THE OXIDATION OF FE(OH)(2) IN THE PRESENCE OF CARBONATE IONS - STRUCTURE OF CARBONATE GREEN RUST ONE

Sodium carbonate Na2CO3 is added to a solution containing an Fe(OH)(2) precipitate in order to study the influence of CO32- ions on the oxid ation of ferrous hydroxide. The first stage of the reaction leads to a ferrous-ferric compound, the carbonate green rust one (GR1), identifi ed by its X-ray diffraction pattern. The Mossbauer spectrum at 78 K of this GR1 displays two ferrous doublets and one ferric doublet in the 3: 1:2 abundance ratio. The quadrupole splittings QS are 2.91, 2.58 an d 0.42 mm/s, respectively, and the isomer shifts IS are 1.25, 1.25 and 0.47 mm/s respectively. These values are very dose to those of the th ree doublets of the chloride GR1, 3Fe(OH)(2)Cl.Fe(OH)(2)Cl.nH(2)O. Thi s fact confirms that the crystallographic structures of these two GR1s are similar, formed by the stacking of hydroxide layers and interlaye rs containing the considered anions (Cl- or CO32-) and water molecules . The chemical formula of carbonate Gel is Fe-4((II))Fe-2((III))(OH)(1 2)CO3.nH(2)O, and its standard chemical potential -853 900 cal/mol if n = 0. The second stage of the reaction is the oxidation of GR1, which leads to alpha-FeOOH goethite.