Fe Electron Transfer and Atom Exchange in Goethite: Influence of Al-Substitution and Anion Sorption

The reaction of Fe­(II) with Fe­(III) oxides and hydroxides is complex and includes sorption of Fe­(II) to the oxide, electron transfer between sorbed Fe­(II) and structural Fe­(III), reductive dissolution coupled to Fe atom exchange, and, in some cases mineral phase transformation. Much of the work investigating electron transfer and atom exchange between aqueous Fe­(II) and Fe­(III) oxides has been done under relatively simple aqueous conditions in organic buffers to control pH and background electrolytes to control ionic strength. Here, we investigate whether electron transfer is influenced by cation substitution of Al­(III) in goethite and the presence of anions such as phosphate, carbonate, silicate, and natural organic matter. Results from ⁵⁷Fe Mössbauer spectroscopy indicate that both Al-substitution (up to 9%) and the presence of common anions (PO₄^3‑, CO₃^2‑, SiO₄^4‑, and humic acid) does not inhibit electron transfer between aqueous Fe­(II) and Fe­(III) in goethite under the conditions we studied. In contrast, sorption of a long-chain phospholipid completely shuts down electron transfer. Using an enriched isotope tracer method, we found that Al-substitution in goethite (10%), does, however, significantly decrease the extent of atom exchange between Fe­(II) and goethite (from 43 to 12%) over a month’s time. Phosphate, somewhat surprisingly, appears to have little effect on the rate and extent of atom exchange between aqueous Fe­(II) and goethite. Our results show that electron transfer between aqueous Fe­(II) and solid Fe­(III) in goethite can occur under wide range of geochemical conditions, but that the extent of redox-driven Fe atom exchange may be dependent on the presence of substituting cations such as Al