Cadmium Immobilization on Fe Oxyhydroxides Enhanced by DOM Using Single-Molecule Determinations

The groups from dissolved organic matter (DOM) enhance cadmium (Cd) immobilization on Fe oxyhydroxide, while it is difficult to evaluate the contributions of different groups on the binding configurations and strength between Cd and Fe oxyhydroxides because of DOM's complex composition and lack of in situ methods. Here, we selected organic small molecules with representative functional groups to investigate the molecular mechanisms of Cd immobilization on goethite using batch experiments, solid characterization, theoretical calculations, and single-molecule force spectroscopy (SMFS) combined with K-means analysis. These organic molecules increase Cd adsorption on goethite, with carboxyl groups showing the most substantial enhancement (increased by 81.7%). Solid-state characterization reveals that the adsorption of organic molecules is the primary driver of enhanced Cd immobilization, promoting the formation of new Cd-O(C) and Cd-O(Fe/C) bonds. Especially, thermodynamic analysis indicates that Cd-O(C) and Cd-O(Fe/C) bonds represent 75%-80% of total Cd binding configurations in the presence of organic molecules. Notably, the newly developed thermodynamic results show a strong correlation with the adsorption capacity, which may deepen the understanding of DOM-mediated Cd immobilization on Fe oxyhydroxides, offering crucial insights into Cd behavior and providing a theoretical basis for pollution control in subsurface and superficial environments.