Abstract
Adsorption of Zn2+ at the rutile TiO2 (110)-aqueous interface was studied with Bragg-reflection X-ray standing waves (XSW), polarization-dependent surface extended X-ray absorption fine structure (EXAFS) spectroscopy, and density functional theory (DFT) calculations to understand the interrelated issues of adsorption site, its occupancy, ion-oxygen coordination and hydrolysis. At pH 8, Zn2+ was found to adsorb as an inner-sphere complex at two different sites, i.e., monodentate above the bridging O site and bidentate between two neighboring terminal O sites. EXAFS results directly revealed a four or fivefold first shell coordination environment for adsorbed Zn2+ instead of the sixfold coordination found for aqueous species at this pH. DFT calculations confirmed the energetic stability of a lower coordination environment for the adsorbed species and revealed that the change to this coordination environment is correlated with the hydrolysis of adsorbed Zn2+. In addition, the derived adsorption locations and the occupancy factors of both sites from three methods agree well, with some quantitative discrepancies in the minor site location among the XSW, EXAFS, and DFT methods. Additional XSW measurements showed that the adsorption sites of Zn2+ were unchanged at pH 6. However, the Zn2+ partitioning between the two sites changed substantially, with an almost equal distribution between the two types of sites at pH 6 compared to predominantly monodentate occupation at pH 8.
Original language | English (US) |
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Pages (from-to) | 4039-4056 |
Number of pages | 18 |
Journal | Geochimica et Cosmochimica Acta |
Volume | 70 |
Issue number | 16 |
DOIs | |
State | Published - Aug 15 2006 |
Funding
We would like to thank Dr. Lawrence Anovitz for rutile crystal treatments and beamline staff (Drs. Klaus Attenkopfer and Jennifer Linton) for help with the experimental setup and Dr. Milan Predota for sharing preliminary results of MD simulations of Li + adsorption. General user support to SDK for extended X-ray absorption fine structure analysis was provided by DOE’s Environmental Remediation Sciences Division. This work was supported by U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, and through its support for the Advanced Photon Source, under contract W-31-109-ENG-38. The National Synchrotron Light Source was supported under Contract No. DE-AC02-98CH10886. Computational support was provided by the Materials Simulation Center, a Penn State MRSEC facility, and the Center for Environmental Kinetics Analysis (CEKA), an NSF/DOE Environmental Molecular Sciences Institute. Work performed under the auspices of the Office of Science, Division of Chemical Science, US-DOE under Contract No. W-31-109-ENG-38.
ASJC Scopus subject areas
- Geochemistry and Petrology