The oxidation of Fe(OH)2 in the presence of carbonate ions: Structure of carbonate green rust one

Abstract

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 oxidation of ferrous hydroxide. The first stage of the reaction leads to a ferrous-ferric compound, the carbonate green rust one (GR1), identified 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 splittingsQS are 2.91, 2.58 and 0.42 mm/s, respectively, and the isomer shifts 75 are 1.25, 1.25 and 0.47 mm/s respectively. These values are very close to those of the three doublets of the chloride GR1, 3Fe(OH)2Cl·Fe(OH)2Cl·nH2O. This fact confirms that the crystallographic structures of these two GR1s are similar, formed by the stacking of hydroxide layers and interlayers containing the considered anions (Cl− or CO32−) and water molecules. The chemical formula of carbonate GR1 is Fe4(II)Fe2(III)(OH)12CO3·nH2O, and its standard chemical potential -853 900 cal/mol ifn=0. The second stage of the reaction is the oxidation of GR1, which leads to α-FeOOH goethite.