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
MnII–nitrilotriacetic acid (NTA) complex, which
outperformed iron- and cobalt-based PAA activation processes for rapidly
degrading phenolic and aniline contaminants from water. The proposed
MnII/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 MnII center to elongate the O–O bond of the complexed PAA. Additionally,
the NTA-MnII-PAA* molecular cluster presented a lower energy
gap than the MnII–PAA complex, indicating that the
MnII–peroxy complex was more reactive in the presence
of NTA. Thus, the NTA-MnII-PAA* complex exhibited a stronger
oxidation potential than PAA, which could rapidly oxidize organic
contaminants from water. Further, we generalized our findings to the
CoII/PAA oxidation process and highlighted that the CoII–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.