Structural Influences in Relative Sorptivity of Chloroacetanilide Herbicides on Soil
Total Page：16
File Type：pdf, Size：1020Kb
4320 J. Agric. Food Chem. 2000, 48, 4320−4325 Structural Influences in Relative Sorptivity of Chloroacetanilide Herbicides on Soil Weiping Liu,† Jianying Gan,* Sharon K. Papiernik, and Scott R. Yates Soil Physics and Pesticide Research Unit, U.S. Salinity Laboratory, Agricultural Research Service, U.S. Department of Agriculture, 450 West Big Springs Road, Riverside, California 92507 Adsorption of the chloroacetanilide herbicides acetochlor, alachlor, metolachlor, and propachlor was determined on soils and soil components, and their structural differences were used to explain their sorptivity orders. On all soils and soil humic acids, adsorption decreased in the order: metolachlor > acetochlor > propachlor > alachlor. On Ca2+-saturated montmorillonite, the order changed to metolachlor > acetochlor > alachlor > propachlor. FT-IR differential spectra of herbicide-clay or herbicide-humic acid-clay showed possible formation of hydrogen bonds and charge-transfer bonds between herbicides and adsorbents. The different substitutions and their spatial arrangement in the herbicide molecule were found to affect the relative sorptivity of these herbicides by influencing the reactivity of functional groups participating in these bond interactions. It was further suggested that structural characteristics of pesticides from the same class could be used to improve prediction of pesticide adsorption on soil. Keywords: Adsorption; sorption; acetanilide herbicides; chloroacetanilide herbicides; acetochlor; alachlor; metolachlor; propachlor; organic matter; montmorillonite; humic acid INTRODUCTION Chloroacetanilide herbicides are used in large quanti- ties for preemergence control of annual grasses and broadleaf weeds in corn, soybeans, and many other crops. The most commonly used herbicides from this class are acetochlor [2-chloro-N-(ethoxymethyl)-N-(2- ethyl-6-methylphenyl)acetamide], alachlor [2-chloro-N- (2,6-diethylphenyl)-N-(methoxymethyl)acetamide], me- tolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2- methoxy-1-methyethyl)acetamide], and propachlor [2- chloro-N-(1-methylethyl)-N-phenylacetamide]. The com- bined sales of acetochlor, alachlor, and metolachlor in 1995 reached 100 to 115 million pounds in the United States (U. S. Environmental Protection Agency, 1997). Because of their extensive usage and also their char- Figure 1. Molecular structures of acetochlor, alachlor, me- acteristics, these herbicides have been frequently de- tolachlor, and propachlor. tected in ground and surface waters (e.g., Cohen et al., 1986; Chesters et al., 1989; Hallberg, 1989; Ritter, 1990; sorptivity cannot be inferred from existing literature for Potter and Carpenter, 1995). these herbicides. Thus, the first purpose of this study was to determine the relative sorptivity of acetochlor, Adsorption on soil is one of the most important factors alachlor, metolachlor, and propachlor on soils, and soil for controlling pesticide movement toward groundwater mineral and humic acid components. (Koskinen and Harper, 1990). Thus, the relative sus- Chloroacetanilide herbicides share the same molec- ceptibility of groundwater to contamination from chlo- ular core of 2-chloroacetanilide and differ only in the roacetanilide herbicides may depend closely on their type and arrangement of substitutions (Figure 1). These relative sorptivity. Adsorption of chloroacetanilide her- structural differences should affect the reactivity of bicides on soil has been extensively studied (Weber and functional groups of these herbicides, and ultimately Peter, 1982; Kozak et al., 1983; Peter and Weber, 1985; their relative sorptivity. The second purpose of this Senesi et al., 1994). In these studies, however, adsorp- study was to identify the functional groups that may tion of only one or two of these herbicides was simul- be involved in adsorption, and to evaluate possible taneously investigated. Because of the use of different interactions between the substitutions and these func- soils and methodologies for these studies, an order of tional groups, and the influences of such interactions on herbicide sorptivity. * To whom correspondence should be addressed. Tele- phone: (909) 369-4804. Fax: (909) 342-4964. E-mail: jgan@ MATERIALS AND METHODS ussl.ars.usda.gov. † Current address: Chemistry Department, Zhejiang Uni- Chemicals and Soils. Acetochlor (purity 98.1%), alachlor versity, Hangzhou, China. (99.5%), metolachlor (purity 98.7%), and propachlor (purity 10.1021/jf990970+ CCC: $19.00 © 2000 American Chemical Society Published on Web 08/05/2000 Structure & Sorptivity: Chloracetanilide Herbicides J. Agric. Food Chem., Vol. 48, No. 9, 2000 4321 Table 1. Selected Properties of Chloroacetanilide C ) K C 1/n (1) Herbicides Useda s f e -1 solubility where Cs (µmol kg ) is herbicide concentration in the solid b -1 c -1 herbicide MW (mg L ) Kow phase at equilibrium, Ce (µmol L ) is herbicide concentration metolachlor 284 530 398 in the solution phase at equilibrium, and Kf and 1/n are -1 acetochlor 270 223 1082 empirical constants. Koc (g mL ), the adsorption constant after propachlor 212 618 200 normalization over soil organic carbon content, was calculated alachlor 270 242 794 by dividing Kf by soil organic carbon content. a The solubility values are from Worthing and Hance (1991), ) × Koc (Kf/OC%) 100 (2) and Kow values are from the USDA-ARS Pesticide Property b c Database (1998). MW ) molecular weight. Kow ) octanol-water partition coefficient. Herbicide adsorption at a single concentration was sepa- rately measured for each herbicide on Ca2+-clay, HA, and a 2+ ( 99.5%) were all purchased from Chem Service (West Chester, mixture of HA and Ca -clay at 20 1 °C. Adsorption was determined using 50 mg of adsorbent and 5.0 mL of 0.01 M PA) and used as received. The structures of these compounds -1 CaCl2-herbicide solution (100 µmol L ), using three replicates are shown in Figure 1, and selected properties are given in - Table 1. for each herbicide sorbent combination. For adsorption on HA-clay mixtures, 5 mg of HA and 45 mg of Ca2+-clay were Three soils, Arlington sandy loam (coarse loamy, mixed, added into the herbicide solution before shaking. After the thermic, haplic Durixeralf; Riverside, CA), Linne clay loam suspension was shaken for 24 h, it was centrifuged at 20 000g (fine loamy, mixed, thermic, calcic pachic Haploxerolls; Paso for 15 min and filtered through 0.2-µm syringe filters. The final Robles, CA), and Webster clay loam (fine loamy, mixed, mesic, solution was analyzed on HPLC for herbicide concentration. typic Endoaquolls; Waseca, MN), were used in this study. Soils - Analysis of herbicide concentration in supernatant was were collected from the surface (0 15 cm), air-dried, and conducted on a HP 1090 HPLC (Hewlett-Packard, Wilmington, passed through a 2-mm sieve. Particle size distribution of these DE) equipped with an auto-injection system and a diode-array soils was measured using the pipet method (Gee and Bauder, detector (DAD). The column was a 250-mm × 4.6-mm (i.d.) 1986), organic carbon content (OC) was measured by the - reverse-phase Adsorbosphere HS C18 (5µm, Alltech, Deerfield, modified Walkley Black method (Nelson and Sommers, 1982), IL), injection volume was 20 µL, and wavelength of detection and cation exchange capacity (CEC) was determined by the was 230 nm. Mobile phase was a mixture of acetonitrile- procedure of Rhoades (1982). Soil pH was determined in methanol-water (60:10:30) that was acidified with 0.5% slurries of soil and water (1:1, w/w). The measured soil phosphoric acid. The flow rate was maintained at 1.0 mL min-1 properties are given in Table 2. for all herbicides. External calibration was used for quantifica- Clay and Humic Acids. The clay was a montmorillonite tion. Swy-2 from Crook County, WY, and was purchased from the FT-IR Analysis. Probable bond interactions between her- Source Clay Minerals Repository at University of Missouri, bicides and sorbents were investigated by comparing FT-IR Columbia, MO. The <2-µm fraction was obtained by sedimen- spectra of herbicides adsorbed on thin films of Ca2+-clay and + tation. Ca2 -saturated clay was prepared by repetitive treat- HA-Ca2+-clay mixture. Self-supporting films of clay or HA- + ment of the clay with 0.5 M CaCl2 solution. The prepared clay clay were prepared by evaporating 5 mL of Ca2 -clay or HA- sample was centrifuged, washed repeatedly with deionized (10%) + Ca2+-clay(90%) suspension in a 5-cm (i.d.) ring on a - water until Cl free, and ground to a fine powder after drying polyethylene sheet at room temperature. These air-dried films, at room temperature. Humic acid (HA) was prepared from the about 50 mg in weight, 5 cm in diameter, 10 µm in thickness, Webster clay loam using modified procedures from Schnitzer were cut into two halves. One half was treated with herbicides (1982). Briefly, 0.8 kg of air-dried soil was shaken with 3.0 L by immersing the film in 2% herbicide-CHCl3 solution. After of 0.5 M NaOH solution under N2 gas in a capped bottle for 1 d of treatment for clay films or 2 d for HA-clay films, films 24 h. The alkaline upper solution was centrifuged at 8500 g were removed from the solution and rinsed several times with for 15 min, and the supernatant was acidified with6MHCl CHCl3. The other half of the film was not treated with to pH 1, followed by standing for 24 h to permit coagulation herbicide, but was similarly washed with CHCl3. FT-IR spectra of the HA fraction. The precipitated HA was separated from of the treated films were recorded at 3500-600 cm-1 by using the solution by centrifugation at 20 000g for 25 min, and then a Galaxy 4020 FT-IR spectrometer (Mattson Instrument Co., redissolved in a small amount of 0.5 M NaOH solution under Madison, WI). All FT-IR spectra of the pure and adsorbed N2 gas. The NaOH-dissolution and HCl-precipitation of HA compounds were measured under the same conditions. Dif- was repeated two more times. Finally, HA was dialyzed in ferential spectra of adsorbed compounds were obtained by distilled water until salt-free and ground to a fine powder after subtracting on the same scale the spectra of the herbicide- being dried at 40 °C.