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With the use of bipolar membranes (BPMs) in an expanding range of applications, there is an urgent need to understand and improve the catalytic performance of BPMs for water dissociation, as well as to increase their physical and chemical stability. In this regard, electrospinning BPMs with 2D and 3D junction structures have been suggested as a promising route to produce high-performance BPMs. In this work, we investigate the effect of entangling anion and cation exchange nanofibers at the junction of bipolar membranes on the water dissociation rate. In particular, we compare the performance of different tailor-made BPMs with a laminated 2D junction and a 3D electrospun entangled junction, while using the same type of anion and cation exchange polymers in a single/dual continuous electrospinning manufacturing method. The bipolar membrane with a 3D entangled junction shows an enhanced water dissociation rate as compared to the bipolar membrane with laminated 2D junction, as measured by the decreased bipolar membrane potential. Moreover, we investigate the use of a third polymer, that is, poly(4-vinylpyrrolidine) (P4VP), as a catalyst for water dissociation. This polymer confirmed that a 3D entangled junction BPM (with incorporated P4VP) gives a higher water dissociation rate than does a 2D laminated junction BPM with P4VP as the water dissociation catalyst. This work demonstrates that the entanglement of the anion exchange polymer with P4VP as the water dissociation catalyst in a 3D junction is promising to develop bipolar membranes with enhanced performance as compared to the conventionally laminated membranes.

Bottom-up methods for the fabrication of nanoporous polymer membranes have numerous advantages. However, it remains challenging to fabricate nanoporous membranes that are mechanically robust and have aligned pores, that is, with a low tortuosity. Here, a mechanically robust thin-film composite membrane was fabricated consisting of a two-dimensional (2D) porous smectic liquid crystalline polymer network inside an anisotropic, microporous polymer scaffold. The polymer scaffold allows for relatively straightforward planar alignment of the smectic liquid crystalline mixture, which consisted of a diacrylate cross-linker and a dimer forming benzoic acid-based monoacrylate. Polymerized samples displayed a smectic A (SmA) phase, which formed the eventual 2D porous channels after base treatment. The aligned 2D nanoporous membranes showed a high rejection of anionic solutes bigger than 322 g/mol. Cleaning and reusability of the system were demonstrated by intentionally fouling the porous channels with a cationic dye and subsequently cleaning the membrane with an acidic solution. After cleaning, the membrane properties were unaffected; this, combined with numerous pressurizing cycles, demonstrated reusability of the system.

To prevent greenhouse emissions into the atmosphere, separations like CO2/CH4 and CO2/N2 from natural gas, biogas, and flue gasses are crucial. Polymer membranes gained a key role in gas separations over the past decades, but these polymers are often not organized at a molecular level, which results in a trade-off between permeability and selectivity. In this work, the effect of molecular order and orientation in liquid crystals (LCs) polymer membranes for gas permeation is demonstrated. Using the self-assembly of polymerizable LCs to prepare membranes ensures control over the supramolecular organization and alignment of the building blocks at a molecular level. Robust freestanding LC membranes were fabricated that have various, distinct morphologies (isotropic, nematic cybotactic, and smectic C) and alignment (planar and homeotropic), while using the same chemical composition. Single gas permeation data show that the permeability decreases with increasing molecular order while the ideal gas selectivity of He and CO2 over N2 increases tremendously (36-fold for He/N2 and 21-fold for CO2/N2) when going from randomly ordered to the highly ordered smectic C morphology. The calculated diffusion coefficients showed a 10-fold decrease when going from randomly ordered membranes to ordered smectic C membranes. It is proposed that with increasing molecular order, the free volume elements in the membrane become smaller, which hinders gasses with larger kinetic diameters (Ar, N2) more than gasses with smaller kinetic diameters (He, CO2), inducing selectivity. Comparison of gas sorption and permeation performances of planar and homeotropic aligned smectic C membranes shows the effect of molecular orientation by a 3-fold decrease of the diffusion coefficient of homeotropic aligned smectic C membranes resulting in a diminished gas permeation and increased ideal gas selectivities. These results strongly highlight the importance of molecular order and orientation in LC polymer membranes for gas separation.

Controlling the metal-organic framework (MOF)-matrix interface is a useful strategy to improve the gas separation performance of mixed matrix membranes (MMMs). Although polymer blending has been investigated to enhance MMM performances, its true strength, i.e., aligning polymer and additive chemistries to improve interfacial compatibility and ultimately the separation performance, is only poorly investigated. In this work we demonstrate how controlling interfacial chemistries by polymer blending is an effective tool to tune the membrane performance. Three isoreticular zeolitic imidazolate frameworks (ZIFs) and two matrix polymers (Matrimid and polybenzimidazole oPBI (PBI)) were used to prepare the MMMs. The ZIF linker functionality strongly determined the extent of the effect of PBI addition in the MMMs. For both the hydrophobic ZIF-7 and ZIF-8-based MMMs, PBI compatibilized the MOF-matrix interface and increased the CO2/N2 separation factor while slightly decreasing the permeability. Contrarily, the separation performance of the hydrophilic ZIF-90 MMM was not affected by PBI incorporation. Additionally, the MMM permeability followed the trend of the ZIF pore geometries and linker flexibilities. These results proved that MOF-matrix interfacial compatibility can effectively be controlled by polymer blending and that the extent of control is determined by a subtle balance between the MOF linker functionality and matrix chemistry.

In this work, the effectiveness of metal-organic frameworks (MOFs) with sheet-like morphologies was assessed as function of the MOF microporosity and MOF-matrix compatibility. Zeolitic imidazolate frameworks (ZIFs, a MOF subclass) with sheet-like/platelet morphologies were incorporated in Matrimid/PBI matrices resulting in mixed matrix membranes (MMMs). The ZIFs were either permeable (ZIF-301) or impermeable (ZIF-95X) for gases with a kinetic diameter bigger than or equal to the kinetic diameter of CO2. Additionally, MMMs containing impermeable graphene nanosheets were fabricated as reference to confirm the ZIF-95X impermeability. The MMMs containing the ZIF-301 nanoplatelets showed enhanced CO2 permeabilities. Analysis of the N2 and CO2 solubility and diffusivity showed that this permeable additive enhances the solubility of both gases, but only increases the N2 diffusivity through the ZIF-301 MMM. This signified that for MMMs with sheet-like MOFs the MOF micropore volume should only be accessible for one gas such that the sheet-like morphology effectively increases the tortuosity for the other species. Contrarily, the MMMs containing impermeable ZIF-95X and graphene showed declined MMM separation performances. Analysis of the N2 and CO2 diffusivities showed the presence of defective interfaces in these MMMs and proved that an increased tortuosity is only effective if the additive-matrix interface is defect free.

The time-dependent CO2-induced plasticization behavior of glassy Matrimid® 5218 polymer membranes at supercritical conditions up to 120 bar was investigated. Glassy polyimide membranes were conditioned with both gaseous CO2 and liquid-like sc-CO2. The plasticization behavior during permeation and sorption was correlated with the intrinsic membrane properties and the CO2 fluid properties. In the gaseous region the CO2 concentration increased slightly over time, while in the liquid-like sc-CO2 region the CO2 concentration remained constant over time and showed no hysteresis, indicating an induced glass transition. Contrary to the CO2 sorption the CO2 permeability showed more pronounced time-dependent behavior which increases with feed pressure because of polymer membrane plasticization. Despite the strong time-dependency, the CO2 permeability was independent of the feed pressure in the liquid-like sc-CO2 region. This difference in time-dependent behavior between sorption and permeation is due to the presence of a concentration gradient during permeation experiments. In addition, the permeability showed significant hysteresis. Exposure to liquid-like sc-CO2 resulted in a highly plasticized membrane and changed the permeation behavior at all subsequent feed pressures, due to slow polymer chain relaxation rates. Clearly, these relationships proof that the permeation history is a critical aspect for time-dependent plasticization phenomena at high CO2 pressures.

Reverse electrodialysis (RED) is an electro-membrane process to harvest renewable energy from salinity gradients. RED process models have been developed in the past, but they mostly assume that only NaCl is present in the feedwaters, which results in unrealistically high predictions. In the present work, an existing simple model is extended to accommodate the presence of magnesium ions and sulfate in the feedwaters, and potentially even more complex mixtures. All power loss mechanisms deriving from the presence of multivalent ions are included in the new model: increased membrane electrical resistance, uphill transport of multivalent ions from the river to the seawater compartment, and membrane permselectivity loss. This new model is validated with experimental and literature data of membrane electrical resistance (at 10 mol. % MgCl2 for the CEMs and 25 mol. % Na2SO4 for the AEMs), RED stack performance (up to 50 mol. % MgCl2 or Na2SO4 in the feedwaters), and ion transport (at 10 mol. % MgCl2 or Na2SO4 in the feedwaters) showing very good agreement between model predictions and experimental data. Finally, we showed that the developed model not only describes experimental data but can also predict RED performances under a variety of conditions and cross-flow configurations (single-stage with and without electrode segmentation, multi-stage in co-current and counter-current mode) and feedwater compositions (only NaCl, with Na2SO4, with MgCl2, and with MgSO4). It thus provides a very valuable tool to design and evaluate RED process systems.

Supercritical CO 2 (scCO 2 ), used in the food industry as a water extraction agent, requires dehydration units for regeneration. The present study assesses the economics of membrane-based dehydration of scCO 2 . In contrast to earlier studies, the contribution, next to the membrane, also the contributions of the mass transfer resistances of the feed and permeate boundary are included, which have a dominant effect on the final process design and economics. In addition, our work also extrapolates the process to industrial scale evaluating different configurations and process conditions. Specifically, the contribution of the membrane and membrane unit costs is discussed in more detail. Including the mass transfer resistances of feed and permeate boundary layer reduces the water flux across the membrane up to a factor 150, implying a larger required membrane surface area for a given water removal rate, and thus higher costs. Using a SPEEK-based membrane, the total drying costs, normalized for the amount of water removed, minimize around a skin layer thickness of 1 μm, i.e., not too thin to permeate and thus spill too much CO 2 and not too thick to hamper the H 2 O flux. Because the feed boundary layer dominates water transport, conditions that minimize its thickness reduce total costs. A reduction of the feed boundary layer dominance can be achieved by adjusting channel height, cross-flow velocity and the density and viscosity of scCO 2 , the latter two by increasing the operational temperature from 45 to 65 °C (at 130 bar). Compared to the benchmark zeolite process currently available, the membrane-based process for drying scCO 2 outlined and optimized in the present study results in a 50% saving of total drying costs. These savings can be achieved by using a dense polymeric membrane with a H 2 O permeability of at least 10,000 Barrer and a CO 2 permeability of at most 10 Barrer.

Supercritical carbon dioxide (scCO2) is used in the food industry as a water-extracting drying agent. Once saturated with water, the scCO2 needs to be regenerated. A promising way of drying scCO2 is by using H2O permeable membranes. Ideally, these membranes demonstrate low CO2 permeability. Here, we investigated the CO2 permeability of three types of dense membranes, Nafion, Natural Rubber and PDMS, of which the latter in more detail because of its ease of handling. The experimental conditions, temperature and pressure, resulting in minimum CO2 permeability (=losses) were explored. Even though the absolute CO2 permeability depends on the intrinsic membrane material properties, its trend with increasing feed pressure is defined by the (supercritical) behavior of CO2, notably its density as a function of temperature and pressure. The data points to transitions within the supercritical regime, from the gaseous-like supercritical state to the liquid-like supercritical state, graphically visualized by the Widom line for CO2 density. Sorption measurements with PDMS membranes confirm this behavior that follows the diffusion-solution theory. In the gaseous state, the (normalized) permeability follows the (normalized) solubility, indicating a constant CO2 diffusivity. With increasing pressure and when entering the liquid-like (supercritical) regime, the diffusivity drops, resulting in a (normalized) permeability that starts to lag behind the (normalized) solubility.

Continuous drying processes using supercritical CO2 (scCO2) as a water extraction agent require 24/7 operational dehydration units for scCO2 regeneration. Dehydration units using dense polymeric membranes are considered a cost effective, sustainable alternative to the current zeolite-based units. The focus of previous studies on the membrane-based dehydration of scCO2 was always on the membrane itself whereas boundary layer effects, e.g., concentration polarization, were not taken into account. To quantify the boundary layer effects, simulations were performed using three different membrane materials: SPEEK, Nafion® 117, and PEBAX® 1074. Process conditions during the simulations ranged from 8.0 to 18.0 MPa and 40 to 100 °C. Even though the three types of membranes examined differ in their H2O permeability and H2O over CO2 selectivity, in all cases 80% of the total mass-transfer resistance can be assigned to concentration polarization effects, making it the dominant parameter for water transport. Despite high but differing intrinsic water permeabilities of all three membranes materials, the H2O transport, thus H2O flux through the membrane is significantly reduced by concentration polarization down to similar levels. This makes it necessary to use larger membrane areas, that result in higher CO2 fluxes. As a consequence, material selection is predominantly based on the ability to reject CO2. Optimization of process conditions other than membrane material is briefly discussed.

Reverse electrodialysis (RED) is a process to harvest renewable energy from salinity gradients. Under lab conditions with artificial salt solutions, promising results have been achieved in recent years. However, in large scale industrial applications, natural waters are used and that poses challenges such as fouling. Fouling of anion exchange membranes (AEMs) by organic matter (e.g. humic acids) has been identified as a possible cause that lowers RED performance with natural waters. In this work, natural river and seawater at the Afsluitdijk (The Netherlands) are used to study the RED performance of six different AEMs. These AEMs are characterized before and after RED experiments with natural waters. The effect of natural fouling is found to be specific for each AEM and highly dependent on their respective chemistries and associated membrane properties. Firstly, aromatic AEMs with a low swelling degree showed a permselectivity decrease as well as membrane resistance increase. Secondly, aliphatic AEMs with a medium swelling degree experienced only a membrane resistance increase. Finally, only a decrease in permselectivity was observed for aliphatic AEMs with large swelling degrees. Subsequently, the effect of AEM fouling is compared to the observed decrease in RED performance and this shows that AEM fouling can only explain a minor part of the losses in open circuit voltage (OCV). The RED power densities dropped by 15–20% over 12 days, independent of the AEMs selected, while the reduced AEM performance could only explain 2–4% of this reduction in power density. This demonstrates that next to AEM fouling, also other factors, such as spacer fouling, are expected to be the dominant fouling mechanism, reducing the performance to a much larger extent.

Nanofiltration (NF) membrane processes are attractive to remove multivalent ions. As ion retention in NF membranes is determined by both size and charge exclusion, negatively charged membranes are required to reject negatively charged ions. Layer-by-layer assembly of alternating polycation (PC) and polyanion layers on top of a support is a versatile method to produce membranes. Especially the polyelectrolyte (PE) couple polydiallyldimethylammoniumchloride and poly(sodium-4-styrenesulfonate) (PDADMAC/PSS) is extensively investigated. This PE couple cannot form highly negatively charged membrane surfaces, due to interdiffusion and charge overcompensation of PDADMAC into the PSS layers, which limits the operational window to tailor membrane properties. We propose the use of asymmetric layer formation and show how combining two charge densities of one PC can produce negatively charged NF membranes. Starting from hollow fiber ultrafiltration supports coated with base layers of PDADMAC/PSS, they are coated with PDADMAC/PSS or poly(acrylamide-co-diallyldimethylammoniumchloride), P(AM-co-DADMAC)/PSS layers. P(AM-co-DADMAC) has a charge density of only 32% compared to 100% for PDADMAC. The particular novel membranes coated with P(AM-co-DADMAC) have a highly negatively charged surface and high permeabilities (7–19 L/[m2hbar]), with high retentions for Na2SO4 of up to 95%. These values position the developed membranes in the top range compared to commercial and other layer-by-layer membranes.

Cost-effective dense membranes were developed by blending proton-conductive sulfonated poly(ether ether ketone) (SPEEK) with inert, mechanically stable poly(vinylidene fluoride) (PVDF) for hydrogen-bromine flow batteries (HBFBs). Wire-electrospinning followed by hot-pressing was employed to prepare dense membranes. Their properties and performance were compared to solution-cast membranes of similar composition and thickness. Electrospinning improved the ionic conductivity and bromine diffusion properties by providing interconnected ion-conductive SPEEK nanofiber pathways through a PVDF matrix. Relatively thin (~50–60 μm) electrospun membranes with a SPEEK/PVDF ratio (wt%/wt%) of 90/10 and 80/20 showed comparable Br3− diffusion rates as the relatively thick and commercially available perfluorosulfonic acid (PFSA) membrane (~100 μm) at a 35%–42% lower proton conductivity. The latter can be attributed to the poorer ion conductivity of SPEEK compared to PFSA and the presence of PVDF. The HBFB single cell featured the best polarization behavior and ohmic area resistance with the electrospun membrane containing 80/20 (wt%/wt%) SPEEK/PVDF. However, the low thickness and insufficient chemical/mechanical stability of the ES 80/20 causes a rapid decay in the HBFB cycling performance. This study promotes a life-time comparison study between the low-cost wire-electrospun SPEEK/PVDF blend membranes (~€100 m−2) and the typically used PFSA membranes (~€400 m−2) for a long-term HBFB performance.

Electrodialysis (ED) is receiving increasing attention as promising technology for seawater desalination. However, most of the ED investigations are typically performed using artificial NaCl solutions, while the effect of multivalent ions (such as Mg2+ and Ca2+) on membrane scaling and resistance has been so far overlooked. In this work, we investigate the influence of multivalent ions in seawater on the desalination performance of multistage ED. In particular, natural seawater was used as feed solution, and two different strategies were compared, i.e. by using conventional cation exchange membranes (CEMs), as well as CEMs with preferential removal of multivalent ions. For both CEMs, we found that the removal of calcium and magnesium was higher compared to that of sodium and no effect due to operation at low current density was observed. More magnesium was removed with the multivalent ion permeable CEM. Starting from ~27 g/l (i.e. inlet concentration of the natural seawater source), the upscaled multistage ED system produced a continuous diluate concentration of 1.9 g/l. The system performance was stable over 18 days, with an average energy consumption of 3 kWh/m3, demonstrating the potential of multistage ED seawater desalination.

Fouling is a pressing issue for harvesting salinity gradient energy with reverse electrodialysis (RED). In this work, antifouling membranes were fabricated by surface modification of a commercial anion exchange membrane with zwitterionic layers. Either zwitterionic monomers or zwitterionic brushes were applied on the surface. Zwitterionic monomers were grafted to the surface by deposition of a polydopamine layer followed by an aza-Michael reaction with sulfobetaine. Zwitterionic brushes were grafted on the surface by deposition of polydopamine modified with a surface initiator for subsequent atom transfer radical polymerization to obtain polysulfobetaine. As expected, the zwitterionic layers did increase the membrane hydrophilicity. The antifouling behavior of the membranes in RED was evaluated using artificial river and seawater and sodium dodecylbenzenesulfonate as the model foulant. The zwitterionic monomers are effective in delaying the fouling onset, but the further build-up of the fouling layer is hardly affected, resulting in similar power density losses as for the unmodified membranes. Membranes modified with zwitterionic brushes show a high potential for application in RED as they not only delay the onset of fouling but they also slow down the growth of the fouling layer, thus retaining higher power density outputs.

In this work, the influence of the zeolitic imidazolate framework 78 (ZIF‑78) morphology, with 1D pores, on the mixed matrix membrane (MMM) CO2/N2 mixed gas separation performance is investigated as well as the influence of the feed composition and pressure. Low aspect ratio and a high aspect ratio ZIF‑78 particles are synthesized and incorporated in Matrimid with 10 and 20 wt% additive content. High pressure CO2 and N2 sorption measurements show that both the low and high aspect ratio ZIF‑78 metal–organic frameworks (MOFs) exhibit similar sorption behavior. The incorporation of ZIF‑78 into Matrimid results in improved CO2 permeabilities up to 39%, without compromising the selectivity, relative to native Matrimid membranes. Both at low and high CO2 partial pressures, the MMMs containing the different ZIF‑78 morphologies show equal CO2 permeabilities, indicating that the ZIF‑78 morphology and consequently the aspect ratio are insignificant for these particular MMMs, contradicting previous observations in literature. Thus, depending on the MOF/polymer system, the MOF morphology and aspect ratio can be considered as a design aspect. Finally, a clear influence of feed composition and pressure on the MMM solubility coefficient and diffusivity is observed, emphasizing the importance of the mixed gas composition and measurement conditions.

The high-pressure permeation and sorption behavior of supercritical carbon dioxide (sc-CO2) in glassy Matrimid® 5218 polymer membranes were extensively investigated. The effect of pressure (0–120 bar) and temperature (25–55 °C) was examined. The observations were related to the intrinsic membrane properties, plasticization phenomena and the CO2 fluid properties. The phase transition from gaseous (-like sc) CO2 to liquid (-like sc) CO2 has the largest influence on the CO2 fluid properties and therefore was found to have the most influence on the CO2 sorption and CO2 permeability. The CO2 sorption was directly dependent on the CO2 density in the liquid (-like sc) regime. The CO2 permeability of Matrimid® 5218 showed typical CO2-induced plasticization behavior in the gaseous (-like sc) CO2 regime. When entering the liquid (-like sc) CO2 regime, the extent of plasticization was found to be independent of the applied feed pressure in this regime. The membranes showed strong hysteresis with pressure. The permeation history of the membrane thus has a large influence on the time-dependent permeability behavior. Clearly, the CO2 permeability behavior at these high pressures in glassy Matrimid® 5218 is determined by a combination of the CO2 fluid density and plasticization phenomena.

Although the layer-by-layer technique to produce nanofiltration membranes has been studied frequently, the influence of ionic strength of the coating solution in relation to the charge density lacks understanding. Here, we investigate the effect of both parameters systematically by comparing two strong polyelectrolyte sets: poly(acrylamide-co-diallyldimethylammoniumchloride) (P(AM-co-DADMAC))/ poly(sodium-4-styrenesulfonate) (PSS) and polydiallyldimethylammoniumchloride (PDADMAC)/PSS, with a polycation charge density of 32 and 100%, respectively. The influence of the charge density and the ionic strength during layer formation (5∙10−5 M – 1 M) has been researched in terms of polyelectrolyte adsorbance, membrane surface charge, and filtration performance. A low charge density is a limiting parameter for polyelectrolyte adsorption and retention, due to limited uptake of two bilayers. Consequently, it is impossible to form NF membranes with solely P(AM-co-DADMAC)/PSS layers. For high charge density polyelectrolytes, increasing the ionic strength is crucial to increase PE adsorption and MgSO4 retention. However, there is an opposite effect of ionic strength and charge density on membrane surface charge and pure water permeability. An increasing ionic strength results in a decreasing surface charge and increasing permeability for P(AM-co-DADMAC)-based membranes, and vice versa for PDADMAC. These opposite trends undoubtedly show the importance of simultaneously taking into account the charge density and ionic strength.

A main component of a hydrogen-bromine flow battery (HBFB) is the ion exchange membrane. Available membranes have a trade-off between the major requirements: high proton conductivity, low bromine species crossover, and high mechanical and chemical stability. To overcome this, electrospinning of a highly proton conductive polymer (short side chain perfluorosulfonic acid (SSC PFSA)) and a hydrophobic inert polymer (polyvinylidene fluoride (PVDF)) was used to electrospin composite polymer fiber mats. Piles of multiple mats were hot pressed resulting in dense ion exchange membranes. Membranes with three different SSC PFSA/PVDF ratios were prepared, characterized, and subjected to short and long term (1500 h) HBFB testing. The electrospun membranes have performances very comparable to those of commercial membranes. For the SSC PFSA/PVDF electrospun membrane, a higher SSC PFSA loading gives a higher membrane proton conductivity compared to a lower loading, but at the expense of a higher bromine species crossover. The SSC PFSA/PVDF (50/50 wt%) membrane shows a coulombic efficiency of 98%, a voltaic efficiency of 80% and an initial available capacity of 105 Ah L−1 at a current density of 150 mA cm−2, which equals that of the current benchmark long side chain PFSA membrane. This performance is constant over 200 cycles during 2 months of continuous HBFB operation.

Thermally induced decarboxylation cross-linking of carboxylic acid bearing polyimides has been recently introduced to create cross-linked polymer membranes with enhanced gas permeability. The main focus has been on the cross-linking of high permeability/low selectivity 6FDA copolymers with a relatively low amount of carboxylic acid groups. In contrast, decarboxylation cross-linking was applied in this work on a low permeability/high selectivity polymer with a large amount of –COOH groups (pure 6FDA-DABA). 6FDA-DABA membranes were thermally treated at different temperatures (100 °C, 180 °C, 250 °C, 350 °C and 400 °C and tested for CO2/CH4 and CO2/N2 separation and thoroughly characterized by ATR-FTIR, TGA-MS, DSC, EDX, XRD, density measurements, fluorescence spectroscopy and gas sorption measurements. Two counteracting mechanisms defined the overall performance of the cured membranes. Physical tightening of the membrane significantly enhanced the CO2/CH4 separation factor with increasing annealing temperature due to an improved polymer chain packing efficiency, which was confirmed by fluorescence spectroscopy and membrane density experiments. In contrast, cross-linking through decarboxylation occurred from 330 °C onward and induced a more open polymer structure for 6FDA-DABA-350 and 6FDA-DABA-400. Consequently, the more open polymer structure resulted in an increase in permeability of all gases. For 6FDA-DABA-350, a synergy was observed between the dilation effect of cross-linking and the tightening effect, causing a simultaneous, strong improvement of both separation factor (+100%) and permeability (+40%). As a result, the membrane performance crossed the 2018 mixed-gas upper bound and scored very close to the 2008 Robeson upper bound. Moreover, the cross-linked 6FDA-DABA-350 showed an increased resistance to CO2-induced plasticization thanks to covalent cross-linking.

Metal-organic frameworks (MOFs) as additives in mixed matrix membranes (MMMs) for gas separation have gained significant attention over the past decades. Many design parameters have been investigated for MOF based MMMs, but the spatial distribution of the MOF throughout MMMs lacks investigation. Therefore, magnetically aligned and enriched pathways of zeolitic imidazolate framework 8 (ZIF&minus, 8) in Matrimid MMMs were synthesized and investigated by means of their N2 and CO2 permeability. Magnetic ZIF&minus, 8 (m&ndash, ZIF&minus, 8) was synthesized by incorporating Fe3O4 in the ZIF&minus, 8 structure. The presence of Fe3O4 in m&ndash, 8 showed a decrease in surface area and N2 and CO2 uptake, with respect to pure ZIF&minus, 8. Alignment of m&ndash, 8 in Matrimid showed the presence of enriched pathways of m&ndash, 8 through the MMMs. At 10 wt.% m&ndash, 8 incorporation, no effect of alignment was observed for the N2 and CO2 permeability, which was ascribed anon-ideal tortuous alignment. However, alignment of 20 wt.% m&ndash, 8 in Matrimid showed to increase the CO2 diffusivity and permeability (19%) at 7 bar, while no loss in ideal selectivity was observed, with respect to homogeneously dispersed m&ndash, 8 membranes. Thus, the alignment of MOF particles throughout the matrix was shown to enhance the CO2 permeability at a certain weight content of MOF.

Application of forward osmosis (FO) is limited due to membrane fouling and, most importantly, high reverse salt fluxes that deteriorate the concentrated product. Polydopamine (PDA) is a widely used, easily applicable, hydrophilic, adhesive antifouling coating. Among the coating parameters, surprisingly, the effect of PDA coating temperature on the membrane properties has not been well studied. Polyethersulfone (PES) 30 kDa ultrafiltration membranes were PDA-coated with varying dopamine concentrations (0.5&ndash, 3 g/L) and coating temperatures (4&ndash, 55 &deg, C). The quality of the applied coating has been determined by surface properties, water permeability and reverse salt flux using a 1.2 M MgSO4 draw solution. The coating thickness increased both with the dopamine concentration and coating temperature, the latter having a remarkably stronger effect resulting in a higher PDA deposition speed and smaller PDA aggregates. In dead‑end stirred cell, the membranes coated at 55 &deg, C with 2.0 g/L dopamine showed NaCl and MgSO4 retentions of 41% and 93%, respectively. In crossflow FO, a low reverse MgSO4 flux (0.34 g/m2&middot, h) was found making a very low specific reverse salt flux (Js/Jw) of 0.08 g/L, which outperformed the commercial CTA FO membranes, showing the strong benefit of high temperature PDA-coated PES membranes to assure high quality products.

A variety of biomedical applications requires tailored membranes; fabrication through a mix-and-match approach is simple and desired. Polymers based on supramolecular bis-urea (BU) moieties are capable of modular integration through directed non-covalent stacking. Here, it is proposed that non-cell adhesive properties can be introduced in polycaprolactone-BU-based membranes by the addition of poly(ethylene glycol) (PEG)-BU during immersion precipitation membrane fabrication, while unmodified PEG is not retained in the membrane. PEG-BU addition results in denser membranes with a similar pore size compared to pristine membranes, while PEG addition induces defect formation. Infrared spectroscopy and surface hydrophobicity measurements indicate that PEG-BU is retained during membrane processing. Additionally, PEG-BU incorporation successfully leads to poor cell adhesive surfaces. No evidence is observed to indicate PEG retention. The results obtained indicate that the BU system enables intimate mixing of BU-modified polymers after processing. Collectively, the results provide the first steps toward BU-based immersion precipitated supramolecular membranes for biomedical applications.

Reverse electrodialysis (RED) is a technology producing renewable energy from the mixing of river and seawater. In natural salinity gradients, multivalent ions are present, which lead to a reduced RED power output. Transport of multivalent ions against the concentration gradient and their trapping inside the membranes leads to a lower driving force and increased membrane resistance. The present work focuses on the effect of sulfate ions on anion exchange membranes in RED. A monovalent ion selective membrane ability to retain a higher open circuit voltage is offset by the higher resistance in the presence of sulfate, leading to losses in normalized power outputs (−25%) comparable to a standard grade membrane. Longer term experiments revealed that membrane resistance increases over time. This study highlights the need to address uphill transport, resistance increase, and decreased permselectivity of anion exchange membranes in presence of multivalent ions.

Because of the impracticality of long life time testing of hydrogen bromine flow batteries (HBFBs) under real, cyclic operating conditions, we utilized a high-frequency cycling load accelerated lifetime test (ALT) to evaluate the membrane electrode assembly performance in HBFBs. Four different membrane chemistries were tested to assess the relative long-term performance and durability of the corresponding HBFBs and to understand the main long-term failure mechanism in HBFB technology. The high-frequency cycling load ALT is a highly valuable method to assess long-term HBFB (components) stability and to understand the degradation/failure mechanism. The results showed that the utilization of long side chain perfluorosulfonic acid (LSC PFSA, i.e. Nafion®) membranes result in stable HBFB operation until a sudden cell failure at the end of the cycle life. The use of a more selective (lower bromine species crossover) reinforced LSC PFSA membrane results in approx. five times longer life times. In contrast, the use of a grafted polyvinylidene fluoride (SPVDF) membrane results in a slow incremental performance decrease (or area resistance increase) during the ALT and, again, a sudden cell failure at the end of the cycle life. After the ALT, the LSC PFSA membrane still shows high chemical stability. On the other hand, the grafted SPVDF and SPE membranes show clear membrane degradation visible as a strong decrease in IEC and an increase in ohmic AR. Gradual Pt catalyst degradation-dissolution, that results in insufficient Pt catalyst loading and subsequently low hydrogen reaction kinetics, is the main failure mechanism of the HBFBs. The membrane bromine species crossover rate is directly related to the rate of Pt catalyst degradation-dissolution and scales almost linearly with the cell total ampere-hours. As long as the catalyst loading is sufficient and does not reach a minimum value, this observed Pt degradation-dissolution rate does not significantly impact the cell performance.

Electricity storage is essential for the transition to sustainable energy sources. Hydrogen-bromine flow batteries (HBFBs) are promising cost-effective energy storage systems. In HBFBs, proton exchange membranes are required to separate the two reactive materials, enabling proton transport for charge balancing. In this paper, we present a comprehensive overview of the key properties and an experimental performance map of cation exchange membranes for HBFBs. Our study shows that membrane water uptake is an important property due to its strong correlation with membrane resistance and bromide species crossover. Long chain perfluorosulfonic acid (LC PFSA) membranes are shown to have a better power density–crossover tradeoff and a higher stability than other types of functionalized membranes. The good power density-crossover tradeoff of LC PFSA membranes is the result of the high level of separation of hydrophobic and hydrophilic domains in the membrane, leading to well-connected ionic pathways for proton transport. Reinforcement of long chain LC PFSA membranes further improves their tradeoff because it mechanically constrains the swelling (lower water uptake), resulting in a lower crossover but a similar peak power density. Consequently, reinforced LC PFSA membranes are the most promising option for HBFBs.

Divalent cations in feedwater can cause significant decreases in efficiencies for membrane processes, such as reverse electrodialysis (RED). In RED, power is harvested from the mixing of river and seawater, and the obtainable voltage is reduced and the resistance is increased if divalent cations are present. The power density of the RED process can be improved by removing divalent cations from the fresh water. Here, we study divalent cation removal from fresh water using seawater as draw solution in a Donnan dialysis (DD) process. In this way, a membrane system with neither chemicals nor electrodes but only natural salinity gradients can be used to exchange divalent cations. For DD, the permselectivity of the cation exchange membrane is found to be crucial as it determines the ability to block salt leakage (also referred to as co-ion transport). Operating DD using a membrane stack achieved a 76% reduction in the divalent cation content in natural fresh water with residence times of just a few seconds. DD pretreated fresh water was then used in a RED process, which showed improved gross and net power densities of 9.0 and 6.3%, respectively. This improvement is caused by a lower fresh water resistance (at similar open circuit voltages), due to exchange of divalent for monovalent cations., This paper described a sustainable method of divalent cation exchange that avoids chemical and power usage to enhance renewable energy harvesting from natural salinity gradients.

Reverse electrodialysis (RED) is a sustainable method to harvest energy using the salinity gradient between fresh and seawater. RED technology is developing but efficiencies are still limited when using natural feed water sources. One significant constraint is induced by the presence of multivalent ions in sea and river water (i.e. Mg2+, Ca2+, SO4 2-). Uphill transport and an increase in membrane resistance in the presence of magnesium ions significantly reduce the power density output obtainable. The choice of cation exchange membrane determines the magnesium transport and as such the power density. Here we investigate four cation exchange membrane types and relate their properties to the stack performance using three different magnesium concentrations on either river and/or seawater side: 1) a highly cross-linked styrene-divinyl benzene monovalent selective cation exchange membrane (Neosepta CMS); 2) a monovalent selective cation exchange membrane that contains a thin polyethyleneimine (PEI) anion exchange layer (Selemion CSO); 3) a multivalent ion (e.g. magnesium) permeable cation exchange membrane with an engineered molecularly open structure facilitating the transport of multivalent ions as recently developed (T1 Fujifilm); 4) a standard cation exchange membrane (Type I Fujifilm (reference)). The first two membranes both retain magnesium ions, while the other two membranes are considered permeable for magnesium ions. The results show that power density strongly depends on the composition of both river and seawater. Power density decreases in the presence of magnesium, an effect being strongest with magnesium at both river and seawater side, followed by the river water side and the seawater side. The negative effect of multivalent ion transport against the concentration gradient, so called uphill transport, in RED can be significantly minimized when monovalent selective membranes such as the highly cross-linked Neosepta CMS membrane or the AEM coated Selemion CSO membrane are used. However, the use of such membranes directly results in a strong increase in membrane resistance due to the lower ion mobility of magnesium ions inside these membranes. As a consequence, power densities in RED are not improved. Especially at high magnesium concentrations, this effect is very strong at higher concentrations, the membranes are no longer able to retain magnesium ions effectively. More beneficial is the application of multivalent permeable membranes with a more ‘open’ structure that allow the free movement of both sodium and magnesium ions through the membrane. Maybe somewhat counter intuitively, such membranes (especially the Fujifilm multivalent permeable T1 membrane) have low resistance values combined with reasonable OCV values leading to high power densities under almost all magnesium concentrations, especially at long term applications. Highest power densities well exceeding 0.3 W/m2 are still obtained when only sodium is present. However, when magnesium ions are present power densities in the order of 0.2–0.25 W/m2 can still be obtained for these membranes.

Forward osmosis (FO) is an emerging process to dewater whey streams energy efficiently. The driving force for the process is the concentration gradient between the feed (FS) and the concentrated draw (DS) solution. Here we investigate not only the effect of the DS concentration on the performance, but also that of the FS is varied to maintain equal driving force at different absolute concentrations. Experiments with clean water as feed reveal a flux increase at higher osmotic pressure. When high product purities and thus low reverse salt fluxes are required, operation at lower DS concentrations is preferred. Whey as FS induces severe initial flux decline due to instantaneous protein fouling of the membrane. This is mostly due to reversible fouling, and to a smaller extent to irreversible fouling. Concentration factors in the range of 1.2–1.3 are obtained. When 0.5 M NaCl is added to whey as FS, clearly lower fluxes are obtained due to more severe concentration polarization. Multiple runs over longer times show though that irreversible fouling is fully suppressed due to salting in/out effects and flux decline is the result of reversible fouling only.

Reverse electrodialysis (RED) is a membrane-based renewable energy technology that can harvest energy from salinity gradients. The anticipated feed streams are natural river and seawater, both of which contain not only monovalent ions but also divalent ions. However, RED using feed streams containing divalent ions experiences lower power densities because of both uphill transport and increased membrane resistance. In this study, we investigate the effects of divalent cations (Mg(2+) and Ca(2+)) on RED and demonstrate the mitigation of those effects using both novel and existing commercial cation exchange membranes (CEMs). Monovalent-selective Neosepta CMS is known to block divalent cations transport and can therefore mitigate reductions in stack voltage. The new multivalent-permeable Fuji T1 is able to transport divalent cations without a major increase in resistance. Both strategies significantly improve power densities compared to standard-grade CEMs when performing RED using streams containing divalent cations.

To understand and mitigate the role of surface water composition and associated membrane fouling in the removal of nC60 nanoparticles by low-pressure membranes, experiments were carried out with microfiltration membranes using natural feed waters, mimicking separation in real industrial water treatment plants. The effects of water composition, the presence of nanoparticles, and membrane fouling were investigated with a dead-end bench-scale system operated under constant flux conditions including a hydraulic backwash cleaning procedure. nC60 nanoparticles can be efficiently removed by microfiltration and the removal efficiency is found to be independent of the water surface composition. However, the water composition controls the extent of fouling occurring during filtration. A synergistic effect on membrane fouling between nC60 and surface water constituents such as natural organic matter (NOM) and its fractions is observed: the synergistic effect resulted in a transmembrane pressure (TMP) increase always higher than the sum of the TMP increase due to the filtration of nC60 in ultrapure water and the TMP increase due to the surface water without nC60.

Spacer-less RED stacks using membranes with integrated spacer profiles have been investigated during the last years to eliminate the spacer shadow effect. The presence of spacers partially blocks the membrane surface and creates a tortuous and thus longer path for ions in the channel, meaning higher ohmic resistance. Consequently, power outputs are reduced. Profiled membranes may solve this problem as they provide flow channels for the feed streams, while the relief formed on their surfaces keeps the membranes separated. Although the geometry and arrangement of so far used profiles led to lower ohmic resistance, it did not grant an efficient fluid mixing. Recently, so-called chevron profiles, with enhanced mixing, were proposed based on computational fluid dynamics (CFD) simulations. In the present study, the performance of such chevron-profiled membranes, prepared by thermal pressing, was experimentally validated in a reverse electrodialysis (RED) stack. According to the obtained experimental values of non-ohmic resistance and total pressure drop across the RED stack, the chevron-profiled membranes assure efficient fluid mixing at comparatively low hydraulic losses. The net power density obtained with chevron-profiled membranes was the highest obtained for the present stack design. It outperformed the alternative RED stack configurations investigated in this study, such as channels with optimized spacers and channels formed by pillar-profiled membranes. To allow for an even more straightforward and efficient RED stack assembling with chevron-profiled membranes, recommendations for a further simplified design, consisting of diagonal ridges that are assembled perpendicularly, are provided.

A homologous series of 12 all-aromatic PEI membranes was investigated with the aim to understand how subtle changes in the PEI main-chain affect the carbon dioxide/methane (CO2/CH4) gas separation performance. The 3-ring diamines selected for this study are either para-, meta- or ortho-aryloxy substituted with respect to the central benzene ring, i.e. 1,4-bis(4-aminophenoxy)benzene (P1), 1,3-bis(4-aminophenoxy)benzene (M1) and 1,2-bis(4-aminophenoxy)benzene (O1). Doing so changes the backbone geometry from a more linear to a more kinked conformation. In addition, four dianhydrides were selected with the aim to tailor the segmental mobility and hence the free volume of the PEIs, i.e. pyromellitic dianhydride (PMDA), 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA) and 3,3′,4,4′-oxydiphthalic dianhydride (ODPA). We have investigated how subtle changes in these prototypical PEIs affect membrane critical performance criteria such as CO2 permeability, CO2/CH4 selectivity and ability to withstand high operating pressures. In ODPA-based membranes the CO2 permeability decreases in the order P1 > O1 > M1 and remains steady throughout measurements with mixed feed pressures up to 40 bar, however, the selectivity decreases for ODPA-O1 and ODPA-M1. For high-pressure applications, the OPDA-P1 membrane is a good candidate with a selectivity of 48, permeability of CO2 of 0.74 Barrer and ability to resist plasticization up to 40 bar of total pressure (16 bar of CO2 partial pressure). Alternatively, for applications up to 10 bar of total mixed feed (5 bar of CO2 partial pressure), BPDA-O1 is a promising candidate because this membrane displays a high selectivity of 70 and permeability of 1.3 Barrer.

Electrodialysis (ED) is an important process to desalinate brackish water, which contains scale-forming divalent ions. Hence, there is an interest in separating mono- and divalent ions. In recent years, layer-by-layer (LbL) polyelectrolyte coatings on ion exchange membranes (IEMs) have provided high monovalent ion selectivities. However, there is a lack of understanding on the structure-property relationships for polyelectrolyte multilayers for membrane applications and lack of evaluation under practical conditions. In this work, we evaluate polyelectrolyte multilayer properties by optical techniques, connect them to monovalent-selectivity using resistance measurements and upscale this to practical monovalent-selectivities for an ED unit. Excess positive poly(allyl amine) (PAH) was observed in the PAH/PSS (polystyrene sulfonate) multilayer, resulting in a positively charged multilayer. Coated multilayers with low hydrations (0.2) are able to achieve high monovalent-selectivities (of 5.7 up to 7.8) on CEMs comparable to the commercial monovalent-selective CSO (with 6.9). This enhanced selectivity was only observed for cations with LbL-coated CEMs and not for anions with LbL-coated AEMs, which we argue is due to the excess positive charge of the PAH/PSS multilayers. Finally, LbL-coatings show an improved monovalent-selectivity in ED stack experiments with artificial brackish water of 1.7 for LbL-coated CEMs compared to uncoated CEMs (with a monovalent-selectivity of 0.5).

Zeolitic imidazolate frameworks (ZIFs), a subclass of metal–organic frameworks (MOFs), have been widely investigated as additive in mixed matrix membranes (MMMs), but systematic studies to identify critical intrinsic ZIF-type parameters that determine the MMM performance are lacking. Therefore, three isoreticular gmelinite (GME) ZIFs, i.e., ZIF-68, 69 and 78 that are distinct by their partial different imidazolate linkers, were incorporated in Matrimid matrices to systematically elucidate the effects of the varying pore aperture, pore diameter, pore volume and functionality of the ZIFs on the CO2/N2 and CO2/CH4 MMM mixed gas permeability. The incorporation of the ZIFs in Matrimid increases the CO2 permeability for all MMMs for both CO2/N2 and CO2/CH4 feed mixtures without compromising the selectivity. The highest increase in CO2 permeability is observed for the 20 wt% ZIF-68 MMM, with an increase in CO2 permeability of 116% for CO2/N2 feed and 122% for CO2/CH4 feed relative to Matrimid was achieved and surpassed the permeability of conventional ZIF-8/Matrimid MMMs. Regarding the isoreticular ZIF series, the results showed that for a higher permeability a larger pore aperture, diameter and volume of the ZIFs in the MMMs are more beneficial than a more polar functionality with smaller pore size, aperture and volume.

International audience; This manuscript encompasses the investigation into the solvent resistant nanofiltration (SRNF) performance of multilayer membranes prepared from weak polyelectrolytes. These weak polyelectrolytes are unique in that the charge density is not fixed and depends on the coating pH, adding an extra variable as tuning parameter for SRNF performance. The weak polyelectrolyte based multilayers (PEMs) were prepared on a hydrolyzed PAN support membrane from poly(allylamine hydrochloride) (PAH) as polycation and poly(acrylic acid) (PAA) as polyanion. Detailed investigations on the role of the pH of the coating solution on the performance of the prepared SRNF-membranes were carried out with organic dyes of different size (~300–1000 Da) and charge. Variation in pH of the coating solutions was found to lead to a large degree of control over the separation performance of the prepared SRNF-membranes for the different dyes. The solvent permeabilities and the dye retentions were measured and correlated to variations in the PEM membrane structures, with more dye adsorption being found for membranes with more free acid and amine groups. The membranes were also found to be stable for long term-filtrations in solvents such as isopropyl alcohol (IPA), acetonitrile (ACN), tetrahydrofuran (THF) and in the challenging polar aprotic solvent, N,N-dimethylformamide (DMF). Results of this study clearly demonstrate the potential of using pH as tuning parameter for weak PEMs to prepare SRNF-membranes optimized for specific applications

Block copolymer (BCP) membranes are a very promising new type of membranes, but often have a difficult fabrication method that involves a long evaporation step prior to phase inversion. In this work, we study new novel BCP phase inversion recipes for the fabrication of asymmetric membranes with a thin, ordered isoporous selective layer on top of a highly interconnected porous support layer. A key aim is to shorten the evaporation time while simultaneously allowing the formation of even thinner selective top layers. Asymmetric membranes were fabricated via the combination of polystyrene-block-poly(4-vinyl pyridine) self-assembly, solvent evaporation and liquid induced phase separation. Using a solvent mixture of THF and NMP, a selective top layer of just 60 nm thick was formed with an ordered honeycomb-like pore structure. The formed structure depended on several parameters, such as THF/NMP ratio, polymer concentration of the polymer solution and the duration of solvent evaporation. When a high THF/NMP ratio was used (more THF than NMP) the solvent evaporation step could be reduced to only 1 s, a clear advantage when considering scale up of this approach. The THF/NMP ratio also influenced the morphology of the support layer, which translated into a variety of permeabilities (270–1320 L m−2 h−1 bar−1). Filtration experiments showed that the different top layer structures result in different filtration performance, with more ordered pores resulting in more selective filtration.

Blend membranes of sulfonated poly(ether ether ketone) (SPEEK) and poly(arylene ether ketone) (PAEK) derivatives containing crown ether units in the main chain (CPAEK) were prepared and characterized in terms of water swelling and ion exchange capacity (IEC). The miscibility of the polymers was verified by DSC and HR-SEM. Ion transport characteristics of the membranes were established for the monovalent ions Li+ and K+ and the separation of these ions by the cation exchange membranes was investigated. Diffusion experiments for aqueous KCl, LiCl and their mixtures were carried out with pure SPEEK membranes as well as with the CPAEK/SPEEK membranes. Blending significantly decreased the ion permeability due to cation-crown ether complexation and increased the hydrophobicity of the matrix. The K+ over Li+ selectivity of the SPEEK membrane was enhanced by blending SPEEK with CPAEK by a factor of nearly 4, indicating that the presence of a crown ether polymer changes the relative transport of the ions in the membrane.

Energy consumption for seawater desalination by multistage electrodialysis (ED) is lowered last decade from 6.6 kWh/m3 to 3.6 kWh/m3. In multistage ED the driving force can be adapted to the conditions of that specific desalination stage. In this study however, we varied the membrane area, the residence time, and the membrane properties in the different stages to investigate the transport mechanisms of salt and osmotic water to improve the desalination performance of multistage ED even further. Residence time affects both salt fluxes and osmotic transport. We showed that a longer residence time is beneficial in the first stages, in combination with a shorter residence time in the later stages. The gradient in the later stages between the diluate and concentrate compartments is large, and a short residence time decreases the undesired osmotic flow, resulting in lower product (diluate) loss. Moreover, the use of membranes with lower water permeability in the last three stages results in state-of-the-art energy consumptions of 2.2 kWh/m3 for the desalination of 510 mM NaCl to 5.4 mM NaCl using multistage ED. This obtained diluate salinity more than meets the WHO standard for drinking water of 8.5 mM NaCl.

This work presents a detailed investigation into the long term stability of polyelectrolyte multilayer (PEM) modified membranes, a key factor for the application of these membranes in water purification processes. Although PEM modified membranes have been frequently investigated, their long term stability, critical for application, has not been considered up till now. We focus on both the physical stability of the multilayer on different membranes as well as on the chemical degradation of two different multilayers in the presence of sodium hypochlorite. Two different polymeric ultrafiltration membranes are modified to become dense nanofiltration membranes by applying a thin (PEM) coating on the membrane via the Layer-by-Layer technique. During sequential backwash cycles, no performance loss is observed for PEM modified membranes based on sulfonated poly(ether sulfone) (SPES). On the other hand, PEM modified membranes based on the non-ionic poly(ether sulfone) (PES) show a gradual increase in permeability and loss in retention after each backwash cycle. We demonstrate that a PEM on an ultrafiltration membrane that bears ionic charges has superior adhesion to the substrate, ensuring long term stability. In addition, the chemical stability of two different multilayers is assessed by means of the resistance against sodium hypochlorite degradation. An important factor in the chemical stability is the type of polycation. Membranes coated with multilayers based on the primary polycation poly(allylamine) hydrochloride (PAH) show a loss in performance after 24,000. ppm hours NaOCl (pH 8). Membranes coated with multilayers based on the quaternary polycation poly(diallyldimethylammonium) chloride (PDADMAC) are stable for more than 100,000. ppm hours NaOCl (pH 8), which is an excellent stability, comparable to that of commercial PES ultra- and microfiltration membranes.

Membrane technology proves to be effective in the removal of nano-sized contaminants from water. However, not much is known on the filtration and fouling behavior of manufactured nanoparticles.The high surface-area-to-volume ratio of nanoparticles, significantly increases the effect of surface interactions on the stability of nanoparticle suspensions. Also, the stability of nanoparticle suspensions and their tendency to aggregate strongly affects the fouling mechanism during membrane filtration of nanoparticles. In this experimental study, fouling development and rejection mechanisms of mono-disperse silica model nanoparticles were investigated in great detail.A microfiltration hollow fiber membrane was employed in dead-end filtration mode for the filtration of commercially available silica nanoparticles under constant pressure. By applying a low concentration of nanoparticles and a large difference between the membrane pore size (∼200 nm) and the nominal size of the nanoparticles (22 nm), a detailed investigation of the fouling mechanisms was allowed. Five subsequent fouling stages were postulated: adsorption, unrestricted transport through pores, pore blocking, cake filtration and cake maturation. Higher concentrations of nanoparticles did not change the behavior of these fouling stages, but were found to lead to their acceleration. Fouling severity and occurrence of dynamic transitions between these fouling stages were quantitatively evaluated. The presence of salts, pH and valency of the cation strongly influenced nanoparticle properties and interactions and thus occurrence and character of the fouling stages. Lower repulsive interactions between the nanoparticles accelerate fouling by faster pore blockage and aggregation on the membrane surface. Porosity and permeability of the formed filtration cake layer are strongly dependent on the repulsive interactions between nanoparticles, with a lower repulsion leading to denser cake layers. This paper clearly shows that fouling development and rejection of nanoparticles by microfiltration membranes easily can be adjusted by tuning the electrostatic interactions between the suspended nanoparticles

Biofouling is still a major challenge in the application of nanofiltration and reverse osmosis membranes. Here we present a platform approach for environmentally friendly biofouling control using a combination of a hydrogel-coated feed spacer and two-phase flow cleaning. Neutral (polyHEMA-co-PEG(10)MA), cationic (polyDMAEMA) and anionic (polySPMA) hydrogels have been successfully grafted onto polypropylene (PP) feed spacers via plasma-mediated UV-polymerization. These coatings maintained their chemical stability after 7 days incubation in neutral (pH 7), acidic (pH 5) and basic (pH 9) environments. Anti-biofouling properties of these coatings were evaluated by Escherichia coli attachment assay and nanofiltration experiments at a TMP of 600 kPag using tap water with additional nutrients as feed and by using optical coherence tomography. Especially the anionic polySPMA-coated PP feed spacer shows reduced attachment of E. coli and biofouling in the spacer-filled narrow channels resulting in delayed biofilm growth. Employing this highly hydrophilic coating during removal of biofouling by two-phase flow cleaning also showed enhanced cleaning efficiency, feed channel pressure drop and flux recoveries. The strong hydrophilic nature and the presence of negative charge on polySPMA are most probably responsible for the improved antifouling behavior. A combination of polySPMA-coated PP feed spacers and two-phase flow cleaning therefore is promising and an environmentally friendly approach to control biofouling in NF/RO systems employing spiral-wound membrane modules. Funding Agencies|Dutch Ministry of Economic Affairs; Ministry of Infrastructure and Environment; European Union Regional Development Fund; Province of Fryslan; Northern Netherlands Provinces; European Community [237997]

The increasing use of engineered nanoparticles in customer products results in their accumulation in water sources. In this experimental study, we investigated the role of surfactant type (cationic, anionic and non-ionic) and concentration on fouling development, nanoparticle rejection and fouling irreversibility during dead-end ultrafiltration of model silica nanoparticles. Our work demonstrates that the type of surfactant influences the nanoparticle stability, which in turn is responsible for differences in fouling behavior of the nanoparticles. Moreover, the surfactant itself interacts with the PES-PVP membrane and contributes to the fouling as well. We have shown that anionic SDS (sodium dodecylsulfate) does not interact extensively with the negatively charged silica nanoparticles and does not change significantly the surface charge and size of these nanoparticles. Adsorption of the cationic CTAB (cetyltrimethylammonium bromide) onto the silica nanoparticles causes charge transition and nanoparticle aggregation, whereas non-ionic TX-100 (Triton X-100) neutralizes the surface charge of the nanoparticles but does not change significantly the nanoparticle size. The most severe fouling development was observed for the silica nanoparticle – TX-100 system, where nanoparticles in the filtration cake formed exhibited the lowest repulsive interactions. Rejection of the nanoparticles was also highest for the mixture containing silica nanoparticles and TX-100.

This study introduces the concept of ion selective actuation in polymer metal composite actuators, employing crown ether bearing aromatic polyether materials. For this purpose, sulfonated poly(arylene ether ketone) (SPAEK) and crown ether containing SPAEK with molar masses suitable for membrane preparation are synthesized. The synthesized polymers are characterized using Nuclear magnetic resonance (NMR) and fourier transform infrared spectroscopy (FTIR) spectroscopy, thermal gravimetric analysis (TGA), and differential scanning calorimetry (DSC). Ionic polymer metal composite (IPMC) actuators are fabricated by electroless chemical deposition of a platinum (Pt) layer on both sides of SPAEK and crown-ether containing SPAEK membranes, resulting in electrode layers of around 120 nm thickness. Actuation experiments demonstrate cation specific responses and bending degrees of the IPMC actuators. Incorporation of crown ether units in the polymer backbone results in an improved and ion-selective bending displacement compared with SPAEK actuators. S(25)C(50)PAEK actuators show an increased bending displacement of 28% for Na+ and 20% for K+ ions.

This paper describes fouling in simulated Point-of-Use (PoU) systems based on low pressure hollow fiber ultrafiltration membranes. Various operational configurations such as recirculation of feed, discontinuous vs. continuous filtration, and inside/out vs. outside/in were compared to study their effects on fouling and permeate production. Flux values stabilized around 2L/m2h for gravity driven (100mbar) ultrafiltration. Intermittent operation resulted in lower overall hydraulic resistances compared to continuously operated systems. This was due to the low organic loading and relaxation of the fouling layer during periods of standstill. In most experiments the fouling layer mainly consisted of diatoms, inorganic particles and few microbial clusters. The PoU systems investigated can be operated for longer duration without the need for strong chemical cleaning. &nbsp

Hollow fiber nanofiltration membranes can withstand much higher foulant concentrations than their spiral wound counterparts and can be used in water purification without pretreatment. Still, the preparation of hollow fiber nanofiltration membranes is much less established. In this work, we demonstrate the design of a hollow fiber nanofiltration membrane with excellent rejection properties by alternatively coating a porous ultrafiltration membrane with a polycation, a polyzwitterion, and a polyanion. On model surfaces, we show, for the first time, that the polyzwitterion poly N-(3-sulfopropyl)-N-(methacryloxyethyl)-N,N-dimethylammonium betaine (PSBMA) can be incorporated into traditional polyelectrolyte multilayers based on poly(styrenesulfonate) (PSS) and poly(diallyldimethylammonium chloride) (PDADMAC). Furthermore, work on model surfaces allows a good characterization of, and insight into, the layer build-up and helps to establish the optimal membrane coating conditions. Membranes coated with these multilayers have high salt rejection of up to 42% NaCl, 72% CaCl2, and 98% Na2SO4 with permeabilities of 3.7-4.5 l·m-2·h-1·bar-1. In addition to the salt rejections, the rejection of six distinctively different micropollutants, with molecular weights between 215 and 362 g·mol-1, was investigated. Depending on the terminating layer, the incorporation of the polyzwitterion in the multilayer results in nanofiltration membranes that show excellent retentions for both positively and negatively charged micropollutants, a behavior that is attributed to dielectric exclusion of the solutes. Our approach of combining model surfaces with membrane performance measurements provides unique insights into the properties of polyzwitterion-containing multilayers and their applications.

Reverse electrodialysis (RED) is a technology for extracting salinity gradient power by contacting waters with different salinity, i.e. seawater and river water, through ion exchange membranes. Conventionally, non-conductive spacers are used to separate these ion exchange membranes from each other in RED. The power output is hampered by these non-conductive elements which increase electrical resistance in the RED stack. To eliminate the use of these spacers, structured anion exchange membranes with a structure height of 100 µm were prepared by casting a polymer solution on stainless steel molds followed by solvent evaporation. These self-standing membranes with straight-ridge, wave and pillar structures as well as similarly prepared flat membranes were installed on the river water side in a RED stack (where electrical resistance is the highest). 38% higher gross power density and 20% higher net power density were achieved with the pillar-structured membranes when compared to that of flat membranes with spacers. Further optimization of the structure geometry in combination with the possibility to cast membranes of different chemistries offer a huge potential for further development of homogeneous membranes with the desired electrochemical and physical properties, which could provide high power densities in RED

Reverse electrodialysis (RED) is a process that can be used to generate energy from salinity gradients. Since its application in practice requires the use of natural seawater and river water, the presence of multivalent ions is inevitable, but this currently limits RED performance. Membranes with selectivity for monovalent ions may overcome this limitation. Standard ion exchange membranes have low monovalent-ion selectivity. We used a relatively fast method to coat a standard commercial anion exchange membrane to improve its monovalent-ion selectivity. The coating layer was formed by copolymerization of 2-acryloylamido-2-methylpropanesulfonic acid (AMPS) as the active polymer and N,N-methylenebis(acrylamide) (MBA) as the crosslinker, using UV irradiation. The monovalent ion selectivity of the resulting membranes was comparable to that of commercial monovalent-selective membranes. Furthermore, the modified membranes with their negatively charged coating showed increased hydrophilicity and exhibited sufficient antifouling potential against organic foulants. When they were tested in an RED stack, their performance was found to depend especially on the proportion of the divalent ions (sulfate) in the river water stream. However, the use of the currently available monovalent selective membranes was not found to be very effective for obtaining higher gross power densities in RED

Energy is released when feed waters with different salinity mix. This energy can be captured in reverse electrodialysis (RED). This paper examines experimentally the effect of varying feed water concentrations on a RED system in terms of permselectivity of the membrane, energy efficiency, power density and electrical resistance. Salt concentrations ranging from 0.01 M to 5 M were used simultaneously in two stacks with identical specifications, providing an overview of potential applications. Results show a decrease of both permselectivity and energy efficiency with higher salt concentrations and higher gradients. Conversely, power density increases when higher gradients are used. The resistance contribution of concentration change in the bulk solution, spacers and the boundary layer is more significant for lower concentrations and gradients, while membrane resistance is dominant for high concentrations. Increasing temperature has a negative effect on permselectivity and energy efficiency, but is beneficial for power density. A power density of 6.7 W/m(2) is achieved using 0.01 M against 5 Mat 60 degrees C. The results suggest that there is no single way to improve the performance of a RED system for all concentrations. Improvements are therefore subject to the specific priorities of the application and the salt concentration levels used. Regarding ion exchange membranes, higher salinity gradients would benefit most from a higher fixed charge density to reduce co-ion transport, while lower salinity gradients benefit from a thicker membrane to decrease the osmotic flux. (C) 2013 Elsevier Ltd. All rights reserved.

This paper presents a thorough investigation of a simple method to decrease the dimensions of polymeric microsieves. Pore sizes of microsieves are usually in the micrometer scale, but need to be reduced to below 1 µm to make the microsieves attractive for aqueous filtration applications. In this work polyethersulfone/polyvinylpyrrolidone microsieves were prepared with pores with diameters in the range of 2–8 µm (perforated pores) and a very open internal structure containing many voids. Subsequently, size reduction in terms of pore size and periodicity of the perforated pores was performed by immersing microsieves in mixtures of acetone and N-methylpyrrolidone. Microsieves shrunk because of swelling and weakening of the polymers, and subsequently collapse of the open internal structure. Shrinking typically occurred in two stages: first, a stage where both the perforated pores and periodicity shrink, and second, a stage where the perforated pores continue to shrink while the periodicity remains constant. Microsieves with initial pore diameters of 2.6 µm were reduced down to only 0.2 µm. The maximum isotropic shrinkage was ~35%, which was determined by the amount of voids in the polymer matrix. We propose that to come to higher (nearly) isotropic shrinkage, the amount of voids should be further increased.