Chlorate Formation Mechanism in the Presence of Sulfate Radical, Chloride, Bromide and Natural Organic Matter

Halides and natural organic matter (NOM) are inevitable in aquatic environment and influence the degradation of contaminants in sulfate radical (SO₄^•-)-based advanced oxidation processes. This study investigated the formation of chlorate in the coexposure of SO₄^•-, chloride (Cl^-), bromide (Br^-) and/or NOM in UV/persulfate (UV/PDS) and cobalt(II)/peroxymonosulfate (Co/PMS) systems. The formation of chlorate increased with increasing Cl^- concentration in the UV/PDS system, however, in the Co/PMS system, it initially increased and then decreased. The chlorate formation involved the formation of hypochlorous acid/hypochlorite (HOCl/OCl^-) as an intermediate in both systems. The formation was primarily attributable to SO₄^•- in the UV/PDS system, whereas Co(III) played a significant role in the oxidation of Cl^- to HOCl/OCl^- and SO₄^•- was important for the oxidation of HOCl/OCl^- to chlorate in the Co/PMS system. The pseudo-first-order rate constants ( k') of the transformation from Cl^- to HOCl/OCl^- were 3.32 × 10^-6 s^-1 and 9.23 × 10^-3 s^-1 in UV/PDS and Co/PMS, respectively. Meanwhile, k' of HOCl/OCl^- to chlorate in UV/PDS and Co/PMS were 2.43 × 10^-3 s^-1 and 2.70 × 10^-4 s^-1, respectively. Br^- completely inhibited the chlorate formation in UV/PDS, but inhibited it by 45.2% in Co/PMS. The k' of SO₄^•- reacting with Br^- to form hypobromous acid/hypobromite (HOBr/OBr^-) was calculated to be 378 times higher than that of Cl^- to HOCl/OCl^-, but the k' of Co(III) reacting with Br^- to form HOBr/OBr^- was comparable to that of Cl^- to HOCl/OCl^-. NOM also significantly inhibited the chlorate formation, due to the consumption of SO₄^•- and reactive chlorine species (RCS, such as Cl·, ClO· and HOCl/OCl^-). This study demonstrated the formation of chlorate in SO₄^•--based AOPs, which should to be considered in their application in water treatment.