THE PREPARATION AND THERMODYNAMIC PROPERTIES OF FE(II)-FE(III) HYDROXIDE-CARBONATE (GREEN-RUST-1) - POURBAIX DIAGRAM OF IRON IN CARBONATE-CONTAINING AQUEOUS-MEDIA

Carbonate-containing green rust 1, GR1(CO32-), is prepared by oxidatio n of Fe(OH)(2) in aqueous solution. Ferrous hydroxide is precipitated from NaOH and FeSO4 . 7H(2)O solutions and carbonate ions are added as a Na2CO3 solution. For sufficiently large concentrations of sodium ca rbonate, 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 int o GR1(CO32-). In the second stage of reaction, GR1(CO32-) oxidizes int o alpha-FeOOH goethite except when the transformation of ferrous hydro xide is partial, which leads to the formation of magnetite. From the X -ray diffraction analysis of GR1(CO32-), lattice parameters of its hex agonal cell are found to be a=3.160+/-0.005 Angstrom and c=22.45+/-0.0 5 Angstrom. From the Mossbauer analysis of the stoichiometric GR1(CO32 -), which leads to a Fe2+:Fe3+ ratio of 2:1, the chemical formula is e stablished to be: [Fe-4((II))Fe-2((III))(OH)(12)][CO3 . 2H(2)O]. The 7 8 K Mossbauer spectrum of the compound can be fitted with three quadru pole doublers, two Fe2+. doublers D-1 and D-2 corresponding to isomer shifts (IS) of 1.27 and 1.28 mm s(-1) and quadrupole splittings (QS) o f 2.93 and 2.67 mm s(-1), respectively, and one Fe3+ doublet D-3 with an IS of 0.47 mm s(-1) and QS of 0.43 mm s(-1). These three doublets w ere already used to fit the Mossbauer spectrum of chloride-containing GR1(Cl-) [see J.M.R. Genin et at., Mat. Sci. Forum 8, 477 (1986) and J .M.R. Genin et al., Hyp. Int. 29, 1355 (1986)] and therefore are chara cteristic of GR1 compounds. From the recording of electrode potential E and the pH of the suspension versus time during the oxidation, the s tandard free enthalpy of formation of stoichiometric GR1(CO32-) is est imated to be Delta G degrees(f) = -966.250 cal mol(-1). Knowing the ch emical formula and Delta G degrees(f) of GR1(CO32-) the Pourbaix diagr am of iron in carbonate-containing aqueous solutions is drawn.