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. , Grolimund, Daniel , Gustin, Mae Sexauer , Ireland, Trevor R. , Kim, Christopher S. , Rytuba, James J.
Current Investigators: Brown Jr., Gordon E. , Coolbaugh, Mark , Engle, Mark , Fitzgerald, Brian , Giglini, Anthony , Gustin, Mae Sexauer , Johnson, Stephen B. , Kim, Christopher S. , Lowry, Gregory V. , Nacht, David M. , Rytuba, James J. , Shaw, Samuel , Sladek, Chris , Slowey, Aaron J. , Vette, Alan , Zehner, Richard E.
Institution: Stanford University , University of Nevada - Reno
Current Institution: Stanford University , United States Geological Survey [USGS]
EPA Project Officer: Hiscock, Michael
Project Period: October 1, 1999 through September 30, 2002
Project Amount: $708,634
RFA: Mercury: Transport and Fate through a Watershed (1999) RFA Text | Recipients Lists
Research Category: Water and Watersheds , Mercury , Water , Safer Chemicals
Description:Extensive mining of mercury ore has left a legacy of contaminated mine wastes distributed throughout the Hg mineral belts of the United States. The problem is compounded by weathering of waste materials, which can redistribute Hg in additional chemical forms, facilitate its dispersal in watersheds or through atmospheric emissions, and increase its bioavailability to organisms. An understanding of the physical and chemical processes that control the speciation and distribution of Hg in mine wastes and its release from mine sites is critical to establishing the risk these sites pose to local and regional ecosystems. To address these issues, we propose a multidisciplinary, multi-task project that will utilize state-of-the-art spectroscopic, isotopic, and analytical methods to (1) determine the chemical and isotopic speciation of Hg in natural samples; (2) examine the transport of Hg on colloidal particles in laboratory column experiments; (3) examine sorption processes of Hg on mineral particles common in sediments downstream from mine sites, as well as the effects of the common aqueous ligands sulfate and chloride on Hg sorption processes; and (4) monitor the atmospheric emission of Hg from selected mine waste sites representing different weathering and climatic regimes with the objective of correlating emission levels with the chemical speciation of Hg in the mine wastes.
Chemical and isotopic speciation of Hg in natural samples. We propose to determine the chemical and isotopic forms of Hg associated with mining wastes from selected sites in the western U.S. using modern spectroscopic and isotopic methods, as well as other techniques such as X-ray diffraction (XRD) and electron probe microanalysis (EPMA). More specifically, we will utilize synchrotron radiation-based X-ray absorption fine structure (XAFS) spectroscopy to determine quantitatively and directly the chemical speciation of Hg in contaminated natural samples. These species may range from relatively soluble and insoluble Hg-containing solids to Hg species sorbed on particle surfaces, which may be significantly more mobile than Hg in the solid phase. Our extensive experience using XAFS spectroscopy to determine the chemical forms of contaminants like As, Se, Pb, and Hg in natural samples indicates that this technique can provide unique information on elemental speciation in complex, multiphase mixtures such as mine wastes and related sediments. The species-specific information derived from the proposed XAFS studies will place important constraints on the potential mobility and bioavailability of Hg in mining wastes. We also propose to determine the isotopic fractionation of Hg in natural samples at spatial resolutions of one µm or less using a new state-of-the-art secondary ion mass spectrometer at Stanford University. This exploratory work involves measuring Hg isotopic signatures in mine samples (including Hg isotopes in the primary ores before and after processing), the weathering products in calcine piles, the sediments in which Hg is concentrated downstream from mining sites, and selected biological organisms, particularly fish, in which Hg is bioaccumulated, and is designed to provide constraints on the sources of Hg contamination and potential changes in Hg isotopic fractionation by secondary transformation processes, including methylation by sulfate-reducing bacteria. Such speciation information should be of direct relevance to regulatory agencies in the identification and prioritization of Hg-contaminated sites requiring remediation.
Colloidal transport of Hg. Laboratory column experiments will be conducted to examine the transport of Hg by colloids (sub-micron to nanometer-scale inorganic or organic particles). Transport of Hg and other heavy metals by colloids has been implicated in a number of studies, yet little is known about the association of such metals with colloidal particles or how the particles move through the soil-water column. Our research will address the sources and characteristics of mobile colloidal particles, colloid-Hg interactions, and phenomena related to the persistence of suspended particles in aquatic systems. Chromatographic columns filled with waste-pile and sediment material will be used as porous media through which systematic variations of feed solutions will induce the release of colloids. Column effluents and mobilized colloidal particles will be analyzed using acid digestion, inductively coupled plasma (ICP), EPMA, XRD, ultrafiltration, ultracentrifugation, transmission electron microscopy, atomic force microscopy, BET surface area measurements, and light scattering. Hg sorption to and desorption from colloidal particles will be studied with macroscopic batch uptake experiments, using cold vapor atomic absorption spectroscopy to measure Hg uptake, and XAFS methods to determine the molecular-scale mode of Hg sorption. Precipitation phenomena will be assessed by thermodynamic modeling and laboratory batch reactor studies that induce precipitation. An improved knowledge of colloidal phenomena and multi-component interfacial reactions involving Hg will be critical in understanding the transport of Hg in surface environments and predicting the importance of mobile colloidal particles at various stages during the transport of Hg from mine sites.
Sorption of Hg on mineral particles. A primary mechanism by which Hg can be removed from solution is through sorption to larger solid particles, which then settle out into the sediment bed. Identifying the molecular-scale mode of Hg sorption to particle surfaces is essential in assessing the potential mobility of sorbed Hg over time. The proposed research will test the hypotheses that Hg sorbs in a strong inner-sphere fashion to iron oxyhydroxides and clays, which are commonly present downstream from Hg mine sites, and that the aqueous ligands chloride and sulfate inhibit Hg sorption through formation of stable aqueous Hg complexes. Batch sorption experiments of aqueous Hg to metal oxides are planned using goethite and -alumina, which will first be characterized using XRD, TEM and BET methods. The Hg/substrate sorption samples will then be characterized at the molecular level using XAFS spectroscopy, and the resulting data used to generate a molecular-scale model for Hg sorption. Selected sorption samples will also be characterized with TEM to determine the presence and identity of solid precipitates containing Hg. To investigate Hg sorption processes in the natural environment, samples will be collected in areas where sorption is expected; iron oxyhydroxide and aluminosilicate precipitates will be specifically targeted. After initial characterization of the natural substrates, samples containing sufficient Hg (>20-50 ppm) will be analyzed directly using XAFS methods, while those with lower levels will be used as natural substrates for Hg sorption experiments followed by XAFS studies of sorption products. Sorption experiments conducted in the presence of sulfate and chloride will determine if complexing ligands reduce sorption of Hg and/or encourage desorption by forming aqueous Hg complexes or by competing for sorption sites on mineral surfaces. These studies are necessary to understand chemically complex natural systems featuring high ligand concentrations, such as hot-spring Hg environments. Knowledge of Hg sorption processes and their consequences for the mobility and sequestration of Hg will be useful in developing future remediation strategies that employ sorption as a means of removing Hg from aqueous solutions.
Atmospheric emission of Hg. Measured emissions from Hg-contaminated mine wastes are well above those measured in background areas. Such emissions may have significant regional environmental impacts, since Hg can travel via the atmosphere to pristine uncontaminated ecosystems located far from point sources. We will test the hypothesis that atmospheric Hg emissions are closely linked to Hg speciation in mine wastes (as determined by XAFS spectroscopy), the amount of available elemental Hg, the host material (as characterized by imaging spectroscopy), and climate. The atmospheric release of Hg from contaminated mine wastes and soils (representing diverse Hg mineralogies and climatic settings and for which the Hg speciation has been determined using XAFS) will be studied by micrometeorological and flux chamber methods. Emissions will be measured from one representative locality at each site using micrometeorological methods and from multiple sites using a field flux chamber. Total gaseous Hg will be collected either on gold-coated quartz sand traps, which are analyzed by a dual amalgamation and cold vapor atomic fluorescence spectrophotometry, or determined using a Tekran Hg analyzer. In order to develop an area emission model that accounts for factors controlling Hg emissions, we will use AVIRIS (Advanced Visible Infrared Imaging Spectroscopy) data to delineate mineral alteration assemblages in Hg mining districts. The AVIRIS data will be correlated using a Geographical Information System in order to scale up Hg emissions over entire contaminated mine sites. The Hg fluxes measured and the Hg species that contribute to the total Hg flux are expected to correlate with Hg speciation at a variety of field sites. Understanding the contribution of natural Hg sources to local, regional, and global atmospheric Hg budgets will help assess whether regulatory controls on point sources will be effective.