Journal of Membrane Science Letters最新文献

Pore wetting is a major constraint to the performance of membrane distillation (MD) for hypersaline brine treatment. Despite the existence of surfactants with diverse properties, an explicit relationship between the properties of surfactants and their capabilities of inducing pore wetting has yet to be established. In this study, we perform a comparative analysis of the wetting behaviors of various surfactants with different charges and molecular weights in MD desalination. The induction time of surfactants to initiate pore wetting was correlated to the apparent contact angle and surface tension of the feedwater. Our results show that different surfactants resulting in similar feedwater surface tensions can lead to drastically different wetting potential, suggesting that both charge of the head group and molecular weight of surfactants have a significant influence on membrane pore wetting. Further, we demonstrate that parameters that have been commonly used to indicate wetting potential, including apparent contact angle and solution surface tension, are not reliable in predicting the wetting behavior of MD membranes, which is intricately linked with surfactant properties such as charge and molecular size. We envision that our results not only improve our fundamental understanding of surfactant-induced wetting but also provide valuable insights that necessitate thorough consideration of surfactant properties in evaluating wetting potential and membrane wetting resistance for MD desalination.

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.

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.

Vanadium acetylacetonate (V(acac)₃) disproportionation electrochemistry promises a crossover-tolerant, high-voltage flow battery, but exhibits low efficiency and short cycle life. We show that membrane fouling, rather than a parasitic side reaction, dominates early performance fade. Crossover rates through porous membranes were estimated from voltage transients with an adaptive observer while cycling flow-through reactors. For $0.1\text{M}$ V(acac)₃ and $0.3\text{M}$ TEABF₄ in acetonitrile flowed countercurrently at $5.0\text{cm}{\text{s}}^{-1}$ parallel to the separator, fresh Daramic 175 and Celgard 4650 afforded active-species mass-transfer coefficients of $3.8\mu \text{m}{\text{s}}^{-1}$ and $7.5\mu \text{m}{\text{s}}^{-1}$, respectively, which decreased and became non-Fickian as cycling progressed. At $\pm 10\text{mA}{\text{cm}}^{-2}$ from 0%–20% state of charge, voltage efficiency with Celgard fell from 96% to 60% over 27 cycles. Separator replacement restored the coulombic and voltage efficiencies, which repeated their first progression. Impedance spectra from series-connected canary cells reveal that separator resistances remain stable during open-circuit exposure to charged single electrolytes, but increase under applied current or open-circuit contact with differently charged electrolytes.

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.

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.

New method of emulsion synthesis of Nafion®-type copolymer composition by using nanodiamond platform has been proposed and implemented. Produced polymeric coagulate saturated with diamonds (4.1 % wt.) possessed increased ionic capacity of the copolymer comparative to the analogue without diamonds. SEM patterns for coagulate membranes showed labyrinthine structures with diamonds integrated into copolymer without any segregation. This structuring provided necessary elastic and strength properties of new type membranes for hydrogen fuel cells. In new membranes synchrotron experiments exhibited a network of ionic channels which ensured a proton conductivity by one order of magnitude higher than that for the analogue produced of premade components.

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.

Membrane distillation (MD) is a separation technology for many industries including desalination, pharmaceuticals, and food processing. However, MD technology readiness has not reached the required level to penetrate the desalination and water treatment market. One of the challenges to commercialization is the limited development and inaccurate assessment of MD-specific membranes. In fact, measuring the performance of MD membranes is challenging because it is dependent on process parameters, making it difficult to separate the individual influences of the process operating conditions and the membranes’ intrinsic properties. These shortcomings drive the need for a standardized methodology to compare and report membrane performance independently of the process parameters. In this work, we propose a standardized methodology for measuring the permeance of MD membranes using a reduced scale direct contact membrane distillation (DCMD) setup. This methodology has the potential to streamline membrane assessment and support ongoing efforts in MD membrane development and process scale-up.

Controlling the internal concentration polarization in forward osmosis (FO) membranes by minimizing the substrate thickness is critical to enhancing the water flux. This study aimed to achieve the fabrication of an ultra-thin FO membrane by forming the polyamide (PA) active layer on a thin and straight-bore film, a so-called track-etched (TE) membrane. The polycarbonate TE membrane had a uniform pore size of 0.22 µm and a thickness of 25 µm. The PA active layer was successfully formed only by creating a thin m-phenylenediamine solution layer on the smooth TE membrane surface before interfacial polymerization. The TE- FO membrane with low porosity (14 %) provided a water flux of 21 L/m²h and a reverse salt flux of 8.0 g/m²h when evaluated with a 1.0 M NaCl draw solution. Further evaluations showed the potential of increasing water flux by increasing the TE substrate porosity (14 %) and reducing the apparent PA active layer thickness (504 nm). These results suggest the potential of achieving a high-water flux FO membrane using a thin TE substrate and ultimately improving the validity of FO membrane-based water treatment.