Project 6

Geochemical Controls on the Sorption, Bioavailability, Formation and Long-term Environmental Fate of Polychlorinated Dibenzo-p-Dioxins (PCDDs) 

Due to their exceptionally low water solubilities polychlorinated dibenzodioxins and furans (PCDD/Fs) are strongly and extensively bound to soil and sediment particles.  Sorbed PCDD/Fs are distributed among the primary component geosorbents, namely char-like carbonaceous materials, amorphous organic matter, and clays, and the fractional distribution among these geosorbents is hypothesized to change with the total PCDD/F load.  Since PCDD/Fs are also highly resistant to decomposition, sorption is a primary determinant of their environmental fates and impacts.  Importantly, sorption to soil/sediment particles may modify the bioavailabilities and toxicities of PCDD/Fs in unknown ways, and bioavailability is expected to be geosorbent-specific.  

The major goals are:

  1. to advance fundamental understanding of PCDD/F sorption by the dominant geosorbents comprising soils/sediments, especially at very low environmentally relevant (ppt-ppb) concentrations where carbonaceous materials (e.g. chars) are hypothesized to control soil-water distribution
  2. to determine the differential bioaccessibilities/bioavailabilities of PCDD/Fs sorbed to each key geosorbent type using physiologically based extraction fluid, and mammalian models
  3. to test the hypothesis that mechanistic knowledge of sorption/desorption reactions for PCDD/Fs with individual component geosorbents can be extrapolated to predict site-specific bioaccessibilites and bioavailabilities for contaminated whole soils/sediments
  4. to evaluate the clay-facilitated formation of PCDD/Fs, and corresponding predioxins/furans, from precursor chlorophenols, and elucidate the underlying mechanistic basis for these reactions

Estimates of PCDD/F bioavailability in soils/sediments are few and inconsistent, hence most risk assessment models for exposure to environmental PCDD/Fs make generic assumptions of 100% bioavailability, irrespective of soil/sediment characteristics.  The results of the proposed research will provide the basis for (1) a more mechanistic understanding of the relationship between soil/sediment composition and the human and ecological risks posed by a given total PCDD/F load in soil/sediment, and (2) understanding the prevalence of clay-facilitated PCDD/F formation as an ongoing in-situ process leading to unexpected PCDD/F accumulations that threaten human health.  Further, it would be of great economic and environmental benefit if certain chars, such as those produced as intentional byproducts of biofuels/C-sequestration technologies, were shown to be effective as soil/sediment amendments to diminish bioavailability of PCDD/Fs.