Jerden and Sinha reported preliminary results in 2003 and the the final results in 2006 that the mineralogy and geochemistry of soils developed over the unmined Coles Hill uranium deposit (Virginia) were studied to determine how phosphorous influences the speciation of uranium in oxidizing soil/saprolite systems typical of the eastern US. Results from this study have implications for both uranium remediation (e.g. in situ stabilization) and uranium resource exploration (e.g. near-surface geochemical sampling).
The primary uranium ore (coffinite and uraninite hosted in quartzo-feldspathic gneiss) weathers to saprolites containing the same uranium concentration as the underlying ore (approximately 1000 mg U/kg saprolites). In these water saturated (below water table) saprolites the uranium is retained as uranyl phosphates of the meta-autunite group (mainly meta-uranocircite). Above the water table the soils overlying the deposit contain approximately 200 mg uranium per kg soil (20 times higher than uranium concentrations in similar soils formed from unmineralized rocks adjacent to the deposit).
In these unsaturated zone soils uranium is retained by two processes:
1) incorporation into barium–strontium–calcium aluminum phosphate minerals of the crandallite group (mainly gorceixite), and
2) sorption of uranium with phosphorous onto iron oxides that coat the surfaces of other soil minerals.
Thermodynamic calculations suggest that the meta-autunite group minerals present in the saprolites below the water table are not stable in the unsaturated zone soils overlying the deposit due to the drop in soil pH from ~6.0 down to ~4.5. Mineralogical observations suggest that, once exposed to the unsaturated environment, the meta-autunite group minerals react to form U(VI)- bearing aluminum phosphates and U(VI) surface complexes or nano-precipitates associated with ferric oxides. These results therefore indicate that models predicting U(VI) speciation in phosphate amended soils must simultaneously account for variations in pH, ion activities (aluminum appears to be particularly important) and surface complexation with iron oxide mineral surfaces.
The authors concluded that soils developed from the Coles Hill uranium deposit (hosted in quartzo-feldspathic gneiss) are enriched in uranium relative to soil profiles developed from unmineralized bedrock adjacent to the deposit (200–1000 mg/kg U in soils overlying the deposit vs. 5–15 mg/kg U in adjacent soil profiles).
Uranium enrichment is greatest in the saturated zone saprolites where uranium concentrations are equal to, or higher than, the average concentration in the primary ore (~1000mg/kg U in the saturated saprolite vs. 960 U mg/kg in primary ore).
Uranium in the primary ore is present as coffinite and uraninite (Jerden and Sinha, 2003) and in the saturated saprolites, as meta-autunite group mineral (Jerden et al., 2003). The authors focused on the unsaturated soil horizons developed from the uranium rich saprolites. Bulk chemical analyses show that the unsaturated zone soils overlying the Coles Hill deposit are also enriched in uranium relative to surrounding soil profiles, however, the level of enrichment is lower than in the underlying saprolites (~200 mg/kg U in the unsaturated soils vs. ~1000 mg/kg U in the saturated saprolites). This discontinuity in uranium concentration corresponds to an abrupt change in uranium mineralogy at the water table. In soils above the water table uranyl phosphates are absent; however, uranium is still closely associated with phosphate. The two main uranium sinks in the unsaturated zone soils are:
1) a barium–strontium–calcium aluminum phosphate of the crandallite group (composition and XRD patterns most closely match that of gorceixite), which contains 1.1 wt.% uranium, and
2) phosphate-rich iron oxide mineral coatings which contain up to 6.3 wt.% uranium (uranium concentrations determined by EMP).
In the iron oxide mineral coatings, uranium is positively correlated with phosphorous. Electron microprobe analyses of manganese oxide inclusions within the uranium-bearing gorceixite show that uranium is preferentially partitioned
into the aluminum phosphate relative to the intermixed manganese oxides.
Thermodynamic calculations suggest that the absence of meta-autunite group minerals in the unsaturated soil horizons is primarily due to a drop in pH of the pore fluids (soil pH in unsaturated zone is ~6.0 pH of saprolites ground waters in saprolites ~6.0). One indication of this is that the modeled pore
water solutions become understaturated with respect to meta-uranocircite at a pH of approximately 6.5
Results from the authors' mineralogical characterizations of Coles Hill soils suggest that a potentially important type of surface complex that is not accounted for in our modeling are uranyl-phosphato complexes. It is possible that aqueous complexes UO2PO4− or UO2H2PO4+ may interact with the iron oxide substrate to form a stable uranyl-phosphate surface species that may contribute to the retention of uranium in the
unsaturated zone.
Overall, the authors' thermodynamic modeling supports the field observations showing that, in well developed saprolites/soil profiles typical of the mid-Atlantic and south eastern U.S., the presence of phosphorous will strongly influence both the solid and aqueous phase speciation of uranium. The results of this study therefore have important implications for uranium stabilization strategies involving phosphate amendments and for
geochemical exploration for uranium resources in phosphate-rich weathered regions.
The primary implications of this work are:
1) secondary uranyl phosphates (e.g. autunite or metaautunite group minerals) may not be stable in unsaturated soils with relatively low pH, and
2) even if uranyl phosphates are not stable, uranium may be retained in relatively low pH soils (e.g. pH 4–6) through incorporation into aluminum phosphates and the formation of uranium–phosphate surface complexes (or nano-precipitates) associated with iron oxides.
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