Partition Equilibria of Nonionic Organic Compounds between Soil Organic Matter and Water†

Equilibrium isotherms for the simultaneous uptake of binary nonionic organic compounds from water on soil indicated no competitive effect between the two solutes. This observation supports the hypothesis that partition to the soil organic phase is the primary process for sorption of nonionic organic compounds from water on soil. The partition process between soil organic matter and water was analyzed by using the conventional solution concept for solutes in water and the Flory-Huggins treatment for solutes in the polymeric humic phase. Sorption determined for 12 aromatic compounds on a Woodburn soil shows that the extent of solute insolubility in water (S) is the primary factor affecting the soil organic matter-water partition coefficient (Kom) and that the effect of solute incompatibility with soil organic matter is significant but secondary. This explains the commonly observed correlations of log Kom vs. log S and log Kom vs. log Kow (octanol-water).