Unexpected roles of ascorbic acid in terminating permanganate oxidation: Quencher and/or secondary oxidant precursor?

Ascorbic acid has been widely used as a quencher to terminate oxidation processes like Mn(VII) oxidation, ozonation and chlorination, aiming to achieve specific oxidant exposure without interfering with the reactions. However, this study found that quenching with excess ascorbic acid ([AH^-]₀: [Mn(VII)]₀ = 4.5-25) unexpectedly increased the degradation efficiency of micropollutants by approximately 1-5 times during Mn(VII) oxidation, compared to the theoretical degradation and quenching with hydroxylamine. Interestingly, this enhanced degradation weakened at higher ascorbic acid dosage ([AH^-]₀: [Mn(VII)]₀ = 50-200) and exhibited two stages: an initial rapid degradation immediately after quenching (up to 3-fold), followed by a gradual degradation lasting over days (up to 50 % degraded within 15 h). Combining scavenging experiments with spin-trapping and UV-vis absorption spectroscopy analyses, multiple reactive species including ^•OH, O₂^•-, ascorbyl radical (A^•-), and Mn(III) were identified, with ^•OH mainly contributing to the enhanced degradation during the entire reaction course and A^•- playing a key modulatory role. Initially, Mn(VII) oxidized ascorbic acid to form A^•-, which further reacted with O₂ to form O₂^•- that disproportionated into H₂O₂; then the latter was activated by Mn(II) to produce ^•OH and stabilization of Mn(III) via coordination significantly accelerated this process. After Mn(VII) was depleted, A^•- acted as an electron shuttle, catalyzing the Fenton-like cycle of Mn(II)/Mn(III) via ligands and contributing to the prolonged gradual degradation of micropollutants. These findings highlight the proper use of quenchers in kinetic studies of one-electron oxidants including but not limited to Mn(VII), and imply the potential formation of secondary oxidants modulated by electron donors widespread in natural waters.