Gallium speciation in aqueous solution. Experimental study and modelling .2. Solubility of alpha-GaOOH in acidic solutions from 150 to 250 degrees C and hydrolysis constants of gallium (III) to 300 degrees C

The solubility of alpha-GaOOH was measured in low ionic strength (0.025 M) HCl-NaCl solutions over the pH range 1.6-4.1 at 150, 200, and 250 degrees C. A least squares linear regression of these data was used to determine the dissociation constants (K-s0* to K-s3*) of the mineral from which hydrolysis constants of aqueous gallium species were derived. The dissociation constants generated in this study were used together with the thermodynamic properties of alpha-GaOOH (Pokrovski et al., 1997) to calculate the standard partial molal thermodynamic properties at 25 degrees C, 1 bar and the equations of state parameters for Ga3+, Ga(OH)(2+), Ga(OH)(2)(+), and Ga(OH)(3)(0) within the framework of the revised HKF model (Tanger and Helgeson, 1988; Shock and Helgeson, 1988; Shock et al., 1992). These thermodynamic properties enabled calculation of gallium speciation as a function of pH and temperature. Our results indicate that the hydrolysis of Ga occurs at low pH and its speciation is strongly dominated by the negatively-charged species Ga(OH)(4)(-) in the pH range of natural solutions at 25 degrees C. At increasing temperature, the amount of Ga(OH)(2)(+) and Ga(OH)(3)(0) becomes significant. These data are used to compare the geochemical behavior of Ga and Al in natural fluids. Copyright (C) 1997 Elsevier Science Ltd.