A geochemical investigation of heterogeneous redox reactions between Fe(II), Fe(III), and uranium

Iron (Fe) minerals and ferrous iron (Fe(II)) play an important role in the several natural elemental cycles, including the carbon cycle, nutrient cycles, and the cycling of metals. In this work we have characterized the reactivity structural Fe(II) in several Fe minerals and in natural soil with uranium. We have studied the reactivity of Fe(II) in solution with the Fe oxide goethite conditions relevant to many natural systems.

Green rusts are widely recognized as an intermediate phase in the Fe cycle. Here we investigate the reactivity of green rusts containing different structural anions with uranium^VI (U^VI). We have also investigated the effect of aqueous bicarbonate on U^VI sorption and reduction by green rusts. Our findings indicate that green rusts reduce U^VI to U^IV, and that environmentally relevant carbonate concentrations have little effect the rate and extent on this reaction.

We have also investigated U^VI reduction by structural Fe(II) in magnetite. Magnetite with varying stoichiometry (x = Fe²⁺/Fe³⁺) was reacted with U^VI. Results from x-ray absorption spectroscopy indicate that the redox properties of magnetite dictate whether magnetite reduces U^VI. In addition, magnetite reactivity can be "recharged" by electron transfer from aqueous Fe(II).

There is little evidence of the reactivity of structural Fe(II) towards U^VI in natural materials. We have characterized a naturally reduced soil and found it contains structural Fe(II) in clay minerals and a possible green rust-like phase. When this soil is exposed to U^VI we find that Fe(II) reduces a portion of the U added. Our work highlights the potential for abiotic reduction of U^VI by Fe(II) in reduced, Fe-rich environments.

We have used ⁵⁷Fe Mössbauer spectroscopy to study redox reactions of Fe(II) with goethite under biogeochemical conditions relevant to natural systems. When Fe(III) in goethite is substituted with aluminum or anions such as phosphate, silicate, carbonate, and natural organic matter are sorbed onto the surface of goethite, interfacial electron transfer occurs between sorbed Fe(II) and goethite. These results indicate that electron transfer between Fe(II) and Fe oxides occurs under environmentally relevant conditions. Electron transfer was blocked by phospholipids, however, suggesting electron transfer may be inhibited under eutrophic conditions.