Assessing Iron Complexation by Dissolved Organic Matter Using Mediated Electrochemical Oxidation

Abstract

Fe^II is an abundant reductant in the environment that participates in numerous biogeochemical cycles and pollutant attenuation. Fe^II in aquatic environments can exist as a complex with dissolved organic matter (DOM), where organic ligands in DOM can modulate iron’s redox potential (E_H) and henceforth reactivity as a reductant. Previous studies have assessed the reactivity of Fe^II-complexes using probe compounds, although these compounds are limited in their ability to profile Fe^II oxidation across multiple thermodynamic conditions (i.e., both pH and E_H) and fail to validate the E_H of Fe(II)-complexes via their direct measurement. This study elucidated the redox potentials of Fe^II-DOM complexes via mediated electrochemical oxidation (MEO) and assessed the extent of Fe^II oxidation at two different applied E_H and pH regimes. Furthermore, we used a Nernstian-based model calibrated with a training set between known iron-ligand thermodynamic stability constants and their respective measured potentials to indirectly determine the stability constants of both Fe^II and Fe^III-DOM complexes as a function of E_H and pH. This work highlights the versatility of MEO as an electrochemical technique and is the first to assess stability constants of Fe-complexes with aquatic DOM isolates. We also discuss linkages between speciation modeling and redox reactivity of Fe^II.