Watching Iron Geochemistry at the Nanosecond Timescale
The importance of determining molecular-scale reaction mechanisms in geochemistry
At the molecular scale, most geochemical reactions are complex, multistep processes in which the elementary steps occur too quickly for direct observation. Together, these steps define an overall reaction mechanism. Although thermodynamic descriptions of geochemical processes are well established in many cases, knowledge of the mechanism is essential for understanding the chemical controls on reaction rates, and for the ability to predict geochemical reaction dynamics. Thus, determining and understanding reaction mechanisms are forefront challenges of low-temperature geochemistry.
Studying chemical reactions with time-resolved methods
In recent years, developments in the field of ultrafast spectroscopy have begun to offer new ways to observe chemical reactions at the timescales that they occur. For example, time-resolved optical spectroscopy in the fast (subnanosecond) and ultrafast (subpicosecond) regimes have identified and characterized transient electronic or vibrational states. More recently, time-resolved x-ray spectroscopy has become a powerful technique for studying changes in metal atom oxidation state and coordination geometry during chemical and photochemical reactions. ESD’s Ben Gilbert and coworkers are developing time-resolved methods to determine the mechanisms of interfacial reactions relevant to geochemical and biogeochemical systems. Their first project, performed with recent post-doctoral scholar Jordan Katz, is the study of the formation and fate of ferrous iron sites created at the surfaces of iron oxide and oxyhydroxide minerals by interfacial electron transfer.
Interfacial redox reactions in the environment
Electron transfer at the mineral-water interfaces is central to many important natural cycles, particularly the geochemical and biological redox cycling of common transition metals, such as iron and manganese. In subsurface environments such as soils and sediments, the transfer of electrons from reductants (including microbial proteins) to redox active minerals causes substantial changes in geochemistry and mineralogy. As depicted in Figure 1, electron transfer is just the first step in a complex reaction pathway that can lead to diverse outcomes—such as mineral dissolution or phase transformation, or the secondary reduction of adsorbates. Currently, the factors that determine the outcome of such mineral redox reactions—that is, the fate of the electrons transferred to the mineral—are very poorly understood.
Figure 1. Environmental electron donors readily transfer electrons to ferric iron (oxyhydr)oxide minerals and nanoparticles leading to mineral dissolution or phase transformation, or secondary adsorbate reduction.
Studying redox reactions with the “pump-probe” method
Elementary redox reaction steps occur on femtosecond to nanosecond timescales—much faster than the timescales for mixing solutions. In order to study such fast processes, Gilbert and his team have developed a method to initiate interfacial redox reactions with a fast pulse of light. Taking inspiration from the development of dye-sensitized semiconductor nanoparticles for solar energy conversion, they identified organic molecules that bind strongly to iron (oxyhydr)oxide surfaces and readily transfer an electron to surface iron sites when excited by green light. As shown in the animated scheme of Figure 2, this enables the formation ferrous iron species in solid-phase minerals to be controlled by light and studied by time-resolved x-ray spectroscopy using the pump-probe method.
Figure 2. Animated scheme demonstrating the pump-probe method for performing time-resolved X-ray spectroscopy to follow the fate of ferrous iron sites in ferric iron oxide and oxyhydroxide nanoparticles. The Fe2+ sites are created within 10 ps by light-initiated electron transfer from surface-bound dye molecule (a fluorescein derivative). The X-ray spectroscopy is acquires snapshots of the subsequent iron chemistry from 100 ps to 50 ns after the electron transfer event.
Gilbert and his team recently studied the formation and lifetime of ferrous iron sites in ~3-nm ferrihydrite nanoparticles coated with the molecular sensitizer 2’,7’-dichlorofluorescein. As shown in Figure 3, optical-pump/x-ray-probe spectroscopy performed at 150 ps following the triggering laser pulse detected a signal associated with the formation of ferrous iron sites in the nanoparticles. This signal exhibited two-phase decay kinetics, interpreted as a competition between (1) the reverse electron transfer process and (2) electron hopping into the nanoparticle interior. Using this approach, the team has quantified the rate for interfacial electron transfer and the fate and lifetime of mobile ferrous iron sites in the nanoparticle. In the current system, about 1 in 300 ferrous iron sites created by the laser eventually are released into solution (dissolution). By observing the pump-probe method the team seek to observe all stages of the reductive dissolution to identify the rate limiting step.
Figure 3. The lifetime of ferrous iron sites created in ferrihydrite nanoparticles (a) Differential X-ray absorption spectra obtained by taking the difference between XANES signals acquired before and after a known delay (0.1 – 10 ns) from the 2-ps laser pulse. (b) The same traces shown in (a), with fits to estimate the magnitude of the differential signal. (c) The time-dependence of the differential signal following electron injection.
Future Work
Experiments are under way to measure the timescales for interfacial electron transfer for different phase of iron (oxyhdr)oxides. Two DOE proposals are pending to apply this approach to other topics in interface geochemistry: determining the mechanisms of redox reactions of adsorbate on mineral surfaces; and determining the factors controlling the rate of protein-mineral electron transfer.
Acknowledgements
This research was supported by the DOE BES Chemical Imaging program and the DOE BES (Geochemistry) program.
References
This research is described in the following publications:
- J. E. Katz, B. Gilbert, X. Zhang, K. Attenkofer, R. W. Falcone and G. A. Waychunas. Observation of transient ferrous iron formation in dye-sensitized iron oxide nanoparticles by time-resolved X-ray spectroscopy. J. Phys. Chem. Letters 1, 1372-1376 (2010).
- J.E. Katz, X. Zhang, K. Attenkofer, K. Chapman, P. Zarzycki, K. M. Rosso, R. W. Falcone, G. A. Waychunas and B. Gilbert. Dynamic, electronic and structural controls on the reductive dissolution of iron (oxyhydr)oxide nanoparticles. Submitted
