Enhanced darkening effect from the interaction of MnO₂ and oxygen on the component evolution of amino-phenolic humic-like substances

Humification is greatly enhanced by metallic oxides in nature, and the related products are critical to various environmental processes. However, little is known about the interaction between metallic oxides and oxygen in promoting the oxidative polymerization of small organic molecules during the humification process. The synthesis of humic-like acids (HLAs) with MnO2 was performed in the presence and absence of oxygen, and the influence of oxygen and MnO2 on the composition evolution of aminophenolic HLAs was illustrated. The results of ultraviolet-visible (UV-Vis) spectra of reaction mixtures associated with two-dimensional correlation spectroscopy (2D-COS) combined with the XPS spectra of N 1s content changes in HLAs demonstrated that MnO2 induced pyrrole-type nitrogen formation and enhanced darkening. Furthermore, MnO2 mainly acted as a catalyst, and oxygen activated the regeneration of MnO2 by oxidizing free manganese ions, thus substantially promoting the formation and accumulation of HLAs, whereas it decreased the reaction rate of HLAs formation. Moreover, carbon dioxide release was found during the process of the formation of fulvic-like acids (FLAs), and the reaction was oxygen-independent. Additionally, the formation and transformation of products without MnO2 do not obey kinetics equations, whereas the darkening reaction with MnO2 followed the pseudo-secondorder and pseudo-zero-order kinetics equations. These findings provide new insights into the behaviours and fate of the oxygen-mediated humification process and related reaction products. (C) 2020 Elsevier Ltd. All rights reserved.