Black carbon such as biochar has been shown to possess electron storage capacity (ESC), which enables black carbon to store and reversibly exchange electrons with its surroundings. However, the origin of black carbon ESC remains unknown to date. To answer this question, we measured the ESC of cellulose, lignin, hemicellulose, and biochars prepared from these biopolymers, their mixture, and a pinewood, through pyrolysis at 350−650 °C. Mediated electrochemical analysis (MEA) with ABTS and diquat as mediators and chemical redox titration (CRT) with O 2 and titanium(III) citrate were used to quantify ESC. MEA could measure lignin ESC but significantly underestimated biochar ESC; CRT could measure the total reversible ESC of biochars but not that of lignin. Lignin was the only biopolymer that possessed ESC, which was largely destroyed during pyrolysis at 350 °C. After pyrolysis at ≥450 °C, the three biopolymers, their mixture, and pinewood all yielded biochars that possessed a highly reversible ESC of 1−2 mmol e − /g. This indicates that pyrolysis created reversible ESC of biochar from lignocellulosic biomass, presumably by converting oxygen-containing moieties of the biopolymers into (hydro)quinones in biochar. The implications of our findings for biogeochemistry, climate, contaminant fate, and engineering applications are discussed.

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Fe­(II) has been extensively studied due to its importance as a reductant in biogeochemical processes and contaminant attenuation. Previous studies have shown that ligands can alter aqueous Fe­(II) redox reactivity but their data interpretation is constrained by the use of probe compounds. Here, we employed mediated electrochemical oxidation (MEO) as an approach to directly quantify the extent of Fe­(II) oxidation in the absence and presence of three model organic ligands (citrate, nitrilotriacetic acid, and ferrozine) across a range of potentials (E H) and pH, thereby manipulating oxidation over a broad range of fixed thermodynamic conditions. Fe­(III)-stabilizing ligands enhanced Fe­(II) reactivity in thermodynamically unfavorable regions (i.e., low pH and E H) while an Fe­(II) stabilizing ligand (ferrozine) prevented oxidation across all thermodynamic regions. We experimentally derived apparent standard redox potentials, E H ϕ, for these and other (oxalate, oxalate2, NTA2, EDTA, and OH2) Fe-ligand redox couples via oxidative current integration. Preferential stabilization of Fe­(III) over Fe­(II) decreased E H ϕ values, and a Nernstian correlation between E H ϕ and log­(K Fe(III)/K Fe(II)) exists across a wide range of potentials and stability constants. We used this correlation to estimate log­(K Fe(III)/K Fe(II)) for a natural organic matter isolate, demonstrating that MEO can be used to measure iron stability constant ratios for unknown ligands.

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