Heterogeneous reduction of uranyl by micas: Crystal chemical and solution controls

Ilton, Eugene S.; Haiduc, Anca; Moses, Carl O.; Heald, Steve M.; Elbert, David C.; Veblen, David R.

doi:10.1016/j.gca.2003.08.010

Cited by 96 publications

(83 citation statements)

References 74 publications

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“…The bulk of the data was determined by XPS of edge and basal plane orientations. Some of our results have been reported in Ilton et al (2003) and Ilton and Simmons (2003). However, we made three unexpected discoveries that have expanded the possible pathways for sorption-reduction: 1) XAS has shown that uranyl may be reduced in the interlayer region, 2) XPS yielded evidence that pentavalent uranium is stabilized on the mica edge orientations, and 3) HRTEM recorded the presence of U/Fe-rich nano-scale zones on interior basal surfaces.…”

supporting

confidence: 82%

Reactions and Transport of Toxic Metals in Rock-Forming Silicates at 25 degrees C

Ilton¹

2003

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Sorption-reduction of aqueous Uv' by ferrous micasThe primary objective of our work in the present grant period was to extend our earliest results to a wider range of solution and mica compositions, and reaction times. These objectives were pursued through a series of systematic, batch sorption experiments, performed under argon at 25C. Single crystal run products were analyzed by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The use of large single crystals and high waterhock ratios had advantages, but precluded changes in solution composition. The bulk of the data was determined by XPS of edge and basal plane orientations. Some of our results have been reported in Ilton et al. (2003) and Ilton and Simmons (2003). However, we made three unexpected discoveries that have expanded the possible pathways for sorption-reduction: 1) XAS has shown that uranyl may be reduced in the interlayer region, 2) XPS yielded evidence that pentavalent uranium is stabilized on the mica edge orientations, and 3) HRTEM recorded the presence of U/Fe-rich nano-scale zones on interior basal surfaces. All of our results are briefly summarized here.

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supporting

confidence: 82%

Reactions and Transport of Toxic Metals in Rock-Forming Silicates at 25 degrees C

Ilton¹

2003

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“…This process may be significant in that, As sorption (and to a lesser extent Pb, and U) with Fe-rich phases (including but not limited to clays) appears to play a major role in As fate in the shallow subsurface at Chimayo. Localized structural site-tosite electron exchange has been shown to be potentially significant related to sorption-reduction of U (Ilton et al, 2004;Singer et al, 2009;Chakraborty et al, 2010), and may be significant for several other redox-sensitive elements (Peterson et al, 1997;Felmy et al, 2011). While this process has been shown by Chakraborty et al (2011) to not be as significant for As on phases such as biotite, it could have additional impacts on how to predict controls on metal mobility in the Chimayo system in general.…”

Section: Discussionmentioning

confidence: 99%

Developing a robust geochemical and reactive transport model to evaluate possible sources of arsenic at the CO2 sequestration natural analog site in Chimayo, New Mexico

Viswanathan

Dai

Lopano

et al. 2012

International Journal of Greenhouse Gas Control

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a b s t r a c tMigration of carbon dioxide (CO 2 ) from deep storage formations into shallow drinking water aquifers is a possible system failure related to geologic CO 2 sequestration. A CO 2 leak may cause mineral precipitation/dissolution reactions, changes in aqueous speciation, and alteration of pH and redox conditions leading to potential increases of trace metal concentrations above EPA National Primary Drinking Water Standards. In this study, the Chimayo site (NM) was examined for site-specific impacts of shallow groundwater interacting with CO 2 from deep storage formations. Major ion and trace element chemistry for the site have been previously studied. This work focuses on arsenic (As), which is regulated by the EPA under the Safe Drinking Water Act and for which some wells in the Chimayo area have concentrations higher than the maximum contaminant level (MCL). Statistical analysis of the existing Chimayo groundwater data indicates that As is strongly correlated with trace metals U and Pb indicating that their source may be from the same deep subsurface water. Batch experiments and materials characterization, such as: X-ray diffraction (XRD), scanning electron microscopy (SEM), and synchrotron micro X-ray fluorescence (-XRF), were used to identify As association with Fe-rich phases, such as clays or oxides, in the Chimayo sediments as the major factor controlling As fate in the subsurface. Batch laboratory experiments with Chimayo sediments and groundwater show that pH decreases as CO 2 is introduced into the system and buffered by calcite. The introduction of CO 2 causes an immediate increase in As solution concentration, which then decreases over time. A geochemical model was developed to simulate these batch experiments and successfully predicted the pH drop once CO 2 was introduced into the experiment. In the model, sorption of As to illite, kaolinite and smectite through surface complexation proved to be the key reactions in simulating the drop in As concentration as a function of time in the batch experiments. Based on modeling, kaolinite precipitation is anticipated to occur during the experiment, which allows for additional sorption sites to form with time resulting in the slow decrease in As concentration. This mechanism can be viewed as trace metal "scavenging" due to sorption caused secondary mineral precipitation. Since deep geologic transport of these trace metals to the shallow subsurface by brine or CO 2 intrusion is critical to assessing environmental impacts, the effective retardation of trace metal transport is an important parameter to estimate and it is dependent on multiple coupled reactions. At the field scale, As mobility is retarded due to the influence of sorption reactions, which can affect environmental performance assessment studies of a sequestration site.

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“…15 More generally, the availability of Fe(II) at the mineral-water interface, either as the result of adsorption or in surface sites, can impact the extent of U(VI) reduction, ranging from complete to partial reduction. 13,[16][17][18][19][20][21] Studies to date have provided U(VI) reduction rates for a wide range of solution conditions and mineral characteristics. However, a molecular-scale view of the process is lacking.…”

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confidence: 99%

U(VI) Sorption and Reduction Kinetics on the Magnetite (111) Surface

Singer

Chatman

Ilton

et al. 2012

Environ. Sci. Technol.

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continuous batch-flow conditions. We observed, in real-time and in situ, adsorption and reduction of U(VI) and subsequent growth of UO 2 nanoprecipitates using atomic force microscopy (AFM) and newly developed batch-flow U L III -edge grazing-incidence x-ray absorption spectroscopy near-edge structure (GI-XANES) spectroscopy. U(VI) reduction occurred with and without CO 3 present, and coincided with nucleation and growth of UO 2 particles. When Ca and CO 3 were both present no U(VI) reduction occurred and the U surface loading was lower. In situ batch-flow AFM data indicated that UO 2 particles achieved a maximum height of 4-5 nm after about 8 hours of exposure, however, aggregates continued to grow laterally after 8 hours reaching up to about 300 nm in diameter. The combination of techniques indicated that U uptake is divided into three-stages; (1) initial adsorption of U(VI),(2) reduction of U(VI) to UO 2 nanoprecipitates at surface-specific sites after 2-3 hours of exposure, and (3) completion of U(VI) reduction after ~6-8 hours. U(VI) reduction also corresponded to detectable increases in Fe released to solution and surface topography changes.Redox reactions are proposed that explicitly couple the reduction of U(VI) to enhanced release of Fe(II) from magnetite. Although counter intuitive, the proposed reaction stoichiometry was shown to be largely consistent with the experimental results. In addition to providing molecularscale details about U sorption on magnetite, this work also presents novel advances for collecting surface sensitive molecular-scale information in real-time under batch-flow conditions. 3

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Heterogeneous reduction of uranyl by micas: Crystal chemical and solution controls

Cited by 96 publications

References 74 publications

Reactions and Transport of Toxic Metals in Rock-Forming Silicates at 25 degrees C

Reactions and Transport of Toxic Metals in Rock-Forming Silicates at 25 degrees C

Developing a robust geochemical and reactive transport model to evaluate possible sources of arsenic at the CO2 sequestration natural analog site in Chimayo, New Mexico

U(VI) Sorption and Reduction Kinetics on the Magnetite (111) Surface

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