Solar Purification and Potabilization of Water Containing Dyes.

International audienc

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 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

Environmental green chemistry as defined by photocatalysis

International audiencePhotocatalysis is efficient in several fields. Firstly, in selective mild oxidation: oxidation of gas and liquid hydrocarbons (alkanes, alkenes, cyclo-alkanes, aromatics) into aldehydes and ketons. Primary and secondary alcohols are also oxidized into their corresponding aldehydes or ketones. The high selectivity was ascribed to a photoactive neutral, atomic oxygen species. Once platinized (only 0.5 wt.% Pt) titania may catalyze reactions involving hydrogen (deuterium-alkane isotopic exchange and alcohol dehydrogenation). For fine chemicals, high initial selectivities enable titania to address most of the twelve principles of “green chemistry”, such as the synthesis of 4-tert-butyl-benzaldehyde, an important intermediate in perfume industry by direct selective oxidation of 4-tert-butyl-toluene with air. A new field recently appeared: thio-photocatalysis. Oxygen was replaced by sulfur, using H2S as a convenient and reactive source. For instance, the conversion of propene in 1-propanthiol was successfully obtained. The reaction was performed using either CdS or TiO2. The latter was much more active than CdS. In environmental photocatalysis, titania becomes a total oxidation catalyst once in presence of water because of the photogeneration of OH radicals by neutralization of OH− surface groups by positive holes. Many toxic inorganic ions are oxidized in their harmless upper oxidized state. The total degradation of organic pollutants (pesticides, herbicides, insecticides, fungicides, dyes, etc. ...) is the main field of water photocatalytic decontamination. The UVA solar spectrum can de advantageously used as demonstrated by many campaigns performed in the solar pilot plant at the “Plataforma Solar de Almeria” (Spain)