Raman analysis of perrhenate and pertechnetate in alkali salts and borosilicate glasses

Sodium borosilicate glasses containing rhenium or technetium were fabricated and their vibrational spectra studied using confocal Raman microscopy. Glass spectra were interpreted relative to new high‐resolution spectra of pure crystalline NaReO4, KReO4, NaTcO4, and KTcO4 salts. Spectra of perrhenate and pertechnetate glasses exhibited sharp Raman bands, characteristic of crystalline salt species, superimposed on spectral features of the borosilicate matrix. At low concentrations of added KReO4 or KTcO4, the characteristic pertechnetate and perrhenate features are weak, whereas at high additions, sharp peaks from crystal field‐splitting and C4h symmetry dominate glass spectra, clearly indicating ReO4− or TcO4− is locally coordinated with K and/or Na. Peaks indicative of both K and Na salts are evident in many Raman spectra, with the Na form being favored at high concentrations of the source chemicals, where more K+ is available for ion exchange with Na+ from the base glass. The observed ion exchange likely occurred within depolymerized channels where nonbridging oxygens create segregation from the glass network in regions containing anions such as ReO4− and TcO4− as well as excess alkali cations. Although this anion exchange provides evidence of chemical mixing in the glass, it does not prove the added salts were homogeneously incorporated in the glass. The susceptibility to ion exchange from the base glass indicates that long‐term immobilization of Tc in borosilicate glass must account for excess charge compensating alkali cations in melt glass formulations. Published 2014. This article is a U. S. Government work and is in the public domain in the USA. Raman spectra of alkali perrhenates and pertechnetates were used to better understand the incorporation of these salts in sodium borosilicate glasses. Ion exchange by Na+ from the base glass caused crystalline NaReO4 and NaTcO4 to form, indicating the importance of segregation effects by network‐modifying alkali cations on long‐term performance of radioactive waste glasses containing 99Tc.