Chlorine Based Oxidants for Water Purification and ... - ACS Publications

Chapter 11

Chlorine Based Oxidants for Water Purification and Disinfection Downloaded by UNIV LAVAL on April 25, 2013 | http://pubs.acs.org Publication Date (Web): September 2, 2011 | doi: 10.1021/bk-2011-1071.ch011

Gregory V. Korshin* Department of Civil and Environmental Engineering, University of Washington, Seattle, WA 98195-2700 *[email protected]

This chapter discusses the main aspects of chlorine and bromine speciation in systems with varying pHs, concentrations of bromide, chloride and total active chlorine. In the absence of ammonia, formal consideration of equilibria in solutions containing hypohalogenous acids, Cl2, Br2, BrCl and trihalogenide ions BriCl3-i- can be carried out based on two reference species (OCl-, Br-). Formal constants necessary for implementing such an approach are presented in the paper. While haloamine formation constants can be determined based on the consideration of OCl-, Br- and NH4+ as reference species, this approach is deemed to be applicable only when monochloramine is prevalent. Properties of systems with halogens can be examined based on the electrochemical potential of the HOCl/Cl- couple.

Introduction The use of chlorine-based species for oxidation and disinfection purposes has had a long and in some respects extraordinary history. For instance, the elimination of waterborne disease and ensuing notable increase of averaged life expectancy have been largely a result of use of chlorine to disinfect drinking water. While this fact is common knowledge, other aspects of water treatment processes that involve halogen-driven oxidations and related reactions remain a matter of continuing research and public debate. For instance, there are numerous issues related to further exploration of formation pathways, speciation and identification of disinfection by-products (DBPs) found in drinking water and © 2011 American Chemical Society In Aquatic Redox Chemistry; Tratnyek, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

Downloaded by UNIV LAVAL on April 25, 2013 | http://pubs.acs.org Publication Date (Web): September 2, 2011 | doi: 10.1021/bk-2011-1071.ch011

their health effects (1–3). Formation and environmental effects of halogenated species generated in chlorinated wastewater or seawater used for desalination or cooling of nuclear reactors also need to be ascertained in more detail (4). Effects of chlorine and allied species on emerging trace-level organic species grouped into the operationally defined classes of endocrine disrupting chemicals, pharmaceuticals and personal care products (EDC/PPCP) have also attracted considerable attention (5–10). Roles of dissimilar halogen species in the deactivation of various microorganisms (e.g., (11–13)) and their influence on the kinetics of removal of trace-level organic contaminants are also subject of continuing research (5–10, 14–18). While effects of concentrations and speciation of aqueous halogen species on the viability of known and emerging pathogens, formation of DBPs caused by reactions of halogen species with natural and effluent organic matter found in surface waters and wastewater (NOM and EfOM, respectively), and the degradation of EDC/PPCP compounds are enormously important, the sheer complexity of these subjects preclude their review in this document. Likewise, this document will not cover issues related to the generation of iodine-containing oxidants, their reactions with NOM or EfOM and ensuing formation of iodinated DBPs. The intent of this chapter is to provide a reasonably detailed picture of the equilibrium chemistry of chlorine- and bromine-containing oxidants common in water treatment operation. This goal will be pursued based on a consistent description of reactions that involve diverse halogen compounds and define both the species that dominate in representative situations and also the nominal electrochemical potentials associated with the presence of these oxidants.

General Aspects of Halogen Concentration and Speciation In most cases pertaining to water treatment operations, chlorine is added as chlorine gas Cl2, or concentrated sodium hypochlorite NaOCl (bleach), or in some cases solid calcium hypochlorite Ca(OCl)2. Monochloramine NH2Cl is also frequently used for disinfection. The concentration of total active chlorine in water treatment operations does not normally exceed the threshold of 4 mg/L as Cl2 (or 5.64·10-5 M) The introduction of chlorine can be accompanied by the formation of species of bromine (e.g., HOBr, OBr- and others) generated via the relatively rapid oxidation of bromide ion (Br-) by HOCl or OCl- (19). The actual concentration and speciation of bromine species released as a result of the oxidation of Brdepends on the bromide concentration in any particular water source and, as is discussed in more detail below, on the solution pH and chloride ion concentration. The concentration of bromide in surface waters is variable and can be from 7) are shown in Figure 8. The data of numerical modeling indicate that NH2Cl formation will cause a significant decrease in the redox conditions at practically all pHs relevant to water treatment. Calculations for varying ammonia levels show that the electrochemical potential of the HOCl/Cl- couple at a fixed pH typical for water treatment processes (e.g., pH 8) is expected to decrease when the molar concentration of ammonia approaches that of the total chlorine (Figure 9). At equimolar concentrations of ammonia and chlorine, the decrease of the electrochemical potential at pH 8 is expected to be close to 0.075 V. This decrease in the electrochemical potential of the system has been shown to be significant enough to affect NOM oxidation and disinfection processes and destabilize some of the solids, for instance PbO2, formed in drinking water distribution systems in the presence of chlorine (38, 46–48).

240 In Aquatic Redox Chemistry; Tratnyek, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


Figure 8. Effects of varying ammonia/total chlorine molar ratios on the electrochemical potential of the HOCl/Cl- couple. 150 mg/L chloride, no bromide, ionic strength 0.01 M. Total active chlorine concentration 4 mg/L as Cl2.

Figure 9. Change of the electrochemical potentials of the HOCl/Cl- couple at varying ammonia/total chlorine molar ratios at pH 8. 150 mg/L chloride, no bromide, ionic strength 0.01 M. Total active chlorine concentration 4 mg/L as Cl2. NOM is assumed to be absent.

Results shown in Figure 8 and Figure 9 are likely to underestimate effects of haloamine formation on the redox potential of water since these calculations do not account neither for the formation of NHCl2 and NCl3 nor the effects of NOM on the equilibrium concentration of HOCl that ultimately defines the electrochemical potential of systems with halogens.


The formation of di- and trichloramine as well as the engagement of HOCl and OCl- in reactions with amine groups (and other reactive functionalities) present in NOM will undoubtedly decrease the concentration of these oxidants. These effects can be quantified via detailed examination and subsequent interpretation of the redox potential of waters containing varying levels of chlorine, ammonia and NOM. These measurements have not been done with adequate consistency and need to be carried out in the future.


Conclusions Major aspects of the speciation of chlorine and bromine in aquatic systems with widely varying pHs, concentrations of bromide, chloride and total active chlorine are discussed in this chapter. In the absence of ammonia, formal consideration of equilibria involving three main types of halogen species (hypohalogenous acids HOX and their anionic forms, molecular forms of halogens Cl2, Br2 and BrCl and trihalogenide ions BriCl3-i-) can be carried out based on two major reference species (OCl-, Br-) and accounting for effects of free Br-, Cl-, pH and ionic strength of the relevant reactions. Formal equilibria constants that are necessary for implementing such an approach were calculated in the paper and compiled in Table 2. Formal haloamine formation constants can also be defined based on the consideration of OCl-, Br- and NH4+ as reference species (Table 4) but this approach is deemed to have a limited applicability, primarily to conditions when monochloramine is expected to be prevalent. Detailed kinetic modeling that takes into account forward and reverse reactions associated with haloamine formation as well as redox transformation yielding Cl-, N2, NO3- and other species needs to be employed to model actual aquatic systems containing both ammonia and halogens. Redox conditions in such systems can be examined based on the electrochemical potential of the HOCl/Cl- couple that be modeled numerically and quantified experimentally.

Acknowledgments This work was partially supported by the Chemical, Bioengineering, Environmental and Transport Systems Program (CBET) of the National Science Foundation (project # 0931676) and WateReuse Foundation (project # WRF09-10).

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Chlorine Based Oxidants for Water Purification and ... - ACS Publications

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