November 1, 2005

NSLS 2005 Annual Users’ Meeting Workshop

In-Situ Kinetic Analyses in Environmental and Chemical Systems

Natural environmental systems, such as soils, sediments, and subsurface aquifers, are always in a state of disequilibrium. Changes in porewater chemistry, biological activity, and chemical properties, such as particle-water interfacial composition, redox potential, and pH, occur over time scales ranging from microseconds to years. Research on the kinetics of chemical and physical processes at different time scales is essential for understanding the fate and transport of environmental contaminants. A workshop organized by Dean Hesterberg and Jeff Fitts for the NSLS Annual Users' Meeting on May 25, 2005, brought together a multidisciplinary group of scientists who discussed in-situ approaches for studying time-dependent chemical processes.

The workshop was dedicated to the memory of the late Dale E. Sayers. Ed Stern (University of Washington) provided a historical perspective on Sayers, discussing how, as a Ph.D. student in 1971, Sayers pioneered (along with Stern and Farrell Lytle) the first correct physical model explaining the extended x-ray absorption fine structure (EXAFS). This breakthrough led to development of XAFS spectroscopy as an analytical tool for determining the local molecular structure of condensed phases. Stern discussed the unique advantages of XAFS spectroscopy in determining short-range structure, valence, local symmetry, and angular deviation of atoms from collinearity. He also described nanosecond-scale time-dependent studies involving laser excitations of atoms in tandem with photon pulses inherent in synchrotron radiation.

Donald L. Sparks (University of Delaware) gave an overview of the importance of understanding the kinetics of reactions and processes in soils and sediments. Given the extreme complexity of such matrices, it is challenging to measure chemical kinetics separate from diffusion. Soil chemical reactions involving inorganic and organic contaminants often show an initially rapid rate followed by a slow approach to steady state, with hysteresis upon reversal. Very short, real-time, molecular-scale analyses are needed to determine chemical kinetics in isolation from transport, and elucidate reaction mechanisms that determine the fate of environmental contaminants.

One promising method for measuring short-term kinetics is Quick-EXAFS. Wolfgang Caliebe (NSLS) compared various Quick-EXAFS systems and showed Quick-EXAFS spectra collected at the NSLS on time scales of seconds. The rate-limiting component for Quick-EXAFS is typically the detector. The application of time-resolved in-situ EXAFS analysis was illustrated for battery materials by Mali Balasubramanian (Advanced Photon Source). These examples showed how quasi-equilibrium states of chemical reactions could be probed on times scales of three to 30 minutes. For example, XAFS spectra revealed how chromium ions migrate between octahedral and tetrahedral positions in Mn-oxide systems.

The precipitation, growth, and crystallization of naturally occurring iron oxides under various redox and pH conditions regulate the mobility of chemicals in the environment by, for example, adsorption and encapsulation. Sam Shaw (Oxford University) described in-situ, synchrotron-based time-resolved small-angle x-ray scattering (SAXS) and time-resolved x-ray diffraction (TRXRD) studies that followed the growth of poorly ordered iron oxyhydroxide (ferrihydrite) and its transformation into goethite and hematite, which are more stable crystalline endproducts. Rapid (seconds to minutes) precipitation and crystal growth of ferrihydrite was followed with SAXS using a stopped-flow reaction cell. Adsorption of chemical species such as phosphate on freshly precipitated mineral phases slows the overall crystal growth and retards growth on specific crystal faces. Based on activation energies calculated from TRXRD, ferrihydrite apparently transforms to goethite by dissolution and precipitation and to hematite by solid-state transformations.

John Parise (Stony Brook University) and James D. Martin (North Carolina State University) gave additional examples of using TRXRD to follow chemical kinetics. Parise showed how this technique could be used to optimize the synthesis of a titanium silicate material to maximize its selectivity for cesium, an important radioactive contaminant in nuclear waste. Highly selective binding of this element by ion exchange was attributed to structural distortions in the material. By following total scattering from nanocrystalline materials over time, evolution of the pair-distribution functions for FeS minerals were also determined. Martin used TRXRD coupled with differential scanning calorimetry to study the nucleation and growth of ZnCl₂ from melts. The rate of crystal growth was inversely related to the initial temperature of the melt. For sodalite, templating was found to be critical to crystal growth, with isothermal crystallization only occurring if the melt reached a critical temperature below the melting temperature, which allowed proper ordering of the templating cations.

In summary, in-situ synchrotron-based x-ray absorption, scattering, and diffraction are successfully used to follow the kinetics and mechanisms of chemical processes in a variety of systems. Further developments and applications of these methods to probe reactions at increasingly shorter time scales in highly heterogeneous natural systems will advance our understanding of the short- and long-term fate of chemical contaminants in the environment.

ACKNOWLEDGEMENTS
The authors are grateful to the NSLS User Executive Committee for funding this workshop.

FOR MORE INFORMATION
Dean Hesterberg
Department of Soil Science
NC State University
Raleigh, NC Email: dean_hesterberg@ncsu.edu

Jeffrey P. Fitts
Environmental Sciences Department
Brookhaven National Laboratory
Upton, NY
Email: fitts@bnl.gov