THERMODYNAMICS OF ORGANIC-CHEMICAL PARTITION IN SOILS .2. NONLINEAR PARTITION OF SUBSTITUTED PHENYLUREAS FROM AQUEOUS-SOLUTION

A study was conducted to evaluate the sorption behavior of five substituted phenylureas of varying hydrophobicity over wide ranges in equilibrium solution concentration. The three sorbents were of similar texture with soil organic carbon ranging from 1 to 5%. Sorption was thermodynamically evaluated in terms of nonlinear partitioning into soil organic matter. Similar to observations of linear sorption of nonpolar compounds, sorbed-phase activity coefficients calculated at the limit of sorbate solubility (corrected for crystal energy) were well correlated with molar volume of the sorbates. In all cases, sorbate residual free energies were found to decrease dramatically as sorbed-phase concentration decreased, demonstrating the presence of strong specific sorbate-sorbent interactions at low sorbed-phase loadings. Nonlinearity was quantitatively related to the variation of sorbate partial molar free energy in the sorbed phase.