Speciation and Oxidative Stability of Alkaline Soluble, Non-Pertechnetate Technetium

Abstract

Tc(CO) 3 ] + /gluconate complex was considerably slower in the supernatant simulant than in the simple 5 M NaNO 3 /0.5 M NaOH/0.5 M NaGluconate solution. 4. These results indicate that a carbonyl complex is a viable candidate for the source of non-pertechnetate in tank waste. As indicated, testing has identified a range of conditions under which a significant portion of the carbonyl complex is stable for extended periods of time. Preliminary studies have shown a viable route for the formation of this complex under tank waste conditions. Also, since the range of concentration in tank waste varies from less than 0.1% to greater than 50%, the rates of production and destruction under these varying conditions obviously vary dramatically, and as such, the balance of these two reactions will be highly dependent upon tank waste chemistry. 5. A proof-of-principle demonstration corroborating the mechanism and feasibility of [Tc(CO) 3 ] + formation from pertechnetate using CO/H 2 reductant in the presence of an organic chelator and catalytic noble metals in the tank waste simulant was performed. The simulant used was based on the previously used Pretreatment Engineering Platform (PEP) simulant formulation, albeit with altered free hydroxide concentrations and with the addition of some noble metals (simulating fission products) to catalyze any needed reduction of Tc(VII) by hydrogen and gluconate as a reductant/complexant. Reaction conditions were approximately 1300 psi atmosphere, at 80 °C for about 12 days. The bulk of the pertechnetate was reduced following this treatment and the Tc(I)-tricarbonyl/gluconate compound was observed by 99 Tc NMR spectroscopy. The same product can be prepared independently by the reaction of [Tc(CO) 3 ] + with gluconate in water, sodium nitrate, or supernatant simulant solutions. In short, this proof-of-principle test supports the concept of alkaline-soluble, low-valent Tc being formed by pertechnetate reduction under conditions consistent with those found in Hanford tank supernatants, albeit at intentionally high concentrations of carbon monoxide in this first proof-of-principle test. 6. To understand Tc speciation in the alkaline solutions, significant effort was placed on the expansion of the Tc characterization techniques and development of computational approaches to enhance interpretation of the experimental observations. In this work, considerable achievements were made toward verifying that the Tc(I)-tricarbonyl species is a viable candidate for the source of alkaline-soluble, non-pertechnetate Tc in the Hanford tank supernatants. This work confirmed that the Tc species based on the [Tc(CO) 3 ] + center can be obtained by the laboratory synthetic route and that a potential route exists for their production in the alkaline tank wastes. These non-pertechnetate species are sufficiently stable under the conditions associated with Hanford tank supernatants. However, considerable work remains, specifically to achieve control over Tc redox behavior in the alkaline media, and to develop metho...

Spectroscopic Properties of Tc(I) Tricarbonyl Species Relevant to the Hanford Tank Waste

“…Previous analysis of the SY-101 and SY-103 tank waste samples provided strong evidence that non-pertechnetate can be comprised of [Tc(CO) 3 ] + complexes containing Tc in oxidation state +1 (Lukens et al 2004). During the last two years, our team has expanded this work and demonstrated that high-ionic-strength solutions typifying tank waste supernatants promote oxidative stability of the [Tc(CO) 3 ] + species (Rapko et al 2013;Levitskaia et al 2014). It also was observed that high-ionic-strength alkaline matrices stabilize Tc(VI) and potentially Tc(IV) oxidation states, particularly in presence organic chelators, suggesting that the relevant Tc compounds can serve as important redox intermediates facilitating the reduction of Tc(VII) to Tc(I).…”

Synthesis and Characterization of Tc(I) Carbonyl Nitrosyl Species Relevant to the Hanford Tank Waste: FY 2016 Status Report

“…It is significant that only about 5-10% of total Tc present in the sample got oxidized to TcO 4 in 30 days ( Figure 5; green trace [second from bottom]) as determined based on the integration of signal intensity from a TcO 4 standard of known concentration. This is in contrast to the Tc(I) [Tc(CO) 3 ] + species, which undergo fast oxidative hydrolysis to form TcO 4 in aqueous solutions with low ionic strength (Rapko et al 2013;Levitskaia et al 2014). When the yellow reaction precipitate was dissolved in 5 M NaNO 3 , it demonstrated similar but significantly less intense color change from yellow to red.…”

“…It was recently proposed based on the x-ray absorption studies of the actual SY-101 and SY-103 tank waste supernatants that non-pertechnetate species are the derivatives of low-valent Tc(I) carbonyl moiety and derived from fac-[Tc(CO) 3 ] + , 2 (Lukens et al 2004). Recent work by our group has significantly expanded this hypothesis, demonstrating that Tc(I) carbonyl compounds can be generated from reduction of TcO 4 in simulated Hanford tank waste in the presence CO at elevated temperature (Levitskaia et al 2014), and ongoing monitoring of these samples indicates long-term stability of Tc(I) carbonyl non-pertechnetate 3 . These results are consistent with the proposal that [Tc(CO) 3 ] + species can be present in the Hanford tank waste and suggest that the low Tc(I) oxidation state is stabilized by the π-accepting ability of the CO ligands.…”

“…(Et 4 N) 2 [Tc(CO) 3 Cl 3 ] was prepared by the two-step reduction of ammonium pertechnetate as described elsewhere (Levitskaia et al 2014). It was used to generate analytically pure tetrameric [Tc(CO) 3 (OH)] 4 species according to modified literature procedure (Alberto et al 1998) by dissolution in NaOH solution and extraction of the product into diethyl ether.…”

“…It has been recently proposed that the non-pertechnetate species can comprise Tc(I) metal center and carbonyl or mixed carbonyl nitrosyl ligands stabilizing low-valent Tc. Recent work by our group has significantly expanded this previous work, generating a series of Tc(I) carbonyl compounds and demonstrating that they can be generated from reduction of TcO 4 in the simulated Hanford tank waste in presence of CO at elevated temperature (Levitskaia et al 2014). These results are consistent with the previous proposal that [Tc(CO) 3 ] + species can be present in the Hanford tank waste and suggest that the low Tc(I) oxidation state is stabilized by the π-accepting ability of the CO ligands.…”