Optimization of the photocatalytic degradation of commercial azo dyes in aqueous TiO2 suspensions

The photocatalytic degradation of three textile azo dyes, Remazol Yellow Gold RGB, Remazol Blue RGB and Remazol Carbon RGB has been studied in model effluents regarding the variation of parameters as concentration of titanium dioxide (TiO2), concentration of the dyes, effect of hydrogen peroxide (H2O2), effect of pH, recycling of TiO2. The concentration of 1 g L-1 of TiO2 degraded around 100 % of Remazol Blue RGB and Remazol Carbon RGB and around 90 % for the Remazol Yellow Gold RGB after 120 min of photodegradation. The remaining concentrations of added TiO2 were found to be less efficient, except for Remazol Blue RGB, which still reaches about 80 % photodegradation, for 0.1 g L-1 of TiO2. The efficiency of photodegradation of all dyes decreased with an increase of the initial dye concentration. For a concentration of 30 mg L-1, all dyes showed around 100 %. However, upon increasing the concentration of the dye until 150 mg L-1, the photodegradation becomes very slow, with photodegradation rates off only 11 % for Remazol Yellow Gold RGB and Remazol Blue RGB and 9.7 % for the Remazol Carbon RGB. Photodegradation of the three dyes was found to be pH dependent. The maximum reaction rate was observed with 0.3 x 10(-3) mol L-1 of H2O2 for the three azo dyes (achieving more than 90 % of degradation in 120 min). At higher concentrations of H2O2, the degradation efficiency decreased significantly for all dyes. In what regards the reuse of the photocatalyst, the first cycle degraded 100 % of the Remazol Carbon RGB after 60 min of irradiation and the second and third cycles degraded 44 and 36 % of the dye. The effect of the simultaneous photodegradation of the three azo dyes in water was further investigated for distilled and natural lake water. It was observed that the degradation rates achieved in mono and three-component system were identical. The degradation yield in lake water observed to be smaller than in distillated water with 75, 72 and 53 % (99, 98 and 88 % in distilled water) of degradation for Remazol Blue RGB, Remazol Carbon RGB and Remazol Yellow Gold RGB, respectively.