Arsenic-rich uranium mine tailings from the Rabbit Lake in-pit tailings management facility (RLITMF) in northern Saskatchewan, Canada, were investigated to determine the mineralogy and long-term stability of secondary arsenic precipitates formed from iron-rich hydrometallurgical solutions. Total arsenic and iron concentrations in six iron-rich samples of the mine tailings ranged from 56 to 6000 μg/g and from 12 600 to 30 200 μg/g, respectively (Fe/As molar ratios of 5.3-303). On the basis of stability field diagrams generated from pH, E{sub}h, and temperature measurements on tailings samples (mean values of 9.79, +162 mV, and 2.8℃, respectively), it was concluded that arsenic and iron in the tailings were stable as As{sup}5+ and Fe{sup}3+. Synchrotron-based X-ray absorption spectroscopic studies of tailings samples, fresh mill precipitates, and reference compounds showed that the arsenic in iron-rich areas of the tailings existed as the stable As{sup}5+ and was adsorbed to 2-line ferrihydrite through inner-sphere bidentate linkages. Furthermore, under the conditions in the RLITMF, the 2-line ferrihydrite did not undergo any measurable conversion to more crystalline goethite or hematite, even in tailings discharged to the RLITMF 10 yr prior to sampling.
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