Konstantin Rozov

PREPARATION AND CHARACTERIZATION OF Fe-, Co-, AND Ni-CONTAINING Mg-Al-LAYERED DOUBLE HYDROXIDES

Syntheses of Fe-, Co-, and Ni-containing Mg-Al-layered double hydroxides (LDHs) are described here because Fe, Co, and Ni represent the major constituents in steel containers used for storing spent nuclear fuel. Much evidence exists for the formation of LDHs during the corrosion of such containers under repository-relevant conditions. Because of their anion-exchange properties, LDHs can be considered as materials with the potential to retain and immobilize anionic radionuclides. Evaluation of the thermodynamic properties of LDHs is essential for reliable prediction of their behavior (solubility, anion exchange properties) in geochemical environments. The impact on the thermodynamic properties of the isostructural incorporation of divalent cations into the LDH was the main focus of the present study.Mg-Al-Cl-LDH and the Fe-, Co-, and Ni-doped LDHs were synthesized by the co-precipitation method and then characterized (using powder X-ray diffraction (PXRD), extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy energy-dispersive X-ray spectroscopy (SEM-EDX), and thermogravimetric analyses (TGA)).The PXRD and EXAFS analyses indicated that all synthesized samples were pure LDHs where Co, Ni, and Fe were incorporated isostructurally. The EXAFS and XANES results demonstrated that Ni and Co were incorporated as divalent cations and Fe as a trivalent cation. Thermodynamic calculations were performed assuming an equilibrium state between aqueous solutions and corresponding precipitates after synthesis. The first estimates of the molar Gibbs free energies for Fe-, Co-, and Ni-containing LDHs at 70ºC were provided. The calculated Gibbs free energy of the pure Mg-Al-LDH (-3629 kJ/mol) was slightly less than those for Fe-, Co-, and Ni-containing compositions (-3612±50, -3604±50, -3593±50kJ/mol).

Preparation, characterization and thermodynamic properties of Zr-containing Cl-bearing layered double hydroxides (LDHs)

Abstract Zr-containing layered double hydroxides (LDHs) with variable xZrsolid = Zr/(Zr + Al) mole fractions were synthesized by a co-precipitation method at ambient conditions. The chemical compositions of samples and corresponding aqueous solutions after syntheses were analyzed by ICP-OES, EDX (Mg, Al, Zr) and ion chromatography (Cl–). Results of PXRD technique demonstrated that solids with 0 ≤ xZrsolid ≤ 0.5 show only X-ray reflexes typical for pure LDH compositions, while products of syntheses with xZrsolid > 0.5 display additional patterns attributed to brucite. ICP-OES and EDX techniques shown that in pure Zr-containing LDHs the Mg/(Al + Zr) ratio is reducing with increase of xZrsolid and the stoichiometry of brucite-like layers corresponds to [Mg3–2xAl1–xZrx]. This fact may indicate that the incorporation of 1 Zr-containing specie results in the removal of 1 Al- and 2 Mg-containing species from the pure Mg-Al-composition. Such mechanism may be confirmed by the observation that measured a0 = b0 distances are generally consistent with theoretical estimates obtained from [Mg3–2xAl1–xZrx]-stoichiometry. The presence of predominant Mg2+, Al(OH)4– and Zr(OH)5– complexes in aqueous solutions after syntheses was established in thermodynamic calculations by applying GEMS-Selektor v.3. code and, therefore, the reaction: Mg3Al1(OH)8Cl1 + Zr(OH)5– = Mg1Zr1(OH)5Cl1 + Al(OH)4– + 2 Mg2+ + 4 OH– can describe a mechanism of Zr-substitution. Estimates of the molar Gibbs free energies of Zr-containing LDHs with 0 ≤ xZrsolid ≤ 0.5 show that the incorporation of Zr into the LDH increasing significantly their aqueous solubility. Thus, it is not possible to neglect that Zr can be partly localized as Zr(OH)5–-ligands in the interlayer space of the LDH structure.

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

Abstract A layered double hydroxides (LDH) hydrotalcite-pyroaurite solid solution series (Mg1−xFe(II)x)3Al1Cl1·nH2O with variable xFesolid = 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 ao = bo and co from Bragg evaluation. Syntheses products containing xFesolid > 0.13 display additional X-ray patterns attributed to the mixture of iron oxides and hydroxides. On the other side, precipitates with 0 ≤ xFesolid ≤ 0.13 show only X-ray reflexes typical for pure LDH compositions. Moreover, in this case unit-cell parameters ao = bo as a function of xFesolid follow Vegards 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 xFesolid 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 Gºf (Hydrotalcite) = −3619.04 ± 15.27 kJ/mol and Gºf(Pyroaurite) = −2703.61 ± 191.93 kJ/mol were found at 298.15 K. A comparison of our estimate with Gºf 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 Helgesons methods and using results of co-precipitation experiments at variable temperatures let us to conclude that derivation of more precise Sºf values would require calorimetric measurements.