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

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.