DPB Formation

Volatile Disinfection Byproduct
Formation Resulting from
Chlorination of Organic-Nitrogen
Precursors in Swimming Pools
J I N G L I A N D E R N E S T R . B L A T C H L E Y I I I *
School of Civil Engineering, Purdue University,
West Lafayette, Indiana 47907-2051
Clinical studies have documented the promotion of
respiratory ailments (e.g., asthma) among swimmers,
especially in indoor swimming pools. Most studies of this
behavior have identified trichloramine (NCl3) as the
causative agent for these respiratory ailments; however,
the analytical methods employed in these studies were not
suited for identification or quantification of other volatile
disinfection byproducts (DPBs) that could also contribute to
this process. To address this issue, volatile DBP formation
resulting from the chlorination of four model compounds
(creatinine, urea, L-histidine, and L-arginine) was investigated
over a range of chlorine/precursor (Cl/P) molar ratios.
Trichloramine was observed to result from chlorination of
all four model organic-nitrogen compounds. In addition
to trichloramine, dichloromethylamine (CH3NCl2) was detected
in the chlorination of creatinine, while cyanogen chloride
(CNCl) and dichoroacetonitrile (CNCHCl2) were identified
in the chlorination of L-histidine. Roughly 0.1 mg/L (as Cl2)
NCl3, 0.01 mg/L CNCHCl2, and 0.01 mg/L CH3NCl2 were
also observed in actual swimming pool water samples. DPD/
FAS titration and MIMS (membrane introduction mass
spectrometry) were both employed to measure residual
chlorine and DBPs. The combined application of these
methods allowed for identification of sources of interference
in the conventional method (DPD/FAS), as well as structural
information about the volatile DBPs that formed. The
analysis by MIMS clearly indicates that volatile DBP
formation in swimming pools is not limited to inorganic
chloramines and haloforms. Additional experimentation
allowed for the identification of possible reaction pathways
to describe the formation of these DBPs from the precursor
compounds used in this study.
Chlorination is the most common method for disinfection
of recreational water, such as swimming pools. Although +1
valent chlorine has been demonstrated to be effective for
the control of many microbial pathogens, it is also known
to react with aqueous constituents to yield disinfection
byproducts (DBPs) (1).Manycommonaqueous constituents
can react with chlorine to yield DBPs; however, the focus of
this investigation is on the chlorination of organic-nitrogen
Important sources of organic-nitrogen compounds in
recreational waters include sweat and urine, and relevant
organic-nitrogen precursors may include urea, creatinine,
and amino acids. These compounds will consume free
chlorine (2, 3) and also act as precursors in the formation of
DBPs, including some that are sufficiently volatile to be
transferred to the gas phase (4). For example, trihalomethanes
(THMs) have been reported to be formed by chlorination of
materials ofhumanorigin (hair, lotion, saliva, skin, and urine)
(5) and by chlorination ofhumanurine analogues containing
urea, creatinine, and citric acid (6).
Past studies have suggested that the smell and irritant
properties of swimming pool air are largely attributable to
inorganic chloramines. These compounds, particularly
trichloramine (NCl3), are volatile (7, 8). Trichloramine has
been reported to function as an irritant to the eyes and upper
respiratory tract, and it may contribute to acute lung injury
in accidental, occupational, or recreational exposures to
chlorine-based disinfectants (9-11). However, relatively little
information is available to describe the specific chemistry
responsible for NCl3 formation in recreational waters. Additionally,
most available literature related to DBPs in
recreational water has focused on inorganic chloramines.
Comparatively little information is available to describe the
formation of other volatile DBPs that may be found in
swimming pool settings.
The objective of this study was to identify volatile DBPs
that will result from the chlorination of organic-nitrogen
compoundsthat are likely to be present in recreational waters.
Four model organic-nitrogen compounds, urea, creatinine,
L-histidine, and L-arginine, were selected as representative
organic pollutants of recreational water (refer to Figure 1).
Urea is the major nitrogenous end product of protein
metabolism and is the chief nitrogenous component of urine
and sweat in mammals. Creatinine is a constituent of
perspiration and urine formed by the metabolism of creatine;
it is found in muscle tissue and blood and is normally excreted
in urine and sweat as a metabolic waste product. L-Histidine
and L-arginine are amino acids that are commonly found in
human sweat. Urea, creatinine, and L-histidine are also the
primary constituents of “body fluid analogue” (BFA), which
has been used as a surrogate mixture of organic-nitrogen
compounds in previous studies involving chlorination of
recreational waters (12, 13). However, the focus of the
previously published research involving the chlorination of
BFA was on the measurement of THMs and chloramines.
Moreover, these investigations were based on analytical
measurements that are known to be susceptible to common
forms of analytical interference (e.g., diethyl-p-phenylenediamine/
ferrous ammonium sulfate (DPD/FAS) titration).
Membrane introduction mass spectrometry (MIMS) was
employed to monitor theDBPsof chlorination becauseMIMS
has been shown to be a suitable method for the analysis of
volatile compounds in aqueous samples. Also, MIMS-based
analytical procedures have the potential to be used to
characterizemanyaspects of chlorine-based water treatment
applications. An important advantage of the method is that
it can yield quantitative and structural information about
volatile DBPs present in an aqueous sample. MIMS measurements
were also conducted in parallel with measurements
by DPD/FAS titration, so as to allow for comparisons
between the two methods. In conjunction with previously
published information, the results of these measurements
allowed for the development of hypothesized reaction
mechanisms to describe volatile DBP formation from chlorination
of several model organic-nitrogen compounds. Last,
pool water samples were collected and analyzed using these
same methods so that comparisons between DBP formation
* Corresponding author phone: (765)494-0316; fax: (765)494-0395;
e-mail: blatch@purdue.edu.
Environ. Sci. Technol. 2007, 41, 6732-6739
6732 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 19, 2007 10.1021/es070871+ CCC: $37.00 ã 2007 American Chemical Society
Published on Web 09/05/2007
in actual recreational water could be made with the results
of themorecontrolled experiments involving model organicnitrogen
Experimental Procedures
Materials and Methods. All chemicals used in this study,
unless otherwise noted, were reagent-grade, purchased from
Sigma-Aldrich,andused without further purification. Dilution
to target aqueous-phase concentrations was accomplished
with distilled, deionized water. Free chlorine stock solutions
and standard solutions of inorganic chloramines were
prepared in the same manner as described previously (14).
Standard solutions of cyanogen chloride (CNCl) were prepared
daily from a CNCl stock solution (2000 mg/L Protocol
Analytical Supplies, Inc.). Standard solutions of chloroform
(CHCl3) and dichloroacetonitrile (CNCHCl2) were prepared
gravimetrically from pure compounds. Standard dichloromethylamine
(CH3NCl2) solutions were prepared by chlorination
of methylamine (CH3NH2) at a Cl/N molar ratio of
2.0; the concentration of CH3NCl2 was defined by DPD/FAS
titration as apparent dichloramine.
The MIMS system was based on a modification of an HP
5892 benchtop GC/MS comprising an HP 5972A mass
selective detector (MSD) equipped with electron (70 ev)
ionization (EI). A mass spectrum scan mode (49 e m/z e
200) coupled with EI was used to identify possible DBPs,
while a selected ion monitoring (SIM) mode was used for
quantification of volatile DBPs. Other details of the configuration
and setup for the MIMS system and operational
conditions can be obtained from ref 14. The concentration
of volatile DBPs was determined by comparison of ion
abundance measurements with those developed from a series
of standard solutions. Ions atm/z 61, 74, 85, 98, and 119 amu
were monitored for quantification of CNCl,CNCHCl2, CHCl3,
CH3NCl2,andNCl3, respectively. Allcompoundidentifications
by MIMS were confirmed by analysis of standard solutions.
All experiments were conducted using a bicarbonate buffer
system (120 mg/L CaCO3) at pH 7.5; this water chemistry
was selected to provide an aquatic matrix that was representative
of swimming pool chemistry. Chlorination experiments
were carried out in well-mixed, glass-stoppered, 250
mLflasks in the dark. For experiments in which the objective
was the identification of volatile DBPs, organic-nitrogen
compounds were added to achieve a concentration of 1.8  10-4 M. Solutions were chlorinated at a chlorine/precursor
molar ratio (Cl/P) of 5.0 for 24 h. This Cl/P molar ratio was
selected because it is well beyond the breakpoint based on
reduced N (in the form of NH3) and is representative of the
conditions of chlorination that are often used in swimming
pools. The 24hreaction period was selected because previous
experiments had demonstrated that most (detectable) changes
in water chemistry were complete in a 24 h period. The
resulting solutions were then subjected to MIMS analysis.
For experiments in which the objective was the quantification
of volatile DBPs, aqueous precursor solutions were
freshly prepared by the addition of 1.0 mL of a model
compound stock solution to 200 mL of an aqueous bicarbonate
buffer solution to achieve a target precursor concentration
of 1.810-5 M. Aliquots of a standardized sodium
hypochlorite (NaOCl) stock solution were then added to the
flasks. The 0.01MHCl and 0.01MNaOH were used to adjust
the initial pH of the solution to 7.5. The reaction vessels had
little headspace, and they were sealed to avoid volatilization
and kept in the dark atroomtemperature. The concentrations
of free and (apparent) combined chlorine were measured by
DPD/FAS titration; MIMS was used to measure residual
chlorine and volatile DBPs.
Recreational water samples were collected from indoor
and outdoor municipal swimming facilities. Pool water
samples were immediately transported to the laboratory in
sealed plastic bottles with little headspace for analysis by
DPD/FAS titration and MIMS. The bottles used to collect
and transport the pool water samples were opaque to
ultraviolet radiation.
Results and Discussion
Identification of Volatile DBPs. Initial experiments were
conducted using relatively high concentrations of free
chlorine and model organic-nitrogen compounds to aid in
volatile DBP identification. A summary of the volatile DBPs
identified in this study is provided in Figure 1. Figure 2
illustrates typical mass spectra obtained by MIMS analysis
of chlorinated solutions of the four organic-nitrogen precursors.
Trichloramine was identified in all four cases by
the existence of a peak cluster at m/z 84 (N35Cl2
¥+), 86
(N35Cl37Cl¥+), and 88 (N37Cl2
¥+) at an abundance ratio of 9:6:1,
as well as a molecular ion peak at m/z 119 (N35Cl3
¥+). This
spectral pattern was in agreement with spectra that were
developed from NCl3 standard solutions, as well as previously
published spectra developed using MIMS (14). As shown in
Figure 2, urea appears to be the most active NCl3 precursor
among these four model organic-nitrogen compounds.
Chlorination of creatinine and L-histidine also yielded
other volatile DBPs. Figure 2a presents the MIMS mass
spectrum resulting from chlorination of aqueous creatinine.
In addition to the peak clusters described previously
that were associated with NCl3, three additional clusters
were observed. The peak clusters located at m/z 98
¥+)-100 (CH2N35Cl37Cl¥+)-102 (CH2N37Cl2
¥+) and
99 (CH3N35Cl2
¥+)-101 (CH3N35Cl37Cl¥+)-103 (CH3N37Cl2
each with abundance ratios of 9:6:1, and m/z 62
(CHN35Cl¥+), 63 (CH2N35Cl¥+), 64(CHN37Cl¥+ andCH3N35Cl¥+),
65 (CH2N37Cl¥+), and 66 (CH3N37Cl¥+) suggested the formation
of N,N-dichloromethylamine (dichloromethylamine).
MIMSanalysis of a standard solution of dichloromethylamine
confirmed this finding. Although no reports of dichloromethylamine
formation from the chlorination of creatinine
were found in the literature, methylamine has been reported
as a disinfection byproduct and has also been detected in
swimming pool water samples (15). Methylamine will react
with free chlorine to generate monochloromethylamine
immediately by chlorine substitution; however, the results
of these experiments described herein indicate that the formation
of dichloromethylamine by chlorination of methylamine
requires several hours to complete, as well as a Cl/P
ratio of greater than 1.
Unlikemanyother organic chloramines, which often yield
DPD/FAS signals that correspond to inorganic dichloramine
(NHCl2), monochloromethylamineanddichloromethylamine
behave much like their inorganic chloramine analogues
(NH2Cl and NHCl2, respectively) in the DPD/FAS titrimetric
FIGURE 1. Illustration of the structures of model organic-nitrogen
compounds and possible volatile DBPs that result from chlorination
of recreational waters.
method. Specifically, monochloromethylamine yields an
apparent NH2Cl signal in the DPD/FAS method, while
dichloromethylamine yields an apparent NHCl2 signal in the
same method. The health effects of dichloromethylamine
are not known; however, exposure to this chemical during
this research indicated a characteristic malodor, with a smell
similar to that of trichloramine.
In the case of L-histidine (Figure 2b), a peak cluster atm/z
74 (CNCH35Cl¥+) and 76 (CNCH37Cl¥+), with an abundance
ratio of 3:1, along with a second cluster atm/z 82 (C35Cl2
¥+) indicated the formation of dichloroacetonitrile.
A peak cluster at m/z 61 (CN35Cl¥+) and 63
(CN37Cl¥+), with an abundance ratio of 3:1, suggested the
possible formation of cyanogen chloride. These peaksshowed
good agreement with mass spectra developed fromCNCHCl2
andCNClstandard solutions, respectively. However, the peak
corresponding to trichloramine was not obvious in this
spectrum because trichloramine was present at a relatively
low concentration under this condition as compared to the
other volatile compounds.
CNCHCl2 and CNCl have both been reported as DBPs
that result from the reaction of free chlorine with natural
organic-nitrogen compounds (16). CNCHCl2 has been
identified as an irritant of the respiratory system and skin
and a possible mutagen in humans (17). CNCl is also highly
toxic, even at very low concentrations. The World Health
Organization (WHO) has recommended that a maximum
concentration of 70 íg/L (as cyanide) be used as a guideline
for total cyanogen compounds in drinking water.
As shown in Figure 2b, small amounts of chloroform were
formed as a result of chlorination of L-histidine, as evidenced
by the peak cluster at 83 (CH35Cl2
¥+).CNCHCl2 and NCl3 will also yield peaks at these
same m/z values; however, the abundance signature at
83:85:87 was different than expected on the basis of any of
these compounds individually. This suggests that some
combination of CHCl3, CNCHCl2, and NCl3 is formed as a
result of the chlorination of L-histidine.
TheMIMSconfiguration used in this research yields mass
spectral signals only for compounds that are able to pervaporate
through a tubular silicone membrane. Therefore,
this method allows for selective identification and quantification
of volatile and semi-volatile constituents. As such,
it is well-suited for analysis of volatile DBPs thatmaydevelop
in a recreational water setting. Given that all the DBPs
identified in these experiments are volatile, it is reasonable
to hypothesize that they could also affect air quality in
recreational water settings.
Quantification of Volatile DBPs and Residual Chlorine.
DPD/FAS titration and MIMS were employed in parallel to
measure the residual chlorine and volatile DBPs that resulted
from the chlorination of aqueous solutions containing
organic-nitrogen precursors at a concentration of 1.810-5
M. These experiments were conducted over a range of Cl/P
molar ratios (1.6 e Cl/P e 9.6) that is believed to be
representative of those employed in recreational water
settings.DPD/FAStitration consistently yielded false-positive
measurements of inorganic combined chlorine residuals,
which were attributed to the formation of organic chloramines.
The volatileDBPsthat resulted from reaction of these
four organic-nitrogen precursors and free chlorine responded
in theDPD/FAStitration predominantly as apparent
dichloramine, with smaller amounts of apparent monochloramine;
these observations were consistent with previous
findings (14, 18). Free chlorine measurements by DPD/FAS
titration andMIMSwere in good agreement. The four volatile
N-containing DBPs that were identified in the experiments
involving relatively high precursor concentrations (1.810-4
M) were also detected by MIMS at low precursor concentrations
(1.8  10-5 M).
FIGURE 2. Mass spectra of chlorinated samples of aqueous solutions of organic-nitrogen compounds: (a) creatinine, (b) L-histidine, (c)
urea, and (d) L-arginine after 24 h. In each case, organic-nitrogen compounds were present at an initial concentration of 1.8  10-4 M;
initial Cl/P ) 8.0 for L-arginine and Cl/P ) 5.0 for all other cases; pH ) 7.5. Note that the scales on the vertical (abundance) axes in these
mass spectra have been adjusted to coincide with the largest abundance value in each spectrum.
Residual chlorine (here defined as the sum of the
concentrations of free chlorine and [organic or inorganic]
chloramines) and volatile DBPs generated in an aqueous
solution of creatinine (1.810-5M)treated with free chlorine
at pH 7.5 were quantitatively analyzed by both DPD/FAS
titration and MIMS. Figure 3 illustrates the results of these
measurements for 1 and 96 h of chlorination of creatinine.
As described previously, dichloromethylamine yielded a
signal that corresponded with NHCl2 in the DPD/FAS
titration. According to the DPD/FAS titration results, the
concentration of apparent dichloramine did not change
substantially from 1 to 96 h post-chlorination. However, the
MIMS results showed a significant increase of dichloromethylamine
from 1 to 96 h chlorination. The concentration
of dichloromethylamine measured by MIMS accounted for
80-90% of apparent dichloramine by the DPD/FAS method
after 96 h of chlorination. These data also suggest that there
aresomenonvolatile (or perhaps low volatility) intermediates
that are formed in the production of CH3NCl2. These data
support previous research efforts that have revealed that the
DPD/FAS method does not discriminate between volatile
and nonvolatile aqueous constituents, nor does it discriminate
among many organic chloramines or inorganic chloramines.
In contrast, the MIMS configuration used in this
work allows selective measurement of constituents that will
diffuse through a silicon polymer membrane (i.e., volatile
and semi-volatile compounds). However, this method does
not allow measurement of low volatility compounds. The
MIMS results for total residual chlorine were consistently
lower than the corresponding measurements from DPD/
FAS titration. The difference between these total residual
chlorine measurements was assumed to be attributable to
low volatility DBPs.
The time-course concentrations of free chlorine, trichloramine,
and dichloromethylamine are shown in Figure 4 for
chlorination of an aqueous solution containing creatinine.
MIMS measurements indicated a dichloromethylamine
concentration of less than 10 íg/L (as Cl2) after 1 h
chlorination; however, the dichloromethylamine concentration
increased steadily during the 24 h chlorination period,
ultimately yielding a concentration of approximately 1.5mg/L
Cl2. The maximum yield of dichloromethylamine was approximately
66% of the initial creatinine, on a molar basis.
The experiment was repeated at different Cl/P molar ratios,
and dichloromethylamine formation was observed to be
directly related to the initial concentration free chlorine (see
Figure S1 in Supporting Information). Free chlorine decayed
more rapidly at the beginning of the experiment than at the
end. Approximately 0.15 mg/L (Cl2) trichloramine was
formed after 4 h chlorination of creatinine; the concentration
of trichloramine was fairly stable after that point.
In general, roughly 0.1 mg/L trichloramine (Cl2) was
detected by MIMS after 96 h creatinine chlorination at Cl/P
> 3.2. At these low concentrations, DPD/FAS titration
generally does not yield accurate measurements of NCl3
because it is near the detection limit and because of
interference by organic chloramines. As will be described
next, the concentration of NCl3 in recreational water is often
in the vicinity of 0.1 mg/L Cl2. Therefore, accurate quantification
and identification of NCl3 in recreational water by
DPD/FAS is likely to be difficult. In contrast, the detection
limit of NCl3 byMIMSis less than 0.06 mg/L Cl2, and the NCl3
signal provided by a mass spectrometer is unlikely to be
interfered with by common constituents in recreational
Among the organic precursors investigated in this study,
L-histidine was found to be the most reactive toward free
chlorine, as defined by the rate of free chlorine consumption
(compare Figure 5 with Figure 3 and Supporting Information
Figure S2). Two volatile DBPs, CNCHCl2 and CNCl, were
detected by MIMS after L-histidine chlorination. At the end
of the experiment (t ) 96 h), the MIMS signal for CNCHCl2
accounted for roughly 50-80% of the apparent dichloramine
signal from DPD/FAS, depending on the Cl/P ratio. These
results imply that CNCHCl2 was the major volatile DBP to
FIGURE 3. Residual chlorine concentration as a function of Cl/P
molar ratio after 1 h (bottom) and 96 h (top) chlorination of an
aqueous solution containing creatinine (1.8  10-5 M) at pH 7.5.
FIGURE 4. Residual chlorine concentration as a function of time
at Cl/P ) 8.0 chlorination of an aqueous solution containing
creatinine (1.8  10-5 M) at pH 7.5.
FIGURE 5. Residual chlorine concentration as a function of Cl/P
molar ratio after 1 h (bottom) and 96 h (top) chlorination of an
aqueous solution containing L-histidine (1.8  10-5 M) at pH 7.5.
result from L-histidine chlorination. Approximately 0.2 mg/L
(Cl2) CNCHCl2 and 0.02 mg/L (Cl2) CNCl were found to be
generated at a molar ratio Cl/P of 8.0. As shown in Figure 6,
the free chlorine concentration decreased steadily over the
course of 96hchlorination, while theCNCHCl2 concentration
generally increased. NCl3 and CNCl concentrations both
increased early in the reaction, reached a plateau, and then
decreased (see inset in Figure 6). Roughly 0.22 mg/L (Cl2)
NCl3 was formed after 4 h chlorination of L-histidine, followed
by decay.
Urea was found to yield relatively high concentrations of
NCl3, even at molar ratios as low as Cl/P ) 1.6 (see Figure
S2A in Supporting Information). For example, roughly 0.1
mg/L (Cl2) NCl3 was detected by MIMS after 1 h chlorination
of urea at Cl/P ) 1.6. This observation was consistent with
results from experiments involving high precursor concentrations.
No other forms of residual chlorine were evident in
chlorinated urea samples. Trichloramine was the only DBP
that was identified by MIMS to result from chlorination of
L-arginine (see Figure S2B in Supporting Information). No
NCl3 was detected after 1 h chlorination of L-arginine at a
Cl/P molar ratio from 1.6 to 9.6, and NCl3 was detectable
only under conditions of Cl/Pg6.4. This behavior suggested
that L-arginine is less active as a precursor to volatile DBP
production than the other three model precursors.
In general, chlorination of all four organic-nitrogen
precursors was shown to yield volatile DBPs, even at low
precursor concentrations that are believed to be representative
of recreational waters. Inorganic chloramine, which was
always observed as trichloramine (always in the presence of
free chlorine), was found as a common byproduct in all
cases. This finding, together with previously published
results (19-27), suggests that many organic-nitrogen compounds
can act as precursors to NCl3 formation. The
formation of trichloramine was strongly dependent on the
Cl/P molar ratio, the structure of the precursor, and the
reaction time.
It is also important to recognize that inorganic monochloramine
(NH2Cl) and dichloramine (NHCl2) were always
present below the detection limits for MIMS in these
laboratory experiments. The MIMS detection limits for
NH2Cl and NHCl2 were approximately 0.1 and 0.02 mg/L
(Cl2), respectively (14). The low concentrations of these
inorganic chloramines were probably attributable to the
relatively high Cl/P molar ratios used in these experiments.
Mechanisms of Volatile DBP Formation. Reactions
between free chlorine and organic-nitrogen precursors have
been generalized to proceed via electrophilic substitution
(19), combined with hydrolysis. In the initial stage, organicnitrogen
precursors are generally chlorinated to N-monoor
N,N-di-chlorinated forms by electrophilic attack of
OCl- or HOCl. When the molar ratio of free chlorine to
organic precursor exceeds the stoichiometric requirement
for dichloro substitution, the organic N-mono- or N,Ndichloramine
will often decompose to yield carbon dioxide,
inorganic chloramines, nitriles, aldehydes, and chloroaldimines
Some generalizations can be identified in chlorination
mechanisms. For example, electron-withdrawing groups can
increase the acidity of a proton of C orN, thereby promoting
chlorination of C orNat those positions by substitution (20).
Good leaving groups (e.g., carboxylate) can also enhance
electrophilic substitution. Furthermore, a leaving group near
a chlorinated N atom in an organic-nitrogen precursor
(e.g., carboxylate) will enhance the formation of nitriles or
chloroaldimines. As an illustration of this behavior, it has
been reported that nitriles and chloroaldimines are formed
as a result of chlorination of R-amino acids (23-25).
The mechanism of trichloramine formation can be
assumed to involve simple substitution of Cl+. Most organic
amines, including primary, secondary, and tertiary amines,
can act as precursors to trichloramine formation. Nitriles
can also function as trichloramine precursors. For example,
trichloramine can be produced when cyanogen chloride
reacts with free chlorine in water (26). As part of this study,
we also found that detectable trichloramine was formed as
a volatile byproduct when dichloroacetonitrile reacted with
free chlorine at a molar ratio Cl/P ) 50 (see Figure S3 in
Supporting Information). This observation suggests that NCl3
formation could be a minor reaction pathway of chlorination
of dichloroacetonitrile.
A proposed mechanism for the formation of dichloromethylamine
from the chlorination of creatinine is illustrated
in Scheme 1. The mechanism begins with chlorine substitution,
followed by hydrolysis to yield urea and N-chlorosarcosine
(CH3NClCH2COOH) as intermediates. Urea then
reacts through a sequence of chlorine substitution and
hydrolysis steps to yield trichloramine. N-Chloro-sarcosine
is hypothesized to first undergo decarboxylation and dehydrochlorination
to yield CH3NdCH2 as an intermediate,
which itself undergoes hydrolysis to yield methylamine,
which in turn undergoes N-dichloro substitution to yield
To test the validity of this hypothesized mechanism, an
aqueous solution of sarcosine was exposed to free chlorine
at a molar ratio of Cl/P ) 5.0, and dichloromethylamine was
found as the major DBP (see Figure S4 in Supporting
Information). Urea was observed as a byproduct of chlorination
of creatinine by Tachikawa et al. (15). Dichlorourea was
found to be formed in the chlorination of urea, and when a
solution of dichlorourea was allowed to stand, it decomposed,
yielding trichloramine as one of the products (27).
A proposed mechanism for the formation of CNCl and
CNCHCl2 from the chlorination of L-histidine (I) is illustrated
in Scheme 2. As with other R-amino acids (22, 28), an initial
chlorine substitution step is followed by dechlorination and
decarboxylation to yield a nitrile (II). It is hypothesized that
the two electron-withdrawing groups, sCtN and a heterocycle,
will promote electrophilic attack of OCl- on the
â-carbon of the intermediate (II). As a result, one of the
protons of the â-carbon of nitrile (II) is substituted by chlorine
to generate intermediate (III), and then with a further
electrophilic attack of OCl- on this position, the C-C bond
is broken to form dichloroacetonitrile and the heterocycle
To test the validity of this hypothesized mechanism, an
experiment was conducted involving aspartic acid (HOOCFIGURE
6. Residual chlorine concentration as a function of time
for Cl/P ) 8.0 in chlorination of an aqueous solution containing
L-histidine (1.8  10-5 M) at pH 7.5. Inset illustrates the dynamics
of volatile DBP formation in greater detail.
CH2CHNH2COOH)as the precursor, for which the carboxylate
group on the â-carbon would also be expected to act as an
electron-withdrawing group and a good leaving group.
Chlorination of aspartic acid also resulted in the formation
of dichloroacetonitrile. On the other hand, no dichloroacetonitrile
was detected by chlorination of alanine
(CH3CHNH2COOH) (see Figure S5 in Supporting Information).
With free chlorine, dichloroglycine (V) could be formed
by the cleavage of the heterocycle (IV) through the rupture
of two C-N bonds. Several studies have documented
CNCl formation to result from the chlorination of gly-
SCHEME 1. Proposed Mechanism for Formation of Trichloramine and Dichloromethylamine from Chlorination of Creatinine
SCHEME 2. Proposed Mechanism for the Formation of Dichloroacetonitrile and Cyanogen Chloride from Chlorination of
TABLE 1. Volatile DBP Measurement in Samples of Recreational Water
(mg/L Cl2)
free chlorinec
(mg/L Cl2)
combined chlorinec
(mg/L Cl2)
free chlorine to combined
chlorine (molar ratio)
A 0.08 0.07 0.01 1.5 1.34 1.12
B 0.07 0.13 0.03 1.95 0.25 7.80
C 0.09 0.14 0.01 0.68 1.36 0.50
D 0.16 0.08 0.02 6.52 1.76 3.70
E 0.1 0.13 0.01 5.92 1.28 4.63
F 0.07 0.08 0.01 1.72 0.76 2.26
a A, C, E, and F: indoor lap swimming pool; B: outdoor general use swimming pool; and D: outdoor recreation park. b Analysis by MIMS.
c Analysis by DPD/FAS titration.
cine (16, 23) by the mechanism illustrated at the end of
Scheme 2.
Analysis of Recreational Water Samples. MIMS was also
employed to analyze volatile DBPs in samples collected from
public recreational water facilities. Water samples were
collected from indoor and outdoor swimming pools in screwcapped
bottles and transported back to the Environmental
Engineering Laboratories at Purdue University to allow
initiation of MIMS analyses within 1 h of collection. Table
1 provides a summary of volatile DBP measurements for
samples collected from six public pools. These data are
accompanied by measurements of residual chlorine concentration
by DPD/FAS titration.
Three volatile DBPs (chloroform [CHCl3], trichloramine,
and dichloroacetonitrile) were detected and quantified from
all six recreational water samples by MIMS. Among them,
trichloramine and dichloroacetonitrile were also detected in
the chlorination of model organic-nitrogen compounds.
Dichloromethylamine was detected only in sample A and
was present at a concentration of approximately 10 íg/L.
Interestingly, sample A was collected from a natatorium that
is used almost exclusively for lap swimming and high-level
competitive swimming. Although no measurements of
precursor concentrations were performed for the swimming
pool samples, the use pattern for swimming pool A is
consistent with a scenario where relatively high creatinine
concentrations might be expected. Creatinine was the only
precursor in this research that yielded dichloromethylamine
as a result of chlorination. More generally, the volatile DBPs
that were identified in the experiments with model organicnitrogen
compounds were also detected in municipal pool
water samples. However, no CNCl was detected in swimming
pool samples, which may be attributed to the short half-life
(about 1 h in the presence of 0.5 mg/L free chlorine, at 25
°C, pH 7) of CNCl in the presence of free chlorine (26).
The data presented in Table 1 also suggest that the
concentration ranges of these volatile products in actual
recreational water facilities were relatively narrow, regardless
of the concentrations of free chlorine and combined chlorine.
This suggests that these volatile DBPs may be ubiquitous in
chlorinated swimming pools, even in well-maintained facilities.
These compounds are difficult to eliminate by simple
chlorination, even shock chlorination. Therefore, additional
treatment operations may be needed in recreational water
settings to improve water and air quality, relative to these
volatile DBPs.
Collectively, the experiments involving chlorination of
organic precursors and the analysis of swimming pool
samples by MIMS indicate the presence of volatile DBPs,
includingNCl3andseveral organic chloramines. Interestingly,
NH2Cl and NHCl2 were not detected in samples from
laboratory experiments or swimming pools. While the results
of these experiments do not allow for the prediction of the
rates of volatile DBP formation in swimming pools, the
analysis by MIMS provides a clear indication that DBP
formation in swimming pools is not limited to inorganic
chloramines and haloforms. Therefore, future research
should be aimed at further defining the reactions that are
responsible for volatile DBP formation in swimming pools,
as well as processes that may be used for removal or control
of these compounds.
The authors are grateful to the DuPont Experimental Station
(Wilmington, DE) and the National Swimming Pool Foundation
for financial support of this research.
Supporting Information Available
Detailed descriptions of volatile DBP formation as well as
additional information to support the hypothesized reaction
mechanisms. This material is available free of charge via the
Internet at http://pubs.acs.org.
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Received for review April 13, 2007. Revised manuscript received
July 20, 2007. Accepted July 30, 2007.


Water Quality Laboratory, Metropolitan Water District of Southern California, 700 Moreno Avenue, La Verne, California 91750, Department of Civil, Environmental and Sustainable Engineering, Engineering Center (G-Wing), Arizona State University, Room ECG-252, Tempe, Arizona 85287-5306, Center for Environmental Biotechnology, Biodesign Institute, Arizona State University, P.O. Box 875701, Tempe, Arizona 85287-5701, and King Abdullah University of Science and Technology, Box 55455, Jeddah, 21534, Saudi Arabia.
Effluents from wastewater treatment plants (WWTPs) contain disinfection byproducts (DBPs) of health concern when the water is utilized downstream as a potable water supply. The pattern of DBP formation was strongly affected by whether or not the WWTP achieved good nitrification. Chlorine addition to poorly nitrified effluents formed low levels of halogenated DBPs, except for (in some cases) dihalogenated acetic acids, but often substantial amounts of N-nitrosodimethyamine (NDMA). Chlorination of well-nitrified effluent typically resulted in substantial formation of halogenated DBPs but much less NDMA. For example, on a median basis after chlorine addition, the well-nitrified effluents had 57 mug/L of trihalomethanes [THMs] and 3 ng/L of NDMA, while the poorly nitrified effluents had 2 mug/L of THMs and 11 ng/L of NDMA. DBPs with amino acid precursors (haloacetonitriles, haloacetaldehydes) formed at substantial levels after chlorination of well-nitrified effluent. The formation of halogenated DBPs but not that of NDMA correlated with the formation of THMs in WWTP effluents disinfected with free chlorine. However, THM formation did not correlate with the formation of other DBPs in effluents disinfected with chloramines. Because of the relatively high levels of bromide in treated wastewater, bromine incorporation was observed in various classes of DBPs.

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