Reversible Fe(II) uptake/release by magnetite nanoparticles

Magnetite commonly coexists with aqueous Fe2+ (Fe-(aq)(2+)) in anoxic subsurface environments. Complex interactions between magnetite and Fe-(aq)(2+) profoundly impact redox potential fluctuations in surrounding environment and biogeochemical cycles of important elements and contaminants. However, the ability of magnetite to act as a source/sink of electron equivalents through fluctuations in solution pH or the activity of Fe-(aq)(2+) remains poorly quantified. We systematically studied the interrelationships between equilibrium Fe-(aq)(2+) concentrations and structural versus surface-localized Fe(II)/Fe(III) ratios in magnetite using micro X-ray diffraction and synchrotron-based X-ray magnetic circular dichroism, respectively, under different controlled experimental conditions. Relative to pH 7, at pH 6 proton-promoted dissolution yields Fe-(aq)(2+) release from magnetite nanoparticles, coupled to a decrease in the structural Fe(II)/Fe(III) ratio by electron hopping along the octahedral sublattice from the particle interior to the surface. At pH 8, magnetite sorbs Fe-(aq)(2+), increasing both the structural and surface-localized Fe(II)/Fe(III) ratio. Amendments of Fe-(aq)(2+) inhibit acidic Fe-(aq)(2+) release and promote Fe-(aq)(2+) uptake at more basic conditions, whereas increasing magnetite loading facilitates Fe-(aq)(2+)-magnetite interaction at the same respective pH extremes. The reversible flow of Fe(II) across the magnetite-solution interface under different conditions implies that the redox reactivity of magnetite nanoparticles is quickly responsive to changes in environmental conditions, such as an increase in pH due to groundwater passing through carbonate-rich rocks, via a dynamic redistribution of electron equivalents between particle interiors and the solid/water interface.