The function of Sn(II)-apatite as a Tc immobilizing agent

Asmussen, R. Matthew; Neeway, James J.; Lawter, Amanda R.; Levitskaia, Tatiana G.; Lukens, Wayne W.; Qafoku, Nikolla P.

doi:10.1016/j.jnucmat.2016.09.002

Cited by 19 publications

(8 citation statements)

References 35 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…chromate, Cr(VI) 2 O 7 2-, and nitrate/nitrite, NO 3 -/NO 2 -). Examples of this include the decrease in Tc removal capability observed for many compounds such as Sn(II)-treated apatite, zerovalent Fe and others in simulated low activity waste (LAW) or low level waste (LLW) (Darab et al 2007;Qafoku et al 2014;Asmussen et al 2016). Despite this, Fe-sulfides have been suggested as possible treatments for Tc containing nuclear wastes (Watson and Ellwood 2003) and recent work has shown that sulfide based materials are adept at sequestering Tc from LAW.…”

Section: Technetium Sulfide In Waste Formsmentioning

confidence: 99%

Technetium Stabilization in Low-Solubility Sulfide Phases: A Review

Pearce

Icenhower²,

Asmussen

et al. 2018

ACS Earth Space Chem.

Self Cite

View full text Add to dashboard Cite

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.

show abstract

Section: Technetium Sulfide In Waste Formsmentioning

confidence: 99%

Technetium Stabilization in Low-Solubility Sulfide Phases: A Review

Pearce

Icenhower²,

Asmussen

et al. 2018

ACS Earth Space Chem.

Self Cite

View full text Add to dashboard Cite

show abstract

“…The reduction of 99-Tc from its soluble Tc(VII)O 4 form to a low solubility Tc(IV) species can be utilized to extract 99-Tc from liquid waste streams (Serne et al 2016a). Several candidate reductants exist which can drive the reaction of Tc(VII) including Sn(II) (Asmussen et al 2016b;Duncan et al 2016;Taylor-Pashow et al 2018), Fe (0 or II) (Del Cul et al 1995) and S(0) (Pearce et al 2018). In this approach a simple chemical form of the reductant would be introduced to the LAW to precipitate 99-Tc.…”

Section: Line 6 -Redox Strike/co-precipitationmentioning

confidence: 99%

“…The primary challenges with the approach are the presence of excess Cr(VI) in the LAW (along with other oxidants such as NO 3 ) that would compete for reductants (~50 × higher than 99-Tc), decreased efficiency of some reductants in alkaline conditions (Asmussen et al 2016b), and the potential need for a filtration step to collect the precipitated 99-Tc. The strike could be performed as a direct chemical addition or in a column.…”

Section: Line 6 -Redox Strike/co-precipitationmentioning

confidence: 99%

Evaluation of Technologies for Enhancing Grout for Immobilizing Hanford Supplemental Low-Activity Waste (SLAW)

Skeen

Langston

McCabe

et al. 2020

View full text Add to dashboard Cite

An expert panel consisting of the document authors was assembled by Washington River Protection Solutions, LLC (WRPS) and tasked with (1) identifying technical and regulatory limitations to treating and disposing of grouted low-activity waste (LAW), and (2) with identifying and evaluating a wide range of technical solutions to these limitations. The analysis included technologies to allow inventory management and/or improve retention of iodine, technetium, and nitrate either in the waste packages, or within the Integrated Disposal Facility (IDF), and, technologies to ensure regulated organics are below Resource Conservation and Recovery Act of 1976 (RCRA) Land Disposal Restriction (LDR) levels.The expert panel completed their analysis in three distinct phases. In Phase I, the team reviewed the chemical and physical characteristics of the most recent projected supplemental LAW (SLAW) feed vector along with disposal site waste acceptance criteria and the fiscal year 2019 results from Pacific Northwest National Laboratory that identified the level of improvement that would be needed to meet onsite disposal requirements in the IDF (Asmussen et al. 2019a). The two regulatory constraints targeted for technology solutions by the expert panel were (1) current estimates based on performance assessment fate-andtransport modeling indicate that if the entire quantity of LAW currently designated for supplemental treatment were solidified as a cementitious material, nitrate, 99-Tc, and 129-I could exceed groundwater standards after DOE's time of compliance (1,000 years) but within 10,000 years after IDF closure and (2) LAW may contain RCRA LDR organics at levels that require removal to meet LDR concentration-based limits. It should also be noted that the first issue is only applicable to on-site disposal, not offsite at WCS and is based on calculations that assume either a low-performing grout is used, or high-performance grout does not maintain its performance (SRNL-RP-2018-00687). Asmussen et al. (2019a) estimated that the inventory of grouted SLAW could meet IDF groundwater requirements if the release rate of nitrate, 99-Tc, and 129-I were reduced by a factor of approximately 3.2, 10, and 32, respectively versus baseline formulation performance.In Phase II, an extensive list of technologies and engineering approaches was identified and subjected to an initial assessment by the group. Current knowledge associated with technology maturation needs and constraints to implementation at Hanford were recorded in 13 distinct categories. In the final phase of the process (Phase III), the expert panel selected 7 of the 13 categories for qualitative rating and a series of meetings were held between the expert panel to discuss and rate each technology in each category. All ratings were developed by consensus. After the qualitative rating was complete, a numerical scoring criterion for each category was applied and a composite score for the individual technologies was calculated. An uncertainty score was also developed for each technolo...

show abstract

“…Granular activated carbon (GAC; Nuchar WV-G) from West Virginia Pulp and Paper was evaluated for TcO4removal from groundwater, but the available data demonstrated that its adsorption capacity was low [8]. For high TcO4waste streams, reductant materials (i.e., Fe sulfides [9,10], layered potassium metal sulfides [11], tin-apatite [12], adsorbed or structural Fe(II) [13][14][15][16][17], zero valent iron [18]), and reducing bacteria [9,10,19] can chemically reduce Tc(VII) to sparingly soluble Tc(IV). However, the solubility of the precipitated reduction product, Tc(IV)O2•1.6H2O, is still relatively high (i.e., 1.5×10 -8 M) in groundwater [20,21], greatly exceeding the EPA's maximum contaminant level (MCL) of 5×10 -10 M Tc.…”

Section: Introductionmentioning

confidence: 99%

Pertechnetate (TcO4−) sequestration from groundwater by cost-effective organoclays and granular activated carbon under oxic environmental conditions

Seaman

Kaplan

et al. 2019

Chemical Engineering Journal

View full text Add to dashboard Cite

Technetium-99 ( 99 Tc) is a major risk driver at nuclear power plants and several US Department of Energy (DOE) sites. Pertechnetate (TcO4 -), the most common chemical form present in liquid nuclear waste and the environment, displays limited adsorption onto sediments, making it highly mobile and difficult to immobilize. In this work, inexpensive organoclays and granular activated carbon (GAC) were investigated for TcO4sequestration from artificial groundwater (AGW).The Tc adsorption capacities were 15 mg/g for organoclays and 25 mg/g for GAC from pH 3-12 AGW under oxic conditions. Competitive NO3at two orders of magnitude greater concentration than TcO4reduced the Tc adsorption capacity by only 9-25%. In addition, the adsorbed Tc was effectively desorbed by KI, but not Na2SO4, and the regenerated sorbents retained their Tc removal capacity. X-ray absorption spectroscopy demonstrated that the Tc species bound to the 2 organoclays and GAC was Tc(VII) (i.e., TcO4 -), rather than Tc(IV). Thus, the inexpensive organoclays and GAC are highly effective at sequestering 99 Tc in its naturally existing TcO4without requiring costly reductive systems, which may provide a practical solution for removing 99 TcO4from environmental systems.

show abstract

The function of Sn(II)-apatite as a Tc immobilizing agent

Cited by 19 publications

References 35 publications

Technetium Stabilization in Low-Solubility Sulfide Phases: A Review

Technetium Stabilization in Low-Solubility Sulfide Phases: A Review

Evaluation of Technologies for Enhancing Grout for Immobilizing Hanford Supplemental Low-Activity Waste (SLAW)

Pertechnetate (TcO4−) sequestration from groundwater by cost-effective organoclays and granular activated carbon under oxic environmental conditions

Contact Info

Product

Resources

About