Biogeochemical iron cycling is controlled by the dissolution and precipitation of iron-bearing minerals. These reactions are regulated by the structure and dynamics of water at the mineral surface. Atomic-scale models of mineral-water interfaces are needed to predict the behavior of ferric minerals such as hematite (α-Fe2O3) in soils and natural waters. We present the most accurate models to date of the interface between the naturally abundant hematite (1-102) (r-cut) surface and bulk water or adsorbed ultrathin water films. X-ray crystal truncation rod (CTR) analysis indicates that adsorbed water closely coordinates aquo groups terminating the hematite surface. Large-scale ab initio molecular dynamics (AIMD) simulations reveal frequent molecular exchange between aquo groups and adsorbed water. Experiment and theory agree that adsorbed water is laterally confined in a ridged structure atop -FeOH2 groups, possibly screening underlying -Fe2OH groups from adsorption events. We also explore the relationships between interface structure, water ordering, and reductive dissolution through structure measurements as a function of solution pH and applied electrochemical potential. Our results call into question previous conclusions from classical molecular dynamics but validate several hypotheses of the behavior of hematite-water interfaces for which only indirect experimental evidence had previously been available.

Water structure and dynamics at the hematite (1-102) surface

253rd National Meeting (2017)

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