Electrochemistry of natural organic matter

Natural organic matter (NOM) plays an important role in a variety of environmental redox processes, ranging from fueling the global carbon cycle to mediating microbial interactions with minerals. However, the complex and indeterminant composition of NOM makes characterization of its redox activity challenging. Approaches that have been taken to address these challenges include chemical probe reactions, potentiometric titrations, chronocoulometry, and voltammetry. Advantages of the latter include that it can be diagnostic and quantitative, but applying voltammetry to the characterization of NOM has been challenging. Improved results have been obtained recently by using aprotic solvents, microelectrodes, and various applied potential waveforms. Results obtained with several voltammetric methods and DMSO as the solvent strongly suggest that quinone-like moieties are the dominant redox active groups. Correcting the associated peak potentials for comparison with estimates of NOM redox potentials in aqueous solutions shows that the range of peak potentials resolved by voltammetry spans most of the redox potentials obtained by other means that have been reported for various types of NOM or NOM model compounds. The multiplicity of electron-transfer steps that are distinguishable by voltammetry, and the likelihood that there is a degree of redox-coupling among these moieties, suggests that the redox potential of NOM might best be modeled as a continuum of redox potentials. The kinetics of electron exchange along this continuum will vary with factors such as the complex tertiary structure of NOM. The kinetic limitations created by this tertiary structure may be overcome with organic solvents (which allow the structure to unravel) or electron shuttles (which can pass into the structure), which accounts for the improved resolution of methods that use these strategies in electrochemical characterization of NOM.