Analysis of Biochar's Acid-Base Properties.

Detailed knowledge of biochar’s intrinsic acid-base properties will be necessary to understand long-term soil-biochar interactions and to develop a working biochar classification scheme.  Knowledge of acid-base properties is important because biochar chemistry and its effects on soil pH will have cascading impacts on other aspects of soil chemistry, including cation exchange capacity, nutrient availability, and aluminum toxicity.  In this study, acid neutralization kinetics, Boehm titrations and carbonate analysis are used to characterize and quantify the acids and bases present in 500°C corn stover and red oak fast pyrolysis biochars. We hypothesize that (1) organic functional groups, oxides and hydroxides of base cations, and carbonates all contribute significantly to the acidity and/or alkalinity of biochar, and (2) alkalinity and acidity are regulated in part by feedstock. Preliminary results show that the corn stover char is more alkaline than the red oak char, in part due to a greater carbonate concentration. Reaction kinetics were found to be dependent on both feedstock composition and the initial concentration of H⁺. As measured using 0.05M HCl, the corn stover and red oak chars neutralized 1.43 and 0.23 mmol H⁺ per gram of char (equivalent to 7.2 and 1.08 CCE), and contained 0.388 and 0.095 mmol of carbonates per gram of char, respectively. The amount of CO₂ released by the reaction of char with H⁺ increased proportionally with the initial concentration of H⁺, and stoichiometrically exceeded the amount of acid neutralized. Therefore, the data also suggest that quantification of carbonates may be complicated by release of CO₂ during acid-catalyzed decarboxylation and reactions of H⁺ with other organic functional groups.