Insight into the synergistic acceleration effects of EDTA and hydroxylamine in heterogeneous Fenton-like reaction over a wide initial pH range

Chelating agent modification and reductant enhancement are effective strategies to improve the narrow working pH and Fe3+/Fe2+ cycle delay of a heterogeneous Fenton system. However, the different mechanisms for the promotional effects between disodium ethylenediaminetetraacetate (EDTA) and hydroxylamine (HA) are rarely reported. Herein, the effects of EDTA and HA on the degradation kinetics of the selected probe compounds were compared in the CoFe2O4/H2O2 system for the first time. This paper found that the EDTA-chelated CoFe2O4 Fenton-like system enhanced the activation rate of H2O2 via reducing the redox potential of Fe2+/Fe3+ and Co2+/Co3+ pairs. The apparent rate constant of methyl orange (MO) in the CoFe2O4/H2O2/HA system was approximately 84-fold higher than that in the CoFe2O4/H2O2 system under the reaction conditions with 1.0 mM of HA and pH 6.4 within 180 min. This result was attributed to the dynamic equilibrium of Fe3+/Fe2+ and Co3+/Co2+ redox cycles after the addition of HA. Based on quenching experiments and electron paramagnetic resonance (EPR), these superior activities are due to the hydroxyl radicals (•OH) and singlet oxygen (1O2) under circumneutral pH, whereas •OH is the major reactive oxygen species (ROS) at pH values <4 or >10. The potential mechanism of CoFe2O4/H2O2/EDTA/HA systems was elucidated.