Fe(VI) enhanced coagulation-ultrafiltration for simultaneous ibuprofen removal and membrane fouling alleviation

The potential of Fe(VI) enhanced coagulation-ultrafiltration (C-UF) for the simultaneous enhancement of ibuprofen (IBP) removal and membrane alleviation was investigated by comparing Fe(VI)-UF, FeCl₃-UF, AlCl₃-UF, Fe(VI)-FeCl₃-UF, and Fe(VI)-AlCl₃-UF. The results showed that Fe(VI)-FeCl₃-UF and Fe(VI)-AlCl₃-UF achieved IBP removal efficiencies of 66.6% and 76.9%, respectively, which were significantly higher than those obtained with Fe(VI)-UF, FeCl₃-UF, AlCl₃-UF. This confirmed that Fe(VI) enhanced C-UF could effectively improve IBP removal compared to individual Fe(VI)-UF or conventional C-UF. This improvement was attributed to additional highly reactive Fe(V)/Fe(IV) intermediates generated by adding metal coagulants to instantaneously activate residual Fe(VI) during the Fe(VI) enhanced C-UF process. In addition, Fe(VI)-FeCl₃-UF and Fe(VI)-AlCl₃-UF further alleviated total membrane fouling compared to FeCl₃-UF and AlCl₃-UF, with a significant shift of membrane fouling toward reversible fouling. Floc properties, NOM characteristics, and membrane surface characterization were comprehensively analyzed to reveal membrane fouling alleviation mechanisms in Fe(VI) enhanced C-UF. Simultaneous cleavage into smaller molecules and polymerization into larger fractions of NOM by Fe(VI) could reduce the affinity of foulants for membrane surface adsorption and consequently lower irreversible fouling. While Fe(VI) reduction would produce iron-based hydroxides to participate in the growth process of flocs, leading to the formation of flocs with higher density. These flocs would accumulate on membrane surface to form a denser cake layer, which contributed to severe membrane fouling.