The electron storage capacity (ESC) of black carbon (BC); i.e., the quantity of electrons BC can store and reversibly donate/accept, determines its capacity to mediate abiotic and microbial redox processes, many of which are relevant to biogeochemistry, greenhouse gas production, contaminant fate and remediation. Electrochemical and chemical analyses have shown the ESC of BC (such as plant-derived biochar) was on the order of a few mmol/g. However, it remains unknown where ESC is located. The spatial distribution of ESC is important because it controls the accessibility of ESC and the kinetics of redox reactions involving BC. In this study, we fully saturated the ESC of a model BC (a wood-derived biochar) and exposed it to silver ion (Ag+), a tagging agent that can diffuse into BC, react with the stored electrons, and be immobilized in situ as elemental silver nanoparticles (nAg). Up to 2.49 mmol Ag+ per gram BC was reduced to nAg, which served as an ESC marker and was visualized by electron microscopy. Abundant and dense nAg were observed on the BC surface. Furthermore, the cross-section images of a microtomed BC particle showed ubiquitous and well-dispersed nAg in the particle interior. The finding that a significant portion of the ESC resided in the interior of BC explains the partial (bio)accessibility of the ESC and the pore diffusion-controlled nature of the electron exchange reactions in BC previously reported. These results further suggest the silver tagging method can be a new and useful tool for investigating ESC and elucidating how black carbon mediates biogeochemical redox reactions.

Quantifying and visualizing the spatial distribution of black carbon's electron storage capacity

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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