Advances in contaminants abatement by ferrate(VI)

Abstract

Ferrate(VI) (FeVIO42−, Fe(VI)) has received increasing attention as a potential water treatment chemical since it possesses oxidation, disinfection, and coagulation properties, which can be used in simultaneously remediating organic contaminants (e.g., endocrine disruptors and pharmaceuticals), inactivating microorganisms (e.g., Escherichia coli and bacteriophage MS2), and removing toxic metals (e.g., Cu(II) and As(III)) in water. This paper introduces the properties of Fe(VI) and its oxidation mechanisms, then reviews the research on Fe(VI)-based water treatment technologies in recent years. The benefits of using Fe(VI) as an oxidizing agent are that it produces an environmentally benign byproduct (Fe(III)) while exhibiting high efficiency in abating pollutants in water, although Fe(VI) oxidation efficiency is limited to its stability and selectivity. The recent research results on the degradation of emerging organic contaminants by enhanced Fe(VI) oxidation technologies are presented. The induction of Fe(V) and Fe(IV) species generation in organic contaminants oxidation by Fe(VI) is the most effective method that may contribute to enhanced oxidation of organic contaminants that are less reactive with Fe(VI). Because Fe(V) and Fe(IV) have their higher oxidation reactivity than Fe(VI), and the rate constants of the reactions of Fe(IV) and Fe(V) with compounds are 2–4 orders of magnitude higher than that of Fe(VI) with compounds. It is generally accepted that no harmful byproducts such as halogenated organics and bromate would be formed during Fe(VI) oxidation. However, some recent studies have shown that disinfection by-products (DBPs) might be generated during Fe(VI) oxidation in the presence of Br– or I–. Under environmentally relevant conditions, the formation of bromate and DBPs would not be a problem for Fe(VI) application as their concentration levels are quite low. Meanwhile, the characteristics of newly formed iron(III) oxides/hydroxide during Fe(VI) oxidation and self-decomposition, and the mechanisms for metals removal by newly formed iron(III) oxides/hydroxide are summarized in this paper. Besides Fe(VI) oxidation, adsorption of organic pollutants and metals with Fe(VI) resultant nanoparticles could also be an effective method for water treatment and environmental remediation. Finally, the current knowledge gaps and future research needs are proposed. The economic synthesis method for ferrate remains a major challenge. The mechanism of the oxidation of organic pollutants by Fe(VI), especially the involvement of Fe(V) and Fe(IV), remains to be further clarified. The oxidation performance of Fe(VI) should be improved by greener and more efficient activation means. The oxidation and adsorption/coagulation synergetic effect of Fe(VI) should also be explored to promote the application of Fe(VI) in water pollution control. We believe that this review could inspire scientists, inventors, researchers, and engineers in this field.