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 have been investigated both in aqueous heterogeneous suspensions and in the solid state. In addition to prompt removal of the color, TiO2/UV-based photocatalysis was simultaneously able to oxidize the dye, with almost complete mineralization of carbon and of nitrogen and sulfur heteroatoms into CO2, NH+4, NO−3, and SO2−4, respectively. A detailed degradation pathway has been determined by careful identification of intermediate products, in particular, carboxylic acids, whose decarboxylation by photo-Kolbe reactions constitutes 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 for treatment of diluted wastewaters in textile industries. The irradiation of titania with visible light did produce a photoinduced decolorization of the dye, probably induced by the breaking of the double-bond conjugation system of the chromophoric group. However, this decolorization was not accompanied by any degradation of the molecule since no loss of total organic carbon (TOC) nor release of inorganic ions were observed. This corresponded to a stoichiometric reaction of an electron transfer from the dye molecule excited in visible irradiation to titania. Because indigo is very poorly soluble (≈2 ppm), it was tentatively degraded in its solid state, mixed with titania in a photocatalytic solid-solid-type reaction. Observation of the decolorization and of the degradation of solid indigo constitutes a surprising and encouraging result for the development of self-cleaning titania-coated objects (glasses, steel, aluminium, metals, walls, etc.) fouled by solid dirt particles.