Bioavailability of hydrophobic organic compounds from nonaqueous-phase liquids: The biodegradation of naphthalene from coal tar

The literature on microbial degradation of polycyclic aromatic hydrocarbon (PAH) compounds in soils and sediments shows that microbial biotreatment rates are highly variable. Although it is believed that these disparate results are somehow attributable to the bioavailability of PAH compounds, current understanding does not allow prediction of this phenomenon. This work evaluated relationships between coal tar composition and aqueous naphthalene concentration and between the rate of naphthalene mass transfer and microbial mineralization of naphthalene. The use of a dissolution-degradation framework for identifying rate-controlling phenomena in slurry biotreatment of coal tar is discussed. Experiments were performed in slurry systems with two coal tars for which naphthalene was the principal component. In slurry systems coal tar was present either as a single 0.7-ml globule or as coated onto 250-mu m-diameter microporous silica beads. Independent tests were conducted with these systems to assess the rates of naphthalene mass transfer and rates of naphthalene biomineralization. The area-dependent mass transfer coefficient of naphthalene in the coal tar-silica beads system was three orders of magnitude greater than for the single coal tar globule. A coupled, solute-mass transfer biodegradation model was used to compare initial biomineralization rates with rates of naphthalene dissolution. Analysis of the results showed that the rate of biomineralization of naphthalene in the systems containing single coal tar globules was influenced by the rate of mass transfer of naphthalene.