Impact of Different Oxygen Concentrations on the Spectral Characteristics of Iron Oxide-Enhanced Abiotic Humification Products

Humification is a carbon sequestration process where organic residues are transformed into humic substances (HS) through microbial conversion or chemical oxidation and polymerization. The active functional groups present in the resulting humic acids (HA) significantly impact the environmental chemical behavior of pollutants. Natural minerals containing Mn/Fe/Al/Si oxides can facilitate the oxidative polymerization of organic monomeric small molecules (e.g., polyphenols, reducing sugars, amino acids) into HA. Oxygen, as a natural oxidant, can synergistically enhance non-biological humification in the presence of metal oxides and improve the quality of the resulting HA. However, the mechanisms of abiotic humification enhancement by metal oxides under varying oxygen atmospheres and the impact on the evolution of organic matter remain unclear. This study used catechol and glycine as representative precursors for polyphenol and protein degradation, respectively, with iron oxide enhancing humification reactions. Solution and extracted HA samples were analyzed by ultraviolet-visible spectroscopy (UV-Vis), Fourier-transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) during the reaction process. The results indicated that the aromatization of organic matter, as indicated by UV254, followed the order of 21%>40%>0% oxygen concentration. HA yields mirrored the UV254 findings, with an approximately 25% increase in HA production observed at 21% oxygen concentration compared to 40% and 0% oxygen concentrations. XPS spectroscopic analysis revealed that 21% oxygen concentration promoted the conversion of amino group N to pyrrole N in HA. In comparison, 40% oxygen concentration facilitated the conversion of amino group N to amide N, and under 0% oxygen concentration, a substantial amount of amino group N remained in HA. FTIR analysis indicated that center dot OH generated from the aromatic compound ring cleavage contributed to oxygen-containing functional groups(C-O, COOH) in HA. FTIR combined with hyperspectral 2D maps from UV-Vis suggested that oxygen's involvement promoted the conversion of amino acids into C-O in HA and the transformation of amides into aromatic and aliphatic moieties in HA. Structural equation modeling (SEM) indicated that releasing iron ions was a key factor in promoting HA yield, and 21% oxygen concentration accelerated organic matter polymerization by enhancing iron ion release. Furthermore, the divalent iron/trivalent iron ratio was negatively correlated with HA yield, implying that oxygen indirectly affected humification by promoting iron ion conversion. In conclusion, changes in oxygen concentration affect the transformation of humification products, subsequently influencing HA yield and functional group composition. This study reveals the impact of oxygen concentration on humification products, offering references for refining humification theory.