Persistent U(IV) and U(VI) following in-situ recovery (ISR) mining of a sandstone uranium deposit, Wyoming, USA

Abstract

Drill-core samples from a sandstone-hosted uranium (U) deposit in Wyoming were characterized to determine the abundance and distribution of uranium following in-situ recovery (ISR) mining with oxygen-and carbon dioxide-enriched water. Concentrations of uranium, collected from ten depth intervals, ranged from 5 to 1920 ppm. A composite sample contained 750 ppm uranium with an average oxidation state of 54% U(VI) and 46% U(IV). Scanning electron microscopy (SEM) indicated rare high uranium (~1000 ppm U) in spatial association with P/Ca and Si/O attributed to relict uranium minerals, possibly coffinite, uraninite, and autunite, trapped within low permeability layers bypassed during ISR mining. Fission track analysis revealed lower but still elevated

Biogenic non-crystalline U(IV) revealed as major component in uranium ore deposits

“…Both the sequential extraction and the shell-by-shell EXAFS data suggest that a large fraction of U in the solid phase is bound to the organic sediment fraction (representing 3.6-4.9 % of the dry sediment w/w, Supplementary Table 4), represented by oxalate-like functional groups. Previous characterization of post-mining ore zone samples also showed association between U and carbonaceous material in the deposit24333435. Thus, the results presented show direct evidence for the presence of non-crystalline U (IV) within undisturbed roll-front deposits, a revision to the established paradigm2936.…”

“…However, non-crystalline U (IV) lacks long-range order, thus eluding identification via standard mineralogical characterization 17 . We combine advanced techniques in order to fully probe the potential presence of biogenic non-crystalline U (IV) in an unmined ore zone located 200 m below ground surface (m-bgs): X-ray absorption spectroscopy (XAS) was used to determine the valence state and the average local atomic coordination environment of U; sequential extractions were used to determine the sediment fractions hosting U (refs 30 , 31 ); a multicollector inductively coupled plasma mass spectrometer was used to measure the 238 U/ 235 U isotope ratio to discriminate between abiotic and biotic U transformation mechanisms 27 ; and DNA-based (16S rRNA) microbial community analysis was performed in order to characterize its diversity and metabolic potential 32 33 ( Supplementary Table 1 ). The experimental findings for this deposit may help shed light on other deposits of the same type and augment our current understanding of roll-front deposit formation.…”

To the Question of Recovery of Uranium From Raw Materials

“…The most common in the ores of almost all genetic types are simple oxides of U (IV). Minerals of hexavalent uranium are of lesser industrial importance [1][2][3].…”

Potential aquifer vulnerability in regions down-gradient from uranium in situ recovery (ISR) sites

“…Parameters related to geochemical rates are generally lacking, particularly kinetic data on mineral-water reactions involving metals and metalloids. Petrographic, mineralogical, and geochemical studies of core samples from ISR sites are needed to feed into the model architecture (see WoldeGabriel et al, 2014;Gallegos et al, 2015).…”

“…Several approaches have been used to restore groundwater quality, including groundwater sweeping, reverse osmosis treatment, subsurface injection of chemical reductants, bioremediation, and monitored natural attenuation (MNA) (e.g., Borch et al, 2012;Gallegos et al, 2015). Groundwater sweep restoration involves pumping one or more pore volumes from the targeted mining zone followed by treatment or disposal of the extracted water.…”