Stephen R. Walker

Effects of Soil Composition and Mineralogy on the Bioaccessibility of Arsenic from Tailings and Soil in Gold Mine Districts of Nova Scotia

Bioaccessibility tests and mineralogical analyses were performed on arsenic-contaminated tailings and soils from gold mine districts of Nova Scotia, Canada, to examine the links between soil composition, mineralogy, and arsenic bioaccessibility. Arsenic bioaccessibility ranges from 0.1% to 49%. A weak correlation was observed between total and bioaccessible arsenic concentrations, and the arsenic bioaccessibility was not correlated with other elements. Bulk X-ray absorption near-edge structure analysis shows arsenic in these near-surface samples is mainly in the pentavalent form, indicating that most of the arsenopyrite (As(1-)) originally present in the tailings and soils has been oxidized during weathering reactions. Detailed mineralogical analyses of individual samples have identified up to seven arsenic species, the relative proportions of which appear to affect arsenic bioaccessibility. The highest arsenic bioaccessibility (up to 49%) is associated with the presence of calcium-iron arsenate. Samples containing arsenic predominantly as arsenopyrite or scorodite have the lowest bioaccessibility (<1%). Other arsenic species identified (predominantly amorphous iron arsenates and arsenic-bearing iron(oxy)hydroxides) are associated with intermediate bioaccessibility (1 to 10%). The presence of a more soluble arsenic phase, even at low concentrations, results in increased arsenic bioaccessibility from the mixed arsenic phases associated with tailings and mine-impacted soils.

Application of Synchrotron Microprobe Methods to Solid-Phase Speciation of Metals and Metalloids in House Dust

Determination of the source and form of metals in house dust is important to those working to understand human and particularly childhood exposure to metals in residential environments. We report the development of a synchrotron microprobe technique for characterization of multiple metal hosts in house dust. We have applied X-ray fluorescence for chemical characterization and X-ray diffraction for crystal structure identification using microfocused synchrotron X-rays at a less than 10 μm spot size. The technique has been evaluated by application to archived house dust samples containing elevated concentrations of Pb, Zn, and Ba in bedroom dust, and Pb and As in living room dust. The technique was also applied to a sample of soil from the corresponding garden to identify linkages between indoor and outdoor sources of metals. Paint pigments including white lead (hydrocerussite) and lithopone (wurtzite and barite) are the primary source of Pb, Zn, and Ba in bedroom dust, probably related to renovation activity in the home at the time of sampling. The much lower Pb content in the living room dust shows a relationship to the exterior soil and no specific evidence of Pb and Zn from the bedroom paint pigments. The technique was also successful at confirming the presence of chromated copper arsenate treated wood as a source of As in the living room dust. The results of the study have confirmed the utility of this approach in identifying specific metal forms within the dust.

The effect of ore roasting on arsenic oxidation state and solid phase speciation in gold mine tailings

Metallurgical pretreatment of As-bearing ores involves oxidation of sulphides (most often As-bearing pyrite, arsenopyrite or enargite) resulting in complex oxidized As-bearing products. We have evaluated roasting pretreatment of arsenic-bearing ores in a broad context and related this to the specific operations at the Giant mine, Yellowknife, NWT, Canada, which roasted arsenopyrite (FeAsS)-rich gold ore concentrates during 50 years of operations. A large portion of the As was collected and stored in underground vaults as As2O3 dust; however, some of the As was also released with tailings which contain concentrations between 1000 to 5000 ppm. Powder X-ray diffraction (XRD) and sequential extractions have been completed on samples of mill products and various ages of tailings at the Giant mine. These data along with petrographic and synchrotron μXRD and μX-ray absorption near-edge spectroscopy (μXANES) indicate that the largely oxidized roaster products (calcine) and electrostatic precipitator (ESP) dust host most of the As in the tailings with a lesser component of sulphide arsenic. The fine-grained nature of these oxidized products has led to hydraulic sorting within the tailings impounds and dispersal to downstream creek and lake sediments.

Identification and Characterization of Arsenic and Metal Compounds in Contaminated Soil, Mine Tailings, and House Dust Using Synchrotron-Based Microanalysis

ABSTRACT A comprehensive understanding of the risk associated with metal-rich soils and other materials includes identification of the solid phases hosting the metals. Synchrotron microanalysis provides a powerful diagnostic tool to characterize metal-bearing particles in mine tailings, soils, lake sediments, windblown dust, and household dust. A near simultaneous combination of X-ray fluorescence, diffraction, and absorption experiments using a microfocused beam can provide information on elemental concentrations, crystal structure, and oxidation state of individual particles. This approach can distinguish multiple metal-hosting minerals and industrial compounds in a single sample. Our objective is to provide examples of the application of this technique to a range of materials representing potential risk to human or ecosystem health. These examples include arsenic-contaminated materials and metal-rich household dust. We have identified grains of scorodite and other arsenate minerals in mine tailings and associated airborne dust, arsenic trioxide in organic soils near an ore roaster, metallurgical products dispersed to the environment, and various metal-rich particles in household dust. A comparison of chemical analysis of individual particles using electron microprobe analysis and synchrotron-based X-ray fluorescence analysis is provided.