Biodegradation of sediment-bound PAHs in field-contaminated sediment

Abstract

The biodegradation of polycyclic aromatic hydrocarbons (PAHs) has been reported to occur under aerobic, sulfate reducing, and denitrifying conditions. PAHs present in contaminated sites, however, are known for their persistence. Most published studies were conducted in systems where PAHs were freshly spiked, and biodegradation was often tested using pure cultures or enrichments. This paper investigated the degradation potentials of PAHs that were present in aged contaminated sediment by indigenous bacteria, where the limited bioavailability of PAHs due to aging played an important role. The sediment and the overlaying water were collected from a contaminated site to prepare sediment-water slurries, and the sediment served as both the media containing PAH substrates and the habitat for the indigenous microorganisms. Reduced sulfur compounds present in the sediment caused rapid oxygen depletion due to extensive activities of sulfur-oxidizing bacteria and could result in a dramatic pH drop. Once oxygen depletion and acidification problems were avoided, substantial removals of two-, three-, four-, and five-ring PAHs were achieved aerobically, though the extent of degradations was smaller than what was reported for freshly spiked PAHs. The amendment of inorganic N and P, co-substrates, or surfactant Triton X-100 did not enhance the level of degradations appreciably. Under denitrifying conditions, no distinct PAH degradation was observed, while the complete denitrification of nitrate to nitrogen occurred stoichiometrically with a concomitant increase in sulfate concentration, indicating the dominance of autotrophic denitrifiers. The addition of ethanol or acetic acid did not stimulate PAH degradation. Substantial PAH degradation attributed to sulfate reduction was only observed for phenanthrene, the low-ring PAH existing in a highest initial concentration. Addition of ethanol or acetic acid did not change this finding. This is the first study to our knowledge that revealed the importance of indigenous bacteria involved in natural sulfur cycling in determining degradation behavior of PAHs.