Stability and Speciation of Hydrated Magnetite 111 Surfaces from Ab Initio Simulations with Relevance for Geochemical Redox Processes

AS Katheras and K Karalis and M Krack and AC Scheinost and SV Churakov, ENVIRONMENTAL SCIENCE & TECHNOLOGY, 58, 935-946 (2023).

DOI: 10.1021/acs.est.3c07202

Magnetite is a common mixed Fe-(II,III) iron oxide in mineral deposits and the product of (anaerobic) iron corrosion. In various Earth systems, magnetite surfaces participate in surface-mediated redox reactions. The reactivity and redox properties of the magnetite surface depend on the surface speciation, which varies with environmental conditions. In this study, Kohn-Sham density functional theory (DFT + U method) was used to examine the stability and speciation of the prevalent magnetite crystal face 111 in a wide range of pH and E- h conditions. The simulations reveal that the oxidation state and speciation of the surface depend strongly on imposed redox conditions and, in general, may differ from those of the bulk state. Corresponding predominant phase diagrams for the surface speciation and structure were calculated from first principles. Furthermore, classical molecular dynamics simulations were conducted investigating the mobility of water near the magnetite surface. The obtained knowledge of the surface structure and oxidation state of iron is essential for modeling retention of redox-sensitive nuclides.

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