Characterization of complexation between biochar-derived dissolved organic matter and two typical heavy metals by a multi-spectral technique

Biochar-derived dissolved organic matter (BDOM) is important in the migration and transformation of heavy metals (HMs) within aqueous environments by directly influencing their morphologies and environmental behavior. This study systematically investigated the binding characteristics and mechanisms between biochar-derived dissolved organic matter (BDOM) and heavy metals (Pb(II)/Cd(II)) using multi-spectroscopic techniques. BDOM extracted from four biochar types (pig manure, cow manure, corn stalk, and sewage sludge) was characterized by UV–Vis, EEM-PARAFAC, SFS, FTIR, and 2D-COS analyses. Results revealed that BDOM primarily consisted of three humic-like components and one protein-like component, with sewage sludge BDOM exhibiting the highest fluorescence intensity. Metal titration experiments demonstrated stronger fluorescence quenching for Pb(II) (logK_M = 3.89–5.15) than Cd(II), though the Ryan-Weber model only successfully fitted Pb(II) binding parameters. 2D-COS analysis identified polysaccharides as the primary binding sites, followed by aliphatic compounds. Notably, despite their low abundance, protein-like components and non-fluorescent polysaccharides played significant roles in HMs binding. These findings provide molecular-level insights into the source-dependent interactions between BDOM and HMs, offering a theoretical basis for predicting HMs mobility and transformation in biochar-amended environments.