The impact of nutrient and biodiesel amendments on the biodegradation of hydrocarbons in contaminated soil

Abstract

The effect of nutrient, biodiesel and biochar amendments on the biodegradation of pollutant hydrocarbons was investigated in soil spiked with crude oil and coal tar over periods of up to 180 days. Biodiesel was chosen as an amendment for increasing the bioavailability of hydrocarbon pollutants because of its good solvent properties but low toxicity. Results from laboratory microcosm experiments on soil spiked with weathered and un-weathered tar showed that after 60 days, the concentrations of the potent carcinogen benzo[a]pyrene (BaP) were significantly reduced by 92 and 81%, respectively, in the biodiesel amended samples compared to the 17 and 26% reduction in the controls, and 8 and 34% depletion observed in the nutrient-only amended microcosms, respectively. The 3-ring PAH anthracene was also almost completely biodegraded in all the biodiesel amended experiments. However, phenanthrene degradation was significantly inhibited in these samples as only 0-2% reduction occurred after 180 days as opposed to the losses (>70%) observed in the control and the nutrient amended experiments. Apparent increases in concentrations were observed for some 4-ring PAHs, while the degradation of other 5 and 6-ring compounds was enhanced in the biodiesel amended samples probably due to their increased solubilisation by biodiesel. A stepwise treatment approach conducted on tar spiked soil revealed a higher reduction in BaP (98%) in the biodiesel amended microcosms compared to the control (29%) and phenanthrene depletion was also enhanced by 51% after 60 days of adding biodiesel to soil initially treated with nutrients. A similar trend in PAH degradation was observed for the crude oil spiked soil, but in these experiments the removal of the n-alkanes was significantly enhanced by nutrient amendment alone, while degradation of the branched alkanes was increased in the biodiesel treatments. Toxicity assays showed that biodiesel amended microcosms stimulated phosphatase enzyme activity and exhibited a lower toxic response to Microtox Vibrio fischeri. In this study, biochar amendment did not significantly reduce residual pollutant concentrations. Overall, the pattern observed in the removal of the PAHs using biodiesel, suggests the cometabolic action of ligninolytic fungi, probably via lignin peroxidases, as also evidenced from the visible growth of moulds after 7-14 days of amendment. The enhanced removal of carcinogenic PAH and the reduced toxicity observed in soil after biodiesel amendment, indicates that this bioremediation technique has potential for full scale field trials.