A novel photochemical system of ferrous sulfite complex: Kinetics and mechanisms of rapid decolorization of Acid Orange 7 in aqueous solutions

•Irradiation increases steady-state concentration of SO4−.•The reaction between AO7 and SO4− involves electron transfer and SO4− addition.•The second-order rate constant of the reaction between AO7 and SO4− was determined.•Sulfate adduct AO7-SO4 forms in the photo-iron(II) sulfite system.•The mechanism explains the high decolorization and low TOC removal of AO7.

We previously reported the decolorization of the azo dye Acid Orange 7 (AO7) by sulfate radical (SO4−) in the presence of iron(II) sulfite complex and oxygen under UV-vis irradiation (photo-iron(II) sulfite system). This system, however, achieves very limited mineralization of AO7 (in terms of total organic carbon (TOC) removal), which is not in accordance with literature reports on the oxidation of organic contaminants by SO4−. In the present work, kinetics and products under irradiation of xenon lamp (350 W) were analyzed to reveal the reaction pathway of decolorization of AO7. Steady-state approximation (SSA) of SO4− radicals and apparent kinetics of AO7 were examined. The reaction between AO7 and SO4− was found to proceed in two steps, namely, electron transfer and SO4− addition. The second-order rate constant for the reaction between AO7 and SO4− was found to be 8.07 ± 1.07 × 109 M−1 s−1 by SSA and 6.80 ± 0.68 × 109 M−1 s−1 by competition kinetics method. The apparent kinetics of the decolorization of AO7 under irradiation closely fits the mechanism of radical chain reactions of various reactive sulfur species. By liquid chromatography coupled with mass spectrometry, we identified the sulfate adduct AO7-SO4 and confirmed the two-step reaction between AO7 and SO4−. This stable sulfate adduct provides a good explanation of the poor TOC removal during decolorization of AO7 by the photo-iron(II) sulfite system.

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