Argonne researchers have directly visualized, in three-dimensions, ion site distributions at the mineral-water interface using a technique called X-ray standing wave (XSW) imaging. Their finding demonstrates a new capability for revealing complex reactions at mineral-water interfaces that previously could be understood only through more indirect approaches.

XSW, in contrast to X-ray crystallography, measures both the amplitude and phase information that completely describe the molecular-scale structure of interest. In standard crystallography only the amplitudes can be measured and consequently an elaborate approach is needed to determine the structure.

"We are the first to show that images of the atom site distributions can be directly obtained with XSW. In this case of adsorption at the mineral-water interface, we were able to fully resolve ion-specific sorption sites and distinguish different coordination environments for each ion," said Paul Fenter of Argonne's Environmental Research Division.

The XSW imaging approach allows scientists to streamline the tedious process of structure determination. "With XSW imaging, data acquisition and analysis can be completed in less than 24 hours. Previously, surface structure determination would take weeks or months to complete" said Fenter.

Authors on the report, in addition to Fenter, are Zhan Zhang of Northwestern University, Likwan Cheng of Argonne, Neil Sturchio of the University of Illinois at Chicago and Argonne, Michael Bedzyk of Northwestern University and Argonne, Michael Machesky of the Illinois Water Survey, and David Wesolowski of Oak Ridge National Laboratory.

Solid-liquid interface structure is essential to many natural and technological processes. The interaction of mineral surfaces with fluids controls rock weathering, evolution of petroleum reservoirs and ore deposits, and the transport and remediation of contaminants in groundwater aquifers. "Our long term goal is to learn how to use X-rays to 'see' geochemical processes in action at the molecular level," said Fenter.

The research was conducted at the Basic Energy Sciences Synchrotron Radiation Center at the Advanced Photon Source (APS) at Argonne. The APS, which produces the most brilliant X-rays for research in the Western Hemisphere, is ideal for this type of research. "The APS brilliance allows us to illuminate a small, nearly-perfect region of a larger imperfect crystal and still be able to do the measurement," said Fenter.

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