Photodegradation of Metolachlor in Water in the Presence of Soil Mineral and Organic Constituents

Article abstract of DOI:10.1021/jf960123w

Photodegradation of metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)-acetamide] at 253.7 nm was carried out in water containing soil minerals (kaolinite, montmorillonite, and goethite) and fulvic acid under two different pH conditions. The rate of photolysis was dependent on the nature of the soil constituents and the initial pH of the medium. Based on the regression analysis, it was shown that the photodegradation followed the first-order kinetics with respect to the metolachlor concentration, and the half-life of the herbicide under UV irradiation was longer in the absence of soil constituents. Hydroxylation, dehalogenation, oxoquinoline formation, and demethylation were the main processes observed during the photolysis of metolachlor. More degradation products were formed in the presence of kaolinite, montmorillonite, and goethite than with fulvic acid and water alone. The major degradation product formed under UV irradiation in all the treatments was identified as 4-(2-ethyl-6-methylphenyl)-5-methyl-3-morpholine.

Full text of DOI:10.1021/jf960123w

3996
J. Agric. Food Chem. 1996, 44, 3996−4000
P h otod egr a d a tion of Metola ch lor in Wa ter in th e P r esen ce of Soil
Min er a l a n d Or ga n ic Con stitu en ts†
Regi Mathew* and Shahamat U. Khan
Centre for Land and Biological Resources Research, Research Branch, Agriculture and Agri-Food Canada,
Ottawa, Ontario, Canada K1A 0C6
Photodegradation of metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)-
acetamide] at 253.7 nm was carried out in water containing soil minerals (kaolinite, montmorillonite,
and goethite) and fulvic acid under two different pH conditions. The rate of photolysis was dependent
on the nature of the soil constituents and the initial pH of the medium. Based on the regression
analysis, it was shown that the photodegradation followed the first-order kinetics with respect to
the metolachlor concentration, and the half-life of the herbicide under UV irradiation was longer in
the absence of soil constituents. Hydroxylation, dehalogenation, oxoquinoline formation, and
demethylation were the main processes observed during the photolysis of metolachlor. More
degradation products were formed in the presence of kaolinite, montmorillonite, and goethite than
with fulvic acid and water alone. The major degradation product formed under UV irradiation in
all the treatments was identified as 4-(2-ethyl-6-methylphenyl)-5-methyl-3-morpholine.
Keyw or d s: Photodegradation; metolachlor; soil minerals; fulvic acid; degradation products
INTRODUCTION
Katagi, 1991; Larson et al., 1991; Kochany, 1992; Katagi
1993). It is possible that some of the degradation
products thus formed in the presence of soil constituents
may be nontoxic or less toxic to the aquatic organisms.
Therefore, this experiment was undertaken to study the
photodegradation of metolachlor in water in the pres-
ence of soil mineral and organic constituents at different
pH values. Various degradation products formed under
UV irradiation were also identified, and regression
analysis was carried out to determine the order of
reaction.
Metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-
(2-methoxy-1-methylethyl)acetamide] is one of the most
widely used herbicides for the control of grasses in many
crops (Lebaron et al., 1988; Chesters et al., 1989).
Although metolachlor is not registered as an aquatic
herbicide, its residues carried from terrestrial applica-
tions are reported in surface waters and groundwater
(Frank and Logan, 1988; Maguire and Tkacz, 1993;
Richards and Baker, 1993). When it reaches the various
aquatic systems without transformation/degradation to
less toxic products, the herbicide may pose a threat to
our environment. Generally, volatilization, hydrolysis,
and photolysis are the major chemical transformation
pathways of a pesticide in aqueous solution. Among
these, photolysis is known to be the important degrada-
tion pathway for metolachlor in water under normal
conditions (Kochany and Maguire, 1994).
Natural waters contain a variety of dissolved, col-
loidal, and suspended organic and mineral constituents
from soil. Therefore, a part of the pesticides present in
the aquatic systems will be associated with these
materials. In addition, metolachlor is a soil-applied
pesticide with a relatively high sorption affinity for soil
and various soil organic and mineral constituents
(Obrigawitch et al., 1981; Bouchard et al., 1982; Weber
and Peter, 1982; Kozak et al., 1983; Peter and Weber,
1985; Wood et al., 1987). Hence, it is possible that a
major part of the soil-applied metolachlor reaching the
natural waters through leaching and surface runoff is
adsorbed on the soil mineral and organic constituents.
Soil organic and inorganic materials may influence
the photolysis of chemicals in water in different ways.
They may reduce the photolysis by quenching the
excited states of organic molecules or by shielding them
from incident radiation (Oliver et al., 1979). These
materials may accelerate the photodegradation by en-
ergy transfer reactions, by photoinduced oxidation, or
by efficient light scattering (Miller and Zepp, 1979a;
MATERIALS AND METHODS
Ma ter ia ls. Analytical standards of metolachlor and deg-
radation products were gifts from Ciba-Geigy Co. (Greensboro,
NC). All the organic solvents used in this study were of HPLC
grade.
The organic constituent and soil minerals used in this study
were as follows: (i) Fulvic acid originated from the Bh horizon
of a Podzol soil in Prince Edward Island, Canada. Methods
of extraction, purification, and functional group analysis were
the same as those described by Schnitzer and Skinner (1968)
and Gamble (1972). It contained 1.0% ash, 50.9% C, 3.3% H,
0.7% N, 0.3% S, and 44.7% O on a moisture- and ash-free basis.
Functional group analysis showed the presence of COOH (5.0
mequiv/g), phenolic OH (3.3 mequiv/g), alcoholic OH (3.6
mequiv/g), CdO (0.6 mequiv/g), and OCH3 (2.5 mequiv/g)
groups. (ii) Montmorillonite mineral from Wyoming (4.4% Fe
as Fe2O3 and 0.15% Ti as TiO2). (iii) Kaolinite from Georgia
(0.35% Fe as Fe2O3 and 1.26% Ti as TiO2). (iv) Goethite from
Biwabik, MN (2-5 µm size; 86.04% Fe as Fe2O3).
Ad sor p tion Stu d y. Adsorption of chemicals to clays
during photolysis may interfere with the kinetics of photodeg-
radation (Zepp and Schlotzhauer, 1981). Therefore, the extent
of metolachlor adsorption on various soil constituents prior to
photolysis was determined. Fulvic acid was dissolved, miner-
als were suspended in 10 mL of deionized water for 8 h, and
metolachlor stock solution (300 mL) was added to the mineral
(0.05% w/v) suspensions and fulvic acid (0.01% w/v) solution
to get a final concentration of 30 µg of metolachlor/mL. The
samples were shaken over a reciprocating shaker for 12 h in
the dark. Metolachlor in water was also shaken for 12 h in
the dark prior to the photolysis experiment. In one set of
experiments, the initial pH of the medium before and after
† Contribution No. 95-70.
S0021-8561(96)00123-9 CCC: $12.00
Published 1996 by the American Chemical Society

Photodegradation of Metolachlor
J. Agric. Food Chem., Vol. 44, No. 12, 1996 3997
Ta ble 1. In itia l p H a n d Am ou n t of Metola ch lor Ad sor bed (m g/100 m g of Ad sor ba te), P a r tition Coefficien t (Kd , m L/g) of
Ad sor p tion , a n d Ra te Con sta n ts (m in ), Ha lf-Life (h ), a n d Cova r ia n ce of Metola ch lor P h otod egr a d a tion in Differ en t
Tr ea tm en ts
photodegradation
rate constant
treatment
initial pH
amount adsorbed
Kd
(×10-3
)
half-life
covariancea
water
water
FA
5.35b
7.00
7.76
4.48
9.05
9.89
11.2
8.09
8.82
9.00
9.69
8.46
1.49
2.58
1.28
1.17
1.03
1.43
1.31
1.28
1.19
1.37
12.9
9.66
69.1
64.8
27.3
23.2
23.7
21.1
31.8
19.4
3.46b
7.00
19.7 (1.44)c
15.6 (0.28)
13.4 (0.17)
8.68 (0.50)
12.2 (0.31)
7.42 (0.25)
1.96 (0.16)
0.74 (0.08)
0.071 (0.001)
0.055 (0.01)
0.047 (0.001)
0.030 (0.003)
0.043 (0.002)
0.025 (0.001)
0.007 (0.000)
0.002 (0.001)
FA
MON
MON
KAO
KAO
GEO
GEO
5.47b
7.00
5.54b
7.00
4.41b
7.00
a
Covariance of the linearized form of metolachlor degraded. Abbreviations: FA, fulvic acid; MON, montmorillonite; KAO, kaolinite;
GEO, goethite. Unadjusted pH. c Standard error of mean.
b
shaking with metolachlor was adjusted to 7.00 ( 0.02, using
0.01 N NaOH or HCl. In the other set, the initial pH was not
adjusted after adding metolachlor. Determination of meto-
lachlor concentration in the solution at regular intervals
showed that the amount of metolachlor adsorbed remained
constant after 8-10 h of reaction. Therefore, at the end of 12
h of shaking, a 25-mL aliquot was taken from all the treat-
ments and centrifuged at 14000g for 20 min, and the super-
natant was used for determining the concentration of meto-
lachlor. The amount of metolachlor adsorbed to each adsorbate
is estimated by subtracting the final solution concentration
from the initial concentration. The starting concentration in
each treatment at the beginning of the photolysis study is the
amount of metolachlor present after the adsorption. Partition
coefficient of metolachlor adsorption is determined as the ratio
of the amount adsorbed to the concentration in the equilibrium
solution.
P h otolysis Stu d y. About 150 mL of metolachlor in deion-
ized water and fulvic acid solution or mineral suspensions at
the end of the adsorption study were transferred to a 200-mL
pear-shaped necked flask and photolyzed under UV light for
a period of 6 h. For photolysis, the light source used was a
UV Pen Ray Lamp (Model II, SC-IL, 5.5 W, manufactured by
Ultraviolet Products, Inc., San Gabriel, CA) with the 253.7-
nm line comprising 92% of the total irradiation. The Pen Ray
Lamp was inserted into the solution or suspension through
the neck of the reaction flask, and the apparatus was placed
in a dark room. The temperature of the irradiation solution
was maintained at 22 ( 2 °C by circulating water. A magnetic
stirrer was used to agitate the solutions or suspension during
irradiation. The control experiment was conducted in a similar
manner in the dark for all the treatments. An aliquot (15 mL)
at 0, 15, 30, 60, 120, 240, and 360 min of irradiation was
withdrawn for the determination of metolachlor using high-
pressure liquid chromatography (HPLC) and for the identifica-
tion of photodegradation products by gas chromatography-
mass spectrometry (GC-MS). After 6 h of photodegradation,
the amount of metolachlor remaining for further photodegra-
dation was negligible. Therefore, photodegradation during the
6-h period was taken for treatment comparison. All experi-
ments were carried out in duplicate.
1.9, and 3.7 min, respectively. Furthermore, the lower limit
of detection for metolachlor under the above HPLC conditions
was 0.1 µg/mL, and for the metabolites it ranged from 0.1 to
0.8 µg/mL. The amount of these compounds was calculated
on the basis of the peak areas obtained with standardized
authentic samples analyzed under the same HPLC conditions.
Ga s Ch r om a togr a p h y. For the GC-MS analysis, an
aliquot (3 mL) of the supernatant was taken to dryness under
reduced pressure at room temperature, and the residue was
dissolved in 50 µL of methanol. Electron impact (EI) mass
spectra were recorded on a Finnigan MAT 4500 instrument
coupled to an INCOS data system. A DB-5 column (12m ×
0.32 mm i.d.; 1 µm film thickness) was used. The column
temperature was programmed from 100 to 280 °C at a rate of
15 °C/min. The inlet temperature was 250 °C. The flow rate
of helium carrier gas was 1 mL/min, and the inlet pressure
was 10 psi. For the EI mass spectrum, the electron energy
was set at 70 eV and a source temperature of 190 °C. The
scan range was 30-500 amu. The identity of the photodeg-
radation products was confirmed by comparing the HPLC
retention times with those of the available authentic samples,
by co-chromatography, and by GC-MS.
Sta tistica l An a lysis of Da ta . Based on R2
values, the
first-order kinetic equation was used to depict the photodeg-
radation of metolachlor. The integrated form of the first-order
kinetic equation (Atkins, 1994) is
[Ct] ) [C0]e-kt
where t is time (min), [C0] is the metolachlor concentration at
time 0, [Ct] is the metolachlor concentration at time t, and k
is the rate constant (min). Half-life (t1/2) of photodegradation
is determined from the rate constants (k).
Covariance analysis was used to measure the joint variation
of UV irradiation and the presence of soil constituents on
photodegradation. It was done on the linearized form (log
transformation) of the metolachlor degraded at different times
of UV irradiation for all the treatments.
RESULTS AND DISCUSSION
High -P r essu r e Liqu id Ch r om a togr a p h y. Samples for
the determination of metolachlor were obtained by centrifuging
the minerals and fulvic acid from the photolyzed mixture
(14000g, 15 min), and an aliquot (25 µL) of the supernatant
was injected directly into a HPLC pump (Waters 501). The
column was a Beckman Ultrasphere ODS 5 µm (25 cm long ×
4.5 mm i.d.) and was preceded by a adsorbosphere C18 5µm
guard column. Methanol-water (80/20 v/v) was used as the
eluting solvent at a flow rate of 1 mL/min. Varian Model 9050
UV detector was used for measuring the absorbance at 220
nm. Under these conditions, the retention times for meto-
lachlor, 4-(2-ethyl-6-methylphenyl)-5-methyl-3-morpholine),
8-ethyl-3-hydroxy-N-(2-methoxy-1-methylethyl)-2-oxo-1,2,3,4
tetrahydroquinoline, and 2-hydroxy-N-(2-ethyl-6-methyl-
phenyl)-N-(2-methoxy-1-methylethyl)acetamide were 6.7, 4.6,
Ad sor p tion . The extent of the adsorption of meto-
lachlor varied with the nature of the adsorbate and the
initial pH of the adsorbing medium (Table 1). Sorption
was higher at the unadjusted pH values than at pH 7
in all the treatments. Among the various treatments,
fulvic acid adsorbed the highest amount of metolachlor
at both pH values, and adsorption on the goethite
surface was relatively very small. The partition coef-
ficient (Kd) was higher for fulvic acid treatments than
others at both pH values (Table 1).
The higher Kd in the fulvic acid solution indicates that
the added metolachlor was retained with greater affinity
than when montmorillonite or kaolinite were present.

3998 J. Agric. Food Chem., Vol. 44, No. 12, 1996
Mathew and Khan
F igu r e 1. Influence of various treatments on the percentage
of metolachlor photodegraded at different times of UV irradia-
tion (pH not adjusted).
F igu r e 2. Influence of various treatments on the percentage
of metolachlor photodegraded at different times of UV irradia-
tion (pH 7).
Higher amount of metolachlor adsorbed from the mont-
morillonite suspension than from kaolinite suspension
suggests that some of the metolachlor is also adsorbed
on the interlayer surfaces of montmorillonite. Webber
and Peter (1982) showed that the metolachlor is ad-
sorbed to the Ca-saturated montmorillonite surfaces by
coordination between the carbonyl oxygen atoms of
metolachlor and the positively charged mineral surface.
It is adsorbed by the fulvic acid involving hydrogen
bonding and through charge transfer bonds between the
aromatic nucleus of metolachlor and the aromatic rings
at fulvic acid surface (Kozak et al., 1983). The number
of positive sites in montmorillonite, kaolinite, and
goethite surfaces is higher at lower pH (Bohn et al.,
1979). This results in more binding between the car-
bonyls of metolachlor and the positive sites at clay
surfaces at lower pH values. As a result, it is adsorbed
more on the inorganic constituents at unadjusted pH
values than at pH 7.
P h otolysis. On exposure to UV light, a major part
of the added metolachlor was photodegraded at both pH
conditions in all the treatments (Figures 1 and 2).
However, the rate of photolysis and the degradation
products formed were dependent on the duration of UV
irradiation, the initial pH of the medium, and the nature
of the suspended/dissolved material. Regression analy-
sis showed that in all the treatments the photolysis
followed the first-order kinetics with respect to the
herbicide concentration (Table 1).
Covariance analysis showed that the presence of soil
constituents in water during UV irradiation has greater
impact on the photodegradation of metolachlor than in
their absence (Table 1). Among the soil constituents,
FA treatment had the highest covariance at both pH
conditions, indicating greater interaction than the in-
organic constituents for photodegradation. The per-
centage of metolachlor degraded during the initial 15-
min period was greater in water alone than in the
presence of soil constituents at both pH conditions
(Figures 1 and 2). However, with increasing time of UV
irradiation, the effect of soil constituents on the photo-
degradation became more pronounced. After 6 h of
photolysis, the amount of metolachlor degraded was
more in the presence of soil minerals and fulvic acid
than with water alone at pH 7. When the original pH
was not adjusted, the amount of metolachlor degraded
in the 6-h period was more than 94% in all the
treatments. The lowest degradation of metolachlor in
the first 1-h period of UV irradiation occurred in the
fulvic acid treatments at both pH conditions.
The rate of photodegradation of metolachlor in water
alone, montmorillonite, and goethite treatment was
higher when the initial pH was not adjusted (Table 1).
Under both pH conditions, the half-lives of photodeg-
radation was longer in the absence of soil constituents
(Table 1). For example, the calculated half-life in water
alone at pH 7 was 2.58 h, and at unadjusted pH
condition, it was 1.49 h. The shortest half-life at pH 7
was in fulvic acid treatment (1.17 h), and at unadjusted
pH, it was in montmorillonite treatment (1.03 h).
The loss of metolachlor from various treatments under
UV irradiation was mainly attributed to photodegra-
dation because of the following reasons: (1) transforma-
tion of metolachlor in the dark due to hydrolysis was
negligible; (2) before starting the photolysis experiment,
metolachlor was adsorbed to the soil constituents, and
therefore the possibility of loss of pesticide from solution
due to continuous adsorption to the soil constituents is
ruled out. Metolachlor adsorbed on the clay surface as
well as that present in the bulk water phase may affect
the photolysis as shown for other pesticides (Miller and
Zepp, 1979a; Katagi, 1990). However, no differentiation
was made in this study to find the effect of adsorbed
metolachlor and that present in the bulk solution on
the photolysis rate and the nature of the photoproducts
formed. The rate of photolysis in this study constituted
that from the bulk solution and the metolachlor ad-
sorbed on the surface of soil constituents.
The initial slow degradation of metolachlor (15 min)
in the presence of soil constituents might be due to their
light shielding effect (Oliver et al., 1979). Also, the 15
min of UV irradiation may not be enough for the
intensive indirect photochemical reactions that occur in
the presence of soil suspended sediments (Katagi, 1990).
This initial light shielding effect was more with fulvic
acid than with other inorganic constituents. However,

Photodegradation of Metolachlor
J. Agric. Food Chem., Vol. 44, No. 12, 1996 3999
Ta ble 2. Rela tive Distr ibu tion of P h otop r od u cts (%)
F or m ed a t Differ en t Tim es of UV Ir r a d ia tion (In itia l p H
Not Ad ju sted )
Ta ble 3. Rela tive Distr ibu tion of P h otop r od u cts (%)
F or m ed a t Differ en t Tim es of UV Ir r a d ia tion (In itia l p H
a t 7)
water
(%)
FA
(%)
MON
(%)
KAO
(%)
GEO
(%)
water
(%)
FA
(%)
MON
(%)
KAO
(%)
GEO
(%)
photoproducts
photoproducts
At 15 Minutes
At 15 Minutes
a
P1
P2
P3
P4
P5
18.8
37.5
12.9
-
-
-
9.80
76.5
-
-
P1
P2
P3
P4
P5
-
-
-
7.58
63.6
9.09
-
-
100
-
100
-
31.9
52.2
-
75.0
25.0
-
100
-
80.8
19.2
-
45.8
35.4
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
At 30 Minutes
At 30 Minutes
P1
P2
P3
P4
P5
18.2
33.3
6.1
-
-
100
-
3.1
47.7
4.6
-
9.5
57.9
4.2
9.5
-
-
P1
P2
P3
P4
P5
14.8
70.4
14.8
-
-
-
8.60
67.7
6.45
-
2.35
38.8
49.4
-
32.8
36.0
4.8
91.4
92.7
7.32
8.62
-
-
-
-
-
-
-
-
12.8
-
-
4.71
At 60 Minutes
At 60 Minutes
P1
P2
P3
P4
P5
13.8
53.8
3.08
-
-
94.4
-
-
7.03
59.4
7.03
-
3.07
26.6
32.0
4.78
21.8
P1
P2
P3
P4
P5
-
-
-
4.86
56.2
3.24
-
1.33
50.0
30.0
7.33
7.33
50.0
83.6
-
86.2
97.1
3.91
4.69
4.69
4.62
-
9.23
2.94
-
-
-
16.4
-
-
-
-
24.9
At 120 Minutes
At 120 Minutes
4.35
95.7
P1
P2
P3
P4
P5
3.2
83.9
6.5
-
-
97.7
-
-
2.31
-
-
1.66
31.5
36.5
4.56
24.5
P1
P2
P3
P4
P5
13.9
86.1
-
-
2.50
54.7
7.40
7.88
23.7
4.29
53.4
22.1
4.29
12.9
50.8
2.09
5.76
12.1
82.2
5.93
8.89
-
89.7
10.3
-
-
-
-
-
-
-
-
At 240 Minutes
At 240 Minutes
6.66
100
P1
P2
P3
P4
P5
80.0
20.0
-
-
96.9
-
-
3.08
7.41
49.7
2.12
8.99
-
2.13
28.6
35.6
4.86
21.6
P1
P2
P3
P4
P5
-
100
-
-
-
100
-
4.14
65.5
-
4.14
26.2
53.0
3.28
9.84
-
90.0
3.33
-
-
-
-
-
-
-
-
-
-
-
-
At 360 Minutes
At 360 Minutes
P1
P2
P3
P4
P5
25.6
74.4
-
-
100
-
4.9
48.2
-
22.6
67.8
-
9.58
-
2.26
26.8
40.4
4.15
18.9
P1
P2
P3
P4
P5
15.8
84.4
-
-
100
-
11.8
85.3
2.94
-
-
11.9
80.4
-
7.70
-
7.60
40.0
29.1
5.70
-
-
-
7.93
-
-
-
-
-
-
-
a
a
-, not detected
-, not detected.
with increasing time of UV irradiation, the light shield-
ing effect of fulvic acid decreased. When fulvic acid is
UV irradiated, the solution turned colorless. Khan and
Schnitzer (1978) found that irradiation of fulvic acid
results in the destruction of the phenols that are the
chromophores that give fulvic acid solutions their
distinctive color. Therefore, once the phenolic hydroxyls
are destroyed by UV irradiation in this experiment,
more of the adsorbed metolachlor (metolachlor forms H
bonds with the phenolic groups) is released into the bulk
solution for further direct photodegradation. This re-
sults in a higher photodegradation of metolachlor with
time in the presence of fulvic acid. While, as the time
of UV irradiation increased, the light shielding effect
of the suspended inorganic constituents was overcome
by light scattering effect and/or by various photochemi-
cal reactions (Miller and Zepp, 1979b). This was clear
from the increased photolysis of metolachlor in the
presence of inorganic constituents after 15 min of UV
irradiation. Iron and/or Ti present on the surfaces of
montmorillonite, kaolinite, and goethite might have
generated hydroxyl radicals and other active oxygen
species that assisted in the increased rate of photodeg-
radation of metolachlor (Fugihira et al., 1981; Katagi,
1989, 1990, 1992, 1993; Larson et al., 1991).
methylphenyl)-5-methyl-3-morpholine, MW 233, [P2];
(3) 8-ethyl-3-hydroxy-N-(2-methoxy-1-methylethyl)-2-
oxo-1,2,3,4-tetrahydroquinoline, MW 263, [P3]; (4) 2-chlo-
ro-N-[2-(1-hydroxyethyl)-6-methylphenyl]-N-(2-hydroxy-
1-methylethyl)acetamide, MW 285, [P4]; (5) 2-chloro-
N-(2-ethyl-6-hydroxymethylphenyl)-N-(2-methoxy-1-
methylethyl)acetamide, MW 299, [P5]. The number of
photodegradation products formed were higher with
inorganic constituents than with fulvic acid and water
alone. In all the treatments, the major degradation
product identified was P2. For example, after 6 h of
photolysis, in water alone treatment, P2 constituted
about 84% of the photodegradation products formed
when the initial pH was 7, and it was 74% when the
pH was not adjusted (Tables 2 and 3).
Hydroxylation, dehalogenation (dechlorination), oxo-
quinoline formation, and demethylation were the pro-
cesses observed during the photodegradation of meto-
lachlor in this study. P1, P2, and P3 were dechlorinated
compounds. In P3, in addition to dehalogenation,
oxoquinoline formation was observed. Formation of P1
and P2 was identified in earlier photolytic studies of
metolachlor in water (Kochany and Maguire, 1994). P2
was also identified as a degradation product in bacterial
cultures (Liu et al., 1989). P3 was identified as a fungal
degradation product (McGahen and Tiedje, 1978). No
dehalogenation occurred in the formation of P4 and P5.
Benzylic hydroxylation occurred at methyl (P4) or at
ethyl (P5) substituents. No change in N-alkyl and
P h otod egr a d a tion P r od u cts. The various photo-
degradation products identified were as follows: (1)
2-hydroxy-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-
methylethyl)acetamide, MW 265, [P1]; (2) 4-(2-ethyl-6-

4000 J. Agric. Food Chem., Vol. 44, No. 12, 1996
Mathew and Khan
Krause, A.; Hancock, W. G.; Minard, R. D.; Freyer, A. J .;
Honeycutt, R. C.; Lebaron, H. M.; Paulson, D. L.; Liu, S,
Y.; Bollag, J . M. Microbial transformation of the herbicide
metolachlor by a soil actinomycete. J . Agric. Food Chem.
1985, 33, 584-589.
Larson, R. A.; Schlauch, B.; Marley, K. Ferric ion promoted
photodecomposition of triazines. J . Agric. Food Chem. 1991,
39, 2057-2062.
Lebaron, H. M.; Mcfarland, J . E.; Simoneaux, B. J . Meto-
lachlor. In Herbicides: Chemistry, Degradation and Mode
of Action; Kearney, P. C., Kaufman, D. D., Eds.; Dekker:
New York, 1988; Vol. 3, Chapter 7.
Liu, S, Y.; Zheng, Z.; Zhang, R.; Bollag, J . M. Sorption and
metabolism of metolachlor by a bacterial community. Appl.
Environ. Microbiol. 1989, 55, 733-740.
N-chloroacetyl groups was found in P5. In P4, the
methoxyl group of the N-alkyl substituent was de-
methylated, and a further hydroxylation of the ethyl
group at the aromatic side chain occurred. P4 and P5
were identified as the degradation products in actino-
mycete cultures (Krause et al., 1985). The hydroxylated
compounds have higher water solubility and are more
easily degraded. Thus, hydroxylation occurring during
photodegradation of metolachlor is associated with
detoxification.
LITERATURE CITED
Atkins, P. W. The rates of chemical reactions. In Physical
Chemistry; W. H. Freeman: New York, 1994; pp 870-872.
Bohn, H.; McNeal, B.; O’Connor, G. Anion and molecular
retention. In Soil Chemistry; Wiley: New York, 1979; pp
171-187.
Maguire, R. J .; Tkacz, R. J . Occurrence of pesticides in the
Yamasaka River, Quebec. Arch. Environ. Contam. Toxicol.
1993, 25, 220-226.
Bouchard, D. C.; Lavy, T. L.; Marx D. B. Fate of metribuzin,
metolachlor and fluometuron in soil. Weed Sci. 1982, 30,
629-632.
Chesters, G.; Simsiman, G. V.; Levy, J .; Alhajjar, B. L.;
Fathulla, R. N.; Harkin, J . M. Environmental fate of alachlor
and metolachlor. In Reviews of Environmental Contamina-
tion and Toxicology; Ware, G. W., Ed.; Springer-Verlag:
New York, 1989; Vol. 10, Chapter 1.
Frank, R.; Logan, L. Pesticides and industrial chemical
residues at the mouth of the Grand, Saugeen and Thames
Rivers, Ontario, Canada, 1981-1985. Arch. Environ. Con-
tam. Toxicol. 1988, 17, 741-754.
Fujihira, M.; Satoh,Y.; Osa, T. Heterogeneous photocatalytic
oxidation of aromatic compounds on TiO2. Nature 1981, 293,
206-208.
Gamble, D. S. Potentiometric titration of fulvic acid: equiva-
lence point calculations and acidic functional groups. Can.
J . Chem. 1972, 50, 2680-2690.
Katagi, T. Molecular approaches to the photolysis of organo-
phosphorus insecticide fenitrothion. J . Agric. Food Chem.
1989, 37, 1124-1128.
Katagi, T. Photoinduced oxidation of the organophosphorus
fungicide tolclofos-methyl on clay minerals. J . Agric. Food
Chem. 1990, 38, 1595-1600.
Katagi, T. Photodegradation of the pyrethroid insecticide
esfenvalerate on soil, clay minerals and humic acid surfaces.
J . Agric. Food Chem. 1991, 39, 1351-1356.
Katagi, T. Photodegradation of 3-phenoxybenzoic acid in water
and on solid surfaces. J . Agric. Food Chem. 1992, 40, 1269-
1274.
Katagi, T. Photodegradation of esfenvalerate in clay suspen-
sions. J . Agric. Food Chem. 1993, 41, 2178-2183.
Khan, S. U.; Schnitzer, M. UV irradiation of atrazine in
aqueous fulvic acid solution. J . Environ. Sci. Health B13
1978, 3, 299-310.
Kochany, J . Effects of iron(III) and manganese(II) ions on the
aquatic photodegradation rate of bromoxynil (3,5-dibromo-
4-hydroxybenzonitrile) herbicide. Chemosphere 1992, 3,
261-270.
Kochany, J .; Maguire, R. J . Sunlight photodegradation of
metolachlor in water. J . Agric. Food Chem. 1994, 42, 406-
412.
McGahen, L. L.; Tiedje, M. Metabolism of two new acylanilide
herbicides, Antor herbicide (H-22234) and Dual (meto-
lachlor) by the soil fungus Chaetomium globosum. J . Agric.
Food Chem. 1978, 26, 414-419.
Miller, G. C.; Zepp, R, G. Photoreactivity of aquatic pollutants
sorbed on suspended sediments. Environ. Sci. Technol.
1979a , 13, 860-863.
Miller, G. C.; Zepp, R. G. Effects of suspended sediments on
photolysis rates of dissolved pollutants. Water Res. 1979b,
13, 453-459.
Obrigawitch, T.; Hons, F. M.; Abernathy, J . R.; Gipson, J . R.
Adsorption, desorption and mobility of metolachlor in soils.
Weed Sci. 1981, 29, 332-336.
Oliver, G. J .; Cosgrove, E. G.; Carey, J . H. Effect of suspended
sediments on the photolysis of organics in water. Environ.
Sci. Technol. 1979, 13, 1075-1077.
Peter, J . C.; Weber, J . B. Adsorption, mobility, and efficacy of
alachlor and metolachlor as influenced by soil properties.
Weed Sci. 1985, 33, 874-881.
Richards, R. P.; Baker, D. B. Pesticide concentration patterns
in agricultural drainage networks in the Lake Erie basin.
Environ. Toxicol. Chem. 1993, 12, 13-26.
Schnitzer, M.; Skinner, S. I. M. Alkali verses acid extraction
of soil organic matter. Soil Sci. 1968, 105, 392-396.
Weber, J . B.; Peter, C. J . Adsorption, bioactivity and evaluation
of soil tests for alachlor, acetochlor and metolachlor. Weed
Sci. 1982, 30, 14-20.
Wood, L. S.; Scott, H. D.; Marx, D. B; Lavy, T. L. Variability
in sorption coefficients of metolachlor on a captina silt loam.
J . Environ. Qual. 1987, 16, 251-256.
Zepp, R. G.; Schlotzhauer, P. F. Effects of equilibrium time
on photoreactivity of the pollutant DDE sorbed on natural
sediments. Chemosphere 1981, 10, 453-460.
Received for review February 22, 1996. Revised manuscript
received August 14, 1996. Accepted September 13, 1996.X
J F960123W
Kozak, J .; Weber, J . B.; Sheets, T. J . Adsorption of prometryn
and metolachlor by selected soil organic matter fractions.
Soil Sci. 1983, 136, 94-10.
X Abstract published in Advance ACS Abstracts, No-
vember 1, 1996.