Aqueous ferrous iron (Fe(II)) has been extensively studied due to its importance as a reductant in both biogeochemical processes and contaminant attenuation. Previous studies have shown that organic ligands can alter the redox reactivity of Fe(II) by linking the reduction kinetics of probe compounds to Fe-ligand stability complexes (log K). Here, we use mediated electrochemical oxidation (MEO) as a tool to profile and directly quantify the extent of iron oxidation in the presence of model organic ligands and Suwannee River natural organic matter (SRNOM) across a range of oxidizing potentials (EHMEO) and pH. At low pH, Fe(III)-stabilizing organic ligands and SRNOM expanded the range of potentials over which Fe(II) was oxidized. Conversely, an Fe(II) stabilizing ligand prevented oxidation across all ranges of EHMEO and pH. By integrating peaks from the Fe(II) oxidative current response at a fixed EHMEO, we were able to experimentally derive apparent standard redox potentials, EΦH, for specific Fe-ligand redox couples. Preferential stabilization of Fe(III) over Fe(II) decreased EΦH, and a linear correlation between EΦH and log(KFe(III)L/KFe(II)L) exists across a broad range of potentials and stability constants. Modeling of Fe EΦH in the presence of SRNOM showed increased reactivity of Fe(II). The results of this study demonstrate how speciation influences Fe(II) redox-reactivity, particularly in environmental compartments rich in natural organic matter. Further, our work also demonstrates that MEO can be used as an approach to measure iron stability constants for specific ligand complexes.

Influence of organic ligands and dissolved organic matter on redox properties of Fe(II) as determined by mediated electrochemical oxidation

ACS Spring Meeting 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.