Characterization of Non-pertechnetate Species Relevant to the Hanford Tank Waste

Abstract

Among radioactive constituents present in the tank waste stored at the U.S. DOE Hanford Site, technetium-99 (Tc), which is generated from the fission of 235 U and 239 Pu in high yields, presents a unique challenge in that it has a long half-life ( = 292 keV; T 1/2 = 2.1110 5 y) and exists predominately in soluble forms in the liquid supernatant and salt cake fractions of the waste. In the strongly alkaline environments prevalent in most of the tank waste, its dominant chemical form is pertechnetate

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

“…The most comprehensive experimental determination of the AN-102 supernatant has been reported by Urie et al It has high sodium and aluminum concentrations as well as several anionic constituents. , Several organic chelators were also reported, noteworthy among which are citrate, formate, gluconate, glycolate, and IDA (Table S2). These chelators (most notably gluconate and IDA) have been observed to form coordination complexes with low-valent [ fac -Tc I (CO) 3 ] + leading to their stabilization in high-ionic-strength matrices . The formation of [ fac- Tc­(CO) 3 (L) x ] n − chelates with different chelators present in AN-102 has been previously observed , (Table S2).…”

“…In light of the observation of the chelated [ fac -Tc­(CO) 3 ] + complex in the SY-101 and SY-103 Hanford tank wastes, a spectroscopic library of relevant [ fac -Tc­(CO) 3 ] + compounds was developed to allow differentiation between the various aqua species with the general formulae [ fac -Tc­(CO) 3 (H 2 O) 3– n (OH) n ] 1– n ( n = 0–3) , or those chelated by organic complexants [ fac- Tc­(CO) 3 (L) 3 ] n − . Among organic chelators reported in AN-102, polyaminocarboxylates and gluconate were observed to form [ fac- Tc­(CO) 3 (L) x ] n − coordination complexes in our ex situ studies documented elsewhere; , the observed 99 Tc NMR chemical shifts are shown in Table S2. Notably, in the presence of citrate, succinate, glycolate, or formate, no complexation was observed.…”

“…(Left panel) Interpretation of the AN-102 XAS spectrum based on the linear combination: (black circles) Tc K-edge XANES spectrum of the Cs-depleted fraction, (black trace) spectrum of TcO 2 · n H 2 O (Lukens et al), (green trace) spectrum of Tc­(IV)·gluconate (Shuh et al), (violet trace) spectrum of [ fac -Tc­(CO) 3 ] + ·gluconate, (red trace) spectrum of TcO 4 – , and (blue trace) linear combination fitting of all of these spectra. The relative contributions of each species to the composite fit are 44% for TcO 4 – , 32% for [ fac -Tc­(CO) 3 ] + ·gluconate, 13% for Tc­(IV)·gluconate, and 11% for TcO 2 · n H 2 O.…”

Synthesis and reactivity of low-valent uranium and neptunium complexes

“…A previously unknown Tc(I) complex was discovered in the waste tanks due to the highly reducing conditions generated from radiolysis products. 12,13 This complex was not able to be removed by ion-exchange, which is effective for the removal of TcO4 -, and was found to be difficult to oxidize. 14 Examining these complexes with unique oxidation states can help elucidate the chemistry of elements in nuclear waste, allowing for further methods to be developed for effective separation.…”

“…Furthermore, under highly reducing conditions, like the ones found in storage tanks, it is conceivable that Np(III) and Np(IV) species may persist. 12 Hence, the coordination chemistry and redox properties of Np IV/III complexes can provide valuable insight into advanced separation techniques, in addition to advancing our understanding of this understudied element.…”