Oxidation of carbaryl in aqueous solution by membrane anodic fenton treatment.

Article abstract of DOI:10.1021/jf011434w

Carbaryl, a commonly used insecticide, was used in this study as a probe to investigate a new Fenton treatment technology, ion exchange membrane anodic Fenton treatment (membrane AFT). It was found that the degradation kinetics of carbaryl by membrane AFT obeys a previously published AFT model quite well. The NaCl (electrolyte) concentration in two half-cells was optimized for two kinds of membrane. Effects of the H(2)O(2)/Fe(2+) ratio and the Fenton reagent delivery rate were also investigated. The treatment efficiency for anion membrane AFT is higher than for salt-bridge AFT under the same operating conditions. Decreasing the delivery rate of Fenton reagents and increasing the treatment temperature also increase the treatment efficiency. The activation energy for carbaryl degradation by anion membrane AFT was estimated to be 14.7 kJ x mol(-1). 1-Naphthol, 1,4-naphthoquinone, and (phthalic acid-O-)yl N-methylcarbamate were detected by GC-MS as the degradation products of carbaryl by Fenton treatment. No decrease in carbaryl degradation rate was found during repeated use (100 times) of the anion exchange membrane. High and stable treatment efficiency can be achieved using an anion exchange membrane rather than a salt-bridge in the AFT system. Because of its effectiveness and convenience, the use of an ion exchange membrane as a substitute for the salt-bridge used in the previous AFT system has brought the AFT technology a major step closer to practical application.

Full text of DOI:10.1021/jf011434w

J. Agric. Food Chem. 2002, 50, 2331−2337 2331
Oxidation of Carbaryl in Aqueous Solution by Membrane
Anodic Fenton Treatment
QIQUAN WANG AND ANN T. LEMLEY*
Graduate Field of Environmental Toxicology, TXA, MVR Hall, Cornell University,
Ithaca, New York 14853-4401
Carbaryl, a commonly used insecticide, was used in this study as a probe to investigate a new Fenton
treatment technology, ion exchange membrane anodic Fenton treatment (membrane AFT). It was
found that the degradation kinetics of carbaryl by membrane AFT obeys a previously published AFT
model quite well. The NaCl (electrolyte) concentration in two half-cells was optimized for two kinds
of membrane. Effects of the H2O2/Fe2+ ratio and the Fenton reagent delivery rate were also
investigated. The treatment efficiency for anion membrane AFT is higher than for salt-bridge AFT
under the same operating conditions. Decreasing the delivery rate of Fenton reagents and increasing
the treatment temperature also increase the treatment efficiency. The activation energy for carbaryl
degradation by anion membrane AFT was estimated to be 14.7 kJ‚mol-1. 1-Naphthol, 1,4-
naphthoquinone, and (phthalic acid-O-)yl N-methylcarbamate were detected by GC-MS as the
degradation products of carbaryl by Fenton treatment. No decrease in carbaryl degradation rate was
found during repeated use (100 times) of the anion exchange membrane. High and stable treatment
efficiency can be achieved using an anion exchange membrane rather than a salt-bridge in the AFT
system. Because of its effectiveness and convenience, the use of an ion exchange membrane as a
substitute for the salt-bridge used in the previous AFT system has brought the AFT technology a
major step closer to practical application.
KEYWORDS: Carbaryl; Fenton treatment; anodic; ion exchange membrane; hydrogen peroxide; oxidation;
waste water; pesticide
INTRODUCTION
is the low pH of the effluent, requiring neutralization prior to
drainage. The optimal pH for the Fenton reaction is around 3
(6, 10). The use of iron(III) chelates in place of Fe2+ to
overcome acidic reaction conditions has been explored, but the
treatment rate is much lower (11). The other difficulty is the
handling of ferrous salts and accurately delivering the ferrous
solution into the reaction system, because ferrous salt is
hygroscopic and aqueous ferrous ion is readily oxidized.
To overcome these difficulties, anodic Fenton treatment
(AFT) was proposed as an improvement to classic Fenton
treatment (CFT) (12). The treatment system is separated into
two half-cells, which are connected via a salt-bridge. The ferrous
ion is delivered into the anodic half-cell by electrolysis from
an iron anode (eq 2). Hydrogen peroxide is constantly added to
Water contamination by pesticides is an undesirable effect
of chemical use in production agriculture. This is a serious
problem and one that compounds itself because some pesticides
and their degradation products bioaccumulate and persist in the
environment (1) and may even be toxic and carcinogenic (2).
The handling of waste water, disposing and recycling of
containers, and remediation of contaminated soils have become
very important issues in U.S. pesticide waste management (3).
Effective, low-cost, and easily operated technologies are greatly
needed for treating small-scale waste generated by individual
farmers or commercial agricultural applicators.
As one of the advanced oxidation technologies, Fenton
treatment has recently received extensive attention (4-9). The
hydroxyl radical (•OH) generated from the Fenton reaction (eq
1) is found to be a strong oxidant to many toxic or non-
anode:
Fe f Fe2+ + 2e
(2)
(3)
cathode:
2H2O + 2e f H2 + 2OH-
Fe2+ + H2O2 ) Fe3+ + OH- + •OH
(1)
the anodic half-cell by a pump. In the cathodic half-cell, water
is reduced on a graphite cathode (eq 3). The most significant
advantages of AFT over other Fenton treatment technologies
are that the Fenton reaction occurs in self-developed optimal
acidic conditions (pH ∼3) and that the pH of the treatment
biodegradable organics in waste water. However, the direct use
of the Fenton reaction in treatment creates two difficulties. One
* Author to whom correspondence should be addressed [telephone (607)
255-3151; fax (607) 255-1093; e-mail ATL2@cornell.edu].
10.1021/jf011434w CCC: $22.00 © 2002 American Chemical Society
Published on Web 03/03/2002

2332 J. Agric. Food Chem., Vol. 50, No. 8, 2002
Wang and Lemley
effluent can be partially neutralized from 3 to 5 by combining
the solutions from the two half-cells. Also, handling large
amounts of ferrous salt in practical applications is no longer
needed.
Degradation of 2,4-D by AFT has been investigated and
published elsewhere (13). A kinetic model was established to
describe the concentration changes of the target organic during
AFT treatment and to optimize the operating conditions.
However, there are still practical inconveniences when using
AFT due to the use of the NaCl saturated salt-bridge. The
saturated NaCl solution inside the salt-bridge can leak from the
glass frits and be easily diluted by the treatment solution. It
always needs to be replaced after each treatment to maintain
treatment efficiency. To make this technology more practical,
our goal was to substitute an ion exchange membrane for the
salt-bridge and test its effectiveness.
Carbaryl (1-naphthyl N-methylcarbamate), a carbamate in-
secticide with a wide range of activity and a relatively low
degree of animal toxicity, was one of the most commonly used
insecticides. According to U.S. EPA estimates, nonagricultural
usage alone of carbaryl in 1995-1996 reached 1-3 million
pounds per year (14). Carbaryl has been found to react with
nitrite under certain conditions to give rise to N-nitrosocarbaryl,
which can be considered to be one of the strongest genetically
active agents, a potent mutagen, and a carcinogen (15-17).
Carbaryl was chosen as the target compound in this study to
investigate the AFT system with an ion exchange membrane.
The objectives of this work were (i) to optimize the NaCl
(electrolyte) concentration in two half-cells with either a cation
or anion exchange membrane based on an optimized degradation
rate, (ii) to investigate the effect of H2O2/Fe2+ ratio and
treatment temperature on the degradation rate, (iii) to study the
effect of Fenton reagent delivery rate on treatment rate, (iv) to
probe the influence of repeated use of the membrane on
treatments, and (v) to identify the degradation products of
carbaryl upon AFT.
Figure 1. Apparatus of membrane anodic Fenton treatment: (1) anodic
half-cell; (2) cathodic half-cell; (3) ion exchange membrane; (4) iron plate;
(5) graphite stick; (6) magnetic stirring plate.
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 of methanol (to quench the subsequently
generated hydroxyl radical) for HPLC analysis. Treatments were
repeated two times for a total of three replications.
To optimize NaCl concentration in the two half-cells, cation and
anion exchange membranes were tested separately. Three anodic NaCl
concentrations, 0.02, 0.04, and 0.08 M, were chosen. The NaCl
concentration ratios of cathode/anode ranged from 0.125:1 to 5:1. To
investigate the effect of the H2O2/Fe2+ ratio on the carbaryl degradation
rate, H2O2 solutions of different concentrations were used in different
treatments while the electrolysis current was kept at 0.050 A. The H2O2/
Fe2+ ratio ranged from 1:1 to 50:1. The anion exchange membrane
was used in these experiments.
To investigate the effect of different delivery rates of Fenton reagents,
the H2O2/Fe2+ ratio was kept at 10:1. The electrolysis current was varied
from 0.01 to 0.100 A. To find out the effect of temperature on the
treatment, water baths of different temperatures (12.3 ( 1.0, 19.3 (
0.3, 24.7 ( 0.5, and 29.7 ( 0.2 °C) were used. The anion exchange
membrane was also used in these two studies.
Degradation of Carbaryl by Salt-Bridge AFT. Treatments were
carried out in two 250-mL beakers connected by a salt-bridge filled
with NaCl saturated solution. The operating conditions were the same
as those of the typical membrane AFT.
Degradation of Carbaryl by CFT. Treatments were performed in
a 250-mL beaker. Two hundred milliliters of 100 µM carbaryl solution
(with and without 0.02 M NaCl) was added to the beaker. One milliliter
of 0.0311 M FeSO4 solution and 1.00 mL of 0.311 M of H2O2 solution,
corresponding to concentrations of a 5-min AFT treatment at 0.020 A,
were added to the reaction system. The other operating conditions were
the same as those of a typical membrane AFT.
Analysis of Carbaryl and Hydrogen Peroxide Concentration. The
concentration of carbaryl was determined by an HP 1090 HPLC
equipped with a diode array detector. The mobile phase of HPLC was
composed of acetonitrile and water (50:50, pH was adjusted to 3 by
adding phosphoric acid). A C18 5 µm, 250 mm × 4.6 mm (i.d.) PRISM
RP column was used for analysis. The chosen wavelength for
quantification was 280 ( 20 nm. Under these conditions, carbaryl
retention time is 6.90 min. The concentration of hydrogen peroxide
was analyzed by titration in acidic solution using potassium perman-
ganate standard solution (18).
Degradation Product Identification by GC-MS. After 3 min of
treatment by membrane AFT under typical operating conditions, 15
mL of anodic solution was taken out and immediately extracted using
3 mL of hexane. The sample was analyzed by an Agilent 6890N
Network GC system equipped with an Agilent 5973 Network mass
selective detector and Agilent 7683 series injector. The GC-MS
conditions were as follows: a 30 m × 0.25 mm (i.d.) fused silica
capillary column with 0.25 µm film thickness (HP 19091S-433) and a
carrier gas of helium (10.50 psi) were used; initial temperature was
100 °C, increasing at 25.8 °C/min to 305 °C and kept at this temperature
for 14 min; injector port temperature was 220 °C; detector temperature
was 250 °C.
MATERIALS AND METHODS
Chemicals. Carbaryl (99%) was purchased from Chem Services
(West Chester, PA). Hydrogen peroxide (analytical grade), acetonitrile
(HPLC grade), and water (HPLC grade) were purchased from Mallinck-
rodt (Paris, KY). Sodium chloride (certified), hydrated ferrous sulfate
(certified), phosphoric acid (analytical grade), and hexanes (HPLC
grade) were purchased from Fisher Scientific (Fair Lawn, NJ).
Membranes. Cation exchange membrane (ESC-7000) and anion
exchange membrane (ESC-7001) were purchased from Electrosynthesis
(Lancaster, NY). The electrical resistances in 1 M NaCl solution at 25
°C are 6 and 8 ohm‚cm-2, respectively, for the cation and anion
membranes.
Degradation of Carbaryl by Membrane AFT. Degradation experi-
ments were carried out in two 300-mL glass half-cells that served as
anodic and cathodic half-cells. These two half-cells were separated by
an ion exchange membrane. The experimental apparatus is shown in
Figure 1. Typically 200 mL of 100 µM carbaryl solution with 0.02 M
NaCl was added into the anodic half-cell, and the same volume of 0.08
M NaCl aqueous solution was added into the cathodic half-cell. Each
of the two half-cells was stirred using a magnetic stirring bar. A 2 cm
× 10 cm × 0.2 cm iron plate and a 1 cm (i.d.) × 10 cm (l) graphite
rod were used as anode and cathode, respectively. The electrolysis
current was supplied by a BK Precision DC power supply 1610 and
was controlled at 0.050 A. Hydrogen peroxide solution of 0.311 M
(prepared by dilution from a solution of known concentration) was
delivered into the anodic half-cell using a Fisher Scientific peristaltic
pump at a rate of 0.50 mL‚min-1. The delivery rate of ferrous ion was
calculated from the electrolysis current that can be adjusted through
the power supply. The ratio of H2O2/Fe2+ was 10:1. The temperature
was kept at 24.7 ( 0.5 °C. The electrolysis was started by turning on
Degradation Kinetic Model and Definition of Treatment Ef-
ficiency. A detailed derivation of a universal kinetic model for the

Oxidation of Carbaryl
J. Agric. Food Chem., Vol. 50, No. 8, 2002 2333
treatment is due to hydroxyl radical oxidation, rather than
electrolysis or hydrogen peroxide oxidation.
Optimization of NaCl Concentration in Two Half-Cells.
NaCl was used in the treatment system as an electrolyte. It can
affect not only the electrolysis voltage between the two
electrodes but also the treatment efficiency (19). Degradation
of carbaryl by AFT with cation and anion exchange membranes
under different concentrations of NaCl was studied. All
degradation kinetics of carbaryl obeyed the AFT model quite
well (data not shown), with regression coefficients (r) >0.99.
Kλπω can be used as a rate parameter to estimate the effect on
the treatment efficiency of NaCl concentration in the two half-
cells because the electrolysis current and the delivery rate of
hydrogen peroxide are fixed. The changes in Kλπω and
electrolysis voltage with NaCl concentration ratio (cathode
/anode) at three anodic NaCl concentrations are shown in Figure
3. For the cation membrane AFT system, the treatment
efficiency with 0.04 M anodic NaCl is higher than it is with
the other two anodic NaCl concentrations, 0.02 and 0.08 M
(Figure 3a). However, for anion membrane AFT, the highest
treatment efficiency is reached by using 0.02 M anodic NaCl
concentration (Figure 3b).
With an increase in the NaCl concentration ratio (cathode/
anode), the electrolysis voltage decreases for both membrane
types and all anodic NaCl concentration levels (Figure 3a′,b′).
For each anodic NaCl concentration, it appears that the lower
the voltage, the higher the treatment efficiency. For a given
anodic NaCl concentration, there will be a lower total electrolyte
concentration at a lower NaCl concentration ratio. This can result
in a higher electrolysis voltage and the oxidation of other
substances at the anode, therefore causing a low electrolysis
current efficiency resulting in low treatment efficiency. Appar-
ently, the electrolysis voltage is not the only factor that affects
the treatment efficiency. Because the hydroxyl radical can react
with the chloride ion (eq 5) (20), higher anodic NaCl concentra-
Figure 2. Concentration changes of carbaryl by electrolysis without H O2,
2
H O2 without electrolysis, and anion membrane AFT. Experimental data
2
points are fitted by the AFT kinetic model.
degradation of different organics by AFT has been published elsewhere
(13). Here we briefly summarize this established model. During AFT
treatment, 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 simultaneously
into the reaction system. We assume (i) the concentration of ferrous
iron in the reaction system is constant, (ii) hydrogen peroxide can be
accumulated in the reaction system when the ratio of hydrogen peroxide
to ferrous ion is >1, (iii) the Fenton reaction obeys second-order
kinetics, (iv) the instantaneous concentration of hydroxyl radical is
proportional to its generation rate, and (v) the kinetics of the hydroxyl
radical reaction with organics are second order. On the basis of these
assumptions, the following equation can be established to describe the
changes of carbaryl concentration during AFT:
[C]t
[C]0
1
2
2 2
ln
) - Kλπων0 t
(4)
•OH + Cl- ) ClOH-,
k ) 4.3 × 109 M-1 s-1 (pH ∼2) (5)
In eq 4, K ) kk1 (µM-2‚min-2), k (µM-1‚min-1), and k1 (µM-1‚min-1
)
are the second-order rate constants of the Fenton reaction and the
reaction between hydroxyl radical and target compound, respectively;
[C]0 (µM) and [C]t (µM) are the concentrations of the target compound
at 0 and t min, respectively; λ (min) and π (min) are the average
lifetimes of the hydroxyl radical and ferrous ion, respectively; ω is a
constant related to the delivery ratio of hydrogen peroxide to ferrous
tion could also decrease treatment efficiency. This may be the
reason the treatment efficiency corresponding to the 0.08 M
anodic NaCl concentration in the cation membrane AFT system
is lower than that of the other two concentrations.
The ion exchange membrane property may be the third factor
affecting the treatment efficiency. In the cation membrane AFT
system, the membrane allows only anodic Na+ and H+ to pass
through to the cathodic half-cell. With an increase of cathodic
NaCl concentration, the movement of Na+ from anode to
cathode may become more difficult because of the concentration
gradient between the two half-cells. More H+ than Na+ may
move into the cathodic half-cell. To regenerate H+ and keep
the optimal pH for the Fenton reaction, some of the Fenton
reagent may be consumed. This can result in a decrease of
treatment efficiency in the cation membrane AFT system when
the NaCl concentration ratio (cathode/anode) is high. In contrast,
the anion membrane allows only cathodic Cl- and OH- to pass
through to the anodic half-cell. The increased NaCl concentra-
tion ratio (cathode/anode) is beneficial for Cl- to compete with
OH- in movement across the membrane, exerting less effect
on the anodic pH. Thus, the treatment efficiency should increase
with the increase of NaCl concentration ratio (cathode/anode)
in the anion membrane system.
ion and to the consumption ratio of hydrogen peroxide; ν0 (µM‚min-1
)
is the delivery rate of ferrous ion by electrolysis; and t (min) is time.
We call this kinetic model the AFT model.
Treatment efficiency is defined as the removal rate of the target
compound per unit of Fenton reagent. Because the Fenton reaction and
the hydroxyl radical reaction are fast reactions, the degradation rate
can be used as a parameter of treatment efficiency to compare treatments
with the same delivery rate of the Fenton reagents.
RESULTS AND DISCUSSION
Degradation of Carbaryl and Model Fitting. Carbaryl can
be rapidly degraded by anion membrane AFT (Figure 2). Five
minutes of treatment (0.050-A electrolysis and 10:1 ratio of
H2O2/Fe2+) can totally degrade 100 µM carbaryl in a 200-mL
solution. As shown by the agreement between the data points
and the model regression line in Figure 2, the degradation
kinetics of carbaryl can be fitted by the AFT model very well.
A high regression coefficient (r), 0.99, was obtained. If only
electrolysis or hydrogen peroxide is applied to the same solution,
no significant degradation of carbaryl is found. This implies
that the dissipation of carbaryl in anion membrane AFT
In terms of treatment efficiency, the optimal NaCl concentra-
tion in the cation membrane AFT system is 0.04 M in the anodic
half-cell and 0.08-0.12 M in the cathodic half-cell. For the

2334 J. Agric. Food Chem., Vol. 50, No. 8, 2002
Wang and Lemley
Figure 3. Changes in degradation rate (Kλπω) and electrolysis voltage with NaCl concentration in two half-cells: (a, a′) cation exchange membrane
AFT; (b, b′) anion exchange membrane AFT.
anion membrane system, 0.02 M in the anodic half-cell and
0.08-0.10 M in the cathodic half-cell are the optimal NaCl
concentrations. Because it is advisable to introduce the least
amount of NaCl into the treatment system, the optimum
treatment conditions for the experiments in this work include
the anion membrane AFT with an anodic NaCl concentration
of 0.02 M and a cathodic NaCl concentration of 0.08 M.
Effect of H2O2/Fe2+ Ratio. Degradation of carbaryl by the
salt-bridge AFT and the anion membrane AFT with different
H2O2/Fe2+ ratios can also be fitted by the AFT kinetic model
very well (data not shown). All regression coefficients (r) are
>0.99. Because the delivery rate of ferrous ion (ν0) is fixed,
Kλπω can be used as a degradation rate parameter under
different H2O2/Fe2+ ratios. For both anion membrane AFT and
salt-bridge AFT, the degradation rate of carbaryl initially
increased rapidly with the increase of H2O2/Fe2+ ratio (Figure
4). After the ratio reaches 15:1, the degradation rate becomes
almost constant. However, when the ratio reaches 100:1 for the
anion membrane AFT and 50:1 for the salt-bridge AFT, the
degradation kinetics no longer obey the AFT model, and the
degradation rate significantly decreases (data not shown). The
excessive concentration of hydrogen peroxide may decrease its
electrode potential. Hydrogen peroxide can compete with iron
to be oxidized at the anode (eq 6) (21), decreasing the
electrolysis current efficiency.
Figure 4. Effect of H O2/Fe2+ ratio on carbaryl degradation rate by
membrane AFT and salt-bridge AFT.
2
application convenience but also potentially in a higher treatment
efficiency when a high ratio of H2O2/Fe2+ is applied.
Effect of Delivery Rates of Fenton Reagent. The degrada-
tion rate of carbaryl using the anion membrane AFT increases
with the increase of Fenton reagent delivery rate (Figure 5).
As shown by the regression coefficients in Table 1, the
degradation kinetics obey the AFT model very well at all
2
delivery rates. Kλπων0 , the appropriate rate parameter for
H2O2 f O2 + 2H+ + 2e
(6)
variable reagent delivery rates, increases with an increase in
Fenton reagent delivery rates, resulting in an accelerated
degradation of carbaryl. However, if the increase of Fenton
reagent delivery rate were directly proportional to the degrada-
tion rate of carbaryl, the half-life of carbaryl (t1/2) would be
inversely proportional to the Fenton reagent delivery rate (ν0).
As also shown in Figure 4, the anion membrane appears to
have a higher treatment efficiency than the salt-bridge AFT
under the same operating conditions. The substitution of the
ion exchange membrane for the salt-bridge results not only in

Oxidation of Carbaryl
J. Agric. Food Chem., Vol. 50, No. 8, 2002 2335
Figure 5. Degradation of carbaryl by anion membrane AFT at different
delivery rates of Fenton reagent. Experimental data points are fitted by
the AFT model.
Figure 6. Degradation of carbaryl by anion membrane AFT and CFT
with the same amount of Fenton reagent.
-2
Table 2. Values of Kλπω (µM ) at Different Temperatures and
Activation Energies, Ea, of Carbaryl Degradation by Anion Membrane
Table 1. Regression Results of Carbaryl Degradation Kinetics at
Different Delivery Rates of Fenton Reagent Using the AFT Model
AFT
Kλπω
(µM )
delivery rate
-2
of Fe2+, ν0
T (K)
regression results
electrolysis
current (A)
(µM‚min-1
)
regression results
285.4
292.4
297.9
302.8
8.126 ± 0.060
9.745 ± 0.146
10.821 ± 0.126
11.589 ± 0.139
ln(Kλπω) ) −(3.303 ± 0.507) − (1770 ± 149)(1/T)
r ) 0.99
0.010
0.020
0.050
0.080
0.100
15.6
ln [C] /[C] ) −(0.0265 ± 0.0009)t2, r ) 0.99
t
0
Kλωπ ) (2.19 ± 0.07) × 10-4 (µM )
-2
Ea ) 14.7 ± 1.2 kJ‚mol-1
31.1
77.7
125
ln [C] /[C] ) −(0.0765 ± 0.0018)t2, r ) 0.99
t
0
Kλωπ ) (1.58 ± 0.04) × 10-4 (µM )
-2
ln [C] /[C] ) −(0.304 ± 0.001)t2, r ) 1.00
t
0
Kλωπ ) (1.06 ± 0.003) × 10-4 (µM )
-2
that added during 5 min of membrane AFT. By comparing the
residual carbaryl concentration, it appears that the treatment
efficiency of CFT without NaCl is slightly higher than that with
0.02 M NaCl. This result is in accord with the reaction in eq 5.
Although 0.02 M NaCl was present in the membrane AFT
system, its treatment efficiency is still higher than both versions
of CFT. This result indicates that a higher delivery rate of Fenton
reagent results in a lower treatment efficiency. If time permits,
a low delivery rate should be chosen to achieve high efficiency
in practical applications of this membrane AFT technology.
Degradation at Different Temperatures. Hydroxyl radical
reactions are usually very fast (k ) 107-1010 M-1 s-1) (22),
making it very difficult to investigate the effect of temperature
kinetically. Because AFT is a controlled Fenton process and
its kinetic model has been established, it is simple to estimate
the effect of temperature on the Fenton treatment. Assuming
that λπω is a constant parameter with variable temperature, then
Kλπω can be used as a parameter to signify K at different
temperatures. Activation energy (Ea) can be obtained by using
the Arrhenius equation
ln [C] /[C] ) −(0.671 ± 0.004)t2, r ) 0.99
t
0
Kλωπ ) (8.67 ± 0.05) × 10-5 (µM )
-2
156
ln [C] /[C] ) −(1.019 ± 0.011)t2, r ) 0.99
t
0
Kλωπ ) (8.43 ± 0.09) × 10-5 (µM )
-2
Then Kλπω should be constant under different delivery rates.
This can be expressed as follows:
2
2
ln(1/2) ) -(1/2)kλπων0 t1/2
(7)
Then
t ν ) 2 ln 2(Kλπω)-1 ) constant
(8)
x
1/2
0
To understand these results, the linear correlation between
Kλπω and ν0 was analyzed, and it was found that Kλπω
significantly decreases with the increase of reagent delivery rate
(eq 9), implying that the degradation rate of carbaryl is not
directly proportional to the Fenton reagent delivery rate.
6.94 × 10-4
Ea
Kλπω )
,
r ) 0.99
(9)
ln k′ ) ln A +
(10)
ν0.43(0.03
RT
The higher the delivery rate, the lower the treatment efficiency
will be.
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 (J‚K-1‚mol-1), and T is the
temperature (K).
Our results show that the degradation rate of carbaryl
increases with an increase in temperature (Table 2). Increasing
treatment temperature also increases treatment efficiency. All
degradation kinetics of carbaryl at four temperatures can be fitted
by the AFT model quite well, and regression coefficients (r)
are all >0.99 (data not shown). The activation energy (Ea) for
degradation of carbaryl by the anion membrane AFT is estimated
This conclusion was also confirmed by comparing the anion
membrane AFT with CFT. Because ferrous ion and hydrogen
peroxide are added all at once during CFT, it can be regarded
as a special kind of AFT with an extremely high Fenton reagent
delivery rate. The degradation of carbaryl for 5 min of anion
membrane AFT at 0.020 A under typical membrane AFT
operating conditions is shown in Figure 6. The degradation of
carbaryl by CFT with and without 0.02 M NaCl is also shown.
The amount of Fenton reagent added in the CFT is the same as

2336 J. Agric. Food Chem., Vol. 50, No. 8, 2002
Wang and Lemley
Figure 7. Changes in degradation rate of carbaryl with repeated use of
anion membrane.
Figure 9. Degradation pathways of carbaryl by hydroxyl radical attack.
be achieved using such a membrane rather than the salt-bridge
in the AFT system.
The functional stability of the membrane and the resulting
high treatment efficiency demonstrate the advantage of mem-
brane AFT over salt-bridge AFT, particularly because the salt
bridge needs frequent replacement of the saturated electrolyte
solution. The successful application of membrane technology
to AFT brings it a major step closer to practical application. It
is well-known that the hydroxyl radical is the essential strong
oxidant in all Fenton treatments and all Fenton technologies
focus on efficiently generating hydroxyl radicals and allowing
them to react with pollutants efficiently. Thus, we believe that
membrane AFT is effective and efficient when applied not only
to carbaryl but also to many other kinds of organics.
Degradation Products. Degradation products formed after
3 min of treatment of carbaryl solution (at 0.050 A) by anion
membrane AFT were determined using GC-MS. Four peaks
were found in the total ion current (TIC) spectrum (Figure 8a).
According to the MS spectrum (Figure 8d), the third peak
(retention time ) 6.84 min) can be attributed to the residual
carbaryl. Interpretation of the MS spectrum (Figure 8c) suggests
that the second peak (retention time ) 5.00 min) in the TIC
spectrum be attributed to 1-naphthol. This product has been
found to be the main degradation product of carbaryl in water,
soil, and some microorganism media (23-26). Confirmed by
the MS spectrum (Figure 8b), the first peak in the TIC spectrum
(retention time ) 4.50 min) can be attributed to 1,4-naphtho-
quinone, which comes from the oxidation of 1-naphthol. This
product has also been found in the TiO2 photocatalytic degrada-
tion of carbaryl (27). By interpreting the MS spectrum (Figure
8e), we attributed the fourth peak in the TIC (retention time )
Figure 8. TIC and MS spectra of carbaryl degradation products.
to be 14.7 kJ‚mol-1. This value is in the same range as that
found for diazinon with salt-bridge AFT (19).
Functional Stability of the Membrane with Repeated Use.
A major advantage of the membrane AFT over the salt-bridge
AFT is its convenience in practical application. Under these
circumstances, the stability and reusability of the membrane may
be an important concern. A repeat-use test of the anion
membrane was investigated. As shown in Figure 7, the
treatment efficiency increased rapidly for the first uses (∼10)
and soon became stable after that. No decrease of treatment
efficiency was found with 100 repeated uses (each treatment
time was 5-8 min), indicating that the ion exchange function
of the membrane has been maintained during the repeated
treatment processes. High and stable treatment efficiency can

Oxidation of Carbaryl
J. Agric. Food Chem., Vol. 50, No. 8, 2002 2337
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6.98 min) to a product formed by a partial breakdown of the
naphthalene ring in the carbaryl molecule, (phthalic acid-O-)yl
N-methylcarbamate.
It is possible that there were other, undetermined degradation
products not found because of their evaporation and ionization
properties in the GC-MS system, but those that were found show
two possible oxidation pathways of carbaryl by the hydroxyl
radical in the Fenton treatment (Figure 9). One path involves
breaking off the carbamate branch; another involves direct
breakdown of the naphthalene ring. The carbamate branch group
is therefore not the only position attacked and not necessarily
the first step for carbaryl degradation by the hydroxyl radical.
Conclusion. Degradation kinetics of carbaryl by ion exchange
membrane AFT with different NaCl concentrations in both half-
cells can be fitted by the AFT model quite well. The concentra-
tion of NaCl in both half-cells and its ratio can affect the
treatment efficiency. The optimal NaCl concentrations for anion
membrane AFT are 0.02 and 0.08 M, respectively, for the anodic
and cathodic half-cell.
Compared with salt-bridge AFT, anion membrane AFT shows
the same efficiency when the H2O2/Fe2+ ratio is less than 10:1
and higher efficiency when this ratio goes above 10:1. For
membrane AFT, the lower the delivery rate of Fenton reagent
and the higher the treatment temperature, the higher the
treatment efficiency will be. The functional stability of the
membrane during repeated use demonstrates the advantage of
membrane AFT over salt-bridge AFT.
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ABBREVIATIONS USED
AFT, anodic Fenton treatment; CFT, classic Fenton treatment;
TIC, total ion current; HPLC, high-performance liquid chro-
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ACKNOWLEDGMENT
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Received for review October 30, 2001. Revised manuscript received
January 22, 2002. Accepted January 23, 2002. This work was supported
in part by the NRI Competitive Grants Program/USDA Award 99-
35102-8238 and in part by the Cornell University Agricultural Experi-
ment Station federal formula funds, Project 329423, received from
CSREES, USDA.
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JF011434W