Synthesis, characterization and stability properties of Cl-bearing hydrotalcite-pyroaurite solids

Abstract

A layered double hydroxides (LDH) hydrotalcite-pyroaurite solid solution series (Mg_1−xFe(II)_x)₃Al₁Cl₁·nH₂O with variable xFe_solid = Fe²⁺/(Fe²⁺+Mg²⁺) iron mole fractions were studied in co-precipitation experiments at T = 25, 40, 45, 50, 55 and 60 ºC and pH = 10.00 ± 0.05. The compositions of the solids and reaction solutions were determined using ICP-OES, EDX (Mg, Al, Fe) and TGA techniques (Cl⁻, OH⁻, H₂O). Powder X-ray diffraction was applied for phase identification and determination of unit-cell parameters a_o = b_o and c_o from Bragg evaluation. Syntheses products containing xFe_solid > 0.13 display additional X-ray patterns attributed to the mixture of iron oxides and hydroxides. On the other side, precipitates with 0 ≤ xFe_solid ≤ 0.13 show only X-ray reflexes typical for pure LDH compositions. Moreover, in this case unit-cell parameters a_o = b_o as a function of xFe_solid follow Vegard's law corroborating the existence of a continuous solid solution series. TGA data demonstrated the temperatures at which interlayer H₂O molecules and Cl⁻-anions are lost, and at which temperatures dehydroxylation of brucite-like layer occurs. Based on detailed analyses of TGA curves it was established that the increase of xFe_solid does not result in a visible change of the thermal stability of hydrotalcite-pyroaurite solids. From the chemical analyses of both the solids and the reaction solutions after syntheses, preliminary Gibbs free energies of formation were estimated by using GEMS-PSI code package. Values of Gº_f (Hydrotalcite) = −3619.04 ± 15.27 kJ/mol and Gº_f(Pyroaurite) = −2703.61 ± 191.93 kJ/mol were found at 298.15 K. A comparison of our estimate with Gº_f value −3746.90 ± 11.00 kJ/mol for CO₃²⁻-bearing hydrotalcite presented in our previous studies, denotes the effect of intercalated anion on the aqueous solubilities of LDH when Cl-containing solids have to be more soluble than CO₃²⁻-bearing substances. Estimation of the standard molar entropy of the hydrotalcite end-member by applying Helgeson's methods and using results of co-precipitation experiments at variable temperatures let us to conclude that derivation of more precise Sº_f values would require calorimetric measurements.