Uranium (U) ore zones in abandoned mines continue to impact groundwater systems as exposed U(IV)-bearing ore material (e.g. uraninite) oxidizes to aqueous U(VI). The addition of phosphate to uranium-rich mine systems is often employed to remove aqueous U(VI) from solution by precipitating solid-phase uranyl phosphate minerals. We present a laboratory investigation of a remediation technique designed to remove U(VI) from mine-impacted groundwater while precipitating a passivating mineral coating onto ore surfaces, the latter aimed at reducing the rate of future U(IV) oxidative dissolution. Initial experiments have revealed complete oxidation of a biogenic uraninite slurry within two weeks of atmospheric exposure without added phosphate. Batch experiments introducing phosphate to a synthetic U(VI)-contaminated groundwater demonstrated swift removal (within 1-2 hours) of aqueous U(VI) from solution by the precipitation of a microcrystalline Ca-U(VI)-PO4 phase, as identified by powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). The microcrystalline Ca-U(VI)-PO4 phase matures over several weeks to form a more ordered coating on uraninite surfaces. The formation of a Ca-U(VI)-PO4 coating on uraninite significantly reduced the rate of U(IV) oxidative dissolution. Finally, we assessed the stability of the uranyl phosphate coating and passivated uraninite to oxidative dissolution. Column experiments quantified the release of U(VI) from these materials by simulating the return to mine conditions after the cessation of phosphate treatment. These findings provide new insight into uranium ore surface passivation, and could help advance the efficacy of in situ mine remediation efforts.

Passivation of uranium ore material surfaces with uranyl phosphate precipitation

253rd National Meeting (2017)

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