Roles of oxygen and Mn (IV) oxide in abiotic formation of humic substances by oxidative polymerization of polyphenol and amino acid

Abstract

Abstract The metal oxide-facilitated transformation of polyphenols and amino acids to humic substances (HS) is an essential process in soil and compost maturing processes. The objective of this research was to examine the specific roles of MnO2 and O2 in the formation of HS by oxidative polymerization of catechin (a polyphenol) and glycine (an amino acid). The results demonstrated that large amounts of fulvic acid (FA) and humic acid (HA) only formed in the presence of O2, while the presence of both O2 and MnO2 significantly enhanced HA formation, demonstrating their synergistic effects. The size of HA only increased in the presence of MnO2, but MnO2 without O2 was unable to convert FA to HA. Two-dimensional correlation spectroscopy results suggested that the sequence of the formation of HA functional groups was: C–OH in carboxylic acids >C–O in phenolic-OH > amide II and aromatic C–C > amide and carboxylate C=O > COO− >NH > hydrogen bond. Finally, XPS spectra of the MnO2 before and after the reaction showed that the Mn(IV) content decreased from >99% to 89.98% and 80.89% while the Mn(III) content increased from