Christian Ostertag-Henning

The transformation between iron oxide minerals is a topic of interst in many different fields: From high-temperature conversion of hematite to magnetite to metallic iron in steel production, to natural ore formation in e.g. hydrothermal systems and a multitude of oxidation-reduction reactions in the soil system. For steel manufacturing usually a gas phase-solid system at ambient pressure is investigated, for the hydrothermal/groundwater system studies a liquid-solid system. However, for both the subsurface storage of molecular hydrogen and the formation, transport or accumulation of natural hydrogen, a gas-fluid-solid system must be considered. Considering porous rocks for subsurface storage, mineralogical investigations have revealed that many possible formations in Germany contain hematite – in low amounts, but as a reactive surface that could oxidize molecular hydrogen in the storage complex. Unlike the gas-solid phase reactions in steel manufacturing, the processes involved in the transformation of hematite to magnetite and vice versa in liquid water remain unclear, with various theories proposed. Many researchers [e.g. 2] favour the reduction of surficial ferric iron in the crystal lattice during oxidation of dissolved H2 adsorbed onto the surface, and subsequent migration of either ferrous iron into the lattice or oxygen out of the lattice - for production of water as oxidation product of H2 during the processes However, Otake et al. [3] and others [4] suggest a non-redox process driven by hematite’s interaction with dissolved iron, where hematite transforms to magnetite through a dissolution and reprecipitation-driven replacement mechanism. Evidence is mounting for the attainment of partial (local) equilibria in the evolving system during the redox reactions. In the context of subsurface hydrogen storage in the project BiMiAb_H2 we investigated the reactions of molecular hydrogen with hematite and magnetite in the aqueous system near in situ conditions, i.e., at temperatures of 80-200°C and a pressure of 120-200 bar in high pressure reactors for up to four weeks. We used sealed gold capsules with natural hematite or magnetite, water (H2O) and H2 as starting material and quantified the conversion of H2 to H2O as well as the reduction of hematite to magnetite or the back reaction by X-ray diffraction and Raman spectroscopic analyses. In addition, we employed isotope tracer techniques to assess the contribution of oxygen from the hematite to the water by using mass spectrometry and Raman microscopy. The rate of H2 oxidation could not be described by a simple first-order reaction. For high-resolution insights, X-ray microscopy and TEM analyses on FIB sections were used to investigate the reaction progress within the grains at various locations. By examining the unique features at different structural locations on a single grain and by using Raman spectroscopy to analyse isotope label incorporation, we explored various theories on how hematite is replaced by magnetite in water. This includes considering whether the process is a solid-state reaction or driven by dissolution-reprecipitation, potentially involving tiny clusters of amorphous iron oxide [5]. References: [1] Abd Elhamid et al. (1996) J Solid State Chem 123:249-254 [2] Hallström et al. (11) Acta Mater 59: 53-60 [3] Otake et al. (2007) EPSL 257: 60-70 [4] Yin et al. (2022) EPSL 577: 117282 [5] Sun et al. (2007) Angew. Chem. 56: 4042-4046