Phenanthrene desorption from soil in the presence of bacterial extracellular polymer: observations and model...

The extracellular polymer produced by a bacterium isolated from soil was employed in laboratory studies of desorption of a model polynuelear aromatic hydrocarbon (PAH), phenanthrene. The experimental results show that the selected extracellular polymer enhances the extent of release of soil-bound phenanthrene. A kinetic model was developed as an aid in interpreting the alterations in phenanthrene desorption resulting from polymer addition. The model employs a statistical gamma (gamma) distribution to describe spectrum of rate constants for transfer of phenanthrene from soil to water, and assumes instantaneous binding of phenanthrene to polymer and of polymer to the test soil. The relevant distribution coefficients and statistical parameters of the gamma distribution needed for the model were evaluated in independent experiments. Using these measured parameters, the model provides a satisfactory independent prediction of phenanthrene release from soil to aqueous phase at two test polymer concentrations, 50 mg TOC/L and 100 mg TOC/L. The success of the independent model predictions suggests a mechanism for the influence of extracellular polymer on phenanthrene desorption. The intrinsic, soil-specific, rate constants for solid to solution transfer of phenanthrene do not appear to be changed by bacterial polymer. Instead, polymer binding of phenanthrene in solution results in an increase in driving force for desorption by decreasing the solution concentration of the free, unbound, PAH molecule.