Molecular transformation of dissolved organic matter during persulfate-based advanced oxidation: Response of reaction pathways to structure

Persulfate (PS)-based advanced oxidation is an effective technology for remediating organically contaminated groundwater, which could induce the structural transformation of dissolved organic matter (DOM). Although the influences of DOM components on its structural transformation have been studied extensively, a comprehensive understanding, particularly at the molecular level, is still lacking. Here, selecting different DOM components including humic acid (HA), fulvic acid (FA), and natural organic matter (NOM), we comparatively analyzed the changes in structural characteristics during PS oxidation using optical spectroscopy and high-resolution mass spectrometry. The results showed a reduction in aromaticity and molecular weight of DOM during PS oxidation, with the largest decrease observed in NOM. The fluorescence intensity in HA increased, that of FA showed a decreasing and then increasing trend, while that of NOM significantly decreased after PS oxidation. Sequential oxidation responses of functional groups in DOM followed a specific order, with C-O of aromatic esters → C = O of polysaccharides or alcohols → C-C of quinone rings → C-O of aromatic ethers. The removal of low oxygen molecules (O/C < 0.4) and the formation of high oxygen molecules containing N and S, primarily composed of lignin and tannin molecules in DOM. The transformation pathways of DOM were further uncovered by mass difference analysis based on 26 types of transformation reactions. Compared to HA, FA is more prone to the reactions of carboxylic acid (−CH2O and − CO), while the amine reactions (−CONH + H2O) was dominant in NOM. Correlation analysis indicated that SO4•− and •OH reduced the number of aromatic molecules in the range of 500–600 Da by breaking C = C bonds, thereby decreasing the aromaticity, molecular weight, and fulvic-like acid content of DOM.