Oxidation of odor compound indole in aqueous solution with ferrate (VI): Kinetics, pathway, and the variation of assimilable organic carbon

- The oxidation of indole by ferrate(VI) was investigated for the first time.
- Fe(VI) exhibited high efficiency for the oxidation of indole in real source waters.
- The degradation pathway of indole by Fe(VI) was proposed.

Indole is a typical fecal odor compound and has negative impact on water quality at low concentrations. Here, the oxidation of indole by ferrate [Fe(VI)] was investigated, in terms of the reaction kinetics, the oxidation products, and the variation of assimilable organic carbon (AOC). The oxidation of indole was pH-dependent (6.0-9.5), and the pH-dependent profile could be well explained by the acid-base equilibrium of Fe(VI). The apparent second-order rate constants (kapp) for indole oxidation with Fe(VI) ranged from 692 ± 26 M−1 s−1 at pH 6.0 to 3.08 ± 0.15 M−1 s−1 at pH 9.5. The results of experiments conducted under real source water background showed that indole could be effectively removed by Fe(VI) from actual polluted waters. After analyzing the reaction pathway, it was found that the initial steps in the oxidation of indole were the hydroxylation of indole to 2,3-dihydroxyindole, followed by the cleavage of N-heterocyclic ring. Formanilide was a major oxidation product in this process, and it is less reactive with Fe(VI) than that of indole. High concentrations of Fe(VI) could effectively oxidize indole in the water and has little impact on the microbiological stability of the treated water. Based on the results, Fe(VI) can be an effective agent for the control of indole in water treatment.

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