Role of Ferrate(IV) and Ferrate(V) in Activating Ferrate(VI) by Calcium Sulfite for Enhanced Oxidation of Organic Contaminants

Although the Fe(VI)-sulfite process has shown great potential for the rapid removal of organic contaminants, the major active oxidants (Fe(IV)/Fe(V) versus SO₄^•-/^•OH) involved in this process are still under debate. By employing sparingly soluble CaSO₃ as a slow-releasing source of SO₃^2-, this study evaluated the oxidation performance of the Fe(VI)-CaSO₃ process and identified the active oxidants involved in this process. The process exhibited efficient oxidation of a variety of compounds, including antibiotics, pharmaceuticals, and pesticides, at rates that were 6.1-173.7-fold faster than those measured for Fe(VI) alone, depending on pH, CaSO₃ dosage, and the properties of organic contaminants. Many lines of evidence verified that neither SO₄^•- nor ^•OH was the active species in the Fe(VI)-CaSO₃ process. The accelerating effect of CaSO₃ was ascribed to the direct generation of Fe(IV)/Fe(V) species from the reaction of Fe(VI) with soluble SO₃^2- via one-electron steps as well as the indirect generation of Fe(IV)/Fe(V) species from the self-decay of Fe(VI) and Fe(VI) reaction with H₂O₂, which could be catalyzed by uncomplexed Fe(III). Besides, the Fe(VI)-CaSO₃ process exhibited satisfactory removal of organic contaminants in real water, and inorganic anions showed negligible effects on organic contaminant decomposition in this process. Thus, the Fe(VI)-CaSO₃ process with Fe(IV)/Fe(V) as reactive oxidants may be a promising method for abating various micropollutants in water treatment.