BOEHMITE SOLUBILITY AND AQUEOUS ALUMINUM SPECIATION IN HYDROTHERMAL SOLUTIONS (90-350-DEGREES-C) - EXPERIMENTAL-STUDY AND MODELING

The solubility of a pure synthetic boehmite was measured in noncomplexing solutions over a wide range of pH (2-9) and temperatures (90-350 degrees C). A least-squares linear regression of these data was used to determine the dissociation constants (K*s(0) to K*s(4)) of the mineral. Values obtained in basic solutions (K*s(4)) are in close agreement with published data. In weakly acidic and neutral media, however, the values are lower than those reported previously in the literature. These differences are the result of the use of improved analytical techniques with lower detection limits and a more thermodynamically pure and stable solid material. The hydrolysis constants deduced in the present study indicate that, as temperature increases, aluminum hydrolysis becomes stronger, and the stability fields of Al(OH)(4)(-) and Al(OH)(3)(0) expand at the expense of the other species. A density model( FRANCK, 1956; ANDERSON et al., 1991)was applied to extrapolate the experimental data to 25 degrees C. This extrapolation permitted generation of a consistent set of thermodynamic data (Delta G(f,298.15)(0) and Delta H-f,298.15(0)) for the aqueous aluminum monomer species. These data have been used to calculate the solubility of corundum at high temperature and pressure. The close agreement obtained with available experimental data indicates that the aluminate ion is dominant in most crustal fluids, in the absence of large amounts of complexing species.