Aqueous high-temperature solubility studies. I. The solubility of boehmite as functions of ionic strength (to 5 molal, NaCl), temperature (100–290°C), and pH as determined by in situ measurements

Abstract

The solubility of boehmite, γ-AlOOH, was measured in a modified hydrogen-electrode concentration cell, which provided continuous in situ measurements of hydrogen ion molality over the range of ionic strengths from 0.1 to 5.0 mol · kg −1 (NaCl) at temperatures from 100 to 290°C. A series of conventional solubility measurements was also carried out in acidic solutions over the same temperature range (i.e., pH was not monitored, but rather calculated from mass balance). The combined results yielded the molal solubility quotients, Q s0 and Q s4 for the equilibria: AlOOH (cr) + 3H + ⇄ Al 3+ + 2H 2O Q s0 = [Al 3+]/[H +] 3 AlOOH (cr) + 2H 2O ⇄ Al(OH) 4 − + H + Q s4 = [Al(OH) 4 −][H +]In the regression of each isothermal data set, the values for the first hydrolysis quotient, Al 3+ + H 2O ⇄ Al(OH) 2+ + H + Q 1,1 = [Al(OH) 2+][H +]/[Al 3+]were fixed according to a previous potentiometric study (Palmer and Wesolowski, 1993). Moreover, for one series of titrations at 0.1 mol · kg −1 ionic strength at 150°C, the remaining two solubility quotients, Q s2 and Q s3, were determined simultaneously from the regression. However, at all other conditions, the values of Q s2 and Q s3 were also fixed at values consistent with the corresponding 0.03 mol · kg −1 ionic strength results (Bénézeth et al., 2001) by invoking the isocoulombic assumption (i.e., an assumption of minimal ionic strength dependence for reactions with no net change in charge). The stability field with respect to pH of the Al(OH) 2 + and Al(OH) 3 0 aqueous species were found to be very narrow, and hence assumptions concerning their stabilities had little effect on the predicted shape of the solubility profiles at high ionic strength. Global fits were made of the log Q s0 and log Q s4 values as functions of temperature and ionic strength after combining with corresponding values from an analogous study at 0.03 mol · kg −1 ionic strength (Bénézeth et al., 2001), as well as appropriate constants taken from the literature. The fits were further constrained by inclusion of gibbsite solubility data (<100°C) in NaCl media (Wesolowski, 1992; Palmer and Wesolowski, 1992) after adjustment for the relative free energies of formation of gibbsite and boehmite. This treatment ensured that a continuous empirical model exists for Al 3+ and Al(OH) 4 − from ambient conditions to 300°C and infinite dilution to five mol · kg −1 ionic strength.