Photodegradation of methyl red by advanced and homogeneous photo-Fenton’s processes: A comparative study and kinetic approach

Abstract

The degradation of methyl red (MR), an azo dye, was carried out by the homogeneous photo-Fenton’s process (HPFP) and the advanced photo-Fenton’s process (APFP) using symmetrical peroxides such as hydrogen peroxide and ammonium persulfate (APS) as oxidants. The APFP showed higher efficiency than their homogeneous counterparts even at high dye concentrations due to the faster reduction of Fe³⁺ to Fe²⁺ ions on the iron surface. H₂O₂ proved to be a better oxidant for both the processes. However, APS efficiently inhibited the precipitation of iron oxy hydroxides at higher dosage of iron powder compared to H₂O₂ by providing excess acidity to the reaction medium. The rate constant for the kinetics of decolorisation by various oxidation processes is of the order: Fe⁰/H₂O₂/UV > Fe⁰/H₂O₂/dark > Fe⁰/APS/UV > Fe²⁺/H₂O₂/UV > Fe⁰/UV > Fe⁰/APS/dark > Fe⁰/dark ≈ H₂O₂/UV > Fe²⁺/APS/UV > APS/UV > Fe²⁺/H₂O₂/dark > Fe²⁺/APS/dark ≈ Fe²⁺/UV. The degradation reaction was followed by UV-visible and GC-MS spectroscopic techniques. Based on the intermediates obtained, probable degradation mechanisms have been proposed. It was found that the initial mechanism in the APFP involves the reduction of azo groups to amines while in the case of HPFP it leads to the formation of hydroxylated products due to the oxidation of azo groups.