CHARACTERIZATION OF U(VI) MINERAL PHASES FORMED BY HOMOGENEOUS NUCLEATION FROM ACIDIC CRIB WASTE

The 216-U-8 and U-12 Cribs, inactive wastewater disposal sites, at the Hanford Site (WA, USA) received large amount of acidic wastewater containing uranium (U) and other contaminants. The acidic wastewaters were percolated directly into the vadose zone and contaminated underlying sediment and groundwater. To mitigate impact at the site and help predict the long term fate and transport of U, it is critical to identify and characterize U mineral phases that may have formed in acidic conditions. Here, we studied U(VI) precipitation products from aqueous solutions after 30 days of reaction time, with and without dissolved phosphate and silica, that simulate acidic and neutralized wastewater at the Hanford site. Powder X-ray diffraction, X-ray absorption spectroscopy, and BET surface area analyses, thermogravimetric analyses, and bulk chemical analyses were used to characterize U(VI) solid phases. In the absence of dissolved phosphate and silicate, U(VI) was precipitated as compreignacite at pH 5 and metaschoepite at pH 7. In the presence of dissolved silica (1 mM), XRD and EXAFS results indicated poorly-crystalline boltwoodite was the primary precipitation product together with becquerelite at pH 5-7. In the presence of phosphate (3 mM), U was precipitated as K-autunite at pH 3 and meta-ankoleite at pH 5-7 regardless of the presence of dissolved silica. Thermodynamic calculations were conducted to study the effects of relative concentrations of dissolved constituents on thermodynamically stable U(VI) phases. Since U(VI) phosphates have very low solubility in acidic conditions, thermodynamic calculations predicted that U(VI) phosphates are stable even in the presence of low phosphate concentrations. In the absence of phosphate, dissolved silica in Hanford porewaters is derived from kinetically controlled silicate mineral dissolution and acid neutralization. These kinetics factors are also important for predicting the formation of U(VI) phases, especially the formation of poorly-crystalline phases. This study suggests that the initial composition of the acidic crib waste and constituents derived from the sediment-acid neutralization process significantly alter the speciation and physical properties of U(VI) precipitates, which can influence long-term fate and transport of U at the site.