Superfund Research Program

Advances Made in Determining Fate and Bioavailability of PAH in Soil

Release Date: 08/20/1997

Polycyclic aromatic hydrocarbon compounds (PAHs) are of great concern to the environmental community because of their widespread distribution in the environment, their persistence and their known toxic properties. One specific aspect of addressing this multifaceted public health problem involves understanding the mechanisms that control the fate of PAHs in the environment. Understanding of such mechanisms not only provides some predictive power for the concentrations that may be bioavailable for human and environmental exposure, but also supplies clues as to bioremediation strategies that may be environmentally sound and cost effective.

In an ongoing effort to elucidate the mechanisms that control the fate of PAHs in the environment, researchers at the University of North Carolina recently determined that microbial metabolism is a major mechanism for transforming high molecular weight PAHs - being a more important fate determinant than previously realized. Studies also show that the bioavailability of PAHs in soil can be affected by different soil conditions. It was found that when soil not previously exposed to PAHs (unexposed soil) was mixed with PAHs, the resultant metabolism of the PAHs by unadapted microbes led primarily to the formation of non-polar products. These by-products were bound to the soil, making them biologically unavailable to other organisms. Very little of the parent PAH was completely degraded to carbon dioxide and water, i.e. mineralized, under these soil conditions. When the unexposed soil was mixed with PAHs and PAH adapted microorganisms, there was a large increase in mineralization with concomitant decreases in products sequestered in the soil.

Analysis of the PAHs and/or its by-products associated with soil suggest that in sterile soil, adsorption is responsible for most material bound and the predominate contaminant is the parent compound. Therefore, in sterile soil systems, PAH is still biologically available. When living microbes are present, either adapted or unadapted to PAHs, microbial products dominate with little parent compound remaining. Measurement of the toxicity of the soil fractions suggests that most of the toxicity remains in the sterile systems, due to the fact that the parent compound has not been degraded. Whereas, in soils with active microbial metabolism the toxicity has largely disappeared because the PAHs are either mineralized or broken down into products that are sequestered in the soil and are not bioavailable.

These findings contribute significantly to the fundamental understanding of the fate of PAHs in soil. The soil system is more complex than previously believed. These results demonstrate that the microbial activity may have a much more significant role than merely mineralizing part of the PAHs - the microbes are also degrading PAHs into products that become unavailable thereby reducing the threat to human health. The work so far has dealt with soils subjected to a single exposure. This does not represent the situation at most waste sites where there have been multiple additions of PAHs over time. This scenario will be addressed in upcoming experiments.

For More Information Contact:

Frederic K. Pfaender
University of North Carolina-Chapel Hill
Department of Environmental Sciences and Engineering
CB#7431, 008 Rosenau Hall
Chapel Hill, North Carolina 27599-7431
Phone: 919-966-3842

To learn more about this research, please refer to the following sources:

  • Carmichael LM, Christman RF, Pfaender FK. 1997. Desorption and mineralization kinetics of phenanthrene and chrysene in contaminated soils. Environ Sci Technol 31:126-132.
  • Carmichael LM, Pfaender FK. 1997. Polynuclear aromatic hydrocarbon metabolism in soils: relationships to soil characteristics and preexposure. Environ Toxicol Chem 16(4):666-675.
  • Carmichael LM, Pfaender FK. 1997. The effect of inorganic and organic supplements on the microbial degradation of phenanthrene and pyrene in soils. Biodegradation 8:1-13. PMID:9290252
  • Guthrie EA, Pfaender FK. 1997. Microbial role in regulation of PAH bioavailability. OECD Workshop Mexico on Biotechnology for Water Use and Conservation, Oct. 20, Cocoyoc, Mexico. OECD Proceedings 221-238.

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