Enhanced oxidation of aniline using Fe(III)-S(IV) system: Role of different oxysulfur radicals

In this paper, the efficiency of Fe(III)-S(IV) system used for advanced oxidation processes (AOPs) has been investigated using aniline as a pollutant model compound in water. The chemical kinetics, influencing factors, and mechanism of aniline oxidation are examined with an emphasis on the contribution of the different oxysulfur radicals (mainly SO4− and SO5−). Our results show a significant enhancement in the efficiency of aniline oxidation observed at pH 4.0 with 1.0 mM S(IV) and 0.1 mM Fe(III) concentrations. Moreover, the degradation efficiency drastically decreases to 10% in the absence of oxygen indicating the significant role of oxygen in this type of process. Through competition kinetic experiments and radical scavenger experiments, it is shown that SO5− is responsible for about 60% of the aniline oxidation in the Fe(III)-S(IV) system under the typical conditions investigated in this work. For the first time we have determined the second order rate constant between SO5− and aniline (5.8 ± 0.6 × 106 M−1 s−1 (at pH 3.0) and SO4− and aniline 7.7 ± 0.5 × 109 M−1 s−1 (at pH 3.0)). Sequential experiments with successive additions of sulfite drastically improve the oxidation efficiency. These findings may provide a precise understanding of the overall mechanism and may have promising implications in developing a new cost-effective technology for the treatment of organic compounds-containing water. Furthermore, the results of this work help to understand the relevance and mechanism of organic contaminants oxidation by SO5−, which has not been given much attention in conventional SR-AOPs using peroxymonosulfate.