REDUCTION OF TOXICITY AND PAH BIOACCUMULATION POTENTIAL DURING BIOREMEDIATION OF PETROLEUM-CONTAMINATED SOILS

摘要

A set of model soils (e.g., quartz sands, loams, clays, peat, silica gels) was spiked with crude oil, aged for 27 months in the laboratory, and subjected to bench-scale slurry bioremediation treatment for 90 weeks. At various times, slurry samples were removed from the bioreactors, analyzed for polynuclear aromatic hydrocarbons (PAHs), and assayed for toxicity using the solid-phase MicrotoxTM test. In addition, the PAH bioaccumulation potential in tissues of soil dwelling organisms was estimated by exposing semi-permeable membrane devices (SPMDs) for 14 days to initial (t=0) and final (t=90 wks) slurry samples. In most cases, soil toxicity (EC50) was reduced 5- to 10 fold during the first 20 weeks of bioremediation treatment but it rarely reached the EC50 value of the respective clean reference soils, indicating that some residual toxicity still remained after 20 weeks of bioremediation. The reduction of soil toxicity most closely correlated with the biodegradation of the sum of 4, 5, and 6 ring PAHs. A comparison of PAH concentrations in SPMDs exposed to initial and final slurry samples revealed that the potential of PAH accumulation in exposed animal tissues was reduced 50 to 300 fold as a result of slurry biotreatment. While in most soils the bioavailable PAH fraction was preferentially removed by the microorganisms, several high molecular weight PAHs that had reached an apparent concentration asymptote after 90 weeks of treatment were still bioavailable to a significant extent in several soils since they were readily taken up by the SPMDs. In summary, it can be concluded that bioremediation significantly reduces both toxicity and PAH bioaccumulation potential. However, the remaining undegraded contaminants are likely to pose some residual risk to environmental receptors since they still exhibit toxicity relative to uncontaminated background soils and are bioavailable to animal tissues.