Photochemical and photocatalytic degradation of an azo dye in aqueous solution by UV irradiation

The photochemical and photocatalytic degradation of aqueous solutions of Solophenyl Green (SG) BLE 155%, an azo dye preparation very persistent in heavy colored textile waters, has been investigated by means of ultraviolet (UV) irradiation. The pure photochemical process demonstrated to be very efficient for low initial concentrations of the dyestuff. For higher concentrations the photocatalyitic degradation was carried on using commercial titanium dioxide, and mixtures of this semiconductor with different activated carbons (AC) suspended in the solution. The kinetics of photocatalytic dyestuff degradation were found to follow a first-order rate law. It was observed that the presence of the activated carbon enhanced the photoefficiency of the titanium dioxide catalyst. Differently activated carbon materials induced different increases in the apparent first-order rate constant of the process. The effect was quantified in terms of a synergy factor (R) already described in the literature. The kinetic behavior could be described in terms of a modified Langmuir–Hinshelwood model. The values of the adsorption equilibrium constants for the organic molecules, KC, and for the reaction rate constants, kC, were 0.0923 l mg−1 and 1.58 mg l−1 min−1 for the TiO2/UV process and 0.0928 l mg−1 and 2.64 mg l−1 min−1 for the TiO2+AC/UV system with highest synergy factor, respectively. The mechanism of degradation was discussed in terms of the titanium dioxide photosensitization by the activated carbon.