Potential interferences on the pertechnetate-sulfide immobilization reaction

Liu, Yongjian; Terry, Jeff; Jurisson, Silvia S.

doi:10.1524/ract.2009.1569

Cited by 11 publications

(22 citation statements)

References 45 publications

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“…To examine the influence of environmental conditions on the reductive immobilization reaction between TcO 4 and H 2 S, the effect of competing inorganic anions (nitrate), chelating agents (ethylenediaminetetraacetic acid, EDTA), low molecular weight organic acids, and surface effects of non-reducing minerals (goethite) were subsequently investigated (Liu et al, 2009). The presence of nitrate, EDTA and goethite did not affect the immobilization reaction, and the product was identified as Tc(IV) 2 S 7 in all cases by XANES and EXAFS.…”

Section: Sulfides For Remediationmentioning

confidence: 99%

“…The presence of nitrate, EDTA and goethite did not affect the immobilization reaction, and the product was identified as Tc(IV) 2 S 7 in all cases by XANES and EXAFS. Only maleic acid and fumaric acid inhibited the immobilization process by forming anionic water-soluble complexes with TcO 4 - (Liu et al, 2009). Szecsody et al (2014) found that alkaline and acidic waste streams had an impact on Hanford sediments and that the waste composition significantly altered contaminant speciation and mineralogy.…”

Section: Sulfides For Remediationmentioning

confidence: 99%

See 1 more Smart Citation

Technetium Stabilization in Low-Solubility Sulfide Phases: A Review

Pearce

Icenhower²,

Asmussen

et al. 2018

ACS Earth Space Chem.

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Technetium (Tc) contamination remains a major environmental problem at nuclear reprocessing sites, e.g., the Hanford Site, Washington State, USA. At these site, Tc is present in liquid waste destined for immobilization in a waste form or has been released into the subsurface environment. The high environmental risk associated with Tc is due to its long half-life (213,000 years) and the mobility of the oxidized anionic species Tc(VII)O 4-. Under reducing conditions, TcO 4 is readily reduced to Tc(IV), which commonly exists as a relatively insoluble and therefore immobile, hydrous Tc oxide (TcO 2 •nH 2 O). The stability of Tc(IV) sequestered as solid phases depends on the solubility of the solid and susceptibility to re-oxidation to TcO 4-, which in turn depend on the (bio-geo)chemical conditions of the environment and/or nuclear waste streams. Unfortunately, the solubility of crystalline TcO 2 or amorphous TcO 2 •H 2 O is still above the maximum contaminant level (MCL) established by the US EPA (900 pCi/L), and the kinetics of TcO 2 oxidative dissolution can be on the order of days to years. In addition to oxygen, sulfur can form complexes that significantly affect the adsorption, solubility and re-oxidation potential of Tc, especially Tc(IV). The principal technetium sulfides areTcS 2 and Tc 2 S 7 but much less is known about the mechanisms of formation, stabilization and re-oxidation of Tc sulfides. A common assumption is that sulfides are less soluble that their oxyhydrous counterparts. Determination of the molecular structure of Tc 2 S 7 in particular has been hampered by the propensity of this phase to precipitate as an amorphous substance. Recent work indicates that the oxidation state of Tc in Tc 2 S 7 is Tc(IV), in apparent contradiction to its nominal stoichiometry. Technetium is relatively immobile in reduced sediments and soils, but in many cases the exact sink for Tc has not been identified. Experiments and modeling have demonstrated that both abiotic and biologic mechanisms can exert strong controls on Tc mobility and that Tc binding or uptake into sulfide phases can occur. These and similar investigations also show that extended exposure to oxidizing conditions results in transformation of sulfide-stabilized Tc(IV) to a Tc(IV)O 2-like phase without formation of measurable dissolved TcO 4-, suggesting a solid-state transformation in which Tc(IV)-associated sulfide is preferentially oxidized before the Tc(IV) cation. This transformation of Tc(IV) sulfides to Tc(IV) oxides may be the main process that limits remobilization of Tc as Tc(VII)O 4-. The efficacy of the final waste form to retain Tc also strongly depends on the ability of oxidizing species to enter the waste and convert Tc(IV) to Tc(VII). Many waste form designs are reducing (e.g., cementitious waste forms such as salt stone), therefore, attempt to restrict access of oxidizing species such that diffusion is the ratelimiting step in remobilization of Tc.

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Section: Sulfides For Remediationmentioning

confidence: 99%

Section: Sulfides For Remediationmentioning

confidence: 99%

Technetium Stabilization in Low-Solubility Sulfide Phases: A Review

Pearce

Icenhower²,

Asmussen

et al. 2018

ACS Earth Space Chem.

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“…When dissolved in water, sodium sulfide, an ingredient of DuraLith, produces HS -, which promotes the formation of many metal sulfide precipitates. Liu et al (2008Liu et al ( , 2009 reported that the reaction between sulfide and TcO 4 in pH 9 aqueous solution under anaerobic conditions produces Tc 2 S 7 according to the reaction 2TcO 4 -+ 7HS -+ H 2 O → Tc 2 S 7 + 9OH -. Note that the sulfur in this compound is a mix of polysulfides such that each sulfur is not -2 charged.…”

Section: 9mentioning

confidence: 99%

Radionuclide Retention Mechanisms in Secondary Waste-Form Testing: Phase II

Um¹,

Valenta²,

Chung³

et al. 2011

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v compared to the uncarbonated Cast Stone. Slightly increased porosity, ranging from 8.6% to 10.7% after carbonation using gas absorption analysis, also was measured in the weathered Cast Stone. A similar increased porosity was found in Cast Stone after carbonation (14 days) using the XMT analysis. However, smaller values of porosity were measured by XMT in Cast Stone before and after carbonation (14 days) because of the limited capability of the XMT technique to separate water-filled pore areas from cement solid materials-that is, the XMT was delineating mostly the small volume of air-filled pores and not the total porosity. The EPA 1314 method (up-flow percolation column leaching test) was used for the Cast Stone and DuraLith geopolymer (Batch #2) waste forms available from Phase I and for the Geo-7 encapsulated FBSR product recently available from Savannah River National Laboratory. The Tc concentrations leached from the DuraLith geopolymer packed column were slightly higher than the Tc concentrations leached from the Cast Stone packed column. Significantly higher Re concentrations were found in the leachates from the FBSR packed column. The stop-flow technique was also applied to the Cast Stone and DuraLith geopolymer packed columns. When the first effluents were obtained after flow was resumed, the Tc concentrations were significantly higher, which confirms that Tc releases from both Cast Stone and DuraLith were controlled by diffusion processes. Leachate concentrations for RCRA-listed metals from the packed columns were very low, a result that was similar for the Cast Stone and DuraLith waste forms. That is, none of the RCRA-listed metals present in the secondary waste simulants leached out of the Cast Stone or DuraLith in significant concentrations. Concentrations of RCRA-listed metals and major cations (including Re), electrical conductivity, and alkalinity were higher in the effluents from the packed column with FBSR encapsulated with Geo-7 monolith chunks using the EPA 1314 method. These high concentrations may be attributable to the higher concentrations of RCRA metals in the FBSR product that, after encapsulation in Geo-7, was used for the packed column test. EPA 1315 leach tests for Cast Stone and DuraLith monoliths from the Phase-I project were extended beyond 63 days to 90 days. The leach test results showed that the Tc diffusivity for the DuraLith Batch #1 and Batch #2 waste forms at 90 days were a bit higher than that observed for the Cast Stone. There also was a minor decreasing Tc diffusivity trend for the Cast Stone over the entire 90-day leaching period. The decreasing Tc diffusivity trend might be caused by ongoing carbonation reactions-that is, formation of calcium carbonate on the surface and within near-surface micro cracks that might be reducing the porosity of the monolith during the leach testing. The slightly increased Tc diffusivity in the DuraLith waste form near the 90-day leach interval might result from more dissolution of the DuraLith matrix with extended contact with the soluti...

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“…Another approach employs Sn II incorporated into aluminophosphate nanocomposites for the reduction of TcO 4 – followed by stabilization within the nanocomposite , . Other approaches use sulfides or iron sulfides for reduction of TcO 4 – and subsequent sequestration of the resulting low valent Tc species …”

Section: Introductionmentioning

confidence: 99%

Strategies for the Photoreduction of Tc‐99 Pertechnetate to Low‐Valent Tc by Keggin Polyoxometalates

et al. 2020

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Technetium‐99 (half‐life of 2.1 × 105 yrs, βmax = 0.29 MeV) is a hazardous radiological contaminant, which, in its predominant form of pertechnetate (TcO4–), is highly mobile in the environment. Most strategies for the removal of pertechnetate from the environment involve uptake and/or absorption of pertechnetate using resins, clays, cationic metal‐organic frameworks and even thorium borate ceramic like materials. Alternative approaches have involved the reduction and subsequent sequestration of lower valent technetium species using iron, sulfides, or iron sulfides. Our lab has explored this strategy using the lacunary alpha‐2 Wells–Dawson polyoxometalate (α2‐[P2W17O61]10–) to both reduce and sequester lower valent technetium, and we have reported on the ligand features that stabilize the reduced species. In this work we investigate the potential of “plenary” Keggin POMs (XW12O40n–) (X = P, Si, Al, n = 3, 4, 5, respectively) to both reduce TcO4– and stabilize the reduced Tc species. Specifically, we report on the mechanism by which the reduction of technetium occurs and find that PW12, SiW12, and AlW12 promote the reduction of TcO4– to lower valent states. X‐ray absorption spectroscopy was used to confirm a combination of TcIV {in the form of TcO2·2H2O and Tc2(µ‐O)24+} and TcV, which is subsequently complexed into a POM defect as a TcV=O species.

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Potential interferences on the pertechnetate-sulfide immobilization reaction

Cited by 11 publications

References 45 publications

Technetium Stabilization in Low-Solubility Sulfide Phases: A Review

Technetium Stabilization in Low-Solubility Sulfide Phases: A Review

Radionuclide Retention Mechanisms in Secondary Waste-Form Testing: Phase II

Strategies for the Photoreduction of Tc‐99 Pertechnetate to Low‐Valent Tc by Keggin Polyoxometalates

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