Effect of calcium and sodium on membrane fouling behavior in the presence of microplastics and humic acid

This study examined the adsorption-desorption behavior of polystyrene (PS) on polyvinylidene fluoride (PVDF) membranes, focusing on the effects of humic acid (HA) and Ca2+/Na+ ions. The adsorption-desorption dynamics of HA were initially characterized to establish its role in modulating PS behavior. PS was selected as the representative microplastic (MP). Fouling experiments were conducted under the conditions with and without calcium and sodium ions. Quartz crystal microbalance with dissipation (QCM-D) indicated that the adsorption of PS on the membrane surface increased with increasing ion concentration, both in Ca2+ and Na+ conditions. And the adsorption of PS under Na+ conditions was apparently more than that under Ca2+, which is mainly due to the greater attraction between PS and membrane surface under Na+ condition. In the presence of HA, the adsorption of PS led to a looser structure of the fouling layer and made the fouling more reversible. Our results will help to enrich the understanding of the adsorption behavior of MPs under typical water chemistry conditions, and provide new insights into membrane fouling caused by their synergistic effects.