Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/22/2002
Publication Date: 7/3/2002
Citation: Liu, W.P., Gan, J.Y., Yates, S.R. 2002. Influence of Herbicide Structure, clay Acidity, and Humic Acid Coating on Acetanilide Herbicide Adsorpion on Homoionic Clays. Journal of Agricultural and Food Chemistry. 50(14):4003-4008. Interpretive Summary: Adsorption of pesticides on soil is one of the most important processes affecting their environmental fate. The clay fraction and associated organic matter have been shown to be the primary adsorbents for pesticides in soil. A number of studies show that interactions between soil constituents can greatly affect the overall adsorption and that the total adsorption is not the sum of adsorption to individual components assuming independent and additive behavior. Although efforts have been made to derive equations to describe pesticide adsorption from pesticide properties such as solubility or Kow, a widely applicable relationship has not been found. This indicates that solubility and Kow values of pesticides may be too empirical in nature, and more descriptive, likely molecular structure or charge distribution based descriptors, should be incorporated into the models. Developing a quantitative structure adsorption relationships is essential to better understand how a pesticide's adsorption is affected by its structure and charge characteristics at the molecular level.
Technical Abstract: Adsorption of chloroacetanilide herbicides on homoionic montmorillonite was studied by coupling batch equilibration and FT-IR analysis. Adsorption decreased in the order metolachlor > acetochlor > alachlor > propachlor on Ca2+- or Mg2+-saturated clays and in the order metolachlor > alachlor > acetachlor > propachlor on Al3+- or Fe3+-saturated clays. FT-IR spectra showed that the carbonyl group of the herbicide molecule was involved in bonding. For the same herbicide, adsorption of alachlor, acetachlor, and metolachlor on clay followed the order Ca2+ ~ Mg2+ < Al3+ =< Fe3+, which coincided with the increasing acidity of homoionic clays. Adsorption of propachlor, however, showed an opposite dependence, suggesting a different governing interaction. In clay and humic acid mixtures, herbicide adsorption was less than that expected from independent additive adsorption by the individual constituents, and the deviation was dependent on the clay-to-humic acid ratio, with the greatest deviation consistently occurring at a 60:40 clay-to-humic acid ratio.