Structure and Charge Hopping Dynamics in Green Rust

Abstract

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.

Neptunyl (Np) interaction with green rust,

“…The alignment of the secondary phase along the edges of the GR suggests that cationic NpO þ 2 aquo ions interact with the electron donor sites at the edges for efficient electron transfer and finally forms colloidal NpO 2 . The preferred redox interaction along GR particle edges is consistent with the electron transfer study by Wander et al (2007), where it was established that electrons are transferred several orders of magnitude faster near the edges. If we assume homogeneous reduction and precipitation from solution, the experimentally observed non-random distribution of the insoluble Np end product can only be explained by consideration of opposing surface charges of GR and colloidal NpO 2 .…”

The inevitable journey to being

“…The changes in local redox state locally distort the crystal structure inducing, via phonon interactions, the formation of localized polaron quasi-particles [171,172]. The polaron-based local dilations produced by the oxidation of green rust migrate through Fe 3þ hole polaron transport, charge hopping interdependently correlating with anion migration along the interlayer towards the interior [173]. The double valent anions such as orthophosphate ( Pi 22 ), confined within localized polaron distorions, are also pushed along the interlayer (acting like a protein channel or pore) which thereby acts as a peristaltic pump.…”

Green Rust: The Simple Organizing &lsquo;Seed&rsquo; of All Life?&nbsp;&nbsp;<strong> </strong>

“…And the Fe 2+ -Fe 3+ distance of ~0.33 nm along the iron layers would allow electron hopping by quantum tunneling, though a small proportion of electrons could also jump across the layers. [18,19] The brucite layers trap a monomolecular interlayer of viscous solution producing solutions concentrated to around 14 molar! [7] Green rust is redox active and can rapidly reduce nitrate to the ammonium ion as well as the redox sensitive ions such as Mo VI to Mo IV .…”

“…[15,16] That green rust is capable of conformational changes during oxidative-reductive and protonation-deprotonation oscillations lends itself to such investigations. [7,17,18,19] We also speculate on the crucial steps taken toward what must have been the break-out metabolism and an organic takeover of the mineral precursor disequilibrium or 'free energy' converters, as they acquire organic frameworks in a process that we have called 'minerobiolization'. We then review the status of efforts to test these and related ideas bearing on the issue and conclude with a rather long list of falsifiable predictions of the submarine AHV as it now stands!…”