CORUNDUM SOLUBILITY AND ALUMINUM SPECIATION IN KOH AQUEOUS-SOLUTIONS AT 400-DEGREES-C FROM 0.5 TO 2.0 KBAR

Corundum solubility has been measured in KOH aqueous solutions under s upercritical conditions (400 degrees C, from 0.5 to 2 kbar). The inves tigated concentration range (0.001 < m(KOH) < 0.1 mol/kg H2O) allowed to vary the species balance between diluted solutions dominated by alu minate ion, AI(OH)I, and more concentrated ones in which association o f aluminate ion with K+ ion is appreciable. The dissociation constant of the complex KAl(OH)(4)(0) and the equilibrium constant for hydrolys is of corundum have been simultaneously fitted to the measured solubil ities through a speciation calculation. The corundum hydrolysis (1/2 A l2O3+5/2 H2O double left right arrow Al(OH)(4)(-)+H+) equilibrium cons tant is in close agreement with the values calculated from thermodynam ical data for corundum (Berman, 1988) and aqueous species (Johnson et al., 1992). Using the Density Model (Anderson et al., 1991) for extrap olating thermodynamical properties of aqueous species results in appre ciably higher values for this equilibrium constant, not consistent wit h experimental data, due to the inadequacy of the Density Model for ex trapolating water properties in the supercritical P-T range. For the K Al(OH)(4)(0) dissociation reaction, the following values of log K: -1. 94+/-0.17, -1.73+/-0.14, and -1.56+/-0.11, are obtained at 0.5, 1.0, a nd 2.0 kbar, respectively. These constants correspond to equal contrib utions of the aluminate ion and of the KAl(OH)(4)(0) complex to the to tal dissolved Al in 0.1 m KOH solutions. At 600 and 700 degrees C, 2 k bar, the same equilibrium constants calculated from literature experim ental corundum solubilities are -2.43 and -3.16, respectively. From th e calculated aluminate ion concentration in the system Al2O3-H2O and t he literature values of corundum solubility in water, the neutral hydr oxyde AI(OH)(3) is inferred to be probably the dominant Al aqueous spe cies in the neutral pH range in this system.