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

Abstract

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 × 10(6) 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 = ∼10(7) 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).

Oxidation of manganese(II) with ferrate: Stoichiometry, kinetics, products and impact of organic carbon

“…At pH 7.5 and 6.2, the apparent reaction constant is faster than the maximum detectable rate of the stopped flow system, 9 Â 10 4 M À1 s À1 . This reaction rate constant is orders of magnitude larger than the reaction rate constant of ferrate self-decay of 1 Â 10 2 M À1 s À1 and 2 Â 10 2 M À1 s À1 , at pH 7.5 and 6.2, respectively (Lee et al, 2014). This supports the conclusion that ferrate self-decay is not appreciable in the stoichiometry of Mn(II) oxidation.…”

“…In other reactions, Fe(VI) may undergo a one or two electron transfer, momentarily resulting in perferryl(V) or ferryl(IV) (Lee et al, 2005;Sharma, 2010). These intermediates are much more reactive than Fe(VI), and may react rapidly and indiscriminately with other solutes, including H 2 O (Lee et al, 2014). The production of these intermediates is difficult to Fig.…”

Preliminary Assessment of Ferrate Treatment of Metals in Acid Mine Drainage

“…Figure 2 includes the results from experiments up to the previously described optimal Fe(VI) dose of 25 μM, in terms Fe and Mn precipitated on a molar basis. Figure 2 also includes the theoretical stoichiometry of the oxidation reaction between Fe(II) and Fe(VI) (Lee et al, 2014), and Mn(II) and Fe(VI) (Goodwill et al, 2016b), respectively. Lee et al (2014) demonstrated a 3:1 stoichiometry in an N 2 –purged, circumneutral pH solution with 5 mM carbonate buffer (see Fig.…”

“…This suggests that Fe(VI) auto‐decay (e.g., oxidation of water) competes with other redox pathways, or that the Fe and Mn were already in an oxidized state prior to Fe(VI) addition. The apparent rate constant ( k app ) of Fe(VI) auto‐decay has been quantified by Lee et al (2014) and ranges from ∼5 × 10 4 M −1 s −1 at pH 3.0 to ∼1 × 10 4 M −1 s −1 at pH 3.5. However, Lee et al used phosphate buffers to sequester Fe(III) iron particles, which retards the rate of Fe(VI) decay (Jiang et al, 2015).…”

Spectrophotometric Method for Determination of Ozone Residual in Water Using ABTS: 2.2’-Azino-Bis (3-Ethylbenzothiazoline-6-Sulfonate)

“…The direct ultraviolet (UV) absorbance method, the iodometric method, the DPD method, a membrane electrode method and the indigo method are those most commonly used for determination of aqueous ozone in the laboratory. Although these methods have been used for ozone determination for many years, there are still significant disadvantages that limit their application (Lee, Kissner, and Von Gunten 2014;Von Gunten 2003b).…”