Natural organic matter (OM), including humic substances, have long been known to be redox-active and to play important roles in electron transfer reactions in natural and engineered systems. These reactions include but are not limited to microbial OM reduction under anoxic conditions, reductive transformation of pollutants by reduced OM, enzymatic and photochemical OM oxidation, and chemical OM oxidation in water treatment. The redox properties and reactivities of OM have consequently received and will continue to receive considerable research interest in various scientific fields on both fundamental and applied levels.  This contribution will focus on novel insights into the redox properties and reactivities of OM that were recently gained by using mediated electrochemical analysis, a novel analytical approach. The first part of this contribution will introduce mediated electrochemical analysis and show that dissolved redox mediators can be used to facilitate redox equilibration between OM samples and the working electrodes in electrochemical cells. The unique features of the approach and selected results of the electrochemical analysis of a large and diverse set of OM samples will be presented. The second part of the contribution will highlight the biogeochemical importance of electron transfer to and from OM by results of two mechanistic laboratory studies. In the first study, we quantified the changes in the redox states of different humic substances over successive microbial reduction and O2-oxidation cycles. We show that electron transfer to OM was fully reversible and sustainable, that system thermodynamics controlled the extents of microbial OM reduction, and that OM accepted electrons over wide reduction potential ranges. In the second study, we used mediated electrochemical analysis to demonstrate fast, extensive and irreversible enzymatic oxidation of phenolic moieties in the OM in the presence of laccase. The implications of these studies for carbon cycling in temporarily anoxic wetlands will be discussed.

Humic substances redox redux: From molecular scale electron transfer to global biogeochemical cycles

248th National Meeting (2014)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.