Mechanism of azo dye degradation in Advanced Oxidation Processes: Degradation of Sulfanilic Acid Azochromotrop and its parent compounds in aqueous solution by ionizing radiation

Mechanistic studies were made on hydroxyl radical and hydrated electron reaction with Sulfanilic Acid Azochromotrop (SPADNS) as model azo dye in dilute aqueous solution. SPADNS contains 4,5-dihydroxynaphthalene-2,7-disulfonic acid part and 4-sulfophenylazo group. To establish the details of the reaction mechanism the reactions of two simpler molecules without 4-sulfophenylazo part were also studied: one of them contained one (in position 4, II), the other two (in positions 4 and 5, III) –OH groups. Hydroxyl radicals react with these molecules with radical addition to the naphthalene-2,7-disulfonic acid part. The adduct hydroxycyclohexadienyl type radical decays in radical–radical reactions, or undergoes a (pH dependent) water elimination to yield naphthoxy radical. The radical decay takes place on the ms timescale. Degradation efficiencies are 0.6−0.8. Hydrated electron in the case of the two simpler molecules reacts with the rings, while in the case of dye with the azo bond. Electron scavenging is followed by protonation, this reaction in the case of II and III yields cyclohexadienyl, while with the dye hydrazo radical. The efficiency of degradation with II and III is 0.2−0.6, while for SPADNS it is close to 1.