S. Drissi, P. Refait, M. Abdelmoula
Dec 1, 1995
Citations
5
Influential Citations
243
Citations
Quality indicators
Journal
Corrosion Science
Abstract
Abstract Carbonate-containing green rust 1, GR1(CO32−), is prepared by oxidation of Fe(OH)2 in aqueous solution. Ferrous hydroxide is precipitated from NaOH and FeSO4·7H2O solutions and carbonate ions are added as a Na2CO3 solution. For sufficiently large concentrations of sodium carbonate, SO42− 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(CO32−). In the second stage of reaction, GR1(CO32−) 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(CO32−), 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 Mossbauer analysis of the stoichiometric GR1(CO32−), which leads to a Fe2+:Fe3+ ratio of 2:1, the chemical formula is established to be: [Fe4(II)Fe2(III)(OH)12][CO3·2H2O]. The 78 K Mossbauer spectrum of the compound can be fitted with three quadrupole doublets, two Fe2+ doublets d1 and D2 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 Fe3+ doublet D3 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 Mossbauer spectrum of chloride-containing GR1(Cl−) [see J.M.R. Genin et al., Mat. Sci. Forum8, 477 (1986) and J.M.R. Genin 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(CO32−) is estimated to be ΔG °f = − 966.250 cal mol−1. Knowing the chemical formula and ΔG °f of GR1(CO32−) the Pourbaix diagram of iron in carbonate-containing aqueous solutions is drawn.