Methodological limitations to determining acidic groups at biochar surfaces via the Boehm titration

The Boehm titration is frequently employed to characterize acidic groups at biochar surfaces. However, biochars contain inorganic basic components (carbonates, oxides, hydroxides), inorganic acidic species (silica, alumina) and organic acids (carboxylic acids, phenols, humic-like substances) that can be differentially solubilized in the Boehm bases, rendering invalid results. This is demonstrated for two biochars. Prior to titrations (Boehm or continuous), biochar should be pretreated with NaOH to remove solubilizable acidic species, and then with HCl to remove solubilizable basic components and protonate carbon acid sites. Pretreatment is successful when direct and indirect titrations yield identical results and no precipitate is observed.