Electrochemical Characterization of Pyrogenic Carbons: Implications for Degradation of Groundwater Contaminants with Carbon-Based Amendments

Pyrogenic carbons (PCs) are important mediators of biogeochemical redox reactions and contribute to contaminant degradation in some remediation methods. Contaminant degradation by electron transfer from PC is influenced by its electron conductivity and storage capacity, and atom transfer reactions involving atomic hydrogen, but resolving their contributions is difficult. In this study, we developed a method to modify working electrodes with agarose-stabilized thin films of PCs and used them to characterize PC reactivity using 2,4,6-trinitrotoluene (TNT) as a model contaminant and several electrochemical methods. Chronoamperometry showed that TNT reduction rates increase with increasing PC temperature, and electrochemical impedance spectroscopy showed that this is because the heterogeneous electron transfer rate constants (<i>k</i>⁰) are inversely correlated to resistivity (<i>R</i>_p) of the PC films. As a result, PCs like biochars, which have higher charge transfer resistance (<i>R</i>_ct), give lower TNT reduction rates, whereas lower <i>R</i>_ct PCs like activated carbons give faster TNT reduction. Current from reduction of H₂O/H⁺ to H₂ (HER) was significant only for low <i>R</i>_p PC-films on Pt working electrodes. The reactive intermediates involved in HER react preferentially with quinonoid moieties on the PC, thereby competing with and limiting TNT reduction at low overpotentials (<ca. -0.6 V). Overall, contaminant reduction on PCs is more favorable by electron transfer than H atom mediated reactions under the conditions of this study.