The reactions of chlorine dioxide with inorganic and organic compounds in water treatment: kinetics and mechanisms

Abstract

Chlorine dioxide (ClO₂), as an alternative to chlorine, has been widely applied in water treatment. In order to better understand the performance of ClO₂ in water treatment, the kinetics and mechanisms of ClO₂ reactions with inorganic and organic compounds found in waters are critically reviewed. In the case of inorganic compounds, ClO₂ reacts with I⁻, CN⁻, NO₂⁻, SO₃²⁻, Fe(II) and Mn(II) rapidly at apparent second-order reaction rate constants (k_app) of 10²–10⁶ M⁻¹ s⁻¹ at pH 7.0 and barely reacts with NH₄⁺ and Br⁻. In the case of organic compounds, ClO₂ selectively reacts with compounds with electron-rich moieties, such as phenols (k_app = 10³–10⁹ M⁻¹ s⁻¹), anilines (k_app = 10⁵–10⁸ M⁻¹ s⁻¹), and thiols (k_app > 10⁸ M⁻¹ s⁻¹). ClO₂ also shows high reactivity towards aliphatic tertiary amines and heterocyclic nitrogenous compounds (i.e., indoles and piperidines) with k_app of 10¹–10⁶ M⁻¹ s⁻¹ at pH 7.0, but low reactivity with unsaturated structures (i.e., olefins and aldehydes). The k_app values at pH 7.0 in ClO₂ oxidation vary over 14 orders of magnitude. Electron transfer is the dominant pathway for ClO₂ reactions. Quantitative structure–activity relationships (QSARs) can be used to predict the species-specific secondary reaction rate constants for ClO₂ oxidation of compounds containing phenolic and amine structures. Little modifications are expected on the structure of the parent compounds upon the primary attack of ClO₂, but further oxidation generally leads to the formation of quinones, aldehydes and carboxylic acids. Furthermore, the transformation kinetics of inorganic compounds, typical organic compounds and emerging micropollutants are compared and their half-life times under typical water treatment conditions during ClO₂ oxidation are calculated.