Redox properties of Fe-OM aggregates: Linking structure to redox reactivity

The biogeochemical cycling of carbon is closely intertwined with iron processes. At aquatic redox interfaces, Fe(II) oxidizes and rapidly undergoes hydrolysis, coprecipitating as Fe(III) with natural organic matter (NOM) and other ions to form short-range-ordered (SRO) Fe minerals that can protect carbon from biotic and abiotic transformations. Relevant Fe-NOM research is dominated by studies of organic carbon fate, yet comparatively little attention has been given to the influence of organic carbon chemistry on iron mineral structure and properties. Organic ligands in NOM bind with Fe and are known to interfere with Fe(III) polymerization, with the extent of interference depending on ligand type and concentration. In comparison to their “pure” counterparts, SRO Fe(III) (oxyhydr)oxide minerals associated with NOM may have altered redox potentials (Eh) due to changes in Fe speciation, coordination environment, and/or particle size. However, it remains unclear if a link exists between coprecipitate mineral properties (e.g., range of order), Eh, and macro-scale redox processes (i.e., extents and rates of redox reactions). In this project, we investigate and link bulk and atomic-scale structural properties of different coprecipitates to their redox properties measured via mediated electrochemistry. Coprecipitates were synthesized by titrating Fe(III) solutions in the presence of 4 model organic ligands at varying Fe:ligand molar ratios. Model ligands were chosen as NOM binding analogs based on binding strength (log K) and type (carboxylate vs phenolic). High resolution X-ray diffraction (HR-XRD) analysis of coprecipitates synthesized with carboxylate ligands show a decrease in coherent scattering domain both with increasing ligand concentration and number of carboxylate functional groups, indicating that carboxylates decrease Fe(III) crystallinity in a systematic fashion. These results were confirmed via Mössbauer spectroscopy (MBS), which showed a decrease in blocking temperature with increasing ligand and/or carboxylate content.  Interestingly, while coprecipitates synthesized with catechol followed this trend at high ligand ratios, lower ligand ratios promoted the transformation towards lepidocrocite and spinel-like phases, suggesting that the electron-donating properties of catechol steer early Fe(III) transformation pathways more rapidly than carboxylate ligands. Electrochemically, the Fe(III)-NOM coprecipitates were more reducible (i.e., greater reduction extent and rate) than ligand-free ferrihydrite controls. When paired with results from HR-XRD and MBS, this finding suggests that coprecipitate redox reactivity is controlled by crystallinity. Ongoing work is investigating the role of particle size in this relationship. Overall, these preliminary mechanistic results may help link the importance of reactive iron phases to carbon dynamics (persistence vs. mineralization) in the environment. Future microbial reduction experiments will be employed to understand how coprecipitate thermodynamics influence biological redox reactivities.