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

Abstract

Hydroxycarbonate green rust GR(CO 3 2−) has been synthesized by oxidation of aqueous suspensions of Fe(OH) 2 by aeration at the air–liquid interface, in the presence of HCO 3 − ions at pH 7.5 to 9. During the oxidation of GR(CO 3 2−), ferrihydrite formed first and then turned into goethite by dissolution and precipitation. The oxidation of GR(CO 3 2−) in the presence of orthophosphate ions, which were added as Na 2HPO 4 · 7H 2O salt, also involved the formation of ferrihydrite but not that of goethite, because the dissolution of ferrihydrite is inhibited by the adsorption of phosphate ions on its surface. The oxidation was slowed down because of the suppression of the catalytic effect of iron(III) hydroxide on the oxidation of Fe(II). In anoxic conditions without phosphate, a mixture of GR(CO 3 2−), goethite, and ferrihydrite was observed to transform spontaneously into a mixture of siderite and magnetite. It is thermodynamically consistent, which shows that GR(CO 3 2−) is metastable with respect to the two-phase system FeCO 3–Fe 3O 4. In the presence of phosphate, this transformation was inhibited and GR(CO 3 2−) did not transform in anoxic conditions. Anionic phosphate species dissolved in solution 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 phosphates did not appear, neither during the oxidation of GR(CO 3 2−) in the presence of oxygen nor in anoxic conditions.