Photocatalytic degradation of various types of dyes (alizarin s, crocein orange g, methyl red, congo red, methylene blue) in water by uv-irradiated titania

TLDR: In this article, the photocatalytic degradation of five various dyes has been investigated in TiO 2 /UV aqueous suspensions, and it was shown that the dyes can be degraded by varying the chemical structures, either anthraquinonic (Alizarin S), or azoic (Crocein Orange G (OG), Methyl Red (MR), Congo Red (CR)) or heteropolyaromatic (Methylene Blue (MB)).

Abstract: The photocatalytic degradation of five various dyes has been investigated in TiO 2 /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, TiO 2 /UV-based photocatalysis was simultaneously able to fully oxidize the dyes, with a complete mineralization of carbon into CO 2 . Sulfur heteroatoms were converted into innocuous SO 4 2− 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 NH 4 + cations, subsequently and very slowly converted into NO 3 − 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 CO 2 induced by repeated subsequent “photo-Kolbe” reactions with carboxylic intermediates. These results suggest that TiO 2 /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.