MECHANISMS OF URANIUM ORE PASSIVATION DURING IN SITU PHOSPHATE INJECTIONS

The geochemical behavior of uranium (U) in environmental systems is governed by its redox biogeochemistry, groundwater geochemistry, and the very low solubilities of U-bearing minerals. Unreacted U ore zones continue to impact groundwater after cessation of mining as exposed U(IV)-bearing ore material (e.g., uraninite) oxidizes to release aqueous U(VI). Traditional restoration actions can be expensive and are not guaranteed to maintain low U concentrations. In situ treatment options include phosphate amendment which has been demonstrated as an effective strategy for removing aqueous U(VI) by precipitating U(VI)-phosphate phases. We present a laboratory investigation demonstrating that in addition to removing U(VI) from solution, a passivating mineral coating forms on ore surfaces during phosphate amendment, decreasing the rate of on-going U(IV) oxidative dissolution. Mechanisms of precipitation and passivation were studied using batch experiments with biogenic uraninite in an artificial groundwater and column experiments with ore zone material from an in-situ recovery (ISR) uranium mine. Introducing dissolved phosphate to artificial groundwater demonstrated swift removal (within 1-3 days) of aqueous U(VI) from solution via the precipitation of a U(VI)-bearing hydroxyapatite-like phase, which was identified using powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). In addition, uraninite oxidation rates were significantly decreased in phosphate-amended reaction vessels. Phosphate-amended artificial groundwater was also passed through a column packed with sediment from an unmined U roll-front ore zone for four weeks. Effluent U(VI) and PO₄^3- concentrations indicated that Ca-U(VI)-PO₄ precipitation throughout the sediment decreased the release rate of dissolved U(VI) by a factor of 5. This effect was sustained for more than 4 weeks after the phosphate amendment had ended. Furthermore, SEM and EDS (energy dispersive X-ray spectroscopy) scans suggest that all U-bearing particles were associated with Ca and P in post-experiment sediment, which is evidence for surface passivation. These findings provide new insight into uranium ore surface passivation and could help advance the efficacy of uranium mine remediation efforts.