Recent Advances in Disinfection By-Products ... - ACS Publications

Chapter 14

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Catalysis of DBP-Precursor Bromination by Halides and Hypochlorous Acid John D. Sivey,* Mark A. Bickley, and Daniel A. Victor Department of Chemistry and Urban Environmental Biogeochemistry Laboratory, Towson University, 8000 York Road, Towson, Maryland 21252, United States *E-mail: [email protected].

When bromide-containing waters are disinfected with free chlorine, bromide can be converted into free bromine (principally HOBr). Although often overlooked, chloride and bromide are capable of catalyzing bromination reactions of organic compounds by promoting the formation of BrCl and Br2, respectively. Similarly, HOCl can catalyze bromination reactions by enhancing concentrations of BrOCl. As brominating agents, BrCl, Br2, BrOCl (and the related species Br2O) have been shown to be several orders of magnitude more inherently reactive than HOBr toward several non-ionizable aromatic compounds. Under conditions representative of chlorinated drinking water, BrCl and BrOCl can contribute more than HOBr to overall bromination rates of modestly nucleophilic aromatic compounds. This chapter reviews the literature regarding these reactive brominating agents and discusses their potential influence on bromination rates of disinfection by-product precursors.

Introduction In 1902, for the first time in the United States, chlorine was added to drinking water (DW) to protect the public from water-borne diseases (1). Since then, countless lives have been saved by this potent microbicidal agent. Not until the 1970s, however, were the unintended consequences of DW chlorination discovered (2, 3). In addition to inactivating bacteria and viruses, chlorine can © 2015 American Chemical Society In Recent Advances in Disinfection By-Products; Xie, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


also react with natural organic matter (4–7) to form potentially toxic disinfection by-products (DBPs) (8). In water with low nitrogen levels, chlorine consists primarily of hypochlorous acid (HOCl, pKa = 7.58 at 20 °C) (9) and ClO-; the sum of such species is termed free chlorine. In addition to reactions with organic DBP precursors, free chlorine can rapidly oxidize bromide (eq 1) (10).

HOCl-mediated oxidation of bromide (eq 1) has a large equilibrium constant (1.5 x 105, 25 °C, µ = 0) (11) and will proceed “to completion” in the presence of excess HOCl. The forward-reaction rate associated with eq 1 is proportional to [HOCl] (k1+ = forward-reaction rate constant = 1.55 x 103 M-1 s-1, 25 °C, µ = 1 M) (10) and is therefore anticipated to decrease with increasing pH. As with HOCl, the presence of HOBr in DW is concurrently beneficial and problematic. Like its chlorinated analogue, HOBr is both a potent microbicide (1, 12–14) and a reactive electrophile capable of forming organobromine compounds in DW (1, 5, 15–22), wastewater (23, 24), and recreational waters (e.g., pools and spas) (25–27). Some of these organobromine compounds are DBPs whose concentrations are regulated in drinking water (28–31) due to their potential cytotoxic (32, 33) and genotoxic effects (34–36). Brominated DBPs are generally more toxic than their chlorinated analogs (34). Therefore, strategies for minimizing their formation are highly desirable. Such DBP-minimization strategies necessitate a comprehensive understanding of bromine chemistry. The purpose of this chapter is to review recent discoveries in the literature pertaining to the chemistry of free bromine, particularly as it relates to the bromination of aromatic DBP precursors. We begin with a brief overview of bromide occurrence in natural and engineered aquatic systems and then discuss the chemistry of free bromine generated when bromide-containing source waters undergo disinfection (e.g., with free chlorine). The bulk of our discussion, however, is devoted to the often-overlooked ability of halides (chloride and bromide) and HOCl to catalyze bromination reactions. We conclude with an analysis of how these catalytic reactions can influence DBP precursor bromination under model DW treatment conditions.

Natural and Anthropogenic Sources of Bromide Bromide is a ubiquitous constituent of natural waters (Table 1), with median concentrations ranging from low µg/L in precipitation (37) to 65 mg/L in seawater (38). Bromide concentrations in groundwater range from low µg/L to low mg/L (39, 40). Brackish-water or seawater intrusion can, however, increase bromide concentrations in aquifers near the coasts (41). Bromide levels measured in DW (39) and wastewater (23) influents (50–250 µg/L) align with the upper levels measured in fresh surface waters. Even higher levels have been reported for surface waters impacted by produced water from hydraulic fracturing operations (Table 1) (42–44). 252 In Recent Advances in Disinfection By-Products; Xie, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Table 1. Bromide Concentrations in Natural Waters, Drinking Water, and Wastewaters [Br–], (µg/L)

Ref


Natural Waters median

maximum

Precipitation

~6

12

(37)

U.S. groundwater (potable waters only)

16

58

(40)

U.S. freshwater lakes

23

322

(39)

U.S. rivers

63

426

(39)

U.S. groundwater (includes non-potable waters)

62

2.7 x 103

(39)

Seawater

6.5 x 104

not reported

(38)

Surface water impacted by produced water from hydraulic fracturing

not reported

7.5 x 104

(43)

Reported Ranges of [Br–], (µg/L)

Ref

U.S. drinking water influent

50–200

(39)

Municipal wastewater influent

100–250

(23)

Drinking Water and Wastewater

Chemistry of Free Bromine The oxidation state of bromine is +I in species such as HOBr (pKa = 8.70 at 20 °C) (45) and BrO-; the sum of all Br(+I) species is referred to as free bromine. The direct application of free bromine as a disinfectant of DW is uncommon due to the increased costs, increased DBP formation, and difficulty in maintaining a disinfectant residual throughout the distribution system relative to free chlorine (1). Free bromine is, however, employed as a disinfectant in some recreational waters (e.g., spas) (1). In waters disinfected with ozone (O3), free bromine can form via the incomplete oxidation of bromide (46–48). Bromide can also be oxidized into free bromine in solutions dosed with chloramines (49–51). Conventional wisdom in the literature generally assumes HOBr is the predominant brominating agent in solutions of free bromine (52). Under this assumption, bromination rates can be described via eq 2:

where kHOBr is a second-order rate constant (M-1 s-1), [HOBr] is the concentration of HOBr (mol/L), [Org-H] is the concentration of the parental organic compound (mol/L), and [Org-Br] is the concentration of the brominated product (mol/L). When HOBr is present in large excess, kHOBr can be calculated by eq 3: 253 In Recent Advances in Disinfection By-Products; Xie, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


where kobs is a pseudo-first-order rate constant (s-1). Apparent rate constants (kapp) are also commonly reported for bromination reactions:

where [Br(I)]tot denotes the total concentration of free bromine. Values of kapp typically vary as a function of pH (e.g., due to the weak-acid character of HOBr). That BrO- possesses a net negative charge and requires expulsion of a weaklylabile leaving group (O2-) suggests HOBr will be a more reactive brominating agent than BrO- (53). The kinetic models represented by eqs 3 and 4 do not account for the potential influence of bromination catalysts. A catalyst is “a substance that affects the rate of a reaction but emerges from the process unchanged (54).” Below, we describe the chemistry by which chloride, bromide, and HOCl can catalyze bromination reactions. Each of these reagents can promote the formation of reactive brominating agents via eq 5:

where HX = HCl, HBr, or HOCl. BrX ostensibly represents a more inherently reactive brominating agent relative to HOBr (55). BrX can participate in bromine substitution reactions with organic compounds via eq 6:

Because HX emerges from this series of reactions (eqs 5 and 6) unchanged, HX species can be viewed as catalysts of bromination. Although the examples herein focus on bromination of aromatic compounds, the chemistry described below conceivably also applies to other organic nucleophiles (e.g., alkenes, ketones, and amines) present in DW and wastewater.

Catalysis by Chloride Although commonly overlooked, bromination rates of organic compounds can depend on the concentration of chloride ions (53, 55–57). For example, a recent investigation involving the herbicide dimethenamid (a substituted thiophene) revealed that rates of bromination in solutions of free bromine increased linearly with the concentration of added chloride (Figure 1) (53). The influence of chloride on rates of dimethenamid bromination (represented by the slopes in Figure 1) increased with decreasing pH. These findings are consistent with the participation of BrCl as a brominating agent, whose concentration is proportional to the concentration of chloride (eq 7):

254 In Recent Advances in Disinfection By-Products; Xie, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Although an additional free bromine species (BrCl2-) can also form in the presence of chloride (58), the formal negative charge on BrCl2- is anticipated to render this species significantly less electrophilic compared to neutral free bromine species (e.g., BrCl and HOBr) (55).

Figure 1. Pseudo-first-order rate constants of dimethenamid bromination as a function of chloride concentration. Arrow denotes site of bromination on dimethenamid. Error estimates denote 95% confidence intervals. Conditions: free chlorine dose = 0.43 mM = 30 mg/L as Cl2, [Br-]o = 0.10 mM = 8.0 mg/L, [dimethenamid]o = 9 µM, [NaNO3] + [NaCl] = 0.1 M, [borate buffer] = 10 mM, T = 20.0 °C. Adapted from reference (53). Copyright 2013, ACS.

The greater electronegativity of chlorine imparts a partial positive charge on bromine in BrCl. Consequently, BrCl is anticipated to function as a brominating agent. During the course of aromatic substitutions involving BrCl as the electrophile, chloride is released as a leaving group from BrCl (Scheme 1). Accordingly, chloride can be viewed as a catalyst of bromination reactions. Chloride catalysis via BrCl formation has also been quantified for bromination of p-xylene (56, 57) and anisole (55).

Scheme 1. Proposed Mechanism of Aromatic Compound Bromination by Free Bromine Species (X = Cl, Br, OCl, OBr, or OH) Using Anisole as a Model Compound. From Reference (55). Copyright 2015, ACS. 255 In Recent Advances in Disinfection By-Products; Xie, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


BrCl has an inherent reactivity 3.6 x 106- and 1.4 x 107-times greater than HOBr toward dimethenamid (53) and the para position of anisole, respectively (55). As such, the potency of chloride as a bromination catalyst can be substantial. The median chloride concentration in raw DW is approximately 10 mg/L (59). Treatment processes (e.g., coagulation) can increase the chloride concentration of DW. For example, addition of FeCl3 at 2 mM (~300 mg/L) as a coagulant concomitantly increases the chloride concentration by ~200 mg/L. This additional chloride is sufficient to increase bromination rates of anisole and dimethenamid by more than a factor of 11 (Figure 2).

Figure 2. Catalysis of bromination of anisole (to give 4-bromoanisole (55)) and of dimethenamid (53) as a function of chloride concentration. Bromination rates are normalized to values calculated at [Cl-] = 10 mg/L. Conditions: pH = 7.00, free chlorine dose = 2.0 mg/L as Cl2,[Br-]o = 0.10 mg/L, T = 20.0 °C. In addition to catalyzing bromination reactions, chloride can similarly catalyze chlorination reactions of several organic compounds (including p-xylene (56, 57), the antimicrobial agent trimethoprim (60), polycyclic aromatic hydrocarbons (61), dimethenamid (62), phenolic compounds (63, 64), and aromatic ethers (65)) by promoting the formation of aqueous molecular chlorine (Cl2). 256 In Recent Advances in Disinfection By-Products; Xie, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Catalysis by Bromide Unoxidized bromide can coexist in solutions of free bromine, including when (1) free chlorine is added substoichiometrically relative to bromide, (2) bromide is incompletely oxidized by ozone, or (3) free bromine is added directly to aqueous solutions containing bromide (e.g., in some pools and spas). Under these conditions, unoxidized bromide can enhance rates of organic compound bromination by promoting the formation of molecular bromine (eq 9):

When Br2 serves as a brominating agent via transfer of Br(I) to an organic compound, bromide is released as a leaving group from Br2. As such, bromide can function as a catalyst of bromination reactions involving Br2. Additional free bromine species (i.e., Br2Cl- and Br3–) can also form in the presence of excess bromide (58, 68). The formal negative charge on these species suggests that they will be significantly less reactive as brominating agents relative to neutral free bromine species (e.g., BrCl and HOBr) (55). The ability of (unoxidized) bromide to increase bromination rates has been demonstrated for the sequential bromination of anisole to give mono- and dibrominated anisoles (55). Figure 3 demonstrates the catalytic effect of bromide on reactions of anisole with free bromine. Bromide catalysis of bromination has also been reported for other aromatic compounds, including p-xylene (56), phenols (69), and dimethenamid (53). The ratio of second-order rate constants for Br2 and HOBr (i.e., kBr2/kHOBr) is a means of quantifying the catalytic efficacy of bromide during bromination reactions involving free bromine. For bromination of dimethenamid (53) and anisole (para position) (55) at 20 °C, kBr2/kHOBr = 6.6 x 104 and 3.2 x 105, respectively. The greater catalytic potency of bromide toward bromination of anisole (relative to the more nucleophilic thiophene ring of dimethenamid) is consistent with the reactivity-selectivity principle, which predicts an increase in selectivity toward more reactive (brominating) agents as the reactivity of a substrate (here, an aromatic compound) decreases (70). The increased catalytic potency of bromide toward bromination of anisole relative to dimethenamid is also evident in Figure 4, which depicts normalized bromination rates as a function of the excess bromide concentration under conditions representative of DW treatment.



Figure 3. Effects of excess bromide (calculated as [Br-]xs = [NaBr]o – [free chlorine]o) on pseudo-first-order rate constants of para bromination of anisole in solutions of free bromine. Arrow denotes site of bromination. Error bars represent 95% confidence intervals. Conditions: pH = 7.31, [bicarbonate buffer] = 20 mM, [free chlorine]o = 119 µM, [Br-]o = 197-588 µM, [anisole]o = 10 µM, [NaNO3] = 98 mM, no added NaCl, T = 20.0 °C. Adapted from reference (55). Copyright 2015, ACS.

Catalysis by Hypochlorous Acid As with chloride and bromide, HOCl can also serve as a catalyst of bromination reactions. For example, bromination rates of dimethenamid in model laboratory reactors were previously shown to increase as the dose of free chlorine increased (53). The authors rationalized this finding by invoking BrOCl as a reactive brominating agent:



As with BrCl, the partial positive charge on the bromine atom of BrOCl and the greater leaving group ability of ClO- (relative to BrO-) suggest this mixed halogen will function as a brominating agent (53). As HOCl is a product of aromatic bromination reactions involving BrOCl (53, 55), HOCl can be viewed as a bromination catalyst. Catalysis of bromination by HOCl has also been reported for reactions of anisole in solutions prepared by adding excess free chlorine to sodium bromide (Figure 5) (55).

Figure 4. Catalysis of bromination of anisole (to give 4-bromoanisole (55)) and of dimethenamid (53) as a function of excess bromide concentration (calculated as [Br-]xs = [NaBr]o – [free chlorine]o). Bromination rates are normalized to values calculated at [Br-]xs = 0.1 mg/L. Conditions: pH = 7.00, [free chlorine]o = 1.2 µM, [Cl-] = 0.3 mM = 11 mg/L, T = 20.0 °C. 259 In Recent Advances in Disinfection By-Products; Xie, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Figure 5. Influence of free chlorine concentration on pseudo-first-order rate constants of para bromination of anisole in solutions of free bromine. Arrow denotes site of bromination. Error estimates denote 95% confidence intervals. Conditions: pH 8.49, [Br-]o = 230 µM, [NaNO3] = 93 µM, [Cl-] = 3.9 mM, [anisole]o = 19 µM, and T = 20.0 °C. Adapted from reference (55). Copyright 2015, ACS.

The influence of free chlorine dose on bromination rates of anisole and dimethenamid are shown in Figure 6 for solution conditions typical of DW chlorination. A ten-fold increase in free chlorine dose (from 1 to 10 mg/L as Cl2) results in a predicted 2.6- and 1.8-fold increase in bromination rates of anisole (para position) and dimethenamid, respectively. In addition to promoting the formation of BrOCl, residual free chlorine may also increase observed rates of bromination in natural waters by facilitating reoxidation of bromide (e.g., formed via reduction of free bromine by endogenous reductants) (71). 260 In Recent Advances in Disinfection By-Products; Xie, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Figure 6. Catalysis of bromination of anisole (to give 4-bromoanisole (55)) and of dimethenamid (53) as a function of initial free chlorine concentration. Conditions: pH 7.00, [Br-]o = 0.1 mg/L = 1.25 µM, [Cl-] = 11 mg/L = 0.3 mM, T = 20.0 °C.

In addition to catalyzing bromination reactions, HOCl can also enhance chlorination rates of organic compounds (including anisole (72), p-xylene (56), biphenyl (73), trans-2-butenoic acid (74), dimethenamid (62), and aromatic ethers (65)) by promoting the formation of Cl2O (eq 11).

Br2O as a Putative Brominating Agent Bromination reactions of organic compounds in solutions of free bromine are often assumed to be first-order in [Br(I)]tot. This assumption was recently tested for bromination reactions of dimethenamid (53). Sivey et al. (53) discovered that bromination rates of dimethenamid had reaction orders in [HOBr] ranging from near 1.0 to approximately 1.7 (Figure 7). 261 In Recent Advances in Disinfection By-Products; Xie, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Figure 7. Reaction order in total free bromine concentration, [Br(I)], as a function of pH for reactions of dimethenamid with free bromine. Conditions:[dimethenamid]o = 8 µM, [Br-]o = (1.0 – 4.0) x 10-4 M, initial free chlorine concentration = 430 µM, [borate buffer] = 10 mM, T = 20.0 °C. Adapted from reference (53). Copyright 2013, ACS. Sivey et al. (53) suggested that the participation of Br2O (eq 12) as a brominating agent could account for the measured reaction rates exhibiting a greater-than-first-order dependence on total free bromine.

Eq 12 indicates [Br2O] is proportional to [HOBr]2. Unlike Br2O, all other brominating agents discussed herein (HOBr, BrCl, Br2, and BrOCl) have concentrations that scale linearly with HOBr. Accordingly, Br2O is the only examined free bromine species anticipated to produce reaction orders in [Br(I)]tot greater than 1.0. Br2O is 7100- and 4100-times more inherently reactive than is HOBr in reactions with dimethenamid (53) and anisole (55), respectively.

A More Complete View of Free Bromine Speciation Although HOBr and BrO- are typically the most abundant constituents of free bromine (Figure 8), previous reports (53, 55–57) indicate the generally less-abundant free bromine species (BrCl, Br2, BrOCl, and Br2O) are sufficiently reactive as to influence overall bromination rates of modestly nucleophilic aromatic compounds. 262 In Recent Advances in Disinfection By-Products; Xie, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Figure 8. Speciation of free bromine for a model water containing bromide (160 µg/L) that undergoes disinfection by (A) free chlorine and (B) a sufficient amount of ozone to convert 95% of initial bromide into free bromine, Br(I), and assuming higher oxidation products of bromine (e.g., bromate) are not formed. Data from reference (55) and sources cited therein.

The extent to which these often-overlooked brominating agents influence bromination rates under conditions representative of drinking water treatment can be calculated using second-order bromination rate constants reported for dimethenamid (53) and anisole (Figure 9) (55). As shown in Figure 9A, HOBr contributes

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