Interfacial Processes

The Interfacial Processes Group studies the reactions that occur at mineral surfaces in geological repositories—the mineral/water interface. Such reactions can determine repository performance and the fate of repository contents but are not well understood.  Our research is aimed at a better understanding of reactions at the mineral/water interface through direct observation of structures and processes at the molecular level so that we can predict the performance of repositories for carbon dioxide sequestration and high-level nuclear waste storage.

Recent Research Results

Recent studies have explored the molecular structure of mineral surfaces, the ordering of fluids adjacent to these surfaces (e.g., interfacial hydration layers), and the distribution of adsorbed ions at charged mineral surfaces (e.g., electrical double-layer structure), as well as dynamical processes such as dissolution and heterogeneous growth processes through real-time observations.

These observations lead to new insights into the specific reaction mechanisms at mineral/fluid interfaces, define the kinetics and reaction mechanisms at the atomic scale in key mineral/fluid systems, and provide critical tests of our understanding of mineral/water reactivity though comparison with predictions of high-level theoretical studies.

Tools and Technical Approaches

The ability to make robust observations of these processes relies on the application and development of advanced synchrotron-based interfacial x-ray tools for in-situ studies of mineral/fluid interfaces. These approaches take advantage of the unique characteristics of synchrotron radiation at the Advanced Photon Source (APS), including temporal and spatial resolution afforded by the high APS beam brilliance and the tunability of the x-ray photon energy that facilitates spectroscopic sensitivities, leading to fundamentally new types of in situ experiments.

Projects primarily use high (<1 Å) resolution X-ray scattering techniques, including surface x-ray scattering (e.g., X-ray reflectivity, XR), resonant anomalous x-ray reflectivity (RAXR), X-ray standing waves (XSW), and x-ray reflection interface microscopy (XRIM).

The relatively high complexity of mineral/water interfaces has also led to the development and extension of various model-independent data analysis techniques, including the ability to image directly: element-specific sub-profiles (e.g., from phase-sensitive XSW and RAXR data) and interfacial density profiles from XR data (e.g., using error correction algorithms).

The X-ray standing wave and X-ray reflection interface microscopy measurements are performed on a spectrometer that built and located at station 33-ID at the APS. In many cases, complementary studies are performed using atomic force microscopy (AFM) and/or X-ray absorption spectroscopy.

Research Partners

This program has ongoing collaborations with scientists at many organizations, including the University of Illinois at Chicago, Northwestern University, the Illinois State Water Survey, and Oak Ridge National Laboratory.

Funding

This research is funded by the Geoscience Research program of the Department of Energy’s Office of Basic Energy Sciences.

September 2009