Thermochemistry and Aqueous Solubilities of Hydrotalcite-Like Solids

Abstract

Hydrotalcites are used in technology as catalysts and anion exchangers and are important sinks for environmental contaminants. Their compositional variability makes it important, but difficult, to estimate their aqueous solubility. We report calorimetric measurements of the heats of formation of cobalt-aluminum hydrotalcite phases. The heat and free energy of formation from the elements are equal to those of mechanical mixtures of binary compounds, namely hydroxides and carbonates. The interlayer anion is much more important than the cation in determining the solubility of the hydrotalcite phase and its ability to contain or release heavy metals to the environment. Because hydrotalcites do not have an unreactive polymer as a structural core, their aqueous stability will change dramatically with composition, particularly with anion content. This simple mechanical mixture model allows prediction of aqueous solubilities and trace metal retention in a variety of geochemical settings.

Review of the synthesis of layered double hydroxides: a thermodynamic approach

“…This result is not surprising if we account for the assumptions in the mixture model, as seen in Equations 5 and 7 and stated by Allada et al 78 : "Metal and anion coordination environments in the hydrotalcite are structurally similar, and therefore probably energetically similar, to those in the simple minerals used as components".…”

“…However, from the results in Table 3, carbonate anions are in some cases slightly unfavorable to the formation of LDHs when compared to hydroxyl anions as in Mg 2+ -M 3+ LDHs. Allada et al 78 estimated the errors associated with the predictions of thermodynamic data by the mixture model to be within 5 to 10 kJ mol -1 . Therefore, differences found in the anion contributions from Table 3 between carbonate and hydroxyl anions fall within the error range given by the model.…”

“…The influence of interlamellar anions on the solubility of LDHs has been little studied. Allada et al 78 reported that nitrate and sulfate anions could increase the solubility of hydrotalcite relative to the carbonate hydrotalcite. The same trend is obtained from ∆ C G o m {298.15 K, MgA} data for NO 3 -, SO 4 2-and CO 3 2-in Table 3 …”

“…In spite of the importance of the solubility of LDHs, related thermodynamic data in the literature are rather scarce 78 . .…”

“…However, this is no easy task, due to the great amount of thermodynamic data required including standard free energies, enthalpies and entropies of formation for all the wide range of compositional variations of LDHs. Moreover, very few articles are found in the literature [78][79][80] dealing with all the necessary data to make this work feasible. We have recently proposed 81 and evaluated five different models for the estimation of thermodynamic properties of LDH compounds.…”