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The reaction between aqueous Fe(II) and Fe(III) oxides is extremely complex, and can catalyze Fe(II)-Fe(III) electron transfer, exchange of Fe atoms between the aqueous and solid phases, mineral transformation, and contaminant reduction. Together, these processes represent a phenomenon referred to as Fe(II)-catalyzed Fe oxide recrystallization, which has been observed under controlled conditions in the laboratory for numerous Fe oxides. In the environment, Fe oxides are likely surrounded by organic carbon in various forms, but their potential to interfere with Fe(II)-catalyzed Fe oxide recrystallization, and its subsequent environmental relevance has not been well studied.
The Fe(II)-catalyzed recrystallization of stable Fe oxides goethite and magnetite was studied in the presence of several environmentally relevant classes of organic carbon. For both goethite and magnetite, Fe(II)-catalyzed recrystallization continued relatively undeterred in the presence of electron shuttling compounds, natural organic matter isolates, and extracellular polysaccharides. Slight inhibition was observed when spent media from dissimilatory iron-reducing cultures was present, but only by sorbing a long-chain phospholipid to the oxides was significant inhibition observed. The lack of interference by organic carbon indicates that Fe(II)-catalyzed Fe oxide recrystallization is likely to be relevant throughout a wide range of environments, and represents a significant process with regards to the geochemical cycling of Fe atoms, a claim supported by evidence of Fe(II)-driven isotope mixing in real soils. The movement of atoms during Fe(II)-catalyzed Fe oxide recrystallization is not limited to just Fe however. Multiple trace elements have been shown to exchange between the aqueous and solid phases along with Fe during the Fe(II)-catalyzed recrystallization of Fe oxides. The effect of organic carbon, both sorbed to the oxide surface and coprecipitated with the oxide, on Fe(II)-catalyzed atom exchange and transformation of ferrihydrite was studied. Again, the presence of organic carbon did not appear to influence Fe atom exchange kinetics. It also did not appear to influence the rapid transformation of ferrihydrite to lepidocrocite. The presence of organic carbon does appear to ultimately have implications for mineral transformation, as over longer time periods it stabilized lepidocrocite, preventing its subsequent transformation to magnetite or goethite.