The Fe(II)-accelerated recrystallization of Fe(III) oxides is postulated to occur via coupled oxidative growth and reductive dissolution reactions enabled by surface potential differences between specific crystallographic terminations. To obtain a detailed understanding of these recrystallization mechanisms and atom exchange pathways, hematite microparticles (dominated by (001) and truncated by (012) surfaces) were reacted with Fe(II) and Fe(II)-oxalate at acidic pH to probe face-specific growth and dissolution fronts, respectively, using a variety of techniques. Atomic force microscopy analyses of the particles before and after interaction with the solutes were performed to correlate the reactions to morphologic changes at the surfaces. Significant changes after reaction with Fe(II) were not observed though substantial face-specific morphologic changes were evident after reaction with Fe(II)-oxalate, as the basal surface was found to be covered in hillocks instead of its characteristic large atomically flat terraces. These features appeared largely consistent with preferential etching of the (001) face relative to the (012) edges, by reductive dissolution by adsorbed Fe(II)-oxalate complexes. In an attempt to resolve Fe cycling pathways we reacted the hematite (Fe2O3, naturally abundant in 56Fe) microparticles with the same solutes except with the ferrous iron component isotopically-enriched in 57Fe. Using isotopic mapping probes, such as nanoscale secondary ion mass spectrometry, sorbed 57Fe distributions were visualized to reveal preferential locations of Fe(II) sorption and oxidative growth, and also regions consistent with dissolution in relation to morphological features. These results shed new light on surface reactivity and surface-mediated reaction mechanisms. Furthermore the collective findings provide insight into potential atom exchange pathways between Fe(II)aq and dominant Fe(III) (oxyhydr)oxides minerals during conditions that cycle Fe between the solid and solution phases, with important and broad implications on our understanding biogeochemical cycling of elements in the environment.

Understanding face-specific interaction of Fe(II) and oxalate with hematite using AFM coupled to NanoSIMS Fe-tracer imaging

256th National Meeting (2018)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.