Effect of slow desorption on the kinetics of biodegradation of polycyclic aromatic hydrocarbons

The bioavailability to bacteria of C-14-labeled polycyclic aromatic hydrocarbons (PAHs) sorbed onto lake sediments was assessed using a mathematical model and three experimental series. The experiments were performed under similar conditions and included: (1) abiotic desorption of PAHs from sediments by Tenax extraction, (2) mineralization of dissolved PAHs with no sediment present, and (3) mineralization of PAHs sorbed onto sediments. Results obtained from the first two series were used to obtain the parameter values for the model, and the experimental results of the third series were compared to model results. We found that microorganisms were able to promote desorption of the more-labile fractions, but were unable to increase the desorption rate of the slow- and very slow-desorbing fractions. Also, our model predictions indicate that, after very long contact times, and in the concurrence of biodegradation, sorbed PAHs remain not under equilibrium conditions, but rather in a steady state. The net rates of PAH desorption from the three sediment domains considered (fast, slow, and very slow) become similar, and the ratio between the aqueous and the sediment concentration remains constant with time.