Electron exchange capacity of dissolved natural organic matter: further method development and interpretation using square wave voltammetry in dimethyl sulfoxide†

Abstract

Most measurements of the electron exchange capacity (EEC) of natural organic matter (NOM) have been done in water using mediated chronoamperometry (MCA), which gives precise results that are believed to be representative of the samples' current redox condition, but the broader significance of these EECs is less clear. In a recent study, we described a novel but complementary electrochemical approach to quantify EECs of 10 pyrogenic dissolved organic matter (pyDOM) and 6 standard/reference natural organic matter (NOM) materials without mediation using square-wave voltammetry (SWV) in dimethyl sulfoxide (DMSO). Comparison of the results obtained by MCA and SWV showed that SWV in DMSO gave larger EECs than MCA, by several-fold for NOM and 1–2 orders of magnitude for pyDOM. In this study, we describe an improved protocol for calibration of the SWV/DMSO method, which largely eliminates the difference in EECs from SWV and MCA for the standard/reference NOM samples. The results show that values obtained via the SWV method depend on the specific redox standards used for calibration (i.e., calibrant model compounds), with slopes that span 1.5 orders of magnitude due to variations in current response factors. For pyDOM, the higher values of EEC obtained by SWV were further verified and rationalized. Like the calibrant model compounds, it is proposed that the relatively large EECs for some pyDOM samples from high-temperature chars reflect a combination of hydrodynamic influences in our electrochemical cell, primarily related to electrode surface area to volume ratio and pyDOM size. A detailed explanation of the calibration method, choice of working electrode, DOM sorption effects, and cosolvent effects are discussed. The results obtained with this method suggest that the capacity of NOM for donating, accepting, and storing elections is an operationally defined property, the significance of which will depend on application, e.g., to carbon, metal, or nutrient cycling, pollutant attenuation, etc.