Degradation of indigo carmine by photo-Fenton, Fenton, H2O2/UV-C and direct UV-C: Comparison of pathways, products and kinetics

Abstract

The kinetics and pathways associated with the photo-Fenton oxidation of indigo carmine (IC; 50 mg L−1) have been determined and compared with those of the Fenton, H2O2/UV-C and direct UV-C processes. The most efficient removal of total organic carbon (75 % in 60 min) was obtained with Fe2+ at 4 mmol L−1 and H2O2/Fe2+ ratio of 6:1. Examination of the kinetics of these four advanced oxidation processes (AOPs) indicated that the degradation of IC involves a rapid step (0–1 min) and a slow step (1–60 min) with both presenting significant differences regarding reaction rates. In the photo-Fenton process, the effect of Fe2+ concentration on the kinetic constant was significantly higher than that of H2O2/Fe2+ ratio, while the excess of H2O2 reduced the reaction rate. The effects of these parameters on the kinetic constant of the slow step were best described by an integrated pseudo second-order model. Photo-Fenton and Fenton processes were more efficient than H2O2/UV-C and direct UV-C in terms of complete mineralization as demonstrated by the mass spectra obtained after each treatment. The mechanisms involved in the mineralization of IC by the tested AOPs were characterized by the action of OH and UV-C radiation and comprised abstraction of electrons, OH and H as well as substitution, electron displacement and addition reactions. Furthermore, the rapid step was associated mainly with the breakdown of chromophore groups, while the slow step involved mineralization of aromatic rings and carbon abstractions.