Geochemical characteristics of aluminum and magnesium secondary mineral phases in uranium mill tailings

The Athabasca Basin in northern Saskatchewan, Canada is a major source of uranium (U) and an important economic driver for the province and country. The Athabasca Basin U deposits contain elevated concentrations of As, Se, Mo, Ni, and 226Ra (elements of concern; EOCs). The Key Lake U mill uses a stepwise Ca(OH)2 neutralization process (pH 4.0, 6.5, 9.5, and 10.5) to precipitate EOCs from raffinate (acidic, metal-rich wastewater) prior to releasing the effluent to the environment. The neutralization process precipitates a complex mixture of secondary minerals concentrated with EOCs that are deposited to an in-pit tailings management facility (TMF) at pH ≈ 10.1. Extensive studies show adsorption to ferrihydrite is a primary control on aqueous EOC concentrations. Recent studies suggest poorly characterized Al and Mg precipitates could also influence EOC concentrations. The objectives of this thesis were to characterize the Al and Mg secondary mineralogy in the Key Lake neutralization process and quantify controls exerted by these minerals on EOC concentrations. Additionally, a review of all geochemical literature pertaining to in-pit TMFs in the Athabasca Basin was conducted to synthesize the collective science. Laboratory and mill samples were used for the study. The Al and Mg secondary minerals identified were amorphous AlOHSO4 (a hydrobasaluminite-like phase, existing below neutral pH), amorphous Al(OH)3, MgAl-hydrotalcite, and Al-substituted ferrihydrite. In the absence of ferrihydrite, Al and Mg mineral phases sequestered As, Se, Mo, and Ni. Batch adsorption experiments showed MgAl-hydrotalcite adsorbs As, Se, and Mo to the same order of magnitude as ferrihydrite. Arsenic and Ni XAS experiments showed Al and Mg minerals sequester As and Ni in concert with ferrihydrite. Arsenic(V) formed bidentate-binuclear complexes on the surface of ferrihydrite and amorphous Al(OH)3. Hydrotalcite adsorbed As(V) on the mineral surface and/or within the mineral interlayer through bidentate complexation. Nickel was adsorbed to amorphous Al(OH)3 through edge-sharing bidentate-mononuclear complexes and precipitated as a Ni-Al layered double hydroxide at pH 10.5. These results demonstrate that mineralogically complex tailings rich in Fe, Al, and Mg exert multiple mechanisms of controls on EOC solubility and further the understanding of the long-term fate of EOCs in tailings