ELECTRON TRANSFER AT OXIDE-WATER INTERFACES

Reductive transformation of Fe(III)-oxides such as hematite by aqueous dissolution/re-precipitation is a long-studied but poorly understood part of the biogeochemical iron cycle. The transformation involves conversion of surface Fe(III) to Fe(II) by electron transfer from adsorbed reductant molecules followed by Fe(II) solubilization and precipitation of new phases. In the absence of direct scrutiny, single site models of this dissolution process have prevailed. Some Fe(III)-oxides such as hematite are semiconductors with a propensity for fast charge redistribution and limited electron diffusion by solid-state electron transport. Microscopic evidence suggests that sites of Fe(II) release are coupled to a process of mobile charge segregation through the surface to specific crystallographic regions. At room temperature charge carrier mobility depends on the diffusion of small polarons occurring by Fe(II/III) valence interchange. Molecular dynamics simulations and ab initio calculations predict high rates of small polaron hopping in the hematite bulk and at (001) and (012) surfaces in vacuum and in equilibrium with an overlying bulk water phase. The collective results suggest surface-specific Fe(II/III) valence cycling via oxidative Fe(II) adsorption and solid-state electron migration as a mechanism for oxide transformation. X-ray reflectivity measurements of Fe(II) interaction with hematite (001) show growth of an Fe(III) surface phase, confirming this view. The emerging picture for iron oxide transformation is one of multiple aproximal site communication by electron transport. This model has important implications for our understanding of chemical behavior at oxide-water interfaces, and therefore also the chemistry of soils and sediments. The case is an exemplary one justifying sustained research on the complexity and dynamics of mineral-water interfaces using advanced analytical probes combined with theory.