Aqueous Acid Orange 7 dye removal by clay and red mud mixes

Abstract In this study, Portuguese clay, Fe-impregnated clay, red mud and clay/red mud mixtures were used in the removal of Acid Orange 7 by Fenton and photo-Fenton (under UV light) oxidation processes. In comparison with pure adsorption, the catalytic activity of Fe-loaded clay showed an optimum removal rate (98%). This photo-assisted Fenton degradation of Acid Orange 7 azo-dye molecules was exploiting HO radicals from generated H2O2 and clay supported iron species, following the pseudo-first order kinetic mechanism. By using red mud pre-calcined at 400 °C, 10% improvement in overall discolouration was observed in comparison to the untreated clay. This improvement is attributed to the partial reduction of Fe3 + to Fe2 + species on the surface of the catalyst, and to the reaction with H2O2 to generate highly oxidative hydroxyl radicals. It was seen that the synergistic effect of photocatalysis due to the presence of TiO2 in the red mud also contributed in this photo-Fenton process. Furthermore, the use of red mud/clay catalyst mixes assured 38% dye discolouration at pH 7, but a lowering of solution pH to 3 resulted in a much higher discolouration rate (over 80% after 1 h). The good fitting with a pseudo-second-order kinetic model (R2 equals to 0.99) shows that adsorption processes could strongly contribute in the dye removal efficiency.