Kinetic and mechanistic analysis of azo dyes decolorization by ZVI-assisted Fenton systems: pH-dependent shift in the contributions of reductive and oxidative transformation pathways

The chemical decolorization of the azo-dye Acid Black 1 (AB1) by ZVI-assisted Fenton systems was investigated for assessing the relative importance of reductive and oxidative pathways. To this end, system evolution was followed by UV-vis, HPLC, CG-MS, TOC and toxicity measurements. The effects of reaction conditions including initial pH, oxidants (O-2 and H2O2) concentrations, ZVI loading, and the presence of SO42- were studied. Mechanistic issues were addressed by following H2O2, Fe(II), Fe(III) and pH profiles, as well as by performing experiments in the presence of iron-complexing agents (o-phenanthroline and EDTA) or reactive species scavengers (2-propanol and DMSO). Results show that AB1 reduction occurs through heterogeneous processes with moderate to low pH dependent rates, whereas AB1 oxidation is ruled by the Fenton reaction with significantly pH dependent rates. Mechanistic studies demonstrated that AB1 decolorization in ZVI-assisted Fenton systems involves both oxidative and reductive pathways, whose relative contributions significantly change as the initial pH is raised from 3 to 5. In acidic media, AB1 is mostly transformed by oxidative pathways linked to H2O2 consumption. In contrast, in circumneutral media, a rather inefficient H2O2 consumption leads to similar contributions of oxidative and reductive AB1 transformation pathways. A detailed analysis of the pH dependence of the key reactions involved suggests that the overall system behavior is ruled by a shift in the oxidation mechanism of Fe(II) species, as a consequence of the development of a thigh corrosion layer onto ZVI particles at circumneutral pH values.