Solubility and thermodynamic properties of carbonate-bearing hydrotalcite/copyroaurite solid solutions with a 3:1 Mg/(Al+Fe) mole ratio

The naturally occurring layered double hydroxides (LDH, or anionic clays) are of particular interest in environmental geochemistry because of their ability to retain hazardous cations and especially anions. However, incorporation of these minerals into predictive models of water/C0 rock interaction in contaminant environments, including radioactive-waste repositories, is hampered by a lack of thermodynamic and stability data. To fill part of this gap the present authors have derived properties of one of the complex multicomponent solid solutions within the LDH family: the hydrotalcite/C0pyroaurite series, Mg3(Al1/C0xFex)(OH)8(CO3)0.5·2.5H2O. Members of the hydrotalcite/C0pyroaurite series with fixed MgII/(AlIII+FeIII) = 3 and various FeIII/(FeIII+AlIII) ratios were synthesized by co-precipitation and dissolved in long-term experiments at 23°C and pH = 11.40±0.03. The chemical compositions of co-existing solid and aqueous phases were determined by inductively coupled plasma-optical emission spectroscopy, thermogravimetric analysis, and liquid scintillation counting of 55Fe tracers; X-ray diffraction and Raman were used to characterize the solids. Overall, good evidence for reversible equilibrium in the experiments led to the examination of the thermodynamic properties of the solid solution using total-scale Lippmann solubility products, resulting in an estimate of the standard Gibbs free energy of the pyroaurite end member.