The effects of (di-,tri-valent)-cation partitioning and intercalant anion-type on the solubility of hydrotalcites

Synthetic hydrotalcites were produced by a co-precipitation method. The hydrotalcites are represented by the general formula [(M(1-x)M(x)III)-M-II(OH)(2)][A(n-)](x/n)center dot zH(2)O, where M(II)is a divalent cation (eg, Mg(2+)or Ca2+), M(III)is a trivalent cation (eg, Al3+) and A(n-)is the interlayer anion. Herein, M-II = Mg, and M-III = Al such that [Mg/Al] = [2, 3] (atomic units) and A(n-), represents intercalant species including: OH-, SO(4)(2-)and CO(3)(2-)anions. The thermochemical data of each compound including their solubility constants (K-so), density and molar volume were quantified at T = 25 +/- 0.5 degrees C, andP = 1 bar. The solubilities of the synthetic hydrotalcites, irrespective of their divalent-trivalent cation partitioning ratio, scaled as CO32- < SO42- < OH-; in order of decreasing solubility. The type of anion, very slightly, affected the solubility with less than +/- 1 log unit of variation for [Mg/Al] = 2, and +/- 2 log units of variation for [Mg/Al] = 3. The solubilities of these phases were strongly correlated with that of gibbsite (Al(OH)(3)); such that activity of the [AlO2-] species wassolubility determiningwith increasing pH. The tabulated thermodynamic data were used to construct solid-solution models for phases encompassing both cation distribution ratios and to calculate stable phase equilibria relevant to alkali-activated slag (AAS) systems for diverse activator compositions.