The role of glucose in abiotic humification pathways as catalyzed by birnessite

Sugars are among the most abundant biomolecules in the environment, however, their role in abiotic humification reactions is not yet fully understood. This paper describes the effect of the molar ratio of glucose to catechol (polyphenol pathway) and glucose to catechol and glycine (integrated catechol–Maillard pathway) on humification processes and reaction products, as catalyzed by birnessite (δ-MnO2), a ubiquitous soil mineral. Our results show that glucose enhances humification in the catechol and especially in the integrated catechol–Maillard reaction systems under the catalysis of birnessite. The visible, FTIR and NEXAFS spectroscopic data indicate that increasing the molar ratio of glucose to catechol and glycine in the integrated catechol–Maillard system enhances the formation of low-molecular weight, strongly aliphatic carboxylic Maillard reaction products in the supernatant, which are similar to natural humic acids. The NEXAFS data show that glucose also promotes the formation of carbonate (MnCO3) at the expense of the aliphatic carboxylic groups in the solid residues of the birnessite-catalyzed catechol–glucose and integrated catechol–Maillard reaction systems. It is thus concluded that sugars, such as glucose, can make a significant contribution in influencing the formation and nature of humic substances and the genesis of carbonates. Our findings demonstrate the important role of sugars in affecting abiotic humification pathways and related products in natural environments.