Sonophotocatalytic/H2O2 degradation of phenolic compounds in agro-industrial effluents

The treatment of a model solution containing 13 compounds typically found in olive mill wastewaters (OMW), at a concentration of 50 mg/L each, by means of sonophotocatalysis over 0.75 g/L Degussa TiO2 suspensions was studied. Experiments were conducted at an ultrasound frequency and intensity of 80 kHz and 120 W, respectively, ultraviolet power of 9, 250 and 400 W with or without the addition of 0.118 mol/L H2O2. Treatment efficiency was assessed following changes in total phenols (TPh) concentration, individual species concentration, chemical oxygen demand (COD), total organic carbon (TOC) and ecotoxicity. In general, photocatalytic degradation increased with increasing UVA power, while sonolysis alone failed to cause any degradation. Process coupling and addition of extra oxidant resulted in substantial levels of degradation. For instance, sonophotocatalytic treatment at 400 W UVA power with H2O2 for 120 min resulted in complete mineralization followed by significant toxicity reduction. TiO2 characterization before and after use showed that the catalyst suffered no composition or morphology changes during treatment. However, a substantial surface area increase was noted and this was attributed to the ultrasound de-aggregating catalyst particles. Preliminary tests with an actual OMW showed that the sonophotocatalytic/H2O2 treatment is a promising technology for this type of effluents.