E. Puzenat, H. Lachheb, M. Karkmaz, A. Houas, C. Guillard, J. M. Herrmann, "Fate of nitrogen atoms in the photocatalytic degradation of industrial (congo red) and alimentary (amaranth) azo dyes. Evidence for mineralization into gaseous dinitrogen", International Journal of Photoenergy, vol. 5, Article ID 895423, 8 pages, 2003. https://doi.org/10.1155/S1110662X03000138
Fate of nitrogen atoms in the photocatalytic degradation of industrial (congo red) and alimentary (amaranth) azo dyes. Evidence for mineralization into gaseous dinitrogen
The photocatalytic degradation of two azo-dyes–an industrial one (Congo Red (CR)), and an alimentary one (Amaranth (AM))–has been investigated in /UV aqueous suspensions. In addition to a prompt removal of the colors, /UV-based photocatalysis was simultaneously able to fully oxidize the dyes, with a complete mineralization of organic carbon into . In particular, the aromatic rings were submitted to successive attacks by photogenerated radicals leading to hydroxylated metabolites before the ring opening and the final evolution of induced by repeated subsequent “photo-Kolbe” reactions with carboxylic intermediates. Simultaneously, sulfur heteroatoms were converted into innocuous ions. The mineralization of nitrogen was more complex to analyze. Nitrogen atoms in the -3 oxidation state, such as in the amino-groups of CR, initially remained at this reduction degree and produced cations, subsequently and very slowly converted into anions. For both azo-dyes (CR and AM) degradation, the overall mass balance in nitrogen was always found incomplete. Various experiments performed in pure oxygen in a vacuum-tight cell and then in an air-free photoreactor (but filled with pure oxygen) enabled us to put in evidence the formation of . Quantitative measurements clearly indicated that gaseous dinitrogen evolved stoichiometrically corresponded to the mineralization of the central –N=N– azo-group. This constitutes the ideal issue for the elimination of nitrogen-containing pollutants, not only for environmental photocatalysis but also for any other physicochemical method. These results suggest that /UV photocatalysis may be envisaged as a method for treatment of diluted colored waste waters not only for decolorization but also for total detoxification, in particular in textile industries in semi-arid countries.
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