Dissolved organic matter-metal interactions in industrial soils: Insights from integrated spectroscopic analyses

Soil heavy metals behavior is substantially influenced by their interaction with dissolved organic matter (DOM). Here, the different spectral characteristics of soil DOM in typical industrial agglomeration, and binding characteristics of copper (Cu) and its co-occurring lanthanide europium (Eu) with DOM were investigated by multi-spectroscopic techniques. Excitation-emission matrix combined parallel factor analysis (EEM-PARAFAC) identified four primary soil DOM. In-situ scanning transmission soft X-ray microscopy (STXM) imaging offered a microscopic characterization of the co-precipitation of Cu and Eu on DOM. Fluorescence titration shows that Cu(Ⅱ) created more stable complexes with DOM, with higher conditional stability constants (log K M = 5.53–5.60) compared to Eu(Ⅲ) (log K M = 4.87–4.92), likely because of stronger site-specific interactions. In contrast, ligand concentrations of DOM-Eu(Ⅲ) were about 8–10 times those of DOM-Cu(Ⅱ), mainly due to non-specific binding. Finally, 2D-FTIR-COS analysis revealed that both Cu(Ⅱ) and Eu(Ⅲ) interacted with similar functional groups in the soil DOM, especially carboxylic O–H and phenolic C–O, although Cu(Ⅱ) showed a stronger connection with these groups. This research highlights the key properties of soil DOM and their binding behaviors with Cu(Ⅱ) and Eu(Ⅲ) at the molecular level, providing insights for predicting the environmental behavior of heavy metals in industrial contaminated soils, and providing a scientific basis for the remediation of Cu-contaminated soils and resource recovery strategies for rare earth elements such as Eu. • Cu and Eu concentrations in DOM < 0.45 μm were 5 and 2 times higher than in 50–250 μm. • STXM observed co-adsorption of Cu and Eu in DOM, with Eu showing greater enrichment. • Cu(Ⅱ) forms more stable specific complexes with soil DOM than Eu(Ⅲ). • Eu(Ⅲ) exhibits higher non-specific interactions and greater overall binding capacity. • Phenolic and carboxylic are primary binding sites for Cu(Ⅱ) and Eu(Ⅲ) in soil DOM.