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Reactive dyes are extensively used in the last years due to their superior performance, but they are environmentally hazardous. In the present work, the decolorization and degradation of commercial reactive azo dyes (Procion Navy H-exl, Procion Crimson H-exl and Procion Yellow H-exl) were studied using five advanced oxidation processes (AOPs): H2O2/UV, Fenton, UV/Fenton, TiO2/UV and TiO2/UV/H2O2. The dependence of the decolorization on the system parameters (solution pH, dye and reactants initial concentrations, and TiO2 loading) and on the presence of salts (NaCl, Na2CO3, NaHCO3, Na2SO4, NaNO3 and Na3PO4) was investigated. The decolorization (determined by spectrophotometric analysis) and the degradation (determined as TOC reduction) were compared for the different processes examined. The decolorization of the Procion H-exl solutions considered was found to strongly depend on the system parameters in all five AOPs. Although decolorization is very fast for the Fenton process and becomes even faster for the UV/Fenton process, degradation rates are relatively low for these two methods. Addition of H2O2 increases the decolorization and especially the degradation rates for the TiO2/UV process. The H2O2/UV and *Corresponding author. Presented at the 9th Environmental Science and Technology Symposium, September 1–3, 2005, Rhodes, Greece. Organized by the Global NEST organization and prepared with the editorial help of the University of Aegean, Mytilene, Greece and the University of Salerno, Fisciano (SA), Italy. 0011-9164/07/$– See front matter © 2007 Elsevier B.V. All rights reserved

The decolorization of the Procion H-exl dilute solutions examined strongly depends on the system parameters (solution pH, dye and reactants initial concentrations, and TiO2 loading) in all five AOPs studied. • • • • The following conclusions can also be made: Decolorization is very fast for the Fenton process and becomes even faster for the UV/ Fenton process. However, degradation rates (TOC reduction) are relatively low for these two methods. The Fenton process degrades the dye only up to 20%, while, irradiating the solution, greatly increases the degradation efficiency (about 60%). Decolorization rates increase in the TiO2/UV process with the increase of TiO2 loading, especially up to about 1000 mg/L. Adsorption of all three dyes onto the catalyst surface is very pronounced at pH = 3. Addition of H2O2 increases the decolorization and especially the degradation rates. The H2O2/UV process results in the fastest dye degradation, followed by the TiO2/UV/H2O2 process. The addition of Cl–, CO2–, HCO–, SO2–, H2PO– or NO– in the dye solutions examined has in general an adverse effect on the decolorization rates, but to a varying degree depending on the salt used.