Photocatalytic degradation of triazine-containing azo dyes in aqueous TiO2 suspensions

The photocatalytic degradation of triazine-containing azo dyes, Procion Red MX-5B and Reactive Brilliant Red K-2G, in aqueous TiO2 dispersions was investigated. Total organic carbon (TOC), ion chromatography (IC), HPLC, UV-Vis, FTIR and GC-MS analyses were employed to obtain the details of the photodegradation of the selected dyes. The results verified that decolorization and desulfuration occurred at almost the same rate in the first step of photo-oxidation. The substituents attached to the naphthalene group of a dye molecule were hydroxylated more easily than those linked to the triazine group. Intermediates include mainly organic aromatic and aliphatic carboxylic acids, which were further oxidized slowly to CO2. TiO2/UV-based photocatalysis was able to oxidize the dyes with partial mineralization of carbon, nitrogen and sulfur heteroatoms into CO2, NH4+, NO3- and SO42-, respectively. Cyanuric acid (OOOT) was one of the final products for both azo dyes, showing no sign of decomposition by heterogeneous photocatalysis. The residual nitrogen-containing organics after prolonged-photocatalytic oxidation (PCO) appeared to be the PCO-resistant OOOT. There was a similar degradation pathway for photodegradation of the two dyes. The selected dyes were completely detoxified as indicated by the results of Microtox(R) tests. (C) 2002 Elsevier Science B.V. All rights reserved.