Photocatalytic degradation of dyes in water: Case study of indigo and of indigo carmine

The TiO2/UV photocatalytic degradations of indigo and of indigo carmine hav e been investigated both in aqueous heterogeneous suspensions and in the so lid state. In addition to prompt removal of the color, TiO2/UV-based photoc atalysis was simultaneously able to oxidize the dye, with almost complete m ineralization of carbon and of nitrogen and sulfur heteroatoms into CO2, NH 4+, NO;, and SO42-, respectively. A detailed degradation pathway has been d etermined by careful identification of intermediate products, in particular , carboxylic acids, whose decarboxylation by photo-Kolbe reactions constitu tes the main source of CO2 evolution. The only persistent organic compound was acetic acid, whose degradation required a longer period of time. These results suggest that TiO2/UV photocatalysis may be envisaged as a method fo r treatment of diluted wastewaters in textile industries. The irradiation o f titania with visible light did produce a photoinduced decolorization of t he dye, probably induced by the breaking of the double-bond conjugation sys tem of the chromophoric group. However, this decolorization was not accompa nied by any degradation of the molecule since no loss of total organic carb on (TOC) nor release of inorganic ions were observed. This corresponded to a stoichiometric reaction of an electron transfer from the dye molecule exc ited in visible irradiation to titania. Because indigo is very poorly solub le (approximate to2 ppm), it was tentatively degraded in its solid state, m ixed with titania in a photocatalytic solid-solid-type reaction. Observatio n of the decolorization and of the degradation of solid indigo constitutes a surprising and encouraging result for the development of self-cleaning ti tania-coated objects (glasses, steel, aluminium, metals, walls, etc.) foule d by solid dirt particles. (C) 2001 Academic Press.