Sodium meta-autunite colloids: Synthesis, characterization, and stability

Abstract

Waste forms of U such as those in the United States Department of Energy's Hanford Site often contain high concentrations of Na and P. Low solubility sodium uranyl phosphates such as sodium meta-autunite have the potential to form mobile colloids that can facilitate transport of this radionuclide. In order to understand the geochemical behavior of uranyl phosphate colloids, we synthesized sodium meta-autunite colloids, and characterized their morphology, chemical composition, structure, dehydration, and surface charge. The stability of these synthetic plate-shaped colloids was tested with respect to time and pH. The highest aggregation rate was observed at pH 3, and the rate decreases as pH increases, indicating that higher stability of colloid dispersion under neutral and alkaline pH conditions. The synthetic colloids are all negatively charged and no isoelectric points were found over a pH range of 3 to 9. The zeta-potentials of the colloids in the phosphate solution show a strong pH-dependence in the more acidic range over time, but are relatively constant in the neutral and alkaline pH range. The geochemical behavior of the synthetic colloids can be interpreted using DLVO theory.The results suggest that formation of mobile sodium meta-autunite colloids can enhance the transport of U in some contaminated sediments.

Cloning and Overexpression of Alkaline Phosphatase PhoK from Sphingomonas sp. Strain BSAR-1 for Bioprecipitation of Uranium from Alkaline Solutions

“…It is rather difficult to precipitate uranium as phosphate in the presence of excess carbonate at pH 9.0. However, earlier studies (29) have shown that it is feasible only at log(PO 4 Ϫ3 /CO 3 Ϫ2 ) values of ϾϪ3. We determined the amount of P i released by our strains in the absence of uranium and calculated the log(PO 4 Ϫ3 /CO 3 Ϫ2 ) value in assays containing 0.5 to 5 mM uranium concentrations.…”

The effect of pH and natural microbial phosphatase activity on the speciation of uranium in subsurface soils

“…Before uranium bioreduction could be stimulated during a pilot test study, however, groundwater had to be treated above ground to remove nitrate (a more favorable terminal electron acceptor), the pH adjusted to circumneutral values to promote the growth of iron-and sulfate-reducing bacteria (Finneran et al, 2002;Senko et al, 2002;Istok et al, 2004), while simultaneously removing aqueous aluminum and calcium to prevent formation of Al-hydroxide precipitates at pH > 4.5 and stable aqueous Ca-U-CO 3 complexes. U(VI) also forms highly insoluble phosphate minerals with a 1:1 stoichiometry in a wide range of pH (Ohnuki et al, 2004;Zheng et al, 2006;Wellman et al, 2007). These minerals comprise members of the autunite/meta-autunite group, including calcium autunite [Ca(UO 2 ) 2 (PO 4 ) 2 ], chernikovite [H 2 (UO 2 ) 2 (PO 4 ) 2 ], saléeite [Mg(UO 2 ) 2 (PO 4 ) 2 ], and ankoleite [K 2 (UO 2 ) 2 (PO 4 ) 2 ], and remain stable for long periods of time (Jerden and Sinha, 2003), suggesting that uranyl phosphates may provide a long-term sink for uranium in contaminated environments.…”

Test Plan: Sludge Treatment Project Corrosion Process Chemistry Follow-on Testing

“…The solubility of sodium autunite {(Na 2 ,Ca)[(UO 2 )(PO 4 )] 2 (H 2 O) 8 }, about 2.5×10 -7 molar at pH 4.5-8.5 (Zheng et al 2006), is lower than that of metaschoepite. The conversion of metaschoepite to sodium autunite can occur by the addition of sodium phosphate salts to the metaschoepite-bearing sludge and by providing, as required, a sufficient source of calcium.…”