Novel Electrochemical Approach to Assess the Redox Properties of Humic Substances

Abstract

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.

Probing the pH Dependent Optical Properties of Aquatic, Terrestrial and Microbial Humic Substances by Sodium Borohydride Reduction

“…This is exemplified by the loss of long wavelength fluorescence emission in the HS (LHA and EHA) ( Figure 5) and by the short wavelength absorbance recovery (< 350 nm) with no concurrent long wavelength recovery (Figures 1-3). The quinone derived recovery of reoxidized HS is small, much lower than the expected concentration observed by electrochemical means (Aeschbacher et al, 2010) indicating quinones although an important redox buffer may not play a substantial role as a charge transfer acceptor and therefore do not play large role in the optics of terrestrially sourced material (EHA, LHA, SRFA, SRHA). Soil sourced and aquatic sources of HA/FA should fit into a hierarchy of digenesis.…”

Electron transfer capacity of soil dissolved organic matter and its potential impact on soil respiration

“…The ability of soil DOM to shuttle electrons from donors to acceptors was mainly due to the electroactive moieties (Scott et al 1998), which are ubiquitous in living cells, extracellular material, humic substances, and dissolved organic matter (Cory and Mcknight 2005). DOM can be reversibly oxidized and reduced (Aeschbacher et al 2010;Yuan et al 2012), by which electron transfer is sustained, and reversibility through multiple reduction-oxidation cycles is a key requirement for environmentally sustainable ETC (Ratasuk and Nanny 2007). The percentage of electrons still involving in the next redox cycle can be evaluated with ERR.…”

“…The redox properties of DOM, including the electron acceptor capacity (EAC) and electron donor capacity (EDC) of DOM and its reversibility in electron transfer, have been widely investigated with electrochemical techniques (Aeschbacher et al 2010;Bauer et al 2007;Nurmi and Tratnyek 2002).…”

Redox activity and accelerating capacity of model redox mediators during biodenitrification

“…[6,14À16] For example, the thermodynamics and kinetics of electron transfer reactions with both the electron donor (e.g., hydrogen sulphide as a bulk reductant) and the terminal electron acceptor (e.g., nitro aromatic explosives in contaminant degradation) and the stability toward irreversible side reactions are key factors in determining anaerobic biotransformation or biodegradation. [1,21,22] Aeschbacher et al [23] investigated two electrochemical methods to assess the redox properties of humic substances. A key question in the successful application of redox mediators is the manner of assessing, predicting and establishing the relationship between characteristics of redox mediators and their catalytic capabilities in various reactions.…”