Complexation of humic acid with Fe ions upon persulfate/ferrous oxidation: Further insight from spectral analysis

The complexation of humic acid (HA) with dissolved Fe ions is beneficial to 2,4-dinitrotoluene degradation by PS/Fe2+, while the mechanism on HA binding with Fe ions is still unclear and warrants further exploration. In this study, the binding characteristics of HA with Fe ions and structural variations of HA during the complexation with Fe ions were investigated. Synchronous fluorescence analysis showed that the complexation ability of HA with Fe species at acid (pH = 5.0) and neutral condition (pH = 7.0) is higher than that of alkaline condition (pH = 9.0 and 11.0). Different components in HA including humic-like fraction (C1), fulvic-like fraction (C2), protein-like fraction (C3), and microbial-derived humic-like fraction (C4) were identified by excitation emission matrix-parallel factor analysis (EEM-PARAFAC). The complexation ability of C1, C2, and C4 with Fe species is higher than that of C3, and C1 and C4 primarily contributed to the complexation of HA with Fe species. Moreover, the sequence of HA structural variation during the complexation with Fe species was elucidated by Fourier transform infrared spectroscopy coupled with two-dimensional correlation spectroscopy analysis (2D FTIR COS), and could be concluded as follows: ester -> quinoid rings -> aromatic groups -> aliphatic groups -> phenolic groups.