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 substi tuted phenylureas of varying hydrophobicity over wide ranges in equili brium solution concentration. The three sorbents were of similar textu re with soil organic carbon ranging from 1 to 5%. Sorption was thermod ynamically evaluated in terms of nonlinear partitioning into soil orga nic matter. Similar to observations of linear sorption of nonpolar com pounds, 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 fre e energies were found to decrease dramatically as sorbed-phase concent ration decreased, demonstrating the presence of strong specific sorbat e-sorbent interactions at low sorbed-phase loadings. Nonlinearity was quantitatively related to the variation of sorbate partial molar free energy in the sorbed phase.