Bioavailability of polycyclic aromatic hydrocarbons as sediment-associated, desorption-resistant contaminants

Abstract

This study investigated the bioavailability of sediment-sorbed polycyclic aromatic hydrocarbons (PAHs), specifically naphthalene, phenanthrene and fluoranthene to a mixed microbial culture after extensive step desorption. The culture was tested for its ability to produce extracellular biomolecules that could affect PAR partitioning. Also, the relative importance of mass transfer and biodegradation rates to PAR bioavailability was analyzed. The study was designed to overcome certain problems associated with previous bioavailability studies. The use of a unified approach (same sediments, contaminants and inocula) made the effects of each parameter on bioavailability more readily identifiable. Also, because the effect of extracellular biomolecules on PAH partitioning was identified separately, an analysis of abiotic desorption and biodegradation was possible. The PAHs were all bioavailable, even at low aqueous concentrations. Naphthalene and phenanthrene mineralization could be enhanced by adding excess naphthalene. The excess naphthalene supported higher biomass concentrations in the phenanthrene systems. Fluoranthene mineralization was enhanced for only a short time, then was statistically indistinguishable from systems without excess naphthalene. After biodegradation, the final sediment concentration of all PAHs became independent of the initial concentration. The desorption of all the sediment-sorbed PAHs was not affected by either biologically produced molecules or the presence of excess naphthalene. The PAHs all exhibited biphasic abiotic desorption, and the extent of desorption varied inversely with the Kow of the contaminant. The amount of phenanthrene and fluoranthene biodegraded was less than that available through abiotic desorption. Naphthalene biodegradation differed. Without excess naphthalene, biodegradation closely followed abiotic desorption. With excess naphthalene, biodegradation exceeded predicted abiotic desorption. In conclusion, PAHs are bioavailable, even at low initial aqueous concentrations. The mixed culture produced consistent, low final sediment concentrations for all three PAHs. While additional naphthalene can increase PAH bioavailability, the effect is PAH specific, and probably due to higher biomass concentrations. Because the partitioning of the sorbed PAHs is the same in abiotic and biotic systems, a direct comparison of mass transfer and biodegradation is valid. This analysis determined that the biodegradation of the higher molecular weight PAHs is controlled by biological activity, not mass transfer rates.