Rate constants of dichloride radical anion reactions with molecules of environmental interest in aqueous solution: a review

Abstract

Natural waters, water droplets in the air at coastal regions and wastewaters usually contain chloride ions (Cl-) in relatively high concentrations in the milimolar range. In the reactions of highly oxidizing radicals (e.g., •OH, •NO3, or SO4•-) in the nature or during wastewater treatment in advanced oxidation processes the chloride ions easily transform to chlorine containing radicals, such as Cl•, Cl2•-, and ClO•. This transformation basically affects the degradation of organic molecules. In this review about 400 rate constants of the dichloride radical anion (Cl2•-) with about 300 organic molecules is discussed together with the reaction mechanisms. The reactions with phenols, anilines, sulfur compounds (with sulfur atom in lower oxidation state), and molecules with conjugated electron systems are suggested to take place with electron transfer mechanism. The rate constant is high (107–109 M-1 s-1) when the reduction potential the one-electron oxidized species/molecule couple is well below that of the Cl2•-/2Cl- couple.

Toward Selective Oxidation of Contaminants in Aqueous Systems

“…•− /CO 3 •− and the electron-rich aromatic compounds, which is proved by the generation of cation radicals. 13,44,45,[48][49][50]54 Such radical cations could either undergo deprotonation or transform to hydroxycyclohexadienyl radicals after a nucleophilic attack by H 2 O, which are the same intermediates from HO • addition. 47,55 Cl 2…”

“…− , FeO 4 2− , ClO 2 , and HOCl are sensitive to electron-rich C, N, and S sites, and are thought to be more selective with varied rate constants from ∼0 to 10 9 M −1 s −1 . 35,36,40,42,45,50,51,94 It is important to address that the target compound normally bears more than one functional group, in which the oxidation might occur with different rate constants and even via different mechanisms. QSAR has been developed to resolve the complex mechanisms and predict the bimolecular rate constants by establishing the suitable descriptors of structures, such as redox 29,43,49,88,91 We then consider the selectivity of these oxidants toward target pollutants in the complex background matrix, rather than the reactivity of the oxidants toward a variety of compounds, which has been well examined and analyzed by comparing the values of rate constants in previous works.…”

“…QSAR has been developed to resolve the complex mechanisms and predict the bimolecular rate constants by establishing the suitable descriptors of structures, such as redox 29,43,49,88,91 We then consider the selectivity of these oxidants toward target pollutants in the complex background matrix, rather than the reactivity of the oxidants toward a variety of compounds, which has been well examined and analyzed by comparing the values of rate constants in previous works. 10,35,36,40,43,45,47,50,51,95 In real wastewater, DOM, (bi)carbonate, halide ions, and other coexisting ions with reducibility are major consumers for active oxidants. The values of second-order rate constants of these background compounds are summarized in SI Table S2, and the scavenging reactivity of these constituents could be calculated according to eq 1:…”

Photo- and Radiation-Induced One-Electron Oxidation of Methionine in Various Structural Environments Studied by Time-Resolved Techniques

“…Although Cl2 •− radicals exhibit strong oxidation properties (the standard reduction potential of the Cl2 •− /2Cl − redox couple is equal to E 0 = +2.13 V vs. NHE [47]), their oxida tion action is not an adduct mediated one-electron transfer process (an inner-sphere elec tron transfer) such as in the case of • OH radicals. Moreover, this reaction could be studied only in very acid solutions owing to the complex reaction mechanism leading to Cl2 •− for mation in aqueous solutions and involving • OH radicals, HOCl •− radical anions, proton (H + ) and chlorine atoms (Cl • ) [51]. The primary step of Met oxidation by Cl2 •− radicals wa found to be, in principle, similar to that established for the • OH radicals and occurs with absolute rate constant k = 3.9 × 10 10 M −1 s −1 (see Table 1) [52].…”

“…•− formation in aqueous solutions and involving • OH radicals, HOCl •− radical anions, protons (H + ) and chlorine atoms (Cl • ) [51]. The primary step of Met oxidation by Cl 2…”