Journal of Membrane Science Letters最新文献

Removing salinity has always been a challenge for wastewater treatment. Utilizing nanofiltration (NF) membranes is a promising approach. However, currently available NF membranes are less effective in monovalent salt removal. In this study, work toward the initial aim of fabricating charge mosaic membranes led to charge-patterned NF-selective films on polyether sulfone (PES) or polyacrylonitrile (PAN) support membranes, with similar rejection for mono- and divalent salts. The membranes were fabricated by a two-step layer assembly of first negatively charged polystyrene sulfonate (PSS) particles immobilized in a polyvinyl alcohol (PVA) layer, followed by coating a positively charged polyethyleneimine (PEI) layer. Both PVA and PEI were crosslinked using glutaraldehyde that had initially been impregnated into the support membrane. The type of support membrane, nanoparticle, PVA, and PEI concentrations during fabrication, as well as feed pH and salt concentration, play significant roles in separation performance of obtained composite membranes. Charge-patterned NF membranes fabricated using 0.5 wt.% PVA and 0.05 wt.% PSS for assembly of the first layer followed by coating 0.5 wt.% PEI solution had even somewhat higher rejection for monovalent salt (NaCl; ∼82%) compared to multivalent salts (Na₂SO₄, MgSO₄, and MgCl₂; ∼74%), at a permeance of 5.5 LMH/bar on the PES and 3.1 LMH/bar on the PAN support membrane.

As a vapor pressure-driven process, pervaporation (PV) shares several of the advantages of membrane distillation (MD), such as the ability to tackle high salinity waters and the possibility of integrating low grade heat sources to reduce energy consumption. Membrane scaling and pore wetting remain strong limitations to the implementation of MD desalination. In comparison, dense, non-porous PV membranes are considered. In this study, PV membranes made from NEXAR^TM, a sulfonated pentablock copolymer, were evaluated and compared to polytetrafluoroethylene (PTFE) MD membranes in a vacuum configuration. The membranes were tested using three solutions: 32 g L^-1 sodium chloride (NaCl), a brackish water (8.4 g L^-1) of high scaling potential, and 5.5 g L^-1 NaCl with 1 mM sodium dodecyl sulfate. The NEXAR^TM membrane achieved a permeance of 93.1±44.6 kg m^-2 h^-1 bar^-1 for the 32 g L^-1 brine, which was almost 20% higher than the PTFE MD membrane. This permeance decreased in the presence of foulants; however, in contrast with the MD membrane, where scaling and surfactants induced pore wetting, the salt rejection for the NEXAR^TM PV membrane was constant at >99% for all water types. These results emphasize the robustness of PV as a process to deal with challenging saline waters.

New blend membranes consisting of a tuned ratio of polyvinylidene fluoride (PVDF) and alkali lignin (AL) were studied. Through the use of a green solvent like dimethyl sulfoxide, effective mixing between PVDF and AL was achieved, leading to the development of highly hydrophilic membranes with robust mechanical stability. Characterization methods confirmed the suitability of the blend for membrane preparation and its hydrophilic nature.

A key aspect of the strategy involved hydrophilizing PVDF during the preparation process by blending it with AL in the pot. This approach aimed to streamline production by reducing the number of steps compared to post-treatment methods such as grafting or coating. The presence of hydrophobic/hydrophilic groups in the AL structure addressed the challenge of compatibility between PVDF and conventional hydrophilic polymers, enhancing interaction between the components.

The resulting hydrophilic material exhibited improved pure water permeance and demonstrated resistance to irreversible fouling. The membrane's ability to process wastewater streams and its resistance to fouling was demonstrated by separating stable and uniform submicron oil-in-water emulsions with high rejection (>99.9 %) up to a volume reduction factor (VRF) of 7.7.

Electrokinetic measurements to determine the electrical properties (zeta potential) of membrane surfaces have become increasingly popular in the toolbox of characterization techniques. However, it has been established in the literature that parasitic phenomena such as electrokinetic leakage can hamper data interpretation, leading to not only quantitative but also qualitative errors in membrane zeta potential determination. To date, the only method for highlighting and accounting for electrokinetic leakage is limited to flat-sheet membranes. In this letter, we propose an alternative method that is much less time-consuming and applicable to all membrane geometries. This method is based on the determination of the electrokinetic index, which we define as the ratio of the apparent zeta potentials determined from single measurements of the streaming current and streaming potential coefficients. We show that variation in the electrokinetic index reflects modifications occurring within the membrane matrix (in addition to surface properties alteration). The chemical degradation of polyethersulfone (PES)-based flat-sheet and hollow-fiber membranes is used as a proof of concept, but the proposed approach is readily transposable to other problems of practical interest, such as e.g. membrane fouling. This work also paves the way for the development of a new type of electrokinetic sensors for on-line monitoring of membrane operations.

While membrane separation has proven to be an outstanding method for separating oil in water (O/W) emulsions containing droplets smaller than 20 µm, it is severely limited due to fouling. Much research has been aimed at understanding the mechanism behind membrane fouling, particularly for ultrafiltration (UF) membranes. Interestingly, studies pointed out that the emulsifier, namely surfactant, is the main source of fouling in nanofiltration and reverse osmosis, while in the case of UF, oil is generally regarded as the main source of fouling. Herein, we study the fouling of UF membranes during separation of O/W emulsions stabilized by surfactants, with the explicit goal of determining the relative impact of the oil and surfactant present on fouling severity and dynamics. Results obtained from flux decline measurements, complimented by visualization using confocal microscopy, show that oil causes irreversible fouling to a certain extent, however, surfactant fouling dominates the observed membrane performance. The degree of fouling and flux recovery appears to be closely related to the properties the surfactant, namely charge and molecular weight, as has been observed in the past but attributed to the oil-membrane interactions, mediated by the surfactant. Further, to visualize the fouling mechanisms, direct observation via a confocal microscope set-up is used to capture real-time images of the membrane surface, which reveal that surface coverage of oil is not directly related to flux decline during the separation process. Our results suggest that membrane flux decline and fouling is dominated by membrane-surfactant interactions, the exact nature of which is a topic for future extensions of this work.

Novel membrane materials developed in research labs struggle to gain widespread industrial adoption due in part to insufficient reproducibility and unreliable performance data. Open-source hardware approaches, especially 3D printing, enable the democratization of automation within a laboratory setting, and in the context of membranes, can minimize the inherent variability associated with manual methods of membrane casting. In this study, the native hardware and firmware of an inexpensive, conventional 3D printer was extensively modified for the purpose of flat-sheet membrane casting. Replicate poly (ether-ether ketone) (PEEK) membranes were cast with a thickness coefficient of variation of ∼10 % using the modified device. Cast membranes were used to assess the importance of controlling shear rate by characterizing both intra- and inter-film variability. Statistical differences in pure water permeability were observed across tested shear rates, with distinct morphological changes occurring to the membrane substructure. Overall, the technology developed in this study is shown to be an extremely useful approach for improving the process of developing membranes at the bench-scale.

Solid and liquid products can form in the gas phase of membrane contactors applied to reactive ternary systems for CO₂ absorption, which poses a critical barrier for carbon capture applications. The mechanism initiating these unwanted phase changes in the gas phase is unclear. This study therefore systematically characterises CO₂ absorption in distinct regions of the vapour-liquid equilibrium (VLE) within an illustrative ternary system (CO₂-NH₃-H₂O), to provide an explanation for the formation and mitigation of these solid and liquid products in the gas-phase. Unstable CO₂ absorption and increased pressure drop indicated product formation within the gas-phase, which occurred at high CO₂ capture ratios. Temporal analysis of gas-phase composition enabled gas-phase products to be related to the relative ternary composition. This was subsequently correlated to distinct regions of the VLE. Consequently, mitigation strategies can be developed with recognition for where products are least likely to form. Pressurisation was proposed to modify the relative gas-phase ammonia composition to reposition conditions within the VLE. The commensurate increase of CO₂ into the solvent shifts the ammonia-ammonium equilibrium towards ammonium to indirectly reduce vapour pressure. This synergistic strategy allows sustained operation of membrane contactors for CO₂ separation within reactive ternary systems which are critical to delivering carbon capture economically at scale.

This study systematically examines the influence of polymeric membrane sample preparation techniques on their morphologies and structures as revealed by scanning electron microscopy (SEM). We address the variability introduced by diverse preparation methods in research, which leads to subjective qualitative and quantitative SEM interpretations. Our investigation encompasses various preparation techniques, focusing on cryogenic sectioning—alongside SEM operational parameters including accelerating voltage and conductive sputter coating thickness. We demonstrate that surface morphology analysis via SEM is significantly affected by coating thickness and accelerating voltage, while cross-sectional images (typically, at much higher magnification) exhibit little difference in morphology. However, improper preparation can damage membranes, compromising cross-sectional imaging. We provide a detailed exploration of the cryogenic-sectioning and its effects on SEM image quality. Our findings indicate one's selection of preparation procedure can create significant biases in SEM analyses of microfiltration, ultrafiltration, and reverse osmosis polymeric membranes.