ALUMINUM SPECIATION AND EQUILIBRIA IN AQUEOUS-SOLUTION .5. GIBBSITE SOLUBILITY AT 50-DEGREES-C AND PH 3-9 IN 0.1 MOLAL NACL SOLUTIONS (A GENERAL-MODEL FOR ALUMINUM SPECIATION - ANALYTICAL METHODS)

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 acet ate, bistris, and tris buffers with hydrogen ion concentrations rangin g from 10(-3) to 10(-9) molal. Samples collected at 35, 63, 66, 120, a nd 144 days show no detectable difference in the total aluminum at sim ilar pH values. Correction of the measured solubilities for complexati on reactions involving Al3+ with acetate and Al(OH)4- with bistris giv es the solubility curve due to Al(OH)y3-y species alone, which is smoo th 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 w ith 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 P ALMER and WESOLOWSKI (1993). An additional species, Al(OH)2+, was intr oduced 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+ reversi ble 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 combin ed with our previous results and the new boehmite solubility data of C ASTET et al. (1993) to provide a consistent model for the distribution of monomeric aluminum hydrolysis species and the solubility of gibbsi te 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 m inerals in solutions 1 to 2 units more acidic than the neutral pH at t emperatures of 0 to 100-degrees-C. Acetate complexation is modeled fro m the results of this study and PALMER and BELL (1994), and is shown t o enhance the solubility of gibbsite by more than an order of magnitud e in mildly acidic brines containing a few thousand parts per million total acetate, in the absence of competition by other metal ions. A mo del is also presented for the aluminum hydrolysis constants at higher temperatures at infinite dilution which is quantitatively consistent w ith the low temperature data. Detailed aluminum analysis techniques em ploying ion chromatography are discussed in the Appendix.