Molecular insights into the Tl(I) binding capacity and response sequences of soil humic acids from different sources

Humic acid (HA) significantly affects the migration, transformation, and environmental fate of Tl(I) through complexation. However, knowledge of the interaction processes and interfacial mechanisms between HA and Tl(I) remains lacking. Here, we investigated the Tl(I) binding characteristics of Sigma-HA and soil HAs from representative watersheds in China at the molecular level using adsorption models, an excitation-emission matrix with parallel factor analysis (EEM-PARAFAC), and two-dimensional correlation spectroscopy (2D-COS). According to the isothermal adsorption modeling results, SSHA exhibited the greatest attraction for Tl(I). However, YSHA exhibited the lowest value. On the one hand, the content of oxygen-containing functional groups on HAs may influence the adsorption capacity for Tl(I). On the other hand, the EEM-PARAFAC analysis results revealed that the UV humic-like component (C3), which is unique to SSHA, plays a crucial role in determining Tl(I) binding as a more effective complexing species (log K_M = 5.248). For the binding responsiveness of HAs, the 2D-COS results indicated that the carboxyl and phenolic hydroxyl groups associated with humic-like components in SSHA are the optimal structures for Tl(I) binding, whereas the polysaccharides and aliphatics in YSHA and PSHA are more sensitive. These findings increase our understanding of environmental behavior of Tl(I) and provide a solid theoretical foundation for evaluating the effectiveness of HA remediation in Tl-contaminated soils.