Pore Size Distribution Of Membranes Research Articles

Charge‐Sign‐Independent Separation of Mono‐ and Divalent Ions With Nanofiltration Membranes

AbstractAchieving precise selective separation of monovalent and divalent cations, as well as anions, is vital yet challenging in practical applications involving complex component treatments. Current membranes are typically effective for separating either cation pairs or anion pairs. To address this issue, a straightforward strategy for fabricating a nanofiltration (NF) membrane is developed that selectively permeates monovalent ions. This study focused on neutralizing the surface charge and tuning the pore size distribution of the polyamide membranes through a secondary interfacial polymerization using a zwitterionic copolymer consisting of 2‐methacryloyloxyethyl phosphorylcholine and 2‐aminoethyl methacrylate hydrochloride. The optimized NF membrane prepared in this study, with a near‐neutrally charged membrane surface and appropriate pore size distribution, demonstrates favorable performance in precisely separating monovalent and divalent ions, irrespective of the ion charge sign. The optimum NF membrane features high selectivity for both Cl−/SO42− (93) and Li+/Mg2+ (67) ion pairs, along with high water permeance of 8.5 L m−2 h−1 bar−1, making it competitive with many reported membranes. This study offers new insights into the ion‐selective mechanisms of polyamide membranes for monovalent/divalent ions and may guide the development of advanced membranes with single‐solute selectivity.

Insights into the pore size distribution of amorphous silica membranes using gas permeation activation energies

Silica-based membranes exhibit significant promise for the separation of hydrogen from other larger gas molecules based on size sieving mechanism, particularly when employed in membrane reactors. Nevertheless, the migration of silanol bonds (Si–OH) formed under hydrothermal conditions leads to alterations in pore size, ultimately compromising the performance of the membrane. Therefore, this study focuses on determining the pore size of cobalt-doped silica membranes before and after hydrothermal treatment by the apparent activation energy of gas permeation. The Oscillator model and the effective medium theory are employed to estimate the potential pore size distribution, as well as to calculate the apparent activation energy and permeability. The calculated apparent activation energy is compared with experimental data to identify the most probable pore size distribution, which showed the minimum activation energy error to the experimental value. The calculated permeability based on the identified pore size distribution is in line with experimental permeability, which validated the identified pore size distribution. Since silica-based membrane is generally applied in hydrothermal conditions, our model successfully identifies the changes in pore size of silica-based membranes after hydrothermal treatment. The results demonstrated that hydrothermal treatment significantly impacts the pore size of silica-based membranes. Specifically, 5-membered rings are prevalent in the intact membrane, but after hydrothermal treatment, there is a gradual shift of the pore size distribution towards larger pores, potentially leading to a decrease in sieving performance. This methodology presents a promising approach for determining intriguing pore size information of porous materials.

Mechanistic insights into the separation behaviors of polyamide desalination membranes for emerging micropollutants removal

As emerging micropollutants (EMPs) become a global environmental concern, it is crucial to devise energy-efficient methods that can effectively remove EMPs from water. Polyamide thin-film composite (PA-TFC) membranes are state-of-the-art water separation membranes and their applications for EMPs separations have been increasingly conscripted in recent years. However, a comprehensive fundamental understanding of the underlying relationship between membrane microstructures and EMPs transport across the PA-TFC membranes remains enigmatic. To address this gap, we thoroughly investigated the separation behaviors of two PA-TFC membranes with distinct PA microstructures toward a wide spectrum of EMPs with various dimensions and chemistries. By reconciling experimental evidence and density-functional theory (DFT), we disclosed that the water permeance of the PA-TFC membrane is predominately governed by the effective filtration area associated with surface protuberances and the intrinsic wall thickness of the PA microlayers. Precise control of the solute–PA interactions is explicitly important for membrane selectivity toward EMPs, which deserves more attention in future membrane design besides synchronously regulating membrane pore size and size distribution. This work represents a conspicuous step forward in the fundamental understanding of the structure–performance relationship of PA-TFC membranes for EMPs separations, which would inspire the rational construction and structural regulation of high-performance membranes for new pollutants removal.

Custom-tailoring zwitterion-based nanofiltration membranes for organic molecule desalination with sub-200 Da molecular weight

Organic molecule desalination with nanofiltration has important practical application significance. Zwitterion-based nanofiltration membranes (ZNFMs) with selective channels for monovalent salts have been prepared to improve the separation performance. However, most ZNFMs tend to be restricted by their loose structures, which results in unsatisfactory rejection for small molecules with sub-200 Da molecular weight. In this study, a synchronous modification strategy during the heat treatment of interfacial polymerization was proposed, in which various multi-amine solutions were employed as the novel medium of heat treatment to regulate the structures of ZNFMs. The pore size distribution of membranes could be narrowed to a certain degree through cross-linking by corresponding multi-amines. The optimized ZNFMs exhibited excellent retention to small molecules. For example, the retention of glucose was dramatically increased to 96.5%. Meanwhile, the ZNFMs also had low rejection for monovalent salts, such as KCl (26.7 %). Consequently, the separation factor between glucose and KCl reached 21.2, which demonstrated a great potential for organic molecule desalination with sub-200 Da molecular weight.

Novel bentonite modified PVDF membrane for desalination of synthetic-produced water by vacuum membrane distillation

ABSTRACT Membrane distillation (MD) has emerged as a highly promising method for the treatment of intricate and heavily contaminated water sources, such as those containing oily wastewater. The present study endeavors to advance the field by designing and evaluating a novel nanocomposite membrane comprised polyvinylidene fluoride (PVDF) and Bentonite (BNT) for application in vacuum membrane distillation (VMD) processes targeted at treating oily wastewater. A series of five nanocomposite flat sheet membranes were meticulously fabricated, incorporating varying quantities of bentonite (ranging from 0 to 0.8 g). Systematic characterizations were meticulously undertaken to comprehensively elucidate the influence of BNT incorporation on the structural and functional attributes of the developed membranes. These encompassed an array of analytical techniques, including Fourier Transform Infrared Spectroscopy (FTIR), atomic Force Microscopy (AFM), X-Ray diffraction (XRD), field emission scanning electron microscopy (FESEM), contact angle measurements (CA), as well as assessments of membrane thickness, porosity, pore size, and pore size distribution. The findings distinctly showcased the successful integration of BNT within the PVDF matrix, which correspondingly led to discernable augmentations in membrane surface roughness, hydrophobicity, and porosity. Employing consistent operational conditions involving a feed temperature of 50°C, a vacuum pressure of 30 kPa, and a NaCl solution with a concentration of 14.75 g/L, VMD experiments were systematically conducted. The empirical outcomes revealed that the incorporation of BNT notably conferred substantial enhancements upon the nanocomposite membrane’s performance. The pinnacle achievement was evidenced in a flux rate of 34 L/m2.h and a remarkable NaCl rejection rate of 99.92%, a result achieved by the nanocomposite membrane incorporating the highest content of BNT additives.

Chlorine degradation of semi-aromatic polypiperazine-amide membranes and the mechanisms

Semi-aromatic polypiperazine-amide (PPA) nanofiltration (NF) membranes are widely used in water treatment applications, but the resistance to free chlorine and other biocidal agents need to be improved. Understanding the chlorination mechanisms of PPA NF membranes is of great significance for avoiding membrane degradation and developing new chlorine-resistant membranes. In this paper, the effects of chlorination on the rejection selectivity as well as the surface charge, hydrophilicity and roughness, and pore size distribution of PPA NF membranes are presented. In addition, the chlorination mechanisms of the membranes are proposed through functional groups characterization of the stripped PPA active layer, element composition analysis, quantum mechanical calculations based on density-functional theory, and use of model compound ((3,5-dimethylphenyl)(4-methylpiperazin-1-yl)methanone) for PPA material. The molecular active chlorine species, HOCl, predominantly induces the cleavage of C–N bonds, whereas the primary reaction pathway caused by the ionic active chlorine species, ClO−, involves the ring-opening of benzene. Under the same chlorination conditions, HClO has a more severe impact on membrane properties and performance than ClO−. NF membranes with a looser polyamide layer are more susceptible to the attack by active chlorine species.

Tailoring the pore size distribution of nanofiltration membranes via surfactants with different alkyl chain lengths: Towards efficient molecular separation

The diffusion of amine monomer during interfacial polymerization (IP) directly affects the structure and properties of the polyamide (PA) separation layer. Surfactants can regulate the diffusion rate of monomers and change the microstructure of the PA layer. In this study, a PA nanofiltration (NF) membrane with the tailor-made pore size is fabricated by modulating the diffusion rate of 1, 4, 7, 10-tetraazacyclododecane (Cyclen). Surfactants with different alkyl chain lengths are used to precisely regulate the diffusion rate of Cyclen monomer. The longer the alkyl chain length of the surfactant is, the faster the diffusion rate of the Cyclen monomer is, resulting in the formation of a PA layer with a narrow pore size distribution. The prepared composite membrane is qualified for on-demand desalting and antibiotic/salt separation performance by changing the length of the alkyl chain. For example, surfactants with short alkane chains (C4) endow the NF membranes with a high water permeance of 30.7 L−1 m−2h−1 bar−1 for tetracycline separation. Surfactants with long alkane chains (C12) endow the NF membranes with a high selectivity of 84.8 for tetracycline/NaCl separation. This study provides further evidence for surfactant-regulated IP reactions and offers new ideas for the tailor-made regulation of NF membrane properties.

Development of essential oil-containing antimicrobial and deodorizing nanofibrous membranes for sanitary napkin applications

In this study, antimicrobial and deodorizing nanofibrous composite membranes based on natural substances were developed for use in feminine sanitary napkins. Plant-derived natural essential oils, such as lemongrass oil and May Chang oil, were incorporated into polyvinyl alcohol nanofibers using emulsion electrospinning. The fiber morphology, oil distribution, and pore size distribution of the nanofibrous composite membranes were examined. The antimicrobial and deodorizing effects, tensile properties, and release behavior of the functional ingredients from the composite membranes were investigated to examine their use in sanitary napkins. Core–sheath nanofibers, in which lemongrass oil or May Chang oil was uniformly distributed within the fiber core, were fabricated using various oil contents. The essential oil-loaded nanofibrous composite membranes contained pores ranging from 0.130 to 1.349 µm in size. The release profiles of the essential oils from the composite membranes over an 8 h period demonstrated a continuous release of citral and limonene. Composite membranes containing either lemongrass oil or May Chang oil exhibited outstanding antimicrobial effects against vaginal pathogens, such as Candida albicans and Staphylococcus aureus, despite their low oil content. Deodorizing effects against ammonia gas were observed at oil contents higher than that required to achieve antimicrobial effects. The composite membranes exhibited lower Young’s modulus and greater elongation at break values under wet conditions compared to dry conditions. Our findings demonstrate that antimicrobial and deodorizing nanofibrous composite membranes based on plant-derived essential oils have the potential for use in feminine hygiene products.

New method for measuring the pore sizes and pore size distributions of filter membranes-the fluorescence probe method.

Anovel, simple, and rapid method is demonstratedfor measuring the pore size and pore size distribution of filtration membranes (FMs) used in aqueous applications with fluorescence probes. Because the selected fluorescent probes are mixable and have strong signals, combined with the operation of dead-end filtration, this method only requires small amounts of reagents; additionally, it is time-efficient by avoiding multiple rounds of filtration. This method detects the size of a FM pore throat (i.e., the narrowest position of a pore tunnel), which is more consistent with the actual filtration situation. The conditions, such as probe concentration, temperature, transmembrane pressure difference, and types of surfactants, have been optimized. The experimental results show that the fluorescence probe method has good accuracy and reproducibility for measuring the pore size and pore size distribution of both organic and inorganic FMs. The method is particularly suitable for rapid testing of the filtration performance (nominal pore size≥0.02 μm) of purchased or synthetic membranes in the laboratory.

Proton sponge catalyzes interfacial polymerization in organic phase to enhance nanofiltration membrane performance

Interfacial polymerization (IP) plays a vital role in the manufacturing process of polyamide (PA) nanofiltration (NF), which is of great value in a wide variety of separation processes. In this study, we proposed a catalytic strategy to mix proton sponge/hexane solution and TMC/hexane solution during the interfacial polymerization reaction, thus achieving a facilitative modulating effect on the interfacial polymerization process. The nanofiltration membranes prepared by the catalytic strategy showed reduced thickness of the separation layer, reduced roughness, and more uniform pore size distribution than the blank membrane. As a result, the modified membranes showed nearly double permeate flux (from 8.0 to 15.9 Lm−2h−1bar−1) and slightly increased Na2SO4 retention, as well as improved separation selectivity for monovalent/divalent salts and stronger antifouling ability. At the same time, this strategy also improved the uniformity of membrane pore size distribution while loosening the membrane structure. The illustrated strategy based on the catalytic modulation of interfacial polymerization by proton sponge achieved an enhancement of all aspects of nanofiltration membrane performance through such a simple step, showing good prospects for application in the manufacture of membrane for high performance nanofiltration.

CO2 capture performance of ceramic membrane with superhydrophobic modification based on deposited SiO2 particles

Membrane absorption is a promising CO2 capture technology, which is limited by the high cost of membrane materials. Coal fly ash (CFA)-based ceramic membrane as an alternative material for membrane modules can effectively reduce the preparation cost. In this study, CFA-based ceramic membrane with superhydrophobic properties is prepared by depositing SiO2 nanoparticles and grafting 1H, 1H, 2H, 2H-perfluorooctyltriethoxysilane (POTS). Characters including pore size distribution, porosity and N2 flux of superhydrophobic membrane are performed. The superhydrophobic modified ceramic membrane is applied to CO2 capture experiments using ethanolamine (MEA) solution as the absorbent. The result shows that the contact angle of the modified ceramic membrane reaches 155.0°, which significantly improves the anti-wetting properties of the membrane surface. The CO2 capture experiment demonstrates a maximum capture efficiency of 97.45% and a mass transfer rate of 21.17 mol/(m2·h). In addition, the superhydrophobic modified ceramic membrane exhibits excellent thermal and chemical stability, sustaining high performance even after 8 h of continuous operation. This study provides the theoretical and empirical foundation for the application of CFA-based superhydrophobic ceramic membranes in CO2 capture.

Membrane Pore Size Distribution by Design via Kinetic Engineering Using Initiated Chemical Vapor Deposition

Membrane Pore Size Distribution by Design via Kinetic Engineering Using Initiated Chemical Vapor Deposition

On the balance between albumin loss and removal of middle molecules in dialyzers

Tuning the pore size distribution of hemodialysis membranes is essential for the membrane’s selectivity and significantly affects the quality of the dialysis treatment. Tailoring the membrane’s molecular weight cut-off appropriately balances the removal of middle-molecular-weight uremic toxins while avoiding albumin loss. This undesirable albumin loss is a potential side effect and concern for clinical use when aiming at increased removal of middle molecular weight molecules (middle molecules). It is hypothesized that control of the position of a narrow pore size distribution allows middle molecule removal while simultaneously counteracting the unwanted albumin loss. This study presents a comprehensive ex-vivo methodology and novel data on the balance of the clearance of middle molecules and albumin loss at different blood and dialysate flow rates using novel dialyzers. The outcomes hold significance for dialysis therapy, while the insights acquired have broader implications for the selectivity of ultrafiltration membranes.The Theranova dialyzer shows the largest clearance for small-middle molecules. Phylther stands out with higher removal of the middle molecule YKL-40 than the other dialyzers but exhibits a significant albumin loss. Theranova demonstrates the best compromise between low albumin loss and good clearances of middle molecules.

Carbon molecular sieve membranes derived from hydrogen-bonded organic frameworks for CO2/CH4 separation

Carbon molecular sieve (CMS) membranes are ideal candidates for natural gas separation due to their rigid pore structures and stability. The chemical composite and spatial configuration of the used precursors for deriving CMS membranes can greatly influence the final microporous structure and separation performance of the membranes. Most CMS membranes are transformed from amorphous precursors such as polymers nowadays. Here, for the first time, we report a new type of CMS membrane derived from a crystalline porous hydrogen-bonded organic framework membrane (named HCMS). The conversion process has been studied with the characterization of TGA, FTIR, XRD, SEM, and pore size distribution. The effect of pyrolysis temperature on the membrane structure has been investigated to achieve optimized CO2/CH4 separation performance. On the one hand, as the pyrolysis temperature increases from 550 to 650 °C, the pore size distribution of HCMS membrane is narrowed, which is conducive to improving the molecular sieving effect of the membrane. On the other hand, higher ratios of graphitic N and pyrrolic N, which show better affinity for CO2 molecules than pyridinic N, are obtained on the HCMS membranes, leading to more favorable adsorption of CO2. The optimized HCMS membrane pyrolyzed at 600 °C shows a remarkable CO2/CH4 selectivity of 128 and excellent separation stability at varying temperatures and pressures. The HCMS membranes derived from the crystalline HOF precursor can also maintain a stable separation performance against physical aging. The results reported in this work may open the door to constructing CMS membranes with the precursor of crystalline porous membranes.

Porous ceramic membranes from coal fly ash with addition of various pore-forming agents for oil-in-water emulsion separation

Coal fly ash (CFA) is a substitute material for ceramic membrane which can effectively reduce the preparation cost. However, CFA based ceramic membranes usually show insufficient mechanical strength for application. Revealing pore-forming mechanisms of different pore-forming agents in membranes preparation is of great significance to improve mechanical strength without harming permeability. In this work, three different pore-forming agents, carbon powder, dolomite and dextrin, were adopted to prepare CFA membranes. Characters including porosity, pore size distribution and permeability of membranes were comparatively analyzed. Effects of pore-forming agents on membranes were discussed. The membrane with dextrin at 1200 °C sintering showed the best comprehensive properties. Its porosity, flexural strength and Darcian permeability coefficient were 39.46 %, 35.56 MPa and 3.84 × 10−14 m2, respectively. Application in oil-in-water emulsion separation indicated higher than 94 % oil rejection. This research provides experimental bases for improving performances of low-cost CFA membranes, thereby conducive to engineering applications.

Modeling virus filtration based on a multilayer membrane morphology and pore size distribution

Obtaining adequate viral clearance in the manufacture of biopharmaceuticals is a major challenge. While virus filtration is routinely used for this purpose, there have been few attempts to model virus filter performance. Here, virus filter performance is modeled in consideration of a multilayered membrane structure with a membrane pore size distribution that is assumed to be the same log-normal distribution in each membrane layer. Comparison of experimental data both for constant pressure and constant flux filtration to the model calculations showed good agreement. This method relies on three fitted parameters that can easily be determined from experimental data: particle size that blocks the membrane pores, particle density and the maximum pore size that can capture a particle. In addition, high virus removability was confirmed by the model calculations. By applying the calculation method introduced here, it becomes possible to quantitatively reproduce and analyze the filtration characteristics of virus filters. Based on analysis using the numerical calculation method presented in this study, we demonstrated the equivalence of constant pressure filtration and constant flux filtration. This method also shows potential for predicting filtration behavior based on minimal experimental data.

CO2 capture based on Al2O3 ceramic membrane with hydrophobic modification

In this paper, Al2O3 ceramic membrane is modified to hydrophobicity by grafting 1 H,1 H,2 H,2 H-perfluorodecyltriethoxysilane. And its properties are characterized in detail. CO2 capture performance of ceramic membrane is investigated by experiments. Results show that wetting resistance after modification is significantly improved, and contact angle increases from the initial 49.8–130.9°. However, hydrophobic modification has no significant effect on the crystalline phase, surface morphology and pore size distribution of the ceramic membrane. With ethanolamine (MEA) as absorbent, CO2 mass transfer rate and capture efficiency using modified hydrophobic ceramic membrane are 46.6 × 10−3 mol/(m2·s) and 98.0%, showing significantly increase compared to the original membrane. After 72 h immersion in MEA solution, quality of ceramic membrane does not change significantly. And there is almost no change in average pore size. We believe this study will provide a reference for the industrial application for CO2 capture by gas-liquid membrane contactor with ceramic membrane.

Superior separation of industrial oil-in-water emulsions utilizing surface patterned isotropic PES membranes

Developing polymeric membranes for oily wastewater treatment has attracted great interest over the last few years. However, the low permeance and poor fouling resistance are considered the most challenging issues. Introducing topographic patterns on the membrane surface to increase active area and surface roughness is one practical strategy for reducing membrane fouling. Here, isotropic polyethersulfone (PES) membranes with line patterns and continuous sponge-like porous structure were prepared using phase separation micro-molding processes. Various techniques were used to characterize the morphologies and physicochemical properties of the as-prepared membranes. The surface wettability of the membranes was evaluated by measuring contact angle of a water drop in air and of industrial crude oil under water. The as-prepared membranes separation and antifouling performance were assessed using water permeance, oil rejection, and flux recovery ratio after emulsion filtration. The results showed that the patterned PES membrane has superior hydrophilic and antifouling capabilities, with a lower static water contact angle (CA) of 32° as compared to a flat PES membrane (CA = 39°). Furthermore, the patterned membrane had pure water permeance (PWP) of 8036 ± 65 L·h−1·m−2·bar−1, which was ∼ 68 % greater than that of the flat PES membrane. This is mainly attributable to the discernible increase in the membrane pore size distribution and the enhancement in membrane surface hydrophilicity due to the increase in surface roughness as a result of micro-patterning. The patterned membranes showed superior separation efficiency (>99.0 %) and permeance (7031 ± 65 L·h−1·m−2·bar−1) for the oil-in-water emulsion of 0.1 g·L−1. At a higher concentration of crude oil-in-water emulsion (50 g·L−1), the patterned PES membranes achieved promising permeance of 90 L/h.m2.bar and 99.9 % rejection. Furthermore, the flux recovery ratio of the patterned membrane exceeded 91 % after three consecutive cycles of dead-end filtration and backwashing with water, demonstrating the benefit of using micro-patterned membranes for treating high concentration and industrial oil-in-water emulsions.

A New Interpretation of Gas Viscosity for Flow through Micro-Capillaries and Pores.

The Hagen-Poiseuille equation for gas flowhad never been derived theoretically; it is rather a simple analogy of the same for liquid flow, and "gas viscosity" is a measure for overall resistance to flow. In this work, experimental flow data for different gases through capillaries and porous media, reported in literature by different groups, including those measured and treated by Knudsen are treated with Hagen-Poiseuille equation, but taking "gas viscosity" as an adjustable parameter. It is found that, at constant temperature, there exists an unambiguous relation between the viscosity (µ) of a given gas, and the product of average pressure (Pav ) and capillary diameter (D). In addition, for Pav * D<0.01, a universal linear relation exists between µ/M0.5 (where M is molecular mass) for different gases and the parameter Pav * D. The new interpretation of gas viscosity avoids the differentiation of regimes into "Knudsen" and "viscous" flow as it is frequently done in literature. The concept can be applied to obtain a reliable data base for gas viscosities in different fields of applications, for example in microfluidic systems or the analysis of pore size distributions of filters and membranes by gas flow porometry.

Rigorous determination of pore size non-uniformity for nanofiltration membranes by incorporating the effects on mass transport

The pore size uniformity of a nanofiltration (NF) membrane has an important impact on its rejection capacity and separation selectivity, which remains yet difficult to be scientifically and accurately characterized. In this study, a method was proposed for characterizing the pore size distribution of NF membranes, by assuming its obeyance of the log-normal distribution and incorporating the Donnan and steric pore model to describe water permeation and solute rejection. The distribution parameters are derived through data-fitting the experimental rejections of reference solutes in the model framework. The non-uniformity index is newly defined as the degree of deviation of a special pore size from the median pore size. The index makes that possible to quantitatively compare the non-uniformity among different NF membranes. The fast convergence of the method was verified. Furthermore, we put forward some suggestions on the use of the proposed method for pore size distribution characterization. The determination for the rejections of highly-rejected (∼90 %) solutes and the radii of the lowly-rejected (∼30 %) solutes must be sufficiently accurate. The proposed method is more advantageous over the conventionally adopted method due to the higher accuracy and more convenience in the assessment of membrane pore size distribution as well as membrane performance.