Although goethite-water interfaces have been studied extensively using a variety of experimental and computational techniques, the exact nature of metal surface complexes remains unresolved. Regardless of the level of detail incorporated into a surface complexation model (SCM), the model is used to fit bulk adsorption data, limiting our ability to validate the SCM predictions for each surface. Classical molecular simulations provide more detailed information regarding adsorption on each surface; however, these simulations are limited because they do not allow changes in surface hydroxylation or the formation of aqueous metal-hydroxide complexes. To date, SCM fits for alkaline earth metal adsorption suggest that some Mg2+ adsorbs as an inner-sphere complex, and Ba2+ adsorbs as an outer-sphere complex. In contrast, molecular simulations suggest that Mg2+ will always adsorb as an outer-sphere complex, and Ba2+ forms both inner- and outer-sphere complexes. Reasons for this discrepancy will be discussed. SCM modeling by Mangold and Katz suggests that in order to fit bulk data for the adsorption of both contaminant cations (e.g., Pb2+) and anions (e.g., SeO32-) from the same aqueous solution, these ions must form ternary complexes on the goethite surface. Using ab initio molecular dynamics, we have found that a PbSeO3 ion pair may be stable on the (210) surface. This would be consistent with the complex proposed through data-fitting with the CD-MUSIC model. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Adsorption of divalent metals and oxyanions to goethite-water interfaces

251st National Meeting (2016)

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