2006 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, 2006 through March 31, 2007
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 experiments have been conducted in order to investigate the kinetic and thermodynamic parameters involved in Hg(II) immobilization by sulfide minerals. Potential candidates have been evaluated in terms of removal efficiency and capacity. It has been observed that various sulfide minerals have the potential to effectively immobilize Hg. Parameters such as pH, reaction time, and initial Hg(II) concentration were varied to determine optimal conditions. Batch experiments demonstrate that FeS, which has the greatest potential of the minerals studied, can remove more than 99 percent of Hg over the total pH range. Pyrite also exhibited great potential, with over 95 percent Hg removal throughout the pH range of typical groundwaters. For pyrite, the mechanism of removal was most likely adsorption, possibly followed by solid formation. The mechanism of Hg(II) removal by FeS(s) was most likely HgS(s) formation at neutral pH and adsorption at basic pH. Moreover, an Fe hydroxide layer is postulated to form on iron sulfides at basic pH, resulting in additional adsorption sites. Therefore, faster sorption is achieved at high pH. FeS(s) was found to be more efficient at removing Hg(II), most likely due to the formation of HgS(s).
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. Column experiments were conducted to provide insight into the environmental behavior of Hg(II) under dynamic (flow) scenarios. Furthermore, models were generated using CXTFIT to aid in the development of long-term barrier systems, such as permeable reactive barriers. Because of pyrite’s vast abundance and high capacity, it also was chosen as a potential candidate for column studies. Column studies revealed that transport of the Hg(II) was significantly retarded in the presence of pyrite, indicating its ability as a potential barrier material. Due to nonequilibrium, the local linear equilibrium (LLE) model over predicted the Hg break through; however, the presence of an irreversible fraction of Hg(II) on the pyrite acted to counteract the increased mobility. The asymmetric shape of the Hg break-through curves which is indicative of rate-limited and/or nonlinear adsorption, corresponded with the findings of the kinetic and equilibrium batch experiments.
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 of Hg between the aqueous and solid phase 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. A bulk EXAFS technique was conducted which complements the microprobe technique. Spectra of mixtures were deconvoluted to assess the relative contributions to Hg sequestration by different mineral substrates. Aging studies were also conducted to assess the long-term stability of Hg sequestration on the sulfide minerals. Least squares fitting algorithms of the EXAFS function were 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 strengthened the preliminary analysis by visual comparison of spectra. X-ray diffraction data were processed using FIT2D, and results were compared to reference minerals in the International Centre for Diffraction Data database. EXAFS analysis showed that a transformation in the coordination environment at low pH occurred during aging over two weeks, forming an ordered monolayer of Hg-Cl complexes on pyrite, while at high pH, Hg is bound to S on the pyrite surface as Hg-OH.
In order to determine the potential of FeS(s) as a barrier material for Hg(II), further column studies will be conducted under both aerobic and anaerobic conditions using this mineral. Additionally, mercury contamination is most often released as Hg0 and recent studies have detected traces of HgS(s) in samples containing Hg0 and pyrite as well as FeS(s); therefore, further experiments will be carried out using these materials. Finally, in order to assess the ability of the barrier materials to immobilize Hg effectively, column experiments will be conducted using contaminated groundwater from polluted sites.