyethersulfone (PES)/poly (vinyl alcohol) (PVA)/titanium dioxide (TiO₂) composite nanofiltration membranes were prepared by dip-coating of PES membrane in PVA and TiO₂ nanoparticles aqueous solution. Glutaraldehyde (GA) was used as a cross-linker for the composite polymer membrane in order to enhance the chemical, thermal as well as mechanical stabilities. TiO₂ nanoparticles with different concentrations (0, 0.05, 0.1, 0.5 wt.%) were coated on the surface of PVA/PES composite membrane. The morphological study was investigated by atomic force microscopy (AFM), scanning surface microscopy (SEM) and along with X-ray diffraction (XRD). In addition, the membranes performances, in terms of permeate flux, ion rejection and swelling factor were also investigated. It was found that the increase in TiO₂ solution concentration can highly affect the surface morphology and filtration performance of coated membranes. The contact angle measurement and XRD studies indicated that the TiO₂ nanoparticles successfully were coated on the surface of PVA/PES composite membranes. However, rougher surface was obtained for membranes by TiO₂ coating. The filtration performance data showed that the 0.1 wt.% TiO₂-modified membrane presents higher performance in terms of flux and NaCl salt rejection. Finally, TiO₂ modified membranes demonstrated the lower degree of swelling.     

Effects of hypochlorite exposure on flux through polyethersulphone ultrafiltration membranes

Polyethersulphone ultrafiltration membranes with a nominal molecular weight cut off of 10 kDa were degraded in solutions of sodium hypochlorite over a range of pH values at 55 °C to achieve exposure measured in ppm-days of chlorine exposure. The degraded membranes were tested, using an ÄKTAcrossflow? system, for clean water flux, demineralised whey flux and protein rejection. The water fluxes for three membranes (new, 10,000 ppm-day pH 12, and 10,000 ppm-day pH 9) were found to be about 100, 200 and 400 L m^?2 h^?1, respectively with cross flow at 1 bar transmembrane pressure. However whey fluxes were about 23, 5, and 6 L m^?2 h^?1 for the same three membranes. Size exclusion chromatography of the permeates showed significant permeation of α-lactalbumin and β-lactoglobulin through membranes degraded at pH 9 for 20,000 ppm-days, while almost no permeation was found for degradation at pH 12.These results show that hypochlorite degradation affected fluxes by at least two mechanisms. It was likely that membrane pitting increased the pore size causing increased water flux and reduced protein rejection. However hypochlorite also seemed to alter the membrane surface properties, causing the protein to form a less permeable layer that reduced the flux of whey.     

Hydrophilic modification of polyacrylonitrile membranes by oxyfluorination

The surface of polyacrylonitrile (PAN) membranes was modified by oxyfluorination with various conditions to improve its wettability. The membranes were characterized in terms of morphology, structure, hydrophilicity, and membrane performance. The properties and functional groups on the surface of PAN membranes were investigated by contact angle, SEM, ATR-IR and XPS. And permeability of PAN membranes was compared by permeating pure water flux through membrane surface under 100, 150 and 200 kPa pressure. Oxyfluorination introduced oxygen contained functional groups such as the carboxylic acid groups that help increment of wettability on the surface of PAN membrane. Water flux of oxyfluorinated PAN UF membrane increased 20% at pure water permeation pressure 200 kPa compared to that of untreated PAN UF membrane.     

Preparation of mullite ceramic microfilter membranes using Response surface methodology based on central composite design

In this paper effect of free silica removal from mullite microfilter membranes using different sodium hydroxide (NaOH) concentrations at different temperatures and for different removal times was studied. The prepared membranes were subjected to XRD, SEM, porosity analysis, and mechanical strength measurement. Response surface methodology (RSM) based on central composite design (CCD) was used to design the experiments and analyze three operating parameters including; NaOH solution concentration, NaOH solution temperature and removal time. The optimum porosity of 49.4 was obtained with NaOH solution concentration of 35 wt% at temperature 75 °C and removal time equal to 8 h.Water flux and mechanical strength as important characteristics were measured for all the membranes. For the membrane with the optimum porosity, water flux, mechanical strength, and free silica removal percentage were 61.7 kg/m² h, 21.6 MPa, and 28.2%, respectively. The maximum rejection percentage was 97.2% and emulsion flux for this state was 15.6 kg/m² h at temperature 25 °C and cross flow velocity of 1.5 m/s.     

Extraction of anthocyanins from Arrabidaea chica in fixed bed using CO2 and CO2/ethanol/water mixtures as solvents

Leaves of Arrabidaea chica (Humb. Bonpl.) Verlot are rich in anthocyanins and have been used as a medicinal plant in the Amazon region. In order to obtain different extracts from this plant, a sequential extraction in fixed bed was carried out at 40 °C and 300 bar, using supercritical carbon dioxide (scCO₂) in a first step, and a mixture containing CO₂/ethanol/water at mass ratios of approximately 80/20/0, 80/14/6 and 80/10/10 in a second extraction step. The residue from the second step was extracted with water at 40 °C and atmospheric pressure. Ethanolic, aqueous and hydroalcoholic (70:30, v:v) extracts were also obtained by conventional extraction methods at atmospheric pressure. All extracts were analyzed for global extraction yield, total phenolic content, total flavonoids, and carajurin content. High performance liquid chromatography (HPLC) was used both to quantify carajurin, which is the main anthocyanin component of A. chica, and to monitor qualitatively two other anthocyanin pigments found in that plant. The extraction yield in the first step was only 0.65% using pure scCO₂, but this extraction was highly selective to extract carajurin from the three main anthocyanins. The accumulated global yield of the two steps ranged from 3% when the solvent ratio (80/20/0) was used in the second step to about 50% when 6 or 10% water was used, showing the highest yield when the extraction was done with water. The highest contents of total phenolic compounds (178 mg GAE/g extract) and total flavonoids (373 mg EC/g extract) were found in the process performed with the extraction mixture (80/20/0), and the highest carajurin content was obtained in the ethanolic extracts.     

Enhanced water desalination performance through hierarchically-structured ceramic membranes

Developments of membrane water desalination are impeded by low water vapor flux across the membrane. We present an innovative membrane design to significantly enhance the water vapor flux. A bilayer zirconia-based membrane with a thick hierarchically-structured support and a thin functional layer is prepared using a combined freeze drying tape casting and screen printing method. The hierarchically-structured YSZ support has a porosity of 42.6%, pores of 4.5 μm or larger, and a relatively low tortuosity of 1.58 along the thickness direction. The bilayer membrane is then converted from naturally hydrophilic to hydrophobic via grafting with a fluoroalkylsilane. A water flux of 28.7 Lm⁻² h⁻¹ and a salt rejection of 99.5% are achieved by exposing the functional layer to 80 °C salt water of 2 wt.% NaCl and the support layer to 20 °C distilled water. These results are the best performing ones for ceramic membranes in direct contact membrane distillation operation.     

Evaluation of commercial PTFE membranes in desalination by direct contact membrane distillation

In this study, nine flat-sheet commercially available hydrophobic PTFE membranes were used in desalination by direct contact membrane distillation and their characteristics were investigated under different operating conditions including feed temperature, feed flow rate, cold stream flow rate, and feed concentration. Membrane properties, i.e. pore size, thickness, support layer, and salt rejection were also studied. Moreover, membrane module designs including flow arrangements (co-current, counter-current and tangential) for process liquid and depth both on hot and cold sides were tested experimentally. Finally, the long-term performance of the selected membranes for direct contact membrane distillation as a stand-alone desalination process was investigated. The results indicated that increasing feed temperature, hot feed flow rate, and module depth on the cold side led to increase permeate flux. On the other hand, increasing membrane thickness and module depth on the hot side (at constant flow rate) had negative effects on the flux. The highest permeation flux and salt rejection was achieved when the membranes with a pore size of 0.22 μm were used in the cross-current follow arrangement of hot and cold streams. In addition, the requirements for support layer for a successful DCMD process has been extensively discussed.     

Extraction of phenolic compounds and anthocyanins from blueberry (Vaccinium myrtillus L.) residues using supercritical CO2 and pressurized liquids

This work explored the potential of subcritical liquids and supercritical carbon dioxide (CO₂) in the recovery of extracts containing phenolic compounds, antioxidants and anthocyanins from residues of blueberry (Vaccinium myrtillus L.) processing. Supercritical CO₂ and pressurized liquids are alternatives to the use of toxic organic solvents or extraction methods that apply high temperatures. Blueberry is the fruit with the highest antioxidant and polyphenol content, which is present in both peel and pulp. In the extraction with pressurized liquids (PLE), water, ethanol and acetone were used at different proportions, with temperature, pressure and solvent flow rate kept constant at 40 °C, 20 MPa and 10 ml/min, respectively. The extracts were analyzed and the highest antioxidant activities and phenolic contents were found in the extracts obtained with pure ethanol and ethanol + water. The highest concentrations of anthocyanins were recovered with acidified water as solvent. In supercritical fluid extraction (SFE) with CO₂, water, acidified water, and ethanol were used as modifiers, and the best condition for all functional components evaluated was SFE with 90% CO₂, 5% water, and 5% ethanol. Sixteen anthocyanins were identified and quantified by ultra performance liquid chromatography (UPLC).     

Fabrication of polyetherimide microporous membrane using supercritical CO2 technology and its application for affinity membrane matrix

Polyetherimide (PEI) microporous membranes with uniform cellular structure, high porosity, and narrow pore size distribution were formed by supercritical CO₂ (ScCO₂) phase inversion method, and the membrane was modified to be a matrix for the preparation of affinity membrane due to its low solvent residue and appropriate porous structure. The effects of ScCO₂ temperature and pressure on the morphology and pure water flux of the membrane were investigated. The membrane prepared at 24 MPa and 45 °C with a large mean cell diameter of 6.0 μm, high porosity of 73%, narrow pore size distribution and a pure water flux of 56 L/(m² h bar) was coated with chitosan to improve its hydrophilicity and coupled with Cibacron Blue F3GA (CB) as a special ligand to form an affinity membrane (PEI-coated chitosan-CB membrane). The PEI-coated chitosan-CB membrane showed a high adsorption capacity of 33.9 mg/g membrane to bovine serum albumin and was higher than most of affinity membranes. Moreover, the tensile strength of PEI-coated chitosan-CB membrane was 11.58 MPa and was much higher than those of affinity membranes. This work demonstrates that ScCO₂ phase inversion method is a potential method to prepare an affinity matrix.     

Dynamic removal of n-hexane from water using nanocomposite membranes: Serial coating of para-aminobenzoate alumoxane,boehmite-epoxide and chitosan on Kevlar fabrics

Nanocomposite membranes were fabricated by dip-coating technique. Para-aminobenzoate alumoxane, boehmite–epoxide and chitosan were consecutively coated on the Kevlar fabric surface. The membranes were utilized for removing n-hexane from water using a gravity-driven dead-end filtration setup. UV–vis, FTIR and SEM analyses were performed for characterization. Water affinity analyses revealed highly hydrophilic nature of the modified membranes. Effect of pH was examined with the pH 2 providing the best condition for oil–water separation. Maximum flux and rejection were obtained as 1128 L m⁻² h⁻¹) and 94%, respectively. Deposited oil layer was effectively removed from the membrane surface using hot citric acid solution.     

Pervaporative desalination of water using natural zeolite membranes

Several deposits of dense natural clinoptilolite, including one from British Columbia, Canada, have been identified which exhibit both high zeolite content and essentially no macroporosity or intercrystalline voids. Sections of zeolite mineral from the British Columbia deposit were machined into thin membranes and used in the pervaporative desalination of water samples with varying salinity levels, including synthetic seawater. Essentially complete rejection of Mg²⁺ and Ca²⁺ (99.99% and 98.52%) and high levels of rejection of Na⁺ and K⁺ (over 97.5%) were observed when using a synthetic seawater feed at 75 °C and 1 atm feed-side pressure. Water flux through the natural zeolite membranes was dependent on the ion concentration in the feed, the operating temperature and the feed salinity. At 93 °C, water fluxes of 2.5 kg/m²·h and 0.39 kg/m²·h were obtained for feed concentrations of 100 mg/L Na⁺ and 5500 mg/L Na⁺, respectively. The high ion rejection and water flux observed for these rugged and economical natural clinoptilolite membranes indicates their potential utility for desalination applications.     

Extraction of phenolic compounds from pitanga (Eugenia uniflora L.) leaves by sequential extraction in fixed bed extractor using supercritical CO2, ethanol and water as solvents

With the goal of maximizing the extraction yield of phenolic compounds from pitanga leaves (Eugenia uniflora L.), a sequential extraction in fixed bed was carried out in three steps at 60 °C and 400 bar, using supercritical CO₂ (non-polar) as solvent in a first step, followed by ethanol (polarity: 5.2) and water (polarity: 9.0) in a second and third steps, respectively. All extracts were evaluated for global extraction yield, concentration and yield of both polyphenols and total flavonoids and antioxidant activity by DPPH method (in terms of EC₅₀). The nature of the solvent significantly influenced the process, since the extraction yield increased with solvent polarity. The aqueous extracts presented higher global extraction yield (22%), followed by ethanolic (16%) and supercritical extracts (5%). The study pointed out that the sequential extraction process is the most effective in terms of global extraction yield and yield of polyphenols and total flavonoids, because it produced the more concentrated extracts on phenolic compounds, since the supercritical ethanolic extract presented the highest phenolics content (240.5 mg GAE/g extract) and antioxidant capacity (EC₅₀ = 9.15 μg/mL). The most volatile fraction from the supercritical extract, which is similar to the essential oils obtained by steam distillation or hydrodistillation, presented as major compounds the germacrenos D and B + bicyclogermacrene (40.75%), selina-1,3,7(11)-trien-8-one + selina-1,3,7(11)-trien-8-one epoxide (27.7%) and trans-caryophyllene (14.18%).     

Thin-film composite crosslinked polythiosemicarbazide membranes for organic solvent nanofiltration (OSN)

In this work we report a new class of solvent stable thin-film composite (TFC) membrane fabricated on crosslinked polythiosemicarbazide (PTSC) as substrate that exhibits superior stability compared with other solvent stable polymeric membranes reported up to now. Integrally skinned asymmetric PTSC membranes were prepared by the phase inversion process and crosslinked with an aromatic bifunctional crosslinker to improve the solvent stability. TFC membranes were obtained via interfacial polymerization using trimesoyl chloride (TMC) and diaminopiperazine (DAP) monomers. The membranes were characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and contact angle measurement.The membranes exhibited high fluxes toward solvents like tetrahydrofuran (THF), dimethylformamide (DMF) and dimethylsulfoxide (DMSO) ranging around 20 L/m² h at 5 bar with a molecular weight cut off (MWCO) of around 1000 g/mol. The PTSC-based thin-film composite membranes are very stable toward polar aprotic solvents and they have potential applications in the petrochemical and pharmaceutical industry.     

Vapor permeation of ethyl acetate,propyl acetate,and butyl acetate with hydrophobic inorganic membrane

This study is focused on the permeation characteristics of ester compounds from aqueous solutions through hydrophobic membrane; ethyl acetate (EA), propyl acetate (PA) and butyl acetate (BA). A surface-modified tube-type alumina membrane (Al₂O₃) was used for ester compounds recovery by vapor permeation. Experiments were performed to evaluate the effects of the feed concentration (0.15–0.60 wt.%) and feed temperature (30–50 °C) on the separation of EA, PA, and BA from dilute aqueous solutions. It was found that the permeation flux increased with increasing feed ester concentration and operating temperature. The fluxes of EA, PA, and BA at 0.60 wt.% feed concentration and 40 °C were 282, 526, 661 g/m² h, which were much higher than those of polymer membranes. The separation factors for the 0.15–0.60 wt.% feed solution of EA, PA, and BA at 40 °C were in the range of 28.1–93.9, 83.6–129.4, and 190.7–209.9, respectively. The membrane tested showed high flux and high selectivity.