Involvements of chloride ion in decolorization of Acid Orange 7 by activated peroxydisulfate or peroxymonosulfate oxidation

The effects of chloride anion (Cl^?) (up to 1.0 mol/L) on the decolorization of a model compound, azo dye Acid Orange 7 (AO7), by sulfate radical (SO₄^?·) based-peroxydisulfate (PS) or peroxymonosulfate (PMS) oxidation under various activated conditions (UV_254nm/PS, Thermal (70°C/PS, UV_254nm/PMS, Co²⁺/PMS) were investigated. Methanol and NH₄⁺ were used as quenching reagents to determine the contributions of active chlorine species (dichloride radical (Cl₂^?·) and hypochlorous acid (HCIO)). The results indicated that the effects of Cl^? on the reaction mechanism were different under various activated conditions. For UV/PS and Thermal/PS, the inhibition tendency became more clear as the Cl^? concentration increased, probably due to the reaction between Cl^? and SO₄^?· and the generation of Cl₂^?· or HCIO. For UV/PMS, Cl^? did not exhibit inhibition when the concentration was below 0.1 mol/L. As Cl^? concentration reached to 1.0 mol/L, the decolorization rate of AO7 was, however, accelerated, possibly because PMS directly reacts with Cl^? to form HCIO. For Co²⁺/PMS, Cl^? exhibited a significant inhibiting effect even at low concentration (≤ 0.01 mol/L). When Cl^? concentration exceeded 0.1 mol/L, the activation of PMS by Co²⁺ was almost completely inhibited. Under this condition, HCIO maybe played a major role in decolorization of AO7. The results implicated that chloride ion is an important factor in SO₄^?·-based degradation of organic contamination in chloride-containing water.