Modelling the formation of brominated trihalomethanes in chlorinated drinking waters

Article abstract of DOI:10.1016/S0043-1354(99)00081-0

The chlorination of water containing bromide and natural organic matter (NOM) leads to the formation of brominated trihalomethanes (THMs). The extent of brominated THM formation depends, inter alia, on the bromide:chlorine concentration ratio ([Br^-]:[chlorine]). A reaction scheme is proposed from which a simple kinetic model is developed that mathematically relates the extent of bromination, and the relative abundances of the four THMs, to the [Br^-]:[chlorine] ratio. In the scheme, the trihalogenated precursors to THMs are formed by three steps each of which substitutes either bromine or chlorine into an activated carbon site in the NOM. This leads to six pairs of competing bromination:chlorination reactions, whose rate constant ratios (k(b):k(c)) have been estimated by using the model to fit THM data obtained from the chlorination of 17 waters from New Zealand. The individual k(b):k(c) ratios range from 4 to 15. From a plot of the ratio of total bromine to total chlorine present in the THMs as a function of the [Br^-]:[chlorine] ratio, an apparent overall k(b):k(c) ratio of 9.1 is obtained. Using USEPA cancer potency factors, the model is used to predict the relative cancer risk associated with THMs as a function of the [Br^-]:[chlorine] ratio. This risk increases steeply to a peak at a [Br^-]:[chlorine] ratio of approximately 0.15, then gradually decreases to the value associated with bromoform alone. The risk associated with THMs may vary considerably through changes in the [Br^-]:[chlorine] ratio as the result of natural variation in the [Br^-], or through poor control of chlorine dosing.

Full text of DOI:10.1016/S0043-1354(99)00081-0

Wat. Res. Vol. 33, No. 17, pp. 3557±3568, 1999
#
1999 Elsevier Science Ltd. All rights reserved
Printed in Great Britain
0043-1354/99/$ - see front matter
PII: S0043-1354(99)00081-0
www.elsevier.com/locate/watres
MODELLING THE FORMATION OF BROMINATED
TRIHALOMETHANES IN CHLORINATED DRINKING
WATERS
C. J. NOKES*, E. FENTON and C. J. RANDALL
Institute of Environmental Science and Research Limited, P.O. Box 29-181, Christchurch, New Zealand
(
First received 1 November 1998; accepted in revised form 1 February 1999)
AbstractÐThe chlorination of water containing bromide and natural organic matter (NOM) leads to
the formation of brominated trihalomethanes (THMs). The extent of brominated THM formation
�
depends, inter alia, on the bromide:chlorine concentration ratio ([Br ]:[chlorine]). A reaction scheme is
proposed from which a simple kinetic model is developed that mathematically relates the extent of
�
bromination, and the relative abundances of the four THMs, to the [Br ]:[chlorine] ratio. In the
scheme, the trihalogenated precursors to THMs are formed by three steps each of which substitutes
either bromine or chlorine into an activated carbon site in the NOM. This leads to six pairs of
b c
competing bromination:chlorination reactions, whose rate constant ratios (k :k ) have been estimated
by using the model to ®t THM data obtained from the chlorination of 17 waters from New Zealand.
The individual k
b
:k
chlorine present in the THMs as a function of the [Br ]:[chlorine] ratio, an apparent overall k
c
ratios range from 4 to 15. From a plot of the ratio of total bromine to total
ratio
�
b
:k
c
of 9.1 is obtained.
Using USEPA cancer potency factors, the model is used to predict the relative cancer risk associated
with THMs as a function of the [Br ]:[chlorine] ratio. This risk increases steeply to a peak at a
�
�
[
bromoform alone. The risk associated with THMs may vary considerably through changes in the
Br ]:[chlorine] ratio of approximately 0.15, then gradually decreases to the value associated with
�
�
[
Br ]:[chlorine] ratio as the result of natural variation in the [Br ], or through poor control of chlorine
dosing. # 1999 Elsevier Science Ltd. All rights reserved
Key wordsÐbromination, disinfection by-products, drinking water, kinetics, trihalomethanes
INTRODUCTION
leads to competition for substitution at suitable car-
bon atoms in the NOM, with the eventual pro-
duction of DBPs containing chlorine, bromine, or a
mixture of both. The relative rates at which chlorine
and bromine react with NOM determine the extent
to which each halogen appears in the DBPs
Reactions between chlorine (hypochlorous acid and
hypochlorite ion) and natural organic matter
(
NOM) in water form a wide range of halogenated
and non-halogenated compounds (de Leer et al.,
985; Stevens et al., 1989). The halogen-containing
1
(
Morris, 1978). The composition of the NOM, par-
compounds appearing in the highest concentrations
are small organic molecules, such as the trihalo-
methanes (THMs) and haloacetic acids (HAA)
ticularly the degree of aromaticity and the nature
and position of the functionalities within the aro-
matic structures, also have an in¯uence on the rela-
tive amounts of chlorine and bromine incorporated
into the organic matter (Rook, 1980; Boyce and
Hornig, 1983; Heller-Grossman et al., 1993).
(
Reckhow et al., 1990). As might be expected, the
prevalent halogen in these compounds is chlorine.
However, where bromide is present in the raw
water, even at trace levels, disinfection by-products
(
DBPs) with varying degrees of bromine substi- Hypobromous acid (HOBr) has been noted to be a
tution are also found (Cooper et al., 1985).
more powerful halogenating agent than hypochlor-
Bromine substitution is a consequence of the ous acid (HOCl) (Morris, 1978), and this results in
rapid oxidation of bromide to bromine (hypobro- reactions incorporating bromine into NOM being
mous acid and hypobromite ion) by chlorine (Wong faster than those incorporating chlorine (Symons et
and Davidson, 1977). Once formed, bromine is al., 1993).
capable of participating in reactions analogous to
This paper examines the incorporation of bro-
those of chlorine. The presence of both halogens mine into THMs from a kinetic standpoint. It does
so using data collected as part of a programme,
funded by the New Zealand Ministry of Health, the
*
Author to whom all correspondence should be addressed.
Tel.: +64-3-3516-019; fax: +64-3-3510-010; e-mail:
chris.nokes@esr.cri.nz
prime purpose of which is to identify chemical con-
taminants of health signi®cance within the country's
3557

3
558
C. J. Nokes et al.
placed in a thermostated bath, and covered to exclude
light. After the required contact time, the chlorine in the
bottle designated for THM formation was quenched with
drinking-water supplies. As part of this programme,
water samples are collected prior to chlorination for
subsequent distribution system simulation exper-
0.1 ml of 100 g/l sodium sulphite, and the pH lowered
iments. Data from these experiments are the basis with 0.1 ml of conc HCl. The samples were stored in the
of the work reported here.
dark at 48C until analysis.
The chlorination conditions in the simulation ex-
periments, though non-uniform, are de®ned, and
the presence of measurable concentrations of bro-
mide in some of the raw waters has provided the
opportunity to address the following questions:
Analytical reagents and methods
All solutions used in this work were prepared with
Millipore MilliQ (18 MO) deionized water. All reagents
were analytical grade with the exception of the commercial
lithium hypochlorite used to prepare the chlorinating sol-
1
. Can the extent of bromine incorporation, and ution, and the DPD indicator (BDH) used in the FAC ti-
trations. Ultra-violet absorbance, bromide and total
organic carbon (TOC) measurements were made on the
samples before chlorination.
the relative abundances of the four THMs, be
mathematically related to the bromide:chlorine
dose ratio?
pH measurement. pH measurements were made using a
2
. Can measures of the relative rates of bromina- radiometer PHM64 research pH meter with a GK 2401C
electrode, calibrated with BDH ``Colourkey'' pH 4 and 7
bu€er solutions.
tion and chlorination be extracted from the data?
The simple kinetic model described here was
Absorbance at 254 nm (A254). A254 measurements were
made in 1 cm path length quartz cells in a Perkin-Elmer
Lambda 3B UV±VIS spectrophotometer. Measurements
were made directly on the sample without pH adjustment.
TOC. A Dohrmann DC-180 TOC analyser with uv/per-
developed to seek answers to these questions. The
paper discusses: how the model allows estimates of
the relative rate constants for bromination and
chlorination to be obtained from the data; the abil- sulphate digestion and using a 1 or 2 ml injection loop
ity of the model to calculate the relative abundances (depending on the organic content of the water) was used
for the TOC measurements.
Bromide. Bromide concentrations were measured using
chlorine ratio; the model's limitations; and the use
of the four THMs as a function of the bromide:-
suppressed ion chromatography (Dionex 4000i) ®tted with
of the model to estimate how the bromide:chlorine
ratio in¯uences the cancer risk associated with Na
0 ml.
FAC. The FAC was determined by the DPD ferrous
an AS-14 column and an eluent composition of 3.5 mM
CO /1.0 mM NaHCO . Injection loop volume was
2
3
3
5
THMs.
sulphate titration method, 4500-Cl F, APHA et al. (1992).
THM. THM measurements were made following
USEPA Method 551. A Hewlett Packard HP5890 gas
chromatograph with HP5970 mass selective detector was
EXPERIMENTAL PROCEDURES AND METHODS
used for analysis. The column used was
a DB5MS
Chlorination procedure
(30 m  0.25 mm id & 1.0 mm). Operating software was
G1701 AAMS Chemstation.
Waters from New Zealand drinking-water supplies were
sampled prior to their chlorination point, and stored in
the dark at 48C before chlorination in the laboratory.
Chlorination conditions [pH, temperature, contact time
and target free available chlorine (FAC) residual] were set
to approximate distribution system conditions. The chlor-
ine dose required to yield the target FAC was determined
for each water by a chlorine demand measurement.
Chlorine demand measurements. Two sub-samples of
each water were chlorinated, one at the chlorine dose esti-
mated by the water supplier, and the second at twice this
level. Sample chlorination was achieved by the addition of
2
an appropriate volume of 10,000 mg/l (as Cl ) chlorine sol-
ution. During the reaction, the samples were kept dark in
a water bath thermostated at 13, 18 or 238C (20.28C),
whichever was above, and closest to, the highest annual
distribution system water temperature. After the required
contact time the FAC residual was measured, and from
the two sub-sample values an estimate was made of the
chlorine dose required to obtain the target residual.
THM formation. The pH of a 500 ml sample of each
water was adjusted using a small volume of concentrated
phosphate bu€er (based on tables contained in CRC,
1
982). The sample was then chlorinated using the dose
estimated from the chlorine demand measurement, the pH
checked, and if necessary, adjusted by the drop-wise ad-
dition of 0.1 M HCl or 0.1 M NaOH. Sub-samples for
THM formation were then rapidly poured into 100-ml
amber glass bottles and the bottles ®rmly sealed with
te¯on-lined lids. In the same way, a 120 ml sub-sample
was prepared to provide a measure of the chlorine residual
when the reaction was quenched. The bottles were then
Fig. 1. Reaction scheme used as the basis for the devel-
opment of the kinetic model.

Modelling brominated trihalomethane formation
3559
Data processing
mediate has been postulated to be part of a triha-
loacetyl group, and base hydrolysis to be the ®nal
step leading to the THM (Peters et al., 1980; Boyce
and Hornig, 1983; Reckhow and Singer, 1985). The
rate of this ®nal step will not in¯uence the ratio of
1
Model calculations were carried out using Microsoft
EXCEL version 7.0. A least-squares method was used to
t the data in Fig. 2, but the optimization of the model's
®
parameters for Fig. 4 was achieved by judgement of the
goodness of ®t by eye; mathematically derived ®ts were
poorer because of the model's limitations under some con- bromine:chlorine incorporation because it is sub-
ditions, as discussed below.
sequent to the halogenation steps. However, unless
THM formation in a water is complete when the
measurements are made, di€erences in the rates of
hydrolysis of the di€erent intermediates (Aizawa et
al., 1989), and hence rates at which di€erent THMs
are formed, may a€ect the apparent bromine:chlor-
ine incorporation ratio.
The concentrations of the four trihalo-intermedi-
ates are determined by the relative rates of chlori-
nation and bromination at each intermediate.
Twelve rate equations arise from the reaction steps
of Fig. 1. It is assumed that the rate of any step, A,
in the reaction scheme is given by:
THE MODEL
Concepts and mathematical development
The fundamental premise of this model is that
the relative concentrations of the four THM species
are kinetically controlled, i.e. that the relative rates
of the competing halogenation reactions control the
distribution of the reaction products. In practice,
this means that sucient bromide and chlorine
must be present in the water to ensure that neither
halogen is totally consumed during the reaction.
For waters chlorinated to provide a free chlorine re-
Rate of A ˆ kA‰HOX Š‰IntermediateŠ,
ꢀ1†
sidual, as is the case here, this requirement is met where [HOX] is the concentration of the reacting
for chlorine at least.
The conceptual basis for the model is shown of the intermediate (or C ) with which it reacts, and
hypohalous acid, [Intermediate] is the concentration
Ã
schematically in Fig. 1. The starting points are acti- kA is the rate constant for that process. All concen-
Ã
vated carbon atoms, C , in the NOM. These are
carbon atoms that undergo halogen substitution to
trations are molar.
To make the derivation of expressions for the
form the carbon of the THM molecules. The model four trihalo-intermediates tractable, two simplifying
makes no assumptions as to how activated carbon
atoms are formed; this is immaterial to its develop-
ment.
approximations are required. First, changes in the
concentrations of bromine and chlorine during the
period in which the trihalo-intermediates are formed
The scheme allows two possible reaction out- are relatively small, hence the rates of reaction of
Ã
comes from C : either chlorination to incorporate
the intermediates are pseudo-®rst order with respect
one chlorine atom, or bromination to incorporate to the intermediates. Second, the rates of change of
one bromine atom. The mono-halogenated reaction the mono- and dihalogenated intermediate concen-
intermediates may then each undergo either chlori-
trations during the reaction are very small. This
nation or bromination, and so this process con- allows the steady-state approximation to be made
tinues until the trihalo-intermediates, from which and simpli®ed expressions for these two types of in-
the THMs are eventually formed, are produced.
The reaction scheme therefore develops with a
termediate to be derived.
On the basis of these approximations the follow-
`
The trihalogenated carbon atom of the ®nal inter-
`tier'' of intermediates after each halogenation step. ing expressions for the concentrations of the tri-
halo-intermediates are obtained:
3
Ã
0
k1k3k7‰HOClŠ ‰C Š
‰
CCl3Š ˆ
ꢀ2†
ꢀ3†
ꢀ
k1‰HOClŠ ‡ k2‰HOBrŠ†ꢀk3‰HOClŠ ‡ k4‰HOBrŠ†ꢀk7‰HOClŠ ‡ k8‰HOBrŠ†
!
2
ꢀ
k1k3k8‰HOClŠ ‰HOBrŠ†=ꢀꢀk3‰HOClŠ ‡ k4‰HOBrŠ†ꢀk7‰HOClŠ ‡ k8‰HOBrŠ†† ‡ k9‰HOClŠ=a†
‰
CCl2BrŠ ˆ
ꢀk1‰HOClŠ ‡ k2‰HOBrŠ†
!
2
ꢀ
k10‰HOBrŠ=a† ‡ ꢀk2k6k11‰HOClŠ‰HOBrŠ †=ꢀꢀk5‰HOClŠ ‡ k6‰HOBrŠ†ꢀk11‰HOClŠ ‡ k12‰HOBrŠ††
‰
CClBr2Š ˆ
ꢀ
k1‰HOClŠ ‡ k2‰HOBrŠ†
 ‰CÊ
0
ꢀ
4†

3
560
C. J. Nokes et al.
3
Ã
0
k2k6k12‰HOBrŠ ‰C Š
‰
CBr3Š ˆ
ꢀ5†
ꢀ
is the initial C concentration and
k1‰HOClŠ ‡ k2‰HOBrŠ†ꢀk5‰HOClŠ ‡ k6‰HOBrŠ†ꢀk11‰HOClŠ ‡ k12‰HOBrŠ†
Ã
Ã
where [C ]
0
ꢀ
ꢁꢀ
ꢁ
k1k4
k2k5
‰HOClŠ‰HOBrŠ
a ˆ
‡
:
ꢀ
k3‰HOClŠ ‡ k4‰HOBrŠ† ꢀk5‰HOClŠ ‡ k6‰HOBrŠ†
ꢀk9‰HOClŠ ‡ k10‰HOBrŠ†
Fig. 2. The dependence of the molar ratio of bromine to chlorine incorporation into trihalomethanes
on the [Br ]:[chlorine] ratio.
�
Fig. 3. The dependence of the molar ratio of bromine to total halogen incorporation into trihalo-
methanes on the [Br ]:[chlorine] ratio.
�

Modelling brominated trihalomethane formation
3561
The relative concentrations of the four THMs are the hypohalous acid and its dissociated form, the
the same as those of the four trihalo-intermediates hypohalite ion. In equations (1±5) the hypohalous
given by equations (2±5), provided: THM for- acids have been denoted as the species reacting with
mation is complete when the measurements are the NOM, because they are considered to be better
made; and the percentage of each trihalo-intermedi- halogenating agents than the corresponding hypo-
ate that is converted to its corresponding THM halite ions (Morris, 1978). However, better matches
(
termediate.
rather than other products) is the same for each in- between calculated and experimental data are
obtained when the total halogen concentrations
hypohalous acid plus hypohalite ion) are substi-
tuted for the hypohalous acid concentrations. For
(
Halogenating agents
Halogens in drinking waters exist in two forms, this reason, when equations (6 and 7) are derived,
�
Fig. 4. The in¯uence of the [Br ]:[chlorine] ratio on the percentage of the total molar concentration of
trihalomethanes constituted by each of the four trihalomethanes. (a) CHCl
CHClBr ; (d) CHBr . Circles are experimental data; lines are calculated.
3 2
; (b) CHCl Br; (c)
2
3

3
562
C. J. Nokes et al.
chlorine and bromine (the total halogen concen- tration is approximated by the ``average chlorine
trations) are identi®ed as the halogenating agents concentration'', de®ned as:
rather than the hypohalous acids alone.
Estimates of the chlorine and bromine concen-
trations are required for the plots in Figs 2±5, but
ꢀInitial chlorine dose ‡ Final FAC residual†=2
neither halogen concentration was monitored This is abbreviated to [chlorine].
during the course of the reactions. Initial and ®nal
The concentration of bromine during the halo-
chlorine concentrations are known, but neither is a genation reaction is assumed to be approximately
suitable approximation for the concentration of equal to the initial bromide concentration (in molar
�
the chlorinating species; some intermediate con- terms), [Br ], because the possible re-oxidation of
zcentration is likely to be a better estimate. bromide to bromine assists in maintaining the bro-
Consequently, for this work, the chlorine concen- mine concentration.
Fig. 4 (continued)

Modelling brominated trihalomethane formation
3563
�
Fig. 5. Dependence of the cancer risk associated with trihalomethanes on the [Br ]:[chlorine] ratio rela-
tive to that due to chloroform alone.
RESULTS AND DISCUSSION
degree of aromaticity of the organic matter, is pro-
vided by the A254:TOC ratio. Reckhow et al.
Raw waters and their characteristics
(
1990), using organic matter from aquatic sources,
The data sets used here were obtained from 17
waters, all of which originated in the North Island
of New Zealand. The data sets were selected
according to two criteria: the minimum bromide
concentration in the water was to be no less than
reported ratios of 0.03±0.04 and 0.05±0.07 (AU l/
mg-C) for fulvic and humic acid fractions respect-
ively. Some of the waters (5, 9, 13, 17) in Table 1
show ratios considerably less than 0.03. These are
all groundwaters, with the exception of water 5,
and all have low, or relatively low, TOC concen-
trations. All other waters possess A254:TOC ratios
that fall within the 0.025±0.044 (AU l/mg-C) range.
Using the A254:TOC ratios reported by Reckhow's
group as a guide, the humic substances in these
0
.05 mg/l (the method detection limit); and the
minimum total THM concentration was required to
�
4
be 1 Â 10 M to ensure an acceptable percentage
uncertainty in the concentrations. Of the 17 waters,
six were groundwaters, 10 were surface waters and
one was a mix of ground and surface waters. Four
characteristics of the 17 waters are given in Table 1. waters appear to be predominantly fulvic, rather
An indication of the humic and fulvic acid con- than humic, acids, and are probably of low to mod-
tent of the NOM in these waters, and hence the erate aromaticity.
Table 1. Characteristics of the waters studied. Abbreviations: TOC-total organic carbon; A254-absorbance at 254 nm (path-length=1 cm);
AU-absorbance units
�
Supply number
Source type
TOC (mg-C/l)
A
254 (AU)
Br (mg/l)
A
254/TOC (AU/mg-C/l)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
surface
surface
surface
surface
surface
surface
surface
surface
3.5
3.4
1.0
1.2
0.6
3.4
4.2
2.5
1.1
0.8
1.6
1.1
0.6
1.6
1.0
2.2
1.3
0.15
0.15
0.03
0.03
0.01
0.12
0.16
0.11
0.02
0.03
0.06
0.04
0.01
0.06
0.03
0.09
0.02
0.08
0.05
0.05
0.05
0.18
0.08
0.09
0.09
0.80
0.09
0.06
0.06
0.70
0.09
0.05
0.11
0.20
0.043
0.044
0.030
0.025
0.017
0.035
0.038
0.044
0.018
0.038
0.038
0.036
0.017
0.038
0.030
0.041
0.015
ground
ground
ground
surface/ground
ground
ground
surface
0
1
2
3
4
5
6
7
surface
ground

3
564
C. J. Nokes et al.
Table 2. Chlorination conditions used for each water. Abbreviation: FAC-free available chlorine
Supply number
1
Temp (8C)
pH
Contact time (hr)
Cl
2
dose (mg/l as Cl
2
)
2
FAC residual (mg/l as Cl )
23
23
23
23
18
13
18
18
13
13
13
18
18
18
23
23
13
7.5
7.2
7.6
7.3
7.1
7.0
7.8
6.9
8.1
7.9
8.4
7.6
7.8
7.4
7.8
7.2
7.7
24
40
48
36
24
48
10
48
24
168
6
5.0
5.0
3.0
3.0
1.0
5.0
4.5
5.0
2.5
1.2
1.0
1.2
5.0
2.8
2.0
3.5
1.0
0.4
0.0
0.3
0.1
0.3
0.3
0.6
1.0
0.8
0.6
0.3
0.3
2.2
0.5
0.1
0.4
0.2
$
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
6
24
24
36
48
36
$The FAC residual for this water dropped to zero before the end of the contact period.
THM formation
di€erent. However, the simplest hypothesis that can
account for the linear relationship of Fig. 2 is that
all bromination rate constants are the same (i.e.
Listed in Table 2 are the ®ve parameters de®ning
the chlorination conditions for each water. The
range of chlorination conditions is wide, and re¯ects
the conditions water suppliers stated would allow
simulation of their distribution system. The concen-
trations of the four THMs produced in each water
are contained in Table 3.
k
chlorination rate constants (i.e. k =k =k . . .=k ).
2
=k
4 6 b
=k . . .=k ), and that the same is true of the
1
3
5
c
Introducing this assumption and the approxi-
mations for the halogenating agents noted earlier,
yields:
�
�
Br±TTHM
Cl±TTHM
kb‰Br Š
The e€ect of the [Br ]:[chlorine] ratio on the incor-
poration of bromine into THMs
ˆ
:
ꢀ6†
kc‰chlorineŠ
The molar ratio of Br-TTHM (total concen-
tration of bromine in THMs) to Cl-TTHM (total the k
Thus the slope of Fig. 2 provides an estimate of
:k ratio. The value of 9.1 for this ratio is
b
c
concentration of chlorine in THMs) is plotted as a consistent with bromination of NOM being a faster
�
function of the [Br ]:[chlorine] molar ratio in Fig. process than chlorination (Symons et al., 1993).
. While the di€ering experimental conditions have Note that the k :k ratio value is dependent on the
b c
2
contributed to scatter in the data, a straight line, assumptions made in estimating the concentrations
having a slope of approximately 9.1, can be ®t well of bromine and chlorine reacting with the NOM.
2
to the plot (r =0.96). Equation (6), which describes
An expression for the relationship between the
this relationship, can be derived from equations (2± Br±TTHM:X±TTHM (the total molar concen-
5). These equations allow for the possibility that all tration of both halogens present in THMs) ratio
�
bromination and chlorination rate constants are and the [Br ]:[chlorine] ratio can also be obtained:
Table 3. Trihalomethane formation following chlorination of each water
Supply number
CHCl
3
(mg/l)
CHCl
2
Br (mg/l)
CHClBr
2
(mg/l)
3
CHBr (mg/l)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
104
156
52
36
0.7
71
72
154
0.8
10
6.3
8.9
1.5
24
17
24
20
4.1
< 0.3
6.3
5.4
10
4.0
5.3
< 0.3
11
15
3.0
4.6
7.8
9.1
< 0.3
< 0.3
< 0.3
< 0.3
9.8
< 0.3
< 0.3
< 0.3
20
3.1
22
26
6
3.1
13
5.8
9
3.3
12
0
1
2
3
4
5
6
7
4.8
< 0.3
0.3
7.6
11
1.1
17
42
1
13
26
4
6.8
10
16
< 0.3
0.4
24

Modelling brominated trihalomethane formation
3565
�
Br±TTHM
X±TTHM
kb‰Br Š=kc‰chlorineŠ
ˆ
:
ꢀ7† 1989). The chloroform concentration (Fig. 4(a))
�
ꢀ1 ‡ kb‰Br Š=kc‰chlorineŠ†
shows a constant, and initially steep, decline with
increasing [Br ]:[chlorine] ratio as bromination
�
Multiplying the Br±TTHM:X±TTHM ratio by a reactions begin to compete e€ectively with the
factor of three yields the bromine incorporation fac- chlorination reactions. The e€ectiveness of this
tor proposed by Gould et al. (1983) and since used competition increases as ®rst bromodichloro-
by other groups (Symons et al., 1993; Sketchell et methane [Fig. 4(b)], then dibromochloromethane
al., 1995). Equation (7) therefore provides a math- [Fig. 4(c)], reaches a maximum concentration before
ematical basis for the form of the relationship gradually decaying as the formation of the next
between the bromine incorporation factor and the THM in the series increases in probability. The bro-
�
[
Br ]:[chlorine] ratio. The Br±TTHM:X±TTHM moform concentration gradually increases [Fig.
�
�
ratio is plotted as a function of the [Br ]:[chlorine] 4(d)], until, at very high [Br ]:[chlorine] ratios, (not
ratio in Fig. 3, in which the line is the calculated present with these data) bromoform is virtually the
Br±TTHM:X±TTHM ratio obtained by substituting only THM in the water.
the k
b
:k
equation (7).
c
ratio from the slope of Fig. 1 into
The agreement of the experimental data with the
model calculations (shown as lines) in Fig. 4 is
The form of the plot in Fig. 3 is as expected from remarkably good considering the wide range of
equation (7). However, where as the calculated line chlorination conditions used in the experiments and
passes through the origin, extrapolation of the ex- the use of many di€erent source waters. This said,
perimental data would result in a positive intercept the deviations of the experimental data from the
with the horizontal axis. A similar e€ect, not calculated values in Figs 2 and 3 are also evident in
�
expected from equation (6), is evident in Fig. 2; the Fig. 4. Below a [Br ]:[chlorine] ratio of 0.1, all
�
collection of points at low [Br ]:[chlorine] ratios in chloroform concentrations lie above the calculated
this ®gure also pulls the regression ®t to a positive values. The concentrations of all the brominated
intercept with the horizontal axis. In both ®gures THMs, on the other hand, tend to fall below the
�
the amount of bromine incorporated into THMs at model line at low [Br ]:[chlorine] ratios. Again, this
�
low [Br ]:[chlorine] ratios is less than that calcu- is consistent with halogen incorporation into THMs
lated by the model. This behaviour probably arises not being kinetically controlled at these
�
from a signi®cant, or total, depletion of the bro- [Br ]:[chlorine] ratios.
mide during THM formation. This will progress-
ively reduce the rate of bromination during the using equations (2±5). The k
reaction, and should bromide be fully consumed, in- the best ®ts are given in Table 4; absolute values
corporation will be limited by its concentration for k and k do not a€ect the ®t. Although the
rather than its ability to compete kinetically with data in Table 4 show that the k :k ratio is not the
chlorine, as assumed in the model.
same for all reaction pairs in Fig. 1, as assumed in
The ®ts to the plots in Fig. 4 were calculated
:k ratios that yielded
c
b
b
c
b
c
This explanation for the discrepancy between ob- the derivation of equations (6 and 7), this combi-
servation and the model calculations is supported nation of rate constant values still results in a linear
�
by the data in Tables 1 and 2. Waters with a low plot of Br±TTHM:Cl±TTHM vs [Br ]:[chlorine]
�
[
[
Br ]:[chlorine] ratio are seen to have a high when the model is used to calculate Br±TTHM:Cl±
�
�
TOC]:[Br ] ratio, and therefore their bromide con- TTHM values from input [Br ]:[chlorine] ratios.
Ã
tent may not exceed their [C ]. If it is assumed that Furthermore, the slope of the plot is 9.5, which is
the total concentration of THMs produced in a in good agreement with the experimental value of
Ã
water is an indication of the minimum [C ] avail- 9.1. The k
b
:k
c
ratio determined from the slope of
able for reaction with bromine, then for the bro- Fig. 2 is therefore best regarded as an ``overall''
mide to be in excess, its concentration must exceed ratio resulting from the combined e€ects of the var-
the THM concentration by at least a factor of ious reaction steps. The rate at which the halogena-
three. Carrying out this calculation shows that the tion of activated carbon sites in NOM occurs will
�
[
[
Br ] meets this criterion in only four waters with a depend upon the structural features of the NOM
Br]:[chlorine] ratio less than 0.1. The criterion is (Boyce and Hornig, 1983; Heller-Grossman et al.,
�
met by all four waters with [Br ]:[chlorine] ratios
greater than 0.1.
Table 4. Values obtained for k
b c
:k ratios from the optimization
�
of the ®ts to the data plotted in Fig. 4
The e€ect of the [Br ]:[chlorine] ratio on the for-
mation of the individual THMs
k
b
:k
c
ratio
Best ®t value
The relative concentrations of the four THMs,
expressed as molar percentages of the total THM
concentration, are plotted as a function of the
k
k
2
/k
/k
1
8
6
9
4
10
15
4
3
k6/k5
�
k
k
k
/k
8 7
[
Br ]:[chlorine] ratio in Fig. 4. The characteristics
10/
k
9
of these plots are similar to those found by others
Minear and Bird, 1980; Oliver, 1980; Aizawa et al.,
12/k11
(

3
566
C. J. Nokes et al.
1
993). The A254:TOC ratios for the waters in this tal conditions, have found the THM distribution to
study show that most NOM is of low to moderate be pH-independent.
aromaticity. NOM in other waters may possess
Evaluation of the risk associated with THMs
quite di€erent characteristics, and consequently
:k
may show di€erent k
mined here.
b
c
ratios from those deter-
The present understanding of the toxicology of
the four THMs indicates that the brominated
While the ratios of Table 2 are not the same for THMs represent more of a risk to public health
all reaction pairs in Fig. 1, there is a pattern to the than chloroform. The USEPA (1994) has calculated
variation in the ratios. For a given ``tier'' of inter- cancer potency factors for chloroform, bromodi-
�
3
mediates (see Fig. 1), the k
the extent of bromine substitution. This cannot be 6.2 Â 10
explained by steric e€ects controlling the rate of ively. It considers evidence of the carcinogenicity of
reaction, because k :k
increases despite the increas- dibromochloromethane to be limited, and conse-
b
:k
c
ratio increases with chloromethane and bromoform of 6.1 Â 10
,
�
2
� 3
� 1
(mg/kg/day) , respect-
and 7.9 Â 10
b
c
ing bulk of the halogens surrounding the central quently has not calculated a potency factor for it.
carbon atom. However, di€erences in the charge For the purposes of calculating cancer risk associ-
induced on the carbon atom as a result of changes ated with THMs, Black et al. (1996) have assumed
in the bromine:chlorine substitution ratio may the potency factor for dibromochloromethane to be
favour further bromine attack over chlorine attack, the same as that for bromodichloromethane.
so that polarization e€ects dictate the kinetics of
halogen incorporation.
Using the USEPA potency factors, and making
the same assumption as Black et al. regarding the
The rate of hydrolysis of the trihalo-intermediates potency factor for dibromochloromethane, the
to form the corresponding THMs is reported to model developed here can be used to determine
increase with increasing bromine content of the in- how the relative cancer risk associated with THMs
�
termediate (Luong et al., 1982; Aizawa et al., 1989). changes with [Br ]:[chlorine] ratio. The theoretical
The model takes no account of the step from the relative risk associated with each individual THM,
trihalo-intermediate to the THM, hence if the reac- and the summed risk, are plotted in Fig. 5. The cal-
Ã
tion is incomplete when it is quenched for measure- culations assume a ®xed [C ]. The form of these
ment of the THMs, di€erences in the rate of the plots is independent of the TOC concentration pro-
�
Ã
hydrolysis step may result in higher values for vided [Br ] and [chlorine] exceed [C ]. This ensures
k
10:k
9
and k12:k11 being required to ®t the data.
Figure 1 shows that there is a hierarchy of reac-
tion steps: the ®rst halogenation step in¯uences the in
that halogen incorporation is kinetically controlled.
Figure 5 shows that the total risk due to THMs
a
water increases very rapidly as the
�
concentrations of all intermediates, while the in¯u- [Br ]:[chlorine] ratio increases to approximately
ence of each of the ®nal halogenation steps is much 0.15, after which the risk slowly decreases to the
more localized. This has consequences for the sensi- value associated with bromoform alone at high
�
tivity of the ®t to the individual k :k values, and [Br ]:[chlorine] ratios. The form of the plot is pri-
b
c
b c
for the ``overall'' k :k
ratio calculated from the in- marily controlled by the rise and fall in the concen-
dividual ratios. The calculated ®ts to the four plots trations of the two mixed-halogen THMs and their
in Fig. 3 are most sensitive to the k :k ratio, and greater cancer potency factors.
2
1
least sensitive to the k
b
:k
c
ratios of the reaction
Chlorinated waters with low, variable concen-
pairs leading to the trihalo-intermediates. The trations of bromide may therefore undergo signi®-
uncertainties in the k value ratios therefore increase cant changes in the risk associated with THMs.
in the order k
2
:k
1
< k
4
:k
3
, k
6
:k
5
< k
8
:k
7
, k10:k
9
,
Such situations may arise in bores near the coast
k :k . In a similar way, the calculated ``overall'' that are subject to varying degrees of sea-water
1
2
11
b c
k :k
ratio is in¯uenced more strongly by the early intrusion depending on the tide. Poor control of the
reaction steps than the ®nal reaction steps. Hence, a chlorination of any water containing bromide may
�
calculated ``overall'' k :k ratio may be close to the also create changes in the [Br ]:[chlorine] ratio
b
c
value of the k
2
:k
1
ratio despite the k10:k
9
and resulting in marked changes in risk associated with
THMs. In both cases, any changes in absolute
k12:k11 values being much greater.
This work has been concerned primarily with THM concentrations will also lead to changes in
establishing an understanding of how the the risk arising from THMs.
�
[
Br ]:[chlorine] ratio in¯uences bromine incorpor-
ation, and the data set is not ideal for establishing
the e€ects of pH. A pH e€ect would be expected if
the hypohalous acids are the only signi®cant halo-
SUMMARY AND CONCLUSIONS
The chlorination of water containing bromide
genating agents, because of the pH-dependent dis- and natural organic matter leads to the formation
sociation of hypohalous acids, and further work of trihalomethanes (THMs) containing bromine.
would be of value in clarifying the in¯uence of pH One of the factors that a€ects the extent of this in-
�
to assist in re®nement of the model. Ichihashi et al. corporation is the [Br ]:[chlorine] ratio. A reaction
(
1999), although working with di€erent experimen- scheme, in which each step incorporating a halogen

Modelling brominated trihalomethane formation
3567
of trihalomethane formation from the halogenation of
dihydroxyaromatic model compounds for humic acid.
Environ. Sci Technol. 17, 202±211.
Cooper W. J., Zika R. G. and Steinhauer M. S. (1985)
Bromide-oxidant interactions and THM formation: a
literature review. J. Am. Water Wks. Ass. 77(4), 116±
into the organic matter is considered separately, has
been used to develop a simple kinetic model to
mathematically describe the extent of bromine in-
corporation into THMs. For each reaction step, the
rate constant ratio for the competing bromination
and chlorination reactions has been estimated by
using the model to ®t THM data obtained from the
chlorination of 17 waters whose organic matter was
predominantly fulvic rather than humic in charac-
ter. While the parameters obtained from these
waters ®t the experimental data well, they may not
be appropriate for waters with organic matter of a
di€erent nature.
121.
CRC (1982) Handbook of Chemistry and Physics, 62nd
edn, ed. R. C. Weast. CRC Press, Boca Raton, FL.
de Leer E. W. B., Sinninghe Damste J. S., Erkelens C.
and de Galan L. (1985) Identi®cation of intermediates
leading to chloroform and c-4 diacids in the chlori-
nation of humic acid. Environ. Sci. Technol. 19, 512±
522.
Gould J. P., Fitchhorn L. E. and Urheim E. (1983)
Formation of brominated trihalomethanes: extent and
kinetics. In ed. R. L. Jolley, Water Chlorination:
Environmental Impact and Health E€ects, 4. Ann Arbor
Science, Ann Arbor, MI, p. 297.
The model:
.
interprets the slope of the linear relationship
between the Br±TTHM:Cl±TTHM ratio and the Heller-Grossman L., Manka J., Limoni-Relis B. and
�
Rebhun M. (1993) Formation and distribution of haloa-
cetic acids THM and TOX in chlorination of bromide-
rich lake water. Water Res. 27, 1323±1331.
Ichihashi K., Ternahishi K. and Ichimura A. (1999)
Brominated trihalomethane formation in halogenation
of humic acid in the coexistance of hypochlorite and
hypobromite ions. Water Res. 33, 477±483.
Luong T. V., Peters C. J. and Perry R. (1982) In¯uence of
bromide and ammonia upon the formation of trihalo-
methanes under water-treatment conditions. Environ.
Sci. Technol. 16, 473±479.
[
Br ]:[chlorine] ratio in terms of an ``overall''
bromination:chlorination rate constant ratio,
which, on the basis of the data used in this study,
has an approximate value of 9;
.
provides
form of the Br±TTHM:X±TTHM ratio vs
a mathematical expression for the
�
[
Br ]:[chlorine] ratio plot;
.
.
provides expressions for the relative concen-
trations of the four THMs;
calculates relative THM concentrations produced Minear R. A. and Bird J. C. (1980) Trihalomethanes:
�
impact of bromide ion concentration on yield, species
distribution, rate of formation and in¯uence of other
variables. In ed. R. L. Jolley, Water Chlorination:
Environmental Impact and Health E€ects, 3. Ann Arbor
Science, Ann Arbor, MI, p. 151.
Morris J. C. (1978) The chemistry of aqueous chlorine in
relation to water chlorination. In ed. R. L. Jolley,
Water Chlorination: Environmental Impact and Health
E€ects, Vol. 1. Ann Arbor Science, Ann Arbor, MI, p.
21.
in the waters as a function of the [Br ]:[chlorine]
ratio which match the observed data well, despite
the range of chlorination conditions used;
calculates too little bromine incorporation into
THMs where the bromide present is not in excess
of reactive sites in the natural organic matter;
allows the relative cancer risk, with respect to
.
.
chloroform, associated with THMs to be calcu-
�
lated as a function of the [Br ]:[chlorine] ratio, Oliver B. G. (1980) E€ect of temperature, pH and bro-
mide concentration on the trihalomethane reaction of
and shows that situations in which this ratio is
chlorine with aquatic humic material. In ed. R. L.
variable (e.g. seawater-intrusion, poor chlori-
nation control) may lead to marked changes in
Jolley, Water Chlorination: Environmental Impact and
Health E€ects, 3. Ann Arbor Science, Ann Arbor, MI,
the risk associated with THMs.
p. 141.
Peters C. J., Young R. J. and Perry R. (1980) Factors
in¯uencing the formation of haloforms in the chlori-
nation of humic material. Environ. Sci. Technol. 14,
1
391±1395.
AcknowledgementsÐThe authors gratefully acknowledge
permission from the New Zealand Ministry of Health to
use data collected as part of their water quality pro-
gramme. Review of the manuscript by Dr C. Hinton and
Dr J. Gregor, was very much appreciated. Preparation of
the paper was supported by an ESR senior fellowship to
C. J. N.
Reckhow D. A. and Singer P. C. (1985) Mechanisms of
organic halide formation during fulvic acid chlorination
and implications with respect to preozonation. In ed. R.
L. Jolley, Water Chlorination: Chemistry, Environmental
Impact and Health E€ects, 5. Lewis Publishers, Chelsea,
MI, p. 1229.
Reckhow D. A., Singer P. C. and Malcolm R. L. (1990)
Chlorination of humic materials: by-product formation
and chemical interpretations. Environ. Sci. Technol. 24,
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