Autoxidation of S^IV inhibited by chlorophenols reacting with sulfate radicals

Environmental context. Chlorophenols pollute natural waters and soils, as well as urban waste water systems. Although toxic and carcinogenic to animals and humans, a detailed knowledge of their action is limited. A new approach to effective degradation in the environment is advanced oxidation processes with sulfate radicals. The radicals can originate from the oxidation of sulfur dioxide or sulfites to make these common pollutants and food additives interact with chlorophenols. The main goal of this work is to determine rate constants for reactions of these chlorophenols with sulfate radicals in order to shed some light on the chemical kinetics of these reactions.

Abstract. Kinetic experiments have shown that six chlorophenols (CPs) inhibit the autoxidation of S^IV catalysed by Fe(ClO₄)₃ in aqueous solution at 25°C and pH ≈ 3.0. Efficiency of the inhibition decreases with the number of chlorine substituents for all CPs except for 2,5-dichlorophenol (2,5-DCP), which ranked between the tri- and tetrachlorophenols. The inhibition is explained by reactions of chlorophenols with sulfate radicals, the chain carriers in the mechanism of autoxidation. Rate constants for these reactions are determined for the first time, using the reversed-rates method with ethanol as a reference inhibitor: 8.7 × 10⁹ (4-CP), 7.4 × 10⁹ (2,4-DCP), 1.9 × 10⁹ (2,5-DCP), 2.4 × 10⁹ (2,4,5-TCP), 2.9 × 10⁹ (2,4,6-TCP), and 7.5 × 10⁸ (2,3,5,6-TTCP); 4.3 × 10⁷ (ethanol reference) M^–1 s^–1. Linear correlations were derived for the estimation of rate constants for the remaining chlorophenols using sums of Brown substituent coefficients or relative strengths of O–H bonds. The results can be used in the development of advanced oxidation processes that utilise sulfate radicals for mineralisation of chlorophenols in wastewaters, and also demonstrate that chlorophenols can extend the lifetimes of SO₂ and sulfites in natural and atmospheric waters.