Oxidation of diazinon by aqueous chlorine: Kinetics, mechanisms, and product studies

Article abstract of DOI:10.1021/jf981004e

The oxidation kinetics and mechanisms of diazinon, an organophosphorus pesticide, by aqueous chlorine were studied under different conditions. The oxidation is of first order with respect to both diazinon and chlorine. The oxidation rate is found to increase with decreasing pH. The second-order rate constants at pH 9.5, 10.0, 10.5, and 11.0 are determined to be 1.6, 0.64, 0.43, and 0.32 M^-1 s^-1, respectively. Based on the rate constants at different temperatures, the activation energy is calculated to be 30 kJ/mol at pH 10.0 with a chlorine-to-diazinon ratio of 11:1, 33 kJ/mol at pH 11.0 with a 11:1 ratio, and 36 kJ/mol at pH 11.0 with a 5:1 ratio, respectively. Diazoxon is identified as the oxidation product by GC-MS. Ion chromatography analysis shows an increase of sulfate concentration as the reaction proceeds, indicating that sulfur is being oxidized to sulfate. This study indicates that oxidation by aqueous chlorine can significantly affect the fate of diazinon in the environment.

Full text of DOI:10.1021/jf981004e

1760
J. Agric. Food Chem. 1999, 47, 1760−1766
Oxid a tion of Dia zin on by Aqu eou s Ch lor in e: Kin etics, Mech a n ism s,
a n d P r od u ct Stu d ies
Qi Zhang and Simo O. Pehkonen*
Department of Civil and Environmental Engineering, University of Cincinnati,
Cincinnati, Ohio 45221-0071
The oxidation kinetics and mechanisms of diazinon, an organophosphorus pesticide, by aqueous
chlorine were studied under different conditions. The oxidation is of first order with respect to both
diazinon and chlorine. The oxidation rate is found to increase with decreasing pH. The second-
order rate constants at pH 9.5, 10.0, 10.5, and 11.0 are determined to be 1.6, 0.64, 0.43, and 0.32
M-1 s-1, respectively. Based on the rate constants at different temperatures, the activation energy
is calculated to be 30 kJ /mol at pH 10.0 with a chlorine-to-diazinon ratio of 11:1, 33 kJ /mol at pH
11.0 with a 11:1 ratio, and 36 kJ /mol at pH 11.0 with a 5:1 ratio, respectively. Diazoxon is identified
as the oxidation product by GC-MS. Ion chromatography analysis shows an increase of sulfate
concentration as the reaction proceeds, indicating that sulfur is being oxidized to sulfate. This study
indicates that oxidation by aqueous chlorine can significantly affect the fate of diazinon in the
environment.
Keyw or d s: Diazinon; oxidation; chlorine; diazoxon
INTRODUCTION
Esters and thioesters of phosphoric acid and thio-
phosphoric acid are widely used in agricultural practice
as insecticides, herbicides, and, on a more limited scale,
fungicides (Mortimer and Dawson, 1991). Concern about
F igu r e 1. Structure of diazinon.
organophosphorus pesticides arises from the large
amount applied and their persistence in the environ-
ment since pesticides are not specific for insect acetyl-
cholinesterases but can also affect the nervous systems
of humans. Previous studies have been focused on the
hydrolysis of organophosphorus pesticides. Hydrolysis
is pH- and temperature-dependent (Muehlmann and
Schrader, 1957; Faust and Gomaa, 1972; Schwarzen-
bach et al., 1993) and can be catalyzed or inhibited by
oxide surfaces (Torrents and Stone, 1994; Baldwin et
al., 1995), dissolved metals and metal-containing sur-
faces (Mortland and Raman, 1967; Plastourgou and
Hoffmann, 1984; Stone and Torrents, 1995; Stone et al.,
1995) and metal oxides (Dannenberg and Pehkonen,
1998; Hong and Pehkonen, 1998).
Diazinon, a phosphorothioate commercially intro-
duced in 1952, is used as a pesticide for different types
of cultivation such as fruit trees, rice, sugarcane, corn,
tobacco, and horticultural plants because of its inhibi-
tion of the acetylcholinesterases of most kinds of insects.
Figure 1 shows the structure of diazinon. Diazinon is
considered moderately toxic: 48-h LC 50 in killifish is
4.4 mg/L (Tsuda et al., 1997). However under certain
conditions such as the contamination of hydrocarbon
solvent with water (0.1-2%) plus exposure to elevated
temperatures, diazinon can deteriorate to harmful
substances including monothiotepp (O,S-TEPP) and
sulfotepp (S,S-TEPP) which are known to be highly toxic
and to have a strong inhibitory effect on cholinesterase
enzyme systems (Gysin and Margot, 1958; Sovocool et
F igu r e 2. Structure of diazoxon.
al., 1981; Soliman et al., 1982). Previous studies found
the hydrolysis of diazinon to be relatively slow. Hy-
drolysis half-life at pH 7.0 and 25 °C is 178 days
(Schwarzenbach et al., 1993). The hydrolysis rate also
strongly depends on pH and temperature: half-lives at
pH 3.1, 5.0, 7.4, 9.0, and 10.4 at 20 °C are 11.77, 740.7,
4435.8, 3263.0, and 144.9 h, respectively; at pH 3.1, the
rate constant at 40 °C is larger than the one obtained
at 10 °C by a factor of 7 (Faust and Gomaa, 1972). The
presence of catalysts also affects the rate of hydrolysis.
Dannenberg and Pehkonen (1998) applied ferrihydrite,
goethite, and hematite and found both catalytic and
inhibitory effects of these compounds under different
conditions (i.e., different pH, temperatures, and dosages
of catalysts). 2-Isopropyl-4-methyl-6-hydroxypyrimidine
is the identified hydrolysis product under both acidic
and basic conditions (Muhlmann and Schrader, 1957;
Faust and Gomaa, 1972; Dannenberg, 1996).
For the oxidation of diazinon, previous studies were
focused on ozonation. Ohashi et al. (1994) applied
ozonation in the aqueous phase and found that diazoxon
(Figure 2) was the oxidation product. They proposed
that sulfur was oxidized to sulfate in solution after it
was released from the diazinon molecule and that
diazoxon was further hydrolyzed to diethyl phosphate
* Author for correspondence (fax, 513-556-2599; e-mail,
spehkone@uceng.uc.edu).
10.1021/jf981004e CCC: $18.00 © 1999 American Chemical Society
Published on Web 03/06/1999

Oxidation of Diazinon by Aqueous Chlorine
J. Agric. Food Chem., Vol. 47, No. 4, 1999 1761
lower than the rate constants observed in the experiments
using borate as the buffer.
and 2-isopropyl-4-methyl-6-hydroxylpyrimidine; these
two compounds further degraded to acetic acid and
formic acid. Ku et al. (1998) also studied the ozonation
of diazinon in aqueous solution. They concluded that
ozonation had been shown to be feasible for achieving
nearly complete degradation of diazinon within 1 h.
Their kinetic results showed that pH, temperature, and
alkalinity had little influence on the ozonation of
diazinon. According to the analyses by Tsuda et al.
(1997), 48-h LC 50 in killifish for diazoxon is 0.22 mg/
L, meaning that diazoxon is more toxic than diazinon.
Faust and Gomaa (1972) also mentioned diazoxon as
the oxidation product of diazinon. They pointed out that
the anti-acetylcholinesterase activity of diazoxon proved
to be higher than that of diazinon. According to a
thorough literature search, none of the aforementioned
studies investigated the reaction kinetics or mechanisms
of diazinon oxidation by aqueous chlorine.
The current study is intended to evaluate the oxida-
tion kinetics, mechanisms, and product(s) of diazinon
by aqueous chlorine. Hypochlorite is used as the oxidant
since it is the most commonly utilized reagent for
disinfection (Montgomery, 1985) in both wastewater and
drinking water treatment plants. The contact time
usually ranges from 15 to 45 min (Tchobanoglous and
Burton, 1991). Some treatment plants discharge the
treated water without dechlorination. Since Amato et
al. (1992) observed diazinon in municipal wastewater,
it is very possible for diazinon to be oxidized by aqueous
chlorine either in treatment plants or in ambient water
systems immediately after treatment. The rates, mech-
anisms, and product(s) of diazinon oxidation by aqueous
chlorine will assist in assessing the fate of diazinon in
aquatic environments.
Ch lor in e Dem a n d of Wa ter a n d Rea ctor s. Chlorine
demand of deionized and distilled water and reactors can cause
the real amount of chlorine reacting with diazinon to be lower
than the calculated one since the water and reactors can
consume a portion of the chlorine. The procedure for checking
the chlorine demand of deionized and distilled water was as
follows: Sodium hypochlorite at a concentration of 1.34 × 10-4
M was added in 0.01 M phosphate or borate buffer which was
made with distilled and deionized water and stirred for 2-3
h. The solution was left under ultraviolet lamp for 6-8 h to
decompose chlorine because chlorine is highly photoreactive
at ultraviolet wavelengths of about 2600 Å and decomposes
quickly (Scarpino et al., 1972). After the decomposition, the
buffer was tested with the DPD ferrous titrimetric method
(Eaton et al., 1995) to make sure there was no chlorine left in
it. The buffer was chlorine-demand-free and used to conduct
control experiments. To check the chlorine demand of the
amber Pyrex reactors, a buffer with 6.71 × 10-4 M sodium
hypochlorite was filled in a reactor bottle and kept stirring
for 2-3 h. Then the reactor bottle was emptied and dried in
the air. Later the control experiment was carried out in this
chlorine-demand-free reactor.
GC a n d GC-MS An a lyses. HP 5890 series II GC with an
FID detector (Hewlett-Packard Co., Palo Alto, CA) was used
for GC analysis. GC conditions were as follows: 30- × 0.53-
mm i.d. fused silica capillary column with 1.5-µm film thick-
ness (DB-5, J &W Scientific, Folsom, CA) and a carrier gas of
nitrogen (10 psi) were used; initial temperature was 130 °C
for 1 min; temperature increased at 17 °C/min up to 220 °C
for 3 min; injector port temperature was 250 °C; detector
temperature was 300 °C. GC standards for diazinon were
prepared by making aqueous solutions of diazinon at 2.3 ×
10-5, 4.6 × 10-5, 8.2 × 10-5, and 1.4 × 10-4 M; 10 mL of
solution was extracted with 1.5 mL of benzene containing the
internal standard.
HP 5890 series II GC with HP 5970 MS detector (Hewlett-
Packard) was used for GC-MS analysis. GC-MS conditions
were as follows: 30-m × 0.25-mm i.d. fused silica capillary
column with 0.25-µm film thickness (DB-5ms, J &W Scientific)
and a carrier gas of helium (5.5 psi) were used; initial
temperature was 80 °C; temperature increased at 3 °C/min
up to 160 °C; injector port temperature was 225 °C; detector
temperature was 250 °C. After the last sample was taken for
the kinetics determination, the remaining reaction mixtures
were extracted with benzene and served as GC-MS samples.
Neat diazoxon was dissolved in benzene and subjected to GC-
MS analysis as a standard.
Ion Ch r om a togr a p h y An a lysis for Su lfa te Con cen tr a -
tion . Sulfate concentrations in the reaction mixtures were
analyzed with ion chromatography to check the valence state
of sulfur after it was released from the diazinon molecule. The
analysis was performed by Environmental Enterprises Inc.
(Cincinnati, OH). The experiments were conducted at pH 10.5
with 7.79 × 10-4 M sodium hypochlorite. Since sulfite will be
oxidized to sulfate by chlorine and therefore increases the
concentration of sulfate, ferrous bromide was used instead of
sodium sulfite to consume the extra chlorine in the samples.
Kin etic Con cep t. The rate law and its constants for the
following reaction have been determined in this study:
MATERIALS AND METHODS
Oxid a tion Exp er im en ts. Diazinon was obtained from
Supelco (Bellefonte, PA) at 99% purity. Diazoxon was obtained
from Chemservice (Westchester, PA) at 96% purity. Sodium
hypochlorite solution (13%) was purchased from Aldrich
(Milwaukee, WI). The hypochlorite solution was titrated
periodically using DPD ferrous titrimetric method (Eaton et
al., 1995) to measure the accurate concentration of chlorine
because of the concern over the decomposition of chlorine with
time.
Amber Pyrex bottles (to prevent the exposure to light which
can decompose chlorine) of 250-mL size were used as reactors.
Solutions were prepared from 18 MΩ cm water (deionized and
distilled); 0.01 M phosphate (for pH 11.0) or borate buffer (for
pH 9.5, 10.0, and 10.5) was used to cover the pH range. The
initial concentration of diazinon was 6.58 × 10-5 M. Sodium
hypochlorite was added 6-12 h after diazinon was mixed with
buffer to make sure of the thorough dissolution of diazinon.
The pH was monitored throughout the experiments and
adjusted with concentrated sulfuric acid or sodium hydroxide.
Samples were withdrawn periodically covering at least two
half-lives (i.e., the final concentration of diazinon in the
reaction mixture was one-fourth of the original concentration).
Sodium sulfite was used to consume the extra chlorine in
withdrawn samples, thus quenching further oxidation. Oxida-
tion of diazinon was monitored by extracting a 10-mL aliquot
of the stirred reaction mixture with 1.5 mL of benzene
containing 4-chloro-3-methylphenol (Aldrich) as the internal
standard for GC.
a diazinon + b chlorine f c product(s)
(1)
In the above equation, a is the reaction order with respect to
diazinon and b is the reaction order with respect to aqueous
chlorine. To get a and b, various dosages of sodium hypochlo-
rite were used. For all kinetic measurements, initial molar
concentration ratios of sodium hypochlorite to diazinon were
varied from 3:1 to 20:1. Therefore, a and b could be obtained
by eq 2:
One experiment at pH 10.5 and 27 °C with sodium carbon-
ate as the buffer (at a concentration of 0.01 M) was conducted
to check if chlorine radicals were involved in the reactions since
carbonate is an effective radical scavenger (Buxton et al.,
1988). If radicals were involved in the reactions, the rate
constant obtained in this experiment would be significantly
- d[diazinon]
) kr[diazinon]a[chlorine]b
(2)
dt

1762 J. Agric. Food Chem., Vol. 47, No. 4, 1999
Zhang and Pehkonen
For a first-order reaction in terms of diazinon,
ln[diazinon] ) kobs
t
(3)
where kobs equals kr[chlorine]b, and kr is the second-order rate
constant.
Th er m od yn a m ic Ap p r oa ch . Oxidation reactions were
carried out at four temperatures, room temperature (∼26 °C),
low temperatures of ∼0 and 15 °C, and an elevated temper-
ature of 34 °C, in order to study the effect of temperature on
the oxidation rate and to obtain the activation parameters.
Experiments at 0, 15, and 34 °C were conducted in large
beakers filled with ice, a temperature-controlled cold room, and
a thermostatically controlled water bath, respectively. These
experiments were conducted under three conditions: at pH
11.0 with chlorine-to-diazinon ratio of 5:1, at pH 11.0 with a
chlorine-to-diazinon ratio of 11:1, and at pH 10.0 with a
chlorine-to-diazinon ratio of 11:1. According to eq 4:
F igu r e 3. Oxidation of diazinon at pH 10.0 and 25.8 °C with
7.79 × 10-4 M sodium hypochlorite.
HOCl T H+ + OCl-, KA)10-7.5
(4)
the distribution of hypochlorous acid (HOCl) and hypochlorite
ion (OCl-) will change with pH, thus an understanding about
the thermodynamic characteristics of the oxidation of diazinon
with respect to hypochlorous acid and hypochlorite ion can be
obtained. As a result, kr can be expressed as
kr ) k0[HOCl]/[Cl]T + k1[OCl-]/[Cl]T
(5)
The activation enthalpy, ∆Hq, was determined by measuring
the rate of reaction at four temperatures and plotting ln(kr/T)
versus 1/T based on eq 6; the activation entropy, ∆Sq, was
calculated according to eq 7. Arrhenius equation (eq 8) was
used to calculate the activation energy by plotting ln kr versus
1/T (Carey and Sundberg, 1984):
ln(kr/T) ) -∆Hq/RT + constant
∆Sq) (∆Hq/T) + [R ln(hkr/K kT)]
(6)
(7)
(8)
kr ) Ae-E /RT
a
F igu r e 4. Reaction order with respect to chlorine.
here k is Boltzmann constant, h is Planck’s constant, and K
is the transmission coefficient (∼1).
Ta ble 1. Oxid a tion Ra te Con sta n ts a t Differ en t p H
Va lu es a t Room Tem p er a tu r e a t a Ch lor in e Dosa ge of
Effect of p H on Kin etics. pH of 9.5, 10.0, 10.5, and 11.0
were selected to study the effect of pH on the oxidation rate.
The reason for choosing 9.5 as the lowest pH was that
oxidation at pH lower than 9.5 is so quick that the rate
constants cannot be measured. However, based on pH and the
percentage distribution of hypochlorous acid and hypochlorite
ion, the oxidation rate constants of diazinon with these two
species separately were calculated according to eq 4. Finally,
the oxidation rate constant of diazinon by aqueous chlorine
at any pH can be obtained by calculation according to eq 5.
7.79 × 10-4
reaction conditions
M
kr ( SDa (M-1 s-1
)
no. of expt
pH 9.5
1.6 ( 0.42
3
3
3
4
pH 10.0
pH 10.5
pH 11.0
0.64 ( 0.030
0.43 ( 0.080
0.32 ( 0.014
a
SD, standard deviation.
Ta ble 2. Ca lcu la ted Oxid a tion Ra te Con sta n ts
constant
k (M-1 s-1
130
0.27
101
32.0
)
RESULTS AND DISCUSSION
a
k0
k1
Rea ction Or d er . For each experiment, plotting ln
[diazinon] versus time resulted in a linear profile. This
indicates that the oxidation is a first-order reaction with
respect to diazinon. The determination of the rate
constant of the experiment at pH 10.0 and room tem-
perature is shown in Figure 3. Figure 4 is the plot of
log kobs versus log [chlorine]T. Since the slope of the
linear curve is close to 1, one can see that the oxidation
with respect to chlorine is also a first-order reaction.
Oxid a tion Kin etics. Oxidation rate constants at four
pH values at a chlorine dosage of 7.79 × 10-4 M (the
molar concentration ratio of chlorine to diazinon is 11:
1) are listed in Table 1. The calculated rate constants
of k0, k1, and kr at pH 7.0 and 8.0 are listed in Table 2.
At pH 7.0 and 8.0, the second-order rate constant is 101
b
kr at pH 7.0
kr at pH 8.0
a
k0 is the intrinsic rate constant when diazinon is oxidized by
hypochlorous acid. b k1 is the intrinsic rate constant when diazinon
is oxidized by hypochlorite ion.
and 32.0 M-1 s-1, respectively. The typical chlorine
residual after 15 min of contact time in the treatment
plants is 0.5 mg/L which equals 7.0 × 10-6 M (Tchoban-
oglous and Burton, 1991). Using this value for a
calculation, the oxidation half-life of diazinon is just 16.4
min at pH 7.0 and 51.6 min at pH 8.0. Compared to the
hydrolytic half-life at pH 7.0, which is 178 days, oxida-
tion by aqueous chlorine is much faster than hydrolysis.

Oxidation of Diazinon by Aqueous Chlorine
J. Agric. Food Chem., Vol. 47, No. 4, 1999 1763
Ta ble 3. Oxid a tion Ra te Con sta n ts of Ch lor in e Dem a n d
Exp er im en ts
reaction conditions
description
kr (M-1 s-1
)
pH 9.5, 7.79 × 10-4
M
chlorine demand
of the water
chlorine demand
of the water
chlorine demand
of the water
chlorine demand
of the reactor
1.3
NaClO, 27.0 °C
pH 10.0, 7.79 × 10-4
NaClO, 27.0 °C
M
M
M
0.68
0.35
0.37
pH 11.0, 7.79 × 10-4
NaClO, 27.2 °C
pH 10.5, 7.79 × 10-4
NaClO, 27.1 °C
Ta ble 4. Oxid a tion Ra te Con sta n ts u n d er Differ en t
Tem p er a tu r e, p H, a n d Ch lor in e Dosa ges
kr ( SDa
no. of
expt
reaction conditions
(M-1 s-1
)
pH 10.0, 7.79 × 10-4 M NaClO, 0.8 °C
pH 10.0, 7.79 × 10-4 M NaClO,14.5 °C
pH 10.0, 7.79 × 10-4 M NaClO,33.9 °C
pH 11.0, 3.22 × 10-4 M NaClO, 0.6 °C
pH 11.0, 3.22 × 10-4 M NaClO, 13.7 °C
pH 11.0, 3.22 × 10-4 M NaClO, 33.9 °C
pH 11.0, 7.79 × 10-4 M NaClO, 0.6 °C
pH 11.0, 7.79 × 10-4 M NaClO, 12.9 °C
pH 11.0, 7.79 × 10-4 M NaClO, 34.3 °C
0.22 ( 0.028
0.35 ( 0.050
0.94 ( 0.039
0.088 ( 0.024
0.18 ( 0.028
0.48 ( 0.057
0.094 ( 0.027
0.14 ( 0.017
0.48 ( 0.029
3
3
3
3
3
3
3
3
3
F igu r e 5. Determination of activation energies under three
reaction conditions.
a
SD, standard deviation.
The rate constant of the oxidation using sodium
carbonate as the buffer is 0.33 M-1 s-1. This value is
quite close to the average rate constant under the same
temperature and pH conditions with borate as the
buffer, which is 0.43 M-1 s-1. This indicates that there
are probably no chlorine radicals involved in the oxida-
tion of diazinon by aqueous chlorine.
Ch lor in e Dem a n d of Wa ter a n d Rea ctor s. The
observed rate constants using chlorine-demand-free
buffer (see Table 3) at pH 9.5, 10.0, and 11.0 are 1.26,
0.68, and 0.35 M-1 s-1, respectively. Not one of these
values is significantly larger than the corresponding
average rate constants listed in Table 1. This indicates
that the deionized and distilled water has no significant
chlorine demand. The observed rate constant obtained
F igu r e 6. Determination of activation enthalpies under three
reaction conditions.
Ta ble 5. Th er m od yn a m ic P a r a m eter s for Oxid a tion of
Dia zin on
by using chlorine-demand-free reactor is 0.37 M-1 s-1
,
pH 10.0/
ratio 11:1
pH 11.0/
ratio 5:1
pH 11.0/
ratio 11:1
which is even smaller than the average oxidation rate
constant at pH 10.0, which is 0.43 M-1 s-1. Therefore,
the amber glass reactors can also be considered chlorine-
demand-free.
parameter
∆Hq (kJ /mol)
Ea (kJ /mol)
28
30
-0.21
33
36
-0.21
31
33
-0.19
∆Sq (kJ /mol × K)
Activa tion P a r a m eter s. Oxidation rate constants
obtained under three reaction conditions are shown in
Table 4. A plot of ln kr versus 1/T under these conditions
is shown in Figure 5. Figure 6 is the plot of ln kr/T
versus 1/T under the same conditions. R2 of each curve
is greater than 0.95 except the one at pH 11.0 with a
chlorine-to-diazinon ratio of 11:1 in Figure 6. Activation
energies, enthalpies, and entropies are listed in Table
5. Activation parameters for the hydrolysis of diazinon
are shown in Table 6 for comparison. Activation energy
is the energy difference between reactant molecules in
the ground state and the transition state. It determines
how rapidly the reaction occurs: a large activation
energy corresponds to a large energy difference between
ground states and transition states and results in a slow
reaction because few reacting molecules collide with
enough energy to climb the high activation energy
barrier; a small activation energy results in a rapid
reaction since almost all reacting molecules are ener-
getic enough to climb to the transition state (McMurry,
1992). Activation energies obtained in this study (Table
5) are smaller than those of hydrolysis of diazinon (Table
6), which indicates that the energy requirements for the
Ta ble 6. Th er m od yn a m ic P a r a m eter s for Hyd r olysis of
Dia zin on a
parameter
pH 5.7
pH 8.5
∆Hq (kJ /mol)
Ea (kJ /mol)
77
80
-0.11
120
130
0.030
∆Sq (kJ /mol × K)
a
Data from Dannenberg (1996).
formation of intermediates during the oxidation of
diazinon are lower. This is in agreement with the much
faster oxidation rates compared to hydrolysis rates.
Based on the same principle, the activation energy at
pH 10.0 is smaller than that at pH 11.0 indicating that
the reaction of diazinon with hypochlorite ion is more
energy-demanding. It should be noted that the activa-
tion entropies under the three conditions are very
similar (Table 5). Since the entropy of activation is a
measure of the degree of order (i.e., negative ∆Sq) or
disorder (i.e., positive ∆Sq) produced in the formation
of the activated complex, it is reasonable to conclude
that the order produced in the formation of the activated

1764 J. Agric. Food Chem., Vol. 47, No. 4, 1999
Zhang and Pehkonen
F igu r e 7. GC-MS spectra of diazoxon standard and a reaction mixture.
complexes at both pH 10 and 11 is roughly at the same
degree. Furthermore, because of the negativity of the
entropies, we can conclude that these oxidation reac-
tions involve a loss of translational, vibrational, or
rotational degrees of freedom in going to the transition
state since there is a decrease in the activation entropy
of the system (Carey and Sundberg, 1984).
Effect of p H on Kin etics. The second-order rate
constants at pH 9.5, 10.0, 10.5, and 11.0 are listed in
Table 1. Unlike the ozonation of diazinon which is pH-
independent (Ku et al., 1998), the oxidation of diazinon
by aqueous chlorine is very sensitive to pH. The
calculated intrinsic rate constants using hypochlorous
acid (k0) and hypochlorite ion (k1) as the oxidant

Oxidation of Diazinon by Aqueous Chlorine
J. Agric. Food Chem., Vol. 47, No. 4, 1999 1765
and hypochlorite ion separately are 130 and 0.27 M-1
s-1, respectively. At pH 7.0, the half-live of degradation
of diazinon at a chlorine concentration of 0.5 mg/L
(equaling 7.0 × 10-6 M) is just 16 min. Activation
energies and enthalpies of the oxidation are smaller
than those for hydrolysis of diazinon. Diazoxon is the
only product identified by GC-MS. Sulfur is found to
be oxidized to sulfate after it is released from the
diazinon molecule. This study indicates that oxidation
by aqueous chlorine can be another important pathway
for the degradation of diazinon in aquatic systems.
ACKNOWLEDGMENT
We thank Dr. Pasquale Scarpino and Dr. Hans
Zimmer for their helpful advice and Dr. Koka J ayasim-
hulu for technical assistance in GC-MS analysis.
F igu r e 8. Sulfate formation in the oxidation of diazinon at
pH 10.5 and 27.4 °C with 7.79 × 10-4 M sodium hypochlorite.
LITERATURE CITED
separately are listed in Table 2. k0 is larger than k1 by
a factor of 490, explaining why a small change in pH
has a large influence on the observed rate constants.
The reason oxidation of diazinon by hypochlorous acid
is much faster can be explained by bond strength.
Hydrogen atom and oxygen atom, oxygen atom and
chlorine atom are connected by sharing electron pairs.
For hypochlorous acid, the hydrogen atom donates one
electron to the oxygen atom, thus the oxygen atom in
hypochlorous acid is less electrophilic than the oxygen
atom in hypochlorite ion. Therefore, the strength of the
oxygen-chlorine bond in a hypochlorous acid molecule
is weaker than that of the oxygen-chlorine bond in a
hypochlorite ion. The oxygen-chlorine bond in hy-
pochlorous acid is easier to break, therefore resulting
in the fast oxidation rate when diazinon is oxidized by
hypochlorous acid.
P r od u ct Id en tifica tion . Diazoxon was the only
identified product by GC-MS. The retention time of
diazoxon was 32.242 min. Neat diazoxon yielded a very
similar GC retention time (31.995 min). GC-MS spectra
of a reaction mixture and diazoxon standard are shown
in Figure 7. Since all experiments were conducted at
high pH values, hydrolysis of diazoxon was also fast
under those conditions and could not be neglected (the
half-life of hydrolysis for diazoxon at pH 10.4 is 10.1 h;
Faust and Gomaa, 1972); with this complication no mass
balance was carried out for diazoxon. Figure 8 shows
the increase in sulfate concentration in the reaction
mixture as the oxidation progressed. The increase
indicates that sulfur was oxidized to sulfate after it was
released from the diazinon molecule. One may notice
that the sulfate concentration of the last sample (45 µM)
was somewhat lower than the corresponding sulfur
concentration in diazinon molecules (79 µM). This can
be explained by the fact that when ferrous bromide was
added for sample dechlorination, iron(III) hydroxide was
formed and precipitated. A fraction of the sulfate ions
was adsorbed on the precipitate; therefore, the concen-
tration of sulfate in the solution was decreased.
Amato, J . R.; Mount, D. I.; Durhan, E. J .; Lukasewycz, M. T.;
Ankley, G. T.; Robert, E. D. An Example of the Identification
of Diazinon as a Primary Toxicant in an Effluent. Environ.
Toxicol. Chem. 1992, 11, 209-216.
Baldwin, D. S.; Beattle, J . K.; Coleman, L. M.; J ones, D. R.
Phosphate Ester Hydrolysis Facilitated by Mineral Phases.
Environ. Sci. Technol. 1995, 29, 1706-1709.
Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B.
J . Phys. Chem. Ref. Data 1988, 15, 449-456.
Carey, F. A.; Sundberg, R. J . Advanced Organic Chemistry,
2nd ed.; Plenum Press: New York, 1984; pp 165-197.
Dannenberg, A. Investigation of the Homogeneous and Het-
erogeneous Hydrolysis Rates and Mechanisms of Selected
Phosphoric and Thiophosphoric Acid Esters. Masters Thesis,
University of Cincinnati, Cincinnati, OH, 1996.
Dannenberg, A.; Pehkonen, S. O. Investigation of the Hetero-
geneously Catalyzed Hydrolysis of Organophosphorus Pes-
ticides. J . Agric. Food Chem. 1998, 46, 325-334.
Eaton, A. D.; Clesceri, L. S.; Greenberg, A. E. Standard Method
for the Examination of Water and Wastewater, 19th ed.;
American Public Health Association: Washington, DC,
1995.
Faust, S. D.; Gomaa, H. M. Chemical Hydrolysis of Some
Organic Phosphorus and Carbamate Pesticides in Aquatic
Environments. Environ. Lett. 1972, 3, 171-201.
Gysin, H.; Margot, A. Chemistry and Toxicological Properties
of O,O-Diethyl-O-(2-isopropyl-4-methyl-6-pyrimidinyl) Phos-
phorothioate (Diazinon). J . Agric. Food Chem. 1958, 6, 900-
903.
Hong, F.; Pehkonen, S. O. Hydrolysis of Phorate Using
Simulated Environmental Conditions: Rates, Mechanisms,
and Product Analysis. J . Agric. Food Chem. 1998, 46, 1192-
1199.
Ku, Y.; Chang, J . L.; Shen, Y. S.; Lin, S. Y. Decomposition of
Diazinon in Aqueous Solution by Ozonation. Water Res.
1998, 32, 1957-1963.
McMurry, J . Organic Chemistry, 2nd ed.; Brooks/Cole Publish-
ing Co.: Pacific Grove, CA, 1992; pp 153-166.
Montgomery, J . M. Water Treatment Principles and Design;
J ohn Wiley & Sons: New York, 1985; p 556.
Mortland, M. M.; Raman, K. V. Catalytic Hydrolysis of Some
Organic Phosphate Pesticides by Copper (II). J . Agric. Food
Chem. 1967, 15, 163-167.
Muhlmann, R.; Schrader, A. Hydrolyse der Insektiziden Phos-
phora¨ureester. Z. Naturforsch. 1957, 12B, 196-208.
Ohashi, N.; Tsuchiya, Y.; Sasano, T.; Hamada, A. Ozonation
Products of Organophosphorus Pesticides in Water. J pn. J .
Toxicol. Environ. Health 1994, 40, 185-192.
Plastourgou, M.; Hoffmann, M. R. Transformation and Fate
of Organic Esters in Layered-Flow Systems: The Role of
Trace Metal Catalysis. Environ. Sci. Technol. 1984, 18, 756-
764.
CONCLUSIONS
Oxidation rates of diazinon by aqueous chlorine
obtained in this study are much larger than the corre-
sponding hydrolysis rates. The second-order rate con-
stants at pH 9.5, 10.0, 10.5, and 11.0 are 1.6, 0.64, 0.43,
and 0.32 M-1 s-1, respectively. The second-order reac-
tion rate constants of diazinon with hypochlorous acid

1766 J. Agric. Food Chem., Vol. 47, No. 4, 1999
Zhang and Pehkonen
Scarpino, P. V.; Berg, G.; Chang, S. L.; Dahling, D.; Lucas, M.
A Comparative Study of the Inactivation of Viruses in Water
by Chlorine. Water Res. Pergamon Press 1972, 6, 959-965.
Schwarzenbach, R.; Gschwend, P.; Imboden, D. Environmental
Organic Chemistry, 1st ed.; J ohn Wiley & Sons: NewYork,
1993; Chapter 12.
Soliman, S. A.; Sovocool, G. W.; Curley, A.; Ahmed, N. S.; El-
Fiki, S.; El-Sebae, A. K. Two Acute Human Poisoning Cases
Resulting from Exposure to Diazinon Transformation Prod-
ucts in Egypt. Arch. Environ. Health 1982, 37, 207-212.
Sovocool, G. W.; Harless, R. L.; Bradway, D. E.; Wright, L.
H.; Lores, E. M.; Feige, L. E. The Recognition of Diazinon,
an Organophosphorus Pesticide, when Found in Samples
in the Form of Decomposition Products. J . Anal. Toxicol.
1981, 5, 73-80.
M., McGill, W. B., Page, A. L., Eds.; Lewis: Boca Raton,
FL, 1995.
Stone, A. T.; Smolen, J . M.; Huang, C. H.; Torrents, A.
Pesticide Hydrolysis Catalyzed by Dissolved Metal Ions and
Metal-Containing Mineral Surfaces. ACS Conference in
Chicago, IL, 1995.
Tchobanoglous, G.; Burton, F. L. Water Engineering, 3rd ed.;
McGraw-Hill: New York, 1991; p 501.
Torrents, A.; Stone, A. T. Oxide Surface-Catalyzed Hydrolysis
of Carboxylate Esters and Phosphorothioate Esters. Soil Sci.
Soc. Am. J . 1994, 58, 738-745.
Tsuda, T.; Kojima, M.; Harada, H.; Nakajima, A.; Aoki, S.
Acute Toxicity, Accumulation and Excretion of Organophos-
phorous Insecticides and Their Oxidation Products in Kil-
lifish. Chemosphere 1997, 35, 939-949.
Stone, A. T.; Torrents, A. The Role of Dissolved Metals and
Metal-Containing Surfaces in Catalyzing the Hydrolysis of
Organic Pollutant. In Environmental Impact of Soil Com-
ponent Interactions; Huang, P. M., Berthelin, J ., Bollag, J .
Received for review September 8, 1998. Revised manuscript
received J anuary 12, 1999. Accepted J anuary 26, 1999.
J F981004E