2005 Progress Report: Sequestration of Subsurface Elemental Mercury (Hg0)EPA Grant Number: GR832212
Title: Sequestration of Subsurface Elemental Mercury (Hg0)
Investigators: Barnett, Mark , Harper, Willie F. , Hamilton, William P. , Savage, Kaye S.
Institution: Auburn University Main Campus , Vanderbilt University
EPA Project Officer: Hahn, Intaek
Project Period: March 1, 2005 through March 31, 2008 (Extended to March 31, 2009)
Project Period Covered by this Report: March 1, 2005 through March 31, 2006
Project Amount: $324,342
RFA: Greater Research Opportunities: Persistent, Bioaccumulative Chemicals (2004) RFA Text | Recipients Lists
Research Category: Land and Waste Management , Safer Chemicals , Hazardous Waste/Remediation , Human Health
The overall goal of this research project is to develop an improved understanding and predictive capability for the in situ abiotic immobilization of subsurface elemental mercury (Hg0) using sulfide minerals. The specific objectives of this research project are to:
- Elucidate the fundamental thermodynamic and kinetic parameters that control the partitioning (uptake and release) of Hg to these materials;
- Investigate the behavior of these materials under more complex hydrodynamic (i.e., flow-through) and environmental conditions, including the long-term stability of the products;
- Probe the immobilized Hg with state-of-the-art environmental spectroscopic techniques to determine the mechanism(s) responsible for immobilization; and
- Validate our results with materials from contaminated sites.
These objectives are based on our hypotheses that: (1) sustained Hg immobilization can be achieved using sulfide minerals; (2) the product will be kinetically stable over long periods under a range of environmental conditions; (3) the molecular mechanism responsible for the removal of the Hg, which will govern its removal efficiency and long-term stability, can be elucidated through the use of state-of-the-art spectroscopic techniques; and (4) the dynamic behavior of Hg in such systems can be quantified to provide a clearer understanding and long-term predictive capability for Hg immobilization.
Batch studies have been conducted, which afforded the opportunity to explore a number of competitive reactions relatively quickly and easily, with the most important competing reactions singled out for further study. Although batch systems can provide important information with respect to the kinetic and thermodynamic properties of a system, the effects of physical factors such as the structure, porosity, and the distribution of the reactive media, and the effects of residence time and fluid velocity on the efficiency of the system are not detected in such studies. Therefore, it is necessary for a study that aims to provide insight into the environmental behavior of a compound to consider the behavior of the compound under dynamic (flow) scenarios. In addition, column studies, which are ideally suited to investigate the effects of complex (time-dependent, multisolute) chemical conditions on pollutant flux, have been initiated. This information is critical for determining the degree to which equilibrium and kinetic data apply to systems under environmentally relevant pore flux conditions so that accurate reactive transport models can be developed and tested. Potential candidates have been evaluated in terms of removal efficiency and capacity. Samples of the materials from both batch and column experiments have been analyzed to clarify the mechanism responsible for immobilization. The microscopic information obtained has been used to provide a mechanistic base for the kinetic and thermodynamic parameters determined from the batch and column experiments. It has been observed that various sulfide minerals have the potential to effectively immobilize Hg. Batch experiments demonstrate that FeS, which has the greatest potential of the minerals studied, can remove more than 95 percent of Hg over the total pH range of typical groundwaters. Furthermore, because of pyrite’s vast abundance and high capacity, it also was chosen as a potential candidate for column studies. Column experiments reveal that pyrite also has the ability to remove Hg from groundwater.
Synchrotron radiation permits analyses of samples with environmentally relevant Hg concentrations that would be impossible to achieve with standard laboratory X-ray instruments. Extended X-ray absorption fine structure (EXAFS) spectroscopy provides semiquantitative information describing the geochemical associations of Hg with the Fe- and Mn-sulfide mineral substrates of interest. The capability to analyze samples nondestructively, with minimal handling, facilitates meaningful characterization of the materials in the same form as they occur in the environments of interest. Microbeam facilities allow targeted analysis of small particles that are of particular interest for understanding partitioning behavior of Hg in multiphase media such as soils and engineered mixtures of mineral materials. Spectroscopic studies reveal the clear association of Hg with both Fe- and Mn-sulfides and demonstrate that a variety of sulfide minerals may provide potential sequestration mechanisms for Hg.
Additional research is necessary to further understand the parameters that control the partitioning of mercury to sulfide minerals. Kinetic and equilibrium experiments currently are being conducted to aid in accomplishing this task. The results of these experiments will be employed in mechanistic surface complexation and reactive transport models. Additionally, the mechanism of Hg uptake and retention on sulfide minerals will be explored further using the previously mentioned spectroscopic techniques. The techniques used to date appear to be promising for assessing the sequestration of Hg in complex mineral mixtures. Further work to assess fully the effects of aging is planned. Although the microprobe X-ray method has the advantage of clearly distinguishing different mineral grains, it is difficult to get a statistically representative sampling of data points because of time limitations. A bulk EXAFS technique would complement the microprobe technique; spectra of mixtures then may be deconvoluted to assess the relative contributions to Hg sequestration by different mineral substrates. The apparent ion exchange of Hg for Fe in finer-grained samples highlights the need for aging studies to assess the long-term stability of Hg sequestration on the sulfide minerals. The X-ray absorption spectroscopy data require further processing to extract all of the information in the data files. Least squares fitting algorithms of the EXAFS function will be applied to determine nearest and second-nearest neighbor atomic identities, coordination numbers, and distances from the target Hg. Comparison with theoretical models and with spectra from relevant model compounds will strengthen the analysis made on a preliminary basis by visual comparison of spectra. X-ray diffraction data will be processed using FIT2D, and results will be compared to reference minerals in the International Centre for Diffraction Data (ICDD) database.