Phenanthrene desorption from soil in the presence of bacterial extracellular polymer: Observations and model predictions of dynamic behavior

The extracellular polymer produced by a bacterium isolated from soil was em ployed in laboratory studies of desorption of a model polynuclear aromatic hydrocarbon (PAH), phenanthrene. The experimental results show that the sel ected extracellular polymer enhances the extent of release of soil-bound ph enanthrene. A kinetic model was developed as an aid in interpreting the alt erations in phenanthrene desorption resulting from polymer addition. The mo del employs a statistical gamma (gamma) distribution to describe spectrum o f rate constants for transfer of phenanthrene from soil to water, and assum es instantaneous binding of phenanthrene to polymer and of polymer to the t est soil. The relevant distribution coefficients and statistical parameters of the gamma distribution needed for the model were evaluated in independe nt experiments. Using these measured parameters, the model provides a satis factory independent prediction of phenanthrene release from soil to aqueous phase at two test polymer concentrations, 50 mg TOC/L and 100 mg TOC/L. Th e success of the independent model predictions suggests a mechanism for the influence of extracellular polymer on phenanthrene desorption. The intrins ic, soil-specific, rate constants for solid to solution transfer of phenant hrene do not appear to be changed by bacterial polymer. Instead, polymer bi nding of phenanthrene in solution results in an increase in driving force f or desorption by decreasing the solution concentration of the free, unbound , PAH molecule. (C) 2001 Elsevier Science Ltd. All rights reserved.