Odorous products of the chlorination of phenylalanine in water: Formation, evolution, and quantification

Article abstract of DOI:10.1021/es035021i

To explain some of the possible origins of an odor episode which took place in a drinking water supply in the region of Paris (France), the chlorination reaction in water of phenylalanine was studied. This amino acid was chosen for first experiments becau

Full text of DOI:10.1021/es035021i

Environ. Sci. Technol. 2004, 38, 4134-4139
power. But as an oxidizing disinfectant, chlorine can also
Odorous Products of the
Chlorination of Phenylalanine in
Water: Formation, Evolution, and
Quantification
react with natural organic matter (NOM) present in drinking
water (1). The organic compounds composing NOM (polysac-
charides, amino sugars and amino acids, proteins, and
polyhydroxyaromatic compounds) can produce undesirable
disinfection byproducts. Great concern has been focused on
the health risks of drinking water because some of the
disinfection byproducts have been linked to cancer in animals
or are suspected to have possible reproductive and develop-
ment effects (2), but little attention has been paid to off-
flavor properties of treated water. However, drinking water
also needs to have good organoleptic properties. It should
not present any odor, color, or taste because the consumer
often uses sensory evaluation for judging water quality,
assuming, most often wrongly, that water with an unpleasant
smell is unsafe for consumption (3). A better understanding
of the chemical causes of drinking water odor is by this fact
necessary for water operators. The composition of water
before chlorine addition has a great incidence on the
formation of disinfection byproducts. Our study was finan-
cially supported by the Mery-sur-Oise treatment plant,
located in the north suburbs of Paris. The study was based
on an isolated odor episode following the implantation of a
new membrane process. Part of the water (70%) from this
plant has been nanofiltered since 1999 (4). So only very little
organic matter is able to pass the membrane barrier. Only
low-molecular-weight compounds such as amino acids (5)
and short peptides can pass through the membrane.
I N G R I D F R E U Z E , * , †
S T EÄ P H A N B R O S I L L O N , †
D O R I N E H E R M A N , † A L A I N L A P L A N C H E , †
C H R I S T I A N D EÄ M O C R A T E , ‡ A N D
§
J A C Q U E S C A V A R D
Laboratoire de Chimie des Nuisances et Ge´nie de
l’Environnement, Ecole Nationale Supe´rieure de Chimie de
Rennes, Av. du Ge´ne´ral Leclerq, 35700 Rennes, France,
Veolia Water, Compagnie Ge´ne´rale des Eaux, Immeuble Le
Carillon, 6 Esplanade Charles de Gaulle, 92751 Nanterre,
France, and Syndicat des Eaux d’Ile de France (SEDIF),
14 rue Saint Benoˆıt, 75006 Paris, France
To explain some of the possible origins of an odor
episode which took place in a drinking water supply in
the region of Paris (France), the chlorination reaction in
water of phenylalanine was studied. This amino acid was
chosen for first experiments because of its physical and
chemical particular properties. Changes in the different
byproducts formed were followed by high-performance liquid
chromatography (HPLC) over a period of time. N-chlo-
rophenylalanine (mono-N-chlorinated amino acid) and then
phenylacetaldehyde were the major products formed for
the lower chlorine to nitrogen molar ratios. For Cl/N molar
ratios of 1 and beyond, phenylacetonitrile and N-chlo-
rophenylacetaldimine appeared and increased with the
chlorination level. N-chlorophenylacetaldimine was quantified
by using its difference of stability in various organic
solvents. Our attention was first directed to the monochlo-
rinated derivative but further examination indicated that
it could not be responsible for odor troubles: it dissociated
before reaching the consumer’s tap and it was produced
at consistently low yields under conditions relevant to
drinking water treatment. On the contrary, chloroaldimine
appeared to be a very odorous and water-stable product:
it strongly smells of swimming pool with a floral background.
The odor detection threshold is about 3 µg‚L-1 and it
can persist for more than one week at 18 °C. It is now
suspected of being a source of off-flavor concerns among
consumers.
Now odorous chlorinated products, for example, are
formed especially with the amino acids encountered at low
concentration in water, under a dissolved free form or even
more as little peptides. The major amino acids present in
treated water are alanine, glycine, valine, phenylalanine,
serine, threonine, isoleucine, aspartic acid, tyrosine, proline,
glutamic acid, and leucine (6). Amino acid concentrations
in drinking water range from 0.33 to 1.05 µg‚L-1 (7). According
to Dossier Berne (8), free amino acid concentrations deter-
mined in several drinking water factories of western France
would vary between 0 and 30 µg‚L-1. It is well-known that
the reaction of aqueous chlorine with amino acids leads to
the formation of N-chloramines, nitriles, and aldehydes (9),
which are most often odorous products and can cause some
odor problems in distributed water (10). Indeed, amino acid
chlorination is always a subject of great concern and has
received attention for different reasons: chloramines are
believed to be toxic to some fishes (11, 12) and are also
strongly suspected of interfering with the disinfectant
concentration measurements in water (13-15).
Several studies (16-20) presented a new scheme for the
pathway of amino acids chlorination. They demonstrated
that N-chloroaldimines can be produced during this reaction
but no information was available for their odor properties.
The aim of this work was to identify the byproducts formed
during the chlorination reaction of amino acids, which are
suspected of being odorous. It has been known for a long
time that aldehydes have weak odor detection thresholds,
closed to µg‚L-1 concentration (21). The kind of odor
presented by these aldehydes is related to their molecular
weight: low-molecular-weight aldehydes present unpleasant
odors and high-molecular-weight aldehydes present floral
or fruity odors (22). Our study focused on the chloroaldimine,
for which no odor description was available in the literature
but which rapidly appeared odorous at low concentrations.
In this way, the amino acid chlorination reaction was studied
to determine the odor-producing potential of this product
in the drinking water industry.
Introduction
Drinking water disinfection is necessary for eliminating
pathogenic organisms. Indeed, it must be without adverse
effect on any form of life. Chlorine is one of the most
important products used for chemical disinfection of drinking
water because it is effective, cheap, and readily available.
Moreover, it is one of the only disinfectants to have retentive
* Corresponding author phone: (33) 223238048; fax: (33)
223238120; e-mail: ingrid.freuze@ensc-rennes.fr.
† Ecole Nationale Supe´rieure de Chimie de Rennes.
‡ Veolia Water.
§
Syndicat des Eaux d’Ile de France (SEDIF).
9
4 1 3 4 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 15, 2004
10.1021/es035021i CCC: $27.50
2004 Am erican Chem ical Society
Published on Web 06/22/2004

10% A/ 90% B over 15 min and a 5-min isocratic elution with
10% A/ 90% B. These conditions were determined by Nweke
and Scully (16).
Chlorination and Identification of the Byproducts
Form ed. Chlorinations of phenylalanine were first conducted
in both phosphate buffer (pH 7) and water for Cl/ N molar
ratios of 1 and 2. Results obtained were similar. The same
final products were formed and reached the same levels of
concentration. So all experiments were then carried out in
water to simplify the procedure. These conditions were also
chosen to ensure that the pH conditions were quite similar
to industrial conditions. Model solutions of phenylalanine
(50 mL, 10-3 mol‚L-1) were chlorinated to different chlorine
to nitrogen molar ratios by addition of micro volumes of the
hypochlorite solution, incubated in the dark at room tem-
perature, and regularly analyzed by HPLC-UV. Cl/ N molar
ratios of 0.5, 1, 1.5, and 2 will be presented here. All the
compounds formed were collected with the fraction collector.
Once the compounds were isolated, their flavor character-
istics were determined and their oxidizing ability was tested
to determine if they were N-chlorinated products: fractions
obtained were mixed with various quantities of DPD,
phosphate buffer (0.5 mol‚L-1, pH 6.3), and potassium iodide
(24).
The N-chlorophenylacetaldimine was synthesized. A
sodium hypochlorite solution (50 mL, Cl/ N molar ratio of
2.4, providing the best conditions for chloroaldimine forma-
tion, as indicated below) was cooled in an ice bath, and
phenylalanine (0.01 mol‚L-1, 1.65 g) was added while the
mixture was stirred. After 10 min, the reaction mixture, which
became cloudy, was extracted with 3 mL of CDCl3, dried over
anhydrous Na2SO4, and analyzed by both NMR and HPLC.
The stability of chloroaldimine in various solvents was studied
to establish an extraction method for this product and
furthermore to quantify it. A solution of phenylalanine (50
mL, 0.2 mol‚L-1) was chlorinated with a Cl/ N molar ratio of
2.8 and fractionated. Solvent (100 mL ofwater, hexane,
methanol, acetonitrile, or dichloromethane) was added to
10 mL of the reaction mixture. After 24 h, the solution was
analyzed by HPLC (injection of the organic phase for hexane
and dichloromethane).
Because N-chlorophenylacetaldimine is not commercially
available, a quantification method was investigated. Syn-
theses with different initial concentrations in phenylalanine
(0.01, 0.02, 0.05, 0.1, and 0.2 mol‚L-1) were realized, with a
Cl/ N molar ratio of 2.4. Once the reaction stabilized, the
solutions were shared out into two equal parts and each of
them was percolated on a SepPak Cartridge C18, washed
with equal quantities of water, and eluted with the same
volume of solvent (5 mL approximately), hexane for one
sample and acetonitrile for the other. To confirm the validity
of the extraction protocol, the one in acetonitrile was injected
immediately after concentration: it showed exactly the same
quantity of aldehyde, nitrile, and chloroaldimine as the
hexane extract.
The synthesis of imine was realized by an organic way.
Phenylacetaldehyde (46 mg, 0.38 × 10-3 mol‚L-1) was
introduced into 50 mL of anhydrous methanol. This solution
was added to a solution of 5 equiv of ammonium chloride
(NH4Cl, 95 mg) in an additional 50 mL of methanol. Molecular
sieves were used to absorb the water formed during the
reaction. The solution was mixed with a magnetic stirrer for
1 h, then the reaction mixture was injected on HPLC.
Olfaction Tests. The olfaction tests were realized with
standard compounds for aldehyde and nitrile and with
chlorinated amino acid solutions (Cl/ N 2.4) for the chloro-
aldimine. In these conditions, N-chloroaldimine concentra-
tion was considered to be 35% of the initial amino acid
concentration. The following laboratory-made protocol was
employed. First, all the glass was washed with an odorless
FIGURE 1. Simplified scheme for chlorination of phenylalanine,
adaptedfrom Conyers and Scully (18). Principal byproducts formed.
Even if it is not the most abundant amino acid in water,
phenylalanine was chosen for its UV-absorbing properties.
This enabled us to follow the chlorination reactions and the
appearance of byproducts easily. Moreover, this product has
only one nitrogen atom, which restricts its reactivity with
chlorine and thus the complexity of the reaction mixtures
obtained. Various Cl/ N molar ratios and various reaction
times were studied (Figure 1). Other amino acids will be
studied to extend the results.
Materials and Methods
Reagents and Apparatus. D,L-Phenylalanine (99%) was
obtained from Jansen Chimica, phenylacetonitrile (98-99%)
was from EGA Chemie, and phenylacetaldehyde (90%) was
purchased from Aldrich. The sodium hypochlorite solution
(2 mol‚L-1 minimum) was purchased from Prolabo and its
concentration in active chlorine was regularly checked by
thiosulfate titration (23) to record the decay rate of the
solution in consideration. All the buffers and solutions were
prepared in demineralized Millipore water with no chlorine
demand (24). The free and combined chlorine were deter-
mined by reaction with N,N-diethyl-p-phenylenediamine
(DPD) in absence or presence of iodide (24). Indeed, DPD
gives a specific pink color in the presence of chlorine, the
intensity of which can be measured by spectrophotometry
(λ ) 515 nm). Addition of potassium iodide crystals is
necessary to reveal combined chlorine but this method
cannot differentiate organic and mineral chloramines.
Dechlorination was realized by addition of 3 equiv (with
regards to the quantity of chlorine introduced) of sodium
thiosulfate 0.1 mol‚L-1 and 15 min of good stirring.
Measurements of the optical density for the determination
of free and combined chlorine with DPD titration were made
with a spectrofluorimeter Seconam S.750. This titration was
available for chlorine concentrations between 0.05 and 4 mg
Cl2‚L-1. The equation of the curve was OD (optical density)
) 0.1806C (C was the chlorine concentration in mg Cl2‚L-1
)
with a correlation coefficient of 0.9939. Nuclear magnetic
resonance (NMR) data were obtained with a Brucker FT ARX
(1H 400.13 MHz and 13C 100.61 MHz). HPLC was performed
using an Alliance Waters 2690 separation module with a
Waters 2487 UV detector (λ ) 254 nm). A Waters fraction
collector II was added at the outlet of the HPLC apparatus.
The majority of the analyses were first performed after 30
min of contact time and then at various intervals during the
week with an injection volume of 250 µL. Separations were
carried out on a C18 Symmetry column (4.6 × 250 mm)
maintained at 30 °C, with a dual solvent system. Solvent A
was 90% water (pH 4 adjusted with acetic acid) and 10%
acetonitrile and solvent B was 90% acetonitrile and 10% water
(pH 4 adjusted with acetic acid). The solvent program (flow
rate of 1 mL‚min-1) consisted of a 5-min isocratic elution
with 85% A/ 15% B followed by a linear gradient to 55% A/ 45%
B over 20 min. Then a second linear gradient was applied to
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FIGURE 2. High-performance liquid chromatogram on C18 column with UV detection of a solution of phenylalanine (1 mM) Cl/N ) 1.5
after 30 min chlorination. For chlorination rates close to 1 and for reaction times of several hours, another product was observed with
a retention time of 39 min.
TABLE 1. Stable Levels Reached by Phenylalanine,
Phenylacetaldehyde, and Phenylacetonitrile for Differe-n1t Cl/N
Molar Ratios of a Phenylalanine Solution (10-3 mol‚L ) at
22 °C
Cl/N ) 0.5 Cl/N ) 1 Cl/N ) 1.5 Cl/N ) 2
[Phe] (10-3m ol‚L-1
[Ald] (10-3m ol‚L-1
[Nit] (10-3m ol‚L-1
)
)
0.5
0.42
undetected
<0.05
0.88
0.02
<0.05
0.45
0.18
<0.05
0.14
0.45
)
detergent, rinsed 10× with hot tap water, and then rinsed 3×
with unodorous water. Solutions at various concentrations
ranging from 10-10 mol‚L-1 to 10-3 mol‚L-1 in each product
were prepared and one blank of the water used for the dilution
was added. Scent-bottles (250 mL) were totally filled with
each solution and incubated for 1 h at 45 °C. Heated solutions
were then placed in Erlenmeyer flasks and presented
randomly to a panel of testers who had to indicate if they
detected an odor or not. The panel was composed of 4 men
and 3 women, randomly chosen, from 22 to 45 years old.
First, solutions of a wide range of concentrations were used
in order to situate the limit. Then the value was stated
precisely by presenting several times the solutions of
concentrations closest to the pre-estimated limit. The test
took place in a well ventilated place. The odor detection
threshold was determined when the average of positive
answers was lower than 50%.
FIGURE 3. Evolution over time ofN-chlorophenylalanine for different
levels of chlorination of a model solution of phenylalanine (1 mM)
at 25 °C.
FIGURE 4. Evolution over time of N-chlorophenylacetaldimine for
different levels of chlorination of a model solution of phenylalanine
(1 mM) at 25 °C.
Results and Discussion
Identification of the Products Form ed. Samples of phenyl-
alanine (10-3 mol‚L-1) chlorinated at different levels were
periodically injected (250 µL) during a week. The results
obtained were on the whole in agreement with Conyers and
Scully’s proposed scheme of pathways (18). The chromato-
gram of a solution chlorinated to a Cl/ N ratio of 1.5 is shown
in Figure 2, as an example of the results obtained. The
compounds that eluted with retention times of 4.8 and 35
min possessed oxidizing ability that indicated they were
chlorinated products. The compounds with retention times
of 4.8, 19, 23, and 35 min presented odors of respectively
chlorine/ swimming-pool, flowers, “white glue”, and swim-
ming-pool/ flowers. Reference materials were used to identify
and quantify phenylalanine (4.16 min), phenylacetaldehyde
(19 min), and phenylacetonitrile (23 min). Indeed, standards
chromatographed under identical analysis conditions allowed
us to establish calibration curves for a concentration range
between 0.05 × 10-3 and 10-3 mol‚L-1. Whatever the
chlorination rate was, phenylalanine (Phe), -acetaldehyde
(Ald), and -acetonitrile (Nit) levels reached a plateau in about
4 h (Table 1).
The compound with reaction time of 4.8 min appeared
in a broad peak; it was formed quasi instantaneously for the
majority of chlorination rates close to 1 and then it quickly
decreased with time and had totally disappeared after 10 h
(Figure 3). It showed oxidizing ability and was transformed
into phenylalanine by the addition of thiosulfate. The peak
was, for all these reasons, attributed to N-chlorophenyl-
alanine. However, in contrast with findings of previous
authors, it should be emphasized that the N-chlorophenyl-
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FIGURE 5. Parts of the proton NMR spectrum of N-chlorophenylacetaldimine in CDCl3 contaminated by phenylacetonitrile.
alanine was observed even for a chlorination ratio close to
2, yet to a lesser extent and present only during the first 2
h after chlorination. We noticed, in agreement with the
literature, that dichloramine (which should have been
revealed by a different reaction with DPD) was never
observed. Indeed, this observation might be partially ex-
plained by a very quick degradation of the compound, thus
we were unable to analyze it.
The compound with retention time of 35 min increased
for a chlorination rate greater than 1 (it appeared for Cl/ N
molar ratio of 1 and then, the higher the chlorination rate,
the higher the quantity) and decreased with time (Figure 4).
FIGURE 6. Evolution over time of peak with retention time of 39 min
The greatest yields were observed for chlorination rates
thought to be imine or derivative for different levels of chlorination
of a model solution of phenylalanine (1 mM) at 25 °C.
greater than 2. It also possessed an oxidizing ability. It was
believed to be N-chlorophenylacetaldimine, and this struc-
ture was proved by synthesis immediately followed by NMR
known concentration was used as a standard. It was diluted
analysis. The proton spectrum (Figure 5) is consistent with
to determine the analytical detection limit of the product,
a mixture of the two stereoisomers of N-chlorophenyl-
which was found to be 5 × 10-5 mol‚L-1 by this method. This
acetaldimine, anti and syn, contaminated by phenylaceto-
quantification method gave us the same proportions of
nitrile (40/ 20/ 40). This has already been described elsewhere
chloroaldimine formed during the chlorination reactions as
those of Conyers and Scully (18) based on the use of
radioactive chlorine. This enforced the validity of our titration
(18).
RMN1H (CDCl3): 8.28 (t, 0.66 H, HCdN, J ) 6.36 Hz), 8.10
(t, 0.33 H, HCdN, J ) 4.56 Hz), 7.37 (m, 5H, C6H5), 3.93 (d,
0.66H, CH2CdN), 3.74 (d, 1.32H, CH2CdN).
RMN13C (CDCl3): 175 (CdNCl), 173 (CdNCl), 133-127
(aromatic C), 41 (CH2), 40 (CH2).
method. Previously it was believed that N-chloraldimines
were precursors of nitriles (9). Our results suggest, in
agreement with Conyers and Scully (18), that the nitrile and
the chloraldimine are formed by competitive pathways.
Indeed, nitrile was always and only present in water with
chloroaldimine but did not increase when chloroaldimine
decreased.
The compound with a reaction time of 39 min appeared
when N-chlorophenylacetaldimine was treated by a large
excess of thiosulfate, but the reaction was not instantaneous.
This was observed also for chlorination levels close to 1 as
well as the N-chlorophenylalanine but appeared later (Figure
6). It revealed a lot in UV analysis that is consistent with the
presence of a double bond. At first sight it was thus believed
to be the imine. This hypothesis was supported by the fact
that the action of ammonia on phenylacetaldehyde leads to
The chloroaldimine was previously thought to be unstable
in all the organic solvents. Indeed it was entirely decayed in
only aldehyde and nitrile in acetonitrile, but it remained stable
in hexane. This allowed us afterward to relate the peak area
of chloroaldimine, observed for the hexane-stable fraction,
and the concentrations of aldehyde and nitrile obtained for
the acetonitrile fraction injected after 2 h: chloroaldimine
concentrations were deduced from mass balance. A calibra-
tion curve was obtained (area (UA) ) 5.63 × 106 C (10-3
mol‚L-1)) in the range 0.005 to 0.1 mol‚L-1 with a correlation
coefficient of 0.9976 for an injection volume of 250 µL. For
the lower concentrations, one of the previous solutions of
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FIGURE 8. Decay rates of N-chlorophenylacetaldimine for 2 different
temperatures.
3.2 × 10-6 s-1 for the chloroaldimine and 2.1 × 10-4 s-1 for
the monochlorinated derivative. For the same temperature,
the latter decayed 60 times quicker than the chloroaldimine.
Consequently, it must be stressed that if the N-chlorophen-
ylalanine is not stable enough to be involved in water odor
problems, the N-chlorophenylacetaldimine has a sufficient
lifetime to remain several days in the distribution network.
The odor detection thresholds determined with our
olfaction test for phenylacetaldehyde, phenylacetonitrile, and
N-chlorophenylacetaldimine were respectively 30, 1200, and
3 µg‚L-1. It should be emphasized that not only the well-
known aldehyde, but above all the chloroaldimine, for which
no odor characteristic was available, exhibited really low odor
detection thresholds in water. According to amino acid
concentrations in drinking water process, these products
could be involved in odor problems. In contrast, the nitrile
cannot be implicated.
Moreover, Buttery and co-workers (25) announced an odor
detection threshold for phenylacetaldehyde of 4 µg‚L-1 which
led us to suppose that with their protocol the chloroaldimine
could be detected at even lower concentrations. Chloro-
aldimine decay for 2 temperatures (15 and 20 °C) were also
performed (Figure 8). The decomposition kinetics were still
first-order reactions. The half longer lifetimes observed at 15
and 20 °C were respectively about 500 and 50 h. These results
highlight a very significant difference of stability for a variation
in temperature of only 5 °C. No data are available to confirm
our results, yet in winter chloroaldimine may remain in the
distribution network for a long time. This study will now be
extended to other amino acids, such as alanine, leucine, or
valine. They are susceptible to be present in water at higher
concentrations than phenylalanine, and as they have lower
molecular weight, their chloroaldimines will probably be even
more odorous than N-chlorophenylacetaldimine.
FIGURE 7. (a) N-chlorophenylalanine kinetics of decomposition at
22 °C for various chlorination rates. (b) N-chlorophenylacetaldimine
kinetics of decomposition at 22 °C for various chlorination rates.
the formation of a compound with the same reaction time.
Indeed, it is widely known that the action of ammonium
chloride on aldehyde classically leads to the formation of an
imine. In this way a reaction of ammonium chloride on
phenylacetaldehyde was carried out. A regular analysis of
the reaction mixture showed a decrease in the quantity of
aldehyde while the resulting product had a retention time
of 39 min. This partially confirmed our assumption, nev-
ertheless we came up against an impossibility: as the reaction
is a mole per mole one, the amount of disappeared aldehyde
should be equal to the amount of alleged imine present.
However, the detector response attributed to imine was
unsuitable. We tried to use it to quantify the imine in the
chlorination reactions but the quantity of alleged imine
obtained was highly overestimated with regard to the
amounts of introduced phenylalanine and identified and
quantified byproducts. For these reasons, the 39-min com-
pound may be now assumed to be a polymer of imine. Further
studies are needed to address these issues. This compound
concentration seemed to reach a plateau for all the chlori-
nation rates. This partially questions the scheme of pathway
proposed (18) where the degradation of imine in aldehyde
is announced as quantitative. These results suggest that there
may be an equilibrium. However, by considering the total
amount of quantifiable products obtained, the quantity which
could be attributed to the imine or derivative cannot be very
large. For Cl/ N ratio of 1 the imine reached is maximum area
while the total of unreacted phenylalanine, aldehyde, and
nitrile was more than 95% of the initial phenylalanine.
Moreover, this compound is not odorous, so we did not follow
our investigations on it.
Acknowledgments
We are grateful to the Syndicat des Eaux d’Ile-de-France
(SEDIF) for financially supporting this project.
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Received for review September 16, 2003. Revised manuscript
received March 17, 2004. Accepted May 18, 2004.
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