Solid-liquid Phase Equilibria of U(VI) in NaCl Solutions

Details Solid-liquid Phase Equilibria of U(VI) in NaCl Solutions Solid-liquid Phase Equilibria of U(VI) in NaCl Solutions

Jan 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998gecoa..62..245d&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 62, Issue 2, pp.245-263

Computer Science

1

Scientific paper

Solid-liquid phase equilibria and equilibrium phase relationships of U(VI) in up to 5 m NaCl solutions were studied by analyzing the precipitation process in initially oversaturated solutions at different pH values. Comparison to corresponding behavior in NaClO 4 media is made. Solid precipitates and solution concentrations of U were characterized as a function of time and pH. In NaClO 4 media schoepite (UO 3 ·2H 2 O) was found to be the stable phase between pH 4 and 6. By contrast, in NaCl media, sodium polyuranates formed. For a given NaCl concentration and pH, differences in the solubility concentration of about 3 orders of magnitude were observed, as attributed to metastability with respect to crystallinity and Na/U ratio of the precipitates. Average solubility constants log K° so ere calculated for schoepite (log K° so = 5.37 ± 0.25) and for Na 0.33 UO 3.16 ·2H 2 O (log K° so = 7.13 ± 0.15). Based on these data and together with a critical review of literature data on schoepite and polyuranates a solid solution model is developed, describing composition and phase transformation of Na-polyuranates as a function of the activity ratio Na/H in solution. Solid solution formation is rationalized within the structural context of uranyl mineral sheet structure topologies and interlayer water properties.

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