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Radiochimica Acta

International Journal for chemical aspects of nuclear science and technology

Ed. by Qaim, Syed M.

12 Issues per year

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

K. Rozov* / Hilde Curtius1 / Andreas Neumann2 / Dirk. Bosbach3

1Forschungszentrum Jülich, Institut für Energie- und Klimaforschung, Jülich, Deutschland

2Forschungszentrum Jülich, Institut für Energie- und Klimaforschung, Jülich, Deutschland

3Forschungszentrum Jülich, Institut für Energie- und Klimaforschung, Jülich, Deutschland

* Correspondence address: Forschungszentrum Jülich, Institut für Energie- und Klimaforschung, IEK-6, 52425 Jülich, Deutschland,

Citation Information: Radiochimica Acta International journal for chemical aspects of nuclear science and technology. Volume 101, Issue 2, Pages 101–110, ISSN (Print) 2193-3405 , DOI: 10.1524/ract.2013.2007, November 2012

Publication History

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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 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 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 Helgeson's 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.

Keywords: Secondary phases; Layered double hydroxides; Hydrotalcites; Thermodynamic properties

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