In this study, we investigated the volatility of sodium, potassium, and cesium perrhenates using thermogravimetry (TG) performed on single salts and their binary mixed salts. We performed ...simultaneous TG and differential scanning calorimetry under a ramp heating condition at 10 °C/min up to 1200 °C to characterize volatilization behavior as a function of temperature. We also performed isothermal TG over a temperature range of 700–975 °C. To estimate the relative differences in the vapor pressure of the alkali perrhenates we adopted a published equation on the vaporization mass loss rate during diffusion-controlled evaporation, applied a proposed equation on the interdiffusion coefficient of vapors, and used the literature data available on vapor pressure for KReO
4
. Both the temperature ranges of evaporation determined by TG under ramp heating conditions and the vapor pressure estimated from isothermal TG support that the order of volatility of alkali perrhenates is CsReO
4
> KReO
4
> NaReO
4
. This result does not support the unusual volatility order of KReO
4
> CsReO
4
> NaReO
4
observed from Hanford LAW glass feeds, which suggests that the volatility of the perrhenates is likely dependent on the composition of salt in which the perrhenates are dissolved.
We investigated a mass balance of rhenium (used as a surrogate for technetium-99) in a borosilicate glass that was mixed with excess Re source (KReO4) beyond its solubility and heat treated in a ...vacuum-sealed fused silica ampoule. Distribution of Re in the bulk of the glass, in a salt phase formed on the melt surface, and in condensate material deposited on the ampoule wall was evaluated to understand the Re migration into different phases during the reaction between the molten glass and KReO4. The information gained from this study will contribute to an effort to understand the mechanism of technetium retention in, or escape from, glass melt during early stages of glass batch melting, which is a goal of the present series of studies.
•We report Re distribution in a borosilicate glass added with excess KReO4.•Vacuum-sealed fused silica ampoule was used for heat treating the glass sample.•Excess Re beyond its solubility forms a separated salt on glass melt surface.•Surface salt and vapor deposit on ampoule were recovered and analyzed separately.•Ion exchange with glass melt causes the KReO4 salt enriched with Na and S.
Technetium mainly forms during artificial nuclear fission; it exists primarily as TcO4(-) in nuclear waste, and it is among the most hazardous radiation-derived contaminants because of its long ...half-life (t1/2 = 2.13 × 10(5) years) and environmental mobility. The high water solubility of TcO4(-) (11.3 mol L(-1) at 20 °C) and its ability to readily migrate within the upper layer of the Earth's crust make it particularly hazardous. Several types of materials, namely resins, molecular complexes, layered double hydroxides, and pure inorganic and metal-organic materials, have been shown to be capable of capturing TcO4(-) (or other oxoanions) from solution. In this review, we give a brief description about the types of materials that have been used to capture TcO4(-) and closely related oxyanions so far and discuss the possibility of using metal-organic frameworks (MOFs) as next-generation ion-exchange materials for the stated application. In particular, with the advent of ultra-stable MOF materials, in conjunction with their chemical tunability, MOFs can be applied to capture these oxyanions under real-life conditions.
Volatile loss of technetium (Tc) during vitrification of low-activity wastes is a technical challenge for treating and immobilizing the large volumes of radioactive and hazardous wastes stored at the ...U.S. Department of Energy's Hanford Site. Various research efforts are being pursued to develop technologies that can be implemented for cost effective management of Tc, including studies to understand the behavior of Tc during vitrification, with the goal of eventually increasing Tc retention in glass. One of these studies has focused on identifying the form or species of Tc and Re (surrogate for Tc) that evolve during the waste-to-glass conversion process. This information is important for understanding the mechanism of Tc volatilization. In this paper, available information collected from the literature is critically evaluated to clarify the volatile species of Tc and Re and, more specifically, whether they volatilize as alkali pertechnetate and perrhenate or as technetium and rhenium oxides after decomposition of alkali pertechnetate and perrhenate. The evaluated data ranged from mass spectrometric identification of species volatilized from pure and binary alkali pertechnetate and perrhenate salts to structural and chemical analyses of volatilized materials during crucible melting and scaled melter processing of simulated wastes.
•Literature data were critically evaluated to clarify volatile species of Tc and Re.•Tc and Re volatilize as M(Tc,Re)O4 without decomposition for M=Cs, K, Na, Li.•Volatility can be related to concentration and thermodynamic properties of volatile species.
Volatile loss of technetium (Tc) during vitrification of low-activity wastes is a technical challenge for treating and immobilizing the large volumes of radioactive and hazardous wastes stored at the ...U.S. Department of Energy's Hanford Site. There are various research efforts being pursued to develop technologies that can be implemented for cost effective management of Tc, including studies to understand the behavior of Tc during vitrification, with the goal of eventually increasing Tc retention in glass. Furthermore, one of these studies has focused on identifying the form or species of Tc and Re (surrogate for Tc) that evolve during the waste-to-glass conversion process. This information is important for understanding the mechanism of Tc volatilization. In this paper, available information collected from the literature is critically evaluated to clarify the volatile species of Tc and Re and, more specifically, whether they volatilize as alkali pertechnetate and perrhenate or as technetium and rhenium oxides after decomposition of alkali pertechnetate and perrhenate. The evaluated data ranged from mass spectrometric identification of species volatilized from pure and binary alkali pertechnetate and perrhenate salts to structural and chemical analyses of volatilized materials during crucible melting and scaled melter processing of simulated wastes.
Technetium-99 (99Tc) is a major radionuclide of concern in the Hanford low-activity waste (LAW), which will be vitrified into borosilicate glass at the Waste Treatment and Immobilization Plant (WTP). ...Sulfate in LAW has been known to be a critical factor affecting the volatile loss of 99Tc. We investigated rhenium (a nonradioactive surrogate for 99Tc) incorporation by crucible melting tests of two representative simulated LAW glass feeds, each prepared with varied sulfur concentrations in glass. The slurry feeds were dried and heated to 400–1100 °C. Soluble and insoluble phases of heated feeds were analyzed to profile the partitioning of various components during the melting process. The mechanism of how sulfate affects rhenium incorporation during feed-to-glass conversion reactions, and so final retention of rhenium in glass, is proposed based on the two hypotheses that segregated sulfate-rich phase forms during feed conversion reactions and Re preferentially partitions to this sulfate-rich phase.
Volatilization of technetium-99 (99Tc) is of concern during vitrification of Hanford low-activity waste (LAW) to produce a borosilicate glass waste form at the Waste Treatment and Immobilization ...Plant. Addition of reducing agents to LAW glass feed has been known to increase the retention of 99Tc in the final glass product. We investigated rhenium incorporation into glass by performing crucible melting tests on simulated LAW glass feeds, prepared without and with iron oxalate as a reducing agent, to understand the mechanism how the reducing agent influences Re retention in glass. Deionized water leaches were conducted on the dried feed and heat-treated samples to analyze the compositions of soluble and insoluble phases separately and to profile the partitioning of components into different phases during melting. The results in this study do not support the previous hypothesis that the low volatility of reduced Re4+ and Tc4+ is responsible for higher Re and Tc retention in glass when a reducing agent is added. We propose that early reaction of nitrate/nitrite and faster incorporation of sulfate into glass caused by iron oxalate are the two contributing factors that can explain the experimental findings in this study regarding the positive effect of iron oxalate on Re incorporation and retention.
Efficient and cost‐effective removal of radioactive pertechnetate anions from nuclear waste is a key challenge to mitigate long‐term nuclear waste storage issues. Traditional materials such as resins ...and layered double hydroxides (LDHs) were evaluated for their pertechnetate or perrhenate (the non‐radioactive surrogate) removal capacity, but there is room for improvement in terms of capacity, selectivity and kinetics. A series of functionalized hierarchical porous frameworks were evaluated for their perrhenate removal capacity in the presence of other competing anions.
Going nuclear: Functionalized hierarchical porous frameworks were used for the efficient removal of perrhenate from aqueous solution (see figure).
Four consecutive sets of simplified glass feed compositions were designed and tested both with and without rhenium (Re) additions. The objectives of this study concerning the effects of sulfur (S, ...expressed as SO3 in glass) and potassium (K, K2O in glass) on Re retention during melting of low-activity glass feeds were twofold: (i) design a simplified system that reproduces the Re behavior observed in fully simulated feeds, the system will be used to investigate mechanisms related to Re incorporation into glass and (ii) Provide experimental data that support or reject the mechanisms that have been proposed to explain the previous findings from scaled melter and crucible tests. The simplified system of alkali borosilicate glass feeds in this study successfully reproduced the published results from the melter tests with fully simulated feeds on the effects of S and K on Re behavior. The present study also provides experimental evidence on the combined effect of S and K. The findings from this study regarding the effects of S and K on Re retention also are explained by the previously proposed mechanism on the effect of sulfate on Re retention and by the effect of component concentrations and activity coefficients of volatile species on its vapor pressure, also discussed in a recent study.
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