Soils and sediments contain large amounts of pyrogenic carbonaceous materials (PCMs) of natural and industrial origins. Additions of PCMs into soils and sediments are used to sequester pollutants. While pollutant sorption to PCMs has been extensively studied, electron transfer reactions involving PCMs have received significantly less attention and remain only poorly understood at a fundamental level. Yet, a number of recent studies provide evidence that PCMs play an important role in redox reactions in both natural and engineered systems: PCMs may catalyze the reductive transformation of organic pollutants and may lower N2O emissions from soils, possibly due to shuttling electrons towards denitrifying microorganisms. Despite this growing evidence for the importance of electron transfer reactions involving PCMs, a systematic study of their redox properties is missing.  This contribution will present the results of a systematic assessment of the redox properties of biomass-derived chars. We used mediated electrochemical analysis to quantify the electron accepting and donating capacities of a total of 19 chars that were prepared from different feedstock and under different pyrolysis conditions. All tested chars were redox-active and reversibly accepted and donated up to 2 mmol electrons per gram of char. Chars formed at intermediate pyrolysis temperatures (400-500�C) showed the highest total capacities to accept and donate electrons, as compared to chars prepared at lower (200-300�C) and higher (600-700�C) temperatures. Systematic trends between the electron accepting capacities of the chars and their nominal carbon oxidation states and their spectroscopically-determined quinoid C=O contents suggest quinoid groups as major electron accepting moieties. Our results largely substantiate the emerging perception that electron transfer reactions involving PCMs are more important than commonly realized. The results suggest that redox-active PCMs with desired properties may be designed for environmental engineering applications that involve defined electron transfer reactions.

Redox properties of biomass-derived chars as assessed by combined electrochemical and spectroscopic analyses

248th National Meeting (2014)

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