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