Ferrates: greener oxidants with multimodal action in water treatment technologies

CONSPECTUS: One of the biggest challenges for humanity in the 21st century is easy access to purified and potable water. The presence of pathogens and toxins in water causes more than two million deaths annually, mostly among children under the age of five. Identifying and deploying effective and sustainable water treatment technologies is critical to meet the urgent need for clean water globally. Among the various agents used in the purification and treatment of water, iron-based materials have garnered particular attention in view of their special attributes such as their earth-abundant and environmentally friendly nature. In recent years, higher-valent tetraoxy iron(VI) (Fe(VI)O4(2-), Fe(VI)), commonly termed, ferrate, is being explored for a broad portfolio of applications, including a greener oxidant in synthetic organic transformations, a water oxidation catalyst, and an efficient agent for abatement of pollutants in water. The use of Fe(VI) as an oxidant/disinfectant and further utilization of the ensuing iron(III) oxides/hydroxide as coagulants are other additional attributes of ferrate for water treatment. This multimodal action and environmentally benign character of Fe(VI) are key advantages over other commonly used oxidants (e.g., chlorine, chlorine dioxide, permanganate, hydrogen peroxide, and ozone). This Account discusses current state-of-the-art applications of Fe(VI) and the associated unique chemistry of these high-valence states of iron. The main focus centers around the description and salient properties of ferrate species involving various electron transfer and oxygen-atom transfer pathways in terms of presently accepted mechanisms. The mechanisms derive the number of electron equivalents per Fe(VI) (i.e., oxidation capacity) in treating various contaminants. The role of pH in the kinetics of the reactions and in determining the removal efficiency of pollutants is highlighted; the rates of competing reactions of Fe(VI) with itself, water, and the contaminants, which are highly pH dependent, determine the optimum pH range of maximum efficacy. The main emphasis of this account is placed on cases where various modes of ferrate action are utilized, including the treatment of nitrogen- and sulfur-containing waste products, antibiotics, viruses, bacteria, arsenic, and heavy metals. For example, the oxidative degradation of N- and S-bearing contaminants by Fe(VI) yields either Fe(II) or Fe(III) via the intermediacy of Fe(IV) and Fe(V) species, respectively (e.g., Fe(VI) → Fe(IV) → Fe(II) and Fe(VI) → Fe(V) → Fe(III)). Oxidative transformations of antibiotics such as trimethoprim by Fe(VI) generate products with no residual antibiotic activity. Disinfection and inactivation of bacteria and viruses can easily be achieved by Fe(VI). Advanced applications involve the use of ferrate for the degradation of cyanobacteria and microcystin originating from algal blooms and for covalently embedding arsenic and heavy metals into the structure of formed magnetic iron(III) oxides, therefore preventing their leaching. Applications of state-of-the-art analytical techniques, namely, in situ Mössbauer spectroscopy, rapid-freeze electron paramagnetic resonance, nuclear forward scattering of synchrotron radiation, and mass spectrometry will enhance the mechanistic understanding of ferrate species. This will make it possible to unlock the true potential of ferrates for degrading emerging toxins and pollutants, and in the sustainable production and use of nanomaterials in an energy-conserving environment.