CaO2 based Fenton-like reaction at neutral pH: Accelerated reduction of ferric species and production of superoxide radicals

One challenge in H2O2 based Fenton-like reaction is to break through the limitation of slow reduction of ferric species (Fe-III). Present work describes a dramatic acceleration of Fenton-like reaction at neutral pH by using calcium peroxide (CaO2) as a source of hydrogen peroxide (H2O2) and EDTA as a chelating agent of ferric ions. In an optimized condition, phenol degradation in the H2O2 system displayed an initial latent time of 60 min, while phenol can be degraded immediately and removed completely in 30 min in the CaO2 system. Visual MINTEQ analyses indicated Fe-EDTA(-) was the active species in the reaction. The contribution of O-1(2) in CaO2 system was excluded by the poor selectivity in phenol conversion and the comparable (O2-TEMP)-O-1 EPR signals in both CaO2 and H2O2 systems. Kinetic analyses using chloroform as the probe of O-2(-) suggested the high production rate of O-2(-), which is four orders of magnitude higher than that in H2O2 system. The mechanism of the accelerated CaO2 based Fenton-like reactions was featured by that two electrons coming from CaO2 can be utilized to promote reduction of Fe-III: an inner sphere electron transfer takes place to reduce Fe-III-EDTA and produce O-2(-), and subsequently O-2(-) provides an electron to reduce another Fe-III-EDTA. The revealed intrinsic reducibility in CaO2 based Fenton-like reaction represents a new strategy to break through the well-known rate limiting step of Fe-III reduction in Fenton-like reaction and facilitate the removal of organic pollutants at neutral pHs, and also indicates a promising source of O-2(-) for diverse applications. (C) 2018 Elsevier Ltd. All rights reserved.