Transformation of substituted anilines by ferrate(VI): Kinetics, pathways, and effect of dissolved organic matter

Abstract

This study showed that substituted anilines of environmental concerns could be appreciably degraded by potassium ferrate (K2FeO4) in the pH range of 6–10 with apparent second-order rate constants (kapp) varying from 0.32 to 36,500M−1s−1. The pH-dependence rate curves were well fitted via a kinetic model, which involved the formation of intermediates between Fe(VI) and substituted anilines. The plausible degradation pathways were proposed including the generation of aniline radical cations and their further oxidation to nitrobenzene compounds or dimerization to azobenzene compounds. Humic acid (HA) significantly reduced the transformation rates of substituted anilines with Fe(VI) even though considering the effect of oxidant consumption. This suppression effect likely resulted from the reaction between HA and the aniline radical cation, where the aniline radical cation was turned back to parent compound. Meanwhile, it was found that HA also had an impact on the formation kinetics of oxidation products. The effectiveness of Fe(VI) for degradation of substituted anilines in natural water was evaluated, where background matrix constituents could affect both the kinetics of contaminants degradation and products formation, similar to the case of HA. These results have advanced the understanding of Fe(VI) chemistry involved in water and wastewater treatment.