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

Journal Article · 19 December 2018 · Journal of Physical Chemistry. C

DOI: https://doi.org/10.1021/acs.jpcc.8b08848 · OSTI ID:1493716

Bracco, Jacquelyn N.  ^[1]; Lee, Sang Soo  ^[1]; Stubbs, Joanne E.  ^[2]; Eng, Peter J. ^[2]; Jindra, Sarah ^[3]; Warren, D. Morgan ^[4]; Kommu, Anitha ^[5]; Fenter, Paul  ^[1]; Kubicki, James D.  ^[5]; Stack, Andrew G.  ^[6]

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1. Argonne National Lab. (ANL), Lemont, IL (United States)
2. Univ. of Chicago, IL (United States)
3. Wright State University, Dayton, OH (United States)
4. Golder Associates Inc., Lakewood, CO (United States)
5. University of Texas at El Paso, TX (United States)
6. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)

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. In this study, the sorption of strontium ions (Sr²⁺) 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 Sr²⁺ adsorbed on the barite surface as inner-sphere surface complexes and was incorporated within the outermost barite atomic layers. Density functional theory combined with classical molecular dynamics 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 Sr²⁺ 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. Additionally, the energy barriers among those inner-sphere sites were significantly lower compared with those for constituent cation Ba²⁺, implying fast exchange among adsorbed Sr²⁺ species. The Sr²⁺ uptake measured by RAXR followed a Frumkin isotherm defined with an apparent free energy of sorption, Δ≈ G_Sr -22 kJ/mol, and an effective attractive interaction constant, γ ≈ -4.5 kJ/mol, between sorbed Sr²⁺. While the observed free energy can be mostly explained by the Helmholtz free energy of adsorption for Sr²⁺, ΔF_Sr = -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 Sr²⁺ adsorption behavior at the barite (001) surface.

• Research Organization: Argonne National Laboratory (ANL), Argonne, IL (United States); Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES). Chemical Sciences, Geosciences, and Biosciences Division
• Grant/Contract Number: AC02-06CH11357; FG02-94ER14466; AC02-05CH11231; AC05-00OR22725
• OSTI ID: 1493716
• Alternate ID(s): OSTI ID: 1493975
• Journal Information: Journal of Physical Chemistry. C, Vol. 123, Issue 2; ISSN 1932-7447
• Publisher: American Chemical SocietyCopyright Statement
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
X-ray reflectivity
adsorption
barite
cation exchange
density functional theory
interface
isotherm
molecular dynamics
sorption
strontium