THERMODYNAMICS OF ORGANIC-CHEMICAL PARTITION IN SOILS .1. DEVELOPMENT OF A GENERAL PARTITION MODEL AND APPLICATION TO LINEAR ISOTHERMS

A general thermodynamic partition model for organic carbon-based linear and nonlinear sorption from solution was formulated in order to characterize sorbate partial molar free energies. The partial molar free energies are considered as the sum of two components: the Flory combinatorial entropy of mixing and a residual free-energy component. By using appropriate concentration units in the solution and sorbed phases, the conventional Freundlich partition coefficient was found to be related to the aqueous-phase activity coefficient and sorbate solubility (corrected for crystal energy effects, if any) in the humic phase. The Freundlich exponent was related to the variation of sorbate partial molar free energy with loading. Consistent with the behavior of small solutes dissolved in polymers, Flory interaction parameters determined from published soil isotherm sorption data were seen to vary with molar volume within a family of chemically similar compounds. Interaction parameters are shown to be independent of concentration for linear sorption, and residual free energies varied from 5 to 10 kJ/mol in the two literature data sets considered.