Probing the roles of Ca2+ and Mg2+ in humic acids-induced ultrafiltration membrane fouling using an integrated approach

Membrane fouling induced by natural organic matter (NOM) negatively affects the performance of ultrafiltration (UF) technology in producing drinking water. Divalent cation is found to be an important factor that affects the NOM-induced membrane fouling process. In this work, attenuated total reflection-Fourier transformation infrared spectroscopy (ATR-FTIR) coupled with quartz crystal microbalance (QCM), assisted by isothermal titration calorimetry (ITC), is used to explore the contribution of Mg2+ and Ca2+, the two abundant divalent cations in natural water, to the UF membrane fouling caused by humic acid (HA) at a molecular level. The results show that Ca2+ exhibited superior performance in accelerating fouling compared to Mg2+. The hydrophobic polyethersulfone (PES) membrane exhibited greater complexation with HA in the presence of Mg2+ and Ca2+, compared to the hydrophilic cellulose membrane, as evidenced by the more intense polysaccharide C–O, aromatic CC and carboxylic CO bands in the FTIR spectra. The QCM and ITC measurements provide quantitative evidence to support that Ca2+ was more effective than Mg2+ in binding with HA and accumulating foulants on the membrane surfaces. The higher charge neutralization capacity and more favorable binding ability of Ca2+ were found to be responsible for its greater contribution to the NOM-induced membrane fouling than Mg2+. This work offers a new insight into the mechanism of cation-mediated NOM-induced membrane fouling process, and demonstrates that such an integrated ATR-FTIR/QCM/ITC approach could be a useful tool to explore other complicated interaction processes in natural and engineered environments.