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The photocatalytic degradation of two azo-dyes–an industrial one (Congo Red (CR)), and analimentary one (Amaranth (AM))–has been investigated in TiO2/UV aqueous suspensions. In addition to aprompt removal of the colors, TiO2/UV-based photocatalysis was simultaneously able to fully oxidize thedyes, with a complete mineralization of organic carbon into CO2. In particular, the aromatic rings were submittedto successive attacks by photogenerated OH∘ radicals leading to hydroxylated metabolites before thering opening and the final evolution of CO2 induced by repeated subsequent “photo-Kolbe” reactions withcarboxylic intermediates. Simultaneously, sulfur heteroatoms were converted into innocuous SO42− ions. The mineralization of nitrogen was more complex to analyze. Nitrogen atoms in the -3 oxidation state, suchas in the amino-groups of CR, initially remained at this reduction degree and produced NH4+ cations, subsequently and very slowly converted into NO3− anions. For both azo-dyes (CR and AM) degradation, the overallmass balance in nitrogen was always found incomplete. Various experiments performed in pure oxygen ina vacuum-tight cell and then in an air-free photoreactor (but filled with pure oxygen) enabled us to put inevidence the formation of N2. Quantitative measurements clearly indicated that gaseous dinitrogen evolvedstoichiometrically corresponded to the mineralization of the central –N=N– azo-group. This constitutes theideal issue for the elimination of nitrogen-containing pollutants, not only for environmental photocatalysisbut also for any other physicochemical method. These results suggest that TiO2/UV photocatalysis may beenvisaged as a method for treatment of diluted colored waste waters not only for decolorization but also fortotal detoxification, in particular in textile industries in semi-arid countries

international journal of photoenergy

Fate of nitrogen atoms in the photocatalytic degradation of industrial (congo red) and alimentary (amaranth) azo dyes. Evidence for mineralization into gaseous dinitrogen