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TitleMineralogy and bioaccessibility of arsenic in mine tailings and soil
AuthorWalker, S R; Meunier, L; Jamieson, H E; Parsons, M B; Koch, I; Reimer, K R
SourceGeological Association of Canada-Mineralogical Association of Canada, Joint Annual Meeting, Abstracts Volume vol. 33, 2008 p. 180
Alt SeriesEarth Sciences Sector, Contribution Series 20080490
MeetingJoint Meeting of the Geological Association of Canada, Mineralogical Association of Canada, Society of Economic Geologists and the Society for Geology Applied to Mineral Deposits; Québec City; CA; May 26-28, 2008
Mediapaper; digital; on-line
ProvinceNova Scotia
Subjectsmineralogy; environmental geology; Health and Safety; biogeochemistry; biogeology; biogeochemical surveys; arsenic; arsenic geochemistry; tailings; tailings analyses; tailings geochemistry; tailings disposal; environmental analysis; environmental studies; environmental impacts; health hazards; bioaccessibility; human health
AbstractThe risk to human health associated with ingestion of arsenic from publicly-accessible abandoned gold mine tailings in Nova Scotia is expected to be influenced by the mineralogy, grain size, and texture of arsenic-bearing particles. Oxidation of naturally occurring arsenopyrite (FeAsS) has produced a wide variety of secondary minerals. The goal of this research is to examine the links between sample chemistry, mineralogy, texture and arsenic bioaccessibility in a set of 29 samples from six sites. We have developed a micro-analytical method to characterize complex samples at the micron scale combining petrography, electron microprobe, and synchrotron-based grain-by-grain microXANES and microXRD. Application of this novel method has demonstrated that these materials typically contain four or more arsenic-bearing minerals in each sample. These include: sulfides (arsenopyrite and pyrite), Fe-arsenates (scorodite, kankite and amorphous forms), Ca-Fe-arsenates (yukonite and amorphous forms), As-bearing Fe-oxyhydroxides (goethite, lepidocrocite, akaganeite and amorphous forms), tooeleite (ferric arsenite-sulfate) and roaster-generated As-bearing Fe-oxides (hematite and maghemite). At the grain scale, arsenic-bearing phases may exist as discrete grains, coatings, cements, or complex mixtures of these various phases.

The bioaccessibility of arsenic in the samples, as determined by application of a physiologically-based extraction test (PBET), ranges from 0.5 to 48.5% of total As. The variations in bioaccessibility may be controlled by the relative proportions of the arsenic-bearing phases. The higher percent arsenic bioaccessibility values are associated with the presence of Ca-Fe-arsenates. The samples containing arsenic predominantly in the form of arsenopyrite and scorodite return the lowest percent arsenic bioaccessibility. Furthermore, a higher arsenic concentration does not necessarily result in a higher percent bioaccessibility. Identification of specific mineral forms present in a given sample allows a comparison of the geochemical stability of these minerals under the influence of the PBET to the broader geochemical knowledge of mineral stability and kinetic behaviour in the natural environment. For example, the very low bioaccessibility of scorodite is consistent with its known stability at low pH (~2 and in the range of the stomach leach of the PBET). We infer sluggish kinetics with or without transformation of scorodite to a more arsenic-poor amorphous form in the subsequent pH neutral intestinal leach of the PBET, as the reason for the relatively small release of arsenic. Overall, the secondary arsenic minerals which develop in response to local (bio)geochemical conditions can be a predictor of arsenic bioaccessibility.