Prolonged degradation of organic contaminants by Fe(II)/peracetic acid: Unraveling the roles of coexisting H2O2 and pH.

The Fe(II)-activated peracetic acid (PAA) system is a promising advanced oxidation process for the removal of refractory organic pollutants. Although previous studies have reported its mechanism, highlighting the rapid generation of Fe(IV) and radicals (R-O•) within one second, the effects of coexisting H2O2 and pH remain unclear. In this study, we explored these factors in greater detail. In the initial fast stage, the Fe(IV) yield was found to be slightly influenced by water matrix, suggesting a superiority of Fe(II)/PAA in producing Fe(IV) under real water background. Moreover, our findings confirm that coexisting H2O2 is activated by the residual Fe(II) after PAA consumption, broadening the applicability for pollutants removal. The fast stage tends to degrade electron-rich pollutants, while the slow stage can degrade electron-deficient and electron-rich pollutants. During this process, in-situ formed Fe(III) exhibited negligible reactivity towards PAA or H2O2. As the pH increased from 3.0 to 7.0, the overall production of Fe(IV) and •OH drastically declined, reducing the system's oxidizing capacity. Density functional theory (DFT) calculations further suggest that deprotonation at higher pH levels theoretically hinders PAA decomposition by Fe(II). Using sulfamethoxazole (SMX) as a model pollutant, we observed that acidic conditions improved both pollutant removal efficiency and the formation of less toxic by-products. This study significantly advances the understanding of the decontamination mechanisms in the Fe(II)/PAA system.