THE OXIDATION OF CARBONATE GREEN RUST INTO FERRIC PHASES: SOLID-STATE REACTION OR TRANSFORMATION VIA SOLUTION

Abstract

The oxidation of carbonate green rust, GR(CO32−), in NaHCO3 solutions at T = 25°C has been investigated through electrochemical techniques, FTIR, XRD, TEM and SEM. The used GR(CO32−) samples were made of either suspended solid in solution or a thin electrochemically formed layer on the surface of an iron disc. Depending on experimental conditions, oxidation occurs, with or without major modifications of the GR(CO32−) structure, suggesting the existence of two pathways: solid-state oxidation (SSO) leading to a ferric oxyhydroxycarbonate as the end product, and a dissolution-oxidation-precipitation (DOP) mechanism leading to ferric oxihydroxides such as lepidocrocite, goethite, or ferrihydrite. A formula was proposed for this ferric oxyhydroxycarbonate, Fe6IIIO(2+x)(OH)(12-2x)(H2O)x(CO3), assuming that the solid-state oxidation reaction is associated to a deprotonation of the water molecules within the interlayers, or of the hydroxyl groups in the Fe(O,H) octahedra layers. The DOP mechanism involves transformation via solution with the occurrence of soluble ferrous-ferric intermediate species. A discussion about factors influencing the oxidation of carbonate green rust is provided hereafter. The ferric oxyhydroxycarbonate can be reduced back to GR(CO32−) by a reverse solid-state reduction reaction. The potentiality for a solid-state redox cycling of iron to occur may be considered. The stability of the ferric oxyhydroxycarbonate towards thermodynamically stable ferric phases, such as goethite and hematite, was also studied.