Brominated trihalomethane formation in halogenation of humic acid in the coexistence of hypochlorite and hypobromite ions

Article abstract of DOI:10.1016/S0043-1354(98)00227-9

Brominated trihalomethanes (Br-THMs) such as CHCl₂Br, CHClBr₂, and CHBr₃ are produced by the reaction of hypobromite with humic acid in the presence of hypochlorite. In the presence of excess NaOCl, addition of NaOBr enhanced the formation of Br-THMs but reduced the formation of CHCl₃. The product distribution of THMs was affected by the ratio of [NaOBr]/[NaOCl] and was independent of pH and reaction time. In the presence of excess NaOBr, the yield of CHBr₃ only increased linearly with the NaOCl concentration added. However, the other three THMs were hardly produced even though NaOCl concentration was increased up to 0.5 of the [NaOCl]/[NaOBr] molar ratio. Our results suggest that in the process of THM formation, hypochlorite ion reacts effectively with humic acid in the oxidation reaction and hypobromous acid plays a predominant role in the electrophilic substitution when both of hypohalites are present. Brominated trihalomethanes (Br-THMs) such as CHCl₂Br, CHClBr₂, and CHBr₃ are produced by the reaction of hypobromite with humic acid in the presence of hypochlorite. In the presence of excess NaOCl, addition of NaOBr enhanced the formation of Br-THMs but reduced the formation of CHCl₃. The product distribution of THMs was affected by the ratio of [NaOBr]/[NaOCl] and was independent of pH and reaction time. In the presence of excess NaOBr, the yield of CHBr₃ only increased linearly with the NaOCl concentration added. However, the other three THMs were hardly-produced even though NaOCl concentration was increased up to 0.5 of the [NaOCl]/[NaOBr] molar ratio. Our results suggest that in the process of THM formation, hypochlorite ion reacts effectively with humic acid in the oxidation reaction and hypobromous acid plays a predominant role in the electrophilic substitution when both of hypohalites are present.

Full text of DOI:10.1016/S0043-1354(98)00227-9

Wat. Res. Vol. 33, No. 2, pp. 477±483, 1999
# 1998 Elsevier Science Ltd. All rights reserved
Printed in Great Britain
PII: S0043-1354(98)00227-9
0043-1354/98 $19.00 + 0.00
BROMINATED TRIHALOMETHANE FORMATION IN
HALOGENATION OF HUMIC ACID IN THE
COEXISTENCE OF HYPOCHLORITE AND HYPOBROMITE
IONS
KEIKO ICHIHASHI1*, KIYOSHI TERANISHI1 and AKIO ICHIMURA2
1Public Health Institute of Hyogo Prefecture, 2-1-29, Arata-cho, Hyogo-ku, Kobe, 652-0032, Japan and
2Department of Chemistry, Osaka City University, 3-3-138, Sugimoto, Sumiyoshi-ku, Osaka, 558-8585,
Japan
(First received October 1997; accepted in revised form May 1998)
AbstractÐBrominated trihalomethanes (Br-THMs) such as CHCl2Br, CHClBr2, and CHBr3 are pro-
duced by the reaction of hypobromite with humic acid in the presence of hypochlorite. In the presence
of excess NaOCl, addition of NaOBr enhanced the formation of Br-THMs but reduced the formation
of CHCl3. The product distribution of THMs was a€ected by the ratio of [NaOBr]/[NaOCl] and was
independent of pH and reaction time. In the presence of excess NaOBr, the yield of CHBr3 only
increased linearly with the NaOCl concentration added. However, the other three THMs were hardly -
produced even though NaOCl concentration was increased up to 0.5 of the [NaOCl]/[NaOBr] molar
ratio. Our results suggest that in the process of THM formation, hypochlorite ion reacts e€ectively with
humic acid in the oxidation reaction and hypobromous acid plays a predominant role in the electrophi-
lic substitution when both of hypohalites are present. # 1998 Elsevier Science Ltd. All rights reserved
Key words: brominated trihalomethane, redox reaction, hypochlorite, hypobromite, humic acid.
INTRODUCTION
1994). Second, the amounts of Br-THMs formed
were pH dependent when KBr was used as a source
In 1974, brominated trihalomethanes (Br-THMs)
material of bromide (Aizawa et al., 1989). This in-
formation seemed to match with a concept of hypo-
bromite ion (OBr ) and hypobromous acid (HOBr)
acting as reactive entity (Farkas et al., 1949; Rook,
formation, i.e., CHCl2Br, CHClBr2, and CHBr3 in
addition to CHCl3 as main product, was ®rst
reported by Rook (1974) and Bellar et al. (1974) in
the practical water disinfection process using chlor-
1977). The reasons are that bromide can be oxi-
ine. Since then researchers have made intensive
dized readily to highly reactive OBr or HOBr
e€orts to clarify source of bromide of Br-THMs
species, and the composition of these two species is
and also to elucidate the reaction mechanisms.
pH-dependent, and in addition, OBr or HOBr
In contrast, a variety of the candidate for the
species may be formed also from KBr and NaOCl.
source material of bromide can be considered.
Hence, we tested chlorination of the system of
Hence, although much important knowledge has
humic acid and NaOBr (in the presence or absence
already been accumulated concerning reaction
of NaOCl) at ®rst time to obtain new elementary
mechanisms of Br-THM formation. (Cooper et al.,
information about (1) product distribution of
1985; Krasner et al., 1994; Symons et al., 1994),
THMs formed, (2) e€ect of concentration ratio of
there are still uncertainties for the most probable
NaOBr to NaOCl, (3) e€ect of pH, (4) outline of
mechanism.
reaction time-pro®les, all of which will be helpful
Aiming at contribution to the mechanistic stu-
for more detailed mechanistic studies in the future.
dies, we chose a model reaction system with combi-
On the basis of our information, the di€erence in
nation of humic acid and sodium hypobromite
the role of NaOBr from NaOCl is brie¯y discussed.
(NaOBr). The idea of using NaOBr is based on
mainly two types of important observation. First,
Br-THMs were produced even at a considerably
EXPERIMENTAL SECTION
lower concentration of bromide relative to chloride
concentration (Cooper et al., 1985; Krasner et al.,
Reagents and materials
Sodium salt of humic acid was obtained from Aldrich.
This humic acid was composed of 51.85% C, 5.08% H
*The author to whom all correspondence should be
addressed. [Tel.: +81-78-511-6581; Fax: +81-78-531-
7080.
and 0.67% N by elemental analysis. It was observed by
potentiometric titration of humic acid with NaOCH3 in
methanol/benzene in dimethyl sulfoxide (Yonebayashi and

478
Keiko Ichihashi et al.
Hattori, 1985) that the humic acid had the carboxyl group
(4.01 milli equivalent mol/g humic acid) and phenolic hy-
droxyl group (1.4 milli equivalent mol/g humic acid). The
10 mg/L humic acid solution was used as a precursor of
THMs. This concentration of the humic acid corresponded
to 5.2 mg/L of total organic carbon (TOC) by TOC
measurement (TOC-5000A Shimazu). The concentration
of TOC was within the range of 0.1 to 10 mg/L dissolved
organic carbon contained in groundwater (Ga€rey et al.,
1996). Concentrations of NaOCl (5% solution, Kanto)
and NaOBr (5% solution, Kanto) solutions were deter-
mined by titration with 0.01 M sodium thiosulfate solution
after addition of potassium iodide and sulfuric acid. These
solutions were diluted to given concentrations just before
use. Potassium bromide was used as the bromide ion
source. The 0.258 mmol/L NaOCl corresponded to
18.29 mg/L chlorine (Cl2). This concentration of hypo-
chlorite was two times that of humic acid solution.
Solution of hydrochloric acid (0.02 M) and sodium hy-
droxide (0.02 M) were used to adjust the pH of the react-
ing solution. A 50 g/L solution of sodium L-ascorbate and
2 M HCl were used to quench the reaction. All of these
chemicals i.e., sodium thiosulfate, potassium iodide, sulfu-
ric acid, potassium bromide, hydrochloric acid, sodium hy-
droxide, and sodium L-ascorbate, were reagent grade and
obtained from Wako. Glass-distilled and deionized water
was employed to dilute the solutions of humic acid, re-
agents and reaction mixtures.
Fig. 1. Changes in concentration of CHCl3 and CHBr3
formed in the reaction of humic acid with NaOCl or
NaOBr alone, respectively. ± W ± CHCl3 (NaOCl); ± w ±
CHBr3 (NaOBr). Humic acid: 10 mg/L, pH: 8.0, Reaction
time: 60 min.
monitoring (SIM), and the ionization mode was electron
impact (70 eV).
RESULTS
Halogenation procedure
THMs formed by the reaction of NaOCl or NaOBr
with humic acid
The solution of known concentration of NaOCl and
NaOBr or KBr was added to the humic acid solution and
the pH values of the mixed solutions were adjusted under
stirring by using 0.02 M HCl or 0.02 M NaOH solution as
quick as possible. The pH values were measured by an
Figure 1 shows the relationship between the
amount of THM formed and the concentration of
HM-40V pH meter (Toa Electrics). The initial pH value NaOCl or NaOBr added to the humic acid solution.
was adjusted to be 8, at which the surface water was disin-
fected by chlorination for water supply (Krasner et al.,
tion of humic acid with NaOCl and CHBr3 with
1994). The pH 8 decreased little in 60 min to 7.62±7.75 for
It is well known that CHCl3 is formed in the reac-
NaOBr, and the amounts of CHCl3 and CHBr3
using NaOCl and to 7.96±7.98 for using NaOBr. So, the
depend on the concentration of NaOCl and
NaOBr, respectively. The concentration of CHCl3
formed by using a 0.258 mM NaOCl solution was
0.456 mM after reacted for 60 min at pH 8.0. In
contrast, the concentration of CHBr3 was only
adjusting the initial pH was used for the following result
and discussion in this paper. Rook (1977) and Norwood
et al. (1980) already reported that the two kinds of slope
of the curve for chloroform formation rate were distin-
guished. During the ®rst 15 min, a rapid start was
observed followed by a much slower secondary phase
which continued for 24 h. The linear relationship of 0.218 mM by using a 0.258 mM NaOBr solution
CHBrCl2 produced to reaction time at pH 8 was observed
under the same reaction conditions. Thus, it was
until 60 min (Fig. 7). So, 60 min was adopted for the reac-
de®ned that twice the amount of CHCl3 was yielded
tion time to investigate the initial reaction mechanism of
compared with CHBr3 for the reaction employing
THMs formation. Aliquots of the solutions were poured
the same amounts of the two hypohalites for humic
acid.
into 43 mL vials, which were closed immediately by screw
stoppers ®tted with Te¯on septa. After allowing for an
appropriate reaction time, ascorbic acid solution (0.5 mL)
and 2 M HCl (0.5 mL) were added to the vials to quench
the halogenation reaction. The series of experiments was
carried out at room temperature (22±248C).
E€ect of NaOBr concentration on the product distri-
bution of THMs in the presence of excess NaOCl
E€ects of varying the concentration of NaOBr
(from 0 to 0.03 mM) on the distribution of THMs
formed in the reaction with 0.258 mM of NaOCl
Analysis of THM
THMs formed were analyzed by the fully automated
analytical method of purge and cold trapping/gas chroma- and 10 mg/L humic acid at pH 8 were examined
tography (GC)/mass spectrometry (MS). This method has
been described in detail previously (Ichihashi et al., 1995).
than 0.005 mM, i.e., the ratio of [NaOBr]/[NaOCl]
Then the vials were placed in an AquaTek-50 automatic
(Fig. 2). When the concentration of NaOBr was less
being less than 0.04, CHCl3 was formed predomi-
vial sampler (Tekmer) connected to an automatic concen-
nantly. Lower concentrations of other brominated
THMs (Br-THMs, CHCl2Br>CHClBr2>CHBr3)
were detected. This order is the same as the number
of Br atoms in the Br-THMs by which Cl atoms in
CHCl3 were replaced. However, the pro®le of pro-
trator (LSC 2000, Tekmer) by purging and trapping to a
precolumn. GC/MS measurements of the THMs were car-
ried out with a Hewlett Packard 5890 series II gas chro-
matograph interfaced to a Hewlett Packard 5972A mass
spectrometer. The column used was a fused silica capillary
(60 m Â0.25 mm i.d. Â 1 mm) supplied by
J and W
Scienti®c. The mass acquisition mode was selected ion duct distribution of THMs changes as the [NaOBr]/

Brominated trihalomethane formation
479
Fig. 3. Distribution of THM species as function of con-
centration of KBr added to a reaction mixture of humic
Fig. 2. Distribution of THM species as function of con-
centration of NaOBr added to a reaction mixture of
humic acid and NaOCl. ± W ± CHCl3; ± q ± CHCl2Br; ±
r ± CHClBr2; ± w ± CHBr3; ± Q ± Br-THMs. Humic
acid: 10 mg/L, NaOCl: 0.258 mM, pH: 8.0, Reaction time:
60 min.
acid
CHCl2Br;
and
NaOCl.
± W ±
CHCl3;
± q ±
r
CHClBr2; ± w ± CHBr3. Humic acid:
10 mg/L, NaOCl: 0.258 mM, pH: 8.0, Reaction time:
60 min.
concentration of CHBr3 is seven times higher than
that observed in the reaction with the same concen-
tration of NaOBr in the absence of NaOCl. Such a
highly attractive result was ®rst observed here, and
we suggest that NaOCl and NaOBr di€er in oxi-
dation ability and in rate of substitution in the
course of THM formation from humic acid. If the
oxidation potential and rate of halogenation for
OCl are equal to those of OBr , increase of CHCl3
should be observed with increasing NaOCl concen-
tration under our conditions. Our results clearly
shows that NaOCl promotes the formation of
CHBr3. Namely, NaOCl in the reaction system pro-
motes mainly the oxidation of humic acid to pre-
pare a precursors for THM formation.
[NaOCl] ratio increases. Over the molar ratio
[NaOBr]/[NaOCl] = 0.1, the most predominant
species among the four THMs formed was CHBr3,
followed by CHClBr2, CHCl2Br, and CHCl3 in the
decreasing order. This is opposite to the order
observed at the ratio of [NaOBr]/[NaOCl] < 0.04.
These results suggest that the ratio of [NaOBr]/
[NaOCl] regulates the product distribution of
THMs intensely. As the concentration of NaOBr
increases, the amount of CHCl3 decreases continu-
ously, and those of CHCl2Br and CHClBr2 have
maxima. On the other hand, CHBr3 begins to
increase almost linearly with the concentration of
NaOBr being higher than 0.003 mM. The ®nding
that in spite of the presence of a high concentration
of NaOCl larger amounts of CHBr3 and CHClBr2
are formed compared with those of CHCl3 and
CHCl2Br suggests that the rate of substitution of
bromide by NaOBr is greater than that of chloride
by NaOCl. The addition of KBr in the reaction of
NaOCl and humic acid gave pro®les of the product
distribution of THMs similar to the case of addition
of NaOBr as shown in Fig. 3. These ®ndings
strongly suggest that Br is ®rst oxidized to OBr
by OCl (Farkas et al., 1949) and the resultant
OBr reacts with humic acid.
E€ect of pH on the amount and composition of
THMs
The amount of THM formed in disinfection of
natural water depends on the dose of the disinfec-
tant Cl2, concentration of organic substrates, pH,
and the water temperature (Urano et al., 1983;
Amy et al., 1987; Adin et al., 1991). We investigated
the in¯uence of pH on the formation of Br-THMs
by using the reaction mixture of 0.0258 mM NaOBr
E€ect of NaOCl concentration on THM formation in
the presence of excess NaOBr
E€ects of the varying concentration of NaOCl
(from 0 to 0.129 mM) added to a reaction mixture
of 0.258 mM NaOBr and 10 mg/L humic acid sol-
ution at pH 8.0 was also examined (Fig. 4). The
yield of CHBr3 as solely major product, increases
almost linearly with the concentration of NaOCl.
Small amount of CHClBr2 is formed, but no signi®-
cant formation of CHCl3 and CHCl2Br is observed
under the conditions where both of OCl and OBr
ion can react with humic acid. The concentrations
of CHBr3, CHClBr2, CHCl2Br, and CHCl3
observed at 0.129 mM NaOCl were 1.468, 0.063,
0.007, and 0.006 mM, respectively. Surprisingly, the
Fig. 4. Distribution of THM species as function of con-
centration of NaOCl added to a reaction mixture of humic
acid
CHCl2Br;
and
NaOBr.
± W ±
CHCl3;
± q ±
r
CHClBr2; ± w ± CHBr3. Humic acid:
10 mg/L, NaOBr: 0.258 mM, pH: 8.0, Reaction time:
60 min.

480
Keiko Ichihashi et al.
Fig. 5. Dependence of pH on the total brominated THM
concentration formed by the addition of NaOBr or KBr.
± W ± NaOCl 0.258 mM + NaOBr 0.026 mM; ± w ±
NaOCl 0.258 mM + KBr 0.026 mM. Humic acid: 10 mg/
L, Reaction time: 60 min.
or 0.0258 mM KBr, 0.258 mM NaOCl and humic
acid (Fig. 5). The total concentration of Br-THMs
is found to be the same as those of using either
NaOBr or KBr at up to pH 9.0. This means that
the amount of Br-THMs formed is almost constant
independent of pH in a range from 4 to 6, and
increases with pH in a range from 6 to 9.0, and
then decreases at pH above 9.5 in both cases. These
behaviors correspond to those shown by Boyce and
Hornig (1983). The increases of Br-THMs with the
pH in the range of 6.0 to 9.0 is thought to be attrib-
uted to the increment of alkaline hydrolysis of the
Br-THMs intermediates by OH (Peters et al.,
1980). And also, these decreases at pH above 9.0
may be due to the following disproportionation of
hypobromite ion (Bard et al., 1985).
Fig. 6. Time dependencies of THM concentrations formed
by the addition of OBr or Br at pH 5. (A) CHCl3, (B)
CHCl2Br, (C) CHClBr2, (D) CHBr3, ± W ± NaOCl
0.258 mM + NaOBr 0.01 mM, pH 4.9; ± w ± NaOCl
0.258 mM + KBr 0.01 mM, pH 4.8. Humic acid: 10 mg/L.
remained relatively constant. These results suggest
that the amount of THMs produced depends on the
pH values, while the THM composition, including
CHCl3, is almost independent of pH.
‡
OBr ‡ 2HOBr 4BrO3 ‡ 2Br ‡ 2H
E€ect of reaction time on the composition of THM
products
From this equation, the OBr is disproportio-
nated to bromate and bromide ions, neither of
Figure 6 shows the relationships between the
which reacts with humic acid. The disproportiona- yield of THMs and the reaction time when
tion in the case of KBr proceeds slower than in the 0.01 mM NaOBr or 0.01 mM KBr coexisted with
case of NaOBr. When NaOBr was used, the magni- 0.258 mM NaOCl and humic acid at pH 5. The
tude of the decrease was larger than that when KBr rate of production of each THM species by using
was used. This large decrease may be due to the NaOBr is similar to that using KBr. Under the low
magnitude of the proceeding of above disproportio- pH range, the amount of CHCl2Br produced was
nation reaction. Since this disproportionation may linearly increased with reaction time until 180 min.
proceed as fast as the decrease of the concentration The product distribution of four THMs is fairly
of Br in the reaction systems of our experiment. unchanged at the reaction time ranging from 5 to
Therefore, the degree of decrease in the amount of 180 min. The order of magnitude of the rate of in-
THMs with KBr relative to that with NaOBr is less itial reaction for the formation of THMs
than that of the case where NaOBr is used.
(CHClBr2>CHCl2Br>CHBr3) is the same as the
Yields of each THMs formed at various pH are order of the amounts of individual Br-THM formed
listed in Table 1. The proportions of each THM when [NaOBr]/[NaOCl] or [KBr]/[NaOCl] was 0.04
Table 1. PH dependency of the distribution of THM species formed by addition of NaOBr
CHCl3
(mM%)
CHClBr
(mM%)
CHClBr2
(mM%)
CHBr3
(mM%)
pH
Total THM
NaOBr
0.0258 mM
4.9
6.8
8.0
9.0
9.9
10.7
3.1
3.4
2.7
2.3
4.2
5.8
10.5
11.7
11.7
9.7
13.3
10.7
42.0
40.4
41.0
34.3
33.3
42.7
44.4
44.5
44.6
53.7
49.1
40.8
100
100
100
100
100
100
Humic acid: 10 mg/L, NaOCl: 0.258 mM, Reaction time: 60 min.

Brominated trihalomethane formation
481
ing species in the resulting solutions is largely pH
dependent.
The two redox reactions consisting of Cl(+I)/
Cl(0)/Cl( I) redox couples are expressed by
equations
1 and 2. The redox potential for
equation 2, E(2)8, is referred to ``Standard Potentials
in Aqueous solution'' (Bard et al., 1985). The redox
potential for equation 1, E(1)8, and the acid dis-
sociation constant for HOCl, Ka, can be calculated
on the basis of the data in the same book.
‡
2HOCl‡2H ‡ 2e ˆ Cl2ꢀaq† ‡ H2O
E
8 ˆ 1:592 V
ꢀ1†
ꢀ2†
ꢀ3†
ꢀ1†
Cl2ꢀaq† ‡ 2e ˆ 2Cl
E
ꢀ2†
8 ˆ 1:396 V
‡
HOCl ˆ H ‡ OCl
pKa ˆ 7:543
These available values readily give the potential-
pH diagram for the HOCl±OCl /Cl2/Cl system,
which is represented in Fig. 8(a).
Analogously, Fig. 8(b) shows the potential-pH
diagram for the HOBr-OBr /Br2/Br system, which
is obtained from the following three equations.
‡
2HOBr‡2H ‡ 2e ˆ Br2ꢀaq† ‡ H2O
E
8 ˆ1:582V
ꢀ4†
ꢀ5†
ꢀ6†
ꢀ3†
Fig. 7. Time dependencies of THM concentrations formed
by the addition of NaOBr or KBr at pH 8. (A) CHCl3,
(B) CHCl2Br, (C) CHClBr2, (D) CHBr3, (E) Br-THMs.
± W ± NaOCl 0.258 mM + NaOBr 0.026 mM, pH 8.4;
± w ± NaOCl 0.258 mM + KBr 0.026 mM, pH 8.1.
Humic acid: 10 mg/L.
Br2ꢀaq† ‡ 2e ˆ 2Br
E
ꢀ4†
8 ˆ 1:087 V
‡
HOBr ˆ H ‡ OBr
pKa ˆ 8:558
The redox potential of the Cl2/Cl couple is more
positive than that of the Br2/Br couple in the full
pH range, which means that the humic acid is oxi-
dized more readily by NaOCl than by NaOBr.
At pH 8, the species of OCl and HOBr in the
X(+I) state predominate thermodynamically. The
OCl species is 73.3 mol% of NaOCl and HOBr is
80.6 mol% of NaOBr: the ratio of the molar con-
centrations of OCl to HOBr is 0.909 under an
equimolar amount of NaOCl and NaOBr. The con-
centration of OCl is about nine times higher than
that of HOBr at pH 8 when the amount of NaOCl
is ten molar times higher than that of NaOBr.
at pH 8 for 60 min reaction (Figs 2 and 3). As
shown in Fig. 7, on the other hand, at [NaOBr]/
[NaOCl] of 0.1 and pH 8, the initial rate of reaction
for CHBr3 formation is the highest and that for
CHCl3 is the lowest among the four THMs. The in-
itial rates of the formation of four THMs increase
with the number of Br atoms incorporated in the
THM. These results suggests that the product distri-
bution of the THM is controlled almost solely by
the [NaOBr]/[NaOCl] molar ratio, but not by the
reaction time.
In the presence of excess NaOCl, 0.60 mM CHCl3
was formed by 0.191 mM OCl ion in the reaction
with humic acid in the absence of NaOBr (Fig. 2).
The mixture of 0.191 mM OCl and 0.0242 mM
DISCUSSION
There have been many investigations to elucidate
the mechanism of THM formation in the disinfec-
tion of natural water by using dihydroxybenzenes
as model compounds of humic acid and Cl2 or Br2
as halogenation reagent. This work has aimed to
reveal the mechanism of brominated THM for-
mation particularly in the coexistence of chlorine
and bromine. Both of halogens (X2) undergo rapid
hydrolysis and disproportionation reaction to yield
the corresponding halide ion (X ) and hypohalous
acid (HOX) which can dissociate into hypohalite
anion (OX ). The composition of halogen-contain-
Fig. 8. Redox potential (E8)±pH diagrams. (A) chlorine
system, (B) bromine system.

482
Keiko Ichihashi et al.
HOBr, however, yielded 1.61 mM of total THM
products including 0.134 mM of CHCl3.
In the presence of excess of NaOBr, 0.149 mM of
CHBr3 was formed by the reaction of 0.210 mM
HOBr and humic acid in the absence of NaOCl.
The amount of CHBr3 formed was 0.369 mM by the
addition of 0.0191 mM of OCl to the 0.210 mM
HOBr solution, and increased in parallel with the
amount of OCl added. As the molar ratio of
[HOBr]/[HOCl] corresponded to be 32 in this case
of excess NaOBr, it is reasonable that CHBr3 was a
predominant product relative to CHCl3. However,
only by addition of a little amount of OCl , the
amount of CHBr3 formed under the condition was
larger than 2 times of CHBr3 formed by 0.210 mM
HOBr alone.
It has been reported (Tee et al., 1989) that HOX
is more e€ective than OX ion for electrophilic sub-
stitution of humic acid. Hence, HOCl and HOBr
might control the substitution reaction similarly. In
our study, however, the amount of CHBr3 formed
was found to be 6 times larger than that of CHCl3
in spite of the ratio of [HOCl]/[HOBr] of 2.87.
These observations suggest that HOBr becomes an
electrophile stronger than HOCl when the two
species compete in the substitution reaction. In
other words, the oxidation ability of NaOCl is
twice larger than that of NaOBr, but the rate of
substitution by HOBr is 17 times higher than by
HOCl. Accordingly, it can be assumed that humic
acid is ®rst oxidized by OCl to give monohaloge-
nated carbanion(I), and the resultant carbanion is
substituted by HOBr faster than by HOCl to diha-
logenated carbanion (II) as follows:
These evidences suggest that the OCl promoted
the formation of the monohalogenated carbanions
which have been postulated (Rebenne et al., 1996)
as initial intermediates of THM formation from
humic acid, and that faster electrophilic substitution
of the intermediates of THM is caused by HOBr
subsequently.
Humic acid Oxidatio4n
ꢀNaOX†
The results of our experiments support the im-
portance of the existence of mono halogenated car-
banion formed by OCl as oxidant, predominantly
through the pathways of formation of THM from
humic acid.
ꢀI†
Substitution
Humic acid
4 Halogenated Humic acid
ꢀNaOX†
From this work, we propose a new concept that
when hypochlorite and hypobromite coexist, the
hypochlorite reacts more e€ectively with humic acid
in the oxidization reaction to produce precursors,
ꢀII†
The tentative reaction mechanism for the organic
chemistry is as follows:

Brominated trihalomethane formation
483
Bellar T. A., Lichtenberg J. J. and Kroner R. C. (1974)
Occurrence of organohalides in chlorinated drinking
waters. J. Am. Wat. Wks. Ass. 66, 703±706.
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the subsequent electrophilic substitution reaction.
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CONCLUSION
The results observed by the reaction of humic
acid with NaOBr as bromination reagent in the pre-
sence of NaOCl were as follows;
(1) The product distribution of the THM is gov-
erned almost only by the [NaOBr]/[NaOCl] molar
ratio, but within 60 min hardly by the reaction time
and pH.
(2) The amount of THMs formed in the coexis-
tence of NaOCl and NaOBr was larger than the
sum of the CHCl3 and CHBr3 formed in the re-
spective reactions of NaOCl and NaOBr alone with
humic acid.
(3) In the presence of an excess amount of HOBr
to OCl , the amount of CHBr3 formed increased
with the concentration of OCl , and was 7 times
larger than that formed in the reaction of NaOBr
alone with humic acid.
(4) These observations suggest that OCl ion in
the THM formation reaction promotes the oxi-
dation of humic acid, and that the rate of the sub-
sequent substitution by HOBr seems higher than
that of HOCl.
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Rook J. J. (1974) Formation of haloforms during chlori-
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243.
AcknowledgementsÐThe authors are grateful to Dr. Akio
Yasuhara of National Insutitute of Environmental Studies
and Dr. Sukeo Onodera of Tokyo University of Science
for criticism of the present work, and to Emeritus
Professor Takeo Araki of Kyoto Institute of Technology
for reading the manuscript.
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