UV/H2O2 oxidation of azodyes in aqueous media: evidence of a structure¯¯degradability relationship

Textile finishing wastewaters, discharged to rivers and public sewage treatment plants, are very often highly coloured. Faced with the resulting psychological impact on population and more rigorous standards for rejecting waste waters that has being established, textile industries have to find effective methods for discolouration. In the present paper, we demonstrate that the UV/H2O2 oxidation process is able to destroy totally the chromophore structure of azodyes. However the reaction rate strongly depends on the basic structure of the molecule and on the nature of auxiliary groups attached to the aromatic nuclei of the dyes. Indeed, whereas hydroxy monoazodyes are very reactive, aminoazobenzenes, chromium complex dyes and disazodyes are on the whole less sensitive to OH-induced oxidation in neutral or slightly acid media. Oxidation by hydroxyl radicals is slowed down by chloro groups or alkyl chains in the dye molecule. After a detailed analysis of the degradation rate of some specific dyes we can now propose a mechanism for the first step of the dye degradation.