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

Degradation of acid orange 7 (AO7) by Fe-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 Fe 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 Fe, Fe/H₂O₂, Fe/PMS and Fe/PS were compared. AO7 degradation rate by Fe 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 Fe/H₂O₂ and Fe/PS systems, whilst accelerated dye degradation was observed in the Fe/PMS system. In contrast, mineralization of AO7 in the Fe/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.