THERMODYNAMICS OF ORGANIC-CHEMICAL PARTITION IN SOILS .3. NONLINEAR PARTITION FROM WATER-MISCIBLE COSOLVENT SOLUTIONS

Solubility and partitioning behavior of four substituted phenylureas were investigated in various aqueous methanol (MeOH) and aqueous dimethyl sulfoxide (DMSO) solutions. The solubility data for these compounds were reasonably well described by a log-linear relationship between mole fractional solubility and volume fraction cosolvent. Sorption was evaluated in terms of a thermodynamic nonlinear partition model, and solute sorbed-phase activity coefficients gamma1 were found to be independent of solution-phase composition in solutions containing up to 60% volume fraction MeOH. For the systems examined, it is shown that solution-phase effects are the primary determinant for soil partitioning in the MeOH cosolvent systems and that solvent-humic polymer effects are inconsequential. In contrast, increasing solution volume fraction DMSO was associated with decreasing gamma1 and increasing dissolved solution-phase organic matter for volume fraction DMSO >40 %. The DMSO data suggest that increasing volume fraction DMSO results in swelling of the humic phase.