New insights into the dissolution mechanisms of iron oxides and combusted iron particles in oxalic acid.

The influence of oxidation state and crystalline structure on the dissolution mechanisms of both pure iron oxides and combusted iron particles in aqueous oxalic acid (0.5 mol/l) at 60 °C was systematically investigated. Dissolution experiments were carried out in a temperature-controlled, continuous-flow capillary reactor, allowing for the removal of reaction products and thereby suppressing the autocatalytic reaction mechanism. The non-reductive dissolution of α-Fe2O3 was observed through in situ x-ray absorption measurements. In contrast, the dissolution of spinel-type oxides such as γ-Fe2O3 and Fe3O4 proceeded reductively, indicated by gradual changes in characteristic spectral features. Given that γ-Fe2O3 and Fe3O4 share a similar crystal structure but differ in the nominal oxidation state, this implies that the phase composition is decisive for the reductive dissolution. For mixed-phase particles consisting of spinel and rhombohedral phases (maghemite and hematite), the preferential dissolution of the spinel phase was observed. Despite the similar bulk composition of spinel and rhombohedral phases in the combusted iron particles (as confirmed by Mössbauer spectroscopy and x-ray diffraction analysis), dissolution predominantly follows a non-reductive pathway, with no preferential dissolution of the γ-phase. This unique dissolution behavior of combusted iron particles arises from their layered microstructure.