Fate of Pharmaceuticals in the Environment and in Water Treatment Systems - Chapter 12 pot

Transformation of Antibacterial Agents with Aqueous Chlorine under Relevant Water Treatment Conditions Ching-Hua Huang, Michael C.. 262 Fate of Pharmaceuticals in the Environment and in

Transformation of

Antibacterial Agents

with Aqueous Chlorine under Relevant Water

Treatment Conditions

Ching-Hua Huang,

Michael C Dodd, and Amisha D Shah

12.1 INTRODUCTION

Each year large quantities of antibacterial agents (referred to as antibacterials here-after) are used to treat diseases and infections in humans and animals Applications

of antibacterials in human medicine can ultimately lead to significant discharges of

Contents

12.1 Introduction 261

12.2 Background 263

12.2.1 Antibacterial Agents of Investigation 263

12.2.2 Chemical Oxidation by Aqueous Chlorine 267

12.2.3 Prior Work on the Reaction of Pharmaceuticals with Chlorine 267

12.3 Materials and Methods 268

12.3.1 Chemical Reagents 268

12.3.2 Surface Water and Wastewater Samples 269

12.3.3 Reaction Setup and Monitoring 269

12.4 Results and Discussion 271

12.4.1 Reaction Kinetics and Modeling 271

12.4.2 Identification of Reactive Functional Groups 275

12.4.3 Reaction Pathways and Products’ Biological Implications 276

12.4.4 Reaction in Real Water Matrices 283

12.5 Conclusion 285

References 285

262 Fate of Pharmaceuticals in the Environment and in Water Treatment Systems

such compounds into surface waters, via excretion of the unmetabolized parent pounds into municipal sewage systems and subsequent passage through municipalwastewater treatment facilities.1In addition, antibacterials are utilized for a number

com-of agricultural applications, including use as growth promoters and feed efficiencyenhancers for livestock2and in aquaculture and fruit orchards.3It has been estimatedthat nearly 50% of the total antibacterial usage in the United States was for agricul-ture.4Significant proportions of the administered antibacterials can be excreted fromthe dosed animals with little metabolic transformation.1,5A nationwide reconnais-sance study published by the U.S Geological Survey (USGS) in 2002 reported thepresence of a wide variety of chemicals including many pharmaceuticals and per-sonal-care products in U.S streams.6Other similar findings regarding the ubiquity

of pharmaceuticals in the aquatic environment have also been reported in the UnitedStates and other parts of the world.1,7–19Among the pharmaceuticals, several widelyapplied human-use and veterinary antibacterial classes, such as fluoroquinolone, sul-fonamide, tetracycline, macrolide, and so forth, have been repeatedly detected inconcentrations ranging from low ng/L to low µg/L.6–19

The presence of antibacterial residues in natural surface waters and wastewatereffluents merits concern for a number of reasons First, the continuous exposure ofwastewater-borne or environmental bacterial communities to mixtures of antibacterialresidues may promote induction or dissemination of low-level resistant bacterial phe-notypes, which have significant indirect implications for human health.5,20,21Second,studies have shown that, once present in bacterial populations, numerous resistancephenotypes are stable over many bacterial generations, even in the absence of selec-tive pressure from the antibacterial compounds themselves.22–25Furthermore, someantibacterials and their metabolites are reported to exhibit carcinogenic or genotoxiceffects, which may be of direct significance to human health.26–28To properly evaluatethe risks posed by antibacterial micropollutants, and to ensure provision of safe pota-ble water supplies, the behavior of antibacterials during relevant water treatment pro-cesses should be critically evaluated Chlorination is an important treatment processthat is likely to affect the fate of numerous antibacterials, on account of its commonuse in water and wastewater treatment for disinfection purposes, and because manyantibacterial compounds contain electron-rich functional groups that are susceptible

to reaction with electrophilic chlorine

This chapter summarizes the authors’ recent contribution to developing a damental understanding of the interactions of four structural classes (quinoxaline

fun-N,N’-dioxide,29fluoroquinolone,30sulfonamide,31and pyrimidine32) of antibacterialswith aqueous chlorine under relevant water treatment conditions In contrast to theprevious publications in which each structural class was dealt with separately, thischapter discusses these four structural classes simultaneously, highlighting similar-ity and difference in their interactions with aqueous chlorine The investigationswere undertaken to elucidate the chemical reactivity, reaction kinetics, products, andpathways by which antibacterials are transformed by free chlorine Reaction kineticswere determined over a wide pH range and evaluated by a second-order kinetic modelthat incorporated the acid-base speciation of each reactant (i.e., oxidant and antibac-terial) Various structurally related compounds that resemble the hypothesized reac-tive and nonreactive moieties of the target antibacterials were examined to probe

reactive functional groups Results obtained from kinetic experiments were mented by product identification analyses by liquid chromatography/mass spectrom-etry (LC/MS), gas chromatography/mass spectrometry (GC/MS), nuclear magneticresonance spectroscopy (1H-NMR), and other techniques to facilitate identification

supple-of reaction pathways and mechanisms Additional experiments were conducted inreal municipal wastewater and surface water matrices to assess the field-applicabil-ity of observations obtained for reagent water systems in the laboratory

12.2 BACKGROUND

12 2.1 ANTIBACTERIAL AGENTS OF INVESTIGATION

Representative antibacterial agents from four structural classes—quinoxaline

N,N’-dioxide, fluoroquinolone, sulfonamide, and pyrimidine—were investigated (see

asso-ciated model compounds) Carbadox (CDX) and olaquindox (QDX) represent the

quinoxaline N,N’-dioxide group of veterinary antibacterial agents, which are widely

used in swine production for promoting growth and preventing dysentery and rial enteritis In recent years CDX and its major metabolite desoxycarbadox (DCDX)have been shown to exhibit carcinogenic and genotoxic effects.26–28Such concernsled the European Union to ban the use of CDX in animal feeds in 199933and HealthCanada to issue a ban on CDX sales in 2001 after reports of misuse and accidentalcontamination.28Ciprofloxacin (CIP) and enrofloxacin (ENR) belong to the fluoro-quinolone structural class, a group of synthetic, broad-spectrum antibacterial agentsthat interfere with bacterial DNA replication,5and are used in a multitude of humanand veterinary applications.2CIP is one of the most frequently prescribed human-usefluoroquinolones in North America and Europe,1whereas ENR was popular for dis-ease prevention and control in the U.S poultry production industry until recently.34

bacte-Sulfamethoxazole (SMX) is one of the most popular sulfonamide antibacterials used

to treat diseases and infections in humans.5Sulfonamides, often referred to as sulfadrugs, are synthetic antibacterials widely used in human and veterinary medicinesand as growth promoters in feeds for livestock.5SMX is commonly prescribed intandem with the synthetic pyrimidine antibacterial trimethoprim (TMP) under thename cotrimoxazole.35These two antibacterials function synergistically as inhibi-tors of bacterial folic acid synthesis.5

Recent studies have reported frequent detection of fluoroquinolones, amides, and TMP in the aquatic environment Reported concentrations of variousfluoroquinolones range from ~1 to 125 µg/L in untreated hospital sewage,10,11~70

sulfon-to 500 ng/L in secondary wastewater effluents,7,14–16to ~10 to 120 ng/L in surfacewaters.6,14,15SMX is the most frequently detected sulfonamide antibacterial at con-centrations of 70 to 150 ng/L in surface waters6,12–14and 200 to 2000 ng/L in second-ary wastewater effluents.7,12–14,17Occurrence of TMP was reported at several-hundredng/L in secondary municipal wastewater effluents,18,19and at concentrations fromapproximately 10 to several-hundred ng/L in surface waters,12,17particularly thosereceiving substantial discharges of treated wastewater.8Concentrations of CDX insurface waters and wastewaters were reported in two studies to be below the detec-tion limits of 0.1 µg/L and 5 ng/L, respectively.6,17

264 Fate of Pharmaceuticals in the Environment and in Water Treatment Systems

TABLE 12.1

Structures and Apparent Second-Order Rate Constants (k app) for

Reactions of CIP, ENR, SMX, CDX, and Related Model Compounds with FAC at pH 7–7.2 and 25°C

Compound k app(M -1 s -1 ) Compound k app(M –1 s –1 )

N HN

O F

2.3 × 10 3

ENR

N F O OH O

N N

N OH O

O O

N O

O O N O

O

NH2 1.1 × 10 2

DMI

N O

~ 0

QDX = Quindoxin

QXO = Quinoxaline N-oxide

QNO = Quinoline N-oxide

FLU = Flumequine

MMIB = 4-Methyl-N-(5-methyl-isoxazol-3-yl)-benzenesulfonamide

APMS = 4-aminophenyl methyl sulfone

DMI = 3,5-dimethylisoxazole

As shown inFigure 12.1, each of these antibacterials contains acidic or basicfunctional groups in their structures that undergo proton exchange in aqueous sys-tems CDX and DCDX each contain a hydrazone side-chain, in which an N-H groupcan deprotonate with an estimated pKaof 9.6 for CDX (by Strock et al using Che-maxon).36As illustrated by CIP, fluoroquinolones exhibit pH-dependent speciation

in cationic, neutral, zwitterionic, or anionic forms Because the neutral and ionic microspecies are often difficult to distinguish by simple potentiometric titra-tion techniques, macroscopic constants Ka1and Ka2are often used to describe theequilibrium between the cationic and neutral/zwitterionic forms and the equilibriumbetween the neutral/zwitterionic and anionic forms, respectively Although the mac-roscopic constant is not for a particular functional group, literature has linked Ka1

zwitter-to the carboxylate group and Ka2 to the piperazinyl N4 atom of fluoroquinolonesbecause of similar pKavalues to those of monofunctional analogs.37Although notshown, ENR’s pH speciation pattern is similar to that of CIP with reported pKa1and

pKa2values at 6.1 and 7.7, respectively.38 Sulfonamides contain two moieties nected by way of the characteristic sulfonamide linkage (-NH-S(O2)-); the aniline

con-moiety in para-connection to the sulfonyl S is common among almost all

sulfon-amides, and a variety of different structures may be connected to the sulfonamide

N.5Sulfonamides exhibit two acid dissociation constants: one involving protonation

TABLE 12.2

Structures and Apparent Second-Order Rate

Constants (k app) for Reactions of Trimethoprin

(TMP) and Related Model Compounds with Free

Available Chlorine (FAC) at 25°C

O O

14.2 (pH 4) 48.1 (pH 7) 0.78 (pH 9)

TMT

O O O

59.8 (pH 4) 3.22 (pH 7) 0.24 (pH 9)

TMT = 3,4,5-trimethoxytoluene

DAMP = 2,4-diamino-5-methylpyrimidine

N N

S NH+ 3

H N

N O

O

O

H N

N O

O

O

– N

N O

O

H +

N N1

FIGURE 12.1 Structures and pH speciation of representative antibacterial agents (a) Reference 36; (b) Reference 37; (c) Reference 39; (d) Reference

40; and (e) Reference 41.

© 2008 by Taylor & Francis Group, LLC

of the aniline N and the other corresponding to deprotonation of the sulfonamide

N.39 TMP can undergo protonation at the heterocyclic N1 and N3 nitrogen atomscontained within its 2,4-diamino-5-methylpyrimidinyl moiety, leading to positivelycharged species at circumneutral to lower pHs.40,41As will be discussed in this chap-ter, variations in acid-base speciation of these antibacterial agents under environ-mentally relevant conditions strongly affect their reactivity with aqueous chlorine

12.2.2 CHEMICAL OXIDATION BY AQUEOUS CHLORINE

Aqueous chlorine (HOCl / OCl-) is an important drinking water disinfectant and isused in both drinking water and wastewater treatment to achieve chemical oxida-tion of undesirable taste-, odor-, and color-causing compounds and reduced inor-ganic species.42,43Aqueous chlorine is typically present either as hypochlorous acid(HOCl) or its dissociated form, hypochlorite ion (OCl–), at pH > 5 (Equation 12.1),and may form molecular Cl2(aq)at very low pH or high Cl–concentrations (Equation12.2) The combination of these aqueous chlorine species (generally only HOCl +OCl-under typical water treatment conditions) is referred to as free available chlo-rine (FAC) hereafter in this chapter

HOCl OCl–+ H+ pKa= 7.4 – 7.5 (at 25°C)44,45 (12.1)

Cl2+ H2O HOCl + Cl–+ H+ K = 5.1× 10–4M2(at 25°C)46 (12.2)

In recent decades the electrophilic character of aqueous chlorine has drawn stantial attention due to reactions with natural organic matter (NOM), leading to theformation of harmful chlorinated disinfection byproducts (DBPs) (e.g., trihalometh-anes [THMs] and haloacetic acids [HAAs]).42Substrates such as NOM are readilyoxidized, since they consist of organic molecules with electron-rich sites that aresusceptible toward electrophilic attack The reaction mechanisms of aqueous chlo-rine with NOM are complex and may involve oxidation with oxygen transfer andsubstitutions or additions that lead to chlorinated byproducts.42,47The above reac-tions may then be followed by a number of nonoxidation processes, such as elimina-tion, hydrolysis, and rearrangement reactions,42 further complicating the range ofbyproducts generated

sub-Synthetic organic compounds such as antibacterials can be considered targetsfor transformation by aqueous chlorine Since many antibacterials possess structuralmoieties and functional groups that are electron rich, such as activated aromatic ringsand amines (as seen inFigure 12.1), chemical transformation of these compoundsduring chlorine treatment is likely The rate and extent of such reactions, as well

as the byproducts formed, will be highly dependent on the antibacterials’ chemicalproperties, the applied chlorine dose, and water conditions such as pH, temperature,and concentrations and types of dissolved organic or inorganic species

12.2.3 P RIOR W ORK ON THE R EACTION OF P HARMACEUTICALS WITH C HLORINE

Prior studies indicate that a number of pharmaceuticals are highly susceptible towardchlorine oxidation and are readily transformed under various drinking water andwastewater conditions Thus far, studies have either assessed particular groups of

268 Fate of Pharmaceuticals in the Environment and in Water Treatment Systems

pharmaceuticals that exhibit biological activity (e.g., endocrine disruptors,48,49 blockers,50analgesics,50and antibiotics51,52), or have focused on the detailed reactivity

C-of individual compounds (e.g., 17C-estradiol,53acetaminophen,54 naproxen,55 feine,56and triclosan57) In many of these studies, laboratory-scale experiments wereconducted by assessing the removal of spiked pharmaceuticals by aqueous chlorine

caf-in synthetic water or waters taken from water treatment plants or natural rivers Oneparticular study examined a large number of pharmaceuticals in which degradationvaried greatly (<10 to >90%) after an initial chlorine dose of 2.8 to 6.75 mg/L as Cl2,

a 24-h contact time, and a solution pH of 5.5.58Concentrations of pharmaceuticals infull-scale treatment plants before and after chlorination were also monitored in order

to evaluate the removal of these compounds by chlorine.7In many of these ies, reaction kinetics were examined in synthetic waters to determine whether theselected compounds were likely to be completely depleted during contact times typi-cal of drinking water and wastewater treatment In limited cases, byproduct analyseswere conducted to assist in determining whether reaction products could potentiallyretain the biological activity of the corresponding parent compounds.53

stud-Compared to the broad range of pharmaceuticals detected in surface waters,drinking water supplies, and wastewaters, the number of pharmaceutical compoundsthat have been investigated in depth regarding the mechanisms and products of theirtransformation by chlorine is still quite limited A fundamental understanding of thereactions of pharmaceuticals with chlorine is critical because it enables identification

of reactive functional groups/structural moieties and creates the basis for predictingthe fate of other emerging contaminants on the structural basis For example, many ofthe target pharmaceutical compounds contain aromatic functional groups with elec-tron-donating substituents (e.g., substituted phenols and aromatic ethers)48,50,53,54,57

that are known to react readily with chlorine.59In a study addressing chlorination ofnatural hormones (17C-estradiol, estrone, estriol, and progesterone) and one synthetichormone (17B-ethinylestradiol), all molecules with a phenolic group were rapidlyoxidized (t1/2= 6 to 8 min at pH 7, [chlorine]0= 1 mg/L as Cl2), whereas progesterone,which lacks a phenolic group, remained unchanged over 30 min in the presence ofexcess chlorine.49Another study addressing chlorination of analgesics found that allsuch compounds containing aromatic ether substituents were reactive toward excesschlorine, whereas those lacking such substituents (e.g., ibuprofen and ketoprofen) didnot show any significant losses over 5 days.50Amine-containing compounds such asseveralC-blockers have also been shown to be reactive toward chlorine.51

In this chapter recent contributions by the authors toward elucidating the kineticsand transformation pathways of four structural classes of antibacterials (quinoxaline

N,N’-dioxide, fluoroquinolone, sulfonamide, and pyrimidine) in reactions with free

chlorine are discussed.29–32Significantly, these studies on the reactions of rials with free chlorine are among the first conducted in such detail

antibacte-12.3 MATERIALS AND METHODS

12.3.1 CHEMICAL REAGENTS

The forms and commercial sources of the target antibacterials and structurallyrelated model compounds were described previsouly.29–32DCDX, quindoxin (QDX),

and quinoxaline N-oxide (QXO) were synthesized by methods described

previ-ously.60All commercial chemical standards were of >97% purity and used withoutfurther purification NaOCl was obtained from Fisher Scientific at ~7% by weight.All other reagents used (e.g., buffers, colorimetric agents, reductants, solvents, etc.from Fisher Scientific or Aldrich) were of at least reagent-grade purity Stock solu-tions of antibacterials and model compounds were prepared at 25 to 100 mg/L inreagent water (from a Barnstead or a Millipore water purification system) with 10 to50% (v/v) methanol FAC stocks at 0.1 to 1 g/L as Cl2were prepared by dilution of7% NaOCl solutions and standardized iodometrically61or spectrophotometrically.46

12.3.2 SURFACE WATER AND WASTEWATER SAMPLES

Secondary wastewater effluent samples (collected after activated sludge processesand prior to disinfection) were obtained from pilot-scale or full-scale domestic waste-water treatment plants in Atlanta and Zurich Surface water samples were collectedfrom the Chattahoochee River in Atlanta—near the intake of a regional drinkingwater treatment plant, and from Lake Zurich in Switzerland—at the intake of one

of Zurich’s drinking water treatment plants Samples were filtered through 0.45-µmfilters, stored at 4 to 6°C, and used within 2 days of collection Important charac-teristics of these samples were determined by standard methods or provided by thefacilities where samples were taken, as summarized inTable12.3

12.3.3 REACTION SETUP AND MONITORING

Batch Reactions: For slower reactions, batch kinetic experiments were conducted in

25-mL amber glass vials at pH 3–11 under continuous stirring at 25°C Reactionswere buffered using 10 to 50 mM acetate (pH 4 to 5.5), phosphate (pH 6 to 8), or

aMinimum detectable concentration was ~ 120 µg/L NH3-N (~ 7 × 10-6 mol/L)

n.a = not available

* DOC = dissolved organic carbon

270 Fate of Pharmaceuticals in the Environment and in Water Treatment Systems

tetraborate (pH 9 to 11) The initial concentration of test compounds was typically 1

to 10 µM for kinetic studies Reactions were initiated by adding excess (10×) molaramounts of FAC compared to the target compound For SMX, 4-aminophenyl methylsulfone (APMS), TMP, and 2,4-diamino-5-methylpyrimidine (DAMP), sample ali-quots were periodically taken and quenched by a “soft” quenching technique—using

NH3as a reductant—to minimize potential reversal conversion of reaction diates (e.g., N-chlorinated SMX) back to the parent compounds.31For the other com-pounds, sample aliquots were periodically taken and quenched with excess Na2S2O3

interme-Pseudo-first-order rate constants, k obs , were calculated from linear (0.95 > r2 > 1)plots of ln([antibacterial]) vs time Experiments to monitor the reaction rates of anti-bacterials with chlorine in real water samples were conducted by similar proceduresused to measure rate constants in reagent water matrix In these cases, FAC wasadded at concentrations at least tenfold greater than the corresponding antibacterialconcentrations

Competition Kinetics: The reactions of CDX and DCDX with FAC at pH > 5 and

the reaction of CIP with FAC at pH 4, 5, 6, 10, and 11 were too fast to monitor bybatch techniques Instead, two competition kinetics methods were utilized for thesemeasurements In the method utilized for CDX and DCDX, the antibacterial and a

selected competitor (with known k app) were added at equimolar concentrations tobatch reactors at various pHs Varying substoichiometric amounts of free chlorinewere then added Sample aliquots were taken after each reaction was completed andanalyzed for the concentrations of remaining antibacterial and the competitor Plot-ting the data according to the following linear relationship allows determination ofthe second-order rate constant of the antibacterial:

, ,

competitorcompetitor

T

T t

app k

In the method utilized for CIP, the antibacterial and the competitor roresorcinol were added to vials at varying molar ratios of competitor to antibacte-rial over a range of pH values A fixed substoichiometric dose of free chlorine wasthen added under rapid mixing to each vial After complete consumption of freechlorine, 1-mL sample aliquots were transferred to amber, borosilicate high-per-formance liquid chromatography (HPLC) vials, quenched with Na2S2O3to prevent

4,6-dichlo-reactions of the competitor with a N-chlorinated intermediate formed upon

reac-tion of CIP with free chlorine, and stabilized with ~ 0.1 M H3PO4prior to analysis

by HPLC with fluorescence detection Apparent second-order rate constants for thereaction of CIP with FAC at different pH were determined by monitoring yields

of a product (presumably 2,4,6-trichloresorcinol) formed upon chlorination of the

competitor, 2,4-dichloresorcinol, and plotting measured product yields according toEquation 12.3b:

productproduct

absence presence

app antibacter k

Continuous-Flow Method: FAC decay was measured under pseudo-first-order

conditions in the presence of a large excess of CIP, at pH ranging from 5–11, by

a continuous-flow, quenched-reaction method described previously.30

Pseudo-first-order rate constants, k obs , were calculated from linear (0.97 > r2> 1) plots ofln([FAC]) vs time

Reactant and Reaction Product Analyses: Parent compound loss was monitored

by reverse-phase HPLC with ultraviolet (UV), fluorescence, or mass spectrometrydetection Reaction intermediates and products were analyzed using reverse-phaseHPLC/electrospray MS techniques Details of the HPLC and MS instrumental condi-tions were described previously.29–32Residual oxidant concentrations were measured

at the conclusion of each kinetic experiment by N,N-diethyl-p-phenylenediamine

(DPD) colorimetry or DPD-ferrous ammonium sulfate (FAS) titrimetry.61

12.4.1 REACTION KINETICS AND MODELING

For all of the antibacterials examined, reactions were found to be first order withrespect to the antibacterial and FAC and so can be described by a general second-order rate expression:

where k obs(in s-1) is the observed pseudo-first-order rate constant, T represents the

sum of all acid-base species for a given reactant, and k app(in M-1s-1) is the dent apparent second-order rate constant for the overall reaction, which can be cal-culated from k app k obs

pH-depen-;FAC=T

Kinetic experiments revealed a marked dependence of k appvalues on pH (

importance of specific reactions among the individual acid-base species of FAC andantibacterials The acid-base speciation of FAC (i.e., Equation 12.1) and antibacterials

pH (b)

pH (c)

0.0 0.2 0.4 0.6 0.8 1.0

Continu ou s F low Competition Kinetics Modelkapp

CIP Cation

Neutral/

Zwitterionic CIP

CIP Anion HOCl OCl–

100 101 102 103

ENR Anion HOCl OCl–

pH (d)

pH (e)

0.0 0.2 0.4 0.6 0.8 1.0

0.0 0.2 0.4 0.6 0.8 1.0

0.0 5.0e + 2 1.0e + 3 1.5e + 3 2.0e + 3 2.5e + 3

Meas Rate Constant Calc Rate Constant

SMX Cation

SMX Neutral

SMX Anion HOCl

TMP

TMP++

FIGURE 12.2 Effect of pH on the apparent second-order rate constants for the reactions of (a) CDX, (b) CIP, (c) ENR, (d) SMX, and (e) TMP (Adapted

from Reference 29 through Reference 32.)

© 2008 by Taylor & Francis Group, LLC

(i.e.,Figure 12.1) can be modeled according to mass balance relationships and knownacid-base equilibria:

K K

a,HOCl a,HOCl [H ] for OCl-, B1 [H ][H ] 

[H ] [H ] for diprotic antibacterials CIP, ENR, SMX and TMP.

Incorporating Equation 12.5 and Equation 12.6 into Equation 12.4 can derive tion 12.7,

where k ij represents the specific second-order rate constants for reactions of

each oxidant species i with each antibacterial species j Note that for

fluoroqui-nolones, the zwitterionic and neutral species are combined as a single

“effec-tive” monoprotonated species (according to macroscopic pKa1and pKa2 values)because reliable equilibrium constants are not available for the microspeciation

of CIP and ENR

As shown inFigure 12.2,k appdecreases sharply at pH greater than or equal to

7 for all of the compounds The decrease in k appcan be attributed to deprotonation

of HOCl to yield OCl–, which is generally a much weaker electrophile than HOCl.63

This indicates that reactions among OCl-and various antibacterial species are tively unimportant and can be omitted from Equation 12.7 (as shown by Equation12.8 through Equation 12.13 in Table 12.4) Further simplification of the kineticequation can be conducted for the reactions of CDX and SMX with HOCl, respec-tively For CDX, the reaction with HOCl appears to be dominated by the deproton-ated CDX species (CDX-), and the reaction of neutral CDX with HOCl is negligible

rela-(i.e., Equation 12.8) because: (i) the experimental k app values can only be fitted ifthe neutral CDX species’ specific rate constant (i.e., k11is manually assigned a very

small value or omitted, and (ii) the k appexhibits a well-defined bell-shaped pH file (Figure 12.2a) and reaches a maximum at the pH near the average of the pKas

pro-274 Fate of Pharmaceuticals in the Environment and in Water Treatment Systems

of HOCl and CDX For SMX, the reaction of HOCl with the cationic form of SMX(SMX+) is neglected (i.e., Equation 12.11) on the basis of two assumptions: (i) theabundance of SMX+is rather low within the pH ranges studied (pKafor the aromaticamine is 1.7, which is 2.3 pH units lower than the lowest pH value studied), and (ii)protonation of the aniline’s primary amino group should at the very least retard (ifnot prevent) the reaction between HOCl and SMX by coordinating the lone-pairelectrons associated with this nitrogen

For TMP, reactivity trends above pH 5 were similar to those observed for each ofthe other three antibacterial classes However, in this case,k appincreases substantiallywith increasing acidity at pH below 5 This trend indicates that factors other thanacid-base speciation of TMP and HOCl are playing a role in governing the kinetics

of this reaction, as protonation of TMP should yield less nucleophilic species, whichshould in turn be less reactive toward electrophilic chlorine One possible explanationfor these observations is an acid-catalyzed reaction between HOCl and TMP’s 3,4,5-

to trends previously observed for various phenols and methoxybenzenes.50,57,59,62

Accordingly, an acid catalysis term (kH H

¨ ·) can be added to Equation 12.7 to

yield Equation 12.12 inTable12.4 Alternatively, the increase in reaction rate at pH

TABLE 12.4

Kinetic Models for the Reaction of Antibacterial Agents with FAC

Compound

Kinetic Model for the Apparent

Second-Order Rate Constant k app(M –1 s –1 )

Carbadox (CDX) k app CDX, k12A B1 2 (12.8) Ciprofloxacin (CIP) k app CIP, k11A B1 1 k12A B1 2+k13A B1 3 (12.9) Enrofloxacin (ENR) k app ENR, k11A B1 1 k12A B1 2+k13A B1 3 (12.10) Sulfamethoxazole (SMX) k app SMX, k12A B1 2 k13A B1 3 (12.11) Trimethoprim (TMP) k app,TMPkH ¨H · k k k

k11A B1 1k12A B1 2k13A B1 3 (12.13)

The mathematic expressions of B and C values are discussed in the text.

For TMP: k H+(in M –2 s –1 ) represents the rate constant for the acid-catalyzed reaction between HOCl and

TMP, kCl2(aq)(in M –1 s –1 ) represents the apparent second-order rate constant for the bulk reaction of

Cl2(aq)with all three TMP species, and is the equilibrium constant for the hydrolysis

of Cl 2(aq) (Equation 12.2).