Magnetite - environmental biogeobattery and heavy metal remediator

Abstract

Iron (Fe) is an essential element that is widely distributed on earth and is associated with various geochemical cycles such as oxygen, nitrogen and carbon. Magnetite (Fe3O4) is one of the best known mixed-valent Fe minerals. The rare ability of magnetite to facilitate electron transport and storage between Fe-metabolizing microorganisms has earned it the name biogeobattery. However, it is unclear what consequences continued redox cycling has on the properties of magnetite and whether it can, in principle, continuously serve as an electron source and sink during prolonged exposure to redox cycling. It is also unclear to what extent the interactions of magnetite nanoparticles with heavy metals, which are contaminants in the environment, are altered by biotic magnetite oxidation or reduction. In the presented work it is demonstrated that magnetite nanoparticles could be used as a biogeobattery by the enrichment culture “culture KS” and Geobacter sulfurreducens respectively as an electron source and sink in two consecutive oxidation-reduction cycles over 41 days. This oxidation and reduction of magnetite by microorganisms significantly changes the surface properties, which will affect the ability of magnetite to adsorb heavy metal contaminants. It is presented that magnetite nanoparticles oxidized by culture KS and reduced by G. sulfurreducens exhibit unique adsorption capacities and efficiencies for the two heavy metals copper (Cu2+) and cadmium (Cd2+). Thus, the understanding of the roles that the mixed-valent Fe mineral magnetite can play in the environment is extended. Furthermore, the importance of microbial activity for the fate of magnetite and associated pollutants will be illustrated and that although magnetite can serve as a biogeobattery in successive redox cycles, it is lost over time.