Journal of Membrane Science Letters最新文献

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.

Surfactant-induced wetting impedes the practical implementation of membrane distillation (MD). Addressing this issue demands the development of an effective membrane cleaning strategy that can eliminate surfactants adhering to the membrane surface and restore the membrane hydrophobicity. However, current cleaning methods, such as direct drying and pressurized air backwashing, encounter challenges in thoroughly removing surfactants trapped within the pores while preserving the structural integrity of the membrane. This work presents a refined approach to conquer surfactant-induced wetting in MD by water flushing. Utilizing ultrasonic time domain reflectometry and optical coherence tomography techniques, we identified a critical cleaning depth and showed that the hydrophobicity of a partially wetted membrane can be fully recovered by water flushing when the wetting depth is below the critical threshold. Theoretical models evidenced that in instances of low water temperature and low flow rate conditions, relatively high critical cleaning depths can be realized, thereby expanding the operational scope for achieving complete hydrophobicity recovery. Our results demonstrated the applicability of water flushing to commercial membrane modules without necessitating any modification, emphasizing its substantial potential for advancing MD applications.

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.

Understanding salt and water transport mechanisms in reverse osmosis (RO) under high pressures and salinities is critical to advancing RO-based brine management technologies. In this study, we investigate the dependence of salt permeance and partitioning on feed salinity and applied pressure. Salt partitioning coefficients were determined using a novel high-pressure quartz crystal microbalance (QCM), and salt permeances were collected using a lab-scale high-pressure dead-end cell. Our results show that salt permeance decreases with respect to feed concentration, in contrast to conventional theories for charged RO membranes. We further show salt partitioning coefficients do not change with applied hydrostatic pressure but are dependent on feed salt concentration. We use non-equilibrium molecular dynamics simulations to show that these trends are explained by salinity and pressure-induced changes to the structure of the polyamide layer, namely osmotic deswelling and compaction. Changes in the polyamide layer thickness and pore size alter the frictional interactions of ions, affecting membrane performance at larger salinities and pressures. These results provide new insights on how structure-performance relationships affect salt transport at higher pressures.

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.

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.