Chemical methods for determining the electron storage capacity of black carbon

Electron storage capacity (ESC) is a new and important property that determines the capacity of a black carbon to mediate abiotic and microbial electron transfer reactions in natural and engineered systems. It is necessary to develop accurate and reproducible methods to measure black carbon's ESC in order to understand its redox behavior and to predict its capacity to support redox transformation of contaminants in subsurface environments. In this study, we developed chemical methods that employed combinations of reductants and oxidants of different redox potentials – Ti(III) citrate or dithionite as reductant, and ferricyanide or dissolved O(2) as oxidant – to measure the ESC of a wood-derived biochar. Pore diffusion within biochar particles was rate-limiting and controlled the timescale for redox equilibrium and ESC measurements. • The new methods can handle sample mass on the order of a gram; • Sample pretreatment (e.g., oxidation via aeration) is necessary to produce consistent results; • For a given reductant-oxidant pair, colorimetric (or potentiometric) measurements gave constant and reproducible ESC.