Photocatalytic Discoloration of Dyes: Relation between Effect of Operating Parameters and Dye Structure

The process of TiO2/UV photocatalytic discoloration of wastewaters containing organic dyes is influenced by a number of variables. In this paper, the effects of some of these parameters, i.e., mass of catalyst, initial dye concentration, and pH, were related to the structural nature of the dyes investigated, azo and anthraquinone dyes, two families that are representative of about 75% of the dyes marketed nowadays. Both bibliographical and own experimental data were used to analyze and discuss the photocatalytic discoloration in terms of dye structure and degradation pathways. It is shown that azo dyes (Methyl Orange, Metanil Yellow, Acid Orange 7, and Reactive Red 120) are more easily discolored than Reactive Blue 19 (anthraquinone dye), due to the higher molecular stability of the latter. It is also shown that azo dye discoloration is more influenced by the initial dye concentration, whereas recalcitrant anthraquinone dyes (Reactive Blue 19) mainly demand for basic pH values to generate enough OH center dot radicals. Concerning TiO2 loading, both the bibliographical and current research data showed no effect of this variable in the experimental range analyzed.