Electrochemical Properties of Natural Organic Matter (NOM), Fractions of NOM, and Model Biogeochemical Electron Shuttles

In this study, cyclic voltammetry was used to characterize the redox properties of natural organic matter (NOM). Using a stationary platinum working electrode, minimal concentrations of electrolyte, and dimethyl sulfoxide (DMSO) as the solvent, we were able to resolve two pairs of oxidation and reduction peaks for a fraction of Georgetown NOM that is enriched in polyphenolic moieties (NOM-PP). Applying our method to other fractions of Georgetown NOM, and to samples of NOM from a wide range of other sources, gave cyclic voltammograms (CVs) that generally contained fewer distinguishing features than those obtained with NOM-PP. For comparison, CVs were also obtained using our method on six quinone model compounds: anthraquinone-2,6-disulfonate (AQDS), lawsone, juglone, menadione, menaquinone-4, and ubiquinone-5. The CVs of these quinones were similar in shape to the CV of NOM-PP, consistent with the notion that quinones are the dominant redox-active moieties associated with NOM. Quantitative analysis of the peaks in these CVs showed that the peak potentials (Ep) were separated by more than 0.059 V and that the peak currents (i(p)) were linearly related to the square root of the scan rate (v0.5) and concentration (C) for both NOM-PP and the model quinones. Equivalent results were obtained with a rotating Pt disk electrode. From this we conclude that NOM-PP and the model quinones undergo similar sequences of two one-electron, quasi-reversible, diffusion controlled, electron transfers at the Pt electrode surface in DMSO. Although it is difficult to relate these results to Nernstian standard potentials vs the standard hydrogen electrode (SHE) under aqueous conditions, it is clear that the apparent formal potential for NOM-PP lies between the corresponding potentials for menadione and juglone and well above that of AQDS. Attempts to derive correlations between Ep and i(p) for the NOMs with quantifiable electrode response and other measurable properties of NOM (including trace metal content and UV-vis absorbance) did not yield any strong relationships.