Biogeochemistry of electron transfer to iron oxides in sediments

Iron oxides are abundant minerals in aquatic sediments, aquifer sediments, and soils. In natural environments iron easily changes oxidation state from Fe(III) to Fe(II) and this transition in many ways interrelates biotic and abiotic processes. In aquatic sediments the degradation of organic matter and pollutants is directly coupled to the reduction of iron oxides. Iron oxides may also act as a redox buffer trap for poisonous hydrogen sulfide. Adsorption to the surfaces of iron oxides in sediments controls the mobility of substances like phosphate, arsenic, nickel, lead and pesticides like glyphosat. The toxic and eutrophic state of lakes, coastal waters and aquifers are therefore directly related to the turnover of iron oxides. The coupling between adsorption processes and redox chemistry is very important because as the iron oxide is reductively dissolved, the adsorbed substances are mobilized. Probably this is the mechanism behind the release of arsenic to groundwater which threatens the health of millions of people in SE Asia .
Because of the unique importance of the turnover of iron oxides in sediments, the involved processes have been studied for decades by researchers from different disciplines. Still, surprisingly large gaps remain in our basic understanding of the processes. In particular, fundamental aspects of the pathways of electron transfer during the reduction of iron oxides are inadequately understood. We know that microbial catalysis is important, but there is a poor understanding of how microbes transfer electrons to iron oxides or, for that matter, which microbes perform this process in natural sediments. Neither do we understand in detail how the transfer of an an electron from an aqueous ion to the surface of an iron oxide takes place. Recently we have discovered that aqueous Fe(II) may induce catalytic changes in the structure of solid phase Fe(III) oxides and under certain conditions also highly reactive mixed Fe(II,III) oxides may form . These findings indicate that Fe-oxides are much more dynamic phases than we previously have perceived. The implications for the turnover of iron oxides in natural sediments are at present completely unknown and this uncertainty places some serious question marks at our process understanding and models of for example euthrophication of lakes and coastal waters. The project will bring together an interdisciplinary group of scientist. The group will use new state of the art methods and an integrated approach to hopefully provide some answers.

OVERALL OBJECTIVE
The overall aim of the project is therefore to bring together researchers from different fields to carry out fully integrated research on sediment model systems and in the field to obtain a more detailed understanding of the mechanisms controlling the rate of iron oxide reduction in natural sediments. Particular emphasis will be put on the role of electron exchange during both abiotic iron oxide reduction and microbially catalyzed iron reduction and the interrelationships between the abiotic and biological processes.