Reactions of chlorine with inorganic and organic compounds during water treatment—Kinetics and mechanisms: A critical review

Abstract

Numerous inorganic and organic micropollutants can undergo reactions with chlorine. However, for certain compounds, the expected chlorine reactivity is low and only small modifications in the parent compound's structure are expected under typical water treatment conditions. To better understand/predict chlorine reactions with micropollutants, the kinetic and mechanistic information on chlorine reactivity available in literature was critically reviewed. For most micropollutants, HOCl is the major reactive chlorine species during chlorination processes. In the case of inorganic compounds, a fast reaction of ammonia, halides (Br⁻ and I⁻), SO₃²⁻, CN⁻, NO₂⁻, As(III) and Fe(II) with HOCl is reported (10³–10⁹ M⁻¹ s⁻¹) whereas low chlorine reaction rates with Mn(II) were shown in homogeneous systems. Chlorine reactivity usually results from an initial electrophilic attack of HOCl on inorganic compounds. In the case of organic compounds, second-order rate constants for chlorination vary over 10 orders of magnitude (i.e. <0.1–10⁹ M⁻¹ s⁻¹). Oxidation, addition and electrophilic substitution reactions with organic compounds are possible pathways. However, from a kinetic point of view, usually only electrophilic attack is significant. Chlorine reactivity limited to particular sites (mainly amines, reduced sulfur moieties or activated aromatic systems) is commonly observed during chlorination processes and small modifications in the parent compound's structure are expected for the primary attack. Linear structure–activity relationships can be used to make predictions/estimates of the reactivity of functional groups based on structural analogy. Furthermore, comparison of chlorine to ozone reactivity towards aromatic compounds (electrophilic attack) shows a good correlation, with chlorine rate constants being about four orders of magnitude smaller than those for ozone.

Introduction

Due to their capability for disinfection (microorganisms) and oxidation (e.g. taste and odor control, elimination of micropollutants, etc.), chemical oxidants (i.e. ozone, chlorine, chlorine dioxide, chloramines, etc.) are commonly used in water treatment processes (Hoff and Geldreich, 1981; Wolfe et al., 1984; Morris, 1986; Burlingame et al., 1992; Hoigné, 1998; Gottschalk et al., 2000; von Gunten, 2003; Legube, 2003; Bruchet and Duguet, 2004). However, under certain circumstances, oxidants can induce formation of potentially harmful by-products or transformation products due to their reactivity with water matrix components or micropollutants (Cancho et al., 2000; Bichsel and von Gunten., 2000; Simmons et al., 2002; Richardson et al., 2003; Plewa et al., 2004; Richardson, 2005; Krasner et al., 2006).

Owing to its low cost, chlorine is globally the most used chemical oxidant for drinking water disinfection. Drinking water disinfection commonly involves the use of chlorine at one or two point(s) in the treatment process, i.e., for pre-treatment (to induce a primary disinfection at the beginning of the treatment process) and/or for post-treatment (to maintain a disinfectant residual in the distribution system). Despite its low activity on microorganisms in biofilms, chlorine can lead to a significant removal of the majority of planktonic bacteria (Le Chevallier et al., 1988; Bois et al., 1997). Added near the end of the treatment process, i.e., before water release in the distribution system (post chlorination), chlorine thus plays an important role to limit the growth of heterotrophic microorganisms. As a chemical oxidant, though less reactive than ozone, chlorine can transform numerous inorganic and organic micropollutants found in water (e.g. Fe(II), As(III), NO₂⁻, phenols, pesticides, pharmaceuticals, etc.) (Johnson and Margerum, 1991; Magara et al., 1994; Folkes et al., 1995; Gallard and von Gunten, 2002; Lahoutifard et al., 2003; Diurk and Colette, 2006; Dodd et al., 2006; von Gunten et al., 2006). Chlorination usually represents an efficient process to remove/transform inorganic micropollutants. However, due to the potentially harmful chlorinated transformation products, chlorination is usually not applied for oxidation of organic micropollutants.

Similar to other disinfection processes, chlorination presents certain disadvantages in spite of its broad use and its benefits for the improvement of microbial water quality: (i) Due to its pH-dependent aqueous chemistry, various species of chlorine (HOCl, ClO⁻, Cl₂, etc.) may be present in solution (Doré, 1989). These forms of chlorine show significant differences in their reactivity with microorganisms and micropollutants. Therefore, variability in oxidation or disinfection efficiency can be observed depending on the pH of the water. (ii) Chlorine interacts with dissolved natural organic matter (DNOM). Numerous so-called disinfection by-products (DBPs) can result from the reaction of chlorine with DNOM. Among these DBPs, trihalomethanes (THMs) and haloacetic acids (HAAs) were the first chlorine DBPs reported and are currently regulated in the EU (THMs) and the USA (THMs, HAAs) (Richardson, 2005). Currently, about 600 DBPs are identified, among them some highly toxic compounds such as iodo and bromo compounds (Bichsel and von Gunten, 2000; Richardson et al., 2003; Plewa et al., 2004; Krasner et al., 2006), MX (Onstad and Weinberg, 2005; Krasner et al., 2006), halonitromethanes (Krasner et al., 2006) and N-nitrosodimethylamine (Mitch et al., 2003). These individual DBPs or mixtures of DBPs could represent a potential human health risk (Cancho et al., 2000; Simmons et al., 2002; Richardson et al., 2003; Plewa et al., 2004; Richardson, 2005; Krasner et al., 2006). (iii) Because organic micropollutants are typically not mineralized, numerous transformation products can be formed as a result of the oxidation of organic compounds during water chlorination processes (Magara et al., 1994; Gallard and von Gunten, 2002; Hu et al., 2002a, Hu et al., 2002b, Hu et al., 2003; Moriyama et al., 2004; Dodd and Huang, 2004; Dodd et al., 2005; Rule et al., 2005; Diurk and Colette, 2006; von Gunten et al., 2006). Little is known on the stability and the biological effects of these compounds. However, in some cases, certain transformation products are fairly stable against further transformation and could persist for hours to days even in presence of residual chlorine. Moreover, in the case of some endocrine disruptors (i.e. nonylphenol, bisphenol A and hormones), some pesticides (i.e. chlorpyrifos), some pharmaceuticals (i.e. acetaminophen) and some azo-dyes, potentially toxic or biologically active chlorination products were reported (Hu et al., 2002a, Hu et al., 2002b, Hu et al., 2003; Wu and Laird, 2003; Bedner and MacCrehan, 2006; Moriyama et al., 2004; Oliveira et al., 2006). (iv) In bromide-containing waters, chlorination leads to bromine formation. Bromine is usually more reactive than chlorine, especially with phenolic compounds (Gallard et al., 2003; Acero et al., 2005b). Under these conditions, bromination can be highly significant and brominated products can be formed (Gallard et al., 2003; Acero et al., 2005b; Hu et al., 2006).

This study presents an overview on chlorination of drinking water with an emphasis on kinetics and mechanisms of chlorine reactions. Based on literature data, an overview over chlorine reactivity with inorganic and organic compounds is presented. For typical functional groups, chlorination kinetics and mechanisms are described. By structural analogy, linear structure–activity relationships are proposed. Finally, for some organic micropollutants relevant for urban water management, a discussion on expected and observed chlorine reactivities is provided.