Binding mechanisms of Pb(II) adsorption by biochar-derived dissolved organic matter: unraveling site heterogeneity and kinetics through advanced spectral analysis

Abstract Biochar-derived dissolved organic matter (DOM) is a highly active component that plays a critical and complex role in the immobilization of heavy metals. This study systematically investigated the impact of DOM on Pb(II) adsorption by comparing the adsorption capacities of biochar before and after DOM removal, thereby unveiling the underlying mechanisms through advanced spectroscopic techniques. Adsorption experiments demonstrated that water-washed biochar (WBC) exhibited a markedly reduced adsorption capacity (35.0 mg g −1 ) compared to untreated biochar (BC) (96.2 mg g −1 ), highlighting the essential role of DOM in enhancing Pb(II) adsorption. Kinetic and isothermal analyses revealed that the adsorption process was predominantly chemical in nature, as evidenced by the excellent fit of experimental data to the pseudo-second-order, Freundlich, and Temkin models. FTIR and XPS analyses confirmed that oxygen-containing functional groups, including hydroxyl, carboxyl, carbonyl, and ether groups, actively participated in Pb(II) complexation in BC, WBC, and DOM. Spectral shifts and changes in the relative abundance of C–O and C = O bonds further supported this conclusion. The Pb 4f spectra indicated that Pb(II) was primarily retained as Pb 3 (OH) 2 (CO 3 ) 2 , with complexation identified as the dominant mechanism, followed by co-precipitation. UV differential log-transformed absorption spectra derived from titration experiments, revealed the heterogeneity of Pb(II) binding sites within DOM. Furthermore, excitation-emission matrix fluorescence spectroscopy coupled with parallel factor analysis (EEM-PARAFAC) identified three humic-like components. Among these, component C3 (humic-like and tyrosine substance) exhibited the strongest binding affinity for Pb(II). Hetero-2DCOS analysis, combined with additional spectroscopic techniques, demonstrated that carboxyl groups in humic-like substances were the most reactive sites for Pb(II) binding. These findings provide molecular-level insights into the structural and functional characteristics of biochar-derived DOM-Pb(II) complexes, offering a scientific basis for optimizing biochar-based strategies for heavy metal pollution remediation. Graphical Abstract