(68b) Predicting Soil Sorption Coefficients with the Quantum Mechanical COSMO-SAC Model

The sorption of organic molecules to soil plays an important role in determining their fate in the environment. The key parameter used to quantify this process is the soil sorption coefficient, K_OC. Difficulties associated with accurately measuring K_OC values have prompted a need to develop reliable predictive methods. However, most existing approaches rely on correlations of log K_OC with other experimental data, and are therefore limited to applications where accurate data are available. Group contribution methods have also been proposed, however these are restricted to calculations for molecules containing groups with known parameters. More recently, some studies have examined quantum mechanical (QM) approaches for predicting K_OC, based on continuum solvation models. To date, these approaches have been hindered by the lack of a known molecular composition for the soil phase, due to the heterogeneity and variability of soils. To overcome this, solvent descriptors for the soil have been fitted using correlations with measured K_OC values, with varying degrees of accuracy. In this work we employ a new QM approach, using hypothetical molecular structures for humic and fulvic acid molecules taken from the literature as a representation of the organic matter in the soil phase. K_OC values are calculated using the COSMO-SAC (COnductor-like Screening MOdel - Segment Activity Coefficient) model, which combines density functional theory calculations with statistical mechanics calculations. Preliminary results suggest that this approach is capable of predicting K_OC values with greater accuracy than any of the published QM methods. Furthermore, our method appears to have a predictive accuracy similar to or better than group contribution and other methods based on experimental data, but without the limitations of those methods. Ongoing work is aimed at further improving the predictive accuracy by determining the composition of the soil-phase ? as a mixture of the individual humic and fulvic acid molecules ? that best reproduces experimental K_OC values. Our method should prove to be a valuable tool in the prediction of K_OC values for evaluating measurements, making predictions for existing compounds in cases where data are unavailable, and as a pre-screening tool for assessing the environmental impact of proposed new molecules.