Transformation of Natural Organic Matter in Simulated Abiotic Redox Dynamic Environments: Impact on Fe Cycling

Redox fluctuations within redox dynamic environments influence the redox state of natural organic matter (NOM) and its interaction with redox-active elements, such as iron. In this work, we investigate the changes in the molecular composition of NOM during redox fluctuations as well as the impact of these changes on the Fe-NOM interaction employing Suwannee River Dissolved Organic Matter (SRDOM) as a representative NOM. Characterization of SRDOM using X-ray photoelectron spectroscopy and Fourier transform infrared spectrometry showed that irreversible changes occurred following electrochemical reduction and reoxidation of SRDOM in air. Changes in the redox state of SRDOM impacted its interaction with iron with higher rates of Fe(III) reduction in the presence of reduced and reoxidized SRDOM than in the presence of the original SRDOM. The increased rate of Fe(III) reduction in the presence of reduced SRDOM was due to the formation of reduced organic moieties on SRDOM reduction. The Fe(II) oxidation rate also increased in the presence of reduced SRDOM due to the formation of redox-active moieties that were capable of oxidizing Fe(II). Overall, our study provides useful insights into the changes in SRDOM that may occur in redox dynamic environments and the associated impact of these changes on Fe transformations.