Characterization of redox reactions and phase transformation processes at iron mineral surfaces

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Highly bioavailable iron phases in groundwater form at conditions where surface bound Fe(II) is re-oxidized by abiotic processes. Such iron phases, though mostly present only as surface coatings, are thought to be instrumental in supporting an electron cycle by mediating heterogeneous oxidation and reduction processes. Through their high affinity for dissolved ferrous iron, surfaces of iron (hydr)oxides may act as electron donors and electron acceptors at the same time both for chemical and microbial processes. Only recently the high dynamics of surface properties and surface topologies of iron minerals in reducing environments has been fully recognized. Here, we focus on an improved understanding of heterogeneous redox reactions and subsequent phase transformations taking place at iron minerals. In situ characterization of the type and the dynamics of active redox species at mineral surfaces with existing spectroscopic methods, however, is very difficult if feasible at all. We therefore propose here to develop, validate and apply an indirect method (reactive tracer approach) based on changes of the stable isotopic composition of model oxidants reacting specifically with surface bound Fe(II) species. Using this novel approach we intend to characterize the properties, the dynamics of formation and eventually the significance of reactive iron coatings at geochemically relevant settings with experiments conducted under well controlled conditions in batch and column systems.