Structure and Charge Hopping Dynamics in Green Rust

Green rust is a family of mixed-valent iron phases formed by a number of abiotic and biotic processes under alkaline suboxic conditions. Because of its high Fe2+ content, green rust is a potentially important phase for pollution remediation by serving as a powerful electron donor for reductive transformation. However, mechanisms of oxidation of this material are poorly understood. An essential component of the green rust structure is a mixed-valent brucite-like Fe(OH)2 sheet comprised of a two-dimensional network of edge-sharing iron octahedra. Liquid nitrogen temperature Mössbauer spectra show that any Fe2+−Fe3+ valence interchange reaction must be slower than approximately 107 s-1. Using Fe(OH)2 as structural analogue for reduced green rust, we performed Hartree−Fock calculations on periodic slab models and cluster representations to determine the structure and hopping mobility of Fe3+ hole polarons in this material, providing a first principles assessment of the Fe2+−Fe3+ valence interchange reaction rate. The calculations show that, among three possible symmetry unique iron-to-iron hops within a sheet, a hop to next-nearest neighbors at an intermediate distance of 5.6 Å is the fastest. The predicted rate is on the order of 1010 s-1 (at 300 K) and 103 s-1 (at 70 K), consistent the Mössbauer-based constraint. All other possibilities, including hopping across interlayer spaces, are predicted to be slower than 107 s-1. Collectively, the findings suggest the possibility of hole self-diffusion along sheets as a mechanism for regeneration of lattice Fe2+ sites, consistent with previous experimental observations of edge-inward progressive oxidation of green rust.