Assessment of rhenium as a surrogate for technetium in Hanford low activity waste borosilicate glasses: Speciation, solubility, and redox effects

Re is often used as a nonradioactive surrogate for Tc in borosilicate melts because both elements have similar valences, ionic radii, and coordination environments. Re solubility in high sodium borosilicate waste glasses is sensitive to melter redox conditions; at 1150°C the solubility is ∼4130 ppm under oxidizing atmospheres. Raman, X-ray absorption spectroscopy, and X-ray diffraction show that Re dissolved in waste glasses under oxidizing conditions is in perrhenate tetrahedra; under more reducing conditions, Re⁰ separates from the melt. From air to $P_{O_2}$∼ 10⁻⁴ atm, both Re⁷⁺ and Tc⁷⁺ are dominant species in borosilicate melts. However, Tc and Re behaviors deviate between $P_{O_2}$< 10⁻⁴ atm and $P_{O_2}$> 10⁻⁸ atm, where Tc⁴⁺ becomes dominant, whereas Re⁷⁺ remains stable. Unlike Tc⁴⁺, Re⁴⁺ is not observed in borosilicate melts at 1150°C and associated glasses. Near $P_{O_2}$= 10⁻⁸ atm, Tc⁴⁺ reduces to Tc⁰, whereas below $P_{O_2}$= 10⁻⁸ atm, Re⁷⁺ reduces to Re⁰. Re is found to be a good surrogate for Tc under oxidizing conditions ($P_{O_2}$> 10⁻⁴ atm) and under highly reducing conditions ($P_{O_2}$< 10⁻⁹ atm). Redox and solubility behaviors of Tc and Re are also sensitive to glass chemistry, such that the fair assessment of Re modeling Tc behavior should be glass specific.