Photodegradation of Pesticides. Photolysis Rates and Half-Life of Pirimicarb and Its Metabolites in Reactions in Water and in Solid Phase

Article abstract of DOI:10.1021/jf9501711

The photodegradation of Pirimicarb under three different artificial lights and sunlight was studied in water solutions (buffers pH 5, 6, and 7) and in solid phase. Five photocompounds were formed in solution and two in solid phase. Pirimicarb undergoes fast degradation under all conditions. In buffer solutions it first gave three compounds with a kinetic parallel process. These compounds were assigned the structures of 2-[(methylformyl)amino]-5,6-dimethylpyrimidin-4-yl dimethylcarbamate (II), 2-(methylamino)-5,6-dimethylpyrimidin-4-yl dimethylcarbamate (III), and 2-(dimethylamino)-5,6-dimethyl-4-hydroxypyrimidine (V). V and II were stable to further photolysis (the latter with t_1/2 = 849 h) whereas III undergoes further degradation to 2-amino-5,6-dimethylpyrimidin-4-yl dimethylcarbamate (IV), and to 2-(formylamino)-5,6-dimethylpyrimidin-4-yl dimethylcarbamate (IX). Both compounds were photodegraded to undetectable species, and IX shows a very high t_1/2 (48 h). A different behavior was found in solid phase, and only II and III were formed. A kinetic parallel process was demonstrated. The environmental t_1/2 and t_1/100 calculated for Pirimicarb and its photoproducts suggest their reduced persistence in natural waters.

Full text of DOI:10.1021/jf9501711

J. Agric. Food Chem. 1996, 44, 2417
−2422
2417
P h otod egr a d a tion of P esticid es. P h otolysis Ra tes a n d Ha lf-Life of
P ir im ica r b a n d Its Meta bolites in Rea ction s in Wa ter a n d in Solid
P h a se
Filippo M. Pirisi,* Paolo Cabras, V. Luigi Garau, Marinella Melis, and Enrico Secchi
Dipartimento di Tossicologia, dell’Universit a` di Cagliari, Viale A. Diaz 182, 09126 Cagliari, Italy
The photodegradation of Pirimicarb under three different artificial lights and sunlight was studied
in water solutions (buffers pH 5, 6, and 7) and in solid phase. Five photocompounds were formed
in solution and two in solid phase. Pirimicarb undergoes fast degradation under all conditions. In
buffer solutions it first gave three compounds with a kinetic parallel process. These compounds
were assigned the structures of 2-[(methylformyl)amino]-5,6-dimethylpyrimidin-4-yl dimethylcar-
bamate (II), 2-(methylamino)-5,6-dimethylpyrimidin-4-yl dimethylcarbamate (III), and 2-(dimethyl-
amino)-5,6-dimethyl-4-hydroxypyrimidine (V). V and II were stable to further photolysis (the latter
with t1/2 ) 849 h) whereas III undergoes further degradation to 2-amino-5,6-dimethylpyrimidin-4-
yl dimethylcarbamate (IV), and to 2-(formylamino)-5,6-dimethylpyrimidin-4-yl dimethylcarbamate
(
(
IX). Both compounds were photodegraded to undetectable species, and IX shows a very high t1/2
48 h). A different behavior was found in solid phase, and only II and III were formed. A kinetic
parallel process was demonstrated. The environmental t1/2 and t1/100 calculated for Pirimicarb and
its photoproducts suggest their reduced persistence in natural waters.
Keyw or d s: Pirimicarb; photodegradation; kinetics
INTRODUCTION
supplied by ICI Italia (Milan, Italy). 2-[(Methylformyl)amino]-
,6-dimethyl-4-hydroxypyrimidine (VIII) was prepared from
5
Pirimicarb (I, Figure 1), 2-(dimethylamino)-5,6-di-
methylpyrimidin-4-yl dimethylcarbamate, is a selective
systemic insecticide that is widely employed against
aphids with a contact action. It penetrates the leaves
but is not translocated extensively (Tomlin, 1994). Soil-
applied Pirimicarb was taken up by roots of many plants
II by mild hydrolysis with stoichiometric amounts of metha-
nolic NaOH (3% w/v) at room temperature. After 4 h the
solvent was removed under reduced pressure. The crude oil
was found to be a 1:1 mixture of III and VIII. The products
were separated by semipreparative high-performance liquid
chromatography (HPLC; see Isolation of Photoproducts).
The phosphate buffers at pH 6.0 and 7.0 were prepared with
(
e.g., lettuce), translocated through the xylem system,
and degraded by metabolism to give II-VII (FAO/WHO,
977). Similar transformation products (mainly II and
Na
was prepared with Na
Celentano and Monticelli, 1977); the pH was adjusted with
.10 M NaOH.
Acetonitrile, dichloromethane, diethyl ether, ethyl acetate,
2
HPO
4
and KH
2
PO
4
(both 0.067 M), whereas the pH 5 buffer
2
HPO (0.2 M) and citric acid (0.1 M)
4
1
(
0
III) were also present in water solutions of Pirimicarb
after photoirradiation, and in addition, traces of 1,1-
dimethylguanidine and 1-methylguanidine were found
and acetone were HPLC grade solvents (Carlo Erba, Milan,
(FAO/WHO, 1977).
Italy). Pyridine (>97%, AnalGrade) and cellulose thin layers
Romero et al. (1994) have reported data on the
(thickness 0.02 mm) were also from Carlo Erba. Humic acids
photochemical degradation of Pirimicarb in water and
solid phase under artificial light and sunlight. Their
data were not in agreement with FAO/WHO data (1977)
and, in part, with the preliminary photochemical data
reported by Cabras (1995). Thus, while Romero found
II, III, V, and a new compound (VIII), no photodegra-
dation was observed by Cabras on peach and nectarine
fruits, whereas he found II and III after Pirimicarb’s
exposition to natural sunlight as a thin-layer film. In
this work we studied in further detail the photodegra-
dation of Pirimicarb both in buffered aqueous solutions
and in solid phase. Our goal was to define the photo-
chemical behavior of Pirimicarb and its photoproducts
from a kinetic viewpoint.
(
>95%), p-nitroacetophenone (PNAP, >98%), and 1,1-dimeth-
ylguanidine sulfate (97%) were purchased from Aldrich (Milan,
Italy); water was distilled twice and purified on a MilliQ
apparatus (Millipore, Milan, Italy) before use. The actinom-
eter solution was prepared according to Dulin and Mill (1982)
-
5
with PNAP at the concentration of 1 × 10 M.
Ap p a r a tu s. The following equipment was used in this
study. High-pressure liquid chromatography: modular sys-
tems Varian 5020 (ID 1 Varian, Palo Alto, CA), HP 1050 (ID
2
3
, Hewlett-Packard, Milan, Italy), Spectra Physics 8700 (ID
, Spectra Physics, Milan, Italy), fitted with variable-wave-
length detectors. Their outputs were connected to a diode
array detector (LC-235 with LCI-100 computer integrator,
Perkin Elmer, Newark, CT). ID 1, ID 2, and ID 3 were also
provided with a Valco AH-20 (loop 100 µL) injector or auto-
matic autosampler (loop 100 µL) and connected with HP 3390
A integrators. Analytical HPLC columns: C
8
Spherisorb, 250
EXPERIMENTAL SECTION
×
4.6 mm i.d., 5 µm, (Waddinxveen, The Netherlands). Semi-
Ch em ica ls. Pirimicarb (I) and its derivatives 2-[(methyl-
formyl)amino]-5,6-dimethylpyrimidin-4-yl dimethylcarbamate
preparative HPLC column: Econosil C
8
, 250 × 10 mm i.d.,
10 µm (Carlo Erba). Mobile phase systems: several mobile
phases at different percentages of CH
-2
(II), 2-(methylamino)-5,6-dimethylpyrimidin-4-yl dimethylcar-
3 2
CN/buffer (10 M KH -
bamate (III), and 2-amino-5,6-dimethylpyrimidin-4-yl di-
methylcarbamate (IV), 2-(dimethylamino)-5,6-dimethyl-4-hy-
droxypyrimidine (V), 2-(methylamino)-5,6-dimethyl-4-hydroxy-
pirimidine (VI), and 2-amino-5,6-dimethyl-4-hydroxypyrimi-
dine (VII) were analytical standards (purity >98%) kindly
PO containing 5.0 mL/L acetic acid) were employed at the
4
flow rate of 1.0 (analytical) and 3.0 (semipreparative) mL/min.
Percentage composition of eluent mixtures, retention times,
and wavelengths employed were reported in Table 1. Gas
chromatography-mass spectrometry: GC-MS Hewlett-Pack-
S0021-8561(95)00171-3 CCC: $12.00
© 1996 American Chemical Society

2418 J. Agric. Food Chem., Vol. 44, No. 8, 1996
Pirisi et al.
Ligh t Sou r ces. The lamps were calibrated with PNAP
-
5
solutions (1 × 10 M) by three replicate experiments of
photodegradation in water. The following lamps were used:
(
A) low-pressure mercury lamp (50 W, Helios Italquartz, Milan
-
7
-1 -1
Italy; I
λ
) 4.01 × 10 EL
s
) with λmax ) 254 nm; (B) high-
) 4.62 ×
) with a water-cooled quartz filter; (C) high-
pressure mercury lamp (125 W, Helios Italquartz; I
) 3.97 ×
) jacketed with a water-cooled pyrex; this lamp
pressure mercury lamp (125 W, Helios Italquartz; I
λ
-
7
-1
-1
1
0
EL
s
λ
-
7
-1 -1
1
0
EL
s
emitted only the wavelengths >290 nm.
Ir r a d ia tion . In all experiments nonirradiated samples
were held in the dark as control. Each experiment was
duplicated with four replications.
(A) Under Artificial Lights. (1) In Solution. The irradiation
in buffered solutions (pH 7.0, 6.0, and 5.0) were performed as
follows.
The lamps were suspended into a cylindric vessel (v ) 350
mL, pathlength (l) ) 1.2 cm) and the solution was stirred with
a magnetical stirrer. The solutions of exposed compounds were
at concentrations ranging between 1.0 and 3.0 mg/L [(4.2-
-
6
1
2.6) × 10 M). Samples from reaction solutions were injected
into the chromatograph without any further sample prepara-
tion.
At pH 7.0 some experiments were performed by flushing
N
2
into the reactor to check the dependence of Pirimicarb’s
degradation by the content of O . In these cases, the nitrogen
2
was bubbled into the solution before irradiation for 45 min
and a flow at a pressure of 0.2 µPa kept during the time of
experiment.
(
2) In Solid Phase. A 0.5 mL aliquot of a solution in ethyl
-
5
ether (5 mg/L, 2.0 × 10
M) was placed into 2.0 mL
borosilicate screw-capped vials; the solvent was evaporated to
dryness under a gentle nitrogen stream and the vials were
suspended into a black cylinder containing the lamp with the
Pyrex jacket (lamp C). At selected times one vial was
withdrawn from the cylinder, frozen at -25 °C for 10 min, and
finally taken up with 1.0 mL of eluting mixture (35% CH
5% buffer) and injected for HPLC analysis.
B) Under Natural Sunlight. The outdoor experiments were
3
CN/
6
(
carried out in February 1994 by exposing the test chemicals
and the actinometer to natural sunlight at 39°14′ latitude
north and 3°20′ longitude west from the Rome Monte Mario
meridian.
F igu r e 1. Behavior of Pirimicarb photodegradation in solu-
tion (A) and in solid phase (B). (C) Formulas of Pirimicarb
derivatives known in the literature.
(1) In Solution. Twenty screw-capped vials of borosilicate
containing 1.0 mL of buffered (pH 7.0) Pirimicarb or photo-
product solution were exposed. The vials were withdrawn at
random at selected times, and the samples injected into the
chromatograph without any further sample preparation.
Ta ble 1. HP LC Reten tion Tim es (m in ) a n d λm a x (n m ) in
a
th e An a lytica l a n d Sem ip r ep a r a tive Sep a r a tion of
(2) In Solid Phase. Three different exposure experiments
P ir im ica r b a n d Its P h otolytic P r od u cts w ith Differ en t
CH3CN/Bu ffer (% v/v) Ra tios in th e Mobile P h a se
were performed. In the first the same procedure reported
above (A2) was followed.
analytical
In the second experiment, 0.5 mL of an ethereal solution (5
mg/L) was placed in uncovered borosilicate Petri dishes. The
solvent was evaporated and dishes were exposed. The dry
samples were redissolved with 1.0 mL of eluting mixture and
injected.
In the third experiment, 0.5 mL of the ethereal solution of
compounds was adsorbed on cellulose thin layers (5 × 5 cm).
The area of deposition was marked with a pencil and exposed
to natural sunlight. At selected times the thin layer was
drawn, and the area of cellulose scraped with a lancet into a
beaker. Acetone (1 mL) was added to the cellulose powder.
The resulting suspension was sonicated for 5 min and filtered
semipreparative
compd
35:65
25:75
25:75
λmax (nm)
I
II
10.0
8.0
5.9
4.4
3.9
15.1
18.2
9.5
6.7
5.0
21.4
36.3
11.5
8.8
236
245
230
230
226
230
237
III
IV
V
VIII
IX
b
nd
b
nd
4.3
5.5
15.5
4.7
8.4
a
See Experimental Section for the columns employed. b Not
determined.
with filter paper. The filtrate was evaporated under N
2
at
ard 5890-5971, at 70 eV using electron impact ionization. GC
conditions: Hewlett-Packard capillary column HP-5 (coated
with 5% phenyl ethyl silicone, 30 m × 0.25 mm i.d., film
tickness 0.25 µm); injector temperature 50 °C; carrier gas
helium; temperature program 50-260 °C, 10 °C/min. Sample
injection 2 µL. NMR: Varian VXR-300 spectrometer equipped
room temperature, redissolved with 1.0 mL of eluting mixture,
and analyzed by HPLC. Recovery assays performed with
-
6
-6
-6
known amounts (4.2 × 10 , 8.4 × 10 , and 12.6 × 10 M) of
studied compounds shows recoveries ranging between 93 and
105%.
Isola tion of P h otop r od u cts. The photoproducts were
isolated from the reaction solutions as follow. After irradiation
up to two half-lives (t1/2), the solution (350 mL, concentration
1
with a Sun computer 3/60 at 300 and 100 MHz for H and
1
3
C, respectively. UV: Varian DMS 90 UV/vis spectrometer.
-
3
IR: FT-IR 2000 Perkin-Elmer spectrometer.
about 5 × 10 M), was placed in a separatory funnel and
shaken twice with ethyl acetate (500 mL each). After drying
on anhydrous sodium sulfate, the combined organic layers
were evaporated under reduced pressure and temperature
Ch r om a togr a p h y. The calculation of the concentration in
the chromatograms was made by external standard method
by plotting peak height vs concentrations.

Pirimicarb Photodegradation
J. Agric. Food Chem., Vol. 44, No. 8, 1996 2419
Ta ble 2. NMR, F T-IR, a n d GC-MS of Isola ted P h otop r od u cts
mass spectral
compd
1H NMRa
13C NMRa
npc
GC trb
fragmentatione
FT-IR datad
UV (nm, λ; ꢀ)
I
1.97, s, CH3 on C-3
npc
npc
npc
244; 34362.9
265; 23354.3
270; 4000.3
290; 5333.9
2
3
3
.35, s, CH3 on C-4
.05, d, CH3 on NCO
.10, s, CH3 on NCC
npc
10.03
253, M ; 8
+
1731, CdO in sOsCOsNd 240; 29297.5
II
2.04, s, CH3 on C-3
+
2
2
3
.38, s, CH3 on C-4
.97, d, CH3 on NCO
.24, s, CH3 on NCC
224, M - 2CH3; 32
1680, CdO in CHO
258; 14119.3
270; 10236.5
290; 2117.9
+
207, M - 3CH3; 72
+
152, M - N(CH3)2
CH3NCHO, 43
9
.66, s, H of COH
72, CON(CH3)2; 100
np
npc
npc
npc
c
3285, NsH
234; 29198.6
III
1.99, s, CH3 on C-3
2
3
3
4
.33, s, CH3 on C-4
.06, d, CH3 on NCO
.10, s, CH3 on NCC
.98, s, NH
1716, CdO in sOsCOsNd 245; 18837.8
270; 3767.6
290; 6279.3
IV
IX
1.94, s, CH3 on C-3
npc
npc
npc
npc
2
3
4
.30, s, CH3 on C-4
.04, d, CH3 on NCO
.98, s, NH
+
2.05, s, CH3 on C-3
169.04, NCdO
36.61, N(CH3)2
17.64
238, M ; 2
3290, NH
1720, CdO in OsCOsNd
165, M - OCON(CH3)2; 10 1689, CdO in NCHO
239; 51355.5
250; 46686.6
270; 18674.7
290; 933.7
+
2
3
7
.38, s, CH3 on C-4
.04, d, CH3 on NCO 163.10, NCHO
.86, s, NH
195, M - NHCHO; 4
+
+
138, M - OCON(CH3)2 -
CO + 1, 65
9
np
.35, s, H of COH
72, CON(CH3)2; 100
c
+
VIII
29.00, NCH3
63.51, NCHO
12.64
152, M - CHO; 15
3580, OH
1705, CdO in NCHO
226; 2773.4
250; 1773.4
+
1
124, M - N(COH)CH3; 76
2
2
70; 2080.1
90; 742.9
a
b
ppm from TMS. Spectra were registered in CDCl3 solution. Only VIII was in CD3CN solution. min; for conditions see Experimental
c
d
-1
e
Section. Not performed. cm ; Nujol. m/z, identification; % rel abundance.
(
<40 °C) until dry. The crude oil was dissolved in 3.0 mL of
I k
0
2
t
-k T
an acetone/water mixture (1:1, v/v) and injected (200 µL for
each injection) into a semipreparative C column eluted with
a CH CN/buffer (25:75, v/v) mixture at a flow rate of 3.0 mL/
min. The retention times of the compounds collected are in
Table 1.
[V] )
(1 - e
)
(5)
kt
8
3
[where I was the starting concentration of Pirimicarb, T
0
the time (s), and k ) k + k + k . are (see Figure 1A) the
t
1
2
3
three rate constants of the single steps of the parallel
The compounds were extracted from the eluent mixture as
follows. The aqueous solution (e.g., 10 mL) was saturated with
NaCl (5 g). Acetone and dichloromethane were added in
volumes twice that of water (or mobile phase). The mixture
was shaken in a rotatory stirrer for 10 min at 50 rpm. The
process and k ) k of the disappearance of Pirimicarb.
t
obs
In the case of a first-order reaction, when at t ) 0 the
starting concentrations of II, III, and V are zero, the
concentrations of formed compounds were in proportion to
their rate constants independently of the time, II:III:V )
k :k :k .
organic layer was withdrawn, dried with anhydrous Na
filtered, and then evaporated under N
Id en tifica tion of Isola ted P h otop r od u cts. The isolated
2 4
SO ,
1
2
3
2
.
P h otoch em ica l P a r a m eter s. The environmental mea-
sured rate constant (KE ) and t1/2 and t1/100 were calculated
m
1
13
photoproducts were identified by H and C NMR, GC-MS,
FT-IR, and UV analysis. Spectroscopic data are collected in
Table 2.
from the following equations, according to Weerasinghe et al.
(1992).
m
c
Kin etics. The kinetic studies were performed by HPLC
analysis.
The constant rates of disappearance of compounds and
photoproducts (Kobs), were calculated as pseudo-first-order
constants by the equation
KE ) k (hos)/2.2
p
m
t1/2 ) ln(2)/KE
m
t1/100 ) 4.605/KE
-
kt
I ) I0e
(1)
where k c
p
) Kobs and hos ) hours of sunlight. The daily
average hours of sunlight during our experiments were 11.0.
Each sampling time represented four replicate experiments
and each determination was duplicated. The values of Kobs
show a SD ranging between (4.6 and 8.1.
c
The sunlight reaction quantum yield of photodegradation (Φ )
ꢀ
and the maximum predicted environmental photolytic rate
constant (Kc
E
) were calculated by the following equations
The statistical data were calculated by a computer program
(Choudhry and Barrie Webster, 1985).
(Microsoft Excel 5). The same computer program was em-
ployed for the interpretation of the parallel process according
to Frost and Pearson (1961) by the equations
Kp
c
a
a
ΦE
)
(∑
ꢀ λLλ/ΣꢀλLλ)Φ
(6)
(7)
Ε
(
a
)
Kp
-
ktT
[I] ) I0e
(2)
c
c
KE ) Φ
∑ ꢀλLλ
E
I k
0
1
t
-k T
[II] )
(1 - e
)
(3)
p p
where K were, respectively, the degradation rate
and K a
constants of the compound and of the actinometer. The terms
kt
a
∑
ꢀ λLλ and ∑ꢀλLλ (see Table 6) represent the sum of molar
I k
0
3
t
-k T
[III] )
(1 - e
)
(4)
absorptivity times solar irradiance for the actinometer and
kt
compounds, respectively. Φa was the reaction quantum yield
E

2420 J. Agric. Food Chem., Vol. 44, No. 8, 1996
Pirisi et al.
Ta ble 3. Kobs (×10 s-1) for th e Degr a d a tion of
P ir im ica r b a n d Its P h otop r od u cts in Bu ffer s u n d er
La m p s A, B, a n d C a n d Su n ligh t
-5
Pirimicarb, while (IX) photodegraded slower (t1/2 ) 48
h, Table 3). Both give unknown products.
The five photoproducts were contemporaneously
present in the chromatograms at times relative to the
rate constants found under the different lamps. The
sum of the concentrations of photoproducts formed in
the different experiments was almost stoichiometrically
equivalent to the disappearance of Pirimicarb (see Table
4).
compd
pH
Kobs
r 2
t1/2 (h)
Lamp A
24.1
0.13
I
II
II
7.0
7.0
6.0
0.985
0.977
0.948
0.8
148
175
0.11
Lamp B
110.2
I
7.0
7.0
0.993
0.16
To check this behavior, photocompounds II, III, IV,
and IX were irradiated individually under lamp C. II
Lamp C
2.4
1.6
0.023
0.015
2.5
I
0.997
0.996
0.998
0.975
0.999
0.963
0.975
7.9
11.9
849
1284
7.6
undergoes very slow photodegradation (kobs ) 2.27 ×
6
.0
7.0
.0
-7 -1
1
0
s , t1/2 ) 849 h). It does not give III, and no other
II
peak was present in the chromatograms. Moreover if
II should give III, this could not be detected because it
is photodegraded with a kobs 100-fold higher (see Table
3). Photoproduct III may came from II only by hydroly-
sis after very long times, certainly longer than those of
photodegradation. As a matter of fact, we found III, in
the concentration of about 15%, in blanks stored in the
dark of the formylamino derivative only after 24 days.
The compound V was not extensively studied because,
by preliminary experiments, it was found very stable
to photoirradiation with t1/2 similar to that of II, and
no signals of its degradation compounds were found in
the chromatograms.
6
III
IV
IX
7.0
7.0
7.0
2.4
0.38
8.0
48
Sunlight
7.1
0.08
8.0
2.4
1.0
I
II
III
IV
IX
7.0
7.0
7.0
7.0
7.0
0.944
0.945
0.981
0.963
0.942
2.7
226
2.3
8.0
19
of the actinometer (Choudhry et al., 1985). Solar irradiance
values were taken from the Cagliari Astronomical Observatory
University of Cagliari, Cagliari, Italy) data recorded at days
(
of sunlight experiments.
When pure specimens of III were irradiated, photo-
products IV and IX were formed.
Experiments performed by flushing N2 into the reac-
tor show meaningful variation in Kobs values. This
indicates that the photochemical decomposition of Pir-
imicarb is independent of the O2 contents in the buffer,
and consequently, no free radical reaction was active
RESULTS AND DISCUSSION
In all the experiments, no degradation was observed
after 1 week in the blanks stored in the dark.
Ir r a d ia t ion in Bu ffer Solu t ion s. (1) At pH 7.0.
Pirimicarb (I) undergoes fast degradation in buffer
solutions when irradiated with the three lamps. The
rate of degradation seems to be dependent on the nature
of the light, as reported in Table 3. The maximum rate
was observed with lamp B, while the minimum was
achieved with the lamp C. This indicates that the
wavelengths 254-290 nm were the most active in
degradation according to the absorbances of Pirimicarb
and the Iλ of the lamps.
(Pusino et al., 1992).
(2) At pH 6.0. With lamp C the behavior was similar
to that at pH 7.0. The same photoproducts were formed
and only the formation of II was contemporary with III
and V. Therefore this finding does not allow us to
consider the formylamino derivative as the intermediate
of III, as suggested by Romero et al. (1994). Indeed, II
is more stable to photodegradation than Pirimicarb or
III (Table 3). Yet, the signal of III and also that of II
increase up to the half-time of Pirimicarb degradation.
This indicates that the reaction cannot be a consecutive
process because, if it were, the signal of III would be
not present in the chromatograms because its degrada-
tion rate was 111-fold higher than that of II.
The pathway could be described kinetically (Figure
1A), as a set of two parallel reactions. First from
Pirimicarb to II, III, and V; then from III to IV and IX.
We have applied the kinetic model for a parallel
process (Frost and Pearson, 1961) to our experiments
from I to II, III, and V, from t ) 0 to t ) 165 min. Since
at 165 min the signals of IV and IX were not present in
the chromatograms, the second parallel reaction can be
regarded as negligible. As shown in Table 7, the
concentrations of II, III, and V, are in constant ratio
with each other according to the theory.
In all experiments we found five photoproducts. The
HPLC retention times and UV spectra of four com-
pounds indicate that these are II-V. The signal at tr
)
8.4 min (Table 1) shows a UV spectrum different from
those of known metabolites of Pirimicarb. To this
compound, separated from the reaction medium by
semipreparative HPLC chromatography, on the basis
1
13
of H and C NMR spectra, GC-MS fragmentation, and
FT-IR spectrum (Table 2), could be assigned the struc-
ture of 2-(formylamino)-5,6-dimethyl-4-yl dimethylcar-
bamate (IX).
In our experiments, under all artificial or natural
irradiation, signals attributable to the 1,1-dimeth-
ylguanidine, and to photoproducts VI and VII (FAO/
WHO, 1977) or VIII (Romero et al., 1994), were never
detected in the chromatograms (see Figure 1C). The
photodegradation of Pirimicarb under lamp C was very
similar to those observed in natural sunlight experi-
ments, with the photoproducts III and V appearing first
followed by the photoproduct II. The signals of II and
V always increase during the disappearance of Pirimi-
carb. Photoproduct III undergoes further degradation
and the signals of IV and IX appear. During the first
three t1/2 of Pirimicarb degradation, the increase in the
concentration of IV and IX is not linear. Later (IV)
disappeared with a rate constant similar to that of
The behavior of the photodegradations was the same
under different pH and artificial lights. In the buffer
solutions irradiated with lamp C at different pHs the
-
5
-1
rate of disappearance of I (Kobs × 10
s ) was found
to decrease nonlinearly from 2.4 at pH 7, to 1.6 at pH
6, and to 1.2 at pH 5.
Irradiation in Solid Phase. In Petri dishes, cellulose
plates, and vials exposed to sunlight, I always undergoes
a very fast degradation to give only photoproducts II
and III. Under prolonged irradiation, IV and IX were

Pirimicarb Photodegradation
J. Agric. Food Chem., Vol. 44, No. 8, 1996 2421
-
6
Ta ble 4. Com p a r ison of th e Con cen tr a tion s (×10 M) of P h otop r od u cts F or m ed in P ir im ica r b Degr a d a tion a t p H 7.0
u n d er Differ en t La m p s a t Tim es P r op r otion a l to th e Kobs
lamp
sampling time (min)
I
II
III
IV
V
IX
Σ
I0
∆%
A
B
C
75
20
1350
13.90
9.58
6.41
1.55
2.44
5.56
15.80
17.40
3.79
2.09
2.44
2.17
2.79
2.50
1.77
2.58
2.99
3.06
38.70
37.50
22.80
35.90
34.30
23.70
7.80
9.33
-3.80
Ta ble 5. Kobs (×10 s-1) a n d Ha lf-Life (t1/2, m in ) for th e
Degr a d a tion of P ir im ica r b, II, a n d III Exp osed to
Na tu r a l Su n ligh t a n d to La m p C in Solid P h a se
-5
Ta ble 6. En vir on m en ta l Ra te Con sta n ts, Rea ction
Qu a n tu m Yield (Φa ), Ha lf-Lives, a n d t1/100 for
P ir im ica r b a n d Its Ma jor P h otop r od u cts
c
a
Pirimicarb
II
III
Kp
KE
KEm
t1/2
t1/100
compd (h 1)b ΣꢀλLλc ΦE (days-1) (days-1)d (days) (days)
-
light
nea
solid phase
Kobs
t1/2
Kobs
t1/2
Kobs
t1/2
I
II
III
IX
0.25 284.53 0.075 2.13
0.003 0.78 0.033 0.026
0.29 132.83 0.019 2.52
0.04 0.26 1.32 0.34
1.25
0.015
1.45
0.20
0.55
46.2 307
0.48
3.46 23.0
3.68
a
a
nea
nea
Petri dishes
cellulose plates
vials
120.5
60.5
33.9
18.2
10 ne
19 ne
33 ne
ne
ne
ne
a
a
a
a
a
sun
ne
ne
3.17
a
a
a
a
ne
ne
ne
lamp C vials
64 1.5
785 16.9 68
a
By eq 6; Kpa ) 1.6 × 10 h-1; Σꢀ λLλ ) 81.02. b Kp from Tables
-3
a
a
Not exposed.
3
and 5. c Calculated in the range 297.5-400 nm from experimen-
2
tal molar absorptivities. Irradiance values (Lλ meinstein per cm
not formed and signals at the retention times of VIII
and 1,1-dimethylguanidine never appear.
Under lamp C the degradation rate was slower than
day) were supplied by Cagliari Astronomical Observatory and refer
d
to the exposure period, February 16-21, 1994. Daily average
hours of sunlight in February were 11.0.
that in solution and the formation of II increased up to
Ta ble 7. Com p a r ison betw een Ir r a d ia tion by La m p C of
a
1
90 min. At this time Pirimicarb was 25% of the
th e Exp er im en ta l (Exp ) a n d Ca lcu la ted (Ca l)
-
6
Con cen tr a tion s (×10 ) vs Tim e a n d of Exp er im en ta l
Con cen tr a tion Ra tios F ou n d in th e P ir im ica r b
Degr a d a tion in Solid P h a se a n d Exp er im en ta l
Con cen tr a tion Ra tios in Bu ffer Solu tion a t p H 7 for
P h otop r od u cts II, III, a n d V
starting concentration. III increased up to 60 min and
then decreased. When II and III were irradiated alone,
III showed a half-life similar to that of Pirimicarb, while
II was degraded with a rate 11 times slower (Table 5).
These data indicate that, in solid phase, Pirimicarb
degraded with a parallel process in which, K1 ) 1.1 ×
concns in solid phase
-
4
-1
-4 -1
II
III
V
exp concn ratio
1
0
s
and K3 ) 0.71 × 10
s
. In a simulation of
time
the parallel process from Pirimicarb to III and II (Frost
and Pearson, 1961) up to the beginning of the degrada-
tion of III to unknown products, a very good agreement
was reached between the theoretical and experimental
concentration data (Table 7). Consequently, we suggest
the pathway reported in Figure 1B for the degradation
of Pirimicarb in solid phase.
(min) Exp Cal Exp Cal Exp Cal II/III III/V V/II
2.52 2.59 nfb
nfb
30
45
1.02 1.4
0.4
0.5
0.6
c
c
c
c
c
c
b
b
1.83 1.99 3.61 3.60 nf
nf
6
0
2.79 2.51 4.29 4.45 nfb nf
b
exp concn ratio in buffer
exp concn ratio in buffer
time
min) II/III
time
V/II (min) II/III
(
III/V
III/V
V/II
8
00
0
0.70
0.74
1.84
1.86
0.77
0.73
130
165
0.76
0.79
1.97
1.97
0.67
0.64
The degradation rate constants under sunlight in
uncovered Petri dishes, cellulose plates, and screw-
capped vials were different (Table 5). The lower deg-
radation rate found in the vials could be explained on
the basis of the absorption of light by glass. The
degradation in vials under lamp C was slower than that
under sunlight.
1
a
By eqs 2-5. b In solid phase V was not formed. c Ratios not
computable.
CONCLUSIONS
When exposed to artificial light and to sunlight,
Pirimicarb always undergoes quick decomposition with
a kinetic parallel two-step behavior in buffer solution
and a one-step behavior in solid phase. The degradation
mechanism in solution does not seem a free-radical
process.
In the literature it has been reported that a major
pathway of loss of Pirimicarb was its volatilization
(FAO/WHO, 1977). In our experiments with uncovered
Petri dishes, we found that the disappearance behavior
of Pirimicarb was the same of that in screw-capped
vials; therefore, it was not due to volatilization but to
degradation.
The difference in the wavelengths of the lamps only
affects the degradation rate constants. In the first step
in aqueous solution, the main photoproducts were II,
III, and V, the same previously reported (FAO/WHO,
1977). Photoproducts II and V were more stable than
III. In the second step, III undergoes a further parallel
degradation to IV and to the new photoproduct IX.
Photoproducts IV and IX also undergo slow photodeg-
radation to unknown products. Disappearance of II was
increased by photosensitizers such as acetone or humic
En vir on m en ta l P h otod egr a d a tion Ca lcu la tion s.
The environmental rate constants, t1/2 and t1/100 in the
buffer solutions at pH 7 were calculated according to
the literature (Choudry and Webster, 1985; Weeras-
inghe et al., 1992). Table 6 shows data for Pirimicarb,
II, III, and IX. A good accordance between the calcu-
lated and measured parameters was achieved. These
data indicate that only II could persist in natural
waters. However, many photochemical sensitizers, such
as humic acids, acetone, etc. (Choudhry et al., 1979,
4
acids by a 10 factor on rate constants. The degradation
data in the solution suggest a moderate persistence of
I and its photoproducts in environmental waters.
1
985), could be present in natural waters. As previously
described in buffer solutions the irradiation of II in the
In solid phase Pirimicarb was not lost by volatilization
from uncovered Petri dishes or cellulose plates. The
degradative behavior of I was similar in aqueous solu-
tion and solid phase in the first step but in aqueous
solution V was also formed. The results from this work
presence of acetone (1% w/v) or humic acids (10 mg/L)
4
gives an increase in Kobs values by a factor of 10 .
Therefore photoproduct II should be also weakly per-
sistent in natural waters.

2422 J. Agric. Food Chem., Vol. 44, No. 8, 1996
Pirisi et al.
disagree, in part, with those by Romero et al. (1994).
We think the differences in indoor experiments could
be due to differences between the lamps used by us and
the Suntest apparatus in Romero’s experiments. How-
ever, the differences in the disappearance rates of
Pirimicarb in aqueous solutions under sunlight could
be attributable to the different solar irradiances at the
two latitudes (39° N vs 48° N).
Dulin, D.; Mill, T. Development and evaluation of sunlight
actinometers. Environ. Sci. Technol. 1982, 16, 815-821.
FAO/WHO. Pesticide Evaluations Annual Report 1976; FAO/
WHO: Rome, 1977; p 535.
Frost, A. A.; Pearson, R. G. Kinetics and Mechanism; J . Wiley
&
Sons: New York, 1961, pp 166-178.
Pusino, A.; Gessa, C.; Liu, W. Photolysis of Dimepiperate in
Aqueous Solution. Pestic. Sci. 1992, 34, 269-271.
Romero, E.; Schimtt, P.; Mansour, M. Photolysis of Pirimicarb
in Water under Natural and Simulated Sunlight Conditions.
Pestic. Sci. 1994, 41, 21-26.
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1
992, 40, 1419-1421.
Received for review March 27, 1995. Revised manuscript
X
received J uly 19, 1995. Accepted May 8, 1996.
J F9501711
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X
Abstract published in Advance ACS Abstracts, J uly
1, 1996.