Ionically-bonded minerals are ubiquitous and play a determinative role in controlling the mobility of toxic metals in natural environments. However, little is known about the mechanism of ion uptake by the surfaces of these surfaces. Using high resolution X-ray reflectivity (XR) and resonant anomalous X-ray reflectivity (RAXR), we quantified structural changes at the barite (001) – water interface associated with the extent of Sr2+ or Pb2+ coverage during adsorption. The XR results indicated that sorption of Pb2+ disrupts the barite surface structure more significantly than Sr2+. The RAXR results showed that Pb2+ sorbs at [Pb2+] ≥ 6 μM, above which the coverage increases with increasing [Pb2+]. This sorption behavior of Pb2+ can be described using a classical Langmuir isotherm. In contrast, Sr2+ sorption is only significant at [Sr2+] ≥ 25 µM, upon which the coverage increases rapidly before the coverage saturated. This unusual sorption behavior of Sr2+ can only be described using a Frumkin isotherm. At its maximum coverage, two-thirds of the Pb2+ adsorbs in a broad distribution centered at ~ 2.0 Å, while the other one third is incorporated into the surface structure at ~0.4 Å below the barite surface. Similar to Pb2+, Sr2+ adsorbs and incorporates into the barite surface, but a larger fraction incorporates into the surface (~50%). Based on density functional theory (DFT) calculations, this incorporated Sr2+ likely substitutes for the topmost and second topmost barium ions at the surface. Overall, these results highlight the differences in sorption behavior of Sr2+ and Pb2+, possibly due to differences is the electronegativity of the two ions or the differences in solubility of the two ions. PbSO4 is an order of magnitude less soluble than SrSO4, which may make re-exchange of an incorporated Pb for a Ba in solution less favorable than for an incorporated Sr.

Cation sorption at the Barite (001): Water interface

2019 Spring National Meeting

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