A gel partition model for organic desorption from a pond sediment.

Adsorption at the water-sediment interface has been correlated with the organic carbon content of natural sediment samples and with water-octanol partitioning equilibria. In deriving a new model for sorption in sediment, it is assumed that adsorption is due to humin-kerogen polymers associated with the clay component in the sediment. The model includes solubility parameter theory applied to solute-gel-liquid interactions and a theory of the liquid-polymer interactions that control gel swelling. Partially swollen gels are expected to exhibit impeded diffusion. These concepts are able to explain observations of limited desorption of certain organic compounds from natural sediments and soil minerals. Experiments were performed in which extractive solvents flowed through a liquid chromatographic column packed with dried estuarine and pond sediment samples and the effluent was analyzed for a test lipophilic compound, di(2-ethylhexyl) phthalate. The model predicts that the maximum desorption rate and the maximum extent of gel swelling should coincide and, conversely, that the desorption rate should be diffusion-limited if the polymer gel is only partially swollen. The conditions for maximum desorption and for diffusion-limited desorption were both observed experimentally.