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

Technetium-99 (⁹⁹Tc) (t_1/2: 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.