Molecular Characterization of Water-Column and Sedimentary Dissolved Organic Matter in Coal Mining Subsidence Areas

Dissolved organic matter (DOM) in the water column and sediments in coal mining subsidence areas considerably affects the carbon cycle and ecological environment. This study analyzed the spectral characteristics, material composition, and molecular transformation of DOM in the water column and sediments of a coal mining subsidence area in Huaibei, China, using ultraviolet–visible absorption spectroscopy, three-dimensional fluorescence spectroscopy combined with the parallel factor analysis (PARAFAC) model, and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The DOM concentration (11.99 ± 4.73 mg/L) and the degree of DOM humification in the sediment were higher than those (9.54 ± 0.68 mg/L) in the water column. The PARAFAC model revealed three chemical components: fulvic acid–like, protein-like, and humus-like substances. DOM in the water column is primarily derived from endogenous inputs, characterized by low humification and high biological activity. In contrast, DOM in the sediments primarily originates from external inputs. FT-ICR MS analysis revealed that lignin-like (50.80–67.02%), protein-like (13.18–18.38%), and lipid-like (5.74–10.39%) components are the main DOM constituents in the water column and sediments, with lignin content in the water column (66.16%) being higher than that in the sediments (55.09%). Paired mass difference network analysis identified redox reactions as the predominant reactions in the water column and sediments, converting aldehydes or carbonyl compounds into acids or alcohols. This study elucidates the composition and transformation characteristics of DOM in the water column and sediments in coal mining subsidence areas, providing a scientific foundation for the protection and management of aquatic ecosystems.