Whose Oxygen Atom Is Transferred to the Products? A Case Study of Peracetic Acid Activation via Complexed Mn^II for Organic Contaminant Degradation

Abstract

Identifying reactive species in advanced oxidation process (AOP) is an essential and intriguing topic that is also challenging and requires continuous efforts. In this study, we exploited a novel AOP technology involving peracetic acid (PAA) activation mediated by a Mn^II-nitrilotriacetic acid (NTA) complex, which outperformed iron- and cobalt-based PAA activation processes for rapidly degrading phenolic and aniline contaminants from water. The proposed Mn^II/NTA/PAA system exhibited non-radical oxidation features and could stoichiometrically oxidize sulfoxide probes to the corresponding sulfone products. More importantly, we traced the origin of O atoms from the sulfone products by ¹⁸O isotope-tracing experiments and found that PAA was the only oxygen-donor, which is different from the oxidation process mediated by high-valence manganese-oxo intermediates. According to the results of theoretical calculations, we proposed that NTA could tune the coordination circumstance of the Mn^II center to elongate the O-O bond of the complexed PAA. Additionally, the NTA-Mn^II-PAA* molecular cluster presented a lower energy gap than the Mn^II-PAA complex, indicating that the Mn^II-peroxy complex was more reactive in the presence of NTA. Thus, the NTA-Mn^II-PAA* complex exhibited a stronger oxidation potential than PAA, which could rapidly oxidize organic contaminants from water. Further, we generalized our findings to the Co^II/PAA oxidation process and highlighted that the Co^II-PAA* complex might be the overlooked reactive cobalt species. The significance of this work lies in discovering that sometimes the metal-peroxy complex could directly oxidize the contaminants without the further generation of high-valence metal-oxo intermediates and/or radical species through interspecies oxygen and/or electron transfer.