Nuclear chemistry

Catalysis

Hypochlorous acid

Inorganic chemistry

Sulfate

Sodium chlorate

Chlorate

Chloride

Organic chemistry

Persulfate

Chemistry

Bromide

Bromate

Chlorine

Hypochlorite

Environmental Science & Technology

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.

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