Influence of Natural Dissolved Organic Matter, Temperature, and Mixing on the Abiotic Hydrolysis of Triazine and Organophosphate Pesticides

Article abstract of DOI:10.1021/jf960315r

Abiotic hydrolysis of simazine, atrazine, diazinon, methylparathion, and chlorpyrifos was studied in three different natural waters and buffered Milli-Q water. The triazines showed no detectable decrease in concentration in any of the waters over 43 days at pH 8.0 and 40 °C. The rates of hydrolysis for diazinon and methylparathion were statistically similar in all waters tested. Chlorpyrifos exhibited a ～32% decrease in hydrolysis rate in the presence of a high concentration of dissolved organic matter (DOM) (34.5 mg/L dissolved organic carbon). The activation energies are larger, and thus the predicted hydrolysis rates are significantly slower than those previously reported. The effect of continuous vigorous mixing on hydrolysis rates was investigated and found to have only a minor effect. The results suggest that uncatalyzed abiotic hydrolysis is very slow for these compounds at the temperatures and pH's typical of most natural waters and affirm the need for a greater understanding of the relative influence of DOM, catalysis, and biodegradation on the fate of organophosphate and triazine pesticides.

Full text of DOI:10.1021/jf960315r

J. Agric. Food Chem. 1996, 44, 3685
−3693
3685
In flu en ce of Na tu r a l Dissolved Or ga n ic Ma tter , Tem p er a tu r e, a n d
Mixin g on th e Abiotic Hyd r olysis of Tr ia zin e a n d Or ga n op h osp h a te
P esticid es
J ames A. Noblet, Lynda A. Smith, and I. H. (Mel) Suffet*
Environmental Science and Engineering Program, Environmental Health Sciences Department, School of
Public Health, University of California, Los Angeles, 10833 LeConte Avenue, Los Angeles, California 90095
Abiotic hydrolysis of simazine, atrazine, diazinon, methylparathion, and chlorpyrifos was studied
in three different natural waters and buffered Milli-Q water. The triazines showed no detectable
decrease in concentration in any of the waters over 43 days at pH 8.0 and 40 °C. The rates of
hydrolysis for diazinon and methylparathion were statistically similar in all waters tested.
Chlorpyrifos exhibited a ∼32% decrease in hydrolysis rate in the presence of a high concentration
of dissolved organic matter (DOM) (34.5 mg/L dissolved organic carbon). The activation energies
are larger, and thus the predicted hydrolysis rates are significantly slower than those previously
reported. The effect of continuous vigorous mixing on hydrolysis rates was investigated and found
to have only a minor effect. The results suggest that uncatalyzed abiotic hydrolysis is very slow for
these compounds at the temperatures and pH’s typical of most natural waters and affirm the need
for a greater understanding of the relative influence of DOM, catalysis, and biodegradation on the
fate of organophosphate and triazine pesticides.
Keyw or d s: Pesticides; organophosphates; triazines; hydrolysis; abiotic; DOM
INTRODUCTION
Five pesticides were chosen for this study based upon
their abundant use in the northern San J oaquin Valley,
CA, and included the triazines simazine and atrazine
and three organophosphate insecticides, diazinon, meth-
ylparathion, and chlorpyrifos. The San J oaquin Valley
accounts for about 10% of the total pesticide use in the
United States (Domagalski and Dubrovsky, 1992) and
thus represents a significant source of organic pollution
to the major waterways of the region.
It is important to understand the relative influence
of various environmental and experimental factors
which can affect the observed empirical pesticide deg-
radation rates. In this study, we focused on three
factors: (1) the effect of DOM in different natural
waters, (2) the effect of temperature, and (3) the effect
of mixing. First, experiments were performed in actual
field waters to investigate the possibility of intersite
variation in hydrolysis rates due to differences in the
overall nature of waters emanating from different
watersheds within the region. The use of different
waters evaluates the effect of DOM as well as the
collective effects of pH, ionic strength, and inorganic
species.
Great care was taken to ensure the sterility of the
waters used for these experiments so that only nonbio-
logically mediated effects would be observed. Steriliza-
tion methods are an important consideration when
performing experiments to determine abiotic degrada-
tion rates. Heat sterilization (autoclave) and chemical
sterilizing agents and preservatives can produce experi-
mental artifacts. Autoclaving has been shown to cause
pH changes (Sharom et al., 1980), and the heat may
alter the character of dissolved organic matter. Ster-
ilization methods utilizing reactive chemicals and/or
radiation, such as ozonation and UV, were excluded
because of their potential to degrade or alter the nature
of the DOM. Moreover, chemicals which remain in the
solution during experiments can affect degradation
rates. The chemical sterilant sodium azide (NaN3) has
This study was performed as part of an effort to
provide site-specific input data for environmental mod-
els used to predict the fate of pesticides from selected
watersheds contributing to the San J oaquin River, CA.
Pesticides are used extensively in agricultural and
urban environments and become pollutants when they
are transported beyond their intended area of influence.
In order to predict the environmental fate of a pesticide,
it is necessary to understand its mobility and rate of
degradation under typical environmental conditions.
Degradation processes are generally divided into bio-
degradation (i.e., mediated by microorganisms) and
abiotic chemical transformations (Schwarzenbach et al.,
1
993). Hydrolysis is an important abiotic degradation
process for many pesticides in aquatic environments.
Although a plethora of hydrolysis rate data are available
for many common pesticides (Howard, 1991), much of
the data has been obtained under experimental condi-
tions uncharacteristic of the field. For example, most
hydrolysis experiments have been performed at, or
above, room temperature, whereas water temperatures
in the field are typically below 25 °C. In addition, all
natural surface waters contain at least some dissolved
organic matter (DOM) (Thurman, 1985), and there is
experimental evidence that humic substances may
significantly affect hydrolysis rates (Macalady et al.,
1
989). Lastly, changes in hydrolysis rates due to the
presence of certain inorganic species (e.g., Cu(II)) have
also been observed (Mortland and Raman, 1967; Meikle
and Youngston, 1978; Blanchet and St. George, 1982;
Chapman and Harris, 1984). These factors, as well as
other experimental variables taken collectively, may
account for the wide range of reported hydrolysis rates
for these compounds in the literature.
*
Author to whom correspondence should be ad-
dressed [phone, (310) 206-8230; fax, (310) 206-3358;
e-mail, msuffet@ucla.edu].
S0021-8561(96)00315-9 CCC: $12.00
© 1996 American Chemical Society

3686 J. Agric. Food Chem., Vol. 44, No. 11, 1996
Noblet et al.
been suspected of reacting with diazinon to cause
anomalously fast degradation (Sharom et al., 1980;
Lichtenstien et al., 1968). In addition, mercuric chloride
equipped with a stainless steel bowl (Horowitz et al., 1989).
The centrifuge was spun at 9800 rpm (9500g), and the
supernatant water flowed through Teflon tubing into 4 L
amber bottles with Teflon-lined closures. The centrifuged
creek waters and the Eucalyptus Ave Drain whole water were
transported back to UCLA in large coolers covered with ice.
The Eucalyptus Ave Drain whole water was subsequently
filtered through 0.45 µm polysulfone membrane filters to
removed suspended particulates. All waters were stored at 4
(HgCl2) has been observed to catalyze the degradation
of some organophosphates (Munch and Frebis, 1992).
Filtration through 0.2 µm membrane filters is commonly
used when the other sterilization methods are unavail-
able or unsuitable (Ingraham and Ingraham, 1995).
Although somewhat less reliable, filtration was selected
as the sterilization method for the natural waters so as
to minimize any alteration of their original composition
or “character.”
Second, the natural waters used in this study were
found to exhibit consistent pH values. Therefore, effort
was concentrated on understanding the temperature
dependence of the hydrolysis rates. Many simple reac-
tions are empirically observed to follow the Arrhenius
law, and therefore it is common practice to characterize
the temperature dependence of first- and pseudo-first-
order reaction rates by making Arrhenius plots (Atkins,
°
C until needed.
Ch em ica ls a n d Sta n d a r d Solu tion s. The atrazine, sim-
azine, diazinon, methylparathion, chlorpyrifos, and their
respective hydrolysis products, hydroxyatrazine, hydroxy-
simazine, 2-isopropyl-4-methyl-6-hydroxypyrimidine (IMHP),
p-nitrophenol, and 3,5,6-trichloro-2-pyridinol were purchased
from Chem Service, Inc., and used without further purification.
Stock solutions of the individual pesticides were prepared at
∼
1200 ppm for the organophosphates and at ∼750 and ∼450
ppm for atrazine and simazine, respectively. A mixed organo-
phosphate solution and a mixed triazine solution were pre-
pared from the stock solutions at approximately 400 and 250
ppm, respectively. A third spiking solution was prepared
which contained all five pesticides with nominal concentrations
of 165 and 135 ppm for the triazines and organophosphates,
respectively. The internal standard, 2-nitro-m-xylene, was
purchased from Aldrich Chemical Co. All stock solutions were
prepared in Fisher Optima grade methanol. Standard solu-
tions of the five pesticides were prepared in ethyl acetate at
nominal concentrations of 0.5, 3, 6, 10, 15, and 20 mg/L and
used for the preparation of standard curves.
1
986). Hydrolysis rates at a minimum of two different
temperatures are required to produce an Arrhenius plot
and thus determine the parameters necessary for
estimation of hydrolysis rates at other temperatures.
Last, in some studies described in the literature, the
pesticide solutions were continuously mixed during the
course of the experiment, while in other studies they
were not. Still other investigators did not clearly specify
whether the solutions were mixed or not. Therefore,
the effect of continuous vigorous mixing was studied to
estimate the impact of this variable relative to the
overall experimental methodology and so that the
results from studies with and without continuous mix-
ing could be properly interpreted and compared. Fur-
thermore, the information obtained could help to un-
derstand the influence of naturally varying hydrologic
conditions on hydrolysis rates. To the authors’ knowl-
edge, the effect of mixing on pesticide hydrolysis rates
has not been previously reported.
Hyd r olysis Exp er im en ts in Na tu r a l Wa ter s (40 °C).
Eight 4 L amber bottles and 32 1 L Boston round amber bottles
were sterilized in an autoclave at 121 °C, 22 psig, for 30 min.
A 90 mm Millipore stainless steel filter holder (with the legs
removed) containing a Gelman Supor 0.2 µm polysulfone
membrane was similarly sterilized by autoclaving. A filter
sterilization apparatus was assembled which consisted of a
Millipore 142 mm hazardous waste extractor containing a 0.45
µm Gelman Supor polysulfone membrane, connected by Teflon
tubing to the sterilized 90 mm filter holder with the 0.2 µm
membrane, which then was connected by Teflon tubing to a
sterile 4 L amber bottle sealed by a cap with two fittings. One
fitting served as inlet for the sterile filtrate, and the other was
used for an exhaust outlet. The outlet was fitted with a 47
mm Teflon filter holder containing a presterilized 0.2 µm
Gelman Supor membrane in order to prevent introduction of
microbes through the exhaust port. Eight liters of each
natural water was run through this filtration system. The
entire setup was taken apart and cleaned and membranes
were replaced and sterilized between each of the natural
waters. After sterilization, the waters were stored at 4 °C until
needed. Several small aliquots of water were taken under
flame with a sterile pipet and plated on agar media. Three
Petri dishes for each bottle were incubated at 37 °C for 72 h.
An aliquot of each of the unsterilized waters was plated on
two Petri dishes and incubated along with the others to test
the ability of the media to support microbial growth. The
appearance of colonies with the unsterilized water, and the
absence of growth with the filtered water, was taken as
confirmation of sterility.
EXPERIMENTAL PROCEDURES
Sa m p lin g. Natural waters were collected from Orestimba
Creek and Del Puerto Creek, two major waterways of the
northern San J oaquin Valley. These two creeks drain inten-
sively farmed watersheds and are conduits for large quantities
of agricultural runoff to the San J oaquin River. Water was
also collected from a third location, referred to herein as
Eucalyptus Ave Drain, a small agricultural drain which also
empties into the San J oaquin River. Water from this location
was collected because it was found to have a high dissolved
organic carbon (DOC) content of 34.5 mg/L (∼70 mg/L DOM).
Natural waters were sampled at each site by wading into
the waterway and dipping an 8 L stainless steel bucket
upstream into the water and transferring the contents into
3
1
7.5 L stainless steel milk cans (Domagalski and Kuivila,
993). This process was repeated until approximately 32 L of
water was collected from each of both Orestimba Creek and
Del Puerto Creek. Water from Eucalyptus Ave Drain was
collected with the stainless steel bucket and transferred
directly into 4 L amber bottles with Teflon-lined closures.
Smaller samples were collected in 1 L amber bottles with
Teflon closures for later determination of suspended solids
concentrations. Dissolved oxygen at each location was meas-
ured using a colorimetric field test kit (CHEMetrics, Inc.,
Calverton, VA). Dissolved oxygen concentrations were con-
sistently between 8 and 10 mg/L, suggesting that the waters
as collected were well mixed and in near equilibrium with the
atmosphere.
Experimental samples were prepared by filling each of 24
1 L presterilized amber bottles to the same level, 950 mL, with
the appropriate natural water. Eight bottles were prepared
for each different field water. After each bottle was filled to
the appropriate level, the water was quickly spiked with 350
µL of the five-pesticide methanol solution and the bottle sealed
with a Teflon-lined closure which had just been immersed in
ethanol. The starting concentrations were therefore ap-
proximately 60 and 50 µg/L for the triazines and organophos-
phates, respectively. The closures were further sealed with
Teflon tape around the edge of the caps. All transfer of water
and spiking was carried out under the flames of two large
Bunsen burners. The 24 bottles were then stored at 4 °C while
the Milli-Q water samples were prepared. The remaining
amounts of each water, ∼400 mL, were used for pH, TOC, and
conductivity. The pH, TOC, conductivity, and suspended solids
The waters from Orestimba and Del Puerto Creeks were
subsequently transported back to a USGS facility in Sacra-
mento, CA. Each of the waters were pumped through Teflon
tubing into a Westfalia high-speed continuous flow centrifuge

Influence of DOM, Temp, and Mixing on Pesticide Hydrolysis
J. Agric. Food Chem., Vol. 44, No. 11, 1996 3687
Ta ble 1. Su m m a r y of Wa ter Qu a lity P a r a m eter s for th e
Ta ble 2. Aver a ge Recover ies of P esticid es fr om Bu ffer ed
Milli-Q Wa ter a n d Del P u er to Cr eek Wa ter for Tr ip lica te
Eth yl Aceta te Extr a ction s F ollow ed by Hea ted
Ku d er n a -Da n ish Tu be Con cen tr a tion w ith Nitr ogen
Blow -Dow n a
4
0 °C Hyd r olysis Exp er im en t
water quality
parameter
Milli-Q Orestimba Del Puerto Eucalyptus
water
Creek
Creek
Ave Drain
pH
8.0
0.4
8.0
3.7
1350
8.0
6.5
1600
7.9
34.5
1050
buffered
Milli-Q water
Del Puerto
Creek water
DOC (mg/L)
conductivity
1300a
(
µmhos/cm)
pesticide
simazine
atrazine
diazinon
methylparathion
chlorpyrifos
% recovery
CV
% recovery
CV
b
suspended solids
(
N/A
121
394
702
59.1
69.0
74.6
76.3
75.1
2.9
2.1
2.7
5.4
6.0
65.2
73.9
86.5
90.4
90.9
1.9
3.8
4.8
2.8
4.1
mg/L)
a
The conductivity of the Milli-Q water is due to the 0.02 M ionic
strength, pH 8.0, phosphate buffer; the original resistance was
8 MΩ. Suspended solids concentration of whole water as
collected before centrifugation and/or filtration.
b
1
a
CV, coefficient of variation, percent relative standard devia-
tion.
data for the waters used in this experiment are summarized
in Table 1.
with a nitrogen-phosphorus detector (Detector Engineering
Technology, Inc., TID-2), using splitless 1.25 µL injection on
a 60 m × 0.32 mm i.d. SPB-1 Column (Supelco, Inc.) and He
as carrier gas. The injector and detector temperatures were
Based on the pH and conductivity data, the Milli-Q water
samples were prepared at pH 8.0 with a 0.02 M ionic strength
phosphate buffer. The water was then sterilized by filtration
as previously described. The eight Milli-Q water samples were
prepared and spiked as previously described. All 32 samples
were then placed into two large water baths and maintained
at 40 ( 0.5 °C for the duration of the experiment (43 days).
One bottle from each of the waters was extracted and analyzed
after about 24 h. Thereafter, one bottle of each water was
analyzed at approximately 1 week intervals. Bottles were
inspected frequently for any signs of microbial growth. Any
bottles exhibiting cloudiness, or unusual odors upon opening,
were discarded.
2
50 and 350 °C, respectively. The temperature program used
was as follows: initial temperature 40 °C, 1 min hold; ramp
to 180 °C at 30 °C/min, hold for 5 min; ramp to 220 °C at 5
°
C/min, hold for 3 min; for a total run time of 21.6 min. The
response of the detector was found to be linear in the
concentration range of 0.5-25 ppm. Because of the detector
sensitivity to H
2
flow, standard curves were prepared each day
that samples were analyzed. The identity of each of the
compounds was initially confirmed by GC/MS, and all com-
pounds were identified by retention time thereafter. Organic
carbon measurements were performed on a Dohrmann DC-
Extr a ction a n d Sa m p le P r ep a r a tion . The pesticides
were extracted from the water phase by liquid-liquid extrac-
tion using ethyl acetate as described by Suffet and Faust
8
0 TOC analyzer, using aqueous UV-persulfate oxidation.
Conductivity measurements were performed with a Horizon
Ecology Co. Model 1484 conductivity meter, and pH was
measured with a Fisher Accumet 925 pH meter. Metal
analyses were performed on an ARL ICP/AES instrument
equipped with a 1.5 m grating.
(1972). In contrast to other solvents tested, ethyl acetate
allowed for the extraction of the triazines as well as the
organophosphates in a single step with adequate recovery. A
2
25 mL aliquot of water sample was measured in a graduated
cylinder and poured into a 250 mL volumetric flask, containing
a Teflon-coated stir bar, followed by the addition of 25.0 mL
of Fisher Optima grade ethyl acetate to the flask with a
volumetric pipet. The flask was placed on a magnetic stirrer
and mixed vigorously for 5 min. The stirring was vigorous
enough so as to produce a large vortex encompassing the entire
contents of the flask. After 5 min, the flask was removed from
the stirrer, and the two phases were allowed to separate. All
extractions were performed in triplicate and the results
averaged.
Hyd r olysis Exp er im en ts in Bu ffer ed Milli-Q Wa ter (32
a n d 24 °C). Twenty 1 L amber bottles were filled with 950
mL of 0.02 M ionic strength, pH 8.0, phosphate-buffered
Milli-Q water, sterilized by autoclave, and spiked with the
three organophosphate pesticides as described above. Sim-
azine and atrazine were omitted since no degradation was
observed in the previous experiments. The 32 °C samples (10
bottles) were maintained at 32 ( 0.5 °C in a water bath. The
2
4 °C samples (10 bottles) were placed in a cabinet near the
floor at ambient temperature. A thermometer was placed in
the cabinet to monitor the temperature. The temperature of
the cabinet was observed to be at 24 ( 1 °C for the duration
of the experiment. As before, the bottles were inspected
frequently for any signs of microbial growth. Samples were
taken at 10-15 day intervals.
The extraction procedure produced an upper layer of about
5
mL, which was about 97% ethyl acetate and 3% water. The
upper layer was pipetted off and dried by passing it through
an 8 mm × 5 in. Pasteur pipet containing a small plug of
pesticide grade glass wool and about 2.0 g of granular ACS
reagent grade anhydrous sodium sulfate. The sodium sulfate
column was first cleaned with about 3 mL of dry ethyl acetate
and the eluate discarded. The extract was then applied to the
column with a Pasteur pipet and allowed to percolate through
the column into a 10 mL glass vial. The column was then
rinsed with about 3 mL of ethyl acetate which drained into
the vial for a total extract volume of about 8 mL. The extract
was then stored in a freezer until analyzed (e1 day).
Immediately prior to analysis the extracts were concen-
trated from ∼8 mL ∼0.5 mL in a heated 10 mL Kuderna-
Danish concentrator with a 2.5 cc/s nitrogen blow-down. The
samples were spiked with internal standard (2-nitro-m-xylene)
and then diluted with ethyl acetate to a final volume of exactly
Hyd r olysis P r od u cts. The base-hydrolysis products for
all of the pesticides used in this study are well-known (Howard,
1
991). Standard solutions of these compounds were prepared
and then analyzed using the same analytical procedures as
for the parent pesticides. This was done in order to confirm
that these compounds would not interfere with the analyses
in any way. All five base-hydrolysis products were found to
elute prior to the analytes of interest. The retention times
for all analytes are given in Table 3. In addition, the response
of the nitrogen-phosphorus detector was found to be ap-
proximately 3 times lower for the organophosphate base-
hydrolysis products due to the loss of the phosphorothioate
group. Therefore, these compounds did not affect the results
of the pesticide analyses.
0
.75 mL and finally transferred to 2 mL screw capped vials
with Teflon-lined closures for analysis. Example recoveries
for the extraction-concentration procedure are given in Table
Mixin g Exp er im en t. Eight 500 mL Boston round amber
bottles with Teflon-lined closures were prepared with 450 mL
of pH 8.0 buffered Milli-Q water, 0.02 M ionic strength, and
then sterilized by autoclave. The solutions were spiked under
sterile conditions with the three organophosphates to give a
nominal starting concentration of 50 µg/L. The bottles were
subsequently placed on a Eberbach Corp. 2-speed shaker at
the low speed (∼120 oscillations/min). After 1 h, one bottle
was removed, and the contents were divided into two 225 mL
2
. During the recovery experiments, the empty bottles were
extracted with 10 mL of ethyl acetate to determine whether
there had been significant sorption by any of the analytes to
the container walls. None of the analytes were detected in
the concentrated extract, and thus it was concluded that
sorption to the container walls was insignificant.
An a lytica l Meth od s. All pesticide analyses were per-
formed on a Perkin-Elmer 8500 gas chromatograph equipped

3688 J. Agric. Food Chem., Vol. 44, No. 11, 1996
Noblet et al.
Ta ble 3. Reten tion Tim es of th e P esticid es a n d Th eir
Resp ective Ba se-Ca ta lyzed Hyd r olysis P r od u cts Usin g
th e Ca p illa r y GC-NP D An a lytica l Meth od Em p loyed in
Th is Stu d y
retention
time
(min)
retention
time
(min)
base-hydrolysis
product
pesticide
simazine
atrazine
diazinon
14.09
14.33
15.55
hydroxysimazine
hydroxyatrazine
2-isopropyl-4-methyl-
4.65
4.72
9.33
6
-hydroxypyrimidine
methylparathion
chlorpyrifos
17.19
19.40
4-nitrophenol
3,5,6-trichloropyridinol
10.29
10.50
Ta ble 4. Su m m a r y of Resu lts fr om th e 40 °C Hyd r olysis
a
Exp er im en ts
observed pseudo-first-order rate constants,
-
1
kobs (day ), in natural and buffered Milli-Q
water at pH 8.0 and 40 ( 0.5 °C (r2 values)
F igu r e 2. Observed concentration vs time data for atrazine
in sterile natural and pH 8.0 phosphate-buffered Milli-Q
waters at 40 °C.
Orestimba Del Puerto Eucalyptus
pesticide
diazinon
Milli-Q
0.0411
Creek
Creek
Ave Drain
0.0360
(0.938)
0.0584
(0.981)
0.0571
(0.976)
0.0363
(0.982)
0.0592
(0.995)
0.0572
(0.991)
0.0360
(0.919)
0.0540
(0.989)
0.0417
(0.969)
(
0.976)
methylparathion 0.0680
(
0.989)
0.0619
0.992)
chlorpyrifos
(
a
Atrazine and simazine are excluded as they showed no
detectable decrease in concentration for the duration of the
experiment.
F igu r e 3. Observed concentration vs time data for diazinon
in sterile natural and pH 8.0 phosphate-buffered Milli-Q
waters at 40 °C.
It is noteworthy, however, that the presence of ∼70
mg/L DOM had no catalytic effect on the transformation
of the triazines. This is in contrast to the catalytic effect
of humic substances that has been observed in the
presence of 2% DOM (Li and Feldbeck, 1972) and in
soils (Stevenson, 1994). These data suggest that abiotic
hydrolysis is insignificant for atrazine and simazine at
the pH’s and DOM concentrations typical of most
natural waters.
The hydrolysis data for diazinon, methylparathion,
and chlorpyrifos are shown in Figures 3-5, respectively.
Linear regression of these data reveals that the hy-
drolysis rates are faster in the buffered Milli-Q water
in comparison to all three of the natural waters. Figure
6 shows a comparison of the pseudo-first-order half-lives
for the three pesticides in the four different waters and
the estimated 95% confidence limits. Although the error
bars imply that the differences between the rate con-
stants may not be statistically significant, the fact that
the trend is consistent among the three pesticides
suggests some validity. Two aspects of the data shown
in Figure 6 are particularly important. First, the rates
of hydrolysis appears to be most decreased in the water
with the highest DOC concentration. Second, the
decrease in hydrolysis rates is most pronounced for
chlorpyrifos, the most hydrophobic of the three com-
pounds with a log Kow of 4.96 as compared to 3.81 and
2.86 for diazinon and methylparathion, respectively
F igu r e 1. Observed concentration vs time data for simazine
in sterile natural and pH 8.0 phosphate-buffered Milli-Q
waters at 40 °C.
aliquots, extracted, and analyzed as previously described. The
shaker remained on continuously, one bottle was removed each
week for 7 weeks, and the residual concentration of the
pesticides was determined in duplicate as previously described.
RESULTS AND DISCUSSION
Hyd r olysis Exp er im en ts in Na tu r a l Wa ter s. The
results of the 40 °C hydrolysis experiments are sum-
marized in Table 4. Simazine and atrazine showed no
detectable decrease in concentration in any of the waters
for the 43 day duration of the experiment. This result
is consistent with other studies which have found that
atrazine and simazine should hydrolyze extremely slow
at pH 8.0 (Howard, 1991; Erickson and Lee, 1989).
Plots of the experimental data for simazine and atrazine
was shown in Figures 1 and 2, respectively. Because
there was no observed change in concentration, the
triazines inadvertently acted as recovery surrogates and
reflect the consistency of the extraction-concentration
procedure.

Influence of DOM, Temp, and Mixing on Pesticide Hydrolysis
J. Agric. Food Chem., Vol. 44, No. 11, 1996 3689
chlorpyrifos that was sorbed to sediments. Although the
effect is not as profound in this study, the mechanism
may be analogous. A few examples of an inhibitory
effect on hydrolysis rates by humic substances have
been previously reported (Macalady et al., 1989; Perdue
and Wolfe, 1983).
The results of the natural water hydrolysis experi-
ments are in contrast to the results of several other
studies which found that chlorpyrifos hydrolyzed faster
in natural waters relative to buffered distilled water
(
Racke, 1993). There is evidence to suggest that the
increased hydrolysis rates may be due to the catalytic
effect of certain chelate-forming metal species, such as
2
+
Cu (Blanchet and St. George, 1982; Chapman and
Harris, 1984; Meikle and Youngston, 1978). Still,
Macalady and Wolfe (1985) were unable to explain up
to a 7-fold increase in the observed hydrolysis rate for
chlorpyrifos in natural water as compared to buffered
distilled water at pH 7.0 and 25 °C, on the basis of metal
catalysis, biodegradation, sterilization method, oxida-
tion, or sorption effects.
F igu r e 4. Observed concentration vs time data for methyl-
parathion in sterile natural and pH 8.0 phosphate-buffered
Milli-Q waters at 40 °C.
Effect of DOM on P esticid e Hyd r olysis. The
experiments reported herein were run at pH 8.0; thus,
base hydrolysis would be expected to predominate. At
this pH, the DOM would be negatively charged, due to
deprotonation of the acidic functional groups. Thus, the
ability of a nucleophile (e.g., OH-) to react with DOM-
bound organic compounds might be greatly diminished
or even eliminated due to electrostatic repulsion and/
or steric hindrance.
To test this hypothesis, we used the following kinetic
approach for evaluation of the hydrolysis data:
dCT
)
-k C - k C
b
(1)
f
aq
b
dt
where CT is the total concentration of pesticide in the
system, Caq is µg of freely dissolved chemical/L of
solution, Cb is µg of DOM-bound chemical/L of solution,
and kf and kb are the freely dissolved and DOM-bound
pseudo-first-order hydrolysis rate constants, respec-
tively. Now define that
F igu r e 5. Observed concentration vs time data for chlor-
pyrifos in sterile natural and pH 8.0 phosphate-buffered
Milli-Q waters at 40 °C.
C ) C + Cb
(2)
T
aq
and
Cb
µg of DOM-bound chemical
Caq µg of freely dissolved chemical
K )
)
(3)
B
Now, substituting eqs 2 and 3 into eq 1:
d(Caq + K C )
B
aq
)
-k C - k K C
aq
(4)
(5)
f
aq
b
B
dt
which can be rearranged to
dCaq -(k + k K )
f
b
B
)
Caq
(1 + KB)
F igu r e 6. Comparison of the half-lives in the different waters
for the three organophosphate pesticides at 40 °C and pH 8.0.
The error bars represent the estimated 95% confidence limits.
and integrated to
(
Howard, 1991). These two observations suggest that
-
(k + k K )
f b B
the slowing of hydrolysis rates may be related to the
degree of association or “binding” of the pesticide to the
DOM. This may be analogous to the observations of
Macalady and Wolfe (1985), who found that the rate of
alkaline hydrolysis was slowed by a factor of 7-11 for
ln Caq
)
t + ln Caq,o
(6)
(
1 + KB)
If it is assumed that no hydrolysis occurs in the DOM-
bound state, then kb f 0 and eq 6 reduces to

3690 J. Agric. Food Chem., Vol. 44, No. 11, 1996
Noblet et al.
-
kf
the Kdoc for anthracene was about 14 000. This is
identical with the value implied herein for chlorpyrifos
ln Caq
)
t + ln Caq,o
(7)
(
1 + KB)
(
log Kow ) 4.96).
The lower degree of DOM binding for chlorpyrifos
Now, since the most profound and statistically sig-
relative to anthracene based on the Kow’s can easily be
explained by the differences in the nature of the DOM
used in the experiments. Another consideration is the
effect of temperature, since sorption tends to decrease
as temperature increases (Schwarzenbach et al., 1993).
The experiments in this study were conducted at 40 °C,
and therefore the observed effects could be exacerbated
by a greater degree of DOM binding at lower experi-
mental and typical field temperatures.
nificant difference in hydrolysis rates was observed for
chlorpyrifos in the Eucalyptus Ave water, these data
will be used for evaluation. The observed rate constants
for chlorpyrifos in Milli-Q water and Eucalyptus Ave
-
1
drain water at 40 °C are kobs ) kf ) 0.0619 day and
-
1
k′obs ) 0.0417 day , respectively (Table 4). Since the
slope of the line described by eq 7 is the observed
pseudo-first-order hydrolysis rate constant, we obtain
Deter m in a tion of Ar r h en iu s P a r a m eter s. The
similarities and differences in hydrolysis rates among
the natural and buffered Milli-Q waters observed in
these experiments are important in terms of under-
standing hydrolysis mechanisms and the relative impact
of DOM on the fate of pesticides in natural waters.
However, the most profound decreases in hydrolysis rate
were observed in water from Eucalyptus Ave drain,
which at ∼70 mg/L DOM clearly represents an extreme
case. Orestimba and Del Puerto Creeks are more
representative of agricultural drainage in the region,
and Figure 6 shows that the corresponding decreases
in hydrolysis rates (as increases in half-lives) for these
waters relative to buffered Milli-Q water (12%, 14%, and
8% for diazinon, methylparathion, and chlorpyrifos,
respectively) were within the 95% confidence limits.
Given these caveats on the natural water data, it
seemed appropriate to perform subsequent experiments
for the determination of Arrhenius parameters in
buffered Milli-Q water. In addition to the resulting
Arrhenius data being more generally applicable to all
waters, the use of buffered Milli-Q water would allow
for sterilization by autoclave, thus eliminating the
difficulties associated with the filter sterilization of large
volumes of natural water.
kf
)
k′
(8)
obs
1
+ KB
and inserting the corresponding experimental data:
0
1
.0619
+ KB
)
0.0417
(9)
Therefore, KB ) 0.484.
Thus, in order to explain the observed effect, about
2% of the total chlorpyrifos in the system must be
3
bound to DOM at any given time and the rate of
hydrolysis in the DOM-bound phase must be negligible.
To evaluate the validity of this result, an implicit
organic carbon-normalized partition coefficient for the
binding of chemical to the DOM can be calculated.
Recalling that
-
6
Cb
Cdoc[DOC] × 10
K )
)
(10)
B
Caq
C
aq
where Cdoc (µg of chemical/kg of DOC) is the organic
carbon-normalized concentration of chemical in the
DOM and [DOC] is the dissolved organic carbon con-
centration in mg/L. The factor of 10 is needed to
convert the sorbent mass units from milligram to the
more conventional kilogram. Now defining
The temperature dependence of many reactions can
be described by the empirical Arrhenius equation
-
6
(
Schwarzenbach et al., 1993). The equation can be
written in log-linear form as
µg of chemical/kg of DOC
µg of chemical/L of solution
Cdoc
Ea
1
( )
Kdoc
)
)
(11)
ln kobs ) ln A -
(13)
Caq
R T
Therefore, combining eqs 10 and 11, we get
where A is the pre-exponential or frequency factor, Ea
is the activation energy, R is the gas constant, and T is
the absolute temperature. For a reaction that exhibits
Arrhenius temperature dependence, a plot of ln kobs vs
KB
0.484
Kdoc
)
)
) 14 029 L/kg
-5
-6
[
DOC] × 10
3.45 × 10
1
/T will give a straight line with a slope of Ea/R and
(12)
the intercept equal to ln A. Arrhenius plots of the
hydrolysis data for the three organophosphates are
shown in Figures 7-9. The data do appear to exhibit
an Arrhenius dependence on temperature, and therefore
the parameters derived by linear regression can be used
to predict hydrolysis rates at any temperature of inter-
est. The Arrhenius data obtained are summarized in
Table 5. These Arrhenius parameters can in turn be
used to estimate the hydrolysis rates at the lower
temperatures more characteristic of field conditions.
These results show the importance of performing hy-
drolysis experiments at multiple temperatures. In the
absence of Arrhenius data, there would have been no
way to predict the approximately 7-, 6-, and 5-fold
decreases in hydrolysis rates for the 10 °C drop from
25 °C to the field temperature of 15 °C for diazinon,
methylparathion, and chlorpyrifos, respectively.
This result is well within the range of sediment and
soil Koc values for chlorpyrifos reported in the literature,
i.e., 995-31000 L/kg (Racke, 1993). Furthermore, this
result is reasonable based on the authors’ own data for
chlorpyrifos sorption to Orestimba and Del Puerto Creek
sediments which are in the range of about 15000-30000
L/kg (Noblet and Suffet, 1996). Additional support for
the feasibility of the implicit Kdoc can be obtained by
comparison to the DOM-binding behavior of other
compounds reported in the literature. If it is assumed
that the extent of binding to DOM is related primarily
to the hydrophobicity of the compound, then compounds
of similar hydrophobicity should exhibit similar DOM-
binding behavior. In one such example, McCarthy and
J imenez (1985) determined the DOM-binding behavior
of anthracene (log Kow) 4.45) to Aldrich humic acid (MW
g 1000) using equilibrium dialysis and fluorescence
techniques. They observed that at [DOC] ) ∼35 mg/L,
Although it is difficult to compare hydrolysis rates
directly because of small differences in experimental

Influence of DOM, Temp, and Mixing on Pesticide Hydrolysis
J. Agric. Food Chem., Vol. 44, No. 11, 1996 3691
Ta ble 5. Hyd r olysis Ra te Con sta n ts a n d th e Resu ltin g
Ar r h en iu s Da ta for th e Th r ee Or ga n op h osp h a te
P esticid es in p H 8.0 P h osp h a te-Bu ffer ed Milli-Q Wa ter
Arrhenius
parameters
kobs (day-1)
Ea
pesticide
diazinon
methylparathion 0.068 0 0.007 23 0.004 75
chlorpyrifos 0.061 9 0.007 66 0.005 34
40 °C
32 °C
24 °C
(kcal/mol) ln A
0.041 1 0.007 86 0.002 22
33.7
30.5
28.1
50.83
46.08
42.10
sis rates due to a variety of factors such as biodegrada-
tion and the presence of certain metal species. Most of
the reported half-lives for diazinon range from 54 days
at pH 8.0 and 25 °C (Chapman and Cole, 1982) to 185
days at pH 7.4 and 20 °C. Half-lives for methyl-
parathion range from 16 days at 25 °C and pH 8.5 in
fresh water to 231 days at 25 °C and pH 7.0 in sea water
F igu r e 7. Arrhenius plot of the abiotic hydrolysis rate data
for diazinon in pH 8.0 phosphate-buffered Milli-Q water.
(
Badawy and El-Dib, 1984; Howard, 1991). Chlorpyrifos
has been reported to have hydrolysis half-lives of 23 and
4 days, at pH 8.1, for 25 and 15 °C, respectively (Meikle
5
and Youngston, 1978). Walker et al. (1988), reported
half-lives of 14-24 days for chlorpyrifos in estuarine
water, nonsterile and sterilized with formaldehyde, at
2
5 °C. Macalady and Wolfe (1983) offered a rate
-
6
-1
constant of 6.22 × 10 min as their “best estimate”
for neutral hydrolysis at 25 °C in buffered distilled
water; which corresponds to a half-life of ∼77 days.
Although not explicitly reported, Sharom et al. (1980)
show a plot of diazinon hydrolysis in unsterilized
distilled water at 21 ( 1 °C and pH ∼7, wherein the
half-life appears to be about 70 days. The same study
found that chlorpyrifos and parathion exhibited half-
lives of ∼12 and .16 weeks, respectively. Only about
1
0% of the parathion had hydrolyzed after 16 weeks. A
comparison between methylparathion and parathion is
probably valid since methyl analogs have been shown
to hydrolyze at similar, but somewhat faster, rates
F igu r e 8. Arrhenius plot of the abiotic hydrolysis rate data
for methylparathion in pH 8.0 phosphate-buffered Milli-Q
water.
(Gomaa et al., 1969; Freed et al., 1979). Fewer studies
have determined the Arrhenius parameters for these
compounds. A comparison of the activation energies
determined in this study to those previously reported
is presented in Table 5. Again, the results from this
study show activation energies significantly higher than
those previously reported.
The interpretation of the hydrolysis data is compli-
cated by several factors. First, these compounds are
known to undergo both neutral and acid- and/or base-
catalyzed hydrolysis (Freed et al., 1979). At pH 8.0, it
is reasonable to suspect a significant contribution from
both hydrolytic mechanisms. Thus, the observed hy-
drolysis rates are most likely the result of two competing
hydrolysis mechanisms, each with their own activation
energies. Thus, it would be more appropriate to refer
to the activation energies determined as “apparent
activation energies” since the specific mechanisms are
not known (Brezonik, 1994).
Meta ls. Each of the natural waters used in this
study were analyzed by ICP/AE for the following ele-
ments: P, Na, K, Ca, Mg, Zn, Cu, Fe, Mn, B, Mo, Al, Si,
Ti, V, Co, Ni, Cr, Pb, Sr, Ba, Li, As, Se, Cd, Ag, and Sn.
The results from ICP/AE analyses showed low levels (<5
ppb; Sn < 15 ppb) of heavy metals in the natural waters
used in this experiment. This result is consistent with
the fact that no increase in hydrolysis rates was
observed in the natural waters relative to Milli-Q water.
Therefore, it seems safe to infer that dissolved inorganic
species had little or no influence on the hydrolysis rates
observed in this study, save for their effect on pH and
ionic strength.
F igu r e 9. Arrhenius plot of the abiotic hydrolysis rate data
for chlorpyrifos in pH 8.0 phosphate-buffered Milli-Q water.
conditions, the Arrhenius data from this study predict
hydrolysis rates for these pesticides that are much
slower than those previously reported (Howard, 1991;
Racke, 1993; LaCorte et al., 1995; Lartiges and Garri-
gues, 1995). Indeed, the calculated hydrolysis half-lives
for diazinon, methylparathion, and chlorpyrifos at a
nominal field temperature of 15 °C are 1988, 1000, and
7
69 days, or 5.4, 2.7, and 2.1 years, respectively. These
results are particularly interesting inasmuch as most
of the discussion of these compounds in the literature
has focused on explaining the enhancement of hydroly-

3692 J. Agric. Food Chem., Vol. 44, No. 11, 1996
Noblet et al.
Ta ble 6. Com p a r ison of Ap p a r en t Hyd r olysis Activa tion En er gies Deter m in ed in Th is Stu d y to Selected Va lu es
Rep or ted in th e Liter a tu r e
pesticide
and water type
activation energy,
pH
8.0
3.1
10.4
6.1
Ea (kcal/mol)a
reference
diazinon
buff Milli-Q
buff distilled
buff distilled
Milli-Q
33.7
13.15
14.29
3.1
this study
Gomaa et al., 1969
Gomaa et al., 1969
Lartiges and Garrigues, 1995
Lartiges and Garrigues, 1995
Lartiges and Garrigues, 1995
a
a
river
7.3
3.5
a
filtered river
seawater
7.3
8.1
7.6
3.9
a
Lartiges and Garrigues, 1995
methylparathion
buff Milli-Q
buff distilled
Milli-Q
8.0
11.0
6.1
7.3
7.3
30.5
9.9
7.0
6.1
9.7
8.8
this study
Badawy and El-Dib, 1984
Lartiges and Garrigues, 1995
Lartiges and Garrigues, 1995
Lartiges and Garrigues, 1995
Lartiges and Garrigues, 1995
a
a
river
filtered river
seawater
a
a
8.1
chlorpyrifos
buff Milli-Q
buff distilled
buff distilled
buff distilled
buff distilled
pond water
buff distilled
buff distilled
8.0
7.4
4.7
6.9
8.1
8.0
28.1
14.0
22.8
19.0
21.8
17.5
22.1 ( 0.8
13.0 (1.5
this study
Freed et al., 1979
Meikle and Youngston, 1978
Meikle and Youngston, 1978
Meikle and Youngston, 1978
Macalady and Wolfe, 1983
Macalady and Wolfe, 1983
Macalady and Wolfe, 1983
4.5-7.5
11.25
a
The reported data therein are for nonsterile waters and therefore may include a biodegradation component.
Ta ble 7. Effect of Con tin u ou s Sh a k in g on th e Obser ved
P seu d o-F ir st-Or d er Hyd r olysis Ra tes of th e Th r ee
Or ga n op h osp h a te P esticid es
ment were faster than the predicted nonshaking rates
for all three pesticides. Presumably, vigorous mixing
is analogous to increasing temperature in that the
primary effect is to increase the kinetic energy of the
molecules in solution and thus increase the number of
molecular collisions per unit time. The hydrolysis
mechanism is governed primarily by the pH (Freed et
al., 1979) and thus should not be directly affected by
temperature or mixing. Therefore, to quantify the effect
of shaking on hydrolysis rates, the notion of an “effective
temperature” was employed. Using the Arrhenius data
for nonshaking conditions, the temperature necessary
to produce the observed shaking hydrolysis rates was
estimated and compared to the actual temperature of
the experiment. Thus, an effective temperature change,
T1/2 at 29 °C (days)
with
shaking
without
shaking
apparent
temp (°C)
effective
a
b
c
∆T (°C)d
pesticide
diazinon
methylparathion
chlorpyrifos
108.9
63.4
62.2
130
84
79
30.0
30.7
30.5
+1.0
+1.7
+1.5
a
Elevated temperature due to heat from shaker motor; stable
b
at 29 ( 0.5 °C. Hydrolysis rate at 29 °C estimated using the
Arrhenius parameters determined herein. Apparent temperature
c
to account for observed shaking hydrolysis rates estimated from
d
Arrhenius data. Effective ∆T, estimated equivalent temperature
change necessary to account for the observed hydrolysis rate
increase due to shaking.
∆
Teff, due to mixing alone can be determined, and the
P ossible Exp er im en ta l Ar tifa cts. It is evident
that experimental errors such as losses due to volatil-
ization, biodegradation, pH changes, etc., would cause
results opposite in the sense to those observed. The
erroneous lowering of observed hydrolysis rates could
stem from several possible causes: (1) contamination
of glassware used for extraction, (2) changing of extrac-
tion efficiency as a function of analyte concentration,
relative impact of mixing on the observed hydrolysis
rates can be estimated. These data are shown in Table
6. The estimated effective temperature changes of
∼1-2 °C clearly show that the relative impact of
vigorous mixing on the observed hydrolysis rates is
minimal. Therefore, the absence of mixing cannot
explain the slow hydrolysis rates observed in this study.
In addition, this result has important implication for
environmental modeling, in that the energetics of an
aquatic system need not be considered as an additional
parameter for the accurate prediction of abiotic trans-
formation rates.
Con clu sion s. The results of this study have impor-
tant implications for pesticide fate modeling in aquatic
systems. For the triazines, the results suggest that
dissolved humic substances alone are not sufficient to
catalyze transformations and that a solid substrate, i.e.,
sediment or soil, may play an important role.
The data for the organophosphates suggest that in
addition to facilitating transport, DOM may also impede
the abiotic hydrolytic degradation of these pesticides.
The Arrhenius data derived in this study emphasize the
importance of understanding the temperature depend-
ence of environmental fate processes. The effect of
vigorous mixing on the observed hydrolysis rates of the
organophosphates was found to be minimal, equivalent
to only a 1-2 °C increase in temperature. Thus, the
(3) degradation of pesticide standards and/or internal
standard, (4) degradation products or other compounds
coeluting with analytes, and/or (5) sorption-desortion
to container walls. After careful consideration and
investigation of each of the above factors, the authors
concluded that the experimental controls were sufficient
to preclude these processes from having a significant
effect on the experimental observations. Thus, the
authors firmly believe the data obtained in this study
are valid and not the result of experimental artifacts.
Effect of Mixin g on Hyd r olysis Ra tes. The results
of the continuous shaking hydrolysis experiment are
summarized in Table 7. Due to heat emanating from
the shaker motor, the ambient temperature of the water
was consistently 29 ( 0.5 °C during the course of the
experiment (50 days). In order to evaluate the effect of
continuous mixing, the Arrhenius parameters were used
to estimate the nonshaking hydrolysis rates at 29 °C.
The hydrolysis rates measured for the shaking experi-

Influence of DOM, Temp, and Mixing on Pesticide Hydrolysis
J. Agric. Food Chem., Vol. 44, No. 11, 1996 3693
lack of continuous mixing cannot explain the slow
hydrolysis rates observed. These results suggest that
in the absence of catalytic effects, abiotic hydrolysis is
very slow for these pesticides under typical field condi-
tions and may be insignificant relative to other fate
processes. Moreover, the results underscore the need
for developing a greater understanding of the relative
impact of processes such as catalysis, biodegradation,
and DOM association on the fate of organophosphate
pesticides.
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Received for review April 30, 1996. Revised manuscript
received August 27, 1996. Accepted August 29, 1996. Partial
funding for this study was provided by the Metropolitan Water
District of Southern California.
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Lartiges, S. B.; Garrigues, P. P. Degradation Kinetics of
Organophosphorus and Organonitrogen Pesticides in Dif-
ferent Waters under Various Environmental Conditions.
Environ. Sci. Technol. 1995, 29 (5), 1246-1254.
Li, G. C.; Feldbeck, G. T. Atrazine Hydrolysis as Catalyzed
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Abstract published in Advance ACS Abstracts, Oc-
tober 1, 1996.