LEAD SORPTION AT THE BARITE (001) AND (210)-WATER INTERFACES

Understanding sorption behavior of ions at the mineral-water interface is important to determine the fate of heavy metals in the environment. We used barite as a model substrate to understand heavy metal interactions with ionic crystals. The coverage and location of lead (Pb) sorbed to the (001) and (210) surfaces were studied as a function of aqueous Pb concentration using in situ specular x-ray reflectivity (XR) and resonant anomalous x-ray reflectivity (RAXR). For both surfaces in the absence of Pb, bariums and sulfates move small distances (< 0.15 Å) from their bulk lattice positions and waters adsorb to compensate for missing bonds created during formation of the surface. At [Pb] = 6 – 100 µM, there are small differences in the interfacial structures for both surfaces, primarily in the top 3 Å of the crystal. At [Pb] ≥ 225 µM, the (001) interfacial structures are significantly distorted, especially within the top 5 Å of the crystal and 7 Å of the solution near the surface. At the (210) surface, interfacial distortions are smaller and occur in the top 3 Å of the crystal. These changes in the interfacial structures in the presence of Pb imply Pb sorption. RAXR measurements at both surfaces showed Pb sorption increases as a function of Pb in solution. At the (001) surface, Pb sorption is best described by an incorporated species and an adsorbed or precipitated species while at the (210) surface Pb sorbs through incorporation and inner-sphere adsorption. At [Pb]_aq ≥ 200-225 μM, less than a third of the Pb sorbed to the (001) is incorporated, but up to three-quarters of Pb sorbed to the (210) is incorporated. The total coverage of Pb at each surface varies depending on concentration. At [Pb]_aq = 25-75 μM, the amount of sorbed Pb is ~1.2 Pb per nm² at the (001) and ~0.4-0.5 Pb per nm² at the (210). At [Pb]_aq = 200-225 μM, the Pb coverage is greater: ~2.8 Pb/nm² at the (001) and ~1.0 Pb/nm² at the (210). At the highest concentration of Pb measured, [Pb]_aq = 800-900 μM, ~3.9 Pb/nm² sorbs to the (001), while only ~1.9 Pb/nm² sorbs to the (210). These results suggest that the (001) has a higher sorption capacity than the (210), despite sorption occurring simultaneously through incorporation and adsorption at both surfaces.

This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division.