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United States Department of Agriculture

Agricultural Research Service

Title: Metsulfuron Methyl Sorption-Desorption in Field-Moist Soils

Authors
item Berglof, T - SWEDISH UNIV. AG. SCI.
item Koskinen, William
item Duffy, M - COLLEGE MISERICORDIA
item Norberg, K - UNIV. OF MINNESOTA
item Kylin, H - SWEDISH UNIV. AG. SCI.

Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 13, 2003
Publication Date: May 2, 2003
Citation: Berglof, T., Koskinen, W.C., Duffy, M.J., Norberg, K.A., Kylin, H. Metsulfuron methyl sorption-desorption in field-moist soils. Journal of Agriculture and Food Chemisty. 51(12):3598-3603.

Interpretive Summary: An understanding of the binding mechanisms involved in the sorption of pesticides to soil can help to understand and predict the behavior of pesticides in the soil system, and consequently, the potential risk for contamination of water resources. Pesticide sorption coefficients (Kd) are generally obtained using batch slurry methods. As a consequence, the results may not adequately reflect sorption processes in field-moist or unsaturated soil. The objective of this study was to determine sorption of weak acid herbicides, in field-moist soils using a technique developed in our laboratory. Experiments were performed using low density supercritical carbon dioxide (SFE) to convert the weak acid herbicide to its unionized form and remove it from the soil water phase only, thus allowing calculation of sorption coefficients (Kd) at low water contents. We found that using Kd values of the unionized form, the dissociation constant (pKa) of the acidic herbicide, and the pH of the soil we could successfully predict the Kd values obtained using the traditional batch slurry technique, which typically has a predominance of anionic species in solution during the sorption characterization. This application of SFE to determine sorption coefficients, combined with pKas of herbicides such as sulfonylureas, and the soil pH, will provide an easy method to predict sorption in soil at different pH levels. Scientists will now have a sorption coefficient for use in pesticide environmental fate models that is more reflective of real field situations. Scientists will also not have to do sorption experiments with many soils to characterize the potential mobility of an acidic herbicide.

Technical Abstract: Pesticide sorption coefficients (Kd) are generally obtained using batch slurry methods. As a consequence, the results may not adequately reflect sorption processes in field-moist or unsaturated soil. The objective of this study was to determine sorption of metsulfuron methyl, a weak acid, in field-moist soils. Experiments were performed using low density (i.e., 0.3 g mL-1) supercritical carbon dioxide (SF-CO2) to convert anionic metsulfuron methyl to the molecular species and remove it from the soil water phase only, thus allowing calculation of sorption coefficients (Kd) at low water contents. Kd values for sorption of the metsulfuron methyl molecular species on sandy loam, silt loam, and clay loam soil at 11% water content were 120, 180, and 320 mL g-1, respectively. Using molecular species Kd values and the pKa of metsulfuron methyl and the pH of the soil we could successfully predict the Kd values obtained using the batch slurry technique, which typically has a predominance of anionic species in solution during the sorption characterization. This application of SFE to determine sorption coefficients, combined with sulfonylureas, pKas and the soil pH, will provide an easy method to predict sorption in soil at different pH levels.

Last Modified: 8/27/2014