IN SITU PASSIVATION OF URANIUM ORE MATERIAL SURFACES WITH URANYL PHOSPHATE PRECIPITATION

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 as a means of removing U(VI) from mine-impacted groundwater by precipitating solid-phase uranyl phosphate minerals is currently being explored as a remediation strategy. However, this strategy may potentially have the additional benefit of limiting further U(VI) release through the generation of passivating mineral coatings on U source minerals. We present results of a laboratory investigation on the passivation and prevention of U(VI) oxidative dissolution by phosphate treatment phases. Initial experiments have revealed complete oxidation of a biogenic uraninite slurry within three 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 co-precipitation of hydroxyapatite, as identified by powder X-ray diffraction. Precipitating hydroxyapatite in the presence of biogenic uraninite has resulted in a greatly reduced rate of U(IV) oxidative dissolution. Finally, we assess the stability of the U(VI) extracted from solution to form the mineral coating, as well as that of the U(IV) trapped in the passivated uraninite. These findings provide new insight into U ore surface passivation, and could help advance the efficacy of in situ mine remediation efforts.