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

Abstract

One challenge in H2O2 based Fenton-like reaction is to break through the limitation of slow reduction of ferric species (FeIII). 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 1O2 in CaO2 system was excluded by the poor selectivity in phenol conversion and the comparable 1O2-TEMP EPR signals in both CaO2 and H2O2 systems. Kinetic analyses using chloroform as the probe of O2·− suggested the high production rate of O2·−, 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 FeIII: an inner sphere electron transfer takes place to reduce FeIII-EDTA and produce O2·−, and subsequently O2·− provides an electron to reduce another FeIII-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 FeIII reduction in Fenton-like reaction and facilitate the removal of organic pollutants at neutral pHs, and also indicates a promising source of O2·− for diverse applications.