Kinetics and Mechanisms of Abiotic Degradation of Fipronil (Hydrolysis and Photolysis)

Article abstract of DOI:10.1021/jf970874d

The abiotic degradation of fipronil (compound I), a phenylpyrazole insecticide, was studied in aqueous solution and on the surface of two soils from Niger (Saguia and Banizoumbou) and one Mediterranean soil (Montpellier). The rate of hydrolysis of fipronil in solution was measured at different values of pH and temperature. The pH was an influencing factor: hydrolysis kinetics were pseudo-firstorder, the half-life being 770 h at pH 9.0, 114 h at pH 10.0, 11 h at pH 11.0, and 2.4 h at pH 12.0. Fipronil was stable under acid (pH 5.5) and neutral conditions. The Arrhenius relation was verified over the temperature range 22-45 °C: the activation energy was 62 kJ mol^-1 and the calculated entropy change -32 J mol^-1. Compound II [5-amino-3-carbamoyl-1-(2,6-dichloro-4-(trifluoromethyl)-phenyl)-4- [(trifluoromethyl)sulfinyl]pyrazole] was the only hydrolysis product detected. The hydrolysis reaction mechanism involves nucleophilic addition of OH^- to the polar nitrile bond, producing a hydroxyimine which then undergoes tautomerization into an amide. Fipronil in acidic (pH 5.5) aqueous solution exposed to light from a xenon lamp degraded with first-order kinetics (K^obs, = 1.7 h^-1) with concomitant appearance of 5-amino-3-cyano-1-(2,6-dichloro-4-(trifluoromethyl)phenyl)-4-(trifluoromethyl) pyrazole (compound III) and 5-amino-3-cyano-1-(2,6-dichloro-4-(trifluoromethyl)-phenyl)pyrazole-4-sulfonic acid (compound IV). The photolyte formation rate constants were 0.11 h^-1 (III) and 0.05 h^-1 (IV). The observed degradation process corresponded to a desulfinylation and an oxidation. The reaction mechanisms were not elucidated. The irradiation (xenon lamp) of fipronil adsorbed on three natural soils when dry led to the formation of 4-(trifluoromethyl)pyrazole (III). The K_obs values were 0.0047 h^-1 (Saguia), 0.0039 h^-1 (Banizoumbou), and 0.0032 h^-1 (Montpellier). The degree of photodegradation was inversely proportional to fipronil adsorption: the Freundlich adsorption coefficients were, respectively, 4.3 (Saguia), 7.3 (Banizoumbou), and 45.5 (Montpellier).

Full text of DOI:10.1021/jf970874d

2834
J. Agric. Food Chem. 1998, 46, 2834−2839
Kin etics a n d Mech a n ism s of Abiotic Degr a d a tion of F ip r on il
(Hyd r olysis a n d P h otolysis)
Alain Bobe´,† Pierre Meallier,‡ J ean-Franc¸ois Cooper,*,† and Camille M. Coste§
Laboratoire de Chimie Analytique, Faculte´ de Pharmacie, Avenue Charles Flahault,
34060 Montpellier, France; Laboratoire d’Application de la Chimie a` l’Environnement,
Universite´ Claude Bernard, Boulevard du 11 Novembre 1918, Lyon, France; and Centre de
Phytopharmacie, Avenue de Villeneuve, 66000 Perpignan, France
The abiotic degradation of fipronil (compound I), a phenylpyrazole insecticide, was studied in aqueous
solution and on the surface of two soils from Niger (Saguia and Banizoumbou) and one Mediterranean
soil (Montpellier). The rate of hydrolysis of fipronil in solution was measured at different values of
pH and temperature. The pH was an influencing factor: hydrolysis kinetics were pseudo-first-
order, the half-life being 770 h at pH 9.0, 114 h at pH 10.0, 11 h at pH 11.0, and 2.4 h at pH 12.0.
Fipronil was stable under acid (pH 5.5) and neutral conditions. The Arrhenius relation was verified
over the temperature range 22-45 °C: the activation energy was 62 kJ mol-1 and the calculated
entropy change -32 J mol-1. Compound II [5-amino-3-carbamoyl-1-(2,6-dichloro-4-(trifluoromethyl)-
phenyl)-4-[(trifluoromethyl)sulfinyl]pyrazole] was the only hydrolysis product detected. The hy-
drolysis reaction mechanism involves nucleophilic addition of OH- to the polar nitrile bond, producing
a hydroxyimine which then undergoes tautomerization into an amide. Fipronil in acidic (pH 5.5)
aqueous solution exposed to light from a xenon lamp degraded with first-order kinetics (Kobs ) 1.7
h-1) with concomitant appearance of 5-amino-3-cyano-1-(2,6-dichloro-4-(trifluoromethyl)phenyl)-4-
(trifluoromethyl)pyrazole (compound III) and 5-amino-3-cyano-1-(2,6-dichloro-4-(trifluoromethyl)-
phenyl)pyrazole-4-sulfonic acid (compound IV). The photolyte formation rate constants were 0.11
h-1 (III) and 0.05 h-1 (IV). The observed degradation process corresponded to a desulfinylation
and an oxidation. The reaction mechanisms were not elucidated. The irradiation (xenon lamp) of
fipronil adsorbed on three natural soils when dry led to the formation of 4-(trifluoromethyl)pyrazole
(III). The Kobs values were 0.0047 h-1 (Saguia), 0.0039 h-1 (Banizoumbou), and 0.0032 h-1
(Montpellier). The degree of photodegradation was inversely proportional to fipronil adsorption:
the Freundlich adsorption coefficients were, respectively, 4.3 (Saguia), 7.3 (Banizoumbou), and 45.5
(Montpellier).
Keyw or d s: Fipronil; hydrolysis; photodegradation; kinetics; soils; degradation products
INTRODUCTION
disrupting central nervous system activity and, at
sufficient doses, causing death (Cole et al., 1993).
Fipronil [5-amino-3-cyano-1-[2,6-dichloro-4-(trifluoro-
methyl)phenyl]-4-[(trifluoromethyl)sulfinyl]pyrazole; com-
pound I] is a phenylpyrazole insecticide recently devel-
oped by Rhoˆne-Poulenc Agro. Applied at low doses, it
is active against a wide range of insect pests of crops,
notably rice insects, thrips (citrus, cotton, mango, . . .),
termites (sugar cane, corn), and click beetles (cereals,
corn, sunflower, beets, . . .) (Gaulliard, 1996). It has also
been recommended for use in locust control (Balanc¸a
and De Visscher, 1997). Its wide range of action (locust
and grasshoppers, ready-to-hatch eggs, larvae, and
imagos) and multiple uses (total cover or barrier treat-
ment, possible use in preventive or curative control, in
agricultural, forestry, or pastoral zones) seem to give it
an advantage over growth regulators in chemical locust
control (Rachadi, 1995). Fipronil has been shown to
interfere with the passage of chloride ions through the
γ-aminobutyric acid regulated chloride channel, thereby
Laboratory and field studies on different soil types
(loam, silt loam, clay loam, sand, . . .) under temperate
conditions (United States, Europe) have enabled Rhoˆne-
Poulenc to identify five principal metabolites: 5-amino-
3-carbamoyl-1-[2,6-dichloro-4-(trifluoromethyl)phenyl]-
4-[(trifluoromethyl)sulfinyl]pyrazole (compound II),
5-amino-3-cyano-1-[2,6-dichloro-4-(trifluoromethyl)-
phenyl]-4-(trifluoromethyl)pyrazole (compound III),
5-amino-3-cyano-1-[2,6-dichloro-4-(trifluoromethyl)-
phenyl]pyrazole-4-sulfonic acid (compound IV), 5-amino-
3-cyano-1-[2,6-dichloro-4-(trifluoromethyl)phenyl]-4-(tri-
fluoromethyl)sulfonylpyrazole, and 5-amino-3-cyano-1-
[2,6-dichloro-4-(trifluoromethyl) phenyl]-4-[(trifluoro-
methyl)thio]pyrazole. Different degradation pathways
have been proposed: hydrolysis, photolysis, oxidation,
and reduction.
As part of locust control trials, we set up a study on
the open field behavior of the product under Sahelian
conditions (Niger, Niamey region, October 1995). The
amide (II) and trifluoromethylpyrazole derivative (III)
were the principal degradation products recovered from
the soil (Bobe´ et al., 1998). In the absence of additional
bibliographic data, we wished to confirm in the labora-
* Author to whom correspondence should be addressed (fax
33 4 67 54 45 26; e mail jcooper@pharma.univ-montp1.fr).
† Laboratoire de Chimie Analytique.
‡ Laboratoire d’Application de la Chimie a` l’Environnement.
§ Centre de Phytopharmacie.
S0021-8561(97)00874-1 CCC: $15.00 © 1998 American Chemical Society
Published on Web 06/05/1998

Abiotic Degradation of Fipronil
J. Agric. Food Chem., Vol. 46, No. 7, 1998 2835
Ta ble 1. Som e P h ysica l a n d Ch em ica l P r op er ties of th e
Th r ee Soils
Nigersto the south (Saguia) and the west (Banizoumbou) of
Niameysand one from the region of Montpellier in the south
of France. On the basis of U.S. Soil Taxonomy, Banizoumbou
and Saguia surface soils can be classified as Entisols (tropop-
samments), respectively, loamy sand and sand, while Mont-
pellier soil belongs to the Inceptisols group (eutrochrept).
Samples were air-dried, passed through a 2 mm sieve, and
stored at 4 °C to minimize changes in their microbial popula-
tions. Their physical and chemical properties, determined by
the Soil Laboratory, CIRAD, Montpellier, are summarized in
Table 1.
CEC
pH
sand silt clay (mequiv/
organic
soil
(H2O) (%)
(%) (%)
100 g)
matter (%)
Banizoumbou
Saguia
Montpellier
5.8
5.3
8.3
79.4 18.2
2.4
0.6
22.6
0.7
0.7
23.5
0.3
0.1
6.5
98
1.4
54.4 23
tory the formation of these degradation products during
hydrolysis and photolysis reactions carried out in aque-
ous solution and/or on adsorbed material (Sahelian soils)
and to attempt to propose reaction mechanisms.
The influence of various parameters was studied: pH
and temperature for the hydrolysis reactions and the
Ap p a r a tu s. The following equipment was used in this
study: classical laboratory glassware and equipment (water
bath, rotary evaporator, . . .); a Beckman DU 460 UV-vis
spectrophotometer; a solar simulator (Suntest), fitted with a
xenon lamp (1.8 kW, 14.6 A); UV filters for adjustment of the
UV spectral limit; a parabolic reflector to ensure even distribu-
tion of light in the irradiation chamber (exposed area 500 cm2);
and a ventilation cooling system (21 °C). The emission
spectrum (290-800 nm) was very close to that of sunlight with
optimum simulation up to 460 nm. High-performance liquid
chromatography was run under the following conditions:
Spectra system P1000; 20 µL injection loop; C18 Hypersil
column (200 mm × 4.6 mm × 5 µm); UV detector, Shimadzu
SPD6-A set at 220 nm; mobile phase, acetonitrile (ACN)/water
+ tetrabutylammonium hydroxide (TBAH) (5 mM) + 0.5 M
HCl to adjust pH to 5.0; elution gradient, 0 min ACN/TBAH
(45:55), 6 min ACN/TBAH (60:40), 20 min ACN/TBAH
(60:40), 25 min ACN/TBAH (45:55); flow rate, 1 mL min-1. The
retention times of fipronil and its three degradation products
are given in Table 2. A Hewlett-Packard HP 5890 gas
chromatograph was connected to a mass spectrometric (MS)
detector HP MSD 5971A with the following conditions: elec-
tron impact mode, 70 eV; splitless injection mode (injection
purge off ) 0.75 min); injector temperature, 250 °C; transfer
line temperature, 280 °C; column, SPB-17, 30 m, 0.25 mm i.d.,
0.25 µm film thickness; initial oven temperature, 70 °C for 1
min raised at 50 °C min-1 to 240 °C for 16 min; solvent delay,
nature of soil for photolysis of adsorbed fipronil.
A
comparative study including a Mediterranean soil with
different physical-chemical characteristics was carried
out to complement the evaluation of soil influence.
MATERIALS AND METHODS
Ch em ica ls. Fipronil [5-amino-3-cyano-1-[2,6-dichloro-4-
(trifluoromethyl)phenyl]-4-(trifluoromethyl)sulfinylpyrazole)
(compound I), 5-amino-3-carbamoyl-1-[2,6-dichloro-4-(tri-
fluoromethyl)phenyl]-4-(trifluoromethyl)sulfinylpyrazole (com-
pound II), 5-amino-3-cyano-1-[2,6-dichloro-4-(trifluoromethyl)-
phenyl]-4-(trifluoromethyl)pyrazole (compound III), and
5-amino-3-cyano-1-[2,6-dichloro-4-(trifluoromethyl)phenyl] pyr-
azole-4-sulfonic acid (compound IV) were supplied by Rhoˆne-
Poulenc Agro (Lyon, France) as analytical standards (purity
g 99.4%, except for the derivative acid sulfonic with 94.4%
purity). Stock solutions were prepared at 250 and 25 mg L-1
in methanol and stored at 4 °C. Working solutions were
prepared by dilution in pure water or in toluol. Solvents were
pesticide residue analysis grade (toluol) or HPLC grade
(acetone, methanol, acetonitrile, water). Sodium hydroxide,
hydrochloric acid, anhydrous sodium sulfate, and acetic acid
were analysis grade. Tetrabutylammonium hydroxide, 40 wt
% solution in water, was purchased from Sigma-Aldrich,
France. Reversed phase octadecyl (Sep-Pak Classic, short body
C18) disposable cartridges for solid-phase extraction (SPE) were
obtained from Waters, France. Glass-microfiber filters GF/B
were obtained from Whatman.
3 min; carrier gas, helium 5.5 at a flow rate of 1.2 mL min-1
.
Under such conditions, fipronil and its degradation products
(compounds II and III) were identified by their retention times
on the chromatograms obtained in single-ion mode (SIM) with
characteristic m/z ions, as presented in Table 2.
F ip r on il Hyd r olysis Stu d y. Hydrolysis experiments were
conducted in duplicate using 2.5 mg L-1 (5.7 × 10-6 mol L-1
)
Soil Ch a r a cter istics. Three uncontaminated surface soils
were collected under Sahelian (0-10 cm depth) or Mediter-
ranean (0-30 cm depth) conditions: two from the Republic of
aqueous solutions (2.5% methanol in water), kept in the dark
and frequently shaken. To evaluate the influence of pH on
the degradation of fipronil, tests were carried out at room
Ta ble 2. HP LC a n d GC Reten tion Tim es a n d Ma ss Sp ectr a l a n d UV-Visible Da ta for F ip r on il a n d Degr a d a tion
P r od u cts (II-IV)
chromatographya (tr, min)
UV-visb
λ (nm) ꢀ (L mol-1 cm-1
)
compound
(mol wt)
GC/MS
HPLC
MS, m/z (uma) (rel abundance, %)
fipronil
12H4Cl2F6N4OS
(437)
8.8 ( 0.2
12.9 ( 0.4
367 [M+ - CF3] (100)
209
220
280
35000 ( 5000
28000 ( 8000
9000 ( 1000
C
351 [M+ - CF3 - O] (13)
255 [C6H2(Cl)2(CF3)(NCH2NH2)]+ (14)
213 [M+ - C3N2(CN)(F3CSO)(NH2)] (40)
69 [CF3]+ (16)
other ions: 178 (10), 143 (9), 77 (19)
II
17.5 ( 0.2
7.4 ( 0.2
7.3 ( 0.7
385 [M+ - CF3] (100)
203
278
71000 ( 10000
7000 ( 1000
C
12H6Cl2F6N4O2S
255 [C6H2(Cl)2(CF3)(NCH2NH2)]+ (42)
213 [M+ - C3N2(CONH2)(F3CSO)(NH2)] (26)
69 [CF3]+ (27)
(455)
III
14.4 ( 0.4
388 [M+] (100)
206
280
35000 ( 4000
5000 ( 1000
C
12H4Cl2F6N4
369 [M+ - F] (19)
(389)
333 [M+ - F3] (79)
278 [M+ - CF3 - CN - NH2] (6)
213 [M+ - C3N2(CF3)(CN)(NH2)] (28)
69 [CF3]+ (13)
other ions: 231 (9), 194 (9), 179 (19), 143 (12), 90 (7), 77 (27)
IV
6.2 ( 0.4
206
277
41000 ( 9000
6000 ( 1000
C
11H5Cl2F3N4O3S
(401)
a
b
See Materials and Methods for chromatographic conditions. In a mixture of H2O/MeOH (90:10 v/v).

2836 J. Agric. Food Chem., Vol. 46, No. 7, 1998
Bobe´ et al.
Ta ble 3. Deter m in a tion of Obser ved Ra te Con sta n t
(Kobs) a n d Ha lf-Life (t1/2) for th e Hyd r olysis of F ip r on il
u n d er Alk a lin e Con d ition s a n d th e P h otolysis of
Ad sor bed F ip r on il u n d er a Xen on La m p
Hydrolysis Study
K
obs (h-1
)
r2
t1/2 (h)
pHa
9.0 ( 0.1
0.0009
0.0061
0.062
0.29
0.988
0.982
0.993
0.998
770
114
11
10.0 ( 0.1
11.0 ( 0.1
12.0 ( 0.1
tempb (°C)
30 ( 1
37 ( 1
45 ( 1
2.4
0.0093
0.016
0.039
0.994
0.997
0.983
75
43
18
Photolysis Study
OMd (%) Kobs (h-1
F igu r e 2. Variation of ln Kobs with temperature for the
alkaline hydrolysis of fipronil (pH 10.0 ( 0.1).
pH
)
r2
t1/2 (h)
soil typec
Saguia
Banizoumbou
Montpellier
5.3
5.8
8.3
0.1
0.3
6.5
0.0047
0.0039
0.0032
0.989
0.988
0.991
147
178
217
a
Hydrolysis of fipronil in aqueous solution (C0 ) 5.7 × 10-6
mol L-1) at ambient temperature (22 ( 2 °C), in the dark.
Hydrolysis of fipronil in aqueous solution (C0 ) 5.7 × 10-6 mol
b
L-1) at pH 10.0 ( 0.1, in the dark. c Irradiation of soil samples
(5.7 × 10-6 mol kg-1) in a Suntest solar simulator (xenon lamp).
d
Organic matter.
F igu r e 3. Hydrolysis of fipronil at alkaline pH (10.0 ( 0.1)
at ambient temperature (22 ( 2 °C) in the dark: formation of
amide derivative (compound II).
a uniform distribution of fipronil in the soil at a concentration
of 2.5 mg kg-1 (5.7 × 10-6 mol kg-1). Samples (1 g) of this soil
were irradiated in quartz capsules for 0, 7, 15, 24, 48, 96, 144,
or 196 h. Irradiated samples were kept in the dark at -22 °C
until analysis. To verify that the observed degradation was
due only to photolysis, the experiment was repeated without
irradiation on the three soils kept in the dark for 8 days. All
experiments were performed in duplicate.
An a lytica l P r oced u r e. In aqueous solutions, the decrease
of fipronil concentrations during hydrolysis and photolysis, as
well as the concomitant increase of degradation products, could
be followed by direct injection of the aqueous solution into the
HPLC system. Identification and analysis were performed
using authentic standards for comparison. Limits of quanti-
tation were evaluated at 0.05 mg L-1 for fipronil and compound
III and at 0.03 mg.L-1 for compounds II and IV.
To confirm the results on products I, II, and III by GC/MS,
an extraction procedure was set up: fipronil, amide (II), and
desulfinyl photolyte (III) were extracted using SPE reversed
phase octadecyl (C18) cartridges. The cartridge was first
conditioned with 5 mL of methanol, followed by 5 mL of pure
water, before loading with a 10 mL aliquot of aqueous solution.
Elution of 80% of the amide (II) and >90% of fipronil and
desulfinyl photolyte (III) was obtained with 5 mL of toluene.
This eluate could be concentrated to 1-2 mL under a gentle
stream of nitrogen before being analyzed directly by GC/MS.
The solid-phase extraction of fipronil and its photodegra-
dation products was carried out on irradiated soil. One gram
soil samples were extracted with 2 × 20 mL of acetonitrile/
acetone (70:30 v/v) with magnetic stirring (2 × 30 min.). The
organic phase was filtered under vacuum on a glass fiber filter,
dried over anhydrous sodium sulfate, and evaporated under
vacuum at a temperature e40 °C. The crude extract was
taken up into 0.5-1 mL of toluene, which was injected directly
F igu r e 1. Variation of log Kobs with pH for the alkaline
hydrolysis of fipronil in aqueous solution in the dark at
ambient temperature (22 ( 2 °C).
temperature (22 ( 2 °C) at different values of pH (5.5, 7.0,
9.0, 10.0, 11.0, and 12.0 ( 0.1) adjusted using 0.2-1 M NaOH
or 0.2-1 M HCl aqueous solutions. According to Rhoˆne-
Poulenc (1996), the solubility of the product did not depend
on pH: 2.4 mg L-1 at pH 5.0; 1.9 mg L-1 at pH 9.0. Aliquots
(10 mL) were taken at predetermined times depending on the
operating conditions: 0-20 h (pH 12.0), 0-100 h (pH 11.0),
0-500 h (pH 10.0), and 0-2500 h (pH 9, 7, and 5.5). The
influence of temperature on the rate of hydrolysis was also
examined on aqueous fipronil solutions adjusted to pH 10.0 (
0.1, maintained on a water bath at temperatures of 30 ( 1°
and 37 ( 1 °C (0-250 h) and 45 ( 1 °C (0-80 h).
F ip r on il P h ot olysis St u d y. Aqueous Phase. Ten milli-
liter samples of an aqueous solution (2.5% methanol in water)
of fipronil at 2.5 mg L-1 (5.7 × 10-6 mol L-1) adjusted to pH
5.5 were placed in the quartz capsule of a Suntest solar
simulator and irradiated. Irradiation times were 0, 1, 2, 3, 4,
6, 14, 18, and 24 h.
Adsorbed Phase. Twenty gram of dry soil was placed in a
500 mL round-bottom flask, and 2 mL of methanolic fipronil
solution at 25 mg L-1 and 50 mL of methanol were added. The
suspension was then magnetically stirred for 30 min. Next,
the solvent was removed under vacuum at a temperature e40
°C and the soil dried in the open for 1 h. This procedure gives

Abiotic Degradation of Fipronil
J. Agric. Food Chem., Vol. 46, No. 7, 1998 2837
F igu r e 4. Hydrolysis reaction mechanism of fipronil in alkaline solution.
into the GC/MS system. Extraction recovery rates were
determined: 85 ( 3% for fipronil, 90 ( 5% for the desulfinyl
photolyte (III), and 96 ( 8% for the amide (II). The acid (IV)
was analyzed by HPLC after transfer into 2 mL of redistilled
water (recovery rate ) 85 ( 10%).
RESULTS AND DISCUSSION
Hyd r olysis of F ip r on il. The degradation of fipronil
in aqueous solution in the absence of light at ambient
temperature (22° ( 2 °C) was monitored at different pH
values. Fipronil was stable in acid (pH 5.5 ( 0.1) and
neutral (pH 7.0 ( 0.1) solutions, 80% remaining un-
changed after 100 days in both cases. Under alkaline
conditions (pH 9.0-12.0), degradation increased in
direct proportion to increase in pH. Representation of
F igu r e 5. Direct photodegradation of fipronil in aqueous
solution (pH 5.5): irradiation by xenon lamp (Suntest solar
simulator).
the rate of degradation over time can be given by a
mathematical model illustrating pseudo-first-order trans-
formation kinetics (Atkins, 1994)
obst
C ) C0 e-K
(1)
confirmed the loss of degrees of freedom of the hydroxyl
ion in the transition state.
The hydrolysis of fipronil in alkaline solution led to a
decrease in the concentration of fipronil and a corre-
sponding increase in the concentration of the amide
derivative (II) (Figure 3), identified from its retention
time in different chromatographic systems (HPLC/UV
and GC/MS) and its mass spectrum (ions m/z 385, 255,
and 213 uma) (Table 2). After 1 month, 94% of fipronil
had been transformed into compound II.
The results of the kinetic studiessabsence of general
acid-base catalysis, first-order relative to hydroxyl ion,
negative entropyssuggest a reaction mechanism involv-
ing nucleophilic addition of the hydroxyl ion to a polar
nitrile bond of fipronil, yielding an unstable hydroxy-
imine that undergoes total tautomerization into the
corresponding amide (Figure 4).
where C ) concentration of fipronil aqueous solution
(mol L-1), t ) incubation time (hours), C0 ) initial
concentration of fipronil aqueous solution (mol L-1), and
Kobs ) fipronil hydrolysis rate constant (h-1). The
observed rate constants (Kobs) and calculated half-lives
(t1/2) are given in Table 3. At pH 12.0 ( 0.1, fipronil
degradation was 300 times faster than at pH 9.0 ( 0.1,
the t1/2 values being, respectively, 2.4 and 770 h.
The experimental results fitted well with a pseudo-
first-order hydrolysis of fipronil. For the range of pH
from 9 to 12.0, the graph of the logarithm of observed
rate constant (log Kobs) versus pH (Figure 1) was linear,
with a gradient close to 1 (0.85, r2 ) 0.995). The fipronil
hydrolysis reaction was first-order with respect to the
hydroxyl ion, Kobs ) KOH (OH-).
At temperatures of 22, 30, 37, and 45 °C, the half-
lives were, respectively, 114, 75, 43, and 18 h. From
the rate constants (Table 3) determined at these four
temperatures, the activation energy (Ea) of the hydroly-
sis reaction at pH 10.0 ( 0.1 was calculated according
to Arrhenius’ law
P h otolysis of F ip r on il in Aqu eou s Solu tion . This
study was carried out at pH 5.5 since at this pH fipronil
is stable to hydrolysis (>95% unchanged after 24 h).
Moreover, this pH corresponds to that of the Sahelian
soils used subsequently for the study of photolysis in
the solid phase. With direct excitation in aqueous
solution (pH 5.5), fipronil degraded rapidly with first-
order reaction kinetics (r2 ) 0.995). The observed rate
constant was 0.17 ( 0.01 h-1, and the calculated half-
life was 4.1 h. Rigorously, this rate constant corre-
Kobs ) A e-E /RT
(2)
a
where Kobs ) fipronil hydrolysis rate constant (h-1), A
) constant characteristic of the fipronil hydrolysis
reaction, R ) gas constant (8.314 J K-1 mol-1), T )
temperature (K), and Ea ) activation energy (J mol-1).
Figure 2 represents the graph ln Kobs ) f(1/T), which
sponds to the sum of the photolysis (Kp ) 0.168 h-1
)
and hydrolysis (Kd e 2.15 10-3 h-1) rate constants, but
the latter can be discounted.
The primary photodegradation of fipronil in aqueous
solution is presented in Figure 5 with the concomitant
appearance and evolution of two different compounds
by simultaneous reactions. One compound had a reten-
gave an activation energy of 62 ( 5 kJ mol-1 (r2
)
0.970). The entropy (∆S) was also determined. The
negative sign of the entropy (-32.0 ( 2.0 J mol-1
)

2838 J. Agric. Food Chem., Vol. 46, No. 7, 1998
Bobe´ et al.
F igu r e 6. Possible photolysis reaction mechanism of fipronil in aqueous solution (pH 5.5): irradiation by xenon lamp (Suntest
solar simulator).
obst
C3 ) C0(K3/Kobs)(1 - e-K
C4 ) C0(K4/Kobs)(1 - e-K
)
)
(3)
(4)
tion time in HPLC similar to that of the reference
derivative trifluoromethylpyrazole (III). Identification
was confirmed by GC/MS (ions m/z 388, 369, and 333
uma) (Table 2). The other compound was identified by
HPLC retention time as being the pyrazole-4-sulfonic
acid derivative (IV). Quantitative assessment of the
degradation of fipronil and the appearance of the
photodegradation products gave consistent results at all
incubation times, showing 82-103% of fipronil being
transformed into compounds III and IV. The degrada-
tion processes observed in this study were therefore
desulfonation and oxidation, but other mechanisms of
probable radical nature were not elucidated. We present,
however, in Figure 6 a reaction mechanism (theoretical)
that appears to us a possibility for the phototransfor-
mation of fipronil into products III and IV, although
we were unable to demonstrate passage via the sulfone
derivative. This derivative is a degradation product of
fipronil which was isolated under other conditions
(absence of light), in particular, during the metabolism
of fipronil in soil (Bobe´ et al., 1998).
obst
where C0 ) initial concentration of fipronil, C3 and C4
were the concentrations of compounds III and IV,
respectively, t ) time (h), Kobs ) rate of disappearance
of fipronil, and Kobs ) K3 + K4, where K3 and K4 are the
rate constants of the two single steps of the parallel
process. Initial concentrations of III and IV were zero.
Their appearance followed first-order kinetics, the
concentration of both compounds at any given time
being proportional to their respective rate constants,
such that C3/C4 ) K3/K4. The major photolyte (III) was
the most rapidly formed. The experimental formation
rate constants were 0.11 ( 0.01 h-1 for the trifluoro-
pyrazole derivative (III) and 0.05 ( 0.01 h-1 for the
pyrazole-4-sulfonic acid derivative (compound IV). The
correlation coefficients r2 were 0.979 (compound III) and
0.988 (compound IV).
P h otolysis of F ip r on il in th e Solid P h a se. No
fipronil degradation occurred in the dark since >90%
of the fipronil applied was recovered unchanged after
96 h. Under xenon light, however, degradation occurred
We applied the same kinetic model to the parallel
degradation reactions (Frost and Pearson, 1961)

Abiotic Degradation of Fipronil
J. Agric. Food Chem., Vol. 46, No. 7, 1998 2839
Photolysis of fipronil in aqueous solution under a
xenon lamp in a Suntest solar simulator revealed two
parallel photodegradation reactions with the appearance
of the trifluoromethylpyrazole derivative (III) and the
pyrazole-4-sulfonic acid derivative (IV). The reaction
mechanisms, probably radical in nature, were not
elucidated. The photodegradation studies on soil sur-
faces, under the same operating conditions, revealed the
formation of the trifluoromethylpyrazole derivative (III).
During the plain field study on Sahelian soils, this
photolyte appeared rapidly during the first few days in
a manner concomitant with the degradation of fipronil.
The photolytic reaction was intense as long as the
fipronil was not protected from the light by the solid
soil matrix. Moreover, the study of fipronil photolysis
at the surface of the three soils studied clearly demon-
strated that the screening effect of soil particles and the
adsorption of fipronil by the soil reduced the intensity
of the photodegradation.
F igu r e 7. Photodegradation of fipronil at the surface of
Montpellier soil: irradiation by xenon lamp (Suntest solar
simulator).
ACKNOWLEDGMENT
following first-order kinetics with a correlation coef-
ficient r2 g 0.984. Rate constants and half-life values
for fipronil degradation on different soils are given in
Table 3.
We thank Rhoˆne-Poulenc Agro for their assistance
throughout this study.
LITERATURE CITED
In the solid phase, the half-life times observed were
greater than those obtained in solution; the slow deg-
radation of fipronil in the solid phase could have been
due to a light-shielding effect (Oliver et al., 1979). The
rate of degradation seemed to be dependent on the
nature of the soil. The maximum rate was observed
with Saguia soil and the minimum with Montpellier soil.
In this case, the difference in rate constants seems to
present a possible correlation with the Freundlich
adsorption coefficients (Kf) of fipronil determined on
each of the soils: 4.3, 7.3, and 45.5 for Saguia, Bani-
zoumbou, and Montpellier, respectively (Bobe´ et al.,
1997). Several authors (Yokley et al., 1986; Herbert and
Miller, 1990; Nag and Dureja, 1996) have previously
described the influence of the adsorption capacity of the
soil on the availability of the product subjected to
photolysis. The photodegradation study on the three
soils revealed only one photolyte, compound III (Figure
7). Compound IV, previously found in the photodegra-
dation study on aqueous solutions, was not detected
under these conditions.
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September 1995, we undertook a plain field study on
two acid soils under Sahelian conditionssBanizoumbou
(pH 5.8) and Saguia (pH 5.3). The results obtained
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compound in the two soils revealed a period of latency.
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Received for review October 10, 1997. Revised manuscript
received April 2, 1998. Accepted April 16, 1998.
J F970874D