Unraveling the Immobilization Mechanisms of Biochar and Humic Acid on Heavy Metals: DOM Insights from EEMs-PARAFAC and 2D-COS Analysis

The structural complexity and variability of dissolved organic matter (DOM) significantly affect its binding capacity with heavy metals (HMs). This study evaluated the remediation efficacy of biochar (BC) and humic acid (HA) on Mn- and Cu-contaminated soils using four maize pot treatments: 3% BC (YB3), 6% BC (YB6), 3% BC + 1% HA (YB3H), and 6% BC + 1% HA (YB6H). The results showed that compared to the control (Y), Results showed Mn and Cu concentrations in rhizosphere soil decreased by 11.08–17.76%, while DOM content increased by 44.2–103.83%. BC enhanced DOM aromaticity and humification, further intensified by HA, leading to a more complex and stable DOM structure. PARAFAC identified four DOM components in BC (BC-DOM): C1 (fulvic-like), C2/C3 (humic-like), and C4 (protein-like), and in BC + HA (BC + H-DOM), an enhanced structural complexity with additional aromatic C–H groups was observed. 2D-COS analysis revealed that in BC-DOM, polysaccharides primarily interacted with Mn and Cu, followed by carboxylic acids and phenolic hydroxyl groups, but in BC + H-DOM, aromatic C–H groups preferentially bound Cu before polysaccharides, showing weaker affinity for Mn. These results elucidate the DOM-mediated immobilization mechanisms of BC and HA for HMs, offering insights for soil remediation and carbon sequestration strategies.