The biogeochemical cycles of iron and carbon are tightly coupled in soil, sediment, and aquatic environments. Organic carbon (in the form of natural organic matter (NOM)) is often associated with iron (Fe) via complexation, sorption, or co-precipitation. Via these associations, a large fraction (20-40%) of organic carbon in soils and sediments has been hypothesized to be protected from microbial degradation. In addition, association with NOM has been suggested to control the redox state of Fe, protecting Fe(II) from oxidation to Fe(III) even under oxic conditions. These interactions between NOM and Fe thus affect the bioavailability of organic carbon and Fe(II) and Fe(III). While reactions between Fe and dissolved organic matter (DOM) have been studied extensively, the Fe – particulate organic matter (POM) interface has been poorly studied. This knowledge gap is critical given that (i) the majority of organic carbon is in particulate form and (ii) the functional group composition of POM is different from that of DOM, suggesting that the reactivity of Fe-DOM associates may not be representative of Fe-POM associates. This project investigates interactions between iron (Fe(II) and Fe(III)) and particulate organic matter (POM). We first synthesized Fe-POM associates via sorption under different pH conditions. We observed that Fe(II) sorbed to POM up to 50 mg Fe/g POM. Upon exposure to oxygen, oxidation of Fe(II) was accelerated by association with POM, contrary to previous observations of DOM inhibiting Fe(II) oxidation. The resulting oxidation product was a mixture of poorly crystalline Fe(III) oxyhydroxides and organically complexed Fe(III). We then set up incubations with these oxidized Fe-POM associates and the iron(III)-reducing bacterium Shewanella oneidensis MR-1 to determine the bioavailability of the Fe(III). We repeated this oxidation-reduction over three cycles, and determined Fe speciation, mineralogy, and bonding environment using Mössbauer spectroscopy and synchrotron-based X-ray absorption spectroscopy. To discern whether the bioavailability of Fe was controlled by the functional group composition of POM, we conducted these experiments using two types of POM sourced from bogs and marshes. The results of this work improve our understanding of the stability of Fe-OM phases in the environment, with wide implications for iron and organic carbon biogeochemical cycling.

Redox transformations at the iron (Fe) – particulate organic matter (POM) interface

ACS Spring 2022

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