Understanding the electron storage capacity of pyrogenic black carbon: origin, redox reversibility, spatial distribution, and environmental applications
Date
2021
Authors
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Publisher
University of Delaware
Abstract
Plant-based black carbon is produced globally at a rate of >100 million tons per year through both natural and anthropogenic processes. Electron storage capacity (ESC) is a novel property of black carbon, which determines its capacity to store electrons and reversibly exchange electrons with chemical and microbial agents in its surroundings. Research in recent years has shown that the ESC of plant-based black carbon is considerable, on the order of a few mmol per gram. This suggests that black carbon, which is ubiquitous in soils and sediments, may represent an enormous but previously unrecognized electron reservoir, influencing biogeochemical processes in terrestrial and aquatic environments. In addition, the discovery and understanding of ESC may greatly expand the applications of biochar—a class of man-made black carbon that has received significant interest in recent years for its potential environmental benefits. This is because the ESC may enable biochar to serve as a reactive medium to support the redox transformation of contaminants in natural and engineered systems. ☐ Despite the potential impact of black carbon on biogeochemistry and environmental remediation, prior work on its ESC is very limited. It was hence necessary to develop an improved understanding of the ESC. To this end, this dissertation presents an effort to understand the ESC of black carbon in terms of its origin, redox reversibility, spatial distribution, and environmental applications. Chemical methods were developed to quantify the ESC of black carbon, evaluate its redox reversibility, and characterize its spatial distribution. Based on a chemical redox titration method with Ti(III) citrate and dissolved O2, the ESC of plant-based black carbon ranged from 0.2−7 mmol/g, which was highly reversible over multiple redox cycles. Using a silver tagging method, a sizable fraction of ESC was shown to reside in the interior of black carbon particles, which explains partial microbial accessibility of the ESC and the pore diffusion-limited rate observed in the reactions involving the ESC of black carbon. Furthermore, by comparing the ESC of biopolymers and their corresponding chars, the origin of ESC was identified. The results show that the ESC of black carbon is not derived from its source biomass but created during pyrolysis, suggesting ESC is a common property of black carbon produced through pyrolysis of all lignocellulosic biomass. Finally, fully reduced (i.e., ESC-saturated) biochars were applied to abiotically transform insensitive munitions compounds, demonstrating the potential utility of biochar for removing organic pollutants through its ESC. ☐ This dissertation represents a major step towards understanding the ESC of black carbon. Findings from this dissertation will help explain black carbon-mediated redox processes, lead to new remediation strategies, and shed light on the impacts of black carbon on the cycling of carbon and other elements. The methods developed in this dissertation will be essential tools for further investigation of the ESC of black carbon and evaluating its impacts.
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Keywords
Electron storage capacity, Pyrogenic black carbon, Redox reversibility, Spatial distribution, Environmental applications