The electron donating capacity of biochar is dramatically underestimated

Abstract

Biochars have gathered considerable interest for agronomic and engineering applications. In addition to their high sorption ability, biochars have been shown to accept or donate considerable amounts of electrons to/from their environment via abiotic or microbial processes. Here, we measured the electron accepting (EAC) and electron donating (EDC) capacities of wood-based biochars pyrolyzed at three different highest treatment temperatures (HTTs: 400, 500, 600 °C) via hydrodynamic electrochemical techniques using a rotating disc electrode. EACs and EDCs varied with HTT in accordance with a previous report with a maximal EAC at 500 °C (0.4 mmol(e−).gchar−1) and a large decrease of EDC with HTT. However, while we monitored similar EAC values than in the preceding study, we show that the EDCs have been underestimated by at least 1 order of magnitude, up to 7 mmol(e−).gchar−1 for a HTT of 400 °C. We attribute this existing underestimation to unnoticed slow kinetics of electron transfer from biochars to the dissolved redox mediators used in the monitoring. The EDC of other soil organic constituents such as humic substances may also have been underestimated. These results imply that the redox properties of biochars may have a much bigger impact on soil biogeochemical processes than previously conjectured.

Microstructural and associated chemical changes during the composting of a high temperature biochar: Mechanisms for nitrate, phosphate and other nutrient retention and release

“…Since composting involves complex biotic and abiotic reactions in a fluctuating redox environment (Khalil et al, 2008), the detection of quinones, iron compounds with both Fe 2+ and Fe 3+ and the redox active nature of the surfaces of the composted biochar as revealed by Cyclic Voltammetry, confirms that a series of oxidation and reduction reactions have taken place. These redox reactions could involve both nitrification and denitrification as well as oxidation of organic matter (Klüpfel et al, 2014;Prévoteau et al, 2016;Quin et al, 2015). These reactions could result in an increase in the concentration of nitrates on the surfaces of organomineral layer which could, in turn, be weakly or strongly held in the pores in the organomineral layer.…”

Hydrophilic trace organic contaminants in urban stormwater: occurrence, toxicological relevance, and the need to enhance green stormwater infrastructure

“…195,196 For instance, it is known that biochars are redox-active and reversibly accept and donate electrons. [196][197][198][199][200][201] Numerous studies investigated sorption of hyphil-TrOCs on activated carbon and biochar in controlled laboratory batch experiments. 191,194,202,203 Ulrich et al 202 compared activated carbon and 18 different types of biochars in terms of their performance to remove hyphil-TrOCs (atrazine, benzotriazole, 2,4-D, diuron, fipronil, oryzalin, prometon, and TCPP) from synthetic stormwater.…”

Electrochemical characterization of natural organic matter by direct voltammetry in an aprotic solvent

“…83,104,105 The latter two applications of MER/MEO are indirect measures of the target properties and therefore require careful attention to the assumptions of the method in order to avoid misleading results. 106 Aprotic solvents have long been used in organic electrochemistry, [107][108][109][110][111][112][113][114][115] and also have only recently been shown to be useful for characterization of NOM redox properties. 98,116,117 This approach was rooted in recognition that NOM under aqueous conditions develops tertiary structure, which could result in protection of redox active functional groups from direct interaction with electrodes.…”