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 SO/OH) involved in this process are still under debate. By employing sparingly soluble CaSO as a slow-releasing source of SO, this study evaluated the oxidation performance of the Fe(VI)-CaSO 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, CaSO dosage, and the properties of organic contaminants. Many lines of evidence verified that neither SO nor OH was the active species in the Fe(VI)-CaSO process. The accelerating effect of CaSO was ascribed to the direct generation of Fe(IV)/Fe(V) species from the reaction of Fe(VI) with soluble SO 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 HO, which could be catalyzed by uncomplexed Fe(III). Besides, the Fe(VI)-CaSO 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)-CaSO process with Fe(IV)/Fe(V) as reactive oxidants may be a promising method for abating various micropollutants in water treatment.