Incorporation of Perrhenate (Tc-99 Surrogate) Into Sodalite and the Stability of Tc(VII) to Ion Exchange in the Presence of Competing Ions.

Technetium-99 (Tc), a ubiquitous, long-lived radionuclide at select DOE waste sites, presents a major concern due to its long half-life (211,000 y) and high mobility in most matrices. Technetium contamination has been found in the sediments beneath the Hanford Site C, S, SX, T, and TX Tank Farms after leakage of caustic (OH^- > 8.5 M), Al-rich, high-level waste (HLW) solutions of high ionic strength (up to saturation with respect to NaNO₃). ⁹⁹Tc was also deposited on the Hanford site soil via the BC cribs and trenches, which received in excess of 50-million gallons of reprocessed tank waste during the 1950s.  ⁹⁹Tc resides deep within the vadose zone at the Hanford waste sites and without remedial action its inventory (5.31x10³ Curies) is predicted to leach into the ground water table in a relatively short period of time.  Previous investigations focused on Sr-90 and Cs-137 have postulated the incorporation of these elements into feldspathoid mineral phase(s), such as cancrinite, sodalite [Na₈(Al₆Si₆O₂₄)(NO₃)₂], that formed as a result of contact between Hanford primary silicate minerals and the HLW solutions (Chorover et al., 2008; Deng et al. 2006). The potential for sequestration of ⁹⁹Tc in aluminosilicate minerals formed below leaking tanks as well as through production of mineralized wasteforms further emphasizes the need to understand the long-term stability and release of ⁹⁹Tc from aluminosilicate minerals, specifically sodalite mineral phase(s).

      To elucidate the role of competing anion size on the structure and reactivity of sodalite, we reacted 1.25 M NaOH with zeolite 4A (source of Al and Si), 0.88 and/or 1.76 M solutions of Cl^-, NO₃^-, CO₃^2-, SO₄^2-, MnO₄^-, or WO₄^2-, and 0 to 1.76 M NaReO₄. Initially, we utilized nonradioactive perrhenate (ReO₄^-) as a surrogate for TcO₄^- as their ionic radii are both 0.56 Å, however final results will be confirmed with ⁹⁹Tc. Perrhenate concentrations in the resulting sodalite ranged from 0 to 13 mmol kg^-1 in the 0 to 0.88 M ReO₄^- samples reaching 760 mmol kg^-1 of ReO₄^- in the 1.76 M ReO₄^- sample without competing anions. Using the 211 x-ray diffraction peak, ReO₄^- sodalite was characterized by a lower degree 2θ diffraction peak relative to sodalite that incorporated smaller anions. Prolonged aging time suggests increased crystallinity; however, enclathration of ReO₄^- into the sodalite framework was inconsistent. We concluded that the occupancy of sodalite β-cages by anions is directly related to the size of the effective ionic radii and decreases in the following order: Cl^->NO₃^->CO₃^2->SO₄^2->MnO₄^->ReO₄^->WO₄^2-, suggesting that ⁹⁹Tc found in Hanford tank waste stream loaded with competing anions such as NO₃, NO₂, Cl, SO₄ etc. is unlikely to be sequestered in sodalite. However, as sodalite group mineral possesses a flexible framework structure, which can expand or deform to host guest anions of varying ionic sizes, future work will be directed at elucidating structural changes that sodalite undergoes to accommodate large anions such as ⁹⁹Tc oxyanion.

[1] Chorover, J., Choi, S., Rotenberg, P., Serne, R.J., Rivera, N., Strepka, C., Thompson, A., Mueller, K.T., O'Day, P.A., 2008, Geochimica et Cosmochimica Acta 72, 2024-2047.

[2] Deng, Y., Harsh, J.B., Flury, M., Young, J.S., and Boyle, J.S. 2006, Applied Geochemistry, 21, 1392-1409.