Photocatalytic degradation of various types of dyes (Alizarin S, Crocein Orange G, Methyl Red, Congo Red, Methylene Blue) in water by UV-irradiated titania

Abstract

The photocatalytic degradation of five various dyes has been investigated in TiO2/UV aqueous suspensions. It was attempted to determine the feasibility of such a degradation by varying the chemical structures, either anthraquinonic (Alizarin S (AS)), or azoic (Crocein Orange G (OG), Methyl Red (MR), Congo Red (CR)) or heteropolyaromatic (Methylene Blue (MB)). In addition to a prompt removal of the colors, TiO2/UV-based photocatalysis was simultaneously able to fully oxidize the dyes, with a complete mineralization of carbon into CO2. Sulfur heteroatoms were converted into innocuous SO42−ions. The mineralization of nitrogen was more complex. Nitrogen atoms in the −3 oxidation state, such as in amino-groups, remain at this reduction degree and produced NH4+ cations, subsequently and very slowly converted into NO3− ions. For azo-dye (OG, MR, CR) degradation, the complete mass balance in nitrogen indicated that the central NN azo-group was converted in gaseous dinitrogen, which is the ideal issue for the elimination of nitrogen-containing pollutants, not only for environmental photocatalysis but also for any physicochemical method. The aromatic rings were submitted to successive attacks by photogenerated OH radicals leading to hydroxylated metabolites before the ring opening and the final evolution of CO2 induced by repeated subsequent “photo-Kolbe” reactions with carboxylic intermediates. These results suggest that TiO2/UV photocatalysis may be envisaged as a method for treatment of diluted colored waste waters not only for decolorization, but also for detoxification, in particular in textile industries in semi-arid countries.