A distributed reactivity model for sorption by soils and sediments .9. General isotherm nonlinearity and applicability of the dual reactive domain model

Sorption isotherms for a hydrophobic solute probe, phenanthrene, were measured experimentally for 27 different soils and sediments. The linear and Freundlich isotherm models and the Dual Reactive Domain Model (DRDM) were used to fit the resulting data. The results reveal for all soils and sediments studied that (i) the Freundlich model and the DRDM fit the data well, whereas a linear model fails to do so; (ii) values of the organic carbon-normalized distribution coefficient, K-OC, calculated from individual isotherm points for a specific sorbent-solute system vary significantly with the aqueous-phase solute concentration, C-e; and (iii) all commonly used correlations of K-OC with octanol-water partitioning coefficients and solute solubility limits significantly underestimate K-OC for C-e values smaller than approximately one-tenth of aqueous-phase solute solubility, C-S. The sorption behaviors of all of the soils and sediments studied are thus inconsistent with the simple concept of linear phase partitioning. The general applicability of the DRDM, a polymer-based limiting case form of the Distributed Reactivity Model, for all systems studied supports mechanistic arguments based on polymer sorption theory.