Degradation of azo dye with activated peroxygens: when zero-valent iron meets chloride

Degradation of acid orange 7 (AO7) by Fe⁰-based Advance Oxidation Process (AOPs) with common peroxygens, persulfate (PS), peroxymonosulfate (PMS) and hydrogen peroxide (H₂O₂), was investigated, in which sulfate radicals (SO₄·-) and/or hydroxyl radicals (·OH) are powerful oxidizing species. The effects of Fe⁰ dosage, peroxygen concentration, initial pH and the presence of chloride on the degradation of AO7 were examined. The AO7 degradation efficiencies by four systems, including Fe⁰, Fe⁰/H₂O₂, Fe⁰/PMS and Fe⁰/PS were compared. AO7 degradation rate by Fe⁰ activated AOPs in descending order is H₂O₂ ≧ PS > PMS. Increasing acidity and iron dosage favored a rapid degradation of AO7. The presence of chloride greatly inhibited dye removal in Fe⁰/H₂O₂ and Fe⁰/PS systems, whilst accelerated dye degradation was observed in the Fe⁰/PMS system. In contrast, mineralization of AO7 in the Fe⁰/PMS/Cl- system was minimal, because of formation of lots of refractory chlorinated phenols as identified by GC-MS. These findings are useful for selecting the most appropriate technology for textile wastewater treatment, depending on the wastewater constituents and pH.