Chlorpyrifos transformation by aqueous chlorine in the presence of bromide and natural organic matter

Article abstract of DOI:10.1021/jf072468s

The aqueous chlorination of chlorpyrifos (CP) was investigated in the presence of bromide and natural organic matter (NOM), which were identified as naturally occurring aqueous constituents that could affect CP transformation rates to the toxic product chlorpyrifos oxon (CPO). Bromide can be oxidized by chlorine to form hypobromous acid (HOBr), which was found to oxidize CP at a rate that was 3 orders of magnitude faster than was the case with chlorine: k_HOBr,CP = 1.14 (± 0.21) × 10⁹ M^-1 h^-1 and k_HOCl,CP = 1.72 × 10⁶ M ^-1 h^-1, respectively. Similar to previous findings with the hypochlorite ion, hypobromite (OBr^-) was found to accelerate the hydrolysis of CP and CPO: k_OBr,CP = 965 (± 110) M^-1 h^-1 and k_OBr,CPO = 1390 (± 160) M^-1 h^-1, respectively. Treated water from the Athens-Clarke County (ACC) water treatment plant in Athens, GA, was used in some of the experiments as a NOM source. A mechanistic model was used to adequately predict the loss of CP as well as the formation of CPO and the hydrolysis product 3,5,6-trichloro-2- pyridinol (TCP) in the presence of the ACC water.

Full text of DOI:10.1021/jf072468s

1328 J. Agric. Food Chem. 2008, 56, 1328–1335
Chlorpyrifos Transformation by Aqueous Chlorine in
the Presence of Bromide and Natural Organic Matter
†
STEPHEN E. DUIRK,* J. CHRISTOPHER TARR, AND TIMOTHY W. COLLETTE
Office of Research and Development, National Exposure Research Laboratory, U.S. Environmental
Protection Agency, 960 College Station Road, Athens, Georgia 30605
The aqueous chlorination of chlorpyrifos (CP) was investigated in the presence of bromide and natural
organic matter (NOM), which were identified as naturally occurring aqueous constituents that could
affect CP transformation rates to the toxic product chlorpyrifos oxon (CPO). Bromide can be oxidized
by chlorine to form hypobromous acid (HOBr), which was found to oxidize CP at a rate that was 3
9
-1 -1
orders of magnitude faster than was the case with chlorine: kHOBr,CP ) 1.14 (( 0.21) × 10 M
h
6
-1 -1
and kHOCl,CP ) 1.72 × 10 M
h
, respectively. Similar to previous findings with the hypochlorite
-
ion, hypobromite (OBr ) was found to accelerate the hydrolysis of CP and CPO: kOBr,CP ) 965 ((
10) M-
1
h
-1
and kOBr,CPO ) 1390 (( 160) M
-1 -1
h
, respectively. Treated water from the Athens-
1
Clarke County (ACC) water treatment plant in Athens, GA, was used in some of the experiments as
a NOM source. A mechanistic model was used to adequately predict the loss of CP as well as the
formation of CPO and the hydrolysis product 3,5,6-trichloro-2-pyridinol (TCP) in the presence of the
ACC water.
KEYWORDS: Chlorpyrifos; organophosphorus (OP) pesticides; chlorination; drinking water treatment;
modeling; reaction pathways
INTRODUCTION
curring aqueous constituents such as bromide ion and natural
organic matter (NOM) may affect the transformation rates for
OP pesticides during drinking water treatment.
The Food Quality Protection Act (FQPA) of 1996 requires that
all potential exposure routes be assessed to determine whether
pesticide tolerances will be exceeded. Drinking water is considered
to be a dietary exposure pathway; however, few studies have
examined the fate of pesticides in the presence of free chlorine
under conditions similar to drinking water treatment. Chlorination
is the most commonly used chemical disinfection process for
community water systems (1), and chlorine is known to react with
some pesticides such as s-triazines, carbamates (2–6), and orga-
nophosphorus (OP) pesticides (7–9).
When chlorine reacts with the phosphorothioate subgroup of
OP pesticides, the thiophosphate functionality (PdS) can
be oxidized to its corresponding oxon (PdO) (7–9). The
resulting oxons are typically more potent than the parent as an
inhibitor of acetylcholinesterase, an enzyme necessary for
regulating nerve impulse transmission between nerve fibers (9).
Duirk and Collette (10) elucidated the fate of chlorpyrifos (CP)
and its transformation products over the pH range of 6.3–11 in
the presence of aqueous chlorine. They were able to model the
rapid oxidation of CP by HOCl, resulting in the more toxic
chlorpyrifos oxon (CPO) and hydrolysis of CP and CPO to
Bromide can be easily oxidized to hypobromous acid (HOBr),
which is a stronger oxidant than HOCl, in the presence of
+
aqueous chlorine (11). Upon initial reaction, Cl is transferred
-
to Br , resulting in the formation of bromine chloride (BrCl),
which rapidly hydrolyzes to HOBr (12).
-
-
HOCl + Br h BrCl + OH
-5
K ) 3.0 × 10
(1)
(2)
1
-
+
BrCl + H O h HOBr +Cl + H
2
-4
2
K ) 1.3 × 10
M
2
Under drinking water treatment conditions, the equilibria in
eqs 1 and 2 favors the forward reactions resulting in the overall
stoichiometry and second-order rate coefficient (11).
-
-
HOCl + Br f HOBr + Cl
kHOCl,Br ) 5.6 × 10 M h-1 (3)
6
-1
In the presence of excess chlorine, all bromide present will
be oxidized to hypobromous acid (13). Because the pKa of
hypobromous acid is 8.8 (11), HOBr will be the dominant
brominated oxidant over the drinking water pH range of 6.5–9.
3
,5,6-trichloro-2-pyridinol (TCP), as well as hypochlorite-
assisted hydrolysis of CP and CPO to TCP over this pH range
in buffered deionized water systems. However, naturally oc-
-
+
HOBr +H O h OBr + H O
pK ) 8.8
(4)
2
3
a
*
Corresponding author [e-mail duirk.stephen@epa.gov; telephone
(
706) 355-8206; fax (706) 355-8202].
Therefore, it is believed that the presence of bromide under
drinking water treatment conditions could accelerate the trans-
†
Student Services Authority.
10.1021/jf072468s This article not subject to U.S. Copyright. Published 2008 by the American Chemical Society
Published on Web 02/01/2008

Chlorpyrifos Transformation by Aqueous Chlorine
J. Agric. Food Chem., Vol. 56, No. 4, 2008 1329
formation rate of anthropogenic chemicals. For example, the
rate coefficient for HOBr reacting with pyrene was found to be
Table 1. NOM and Source Water Characteristics for the ACC Water
Collected prior to Chlorination
2
orders of magnitude faster than that for HOCl in aqueous
SUVA254 SUVA280 [NH3]T [Cl ] [NO2 ] [Br ] [PO4]T [SO42-
a
-
-
-
b
]
solutions (14). Westerhoff et al. (15) conducted a nationwide
survey of 101 drinking water sources in the United States and
found an average bromide concentration of ≈1.25 µM. Because
bromide is relatively ubiquitous and 90% of community drinking
water systems use chlorine for both primary and secondary
disinfection, the presence of bromide in drinking water sources
could affect the rate of organophosphorus (OP) pesticide
transformation in chlorinated potable water.
source [L/(m mg)] [L/(m mg)] (µM) (µM) (µM) (µM) (µM)
(µM)
c
ACC
1.61
1.07
2.06 93.46 0.04 0.15 ND
8.30
a
+
b
-
2-
[
NH3]T ) [NH4 ] + [NH3]. [PO4]T ) [H3PO4] + [H2PO4 ] + [HPO4 ] +
PO4 ]. Not detected.
3-
c
[
prerinsed with deionized water. The pH for the experiments was
adjusted with either 1 N H SO or NaOH. All other organic and
2
4
The presence of NOM in drinking water sources has also
been a primary concern for the drinking water industry. The
physical-chemical characteristics of NOM are a function of
decaying organic matter from both terrestrial and aquatic
sources, environmental conditions, and microorganisms present
in the watershed (16). Hence, the composition and concentration
of NOM in source waters fluctuates from source to source and
often varies within each source depending on conditions such
as rainfall, change in seasons, and temperature. When chlorine
is added to treated water containing bromide and NOM, both
brominated and chlorinated oxidants react with NOM, resulting
in the formation of disinfection byproducts (DBPs) (17, 18).
Also, the brominated DBPs are believed to be more carcinogenic
than their chlorinated analogues (19). The electron density (i.e.,
aromaticity) of NOM is often expressed as specific ultraviolet
absorbance (SUVA), which is the ratio of ultraviolet absorbance
of the NOM at a specific wavelength divided by the total organic
carbon concentration (TOC) in milligrams of C per liter. This
specific physical-chemical property has been found to correlate
to reactivity of NOM with free chlorine and serve as a reliable
surrogate for DBP formation (20, 21). Because both chlorinated
and brominated oxidants react with NOM (22), the presence of
NOM can potentially act as a sink for active oxidants in finished
drinking water. Thus, its presence can impact the rate of OP
pesticide transformation under drinking water treatment conditions.
The influence of bromide and NOM was investigated to
determine their effect on the transformation of CP by aqueous
chlorine. The influence of bromide was investigated as a function
of concentration and pH of the aqueous system at chlorine
concentrations representative of drinking water treatment. To
further elucidate transformation pathways, native hypobromous
acid was used to determine intrinsic rate coefficients for the
oxidation of CP to CPO as well as hypobromite-assisted
hydrolysis of CP and CPO to TCP. Treated effluent from the
Athens-Clarke County (ACC) water treatment plant in Athens,
GA, was collected prior to chlorination and used in kinetic
experiments. Experimental results were then compared to
predictions from an existing numerical model that did not
account for the influence of either NOM or bromide (10). The
work reported here serves to further elucidate the OP transfor-
mation pathways under drinking water treatment conditions.
inorganic chemicals were certified ACS reagent grade and used without
further purification. The glassware and polytetrafluoroethylene (PTFE)
septa used in this study were soaked in a concentrated free chlorine
solution for 24 h, rinsed with copious amounts of deionized water, and
dried at 105 °C prior to use. All chlorination experiments were
conducted at constant temperature (25 ( 1 °C).
Water was collected from the Athens-Clarke County (ACC) water
treatment plant in Athens, GA, and used in the experiments investi-
gating the effect of NOM on CP degradation kinetics. The ACC water
treatment facility uses conventional physical-chemical surface water
treatment (i.e., coagulation, flocculation, sedimentation, and dual media
filtration). Water was collected prior to chlorination and was filtered
through a 0.45 µM filter. The ACC water and NOM characteristics are
shown in Table 1. The SUVA254 of the ACC effluent was 1.61 L/(mg
m), which indicates a low aquatic humic substance concentration (20).
For all oxidation experiments, CP was spiked by adding 0.5 mL of
a 4 mM stock solution in ethyl acetate into an empty 4 L borosilicate
glass Erlenmeyer flask. A gentle flow of nitrogen gas was used to
evaporate the ethyl acetate, and then 4 L of deionized or ACC water
was added to the flask. The solution was slowly stirred at a pH of
6
.5–7 and allowed to dissolve for 12 h resulting in an aqueous CP
concentration of 0.5 µM, and hydrolysis of CP over this pH range was
negligible (23). When called for, bromide was added to the aqueous
system prior to chlorination. CP oxidation kinetic experiments were
conducted under pseudo-first-order conditions: total chlorine to chlo-
rpyrifos molar ratios of 20:1, 50:1, 100:1, and 200:1. Chlorine or
bromine was added to solutions under rapid mix conditions achieved
with a magnetic stir plate and a PTFE-coated stir bar. All kinetic
experiments used to determine rate coefficients were conducted until
8
7% loss of parent compound was observed.
Above pH 8, 10 mM carbonate [CO buffer was used to maintain
3 T
]
pH. The commercial free chlorine solution was first diluted to 14 mM
and then added to the aqueous system containing 0.5 µM chlorpyrifos
and carbonate buffer in a 2 L Erlenmeyer flask. Fourteen aliquots from
the large 2 L reactor were then placed into 128 mL amber reaction
vessels with PTFE septa and stored in the dark. In the pH range of
6.5–7.5, the rate of CP loss in the presence of free chlorine was very
rapid. Therefore, 14 100-mL aliquots of the 2 L aqueous system
4 T
containing 10 mM phosphate buffer, [PO ] , and 0.5 µM CP were
placed in 250 mL amber Erlenmeyer flasks. Then each flask was
individually dosed with the appropriate amount of chlorine under rapid
mix conditions. This type of reactor has been successfully shown to
work for investigating halogenated oxidants reacting with aqueous
constituents when the reaction is not diffusion-controlled (24).
At each discrete sampling interval, two reaction vessels were
sacrificed in their entirety. One vessel was used to determine total free
-
chlorine concentration ([HOCl]
T
) [HOCl] +[OCl ]) via Standard
MATERIALS AND METHODS
Method 4500-Cl F DPD-FAS titrimetric method (25). In the other
vessel, free chlorine residuals were quenched with sodium sulfite in
20% excess of the initial free chlorine concentration and the pH of a
100 mL sample was then adjusted to 2 for analysis of CP and its
degradation products. Holding studies have been preformed previously,
and sulfite was found not to interact with CP or its transformation
products (10).
CP and its degradation products were extracted from water using
C-18 solid phase extraction cartridges purchased from Supleco (Belle-
fonte, PA). The 100 mL sample was adjusted to pH e2 to increase the
Chlorpyrifos (99.5%), chlorpyrifos oxon (98.7%), and 3,5,6-trichloro-
2
-pyridinol (99%) were purchased from ChemService (West Chester,
PA). Commercial 10–13% sodium hypochlorite (NaOCl) was purchased
from Aldrich (Milwaukee, WI). Saturated bromine water, which was
approximately 1.3% active bromine, was purchased from Fisher
Scientific (Pittsburgh, PA). Aqueous stock solutions and experiments
utilized laboratory-prepared deionized water (18 MΩ cm- ) from a
Barnstead ROPure Infinity/NANOPure system (Barnstead-Thermolyne
Corp., Dubuque, IA). Phosphate and carbonate salts used for buffer
solutions were dissolved in deionized water and filtered through a 0.45
µM filter purchased from Millipore (Billerica, MA), which was
1
recovery of TCP (pK
the sample was spiked with 1 µM phenthorate (internal standard), mixed
a
) 4.55) on the solid phase adsorbent (26). Then,

1330 J. Agric. Food Chem., Vol. 56, No. 4, 2008
Duirk et al.
Figure 1. Schematic of CP transformation pathways in the presence of
aqueous chlorine and bromide. HOX represents either HOCl or HOBr,
-
-
-
and X can represent either Cl or Br .
Figure 2. Observed first-order rate coefficients for CP loss in the presence
of increasing bromide and [HOCl]
thoroughly by hand for 2 min, passed through the SPE cartridge at an
approximate flow rate of 7 mL/min, and eluted with 3 mL of ethyl
acetate. Quantification for each analyte was compared to eight extracted
standards over the concentration range of 0.01-1 µM. A Hewlett-
Packard 6890 GC equipped with a 5973 MSD was used to analyze
CP, CPO, and TCP. GC conditions were as follows: 30 m Restek Rtx-
T
) 10 µM over the pH range of 7–9.
[CP] ) 0.5 µM, [buffer] ) 10 mM, and temperature ) 25 °C.
0
T
2
00 column with a 0.25 mm i.d. and 0.5 µm film thickness. The
temperature profile was as follows: 100 °C for 5 min, raised from 100
to 250 at 10 °C/min, and then held at 250 °C for 25 min. Mass balances
of 80% or greater were obtained for each experiment.
UV–vis spectra were acquired with a Shimadzu 1700 UV–vis spec-
trometer (Shimadzu Scientific, Columbia, MD). Total organic carbon
(TOC) was measured using a Shimadzu TOC 5000 (Shimadzu Scientific)
and calibrated according to Standard Method 505A (25). Ion chromatog-
raphy was performed on a Metrohm (Houston, TX) MIC-2 ion chromato-
+
graph with chemical suppression. Total ammonia ([NH
NH
]
3 T
) [NH
4
] +
[
3
]), chloride, nitrite, bromide, phosphate, and sulfate were all calibrated
over the concentration range from 0 to 100 µM. All pH measurements
were obtained with an Orion 940 pH-meter using a Ross combination
electrode from Fisher Scientific (Pittsburgh, PA).
RESULTS AND DISCUSSION
Figure 3. Observed first-order rate coefficients for CP loss in the presence
of increasing bromide and [HOCl] ) 50 µM at pH 8 and 9. [CP] ) 0.5
µM, [CO ) 10 mM, and temperature ) 25 °C.
Loss of CP in the Presence of Bromide and Aqueous
Chlorine. The transformation pathways of CP in the presence
of aqueous chlorine and bromide are relatively complicated
T
0
3 T
]
(
Figure 1). The known transformation pathways are (1) HOCl
bromide for oxidation by HOCl. In the presence of excess
chlorine, all bromide present would be oxidized to its active
form (HOBr) (13). Therefore, the oxidation of CP is most likely
carried out by both HOCl and HOBr at low bromide concentra-
tions. As bromide concentrations increased to 5.0 and 10.0 µM,
HOCl is more likely to oxidize bromide than CP as chlorine to
bromide molar ratios approach 1. For example, at the 10 µM
chlorine concentration, addition of 10 µM of bromide induced
∼20- and ∼30-fold increases in kobs at pH 9 and 8, respectively,
compared to the absence of bromide. However, the addition of
10 µM bromide at the 50 µM chlorine increased the kobs only
∼4- and ∼10-fold at pH 9 and 8, respectively, when compared
to the CP loss rates in the absence of bromide. The reaction
-
oxidation of CP resulting in CPO formation, (2) OCl -assisted
hydrolysis of CP and CPO, and (3) hydrolysis of CP and CPO.
Figure 1 also shows the formation of HOBr and the proposed
pathways of brominated oxidants reacting with CP and CPO,
-
which are HOBr oxidation of CP and OBr assisted hydrolysis
of CP and CPO. Prior to the determination of the intrinsic rate
coefficients for these pathways, the influence of bromide on the
overall observed rate of CP loss was determined at bromide
concentrations relevant to drinking water treatment conditions.
Experiments to determine if bromide does accelerate the rate
of CP loss were conducted over the pH range of 7–9, [CP] )
0
.5 µM, [HOCl]T ) 10 and 50 µM, with increasing bromide
concentrations from 0 to 10 µM. At [HOCl]T ) 10 µM (Figure
), kobs was found to increase linearly as bromide concentrations
6
rate coefficient for HOCl with bromide (kHOCl,Br ) 5.6 × 10
-
1
-1
2
M
h ) is approximately 3 times faster than HOCl with CP
6
-1 -1
were increased over the pH range investigated. This was also
observed at the higher chlorine concentration of 50 µM (Figure
3
regression lines in Figures 2 and 3 vary in a systematic way as
a function of pH and bromide concentration. The rate of CP
loss appears to be dependent on the concentration of bromide,
chlorine, and pH. This indicates that bromide could be acting
as a catalyst via HOBr formation.
At lower bromide concentrations (0.1 and 1.0 µM) and
chlorine concentration of 10 µM, CP is competitive with
(kHOCl,CP ) 1.7 × 10
M
h ) (10, 11). When bromide
concentrations are 0.1–1.0 µM, HOCl is the primary oxidant
responsible for the loss of CP. However, when [HOCl]T/
[bromide] molar ratios approach unity, HOBr appears to become
the primary oxidant. Therefore, the magnitude of bromide-
catalyzed oxidation of CP is controlled by the ratio of HOCl to
bromide as well as pH.
). Furthermore, at a given chlorine dose, the slopes of the
HOBr formation is ultimately controlled by the pH of the
aqueous system. Because the pKa of HOCl is 7.5, the percent-
ages of free chlorine present as HOCl were approximately 75,

Chlorpyrifos Transformation by Aqueous Chlorine
J. Agric. Food Chem., Vol. 56, No. 4, 2008 1331
Figure 4. Observed first-order loss rates of CP over the pH range of
6.5–11 with either aqueous bromine or chlorine. [CP]
Figure 5. Second-order rate coefficient for the reaction between
0
) 0.5 µM, [oxidant]
T
hypobromous acid and CP at pH 6.5. [CP]
µM, [PO ) 10 mM, and temperature ) 25 ( 1 °C. Error bars represent
5% confidence intervals.
0
) 0.43 µM, [HOBr]
T
) 10
)
10 µM, [buffer] ) 10 mM, and temperature ) 25 ( 1 °C. The inset
T
4 T
]
shows the first-order observed rate coefficients for the controls and in
the presence of chlorine more clearly. Error bars represent 95% confidence
intervals.
9
2
3, and 3% at pH 7, 8, and 9, respectively. HOCl is the only
chlorine species under these experimental conditions capable
of oxidizing bromide to HOBr (eqs 1–3) (11, 12). The magnitude
of bromide-catalyzed oxidation of CP as a function of pH is
clearly demonstrated by examining the slopes of the regression
lines (Figures 2 and 3). At pH 7, HOCl is the dominant chlorine
species available to react with bromide. In the presence of
increasing bromide concentration, the increased HOBr formation
rate increases the observed rate of CP loss at pH 7. At pH 8
and 9, the slopes of the regression lines are significantly less
than at pH 7. This demonstrates that bromide-catalyzed CP
oxidation is dependent on the concentrations of bromide and
chlorine as well as the pH, which determines chlorine speciation.
Therefore, bromide is a catalyst in the oxidation of CP because
it increased the observed rate of CP loss and does not appear in
the overall stoichiometry of the reaction mechanism. Bromide
is first oxidized by HOCl, resulting in HOBr, which oxidizes
CP, yielding H and Br . The regenerated bromide ion can be
reoxidized by HOCl and participate in additional reactions with
CP (Figure 1).
Bromine Reactions with CP. In the presence of chlorine,
bromide was found to catalyze the loss of CP via the forma-
tion of HOBr. To further investigate this phenomenon, the
reaction of native bromine with CP was investigated over the
pH range of 6.5–11 in the presence of excess bromine (Figure
Figure 6. Second-order rate coefficient for bromine-assisted hydrolysis
of CPO at pH 11. [CPO] ) 0.5 µM, [HOBr] ) 0–250 µM, [buffer]
10 mM, and temperature ) 25 ( 1 °C. Error bars represent 95%
confidence intervals.
0
T
T
)
+
-
indicating that CP was also first-order, yielding an overall
second-order reaction for the loss of CP in the presence of
bromine, which was similar to the results for CP loss in the
presence of aqueous chlorine (10).
The intrinsic rate coefficient for HOBr reacting with CP was
determined at pH 6.5 (Figure 5). At this pH, approximately
99.5% of the active bromine present will be in the HOBr form.
The reaction of HOBr with CP was expected to be 1-3 orders
of magnitude faster than HOCl with CP. This behavior has been
observed by others examining the reactivity of HOCl and HOBr
with pyrene, NOM, and sulfite (14, 22, 27, 28). The reaction of
HOBr with CP was essentially complete after only 10 s. The
second-order rate coefficient for this reaction was determined
by nonlinear regression analysis using Sigma Plot version 8.0
(Point Richmond, CA). The kHOBr,CP was found to be 1.14 ((
4
). At pH 6.5, the first-order observed loss of CP was 3 orders
of magnitude greater in the presence of HOBr than for HOCl
under similar experimental conditions. Even at pH 8 and 9, the
observed rate of CP loss in the presence of bromine was still
significantly greater than with chlorine. Because the pKa of
HOBr is 8.8, approximately 86.3 and 38.7% of bromine would
be in the HOBr form at pH 8 and 9, respectively. At pH 10 and
1
1, bromine does not have as significant an effect on the kobs
because <0.06% of bromine is in the HOBr form. However, as
shown in Figure 4 HOBr reacts significantly more rapidly than
HOCl over the entire drinking water pH range of 6.5–9.
Similarly with chlorine, bromine was added at increasing
concentrations to determine the stoichiometry for both HOBr
and CP. HOBr was found to be first-order with respect to
bromine under pseudo-first-order conditions. When log(kobs) was
plotted versus log[HOBr]T at pH 11 and [HOBr]T ) 10, 25,
9
-1
-1
0.21) × 10
M
h
with 95% confidence intervals in
parentheses, which is 3 orders of magnitude greater than HOCl
6
-1 -1
oxidation of CP (kHOCl,CP ) 1.72 × 10 M
h ). Furthermore,
CPO was found to be the stable transformation product in the
presence of HOBr at pH 6.5. This indicates that no further
oxidation of CPO occurred over a period of 6 h.
-
Hypobromite (OBr ) was then investigated to determine if
5
0, and 100 µM, the slope of the line was approximately 1,
it reacted with either CP or CPO. Previous work found that

1332 J. Agric. Food Chem., Vol. 56, No. 4, 2008
Duirk et al.
a
Table 2. Stoichiometric Equations and Rate Coefficients Used in the Chlorpyrifos Degradation Pathway Model
reaction stoichiometry
kHOX,CP
rate/equilibrium coefficient (25 °C)
ref
9
-1
1
2
+
-
2-
kHOBr,CP ) 1.14 × 10 M h-1
this work
5
HOX + CP
8 CPO + H + X + SO4
kHOCl,CP ) 1.72 × 10 M h-1
6
-1
10
kh,CP
-
kh,CP ) kN,CP + kB,CP[OH ]
23
CP
8 TCP
kN,CP ) 3.72 × 10 h-1
-4
kB,CP ) 37.0 M h-1
-
1
kh,CPO
-
3
4
kh,CPO ) kN,CPO + kB,CPO[OH ]
10
CPO
8 TCP
kN,CPO ) 2.13 × 10 h-1
-3
-
1
-1
kB,CPO ) 230 M
h
kOX, CP
kOBr,CP ) 965 M h-1
-1
-
-
this work
CP + OX
8 TCP + X
kOCl,CP ) 990 M h-1
-
1
10
kOX, CPO
kOBr,CPO ) 1390 M-1h-1
5
6
-
-
this work
CPO + OX
HOX h H+ + OX-
8 TCP + X
kOCl,CPO ) 1340 M h-1
-
1
10
11
34
-
pKa ) 8.8 (HOBr/OBr )
-
pKa ) 7.5 (HOCl/OCl )
a
HOX represents either HOBr or HOCl; OX- represents either OBr- or OCl-.
-
hypochlorite (OCl ) acted as a supernucleophile accelerating
the hydrolysis rate of both CP and CPO (i.e., chlorine-assisted
hydrolysis) (10). Because CPO was found to be stable in the
presence of HOBr, bromine-assisted hydrolysis experiments
were conducted at pH 11, where >99% of bromine would be
in the OBr- form (Figure 6). The observed first-order rate of
CPO loss increased linearly as bromine concentrations increased
from 0 to 250 µM and the only product detected was TCP. The
slope of the regression line represents the intrinsic rate coef-
ficient for the bromine-assisted hydrolysis of CPO, kOBr,CPO )
-1
-1
1
390 ((160) M
h
with 95% confidence intervals in
parentheses.
To determine if bromine also accelerates the hydrolysis of
CP, the model previously developed for chlorine in buffered
aqueous solutions was adopted for bromine. Because the
oxidation of CP to CPO is still fast at pH values >9 due to the
pKa of HOBr being 8.8, the model accounts for the CP oxidation
by HOBr as well as CP and CPO hydrolysis pathways (eqs 5–8).
Table 2 shows the elemental stoichiometric equations used to
develop the following system of ordinary differential equations
Figure 7. TCP experimental and model results for the loss of CP in the
presence of bromine at pH 11. [CP] ) 0.35 µM, [CO ) 10 mM,
) 0-250 µM. Lines represent
0
3 T
]
temperature ) 25 ( 1 °C, and [HOBr]
T
(
ODEs). These ODEs represent a generic form of the model in
model results.
which either bromine or chlorine would be the active oxidant
in the system being investigated.
solver by Micromath (Salt Lake City, UT), was used to fit the
bromine-assisted hydrolysis rate coefficient of CP by hypobro-
mite (kOBr,CP). Scientist uses a modified Powell algorithm to
minimize the unweighted sum of the squares of the residual
error between the predicted and experimentally observed values
to estimate specific parameters in the model. Details of model
development and validation have previously been discussed (10).
To determine kOBr,CP, a pooled data set at pH 11 was collected
in which both CPO and TCP concentrations were used to
determine the hypobromite-assisted hydrolysis rate coefficient.
Figure 7 shows the experimental and model results fitting kOBr,CP
over the [HOBr]T concentration range of 0–250 µM. The loss
of CP proceeded more rapidly at pH 11 in the presence of
bromine than chlorine (10). Although approximately only 0.6%
of bromine present was in the HOBr form, up to one-third of
the initial CP concentration was transformed to CPO within the
first hour of the experiment, which was 7 times greater than in
the presence of chlorine when 0.03% was in the HOCl form at
pH 11. This explains the rapid loss of CP and TCP formation
in the presence of bromine because the oxidation rate is more
rapid than with chlorine. Using the model, kOBr,CP for CP was
d[HOX]T
-
)
-5kHOX,CP[HOX][CP] - kOX,CP[OX ][CP] -
dt
-
kOX,CPO[OX ][CPO] (5)
d[CP]
dt
)
-kHOX,CP[HOX][CP] - kh,CP[CP] -
-
kOX,CP[OX ][CP] (6)
d[CPO]
)
)
kHOX,CP[HOX][CP] - kh,CPO[CPO] -
dt
-
kOX,CPO[OX ][CPO] (7)
d[TCP]
-
kh,CP[CP] + kOX,CP[OX ][CP] + kh,CPO[CPO] +
dt
-
kOX,CPO[OX ][CPO] (8)
In these equations, HOX represents either HOBr or HOCl
and OX- represents either OBr or OCl . Scientist, an ODE
-
-

Chlorpyrifos Transformation by Aqueous Chlorine
J. Agric. Food Chem., Vol. 56, No. 4, 2008 1333
Figure 8. Observed first-order loss rates of CP over the pH range of
Figure 10. Transformation of CP to CPO and degradation of CPO in the
7
.0–9.0 with and without ACC present. [CP]
and 50 µM, [buffer] ) 10 mM, [ACC] ) 1.61 mg of C/L, and temperature
25 ( 1 °C. Error bars represent 95% confidence intervals.
0
) 0.5 µM, [HOCl]
T
) 25
presence of free chlorine at pH 8.5. [CP]
ACC] ) 1.1 mg of C/L, [CO ) 10 mM, and temperature ) 25 ( 1
C. Lines represent model results.
0
) 0.44 µM, [HOCl]
T
) 25 µM,
T
[
3 T
]
)
°
studies exist in the literature examining why halogenated
oxidants act as nucleophiles. Future work is planned to further
investigate the alpha effect of these oxidants with additional
OP pesticides in order to gain greater insight into this
phenomenon.
Modeling CP Transformation in the Presence of Chlorine
and Bromide. With the intrinsic rate coefficients for HOBr and
-
OBr with CP and CPO known, an attempt was made to model
the chlorine/bromide/CP system. The current model assumes
that for the loss of 1 mol of CP there is a subsequent loss of 5
mol of either bromine or chlorine. This assumes that regardless
of either oxidant being present, the sulfur that is lost from the
2
-
parent OP is instantaneously converted to sulfate (SO4 ) due
to the overwhelming oxidant concentration. However, the
chlorine/bromide system produces HOCl, HOBr, and BrCl as
three powerful oxidants not only transforming CP to CPO but
competing to oxidize the sulfur from CP to sulfate. Because
the affinities for the three separate oxidants with each sulfur
Figure 9. Transformation of CP to CPO and degradation of CPO in the
presence of free chlorine at pH 9.0. [CP] ) 0.45 µM, [HOCl] ) 50 µM,
CO ) 10 mM, and temperature ) 25 ( 1 °C. Lines represent model
results.
0
T
[
3 T
]
2-
species (S, SO, SO2, SO3 ) could not be found in literature,
attempts at modeling this system with a mechanistic approach
were unsuccessful.
-1
h-1 with 95% confidence intervals in
9
65 (( 110) M
parentheses. Using nonlinear regression analysis and established
reaction pathways allowed for the parametrization of kOBr,CP in
this complicated yet well-defined reaction system.
Chlorination of CP in the Presence of NOM. The presence
of NOM could potentially decrease the rate of CP degradation
by acting as a sink for oxidants used in drinking water treatment.
Conventional physical-chemical water treatment processes such
as coagulation and lime softening remove a portion of the
hydrophobic fraction of the NOM (31, 32). More advanced
water treatment processes such as granular activated carbon
(GAC) or membrane filtration are capable of removing ad-
ditional NOM fractions (33). However, few community water
systems employ these more advanced technologies. For example,
these processes are not employed at the ACC plant. Therefore,
only a portion of the hydrophobic NOM fraction will be
removed. Chlorine demand studies were conducted over the pH
range of 7–9 on the ACC water to determine chlorine consump-
tion as a function of time. Over the first 0.5 h, approximately 6
µM of the free chlorine initially present ([HOCl]T ) 25 and 50
µM) was consumed by the NOM in a multiphasic reaction and
was found not to be dependent on pH. After the initial chlorine
demand of the NOM was satisfied, the loss of free chlorine
attributable to reaction with NOM was relatively slow (kobs )
The bromine-assisted hydrolysis rate coefficients for both CP
and CPO were found to be very similar to those found for
chlorine (Table 2). This result was unexpected because the
transformation of CP to CPO by HOBr was significantly greater
than for HOCl over the pH range of 6.5–9. With the halogen-
-
assisted hydrolysis (XO ) of CP and CPO, the importance of
the alpha effect for nucleophilic attack at the tetrahedral
phosphorus atom strongly depends on the lone pairs of electrons
on the halogen atom (i.e., Cl or Br). Previous work investigating
-
the nucleophilicity of OCl , repulsions between the lone pairs
of electrons on the chlorine atom and the oxygen atom were
found to enhance the nucleophilicity of hypochlorite toward
tetrahedral phosphorus moieties (29). Because of these repul-
sions, OCl- satisfies the criteria to be considered a supernu-
cleophile (30). The molecular structures of hypochlorite and
hypobromite are similar and the electronegativities of bromine
(
2.96) and chlorine (3.16) are not significantly different, the
-
-1
alpha effect appears to apply to OBr as well. However, few
0.012 h ) over the pH range of 7–9. This suggests that the

1334 J. Agric. Food Chem., Vol. 56, No. 4, 2008
Duirk et al.
NOM present in the ACC water may affect the rate of CP
transformation because chlorine demand is consistent over the
pH range of 7–9.
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would like to thank Dr. Wayne Garrison and Dr. Jackson
Ellington for their consultation and expertise.
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