Transformation of Akaganéite Into Goethite and Hematite in Alkaline Media

Abstract

Abstract--The conversion of akaganrite to goethite and/or hematite in alkaline media has been followed by X-ray powder diffraction and transmission electron microscopy (TEM). The rate of transformation fell and the amount of hematite in the product increased as the [OH-] decreased to < 1 M. Kinetic studies and TEM indicated that the transformation involved dissolution ofakaganrite followed by reprecipitation of goethite and/or hematite. The rate-determining step was the dissolution of akaganrite.Silicate species retarded the formation of goethite + hematite principally by inhibiting dissolution of akaganrite; to a lesser extent, they interfered with the nucleation of goethite. Silicate modified the morphology ofgoethite, but not hematite.Comparison of the transformation behavior ofakaganrite with that previously observed for ferrihydrite indicated that the composition of the reaction product depended strongly on the transformation conditions, i.e., pH and the presence of foreign species. The nature of the solid precursor was important insofar as its degree ofcrystallinity governed the dissolution kinetics and its surface properties influenced interaction with any foreign species in the system.

Transformation of Akaganéite Into Goethite and Hematite in the Presence of Mn

“…This result, together with the observation that Mn did not suppress formation of hematite from ferrihydrite if Mn was added to ferrihydrite (and not coprecipitated with it), indicates that Mn does not influence hematite nucleation directly; instead Mn acts by influencing the rate of formation of the competing product, goethite. Hematite nucleates less readily than goethite (Cornell and Giovanoli, 1990). By retarding the dissolution ofakagan6ite, Mn reduces the supersaturation relative to goethite and, hence, its rate of nucleation, thus enabling hematite to form more competitively.…”

“…Transformation of a related iron oxyhydroxide, ferrihydrite, to hematite and goethite at room temperature and pH 7-8, required over one year to reach completion (Schwertmann and Murad, 1983). Therefore, to obtain transformation products within a reasonable time period and also to facilitate comparison with the previous study of the transformation of akaganrite alone (Cornell and Giovanoli, 1990), the majority of experiments was carried out at high pH. Experiments performed at pH 7.5-8.5 suggested that similar transformation mechanisms were involved both in highly alkaline media and at neutral pH.…”

“…Two specific questions were: (1) does the effect of Mn on the transformation of akagan6ite depend on whether Mn is added as Mn 2 § solution or as a crystalline solid, hausmannite (Mn304); and (2) does Mn directly suppress hematite formation. In the absence ofMn, akagan6ite transforms at pH > 12 into goethite and/or hematite (Cornell and Giovanoli, 1990). Mn could interfere in hematite formation either by stabilizing its precursor or by directly hindering nucleation of hematite.…”

“…Mn could interfere in hematite formation either by stabilizing its precursor or by directly hindering nucleation of hematite. Inasmuch as hematite forms from ferrihydrite by a solid-state transformation (Schwertmann and Fischer, 1966) and from akagan6ite by a dissolution-reprecipitation process (Cornell and Giovanoli, 1990), comparing the effect of Mn on the recrystallization of both ferrihydrite and akagan6ite could indicate at what stage Mn interferes in the formation of hematite.…”

Iron oxyhydroxide nanostructured in montmorillonite clays: Preparation and characterization

“…Akaganéite is used as a precursor for the preparation of other iron oxide phases such as hematite [6], goethite [7], and magnetite [8], in order to obtain particle morphologies that are unusual for these iron oxide phases [9]. The b-FeO(OH) phase has semiconductor (band gap of 2.12 eV), catalytic, and ion-exchange properties [10][11][12][13].…”