Journal of Membrane Science Letters最新文献

The solution-diffusion (SD) model has been instrumental in the advancement of membrane science, due to its simplicity, transparency, and utility in process engineering. However, some doubts have recently been raised, concerning the fundamental validity of SD. These have largely been based on apparent discrepancies between molecular dynamics simulations and several features, deemed inherent to SD, that appeared in early reports — namely, the exact nature of the pressure and concentration distributions within the membrane. Herein, we re-visit the underlying physics of SD in the context of composite membranes, making no a-priori assumptions and, particularly, highlighting the role of polymer thermodynamics and the mechanics of a loaded, swollen film, supported by a porous substrate. The analysis provides a coherent view, linking the solvent concentration profile within the film and the resultant flux-pressure relations with the polymer rigidity and, importantly, the way in which the film is supported. It is shown that, although the flux may generally vary non-linearly with the feed pressure and depend on the film-support geometry, for rigid films – most common in real operations – SD predicts a linear behavior, virtually independent of specific geometry and pressure distribution. Moving forward, we stress the importance and need for further refinements of the SD model, driven by insight from molecular dynamics, thermodynamics and mechanics, while maintaining its applicability to process design.

Background

Superabsorbent polymers (SAPs) have a remarkable ability to absorb significant quantities of water. However, their absorption capacity is significantly reduced when exposed to saline solutions, such as urine, due to the polyelectrolyte effect and charge screening.

Methods

In this study, we demonstrate a zwitterionic superabsorbent polymer (ZSAP) with excellent salt-water absorption and retention capacities. ZSAP was synthesized by grafting a copolymer of p(sulfobetaine methacrylate-co-2-hydroxyethyl methacrylate) (p(SBMA-co-HEMA)) onto an acrylic acid (AA)-based hydrogel via free-radical polymerization. The introduction of zwitterionic SBMA significantly enhances the hydrophilicity of the polymer, particularly in a saline solution due to the anti-polyelectrolyte effect, thereby accelerating the rate of salt absorption. Additionally, the hydroxyl groups from HEMA facilitate the formation of covalent bonds with the AA network membrane through esterification, effectively mitigating polymer leaching. The hydration/dehydration behaviors of linear polymers were measured using the dynamic vapor sorption (DVS) method. Moreover, the salt-water absorption capacity, centrifuge retention capacity (CRC), and absorbency under load (AUL) of ZSAP with various SBMA moieties and copolymer dosages were comprehensively evaluated in a 0.9 wt% sodium chloride solution. Additionally, the water retention under different temperatures and polymer leaching of ZSAP were investigated.

Significant Findings

The copolymer p(SBMA-co-HEMA) not only demonstrates a high salt-water absorption rate at 90% RH in a 0.9 wt% NaCl solution but also exhibits superior water retention at 0% RH compared to the AA polymer. Moreover, the ZSAP exhibits superior salt-water absorption capacity and AUL in a 0.9 wt% NaCl solution compared to conventional AA-based SAP. Additionally, the introduction of the hydroxyl moiety from the p(SBMA-co-HEMA) copolymer reduces free polymer leaching from ZSAP. This work presents an approach for the development of new SAP with high salt-water absorption and retention.

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.