Fe(VI) activation system mediated by a solar-driven TiO2 nanotubes electrode for CLQ degradation: Performances, mechanisms and pathways

Ferrate (Fe(VI), FeO 4 2- ) has been widely used in the degradation of micropollutants with the advantages of high redox potential , no secondary pollution and inhibition of disinfection byproducts . However, the low transformation of Fe(V) and/or Fe(IV) by Fe(VI) and incomplete mineralization of pollutants limit their application. In this work, we designed a photo electric cell with TiO 2 nanotubes (TNTs) and Pt serving as the anode and cathode to enhance the utilization of Fe(VI) (Fe(VI)-TNTs system). TNTs accelerated the generation of •OH via h VB + oxidation of OH - and photogenerated electrons at Pt boosted the transformation of Fe(VI) to Fe(V) and/or Fe(IV), resulting in a 22.2 % enhancement of chloroquine (CLQ) removal compared to Fe(VI) alone. The results from EPR and quenching tests showed that Fe(VI), Fe(V), Fe(IV), •OH, O 2 •- and h VB + coexisted in the Fe(VI)-TNTs system, among which Fe(V) and Fe(IV) were testified as the primary reactive substances accounting for 59 % of CLQ removal. The performance tests and recycling tests demonstrated that the Fe(VI)-TNTs system maintained excellent performance in an authentic water environment. The plausible degradation pathway of CLQ oxidized in the Fe(VI)-TNTs system was proposed with nine identified oxidation products via N-C cleavage, electrophilic addition and carboxylation processes. Based on the ECOSAR calculation, the constructed reaction system allowed a decrease in acute and chronic toxicity. Our findings provide a highly efficient and cost-effective strategy to enhance Fe(VI) application for micropollutant degradation in the future.