Redox property of coordinated iron ion enables activation of O2 via in-situ generated H2O2 and additionally added H2O2 in EDTA-chelated Fenton reaction

Abstract

The Fenton system was a generation system of reactive oxygen species via the chain reactions, which employed H₂O₂ and O₂ as radical precursors and Fe²⁺/Fe³⁺ as electron-donor/acceptor for triggering or terminating the generation of radicals. Recent work mainly emphasized the Fe²⁺- activated H₂O₂ and the application of in-situ generated ^•OH, while neglecting other side-reactions. In this work, EDTA (Ethylene diamine tetraacetic acid) was employed as a chelating agent of iron ions, which simultaneously changed the redox property of coordinated iron. The Fe²⁺-EDTA complexes in the presence of dissolved oxygen enabled the two-electron transfer from Fe²⁺ to O₂ and the in-situ production of H₂O₂, which further activate H₂O₂ for yielding ^•OH. Meanwhile, coordinated Fe³⁺ exhibited non-negligible reactivity toward H₂O₂, which was higher than that of free Fe³⁺ in the traditional Fenton system. The complexation of EDTA with Fe³⁺ could enhance the Fe²⁺ generation reaction by the H₂O₂, accompanied by the O₂^•- formation. The enhancement of O₂^•- formation and Fe²⁺-EDTA regeneration induced the subsequent H₂O₂ activation by Fe²⁺-EDTA, thus accelerating the Fe³⁺-EDTA/Fe²⁺-EDTA cycle for simultaneously producing O₂^•- and ^•OH. To sum up, the EDTA-chelated Fenton system extended the applicable pH range to circumneutral/alkaline level and tuned the redox property of coordinated iron for diversifying the ^•OH production routes. The research reinterpreted the chain reactions in the Fenton system, revealing another way to enhance the radical production or other property of the Fenton/Fenton-like system.

Keywords: Chain reactions; EDTA; Fenton; Hydroxyl radicals; ROS evolution mechanism.