Oxidation pathways for ozonation of azo dyes in a semi‐batch reactor: A kinetic parameters approach

Abstract

In this study ozone and the H2O2/O3 oxidation system are used to decolorize aqueous solutions of Orange II (Or‐II) and Acid Red 27 (AR‐27). Investigations are carried out in a semi‐batch bubble column reactor. A system of series‐parallel reactions is proposed to describe the mechanism of dye oxidation. The stoichiometric ratio for the first reaction is found to be 1 mol dye per mol O3, while the overall ozone demand for both reactions one and two is found to be 5 and 6 moles for Or‐II and AR‐27 respectively. Molecular and radical kinetics are compared: a radical scavenger, t‐butanol, can be added to ensure only the molecular reaction of ozone, or hydrogen peroxide can be supplied through a peristaltic pump, to initiate radical reactivity. Results reveal that colour removal is ensured by direct ozone attack. For both dyes, TOC removal efficiencies of 50 ‐ 60 % are obtained by the action of the hydroxyl free radical. However, this is not improved by addition of H2O2, thus demonstrating that organic species alone ensure HO° radical production during ozonation. Both the mass transfer and the ozone reactivity with the dyes are considered to evaluate the kinetic parameters for the molecular pathway.