Permporometry: the determination of the size distribution of active pores in UF membranes

Bis(triethoxysilyl)ethane (BTESE)-Organosilica Membranes for H2O/DMF Separation in Reverse Osmosis (RO): Evaluation and Correlation of Subnanopores via Nanopermporometry (NPP), Modified Gas Translation (mGT) and RO Performance

A 40 cm length Bis(triethoxysilyl)ethane (BTESE) membrane having different pore sizes was successfully prepared by changing the number of coating times for gas permeation (GP) and organic solvent reverse osmosis (OSRO) separation study. It was found that BTESE-6 membranes prepared through six-time coating consisted of small-sized pores in the range 0.56 to 0.64 nm estimated using modified Gas Translation (mGT) method and 0.59 to 0.67 nm estimated by nanopermporometry (NPP) method, respectively. These membranes demonstrated a high DMF rejection, RDMF > 95% with total flux, Jv total > 5 kg m-2 h-1 at operating condition feed pressure, Pf: 8 MPa; feed temperature, Tf : 50 °C; and feed flowrate, Qf : 30 mL/min; and they exhibited a high degree selectivity of He/SF6 in the range of ~ 260-3400 at a permeation temperature 200 °C. On the other hand, the larger pore sizes of the BTESE-4 membranes (pore size estimates > 0.76 nm to 1.02 nm) exhibited low DMF rejection and a low degree selectivity of He/SF6 around ~30% and 25, respectively, at the same operating condition as BTESE-6. Both GT and NPP methods can be considered as an indicator of the measurement membrane pore size. From this study, it was found that He and SF6 gases can be some of the potential predictors for water and DMF permeance. Furthermore, by comparing our OSRO membrane with other PV membranes for DMF/H2O separation, our BTESE-6 membranes still exhibited high flux in the range of 3-6 kg m-2 h-1 with a separation factor H2O/DMF in the range of 80-120.

Fluorine-Containing, Self-Assembled Graft Copolymer for Tuning the Hydrophilicity and Antifouling Properties of PVDF Ultrafiltration Membranes

Neat poly(vinylidene fluoride) (PVDF) ultrafiltration (UF) membranes exhibit poor water permeance and surface hydrophobicity, resulting in poor antifouling properties. Herein, we report the synthesis of a fluorine-containing amphiphilic graft copolymer, poly(2,2,2-trifluoroethyl methacrylate)-g-poly(ethylene glycol) behenyl ether methacrylate (PTFEMA-g-PEGBEM), hereafter referred to as PTF, and its effect on the structure, morphology, and properties of PVDF membranes. The PTF graft copolymer formed a self-assembled nanostructure with a size of 7-8 nm, benefiting from its amphiphilic nature and microphase separation ability. During the nonsolvent-induced phase separation (NIPS) process, the hydrophilic PEGBEM chains were preferentially oriented towards the membrane surface, whereas the superhydrophobic PTFEMA chains were confined in the hydrophobic PVDF matrix. The PTF graft copolymer not only increased the pore size and porosity but also significantly improved the surface hydrophilicity, flux recovery ratio (FRR), and antifouling properties of the membrane. The membrane performance was optimal at 5 wt.% PTF loading, with a water permeance of 45 L m-2 h-1 bar-1, a BSA rejection of 98.6%, and an FRR of 83.0%, which were much greater than those of the neat PVDF membrane. Notably, the tensile strength of the membrane reached 6.34 MPa, which indicated much better mechanical properties than those reported in the literature. These results highlight the effectiveness of surface modification via the rational design of polymer additives and the precise adjustment of the components for preparing membranes with high performance and excellent mechanical properties.

Effects of the Water Matrix on the Degradation of Micropollutants by a Photocatalytic Ceramic Membrane

The consumption of pharmaceuticals has increased the presence of micropollutants (MPs) in the environment. The removal and degradation of pharmaceutical mixtures in different water matrices are thus of significant importance. The photocatalytic degradation of four micropollutants-diclofenac (DCF), iopamidol (INN), methylene blue (MB), and metoprolol (MTP)-have been analyzed in this study by using a photocatalytic ceramic membrane. We experimentally analyzed the degradation rate by using several water matrices by changing the feed composition of micropollutants in the mixture (from mg· L-1 to μg·L-1), adding different concentrations of inorganic compounds (NaHCO₃ and NaCl), and by using tap water. A maximum degradation of 97% for DCF and MTP, and 85% for INN was observed in a micropollutants (MPs) mixture in tap water at environmentally relevant feed concentrations [1-6 μg·L-1]o; and 86% for MB in an MPs mixture [1-3 mg·L-1]o with 100 mg·L-1 of NaCl. This work provides further insights into the applicability of photocatalytic membranes and illustrates the importance of the water matrix to the photocatalytic degradation of micropollutants.

Mesoporous Membrane Materials Based on Ultra-High-Molecular-Weight Polyethylene: From Synthesis to Applied Aspects

The development of new porous polymeric materials with nanoscale pore dimensions and controlled morphology presents a challenging problem of modern materials and membrane science, which should be based on scientifically justified approaches with the emphasis on ecological issues. This work offers a facile and sustainable strategy allowing preparation of porous nanostructured materials based on ultra-high-molecular-weight polyethylene (UHMWPE) via the mechanism of environmental intercrystallite crazing and their detailed characterization by diverse physicochemical methods, including SEM, TEM, AFM, liquid and gas permeability, DSC, etc. The resultant porous UHMWPE materials are characterized by high porosity (up to ~45%), pore interconnectivity, nanoscale pore dimensions (below 10 nm), high water vapor permeability [1700 g/(m² × day)] and high gas permeability (the Gurley number ~300 s), selectivity, and good mechanical properties. The applied benefits of the advanced UHMWPE mesoporous materials as efficient membranes, breathable, waterproof, and insulating materials, light-weight materials with reduced density, gas capture and storage systems, porous substrates and scaffolds are discussed.

New Method toward a Robust Covalently Attached Cross-Linked Nanofiltration Membrane

As nanofiltration applications increase in diversity, there is a need for new fabrication methods to prepare chemically and thermally stable membranes with high retention performance. In this work, thio-bromo "click" chemistry was adapted for the fabrication of a robust covalently attached and ultrathin nanofiltration membrane. The selective layer was formed on a pre-functionalized porous ceramic surface via a novel, liquid-vapor interfacial polymerization method. Compared to the most common conventional interfacial polymerization procedure, no harmful solvents and a minimal amount of reagents were used. The properties of the membrane selective layer and its free-standing equivalent were characterized by complementary physicochemical analysis. The stability of the thin selective layer was established in water, ethanol, non-polar solvents, and up to 150 °C. The potential as a nanofiltration membrane was confirmed through solvent permeability tests (water, ethanol, hexane, and toluene), PEG-in-water molecular weight cut-off measurements (≈700 g mol^-1), and dye retention measurements.

New Generation of Mesoporous Silica Membranes Prepared by a Stöber-Solution Pore-Growth Approach

Membranes consisting of uniform and vertically organized mesopores are promising systems for molecular filtration because of the possibility to combine high-flux and high-rejection properties. In this work, a new generation of mesoporous silica membranes (MSMs) have been developed, in which an organized mesoporous layer is directly formed on top of a porous ceramic support via a Stöber-solution pore-growth approach. Relevant characterization methods have been used to demonstrate the growth of the membrane separation layer and the effect of reaction time and the concentration of the reactants on the microstructure of the membrane. Compared to previous studies using the evaporation-induced self-assembly method to prepare MSMs, an important increase in water permeability was observed (from 1.0 to at least 3.8 L m^-2 h^-1 bar^-1), indicating an improved pore alignment. The water permeability, cyclohexane permporometry tests, and molecular cut-off measurements (MWCO ≈ 2300 Da) were consistent with membranes composed of 2-3 nm accessible pores.

"Modified" Liquid⁻Liquid Displacement Porometry and Its Applications in Pd-Based Composite Membranes

For H₂ separation by Pd-based composite membranes, the pore mouth size distribution of the porous support immediately affects the quality of the deposited layer, including continuity and defect/pinhole formation. However, there is a lack of convenient and effective methods for characterization of pore mouth size of porous supports as well as of defect distribution of dense Pd-based composite membranes. Here we introduce a novel method by modifying conventional liquid⁻liquid displacement porometry. When the pore tunnels are filled with Liquid B and the outer surface is occupied by Liquid A, the reopening of the pore mouth depends on the pressure of Liquid B and the interfacial tension at the position of the pore mouth, from which the pore mouth size can be determined according to the Young⁻Laplace equation. Our experimental tests using this method with model samples show promising results, which are well supported by those obtained using FESEM (fild emission scanning electron microscope), AFM (atomic force microscope), and conventional liquid⁻liquid displacement porometry. This novel method can provide useful information for not only surface coatings on porous substrates but also for modification of dense membrane defects; thus, broad utilizations of this technique can be expected in future study.

Forward osmosis with a novel thin-film inorganic membrane

Forward osmosis (FO) represents a new promising membrane technology for liquid separation driven by the osmotic pressure of aqueous solution. Organic polymeric FO membranes are subject to severe internal concentration polarization due to asymmetric membrane structure, and low stability due to inherent chemical composition. To address these limitations, this study focuses on the development of a new kind of thin-film inorganic (TFI) membrane made of microporous silica xerogels immobilized onto a stainless steel mesh (SSM) substrate. The FO performances of the TFI membrane were evaluated upon a lab-scale cell-type FO reactor using deionized water as feed solution and sodium chloride (NaCl) as draw solution. The results demonstrated that the TFI membrane could achieve transmembrane water flux of 60.3 L m(-2) h(-1) driven by 2.0 mol L(-1) NaCl draw solution at ambient temperature. Meanwhile, its specific solute flux, i.e. the solute flux normalized by the water flux (0.19 g L(-1)), was 58.7% lower than that obained for a commercial cellulose triacetate (CTA) membrane (0.46 g L(-1)). The quasi-symmetry thin-film microporous structure of the silica membrane is responsible for low-level internal concentration polarization, and thus enhanced water flux during FO process. Moreover, the TFI membrne demonstrated a substantially improved stability in terms of mechanical strength, and resistance to thermal and chemical stimulation. This study not only provides a new method for fabricating quasi-symmetry thin-film inorganic silica membrane, but also suggests an effective strategy using this alternative membrane to achieve improved FO performances for scale-up applications.

Interaction behavior in ultrafiltration of nonionic surfactant micelles by adsorption

Adsorption of nonionic surfactant micelles onto ultrafiltration (UF), membranes was studied. Two homologous series of nonionic surfactants, namely, Tritons (alkylphenol ethoxylates) and Neodols (alcohol ethoxylates), were used to characterize surface properties of two polymeric ultrafiltration membranes with 20,000 nominal cutoff. Particularly, a cellulose acetate and a polysulfone membrane were investigated. Static adsorption experiments were carried out using surfactant solutions at concentrations above their critical micelle concentration. The characterization of surface properties of UF membranes was based on the adsorption behavior of surfactant species. The adsorption extent on UF membranes was affected by the hydrophobicity-to-hydrophilicity ratio mainly determining the interactions developed at the membrane-surfactant species interface. Adsorption experimental data seem generally to fit the Langmuir isotherm model. Atomic force microscopy was used to examine the alteration of the top membrane surface morphology.