Ferrihydrite is a key mineral in many groundwater systems. As the primary precipitate of ferric iron the understanding of its surface structure is key to its chemical reactivity. Recent advances by Marc Michel in the Schoonen group a bulk crystal structure is available for modeling. While the bulk structure requires protentation it is a sufficient starting point for modeling studies of the effect of water on the ferrihydrite surface. The predominant challenge is to rectify the tetrahedral-octahedral rearrangement which occurs surrounding many of the iron centers at the surface of the structure as well as providing key information on proton structure.
Once the rearrangement of atoms on the surface is determined it will be possible to calculate the oxidation reactions with ferrihydrite. For example, it is known that ferrihydrite reacts readily with arsenite. However, preliminary modeling calculations indicate that the reaction with aqueous ferric iron does not proceed due to the instability of the As(IV) intermediate. This suggests that the reaction with ferrihydrite is a strong candidate for a two electron transfer, an extraordinarily unusual event, but not an impossible one. Understanding this chemical process may shed light on a broad category of redox reactions including those involving biological cofactors and nanoscale materials.
This project will use plane wave density functional theory and explicit representation of the solvent interlayer. These are not typically small optimizations and calculations will require taking into account the anti-ferromagnetic behavior of ferrihydrite. Primary work will be performed by Matthew Wander and will bring together multiple research groups within CEMS and the Geosciences department.