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.     

Molecular design of the cellulose ester-based forward osmosis membranes for desalination

This work has investigated the fundamental science of phase inversion and formation mechanism of cellulose ester membranes at the interface between polymer and casting substrate. It also explores the desired membrane preparation conditions for forward osmosis (FO) applications. With the aid of positron annihilation lifetime spectroscopy (PALS), the similarity in physicochemical properties between the polymer and the substrate was found to play a significant role in determining the porosity of the bottom interfacial layer. The structure of the dense interfacial layer was also strongly dependent on membrane thickness and solvent composition. Experimental results surprisingly reveal that the original pore size of the as-cast membrane plays a critical role determining the final performance of the subsequent annealed membrane independently of annealing temperature and time. In addition, since a threshold pore size exists during annealing above which pores become difficult to downsize, we have found that a thin dense selective layer integrated in an asymmetric membrane may not always be the best option for FO. A balanced membrane structure consisting of a thin porous support and a thin dense selective layer has been developed for FO, which shows a low internal concentration polarization (ICP) and a relatively high water flux when seawater was employed as the feed.     

A novel dual-layer forward osmosis membrane for protein enrichment and concentration

We have demonstrated the prospect of dual-layer polybenzimidazole-polyethersulfone/polyvinylpyrrolidone (PBI–PES/PVP) hollow fiber nanofiltration (NF) membranes in the forward osmosis (FO) process for the enrichment and concentration of pharmaceutical products without denaturing the component of interests. The dual-layer hollow fiber membrane via coextrusion technology has an ultra-thin selective skin around 10 μm, fully open-cell water channels underneath and a microporous sponge-like support structure. The self-charged PBI selective skin has an average pore radius at 0.4 nm with a sharp pore size distribution. Experimental results show that the newly developed dual-layer hollow fiber nanofiltration membrane can achieve a high throughput for lysozyme enrichment and less protein fouling when using it as a FO membrane. In addition, the high divalent salt rejection towards Mg²⁺ at around 90% of this dual-layer membrane ensures the enriched lysozyme product with high purity and without change and denaturing.     

Characteristics and potential applications of a novel forward osmosis hollow fiber membrane

There has been a resurgence of interest in forward osmosis (FO) as a potential means of desalination, dewatering and in pressure retarded osmosis, which Sidney Loeb was advocating over 3 decades ago. This paper describes the characteristics and potential applications of a newly developed FO hollow fiber membrane, which was fabricated by interfacial polymerization on the inner surface of a polyethersulfone (PES) hollow fiber. This FO membrane presents excellent intrinsic separation properties, with a water flux of 42.6 L/m² h using 0.5 M NaCl as the draw solution and DI water as the feed with the active layer facing the draw solution orientation at 23 °C. The corresponding ratio of salt flux to water flux was only 0.094 g/L, which is superior to all other FO membranes reported in the open literature. To evaluate different application scenarios, various NaCl solutions (500 ppm (8.6 mM), 1 wt.% (0.17 M) and 3.5 wt.% (0.59 M)) were used as the feed water to test the performance of the FO membrane. The membrane can achieve a water flux of 12.4 L/m² h with 3.5 wt.% NaCl solution as the feed and 2 M NaCl as the draw solution, suggesting it has good potential for seawater desalination.     

Study of an analytical model for hollow fiber reverse osmosis module systems

A number of investigators have made efforts to develop various analytical models for hollow fiber type reverse osmosis (RO) module systems since the 1970s. However, a perfect analytical model, which can precisely explain the observed RO performances under a wide range of operating conditions has not been developed yet. The author previously proposed a precise analytical model called a friction-concentration-polarization model (FCP model) [1], which used the Kimura-Sourirajan model for transport phenomena of solute and water transport through a membrane, taking a mass transfer coefficient as local variables and taking a fiber-bore side pressure drop into account. In the application of this model, fundamental transport parameter of hollow fiber membranes were needed, and they were initially determined by a U-tube membrane test where the effect of concentration polarization could be neglected. Then a local mass transfer coefficient was estimated from experimental data using actual modules as a function of Reynolds and Schimidt numbers by a trial-and-error method for both brackfish water and seawater desalination cases. Using all of the above results, behaviors of hollow fiber modules under various operating conditions were estimated and compared with the results obtained from commercial size module experiments. Compared to other previous models, the FCP model is verified to be the best one to predict actual module performances. This model will be further extended to anlyze a change of transport parameters during long-term runs.     

Two-dimensional MXene hollow fiber membrane for divalent ions exclusion from water

Two-dimensional material membranes with fast transport channels and versatile chemical functionality are promising for molecular separation.Herein,for the first time,we reported design and engineering of two-dimensional Ti3C2Tx MXene (called transition metal carbides and nitrides) membranes supported on asymmetric polymeric hollow fiber substrate for water desalination.The membrane morphology,physic-ochemical properties and ions exclusion performance were systematically investigated.The results demonstrated that surface hydrophilicity and electrostatic repulsion and size sieving effect of interlayer channels synergistically endowed the MXene hollow fiber membrane with fast water permeation and efficient rejection of divalent ions during nanofiltration process.     

Improvements of a hollow fiber reverse osmosis desalination model: Analysis of numerical results

In this work, a rigorous model describing the processes taking place in hollow fiber modules for reverse osmosis desalination is analyzed. The Kimura–Sourirajan model is used for describing transport phenomena through the membrane. The concentration polarization phenomenon is mathematically described using the film theory, while the Hagen–Poiseuille and Ergun equations describe the pressure drop in the fiber bore and on the shell side of the fiber bundle, respectively. Improving the previous model, in this work the salt concentration of the permeate accumulated along the fiber is calculated from appropriate mass balances. Hence, the osmotic pressure and the water and salt fluxes through the membrane that depend on this concentration change through the module; and it also influences indirectly the calculation of other process parameters. The solutions of all the differential equations involved in the model are accurately approximated by the finite differences method applied over an appropriate discretization. The value of the output variables changes less than 1% when the finite difference mesh is increased from 6 to 7 grid points in the range of each domain, axial and radial. The flow rates and salt concentrations profiles obtained by the proposed model are analyzed. The influences of the transmembrane and osmotic pressures over the permeate flow rates and salt concentrations are studied. The effect of incorporating the accumulated permeate salinity is showed. It is proved that errors committed by ignoring the permeate accumulated salinity can be significant. Sensitivity analysis for the permeate flow rate and permeate salt concentration is performed by studying the influence of different kind of data: input variables, physical coefficients and design variables.     

A novel ammonia—carbon dioxide forward (direct) osmosis desalination process

A novel forward (direct) osmosis (FO) desalination process is presented. The process uses an ammonium bicarbonate draw solution to extract water from a saline feed water across a semi-permeable polymeric membrane. Very large osmotic pressures generated by the highly soluble ammonium bicarbonate draw solution yield high water fluxes and can result in very high feed water recoveries. Upon moderate heating, ammonium bicarbonate decomposes into ammonia and carbon dioxide gases that can be separated and recycled as draw solutes, leaving the fresh product water. Experiments with a laboratory-scale FO unit utilizing a flat sheet cellulose tri-acetate membrane demonstrated high product water flux and relatively high salt rejection. The results further revealed that reverse osmosis (RO) membranes are not suitable for the FO process because of relatively low product water fluxes attributed to severe internal concentration polarization in the porous support and fabric layers of the RO membrane.     

An agent-based simulation with NetLogo platform to evaluate forward osmosis process (PRO Mode)

Forward osmosis (FO), as an emerging technology, is influenced by different factors such as operating conditions, module characteristics, and membrane properties. The general aim of this study was to develop a suitable (flexible, comprehensive, and convenient to use) computational tool which is able to simulate osmosis through an asymmetric membrane oriented in pressure retarded osmosis (PRO) mode in a wide variety of scenarios. For this purpose, an agent-based model was created in NetLogo platform, which is an easy-to-use application environment with graphical visualization abilities and well suited for modeling a complex system evolving over time. The simulation results were validated with empirical data obtained from literature and a great agreement was observed. The effect of various parameters on process performance was investigated in terms of temperature, cross-flow velocity, length of the module, pure water permeability coefficient, and structural parameter of the membrane. Results demonstrated that the increase in all parameters, except structural parameter of the membrane and the length of module led to the increase of average water flux. Moreover, nine different draw solutes were selected in order to assess the influence of net bulk osmotic pressure difference between the draw solution (DS) and feed solution (FS) (known as the driving force of FO process) on water flux. Based on the findings of this paper, the performance of FO process (PRO mode) can be efficiently evaluated using the NetLogo platform.     

Poly(vinyl) alcohol coating of the support layer of reverse osmosis membranes to enhance performance in forward osmosis

Membrane hydrophilicity influences the transport of water through the membrane in osmotically driven separations such as forward osmosis. In this paper, we coated the polysulfone support layer of two types of commercially available reverse osmosis membranes (brackish water and seawater) with hydrophilic polyvinyl alcohol (PVA). The aim of this was to increase the support layer hydrophilicity and, correspondingly, the rate of water transport through the membrane. Previous work with polydopamine coatings of the polysulfone support of reverse osmosis membranes has yielded promising results. In this work, we explore more readily available materials. Specifically, we studied the effects of two different PVA crosslinking agents – maleic acid and glutaraldehyde – on the resultant membrane properties and osmotic performance. For seawater membranes we found that PVA crosslinked to a limited degree with maleic acid creates a significant improvement in water flux in RO and FO systems, as compared to membranes with PVA crosslinked by glutaraldehyde. However, brackish water membranes did not have comparably significant changes in membrane performance. We conclude that the smaller pores of the brackish water membrane become clogged, and this effect is magnified by the lack of fractional free volume available within PVA that is highly crosslinked with glutaraldehyde.     

Hydrophobic PVDF hollow fiber membranes with narrow pore size distribution and ultra-thin skin for the fresh water production through membrane distillation

Polyvinylidene fluoride (PVDF) hydrophobic asymmetric hollow fiber membrane was fabricated through the dry-jet wet phase inversion process. It is found that the PVDF hollow fiber has an ultra-thin skin layer and a porous support layer from the morphology study. The fully porous membrane structure has the advantage of decreasing the vapor transport resistance and enhancing the permeation flux. The fabricated PVDF membrane has a mean pore size of in diameter and a narrow pore size distribution. The rough external surface produces an advancing contact angle of 112±3^° with water. During direct contact membrane distillation (MD) of 3.5 wt% salt solution, PVDF hollow fibers produced a water permeation flux of (based on the external diameter of hollow fiber) and a NaCl rejection of 99.99% with a hot salt solution at 79.3 °C and cold distillate water at 17.5 °C. This performance is comparable to or superior to most of commercially available PVDF hollow fiber membranes, indicating that the newly developed PVDF may be suitable for MD applications.     

Comparison of polyamide nanofiltration and low-pressure reverse osmosis membranes on As(III) rejection under various operational conditions

A nanofiltration (NF) membrane and a low-pressure reverse osmosis (LPRO) membrane both with aromatic polyamide selective layer from the same manufacturer were employed for the comparison of their performances in terms of As(III) rejection and filtration flux under a variety of operational conditions. In addition to the smaller membrane pore size, the LPRO membrane possesses much more dissociable functional groups than the NF membrane. When the feed pH was below the pKa₁ value (9.22) of H₃AsO₃, for which the steric hindrance is the only rejection mechanism, the removal efficiencies by NF and LPRO were about 10% and 65%, respectively. When the feed pH was higher, for which electrostatic effect began to take effect, the removal efficiencies could reach 40% and 90% for NF and LPRO, respectively. The rejection performance of LPRO was marginally affected by the feed As(III) concentration or ionic strength, although ionic strength had a strong effect on the filtration flux. In contrast, feed As(III) concentration and ionic strength had little effect on the filtration flux but great influence on the As(III) rejection performance of NF. The filtration flux was enhanced with the increase of transmembrane pressure for either NF or LPRO. The NF model could predict the general trend of the effects of the filtration flux, the feed water chemistry and its own concentration on As(III) rejection ratio by the NF membrane, but the rejection ratios were over-predicted.     

Air-sintered silicon (Si)-bonded silicon carbide (SiC) hollow fiber membranes for oil/water separation

In this work we evaluated the effect of adding Si as sintering additive into SiC for producing air-sintered hollow fiber membranes. According to crystallographic analyses, SiC and Si were converted to SiO₂ after sintering at 1350 °C. The addition of 30 wt% of Si into SiC ceramic material promoted the binding of SiC particles and improved the membrane mechanical resistance to 42.25 ± 3.39 MPa after air sintering at 1350 °C. The produced asymmetric ceramic membrane presented a packed pore-network and micro-voids with pore sizes of 1.73 and 5.29 μm, respectively. The filtration of an oil/water emulsion enabled oil retention 98.75 ± 0.95 %. Cake formation was the main fouling occurrence and membrane regeneration with equivalent oil retention and similar steady sate flux was achieved after water cleaning under ultrasound irradiation. Thus, the use of Si as air-sintering aid was favorable for producing Si-bonded SiC hollow fiber membranes with suitable application for oil/water separation.     

Chromium(VI) removal in a semi-continuous process of hollow fiber membrane with organic extractants

Chromium(VI) extraction from aqueous metal solution was experimentally studied by using a hollow fiber supported liquid membrane module. Organic extractants of D2EHPA, TBP, Lix79 and TOA were used to investigate the extraction performance. The extraction efficiencies of single and multiple component systems were examined with single or multiple extractants, and the role of multiple extraction operations and the difference between batch and continuous processes were inspected to find the optimum extraction condition. The experimental outcome indicates that the extraction process is controlled by aqueous phase mass transfer and multiple extraction operations do not reduce the extraction efficiency. The coexisting elements give no significant effect on chromium removal, while the mixed extractants of D2EHPA and TBP enhance the extraction efficiency by 84%. The efficiencies of batch and continuous extraction processes are nearly identical. This paper was presented at the 2004 Korea/Japan/Taiwan Chemical Engineering Conference held at Busan, Korea between November 3 and 4, 2004.     

Rejection properties of pesticides with a hollow fiber NF membrane (HNF-1)

NF membranes are potentially useful to remove residual hazardous organic pollutants such as pesticides and alkyl phthalates in drinking water treatment processes. In this study, the rejection properties of a hollow fiber NF membrane (HNF-1) for 8 kinds of pesticides were investigated using a bench scale cell: the membrane consisted of polyamide skin layer and polysulfone support membrane with a nominal desalting degree fo 35% at 0.3 MPa. In addition, hydrophilic organic compounds (molecular weight: 74.1-504.5), i.e. alcohols and saccharides, were also used as reference solutes. The hydrophilic compounds were rejected at rates of 9.0-97.8%, revealing that the molecular sieving effect was significant since the logarithm of solute permeability, log B, correlated linearly with the molecular width parameter, MWd, of the solute. Phenylic pesticides, such as alachlor, metolachlor, methoxychlor, and thiobencarb, were rejected at 88.7-99.3%. However, non-phenylic pesticides, such as aldicarb, simazine, atrazine, and pirimicarb, were rejected at lower degrees: 42.2-89.9%. The batch type sorption experiments indicated that all of the pesticides were adsorbed on the membrane, and the adsorption properties were controlled mainly by the hydrophobic property of the pesticides. Sorption properties based on solute recovery rates in the separation experiments, however, were different from those in the batch type experiments.     

非溶剂添加剂对聚醚砜酮中空纤维纳滤膜结构和性能的影响

以新型杂萘联苯聚醚砜酮(PPESK)为膜材料,N-甲基-2-吡咯烷酮(NMP)为溶剂,乙二醇甲醚(EGME)、冰醋酸(AA)以及AA/EGME作为复合添加剂,采用干-湿相转化技术制备了中空纤维非对称纳滤膜,重点考察了非溶剂添加剂对中空纤维膜结构和性能的影响。结果表明非溶剂添加剂的加入导致了中空纤维膜孔结构由指状转变为海绵状,从而引起中空纤维膜性能的变化。当聚合物质量分数为23%,铸膜液溶剂体系为m(AA)∶m(EGME)∶m(NMP)=5.7∶16.5∶54.8时,中空纤维膜对PEG600的截留率高于96%,纯水通量为211 L/(m2.h)。     

Preparation and performances of poly(phthalazinone ether sulfone ketone) hollow fiber membranes with excellent thermotolerance

The cloud points of PPESK/NMP/H₂O ternary system at different temperatures were measured by titrimetric method. The binodal lines in the ternary phase diagram of the poly(phthalazinone ether sulfone ketone (PPESK) dope system was determined, on the basis of the cloud point experimental data being linearly fitted with the semiempirical linear cloud point correlation. Furthermore, phase separation behavior during the phase inversion of PPESK membrane‐forming system was discussed in terms of the phase diagram. Then, dry–wet spinning technique was employed in manufacturing PPESK hollow fiber membranes by immersion precipitation method. The cross‐section morphologies of hollow fibers were observed by scanning electronic microscopy. Also, the effects of dope solution composition and spinning parameters, including the coagulant composition and the spinning temperature on the separation performances of fibers, were evaluated by permeability measurements. The thermotolerance of the PPESK hollow fiber membranes prepared in the work was examined for the permeation operation at different temperatures and pressure differences. The experimental results showed that pure water flux increases several fold along with the temperature increases from 20 to 80°C at different operation pressures, while the solute rejection only decreases slightly. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 878–884, 2006     

Fabrication of harsh environment-tolerant and robust reinforced poly (tetrafluoroethylene) hollow fiber membranes with optimal heat setting technology

The high-temperature industrial waste oil treatment has put forward new requirements for membrane materials. Poly (tetrafluoroethylene) (PTFE) is regarded as the excellent membrane material for membrane process, due to its excellent properties. Herein, the reinforced PTFE hollow fiber membrane (PTFE-HFM) was prepared by the combination of wrapping and heat setting technology. The objection of this study was focusing on the outstanding chemical and thermal resistance of PTEF-HFM for overcoming harsh waste oil treatment. PTFE-HFM exhibited better chemical resistance in the experiment of acid and alkali compared to PVDF-HFM, which the surface of PVDF-HFM corroded obviously and none of PTFE-HFM. The pore size and porosity were precisely adjusted by heat setting technology. Firstly, the morphologies of PTFE-HFM was investigated by scanning electron microscopy, which exhibited favorable interfacial bonding state between PTFE separation layer and supporting layer. Subsequently, the prepared membranes exhibited high-temperature waste oil permeability of 195 L m⁻² h⁻¹ bar⁻¹with only 3% attenuation in long-term operation. Besides, the rejection of lubricants contained activated carbon also reached to 99%. Overall, the facile preparation and excellent performance of robust reinforced PTFE-HFM highlights its great potential in treating high-temperature waste oil.     

Preparation of temperature sensitive poly(vinylidene fluoride) hollow fiber membranes grafted with N‐isopropylacrylamide by a novel approach

In the present study, the temperature sensitive PVDF‐g‐NIPAAm HFM was prepared by grafting N‐isopropylacrylamide (NIPAAm) on poly(vinylidene fluoride) (PVDF) hollow fiber membrane (HFM) using a novel approach, alkaline treatment method. The structures of PVDF‐g‐NIPAAm HFM were characterized by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR), respectively. The effects of alkaline treatment time and grafting yield on the mechanical properties of PVDF HFM were measured and analyzed. In addition, the temperature sensitive behavior of PVDF‐g‐NIPAAm HFM and the effect of grafting yield on the temperature sensitive behavior were investigated by the flux of pure water and the rejection of ovalbumin. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 833–837, 2006