GALLIUM SPECIATION IN AQUEOUS-SOLUTION - EXPERIMENTAL-STUDY AND MODELING .2. SOLUBILITY OF ALPHA-GAOOH IN ACIDIC SOLUTIONS FROM 150-DEGREES-C 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 (0.02 5 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 t o determine the dissociation constants (K-s0 to K-s3*) of the mineral from which hydrolysis constants of aqueous gallium species were deriv ed. The dissociation constants generated in this study were used toget her with the thermodynamic properties of alpha-GaOOH (Pokrovski et al. , 1997) to calculate the standard partial molal thermodynamic properti es at 25 degrees C, 1 bar and the equations of state parameters for Ga 3+, 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 cal culation of gallium speciation as a function of pH and temperature. Ou r 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 inc reasing temperature, the amount of Ga(OH)(2)(+) and Ga(OH)(3)(0) becom es significant. These data are used to compare the geochemical behavio r of Ga and Al in natural fluids. Copyright (C) 1997 Elsevier Science Ltd.