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

This article systematically investigates how crystal structure and oxidation state influence the dissolution mechanisms of pure and mixed-phase iron oxides, including combusted iron particles (CIPs), in aqueous oxalic acid (0.5 mol/l) at 60 °C. Using a continuous-flow capillary reactor combined with in situ X-ray absorption spectroscopy (XAS) and complementary techniques (XRD, Mössbauer spectroscopy, SEM, μCT), the study finds that α-Fe2O3 (hematite) dissolves via a non-reductive pathway, while spinel-type oxides such as γ-Fe2O3 (maghemite) and Fe3O4 (magnetite) undergo reductive dissolution, forming an amorphous structure distinct from known iron carbides or metallic iron. Mixed-phase particles containing both α- and γ-phases exhibit preferential non-reductive dissolution of the γ-phase, and despite similar bulk phase compositions, CIPs predominantly dissolve non-reductively due to their layered microstructure, which limits access to reductive pathways. The research highlights that particle phase composition and internal structure, rather than formal oxidation state alone, primarily govern dissolution behavior under the studied conditions, with implications for optimizing iron leaching and sustainable metal fuel applications.