Surprising formation of quasi-stable Tc(vi) in high ionic strength alkaline media

Abstract

This work demonstrates an aqueous Tc(vi) lifetime 6 orders of magnitude greater than previously suggested.

An electrochemical technique for controlled dissolution of zirconium based components of light water reactors

“…Glassy carbon was eliminated based on the inability to formulate it in the shape, size or orientation needed for effective electrical contact between the electrode and the liner surfaces with convenient ease. Platinum based working electrodes were observed to produce more reproducible voltammograms compared to Ni and Au as observed before, [32][33][34] and were adapted as working electrode for subsequent studies.…”

Identification and Quantification of Technetium Species in Hanford Waste Tank AN-102

“…This result corroborates the previous observation of [ fac -Tc­(CO) 3 ] + species in SY-101 and SY-103 tanks by Lukens et al; however, unlike the latter observations that proposed [ fac -Tc­(CO) 3 ] + to be the only non-TcO 4 – species in these tanks, our results indicate that other oxidation states and chemical forms of Tc, namely, Tc­(IV), are also present in the AN-102 supernatant. It is also worth mentioning that the occurrence of other non-TcO 4 – species, namely, Tc­(VI), may also be possible based on our recent observations of Tc­(VI) species in high alkaline, high-ionic-strength matrices similar to AN-102 supernatant . To summarize, both 99 Tc NMR and XAS results concurrently point to multiple Tc chemical environments and oxidation states being present in the AN-102 supernatant.…”

“…Based on this, the non-TcO 4 – content in NMR is determined to be 3.96 mg (80% of total). The differences in the relative TcO 4 – percentages obtained by NMR and XAS may arise due to limitations in the resolution of the XAS spectrum into multiple components, which relies on the availability of the database library. , …”

“…The long half-life, complex chemical behavior, limited incorporation into glass waste forms, and high mobility in subsurface environments make Tc one of the most difficult contaminants to dispose of and/or remediate. Tc can adopt multiple oxidation states (typically in solution +I through +VII) and exhibits complicated redox and coordination chemistry. − Therefore, identifying chemical and redox speciation of Tc in the Hanford tank waste and quantification of the components will enable projection of its distribution between waste fractions and behavior during processing and support effective treatment and immobilization alternatives such as grouting for safe waste storage and disposition. It will also assist in designing more effective methods of their detection, which has thus far primarily focused on TcO 4 – . − …”

“…The first direct evidence of low-valent non-TcO 4 – forms of Tc in tank waste supernatants was provided by Blanchard Jr. et al and Lukens et al, whose analysis of two SY-101 and SY-103 Hanford tank waste supernatants using X-ray absorption spectroscopy (XAS) showed the presence of low-valent Tc­(I) species derived from [ fac -Tc­(CO) 3 ] + or possibly their mononitrosyl analogue. , While the observations of these low-valent Tc species contradict the expectations of TcO 4 – being the most stable Tc species in the alkaline tank waste supernatants based on the existing knowledge of Tc chemistry, it is not totally surprising based on recent observations that additional factors such as high ionic strength, the presence of chelators, and matrix effects can significantly stabilize the low Tc oxidation states. ,, Tc­(I) carbonyl compounds have been extensively studied because of their applications in nuclear medicine, albeit under conditions drastically different from those typical for the tank waste media, − and the facile substitution of the water molecules in the parent [ fac -Tc­(CO) 3 (H 2 O) 3 ] + compound by organic oxygen and possibly nitrogen donors leading to highly stable complexes is well established. In the alkaline supernatants of Hanford tanks, similar reactions can lead to the formation of stable [ fac- Tc­(CO) 3 (L) x ] n − coordination compounds (L is gluconate, polyaminocarboxylate, or other ligands) or their hydrolyzed aqua products [ fac -Tc­(CO) 3 (OH 2 ) 3– n (OH) n ] 1– n …”