Gibbsite solubility and thermodynamic properties of hydroxy-aluminum ions in aqueous solution at 25°C

Solubility curves were determined for a synthetic gibbsite and a natural gibbsite (Minas Gerais, Brazil) from pH 4 to 9, in 0.2% gibbsite suspensions in 0.01 M NaNO3 that were buffered by low concentrations of non-complexing buffer agents. Equilibrium solubility was approached from oversaturation (in suspensions spiked with Al(NO3)3 solution), and also from undersaturation in some synthetic gibbsite suspensions. Mononuclear Al ion concentrations and pH values were periodically determined. Within 1 month or less, data from over-and undersaturated suspensions of synthetic gibbsite converged to describe an equilibrium solubility curve. A downward shift of the solubility curve, beginning at pH 6.7, indicates that a phase more stable than gibbsite controls Al solubility in alkaline systems. Extrapolation of the initial portion of the high-pH side of the synthetic gibbsite solubility curve provides the first unified equilibrium experimental model of Al ion speciation in waters from pH 4 to 9. The significant mononuclear ion species at equilibrium with gibbsite are Al3+, AlOH2+, Al(OH)+2 and Al(OH)−4, and their ion activity products are ∗K50 = 1.29 × 108, ∗Ks1 = 1.33 × 103, ∗Ks2 = 9.49 × 10−3 and ∗Ks4 = 8.94 × 10−15. The calculated standard Gibbs free energies of formation (ΔG°f) for the synthetic gibbsite and the A1OH2+, Al(OH)+2 and Al(OH)−4 ions are −276.0, −166.9, −216.5 and −313.5 kcal mol−1, respectively. These ΔG°f values are based on the recently revised ΔG°f value for Al3+ (−117.0 ± 0.3 kcal mol−1) and carry the same uncertainty. The ΔG°f of the natural gibbsite is −275.1 ± 0.4 kcal mol−, which suggests that a range of ΔG°f values can exist even for relatively simple natural minerals.