Terrestrial Fate Considerations

PCDDs and PCDFs are largely immobile once adsorbed to particles in the soil column. However, different PCDD and PCDF compounds have different solubilities, and factors such as the soil organic content, clay content, pH and moisture are important. Solute-transport models have shown that mechanisms such as wind and water erosion are likely to be more important than losses by movement within the soil. A soil dissipation rate of 0.069 3 yr_1 (which corresponds to a half-life of c. 10 years) has been postulated under specific experimental conditions.

Rates of volatilization from soil are uncertain. In general, higher-chlorinated dioxins such as octachlorinated dibenzo-/>-dioxins (OCDDs) do not vaporize under environmental conditions and have half-lives in soil measured in years. Some studies suggest that the half-lives of dioxins at the soil surface may be measurable in terms of weeks or months. When mixed in soils to depths generally below 5 mm, however, soil half-lives are likely measured in terms of years.

Rates of photolysis, involving removal of one or more chlorine atoms from the PCDD or PCDF molecule and resulting in higher-chlorinated compounds degrading to less-chlorinated compounds (referred to as dechlorina-tion), are generally believed to be insignificant. Research has shown limited photodegradation of PCDDs and PCDFs from soil surfaces and sediments.

There is scarce information on microbial degradation of PCDDs and PCDFs. Limited data suggest that microbial degradation of PCDDs and PCDFs in soils is generally slow (measured in terms of years); microbial degradation rates generally decrease with increasing chlorination. Recent studies provide limited evidence that some fungi species, most notably white rot fungi, are able to mineralize some PCDD and PCDF congeners.

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