Microbial degradation of acenaphthene and naphthalene under denitrification conditions in soil-water systems

This study examined the microbial degradation of acenaphthene and naphthalene under denitrification conditions at soil-to-water ratios of 1:25 and 1:50 with soil containing approximately 10(5) denitrifying organisms per g of soil. Under nitrate-excess conditions, both acenaphthene and naphthalene were degraded from initial aqueous-phase concentrations of about 1 and several mg/liter respectively, to nondetectable levels (less than 0.01 mg/liter) in less than 9 weeks. Acclimation periods of 12 to 36 days were observed prior to the onset of microbial degradation in tests with soil not previously exposed to polycyclic aromatic hydrocarbon (PAH) compounds, whereas acclimation periods were absent in tests with soil reserved from prior PAH degradation tests. It was judged that the apparent acclimation period resulted from the time required for a small population of organisms capable of PAH degradation to attain sufficient densities to exhibit detectable PAH reduction, rather than being a result of enzyme induction, mutation, or use of preferential substrate. About 0.9% of the naturally occurring soil organic carbon could be mineralized under denitrification conditions, and this accounted for the greater proportion of the nitrate depletion. Mineralization of the labile fraction of the soil organic carbon via microbial denitrification occurred without an observed acclimation period and was rapid compared with PAH degradation. Under nitrate-limiting conditions the PAH compounds were stable owing to the depletion of nitrate via the more rapid process of soil organic carbon mineralization. Soil sorption tests showed at the initiation of a test that the total mass of PAH compound was divided in comparable proportions between solute in the aqueous phase and solute sorbed on the solid phase. The microbial degradation of the PAH compound depends on the interrelationships between (i) the desorption kinetics and the reversibility of desorption of sorbed compound from the soil, (ii) the concentration of PAH-degrading microorganisms, and (iii) the competing reaction for nitrate utilization via mineralization of the labile fraction of naturally occurring soil organic carbon.