Molybdenum Speciation in Uranium Mine Tailings Using X-Ray Absorption Spectroscopy

Abstract

Uranium (U) mill tailings in northern Saskatchewan, Canada, contain elevated concentrations of molybdenum (Mo). The potential for long-term (>10,000 years) mobilization of Mo from the tailings management facilities to regional groundwater systems is an environmental concern. To assist in characterizing long-term stability, X-ray absorption spectroscopy was used to define the chemical (redox and molecular) speciation of Mo in tailings samples from the Deilmann Tailings Management Facility (DTMF) at the Key Lake operations of Cameco Corporation. Comparison of Mo K near-edge X-ray absorption spectra of tailings samples and reference compounds of known oxidation states indicates Mo exists mainly as molybdate (+6 oxidation state). Principal component analysis of tailings samples spectra followed by linear combination fitting using spectra of reference compounds indicates that various proportions of NiMoO(4) and CaMoO(4) complexes, as well as molybdate adsorbed onto ferrihydrite, are the Mo species present in the U mine tailings. Tailings samples with low Fe/Mo (<708) and high Ni/Mo (>113) molar ratios are dominated by NiMoO(4), whereas those with high Fe/Mo (>708) and low Ni/Mo (<113) molar ratios are dominated by molybdate adsorbed onto ferrihydrite. This suggests that the speciation of Mo in the tailings is dependent in part on the chemistry of the original ore.

Kinetics of Molybdenum Adsorption and Desorption in Soils

“…In heterogeneous soil systems, nonequilibrium conditions of Mo adsorption may be related to mechanisms of Mo kinetics because of (i) heterogeneity of sorption sites having different affinities (e.g., Fe or Al oxides, minerals, and organic matter) (Goldberg et al, 1996), and (ii) slow precipitation on mineral surface due to an increase in surface charge on the inner sphere. Common surface precipitates in the environment include powellite (CaMoO 4 ), ferrimolybdite [Fe 2 (MoO 4 ) 3 ⋅8H 2 O], molybdenite (MoS 2 ), and ilsemannite (Mo 3 O 8 ) (Gustafsson, 2003; Essilfie‐Dughan et al, 2011). A third reason is slow diffusion to sites within the soil matrix via intraparticle micropores (Lang and Kaupenjohann, 2003).…”

XAS studies of Se speciation in selenite-fed rats

“…42-44 This process is the preferred method of analysing XANES spectra and it allows the quantification of different chemical species in individual samples. However, the method was not effective with this dataset.…”

Biogeochemical activity of microbial biofilms in the water column overlying uranium mine tailings

“…; Essilfie‐Dughan et al . , ; Shaw et al . ) and Mg‐Al hydrotalcite at alkaline oxic conditions is well understood in the DTMF system (Gomez et al .…”

“…Geochemical controls and spatial/temporal distribution of elements of concern have previously been investigated within the DTMF tailings (Essilfie‐Dughan et al . ; Shaw et al . ) where it has been shown that highly alkaline and oxic conditions exist throughout the deposited tailings mass.…”