Oxidation of the biochar surface or a lower pyrolysis temperature can result in functional groups attached to the surface, composed primarily of oxygen, hydrogen, and carbon. These functional groups can bind with nutrients and minerals, while the fused carbon rings support redox reactions and shuttle electrons around the microbial community attached to the biochar surface, potentially enhancing microbial metabolism and the cycling of nutrients in soils. Most of the biochar samples showed pKa values typical of carboxylic acids and phenols, ranging from 3.74 to 11.40. These pKa (Säurekonstante) values can be tentatively ascribed to various weak acidic groups that form hydrogen bonds29.
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