Electrochemical Analysis of Proton and Electron Transfer Equilibria of the Reducible Moieties in Humic Acids

Abstract

Humic substances play a key role in biogeochemical and pollutant redox reactions. The objective of this work was to characterize the proton and electron transfer equilibria of the reducible moieties in different humic acids (HA). Cyclic voltammetry experiments demonstrated that diquat and ethylviologen mediated electron transfer between carbon working electrodes and HA. These compounds were used also to facilitate attainment of redox equilibria between redox electrodes and HA in potentiometric E(h) measurements. Bulk electrolysis of HA combined with pH-stat acid titration demonstrated that electron transfer to the reducible moieties in HA also resulted in proton uptake, suggesting decreasing reduction potentials E(h) of HA with increasing pH. This was confirmed by potentiometric E(h)-pH titrations of HA at different redox states. E(h) measurements of HA samples prereduced to different redox states by bulk electrolysis revealed reducible moieties in HA that cover a wide range of apparent standard reduction potentials at pH 7 from E(h)(0)* = +0.15 to -0.3 V. Modeling revealed an overall increase in the relative abundance of reducible moieties with decreasing E(h). The wide range of HA is consistent with its involvement in numerous environmental electron transfer reactions under various redox conditions.

Artificial electron acceptors decouple archaeal methane oxidation from sulfate reduction

“…‡ The midpoint reduction potentials of humic acids and melanin are not well defined. Both compounds can act as single electron acceptors due to their quinone moieties as shown experimentally for humic acids (56) and for melanin (57). The melanin used was a gift from Kenneth Nealson (University of Southern California) that is kindly acknowledged.…”

Outer-membrane cytochrome-independent reduction of extracellular electron acceptors in Shewanella oneidensis

“…The redox potential of riboflavin is, at 2210 mV (Stare, 1935), very similar to the potential of AQDS. For naturally occurring humic acids, no defined redox potentials, but rather ranges of standard reduction potentials from +150 to 300 mV (at pH 7), have been determined (Aeschbacher et al, 2011), since they form a supermolecular association of very diverse organic molecules. Still, since redox potentials are most likely not the reason for the observed kinetic data, we hypothesize that substrate specificity causes the observed differences.…”

Humic Substances and Extracellular Electron Transfer

“…Additionally, electrons and protons were consumed in a ratio close to 1:1 during electrochemical reduction of different HS samples, which also points toward the reduction of quinoid functional groups as the active electron acceptors (Aeschbacher et al 2010(Aeschbacher et al , 2011Maurer et al 2010).…”

“…Indications that quinones are the most important redox-active functional groups in HS at environmentally relevant conditions also come from the observation that electron transfer to HS is reversible (Aeschbacher et al 2011;Bauer and Kappler 2009;Ratasuk and Nanny 2007) and from the comparison of electrochemical and spectroscopic properties of HS with those of selected model quinones (Fimmen et al 2007;Nurmi and Tratnyek 2002;Ratasuk and Nanny 2007). Additionally, electrons and protons were consumed in a ratio close to 1:1 during electrochemical reduction of different HS samples, which also points toward the reduction of quinoid functional groups as the active electron acceptors (Aeschbacher et al 2010(Aeschbacher et al , 2011Maurer et al 2010).…”