Influence of Organic Ligands on the Redox Properties of Fe(II) as Determined by Mediated Electrochemical Oxidation

Fe(II) has been extensively studied due to its importance as a reductant in biogeochemical processes and contaminant attenuation. Previous studies have shown that ligands can alter aqueous Fe(II) redox reactivity but their data interpretation is constrained by the use of probe compounds. Here, we employed mediated electrochemical oxidation (MEO) as an approach to directly quantify the extent of Fe(II) oxidation in the absence and presence of three model organic ligands (citrate, nitrilotriacetic acid, and ferrozine) across a range of potentials (E-H) and pH, thereby manipulating oxidation over a broad range of fixed thermodynamic conditions. Fe(III)-stabilizing ligands enhanced Fe(II) reactivity in thermodynamically unfavorable regions (i.e., low pH and E-H) while an Fe(II) stabilizing ligand (ferrozine) prevented oxidation across all thermodynamic regions. We experimentally derived apparent standard redox potentials, E-H(phi), for these and other (oxalate, oxalate(2), NTA(2), EDTA, and OH2) Fe-ligand redox couples via oxidative current integration. Preferential stabilization of Fe(III) over Fe(II) decreased E-H(phi) values, and a Nernstian correlation between E-H(phi) and log(K-Fe(III)/K-Fe(II)) exists across a wide range of potentials and stability constants. We used this correlation to estimate log(K-Fe(III)/K-Fe(II)) for a natural organic matter isolate, demonstrating that MEO can be used to measure iron stability constant ratios for unknown ligands.

Automated summary

This study used mediated electrochemical oxidation to directly measure the extent of Fe(II) oxidation in the presence of different organic ligands, and obtained the apparent standard redox potentials and stability constant ratios of Fe(III)/Fe(II). The experimental results showed that different ligands had different effects on the reactivity of Fe(II), which is of great significance for understanding the role of Fe(II) in biogeochemical processes and contaminant attenuation.