Difficulties arise related to the economy-of-scale and practicability in applying conventional water treatment technologies to unconventional systems, like those for small and remote communities. A primary challenge relates to the supply and use of chemicals for oxidation processes. A possible alternative is that of electro-oxidation (EO), whereby water contaminants are degraded in situ via direct, advanced (●OH) and/or electrosynthesized oxidant mediated reactions. While direct and advanced EO occurs at the electrode surface, the electrosynthesis of oxidant species provides residual oxidation in the bulk water solution. One promising oxidant species is ferrate (Fe(VI)/FeO42-), which is well suited for water treatment applications, as in addition to having a high redox potential, its reduction product is non-toxic (e.g., hydrolysis species of Fe3+) and can function as a coagulant. Only recently has the circumneutral synthesis of ferrate been demonstrated, using high oxygen overpotential (HOP) electrodes, namely boron-doped diamond (BDD). While BDD are robust and have a wide potential range of water stability, several aspects have limited their adoption into industry including their extremely prohibitive costs and limiting growth conditions. In this research, the generation of ferrate using various HOP electrode materials, including Ni-Sb-SnO2, Sb-SnO2 and RuO2-IrO2-TiO2, was investigated. These materials are comparatively facile and inexpensive to prepare, compared to BDD, and have competitive potential ranges for minimizing the oxygen evolution reaction. Ferrate synthesis was investigated in a current density range of 5-20 mA/cm2, using an initial Fe3+ concentration of 10 mM (using FeCl3). While a faradaic efficiency of 2.8% was reported for BDD ferrate synthesis, Ni-Sb-SnO2 yielded efficiencies of 9-21% and a ferrate yield of 12.8, 63.5 and 69.4% during 5, 10 and 20 mA/cm2 operations, respectively. These results suggest mixed metal oxides as a potentially powerful material and novel reaction pathway for ferrate synthesis. Current research is evaluating the efficacy of Sb-SnO2 and RuO2-IrO2-TiO2 for ferrate generation, as well as further determining the kinetics and mechanisms of the circumneutral synthesis pathway.

Circumneutral electrosynthesis of ferrate(VI) oxidants for decentralized water treatment applications

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.