Influence of pH on the adsorptive fouling of ultrafiltration membranes by fatty acid

Fatty acids are found in many solutions treated in ultrafiltration plants, for example, in dairy products, fermentation broth, oily waste water and bl each plant effluents from pulp mills. The influence of fatty acids on the f ouling of membranes is often ignored because their concentration is rather low. However, during recent years the significant influence of fatty acids on the flux reduction of ultrafiltration membranes has attracted much atten tion. The pore radius of the membrane, the concentration of fatty acids and the pH of the solution all have a profound influence on the flux reduction when treating solutions containing fatty acids. In the work presented in this paper, the influence of pH on the flux reduct ion of an ultrafiltration membrane made of polyethersulphone was studied. I t was found that there was no flux reduction under alkaline conditions, whe reas the flux reduction under acidic conditions was severe. In order to elu cidate the adsorption process, the adsorption of octanoic acid on a planar, hydrophobized silica surface was studied by means of in situ null ellipsom etry. In the ellipsometry study a sharp increase in the amount adsorbed was obser ved when the concentration of undissociated acid approached the saturation concentration as the pH was decreased. This explains the observed flux redu ction under acidic conditions. (C) 2000 Elsevier Science B.V. All rights re served.