Novel Synthesis Pathways for Highly Oxidative Iron Species: Generation, Stability, and Treatment Applications of Ferrate(IV/V/VI)

Abstract

Difficulties arise related to the economy-of-scale and practicability in applying conventional water treatment technologies to small and remote systems. A promising oxidation technology better suited for these applications is that of electro-oxidation (EO), whereby contaminants are degraded via direct, advanced, and/or electrosynthesized oxidant-mediated reactions. One species of oxidants of particular interest includes ferrates (Fe(VI)/(V)/(IV)), where only recently has their circumneutral synthesis been demonstrated, using high oxygen overpotential (HOP) electrodes, namely boron-doped diamond (BDD). In this study, the generation of ferrates using various HOP electrodes (BDD, NAT/Ni–Sb–SnO₂, and AT/Sb-SnO₂) was investigated. Ferrate synthesis was pursued in a current density range of 5–15 mA cm⁻² and initial Fe³⁺ concentrations of 10–15 mM. Faradaic efficiencies ranged from 11–23%, depending on operating conditions, with BDD and NAT significantly outperforming AT electrodes. Speciation tests revealed that NAT synthesizes both ferrate(IV/V) and ferrate(VI), while the BDD and AT electrodes synthesized only ferrate(IV/V) species. A number of organic scavenger probes were used to test the relative reactivity, including nitrobenzene, carbamazepine, and fluconazole, whereby ferrate(IV/V) was significantly more oxidative than ferrate(VI). Finally, the ferrate(VI) synthesis mechanism by NAT electrolysis was elucidated, where coproduction of ozone was found to be a key phenomenon for Fe³⁺ oxidation to ferrate(VI).

Additional Information

© 2023 The Authors. Published by American Chemical Society. Attribution 4.0 International (CC BY 4.0). This research was supported by the Resnick Sustainability Institute (RSI) Impact Grant. The authors would also like to thank Heng Dong for help with the Raman spectroscopy work, and Carl K. McBeath for help with the graphical abstract illustration. Author Contributions. Co-first authors: S.T.M. and Y.Z. contributed equally to this paper. The authors declare no competing financial interest.

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