Influence of Organic Ligands on the Redox Properties of Fe(II) as Determined by Mediated Electrochemical Oxidation
Influence of Organic Ligands on the Redox Properties of Fe(II) as Determined by Mediated Electrochemical Oxidation
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DOI：
10.1021/acs.est.2c01782
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发表时间：
2022-06-08
影响因子：
11.4
通讯作者：
Chin, Yu-Ping
中科院分区：
文献类型：
--
作者：
Hudson, Jeffrey M.;Luther, George W., III;Chin, Yu-Ping
由于铁（II）在生物地球化学过程和污染物衰减中作为还原剂的重要性，它已被广泛研究。先前的研究表明，配体可以改变水Fe（II）氧化还原反应性，但其数据解释受到探针化合物使用的限制。在这里，我们采用了介导的电化学氧化（MEO）作为一种方法，直接量化了三种模式有机配体（柠檬酸盐、硝基三乙酸和亚铁锌）在电位（E-H）和pH范围内的Fe（II）氧化程度，从而在广泛的固定热力学条件下操纵氧化。铁（III）稳定配体增强了铁（II）在热力学不利区域（即低pH和E-H）的反应性，而铁（II）稳定配体（亚铁锌）阻止了所有热力学区域的氧化。我们通过氧化电流积分实验推导出这些和其他(草酸、草酸（2)、NTA(2)、EDTA和OH2）铁配体氧化还原对的表观标准氧化还原电位E-H（phi）。铁（III）对铁（II）的优先稳定化降低了E-H（phi）值，并且E-H（phi）与对数(K-Fe（III)/K-Fe(II)）之间存在着广泛的电位和稳定常数范围内的能斯梯相关关系。我们使用这种相关性来估计天然有机物分离物的log(K-Fe(III)/K-Fe(II))，证明MEO可以用于测量未知配体的铁稳定常数比。
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.