Rate-Limited Sorption of Hydrophobic Organic Compounds by Soils with Well-Characterized Organic Matter

This study is focused on the rate-limited sorption of homologous series of polycyclic aromatic hydrocarbons, alkylated benzenes, chlorinated benzenes, and chlorinated alkenes by two soils, one in which the organic matter is dominated by humic acid and the other by fulvic acid. The solutes sorbed more strongly to the humic-dominated soil than the fulvic-dominated soil. The first-order mo lecular connectivity index ( 1 X v ), a molecular solute descriptor, provided slightly better correlations with equilibrium sorp tion coefficients than did the octanol−water partition coefficient ( K ow ), an empirical solute descriptor. Thus, sorbate shape/structure may have a secondary influence on the overall magnitude of equilibrium sorption. However, sorbate shape/structure exhibited a significant influence on sorption kinetics. Distinct differences were observed in the correlations of mass-transfer coefficients to 1 X v for the two soils. The differences were attributed to both sorbate shape/structure and the quantity (path length) and morphology of soil organic matter.