Adsorption and partitioning of organic compounds onto/into fractal sorbents

A model of organic chemical adsorption and partitioning onto/into fractal sorbents is developed in terms of sorption probabilities and location of the sorbate on the sorbent surface. Evidence is presented that the application of a Langmuir derived model to fractal aggregates provides a consistent and mechanistic explanation for both adsorption and partitioning processes. Partitioning-like processes are visualized in terms of adsorption to the inner sites of fractal sorbents, analogous to partitioning of solutes into humic acids. The model for the sorption process exhibits nonlinear dynamics. First, molecules are adsorbed at the outer sites of the aggregates in a fast process that dominates the initial kinetics of sorption. Second, molecules slowly diffuse inward and sorb to the inner sites of the aggregate. For the latter case, sorbate molecules exhibit partitioning-like processes which make a larger contribution to overall sorption at long equilibration times. Implications for the interpretation of partitioning and adsorption processes are discussed.