Effect of Mn(II) incorporation on the transformation of ferrihydrite to goethite

Abstract

A series of Mn-substituted goethites were obtained by the addition of Mn(II) to ferrihydrite in alkaline media, at different times. The total aging period was 24 h. Chemical analysis indicated that the Mn mol fraction ( χ Mn) remained practically constant (ca. 8 mol%) in the oxalate–ammonium non-extracted samples. In the extracted samples, the χ Mn values increased with the earlier addition of the Mn(II) solution to the iron–oxyhydroxide suspension. XRD patterns of the obtained solids showed that the unique phase present was goethite. Although, in most of the experiments, the Mn(II) was added when the goethite phase was already formed, variations in the unit cell parameters were observed. The changes in the cell parameters followed the trend reported for coprecipitated samples. An enlargement of the acicular crystals is in line with the higher Mn incorporation. Lattice parameters and cell volume for the extracted samples were obtained by the Rietveld simulation of XRD data. Kinetics measurements indicate that the initial dissolution rate increases with the Mn content in the goethite structure, except in the samples where Mn was added later. Dissolution–time curves show a better fit with the two-dimensional contracting geometry law than with the cubic root law, this fact is attributed to the presence of two more exposed faces, both of different reactivity, in the acicular crystals of the Mn-substituted goethite. The dissolution has also well been described by the Kabai equation. Deviation from congruence indicates an inhomogeneous distribution of Mn into the goethite crystals due to the more belated addition of Mn to the solid phase.