Use of solvents to enhance PAH biodegradation of coal tar-contaminated soils

Bioremediation of coal tar-contaminated soils containing polycyclic aromatic hydrocarbons (PAHs) is highly challenging because of the low solubility and strong sorption properties of PAHs. Five coal tar-contaminated soils from former manufactured gas plant (MGP) sites were pretreated with two solvents, acetone and ethanol to enhance the bioavailability of the PAH compounds. The biodegradation of various PAHs in the pretreated soils was assessed using soil slurry reactors. The total PAH degradation rates for soils pretreated with solvents were estimated to be about two times faster than soils that were not pretreated with solvents. For example, the total PAH first order degradation rate constants were 0.165+/-0.032, 0.147+/-0.020, and 0.076+/-0.009 day(-1) for Vandalia (EXC) soil that were pretreated with acetone, ethanol, and with no solvent, respectively. A distinctive advantage for soils pretreated with solvents was the enhanced removal of 5-ring PAH compounds such as benzo(a)pyrene and to a limited extent 4-ring compounds such as chrysene. Even for soils with 3.5% or more organic carbon content (two soils out of five), the degradation rate constants of chrysene were found to be two times faster than soils that were not pretreated. The degradation rate constants of benzo(a)pyrene were enhanced by 2-6 times for all five contaminated soils that were pretreated with solvents. To further elucidate trends that control the solvent treatment, the percent improvement in degradation rate constants (100 x rate constants for pretreated soils/rate constants for non-treated soils) for 16 PAHs were found to correlate well with the PAH partition coefficients (K(oc)). Except for phenanthrene and the clay fraction of the soil, correlations between the percent improvement in degradation rates constants and several physical properties of the soils were poor and sporadic. This implies that the enhancement in PAH availability using solvent treatment was driven by the distribution of the PAHs between the solvent and the adsorbed PAHs.