Effect of humic acids on the biodegradation of PAHs by bacteria and fungi.

The objective of this project was to evaluate the potential use of humic acids as a remedial technology to enhance the solubility and biodegradation of residual PAHs due to contamination by heavy fuels. Bacteria and fungi differ in their abilities to degrade PAHs. This study was aimed at studying their behaviour in the presence of humic acid and of petroleum products. In bacterial cultures, increasing humic acid concentrations from 0 to 2,000 mg/L caused a decrease in the rate of production of (CO2)-C-14 from phenanthrene. However, this negative effect was not present when furnace oil or light crude were added instead of pure phenanthrene as a carbon source. In the experiments with the fungus Mucor circinelloides, the concentration of PAHs in the aqueous phase were significantly higher in the presence of increasing concentrations of humic acids and the effect was relatively more pronounced for benzo-a-pyrene and pyrene. The other PAHs were associated with the biomass and bound relatively strongly: 14 to 42% of the radioactivity was liberated only by hydrolysis. All metabolites were liberated into the aqueous phase. Based on the wet weight of biomass, humic acids had no influence on the growth of the micro-organisms. With furnace oil, the amount of activity in the aqueous phase ranged between 5 to 10% of the total activity. In contrast, in the presence of crude oil, phenanthrene partitioned preferentially to the biomass. However, in both cases, the sequential extractions with hexane and acetone liberated most of the activity. Based on GC/MS analysis of a hexane extract of the microcosms at the end of the experiment, M. circinelloides. and Trichoderma hazarnium were able to degrade the hydrocarbons present in furnace and light crude oil.