The preparation and thermodynamic properties of Fe(II)Fe(III) hydroxide-carbonate (green rust 1); Pourbaix diagram of iron in carbonate-containing aqueous media

Abstract

Carbonate-containing green rust 1, GR1(CO 3 2−), is prepared by oxidation of Fe(OH) 2 in aqueous solution. Ferrous hydroxide is precipitated from NaOH and FeSO 4·7H 2O solutions and carbonate ions are added as a Na 2CO 3 solution. For sufficiently large concentrations of sodium carbonate, SO 4 2− ions do not play any role during the oxidation process and, at the end of the first stage of reaction, Fe(OH) 2 oxidizes into GR1(CO 3 2−). In the second stage of reaction, GR1(CO 3 2−) oxidizes into α-FeOOH goethite except when the transformation of ferrous hydroxide is partial, which leads to the formation of magnetite. From the X-ray diffraction analysis of GR1(CO 3 2−), lattice parameters of its hexagonal cell are found to be a = 3.160 ± 0.005 A ̊ and c = 22.45 ± 0.05 A ̊ . From the Mössbauer analysis of the stoichiometric GR1(CO 3 2−), which leads to a Fe 2+:Fe 3+ ratio of 2:1, the chemical formula is established to be: [Fe 4 (II)Fe 2 (III)(OH) 12][CO 3·2H 2O]. The 78 K Mössbauer spectrum of the compound can be fitted with three quadrupole doublets, two Fe 2+ doublets d 1 and D 2 corresponding to isomer shifts (IS) of 1.27 and 1.28 mm s −1 and quadrupole splittings (QS) of 2.93 and 2.67 mm s −1, respectively, and one Fe 3+ doublet D 3 with an IS of 0.47 mm s −1 and QS of 0.43 mm s −1. These three doublets were already used to fit the Mössbauer spectrum of chloride-containing GR1(Cl −) [see J.M.R. Génin et al., Mat. Sci. Forum 8, 477 (1986) and J.M.R. Génin et al., Hyp. Int. 29, 1355 (1986)]and therefore are characteristic of GR1 compounds. From the recording of electrode potential E and the pH of the suspension versus time during the oxidation, the standard free enthalpy of formation of stoichiometric GR1(CO 3 2−) is estimated to be ΔG ° f = − 966.250 cal mol −1. Knowing the chemical formula and ΔG ° f of GR1(CO 3 2−) the Pourbaix diagram of iron in carbonate-containing aqueous solutions is drawn.