Identification of unstable high-valent iron species in electron transfer reactions of ferrate(VI) (Fe^VIO₄ ²⁻, Fe(VI)) has been an important challenge in advancing the understanding of the oxidative mechanisms of ferrates. This paper presents the first example of distinguishing various phases differing in the valence state of iron in the solid state reduction of Fe(VI) to Fe(III) oxides at 235 °C using hyperfine parameters, isomer shift and hyperfine magnetic field, obtained from nuclear forward scattering of synchrotron radiation (NFS). The NFS technique enables a fast data accumulation resulting in high time resolution of in situ experiments. The results suggest a reaction mechanism, involving Fe(V) and Fe(IV) species, in the thermal decomposition of K₂FeO₄ to KFeO₂. The present study opens up an approach to exploring the unambiguous identification of Fe(VI), Fe(V), Fe(IV), and Fe(III) in electron-transfer reaction mechanisms of ferrates in solid and aqueous phase systems.

The kinetics and mechanism of ferrate(IV), (V) and (VI) transformations in water and in polar organic solvents (namely ethanol and tetrahydrofuran) have been investigated by the method of ⁵⁷Fe Mössbauer spectroscopy of frozen solutions. Ethanol with a very limited amount of water under an inert atmosphere, significantly slows down the transformation reactions of ferrates(IV and V) and provides direct proof of the existence of intermediate states. Simultaneously, ethanol is oxidized to caboxylates in the close vicinity of the surface of ferrate crystallites as proven by X-ray photoelectron spectroscopy. On the contrary, any transformation of ferrate(VI) in pure ethanol (with a very limited amount of water) was not observed. Mössbauer spectroscopy of frozen solutions enabled us to experimentally identify and quantify intermediates of ferrate(IV) and ferrate(V) transformations for the first time. Sodium ferrate(IV) in its tetrahedral form, Na₄FeO₄, undergoes a two-step charge disproportionation to Fe(III) and Fe(VI) via a Fe(V) intermediate without any evolution of oxygen in polar protic and aprotic solvents, specifically 2Fe(IV) → Fe(III) + Fe(V), and Fe(IV) + Fe(V) → Fe(III) + Fe(VI), i.e. in sum 3Fe(IV) → 2Fe(III) + Fe(VI). Ferrate(V) (K₃FeO₄) transforms to Fe(III) and Fe(VI) without any indication of the Fe(IV) intermediate within the detection limit of the method. In addition to a charge disproportionation reaction proceeding in polar liquids, 3Fe(V) → Fe(III) + 2Fe(VI), a competitive reduction of Fe(V) directly to Fe(III) accompanied by oxygen evolution takes place in water. Oxygen evolution was also measured for ferrate(IV and VI) transformations in water, but to a higher and a smaller extent compared to ferrate(V), respectively. The thermodynamics of the suggested ferrate(IV) and ferrate(V) transformation pathways was examined by DFT calculations.

Owing to Mössbauer spectroscopy, an advanced characterization technique for iron-containing materials, the present study reveals previously unknown possibilities using l-amino acids for the generation of magnetic particles. Based on our results, a simple choice of the order of l-amino acids addition into a reaction mixture containing ferrous ions leads to either superparamagnetic ferric oxide/oxyhydroxide particles, or magnetically strong Fe⁰-Fe₂O₃/FeOOH core-shell particles after chemical reduction. Conversely, when ferric salts are employed with the addition of selected l-amino acids, only Fe⁰-Fe₂O₃/FeOOH core-shell particles are observed, regardless of the addition order. We explain this phenomenon by a specific transient/intermediate complex formation between Fe²⁺ and l-glutamic acid. This type of complexation prevents ferrous ions from spontaneous oxidation in solutions with full air access. Moreover, due to surface-enhanced Raman scattering spectroscopy we show that the functional groups of l-amino acids are not destroyed during the borohydride-induced reduction. These functionalities can be further exploited for (i) attachment of l-amino acids to the as-prepared magnetic particles, and (ii) for targeted bio- and/or environmental applications where the surface chemistry needs to be tailored and directed toward biocompatible species.