Search

To increase the permeation performance and antifouling properties of polymer membranes, a one-step reaction using electron irradiation was developed. This process combines the surface activation of the membrane polymer and the simultaneous permanent immobilization of hydrophilic molecules. This technology can be applied to various polymers, flat sheet/hollow fiber membranes and all pore ranges. The roll-to-roll system developed for this enables all process steps including inline analysis for quality control of the membrane surface in a continuously operated system. © 2021 The Authors. Chemie Ingenieur Technik published by Wiley-VCH GmbH

Immobilization of different digestive enzymes consisting of proteases, amylases and lipases on polyvinylidene fluoride resulted in biocatalytic active polymer membranes with self-cleaning capability. Activation of enzymes and therefore “switching on” the membrane system was achieved by adjusting pH and temperature leading to an active degradation of fouled substances on its surface. Fouling and self-cleaning experiments with solutions of protein, lipid, carbohydrate, and a mixture were performed and resulted in high recovery of water permeance (99%, 72%, 77%, and 68%, respectively). Furthermore, real samples including river water (75% after first cycle), and household sewage (62%) were examined, as well as first investigations in longtime performance, and stability were performed. Comprehensive membrane characterization was conducted by investigation of the immobilized enzyme concentration, enzyme activity, fouling tests and water permeation monitoring, mercury porosimetry, X-ray photoelectron spectroscopy, scanning electron microscopy, and finally, zeta potential, as well as water contact angle measurements.

This study presents a new method focussing on electrostatic interactions during fouling of microfiltration membranes. Therefore, a new fouling test system was developed. To investigate the impact of surface charge on membrane fouling charged polystyrene beads were used. Also, the tested polyethersulfone membranes were modified with differently charged functional groups. Zeta potential measurements were conducted to describe the charged state of the membranes and particles. Fouling was investigated by filtration of the particle suspensions through the different membranes. It was found that oppositely charged surfaces of the membrane and particle lead to membrane fouling due to electrostatic attraction. Accordingly, no fouling occurred for the combination of evenly charged surfaces due to electrostatic repulsion. Additional experiments with a zwitterionic membrane surface revealed that fouling significantly depends on the pH value. Possessing an isoelectric point of pH 7 this membrane can exists in a positively charged, uncharged, or negatively charged state. Again, fouling occurred when electrostatic attraction was present while electrostatic repulsion resulted in antifouling. In the case of an uncharged surface fouling occurred in general due to the absence of electrostatic interactions. These new findings allowed us to establish a new standard fouling test that reveals the influence of electrostatic interactions in a fouling problem. To further demonstrate the advantages of this system standard protein fouling experiments were conducted and compared. It was found that different types of interactions and changes in the tertiary structure of proteins have to be considered. Thus, the gained results were difficult to interpret and false assumptions may occur. In contrast, electrostatic forces are sufficient to explain the fouling of polystyrene beads. The fouling test system presented by the authors is solely focused on electrostatic interactions and has, therefore, a clear advantage compared to the common protein test system.

The zeta potential of membrane surfaces and the resulting electrostatic interactions are determining factors of membrane fouling. This study investigates the influence of environmental parameters like pH value, salt concentration, or ion valence on the zeta potential of polymer membranes and the resulting fouling. To control electrostatic forces charged polystyrene beads were used as fouling reagents. Also, polyethersulfone and polyvinylidene fluoride membranes were modified to possess an either positive or negative surface charge. Afterwards, suspensions of beads were filtered through the membranes in different electrolytic environments. Fouling occurred when membrane and beads are oppositely charged. Bead adsorption was not observed when both surfaces are evenly charged. The latter was found to be influenced by electrolyte concentration. High salt concentrations or present bivalent ions reduced the electrostatic repulsion between evenly charged surfaces and led to membrane fouling. Low salt concentrations did not influence the electrostatic repulsion. Thus, critical salt concentrations were determined and used to identify the critical zeta potential. In addition, the fouling of a zwitterionic membrane surface was investigated regarding its pH dependence. A critical zeta potential that is associated with membrane fouling was identified. The critical zeta potentials are similar for both pH and salt concentration dependence.

A major goal of membrane science is the improvement of the membrane performance and the reduction of fouling effects, which occur during most aqueous filtration applications. Increasing the surface hydrophilicity can improve the membrane performance (in case of aqueous media) and decelerates membrane fouling. In this study, a PES microfiltration membrane (14,600 L m&minus, 2 h&minus, 1 bar&minus, 1) was hydrophilized using a hydrophilic surface coating based on amide functionalities, converting the hydrophobic membrane surface (water contact angle, WCA: ~90&deg, ) into an extremely hydrophilic one (WCA: ~30&deg, ). The amide layer was created by first immobilizing piperazine to the membrane surface via electron beam irradiation. Subsequently, a reaction with 1,3,5-benzenetricarbonyl trichloride (TMC) was applied to generate an amide structure. The presented approach resulted in a hydrophilic membrane surface, while maintaining permeance of the membrane without pore blocking. All membranes were investigated regarding their permeance, porosity, average pore size, morphology (SEM), chemical composition (XPS), and wettability. Soxhlet extraction was carried out to demonstrate the stability of the applied coating. The improvement of the modified membranes was demonstrated using dead-end filtration of algae solutions. After three fouling cycles, about 60% of the initial permeance remain for the modified membranes, while only ~25% remain for the reference.

A novel technique for the covalent immobilization of trypsin in a one-step reaction using low-energy electron beam is described. The enzyme immobilization was applied on poly(ethersulfone) and poly(vinylidene fluoride) microfiltration membranes. For this purpose, the membranes were dipped in an aqueous solution of trypsin followed by electron beam treatment. The effect of irradiation conditions on the immobilization was investigated, as well as the resulting membrane properties with respect to enzymatic activity, immobilized enzyme concentration, pure water flux, scanning electron microscopy, and porosimetry. This technique shall provide a simple, inexpensive method for enzyme immobilization on various polymer membranes and offer a tool for the application in enzymatic membrane reactors.

Polymer membranes have been modified with hyperbranched polymers with the aim to generate a high density of hydrophilic functional groups at the membrane surface. For this purpose hyperbranched polymers containing amino, alcohol, and carboxylic acid end groups were used for membrane modification, respectively. Thus, surface potential and charges were changed significantly to result in attractive or repulsive interactions towards three different proteins (albumin, lysozyme, myoglobin) that were used to indicate membrane fouling properties. Our studies demonstrated that hydrophilization alone is not effective for avoiding membrane fouling when charged proteins are present. In contrast, electrostatic repulsion seems to be a general key factor.

A new electron beam-based approach for the direct functionalization of polyethersulfone, polyvinylidene fluoride, polysulfone as well as polyacrylonitrile membranes in a one-step procedure is presented. Aqueous solutions of functional molecules were immobilized on the membrane surface by electron beam treatment. The resulting membranes show significantly increased flux and water wettability accompanied by decreased protein adsorption. Stability tests demonstrated the permanence of the modification. This new method neither requires any preceding surface functionalization nor the use of catalysts/photoinitiators or other toxic reagents. In addition, it avoids the synthesis of hydrophilic monomers/polymers, thus avoiding additional synthetic and purification steps as well as the use of organic solvents.

In this work we aim to show that the overall surface potential is a key factor to understand and predict anti-fouling characteristics of a polymer membrane. Therefore, polyvinylidene fluoride membranes were modified by electron beam-induced grafting reactions forming neutral, acidic, alkaline or zwitterionic structures on the membrane surface. The differently charged membranes were investigated regarding their surface properties using diverse analytical methods: zeta potential, static and dynamic water contact angle, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Porosimetry measurements proved that there is no pore blocking due to the modifications. Monodisperse suspensions of differently charged polystyrene beads were synthesized by a radical emulsion polymerization reaction and were used as a model fouling reagent, preventing comparability problems known from current literature. To simulate membrane fouling, different bead suspensions were filtered through the membranes. The fouling characteristics were investigated regarding permeation flux decline and concentration of model fouling reagent in filtrate as well as by SEM. By considering electrostatic interactions equal to hydrophobic interactions we developed a novel fouling test system, which enables the prediction of a membrane’s fouling tendency. Electrostatic forces are dominating, especially when charged fouling reagents are present, and can help to explain fouling characteristics that cannot be explained considering the surface wettability.

Electron-beam-induced grafting of hydrophilic polymers was applied to modify PVDF membranes for biomedical applications. Grafting was performed by immersing the membrane in an aqueous solution of different hydrophilic polymers followed by electron-beam irradiation. The two polymer types are able to cross-link by recombination of adjacent radicals formed via the irradiation. Although the untreated membrane was already quite hydrophilic, the modification resulted in even lower water contact angles at the membrane surface indicating improved water wettability. The presence of different functional groups originating from the hydrophilic polymers was detected on the membrane surface by electrokinetic measurements. SEM investigations as well as porosimetry experiments showed that the grafted hydrophilic polymer layer is very thin; therefore, the membrane pore structure is not negatively affected. Soxhlet extraction revealed the stability of the modification for selected polymers: surface contact angles were comparable after extraction, and total organic carbon investigation of the extraction water revealed no significant loss of organic material. Investigated mechanical properties confirmed an increased stability due to cross-linking of the polymers. Undesired hemolysis was not detected with hemocompatibility tests, and coagulation was decreased with selected hydrophilic polymers. Because of the absence of any toxic material during surface modification and the high stability of the product, this method is believed to be suitable for the modification of membranes for medical applications, e.g. for improving the hemo- or biocompatibility.

This article has been removed: please see Elsevier Policy on Article Withdrawal (http://www.elsevier.com/locate/withdrawalpolicy).This article has been removed at the request of the Executive Publisher.This article has been removed because it was published without the permission of the author(s).