Mechanistic and Kinetic Understanding of the UV₂₅₄ Photolysis of Chlorine and Bromine Species in Water and Formation of Oxyhalides

Abstract

This study investigated the UV254 photolysis of free available chlorine and bromine species in water. The intrinsic quantum yields for •OH and X• (X = Cl or Br) generation were determined by model fitting of formaldehyde formation using a tert-butanol assay to be 0.61/0.45 for HOCl/OCl– and 0.32/0.43 for HOBr/OBr–. The steady-state •OH concentration in UV/HOX was higher than that in UV/OX– by a factor of 23.3 and 7.8 for Cl and Br, respectively. This was attributed to the different •OH consumption rate by HOCl versus OCl–, while for HOBr/OBr–, both the •OH formation and consumption rates were implied. This was supported by a k of 1.4 × 108 M–1 s–1 for the •OH reaction with HOCl, which was >14 times less than the k for •OH reactions with OCl–, HOBr, and OBr–. Formation of ClO3 – and BrO3 – was found to be significant with apparent quantum yields of 0.12–0.23. A detailed mechanistic study on the formation of XO3 – including a new pathway involving XO• is presented, which has important implications as the level of XO3 – can exceed the regulation (BrO3 –) or guideline (ClO3 –) values during UV/halogen oxidant water treatment. Our new kinetic models well simulate the experimental results for the halogen oxidant decomposition, probe compound degradation, and formation of ClO3 – and BrO3 –.

Chloride-Mediated Enhancement in Heat-Induced Activation of Peroxymonosulfate: New Reaction Pathways for Oxidizing Radical Production

“…•− barely contribute to BA oxidation due to their low reactivity toward BA (Table S1), 44,45 although their formation seems kinetically preferred via reactions 2, 6 and 7. The comparison of k(Cl 46 indicates that Cl • decay through the reaction with excess Cl − is kinetically preferred but would not completely rule out the occurrence of the reaction routes leading to • OH production via HOCl •− as an intermediate.…”

“…•− ), 1,44 the presence of Br − in the real wastewater matrix could cause an inhibitory effect on the treatment performance of heat/PMS-Cl − . The molar ratio of Cl − to Br − (R Cl/Br ) varies depending on the water source, with R Cl/Br = 23−225 for urban runoff, R Cl/Br = 225−450 for shallow groundwater, R Cl/Br = 675−1350 for domestic sewage, and R Cl/Br = 380−740 for seawater and brackish water RO concentrates.…”

“…Elevating the temperature may cause the equilibrium of reaction to shift toward the products (i.e., • OH and Cl – ), and the likelihood of heat-assisted conversion of Cl • to • OH under acidic conditions requires further investigation. Chlorine-derived radicals such as ClO • and Cl 2 •– barely contribute to BA oxidation due to their low reactivity toward BA (Table S1), , although their formation seems kinetically preferred via reactions , and . The comparison of k (Cl • + Cl – ) × [Cl – ] (8.5 × 10 7 s –1 ) versus k (Cl • + H 2 O) (2.5 × 10 5 s –1 ) indicates that Cl • decay through the reaction with excess Cl – is kinetically preferred but would not completely rule out the occurrence of the reaction routes leading to • OH production via HOCl •– as an intermediate.…”

“…Br – undergoes more effective conversion to Br • because of the higher reactivity of Br – toward SO 4 •– compared to that of Cl – ( k (Br – ) = 3.5 × 10 9 M –1 s –1 and ( k (Cl – ) = 6.1 × 10 8 M –1 s –1 ). However, since Br • possesses a relatively low oxidation power ( E 0 (Cl • /Cl – ) = 2.5 V NHE and E 0 (Br • /Br – ) = 2.0 V NHE ), Br • underperforms Cl • in BA oxidation, with k (Br • ) = 6.1 × 10 8 M –1 s –1 and k (Cl • ) = 1.8 × 10 10 M –1 s –1 . Furthermore, the reactions involved in the conversion of X • to OH • (reactions –) are much less favored kinetically with X = Br ( k 8 = 1.36 s –1 for Br – and 2.5 × 10 5 s –1 for Cl – ; k 9 = 3.3 × 10 7 s –1 for Br – and 6.1 × 10 9 s –1 for Cl – ; and k 10 = 4.6 × 10 6 s –1 for Br – and 23 s –1 for Cl – ). …”

“…Based on the earlier findings that Cl – and Br – generally co-occur in the natural water systems , and Br – serves as an effective scavenger of highly reactive radicals (e.g., Cl • , • OH, and SO 4 •– ), , the presence of Br – in the real wastewater matrix could cause an inhibitory effect on the treatment performance of heat/PMS-Cl – . The molar ratio of Cl – to Br – ( R Cl/Br ) varies depending on the water source, with R Cl/Br = 23–225 for urban runoff, R Cl/Br = 225–450 for shallow groundwater, R Cl/Br = 675–1350 for domestic sewage, and R Cl/Br = 380–740 for seawater and brackish water RO concentrates. , Accordingly, the efficiency of heat/PMS-Cl – and heat/PDS-Cl – for BA degradation was evaluated at four different R Cl/Br values of 50, 200, 700, and 2500 (Figure S15).…”