Metsulfuron methyl sorption-desorption in field-moist soils

Pesticide sorption coefficients (K-d) 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 fluid 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 (K-d) at low water contents. K-d 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 neutral species K-d values, the pK(a) of metsulfuron methyl, and the pH of the soil, we could successfully predict the K-d values obtained using the batch slurry technique, which typically has a predominance of anionic species in solution during the sorption characterization. This application of supercritical fluid extraction to determine sorption coefficients, combined with sulfonylureas' pK(a) values and the soil pH, will provide an easy method to predict sorption in soil at different pH levels.