Speciation and solubility of rhenium in borosilicate waste glasses

Rhenium is frequently used as a non-radioactive surrogate for technetium. Speciation and solubility of rhenium in borosilicate nuclear waste glasses are investigated, where Raman spectroscopy shows rhenium in perrhenate (ReO ₄ ⁻ ) environments. Perrhenate solubility varied considerably at 1150 °C in ambient air in five different composition glasses, from 10,000 ppm Re for lithium alumino-borosilicate glass to near 4000 ppm for complex U.S. LAW and Japanese HLW glasses. Network modifying cation composition, especially lithium, strongly affect perrhenate solubility. Re-O symmetrical stretch mode (ν ₁ ^Re-O) frequencies shift depending on the alkali cation mix in the glasses, while ν ₁ ^Re-O widths are similar for all glasses studied, indicating random mixing of network modifying cations, especially Li ⁺and Na ⁺, around ReO ₄ ⁻. Perrhenate ν ₁ ^Re-O areas track with Re concentrations, indicating that most, if not all, Re in the glass is in perrhenate environments. Mo added to these glasses did not significantly affect Re solubility, nor ν ₁ ^Re-O behavior.