Assessing\nIron Complexation by Dissolved Organic Matter\nUsing Mediated Electrochemical Oxidation

Fe<sup>II</sup> is an abundant reductant in the environment\nthat\nparticipates in numerous biogeochemical cycles and pollutant attenuation.\nFe<sup>II</sup> in aquatic environments can exist as a complex with\ndissolved organic matter (DOM), where organic ligands in DOM can modulate\niron’s redox potential (<i>E</i><sub>H</sub>) and\nhenceforth reactivity as a reductant. Previous studies have assessed\nthe reactivity of Fe<sup>II</sup>-complexes using probe compounds,\nalthough these compounds are limited in their ability to profile Fe<sup>II</sup> oxidation across multiple thermodynamic conditions (i.e.,\nboth pH and <i>E</i><sub>H</sub>) and fail to validate the <i>E</i><sub>H</sub> of Fe(II)-complexes via their direct measurement.\nThis study elucidated the redox potentials of Fe<sup>II</sup>-DOM\ncomplexes via mediated electrochemical oxidation (MEO) and assessed\nthe extent of Fe<sup>II</sup> oxidation at two different applied <i>E</i><sub>H</sub> and pH regimes. Furthermore, we used a Nernstian-based\nmodel calibrated with a training set between known iron-ligand thermodynamic\nstability constants and their respective measured potentials to indirectly\ndetermine the stability constants of both Fe<sup>II</sup> and Fe<sup>III</sup>-DOM complexes as a function of <i>E</i><sub>H</sub> and pH. This work highlights the versatility of MEO as an electrochemical\ntechnique and is the first to assess stability constants of Fe-complexes\nwith aquatic DOM isolates. We also discuss linkages between speciation\nmodeling and redox reactivity of Fe<sup>II</sup>.