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The utilization of osmosis for engineered applications sparked off various emerging technologies relying on osmotically driven membrane processes (ODMPs). Represented by forward osmosis (FO) and pressure retarded osmosis (PRO), ODMPs show great promise to leverage the global water-energy nexus and have drawn considerable attention in recent years. However, their performance in practical applications is significantly affected by membrane fouling. Membrane fouling is a complex problem and is associated with the foulant deposition, concentration polarization and reverse solute diffusion (RSD) in ODMPs. The current paper provides a comprehensive review on membrane fouling in ODMPs with a focus on the elaboration of the factors and mechanisms governing the fouling behavior. Among those fouling factors and mechanisms, some are also applicable for pressure-driven membrane processes (e.g., reverse osmosis (RO) and nanofiltration (NF)), such as the effects of hydrodynamic conditions, feedwater composition, and membrane material and properties, and the cake-enhanced concentration polarization (CE-CP) mechanism. Others are unique for ODMPs, such as the effects of draw solution composition and membrane orientation, the internal concentration polarization (ICP) self-compensation effect, and the RSD-enhanced fouling. A general osmotic-resistance filtration model for ODMPs is presented in this paper to assist in the interpretation of the intrinsic interrelationships among those fouling factors and mechanisms. The impact and mechanisms of membrane fouling on contaminates removal are also reviewed briefly based on the limited existing literature on this topic. Finally, the available membrane fouling control strategies for ODMPs are summarized upon understanding the cause and effect of fouling. Based on the current review, future research prospects are proposed for further studying the membrane fouling in ODMPs. [ABSTRACT FROM AUTHOR]

Abstract: Aquaporin (AQP) based biomimetic membrane has attracted increasing attention in recent years because of its potential application for water purification and seawater desalination, attributed to the exceptionally high permeability and selectivity of AQPs. Despite its superior performance, AQP based membrane faces several challenges associated with the design and preparation of such membranes for practical application. In this current study, a novel and simple method was introduced to prepare an aquaporin Z (AqpZ) based biomimetic nanofiltration (NF) membrane with a relatively large membrane area of 28.26cm2. The proteoliposome, incorporated with AqpZ, was fully encapsulated into the selective layer through crosslinking of a polyelectrolyte with the membrane substrate made by poly(amide–imde) (PAI). The water flux of the AqpZ based membrane was around 50% higher than the mutant one. At optimal preparation conditions, the AqpZ based membrane could offer a water flux of 36.6Lm−2 h−1 with a MgCl2 rejection of 95% at 0.1MPa. Results from this study showed that AqpZ could maintain its activity even under harsh environmental conditions of thermal treatment at 343K for 2h. These results and findings may provide useful insights on developing next generation of biomimetic membranes. [Copyright &y& Elsevier]

Abstract: Delamination-free ultrafiltration (UF) dual layer hollow fibers have been successfully fabricated by dry jet-wet spinning technique. Two different polymers of poly(amide-imide) (PAI) and polyethersulfone (PES) were utilized as the selective layer on the outer side and the porous support layer on the inner side, respectively. The dual layer substrates were subsequently modified to develop a nanofiltration (NF)-like skin on the outer layer by polyethyleneimine (PEI) cross-linking followed by multilayer polyelectrolyte depositions. These membranes were characterized by a series of standard protocols in terms of membrane structure, permeability and salt rejection, and were applied in FO process. Experiments reveal that the dual-layer NF hollow fiber modified by polyelectrolyte cross-linking and depositions can achieve pure water permeability of 4.1l/m2.h.bar and a high rejection to MgCl2 up to 97%. In FO process, the dual-layer hollow fibers exhibited a water flux of 39 and 21lm−2 h−1 in the orientations of active layer facing draw solution and feed water, respectively, by using 0.5M MgCl2 as draw solution and de-ionized (DI) water as feed at room temperature. It suggests that the combined treatment of polyelectrolyte cross-linking and deposition is an effective way to form a NF-like selective layer for FO dual-layer hollow fiber membranes. [Copyright &y& Elsevier]

Abstract: A novel dual-layer forward osmosis (FO) hollow fiber membrane has been designed and successfully fabricated by using a triple orifice spinneret. The fiber consists of two layers made from polyamide-imide (PAI) polymer for the outer layer and polyethersulfone (PES) polymer for the porous inner layer. Specifically, after obtaining asymmetric microporous PAI/PES dual-layer hollow fibers via non-solvent induced phase inversion, polyethyleneimine (PEI) polyelectrolyte modification on the outer PAI layer was applied to produce a nanofiltration (NF)-like thin layer, while the PES porous inner layer remained intact as PES is inert to PEI. The membrane morphology, structure and surface property were carefully tailored by adjusting polymer dope composition and spinning conditions. These membranes were subsequently characterized by a series of standard protocols in terms of membrane structure, permeability and salt rejection, and were utilized in FO process. It was found that the resultant dual-layer NF hollow fiber membrane can achieve pure water permeability of 15.9lm−2 h−1 bar−1 and a high rejection to divalent cations up to 89%. In FO process, the dual-layer hollow fiber exhibited a water flux of 27.5lm−2 h−1 in the orientation of active layer facing feed water by using 0.5MMgCl2 as draw solution and de-ionized (DI) water as feed at room temperature. The newly developed dual layer hollow fibers outperform all the single layer and dual-layer NF hollow fibers reported in the literature for FO applications. [Copyright &y& Elsevier]

Abstract: A new type of forward osmosis (FO) hollow fiber membranes with an antifouling nanofiltration (NF)-like selective layer has been developed in this work. The fabrication involves a step of porous poly(amide-imide) (PAI) ultrafiltration (UF) hollow fiber spinning via phase inversion, followed by simple polyelectrolyte post-treatments using positively charged polyethyleneimine (PEI) for cross-linking and negatively charged polystyrene sulfonate sodium salt (PSS) for deposition. The membranes are subsequently characterized by a series of standard protocols and evaluated in FO process. The PAI FO membranes possess a pure water permeability of 3.7–4.3Lm−2 h−1 bar−1 and Na2SO4 rejections up to 85% at 1bar. In the FO process, the membranes can achieve water flux of 17 and 12Lm−2 h−1 for active layer facing draw solution (AL-facing-DS) and active layer facing feed water (AL-facing-FW) configurations, respectively, using a 2000ppm Na2SO4 aqueous solution as the feed and 0.5M Na2SO4 as the draw solution. When the feed contained 1000ppm bovine serum albumin (BSA) and 2000ppm Na2SO4, a steady water flux of 11Lm−2 h−1 can be maintained using 0.5M Na2SO4 as the draw solution at ambient temperature of 23°C. The approach of making FO membranes developed in the present study offers the advantages of simple fabrication process, tailorable selective layer and promising membrane performance for protein-contained wastewater treatment by FO process. [Copyright &y& Elsevier]

Abstract: Hollow fiber membranes with a positively charged nanofiltration (NF)-like selective layer have been successfully fabricated by utilizing asymmetric microporous hollow fibers made of Torlon® polyamide–imide (PAI) material as the porous substrate followed by polyelectrolyte post-treatment using polyethyleneimine (PEI), which has not been reported previously. The reaction between PAI and PEI has been confirmed by experiments. The membrane structure and surface property are carefully tailored by adjusting polymer dope composition, spinning conditions and the post-treatment parameters. These membranes are, subsequently, characterized by a series of standard protocols in terms of membrane morphology, structure, permeability and salt rejection, and are applied in forward osmosis (FO) processes. The newly developed PAI hollow fiber membranes show a pure water permeability of 2.19–2.25L/m2 hbar and reasonable NaCl and MgCl2 rejections of 49% and 94% at 1bar, respectively. It is also found that in FO processes, when using 0.5M MgCl2 as a draw solution and DI water as the feed in the active layer facing feed water (AL-facing-FW) configuration at 23°C, the water fluxes of two PAI FO hollow fiber membranes are 8.36 and 9.74L/m2 h, respectively, and the J S /J V of the two membranes is smaller than 0.4g/L, which is lower than the data of 0.85g/L for commercial FO membrane from Hydration Technologies Inc. (HTI). Different from a neutral membrane, the positively charged FO membrane provides double electric repulsions to the salt transfer through the membrane in the AL-facing-FW configuration, leading to a reduction of salt penetration, while in the active layer facing draw solution (AL-facing-DS) configuration, the positive charges facilitate salt transportation. This study provides a simple and effective approach to make hollow fibers suitable for FO applications. [Copyright &y& Elsevier]

Interfacial polymerization is an effective technique to synthesize high performance polyamide thin film membranes. However, it is still very challenging to apply this technique on ceramic hollow fibres or tubular membranes, especially when the substrate pore size is in the microfiltration range. In this study, we demonstrated that thin polyamide layer can be synthesized directly on microfiltration ceramic tubular membranes with a surface pore size of 0.1–0.2 μm via interfacial polymerization without an intermediate layer. A thin polyamide layer with a thickness 30–40 nm was coated on the inner surface of the ceramic substrate by circulating the monomers of branched polyethyleneimine (PEI), piperazine (PIP), and trimesoyl chloride (TMC) through the membrane lumen. The thin film layer showed good integration with the ceramic substrate and could withstand high pressure of at least 10 bar. The mechanical property of the polyamide layer was examined using AFM and the modulus was measured. The thin film composite membranes demonstrated excellent nanofiltration performance with a pure water permeability of 16–18 LMH bar−1 and a molecular weight cut-off of ∼250 Da. The membranes also showed good salt rejections (>90%) to CaCl 2 , MgCl 2 and MgSO 4 , and still maintained high rejections of MgCl 2 and sucrose at elevated temperature of 80 °C. The use of highly inert ceramic substrates has enabled the application of polyamide membranes under more challenging conditions. Image 1 • PEI-based polyamide thin film was synthesized on microfiltration ceramic tubular membranes without an intermediate layer. • The membranes demonstrated excellent nanofiltration performance with a PWP of 16–18 LMH bar−1 and a MWCO of ∼250 Da. • The membranes also showed good divalent salt rejections for CaCl 2 , MgCl 2 and MgSO 4. • The rejections of MgCl 2 and sucrose remained high at elevated temperature of 80 °C. • The mechanical property of the polyamide film was studied using the AFM pinpoint mode and the modulus was measured. [ABSTRACT FROM AUTHOR]

In this study, the glutaraldehyde (GA) crosslinking was adopted in combination of layer-by-layer (LBL) polyelectrolyte deposition to develop a novel hollow fiber NF membrane with low-pressure water softening capability for synthesized hard water containing divalent counterion SO 4 2− . The challenges for the new membrane include (a) the presence of SO 4 2− in the feed water, which tends to deteriorate positively charged NF membrane retention to divalent ions; (b) high salinity feed water up to 10,000 ppm total dissolved salt (TDS); (c) 2 bar operating pressure that requires the membrane to have a good Mg 2+ /Na + selectivity and less mass transfer resistance in the substrate; and (d) better separation performance as compared with the state-of-the-art and commercial NF membranes. Based on the membrane characterization results, the possible structure of the deposited layers and the effect of GA crosslink was proposed. It was found that the crosslinking could tighten the membrane surface pores with increased hydrophilicity while the membrane surface charge was reduced. Therefore, mainly based on the size exclusion, the crosslinked LBL*1.5C membrane showed more than 10 LMH/bar permeability with above 95% rejection to each of 1000 ppm MgCl 2 , MgSO 4 and Na 2 SO 4 solution under 2 bar pressure. It was also proved to be more suitable for the low-pressure water softening application at 2 bar in all tested feed water with the presence of SO 4 2− and TDS up to 10,000 ppm in terms of both permeability and hardness removal efficiency when compared with commercial NF 270 and NF 90 membranes. [ABSTRACT FROM AUTHOR]

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 Mg 2+ and Ca 2+ 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/m 2 h using 0.5 M MgCl 2 as draw solution in active layer facing draw solution configuration) and reduced salt leakage (around 0.5 g/m 2 h using 1 M MgCl 2 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. [ABSTRACT FROM AUTHOR]

Abstract: Nanofiltration (NF) membrane process has become increasingly attractive due to their unique characteristics to selectively remove specific compounds or ions. The most commonly NF membranes are negatively charged which is unsuitable for hardness removal. Therefore, the development of novel NF membranes with a positively charged skin has become a key issue for low pressure water softening. In this study, dual layer microporous hollow fiber membranes were fabricated using Torlon poly(amide-imide) (PAI) as the material of the selective outer layer and polyethersulfone (PES) as the material of the porous support inner layer. A positively charged NF-like selective layer was developed by a simple polyelectrolyte cross-linking using polyallylamine (PAAm). The newly developed (PAI–PAAm)–PES dual layer hollow fiber membranes show a salt water permeability of 15.8 L/m2 h bar and high Mg2+ and Ca2+ rejections of 94.2% and 92.3%, respectively, at operating pressure of 2 bar using a 3000 ppm TDS feed solution. This study provides a simple and effective approach to produce positively charged NF hollow fiber membranes for water softening applications with low energy consumption. [Copyright &y& Elsevier]

[Display omitted] • Reverse IP strategy was proposed to fabricate TFC PA membranes. • Defect-free PA skin layer was constructed on hydrophobic PVDF/PTFE substrates. • The membranes exhibit excellent performance in aqueous and organic solutions. • Pure water permeances are over 11 LMH/bar with MgCl 2 rejections of over 95%. • Methanol permeances are over 3.3 LMH/bar with Congo red rejections of over 98.5%. Polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE) porous membranes are ideal substrates for preparing thin-film composite (TFC) polyamide (PA) membranes with improved solvent resistance. However, it remains difficult to construct PA skin layers on PVDF and PTFE substrates via conventional interfacial polymerization (IP) due to the hydrophobic nature of these materials. To address this challenge, a reverse IP process was proposed to construct a defect-free PA skin layer on PVDF and PTFE substrates, in which the hydrophobic PVDF and PTFE substrates were soaked with the organic phase to ensure uniform coverage of reacted solution on the substrates. This novel strategy can effectively tackle the issue of uneven wetting of the aqueous phase on hydrophobic substrates in the conventional IP process. The resultant TFC membranes (i.e. M rev-PVDF and M rev-PTFE) exhibit excellent nanofiltration (NF) performance in both aqueous and organic solutions. M rev-PVDF and M rev-PTFE have MgCl 2 rejections of 96.12% and 95.17%, alongside the water permeances of 12.8 LMH/bar and 11.2 LMH/bar, respectively (M rev-PTFE was activated with DMF). It is further shown that M rev-PVDF and M rev-PTFE could also reject 99.4% and 98.5% of Congo red in methanol solution at solvent permeances of over 3.3 LMH/bar, while they also perform excellently in other polar protic solvents. Consequently, this work opens a new window for constructing high-performance TFC PA membranes on hydrophobic substrates and expands the application of the TFC PA NF membrane. [ABSTRACT FROM AUTHOR]

Abstract: Composite nanofiltration (NF) hollow fiber membranes desirable for water softening under low operating pressure (<2bar) were successfully developed in the current study. The thin-film selective layer of the composite hollow fiber was formed through interfacial polymerization on the inner surface of a polyethersulfone (PES) ultrafiltration (UF) membrane substrate with branched polyethyleneimine (PEI) and trymesoyl chloride (TMC) employed as the monomers in aqueous and organic phases, respectively. It is found that a proper molecular weight of PEI and the presence of sodium dodecyl sulfate (SDS) in the aqueous phase are important for a successful interfacial polymerization reaction. The resulting membrane prepared with optimized preparation parameters possesses a positively charged thin-film selective layer with pure water permeability (PWP) of about 17l/m2 hbar and a molecular weight cut-off (MWCO) of around 500Da, which translates into the effective pore diameter of about 1.29nm. With combined separation mechanisms of Donnan exclusion and steric hindrance, it was possible to achieve MgCl2 and MgSO4 rejection of 96.7% and 80.6%, respectively, when tested for 1000ppm feed solutions at 2bar operating pressure. In addition, for a 3000ppm TDS feed stream containing salt mixtures, the membrane rejections for Mg2+ and Ca2+ ions were found to be around 90% while the water flux was about 20l/m2 h at 2bar pressure, suggesting the potential of the newly developed composite hollow fibers for effective water softening application. [Copyright &y& Elsevier]

Recent studies have confirmed the advantages of low-pressure nanofiltration (NF) membranes in integrated nanofiltration membrane bioreactor system followed by the reverse osmosis (RO) process for high recovery water reclamation. However, fouling of low-pressure NF membranes restricts their application in providing high quality feed to subsequent RO process. In this study, an antifouling low-pressure NF membrane was designed and fabricated via rapid co-deposition of polydopamine (PDA)/polyethylenimine (PEI) on electrostatic layer-by-layer (LBL) assembled polyethersulfone (PES) flat sheet membrane followed by crosslinking with glutaraldehyde. The optimized resultant membrane (LBL-cPP) presented approximately 11 LMH/bar pure water permeability with more than 90% rejection divalent cations under 2 bar pressure. The newly developed selective layer exhibited excellent antifouling performance and chemical stability ascribing to the inherent hydrophilicity of PDA/PEI and dual covalent reaction of PDA with the LBL layer. By feeding real municipal wastewater, the LBL-cPP membrane also achieved superior permeate quality, leading to lower RO fouling rate in subsequent RO unit as compared to commercial NF 270 membrane. Further organic foulant analysis indicated that 96.8% of dissolved organic carbons were effectively removed by the LBL-cPP membrane. Specifically, the novel NF membrane demonstrated high removal rate (>93%) for foulants with low molecular weights which was largely responsible for RO fouling, while maintaining the desired flux recovery rate and reduced cleaning frequency. This study demonstrates the potential of PDA/PEI modified low-pressure NF membranes in designing efficient and sustainable water reclamation technology. Image 1 • Novel low-pressure antifouling NF membrane was designed. • The selective layer was formed by co-deposition of PDA/PEI on LBL assembly layer. • Structural stability was improved via electrostatic and covalent interactions. • The developed NF membrane exhibited enhanced hydrophilicity and cations rejection. • Antifouling ability in water reclamation was evaluated with municipal wastewater. [ABSTRACT FROM AUTHOR]

Pressure retarded osmosis (PRO) demonstrates great potential in energy harvesting when combining with seawater reverse osmosis. However, the lack of suitable membrane modules and the issue caused by the membrane fouling greatly impede the practical application of PRO to a larger scale. In this study, two-inch thin film composite hollow fiber modules were fabricated by using in-house developed PRO membranes. The produced PRO modules have a maximum effective area of 0.5 m 2 . By assessing the PRO performances of the modules with different sizes, external concentration polarization (ECP) was found to have significant impact on the flux reduction during module scale-up. Different module designs, including fiber bundles, distribution baffles and distribution tubes, were thus adopted as an attempt to boost the membrane performance. A power density of 8.9 W/m 2 at 15 bar was obtained using tap water as feed and 1 M NaCl solution as draw solution. PRO performance tests were also carried out using the developed two-inch modules on a pilot-scale setup with actual wastewater retentate as feed solution. Low pressure nanofiltration was selected as the pretreatment of the wastewater retentate to mitigate fouling. A power density of larger than 8 W/m 2 was obtained when pretreated wastewater retentate was used as the feed solution, implying high potential of PRO in the pilot scale. Nevertheless, full potential of PRO can only be realized by mitigating ECP, which could be achieved by improving the module design in the further endeavor. [ABSTRACT FROM AUTHOR]

The advancement of pressure retarded osmosis (PRO) has drawn much attention recently to its feasibility of practical operation. Membrane fouling is one of the main challenges, especially when real wastewater is used as feed solution in the PRO system. Low pressure nanofiltration (NF) pretreatment was thus adopted in current study to pretreat the wastewater reverse osmosis (RO) brine collected from a wastewater treatment plant prior to feeding to the PRO process. Three NF membranes were compared in terms of the membrane properties, NF permeate (i.e., PRO feed) quality and the PRO membrane performance. Results showed that the PRO water flux could increase to 30.5 L/m 2 /h (LMH) at 16 bar applied pressure by adopting the pretreated solution using an in-house made low-pressure NF hollow fiber membrane module, in contrast to the water flux of 9 LMH for untreated wastewater RO brine. A systematic analysis of water chemistry and membrane characterization (electron dispersed X-ray (EDX) and X-ray photoelectron spectroscopy (XPS) depth profiling) revealed that calcium salts, organic compounds, and silica could be the main contributors to the membrane fouling in PRO. Low-pressure NF was able to mitigate the fouling potential from multivalent ions and organic matters, but silica scaling in PRO needs to be further addressed. [ABSTRACT FROM AUTHOR]

Recent advances in nanotechnology have brought great opportunities to the developing next-generation nanofiltration membranes for addressing the concern of global water scarcity and energy crisis. Carbon dots have been deemed as a promising nanomaterial for enhancing the nanofiltration performance of polymeric membranes. Herein, carbon dots with cationic amine groups (PEI-CDs) and those with anionic sulfonate groups (PS-CDs) are incorporated into the polyamide selective layer of thin-film nanocomposite (TFN) membranes to create charged nanovoids (free volume) between the carbon dots and the surrounding polyamide network for high-efficiency nanofiltration. The positively charged amine group-based and negatively charged sulfonate group-based nanovoids in the resultant TFN-PEI-CDs and TFN-PS-CDs membranes, respectively, provide alternative pathways for efficient water permeation while effectively rejecting divalent ions via the Donnan and dielectric exclusion effects, thereby overcoming the recurring trade-off between permeability and selectivity encountered by dense polymeric membranes. In particular, the creation of negatively charged nanovoids enables the TFN-PS-CDs membrane to achieve one of the best performances among the recently reported nanofiltration membranes, by showing a nearly tripled pure water permeability of 30.9 L m−2 h−1 bar−1 as well as a higher Na 2 SO 4 rejection rate of 99.4% than the membrane with a pristine polyamide selective layer. Moreover, the TFN membranes demonstrate greater resistance to foulants with charges of the same sign as the nanovoids. This work provides insights into the design of nanovoids with desired properties in other polymeric membranes for high-efficiency filtration processes. [Display omitted] • Nanofiltration performance was improved by incorporating cationic and anionic CDs in the PA layer. • The ionic CDs created charged nanochannels between them and the surrounding PA matrix. • High pure water permeability of 30.9 L m−2 h−1 bar−1 and Na 2 SO 4 rejection rate of 99.4%. • Excellent filtration stability under different operating conditions and through a 14-day test. • Improved antifouling performance was demonstrated by using four types of ionic foulants. [ABSTRACT FROM AUTHOR]

• An acid-stable polyamine NF hollow fiber module was prepared for phosphorus recovery via the IP method. • Tannic acid interlayer was first constructed onto the PES substrate to assist the spreading of the aqueous phase. • The resultant NF membrane shows only 9.6% rejection of phosphorus, alongside over 93.5% rejection of metal ions at pH 2. • Membrane water permeability can reach 20.3 LMH/bar. The hazardous heavy metals coexisting with phosphorus-containing sewage sludge seriously limit phosphorus recovery. Nanofiltration (NF) membrane, which can selectively separate heavy metals and phosphorus, has a promising potential for recovering high-quality phosphorus from sewage sludge. In this work, we utilized the undissociated form of H 3 PO 4 at pH <2 and customized an acid-stable polyamine NF membrane via interfacial polymerization (IP) for separating phosphorus and heavy metals under acidic conditions. An interlayer formed via co-deposition of tannic acid (TA) and diethylenetriamine (DETA) was firstly coated onto the polyethersulfone (PES) hollow fiber module to assist the spreading of aqueous phase on the substrate surface. Consequently, a tight and positively charged polyamine layer with a mean pore size of 0.77 nm was successfully constructed on the PES substrate, which is smaller than that of the membrane without interlayer (1.39 nm). This polyamine membrane maintained excellent stability during 15-day acid testing with 0.1 M HCl as feed solution. When it was used to recover phosphorus at acidic condition (pH 2), the phosphorus rejection of the NF membrane was only 9.8 %, much lower than that at neutral pH (46.7 %), which is due to the fact that most phosphorus existed in the undissociated form of H 3 PO 4 at pH 2. Meanwhile, given the fine-tuning pore size and Donnan exclusion effect of this membrane, it could effectively retain 96.9 % of Mg2+ and over 93.5 % of most heavy metal ions such as Pb2+, Cu2+, Fe3+, Zn2+, and Cr3+, alongside outstanding water permeability of 20.3 LMH/bar at 2-bar operation pressure. This result demonstrates the potential of our homemade polyamine membrane in phosphorus recovery. [ABSTRACT FROM AUTHOR]

Recent studies report high concentrations of haloacetic acids (HAAs), a prevalent class of toxic disinfection by-products, in swimming pool water (SPW). We investigated the removal of 9 HAAs by four commercial reverse osmosis (RO) and nanofiltration (NF) membranes. Under typical SPW conditions (pH 7.5 and 50 mM ionic strength), HAA rejections were >60% for NF270 with molecular weight cut-off (MWCO) equal to 266 Da and equal or higher than 90% for XLE, NF90 and SB50 with MWCOs of 96, 118 and 152 Da, respectively, as a result of the combined effects of size exclusion and charge repulsion. We further included 7 neutral hydrophilic surrogates as molecular probes to resolve the rejection mechanisms. In the absence of strong electrostatic interaction (e.g., pH 3.5), the rejection data of HAAs and surrogates by various membranes fall onto an identical size-exclusion (SE) curve when plotted against the relative-size parameter, i.e., the ratio of molecular radius over membrane pore radius. The independence of this SE curve on molecular structures and membrane properties reveals that the relative-size parameter is a more fundamental SE descriptor compared to molecular weight. An effective molecular size with the Stokes radius accounting for size exclusion and the Debye length accounting for electrostatic interaction was further used to evaluate the rejection. The current study provides valuable insights on the rejection of trace contaminants by RO/NF membranes. [ABSTRACT FROM AUTHOR]

We investigated the removal of haloacetic acids (HAAs) from swimming pool waters (SPWs) by two nanofiltration membranes NF270 and NF90. The strong matrix effect (particularly by Ca 2+ ) on membrane rejection prompts us to systematically investigate the mechanistic role of Ca 2+ in HAA rejection. At typical SPW pH of 7.5, NF90 maintained consistently high rejection of HAAs (>95%) with little influence by Ca 2+ , thanks to the dominance of size exclusion effect for this tight membrane (pore radius ∼ 0.31 nm). In contrast, the rejections of both inorganic ions (e.g., Na + and Cl − ) and HAA anions were decreased at higher Ca 2+ concentration for NF270 (pore radius ∼ 0.40 nm). Further tests show that the rejection of neutral hydrophilic molecular probes and the membrane pore size were not affected by Ca 2+ . Although Ca 2+ is unable to form strong complex with HAAs, we observed the binding of Ca 2+ to NF270 together with a reduction in its surface charge. Therefore, the formation of membrane-Ca 2+ complex, which weakens charge interaction effect, was responsible for the reduced HAA rejection. The current study reveals important mechanistic insights of the matrix effect on trace contaminant rejection, which is critical for a better understanding of their fate and removal in membrane-based treatment. [ABSTRACT FROM AUTHOR]

[Display omitted] • Microfiltration PTFE hollow fibres were transformed into durable, high value OSN membranes. • A PEI/PIP based polyamide thin film was successfully synthesized on PTFE hollow fibre substrates. • A two-time interfacial polymerization was conducted to obtain a defect-free polyamide layer. • The resultant membranes could withstand strong aprotic solvents like DMF and DMSO. • The membranes had high solute rejections, of up to 95 % methyl orange (327 Da) rejection in DMF. Polymeric membranes have been extensively studied for organic solvent nanofiltration (OSN) but many fail to show sufficient chemical resistance in strong solvents like dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). This study focuses on harnessing the outstanding chemical resistance of polytetrafluoroethylene (PTFE), to develop polyamide-PTFE thin film composite (TFC) membranes that are resistant to such aprotic solvents. A defect-free polyamide thin film layer was successfully synthesized on the microfiltration PTFE hollow fibre substrates, overcoming challenges such as large and uneven pore size, and hydrophobicity of the PTFE substrates. A simple polydopamine (PDA) coating was carried out to improve the substrate's hydrophilicity prior to the polyamide synthesis via interfacial polymerization (IP). Polyethyleneimine (PEI) and trimesoyl chloride (TMC) were used as the monomers and a two-time IP method was employed to obtain a defect-free polyamide coating. The synthesized membranes showed high acetonitrile (ACN) and DMF permeabilities of 7.94 and 3.70 lm−2h−1bar−1, respectively, with acid fuchsin (585 Da) rejections of > 90 %. The molecular weight cut-off (MWCO) of the membranes could be further reduced to ∼300 Da by the addition of piperazine (PIP) monomers. The membranes also showed excellent stability and performance in a 72-hour OSN test performed in DMF indicating their great potential in effective molecular separation in harsh solvents. This study demonstrates how a simple modification and coating technique can transform commercially available microfiltration PTFE hollow fibres into durable and high value OSN membranes. [ABSTRACT FROM AUTHOR]

Synthetic interlayers with multiple functions and structural stability have a high potential to construct novel thin film composite (TFC) membranes with demanding permselective property for precise separations. Herein, a functional interlayer derived from the polyelectrolyte-based layer-by-layer (LBL) self-assembly is reported to prepare interlayer engineered organic solvent nanofiltration (OSN) membranes using β-cyclodextrin (β-CD) as a monomer via interfacial polymerization (IP) process. Compared to the interlayer-free OSN membrane, a smoother selective layer with higher crosslinking degree is obtained for our developed membranes, enabling an improved organic dye rejection (e.g. , 91.9% for methyl orange and 98.8% for brilliant blue R). Importantly, the LBL-aided β-CD nanofilm exhibits competitive permeances towards both polar (e.g. , methanol, 5.8 LMH/bar) and nonpolar solvents (e.g. , hexane, 7.0 LMH/bar). These performances can be attributed to the multiple roles of LBL-based interlayer as preventing the intrusion of selective layer and creating dual linkages (i.e. , amide and ester), along with tailoring the physicochemical properties of caped β-CD nanofilm. Besides, the newly developed OSN membrane can discriminate between dye molecules with similar molecular weights and same charge but different molecular configurations. Meanwhile, its good structural integrity and long-term stability are also demonstrated by the 7-day filtration test using tetracycline in methanol and β-carotene in hexane, respectively, as evidence of constant solute rejections (≥90%) during the entire filtration process. This work unravels the functions of LBL-based interlayer in enhancing the OSN performance, offering new insights into shaping the development of interlayer engineered OSN membranes for real-life applications. [Display omitted] • An interlayer derived from polyelectrolyte-based LBL is constructed for OSN application. • The multiple roles of LBL-based interlayer in tailoring β-CD nanofilm are investigated. • Interlayered membranes possess high permeances for both polar and nonpolar solvents. • Structural integrity and long-term stability are confirmed by a 7-day filtration test. [ABSTRACT FROM AUTHOR]

The use of nanotechnology in water separation membranes is a promising approach to alleviate the global water crisis. Graphene oxide (GO) with its unique water transport property has exhibited high potential for application in the water treatment processes. In this study, we have designed and fabricated a novel nanofiltration (NF)-like GO surface deposited poly(amide-imide)–polyethyleneimine (PAI–PEI) hollow fiber membrane. The PAI hollow fiber substrate was prepared by phase inversion and modified with PEI to obtain a positively charged membrane surface. The negatively charged GO nanosheets were then electrostatically immobilized onto the membrane surface via an instant dip-coating to form the NF-like selective layer. Our results have shown that the GO nanosheets serve as effective selective barriers and can achieve higher water permeability up to 86% without compromising membrane selectivity when used to substitute part of the PEI cross-linked selective layer. This is attributed to the smaller hydrodynamic resistance of GO nanosheets and their ability to effectively narrow the pore size distribution. Moreover, shortening of the membrane modification time and better mechanical properties of the GO modified membrane can be attained by avoiding excessive PEI cross-linking. The GO deposition also exhibits good stability when subjected to a backwashing pressure of 100,000 Pa and a cross-flow rate of 600 mL/min, which corresponds to a velocity of 14 cm/s and a Reynolds number of ~870. These results have demonstrated a high potential of using this GO modified hollow fiber membrane for large scale water softening applications. [ABSTRACT FROM AUTHOR]

The layer-by-layer (LbL) self-assembly is a common method for fabricating nanofiltration (NF) membranes. However, most multilayers are usually too thick and lack the necessary attributes to endow NF membranes with good permselectivity. In this study, we present a polyelectrolyte multilayered method by integrating dopamine as a "bio-glue". A three-step deposition was applied to the negatively charged PES substrate surface, following an alternate sequence of poly(allylamine hydrochloride) (PAH), dopamine (DA) and poly(sodium 4-styrenesulfonate) (PSS). The dopamine molecules were intercalated between the PAH and PSS layers to achieve a thinner thickness, higher hydrophilicity, smaller surface roughness and narrower pore size distribution. The resultant flat sheet (PAH/DA/PSS)4 polyelectrolyte multilayered (PEM) membranes exhibited enhanced water permeability (more than 60% increment from 13.5 to 21.9 LMH/bar) and higher rejection toward divalent ions (increasing MgCl 2 rejection from 91.0% to 94.8%) as compared with the control (PAH/PSS)4 NF membrane. Furthermore, the developed LbL method was applied on hollow fiber membrane modules and was able to achieve a higher Li+/Mg2+ selectivity from 15.6 to 37.8 and Na+/Mg2+selectivity from 11.0 to 19.1 using different simulated brines as feed solutions. On the whole, this polyelectrolyte multilayered method shows great potential for practical applications, including groundwater softening, seawater pretreatment and lithium extraction. [Display omitted] • A novel three-step LbL self-assembly method was proposed by integrating dopamine. • Dopamine intercalated selective layer exhibited a thinner and smoother surface. • The NF (PAH/DA/PSS)4 membrane showed better water permeability and higher divalent ion rejection. • Higher Na+/Mg2+ and Li+/Mg2+ selectivity was achieved due to the smaller pore size distribution. [ABSTRACT FROM AUTHOR]

Abstract: The layer-by-layer (LBL) polyelectrolyte deposition was applied on polyethersulfone (PES) hollow fiber ultrafiltration membrane to prepare nanofiltration (NF) membrane for low pressure water softening application for the first time. The effects of polyelectrolyte type and molecular weight (MW), deposited bilayer number, polyelectrolyte solution pH and supporting electrolyte concentration on the performance of resultant membranes were studied thoroughly. Experiments revealed that the pH of the polyelectrolyte solution and supporting electrolyte concentration presented the most notable effects. For weak poly-cations, Poly(ethylenimine) (PEI) and Poly(allylamine hydrochloride) (PAH), the solution pH affected the membrane performance significantly, and high water permeability and divalent cation rejection were obtained at optimum pH values. Using a low concentration of supporting electrolyte at underneath layers was proved to be an effective way to increase water permeability while maintaining high salt rejection. A salt water flux (SWP) of 12LMH/bar and 94% Mg2+ rejection was obtained by depositing only two bilayers of PSS/PAH polyelectrolytes on the PES substrate, while for the 3000ppm mixed salt solution, the water permeability was 11LMH/bar with Mg2+ and Ca2+ rejection as high as 90%. These promising results demonstrate that LBL technique is potentially applicable for preparing low-pressure water softening hollow fiber membranes. [Copyright &y& Elsevier]

Electrospun polymeric membranes are promising substrates for thin-film composite (TFC) membranes due to their unique interconnected pores and high porosity. However, it is still challenging to fabricate desirable electrospun substrates for organic solvent nanofiltration (OSN) owing to the relatively complex processing procedures and the organic operating environment. In this work, solvent-resistant electrospun polyimide (PI) nanofiber substrates were successfully fabricated through electrospinning followed by chemical cross-linking and heat-pressing. The cross-linking step improved the solvent tolerance of the membranes, while the heat-pressing step reduced the substrate pore size and surface roughness. However, it was found that heat-pressing at high temperatures (>140 °C) could degrade the cross-linking of PI, undermining their solvent-resistant property. A polyamide thin film layer was then synthesized on the solvent-resistant electrospun nanofibrous substrates via interfacial polymerization using reactant monomers m -phenylenediamine (MPD) and trimesoyl chloride (TMC). The TFC membranes exhibited excellent acetonitrile and acetone permeabilities of 31.28 ± 1.93 and 26.58 ± 1.13 L m−2 h−1 bar−1, respectively, with acid fuchsin (585 Da) and methyl orange (327 Da) rejections of 98.55 ± 1.24% and 92.42 ± 1.66%, respectively, in acetone. This study successfully demonstrated the potential use of electrospun PI nanofibers substrates for TFC membranes in OSN. [Display omitted] • Solvent resistant nanofibrous PI substrates were prepared by electrospinning followed by chemical cross-linking and heat-pressing. • Heat-pressing influenced the PI substrates' surface properties and solvent resistance, and the optimal point was 140 °C. • Polyamide thin film was synthesized on the PI substrates via interfacial polymerization for OSN application. • Acetone permeability was 26.58 L m−2 h−1 bar−1 with high rejection to acid fuchsin and methyl orange. [ABSTRACT FROM AUTHOR]

Abstract: Exploitation of osmotically-driven membrane (OM) technologies and accompanying semi-permeable OM fabrication have become promising topics with numerous potential applications. This paper presents a systematic comparison study of nanofiltration (NF)-like and reverse osmosis (RO)-like thin film composite (TFC) OMs in terms of flux performance and fouling behavior. Major governing mechanisms of OM performance were elucidated and revealed to be related with properties of membrane, draw and feed solutions. Results showed the crucial influence of draw solute reverse diffusion (M ICP-Js ) on OM water flux. Thus the high-rejection RO-like OMs generally performed better than NF-like counterpart in sodium chloride based osmotic processes. On the other hand, the high permeability of NF-like OMs can achieve higher water flux when proper draw solutes were used to minimize M ICP-Js . Fouling tests also suggest that the NF-like TFC OMs tend to be more fouling resistant due to its smooth membrane surface. The current study led to a set of systematic criteria for the selection of osmotically-driven membranes, draw solution and other operational conditions, of which the practicability was further elucidated in potential OM applications. [Copyright &y& Elsevier]

Abstract: Forward osmosis (FO) has attracted wide interest for its many potential applications. Recent developments suggest that double-skinned FO membranes may perform superiorly over conventional single-skinned ones. In this design, a dense rejection skin facing the draw solution is responsible for the separation, while a second skin facing the feed solution is designed to prevent foulants entering into the porous support layer sandwiched between the two skins. For the first time, an analytical model is developed for double-skinned FO membranes and is verified experimentally. Model simulations suggest that an optimal double-skinned membrane shall have a feed skin similar to that of a low rejection nanofiltration membrane to minimize the overall hydraulic resistant and to reduce internal concentration polarization. On the other hand, the optimization of the draw skin calls for a compromise between its water permeability and solute rejection. FO water flux is greater for support layers with higher mass transfer coefficients. In addition, it decreases with decreasing draw solution concentration and increasing feed solution concentration. While membrane orientation plays an important role for applications with low feed solution concentrations (with the denser skin preferentially facing the draw solution), this effect becomes negligible at high feed solution concentrations. [Copyright &y& Elsevier]

Conventional biomimetic membranes for desalination are mostly fabricated by incorporating water channels using self-assembled lipid or polymer vesicles as key platforms for protein or water channel reconstitution. Herein, we propose a thin film composite nanofiltration membrane via an unconventional liposomes-assisted fabrication without the incorporation of protein or water channels. The polyamide skin layer containing liposomes was thinner and filled with wrinkles and nanovoids. As a result, compared with the liposome-free control membrane, our optimized liposome-assisted membrane was able to achieve an increased water permeability from 11.17 to 18.21 LMH/bar, alongside an excellent MgCl 2 rejection of 95.87% and a monovalent/divalent (NaCl/MgCl 2) ion selectivity (α) of 18.2. Extensive membrane characterization showed that the liposome-assisted skin layer indeed exhibited a smaller thickness with an increased effective membrane area and a reduced surface hydrophobicity, which contribute towards a reduction in the hydrodynamic resistance of the polyamide layer. Furthermore, our approach of liposome-assisted thin film composite membrane is simpler and more scalable to fabricate without protein or water channels incorporation, rendering our design more attractive for future nanofiltration using vesicle-embedded biomimetic membranes. Image 1 • Lipid hollow vesicle was utilized to construct a thin film composite membrane. • The pathway of liposomes involved in the interfacial polymerization was confirmed via systematic characterization results. • Thinner polyamide with nanovoids and wrinkled structures was formed to facilitate water transportation. • The LE-TFC membrane exhibited better water permeability and higher divalent ion rejection. [ABSTRACT FROM AUTHOR]

Biocatalytic nanofiltration (NF) membranes incorporated with enzymes show high capacity for micropollutants (MPs) removal. However, there remains significant challenges such as the lack of molecular-level tailoring for skin layer design and effective strategy for enzyme immobilization. In this work, layer-by-layer (LBL) assembly based biocatalytic NF membranes were fabricated for bisphenol (BPA) removal by immobilizing laccase into the skin layer during the LBL polyelectrolytes assembly with controlled crosslinking and immobilization. This strategy enables simultaneous enzyme immobilization and NF skin layer formation. Three laccase immobilization strategies (i.e. , post immobilization, post crosslinking, and post crosslinking and immobilization) on skin layer were explored to prepare NF membrane for evaluating BPA removal efficiency. The post immobilization was identified as the optimal strategy, which endowed the biocatalytic NF membrane with a pure water permeability of 10.9 ± 0.4 LMH/bar and MgCl 2 rejection of 97.2 ± 0.3% under 2 bar pressure, alongside competitive laccase loading (238.8 ± 3.5 μg/cm2) and laccase activity (0.6 U/cm2). The optimal biocatalytic NF membrane exhibited an improvement in BPA removal of 79.5% under an incubation mode and 92.5% under a full recycling mode. The removal efficiencies were ~240% higher than that of the unmodified LBL membrane and clearly comparable to other reported biocatalytic membranes. This performance was attributed to the synergistic effect of membrane rejection, adsorption and laccase catalysis. The optimal biocatalytic NF membrane was found to be robust after six cycles within 14 days, while maintaining a relatively high BPA removal efficiency and salt rejection. Overall, our results open up a new avenue for enzyme immobilization into the skin layer of membranes for designing high-efficient biocatalytic NF membranes for MPs removal. Image 1 • Laccase-decorated LBL skin layer is prepared on lumen side of hollow fiber membrane. • Effects of different laccase immobilization strategies on BPA removal are explored. • Post immobilization strategy endows the membrane with a competitive NF performance. • Desired BPA removal efficiency is achieved with optimal biocatalytic membrane. • The long-term reusability and stability for BPA removal are confirmed. [ABSTRACT FROM AUTHOR]

Polyamide thin film membranes have shown outstanding performance in organic solvent nanofiltration (OSN). However, it is still challenging to produce polyamide hollow fibres for OSN, mainly due to limited solvent resistance of hollow fibre substrates and the difficulty to synthesize polyamide thin film on the surface of hollow fibre substrates. In this study, polyamide-based hollow fibre composite members for low pressure OSN were successfully developed. Solvent resistant polyimide hollow fibre substrates were first prepared through a non-solvent induced phase separation process, followed by chemical cross-linking with hexamethylene diamine. A polyamide thin film layer was then synthesized via interfacial polymerization, by circulating the reactant monomers including polyethyleneimine (PEI), piperazine (PIP) and trimesoyl chloride (TMC) through the hollow fibre lumen. The polyamide thin film with a thickness of ~60 nm was formed on the inner surface of the hollow fibre substrates. The membranes exhibited excellent nanofiltration (NF) performance under 2 bar operating pressure. The permeability of water, acetone and isopropanol was 6.8, 11.6 and 4.5 l m−2 h−1 bar−1, respectively. The membranes also achieved 99.9% and 91.8% rejection to rose bengal (1017 Da) and acid fuchsin (585 Da), respectively, in acetone. Furthermore, a 72-h filtration test was conducted and the membrane showed steady performance throughout the testing period. This study demonstrates the possibility of fabricating polyamide hollow fibre membranes for organic solvent nanofiltration at low pressure, which is desirable for practical applications. • Solvent resistant polyimide hollow fibre membranes were fabricated through chemical cross-linking. • Polyamide-based (PEI/PIP) layer was formed onto polyimide substrate to obtain a relatively loose selective layer. • Membranes achieved 11.6 and 4.5 l m−2 h−1 bar−1 permeability in acetone and isopropanol at 2 bar respectively. • Membranes maintained more than 90% rejection for acid fuchsin in 72-h filtration test, showing stable OSN performance. [ABSTRACT FROM AUTHOR]

In this study, we presented a novel nanofiltration membrane fabrication strategy through rapid co-deposition of a versatile platform – metal phenolic networks (MPNs) and piperazine (PIP) on a porous substrate followed by trimesoyl chloride (TMC) crosslinking. Inspired by the catechol chemistry, tannic acid (TA) was functionalized to tether PIP monomers, and participated in the co-deposition via coordination bonds with Fe3+ ions on a substrate to form Fe3+/TA-PIP complex. A series of analyses (SEM, XPS, FT-IR, water contact angle and zeta potential) confirmed the successful synthesis Fe3+/TA-PIP deposition and polyamide (PA) layer. The resultant membrane exhibited water permeability of 13.73 LMH/bar and 89.52 % for MgSO 4 rejection. The fabrication was further improved by embedding modified graphene oxide (GO) nanosheets into the Fe3+/TA-PIP complex. The optimized nanocomposite membrane achieved a water permeability at 21.66 LMH/bar, along with a well maintained MgSO 4 rejection at 91.25 % and NaCl/MgSO 4 selectivity (α) at 10.02 under 2 bar operation pressure. The rapid co-deposition mediated by nanomaterials incorporation provides an effective approach to design high performance thin film composite (TFC) membrane. Image 1 • A novel fabrication strategy was designed for nanofiltration membrane. • Metal phenolic networks were rapidly co-deposited with piperazine followed by interfacial polymerization. • Graphene oxide was incorporated into the deposited layer without additional fabrication step. • The optimized thin film composite membrane exhibited high separation performance. [ABSTRACT FROM AUTHOR]