Sunlight photocatalytic degradation of azo-dyes in aqueous suspension of polycrystalline TiO2

The photocatalytic oxidation of methyl-orange (C14H14N3SO3Na) and orange 11 (C16H11N2SO4Na) azodyes was carried out in aqueous suspensions of TiO2 powder under solar irradiation. A continuous tubular photoreactor with compound parabolic collectors, installed at the "Plataforma Solar de Almeria" (Spain), was used. Two identical reacting systems were operated at the same time in order to perform photoreactivity trials for the two dyes under the same working conditions. The kinetics of dye photooxidation rate were investigated at natural pH's by varying the initial substrate concentration. The influence of acidic or alkaline conditions and of the presence of strong oxidant (H2O2, S2O82-) and ionic (Cl-, SO42-) species on the process was also studied. The disappearance of colour and substrates together with the abatement of total organic carbon was monitored. The dye photooxidation was successfully achieved under all the experimental conditions. Complete discoloration was obtained within a few hours for both dyes, but mineralisation occurred after longer times with the formation of CO2, nitrates and sulphates. The presence of H2O2 did not appreciably affect the photodegradation rate of methyl-orange whereas the presence Of S2O82- was strongly beneficial. On the contrary, the presence of H2O2 and S2O82- was always beneficial for orange II, but the enhancement of the photooxidation rate was higher in the presence of S2O82-. The presence of Cl- and SO42-, ions did not significantly affect the reaction rates for both dyes.