This study reports 184 new measurements of the solubility of gibbsite at 50-degrees-C and 0.1 molal ionic strength in NaCl solutions of acetate, bistris, and tris buffers with hydrogen ion concentrations ranging from 10(-3) to 10(-9) molal. Samples collected at 35, 63, 66, 120, and 144 days show no detectable difference in the total aluminum at similar pH values. Correction of the measured solubilities for complexation reactions involving Al3+ with acetate and Al(OH)4- with bistris gives the solubility curve due to Al(OH)y3-y species alone, which is smooth and continuous, with a minimum near 10(-8) molal (0.3 ppb) and pH 5.5. The corrected solubilities are shown to be in complete agreement with measurements of the same material in more strongly acidic (PALMER and WESOLOWSKI, 1992) and basic solutions (WESOLOWSKI, 1992) and with the formation constant for Al(OH(2+ determined potentiometrically by PALMER and WESOLOWSKI (1993). An additional species, Al(OH)2+, was introduced in order to explain the solubility at pH values around 5.5, and the molal formation quotient for the reaction Al(OH)3,cr + H+ reversible Al(OH)2+ + H2O was determined to be 10(-3.04+/-0.05) at 50-degrees-C and 0.1 molal ionic strength. The results of this study were combined with our previous results and the new boehmite solubility data of CASTET et al. (1993) to provide a consistent model for the distribution of monomeric aluminum hydrolysis species and the solubility of gibbsite in 0-5 molal NaCl brines in the 0 to 100-degrees-C range. Salinity is shown to be a major factor controlling the solubility of aluminum minerals in solutions 1 to 2 units more acidic than the neutral pH at temperatures of 0 to 100-degrees-C. Acetate complexation is modeled from the results of this study and PALMER and BELL (1994), and is shown to enhance the solubility of gibbsite by more than an order of magnitude in mildly acidic brines containing a few thousand parts per million total acetate, in the absence of competition by other metal ions. A model is also presented for the aluminum hydrolysis constants at higher temperatures at infinite dilution which is quantitatively consistent with the low temperature data. Detailed aluminum analysis techniques employing ion chromatography are discussed in the Appendix.
