Mercury-promoted hydrolysis of parathion-methyl: Effect of chloride and hydrated species

Article abstract of DOI:10.1021/es970972f

Mercury salts are commonly used in laboratory and field experiments as biocides. It has been previously reported that Hg(II) can enhance chemical hydrolysis of a number of pesticides. Earlier studies on metal-promoted hydrolysis have reported overall rate

Full text of DOI:10.1021/es970972f

Environ. Sci. Technol. 1998, 32, 2338-2342
methods can alter the chemical environment to which the
Mercury-Promoted Hydrolysis of
Parathion-methyl: Effect of Chloride
and Hydrated Species
pesticide is exposed; consequently, not only biological
reactions are terminated. These sterilization methods can
also alter homogeneous and heterogeneous chemical pro-
cesses (2). Such alterations need to be addressed before
comparisons and extrapolations can be made.
Previous studies have used various methods to distinguish
between biotic and abiotic transformations (3-7). In a study
to illustrate the pH dependency of chemical processes,
Blumhorst and Weber (8) used autoclaving to distinguish
chemical versus microbial degradation of atrazine and
M A Z YA R Z E I N A L I A N D A L B A T O R R E N T S *
Environmental Engineering Program, Department of
Civil Engineering, University of Maryland,
College Park, Maryland 20742
3
cyanazine in several soil samples. A 0.02% NaN solution
was used by Lion et al. (9) to study the sorption of hydrophobic
compounds on aquifer material, and Xing et al. (10)
minimized biological activity by using a 10 M HgCl solution
2
in their study to predict partition coefficients of nonionic
organic contaminants. Although studies have been con-
ducted to assess the effect of sterilization methods on various
soil properties, little has been reported on any of the possible
secondary effects induced by the use of the biocides (11-
14).
Mercury salts are commonly used in laboratory and field
experiments as biocides. It has been previously reported
that Hg(II) can enhance chemical hydrolysis of a number
of pesticides. Earlier studies on metal-promoted hydrolysis
have reported overall rate constants as a function of
total metal concentration. There are three advantages in
reporting the relative importance of the different species:
-5
(
(
(
1) results can be extrapolated form one situation to another,
2) rates can be predicted for specific conditions, and
3) greater understanding of the catalysis mechanism can
2
In this study, we investigated the use of HgCl to control
biological activity when the organic pollutants contain
hydrolyzable functional groups. OPs represent one class of
such pollutants. OPs are used extensively as insecticides,
and they have replaced organochlorine compounds in many
of their uses. A major transformation pathway controlling
the fate of OPs in the environment is hydrolysis, both chemical
and biotic. Typically, the products of chemical and biological
hydrolysis are the same; thus, scientists rely on the use of
biocides in controlled kinetic studies to distinguish between
the two processes. Some of the most commonly used
biocides can alter the kinetics of chemical degradations.
The use of a divalent metal ion, such as mercury, is of
specific concern with OPs due to possible metal-ion activity
in hydrolysis reactions. The role of various metal ions, such
as Cu(II), Mn(II), and Zn(II), in the hydrolysis of phosphorus
esters has long been established (15-18), yet the degree to
which different aquo-metal species are responsible has not
been evaluated. Normally, the effect of metal catalysis has
be gained. In this study, mercury-promoted hydrolysis
of parathion-methyl (O,O-dimethyl-O-p-nitrophenyl phospho-
rothionate), a probe organophosphate compound (OP),
was studied as a function of Hg(II) speciation. The observed
rate of hydrolysis was a function of specific mercury
species rather than of the total mercury in solution. Second-
order rate constants were determined experimentally at
various pH values. A pH-dependent kinetic expression, kobs
2
+
+
3
)
(RHg k1 + RHgOH k2 + RHg(OH) k ) where ki ) K k′, with
i i
2
Ki representing the Hg:OP equilibrium constant, k′ the rate
i
constant for Hg:OP hydrolysis for the different Hg(II)
species, and R is the fraction of the total Hg(II) present
as specific species, provides a plausible interpretation for
the system. Mercury-chloride species proved to have
2
+
little catalytic power, whereas the contributions for Hg
T
been reported through a lumped parameter, kMe[Me ], that
+
and Hg(OH) were significant. Our results also suggest that
a mixed mechanism (electrophilic and nucleophilic) may
have to be considered for general metal-promoted hydrolysis
of OPs.
is highly pH dependent (19, 20), making it impossible to
extrapolate from one pH to another. For example, in the
study conducted by Smolen and Stone (20), kobs values for
II
II
II
II
II
Co , Ni , Cu , Zn , and Pb catalyzed hydrolysis of five OPs
were reported at different pH values and varied by as much
as 2 orders of magnitude. When only kobs values are reported,
it is difficult to identify the specific metal-ion species involved
Introduction
2+
+
2
in the reaction (e.g., Me , Me(OH) , Me(OH) , ...). The
Many pesticides contain functional groups that make them
susceptible to both biotic and abiotic transformations.
Kinetic studies of pollutant transformations are often con-
founded by the complex array of reactions that can take place
simultaneously in natural environments. To isolate, quantify,
and mechanistically study the different processes controlling
the fate of pollutants in natural systems, laboratory experi-
ments must be performed under various controlled condi-
tions.
separation of the lumped kinetic parameter for contribution
of the Me-aquo species could provide a generalized pH-
dependent rate law.
Different mechanisms for metal-catalyzed hydrolysis have
been postulated where the metal can act as either an
electrophile or a nucleophile (21, 22). In the case where the
metal acts as an electrophile, coordination between the metal
and the hydrolyzable moiety or the leaving group occurs,
which facilitates the attack by a nucleophile (H O). In a
2
One of the first tasks is to determine the relative
importance of biotic versus abiotic processes. To distinguish
between sorption and/ or chemical and microbial degradation
of pesticides in soils, researchers normally use a comparison
between sterile and nonsterile systems (1). Soil-sterilization
nucleophilic mechanism, the metal ion coordinates with the
+
n
nucleophile, forming metal-hydroxo (MeOH ) species that
catalyze the reaction via improved nucleophilicity. Smolen
and Stone (20) concluded that phosphorus-centered nu-
cleophilic attack by Cu and Cd species was the mechanism
for all the esters examined. However, we should expect such
behavior to be metal specific. Sulfur is a good ligand for Hg
that would facilitate the complexation of mercury to the sulfur
*
Corresponding author. Fax: (301) 405-2585; e-mail: alba@eng.
umd.edu.
2
3 3 8
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 15, 1998
S0013-936X(97)00972-3 CCC: $15.00

1998 Am erican Chem ical Society
Published on Web 06/20/1998

moiety in the OPs; thus, mercury would act as an electrophile.
In a recent study, the following mechanism was proposed
for the Hg(II)-promoted hydrolysis of OPs (19):
K
k2
98
Hg(II) + SdP(OR)3 7
9
8 Hg(II)S-P(OR)3
Hg(II)SdP(OR) + ROH
2
|
OH
(1)
where Hg(II) coordinates with sulfur in the thionate moiety
of the pesticide withdrawing electronic density from the “P”
center and thus facilitating a nucleophilic attach by H O. In
2
this mechanism, Hg acts as an electrophile and the observed
hydrolysis is not a complete catalytic process (a 1:1 relation-
ship exists between total mercury added and amount of OP
compound hydrolyzed). However, as in other studies, the
proposed mechanism lumps all mercury species into one
term that fails to consider the contributions of the various
Hg species, and the reported kobs value is pH dependent.
The objectives of our study were (1) to quantify the effects
of pH on Hg-promoted parathion-methyl hydrolysis, (2) to
identify and quantify the rates of the active Hg species, and
(
3) to examine how chloride complexation with Hg affects
hydrolysis rates. Parathion-methyl is used as a probe OP
because of its high solubility and fast hydrolysis as compared
to other OPs.
Materials and Methods
Chem ical Reagents. HgCl
2
(99.5+% purity) and Hg(NO) ‚
3
H
2
O (98+% purity) were purchased from Aldrich Chemical
Co. (Milwaukee, WI). Sodium acetate, acetic acid, sodium
nitrate, sodium chloride, sodium hydroxide, ethylenedi-
aminetetraacetate (EDTA) disodium salt dihydrate, and
hydrochloric acid were purchased from J. T. Baker (Phil-
lipsburg, NJ). Sodium perchlorate was purchased from EM
Science (Gibbstown, NJ). All these chemicals were high-
purity reagent grade and were used without further purifica-
tion. Parathion-methyl (O,O-dimethyl-O-p-nitrophenyl phos-
phorothionate) and paraoxon-methyl (O,O-dimethyl-O-p-
nitrophenyl phosphate) were obtained from Chem Service
FIGURE 1. Mercury speciation in 10 mM NaNO
NO and (b) 65 µM HgCl
3
of (a) 65 µM Hg-
(
3
)
2
2
.
intervals and were analyzed by high-pressure liquid chro-
matography (HPLC). The loss of parathion-methyl and the
formation of p-nitrophenol (hydrolysis product) were moni-
tored using a Waters system equipped with a multiwavelength
programmable UV detector and Millenium software. The
HPLC stationary phase was a 150 mm Altech Altima C18
column, the wavelengths of study were λparathion-methyl ) 277
nm and λp-nitrophenol ) 303 nm. The mobile phase was a
gradient method consisting of acetonitrile/ 0.01 M chloro-
acetic acid. Due to the extremely fast hydrolysis observed
in the presence of Hg(II), EDTA (2:1 EDTA to Hgtotal) was
added to all samples to quench Hg(II) and halt the hydrolysis
reaction at sampling time. To gain insight into the mech-
anism and the selectivity of Hg as a catalyst, experiments
were conducted with the paraoxon-methyl, following the
same procedure.
(
West Chester, PA) at purities greater than 97% and were
used without further purification.
Experim ental Setup. All solutions were prepared from
double-deionized water (DW), Hydro, Durham NC]. All
3
glassware was soaked with 6 N HNO , rinsed several times
with DW, and baked at 550 °C before use. Experiments were
performed in a rotary shaker at room temperature (25 ( 2
°
C). Unless otherwise stated, solutions were prepared in 40
mL amber EPA vials containing 10 mM NaNO as the
3
electrolyte medium. Experiments were conducted at pH
values from 3.5 to 5.5 with acetate buffer. Some experiments
were conducted with Na HPO to compare our results from
2 4
those previously reported by Wan et al. (19). A Fisher
Accumet pH meter with a combination electrode pH was
used to measure pH. Throughout the reaction, the pH was
always maintained within (0.1 pH units of the initial pH.
Hg(II) stock solutions were prepared by dissolving HgCl
2
or
Results and Discussion
HgNO with DW. For all these experiments, acetate buffer
3
was used (with no phosphate), because, in the absence of
chloride, mercury forms a di-metal low-soluble complex with
phosphate. Concentration of Hg in stock solutions was
measured via atomic-absorption spectroscopy. The stock
parathion-methyl solution was prepared by dissolving in
Mercury Speciation. The computer program HYDRAQL (23)
was used to calculate mercury speciation in the absence and
presence of chloride for the studied pH values. The equi-
librium database was modified with equilibrium constants
obtained from Smith and Martell (24). The results are
presented in Figure 1. At pH 5.5, in the absence of chloride,
more than 72% of the mercury exists as hydroxyl species
1
00% methanol (MeOH), final reactor MeOH concentration
of 6.6%. Pilot experiments conducted at lower parathion-
methyl concentrations without MeOH indicated that this
level of MeOH did not alter the reaction rates.
Hydrolysis Experim ents. After the addition of parathion-
methyl, samples were taken at specific predetermined time
[mostly Hg(OH)
49% of the mercury exists as chloride complexes (mostly
HgCl ). At pH 3.5, in the absence of chloride, approximately
84% of mercury is in Hg(OH) form. In the presence of
2
]. In the presence of chloride, approximately
2
2
VOL. 32, NO. 15, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2 3 3 9

FIGURE 3. Concentration dependence of experimentally determined
second-order observed rate constants (kobs) for HgCl
hydrolysis of parathion-methyl at pH 3.5 and 5.5.
2
-promoted
FIGURE 2. HgCl
at pH 3.5 and varying HgCl
2
-promoted hydrolysis of parathion-methyl (60 µM)
concentrations.
2
TABLE 1. Second-Order Observed Rate Constants (kobs) for
Mercury-Promoted Hydrolysis of Parathion-Methyl at Various
pH Values
chloride, 65% of the mercury species is in the di-chloride
species (HgCl ). Therefore, chloride-complexed mercury
2
species can play a significant role in the activity of mercury
in catalyzed hydrolysis, especially at low pH values, where
chloride species have an increased dominance.
kobs
(M-1 s-1
R 2
pH
)
Developm ent of Analytic Method. In the absence of Hg-
4.62
320
330
320
130
170
140
51
0.99
0.99
0.99
0.99
0.99
0.99
0.98
0.90
0.89
4
4
5
5
5
5
.62
.62
.04
.08
.11
.52
(
II), neither parathion-methyl nor paraoxon-methyl signifi-
cantly hydrolyzed over our experimental time (up to 48 h).
Mercury had a high ability to catalyze parathion-methyl
hydrolysis significantly. For example, after 12 min, the typical
time for an HPLC run, the reaction is more than 30%
complete. Due to the fast hydrolysis (reaction is >90%
complete within 30 min), we required a method to quench
the reaction at sampling time before injection to the HPLC.
EDTA had the ability to quench Hg and, hence, stop mercury-
promoted hydrolysis of parathion-methyl. Residual hy-
drolysis was observed with the addition of equimolar
concentrations of EDTA (1:1 EDTA:Hgtotal; data not shown).
Therefore, enough EDTA was added to yield a 2:1 EDTA:Hg
ratio to all samples, at sampling times, to ensure no reaction
during HPLC analysis.
Effects of Chloride on Kinetics. Figure 2 shows the
hydrolysis of parathion-methyl at various mercury-chloride
concentrations at pH 3.5. The disappearance of parent
compound and the appearance of hydrolysis product,
p-nitrophenol, increased with increasing mercury chloride.
A molar-mass balance was maintained through the reaction,
indicating that only one of the two possible hydrolysis
mechanisms operates under our experimental conditions
5.61
5.61
56
73
from 9.8 to 21.4 M- s-1 for initial mercury-chloride to
parathion-methyl ratios of 0.5 and 1.0, respectively. However,
at pH 3.5, kobs decreased from 135.0 to 95.5 M
reduction) for initial mercury-chloride to parathion-methyl
ratios of 0.3 and 1.0, respectively. As shown in Figure 1,
Hg-Cl species are more dominant at pH 3.5 than at pH 5.5,
suggesting that chloride has an inhibitory effect on the Hg-
promoted hydrolysis of parathion-methyl. These results
emphasize the importance of metal speciation rather than
total metal concentration on metal-catalyzed hydrolysis.
Furthermore, we believe that the significant difference for
1
-
1
-1
s
(a 30%
-
1
-1
observed rates at pH 3.5 and 5.5 (21.4 and 95.5 M
respectively) for similar HgCl to parathion-methyl ratios can
also be attributed to different catalytic power by the different
s ,
2
2
+
hydroxy-mercury species present at different pHs (e.g., Hg
,
(
i.e., nucleophilic attack to the P center, and no attack to the
+
Hg(OH) , and Hg(OH)
2
). Thus, the rate expression in eq 2
C center). However, this process was not truly catalytic; as
Figure 2 illustrates, at sufficiently low Hg(II) concentrations,
the reaction rate slows throughout its course without
achieving complete hydrolysis of parathion-methyl. This
slow indicates that mercury is consumed (i.e., complexed in
an unavailable form) during the reaction. Similar findings
were reported by Wan et al. (19) at pH 5.5 for fenitrothion
is inadequate to account for different Hg species.
To assess further the effects of chloride, studies with HgCl
at various NaCl concentrations at pH 5.5 were conducted.
The hydrolysis of parathion-methyl decreased with increasing
NaCl concentration, whereby the mercury-promoted hy-
drolysis of parathion-methyl was virtually nonexistent at NaCl
) 0.1 M. To illustrate that this effect is one of Hg speciation
and not solely an ionic-strength effect, experiments with other
2
(
another OP pesticide). Assuming the mechanism proposed
in ref 1, the following rate law results:
electrolytes (NaNO
3
and NaClO
has a high complexation constant with Hg.
For concentrations ranging from 0.001 to 0.1 M, neither
NaNO nor NaClO showed any effect on the mercury-
4
) were conducted. Neither
- -
3 4
NO nor ClO
-
d[PTM]
dt
)
kobs[PTM][Hg(II)]
(2)
3
4
where kobs ) Kk
2
. We determined the second-order rate
chloride promoted hydrolysis of parathion-methyl, where
complete hydrolysis into p-nitrophenol was observed after
75 min at pH 5.5. However, for NaCl, the formation of the
hydrolysis product (p-nitrophenol) ranged from 50% to less
than 1% for electrolyte concentrations ranging from 0.001 to
0.1 M, respectively. Hence, the presence of chloride can
affect significantly the mercury-promoted hydrolysis of OP
constants (kobs) by fitting the experimental data using
nonlinear kinetic software by Micromath: Scientist. As
illustrated in Figure 3, at pH 3.5, the second-order rate
constant (kobs) decreased with increasing HgCl
at pH 5.5, kobs increased with increasing HgCl
example, at pH 5.5, the second-order kobs more than doubled
2
added while
added. For
2
2
3 4 0
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 15, 1998

pesticides. These results indicate that not all the added Hg
is in a form capable of promoting hydrolysis.
pHs. Figure 4a illustrates the calculated rate law versus the
experimentally measured values and the measured rate for
Rate Law as a Function of Hydrated Mercury Species.
HgCl
pound initial ratio. At pH 3.5, the Hg(NO
a kobs value more than 1 order of magnitude greater than that
measured for HgCl . This prediction is further evidence that
chloride has an inhibitory effect and that the mercury-
chloride species do not contribute to the overall Hg-promoted
2
-promoted hydrolysis at the same mercury to com-
HgNO
3
was used to determine the contributions of the various
3 2
)
model predicts
mercury species to the kobs in the absence of chloride. Nitrate
is not a good ligand for mercury, and mercury-nitrate species
in solution are insignificant.
Metal-catalyzed hydrolysis, such as mercury with OP
pesticides, should be viewed as a series of reactions of the
parent compound with different metal species. A reasonable
mechanism may be postulated as the addition of these
reactions in series:
2
hydrolysis of OP pesticides.
+
The k
1
, k
2
, and k
3
values imply that Hg2 is more active
+
than Hg(OH) and that Hg(OH)
2
is essentially inactive. This
information provides insight into the metal-promoted hy-
drolysis mechanism. As stated in the Introduction, different
mechanisms have been proposed, where the Me species act
as either electrophiles or as nucleophiles:
K1
k′1
Hg+2 + SdP(OR)3 7
+2
98 Hg -SdP(OR) 98
3
+
2
Hg -SdP(OH)(OR) + ROH
2
K2
k′
9
2
8
+
+
Hg(OH) + SdP(OR)3
98 Hg(OH) -SdP(OR)
3
+
Hg(OH) -SdP(OH)(OR) + ROH
2
K3
k′
3
Hg(OH) + SdP(OR) 7
9
8 Hg(OH) -SdP(OR) 98
2
3
2 3
2
Hg(OH) -SdP(OH)(OR) + ROH
2
and, assuming a fast complex formation, a reasonable
mechanisms can be expressed as
The general trends of hydroxy-metal species for nu-
cleophlicity and electrophilicity are as follows:
-
d[parathion-methyl]
)
(RHg2+k1 + RHgOH k2 +
+
dt
RHg(OH)3k3 + ...)[Hg(II)][parathion-methyl] (3)
where k
i
) K k′, and R is the fraction of the total Hg(II)
i
i
present as specific species (ionization fraction) and can be
determined from knowledge of the pH and the equilibrium
constants for the different hydroxy-mercury complexes. The
overall second-order rate constant can then be expressed as
Therefore, a metal-promoted hydrolysis mechanism
dominated by nucleophilic attack by the metal species would
show increased rates with increasing pH; a mechanism
dominated by the metal acting as an electrophile would show
increased rates with decreasing pH. Figure 4b illustrates
this hyphothesis for the mercury-promoted hydrolysis ex-
periments of parathion-methyl and the results published by
Smolen and Stone (20) for copper- and lead-promoted
hydrolysis of parathion-methyl. Although, from the pub-
lished data for the Cu(II)- and Pb(II)-promoted hydrolysis of
parathion-methyl, we could not determine the contribution
of the specific hydroxy-metal species to the overall observed
rate because NaCl was used and the pH values studied
resulted in very small ionization fractions for some metal
species (e.g., RCu(OH)2 at pH 5.5 ) 1.25 × 10 ); the general
trend shown in Figure 4b for Pb and Cu are clear. As the
concentration of more nucleophilic species increases (pH
increase), so does the observed rate for both lead and copper.
Such trends support Smolen and Stone’s (20) conclusion
suggesting that Cu(II) and Pb(II) act mostly as nucleophiles.
However, for the mercury-promoted hydrolysis, the maxi-
mum rates are observed at lower pH values, where the metal
species are more electrophilic.
k
1 2 3
k k + RHg(OH)2k ), assuming that no
obs ) (RHg2+ + RHgOH+
other active species are present, or that if they are present,
they do not contribute significantly to the overall observed
catalysis. A similar approach has been used to describe the
reduction of chromium(VI) by iron(II) (25).
The rate constants k
obs at various pH values by solving the three simultaneous
equations:
1 2 3
, k , and k were extracted from the
k
RHg2+
RHg2+
RHg2+
(pH1)
(pH2)
(pH3)
RHgOH+
RHgOH+
RHgOH+
(pH1)
(pH2)
(pH3)
RHgOH(OH)2(pH1)
k1
kobs(pH1)
RHg(OH)2(pH2)
k
kobs(pH2)
2 )
[
][k3 ] [
]
RHg(OH)2(pH3)
kobs(pH3)
-
13
(
4)
At pHs lower than 4.5, the reaction was too fast to allow an
accurate determination of kobs. At pH above 6, Hg and
Hg(OH) become negligible and the catalytic effect too small
2
+
+1
to measure. kobs values were used with MATLAB to solve for
the k
i
values by minimizing the mean square error with the
g 0. Values
condition that the minimum value for any k
i
obtained from the various sets (Table 1) were used to
determine the standard deviation. The calculated second-
order rate constants are
Experiments with paraoxon-methyl show no catalysis
(data not shown) over the experimental time (24 h). This
result suggests that the sulfur (“S”) was involved in the
interaction with Hg. The high affinity of Hg to coordinate
with “S” over “O” centers has been documented and is evident
from the relative stability constants for mercury sulfide and
k1 ) 5023 ( 210 M s-1
-
1
k2 ) 50 ( 22 M s-1
-
1
-
52.3
-13.3
mercury oxide (Ks,HgS ) 10
vs Ks,HgO ) 10
) (24, 26).
k3 ) 0 ( 0 M s-1
-1
Such a selectivity would play a major role when the dominant
role of the metal is as an electrophile. When a nucleophilic
attack by the metal at the PdSor PdO center is the dominant
mechanism, we would expect the PdO center to yield higher
observed rates, because the oxygen is more electron with-
Clearly, different hydroxy-mercury species have different
powers to promote the hydrolysis of organophosphate esters.
The k values were then used to calculate the kobs at different
i
VOL. 32, NO. 15, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2 3 4 1

The contributions of the various mercury species can be
calculated from the observed rates and presented in a pH-
independent rate law for use in the prediction of catalyzed-
hydrolysis rates. Previous studies of metal-catalyzed or
metal-promoted hydrolysis have reported kobs values without
further examination. There are three advantages to report
the relative importance of the different species in catalyzed
or promoted hydrolysis: (1) results can be extrapolated form
one situation to another, (2) rates can be predicted for specific
environmental conditions, and (3) greater understanding of
the catalysis mechanism can be gained.
Literature Cited
(
(
(
(
(
1) ) Negre, M.; Gennari, M.; Crecchio, C.; Ruggiero, P. Soil Sci.
995, 159, 199-206.
2) Fletcher, C. L.; Kaufman, D. D. J. Agric. Food Chem. 1980, 28,
67-671.
3) Kaufman D. D.; Plimmer, J. R.; Kearney, P. C.; Blake, J.; Guardia,
F. S. Weed Sci. 1968, 16, 266-272.
4) Scott, H. D.; Wold, D. C.; Lavy, T. L. J. Environ. Qual. 1982, 11,
1
6
1
07-112.
5) Von Oepen, B.; Kordel, W.; Klein, W. Chemosphere 1989, 18,
1
495-1511.
(
(
6) Turkington, R.; Klein, E. Can. J. Botany. 1991, 69, 226.
7) Chairidchai, P.; Ritchie, G. S. P. Commun. Soil Sci. Plant Anal.
1
993, 24, 261-275.
(
(
8) Blumhorst, M. R.; Weber, J. B. Pestic. Sci. 1994, 42, 79-84.
9) Lion, L. W.; Stauffer, T. B.; MacIntyre, W. G. J. Contam. Hydrol.
1
990, 5, 215-234.
10) Xing, B.; McGill, W. B.; Dudas, M. J. Chemosphere 1994, 28,
45-153.
(
1
(
(
11) Parochetti, J. V.; Warren, G. F. Weed Sci. 1970, 18, 555-560.
12) Rozycki, M.; Bartha, R. Appl. Environ. Microbiol. 1981, 41, 833-
8
36.
13) Dalton, B. R.; Blum, U.; Weed, S. B. Soil Biol. Biochem. 1989,
1, 1011-1018.
14) Wolf, D. C.; Dao, T. H., Scott, H. D.; Lavy, T. L. J. Environ. Qual.
989, 18, 39-44.
(
(
2
FIGURE 4. (a) pH dependence of kobs from calculated rate law and
experimentally determined values. (b) Predicted kobs values for Hg-
1
(
NO
3
)
2
(this study), and for Cu(II), and Pb(II) [right y-axis, pseudo-
(15) Ketelaar, J. A. A.; Gersmann, H. R.; Beck, M. M. Nature 1956,
177, 392-393.
first-order rate constants from Smolen and Stone, (20)].
(
16) Miekel, R. W.; Youngson, C. R. Arch. Environ. Toxicol. 1978, 7,
3-22.
17) Steffens, J. J.; Sampson, E. J.; Siewers, I. J.; Benkovic, S. J. J. Am.
Chem. Soc. 1973, 95, 936-938.
1
drawing than sulfur and would leave the phosphorus center
more receptive to a nucleophilic attack. Significant differ-
ences in observed hydrolysis rates for OPs between the oxo
and thio forms have also been reported by Smolen and Stone
(
(18) Blanchet, P. F.; St.-George, A. Pestic. Sci. 1982, 13, 85-91.
(19) Wan, H. B.; Wong, M. K.; Mok, C. Y. Pestic. Sci. 1994, 42, 93-99.
(
20) Smolen, J. M.; Stone, A. T.; Environ. Sci. Technol. 1997, 31, 1664-
(
20) for Pb(II) and Cu(II). As discussed above, their data
1
673.
suggest that nucleophilic attack is dominant. However, both
lead and copper showed higher observed hydrolysis rates
for the “thio” than for the “oxo” OPs. This preference for “S”
centers by the metals suggests that the sulfur center also
plays a role in the catalyzed hydrolysis. Thus, although the
metal species can act mostly as electrophiles or nucleophiles,
a mixed mechanism may be involved.
(
(
(
21) Suh, J.; Chun, K. H. J. Am. Chem. Soc. 1986, 108, 3057-3063.
22) Torrents, A.; Stone, A. T. Soil Sci. Soc. Am. J. 1994, 58, 738-745.
23) Papelis, C.; Hayes, K. F.; Leckie, J. O. Hydraql: A program for
the computation of chemical equilibria composition of aqueous
batch systems including surface-complexation modeling of ion
adsorption at the oxide/solution interface; Technical Report 306;
Department of Civil Engineering, Stanford University: Menlo
Park, CA, 1988.
(
(
(
24) Smith, R. M.; Martell, A. E., Eds.; Critical Stability Constants;
Plenum Press: New York, 1976.
Environmental Implications
25) Buerge, I. J.; Hug, S. J. Environ. Sci. Technol. 1997, 31, 1426-
Our results indicate that Hg(II) is capable of promoting the
hydrolysis of organophosphate pesticides, thus, use of Hg(II)
salts as biocides in the presence of hydrolyzable pollutants
may lead to unexpected complications. Furthermore, the
overall hydrolysis rate is a function of the concentration of
specific Hg species rather than of the total Hg(II) concentra-
tion, with the dominant mechanism being electrophilic attach
at the S atom.
1
432.
26) Hogfeldt, E., Ed.; Stability Constants of Metal-Ion Complexes
Part A: Inorganic Ligands; Pergamon Press: New York, 1982.
Received for review November 4, 1997. Revised manuscript
received April 1, 1998. Accepted April 30, 1998.
ES970972F
2
3 4 2
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 15, 1998