In sub- or anoxic conditions, reactions between ferric iron (Fe(III)) minerals with dissolved ferrous iron (Fe(II)) tend to transform minerals with lower crystallinity, such as ferrihydrite, into more crystalline phases, while thermodynamically stable minerals, such as goethite, tend to undergo recrystallization rather than transformation. However, in nature, minerals of varying crystallinity co-exist, and we still lack an understanding of how co-existing minerals could influence transformation or recrystallization pathways in heterogeneous systems. To learn how ferrihydrite (Fh) and goethite (Gt) interact with Fe(II) in a heterogeneous system, we reacted synthetic mixtures of Fh and Gt with 1 mM Fe(II) for 24 hours and followed the transformation products and kinetics using XRD and Rietveld fitting. Our results suggest that the co-existence of goethite with ferrihydrite leads to a faster transformation of ferrihydrite into goethite. To further investigate the mechanism that leads to faster transformation into goethite, in subsequent experiments, we isolated each Fe pool (Fh, Gt, and Fe(II)) with a different stable Fe isotope (e.g., 54Fe, 56Fe, 57Fe) and followed the fate of the atoms using a combination of 57Fe Mössbauer spectroscopy and ICP-MS. In the absence of goethite, atoms initially in the aqueous phase formed mostly lepidocrocite and some goethite, while atoms initially in ferrihydrite formed mostly goethite but also some lepidocrocite. In the presence of goethite, atoms from the aqueous phase formed a much higher fraction of goethite, which can be explained by Fe(II)-goethite electron transfer. Atoms from ferrihydrite still transformed into goethite and lepidocrocite, but a fraction of the formed goethite had very low crystallinity. We hypothesize that the new layer of goethite formed by Fe(II)-goethite electron transfer acts as a template and provides an extra pathway for the transformation of ferrihydrite into goethite. Our results demonstrate that the co-existence of goethite alters ferrihydrite transformation in the presence of Fe(II) and suggest the Fe atoms in the surrounding minerals should be considered when investigating mineral transformation and recrystallization in natural soils.

Co-existing iron minerals influence ferrihydrite Fe(II)-catalyzed transformation pathways

ACS Fall 2021

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