Project Summary (2013-2021)

Due to their exceptionally low water solubilities polychlorinated dibenzodioxin 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 clays, amorphous organic matter, and carbonaceous materials, and the fractional distribution among these geosorbents is hypothesized to change with the total PCDD/F load. Because PCDD/Fs are 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 knowledge of sorption of individual PCDD/F isomers by component geosorbents can be extended to whole soils/sediments by predicting and measuring site-specific domains for PCDD/F sequestration in a series of dioxin contaminated ball clays, and the corresponding PCDD/F bioaccessibilites
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/Fs bioavailability in soils/sediments are few and inconsistent, hence most risk assessment models for exposure to environmental PCDD/Fs make generic and high (e.g. 100%) assumptions of bioavailability irrespective of soil/sediment characteristics. The results of this 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
2. Establishing the importance of clay-facilitated PCDD/F formation as an on-going 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 by-products of biofuels/C-sequestration technologies, were shown to be effective as soil/sediment amendments to diminish bioavailability of PCDD/Fs.

Current risk assessment models typically assume 100% bioavailability of PCDD/Fs in soils. The ability to assign scientifically informed values for PCDD/F bioavailability, that account for soil composition, represents a major advance in understanding the exposure risk of PCDD/F contaminated soils/sediments. Formulating safe and realistic remediation endpoints based on available contaminant concentrations instead of total ones allows limited remediation funds to be better prioritized and needless remediation attempts avoided.