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Reaction of Ferrate(VI) with ABTS and Self-Decay of Ferrate(VI): Kinetics and Mechanisms

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journal contribution
posted on 06.05.2014 by Yunho Lee, Reinhard Kissner, Urs von Gunten
Reactions of ferrate­(VI) during water treatment generate perferryl­(V) or ferryl­(IV) as primary intermediates. To better understand the fate of perferryl­(V) or ferryl­(IV) during ferrate­(VI) oxidation, this study investigates the kinetics, products, and mechanisms for the reaction of ferrate­(VI) with 2,2′-azino-bis­(3-ethylbenzothiazoline-6-sulfonate) (ABTS) and self-decay of ferrate­(VI) in phosphate-buffered solutions. The oxidation of ABTS by ferrate­(VI) via a one-electron transfer process produces ABTS•+ and perferryl­(V) (k = 1.2 × 106 M–1 s–1 at pH 7). The perferryl­(V) mainly self-decays into H2O2 and Fe­(III) in acidic solution while with increasing pH the reaction of perferryl­(V) with H2O2 can compete with the perferryl­(V) self-decay and produces Fe­(III) and O2 as final products. The ferrate­(VI) self-decay generates ferryl­(IV) and H2O2 via a two-electron transfer with the initial step being rate-limiting (k = 26 M–1 s–1 at pH 7). Ferryl­(IV) reacts with H2O2 generating Fe­(II) and O2 and Fe­(II) is oxidized by ferrate­(VI) producing Fe­(III) and perferryl­(V) (k = ∼107 M–1 s–1). Due to these facile transformations of reactive ferrate­(VI), perferryl­(V), and ferryl­(IV) to the much less reactive Fe­(III), H2O2, or O2, the observed oxidation capacity of ferrate­(VI) is typically much lower than expected from theoretical considerations (i.e., three or four electron equivalents per ferrate­(VI)). This should be considered for optimizing water treatment processes using ferrate­(VI).