Visible light and Fe(III)-mediated degradation of Acid Orange 7 in the absence of H₂O₂

The photodegradation of Acid Orange 7 (AO7) was successfully achieved in the presence of Fe(III) ions only under visible light (λ≥420 nm). Upon adding Fe(III) to AO7 solution, ferric ions formed complexes with AO7 mainly through the azo chromophoric group. This AO7-Fe(III) complex formation was highly pH-sensitive and maximized around pH 3.7. The visible light-induced degradation of AO7 was effective only when the complex formation was favored. The AO7 photodegradation accompanied the production of ferrous (Fe²⁺) ions and was not inhibited in the presence of excess OH radical scavenger (2-propanol), which indicated that OH radicals were not responsible for the dye degradation. The proposed mechanism of the dye degradation is the visible light-induced electron transfer from the azo chromophoric group to the iron center in the complex. Therefore, when the formation of AO7-Fe(III) complex was inhibited in the presence of excess interfering anions such as sulfites and sulfates, the photodegradation of the dye was also prevented. The photodegradation of AO7 under visible light produced o-phthalate and 4-hydroxybenzenesulfonate (4-HBS) as major products but did not reduce the total organic carbon (TOC) concentration. Since this process does not require the addition of hydrogen peroxide, it might be developed into an economically viable method to pretreat or decolorize azo-dye wastewaters using sunlight.