Comment on partition equilibria of nonionic organic compounds between soil-organic matter and water

SIR: The article of Chiou et al. (1) stated that sorption of nonionic organic compounds on soils from water solution occurs by equilibrium partitioning into a soil organic phase and that adsorption by minerals is relatively unimportant in wet soils. Evidence presented by Chiou does not prove a partitioning model of sorption to the exclusion of a physical adsorption model. The partitioning model apparently was chosen as a convenient explanation of the effect of soil organic matter on sorption of nonionic compounds and its relation to the sórbate octanol-water partition coefficient. Chiou et al. (1) have assumed without proof that a particular relation between organic content and sorption is valid for all soils. Although there is a correlation between organic content and sorption, considerable variation exists between soils (2, 3). Hamaker et al. (4) observed that Kom values for soils with high organic content are usually lower than Kom values for “normal” soils and that soils with very low organic content give high Kom values.