Ferrates are oxy-anions of iron, and have been synthesized in laboratories since the mid-nineteenth century, with the iron in oxidation states of between two and eight (FeO22- and FeO52-). While several different ferrates have been studied for commercial exploitation, only Ferrate(VI), i.e. (FeO42-) has been utilized for environmental applications. Over 400 published studies have demonstrated the unique treatment capability of Ferrate(VI) for both water and wastewater, as it can simultaneously perform oxidation, and disinfection, and because it is unstable, it is quickly reduced to Fe3+which is a coagulant.
Ferrates can be synthesized in several ways, utilizing an oxidation process to convert metallic, ferrous, or ferric iron to a higher valence. The most efficient and cost-effective way to generate ferrate is by aqueous chemical oxidation of ferric iron utilizing chlorine in a caustic medium. Specifically, a ferric salt, either ferric chloride or ferric sulfate is oxidized to FeO42- by hypochlorite (either calcium or sodium) in the presence of sodium hydroxide, as shown in equation (1) below. The feed stocks are cheap and readily available at most treatment plants in their commercial forms, i.e.. (12% bleach, 40% ferric chloride or sulfate, and 50% caustic)
2FeCl3 + 3NaOCl + 10NaOH -> 2Na2FeO4 + 9NaCl + 5H2O (1)
When added to aqueous systems, Ferrate(VI) is a powerful oxidant which readily decomposes to ferric iron [Fe(OH)3] and oxygen according to:
2FeO42- + 5H2O -> 2Fe(OH)3 + 1.5O2 + 4OH- (2)
Because ferrate has a multi-treatment capacity, it is difficult to generalize optimum doses for each application. For example, when ferrate has been used for disinfection of clean systems such as drinking water, or highly treated wastewater effluent, the doses are usually in the range of 1 to 5 mg per liter (as FeO42-). If ferrate is to be used principally as an oxidant of organics, the dose can vary between 2 and 15 mg per liter. In those applications where ferrate has been utilized for co-precipitation of heavy metals, doses as low as 0.5 mg per liter have proven effective. Industrial wastes, especially those with a high TOC component, can require upwards of a 50 mg per liter dose.
The Center for Ferrate Studies has been actively involved in research to better understand ferrate chemistry in environmental applications. The Center through its cooperating faculty, has initiated research to describe ferrate’s ability to act as: an oxidant, disinfectant, and coagulating agent allowing a multi faceted treatment capability for water and gas.
In addition to the advancement of ferrate technology at the Center for Ferrate Studies, there are other missions: