Competitive Incorporation of Perrhenate and Nitrate into Sodalite

Abstract

Nuclear waste storage tanks at the Hanford site in southeastern Washington have released highly alkaline solutions, containing radioactive and other contaminants, into subsurface sediments. When this waste reacts with subsurface sediments, feldspathoid minerals (sodalite, cancrinite) can form, sequestering pertechnetate (⁹⁹TcO₄^–) and other ions. This study investigates the potential for incorporation of perrhenate (ReO₄^–), a chemical surrogate for ⁹⁹TcO₄^–, into mixed perrhenate/nitrate (ReO₄^–/NO₃^–) sodalite. Mixed-anion sodalites were hydrothermally synthesized in the laboratory from zeolite A in sodium hydroxide, nitrate, and perrhenate solutions at 90 °C for 24 h. The resulting solids were characterized by bulk chemical analysis, X-ray diffraction, scanning electron microscopy, and X-ray absorption near edge structure spectroscopy (XANES) to determine the products’ chemical composition, structure, morphology, and Re oxidation state. The XANES data indicated that nearly all rhenium (Re) was incorporated as Re­(VII)­O₄^–. The nonlinear increase of the unit cell parameter with ReO₄^–/NO₃^– ratios suggests formation of two separate sodalite phases in lieu of a mixed-anion sodalite. The results reveal that the sodalite cage is highly selective toward NO₃^– over ReO₄^–. Calculated enthalpy and Gibbs free energy of formation at 298 K for NO₃- and ReO₄-sodalite suggest that NO₃^– incorporation into the cage is favored over the incorporation of the larger ReO₄^–, due to the smaller ionic radius of NO₃^–. Based on these results, it is expected that NO₃^–, which is present at significantly higher concentrations in alkaline waste solutions than ⁹⁹TcO₄^–, will be strongly preferred for incorporation into the sodalite cage