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

Textile finishing wastewaters, discharged to rivers and public sewage treat ment plants, are very often highly coloured. Faced with the resulting psych ological impact on population and more rigorous standards for rejecting was te waters that has being established, textile industries have to find effec tive methods for discolouration. In the present paper, we demonstrate that the UV/H2O2 oxidation process is able to destroy totally the chromophore st ructure of azodyes. However the reaction rate strongly depends on the basic structure of the molecule and on the nature of auxiliary groups attached t o the aromatic nuclei of the dyes. Indeed, whereas hydroxy monoazodyes are very reactive, aminoazobenzenes, chromium complex dyes and disazodyes are o n the whole less sensitive to OH-induced oxidation in neutral or slightly a cid media. Oxidation by hydroxyl radicals is slowed down by chloro groups o r alkyl chains in the dye molecule. After a detailed analysis of the degrad ation rate of some specific dyes we can now propose a mechanism for the fir st step of the dye degradation. (C) 1999 Elsevier Science Ltd. All rights r eserved.