The results provide fundamental insights into mechanisms by which natural organic matter affects the heavy metal uptake on phyllosilicate minerals through both competitive and cooperative processes.
The characterization of molecular-scale distributions of heavy metals at mineral-water interfaces is crucial to understanding the mechanisms of heavy metal sequestration and transport in the aqueous environments. In-situ observations using resonant anomalous X-ray reflectivity reveal the molecular-scale processes that control competitive adsorption at the muscovite surface between fulvic acid (FA, a major fraction of natural organic matter) and strontium (Sr, a representative heavy metal). We found that the directly-adsorbed portion of FA competes with Sr2+ for surface sorption sites, resulting in a decreased metal uptake adjacent to the surface. However, the outer portion of the FA film also provides additional sorption sites for the metal cation. We also found that pre-sorbed FA films may undergo conformational changes in time, resulting in reduced metal sequestration capacity compared to a fresh organic film.
These results provide new insights into the role that natural organic matter plays in controlling the mobility of Sr2+ in the near-surface environment, and suggests the need for systematic studies of molecular-scale sorption patterns of various metal ions (having a range of organophilicities) at representative mineral surfaces for a better understanding of contaminant mobility in nature.