Superfund Research Program

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

Project Leader: Stephen A. Boyd
Grant Number: P42ES004911
Funding Period: 2006-2021

Project-Specific Links

Final Progress Reports

Year:   2020  2012 

Due to their exceptionally low water solubilities, polychlorodibenzo-p-dioxins (PCDDs) in the environment are strongly and extensively bound to soil and sediment particles. Because PCDDs are highly resistant to microbial degradation, sorption is a primary determinant of their environmental fates and impacts on ecosystem and human health. PCDDs immobilized on geosorbent particles will not move into groundwater or leachates and hence remain near the surface of soils; movement of the particles themselves into surface waters results in PCDD contaminated sediments. Thus, human/biotic exposure to environmental PCDDs will be almost entirely through ingestion or inhalation of adsorbed PCDDs. A major goal of the Geochemical Controls on the Adsorption, Bioavailability, and Long-term Environmental Fate of Dioxins, PCBs, and PAHs project is to advance understanding of bioavailability processes involving the PCDD-contaminated soils and sediments. Because these materials are complex, the researchers seek to evaluate bioavailability processes involving individual geosorbent types, e.g. clays and chars that may sequester PCDDs, and manifest differential bioavailabilities. The researchers' recent studies focus on the relative contributions of carbonaceous geosorbents (CG), amorphous organic carbon (AOC) and clays to sequestration of dibenzo-p-dioxin (DD).

As a first step toward mechanistically relating PCDD sorption by these component geosorbents to the sorption and bioavailability of PCDDs in whole soils, one can approximate the sorption of PCDDs by a whole soil as the simple sum of the sorption to its three primary geosorbents (see Fig. 1). Then, the sorbed concentration (Cs) as a function of the corresponding concentration in water (Cw) can be apportioned to each sorbent phase, and expressed mathematically in a predictive model. This mathematical simulation clearly indicates that at low relative concentration (Cw/Sw), adsorption to CG greatly exceeds partitioning into AOC, and sorption to clay is intermediate. In the environment, PCDD sorption by CGs could be reduced by a factor of ten due to competitive adsorption of native organics, and obscuration of surfaces by AOM, referred to as attenuation. Dioxin sorption by the imaginary soil (Fig. 1) provides a useful working hypothesis or model that the researchersare validating experimentally. If validated, it does point to the potentially dominant role of CGs (e.g. chars) in the sequestration of contaminants, especially those found at low aqueous concentrations such as PCDD/Fs. Dr. Boyd and his research team then plan to assess the differential bioaccessibilities and bioavailabilities of PCDDs bound to CG, AOM, and clays. When establishing remediation goals, if CG-bound contaminants are not available for uptake by higher organisms directly exposed to contaminated soil, and a large portion of the contaminant mass is sequestered by CG, then soil-sediment criteria could be safely relaxed to avoid unnecessary remediation efforts. In fact, in-situ sorbent amendment with CGs is being evaluated as a new direction in contaminated sediment management to effectively and inexpensively reduce bioavailability and hence exposure/risk. Alternatively, if the role of CG in contaminant uptake has been over-emphasized (due to attenuation for instance), or if CG-bound contaminants are found to be bioavailable to mammalian models, then more stringent criteria are warranted to be fully protective of human health.