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

Waste forms of uranium (U), such as those in the U.S. Department of Energyʹs Hanford Site, often contain high concentrations of sodium and phosphorus. Low solubility sodium uranyl phosphates such as sodium meta-autunite have the potential to form mobile colloids that can facilitate transport of this radionuclide. To understand the stability 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. We observed the highest aggregation rate to be at pH 3, with the rate decreasing as pH increases, indicating a 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 3–9. The zeta potentials (ζ) 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. Using Derjaguin–Landau–Verwey–Overbeek theory, the stability behavior of these synthetic colloids was interpreted. The high stability of sodium meta-autunite colloids under neutral–alkaline conditions suggests that formation of sodium meta-autunite colloids can enhance the transport of U in U-contaminated sediments.