Influence of Polyphenols on the Integrated Polyphenol-Maillard Reaction Humification Pathway as Catalyzed by Birnessite
Keywords:abiotic humification, polyphenol-Maillard reaction pathway, pyrogallol, resorcinol, birnessite, NEXAFS
The significance of linking the Maillard reaction and polyphenol pathway as promoted by birnessite (δ-MnO2) into an integrated humification pathway has previously been demonstrated using a system containing catechol, glucose and glycine. The kinds and relative abundance of biomolecules (polyphenols, sugars and amino acids) involved in the proposed integrated humification pathway substantially vary with natural vegetation, microbial populations and activity, and the environment. Therefore, the objective of our study was to examine the effect of two structurally different polyphenols, namely, pyrogallol and resorcinol, in the integrated polyphenol-Maillard reaction system on the humification processes as catalyzed by birnessite. A number of experiments with an increasing concentration of polyphenol (pyrogallol or resorcinol) to a fixed molar ratio of Maillard reagents (glucose and glycine) were conducted under environmentally relevant conditions, and subsequently the degree of humification and the nature of the products formed were examined using XRD, visible absorption, FTIR and C K-edge and Mn L-edge NEXAFS spectroscopies. Our results clearly demonstrate that the structure and functionality of pyrogallol and resorcinol and their molar ratio to Maillard reagents in the polyphenol-Maillard reaction system not only significantly affect the humification processes but also the formation of rhodochrosite (MnCO3). The integrated pyrogallol-Maillard reactions systems formed the least MnCO3 and were more enriched in organic components in the solid phase than the integrated resorcinol-Maillard reaction system. The position and/or number of OH groups on the benzene ring of the polyphenols substantially affects the extent of direct electron transfer reactions, which impacts the reductive dissolution of birnessite, and oxidative polymerization and cleavage of biomolecules. This in turn affects the nature of the humic substances formed. The findings obtained in this study are of fundamental significance in understanding the importance of the nature and abundance of biomolecules in influencing abiotic humification pathways and carbonate formation in natural environments.
Permanent URL: http://hdl.handle.net/2047/d10011924