CRYSTAL STRUCTURE-DEPENDENT DISSOLUTION MECHANISMS OF FE OXIDES/ HYDROXIDES

Changes in the crystal structures of hematite (Fe₂O₃) and goethite (FeOOH) during dissolution were analyzed using in situ, time-resolved X-ray diffraction of powders in 3 M HCl solutions. Rietveld refinements indicated that the crystallite size of hematite decreased as expected during dissolution, but surprisingly, the refined crystallite dimensions for goethite increased. On this basis, we propose different dissolution mechanisms for hematite and goethite. Whereas hematite dissolved by sequential removal of surface layers, the open tunnels of goethite allowed H⁺ ions to diffuse into the structure and substitute for Fe³⁺, generating "hydrogoethite". The exchange of H⁺ for Fe³⁺ was accompanied by an expansion of the unit-cell volume of goethite during dissolution, leading to an overall destabilization of the structure that ultimately led to disintegration. The evolution of Fe occupancy in the octahedral sites of hematite and goethite supports this dissolution model. As goethite dissolution proceeded, Fe occupancy systematically decreased from 0.7984(4) to 0.732(5), whereas the refined Fe occupancy in hematite increased from 0.899(4) to 0.98(1). We attribute the increase in Fe occupancy as hematite dissolved either to sequential removal of defective surface layers and/or to diffusion of Fe³⁺ from the surfaces into the particle cores. In contrast, the decrease in the Fe occupancy as goethite dissolved reflects the leaching of Fe³⁺and the interior diffusion of dissolved H⁺, as facilitated by the open tunnel structure.