Two-Dimensional Heterospectral Correlation Analysis Elucidates Photodegradation Pathways of Riverine Dissolved Organic Matter Using Excitation–Emission Matrix and Ultraviolet Spectroscopy

Photodegradation of dissolved organic matter (DOM) in aquatic environments can alter spectral fingerprints beyond microbial composition changes, which reduces traceback accuracy. Here, photodegradation effects on the fluorescence properties of DOM from typical sources were investigated using fluorescence excitation-emission matrix spectroscopy (EEMs), i.e., sheep excrement (EXC), urban sewage (URB), petrochemical wastewater (IND), and riparian topsoil (tDOM), over a 12-day photodegradation experiment. Six components (C1-C6) were identified: C1: lignin-derived intermediates, C2: tryptophan-like, C3: fulvic-like, C4/C6: tyrosine-like, and C5: photodegradation byproducts. Interestingly, C1, C4 and C6 dominated in EXC/IND-DOM, C5 in tDOM, and C2 in URB. Based on hetero-2D correlation spectroscopy (EEMs-UV) and moving window (MW) analysis, we found that protein-like in EXC-DOM and tDOM and amine-like in IND-DOM containing phenolic, aromatic, and alcoholic-hydroxyl are photosensitive and deeply decompose in 0-4 days during the photodegradation, while protein-like in URB-DOM is continuously decomposed. Moreover, lignin-derived was rapidly decomposed in 0-2 days during the EXC-DOM, URB-DOM, and tDOM degradation. According to the structural equation model, protein-like in EXC-DOM could be deeply degraded indirectly through C5, whereas that in IND, URB and tDOM might be directly degraded. Furthermore, lignin-derived might be directly decomposed in EXC, tDOM, and URB. These findings not only reveal the photodegradation mechanism of DOM from different sources but also are conducive to traceability in natural and engineered water bodies.