Morphological changes and creep recovery behavior of expanded polytetrafluoroethylene (ePTFE) membranes used for membrane distillation

Compaction of Pressure-Driven Water Treatment Membranes: Real-Time Quantification and Analysis

Water treatment membranes play crucial roles in applications such as desalination, wastewater treatment, and potable water reuse. In a prior study, we introduced a novel method, combining electrical impedance spectroscopy with dynamic mechanical analysis, to quantify single-layer homogeneous membrane compaction up to 12.5 psi. Now we extend the method's capabilities to quantify real-time compaction of multilayer heterogeneous nanofiltration and reverse osmosis (RO) membranes up to 330 psi. Our findings demonstrate that membrane compaction does not solely occur in the support/backing layer. The air pockets between the polysulfone support and the polyester backing layers, which were not discussed previously, account for up to 18% and 14% of total membrane compaction for the nanofiltration and RO membranes. For the nanofiltration membrane, the majority of compaction (up to 45%) occurs in the void spaces of the backing layer, while for the RO membrane, the majority of compaction (up to 40%) occurs in the solid material of the backing layer. We also confirm, with experimental results, the importance of using compressive testing instead of tensile testing to accurately characterize compaction. Membrane fatigue is characterized by experimental trends including: increasing irrevocable compaction, increasing creep/instantaneous compaction ratios, and increasing strains in hysteresis experiments.

Design of Robust FEP Porous Ultrafiltration Membranes by Electrospinning-Sintered Technology

Perfluoropolymer membranes are widely used because of their good environmental adaptability. Herein, the ultrafine fibrous FEP porous membranes were fabricated with electrospinning-sintered technology. The effects of PVA content and sintering temperature on the fabricated membranes’ morphologies and properties were investigated. The results indicate that a kind of dimensionally stable network structure was formed in the obtained ultrafine fibrous FEP porous membranes after sintering the nascent ultrafine fibrous FEP/PVA membranes. The optimal sintering conditions were obtained by comparing the membranes’ performance in terms of membrane morphology, hydrophobicity, mechanical strength, and porosity. When the sintering temperature was 300 °C for 10 min, the porosity, water contact angle, and liquid entry pressure of the membrane were 62.7%, 124.2° ± 2.1°, and 0.18 MPa, respectively. Moreover, the ultrafine fibrous FEP porous membrane at the optimal sintering conditions was tested in vacuum membrane distillation with a permeate flux of 15.1 L·m⁻²·h⁻¹ and a salt rejection of 97.99%. Consequently, the ultrafine fibrous FEP porous membrane might be applied in the seawater desalination field.

Interplay of the Factors Affecting Water Flux and Salt Rejection in Membrane Distillation: A State-of-the-Art Critical Review

High water flux and elevated rejection of salts and contaminants are two primary goals for membrane distillation (MD). It is imperative to study the factors affecting water flux and solute transport in MD, the fundamental mechanisms, and practical applications to improve system performance. In this review, we analyzed in-depth the effects of membrane characteristics (e.g., membrane pore size and distribution, porosity, tortuosity, membrane thickness, hydrophobicity, and liquid entry pressure), feed solution composition (e.g., salts, non-volatile and volatile organics, surfactants such as non-ionic and ionic types, trace organic compounds, natural organic matter, and viscosity), and operating conditions (e.g., temperature, flow velocity, and membrane degradation during long-term operation). Intrinsic interactions between the feed solution and the membrane due to hydrophobic interaction and/or electro-interaction (electro-repulsion and adsorption on membrane surface) were also discussed. The interplay among the factors was developed to qualitatively predict water flux and salt rejection considering feed solution, membrane properties, and operating conditions. This review provides a structured understanding of the intrinsic mechanisms of the factors affecting mass transport, heat transfer, and salt rejection in MD and the intra-relationship between these factors from a systematic perspective.

Facile Preparation of a Superhydrophobic iPP Microporous Membrane with Micron-Submicron Hierarchical Structures for Membrane Distillation

A facile method combining micro-molding with thermally-induced phase separation (TIPS) to prepare superhydrophobic isotacticpolypropylene (iPP) microporous membranes with micron-submicron hierarchical structures is proposed in this paper. In this study, the hydrophobicity of the membrane was controlled by changing the size of micro-structures on the micro-structured mold and the temperature of the cooling bath. The best superhydrophobicity was achieved with a high water contact angle (WCA) of 161° and roll-off angle of 2°, which was similar to the lotus effect. The permeability of the membrane was greatly improved and the mechanical properties were maintained. The membrane prepared by the new method and subjected to 60h vacuum membrane distillation (VMD) was compared with a conventional iPP membrane prepared via the TIPS process. The flux of the former membrane was 31.2 kg/m2·h, and salt rejection was always higher than 99.95%, which was obviously higher than that of the latter membrane. The deposition of surface fouling on the former membrane was less and loose, and that of the latter membrane was greater and steady, which was attributed to the micron-submicron hierarchical structure of the former and the single submicron-structure of the latter. Additionally, the new method is expected to become a feasible and economical method for producing an ideal membrane for membrane distillation (MD) on a large scale.