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Reduction Kinetics of Chromium in Magnetite: Implications for Understanding the Reduction Kinetics of Technetium at the Savannah River Saltstone Facility.

Poster Number 1917

Tuesday, November 5, 2013
Tampa Convention Center, East Hall, Third Floor

Amanda Hatfield, Environmental Toxicology, Clemson University, Clemson, SC, Daniel Kaplan, Savannah River National Lab, Aiken, SC, Brian Powell, Environmental Engineering and Earth Sciences, Clemson University, Clemson, SC and Yuji Arai, Clemson University, Urbana, IL
Technetium-99 (99Tc) (t1/2: 2.11 x 105 y) is one of the major risk drivers at DOE Savannah River Site (SRS), Aiken, SC.  It has been immobilized in saltstone to prevent the mobile Tc7+O4- species from leaching to subsurface environments. Reductive stabilization to relatively insoluble Tc4+O2 or Tc4+S2 has often been associated with the reductive power of Fe2+ and/or sulfide bearing minerals in saltstone. To better understand the mechanisms of Tc reduction in saltstone, synthetic magnetite (Fe3O4) was chosen as a model Fe2+ bearing mineral. Accordingly, interactions between Cr6+ (a redox and solubility analogue for Tc) and magnetite were investigated as a function of pH and ionic strength (0.004 – 0.1 M NaNO3 or Na2SO4). Batch sorption experiments showed that Cr6+ sorption decreases with increasing pH. The high Na2SO4 ionic strength effectively reduced sorption whereas lack of ionic strength effect was observed in the NaNO3 media.  Time-resolved X-ray absorption near edge structure spectroscopy (XANES) analysis revealed the slow reduction of Cr6+ at the magnate-water interface during the initial 12 hrs. The electron transfer reaction is likely suppressed by the formation of the Cr3+ and or Fe3+ oxyhydroxide passivation layers. The inhibitory processes were especially pronounced in alkaline solution at high nitrate/sulfate media, resulting in the incomplete reduction of Cr6+. The results of this study pose a question about the presence of potentially unreduced Tc in saltstone matrices.
See more from this Division: SSSA Division: Soil Chemistry
See more from this Session: Chemical Concentrations, Fate, and Distribution in Soils: II

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