Competitive adsorption observed with resonant anomalous X-ray reflectivity

By Steve McGregorApril 1, 2010

We investigated how natural organic matter modifies the adsorption of heavy metals at the muscovite (001)-water interface using in-situ resonant anomalous X-ray reflectivity. 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.