Reaction of chlorpyrifos-methyl in aqueous hydrogen sulfide/bisulfide solutions

Article abstract of DOI:10.1021/jf020955w

The kinetics of the reactions of chlorpyrifos-methyl, an organophosphorus insecticide, with hydrogen sulfide (H₂S) and bisulfide (HS^-) were determined in well-defined aqueous solutions. The resulting pseudo-first-order rate constant for chlorpyrifos-methyl with bisulfide yielded a second-order rate constant of (2.1 ± 0.3) × 10^-3 M^-1 s^-1. The second-order rate constant for chlorpyrifos-methyl with hydrogen sulfide is significantly slower than the second-order rate constant with bisulfide. The contribution of H₂S to the observed degradation rate constant of chlorpyrifos-methyl at concentrations of up to 4 mM H₂S is not significant. The second-order rate constant of chlorpyrifos-methyl with H₂S was too low to be measured in this study. The results indicate that HS^- present at environmentally relevant concentrations may represent an important sink for phosphorothionate triesters in a coastal marine environment, while H₂S reacts too slowly to be environmentally relevant (pH 6-9). Trichloropyridinol, the major product of hydrolysis of chlorpyrifos-methyl, is only a minor product of the reaction of chlorpyrifos-methyl with bisulfide; however, trichloropyridinol was found to be stable under the experimental conditions.

Full text of DOI:10.1021/jf020955w

1956 J. Agric. Food Chem. 2003, 51, 1956−1960
Reaction of Chlorpyrifos-methyl in Aqueous Hydrogen
Sulfide/Bisulfide Solutions
URS JANS* AND M. HASAN MIAH
Chemistry Department, City College of City University of New York, New York, New York 10031
The kinetics of the reactions of chlorpyrifos-methyl, an organophosphorus insecticide, with hydrogen
sulfide (H2S) and bisulfide (HS-) were determined in well-defined aqueous solutions. The resulting
pseudo-first-order rate constant for chlorpyrifos-methyl with bisulfide yielded a second-order rate
constant of (2.1 ( 0.3) × 10-3 M-1 s-1. The second-order rate constant for chlorpyrifos-methyl with
hydrogen sulfide is significantly slower than the second-order rate constant with bisulfide. The
contribution of H2S to the observed degradation rate constant of chlorpyrifos-methyl at concentrations
of up to 4 mM H2S is not significant. The second-order rate constant of chlorpyrifos-methyl with H2S
was too low to be measured in this study. The results indicate that HS- present at environmentally
relevant concentrations may represent an important sink for phosphorothionate triesters in a coastal
marine environment, while H2S reacts too slowly to be environmentally relevant (pH 6-9).
Trichloropyridinol, the major product of hydrolysis of chlorpyrifos-methyl, is only a minor product of
the reaction of chlorpyrifos-methyl with bisulfide; however, trichloropyridinol was found to be stable
under the experimental conditions.
KEYWORDS: Organophosphorus pesticide; degradation; reduced sulfur species; chlorpyrifos-methyl;
trichloropyridinol
INTRODUCTION
others are degraded by homogeneous reactions. Anoxic condi-
tions can give rise to high concentrations of reduced sulfur
species. Reduced sulfur species are capable of reacting with a
wide array of pollutants, including organic contaminants that
undergo nucleophilic reactions (7, 8). Reduced sulfur species
can form whenever O2 is consumed more rapidly than it can be
replenished by mixing processes. Under such conditions, the
microbial reduction of sulfate (abundant in seawater, but limited
in freshwater) gives rise to reduced sulfur species such as HS-.
Concentrations of hydrogen sulfide species (HS-/H2S) as high
as 5 mM have been reported in sediment pore water of estuaries
and salt marshes (9).
Organophosphorus insecticides (OPs) are widely used through-
out the world and have in many cases replaced organochlorine
pesticides. In general, OPs are less persistent in the environment
than organochlorine compounds. The major pathways of
degradation for OPs are hydrolysis, oxidation, photolysis, and
biodegradation (1); the half-lives for OPs in natural water are
in the range of several days to several months (2-4). OPs that
are present in surface water may associate with particles and
can eventually become part of the sediment phase. It is also
likely that some organophosphorus insecticides are transported
into salt marshes and into the anoxic bottom layers of estuaries.
In sediments, salt marshes, and the bottom layer of estuaries,
anoxic conditions are typically prevalent. Transformation pro-
cesses occurring under anoxic conditions may represent an
important sink for OPs and have not yet been extensively
studied.
Investigators have shown that abiotic processes can control
the fate of organic contaminants under anaerobic conditions (5,
6). It is therefore possible that OPs undergo significant
transformation under anoxic conditions by chemical rather than
microbial processes. The possible transformation processes are
compound specific. They typically include reduction reactions
and substitution reactions. While some organic pollutants are
transformed through surface-catalyzed reactions on particles,
Chlorpyrifos-methyl is a typical OP. It contains a PdS moiety
and three ester bonds, which makes it a phosphorothionate
triester. Chlorpyrifos and parathion-methyl are two phospho-
rothionate triesters that are among the most widely used
insecticides in the United States (10). Chlorpyrifos-methyl was
* To whom correspondence should be addressed. Fax: (212) 650-6107.
E-mail: ujans@ccny.cuny.edu.
10.1021/jf020955w CCC: $25.00 © 2003 American Chemical Society
Published on Web 02/28/2003

Chlorpyrifos-methyl
J. Agric. Food Chem., Vol. 51, No. 7, 2003 1957
The syringes contained five glass rings to facilitate mixing. All reaction
solutions contained 5% methanol, 100 mM NaCl, and 50 mM buffer
(sodium phosphate). The glassware for slow hydrolysis experiments
was autoclaved to inhibit biological growth. In addition, the buffer
solutions were filtered (0.2 µM, Anotop 25-sterile, Whatman Ltd.,
Maidstone, England). Filtering of the buffer solution and assembly of
autoclaved glassware were carried out in a biological safety cabinet to
prevent any microbial contamination. The polycarbonate stopcocks used
in the hydrolysis experiments were rinsed with 80% 2-propanol and
air-dried in the biological safety cabinet prior to their use in a hydrolysis
experiment.
The spike solution of chlorpyrifos-methyl was prepared by dissolving
chlorpyrifos-methyl in deoxygenated methanol. Pilot experiments
conducted at varying methanol concentrations (0-15%) indicated that
these levels of methanol did not alter the reaction rates.
An Accumet pH meter (Fisher Scientific) with a Ross combination
pH electrode (ThermoOrion, Beverly, MA) was used to measure the
pH in the bisulfide solutions. The concentration of the hydrogen sulfide/
bisulfide solutions was determined by iodometric titration. The sodium
thiosulfate solution for the titration was standardized against potassium
iodate daily (15).
chosen for this study because it has a higher water solubility
than chlorpyrifos. It has been shown that HO- reacts with
phosphate triesters in a SN2 reactions with attack occurring at
the phosphorus atom, resulting in cleavage of a P-O bond (eq
1) (11, 12). It has also been reported that some phosphate
triesters display cleavage of an O-C bond in water, indicating
that H2O undergoes a nucleophilic substitution reaction at the
carbon atom of a methoxy group (eq 2) (13). On the basis of
these findings, it is hypothesized that phosphorothionate triesters
are likely to undergo displacement reactions with a sulfur
nucleophile (e.g., HS-) either with attack occurring at the carbon
of a methoxy group (eq 3) or with attack occurring at the
phosphorus atom (eq 4) (14). Actual experiments that test this
hypothesis have not yet been reported to the best of our
knowledge.
Kinetic Experiments. Reaction kinetics were measured under
pseudo-first-order conditions, with an initial chlorpyrifos-methyl con-
centration that was typically 0.5-1% of that of the total hydrogen
sulfide concentration. The reaction solutions were spiked with aliquots
(80 µL) of deoxygenated methanol containing the chlorpyrifos-methyl,
yielding an initial concentration of ∼20 µM. Reactors were vigorously
mixed for 30 s in the glovebox and were incubated in a water bath at
25.0 ( 0.1 °C. Aliquots (1 mL) were periodically taken; 2 drops of 6
M HCl was added, and the mixture was extracted into ethyl acetate,
followed by analysis via HPLC. The acidification ensures the extraction
of trichloropyridinol. Pseudo-first-order rate constants were obtained
by performing a linear regression of the natural logarithm of the
chlorpyrifos-methyl concentration versus time. Reactions were moni-
tored over sufficient time (two to three half-lives) to verify pseudo-
first-order kinetics. For selected experiments, the pseudo-first-order rate
constants for the degradation of chlorpyrifos-methyl and for the
formation of trichloropyridinol were concurrently determined via
nonlinear regression techniques using Scientist for Windows, version
2.01 (MicroMath Scientific Software, Salt Lake City, UT).
The primary purpose of this research was to explore the
potential impact of hydrogen sulfide/bisulfide solutions on the
abiotic transformation of chlorpyrifos-methyl. The second-order
rate constant was determined in well-defined systems. In
addition, the formation rates of trichloropyridinol (a degradation
product of this reaction) was measured and compared with the
formation rate of trichloropyridinol in hydrolysis experiments.
Because of differences in reactivity of the different hydrogen sulfide
species (e.g., H2S vs HS-), a pH-dependent pseudo-first-order rate
constant, k′obs, is expected. This rate constant can be corrected for
hydrolysis and divided by the total concentration of hydrogen sulfide
MATERIALS AND METHODS
Chemicals. O,O-Dimethyl O-(3,5,6-trichloro-2-pyridyl)phospho-
rothioate (chlorpyrifos-methyl) (99.7%, CAS registry no. 5598-13-0)
and the hydrolysis product, 3,5,6-trichloropyridinol (98%, CAS registry
no. 6515-38-4), were obtained from Chem Service (West Chester, PA).
All solvents and reagents that were used were analytical grade or
equivalent. They were used without further purification, and were
obtained from Fisher Scientific (Pittsburgh, PA). Ethyl acetate and
methanol were HPLC grade.
species, which results in the apparent second-order rate constant, k′′app
.
This apparent second-order rate constant, k′′app, can be given by the
expression
[HS-]
[H2S]
k′′app ) k′′
+ k′′HS
(5)
H S[H2S]T
-[H2S]T
2
where [H2S]T ) [HS-] + [H2S].
Experimental Setup. All aqueous solutions were prepared from
deionized water (DW) (Milli-Q gradient system, Millipore, Bedford,
MA). All glassware was soaked in 1 M HNO3, rinsed several times
with DW, and dried at 200 °C. Glassware used with bisulfide solutions
was washed with a methanol/NaOH mixture to remove traces of sulfur
impurities prior to acid washing.
Sodium sulfide stock solutions were prepared under argon from
Na2S‚9H2O crystals using deoxygenated DW. Crystals were rinsed with
Ar-purged water to remove surface oxidation products, blotted with a
cellulose wipe, and then transferred to an Ar-purged three-necked flask.
The flask was then connected to an Ar-purged closed glassware system
consisting of a reservoir bottle (containing DW), glass tubing, stopcocks,
and an argon tank. Before rerouting the argon flow and forcing the
reservoir content into the flask containing the washed sodium sulfide
crystals, we purged the DW with argon for 1 h. Then the sodium sulfide
stock solutions were transferred in the anaerobic chamber.
Substituting expressions for [H2S] and [HS-] concentrations as a
function of [H2S]T yields
-1
[HS-]
[H2S]T
[H+]
K′a
-1
[H2S]
K′a
RH S
)
) 1 +
RHS
)
) 1 +
-
+
2
(
)
(
)
[H2S]T
[H ]
(6)
where K′a, the acid dissociation constant of H2S, equals 10-6.98 (16).
After rearrangement, the apparent second-order rate constant k′′app can
-
be given in terms of RHS
:
k′′app ) k′′H S + RHS-‚(k′′HS- - k′′
)
(7)
H2S
2
-
Therefore, a plot of k′′app versus RHS shows a linear correlation, and it
allows extraction of second-order rate constants k′′H S and k′′HS
Liquid Chromatographic Analysis. Ethyl acetate extracts contain-
ing chlorpyrifos-methyl and trichloropyridinol were analyzed using a
Waters 2690 separations module (Waters, Milford, MA) equipped with
-
.
Unless otherwise stated, reaction solutions where prepared in an
anaerobic chamber (5% H2 and 95% N2). The reaction solutions were
prepared in volumetric flasks and then transferred to 20 mL glass
syringes equipped with a polycarbonate stopcock and a Teflon needle.
2

1958 J. Agric. Food Chem., Vol. 51, No. 7, 2003
Jans and Miah
Table 1. Reaction Rate Constants for Chlorpyrifos-methyla
b
b
reaction conditions
k′obs ± SD (s-1
)
k2 ± SD (s-1
)
pH 5.9, hydrolysis
pH 7.6, hydrolysis
pH 6.0, 3.14 mM H S
3.4 × 10-7 ± 1.3 × 10-8
9.7 × 10-7 ± 2.0 × 10-8
1.5 × 10-6 ± 3.3 × 10-8
4.1 × 10-6 ± 1.4 × 10-7
6.2 × 10-6 ± 1.4 × 10-7
1.9 × 10-7 ± 1.2 × 10-8
6.7 × 10-7 ± 2.0 × 10-8
2.1 × 10-7 ± 1.3 × 10-8
6.1 × 10-7 ± 5.2 × 10-8
6.6 × 10-7 ± 5.1 × 10-8
2
tot
pH 7.1, 3.44 mM H S
2
tot
pH 7.9, 3.33 mM H S
2
tot
a Determined via simultaneous nonlinear regression techniques using Scientist
for Windows. b Standard deviation resulting from the nonlinear regression.
explained by the hypothesis that two mechanisms are controlling
the degradation of chlorpyrifos-methyl. While mechanism 2 (eq
4) leads directly to the formation of trichloropyridinol, mech-
anism 1 (eq 3) leads to the formation of desmethyl chlorpyrifos-
methyl. The lack of authentic standards prevented the identi-
fication and quantification of desmethyl chlorpyrifos-methyl.
The deprotonated form of desmethyl chlorpyrifos-methyl, which
is expected to be the dominant species at pH >3 (18), might
not be very susceptible to further hydrolysis, and might be
considered to be stable under the experimental conditions. Only
the nonionic form of desmethyl chlorpyrifos-methyl, which is
present at low pH values, might undergo further hydrolysis. This
hypothesis is based on the observation that at pH >0 the
hydrolysis of dimethyl phosphate is a function of the fraction
of the nonionic form present, indicating that only the nonionic
form is undergoing hydrolysis (18).
In connection with the experiments presented here, the
influence of divalent metal cations on the hydrolysis of
chlorpyrifos-methyl and the product distribution should be
mentioned. It is reported that at pH 4.5, 5 mM acetate, and 10
mM NaCl and in the presence of 1 mM Cu(II) the hydrolysis
of chlorpyrifos-methyl is significantly faster and that the
trichloropyridinol formation is almost 100% of the chlorpyrifos-
methyl loss (19). This result is explained by formation of Cu(II)
complexes that are more susceptible to hydrolysis than the
uncomplexed compound. A similar effect might be expected
for the hydrolysis rate of the desmethyl chlorpyrifos-methyl in
the presence of Cu(II).
Figure 1. Hydrolysis of chlorpyrifos-methyl (b) and formation of trichlo-
ropyridinol (2) in 0.050 M phosphate, 0.10 M NaCl, and 5% methanol at
25 °C and (a) pH 5.9 and (b) pH 7.6.
a photodiode array detector (996, Waters). The stationary phase was
an Xterra MS C18, 3.9 mm × 150 mm, 5 µM column (Waters) with a
guard column (3.9 mm × 20 mm) of the same material. The detector
wavelength was 289 nm for chlorpyrifos-methyl and trichloropyridinol.
The mobile phase was isocratic at 75% methanol and 25% 1 mM H3PO4
in DW.
RESULTS AND DISCUSSION
Hydrolysis Experiments. The hydrolysis of chlorpyrifos-
methyl can be treated as a pseudo-first-order reaction in buffered
solutions with a constant pH; hence, the following equation
applies: dC/dt ) -k′obsC. The observed rate constant for
hydrolysis, k′obs, is equal to the slope in a plot of ln[chlorpyrifos-
methyl] versus reaction time. The hydrolysis of chlorpyrifos-
methyl followed pseudo-first-order kinetics, and chlorpyrifos-
methyl decomposed to yield trichloropyridinol as a major
degradation product (Figure 1). Hydrolysis rates of 0.029 (
0.006 day-1 at pH 5.9 and 0.081 ( 0.009 day-1 at pH 7.6 were
observed. The results are in approximate agreement with that
found in previous studies, 0.040 day-1 at pH 6.7 and 0.055 day-1
at pH 7.8 in 5 mM phosphate buffer at 25 °C (17). This
agreement supports the assumption that the increased buffer
concentration chosen in the present experiments does not have
a significant effect on the hydrolysis rate of chlorpyrifos-methyl.
The data analysis of the hydrolysis experiment was performed
using nonlinear regression allowing the determination of k′obs
and k2 (eq 4) by fitting the degrading chlorpyrifos-methyl
concentration and the forming trichloropyridinol concentration
simultaneously (Table 1). At pH 7.6 (Figure 1b), a pyridinol
formation rate of 6.7 × 10-7 s-1 was determined; this is equal
to 69% of the hydrolysis rate constant of chlorpyrifos-methyl.
At pH 5.9, the determined trichloropyridinol formation rate was
59% of the hydrolysis rate of chlorpyrifos-methyl (Figure 1a).
Meikle et al. (17) report that at pH 6.7 and 35 °C, 46% of the
product that was formed was pyridinol. These results can be
Reaction with Hydrogen Sulfide. The reaction of chlor-
pyrifos-methyl with hydrogen sulfide was assessed at different
pH values. The experiments can be treated as pseudo-first-order
reactions in buffered solutions at a constant pH. The observed
pseudo-first-order rate constant, k′obs, is obtained from the slope
of a plot of ln[chlorpyrifos-methyl] versus time. The apparent
second-order rate constant, k′′app, was determined from k′obs
.
-
Figure 2 shows the plot of k′′app versus RHS [for the calculation
-
of RHS , K′a(H2S) was corrected for ionic strength]. The linear
correlation supports our assumption that HS- has a reactivity
toward chlorpyrifos-methyl different from that of H2S (eq 7).
The second-order reaction rate constant for the reaction of
bisulfide with chlorpyrifos-methyl that can be derived from the
slope of Figure 2 is 7.5 ( 1.0 M-1 h-1 [)(2.1 ( 0.3) × 10-3
M-1 s-1]. The intercept in Figure 2 (-0.01 ( 0.65 M-1 h-1),
which is equal to k′′H S, is small and not significantly different
2
from zero. This indicates that the reaction of chlorpyrifos-methyl
with H2S (at the experimental H2S concentrations) is too slow
compared to hydrolysis and cannot be quantified from the
experiments presented here.
Figure 3 shows time courses of the reaction of chlorpyrifos-
methyl with hydrogen sulfide at pH 6.0, 7.1, and 7.9. Trichlo-
ropyridinol is a product of the reaction and accounts for 11-
15% of the chlorpyrifos-methyl that reacted. However, almost
all of the observed trichloropyridinol formation at this pH can

Chlorpyrifos-methyl
J. Agric. Food Chem., Vol. 51, No. 7, 2003 1959
that the reaction of HS- with chlorpyrifos-methyl does not lead
to the formation of trichloropyridinol. The reactivity of trichlo-
ropyridinol toward HS- was tested in separate experiments,
showing that there is no reaction between trichloropyridinol and
HS- over the time interval relevant for the experiments
presented here. Therefore, it can be concluded that the observed
results can be explained by HS- reacting with chlorpyrifos-
methyl predominantly via an attack at the carbon of a methoxy
group (eq 3) and not via a nucleophilic attack at the phosphorus
atom. Although the results support the postulated hypothesis,
further studies are necessary to elucidate the mechanisms. The
analysis and quantification of desmethyl chlorpyrifos-methyl will
be an important step in this endeavor.
The determined reaction rate constant between HS- and
chlorpyrifos-methyl is environmentally relevant. At a total
hydrogen sulfide concentration of 5 mM, at pH 7, the reaction
of chlorpyrifos-methyl with HS- would result in a half-life of
37 h for chlorpyrifos-methyl; at pH 8, the expected half-life of
chlorpyrifos-methyl would be 21 h. In comparison, the half-
lives due to hydrolysis at these pH values are expected to be
13 and 17 days, respectively. On the basis of these calculations,
it can be expected that in anoxic coastal marine environments
(e.g., sediment pore water of estuaries), the reaction of HS-
with chlorpyrifos-methyl can be an important sink.
Figure 2. Plot of apparent second-order reaction rate constants, k′′app
(M-1 h-1), versus RHS for reactions of chlorpyrifos-methyl with hydrogen
−
sulfide/bisulfide solutions for different pH values in 0.050 M phosphate
buffer, 0.10 M NaCl, and 5% methanol at 25 °C.
ACKNOWLEDGMENT
We thank X. Zeng for conducting the hydrolysis experiments,
T. Wu for conducting additional experiments, and I. Winicov
for allowing us to use the biological safety cabinet. The
manuscript benefited greatly from comments provided by three
anonymous reviewers.
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Received for review September 13, 2002. Revised manuscript received
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