Simultaneous Adsorption and Incorporation of Sr 2+ at the Barite (001)-Water Interface

Jacquelyn N. Bracco, Sang Soo Lee, Joanne E. Stubbs, Peter J. Eng, Sarah Jindra, D. Morgan Warren, Anitha Kommu, Paul Fenter, James D. Kubicki, Andrew G. Stack

Research output: Contribution to journalArticlepeer-review

21 Scopus citations

Abstract

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 these mineral surfaces. Here, the sorption of strontium ions (Sr 2+ ) to the barite (001)-water interface was studied using a combination of synchrotron X-ray scattering and three types of computational simulations (density functional theory, classical molecular dynamics (CMD), and CMD-metadynamics). In situ resonant anomalous X-ray reflectivity (RAXR) revealed that Sr 2+ adsorbed on the barite surface as inner-sphere surface complexes and was incorporated within the outermost barite atomic layers. Density functional theory combined with CMD simulations confirmed the thermodynamic stability of these species, demonstrating almost equal magnitudes in the free energy of sorption between these species. Metadynamics simulations showed a more detailed feature in the free energy landscape for metal adsorption where adsorbed Sr 2+ are stabilized in as many as four distinct inner-sphere sites and additional outer-sphere sites that are more diffuse and less energetically favorable than the inner-sphere sites. All three techniques confirmed Sr 2+ adsorbs inner-sphere and binds to oxygens in the top two surface sulfate groups. The energy barriers among the inner-sphere sites were significantly lower compared with those for constituent cation Ba 2+ , implying fast exchange among adsorbed Sr 2+ species. The Sr 2+ uptake measured by RAXR followed a Frumkin isotherm defined with an apparent free energy of sorption, Î"G Sr â‰-22 kJ/mol, and an effective attractive interaction constant, Î â‰-4.5 kJ/mol, between sorbed Sr 2+ . While the observed free energy can be mostly explained by the (CMD) Helmholtz free energy of adsorption for Sr 2+ , Î"F Sr = â'15.3 kJ/mol, the origin of the sorbate-sorbate correlation could not be fully described by our computational work. Together, these experimental and computational results demonstrate the complexity of Sr 2+ sorption behavior at the barite (001) surface.

Original languageEnglish
Pages (from-to)1194-1207
Number of pages14
JournalJournal of Physical Chemistry C
Volume123
Issue number2
DOIs
StatePublished - Jan 17 2019

Funding

This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division. CTR and RAXR measurements were performed at GeoSoilEnviroCARS (The University of Chicago, Sector 13), Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation−Earth Sciences (EAR-1634415) and Department of Energy−GeoSciences (DE-FG02-94ER14466). A subset of the RAXR measurements (sample 2) were measured at Sector 33-ID-D. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. Computational resources for the DFT and portions of the CMD simulations were provided by The Pennsylvania State University Institute for CyberScience, the NSF XSEDE program, and the University of Texas at El Paso Research and Academic Data Center. Metadynamics simulations were performed using the National Energy Research Scientific Computing Center (NERSC), a U.S. Department of Energy Office of Science User Facility operated under Contract No. DE-AC02-05CH11231. The authors thank Dr. Musa Hussein and Amanda Labrado for assistance performing calculations and analysis of the results.

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