Two-liquid-phase slurry bioreactors to enhance the degradation of high-molecular-weight polycyclic aromatic hydrocarbons in soil

High-molecular-weight (HMW) polycyclic aromatic hydrocarbons (PAHs) are pol lutants that persist in the environment due to their low solubility in wate r and their sequestration by soil and sediments. The addition of a water-im miscible, nonbiodegradable, and biocompatible liquid, silicone oil, to a so il slurry was studied to promote the desorption of PAHs from soil and to in crease their bioavailability. First, the transfer into silicone oil of phen anthrene, pyrene, chrysene, and benzo[a]pyrene added to a sterilized soil ( sandy soil with 0.65% total volatile solids) was measured for 4 days in thr ee two-liquid-phase (TLP) slurry systems each containing 30% (w/v) soil but different volumes of silicone oil (2.5%, 7.5%, and 15% [v/v]). Except for chrysene, a high percentage of these PAHs was transferred from soil to sili cone oil in the TLP slurry system containing 15% silicone oil. Rapid PAH tr ansfer occurred during the first 8 h, probably resulting from the extractio n of nonsolubilized and of poorly sorbed PAHs. This was followed by a perio d in which a slower but constant transfer occurred, suggesting extraction o f more tightly bound PAHs. Second, a HMW PAM-degrading consortium was enric hed in a TLP slurry system with a microbial population isolated from a creo sote-contaminated soil. This consortium was then added to three other TLP s lurry systems each containing 30% (w/v) sterilized soil that had been artif icially contaminated with pyrene, chrysene, and benzo[a]pyrene, but differe nt volumes of silicone oil (10%, 20%, and 30% [v/v]). The resulting TLP slu rry bioreactors were much more efficient than the control slurry bioreactor containing the same contaminated soil but no oil phase. In the TLP slurry bioreactor containing 30% silicone oil, the rate of pyrene degradation was 19 mg L-1 day(-1) and no pyrene was detected after 4 days. The degradation rates of chrysene and benzo[a]pyrene in the 30% TLP slurry bioreactor were, respectively, 3.5 and 0.94 mg L-1 day(-1). Low degradation of pyrene and n o significant degradation of chrysene and benzo[a]pyrene occurred in the sl urry bioreactor. This is the first report in which a TLP system was combine d with a slurry system to improve the biodegradation of PAHs in soil.