2000 Progress Report: Processes Controlling the Chemical/Isotopic Speciation and Distribution of Mercury from Contaminated Mine SitesEPA Grant Number: R827634
Title: Processes Controlling the Chemical/Isotopic Speciation and Distribution of Mercury from Contaminated Mine Sites
Investigators: Brown Jr., Gordon E. , Rytuba, James J. , Kim, Christopher S. , Gustin, Mae Sexauer , Lowry, Greg
Current Investigators: Brown Jr., Gordon E. , Johnson, Stephen B. , Zehner, Richard E. , Slowey, Aaron J. , Rytuba, James J. , Nacht, David M. , Kim, Christopher S. , Gustin, Mae Sexauer , Lowry, Gregory V. , Vette, Alan , Giglini, Anthony , Fitzgerald, Brian , Sladek, Chris , Engle, Mark , Coolbaugh, Mark , Shaw, Samuel
Institution: Stanford University , United States Geological Survey , University of Nevada - Reno
Current Institution: Stanford University , United States Geological Survey
EPA Project Officer: Chung, Serena
Project Period: October 1, 1999 through September 30, 2002
Project Period Covered by this Report: October 1, 1999 through September 30, 2000
Project Amount: $708,634
RFA: Mercury: Transport and Fate through a Watershed (1999) RFA Text | Recipients Lists
Research Category: Watersheds , Heavy Metal Contamination of Soil/Water , Water , Safer Chemicals
Objective:The major objectives of the project are to: (1) determine the chemical speciation and relative abundance of different forms of mercury in mine wastes; (2) investigate the role of colloidal transport as a mechanism for dispersal of mercury from waste sites; (3) identify the mode of mercury sorption in downstream sediments and fine-grained precipitates in the presence of common complexing ligands; (4) determine the effects of aqueous complexing ligands on the desorption and sorption inhibition of mercury; (5) measure and correlate the emission of mercury into the atmosphere with mercury speciation, climate, and geologic factors; and (6) examine mercury isotope fractionation as a potential means of identifying mercury from specific localities and correlate the information on chemical and isotopic speciation of mercury along the various pathways by which mercury may travel. Substantial progress has been made in all six areas of the project as detailed below.
Speciation. Speciation of mercury-bearing samples using x-ray absorption fine structure (XAFS) spectroscopy was conducted on mine waste materials from several mines in the California Coast Range mercury mineral belt and Nevada. Mercuric sulfide, present either as cinnabar (HgS, hex) or metacinnabar (HgS, cub), is the dominant mercury species in all samples; this is consistent with the fact that cinnabar is the primary ore mineral in mercury deposits. Higher than expected proportions of metacinnabar in mining wastes may have resulted from the ore roasting process, which heated ore in excess of the cinnabar-metacinnabar inversion temperature of 345?C. Several minor mercury species also have been identified in the samples, including montroydite (HgO), schuetteite (HgSO4), and a variety of Hg-Cl phases. These species are likely to be disproportionately large contributors of mercury to the surrounding environment due to solubility levels, which are orders of magnitude higher than those of the mercuric sulfides under typical surface oxidizing conditions. Mercuric chloride species were identified only in calcines generated from hot-spring mercury deposits, consistent with elevated levels of chloride in these local hydrothermal systems. In contrast, samples collected from silica-carbonate mercury deposits, where high chloride levels are absent, are distinctly lacking in Hg-Cl phases as determined by XAFS analysis. These results show both a dependence of mercury speciation on the geological origin of the initial mercury ore and the sensitivity of XAFS in distinguishing between samples from the different ore types.
Colloidal Transport of Mercury. Laboratory column experiments examining the transport of Hg by colloids were performed using calcines from the New Idria Hg mine in central California. Calcines were sieved, and the physical/chemical characteristics of different size fractions were analyzed using cold vapor atomic fluorescence (CVAFS), x-ray diffraction (XRD), BET surface area analysis, laser diffraction particle sizing, transmission electron microscopy (TEM/EDAX), transmission FTIR, and EXAFS. BET surface area and Hg concentration increases with decreasing particle size, ranging from 10 m2/g (0.035 percent w/w Hg) for the larger size fraction (2000mm>x>500mm) to 19 m2/g (0.077 percent w/w Hg) for the smallest size fraction (x<45mm). XRD spectra indicate that the sieved fractions predominantly consist of quartz, jarosite, and hematite, with the fraction of quartz decreasing with decreasing particle size. TEM analysis on the x<45 mm size fraction also indicates an amorphous Si/Al phase. FTIR spectra support the presence of jarosite. Chromatographic columns filled with calcines (2000mm<x<500mm size fraction) released significant quantities of colloids on chemical perturbation. The columns were leached with 80 pore volumes of 0.45 M CaCl2, followed by 140 pore volumes of 0.5 M NaCl. Colloid generation was then initiated by leaching with 20 mM NaCl. A pH buffer (15 mM malonic acid, pH = 5.8) and biocide (1 mM sodium azide) were present in all steps. Released colloid particles were predominantly in the 50 nm to 400 nm size range (determined by laser diffraction). TEM/EDAX analysis indicates that colloids consist primarily of crystalline jarosite and iron oxide, as well as an amorphous Si/Al phase. The colloids, therefore, are similar in composition to the bulk material. Total Hg concentration in the column effluent is less than 75 mg/L prior to colloid generation, but greater than 2,000 mg/L in column effluent containing colloidal particles, indicating that mercury associated with colloids may be a significant transport mechanism.
Sorption Processes. Macroscopic uptake studies of Hg(II) to goethite (a-FeOOH) sorption samples show that Hg(II) sorbs strongly to goethite over a pH range of 4.3-7.4; XAFS spectra and the corresponding Fourier transforms of the sorption products are consistent over this pH range. Analysis indicates the presence of iron as a second-shell backscattering atom and the likely presence of Hg(II) as a bidentate inner-sphere sorption complex, forming a corner-sharing arrangement with the Fe(O, OH)6 octahedra of the goethite surface. By comparison, Hg(II) sorbs relatively weakly to g-alumina (Al2O3) from pH 5.2-7.8. XAFS spectra show a distinct change in sorption mode at pH = 6.2. Experiments devised to test the possibility of alumina dissolution and reprecipitation indicate the possibility of two distinct sorption modes in this system: (1) sorption of Hg(II) directly onto g-alumina; and (2) sorption of Hg(II) onto an Al-hydroxide precipitate such as bayerite, b-Al(OH)3, a phase that is known to form at the thermodynamically unstable surface of g-alumina.
Effects of Complexing Ligands on Hg(II) Sorption. Hg(II) uptake to both goethite and g-alumina is strongly reduced as the concentration of chloride in the system increases at pH 6. XAFS spectra and Fourier transforms of the Hg(II)-chloride-goethite system show second-shell features at increasing chloride concentrations that can be fitted as Cl and Fe neighbors. This would indicate the formation of ternary surface complexes, wherein the HgClOH aqueous species forms an inner-sphere bonding arrangement with the goethite surface, as also suggested by thermodynamic constraints. Changes in the sorption mode of Hg(II) sorbed to g-alumina in the presence of chloride are unclear from the EXAFS spectra due to low Hg coverage. However, it appears that the precipitate formation observed in the ligand-free system is suppressed with increasing chloride concentrations.
In contrast to the effects of chloride, sulfate ligands appear to mildly enhance Hg(II) sorption to both goethite and g-alumina at pH 6. Although the EXAFS and FTs of the Hg(II)-sulfate-goethite system show no discernible changes with increasing sulfate concentration, Hg(II) sorption to g-alumina shows a clear trend away from the dual-sorption mode proposed in the ligand-free Hg(II)/alumina system with increasing concentrations of sulfate. One possible reason for this difference is that precipitate formation is suppressed as a result of the increased ionic strength in solution, which through the common ion effect may increase the solubility of this phase. Therefore, at high sulfate concentrations, Hg(II) is expected to sorb primarily to the g-alumina substrate and not to some secondary precipitate. The slight increase in Hg(II) sorption with increasing concentrations of sulfate implies that some association between Hg(II) and sulfate occurs at the surface which therefore enhances uptake, but that this effect is too small to be easily detected with EXAFS spectroscopy.
Atmospheric Emissions of Mercury. In the first year, atmospheric mercury emissions were measured and scaled up for five sites, the Sulphur Bank Superfund Site, CA (n = 22 sites); the Carson River Superfund Site, NV (n = 25); the New Idria District, CA (n = 40); the Ivanhoe District, CA (n = 29); and the Knoxville District, CA (n = 86). All but the Carson River Superfund Site represent areas that are naturally enriched in mercury and have historic mercury mines. The Carson River Superfund Site had mercury imported for gold amalgamation. Both micrometeorological methods and field flux chambers were used to make in situ flux measurements. Area emissions for all of these sites have been estimated using scaling techniques developed as part of this and other projects. Scaling was done primarily using in situ flux measurements, substrate mercury concentrations, and geologic characteristics of each area relevant to mercury mineralization. At the Ivanhoe and New Idria districts, emissions have been scaled up for areas of anthropogenically disturbed substrate associated with mining and from the surrounding undisturbed terrain. For the Carson River and Sulphur Bank Superfund Sites, emissions have been estimated for the areas of mine waste only and more information will be collected in Year 2 to allow for estimation of emissions from the surrounding areas. Some scaling has been done for the Knoxville district and the surrounding area, but this needs to be fine tuned in Year 2 through the collection of additional field data.
At the New Idria mining district, the effect of capping mine waste is being investigated with noncontamined substrate as a method of reducing emissions. Three trips have been made to the district to measure emissions from mine waste in various stages of remediation.
To link mercury speciation with emissions, mercury emissions are being measured using a single pass gas exchange system at UNR from samples that have had mercury speciation determined using EXAFS. The gas exchange chamber allows for the measurement of mercury emissions under precisely controlled conditions. Work has begun determining emissions from approximately15 different substrate exposed to different temperature and light regimes, and ambient air chemistry. These experiments will help us to understand the role of specific mercury species on controlling emissions.
Some general conclusions regarding emissions from mine waste that have been developed so far include the following: (1) substrate mercury concentration is a dominant factor controlling emissions; (2) land disturbance exacerbates emissions; (3) vegetation of contaminated sites as a form of remediation may reduce emissions; and (4) the total annual emissions from naturally Hg-enriched undisturbed substrate surrounding areas of mining and anthropogenic disturbance are typically significantly higher than emissions from the relatively small mining disturbed areas.
Components of two different methods, tubular denudars and annular denudars, used for measurement of reactive gaseous mercury have been purchased. One method, tubular denudars, was applied at Sulphur Bank in the summer of Year 1. Both methods will be applied in Year 2. Annular denudars are endorsed by researchers with the U.S. Environmental Protection Agency at Research Triangle Park.
Isotopic Fractionation of Mercury. Mercury isotopes were measured using the SHRIMP RG ion microprobe at Stanford University. Mercury isotope compositions of cinnabar (mercury sulfide) from four different mercury ore deposits were measured to determine the isotopic variability among the ore deposits. Cinnabar from the Almaden mercury deposit in Spain, the largest mercury deposit in the world, was analyzed as well as cinnabar samples from the Harrison and New Almaden deposits in California, and the Nikitovka deposit in the former Soviet Union. The isotopic composition of the secondary ionization beam was demonstrated to vary over time when the beam was maintained on the same sample spot. To overcome this problem, data were obtained at a given spot only for the period of time that the secondary ionization beam remained constant. The beam was then moved to a new spot, and data were gathered. This process was repeated until sufficient readings were obtained to meet the minimum number of data points necessary to fulfill the requirements of counting statistics. Based on this approach, isotopic compositions were measured with a standard error that ranged from 0.6 to 1 per mil. The Almaden cinnabar samples were shown to be isotopically heterogeneous, so this material does not provide a useful standard to compare isotopic fractionation. The measured weighted mean isotopic ratio of 201Hg/202Hg is: 0.4449 for New Almaden; 0.4453 for Harrison; 0.4464 for Nikitovka; and 0.4456 for Almaden. The maximum isotopic variability among the four deposits is 1.5 per mil, indicating that there is potentially sufficient difference in the isotopic signature of mercury ore deposits to use mercury isotopes to fingerprint the mercury in a deposit.
Future Activities:Excellent progress is being made in each of the six areas of research discussed above. The current research activities will continue in all six areas. The colloidal transport component of our research looks particularly promising. A major effort will be made in Year 2 to study the effects of different types of chemical perturbations on colloidal generation and to identify the types of colloidal particles with which mercury is primarily associated. The characterization studies of the colloidal particles generated in our column experiments using analytical transmission electron microscopy at the National Center for Electron Microscopy at Lawrence Berkeley National Laboratory are generating very useful information on the identity of the colloidal particles and their size, shape, and distribution. The EXAFS spectroscopy studies of Hg(II) speciation in mining wastes have yielded unique information on the types of Hg(II) species present. Comparisons of speciation information from these spectroscopic studies with that from selective chemical extractions performed by Nicholas Bloom and Coworkers at Frontier Geosciences, Seattle, WA, on the same samples are showing good agreement in most cases. The EXAFS spectroscopy studies of Hg(II) sorption processes, including the effects of complexing ligands, are yielding important molecular-level information on sorption reaction products and indicate that sorption is a likely mechanisms for sequestering Hg(II) under some conditions. The isotopic fraction studies have shown that we can achieve a precision level that may allow the detection of different Hg isotopic signatures for different types of mercury deposits.
Mae Gustin and students plan to: (1) complete field work and scaling of emissions for the area encompassing the Knoxville, Wilbur Springs, Clear Lake, and Sulphur Bank Superfund Sites. Scale emissions for the area surrounding the Carson River Superfund Site; (2) quantify the atmospheric mercury species (elemental mercury or Hg2+) being emitted from select field sites; (3) measure mercury emissions from mine sites not associated with economic concentrations of mercury such as precious metal mineralization, massive sulfide mineralization and porphyry copper mineralization; (4) measure mercury emissions from either an anthropogenically contaminated area or naturally enriched area in a humid landscape; and (5) finish laboratory work assessing role of mercury speciation in controlling emissions.
Journal Articles on this Report : 3 Displayed | Download in RIS Format
|Other project views:||All 93 publications||28 publications in selected types||All 27 journal articles|
|| Engle MA, Gustin MS, Zhang H. Quantifying natural source mercury emissions from the Ivanhoe Mining District, north-central Nevada, USA. Atmospheric Environment 2001;35(23):3987-3997.
||Gustin MS, Lindberg SE, Austin K, Coolbaugh M, et al. Assessing the contribution of natural sources to regional atmospheric mercury budgets. Science of the Total Environment 2000; 259(1): 61-71.||
||Kim CS, Brown Jr GE, Rytuba JJ. Characterization and speciation of mercury-bearing mine wastes using X-ray absorption spectroscopy (XAS). Science of the Total Environment 2000;261(1-3):157-168.||