Institute of Biogeochemistry and Pollutant Dynamics (IBP), ETH Zurich, Zurich, Switzerland.
Environ Sci Technol. 2010 Jan 1;44(1):87-93. doi: 10.1021/es902627p.
Two electrochemical methods to assess the redox properties of humic substances (HS) are presented: direct electrochemical reduction (DER) on glassy carbon working electrodes (WE) and mediated electrochemical reduction (MER) and oxidation (MEO) using organic radicals to facilitate electron transfer between HS and the WE. DER allows for continuous monitoring of electron and proton transfer to HS by chronocoulometry and automated acid titration, respectively, and of changes in bulk HS redox potential E(h). Leonardite Humic Acid (LHA) showed an H(+)/e(-) ratio of unity and a decrease in potential from E(h) = +0.18 to -0.23 V upon transfer of 822 mumol(e-) g(LHA)(-1) at pH 7, consistent with quinones as major redox-active functional moieties in LHA. MER and MEO quantitatively detected electrons in LHA samples that were prereduced by DER to different extents. MER and MEO therefore accurately quantify the redox state of HS. Cyclic DER and O(2)-reoxidation revealed that electron transfer to LHA was largely reversible. However, LHA contained a small pool of moieties that were not reoxidized, likely due to endergonic first electron transfer to O(2). Electron accepting capacities of 13 different HS, determined by MER, strongly correlated with their C/H ratios and aromaticities and with previously published values, which, however, were a factor of 3 smaller due to methodological limitations.
本文提出了两种评估腐殖质（HS）氧化还原特性的电化学方法：玻碳工作电极（WE）上的直接电化学还原（DER）和使用有机自由基促进 HS 与 WE 之间电子转移的介导电化学还原（MER）和氧化（MEO）。DER 允许通过计时库仑法和自动酸滴定分别连续监测电子和质子向 HS 的转移，以及 HS 整体氧化还原电位 E(h)的变化。莱纳德特腐殖酸（LHA）在 pH 7 时，当 822 µmole(e-) g(LHA)(-1)转移时，显示出 H(+)/e(-) 比为 1，并且电位从 E(h) = +0.18 降低至-0.23 V，表明醌类是 LHA 中主要的氧化还原活性官能团。MER 和 MEO 定量检测了通过 DER 预还原到不同程度的 LHA 样品中的电子。因此，MER 和 MEO 准确地量化了 HS 的氧化还原状态。循环 DER 和 O(2)-再氧化表明，电子向 LHA 的转移在很大程度上是可逆的。然而，LHA 含有一小部分不能被再氧化的部分，可能是由于首先向 O(2)传递的电子是吸热的。通过 MER 确定的 13 种不同 HS 的电子接受能力与它们的 C/H 比和芳香度以及先前发表的值强烈相关，但由于方法学限制，这些值小了 3 倍。
