Solubility Of Technetium Research Articles

Technetium Complexation with Multidentate Carboxylate-Containing Ligands: Trends in Redox and Solubility Phenomena.

The chemistry of technetium (t1/2(99Tc) = 2.11 × 105 years) is of particular importance in the context of nuclear waste disposal and historic contaminated sites. Polycarboxylate ligands may be present in some sites and are potentially capable of strong complexing interactions, thus increasing the solubility and mobility of 99Tc under environmentally relevant conditions. This work aimed to determine the impact of five organic complexing ligands [L = oxalate, phthalate, citrate, nitrilotriacetate (NTA), and ethylenediaminetetraacetate (EDTA)] under anoxic, alkaline conditions (pH ≈ 9-13) on the solubility of technetium. X-ray absorption spectroscopy confirmed that TcO2(am,hyd) remained the solubility-controlling solid phase in undersaturation solubility experiments. Ligands with maximum coordination numbers (CN) ≥ 3 (EDTA, NTA, and citrate) exhibited an increase in solubility from pH 9 to 11, while ligands with CN ≤ 2 (oxalate and phthalate) at all investigated pH and CN ≥ 3 at pH ≈ 13 were outcompeted by hydrolysis reactions. Though most available thermodynamic values were determined under acidic conditions, these models satisfactorily explained high-pH undersaturation solubility of technetium for citrate and NTA, whereas experimental data for Tc(IV)-EDTA were highly overestimated. This work illustrates the predominance of hydrolysis under hyperalkaline conditions and provides experimental support for existing thermodynamic models of Tc-L except Tc-EDTA, which requires further research regarding aqueous speciation and solubility.

Impact of carbonation on leaching of constituents from a cementitious waste form for treatment of low activity waste at the DOE Hanford site

Carbonation can be a major aging process during disposal of alkaline cementitious waste forms and can impact constituent leaching by changes in material alkalinity, pore structure, and controlling mineral phases. The effect of carbonation on the leaching of major and trace constituents from Cast Stone, a cementitious waste form developed to treat high salt content low activity waste, was studied through a combination of leaching experiments and reactive transport simulations. Diffusive transport of constituents in the waste form was evaluated using reactive transport modeling of diffusion-controlled leaching test results and a geochemical speciation model derived from pH-dependent leaching. Comparisons between Cast Stone materials aged under nitrogen, air, and 2% carbon dioxide in nitrogen showed that carbonation impacts solubility, physical retention and observed diffusivity of major and trace constituents. Carbonation under 2% CO2 decreased the diffusion-controlled leaching of chromium by two orders of magnitude. Modeling results suggest that carbonation may also decrease solubility of technetium while changes to microstructure by carbonation increases effective diffusivity of constituents in Cast Stone.

Redox-dependent solubility of technetium in low activity waste glass

The solubility of technetium was measured in a Hanford low activity waste (LAW) glass simulant, to investigate the extent that technetium solubility controls the incorporation of technetium into LAW glass. A series of LAW glass samples, spiked with 500–6000ppm of Tc as potassium pertechnetate, were melted at 1000°C in sealed fused quartz ampoules. Technetium solubility was determined in the quenched bulk glass to be 2000–2800ppm, with slightly reducing conditions due to choice of milling media resulting in reductant contamination and higher solubility. The chemical form of technetium obtained by X-ray absorption near edge spectroscopy is mainly isolated, octahedrally-coordinated Tc(IV), with a minority of Tc(VII) in some glasses and TcO2 in two glasses. The concentration and speciation of technetium depends on glass redox and amount of technetium added. Salts formed at the top of higher technetium loaded glasses during the melt. The results of this study show that technetium solubility should not be a factor in technetium retention during melting of Hanford LAW glass.

Experimental Study of Contaminant Release from Reducing Grout

A column experiment was conducted to study the release behavior of technetium, uranium, and selenium initially sequestered in reducing grout similar in composition to Savannah River Site (SRS) saltstone, a cementitious waste form made by mixing salt solution from SRS liquid waste storage tanks with a dry mix containing blast furnace slag, fly ash, and Portland cement. The data suggest that uranium was retained in the grout possibly as a CaUO4 phase, whereas most of the selenium was released. Technetium release initially was relatively constant, and then increased significantly after 26 pore volumes. The increase in technetium release was slightly delayed relative to the observed Eh increase. The system Eh-pH started under conditions in which technetium solubility is low, constrained by Tc3O4 solubility, but eventually transitioned into the stability field of the pertechnetate ion. The delay in technetium release relative to the Eh increase was possibly due to slow oxidation of technetium at depth within the grout particles, which in turn was likely controlled by O2 diffusion into the particles. In contrast to technetium and uranium, selenium release was not solubility limited and selenium likely was present in the pore solution initially as a HSe− species.

KURT 지하수 조건에서 셀레늄과 테크네튬의 용해도 및 주요 화학종

장수명 핵분열생성물인 <TEX>$^{79}Se$</TEX>와 <TEX>$^{99}Tc$</TEX>는 자연수 중에서 용해도가 클 뿐더러 음이온으로 존재하여 방사성폐기물 처분장에서 주요 관심핵종들로 고려되고 있다. 본 연구에서는 KURT 지하수의 다양한 pH와 산화-환원 조건에서 셀레늄과 테크네튬의 Solubility Limiting Solid Phase (SLSP)로 알려진 <TEX>$FeSe_2$</TEX>와 <TEX>$TcO_2$</TEX>의 용해도를 측정하였다. 또한, 지화학코드를 이용하여 실험과 유사조건에서 이들의 용해도와 주요 화학종을 계산하였다. 실험 및 계산으로부터 pH 8~9.5와 Eh=-0.3~-0.4 V 조건에서 <TEX>$FeSe_2$</TEX>의 용해도는 <TEX>$1{\times}10^{-6}mol/L$</TEX>이하이며, 주 용해 화학종은 HSe-로 판단된다. <TEX>$TcO_2(s)$</TEX>의 경우는 pH 6~9.5와 Eh<-0.1 V 영역에서 용해도와 주 용해 화학종이 각각 <TEX>$5{\times}10^{-8}{\sim}1{\times}10^{-9}mol/L$</TEX>와 <TEX>$TcO(OH)_2$</TEX>로 나타났지만, Eh=-0.35 V조건에서는 주 용해화학종이 pH가 10.5~12와 12이상에서 각각 <TEX>$TcO(OH)_3^-$</TEX>와 <TEX>$TcO_4^-$</TEX>로 계산되었다. The long-lived fission products <TEX>$^{79}Se$</TEX> and <TEX>$^{99}Tc$</TEX> have been considered as the major concern nuclides for the disposal of radioactive waste because of their high solubilities and the existence of anionic species in natural water. In this study, the solubilities of <TEX>$FeSe_2(s)$</TEX> and <TEX>$TcO_2(s)$</TEX>, known as respective Solubility Limiting Solid Phase (SLSP) of selenium and technetium, were measured in the KURT (KAERI Underground Research Tunnel) groundwater under various pH and redox conditions. And their solubilities and major species were also calculated using geochemical codes under conditions similar to experimental solutions. Experimental results and calculation for <TEX>$FeSe_2$</TEX> show that the solubility of selenium was found to be below <TEX>$1{\times}10^{-6}mol/L$</TEX> under the condition of pH 8~9.5 and Eh=-0.3~-0.4 V while the dominant species was identified as <TEX>$HSe^-$</TEX>. For <TEX>$TcO_2$</TEX>, the solubility of technetium was found to be <TEX>$5{\times}10^{-8}{\sim}1{\times}10^{-9}mol/L$</TEX> in the solutions of pH 6~9.5 and Eh<-0.1 V, while the dominant species was <TEX>$TcO(OH)_2$</TEX>. However, when the Eh of the solution is -0.35 V, <TEX>$TcO(OH)_3^-$</TEX> and <TEX>$TcO_4^-$</TEX> are calculated as the dominant species at pH 10.5~12 and pH>12, respectively.

Determination of technetium-99 in environmental samples: A review

Due to the lack of a stable technetium isotope, and the high mobility and long half-life, 99Tc is considered to be one of the most important radionuclides in safety assessment of environmental radioactivity as well as nuclear waste management. 99Tc is also an important tracer for oceanographic research due to the high technetium solubility in seawater as TcO 4 −. A number of analytical methods, using chemical separation combined with radiometric and mass spectrometric measurement techniques, have been developed over the past decades for determination of 99Tc in different environmental samples. This article summarizes and compares recently reported chemical separation procedures and measurement methods for determination of 99Tc. Due to the extremely low concentration of 99Tc in environmental samples, the sample preparation, pre-concentration, chemical separation and purification for removal of the interferences for detection of 99Tc are the most important issues governing the accurate determination of 99Tc. These aspects are discussed in detail in this article. Meanwhile, the different measurement techniques for 99Tc are also compared with respect to advantages and drawbacks. Novel automated analytical methods for rapid determination of 99Tc using solid extraction or ion exchange chromatography for separation of 99Tc, employing flow injection or sequential injection approaches are also discussed.

Dissolution of Technetium(IV) Oxide by Natural and Synthetic Organic Ligands under both Reducing and Oxidizing Conditions

Technetium-99 (Tc) in nuclear waste is a significant environmental concern due to its long half-life and high mobility in the subsurface. Reductive precipitation of technetium(IV) oxides [TcO(2)(s)] is an effective means of immobilizing Tc, thereby impeding its migration in groundwater. However, technetium(IV) oxides are subject to dissolution by oxidants and/or complexing agents. In this study we ascertain the effects of a synthetic organic ligand, ethylenediaminetetraacetate (EDTA), and two natural humic isolates on the dissolution and solubility of technetium(IV) oxides. Pure synthetic technetium(IV) oxide (0.23 mM) was used in batch experiments to determine dissolution kinetics at pH ∼6 under both reducing and oxidizing conditions. All organic ligands were found to enhance the dissolution of technetium(IV) oxides, increasing their solubility from ∼10(-8) M (without ligands) to 4 × 10(-7) M under strictly anoxic conditions. Reduced Tc(IV) was also found to reoxidize rapidly under oxic conditions, with an observed oxidative dissolution rate approximately an order of magnitude higher than that of ligand-promoted dissolution under reducing conditions. Significantly, oxidative dissolution was inhibited by EDTA but enhanced by humic acid compared to experiments without any complexing agents. The redox functional properties of humics, capable of facilitating intramolecular electron transfer, may account for this increased oxidation rate under oxic conditions. Our results highlight the importance of complex interactions for the stability and mobility of Tc and thus for the long-term fate of Tc in contaminated environments.

The solubility of technetium(IV) at high pH

Summary Technetium-99 is a β-emitting fission product of 238U. It is important when considering a safety case for nuclear waste disposal because of its high yield in radioactive waste and long half-life. The aqueous chemistry of Tc in a cementitious repository is likely to be dominated by Tc(VII), as TcO4 −, in aerobic areas and by Tc(IV), as TcO2 (am), in anaerobic. Some previous studies have shown an increase in Tc(IV) solubility with increasing pH above pH 9.5 whilst others have not observed this increase. This study found that the solubility of Tc(IV) remains independent of pH until around 13.5, when a small increase can be seen which continues to increase linearly with pH. Modelling suggests that this increase occurs as the species TcO(OH)3 − is formed. The formation constant was estimated with data from this study and was found to be logK 2=−21.6±0.3.

Effects of Progressive Anoxia on the Solubility of Technetium in Sediments

Technetium is a significant radioactive contaminant from nuclear fuel cycle operations. It is highly mobile in its oxic form (as Tc(VII)O4-) but is scavenged to sediments in its reduced forms (predominantly Tc(IV)). Here we examine the behavior of Tc at low concentrations and as microbial anoxia develops in sediment microcosms. A cascade of stable-element terminal-electron-accepting processes developed in microcosms due to indigenous microbial activity. TcO4- removal from solution occurred during active microbial Fe(III) reduction, which generated Fe(II) in the sediments and was complete before sulfate reduction began. Microbial community analysis revealed a similar and complex microbial population at all three sample sites. At the intermediate salinity site, PauII, a broad range of NO3-, Mn(IV), Fe(III), and SO4(2-) reducers were present in sediments including microbes with the potential to reduce Fe(III) to Fe(II), although no differences in the microbial population were discerned as anoxia developed. When sterilized sediments were incubated with pure cultures of NO3(-)-, Fe(III)-, and sulfate-reducing bacteria, TcO4- removal occurred during active Fe(III) reduction. X-ray absorption spectroscopy confirmed that TcO4- removal was due to reduction to hydrous Tc(IV)O2 in Fe(III)- and sulfate-reducing estuarine sediments.

Technetium behaviour in Boom Clay – a laboratory and field study

SummaryThis paper describes a study of technetium solubility and migration under chemical conditions representative of those prevailing in a Boom Clay environment. Laboratory andin situmeasurements yielded similar aqueous concentrations of technetium, of about 1×10−8mol dm−3, close to the concentrations measured for hydrated technetium(IV) oxide TcO2·1.6H2O in the solubility studies. From fitting the curves of the Tc concentrations as function of time, distribution coefficient (Kd) values were estimated to lie between 0.8 cm3g−1and 1.8 cm3g−1. Exposure of the system at 80 °C and to γ-radiation dose rates of several hundred Gy h−1resulted in only minor differences in behaviour.

The solubility of technetium in the near-field environment of a radioactive waste repository

Abstract Experimental measurements of the solubility of technetium under conditions similar to those expected to be found in the near-field environment of a radioactive waste repository have been carried out. The measured solubility of technetium under high pH and reducing conditions in contact with hydrated technetium dioxide has been found to be about 10−7 mol dm−3 and is independent of pH in the range 8–13 and the type of cement-equilibrated water used. The measured solubility was insensitive to the solid-liquid separation method used, suggesting that signficant quantities of technetium-containing colloidal species are not present. A limited investigation of the effects of organic decomposition products on the solution concentration of reduced technetium has shown that there is a modest enhancement in technetium solubility.

The system zinc-technetium-99

The Zn-Tc 99 phase diagram was studied by thermal, metallographic, X-ray, sampling, and effusion techniques. The solubility of technetium in liquid zinc is expressed by the empirical equations: (470°–544° C) log ( at.% Tc) = 6.431−7159 T−1 ; (544°–758° C) log ( at.% Tc) = 9.413−1569 T−1 + 4.979.10 6 T −2. Two intermediate phases were found in the system: TcZn 6 and another phase with a Zn-Tc atom ratio between 15 and 18. These phases decompose peritectically at 544° and 950°C, respectively. Mutual solubilities in technetium and zinc appear to be negligible. Technetium metal appears to be unattacked by molten cadmium at 500°C.