Session: Biogeochemical Transformation in the Underground Environment Natural Processes and Engineered Implementations for Contaminant Abatement:

Electron storage capacity (ESC) is a property common to plant-derived biochar. ESC is an important property of biochar as it determines the capacity of biochar to store and reversibly exchange electrons with its surroundings. With growing interest in biochar ESC and its environmental application, it is essential to identify an accurate and reliable method(s) to measure ESC. Two methods have been applied to quantify the ESC of biochar: (1) mediated electrochemical analysis (MEA) and chemical redox titration (CRT). MEA quantifies the electrons transferred between a sample and the working electrode based on the current resulting from the electron transfer carried out by a redox mediator. CRT quantifies the electron transferred between a sample and a redox titrant, i.e., a non-sorbing chemical reductant or oxidant, based on titrant consumption. Although MEA and CRT are different in measurement techniques, time scale, and sensitivity, these methods have never been compared directly, as all ESC studies to date used only MEA or CRT. In this work, we measured the ESC of Soil Reef biochar (SRB) in two particle size fractions, 250-500 µm and <53 µm, using both MEA and CRT. MEA covers the ESC in the range of −0.49 V to +0.61 V (vs. SHE), where diquat and ABTS were used as redox mediators, whereas CRT covers the E_h range of –0.36 V to +0.80 V, where Ti(III) citrate and dissolved O₂ were used as reductant and oxidant, respectively. The ESC of SRB obtained by CRT for the two size fractions were 4.11±0.22 and 4.28±0.19 mmol/g, respectively, identical within measurement error. In contrast, the ESC of SRB measured by MEA was 0.48 mmol/g for 250-500 µm and 1.86 mmol/g for <53 µm, 12% and 43% of the ESC measured with CRT, respectively. These results show that (1) biochar ESC measured by MEA is particle size-dependent, whereas ESC measured by CRT is not, and (2) MEA results are likely underestimates since the method captured only a fraction of the ESC obtained by CRT. This is likely because a significant portion of the ESC resides in the biochar interior and can only be accessed through pore diffusion within biochar particles. Pore diffusion is a slow process that has a time scale of days, too long to be detectable as a current by MEA which has a time scale of an hour or less. Our results suggest that CRT can obtain the full ESC of biochar regardless of particle size and therefore should be the method of choice for measuring the ESC of biochar and other porous materials.

Pei-Chun Chiu

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