Chapter 18
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Simplifying Bromate Formation Kinetic Analysis with a Linear Bromate Yield Concept 1,2
2
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Paul Westerhoff , Gary L. Amy , Rengao Song , and Roger A. Minear 1
Department of Civil and Environmental Engineering, Arizona State University, Tempe, AZ 85287-5306 Department of Civil, Environmental, and Architectural Engineering, University of Colorado, Boulder, CO 80309 Institute for Environmental Studies, Department of Civil Engineering, University of Illinois, 1101 West Peabody Drive, Urbana, IL 61801-4723 2
3
Under completely-mixed batch-reactor conditions, bromate formation exhibits an exponential kinetic relationship that inversely mirrors the exponential loss of ozone. Bromate formation correlates linearly with ozone consumption; this relationship is defined as the bromate yield of a water. The magnitude of the bromate yields varies depending upon water quality characteristics and water treatment processes. Example applications of the bromate yield concept demonstrate techniques for understanding bromate formation mechanisms, evaluation of chemical bromate control options, and evaluation of bromate control strategies in larger-scale, continuous-flow, ozone contactors.
Formation of new inorganic and organic by-products may offset the potential benefits of ozone over the traditional disinfectant or oxidant, namely chlorine, in drinking water treatment. Chlorine reactions with natural organic matter (NOM) precursors and bromide form detectable disinfection by-products (DBPs). Proposed and regulated maximum contaminant levels (MCLs) for these chlorinated, brominated, or mixed halogen species (e.g., trihalomethanes, haloacetic acids) may limit chlorine applications (1-3). With increasing concern for microbial disinfection, ozone emerges as an alternative
0097-6156/96/0649-0322$17.00/0 © 1996 American Chemical Society In Water Disinfection and Natural Organic Matter; Minear, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
18. WESTERHOFF ET A L
Simplifying Bromate Formation Kinetic Analysis 323
disinfectant and oxidant for chlorine. However, alternative oxidants, including ozone, can form new classes of by-products (4). Proposed regulatory constraints on ozonation by-products may preclude use of ozone as an alternative to chlorine under some treatment conditions. Research into the mechanisms and control techniques for these by-products may provide strategies to use ozone and still comply with current and future regulations. As ozone decomposes in water, both molecular ozone (0 ) and hydroxyl (HO) radicals, a daughter product of ozone decomposition, typically exist at levels capable of oxidizing particulate and dissolved phases (5). Both molecular ozone and HO radicals are highly powerful oxidants, although molecular ozone tends to react selectively with solutes while HO radicals react rather unselectively (6-9). In addition to a high disinfection potential, numerous other benefits of ozonation have been reported, among which are the following: degradation of organic taste and odor compounds (e.g., MIB and geosmin) and pesticides (e.g., atrazine), improved particle destabilization, oxidation of NOM, and oxidation of reduced metals (e.g., ferrous iron) (5, 10, 11). However, ozone can produce organic by-products (e.g., aldehydes)fromozone oxidation of NOM and inorganic by-products (e.g., bromate) during ozonation of bromide containing waters (Π Ι 6). Several ozonation by-products, including bromate, may be carcinogenic and regulatory levels have been proposed by the USEPA, European Union, and World Health Organization. By-product formation may preclude the use of ozone in some applications, or at least require additional treatments to control these by-products (15,17). Bromate exhibits carcinogenic effects in two species of laboratory animals and evidence supports its ability to cause chromosome damage in humans (18-21). The USEPA proposes an interim MCL of 10 μg/L for bromate until new epidemiological data and analytical methods become available; the World Health Organization proposes a 25 μg/L maximum bromate concentration in drinking waters. Bromate carcinogenity and regulatory limits have served as the impetus for understanding bromate formation. Potentially high carcinogenicriskfrombromate has spurred research into the mechanisms and levels of bromate formation. Laboratory and field measurements clearly show that bromate forms to levels exceeding regulatory levels during ozonation of some bromide containing waters (15). Current and proposed ozonation facilities nowfindit necessary to consider and evaluate bromate formation in each water supply. To this end, this chapter examines the applicability of a simple technique capable of predicting the kinetics and ultimate level of bromate formation.
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3
Background Since the mechanisms and kinetics of bromate formation depend largely upon the decomposition of ozone and a daughter by-product, HO radicals, a brief summary of ozone decomposition processes is presented prior to discussions on bromide occurrence and bromate formation kinetics. Ozone Decomposition Processes. In pure water, hydroxide initiates ozone decomposition to several highly reactive intermediates, including HO radicals, through a series of electron transfer steps (5). Specific ozone decomposition pathways and rate
In Water Disinfection and Natural Organic Matter; Minear, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
324
WATER DISINFECTION AND NATURAL ORGANIC MATTER
constants differ in the literature, but a generalized series of reactions tend to be representative of the numerous reported models, given as follows: Oj+OH" H0 20 ' + 20 2H + 20 ' H0
=> =>
H0 + 0 " 0* + H 20 " + 20 2HO + 20
it + OW
=>
2
3
+
3
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2
2
(1) (2) (3) (4) (5)
2 +
2
2
3
2 2
Cumulative addition of Equations 1 through 6 results in the following simplified equation: 30 + H 0 3
2
=>
2HO + 40
(6)
2
Equation 6 represents the HO radical yield from ozone decomposition; the theoretical HO radical yield (η) corresponds to a stoichiometric ratio of HO/0 which equals 0.67 mole per mole. Literature reports on η values calculatedfromlaboratory experiments rangefrom0.5 to 1.0 mole per mole (22, 23). Despite the formation of significant HO radical quantities, their rapid reactivity with ozone itself, inorganic solutes, and organic solutes generally lead to HO radical concentrations on the order of 10 times lower than measured ozone residuals (6,24). Inorganic and organic constituents in water complicate the mechanism of ozone consumption and consequentially the mechanisms of oxidation (6-9, 25). For example, carbonate scavenges HO radicals forming a secondary oxidant and resulting in stabilization of ozone consumption (26, 27). Model organic compounds, such as tertiary butanol, likewise scavenge HO radicals, although tertiary butanol reactions with HO radicals do not form secondary oxidants (28). Other model organic compounds influence ozone decomposition through initiation or promotion mechanisms that resultfromdirect reactions with molecular ozone or HO radicals (28). The heterogeneity of structure and functionality which characterizes NOM suggests that NOM can serve as an initiator, promoter, and/or inhibitor of ozone decomposition through reactions with molecular ozone and HO radicals (29-31). Competing reactions among solutes influence the rate of ozone consumption. Overall ozone consumption in "pure" laboratory water typically approaches first- or second-order kinetics with respect to ozone (29, 31, 32). Inorganic and organic by product formation can occur through both direct (molecular ozone) and indirect (HO radical) pathways. Therefore, compounds affecting molecular ozone or HO radical concentrations can influence the rate, and potentially the mechanisms, of by-product formation (33). 3
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Bromide Occurrence. Bromide (Br") serves as the precursor for bromate (Br0") formation. Even drinking waters containing very low bromide levels may form bromate upon ozonation. Amy et al. (34) surveyed 101 drinking water sources and found an average national bromide concentration of almost 100 μg/L (range:
