Effect of humic acids on the bioavailability of PAHs from weathered soils.

This is the last report on a comprehensive multi-year study on the effect of humic acids on the dissolution and biodegradation of aromatic hydrocarbons of petroleum origin. Early phases of the work showed that the addition of humics acids enhanced the dissolution of hydrocarbons from light fuels such as diesel, and thus indirectly helped the bioremediation, because dissolution is often rate limiting. The second phase of the work using radiolabelled phenanthrene, pyrene and benzo-a-pyrene showed that humic acid retarded the degradation of the pure PAHs spiked onto soil, but that this effect was reversed when the petroleum product was also added. This indicated that biodegradation was dependent on the relative sorption of PAHS onto soil, or humic acids. Sandy soils were predominantly used in these early studies. In the current work, humic acids were added to highly weathered clay soils. A subset also including freshly spiked PAHs was included as a control. The hypotheses to be tested were whether humic acids could displace the PAHs on the clay surface and enhance bioremediation and whether weathered PAHs were more difficult to degrade than freshly introduced contaminants. The results showed that humic acids had no noticeable effect on the biodegradation of either weathered or spiked soil. The most important factor was the presence of a microbial population capable of effecting the degradation.