Ion sorption processes are important in understanding and predicting the fate of toxic metal ions in our environment. In this research, we observed sorption of lead (Pb) ions to the barite (001) -water interface using in situ x-ray reflectivity (XR) and resonant anomalous x-ray reflectivity (RAXR) measurements. Previous results on barite (001) surfaces dominated by terraces reveal complex Pb sorption behavior. These results indicate that Pb ions adsorb to and incorporate into the barite (001) surface at ~0-5 Å above the surface and ~0-2.5 Å into the crystal. Increased sorption correlates with increasing [Pb]. Our preliminary data shows that precipitation of a Pb-rich phase occurs at [Pb] ≥ 225 µM, and is resistant to removal. While previous measurements were conducted to understand sorption behavior at terraces on smooth crystal surfaces, the extent of ion sorption at defects is unclear. On freshly cleaved (001) surfaces with natural varying roughness, lead sorption increases as surface roughness increases. Surface roughness arises from defects on the barite surface, which are highly reactive sites where Pb can create additional bonds to the surface as compared to flat terraces. The height of the sorbed Pb from the surface is related to the sorption mechanism (e.g. inner-sphere vs. outer-sphere adsorption). At higher concentrations, there appears to be a small decrease in the height of the sorbed Pb, consistent with adsorption closer to the surface, possibly from sorption at defects. These results emphasize an important role that defects play in the extent of ion adsorption at mineral surfaces, which can control differences in sorption measured on minerals in laboratory versus field settings.

Pb adsorption to the barite (001)-water interface correlates with surface roughness

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.