Colloid–membrane interaction effects on flux decline during cross-flow ultrafiltration of colloidal silica on semi-ceramic membranes

In order to investigate colloid–colloid, as well as colloid–membrane interaction effects on the flux decline occurring during membrane processes, filtration tests were performed at different electrolyte concentrations, as well as at different pH values. The surface properties of the colloidal silica particles were determined by electrophoretic light scattering, whereas the surface charge characteristics of the ultrafiltration membranes were derived from streaming potential measurements. From the flux versus transmembrane pressure curves, it was derived that both high and low salt concentrations were preferable to optimise the permeate flux provided that both particles and membrane had similar charge characteristics. At low salt concentration, this behaviour was due to colloid–colloid as well as colloid–membrane repulsive interactions, which largely reduced concentration polarisation. At high salt concentrations, flocculation of the colloidal dispersions gave raise to large, porous flocs. As a consequence of the latter effect, the concentration–polarisation layer became highly permeable. On the other hand, a severe flux decline was observed at pH conditions where the colloidal particles and the ultrafiltration membrane were oppositely charged, giving rise to attractive electrostatic interactions. The results indicate that the performance of ultrafiltration membranes is strongly affected by its interaction with the colloidal particles in the feed. Optimum conditions are obtained if repulsive interactions prevail, which may be accomplished by proper conditioning of the feed.