Common oxidants activate the reactivity of zero-valent iron (ZVI) and hence remarkably enhance nitrate reduction from water

[Display omitted] •H2O2 and KMnO4 overcome the impedance of ZVI passive layers.•Oxidants (i.e., H2O2, KMnO4) facilitate nitrate reduction by ZVI at non-acid pH.•At pH>∼5, nitrate reduction by ZVI in the absence of O2 was negligible.•Dissolved oxygen initially facilitates but ultimately limits nitrate reduction by ZVI. The surface passivation and the subsequent reduction of ZVI reactivity seriously restricted the application of zero-valent-iron (ZVI) for water remediation. This study comprehensively investigated the critical role of common oxidants, such as O2, H2O2, KMnO4 and CrO42− in activating the reactivity of ZVI surface and hence in facilitating nitrate reduction from the aqueous solution at non-acidic pH. Nitrate reduction by ZVI was negligible in the absence of O2 at pH>5. To the contrary, obvious reduction of nitrate was observed in the presence of O2, although with a clear trend of decreasing nitrate reduction with pH increase. KMnO4, H2O2, and CrO42− were observed to strongly enhance the reactivity of ZVI. Dosing KMnO4 and H2O2 achieved 94% and 97% of nitrate reduction at pH 5.0, and 74% and 98% at pH 7.0. Five cycles of nitrate removal by the combined system of ZVI/oxidant/zeolite demonstrated the overcoming of the impedance of passivation layer, strongly facilitating the ultimate removal of nitrogen species from the aqueous solution.