Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 1/17/1998
Publication Date: N/A
Interpretive Summary: Chloroacetanilide herbicides are used in large quantities for pre-emergence control of annual grasses and broadleaf weeds in corn, soybeans, and many other crops. The most commonly used ones from this class are acetochlor, alachlor, metolachlor, and propachlor. The combined sales of acetochlor, alachlor, and metolachlor in 1995 reached 100 to 115 million pounds in the United States. Because of their extensive usage and also their characteristics, these herbicides have been frequently detected in ground or surface waters. Adsorption on soil is one of the most important factors for controlling pesticide movement toward groundwater. Thus, the relative susceptibility of groundwater to chloroacetanilide herbicide contamination may depend closely on their relative sorptivity. The first purpose of this study was thus to determine the relative sorptivity of acetochlor, alachlor, metolachlor, and propachlor on soils, and soil mineral and humic acid components. The second purpose was to identify herbicide functional groups that may be involved in adsorption, and to evaluate possible interactions between the substitutions and these functional groups and influences of such interactions on herbicide sorptivity.
Technical Abstract: 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 differences of pesticides from the same class may be used as a molecular probe to obtain a better understanding of adsorption mechanisms of pesticides on soil.