New Insights into Natural Polyphenol-Enhanced Fe(III)/Peracetic Acid System under Acidic pH Conditions: The Overlooked Role of Coexisting Hydrogen Peroxide

Natural polyphenols have been extensively utilized as reducing agents to enhance contaminant degradation in the Fe(III)/peracetic acid (PAA) system. However, the roles of coexisting hydrogen peroxide (H₂O₂) remain insufficiently explored. This study, using protocatechuic acid (PCA) as a representative natural polyphenol, demonstrated that contaminant removal within the PCA/Fe(III)/PAA system under acidic pH conditions exhibited two kinetic stages: an initial rapid stage driven by PAA, followed by a slower stage driven by H₂O₂. The presence of H₂O₂ facilitated the complete degradation (100%) of contaminants even at low concentrations (<1.0 μM). Interestingly, these two stages contributed differently to various contaminants' degradation. Mechanistic investigations revealed that Fe(IV) was the major reactive species (RSs) for contaminant degradation during the PAA stage, while ^•OH dominated during the H₂O₂ stage. In brief, H₂O₂ enriched the generation pathways and types of RSs. Notably, besides PCA itself, the reaction intermediates (i.e., phenoxy radicals) formed during the reaction between PCA and RSs also played a key role in reducing Fe(III), which explained why the PCA/Fe(III)/PAA system was able to maintain sufficient Fe(II) to further interact with H₂O₂. Overall, this study highlighted the synergistic role of coexisting H₂O₂ and provided valuable insights for optimizing various contaminants' degradation in actual waters using PAA-based Fenton-like systems.