Modeling the desorptive release of nonionic organic contaminants from heterogenious soil matrices

Several alternative models were developed to describe the rate of desorption of hydrophobic organic contaminants from soil. These include radial diffusion models, multi-compartment models, and models that use a statistical distribution to describe sorption/desorption rate constants or effective diffusion coefficients. With the γ-model, the spectrum of contaminant desorption rate constants, attributed to the heterogeneity of diffusive path lengths and tortuosity in soil pores as well as the diversity of soil organic matter and mineral sorbents, was represented by a distribution first order rate constants. The use of the model to independently predict, with no adjustable parameters, transport of the PAH, naphthalene, in a sterile soil column was demonstrated. An instantaneous equilibrium soil compartment was needed in addition to a distribution of release rates to describe the rapid initial desorption behavior exhibited by naphthalene from the soil used to conduct the experiment. The modified γ-model was used to predict the effect of bacterial polymers on desorption of the PAH, phenanthrene. The results showed good agreement between the model predictions and the experimental data points at both the polymer concentration of 50 mg TOC/L and 100 mg TOC/L. Compared to the experimental data, the root mean square relative error for the predictions at polymer concentrations of 50 and 100 mg TOC/L were 3.08 and 4.71%, respectively.