On the Origins of Some Spectroscopic Properties of “Purple Iron” (the Tetraoxoferrate(VI) Ion) and Its Pourbaix Safe-Space

Abstract

In this work, the authors attempt to interpret the visible, infrared and Raman spectra of ferrate(VI) by means of theoretical physical-inorganic chemistry and historical highlights in this field of interest. In addition, the sacrificial decomposition of ferrate(VI) during water treatment will also be discussed together with a brief mention of how Rayleigh scattering caused by the decomposition of FeVIO42− may render absorbance readings erroneous. This work is not a compendium of all the instrumental methods of analysis which have been deployed to identify ferrate(VI) or to study its plethora of reactions, but mention will be made of the relevant techniques (e.g., Mössbauer Spectroscopy amongst others) which support and advance this overall discourse at appropriate junctures, without undue elaboration on the foundational physics of these techniques.

Direct Detection of FeVI Water Oxidation Intermediates in an Aqueous Solution

“…3 T 2 , respectively. [14] In addition, the two shoulders at 350 and 570 nm and the two minima at 390 and 675 nm in the absorption curves were also shown to be associated with the formation of Fe VI O 4…”

“…The corresponding UV/Vis spectroelectrochemistry in Figure 1c revealed the appearance of distinctive peaks around 505 and 780 nm, which gradually increased with time. The appearance of these peaks is attributed to the electronic transition in the highly oxidized Fe VI complex [Fe VI O 4 2− ], with the absorptions at 505 and 780 nm representing the d‐d transitions of 3 A 2 → 3 T 1 (F) and 3 A 2 → 3 T 2 , respectively [14] . In addition, the two shoulders at 350 and 570 nm and the two minima at 390 and 675 nm in the absorption curves were also shown to be associated with the formation of Fe VI O 4 2− [15] .…”

Degradation of Lignosulfonate to Vanillic Acid Using Ferrate

“…[ 70 ] It is postulated that ferrate can be generated at pH = 14 between the Fe(VI)/Fe(III) standard reduction potential of 0.71 V versus SHE and the necessarily applied potential to oxidize H 2 O at 1.21 V versus SHE, assuming +0.5 V of over‐potential. [ 46,70 ] However, in our practical application, a novel GCI electrode showed an exponential increase in current, reminiscent of OER, starting at 750 mV versus SHE without a distinguishable ferrate peak during LSV (Figure S6, Supporting Information). These findings explain the generally low current efficiencies because ferrate generation competes with OER under the applied conditions.…”

“…[ 38,44 ] However, the practical application remains challenging, mainly since ferrate is only metastable in highly alkaline conditions limiting its use in wastewater treatment. [ 36,45,46 ] Moreover, conventional chemical synthesis of ferrate requires hypochlorite, which diminishes the advantage of using a green oxidizer. [ 36–40 ]…”

“…[38,44] However, the practical application remains challenging, mainly since ferrate is only metastable in highly alkaline conditions limiting its use in wastewater treatment. [36,45,46] Moreover, conventional chemical synthesis of ferrate requires hypochlorite, which diminishes the advantage of using a green oxidizer. [36][37][38][39][40] If prepared via an electrochemical route, ferrate can be obtained by anodic oxidation of iron in alkaline media without needing additional reagents.…”