Degradation of azo dye mixtures through sequential hybrid systems: Evaluation of three advanced oxidation processes for the pre-treatment stage

Homogeneous (Fenton and photo Fenton) and heterogeneous (TiO2-anatase) advanced oxidation process were applied to decolorize a mixture of azo dyes consisted of acid red 151, acid orange 7 and acid blue 113. Under the selected conditions the photo-Fenton process allowed up to 97% decolorization of the model mixture (150 mg/L, 50 mg/L of each dye) in 45 min. Although, Fenton process also provided high decolorization percentages (86%), the presence of residual peroxide and recalcitrant by-products affected the aerobic biodegradability of the pre-treated effluent. The major drawbacks for the TiO2-anatase heterogeneous process were the higher reaction times (4 h) and the lower decolorization percentages. Under the tested conditions, the photo-Fenton process produced a biodegradable (86%) effluent during the pre-treatment of mixtures of 150 mg/L (50 mg/L of each dye) and 600 mg/L (200 mg/L of each dye). The biodegradability was not significant affected due to the change of concentration of the dyes suggesting the feasibility of coupling a photochemical process to biological one.