Validation of a hybrid "two-site gamma" model for naphthalene desorption kinetics

Three models for sorption/desorption of polycyclic aromatic hydrocarbon (PA H) contaminants from soil were compared for their ability to predict the tr ansport of PAH in soil: a "gamma" model, a "two-site/two-region" nonequilib rium model, and a "hybrid" model. In the "hybrid" model, soil organic matte r was conceptually divided into two compartments; a fraction with rapid sor ption/desorption kinetics and a compartment with mass-transfer-limited kine tics. Contaminant sorbed in the rapid compartment was assumed to be in inst antaneous equilibrium with the aqueous phase, while the release of contamin ant from the slow fraction was assumed to be governed by a gamma distributi on of rate coefficients. The "hybrid" model successfully described the init ial rapid release of a model PAH contaminant, naphthalene, from a sieved so il sample of moderate organic content (congruent to 2.3%) as well as the fo llowing slow release observed over 25 days in batch desorption experiments. Other necessary model parameters, such as the hydrodynamic dispersion coef ficient of naphthalene and the macropore porosity, were evaluated in separa te experiments. A transport model incorporating the "hybrid" model for naph thalene sorption/desorption successfully predicted the elution profile of n aphthalene in independent soil-column experiments with no adjustable parame ters. The success of the hybrid model suggests that a wide array of rate co ntrols govern PAH desorption. This conclusion is consistent with the view o f soils as consisting of a mix of different sorptive constituents and heter ogeneous physical constraints on PAH release.