The photocatalytic degradation of two azo-dyes–an industrial one (Congo Red (CR)), and an
alimentary one (Amaranth (AM))–has been investigated in TiO2/UV aqueous suspensions. 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 organic carbon into CO2. In particular, 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. 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, such
as 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 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 N2. 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 TiO2/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.
