Superfund Research Program
Sequestration and Immobilization of Metal and Metalloid Contaminants in Sediments
Project Leader: Peggy A. O'Day
Grant Number: R01ES016201
Funding Period: 2007-2010
Year: 2010 2009
The overall aim of this project is basic research into the use of reactive amendments as an alternative remediation technology for hazardous metal and metalloid contaminants of high priority in sediments at Superfund and other contaminated sites. The researchers' approach is aimed at developing a molecular-chemical understanding of element sequestration mechanisms as a result of in situ reaction between the amendment and contaminated sediments. This research is intended to fill gaps in basic knowledge about the long-term fate of reactive amendments and their sequestered contaminants in soil and sediments, which presently contributes to a lack of acceptance and use of this remediation approach. The research has been focused in three related areas:
- examination of reaction products and mechanisms of arsenic and mercury sequestration by reactive amendments to sediments and soils;
- thermodynamic and kinetic modeling of amendment reactions and scenarios for long-term contaminant transport and fate in sediment caps;
- field investigations of mercury distribution, speciation, potential for methylation, and optimization of remedial amendment approaches in a wetland area at a former mine site (Sulphur Bank Mercury Mine (SBMM), now on the National Priorities List).
In the last year, the researchers focused on mercury as one of the more problematic and less studied inorganic contaminants at National Priorities List (NPL) sites. Spectroscopic studies, bulk characterizations, and geochemical modeling of substrate-amended reaction products show that Portland and super-sulfate cements can effectively immobilize mercury, but that molecular-scale sequestration mechanisms, either precipitation or sorption, are strongly influenced by aqueous mercury complexation. Reactive-transport modeling of mercury behavior in sediment caps similarly demonstrates the importance of aqueous complexation (sulfide, dissolved organic matter) in competition with precipitation of a sulfide phase (metacinnabar (HgS)) on the potential for mercury methylation and transport. Initial field results from SBMM demonstrate significant increases in the proportion of methylmercury to total mercury in seasonal wetland ponds as pH increases from acidic (3.8 - 5.4) to circumneutral (6.5 - 8.6). Spectroscopic results indicate the presence of metacinnabar in surface sediments, probably as weathered residual particles of mine waste. However, surface water and sediment chemistry indicate oxidized conditions and no evidence for active sulfate reduction, suggesting possible transport of methyl mercury from depth.