Synthesis and characterization of sodium meta-autunite, Na[UO ₂PO₄]·3H₂O

Long-chain sodium polyphosphate compounds have been recently proposed as a "time-released" source of phosphate for precipitation of uranium-phosphate minerals. Elevated sodium concentrations presented by this technique promote the formation of sodium autunite relative to the more common calcium autunite mineral phase. In order to evaluate sodium autunite minerals as a long-term "sink" for in-situ immobilization of uranium, it is necessary to quantify their longevity under environmentally relevant conditions. This paper describes a direct method for precipitating sodium autunite and provides a comparative analysis of the structural and chemical properties of direct versus indirectly precipitated sodium autunite. Extended X-ray absorption fine structure (EXAFS) spectroscopy, chemical digestion followed by inductively-coupled plasma-optical emission spectroscopy (ICP-OES) and inductively-coupled plasma-mass spectroscopy (ICP-MS) for elemental analyses, X-ray diffraction (XRD), scanning electron microscopy (SEM), multi point Brunauer-Emmett-Teller (BET) analyses and helium pyconometry were used to characterize the precipitate phases. Morphological differences are discussed in the context of conducting subsequent solubility and dissolution investigations. Research presented here is part of a larger effort to quantify the solubility and dissolution properties of uranium-phosphate minerals.