The role of dissolved Fe(II) concentration in the mineralogical evolution of Fe (hydr)oxides during redox cycling

Iron (Fe) (hydr)oxides play a key role in sediment biogeochemical cycling, particularly in dynamic environments that periodically undergo redox oscillations. The length of redox half cycles varies, depending on the environment, and is often brief in upland tropical soils, the vadose zone and at the sediment/plant root interface. One reaction that impacts Fe (hydr)oxide mineralogy in these environments is the Fe(II) catalyzed transformation of ferrihydrite. This reaction leads to the production of more crystalline phases under strictly anaerobic conditions and in cycling studies with relatively long anaerobic and/or oxic periods (e.g., 7 to 90 days). Ferrihydrite transformation has been investigated under strictly anaerobic conditions, or during a few (1–3), longer redox cycles; however, there is limited information describing the transformation of ferrihydrite, over multiple, short, sequential exposures to chemical reductants (e.g., dissolved Fe(II)) and oxidants (e.g., O₂). Here, we examine the mineralogical evolution of ferrihydrite over eight redox cycles during which it reacts with dissolved Fe(II) in the absence of oxygen (anaerobic conditions) for 48 h and is subsequently exposed to air (oxic conditions) for 18 h. Using X-ray absorption spectroscopy (XAS) and ⁵⁷Fe Mössbauer spectroscopy, the abundance of each mineral phase present after each anaerobic or oxic period, is quantified. Our results demonstrate that rapid redox cycling limits the extent of ferrihydrite transformation leading to disequilibrium between mineral phases. Additionally, magnetite stoichiometry increases throughout the study despite periodic exposure to oxygen. A deeper understanding of poorly crystalline Fe (hydr)oxide transformation during multiple, short redox cycles can provide insight into reactions controlling the speciation of redox active nutrients and contaminants in soils and sediments.