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

A general thermodynamic partition model for organic carbon-based linea r and nonlinear sorption from solution was formulated in order to char acterize sorbate partial molar free energies. The partial molar free e nergies are considered as the sum of two components: the Flory combina torial entropy of mixing and a residual free-energy component. By usin g appropriate concentration units in the solution and sorbed phases, t he conventional Freundlich partition coefficient was found to be relat ed to the aqueous-phase activity coefficient and sorbate solubility (c orrected for crystal energy effects, if any) in the humic phase. The F reundlich exponent was related to the variation of sorbate partial mol ar free energy with loading. Consistent with the behavior of small sol utes dissolved in polymers, Flory interaction parameters determined fr om published soil isotherm sorption data were seen to vary with molar volume within a family of chemically similar compounds. Interaction pa rameters are shown to be independent of concentration for linear sorpt ion, and residual free energies varied from 5 to 10 kJ/mol in the two literature data sets considered.