QSAR Studies Of Soil Sorption Coefficients Of Organic Pollutants

The migration, transformation and behavior of organic chemicals have been concerned by people. An evaluation of soil mobility of chemicals is a primary task in estimating the environmental distribution of chemicals. An important parameter in studying this process for organic chemicals is the soil sorption coefficient(K_oc). It provides a theoretical basis and foundation for the safety evaluation of organic compounds.In this paper, 701 organic compouns were collected from two literature reviews. The correlation analysis was carried out between a number of physicochemical parameters of molecular descriptors and soil sorption coefficient. For most of hydrophobic compounds (i.e. 0.5 < log P < 7.5), organic-carbon content play dominant role in soil sorption and sorption coefficient is linearly related to octanol/water coefficient. On the other hand, for hydrophilic compounds (i.e. log P < 0.5), hydrophobic sorption becomes less significant. The hydrophilic contribution to sorption is equal or higher than the hydrophobic contribution to sorption, resulting in the observed K_oc values higher than that predicted from their log P. For highly hydrophobic compounds (i.e. log > 7.5), log K_oc decreases with increasing hydrophobicity because of a lack of bioavailability. The results show that log K_oc is linearly related to the log P for the compounds with log P in range of 0.5 to 7.5 and non-linearly related to log P for the compounds in a wide range of log P. A non-linear model has been developed between log K_oc and log P for a wide range of compounds in the training set. This model was validated in terms of average error (AE), average absolute error (AAE) and root-mean squared error (RMSE) by using an external test set with 107 compounds. Near same predictive capacity was observed in comparison with the existing models. However, this non-linear model is very simple, only use one parameter. Linear solvation energy relationship shows that the sorption potential decreases with increasing molecular size by increasing the dispersion interactions between the chemical and soil organic phase and decreases with increasing the basicity of hydrophobic compounds by increasing the H-bonding of chemicals with water. Principal component analysis shows that octanol/water system is not a ideal surrogate to describe the soil partitioning sorption for hydrophobic compounds, although octanol/water system is the closest system as compared with other solvent/water partition systems.