Oxidation of diazinon by anodic Fenton treatment

Article abstract of DOI:10.1016/S0043-1354(02)00041-6

Anodic Fenton treatment (AFT) is a new technology that has several advantages over classic Fenton treatment and electrochemical Fenton treatment. The oxidation of diazinon by AFT using different electrolytes has been investigated. NaCl, KCl and Na₂SO₄ show similar effects on the extent and rate of oxidation, and the data can be fitted quite well by the AFT kinetics model. Use of NaNO₃ as the electrolyte causes low-efficiency electrolysis and a subsequent low oxidation rate for diazinon. The NaCl concentration level in the anodic half-cell and the concentration ratio between the two half-cells is optimized at 0.04M and 4:1 (cathodic/anodic), respectively. The activation energy of diazinon oxidation by anodic Fenton treatment is estimated to be 12.6±0.6kJmol^-1, which is less than half of that for aqueous chlorine treatment. Diazoxon is the intermediary oxidation product. The oxidation of diazinon as a formulated product has also been investigated. Its dissipation kinetics can also be fitted quite well by the AFT model. Compared with the oxidation of pure diazinon, the oxidation rate of formulated diazinon is much lower, an indication that many formulation ingredients compete with diazinon for reaction with the hydroxyl radical.

Full text of DOI:10.1016/S0043-1354(02)00041-6

Water Research 36 (2002) 3237–3244
Oxidation of diazinon by anodic Fenton treatment
Qiquan Wang, Ann T. Lemley*
Department of TXA, Graduate Field of Environmental Toxicology, MVR Hall, Cornell University, Ithaca, NY 14853-4401, USA
Received 1 July 2001; received in revised form 1 December 2001
Abstract
Anodic Fenton treatment (AFT) is a new technology that has several advantages over classic Fenton treatment and
electrochemical Fenton treatment. The oxidation of diazinon by AFT using different electrolytes has been investigated.
2 4
NaCl, KCl and Na SO show similar effects on the extent and rate of oxidation, and the data can be fitted quite well by
the AFT kinetics model. Use of NaNO as the electrolyte causes low-efficiency electrolysis and a subsequent low
3
oxidation rate for diazinon. The NaCl concentration level in the anodic half-cell and the concentration ratio between
the two half-cells is optimized at 0.04 M and 4:1 (cathodic/anodic), respectively. The activation energy of diazinon
ꢀ
1
oxidation by anodic Fenton treatment is estimated to be 12.670.6 kJ mol , which is less than half of that for aqueous
chlorine treatment. Diazoxon is the intermediary oxidation product. The oxidation of diazinon as a formulated product
has also been investigated. Its dissipation kinetics can also be fitted quite well by the AFT model. Compared with
the oxidation of pure diazinon, the oxidation rate of formulated diazinon is much lower, an indication that
many formulation ingredients compete with diazinon for reaction with the hydroxyl radical. r 2002 Elsevier Science
Ltd. All rights reserved.
Keywords: Fenton treatment; Diazinon; Oxidation; Pesticide; Anodic
1
. Introduction
enhances treatment efficiency over the classic Fenton
treatment due to the partial regeneration of ferrous ion
in the reaction system. Mineral-catalyzed Fenton treat-
ment overcomes the use of ferrous salts, which are
usually hygroscopic and readily oxidized, but both
mineral-catalyzed and advanced electrochemical Fenton
treatments need addition of sulfuric acid before opera-
tion. In addition, the treatment effluents from these
methods and from classic Fenton treatment are very
acidic; neutralization is always needed for further
treatment. The electrochemical Fenton treatment can
keep the effluent neutral, but the treatment efficiency is
significantly lower than other Fenton treatments.
Hydroxyl radical (ꢁOH) is known as one of the most
active oxidants with a wide variety of organic pollutants
in water. Among all known oxidants, only fluorine has a
higher oxidation potential than hydroxyl radical [1]. The
Fenton reaction has been investigated by many as an
effective method to generate hydroxyl radicals. Several
kinds of technologies based on the Fenton reaction have
been applied to the treatment of many toxic or
nonbiodegradable organics in wastewater and soil. They
include classic Fenton treatment [2–4], photo-assisted
Fenton reaction [5], electrochemical Fenton treatment
[
6], mineral-catalyzed Fenton treatment [7], and ad-
Based on the classic and electrochemical Fenton
treatment, the anodic Fenton treatment (AFT) has been
recently conceptualized in our laboratory [9]. The most
significant advantages of AFT over the other Fenton
treatment technologies are that the Fenton reaction
occurs in self-developed optimal acidic conditions
(pH=2–3), and the pH of the treatment effluent can
be partially neutralized by combining solutions from the
vanced electrochemical Fenton oxidation [8]. All of
these technologies show efficient oxidation of organic
pollutants. The use of photo-assistance significantly
*Corresponding author. Tel. : +1-607-255-3151; fax: +1-
6
07-255-1093.
E-mail address: atl2@cornell.edu (A.T. Lemley).
0043-1354/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 4 3 - 1 3 5 4 ( 0 2 ) 0 0 0 4 1 - 6

3238
Q. Wang, A.T. Lemley / Water Research 36 (2002) 3237–3244
anodic and cathodic half-cells. The other major advan-
tage of AFT is that the ferrous ion is delivered into the
reaction system by electrolysis from a sacrificial iron
anode. This overcomes the need to handle a large
amount of hygroscopic ferrous salt, which is very readily
oxidized.
ferrous sulfate hexahydrate (99+%) was purchased
from Aldrich (Milwaukee, WI 53233). Diazinon for-
mulation ‘‘Diazinon Insect Control’’ was purchased from
Bonide Products, Inc. (Yorkville, NY 13495)
2.2. Oxidation of diazinon by AFT
Diazinon [O,O-diethyl O-2-isopropyl-methylpyrimi-
din-4-yl phosphorothioate] is a phosphorothiate insecti-
cide widely used in home gardens and farms to control a
variety of sucking and leaf eating insects, inactivating
acetylcholinesterase in most kinds of insects [10]. Since
diazinon can also affect the nervous system of humans,
concern about its use arises from the large amount
applied and the production of rinse water from contain-
ers and application equipment. The oxidation of
diazinon by ozone [11] and aqueous chlorine [12] has
been reported. A specific ozone generator is needed for
ozonation treatment, and aqueous chlorine treatment is
suspected of yielding some hazardous chlorinated
organic compounds during the process [13]. Compared
with these two treatment technologies, the Fenton
treatment may be simpler and safer.
Oxidation experiments were carried out in two
250 mL beakers that served as anodic and cathodic
half-cells. A salt-bridge filled with a saturated solution
of the same salt used in both half-cells as electrolyte was
used to connect the half-cells. Typically, 200 mL of
100 mM diazinon solution with 0.04 M NaCl was added
into the anodic half-cell, and the same volume of 0.16 M
NaCl aqueous solution was added into the cathodic
half-cell. Each of the two half-cells was stirred using a
magnetic stirring bar. Ferrous ion was delivered into the
anodic half-cell by electrolysis from a sacrificial iron
anode (2 cm ꢂ 10 cm ꢂ 0.2 cm). A graphite stick (1 cm
(i.d.) ꢂ 10 cm (L)) was put into the cathodic half-cell as a
cathode. The electrolysis current was supplied by a BK
s
Precision DC power supply 1610 and was controlled at
0.050 A. Hydrogen peroxide solution of 0.3109 M was
delivered into the anodic half-cell using a Fisher
The optimal ratio of hydrogen peroxide to ferrous ion
and the relationship between treatment efficiency and
delivery rate of Fenton reagent have been investigated
and published elsewhere [14]. The oxidation of diazinon
by AFT was carried out in this study, and an established
kinetic model was used to fit the data. The objectives of
this work were: (1) to investigate the effect of different
salts as electrolytes for AFT treatment; (2) to optimize
the electrolyte concentration in two half-cells; (3) to
determine the activation energy of diazinon oxidation by
AFT; (4) to determine the degradation products of
diazinon by AFT; and (5) to compare the oxidation of
the active ingredient by AFT with that of diazinon
formulation.
ꢀ
1
Scientific peristaltic pump at a rate of 0.50 mL min
2
.
was 10:1 for all experiments,
+
The ratio of H
2
O
2
:Fe
and the temperature was kept at 25.570.51C. The
electrolysis was started by turning on the power supply
when the first drop of hydrogen peroxide entered the
anodic half-cell. At different time intervals, 1.00 mL of
anodic solution was removed from the anodic half-cell
and added into a 2 mL GC-vial containing 0.10 mL
methanol (to quench the subsequently generated hydro-
xyl radical) for HPLC analysis. Treatments were
repeated two times for a total of three replications.
To investigate the effect of different electrolytes, four
2 4 3
salts (NaCl, KCl, Na SO , and NaNO ) were used. The
concentration of each electrolyte in the anodic and
cathodic half-cells was 0.04 and 0.16 M, respectively.
The degradation rate of diazinon by AFT with these
four electrolytes was compared, and the total iron ion
concentration after 2 min of electrolysis was determined.
In the experiments on the effect of electrolyte concen-
tration in the two half-cells, three anodic NaCl
concentration levels were chosen: 0.02, 0.04, and
0.08 M. For each anodic NaCl concentration, 5 con-
centration ratios of cathodic to anodic NaCl were
investigated. They were 1:0.5, 1:1, 1:2, 1:4, and 1:8. The
total iron ion concentration after 2 min electrolysis of
each treatment was also measured.
2
. Materials and methods
2
.1. Chemicals
Diazinon (99.5%) and diazoxon (>90%) were
purchased from Chem Services (West Chester, PA
9381). Hydrogen peroxide (analytical grade), acetoni-
1
trile (HPLC grade), water (HPLC grade), hydroxyla-
mine hydrochloride (analytical grade), potassium
dichromate (analytical grade) and sodium nitrate
(
(
analytical grade) were purchased from Mallinckrodt
Paris, KY 40361). Sodium chloride (certified), sodium
sulfate (certified), potassium chloride (certified), ammo-
nium acetate (certified), and phosphoric acid (analytical
grade) were purchased from Fisher Scientific (Fair
Lawn, NY 07410). Hydrochloric acid, sulfuric acid
and acetic acid were all analytical grade and purchased
from EM Science (Gibbstown, NJ 08027). Ammonium
To study the effect of temperature on oxidation,
different water baths with different temperatures were
used. The selected temperatures were 10.570.51C,
18.370.51C, 26.070.21C, and 32.970.51C.
When investigating the oxidation of a diazinon
formulation product by AFT, the original formulation

Q. Wang, A.T. Lemley / Water Research 36 (2002) 3237–3244
solution was diluted to change the concentration of AFT:
3239
diazinon from 990 to 100 mM. NaCl was added into
the diluted formulation solution to a concentration
of 0.04 M. At different times the oxidation was
stopped and samples of the solution were taken
for high performance liquid chromatography
½
DŠt
1
2 2
0
ln
¼ ꢀ Klpov t ;
ð1Þ
½
DŠ0
2
where K ¼ kk1 (mM minꢀ2), k (mM minꢀ1) and k1
(mM ꢁ min ) are the second-order rate constants of
the Fenton reaction and the reaction between hydroxyl
ꢀ2
ꢀ1
ꢀ1
ꢀ1
(HPLC) analysis and chemical oxygen demand (COD)
determination.
radical and diazinon, respectively; ½DŠ0 (mM) and ½DŠt
(
mM) are the concentrations of diazinon at 0 and t min,
2.3. Analysis of diazinon concentration, total iron ion,
and determination of COD
respectively; l (min) and p (min) are the average life of
the hydroxyl radical and ferrous ion, respectively; o is a
constant related to the delivery ratio of hydrogen
peroxide to ferrous ion and to the consumption ratio
The concentration of diazinon was analyzed using a
HP 1090 HPLC equipped with a diode-array detector
ꢀ1
of hydrogen peroxide; v0 (mM min ) is the delivery rate
of ferrous ion by electrolysis; and t (min) is time.
(DAD) detector. The mobile phase of HPLC was
composed of acetonitrile and water (70:30, pH was
adjusted to 3 using phosphoric acid). A C18 5 m
2
50 mm ꢂ 4.6 mm (i.d.) PRISM reverse phase (RP)
3
. Results and discussion
column was used for analysis. The chosen wavelength
for the DAD was 254720 nm. Under these conditions,
the retention times of diazinon and diazoxon were 10.1
and 4.9 min, respectively.
3.1. Oxidation of diazinon using different salts as
electrolytes
Total iron ion concentration in the anodic half-cell
was analyzed using the 1,10-phenanthroline method [15].
COD was determined using the dichromate method [16].
The dissipation of diazinon during the AFT treatment
using NaCl, KCl, Na SO , and NaNO as electrolytes is
2
4
3
shown in Fig. 1. The average initial voltages for these
electrolyses were 12.7, 13.7, 13.8, and 14.8 V, respec-
tively. No significant degradation of diazinon was found
2
.4. Oxidation products determination
when control experiments were run using only H
if electrolysis were applied without addition of H
data not shown). Using NaCl, KCl, or Na SO
2 2
O or
An Esquire-LC 00146 HPLC/MS (mass spectrum)
2
O
2
was used for the oxidation products analysis.
For diazinon oxidation by AFT using NaCl as
the electrolyte, 5 mL anodic solution was removed
at 0 and 2 min and mixed with 1 mL of methanol.
Samples were analyzed using direct injection/ion trap
technology.
(
2
4
as
electrolytes, the AFT was very effective and fast in
oxidizing diazinon. The degradation profile is S-shaped,
with a low rate at the beginning and end of the treatment
and a high rate in the middle of the oxidation process.
As shown in Fig. 1, the oxidation kinetics of diazinon
using these three electrolytes can be fitted quite well by
2
.5. Kinetic model for AFT treatment
100
A detailed derivation of the kinetic model for AFT
NaCl
treatment was published elsewhere [14]. Here, we briefly
summarize this established model. During AFT treat-
ment, ferrous ion is constantly delivered into an anodic
half-cell by electrolysis at a fixed rate. It is also
continuously consumed by reacting with hydrogen
peroxide, which is delivered into the reaction system
simultaneously. We assume (1) the concentration of
ferrous iron in the reaction system is constant; (2)
hydrogen peroxide can be accumulated in the reaction
system when the ratio of hydrogen peroxide to ferrous
ion is larger than 1; (3) the Fenton reaction obeys
second-order kinetics; (4) the instantaneous concentra-
tion of hydroxyl radical is proportional to its generation
rate; and (5) the kinetics of hydroxyl radical reaction
with organics obey second order. Based on these
assumptions, the following equation can be established
to describe the changes of diazinon concentration during
Na SO
2
4
80
KCl
NaNO3
60
40
20
0
0
1
2
3
4
5
6
Time (min)
Fig. 1. Oxidation of diazinon by AFT using different salts as
electrolytes. Points are experimental data and lines are the
fitting results using the AFT kinetic model.

3240
Q. Wang, A.T. Lemley / Water Research 36 (2002) 3237–3244
the AFT kinetic model. All regression coefficients are
over 0.99, and the derived oxidation rates using NaCl,
Table 1
Total iron ion concentration and current efficiency from
electrolysis using different electrolytes
KCl and Na
difference in rate is found between NaCl and KCl. The
substitution of Na SO only caused a slight decrease in
2 4
SO are quite similar to each other. No
Electrolyte
Total iron ion concentration
after 2 min of electrolysis
Current
efficiency
(%)
2
4
ꢀ1
the diazinon oxidation rate. Compared to these three
electrolytes, a significant decrease in the oxidation rate
of diazinon was found when NaNO was used as the
(mg L )
NaCl
KCl
Na
8.30
8.23
8.04
0.93
95.3
94.8
92.6
10.7
3
electrolyte. The concentration of diazinon decreased
almost linearly with treatment time, and its oxidation
kinetics cannot be fitted by the AFT kinetic model.
Using AFT, the ferrous ion is delivered into the
anodic half-cell, and hydrogen is generated at the
cathode. The electrode reactions can be expressed as
follows [9]:
2 4
SO
NaNO
3
different electrolysis current efficiencies are the basis for
the different effect of electrolytes on the oxidation rate
of diazinon during AFT. Because of their low current
efficiency, nitrate salts cannot be used as electrolytes for
AFT.
2
Anode: Fe-Fe þ þ 2e E ¼ 0:447 V;
0
ð2Þ
Cathode: 2H2O þ 2e-H2 þ 2OHꢀ
0
E ¼ ꢀ0:8277 V:
ð3Þ
3
.2. Effect of NaCl concentration in two half-cells
The ferrous ion reacts with the constantly added
hydrogen peroxide in the anodic half-cell, generating the
highly effective oxidant, the hydroxyl radical
The oxidation of diazinon by AFT with different
NaCl concentrations in the two half-cells was investi-
2
+
Fe þ H2O2 ¼ Fe3þ þ OH þ ꢁOH:
2
þ
ꢀ
2 2
gated. Since the electrolysis current (v0) and H O :Fe
ð4Þ
ratio (o) are fixed, Klpo can be used as an oxidation
rate parameter to evaluate the effect on diazinon
degradation of the NaCl concentration in the two half-
cells. As shown in Fig. 2(a), the change of Klpo with
NaCl concentration ratio between the two half-cells has
the same M-shape for all three anodic NaCl concentra-
tion levels. Among these three levels, 0.04 M appears to
be the most effective NaCl concentration. Among all
NaCl concentration ratios between the two half-cells, 4:1
During the experiments using NaCl, KCl, and
Na SO , gas bubbles were observed at the cathode,
and the dark yellow color of ferric ion was observed in
the anodic half-cell. But when NaNO was used, only a
2
4
3
slight color of ferric ion was observed, and no bubbles
were seen at the cathode. However, gas bubbles were
generated at the anode. This phenomenon implies that
other electrode reactions are occurring when NaNO
the electrolyte. The possible reactions are [17]:
3
is
(
cathodic:anodic) is the optimal ratio for diazinon
þ
oxidation by AFT. The total iron ion concentration in
the anodic half-cell after 2 min of electrolysis was also
measured, and the calculated current efficiencies are
shown in Fig. 2(b). The change of current efficiency with
NaCl concentration ratio has a pattern similar to the
change in diazinon oxidation rate parameter (Klpo)
with this same ratio as shown in Fig. 2(a). The
agreement in these two patterns signifies that the anodic
NaCl concentration level and the concentration ratio
between the two half-cells can affect the electrode
potentials of relevant compounds and the occurrence
of some secondary electrolysis reactions, resulting in
changes of current efficiency in generating ferrous ion
and thus affecting the production of hydroxyl radicals.
Anode: H2O-2H þ ð1=2ÞO2 þ 2e
0
E ¼ ꢀ1:229 V
ð5Þ
ð6Þ
2
Fe-Fe þ þ 2e E ¼ 0:447 V;
0
ꢀ
3
ꢀ
2
ꢀ
Cathode: NO þ H2O þ 2e-NO þ 2OH
0
E ¼ 0:01 V:
ð7Þ
ꢀ
3
By comparing Eq. (7) with (3), it is obvious that NO
is more easily reduced than H O. Because of the
2
occurrence of reaction (7), the cathode potential
increases, making reaction (5) more likely than reaction
(
6). In order to confirm what is happening, one can look
at the determination of total iron ion concentration in
the anodic half-cell as shown in Table 1. We can assume
that the ratio of real production to theoretical produc-
tion of ferrous ion is the electrolysis current efficiency.
As can be seen, the apparently low current efficiency
3.3. Temperature dependency of diazinon oxidation
by AFT
when using NaNO
3
indicates that reactions other than
The oxidation of diazinon by AFT was further
investigated at different temperatures. The degradation
of diazinon at different temperatures can still be fitted by
the AFT kinetic model quite well. As shown by Klpo
Eq. (5) are occurring at the anode. The current efficiency
of the other electrolytes correlates with the order of
diazinon oxidation rate by AFT, indicating that the

Q. Wang, A.T. Lemley / Water Research 36 (2002) 3237–3244
3241
1.4e-4
1.2e-4
1.0e-4
8.0e-5
6.0e-5
Table 2
Values of Klpo at different temperatures and activation energy
of diazinon oxidation by AFT
Anodic Con. of NaCl=0.02M
Anodic Con. of NaCl=0.04M
Anodic Con. of NaCl=0.08M
(a)
T (K)
Klpo
Regression results
ꢀ
5
283.7
291.5
(6.47170.189) ꢂ 10ꢀ
(7.41570.142) ꢂ 10
5
lnðKlpoÞ ¼
ꢀ
4:313 ꢀ
ð1512:8778:6Þð1=TÞ
r ¼ 0:99
ꢀ
5
2
99.2
06.1
(8.68670.208) ꢂ 10ꢀ
5
Ea ¼ 12:670:6 (kJ molꢀ1)
3
(9.47070.281) ꢂ 10
96
94
92
90
3.4. Oxidation products of diazinon
(b)
Two samples from the anodic half-cell solution,
before oxidation (blank) and after 2 min oxidation
treatment, were analyzed by ion trap/MS in the positive
ion mode. The MS spectra of these two samples are
shown as Fig. 3(a) and (b), respectively. Cations in the
anodic solution can combine with the diazinon molecule
to form positive ions during the ionization process. Ions
of m=z ¼ 305:2 and 327.2 in both spectra were formed in
+
this way, and can be attributed to [Diazinon+H] and
[
+
Diazinon+Na] , respectively. Compared with the
0
2
4
6
8
NaCl concentration ratio of cathodic/anodic
blank spectrum, only one unique peak, at m=z ¼ 343:2;
in the spectrum of the oxidized sample was found. The
MS/MS spectrum of the ion at 343.2 is shown in
Fig. 3(c). By analysis of this MS/MS spectrum and
comparison with the MS/MS spectra of the ions at 305.2
and 327.2, m=z ¼ 343:2; it can be attributed to
Fig. 2. Variation of oxidation rate parameter (Klpo) (a) and
electrolysis current efficiency (b) with NaCl concentration ratio
in the two half-cells.
values listed in Table 2, the oxidation of diazinon is
accelerated with an increase of temperature. This
suggests that the oxidation of diazinon by AFT is an
endothermic process.
+
[Diazoxon-H+Fe] . The structure of diazoxon is
shown in Fig. 3(c). Diazoxon was also reported to be
found in the degradation of diazinon by aqueous
chlorine treatment [12]. This shows that diazoxon is a
relatively stable degradation product during the oxida-
tion process of diazinon. Degradation experiments for
diazinon included analysis for diazoxon (data not
shown). The degradation product that appeared at
0.5 min is 100 times lower concentration than the initial
diazinon and stayed in this low concentration range
until about 3 min into the treatment when it was
completely degraded.
If l; p; and o are assumed to be constant with
temperature, Klpo can be used to signify K in deriving
the oxidation activation energy (Ea) using the Arrhenius
equation
Ea
0
ln k ¼ ln A þ
;
ð8Þ
RT
0
where k is the reaction rate constant; A is an empirical
constant dependent on compound and nonthermal
system conditions; R is the universal gas constant
It has been reported that the oxidative metabolism of
diazinon by microsomes from rat liver and cockroach
fat body [18], and by Drosophila [19] includes three types
of reactions, desulfuration, hydroxylation of the ring-
alkyl side chain, and cleavage of the pyrimidinyl
phosphate bond. Diazoxon, hydroxydiazinon, hydro-
xydiazoxon, pyrimidine, and hydroxypyrimidine are the
identified oxidative metabolites. Among them, diazoxon
and hydroxydiazinon have the same toxicity to Amer-
ican cockroach nymphs as diazinon and are significantly
more toxic than the other metabolites. Several peaks
appeared in the HPLC spectrum during the first 5 min of
the AFT process. As stated above, the diazoxon peak
ꢀ
1
ꢀ1
(
J K mol ); and T is the temperature (K). The
calculated activation energy for diazinon oxidation by
AFT is 12.670.6 kJ molꢀ . Since K ¼ kk1; the obtained
activation energy is the combined activation energy of
the Fenton reaction and diazinon oxidation by the
hydroxyl radical. The oxidation activation energy of
diazinon by aqueous chlorine is reported to be
1
ꢀ
1
3
373 kJ mol
[12]. In comparison, the activation
energy of the Fenton treatment is less than half that of
aqueous chlorine treatment. This implies that the
degradation of diazinon by AFT is more thermodyna-
mically favorable than aqueous chlorine treatment.

3242
Q. Wang, A.T. Lemley / Water Research 36 (2002) 3237–3244
Inten5s.
x10
327.2
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
(
a)
3
05.2
153.1
256.9
250
1
69.1 196.9
372.8
150
200
300
350
27.2
400
450
m/z
3
1
1
0
0
.25 Intens5.
(
b)
x10
.00
.75
.50
3
05.3
0
.25
.00
175.1
256.9
250
343.2
1
98.9
372.8
0
100
150
200
300
350
400
450
m/z
Intens.
191.0
(c)
800
600
400
200
0
315.1
CH3
CH3
H + Fe
O
P
CH
269.0
N
+
CH3CH2O
CH3CH2O
O
N
CH3
1
73.0
2
53.1
297.1
300
50
100
150
200
250
350
400
m/z
Fig. 3. Ion trap/MS spectra of diazinon anodic solution before (a) and after (b) treatment and MS/MS spectrum (c) of ion at
m=z ¼ 343:2 (in (b)).
disappears quickly. It is likely that there is no hydro-
xydiazinon or hydroxydiazoxon in the treatment effluent
since no peaks remained in the range of 3–15 min
retention time, where these peaks would be expected,
after 5 min of treatment. It can be concluded that
diazinon solutions are essentially detoxified by the AFT
treatment.
oxidation of pure diazinon, the oxidation rate of
diazinon in the formulated product is much lower, but
it is completely removed in 14 min. Since the delivery
rate of ferrous ion and hydrogen peroxide and their ratio
were kept the same for these two treatments, the average
life of ferrous ion, p; and the hydrogen peroxide
accumulation constant, o; should be constant. Thus,
the difference in Klpo for these two treatments was due
only to the change in the average life of the hydroxyl
radical, l: This means that l has a much shorter life in
the treatment of diazinon formulation than in pure
diazinon solution, due to the many organic ingredients
in the formulation competing with diazinon for the
hydroxyl radical. The oxidation rate of diazinon in
formulation depends not only on the concentration of
diazinon, but also on the amount and the relative
reactivity of the formulation ingredients.
3
.5. Oxidation of diazinon formulation
Pesticide formulations generally contain inert ingre-
dients in addition to the active compound. Many
formulation ingredients are organics that can consume
hydroxyl radicals through oxidation reactions. The AFT
oxidation of a solution of formulated diazinon was
investigated, and the results are shown in Fig. 4(a). The
oxidation kinetics of formulated diazinon can be fitted
by the AFT model. The values of Klpo for pure
diazinon and formulated diazinon are 11.3970.46 and
The initial COD of the formulation solution is
ꢀ
1
711.672.0 mg L , which is 14.9 times higher than that
of pure diazinon solution with the same initial concen-
ꢀ
2
0
.79470.023 mM , respectively. Compared with the

Q. Wang, A.T. Lemley / Water Research 36 (2002) 3237–3244
3243
1
00
Compared with the dissipation rate of diazinon both
in pure diazinon solution and in the formulation
solution (Fig. 4(a)), the COD removal rate (Fig. 4(b))
is relatively low. Huston and Pignatello [5] have shown
with TOC measurements during photo Fenton oxida-
tion treatments that there is often a time lag in TOC
removal. Our results indicate that AFT is a good
approach for the preliminary oxidation of toxic pesti-
cides, even in formulation, but it is not yet known
whether it is an effective means of COD removal from
wastewater. It is known that the biodegradability of
toxic organic chemicals can be greatly improved by the
Fenton treatment [21].
(
a)
80
60
40
20
0
Pure diazinon
Diazinon formulation
800
(b)
700
4
. Conclusions
(
1) Diazinon can be effectively oxidized by the hydro-
xyl radical during AFT treatment using NaCl, KCl
6
00
00
or Na
described by the AFT model. Use of NaNO
2
SO
4
as the electrolyte, and its kinetics can be
as the
1
3
electrolyte will cause low efficiency of ferrous ion
production and a low oxidation rate of diazinon.
0
0
2
4
6
8
10
12
14
(2) The variation of NaCl concentration in the two
half-cells can also result in changes of current
efficiency that affect the oxidation rate of diazinon.
The optimal NaCl concentration level in the anodic
half-cell and the optimal concentration ratio be-
tween the two half-cells are 0.04 M and 4:1
(cathodic:anodic), respectively.
Time (min)
Fig. 4. Comparison between pure diazinon and diazinon
formulation oxidation (a) and COD changes (b) during AFT.
COD values (b) have been corrected by a blank AFT sample
with no pesticide. Lines (in (a)) are the fitting results using the
AFT kinetic model.
(
3) The activation energy of diazinon oxidation by
Fenton treatment is estimated to be
2.670.6 kJ molꢀ , which is less than the value in
aqueous chlorine treatment.
1
tration of diazinon, confirming the existence of a high
concentration of organic ingredients in the formulation.
As shown in Fig. 4(b), the removal rate of COD for pure
diazinon is lower than that for the formulation solution.
This phenomenon can result from two possible causes.
First, some of the ingredients in the formulation are
more easily oxidized than diazinon. Secondly, in the
presence of a high concentration of organic reactants,
the hydroxyl radical reacts more efficiently with these
compounds such that the percentage of the hydroxyl
radical that is scavenged by other entities in the Fenton
generation system through the following reactions [20]
decreases:
1
(4) Diazoxon is generated as an intermediary oxidation
product during the oxidation of diazinon by AFT.
(5) The oxidation of diazinon by AFT in its formula-
tion solution can also be described by the AFT
kinetic model. Because of the competition of
formulation ingredients with diazinon in reacting
with the hydroxyl radical, the oxidation rate of
diazinon in the formulation solution is much lower
than in the pure diazinon solution.
OH þ Fe þ ¼ Fe3þ þ OHꢀ
2
ꢁ
0
1
8
ꢀ1
ꢀ1Þ;
ðk ¼ 3 ꢂ 10 M
s
ð9Þ
Acknowledgements
ꢁ
OH þ H2O2 ¼ HO2 ꢁ þH2O
This work was supported in part by the NRI
Competitive Grants Program/USDA Award #99-
0
2
7
ꢀ1
ꢀ1Þ;
ðk ¼ 1:2 ꢀ 4:5 ꢂ 10 M
s
ð10Þ
ð11Þ
3
5102-8238 and in part by the Cornell University
Agricultural Experiment Station federal formula funds,
Project No. 329423 received from CSREES, USDA.
0
3
9
ꢀ1
s
ꢀ1Þ:
2
ꢁ OH ¼ H2O2 ðk ¼ 5:3 ꢂ 10 M

3
244
Q. Wang, A.T. Lemley / Water Research 36 (2002) 3237–3244
[12] Zhang Q, Pehkonen SO. Oxidation of diazinon by aqueous
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