Effects of structural heterogeneity in soil humic acids from distinct river basins on Tl(III) binding-reduction: Kinetics and mechanisms

Interactions between thallium (Tl) and natural organic matter (NOM) are key factors influencing the environmental fate of Tl in soil and water systems. As an important component of soil NOM, humic acid (HA) may participate in Tl binding and valence-state transformations. However, the structural composition of soil HAs differs among river basins. The mechanisms by which this variability influences Tl(III) speciation remain poorly understood. Therefore, soil HAs from the Songhua, Yangtze, and Pearl River basins were selected for this study and designated as SSHA, YSHA, and PSHA, respectively. Along with a commercial HA standard (Sigma-HA) used as a control, these samples were employed in kinetic experiments to investigate the coupled processes of Tl(III) binding and reduction. This study shows that the interaction between HA and Tl(III) follows stage-dependent kinetics, with rapid binding dominating the initial stage of the reaction, followed by a gradual increase in the extent of Tl(III) reduction. The pH of the system can modulate the protonation state of HA functional groups and the hydrolytic speciation of Tl(III), thereby influencing the transformation of Tl(III). Results for the ultraviolet parameters indicate that variations in the ultraviolet response of HA are primarily governed by its intrinsic structural characteristics. Spectroscopic analyses showed that HA induced the rapid and sustained reduction of Tl(III), with concurrent restructuring of carbon moieties on the HA surface and alterations in functional groups containing oxygen. These findings provide important insights into how NOM mediates Tl transport and transformation in aquatic environments.