Sorption of hydrophobic organic compounds by soil materials: Application of unit equivalent Freundlich coefficients

Prediction of the fate of hydrophobic organic compounds (HOCs) in the soil environment is difficult due to the highly complex interaction between the organic molecules and the soil components. The Freundlich isotherm is the most commonly used model to describe retention of HOCs by heterogeneous sorbents, Because of inconsistent units of constants calculated from the Freundlich model, it is difficult to strictly compare the sorption of organic molecules by different sorbents. In this study, we modified the Freundlich equation to better assess the effects of organic carbon content and physical heterogeneity on the sorption of HOCs by soils and microaggregate fractions, using naphthalene and phenanthrene as probes. Isotherms for naphthalene and phenanthrene sorption were nonlinear and well described by the Freundlich equation. Unit-equivalent Freundlich coefficients (K'(f)) were calculated by normalizing the liquid-phase equilibrium concentrations (C-e) to the supercooled liquid-state solubility (S-scl) of naphthalene and phenanthrene. The K'(f) values for both naphthalene and phenanthrene sorption correlated well with organic carbon (OC) content, but they did not show a systematic trend with N-2 specific surface area (SSA). Isotherm deviation from linearity, as expressed by the Freundlich exponent (n), was larger for phenanthrene than for naphthalene, indicating that another sorption mechanism in addition to partitioning into soil organic matter was responsible for retention of phenanthrene. Despite the difference in n values, the K'(f) values (mug g(-1)) for naphthalene and phenanthrene sorption were nearly equal, indicating that a given sorbent has same capacity to sorb phenanthrene and naphthalene. Since sorption was nonlinear, this type of comparison could only have been made by using the modified Freundlich equation.