Zr-containing layered double hydroxides: Synthesis, characterization, and evaluation of thermodynamic properties

Abstract

Layered doubled hydroxides (LDH), consisting of positively charged octahedral brucite-type layers and interlayer anions, have been widely studied in the last decades because of their ability to control the mobility of various anions and cations in the environment. LDH may be relevant to the safety case of nuclear waste repositories due to their retention potential of anionic radionuclide species, for example 129I or 79Se. So far few studies were dedicated to Zr incorporation into LDH, which might be a relevant secondary phase in the repository environment due to the possible corrosion of Zr-bearing nuclear materials and the presence of 93Zr, a long-lived fission and activation product, in various nuclear waste streams. The focus of our study was to synthesize and characterize Cl-bearing Mg-Al-LDH (MgxAlyZrz(OH)2Cl(2x+3y+4z−2)) with different Zr-content and to evaluate their thermodynamic properties, especially their solubility as a function of temperature. The LDH were synthesized by a coprecipitation method at temperatures between 298.15 and 343.15K, aiming at Zr/(Zr+Al) ratios of 0.1 and 0.4, respectively. Our analytical techniques combining X-ray diffraction and scanning electron microscopy indicated that up to 5mol% Zr were incorporated into the brucite layer of the LDH. At higher Zr concentrations the precipitation of an amorphous hydrous Zr-oxide was observed. The structural uptake of Zr was lower than the value reported in literature for CO32– bearing LDH, suggesting that the interlayer anions may play a role with regard to the Zr uptake in the brucite layer. At low Zr contents, well defined crystalline LDH with the composition Mg0.72Al0.22Zr0.025(OH)2Cl0.20 were obtained at all syntheses temperatures. The solubility of this LDH decreases slightly with temperature and the stoichiometric saturation constant was found to satisfy the Van't Hoff equation. The thermodynamic properties of the Zr LDH (Gibbs free energy of formation, ΔG°f) were determined by considering an approach based on solubility data and Gibbs energy minimization and a calorimetric approach. The enthalpy of formation (ΔH°f) and the lattice entropy (S°) of the Zr-LDH were determined using calorimetric measurements to be −1181.01±4.98kJmol−1 and 83.9Jmol−1K−1 respectively. Considering contribution of the configurational entropy, the standard entropy, S°, was evaluated at 97±7Jmol−1K−1. The ΔG°f for Mg0.72Al0.22Zr0.025(OH)2Cl0.20*0.69H2O obtained from experimental measurements and that predicted by theory were evaluated at −1046±7 and −1046±13kJmol−1 respectively. Thus, the combination of solubility data and Gibbs energy minimization can be considered as good alternative for the evaluation of ΔG°f of LDH.