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. 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.