Two calculators based on regression analyses between the Freundlich adsorption coefficient (Kf) and the solid solubility (mp &gt; 25°C) or the sub-cooled liquid solubility for pesticide active ingredients on 18 different agricultural soils have been developed. Each calculator provides 18 estimates of Kf and Kfoc (organic carbon normalized Kf), for soil textures ranging from sand to humic (0.36% to 19.7% OC). Predictions of Kfoc values require measured values of the solid solubility and melting point (sub-cooled liquid calculator) or the solid solubility alone (solid solubility calculator). The eighteen soils from North America and Europe are commonly used in batch equilibrium studies submitted to EPA for pesticide registration . Available studies using particular soils were identified by searching for soil series name, and then verifying the country or state of origin, sand/silt/clay content, and organic carbon content. Kf and 1/n values were recorded for each chemical-soil combination. The 18 regression analyses were based on 3 – 10 chemical studies per soil. Solubility and melting point data were obtained from EPA’s Data Evaluation Records for the batch equilibrium studies or from the University of Hertfordshire Pesticide Properties DataBase (PPDB). Sub-cooled liquid solubility was estimated from solid solubility and melting point using a fugacity ratio calculation. Solid solubilities of the pesticides in the regression analyses ranged from 0.02 mg/L to 886,000 mg/L, and melting points ranged from 298K to 510K. Kf values for pesticides included in the regression analyses ranged from 0.02 to 29,900. Mean calculated Kfoc values for 36 pesticides not used in the regression analyses were compared to the range (maximum-minimum limits) of Kfoc values reported in the PPDB. For the sub-cooled liquid calculator, 19 were within the reported range, 10 were within 2x the limits, and only 2 were outside 3x the limits. For the solid solubility calculator, 12 predictions were within the reported range, 14 were within 2x the limits, and 3 were not within 3x the limits. The calculators are expected to be useful in estimating Kfoc for untested chemicals.

QSAR and calculators for Freundlich adsorption coefficient (Kf) based on 18 agricultural soils

254th National Meeting (2017)

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