Task 1.15 - Enhanced Bioremediation of Coal Tar-Contaminated Soil

The remediation of sites where soils have been contaminated with hydrophobic organic compounds is a major problem. This is especially true for manufactured gas plants (MGP) and similar industries. Gasification of fossil fuels has resulted in the production of tars that contain polyaromatic hydrocarbons (PAHs). PAHs are of concern because they are persistent in the environment and because many of them are carcinogenic. When soils contain very high concentrations of PAHs and other tar components, it is generally most feasible to use a chemical-physical remediation technique such as incineration. However, when the contaminant concentrations are medium to low, the most inexpensive technology is generally biological treatment. Biological treatment is an environmentally acceptable method of remediation that is relatively low-cost for many organic contaminants. PAHs and similar hydrophobic compounds, however, present major challenges to the microbial methods. These problems can be broken down into two related areas: mass transfer and bioavailability. The water volubility of most PAW is very small, and they have very low vapor pressures. As a result the diffkion of the sorbed PAHs to a location where a biodegrading microorganism can encounter it is often limiting. Complicating this is that especially in aged samples, the PAHs are bound so strongly to soil components, they are not available for the microorganisms. These two phenomena can be better understood by examination of a typical biodegradation curve, such as might be observed for mixed PAHs. In Figure 1, there are three phases of the biodegradation process plotted with respect to time. In the first phase, concentrations are changing only slowly as the microbes adapt to the conditions, grow to a larger population, and begin to biodegrade the PAHs. The second phase is the phase of rapid, often logarithmic, biodegradation. The third and last phase is that period when available concentrations of PAHs are dropping so that the microorganisms cannot degrade them. The rate observed during the second phase is dependent on the size of the microbial population that can degrade the contaminants, the availability of the contaminants, and suitable nutrients (e.g., phosphorus) and electron acceptors (e.g., oxygen). When readily biodegradable contaminants are present this phase is often very rapid. However, when any one of the above factors is limiting, a reduced rate is observed