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• •OH could be rapidly generated from Fe2+/EDTA system (O 2 atmosphere). • Fe2+-EDTA activate O 2 to induce H 2 O 2 in-situ via direct 2-electron transformation. • EDTA tune the production pathways of radical or intermediate in Fenton system. • The dual effect of •OH and O 2 •− was revealed in Fe3+/EDTA/H 2 O 2 system. The Fenton system was a generation system of reactive oxygen species via the chain reactions, which employed H 2 O 2 and O 2 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 H 2 O 2 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 O 2 and the in-situ production of H 2 O 2 , which further activate H 2 O 2 for yielding •OH. Meanwhile, coordinated Fe3+ exhibited non-negligible reactivity toward H 2 O 2 , 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 H 2 O 2 , accompanied by the O 2 •− formation. The enhancement of O 2 •− formation and Fe2+-EDTA regeneration induced the subsequent H 2 O 2 activation by Fe2+-EDTA, thus accelerating the Fe3+-EDTA/Fe2+-EDTA cycle for simultaneously producing O 2 •− 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. [Display omitted] [ABSTRACT FROM AUTHOR]