Photocatalytic and sonolytic oxidation of acid orange 7 in aqueous solution

Abstract The oxidation of 50 mg/L azodye acid orange 7 (AO7) in water was investigated by means of photocatalysis in the presence of various semiconducting catalysts and ultrasound irradiation. The UVA-induced photocatalytic degradation over TiO 2 anatase suspensions was found to increase with increasing catalyst concentration, decreasing solution pH, as well as in the presence of dissolved oxygen. The presence of 1,4-benzoquinone, sodium azide or sodium chloride in the reaction mixture decreased degradation due to the scavenging of radicals and other reactive moieties. Interestingly, addition of hydrogen peroxide at various concentrations from 21 to 1050 mg/L also inhibited degradation. The sonochemical degradation of AO7 was found to increase with increasing frequency and decreasing temperature. Under similar treatment conditions, ultrasound irradiation resulted in higher conversion than photocatalysis; moreover, the sonochemically irradiated solution consistently contained low aliphatic intermediates, while the photocatalytically treated solution mainly consisted of aromatic intermediates as confirmed by GC–MS analysis. The acute toxicity to marine bacteria V. fischeri decreased following oxidation with either process. Furthermore, deep sonochemical treatment slightly improved the aerobic biodegradability as assessed by shake flask tests.