Nanofiltration and gas separation membrane fabrication using layer by layer assembly of polyelectrolytes

El ensamblaje capa a capa (LBL) de polielectrolitos es un método sencillo y versátil para fabricar membranas con grupos funcionales, que permitan un buen control de las propiedades de la membrana como el grosor. En este estudio, se modificó la membrana de soporte de poliacrilonitrilo (PAN) microporoso con un sistema de multicapas de polielectrolitos débiles (PEM) compuesto por clorhidrato de poli(alilamina) (PAH) y poli(ácido acrílico) (PAA). Para optimizar las condiciones del proceso de recubrimiento PEM, la membrana de soporte PAN fue modificada primero superficialmente mediante hidrólisis alcalina (solución de NaOH), para generar una carga superficial negativa con el fin de mejorar la adhesión de la primera capa. A continuación, el soporte de poliacrilonitrilo hidrolizado (HPAN) fue modificado con polielectrolitos PAH y PAA variando su pH durante el ensamblaje para ajustar la morfología y el rendimiento de la membrana multicapa. Las membranas fueron caracterizadas mediante SEM, potencial zeta de flujo y mediciones del ángulo de contacto (agua). El pH de las soluciones de recubrimiento influyó significativamente en la morfología y el rendimiento de las membranas multicapa de polielectrolitos (PEMM): se observó una película uniformemente densa y fina a un valor de pH de solución de PAH/PAA (6,5/6,5) debido a la elevada compensación de carga intrínseca que se produce entre los dos PE, puesto que ambos PE estuvieron casi completamente ionizados a un pH de 6,5. Asimismo, se obtuvo una película delgada, pero menos densa, a un pH de la solución de PAH/PAA (2,5/8,5) en el que ambos PE estuvieron totalmente ionizados. Por otro lado, la capa más gruesa fue sintetizada mediante las combinaciones de pH de la solución PAH/PAA (2,5/4,5), donde el PAA estuvo parcialmente cargado y el PAH totalmente cargado. Los valores de permeabilidad de agua pura para las membranas PAH2.5/PAA4.5, PAH2.5/PAA8.5 y PAH6.5/PAA6.5 fueron 35,2 8,72 y 2,34 L-m-2-h-1-bar-1, respectivamente, tras el recubrimiento con cuatro capas. Sin embargo, el rendimiento de las membranas en la separación de gases fue muy bajo. Los resultados de este estudio demuestran claramente el potencial de utilizar el pH de la solución de PEs débiles como parámetro de ajuste para preparar PEMMs para aplicaciones específicas<br /

Membranes for Osmotic Power Generation by Reverse Electrodialysis

In recent years, the utilization of the selective ion transport through porous membranes for osmotic power generation (blue energy) has received a lot of attention. The principal of power generation using the porous membranes is same as that of conventional reverse electrodialysis (RED), but nonporous ion exchange membranes are conventionally used for RED. The ion transport mechanisms through the porous and nonporous membranes are considerably different. Unlike the conventional nonporous membranes, the ion transport through the porous membranes is largely dictated by the principles of nanofluidics. This owes to the fact that the osmotic power generation via selective ion transport through porous membranes is often referred to as nanofluidic reverse electrodialysis (NRED) or nanopore-based power generation (NPG). While RED using nonporous membranes has already been implemented on a pilot-plant scale, the progress of NRED/NPG has so far been limited in the development of small-scale, novel, porous membrane materials. The aim of this review is to provide an overview of the membrane design concepts of nanofluidic porous membranes for NPG/NRED. A brief description of material design concepts of conventional nonporous membranes for RED is provided as well

Membrane Separation of Gaseous Hydrocarbons by Semicrystalline Multiblock Copolymers: Role of Cohesive Energy Density and Crystallites of the Polyether Block

The energy-efficient separation of hydrocarbons is critically important for petrochemical industries. As polymeric membranes are ideal candidates for such separation, it is essential to explore the fundamental relationships between the hydrocarbon permeation mechanism and the physical properties of the polymers. In this study, the permeation mechanisms of methane, ethane, ethene, propane, propene and n-butane through three commercial multiblock copolymers PEBAX 2533, PolyActive1500PEGT77PBT23 and PolyActive4000PEGT77PBT23 are thoroughly investigated at 33 °C. This study aims to investigate the influence of cohesive energy density and crystallites of the polyether block of multiblock copolymers on hydrocarbon separation. The hydrocarbon separation behavior of the polymers is explained based on the solution–diffusion model, which is commonly accepted for gas permeation through nonporous polymeric membrane materials

Mass Transport of Dye Solutions through Porous Membrane Containing Tannic Acid/Fe³⁺ Selective Layer

Tannic acid (TA)–Fe3+ membranes have received recent attention due to their sustainable method of fabrication, high water flux and organic solutes rejection performance. In this paper, we present a description of the transport of aqueous solutions of dyes through these membranes using the transport parameters of the Spiegler–Kedem–Katchalsky (SKK) model. The reflection coefficient (σ) and solute permeability (PS) of the considered TA–Fe3+ membranes were estimated from the non-linear model equations to predict the retention of solutes. The coefficients σ and PS depended on the porous medium and dye molecular size as well as the charge. The simulated rejections were in good agreement with the experimental findings. The model was further validated at low permeate fluxes as well as at various feed concentrations. Discrepancies between the observed and simulated data were observed at low fluxes and diluted feed solutions due to limitations of the SKK model. This work provides insights into the mass transport mechanism of dye solutions and allows the prediction of dye rejection by the TFC membranes containing a TA–Fe3+ selective layer using an SKK model

Using the Assembly Time as a Tool to Control the Surface Morphology and Separation Performance of Membranes with a Tannic Acid–Fe³⁺ Selective Layer

Thin-film composite (TFC) membranes containing a metal–polyphenol network (MPN)-based selective layer were fabricated on a porous polyacrylonitrile support. The MPN layer was formed through coordination-based self-assembly between plant-based tannic acid (TA) and an Fe3+ ion. For the first time, we demonstrate that TFC membranes containing TA-Fe3+ selective layers can separate small organic solutes in aqueous media from equimolar mixtures of solutes. The effect of the assembly time on the characteristics and performance of the fabricated selective layer was investigated. An increase in the assembly time led to the formation of selective layers with smaller effective pore sizes. The tannic acid–Fe3+ selective layer exhibited a low rejection towards neutral solutes riboflavin and poly(ethylene glycol) while high rejections were observed for anionic dyes of orange II and naphthol green B. Permeation selectivities in the range of 2–27 were achieved between neutral and charged dyes in both single- and mixed-solute experiments, indicating the significant role of Donnan exclusion and the charge-selective nature of the membranes. The rejection efficiency improved with an increasing assembly time. Overall, this study demonstrates that the assembly time is a vital casting parameter for controlling the permeance, rejection and selectivity of thin-film composite membranes with a tannic acid–Fe3+ selective layer

Frequency and Sensitivity of Extended Spectrum Beta-Lactamase Positive Organisms in a Secondary and Tertiary Level Hospital Network in Dhaka

Background: Extended spectrum β-lactamase (ESBL) positive organisms are now a global health concern including in Bangladesh. These are associated with treatment failure, increased morbidity and mortality and increased health care costs. In this study, frequency of ESBL positive organisms in some health care centres in Dhaka city has been observed and their current status of antibiogram has also been observed. Objective: To observe the current status of antibiogram of ESBL positive organisms. Materials and Methods: This cross-sectional study was done in the Department of Microbiology, Bangladesh Institute of Health Sciences (BIHS) General Hospital, Dhaka, Bangladesh from March, 2012 to February, 2013. Only E. coli and Klebsiella spp. from pus and urine specimens were included in this study. Isolation, identification and antibiotic sensitivity of the organisms were done by standard procedures. Results: Organisms (Escherichia coli and Klebsiella spp.) isolated from urine and pus collected from different sites of 472 subjects were studied. Predominant organisms were Escherichia coli (82.8%) and remaining 17.2% were Klebsiella spp. ESBL positive organisms were higher in Escherichia coli (54.5%) than in Klebsiella spp. (44.4%) and higher in pus (77.0%) than in urine (49.1%) isolates. Imipenem is the most effective drug for treating ESBL positive organisms followed by colistin, tigecycline and piperacillin/tazobactam. Conclusion: Imipenem, colistin, tigecycline and piperacillin/tazobactam drugs should be kept reserved and used only when other effective drugs are not available so that emergence of resistance against these drugs is deferred. While reporting the culture and sensitivity tests, the ESBL positive organisms should be pointed out with comment like this – “The organisms are ESBL positive and resistant to penicillins, cephalosporins and monobactams”

CO₂ Selective PolyActive Membrane: Thermal Transitions and Gas Permeance as a Function of Thickness

It is generally accepted that the melting point of a semicrystalline polymer is associated with the thickness of the crystalline lamellae (Gibbs–Thomson equation). In this study, a commercially available multiblock copolymer PolyActive composed of 77 wt % of poly­(ethylene glycol terephthalate) and 23 wt % of poly­(butylene terephthalate) was dip-coated on top of a multilayer microporous support. The thickness was changed between 0.2 and 8 μm using coating solutions containing 0.75–7.5 wt % PolyActive. The surface temperature of the membrane during dip-coating was monitored using an infrared camera. Single gas permeances of N₂, H₂, CH₄, and CO₂ were measured between 20 and 80 °C at temperature steps of 2 °C. Spherulitic superstructures composed of radially directed lamellae were observed in the polarized light microscope in the prepared membranes. Atomic force microscopy studies showed that the thickness of the crystalline lamellae was in the order of 10 nm or 0.01 μm at the surface of the membrane. Therefore, according to the Gibbs–Thomson equation, the melting point should not change in the thickness range 0.2–8 μm. However, the gas permeance data showed that the melting point of the polyether domains of the 0.2 μm PolyActive layer was 10 °C lower compared to that of the 8 μm layer. The results can be explained by considering that the width of many crystalline lamellae significantly reduces as a function of film thickness, thereby reducing the average fold surface free energy/lateral surface free energy ratio