Application of the dual-mode model for predicting competitive sorption equilibria and rates of polycyclic aromatic hydrocarbons in estuarine sediment suspensions.

We investigated competitive sorption and desorption of phenanthrene, anthracene, and pyrene added to an estuarine sediment from Guilford (CT, USA) on Long Island Sound. The reduction in the equilibrium uptake of a primary solute in the presence of one or both cosolutes was well predicted over the entire isotherm by the dual-mode model, which postulates dissolution and hole-filling domains in sediment organic matter. Isotherms obtained in binary or ternary solute systems were suppressed, and the shapes were more linear than the corresponding single-solute isotherms. In the limit of infinite dilution of the single solute, sorption in the hole-filling domain is a factor of 1.4 to 1.9 times more favorable than sorption in the dissolution domain. A competitive dual-mode radial diffusion model was applied to the batch uptake and sequential-batch desorption rate data. The model captures the uptake and release curves well. The competitive effect slightly accelerates sorption, as predicted. The competitive effect accelerates desorption, but from the existing data it was not possible to decide how much was due to favorable thermodynamic versus favorable kinetic effects. The concentration-independent diffusion rate parameter corresponding to the dissolution domain was approximately the same for sorption and desorption (0.02/h), and little or no sorption hysteresis was observed.