Subtask 1.17 - Subcritical Water Extraction of Mercury From Soils and Sediments

The U.S. Environmental Protection Agency (EPA) "National Sediment Quality Survey" lists the top pollutants responsible for toxicity in watersheds as 1) PCBS (polychlorinated biphenyls), 2) mercury, and 3) other organics such as PAHs polycyclic aromatic hydrocarbons) and pesticides. In addition, these same pollutants are major contributors to chemical pollution on U.S. Department of Energy (DOE) and other contaminated sites (e.g., industrial sites and harbors). An ideal remediation method would allow cost-effective removal of both organic and mercury contamination using a single process. The Energy & Environmental Research Center (EERC) has demonstrated that controlling the temperature (and to a lesser extent, the pressure) of water can dramatically change its ability to extract organics and inorganic from matrices ranging from soils and sediments to waste sludges and coal. The dielectric constant of water can be changed from ca. 80 (a very polar solvent) to <5 (similar to a nonpolar organic solvent) by controlling the temperature (from ca. ambient to ca. 400oC) and pressure (from ea. 5 to 350 bar). The EERC has shown that hazardous organic pollutants such as pesticides, PAHs, and PCBS can be completely removed from soils, sludges, and sediments at temperatures (250"C) and pressures ( c 50 atrn) that are much milder than typically used for supercritical water processes (temperature > 374oC, pressure >221 atm). In addition, the process has been demonstrated to be particularly effective for samples containing very high levels of contaminants (e.g., parts per thousand). The EERC has also demonstrated that mercury can be extracted using supercritical water at much harsher conditions (400"C, and >300 atm). However, the removal of mercury from contaminated solids at the lower temperature and pressure conditions (e. g., 250"C, 50 atm) has not been investigated. If successful, this project will provide the basis for using hot/liquid water to extract both organic contaminants and mercury from contaminated solids in a single-step process.