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

The degrdn. 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 sym. peroxides such as hydrogen peroxide and ammonium persulfate (APS) as oxidants. The APFP showed higher efficiency than their homogeneous counterparts even at high dye concns. due to the faster redn. 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 pptn. of iron oxy hydroxides at higher dosage of iron powder compared to H2O2 by providing excess acidity to the reaction medium. The rate const. for the kinetics of decolorisation by various oxidn. 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 degrdn. reaction was followed by UV-​visible and GC-​MS spectroscopic techniques. Based on the intermediates obtained, probable degrdn. mechanisms have been proposed. It was found that the initial mechanism in the APFP involves the redn. of azo groups to amines while in the case of HPFP it leads to the formation of hydroxylated products due to the oxidn. of azo groups