Semester

Summer

Date of Graduation

2000

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Civil and Environmental Engineering

Committee Chair

Brian E. Reed.

Abstract

High-molecular-weight polynuclear aromatic hydrocarbons (PAHs) are persistent in the environment although a wide variety of microorganisms can metabolize PAHs. In the past decades, laboratory and filed studies have shown that PAH degradation in soil is often limited by poor bioavailability and oxygen availability. Bound residue formation of PAHs with macromolecules of soil organic matter is an important fate mechanism. More recently, phytoremediation for PAH-contaminated soils is being explored. It is believed that PAH degradation may be enhanced in rhizosphere soil due to the improved aeration condition and the active soil microbial community sustained by root exudates. Whether certain root exudates would influence PAH degradation or bioavailability in soil is not adequately understood. Although various plant flavonoids, important components of root exudates, have been found to activate or inhibit PAH metabolism in mammalian cells, research on the interaction between root flavonoids and the soil microbial activities had been few. The effects of root flavonoids on the fate of PAHs in rhizosphere soil was investigated using 14C-labeled B[a]P and pyrene in slurry phase soil microcosms. A compound nested experiment was designed to evaluate the effects of different types of rhizosphere soil and flavonoids at varied concentration levels on PAH fate via mineralization, water leaching, and bound residue formation. Both synthetic nonhydroxylated and natural hydroxylated flavonoids at low concentration (0.1--1 uM) had no statistically significant effects on PAH fate at 95% confidence level. However, higher flavonoid concentration level (>10--100 uM) or complex root-extracts hindered PAH mineralization but enhanced PAH-soil-bound residue formation in biologically active rhizosphere soils. In contrast, mineralization was negligible and bound residues decreased as flavonoid concentration increased in abiotic control soil. A biologically mediated covalent binding between phenolic moieties may be responsible for the enhanced bound residue formation. Relatively high percentage bound residues were found to be associated with higher clay, soil organic matter, and humus contents in soil. Increased bound residue formation may have reduced the amount of PAH available for biodegradation/mineralization. There were little or no water leachable PAHs and their polar metabolites in all the treatments.;This research was supported by Union Carbide Corporation.

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