Uranium Sequestration by Aluminum Phosphate Minerals in Unsaturated Soils
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0985-NN03-05
Uranium Sequestration by Aluminum Phosphate Minerals in Unsaturated Soils James L Jerden Jr. Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL, 60439 ABSTRACT A mineralogical and geochemical study of soils developed from the unmined Coles Hill uranium deposit (Virginia) was undertaken to determine how phosphorous influences the speciation of uranium in an oxidizing soil/saprolite system typical of the eastern United States. This paper presents mineralogical and geochemical results that identify and quantify the processes by which uranium has been sequestered in these soils. It was found that uranium is not leached from the saturated soil zone (saprolites) overlying the deposit due to the formation of a sparingly soluble uranyl phosphate mineral of the meta-autunite group. The concentration of uranium in the saprolites is approximately 1000 mg uranium per kg of saprolite. It was also found that a significant amount of uranium was retained in the unsaturated soil zone overlying uranium-rich saprolites. The uranium concentration in the unsaturated soils is approximately 200 mg uranium per kg of soil (20 times higher than uranium concentrations in similar soils adjacent to the deposit). Mineralogical evidence indicates that uranium in this zone is sequestered by a barium-strontium-calcium aluminum phosphate mineral of the crandallite group (gorceixite). This mineral is intimately inter-grown with iron and manganese oxides that also contain uranium. The amount of uranium associated with both the aluminum phosphates (as much as 1.4 weight percent) has been measured by electron microprobe microanalyses and the geochemical conditions under which these minerals formed has been studied using thermodynamic reaction path modeling. The geochemical data and modeling results suggest the meta-autunite group minerals present in the saprolites overlying the deposit are unstable in the unsaturated zone soils overlying the deposit due to a decrease in soil pH (down to a pH of 4.5) at depths less than 5 meters below the surface. Mineralogical observations suggest that, once exposed to the unsaturated environment, the meta-autunite group minerals react to form U(VI)bearing aluminum phosphates. INTRODUCTION The geochemical coupling of uranium and phosphorous in uranium-rich soils has important implications for both remediation and uranium resource exploration. Natural analogue studies and experimental investigations have shown that the uranium may be stabilized (transport inhibited) in near-surface, oxidizing environments that contain soluble sources of phosphorus (naturally occurring or added as amendments). In some studies it was found that the presence of phosphorus limited uranium mobility due to the formation of sparingly soluble uranyl phosphate minerals (commonly of the autunite or meta-autunite groups, e.g. [1], [2], [3]). However, it has also been shown that in near surface environments with relatively low pH ( 2.9; however, the separation technique was not quantitative as evidenced by minor quartz an
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