How Si and Organic Matter Shape Ferrihydrite Transformation Pathways

Soil minerals such as iron (Fe) oxyhydroxides, are commonly associated with organic matter (OM) and silicon (Si) in complex, heterogeneous mineral frameworks that passivate the mineral surface. Thus, both Si- and OM-associated Fe minerals typically exhibit slower redox-induced transformation kinetics and follow distinct transformation pathways. Interestingly, certain OM coatings can also actively mediate Fe mineral transformations, leading to chemically modified structures and altered OM distribution patterns. However, the balance between the passive and reactive roles of OM, the potential synergistic or antagonistic effects of Si and OM on Fe mineral reactivity, and the impacts of mineral transformation on OM association and distribution remain poorly understood.

In this study, we investigated Fe(II)-catalyzed transformations of ferrihydrite with varying Si/Fe ratios in the presence of small organic compounds with different functional group compositions, under both oxic and anoxic conditions. Abiotic reductive dissolution of ferrihydrite was observed upon incubation with cysteine but not with glutathione, despite both compounds containing redox-active thiol groups. In the presence of cysteine under anoxic conditions, aqueous Fe(II) catalyzed the transformation of ferrihydrite into lepidocrocite, with 50% and 65% transformation observed within 6 and 21 days, respectively, as confirmed by XRD, Mössbauer spectroscopy, and TEM.

In contrast, Si-ferrihydrite (Si/Fe = 0.09) displayed a slower transformation extent (33%) and rate, with transformation products appearing only after 10 days of anoxic incubation. Moreover, both goethite and lepidocrocite formed, indicating that Si also influences the mechanism of mineral transformation. Surprisingly, while no mineral transformation of Si-ferrihydrite was detected by XRD or Mössbauer spectroscopy after 6 days, TEM imaging revealed the presence of a more crystalline and porous intermediate phase. This unidentified phase exhibited bifurcated and non-aligned growth patterns, suggesting it may serve as a precursor to more crystalline structures. These findings provide key insights into how Si and OM co-effects influence Fe mineral evolution pathways.

TEM imaging of 6-day incubated minerals (under anoxic conditions) also revealed differences in the affinities of organic coatings following ferrihydrite transformation. A distinct preference for pristine ferrihydrite over newly formed lepidocrocite was observed. However, in the case of Si-ferrihydrite, a uniform coating was maintained. This highlights how Fe mineral transformations may affect the affinity and distribution of associated OM, influencing its stability and persistence in the environment.