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SEQUESTRATION OF SUBSURFACE ELEMENTAL MERCURY (HG0)
Impact/Purpose:
The primary goal of this proposal is to develop an improved understanding and predictive capability for the in situ abiotic immobilization of subsurface elemental mercury (Hg0) using sulfide minerals. Specific objectives are to 1) elucidate the fundamental thermodynamic and kinetic parameters that control the partitioning (uptake and release) of Hg to these materials; 2) 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; 3) probe the immobilized Hg with state-of-the-art environmental spectroscopic techniques to determine the mechanism(s) responsible for immobilization; and 4) 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 time 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.
Description:
Elemental mercury (Hg0) is a metal with a number of atypical properties, which has resulted in its use in myriad anthropogenic processes. However, these same properties have also led to severe local subsurface contamination at many places where it has been used. As such, we studied the influence of various parameters on Hg(II) sorption onto pyrite (pH, time, Hg(II) concentration), a potential subsurface reactive barrier. Batch sorption studies revealed that total Hg(II) removal increases with both pH and time. X-ray absorption spectroscopy analysis showed that a transformation in the coordination environment at low pH occurred during aging over two weeks, to form an ordered monolayer of monodentate Hg-Cl complexes on pyrite. In column studies packed with pure quartz sand, the transport of Hg(II) was significantly retarded by the presence of a thin pyrite-sand reactive barrier, although dissolved oxygen inhibited Hg(II) sorption onto pyrite in the column.
Batch experiments were conducted in order to investigate the kinetic and thermodynamic parameters involved in Hg(II) immobilization by sulfide minerals. Potential candidates were evaluated in terms of removal efficiency and capacity. Parameters such as pH, reaction time, and initial Hg(II) concentration were varied to determine optimal conditions. Batch experiments demonstrate that FeS(s), which has the greatest potential of the minerals studied, can remove more than ninety-nine percent of Hg over the total pH range. Pyrite also exhibited great potential, with over ninety-five 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 provided important information with respect to the kinetic and thermodynamic properties of the 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 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 permitted 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 provided semi-quantitative 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, facilitated meaningful characterization of the materials in the same form as they occur in the environments of interest. Microbeam facilities allowed targeted analysis of small particles that were of particular interest for understanding partitioning of Hg between the aqueous and solid phase Spectroscopic studies revealed the clear association of Hg with both Fe- and Mn-sulfides and demonstrated 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 the 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.
Mercury contamination exists in the environment primarily as Hg0; therefore column experiments were also conducted in which liquid Hg0 [versus dissolved Hg(II)] was added directly to the columns. FeS(s) was used as a subsurface reactive barrier in Hg0 contaminated columns, because it was found to be more efficient than pyrite at removing Hg. The barrier significantly removed Hg0 contamination from the column effluent concentrations. X-Ray diffraction data revealed that HgS(s) formed in the FeS(s) barrier.