PRECIPITATION OF URANYL PHOSPHATE INFLUENCED BY MINERAL SURFACES

During weathering of natural uranium deposits and uranium-contaminated sediments, association between uranium and phosphate has often led to formation of sparingly soluble uranyl phosphate minerals. Studies of uranyl phosphate in natural systems suggest the formation of uranyl phosphates may be controlled by processes at preexisting mineral surfaces. Precipitation of uranyl phosphate minerals could potentially be used to remediate contaminated sediments due to their low solubility. However, the role of mineral surface processes in controlling the precipitation of thermodynamically stable minerals versus labile sorption complexes or poorly crystalline phases are poorly understood.

Formation of uranyl phosphate minerals on goethite and mica surfaces in comparison with formation in the absence of any mineral surface was investigated. The laboratory study was carried out at a pH of 6 and 275 K. Two different uranium (84 and 210 micromol/L) and phosphate (200 and 800 micromol/L) concentrations were tested in solutions containing a variety of electrolytes. Thermodynamic equilibrium modeling indicated all experiments were supersaturated with respect to uranyl phosphate phases that include autunite group minerals as well as (UO₂)₃(PO₄)₂· 4H₂O. Aqueous uranium and phosphate concentrations rapidly decreased when 4.8 m² goethite and mica were present. In the absence of any added minerals, uranium precipitated but at a slower rate than in the presence of goethite and mica. After 21 days, aqueous concentrations of uranium remained approximately two orders of magnitude higher in treatments not containing goethite or mica. Influences of mineral surfaces on uranyl phosphate precipitation included both as adsorption and nucleation sites. TEM images of goethite-bearing samples showed surface-associated uranyl phosphate precipitation along elongated goethite crystals after 4 days. In the absence of added minerals, precipitates were not observed during the first 14 days, but were found after 42 days. These results suggest that the precipitation of thermodynamically stable autunite-group minerals occurs rapidly in the presence of goethite, but proceeds through a poorly crystalline intermediate in homogenous solution.