Comparing electron donating/accepting capacities (EDC/EAC) between crop residue-derived dissolved black carbon and standard humic substances.

Although dissolved black carbon (DBC) is an important component of dissolved organic matter (DOM), little is known about the electron transfer properties of DBC and their correlation with the structural properties. This study determined the electron donating/accepting capacities (EDC/EAC) of six DBC collected from water extracts of black carbon derived by pyrolyzing crop residues (soybean, wheat, rice, sorghum, peanut, and corn) using an electrochemical approach, along with their chemical and spectroscopic properties. The EDC of the tested DBC ranged from 2.42 to 7.10 mmole-(gC)-1and was generally much higher than the EDC (1.31-3.78 mmole-(gC)-1) of the four standard dissolved humic substances (DHS). In contrast, with the exception of the sorghum DBC, the EAC of all DBC (0.40-0.81 mmole-(gC)-1) was apparently lower than the EAC of DHS (0.87-1.68 mmole-(gC)-1). For the whole pool of DBC and DHS, a strong positive correlation (r = 0.94, P < 0.05) existed between the EDC and the phenolic group content quantified by base titration, suggesting that phenolic groups were the major electron-donating groups. The EAC and EDC of the tested DOM pool were further correlated with the components identified from the fluorescence excitation emission matrices and parallel factor (EEM-PARAFAC) analysis. This is the first study to show that DBC has significantly different electron transfer properties from humic substances and thus likely behaviors differently in many geochemical and environmental aspects. Capsule: Dissolved black carbon (DBC) has significantly different electron-donating/accepting capacities (EDC/EAC) from humic substances.