Oxygen-containing functional groups in land-derived humic acids—II. Changes in the oxygen distribution of humic acids during early diagenesis as revealed by derivatization methods

Comparison of adsorption capacity of young brown coals and humic acids prepared from different coal mines in Anatolia

“…The carboxyl (COOH) group increases in abundance with humidification, reacts readily with metals, and gradually dissociates between pH 2.5 and 7 to form the carboxylate (COO-) group. The phenolic hydroxyl (OH) group is more abundant in the early stages of decomposition, is derived from lignin in woody plants, reacts less with metals, and dissociates between pH 8 and 13.5 [24][25][26][27][28]. The carbonyl (C O) group increases in abundance with humidification, is the main functional group and transforms into the COOH group under oxidizing conditions.…”