Engineering design of outer‐selective tribore hollow fiber membranes for forward osmosis and oil‐water separation

Outer‐selective thin‐film composite (TFC) hollow fiber membranes offer advantages like less fiber blockage in the feed stream and high packing density for industrial applications. However, outer‐selective TFC hollow fiber membranes are rarely commercially available due to the lack of effective ways to remove residual reactants from fiber's outer surface during interfacial polymerization and form a defect‐free polyamide film. A new simplified method to fabricate outer‐selective TFC membranes on tribore hollow fiber substrates is reported. Mechanically robust tribore hollow fiber substrates containing three circular‐sector channels were first prepared by spinning a P84/ethylene glycol mixed dope solution with delayed demixing at the fiber lumen. The thin wall tribore hollow fibers have a large pure water permeability up to 300 L m^?2 h^?1 bar^?1. Outer‐selective TFC tribore hollow fiber membranes were then fabricated by interfacial polymerization with the aid of vacuum sucking to ensure the TFC layer well‐attached to the substrate. Under forward osmosis studies, the TFC tribore hollow fiber membrane exhibits a good water flux and a small flux difference between active‐to‐draw (i.e., the active layer facing the draw solution) and active‐to‐feed (i.e., the active layer facing the feed solution) modes due to the small internal concentration polarization. A hyperbranched polyglycerol was further grafted on top of the newly developed TFC tribore hollow fiber membranes for oily wastewater treatment. The membrane displays low fouling propensity and can fully recover its water flux after a simple 20‐min water wash at 0.5 bar from its lumen side, which makes the membrane preferentially suitable for oil‐water separation. © 2015 American Institute of Chemical Engineers AIChE J, 61: 4491–4501, 2015     

Improved microstructure of asymmetric niobium pentoxide hollow fiber membranes

Asymmetric niobium pentoxide (Nb₂O₅) hollow fiber membranes were prepared by the phase inversion and sintering process at temperatures ranging from 1000 to 1350°C. The effects of extrusion parameters on the morphology and properties of the produced membranes were systematically explored. Asymmetric hollow fibers with regular inner contour were obtained at extrusion flow rates of 15 and 25 ml min⁻¹ of ceramic suspension and internal coagulant, respectively. Hollow fibers sintered at temperatures greater than 1200°C presented modifications in the morphology of Nb₂O₅ grains, which were also evidenced by X-ray diffraction and Raman analyses. Hollow fibers produced with an air gap of 50 mm presented a dense outer sponge-like layer and micro-voids formed from the inner surface. These hollow fibers sintered at 1200°C presented suitable bending resistance and water permeability (24.2 ± 0.60 MPa and 3.00 ± 0.01 L h^-1 m^-2 kPa^-1, respectively). The outer sponge like layer was mitigated when the fibers were produced without air-gap.     

Robust and high performance pressure retarded osmosis hollow fiber membranes for osmotic power generation

Novel fabrication perspectives have been demonstrated to molecularly construct robust hollow fiber membrane supports for high performance thin‐film composite (TFC) pressure retarded osmosis (PRO) membranes. For the first time, we found that the desirable hollow fiber supports should possess high stretch resistance and acceptable ductility. The microstructure strength of the hollow fiber support may have more weights on overall robustness of the TFC PRO membranes than the apparent cross‐section morphology. Effectively manipulating the kinetics of phase inversion during spinning by maneuvering bore fluid chemistry, and polymer solution composition is a promising method to tailor the strength of hollow fiber supports. Prestabilization of the TFC membranes at elevated lumen pressures can significantly improve their PRO performance. The newly developed TFC PRO hollow fiber membranes exhibit a power density as high as 16.5 W/m² and a very low specific reverse salt flux (J_s/J_w) of 0.015 mol/L at a hydraulic pressure of 15 bar using synthetic seawater brine (1.0 M NaCl) as the draw solution. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1107–1119, 2014     

Positively charged hollow‐fiber composite nanofiltration membrane prepared by quaternization crosslinking

Poly(N,N‐dimethylaminoethyl methacrylate) (PDMAEMA) can be crosslinked by interfacial polymerization to develop a positively charged dense network structure. According to this mechanism, a positively charged hollow‐fiber composite nanofiltration (NF) membrane was prepared by quaternization to achieve a crosslinked PDMAEMA gel layer on the outer surface of polysulfone hollow‐fiber ultrafiltration (UF) membranes with a PDMAEMA aqueous solution as a coating solution and p‐xylylene dichloride as an agent. The preparation conditions, including the PDMAEMA concentration, content of additive in the coating solution, catalyzer, alkali, crosslinking temperature, and hollow‐fiber substrate membrane, were studied. Fourier transform infrared spectroscopy and scanning electron microscopy were used to characterize the structure of the membranes. This membrane had a rejection to inorganic salts in aqueous solution. The rejection of MgSO₄ (2 g/L aqueous solution at 0.7 MPa and 25°C) was above 98%, and the flux was about 19.5 L m^?2 h^?1. Moreover, the composite NF membranes showed good stability in the water‐phase filtration process. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Synthesis and characterization of PVC-TFC hollow fibers for forward osmosis application

Forward osmosis (FO) is considered among the most encouraging water desalination processes as a result of its high performance and low energy demand. Thin-film composite (TFC) hollow fibers (HF) were synthesized and examined in the FO process. Three different concentrations of polyvinyl chloride (PVC) support polymer were fabricated via the phase inversion technique. The polyamide layer was synthesized on the outer surface of the PVC-HF substrate via interfacial polymerization (IP) reaction. To the best of our knowledge, PVC HF was used in this research for the first time as a support for TFC-FO membranes. PVC HFs have high-quality specifications that are expected to have outstanding performance in TFC-FO applications, especially for water desalination. The obtained membranes were characterized using contact angle measurement, scanning electron microscopy, atomic force microscope and Fourier-transform Infrared. The performance of the PVC-TFC HF was examined in the FO under standard conditions. Results showed that the membrane fabricated with a lower concentration of PVC substrate exhibited higher water flux in comparison to the higher concentration PVC membrane. Changing the concentration of PVC from 15% to 18% reduced water flux from 25 to 13 L m⁻² h⁻¹; however, salt flux also decreased from 8 to 3 g m⁻² h⁻¹.     

Fabrication of highly permeable PDMS@ZIF-8/PVDF hollow fiber composite membrane in module for ethanol-water separation

The construction of high-performance MOF-based hollow fiber composite membrane (HFCM) modules is a significant, yet challenging task for the biofuel production industry. In this study, a novel approach was taken to fabricate PDMS@ZIF-8/PVDF HFCMs in modules through a facile ZIF-8 self-crystallization synthesis followed by pressure-assisted PDMS infusion for pervaporation ethanol-water separation. The as-prepared HFCMs exhibited an ultrathin separation layer (thickness, 370 ± 35 nm), which was achieved through precise regulation of the ZIF-8 membrane and defect repair by PDMS infusion. Moreover, the strategy utilized in this study resolved the defect issues arising from MOF agglomeration in conventional composite membranes. Impressively, at the optimal packing density, the prepared membrane demonstrated a remarkable ethanol flux (1.11 kg m⁻² h⁻¹) with an PSI value (26.59 kg m⁻² h⁻¹) and showed promising long-term stability for the pervaporation of 5 wt% ethanol aqueous solution at 40°C.     

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.     

A new thin film composite nanofiltration membrane based on PET nanofiber support and polyamide top layer: preparation and characterization

The main objective of this study was to prepare thin film nanofibrous composite (TFNC) membranes based on self-support nanofibrous mats. To this end, polyethylene terephthalate nanofibrous supports were produced by electrospinning technique and subsequently heat treatment was performed to increase mechanical stability of the mats. Then, interfacial polymerization procedure was applied for preparation of TFNC nanofiltration membranes. For comparison, the thin film composite (TFC) nanofiltration membrane was prepared by the same conditions based on polyethersulfone ultrafiltration membrane prepared through phase inversion method. Chemical structure, morphology and mechanical properties were studied by using ATR-FTIR, SEM and tensile tests, respectively. Also, filtration performance was investigated by water flux, rejection, water contact angle and MWCO determination. Results showed that the TFNC nanofiltration membrane had higher salt rejection and four times higher water flux than the TFC nanofiltraion membrane (Na₂SO₄ rejection and pure water flux were (93 ± 3)%, (34 ± 2.3) L./m²h and (67 ± 4)%, (8 ± 0.9) L./m²h for TFNC and TFC, respectively). At the end, the filtration performance of PET TFNC-NF membrane was compared with other nanofibrous nanofiltration membranes.

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Verbesserte Hydrophilie von Hohlfasermembranen mittels funktionalisierter Trennschicht für die Ultrafiltration

Dual layer hollow fiber membranes, based on poly(ether sulfone), were fabricated to achieve hydrophilic membranes with improved fouling resistance. A new triblock copolymer, consisting of two hydrophilic poly(ethylene oxide) blocks and an inner poly(ether sulfone) block, was chosen for the functionalization of the inner layer. The most promising membrane of this study was characterized by an improved hydrophilicity, and a performance in the ultrafiltration studies of 2000 L m⁻²h⁻¹bar⁻¹ and retention of 100 kDa.     

Preparation and characterization of poly(vinyl chloride)/polystyrene/poly(ethylene glycol) hollow‐fiber ultrafiltration membranes

Hollow‐fiber ultrafiltration (UF) membranes were prepared from blends of poly(vinyl chloride) (PVC) and polystyrene (PS) with a dry/wet phase inversion method. Poly(ethylene glycol) (PEG) and N,N‐dimethylacetamide were used as the additive and solvent, respectively. The effects of the PEG concentration in the dope solution as an additive on the cross sections and inner and outer surface morphologies, permeability, and separation performance of the hollow fibers were examined. The mean pore size, pore size distribution, and mean roughness of both the inner and outer surfaces of the produced hollow fibers were determined by atomic force microscopy. Also, the mechanical properties of the hollow‐fiber membranes were investigated. UF experiments were conducted with aqueous solutions of poly(vinyl pyrrolidone) (PVP; K‐90, M_w = 360 kDa). From the results, we found that the PVC/PS hollow‐fiber membranes had two layers with a fingerlike structure. These two layers were changed from a wide and long to a thin and short morphology with increasing PEG concentration. A novel and until now undescribed shape of the nodules in the outer surfaces, which was denoted as a sea‐waves shape, was observed. The outer and inner pore sizes both increased with increasing PEG concentration. The water permeation flux of the hollow fibers increased from 104 to 367 L m^?2 h^?1 bar^?1) at higher PEG concentrations. The PVP rejection reached the highest value at a PEG concentration of 4 wt %, whereas at higher values (from 4 to 9 wt %), the rejection decreased. The same trend was found also for the tensile stress at break, Young's modulus, and elongation at break of the hollow fibers. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 989‐1004, 2013     

Novel thin-film composite nanofiltration membranes fabricated via the incorporation of ssDNA for highly efficient desalination

In this work, the biomacromolecule, single-stranded deoxyribonucleic acid (ssDNA) was innovatively incorporated into the polyamide layer to tailor the permeate flux and antifouling performance of the nanofiltration (NF) membranes. With active amines groups, the ssDNA was as the aqueous phase monomers along with piperazine (PIP), and reacted with trimesoyl chloride on polyethersulfone substrate to fabricate thin-film composite (TFC) NF membranes. The NF membrane prepared under optimal ratio of ssDNA/PIP had a pure water permeability of 75.8 L m⁻² h⁻¹ (improved 58% compared to PIP NF membrane) and Na₂SO₄ rejection of 98.0% at 6.0 bar. The rejections for different inorganic salts were the order: Na₂SO₄ (98.0%) > MgSO₄ (89.2%) > MgCl₂ (72.8%) > NaCl (23.0%). Furthermore, the TFC NF membranes showed good antifouling performance in long-term running with 300 ppm bovine serum albumin and humic acid solution. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 47102.     

A thin film composite membrane supported by a hydrophilic poly(vinyl alcohol‐co‐ethylene) nanofiber membrane: Preparation,characterization, and application in nanofiltration

In this study, a fabricated hydrophilic poly(vinyl alcohol‐co‐ethylene) (PVA‐co‐PE) nanofiber membrane was used as the middle support layer to prepare thin film composite (TFC) membranes for nanofiltration. The effects of the supporting nonwoven layer, grams per square meter (GSM) of nanofiber, reaction time, heat treatment, monomer concentration, operating pressure, and pH value on the separation performance of the TFC membranes were analyzed. These results show that the TFC membranes prepared with the PVA‐co‐PE nanofiber membrane can be used to filtrate different metal ions. For NaCl, Na₂SO₄, CaCl₂, CuCl₂, CuSO₄, and methyl orange solutions, the rejection rates of the TFC membrane with nonwoven polyester as the supporting layer and a nanofiber GSM of 12.8 g/m² are 87.9%, 93.4%, 92.0%, 93.1%, 95.8%, and 100%, respectively. This indicates the potential application of the PVA‐co‐PE nanofiber membrane in the preparation of nanofiltration and reverse‐osmosis TFC membranes. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46261.     

Enhancing the permeability of reverse osmosis membrane by embedding the star-like rigid supports in the substrate

Thin film composite (TFC) reverse osmosis (RO) membranes with high permeability have been prepared by interfacial polymerization based on tailoring the polysulfone (PSf) substrate structure by in situ embedded poly(p-phenylene terephthamide) (PPTA) star-like rigid supports. The star-like rigid supports were observed by the polarizing optical microscopy (POM) and transmission electron microscope (TEM). The surface properties of the substrates were investigated by FTIR, the water contact angle (WCA), FESEM and AFM. The WCA was decreased from 88.5° to 72.3° with the PPTA increasing from 0% to 8%, and the surface roughness increased from 24.2, 25.1, 33.5 and 58.6 nm, respectively. Furthermore, numerous interconnect micro-structures were constructed in the substrate when the PPTA content was up to 8%. The pure water flux of 8% PPTA/92%PSf substrate was up to 377.0 L m⁻² h⁻¹ and the flux decline rate was lowest (64%) after compacted at 5.5 MPa for 30 min. Otherwise, increasing the PPTA contents in the substrate enhanced the roughness, encouraged nanosheet formation and improved the permeability of TFC RO membranes. The pure water flux of the TFC RO membranes increased from 36.32 to 58.42 L m⁻² h⁻¹, where the NaCl rejection was about 99.5% at 5.5 MPa.     

Second interfacial polymerization of thin-film composite hollow fibers with amine-cyclodextrins for pervaporation dehydration

High performance thin-film composite (TFC) hollow fiber membranes have been developed for pervaporation dehydration by second interfacial polymerization (SIP) modification with three kinds of amine-functionalized β-cyclodextrin (amine-CDs), which were synthesized by modifying β-CD with ammonia, ethylenediamine (EDA), and tris(2-aminoethyl)amine, respectively. The chemical properties of amine-CDs and SIP-modified TFC membranes were characterized by various techniques. The effects of amine-CD type and SIP parameters (pH or concentration of CD-EDA solution) were studied systematically to acquire the optimized selective layer of TFC membranes for ethanol dehydration. Among all SIP-modified TFC membranes, the one with SIP by 2 wt% CD-EDA aqueous solution (pH = 2) exhibited the most outstanding separation performance with a ultrahigh permeation flux (3,018.0 ± 12.0 g/m² hr) and permeate concentration (98.7 ± 0.2 wt% water) at 50°C (equivalent to separation factor of 415), contributed by the effectively incorporated CD with rich hydrophilic functional groups and intrinsic nanocavities facilitating the passage of water molecules.     

Investigation on the effects of fabrication parameters on the structure and properties of surface‐modified membranes using response surface methodology

Surface Modifying Macromolecules (SMM) were used to alter the hydrophobicity of polyetherimide (PEI) hollow fiber membranes and the effects of three fabrication parameters, which are the mass fraction of PEI and SMM in the casting dope and air gap, on the properties of fabricated membranes were investigated by application of Response Surface Methodology (RSM). The fabricated membranes were characterized in terms of mean pore size (r_P,m), permeation rate of helium gas at 1 bar transmembrane pressure difference, membrane porosity, and contact angle of water with inner and outer surfaces of membrane. The regression models obtained for mean pore size and permeation rate have good statistical parameters and are accurate. The model for r_P,m predicts that plot of r_P,m versus air gap has a minimum point, whereas the plots of r_P,m versus PEI (wt %) and SMM (wt %) have maximum points. The regression model developed for membrane porosity predicts that membrane porosity decreases when air gap increases. Since water was used as bore fluid, the model developed for inner surface contact angle has low accuracy but the model developed for outer surface contact angle predicts that contact angle increases with SMM concentration in dope solution but there is a maximum point versus air gap. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Poly(ether sulfone) hollow fiber membranes prepared via nonsolvent-induced phase separation using the green solvent Agnique® AMD 3 L

In recent years, the development of sustainable membrane manufacturing processes by the use of environmentally friendly solvents has become a considerable challenge. In this work, poly(ether sulfone) (PES) hollow fiber membranes were manufactured by the nonsolvent-induced phase separation (NIPS) using the green solvent Agnique® AMD 3 L (N,N-dimethyl lactamide; AMD) and N-ethyl-2-pyrrolidone (NEP) as a conventional solvent. The effect of the solvent on the dope solution and membrane properties was investigated. The morphology, mechanical characteristics, barrier pore sizes as well as gas and water permeances of the hollow fibers prepared with AMD were evaluated and compared to membranes that were similarly prepared using NEP as solvent. Membranes prepared with AMD as polymer solvent and NEP as bore liquid exhibit the largest barrier pore size among all variations. Thus, highest water permeance of 406.9 ± 37.4 kg m⁻² h⁻¹ bar⁻¹ was obtained with this combination. Whereas AMD as sole solvent in membrane preparation decreases membrane permeances caused by a denser membrane structure. Nevertheless, AMD is a promising solvent for a sustainable membrane fabrication providing membrane properties that are competitive with membranes manufactured using the conventional solvent NEP.     

Effect of the preparation conditions on the morphology and performance of poly(imide) hollow fiber membranes

Poly(imide) (PI) hollow fiber membranes were prepared by using classical phase inversion process. Effects of different external coagulation bath temperatures (ECBT) and various bore flow rates (BFR) on the morphology and separation performance of the membranes were studied. Cross‐section, inner and outer structures were characterized by using scanning electron microscope and atomic force microscopy (AFM). Mean pore size, pore size distribution, and mean roughness of the PI hollow fibers surfaces were estimated by AFM. It was found that the hollow fibers morphology composed of sponge‐like and finger‐like structures with different ECBT and BFR. A circular shape of the nodules with different sizes was observed in the outer surface of the PI hollow fibers. Mean pore size of the outer surface increases with increasing ECBT and BFR. The important result observed in this study is that the ECBT clearly has the largest effect on hollow fiber PI membrane roughness compared with the BFR. Pure water permeability of the PI hollow fibers was improved with increase of ECBT and BFR. The solute rejection (R%) was reduced when the ECBT and BFR was increased. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40428.     

Dual-layer membrane with hierarchical hydrophobicity and transport channels for nonpolar organic solvent nanofiltration

Nonpolar solvent separation is widely used in petroleum, chemical, food industries, but traditional separation methods consume intensive energy. State-of-art organic solvent nanofiltration membranes require complex modifications for nonpolar solvent transport. For the first time, we propose the concurrent modification of the surface, interface and support layer of dual-layer membranes with three additives (perfluorodecylamine, fluoro substituted aromatic amine, silica nanoparticles) in a one-step cocasting process. A delamination-free dual-layer membrane was obtained with a hierarchical hydrophobicity and transport channels. The novel designed structure elevated the pure n-hexane permeance (28.75 L m⁻² hr⁻¹ bar⁻¹) by 3 orders of magnitude with a high lecithin rejection (98.7%). This method of synergistically controlling the hierarchical structures and properties of dual-layer membranes can significantly shorten the preparation process of high-performance nonpolar solvent nanofiltration membranes.     

Thermo-responsive mixed-matrix hollow fiber membranes

Up to date, preparation of thermo-responsive mixed-matrix membranes (MMM) has only be described as small scale flat membranes or multi-step processes for hollow fiber membranes. In this work, the development of thermo-responsive MMM hollow fibers composed of polyethersulfone as membrane polymer and poly(N-isopropylacrylamide) (PNIPAM) microgel particles via the wet spinning process is presented. PNIPAM particles are synthesized with (NP-S, z_{avg 20°C} = 105 nm) and without (NP-L, z_{avg 20°C} = 250 nm) sodium dodecyl sulfate and their thermo-responsive behavior is characterized by dynamic light scattering. Particle size (NP-S, NP-L), particle content (10%, 15%) and the extrusion pressure in the wet spinning process (1.0–3.0 bar) are investigated as experimental parameters. Reversible thermo-responsive behavior of the hollow fibers is demonstrated by water permeability measurements at different temperatures (20 and 50°C). The largest switching factors (R) are observed for the hollow fibers containing NP-L. For 15% NP-L and 1 bar extrusion pressure, water permeances between 0.5 and 6.0 L m⁻² h⁻¹ bar⁻¹ are observed, corresponding to R = 12 and a dextran (500 kDa) rejection of 91% at 25°C.     

High-flux CO2-stable oxygen transport hollow fiber membranes through surface engineering

The influences of bulk diffusion and surface exchange on oxygen transport of (La_0.6Ca_0.4)(Co_0.8Fe_0.2)O_3-δ (LCCF) hollow fiber membranes were investigated. As an outcome, two strategies for increasing the oxygen permeation were pursued. First, porous LCCF hollow fibers as support were coated with a 22 μm dense LCCF separation layer through dip coating and co-sintering. The oxygen permeation of the porous fiber with dense layer reached up to 5.10 mL min^?1 cm^-2 at 1000 °C in a 50 % CO₂ atmosphere. Second, surface etching of dense LCCF hollow fibers with H₂SO₄ was applied. The surface etching of both inner and outer surfaces leads to a permeation improvement up to 86.0 %. This finding implies that the surface exchange reaction plays a key role in oxygen transport through LCCF hollow fibers. A good long-term (>250 h) stability of the asymmetric hollow fiber in a 50 % CO₂ atmosphere was found at 900 °C.