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.

Figure 1

(A) Cyclic voltammograms recorded at t = 20 d in a suspension of char-400 in 50 mM ferricyanide, 0.1 M PB, 3 M NaCl (“char + ferri”, black curve); in controls in the absence of char (“ferri”, red dotted curve) or in the absence of ferricyanoide (“char”, grey curve). Recorded at a scan rate of 50 mV.s⁻¹ and an electrode rotation speed of 1000 rpm. (B) Zoom on the anodic plateaus corresponding to the mass transfer limiting current density j_la for ferrocyanide oxidation. The amount of electrons donated by the char is proportional to (j_la−j_la,ctrl), difference between the anodic plateau currents recorded in the presence of char and in the control without char.

Figure 2. Electrons donated over time by char-400 (blue circles), char-500 (orange triangles) and char-600 (red squares).

Error bars representing two standard deviations are too small to be visible (n = 2). For comparison with controls, the evolution of j_la values for char suspensions and solutions devoid of char are presented in Supplementary Fig. S2.

Figure 3. EDC (top) and EAC (bottom) of pinewood-based biochars pyrolysed at different HTTs.

Our data (black bars, t = 66 d) are compared with those of Klüpfel et al. (grey bars, with a zoom in inset for their EDC). Error bars represent 2 standard errors (n = 3 for all results except our EDC where n = 2). Note the 10-times difference in scale between the EDC and EAC charts.

Figure 4. Impact of the mass of char-600 initially suspended in the oxidative solution (all data for t = 30 d).

(A) CVs of char-600 suspensions of different masses of char (control devoid of char in red); 1000 rpm, 50 mV.s⁻¹; the inset presents the zoom on anodic plateaus. (B) Corresponding CAs recorded at +0.7 V vs. Ag/AgCl and 1000 rpm. Respective masses of char-600 are stated on the chart. (C) Linearity of the amount of electrons donated (ED) and the mass of char initially suspended. R² is the coefficient of determination for a simple linear regression. (D) Final E_h measured at open circuit potential, 1000 rpm.