Interactions of the products of oxidative polymerization of hydroquinone as catalyzed by birnessite with Fe (hydr)oxides – an implication of the reactive pathway for humic substance formation

Abstract

Polyphenol polymerization (PP) catalyzed by MnO₂ is an important pathway in humification processes in soils. Due to a lack of aliphatic carbons, the products of PP are considered to be humic substance-like materials (HQs), which are subject to adsorption by Fe (hydr)oxide upon their formation. However, the effects of the interactions of HQs with Fe (hydr)oxide in humification processes have received less attention. In this study, hydroquinone was reacted with birnessite for 1 d (HQ-1). After the removal of residual birnessite, the filtrates were incubated for another 7 and 20 days and were denoted as HQ-7, and HQ-20, respectively. The spectroscopic analyses of the HQ samples indicated that oxidative polymerization of hydroquinone occurred within 1 d. With an increase in incubation time, the molecular structure of the resultant HQ continued to change and became increasingly similar to that of natural humic acid (HA), even in the absence of birnessite. Upon adsorption of the HQs by Fe (hydr)oxide, changes in the IR absorption band indicated the complexation of HQ carboxyl groups with metal centers on the mineral surfaces. HQ-20 was preferentially adsorbed on the Fe (hydr)oxides because it contained a higher number of carboxyl groups than their counterparts with a smaller molecular weight. In addition, the steric arrangements and the distributions of the adsorption sites on Fe (hydr)oxide, closely matching the structures of larger molecules, may also contribute to the preferential adsorption. This study implies that HQs with a larger molecular size can be accumulated in soils while reacting with Fe (hydr)oxides during their formation, and the association of HQs with aliphatic carbons derived from the Maillard reaction may contribute to humic substance (HS) formation.