The art of surface modification of synthetic polymeric membranes

Robust Polymer Nanocomposite Membranes Incorporating Discrete TiO2 Nanotubes for Water Treatment

Polyethersulfone (PES) is a polymeric permeable material used in ultrafiltration (UF) membranes due to its high thermomechanical and chemical stability. The hydrophobic nature of PES membranes renders them prone to fouling and restricts the practical applications of PES in the fabrication of water treatment membranes. The present study demonstrates a non-solvent-induced phase separation (NIPS) approach to modifying PES membranes with different concentrations of discrete TiO₂ nanotubes (TNTs). Zeta potential and contact angle measurements showed enhanced hydrophilicity and surface negative charge in TNTs/PES nanocomposite membranes compared to unmodified PES membranes. To discern the antifouling and permeation properties of the TNTs/PES membranes, steam assisted gravity drainage (SAGD) wastewater obtained from the Athabasca oil sands of Alberta was used. The TiO₂ modified polymer nanocomposite membranes resulted in a higher organic matter rejection and water flux than the unmodified PES membrane. The addition of discrete TNTs at 1 wt% afforded maximum water flux (82 L/m² h at 40 psi), organic matter rejection (53.9%), and antifouling properties (29% improvement in comparison to pristine PES membrane). An enhancement in fouling resistance of TNTs/PES nanocomposite membranes was observed in flux recovery ratio experiments.

Preparation and Characterization of Cellulose Acetate Propionate Films Functionalized with Reactive Ionic Liquids

1-(1,3-diethoxy-1,3-dioxopropan-2-ylo)-3-methylimidazolium bromide (RIL1_Br), 1-(2-etoxy-2-oxoethyl)-3-methylimidazolium bromide (RIL2_Br), 1-(2-etoxy-2-oxoethyl)-3-methylimidazolium tetrafluoroborate (RIL3_BF4) ionic liquids were synthesized. Subsequently, the dense cellulose acetate propionate (CAP)-based materials containing from 9 to 28.6 wt % of these reactive ionic liquids were elaborated. Reactive ionic liquids (RILs) were immobilized in CAP as a result of the transesterification reaction. The yield of this reaction was over 90% with respect to the used RIL. The physicochemical properties of resultant films were studied using nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), energy dispersive X-ray (EDX), atomic force microscopy (AFM), and thermogravimetric analysis (TGA). The RIL incorporation influenced the morphology of films by increasing their surface roughness with the rise of RIL content. The thermal stability of CAP-based membranes was dependent on the nature of the ionic liquid. Nevertheless, it was proven that CAP films containing RILs were stable up to 120-150 °C. Transport properties were characterized by water permeation tests. It was found that the type and the amount of the ionic liquid in the CAP matrix substantially influenced the transport properties of the prepared hybrid materials.

Mixed matrix membranes for hydrocarbons separation and recovery: a critical review

The separation and purification of light hydrocarbons are significant challenges in the petrochemical and chemical industries. Because of the growing demand for light hydrocarbons and the environmental and economic issues of traditional separation technologies, much effort has been devoted to developing highly efficient separation techniques. Accordingly, polymeric membranes have gained increasing attention because of their low costs and energy requirements compared with other technologies; however, their industrial exploitation is often hampered because of the trade-off between selectivity and permeability. In this regard, high-performance mixed matrix membranes (MMMs) are prepared by embedding various organic and/or inorganic fillers into polymeric materials. MMMs exhibit the advantageous and disadvantageous properties of both polymer and filler materials. In this review, the influence of filler on polymer chain packing and membrane sieving properties are discussed. Furthermore, the influential parameters affecting MMMs affinity toward hydrocarbons separation are addressed. Selection criteria for a suitable combination of polymer and filler are discussed. Moreover, the challenges arising from polymer/filler interactions are analyzed to allow for the successful implementation of this promising class of membranes.

Immobilization of Perylene-3,4,9,10-Tetracarboxylic Dianhydride on Hollow Polysulfone Fibers: Primary Amine Coupling and Fluorescence Reporting

The fluorescent dye perylene-3,4,9,10-tetracarboxylic dianhydride (PBA) was immobilized onto polysulfone hollow fibers by means of a wet coating procedure. After immobilization, PBA was able to react with primary amines through a double anhydride ring opening reaction. The in situ amine coupling was further revealed by fluorescence analysis. Both emission (534 nm →538 nm) and fluorescence lifetime changes (2.7 ns →3.3 ns) of the dye are a useful tool to detect and visualize the occurrence of the reaction. The successful implementation of amine coupling with a reporting function on polysulfone fibers holds great interest for biomedical applications.

Charge Separating Microfiltration Membrane with pH-Dependent Selectivity

Membrane filters are designed for selective separation of components from a mixture. While separation by size might be the most common approach, other characteristics like charge can also be used for separation as presented in this study. Here, a polyether sulfone membrane was modified to create a zwitterionic surface. Depending on the pH value of the surrounding solution the membrane surface will be either negatively or positively charged. Thus, the charged state can be easily adjusted even by small changes of the pH value of the solution. Charged polystyrene beads were used as model reagent to investigate the pH dependent selectivity of the membrane. It was found that electrostatic forces are dominating the interactions between polystyrene beads and membrane surface during the filtration. This enables a complete control of the membrane's selectivity according to the electrostatic interactions. Furthermore, differently charged beads marked with fluorescent dyes were used to investigate the selectivity of mixtures of charged components. These different components were successfully separated according to their charged state proving the selectivity of the invented membrane.

A novel antifouling technique for the crossflow filtration using porous membranes: Experimental and CFD investigations of the periodic feed pressure technique

Oily water production is one of the many drawbacks of petroleum and several other industries. Finding effective ways for the treatment of produced water remain one of the main areas of interest in membrane sciences. Albeit the many advantages of membrane technology, they suffer from the unavoidable problem of fouling, which results from the accumulation of dispersed materials at the surface of membranes. Membrane modification and operational optimization have been approached as a potential cure of the problem of fouling. In this work we introduce a new and novel method that minimizes the development of fouling and in the same time utilizes no chemicals (i.e., environmentally friendly). The core of this method is based on alternating the pressure in the feed channel in a periodic manner and is therefore named the periodic feed pressure technique, PFPT. The idea is to make pinned droplets at the surface of the membrane lose essential forces that keep them sticking to the surface. The drag force due to permeation flux and the capillary force due to interfacial tension represents the two forces that largely contribute to the pinning of oil droplets at the surface of the membrane. Other forces including buoyancy and lift forces are generally small to be of significant influence. The idea of the PFPT is, therefore, to eliminate the force due to permeation drag. This is done by setting the transmembrane pressure (TMP) to zero at fixed intervals allowing pinned oil droplets to dislodge the surface. When the TMP is set to zero, permeation flux stops and the force due to permeation drag vanishes. This significantly reduces the overall residence time of pinned oil droplets, minimizing the chance for other oil droplets to cluster and coalesce with pinned ones. The PFPT does not cause any damage to the support layer of the polymeric membrane, which is a drawback of back-flushing methodology. The novel PFPT displays minimal membrane fouling and very similar permeation recovery despite only half the cycle time is in filtration mode. In this work, we show how the permeation flux is recovered and provide comparisons between the PFPT and regular filtration methodology. Furthermore, we compare the overall amount of filtrate at the end of the experiments using both methods. It is interesting to note that, the amount of filtrate using the PFPT is very much comparable to that obtained using regular filtration methodology and even higher. By optimizing the frequency of the cycle and the amplitude of the pressure change, it is possible to customize the PFPT to various membrane technologies and to achieve the highest recovery of the flux. Visual inspections of the membranes post operation and post rinsing indicate that membranes undergoing filtration using the PFPT achieves a very clean surface compared with those undergoing regular filtration processes. This method is a promising solution to membrane fouling that is easy to implement without any additional use of chemicals or equipment. Computational fluid dynamics (CFD) investigation is also conducted on microfiltration processes to show why this technique works.

Synthesis of High Crystalline TiO2 Nanoparticles on a Polymer Membrane to Degrade Pollutants from Water

Titanium dioxide (TiO2) is described as an established material to remove pollutants from water. However, TiO2 is still not applied on a large scale due to issues concerning, for example, the form of use or low photocatalytic activity. We present an easily upscalable method to synthesize high active TiO2 nanoparticles on a polyethersulfone microfiltration membrane to remove pollutants in a continuous way. For this purpose, titanium(IV) isopropoxide was mixed with water and hydrochloric acid and treated up to 210 °C. After cooling, the membrane was simply dip-coated into the TiO2 nanoparticle dispersion. Standard characterization was undertaken (i.e., X-ray powder diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, water permeance, contact angle). Degradation of carbamazepine and methylene blue was executed. By increasing synthesis temperature crystallinity and photocatalytic activity elevates. Both ultrasound modification of nanoparticles and membrane pre-modification with carboxyl groups led to fine distribution of nanoparticles. The ultrasound-treated nanoparticles gave the highest photocatalytic activity in degrading carbamazepine and showed no decrease in degradation after nine times of repetition. The TiO2 nanoparticles were strongly bound to the membrane. Photocatalytic TiO2 nanoparticles with high activity were synthesized. The innovative method enables a fast and easy nanoparticle production, which could enable the use in large-scale water cleaning.

Plasma Modification and Synthesis of Membrane Materials-A Mechanistic Review

Although commercial membranes are well established materials for water desalination and wastewater treatment, modification on commercial membranes is still necessary to deliver high-performance with enhanced flux and/or selectivity and fouling resistance. A modification method with plasma techniques has been extensively applied for high-performance membrane production. The paper presents a mechanistic review on the impact of plasma gas and polymerization, at either low pressure or atmospheric pressure on the material properties and performance of the modified membranes. At first, plasma conditions at low-pressure such as plasma power, gas or monomer flow rate, reactor pressure, and treatment duration which affect the chemical structure, surface hydrophilicity, morphology, as well as performance of the membranes have been discussed. The underlying mechanisms of plasma gas and polymerization have been highlighted. Thereafter, the recent research in plasma techniques toward membrane modification at atmospheric environment has been critically evaluated. The research focuses of future plasma-related membrane modification, and fabrication studies have been predicted to closely relate with the implementation of the atmospheric-pressure processes at the large-scale.

Mechanism of bacterial adhesion on ultrafiltration membrane modified by natural antimicrobial polymers (chitosan) and combination with activated carbon (PAC)

Bacterial adhesion to surfaces is related to several factors, such as surface charge, surface energy, and substrate characteristics (leading to the formation of biofilms). Organisms are dominant in most environmental, industrial, and medical problems and processes that are of interest to microbiologists. Biofilm cells are at least 500 times more resistant to antibacterial agents compared to planktonic cells. The usage of ultrafiltration membranes is fast becoming popular for water treatment. Membrane lifetime and permeate flux are primarily affected by the phenomena of microbial accumulation and fouling at the membrane’s surface. This review intends to understand the mechanism of membrane fouling by bacterial attachment on polymeric ultrafiltration membrane modified by natural antimicrobial polymers (chitosan) combined with powder activated carbon. Also, to guide future research on membrane water treatment processes, adhesion prediction using the extended Derjaguin-Landau-Verwey-Overbeek theory is discussed.

Molecular Simulations of the Hydration Behavior of a Zwitterion Brush Array and Its Antifouling Property in an Aqueous Environment

We carried out umbrella sampling and molecular dynamics (MD) simulations to investigate molecular interactions between sulfobetaine zwitterions or between sulfobetaine brushes in different media. Simulation results show that it is more energetically favorable for the two sulfobetaine zwitterions or brushes to be fully hydrated in aqueous solutions than in vacuum where strong ion pairs are formed. Structural properties of the hydrated sulfobetaine brush array and its antifouling behavior against a foulant gel are subsequently studied through steered MD simulations. We find that sulfobetaine brush arrays with different grafting densities have different structures and antifouling mechanisms. At a comparably higher grafting density, the sulfobetaine brush array exhibits a more organized structure which can hold a tightly bound hydration water layer at the interface. Compression of this hydration layer results in a strong repulsive force. However, at a comparably lower grafting density, the brush array exhibits a randomly oriented structure in which the antifouling of the brush array is through the deformation of the sulfobetaine branches.

Functionalized and engineered nanochannels for gas separation

In this work, we present the hydrogen selective gas separation properties of the track-etched poly (ethylene terephthalate) (PET) membranes, which were functionalized with a carboxylic group. Also, Palladium (Pd) nanoparticles of average diameter 5 nm were deposited for a various time on pore walls as well as on the surface of carboxylated membranes. Effect of Pd nanoparticles binding with the increase of deposition time on gas separation and selectivity was studied. For the study of surface morphology of these composite membranes and the confirmation of Pd nanoparticles binding on the surface as well as on pore walls is characterized by scanning electron microscopy (SEM). The gas permeability of carboxylated membrane with increasing Pd deposition timing for hydrogen (H2), carbon dioxide (CO2) and nitrogen (N2) was examined. From the gas permeability data of H2, CO2 and N2 gasses, it was observed that these membranes have higher permeability for H2 as compared with CO2 and N2. Selectivity of H2/CO2 and H2/N2 improves with the increased Pd nanoparticles deposition time. These membranes have effective application in the field of hydrogen based fuel cell.

Development and characterization of antibacterial braided polyamide suture coated with chitosan-citric acid biopolymer

Braided polyamide sutures are frequently used in dermatologic surgery for wound closure. However, braided sutures promote bacteria proliferation. In order to prevent wound complications due to this effect, antibacterial sutures should be used. The main objective of this study is the development of new non-absorbable antibacterial polyamide braided suture. This paper suggests new coating process that leads to obtain suture uniformly covered by antibacterial film enclosing chitosan, which is known for its antibacterial benefit. Mechanical properties and surface morphology of developed sutures were investigated by using mechanical tests. Sutures surfaces were also examined by scanning electron microscope, to perceive spreading of coating product on suture surface. In order to identify potential reactions between chemical compounds present in coating solution and suture material, sutures were analyzed by ATR-IF spectroscopy. It has been demonstrated that many eventual bonds between compounds present in coating solutions and polyamide macromolecular chain may occur. The existence of these bonds implies the fixation of biopolymer coating on suture surface. It has been demonstrated that uniform surface may be obtained by progressively applying coating solution containing little amount of chitosan on suture surface. We have also found that developed coating process has not affected mechanical properties of suture, which still meet United States Pharmacopeia requirement. Finally, antibacterial effects against four colonies, very widespread in hospitals, were studied. Prominent antibacterial effects of braided polyamide suture against two gram-positive ( S Aureus, S epidermidis) and two gram-negative ( E coli and P aeruginosa) colonies are presented. Optimal result of best properties is obtained by applying three layers of biopolymer coating comprising 1% chitosan and 10% citric acid. The new developed suture coating process appears as a promising method for obtaining important antibacterial effect with smooth suture surface.

Charge tunable thin-film composite membranes by gamma-ray triggered surface polymerization

Thin-film composite poly(amide) (PA) membranes have greatly diversified water supplies and food products. However, users would benefit from a control of the electrostatic interactions between the liquid and the net surface charge interface in order to benefit wider application. The ionic selectivity of the 100 nm PA semi-permeable layer is significantly affected by the pH of the solution. In this work, for the first time, a convenient route is presented to configure the surface charge of PA membranes by gamma ray induced surface grafting. This rapid and up-scalable method offers a versatile route for surface grafting by adjusting the irradiation total dose and the monomer concentration. Specifically, thin coatings obtained at low irradiation doses between 1 and 10 kGy and at low monomer concentration of 1 v/v% in methanol/water (1:1) solutions, dramatically altered the net surface charge of the pristine membranes from -25 mV to +45 mV, whilst the isoelectric point of the materials shifted from pH 3 to pH 7. This modification resulted in an improved water flux by over 55%, from 45.9 to up 70 L.m^-2.h^-1, whilst NaCl rejection was found to drop by only 1% compared to pristine membranes.

High Levels of Residue within Polymeric Hollow Fiber Membranes Used for Blood Oxygenation

A number of research teams are developing surface coatings for hollow fiber membrane (HFM) blood oxygenators to improve their biocompatibility and service life. Surface coating techniques can be quite sensitive to the presence of contaminants on the exterior surface of the hollow fibers. We found large amounts of leachable oils associated with several commercial HFMs, i.e., as much as 2.5-7.5 weight percent. Leachable residues were suspected when a surface coating, a surface-initiated atom transfer radical polymerization (s-ATRP) of poly(ethylene glycol) methacrylate, resulted in areas of 100 µm devoid of coatings on the exterior surfaces of HFMs. After leaching residual oils, s-ATRP coatings were uniform and continuous across the hollow fibers. Therefore, removal of residual material should be considered before applying coating technologies to commercial HFMs. The effects of such leachable agents on the performance of blood oxygenators are not known.

Towards Enhanced Performance Thin-film Composite Membranes via Surface Plasma Modification

Advancing the design of thin-film composite membrane surfaces is one of the most promising pathways to deal with treating varying water qualities and increase their long-term stability and permeability. Although plasma technologies have been explored for surface modification of bulk micro and ultrafiltration membrane materials, the modification of thin film composite membranes is yet to be systematically investigated. Here, the performance of commercial thin-film composite desalination membranes has been significantly enhanced by rapid and facile, low pressure, argon plasma activation. Pressure driven water desalination tests showed that at low power density, flux was improved by 22% without compromising salt rejection. Various plasma durations and excitation powers have been systematically evaluated to assess the impact of plasma glow reactions on the physico-chemical properties of these materials associated with permeability. With increasing power density, plasma treatment enhanced the hydrophilicity of the surfaces, where water contact angles decreasing by 70% were strongly correlated with increased negative charge and smooth uniform surface morphology. These results highlight a versatile chemical modification technique for post-treatment of commercial membrane products that provides uniform morphology and chemically altered surface properties.

Molecular Dynamics Simulations of a Poly(ethylene glycol)-Grafted Polyamide Membrane and Its Interaction with a Calcium Alginate Gel

Molecular dynamics simulations are carried out to investigate the antifouling property of a polyethylene glycol (PEG)-grafted polyamide (PA) membrane. Our specific interest is the computational study of the interaction between a grafted PEG coating and an alginate gel foulant by a steered molecular dynamics approach. Simulation results show that the PEG coating can hold a tightly bound hydration water layer. When the alginate gel is dragged to approach the PEG coating surface, a strong repulsive hydration force is observed due to the compression of this hydration layer. Detailed calculations of the potential of mean force (PMF) show that the repulsive interaction between the alginate gel and the hydration water layer around the PEG coating has a dominant contribution to the total repulsive PMF. We have also studied the effect of the PEG coverage on the membrane-foulant interactions. We find that the alginate gel has a strong tendency to drift to the uncovered PA membrane surfaces (namely, the PEG hollows). However, direct attachment of the gel to the PA membrane surface can be avoided if the gel size is slightly larger than the PEG hollow site.

The critical zeta potential of polymer membranes: how electrolytes impact membrane fouling

The zeta potential of membrane surfaces and the resulting electrostatic interactions are determining factors of membrane fouling. This publication presents the impact of salt concentration and pH value on these interactions.

Particle adsorption on a polyether sulfone membrane: how electrostatic interactions dominate membrane fouling

This study presents a new method focussing on electrostatic interactions during fouling of microfiltration membranes.