Decolorization and degradation of reactive azo dyes via heterogeneous photocatalytic processes

Reactive dyes are extensively used in textile industry in the last years due to their superior performance, but they are environmentally hazardous and difficult to treat effectively by classical methods. In the present work, the decolorization and degradation of four commercial reactive azo dyes, namely Remazol Red RR, Remazol Yellow RR, Procion Crimson H-exl and Procion Yellow H-exl, were studied using photocatalytic processes (TiO2/UV and TiO2/UV/H2O2). Decolorization and degradation were found to strongly depend on the system parameters (TiO2 loading, dye and H2O2 initial concentrations, and pH). Decolorization efficiency (%) sharply increases with increasing the TiO2 loading, especially up to 1 g/L, as well as with decreasing the initial dye concentration from 250 down to 50 mg/L. At pH = 3, a > 90% decolorization of all dyes can be achieved in only 15 min. Addition of H2O2 increases the decolorization rates up to an optimum value (97.9% Remazol Red RR decolorization at 12 min irradiation, with a 0.5%w/w initial H2O2 concentration and pH = 3). Among the four dyes examined, significant differences in decolorization and degradation rates were revealed, but decolorization and degradation efficiencies up to 100% (in 25 min and 4 h respectively) are possible with proper combinations of the system parameters.