Electron Transfer between Iron Minerals and Quinones: Estimating the Reduction Potential of the Fe(II)-Goethite Surface from AQDS Speciation

Redox reactions at iron mineral surfaces play an important role in controlling biogeochemical processes of natural porous media such as sediments, soils and aquifers, especially in the presence of recurrent variations in redox conditions. Ferrous iron associated with iron mineral phases forms highly reactive species and is regarded as a key factor in determining pathways, rates, and extent of chemically and microbially driven electron transfer processes across the iron mineral-water interface. Due to their transient nature and heterogeneity a detailed characterization of such surface bound Fe(II) species in terms of redox potential is still missing. To this end, we used the nonsorbing anthraquinone-2,6-disulfonate (AQDS) as a redox probe and studied the thermodynamics of its redox reactions in heterogeneous iron systems, namely goethite-Fe(II). Our results provide a thermodynamic basis for and are consistent with earlier observations on the ability of AQDS to "shuttle" electrons between microbes and iron oxide minerals. On the basis of equilibrium AQDS speciation we reported for the first time robust reduction potential measurements of reactive iron species present at goethite in aqueous systems (EH,Fe-GT ≈ -170 mV). Due to the high redox buffer intensity of heterogeneous mixed valent iron systems, this value might be characteristic for many iron-reducing environments in the subsurface at circumneutral pH. Our results corroborate the picture of a dynamic remodelling of Fe(II)/Fe(III) surface sites at goethite in response to oxidation/reduction events. As quinones play an essential role in the electron transport systems of microbes, the proposed method can be considered as a biomimetic approach to determine "effective" biogeochemical reduction potentials in heterogeneous iron systems.