Combined fouling of forward osmosis membrane by alginate and TiO₂ nanoparticles and fouling mitigation mechanisms

Forward osmosis (FO) is a promising technology for water treatment, but its fouling mechanisms are poorly understood compared to other membrane-based processes. This study focuses on combined fouling caused by alginate (SA) and TiO₂ nanoparticles, which serve as representative organic and inorganic foulants, respectively. The results show that the co-presence of TiO₂ can effectively mitigate membrane fouling by SA under vas feed chemistries (Ca²⁺ concentrations). The negative charge of the SA foulants increased in the presence of TiO₂, alleviating SA aggregation due to electrostatic and steric stabilization. The behavior and mechanisms of membrane fouling were characterized by attenuated total reflection-Fourier transformation infrared spectroscopy (ATR-FTIR) coupled with isothermal titration calorimetry (ITC) and atomic force microscopy (AFM) at the molecular level. Combined SA-TiO₂ had a lower binding affinity to Ca²⁺ than single SA, which was spontaneously exothermic and dominated by electrostatic interaction to reduce membrane fouling. This study provides new insight into the mechanisms of nanoparticles-mediated organic fouling in the FO process. It also demonstrates that an integrated ATR-FTIR/ITC/AFM approach can provide useful information for understanding other complicated interactions between inorganic and organic foulants.