Rapid removal of organic pollutants by activation sulfite with ferrate

With the addition of sulfite, almost complete removal of seven out of eight tested contaminants, i.e. benzotriazole, phenol, ciprofloxacin, methyl blue, rhodamine B, methyl orange and sulfamethoxazole, was observed in 30 s except for sulfamethoxazole at pH 9.0, while Fe(VI) alone only resulted in less than 6% removal. In this study, the mechanism of sulfite/Fe(VI) process was explored for the first time. Firstly, sulfite attacks Fe(VI) via a 1-e⁻ transfer forming the primary sulfite radical (SO₃^[rad]−), then SO₃^[rad]− reacts rapidly with oxygen in a diffusion controlled rate to form a much more reactive peroxyl radical SO₅^[rad]−, which further reacts with HSO₃⁻ and produces SO₄^[rad]−. All the signals of sulfite, sulfate and hydroxyl radicals were observed in the electron spin resonance (ESR) analysis. The DMPO and alcohol quenching experiments confirmed that sulfite/Fe(VI) process involves radicals as the plausible reactive species for decomposition of organic pollutants, and SO₄^·-/·OH instead of SO₃^[rad]−/SO₅^[rad]− are the major reactive oxygen species. Since oxygen participates in the formation of the secondary radicals, i.e. SO₅^[rad]−, SO₄^[rad]− and ·OH, the sulfite/Fe(VI) process open to air displayed better performance than that purged with nitrogen. Compared with previous sulfite/Cr(VI) and sulfite/Mn(VII) processes, which are only effective under acidic conditions, sulfite/Fe(VI) process provides an alternative under neutral and alkaline conditions. The extraordinarily fast oxidation of pollutants in sulfite/Fe(VI) process suggests that activation of sulfite by Fe(VI) might lead to a new class of advanced oxidation processes (AOPs) in water treatment.