Photodegradation of Metolachlor
J. Agric. Food Chem., Vol. 48, No. 3, 2000 945
Aqu eou s Sta bility. The aqueous stability of monochloro-
acetic acid was characterized before photochemical experi-
ments. MCA (13 mg/L) solutions were adjusted to pH 7 and
pH 9 daily using 0.1 N HCl or 0.1 N NaOH and stored in the
dark for a 12 day period. Each pH treatment was done in
duplicate. At various times MCA levels were analyzed using
ion chromatography.
Monochloroacetic acid (MCA) is a polar product that
has been identified as a product of chloroacetanilide
degradation. Pignatello and Sun (1995) reported MCA
as a significant photoproduct of metolachlor resulting
from the photoassisted Fenton Reaction during the
investigation of a potential pesticide waste management
strategy. Mangiapan et al. (1997) identified MCA as a
product of the biodegradation of alachlor incubated in
river water for 28 days. Both studies, while valuable,
do not provide any information regarding the possibility
that direct or indirect photodegradation of chloroaceta-
nilide herbicides could be a source of monochloroacetic
acid in the environment.
The presence of haloacetic acids in the environment
such as MCA is widespread (Scott and Alaee, 1998);
therefore, identification of potential sources is of inter-
est. The chlorination of drinking water (Hirvonen et al.,
1996; Wu and Chadik 1998; Kristiansen et al., 1996) is
one possible source for haloacetic acids in the environ-
ment. Concerns regarding levels of MCA in particular
exist since MCA has proven to be highly phytotoxic; the
48 h effective concentration, 10% (EC10) for the green
algae Scenedesmus subspicatus is 7 µg/L (Kuhn and
Pattard, 1990).
P h otod egr a d a tion . The photochemical stability of four
chloroacetanilides was tested using a Suntest CPS Photosimu-
lator equipped with a xenon arc lamp and a water-cooled tray.
Irradiance was set to maximum, 765 W/m2
( 10%, and
included wavelengths from 290 to 800 nm to mimic the
intensity distribution of natural light. It has been shown that
the behavior of our photosimulator shows similar irradiance
to the sunlight in J uly in Toronto around midday. For each
photolysis experiment, three 80-100 mL samples of 7-12 µM
chloroacetanilide (consistent concentration within each experi-
ment) were placed in quartz test tubes inside the photoreactor
and sealed with rubber stoppers and Teflon tape. Tubes were
removed throughout the course of the run, so that three time
points in addition to the zero time point were analyzed for each
set of photolysis conditions. Samples were stored in bottles
wrapped in aluminum foil in the fridge (4 °C) until analysis.
Plots of ln C/Co (C represents the concentration of the parent
chloroacetanilide at time t and Co is the initial concentration)
vs time for each experiment provided pseudo-first-order kinetic
data including rate constants and half-lives.
The main objective of this investigation was to use a
representative chloroacetanilide herbicide, metolachlor,
to investigate the potential for the production of MCA
as a photoproduct in various matrixes. Further, we
hoped to quantify MCA and determine possible condi-
tions leading to maximum production of the acid. Once
the conditions producing significant acid were realized,
the photolysis of alachlor, butachlor, and a model
chloroacetanilide were used to investigate structural
factors that may play a role in the production of MCA.
Direct Photolysis Experiments. Metolachlor was degraded
in deionized water adjusted to pH 7 and pH 9. The pH was
monitored and adjusted regularly using 0.1 N HCl and 0.1 N
NaOH.
Indirect Photolysis Conditions. Indirect photolysis of meto-
lachlor was examined using two different matrixes chosen for
the potential to yield high concentrations of carbonate radical
upon irradiation. Metolachlor was degraded in the photosimu-
-
lator in a solution of 3 mM H2O2 and 0.092 M HCO3 (pH )
8.23) (Larson and Zepp, 1988). Metolachlor, alachlor, butachlor,
and 2-chloro-N-methylacetanilide were irradiated in a syn-
thetic field water (SFW) matrix in order to mimic possible
environmental sample constituents. SFW consisted of 4.9 mM
MATERIALS AND METHODS
-
HCO3-, 0.5 mg of C/L DOC, and 0.81 mM NO3 (pH ) 8.30).
Ch em ica ls. Metolachlor, alachlor, and butachlor were
obtained from Chem-Service (West Chester, PA). Monochlo-
roacetic acid was of ACS grade obtained from Fischer Scientific
(Fair Lawn, NJ ). High-performance liquid chromatography
(HPLC) grade acetonitrile was obtained from J . T. Baker
(Phillipsburg, NJ ). HPLC grade water was obtained from
Caledon Laboratories (Georgetown, ON). Acetonitrile and
water were passed through a 0.45 µm filter before use.
Organic-free water used for photochemical experiments was
18 MΩ deionized water. Sodium nitrate and sodium bicarbon-
ate used to prepare synthetic field water (SFW) were of reagent
grade. Dissolved organic carbon (DOC) used was prepared by
pre-aging a humic acid stock solution in a homemade photo-
simulator for 8 days. Methylaniline and chloroacetyl chloride
were purchased from Aldrich Chemical Co. (Milwaukee, WI).
Syn th esis. The propyl ester of MCA was synthesized by
refluxing 1 g of MCA, 10 mL of 1-propanol, and 0.5 mL of
concentrated sulfuric acid for 2 h. The ester was identified
using GC/MS (m/z (abundance) [suggested pathway]): 109 (4),
107 (12) [M+ - C2H5], 97 (14), 95 (45) [M+ - C3H5], 79 (32), 77
(100) [M+ - OC3H7], 57 (17).
Metolachlor was also irradiated in a sample of Don River water
(pH ) 8.42). The Don River water sample consisted of 3.7 mM
HCO3-, 5.77 mg of C/L of DOC, and 0.44 mM NO3-. The H2O2/
-
HCO3 and SFW treatments were carried out three and four
times, respectively. All other treatments were performed once.
An a lysis. Routine HPLC-UV analyses to monitor the
photodegradation of the parent chloroacetanilide compound
were done by direct aqueous injection into a Waters 600S
system with a 486 Variable Wavelength Detector set at 220
nm. The column was a 25 cm × 4.6 mm i.d. reversed-phase
Alltech Econosil C18, 5U and was preceded by an Alltech All-
Guard Cartridge system with an Econosil C18 10U guard
cartridge. Various mixtures of acetonitrile and water were used
as the mobile phase. The flow rate was 1.0 mL/min for all
experiments. Calibration was performed daily using external
standards and linear regression analysis.
The aqueous stability of MCA was investigated using ion
chromatography including a PE Series 200 pump, Alltech
ERIS 1000 HP autosuppressor, and a Waters 712 WISP
autosampler. The column was a 25 cm × 4 mm i.d. Dionex
IonPac anion exchange column. The mobile phase and flow
rate were 2.1 mM Na2CO3:0.6 mM NaHCO3 and 1 mL/min,
respectively. Calibration was preformed daily using external
standards and linear regression analysis.
The production of MCA was monitored using a PE Auto-
system XL gas chromatograph with electron capture detection
(ECD). A Simplicity 1701 column (30 m × 0.32 mm i.d., 0.25
µm film thickness) was used to elute the propyl ester of the
acid using the following temperature program: hold for 8 min
at 45 °C, ramp at 30 °C/min to 250 °C. The injector and
detector temperatures were 220 and 300 °C, respectively. The
carrier gas was hydrogen and had a setpoint of 14 psi; the
makeup gas was nitrogen at a flow rate of 30 mL/min. The
injection mode was splitless, and the injection volume was 2.0
The model chloroacetanilide, 2-chloro-N-methylacetanilide,
was synthesized by the method of Biechler et al. (1957): A 5
g sample of chloroacetyl chloride was dissolved in 25 mL
anhydrous diethyl ether. Dropwise, 3.7 g of methylaniline was
added to the solution. The ether layer was washed with water
and evaporated. White crystals were obtained upon recrystal-
lization from ethanol-water. The model was identified using
GC/MS and 1H NMR (Varian 400 MHz spectrometer with
CDCl3 as solvent). GC/MS: 185 (17), 183 (43) [M+], 134 (42)
[M+ - CH2Cl], 107 (46), 106 (100) [M+ - C(O)CH2Cl], 92 (13),
1
90 (43), 79 (16), 77 (68) [M+ - CH3NC(O)CH2Cl]. H NMR (δ,
chemical shift in ppm, s ) singlet, m ) multiplet): δ 7.6-7.2
(m, 5H, H Ar), 3.85 (s, 2H, CH2), 3.30 (s, 3H, CH3).