SPECIATION OF URANIUM IN SEDIMENTARY PHOSPHORITES ASSOCIATED WITH U-AFFECTED GROUNDWATER

Elevated alpha radiation and uranium occur in groundwater of a carbonate-dominated fractured rock aquifer in the Champlain Valley of NW Vermont, and geologic mapping indicates a strong spatial association with dolostones of the late Cambrian Clarendon Springs (Ccs) Formation. Of 131 wells tested within the 12 km² area, 30 contained levels of alpha radiation exceeding the EPA’s Maximum Contaminant Level (MCL) of 15 pCi/L. Of wells that exceeded the alpha MCL, 90% also exceeded the Vermont MCL for uranium of 20 mg/L. Uranium in the CCs occurs in sedimentary breccias and thin wispy bedding bedding planes comprised of black phosphorite which contains up to 420 mg/kg U. X-ray diffraction of the black phosphorites indicates that they are dominated by flouroapatite, a mineral which has the potential to host U(IV) substituted for calcium. New high-resolution XRD analysis also confirms the presence of meta-autunite, a uranium calcium phosphate which contains uranium in the U(VI) oxidation state (i.e. the oxidized form of U). SEM-EDS reveals that uranium is heterogeneously distributed within the black phosphorite clasts and layers and is marked by 5-10 micron “blebs” that contain stoichiometries consistent with meta-autunite. Many of these occur in conjunction with decomposing pyrite, suggesting that sulfuric acid generated by pyrite oxidation may trigger reaction of fluoroapatite (with reduced U(IV)) to meta-autunite (with oxidized U(VI)). Experiments designed to transform flouroapatite to autunite or metaautunite via reaction with solutions of sulfuric acid are currently being carried out – this approach is meant to simulate the acid produced by the oxidation of pyrite within the black chip phosphorite. Sequential chemical extraction of meta-autunite-bearing fluoroapatite-rich phosphorites is also being carried out to further explore the solubility and speciation of uranium in the Ccs. The progressive sequential extraction of the phosphorite samples will progress through solutions of the following: potassium nitrate, acetic acid, hydroxylamine hydrochloride, hydrogen peroxide, nitric acid and lastly aqua regia. XRD and ICPMS analysis following each extraction will reveal the solubility of the various minerals within the black phosphorite chips and their effect on the release of uranium into solution.