Journal of Membrane Science Letters最新文献

This investigation attempts to establish and verify a novel method for quantifying surface porosity of porous polymeric membranes via contact angle measurements. Herein, we fabricate a series of porous membranes via nonsolvent induced phase separation (NIPS) comprising different concentrations of polyvinylidene fluoride (PVDF) and PVDF-poly (methyl methacrylate) block co-polymer (PVDF-PMMA) with different concentrations of water and isopropyl alcohol (IPA) in the coagulation bath. Both sessile drop and captive bubble contact angle measurements are used to determine contact angles (and porosity) for both dry and wet membranes, respectively. The former method is probably applicable for membrane distillation, aeration and de-aeration where liquid water does not saturate the membrane, whereas the latter may be more indicative of pressure-driven aqueous membrane separations where the membrane is saturated through its cross-section. Image analysis of scanning electron microscope (SEM) images quantified dry membrane surface porosity. We propose a simple analytical model to obtain wet and dry membrane surface porosity from contact angle measurements. Our results suggest that the surface porosity calculated from both wet and dry contact angle data correlates strongly with the surface porosity calculated from SEM values. However, wet contact angles of the membranes with high porosities produce significantly higher porosity values, which also establishes the importance of porous membrane swelling in determining membrane porosity for aqueous membrane separations.

Membrane separation is widely used in food, pharmaceutical and water treatment industries but suffers a longstanding challenge of fouling. In this article, acoustically excited microstructures are demonstrated as a new mechanism to mitigate membrane fouling and remove cake layer aggregations formed on a microfluidic membrane-on-chip device. With acoustic streaming induced by oscillating microstructures near the membrane surface, cake layer fouling was effectively broken up and removed on the acoustofluidic membrane separation device within 100 milliseconds. The device is simple to fabricate and offers direct observation of crossflow microfiltration across the device membrane, giving valuable insight to particle fouling events often unobtainable in traditional membrane device configurations. The device bolsters advantages like label-free and reagent-free particle separation and in situ membrane cleaning during separation, providing a new mechanism for membrane separation applications used across industry.

Herein, we report a new class of biosourced nanofiltration membranes based on chemically recyclable aliphatic polyesters (P(4,5-T6GBL)) and the use of green solvents. Given their chemical recyclability and potential biodegradability, these polyester membranes were designed to have a sustainable lifecycle. The effect of membrane thickness and solvent/non-solvent diffusivity on membrane morphology and organic solvent nanofiltration were investigated. Long-term membrane stability was tested in a continuous crow-flow filtration rig over a week, which exhibited stable methanol permeance at 8.6 ± 0.1 L m⁻² h⁻¹ bar⁻¹. The rejection profiles of the pharmaceuticals oleuropein (540 g mol⁻¹) and roxithromycin (837 g mol⁻¹) were also found to be stable at 87% and 100%, respectively.

Reverse osmosis (RO) is the most important membrane technology for the desalination of water. Measured water and salt fluxes are traditionally analyzed in the context of the solution-diffusion (SD) model which leads to a water permeability, A, and a salt permeability, B. However, this parametrization of the salt flux is not correct for water desalination by RO membranes, because these membranes show markedly different retentions for different feed salt concentrations, a classical observation in the literature, and this effect is not captured by the SD model. Thus, the traditional salt permeability B is not an intrinsic property of these membranes. We present a new analysis for desalination of a 1:1 salt, which follows from a transport theory that is based on the assumption that coions are strongly excluded from the membrane, and we demonstrate that it accurately describes a large dataset of salt retention by an RO membrane as function of pressure and feed salt concentration. This analysis leads to unique values of the water and salt permeabilities, A and $B^{″}$, not dependent on salt concentration or permeate water flux. Because we now have an improved parametrization, we can more accurately compare different membranes or study in more detail how membrane performance depends on conditions such as salt type and temperature. The new equation can provide guidance for the design of high-performance desalination membranes and for process modeling of desalination systems.

This work describes a new approach for synthesizing extremely fouling-resistant, zwitterionic membranes with controlled, tunable pore sizes that extend from ion separations (< 1 nm) to the ultrafiltration range (∼2 nm). These membranes are manufactured by the UV treatment of high zwitterion content amphiphilic copolymers with cross-linkable functionality to stabilize the membrane selective layers, preventing excessive swelling and dissolution of copolymers containing as high as 80 wt% zwitterionic repeat units. Zwitterion weight fraction allows the tuning of membrane performance, with effective pore size and permeance both increasing with zwitterion content. The high zwitterion content membranes were remarkably fouling-resistant and demonstrated a salt-responsive behavior not previously observed with self-assembling zwitterionic copolymer membranes.

A novel all-carbon backbone-based membrane is designed by introducing side chains at the non-coplanar site of twisted “ether-free” main chain via Suzuki coupling reaction. The twisted backbone reduces the hindrance effect, providing broader mobile space for the side chains and enhancing the swing ability of the side chains to facilitate the formation of ion channels and the transportation of OH⁻. As a result, the high conductivity is obtained at a relatively low IEC level. The QPS-PB-4 membrane exhibits a superior OH⁻ conductivity of 50.1 to 94.4 mS cm⁻¹ at 30 ℃ to 80 ℃ with an IEC of only 1.48 mmol g⁻¹, and a low swelling ratio of less than 10%. Which show significant advantage among the traditional side-chain-type AEMs reported in recent years. Moreover, the as-prepared membranes have good mechanical and thermal stability, as well as excellent chemical stability because of the all-carbon backbone designed without any sensitive sites that can be attacked by hydroxide. The conductivity of the QPS-PB-4 membrane decrease by only 8% after treatment at 80 ℃ in 1 M NaOH for 1800 h. The fuel cell assembled with the as-prepared membrane has a peak power density of up to 558.8 mW cm⁻², indicating the promising application potential of the membranes.

Three crystalline truxene-based β-ketoenamine COF membranes (TFP-HETTA, TFP-HBTTA and TFP-HHTTA) are fabricated via a de novo monomer design approach to understand the fundamental correlations between pore structure and molecular separation performance. By introducing bulky alkyl groups into the truxene framework, the pore size of TFP-HETTA, TFP-HBTTA, and TFP-HHTTA are systematically tuned from 1.08 to 0.72 nm. Accordingly, the TFP-HETTA showed good water permeance of 47 L m⁻² h⁻¹ bar⁻¹ along with a prominent rejection rate of Reactive Blue (RB, 800 Da) but less than 10% rejection rate of inorganic salts. In contrast, the TFP-HHTTA membrane with pore size of 0.72 nm can reject small dye molecules such as Safranin O (SO, 350 Da) and trivalent salts but with a moderate water permeance of 19 L m⁻² h⁻¹ bar⁻¹. The pore-flow model rooted from the viscous flow could well fit the observed organic solvent nanofiltration results of all three COF membranes.

The fractionation of complex liquid hydrocarbon mixtures is an important and emerging area of membrane science. Polymeric asymmetric hollow fiber membranes have the potential to be used for this purpose, especially if the size and number of defects in the membrane skin layer can be precisely engineered. Here, we fabricated various “defect-engineered” Torlon hollow fiber membranes by modifying hollow fiber spinning conditions and spin dopes to study the role of skin layer defects in the organic solvent reverse osmosis (OSRO) membranes. The quality of the membranes was investigated using several sets of pure gas permeation experiments, which provided input data for a permeation resistance model that estimates the pore size and surface porosity of the asymmetric hollow fiber membrane. We develop and experimentally validate a resistance permeation model for solvent permeation and utilize the surface properties derived from the gas permeation experiments to estimate the relative permeation rates of solvents in a mixture. The approach outlined here highlights the interconnection between gas permeation analysis and OSRO separation performance using Torlon hollow fiber membranes as an exemplar test case. The solvent permeation model is then utilized to provide quantitative insight on the differences between OSRO and organic solvent nanofiltration (OSN), and highlight the important transition region between these two modalities.

The separation of H₂/CH₄ or CO₂/CH₄ is critical to the purification of natural gas. Herein, we report on novel membranes with a metal-organic framework of CAU-10-PDC for the separation of these two mixtures. The dense CAU-10-PDC membranes are fabricated on a porous alumina support using the seeded growth method. An unexpected increase in selectivity was observed while testing mixed gas permeation with either H₂/CH₄ or CO₂/CH₄ at a molar ratio of 50:50. Steady-state selectivity reached 101 for H₂/CH₄ and 62 for CO₂/CH₄. Ideal selectivity measured from single gas permeation reached 475 for H₂/CH₄ and 288 for CO₂/CH₄. Molecular dynamics simulations and time-resolved X-ray diffraction with a synchrotron radiation source were used to probe conformational changes in CAU-10-PDC induced by exposure to CH₄. When exposed to an atmosphere containing CH₄, CAU-10-PDC presented a change in the space group (from I4₁/amd to I4₁), which drastically reduced the pore limiting diameter from 4.15 to 2.95 Å, rendering the channel nearly impermeable to CH₄.