Effect of orthophosphate on the oxidation products of Fe(II)-Fe(III) hydroxycarbonate: The transformation of green rust to ferrihydrite

Hydroxycarbonate green rust GR(CO32-) has been synthesized by oxidation of aqueous suspensions of Fe(OH)(2) by aeration at the air-liquid interface, i n the presence of HCO3- ions at pH 7.5 to 9. During the oxidation of GR(CO3 2-), ferrihydrite formed first and then turned into goethite by dissolution and precipitation. The oxidation of GR(CO32-) in the presence of orthophos phate ions, which were added as Na2HPO4. 7H(2)O salt, also involved the for mation of ferrihydrite but not that of goethite, because the dissolution of ferrihydrite is inhibited by the adsorption of phosphate ions on its surfa ce. The oxidation was slowed down because of the suppression of the catalyt ic effect of iron(III) hydroxide on the oxidation of Fe(II). In anoxic cond itions without phosphate, a mixture of GR(CO32-), goethite, and ferrihydrit e was observed to transform spontaneously into a mixture of siderite and ma gnetite. It is thermodynamically consistent, which shows that GR(CO32-) is metastable with respect to the two-phase system FeCO3-Fe3O4. In the presenc e of phosphate. this transformation was inhibited and GR(CO32-) did not tra nsform in anoxic conditions. Anionic phosphate species dissolved in solutio n did not give rise to a corresponding GR, i.e., phosphate species did not substitute for carbonate inside the interlayers of the GR. Moreover, iron p hosphates did not appear, neither during the oxidation of GR(CO32-) in the presence of oxygen nor in anoxic conditions. Copyright (C) 2001 Elsevier Sc ience Ltd.