Mechanistic Insight into the Reactivity of Chlorine-Derived Radicals in the Aqueous-Phase UV–Chlorine Advanced Oxidation Process: Quantum Mechanical Calculations

Abstract

The combined ultraviolet (UV) and free chlorine (UV-chlorine) advanced oxidation process that produces highly reactive hydroxyl radicals (HO) and chlorine radicals (Cl) is an attractive alternative to UV alone or chlorination for disinfection because of the destruction of a wide variety of organic compounds. However, concerns about the potential formation of chlorinated transformation products require an understanding of the radical-induced elementary reaction mechanisms and their reaction-rate constants. While many studies have revealed the reactivity of oxygenated radicals, the reaction mechanisms of chlorine-derived radicals have not been elucidated due to the data scarcity and discrepancies among experimental observations. We found a linear free-energy relationship quantum mechanically calculated free energies of reaction and the literature-reported experimentally measured reaction rate constants, k, for 22 chlorine-derived inorganic radical reactions in the UV-chlorine process. This relationship highlights the discrepancy among literature-reported rate constants and aids in the determination of the rate constant using quantum mechanical calculations. We also found linear correlations between the theoretically calculated free energies of activation and k for 31 reactions of Cl with organic compounds. The correlation suggests that H-abstraction and Cl-adduct formation are the major reaction mechanisms. This is the first comprehensive study on chlorine-derived radical reactions, and it provides mechanistic insight into the reaction mechanisms for the development of an elementary reaction-based kinetic model.

State of the Art of UV/Chlorine Advanced Oxidation Processes: Their Mechanism, Byproducts Formation, Process Variation, and Applications

“…Recently, it was found that reaction rate constants of HO˙ and chlorine-derived radicals are linearly correlated with theoretically calculated free energies of activation [30,38]. Such a computational chemistry approach will be useful for a deeper understanding of reactivity of reactive radical species in UV/chlorine AOPs.…”

“…Therefore, it is important to elucidate the reactivity of each radical species with chemical compounds for understanding the efficacy of UV/chlorine AOPs. Table 2 summarizes reaction rate constants of HO˙, Cl˙, ClO˙, and Cl 2˙− with some chemicals reported in literatures [18,[24][25][26][27][28][29][30][31][32]. HO˙ has second-order reaction rate constants with approximately 10 9 -order level regardless of saturated compounds or benzene derivatives.…”

“…Boxes depict primary radicals produced by photolysis of chlorine. [24] 6.0×10 8 [24] 1.0×10 9 [32] 1.7×10 9 [32] 1.5-3.2×10 9 [30] 6.2×10 8 [32] ---1.3×10 7 [31] 3.5×10 3 [25] 4.5×10 4 [25] 1.2-1.9×10 5 [25] 0 [26] Aldehyde formaldehyde [24] 5.9×10 9 [24] 5.0×10 9 [24] 7.0×10 9 [24] 7.0×10 9 [24] 6-12×10 9 [28] 0 that RCS show relatively high reactivity with olefins and benzene derivatives and electron-donating groups promote the attack of benzene derivatives [23]. HO˙ is known as a non-selective electrophilic reagent.…”

“…On reaction mechanisms of Cl˙ Buxton et al discussed the reactivity of Cl˙ with oxy-organic compounds like acids, alcohols, aldehydes, and ketones in aqueous solution and concluded that the main mechanism of oxidation by Cl˙ is preferential attack at O-H groups in neutral molecules and electron transfer in anions [32]. Minakata et al suggested that Cl-adduct formation as well as H-abstraction is a major mechanism of oxidation by Cl˙ [30]. Ezell et al reported the reactivity of Cl˙ with alkenes in gas phase and two oxidation mechanisms were proposed; the addition of Cl atom to double bond and abstraction of an allylic hydrogen atom, whereas the latter is slower than that of the analogous alkyl hydrogen atoms in alkanes [35].…”

Participation of the Halogens in Photochemical Reactions in Natural and Treated Waters

“…Ozone also reacts with X 2 −• ( ). The ClO • radical appears to be much less reactive than X • and X 2 −• toward organic compounds [ 65 ].…”