Use of solvents for PAHs extraction and enhancement of the PAHs bioremediation in coal-tar-contaminated soils

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Lee, Pak-Hing
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Say Kee Ong
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Civil, Construction, and Environmental Engineering

Bioremediation of coal-tar-contaminated soils is a challenging task because of the low solubility of the polycyclic aromatic hydrocarbons (PAHs) and their high partitioning with soils. As such, not all the PAHs present in the soil will be available for biodegradation. Enhancing the availability of PAHs will make bioremediation attractive as a treatment technology. Because of the PAH low solubility and high partitioning, the analyses of PAHs in coal-tar-contaminated soils using standard protocols are usually time-consuming and expensive;In the first part of the study, a direct solvent extraction method was evaluated to assess its application for the extraction and analysis of PAHs in coal-tar-contaminated soils. The direct solvent extraction method using acetone as an extraction solvent was shown to be equivalent to the Soxhlet extraction method (EPA Method 3540). In this study, five coal-tar-contaminated soils from four manufactured gas plant (MGP) sites in Iowa were used;In the second part of the study, a mild extraction method was developed as an assessment tool to estimate the extent of PAH degradation or availability in contaminated soils. The percent of PAHs biodegraded after 35 days for 16 individual PAHs using soil slurry reactors were found to correlate well with the percent of PAHs extracted with an acetone-water mixture of 0.6. Two correlations relating the percent biodegraded and the percent of PAHs extracted with 0.6 acetone-water mixture along with the soil properties and PAH properties were developed. This assessment tool using acetone-water mixture of 0.6 may be used to estimate the availability of PAHs in soils;In the third part of the study, two miscible solvents---acetone and ethanol---were used to enhance the biodegradation of PAHs using soil slurry reactors. The degradation rates of PAHs were shown to be significantly enhanced for soils pretreated with solvents. Four- and 5-ring PAHs in pretreated soils were found to degrade by at least 2 to 6 times faster than soils that were not pretreated. The results show that solvents such as acetone and ethanol may enhance the availability of PAHs;Using mixed cultures from a soil-slurry reactor, the degradation of [ 14C]benzo(a)pyrene (BaP) in various aqueous phase and solid phase treatments was studied. The purpose of these studies was to investigate the effects of low molecular weight PAHs on the degradation of BaP. In liquid phase treatments with and without 2- and 3-ring PAHs, no significant mineralization was found for all treatments for 14 weeks even though there was significantly microbial respiration. This indicates that BaP may not be used as a microbial source of carbon and energy and 2- and 3-ring PAHs, most probably, were not used for the cometabolization of BaP. In solid phase treatments, 18.6% of initial 14C were mineralized to CO2 for the land-farming soil from Vandalia road site after 25 weeks incubation. Results of the solid phase treatment showed that the degradation of PAHs was sequential in that the lower molecular weight PAHs were degraded first followed by the higher molecular weight PAHs such as BaP.

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