30 Search Results

Here, the solvation structure of an Np⁴⁺ ion in an aqueous, noncomplexing and nonoxidizing environment of trifluoromethanesulfonic (triflic) acid was investigated with X-ray absorption spectroscopy (XAS) combined with ab initio molecular dynamics (AIMD) and time-dependent density functional theory (TDDFT) calculations. Np L_III-edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) data were collected for Np⁴⁺ in 1, 3, and 7 M triflic acid using a laboratory-scale spectrometer and separately at a synchrotron facility, producing data sets in excellent agreement. TDDFT calculations revealed a weak pre-edge feature not previously reported for Np L_III-edge XANES. AIMD modeling resultsmore » showed differences in the hydration shell of the Np⁴⁺ ion at different concentrations of triflic acid; these results are supported by the experiment. EXAFS fit models to the experiment resulted in similar coordination of Np⁴⁺ in noncomplexing aqueous media as reported in the literature for 1 M perchloric acid but, together with calculations, revealed more than one distance between Np and O atoms in 7 M triflic acid. These results imply monodentate coordination with sulfonate groups in 7 M triflic acid and suggest the possibility of proto-neptunyl species in relatively low-concentration Np⁴⁺ acid solutions.« less

Successful deployment of in situ subsurface remediation strategies requires knowledge of contaminant geochemistry, and the impact of physicochemical sediment properties on remedy performance. Bismuth (Bi) materials can sequester multiple contaminants that are present in the unsaturated zone and groundwater at Department of Energy (DOE) legacy nuclear sites, such as the Hanford Site. Adsorption experiments for individual contaminants (chromate, iodate, pertechnetate, and uranyl carbonate) were conducted with two Bi materials: commercially available Bi-subnitrate (BSN); and laboratory synthesized Bi oxyhydroxide nitrate (BOH). The structure and composition of the Bi material influenced hydrolysis and ion exchange interactions in aqueous solution, with subsequent impactsmore » on solution pH, contaminant speciation, and contaminant uptake. X-ray diffraction revealed that the disordered BOH structure, initially containing charge balancing nitrate and hydroxide anions, rapidly converted to bismutite, Bi₂O₂(CO₃), in the presence of carbonate. During this transformation, BOH removed most contaminant ions from solution. [Bi₆O₅(OH)₃]⁵⁺ clusters in BSN underwent hydrolysis upon exposure to aqueous solutions, substantially reducing pH, and transforming into several mineral phases, including a daubreeite (BiO(OH,Cl)) structure, and an unidentified mineral phase (unk-Bi(NO₃)_x(OH)_yO_z). This transformation decreased uptake efficiency relative to BOH, except for pertechnetate. The adsorption isotherms for the contaminants were fit with a Freundlich model that describes adsorption to Bi materials with dissimilar binding sites. Solid phase characterization after reaction confirmed structural rearrangement of the Bi materials and direct association of the contaminant ions with Bi mineral structures via different mechanisms, including anion exchange or outer-sphere complexation for pertechnetate, and inner-sphere adsorption for all other contaminants. Uranyl carbonate could substitute between the [Bi₂O₂]²⁺ layers, and some iodate was incorporated into a neo-formed δ-Bi₂O₃ phase. This remarkable versatility of Bi-based materials demonstrated here means that they are cost-effective materials with the potential to sequester co-located contaminants at DOE legacy sites.« less

Plutonium uranium reduction extraction (PUREX) is a liquid–liquid extraction process used to recover plutonium (Pu) and uranium (U) from irradiated uranium fuel for various nuclear-related applications. Despite extensive efforts, quantitative prediction of liquid–liquid extraction parameters, i.e., distribution ratios and separation factors, of the process remains challenging. Existing thermodynamic models are difficult to develop and often have limited utility due to the complexity of the aqueous feed. Nitric acid is a critical component of the PUREX system, both as a driving force for dissolving irradiated fuels in preprocessing stages, as well as being efficiently extracted by tributyl phosphate (TBP). Models tomore » understand nitric acid’s distribution behavior is therefore a prerequisite to predict actinide extraction. In this work, we compiled a wealth of solvent extraction literature data and built machine learning (ML) models capable of predicting the organic phase nitric acid equilibrium concentration from initial acid and TBP concentrations across a variety of diluents. Our results demonstrate that ML is highly capable of predicting nitric acid extraction behavior in PUREX systems, and the resultant ML-aided response surfaces demonstrate promising progress as an in silico aid for optimizing the design of experiments for future work with the PUREX process.« less

During nuclear fuel reprocessing, radioiodine, can be released.

We investigated basic bismuth subnitrate for removal of radioactive technetium-99 as pertechnetate (⁹⁹TcO₄^-) from contaminated groundwater. This material removed 93% of the initial concentration of ⁹⁹TcO₄^- within a week via formation of pH-dependent mineral phases that were identified here, but not reported previously. Perrhenate (ReO₄^-) removal was also studied because it is a widely used non-radiological analogue for ⁹⁹TcO₄^-, considering their similar physicochemical properties. We found that removal of ReO₄^- was not identical to removal of ⁹⁹TcO₄^- and led to formation of an additional transitional phase. This demonstrates that perrhenate and pertechnetate have different kinetics of contaminant removal as amore » result of variations in mineral transformation.« less

Here, the review focuses on speciation and migration of anthropogenic ruthenium (Ru) originated from nuclear industry releases and presents updated information regarding Ru in the environment. It provides analysis of the main pathways of Ru species distribution in the aqueous and terrestrial environment, starting from its natural occurrence, generation and release from anthropogenic sources, predominant speciation, and ending with bioaccumulation, which can be directly or indirectly related to human health. Literature sources belonging to the post-Chernobyl time frame were preferentially considered, in which Ru-103 and Ru-106 are the major fission isotopes studied due to their traceability in the environment andmore » their relatively long half-lives.« less

Mechanism of hexavalent chromium removal (Cr(VI) as CrO₄^2-) by the weak-base ion exchange (IX) resin ResinTech® SIR-700-HP (SIR-700) from simulated groundwater is assessed in the presence of radioactive contaminants iodine-129 (as IO₃^-), uranium (U as uranyl UO₂²⁺), and technetium-99 (as TcO₄^-), and common environmental anions sulfate (SO₄^2-) and chloride (Cl-). Batch tests using the acid sulfate form of SIR-700 demonstrated Cr(VI) and U(VI) removal exceeded 97%, except in the presence of high SO₄^2- concentrations (536 mg/L) where Cr(VI) and U(VI) removal decreased to ≥ 80%. However, Cr(VI) removal notably improved with co-mingled U(VI) that complexes with SO₄^2- at the protonatedmore » amine sites. These U–SO₄^2- complexes are integral to U(VI) removal, as confirmed by the decrease in U(VI) removal (<40%) when the acid chloride form of SIR-700 was used instead. Solid phase characterization revealed that CrO₄^2- is removed by IX with SO₄^2- complexes and/or reduced to amorphous Cr(III)(OH)₃ at secondary alcohol sites. Tc(VII)O₄^- and I(V)O₃^- also undergo chemical reduction, following a similar removal mechanism. Oxyanion removal preference is determined by the anion reduction potential (CrO₄^2->TcO₄^->IO₃^-), geometry, and charge density. For these reasons, 39% and 69% of TcO₄^- and 17% and 39% of IO₃^- are removed in the presence and absence of Cr(VI), respectively.« less

Ion exchange (IX) resins are used in pump-and-treat (P&T) facilities to remove soluble groundwater contaminants. However, natural anions present at concentrations orders of magnitude higher than contaminants can compete for IX sites and impact resin lifecycles. Here, the Hanford Site’s 200 West Area P&T facility (Washington State, USA) was selected as a case study because it currently uses two IX resins: Purolite® A532E (A532E) to remove pertechnetate (TcO₄^-) and DOWEX 21K (DOWEX) to remove uranium from groundwater. Nitrate (NO₃^-), sulfate (SO₄^2-), chloride (Cl^-), and carbonate (CO₃^2-) anions have been identified to potentially compete for A532E and DOWEX IX sites. Hanford-relevantmore » anion groundwater concentrations were used to design a series of laboratory-scale batch experiments to evaluate the impact of competing anions on resin performance and potential kinetic effects. These data are then modeled to obtain Cl--normalized equilibrium exchange coefficients (K) needed to predict IX resin performance. The work is presented in two parts, with IX performance evaluated for A532E in Part I and DOWEX in Part II. Part I results demonstrate that TcO₄^- uptake is not impacted by NO₃^-, SO₄^2-, Cl^-, CO₃^2- (as HCO₃^-) and U(VI) carbonate anions, with K_TcO4-/Cl- > 4,000, likely due to the high selectivity of A532E trihexylammonium sites for the large, weakly hydrated TcO₄^- anion. Other anion K values were K_NO3-/Cl- = 20, K_SO4--/Cl- = 0.2, K_HCO3-/Cl- = 0.09, K_U/Cl- = 370–1000. These K values provide conservative parameters for predicting A532E performance, and demonstrate that, under these test conditions, A532E will remove TcO₄^- from current and future influent streams to meet groundwater treatment objectives.« less

The selectivity of ion exchange (IX) resins for aqueous contaminant removal can be impacted by changing concentrations of competing natural groundwater ions. In a two-part investigation, the Hanford Site 200 West Area pump-and-treat (P&T) facility in Washington State, USA is used as a case study to evaluate the performance of two IX resins for groundwater treatment: Purolite® A532E for pertechnetate (TcO₄^-) removal, explored in Part I, and DOWEX 21K (DOWEX) for uranium (U) removal. In Part II, DOWEX selectivity for U, as uranyl carbonate species, and uptake kinetics is quantified in a series of laboratory-scale aqueous batch experiments containing Hanford-relevantmore » concentrations of competing anions nitrate (NO₃^-), sulfate (SO₄^2-), chloride (Cl^-), and carbonate (CO₃^2-), as well as co-mingled contaminant TcO_4-. Here the results demonstrate that DOWEX trimethylammonium functional groups are highly selective for U carbonate species (85–100 % uptake) under all conditions investigated. Only NO_3- concentrations of 100 mM were shown to decrease U removal, with the extent (85–99 %) depending on competing anion concentrations present in solution. However, at the highest NO₃^- concentrations reported for groundwaters treated at the P&T facility (25 mM), the effect on U uptake is minimal. The batch sorption results are modeled to obtain chloride normalized equilibrium exchange coefficients (K) for predicting DOWEX performance: K_SO4--/Cl- = 2.0, K_NO3-/Cl- = 5.0, K_HCO3-/Cl- = 1.5, K_TcO4-/Cl- = 2,000, and K_U/Cl- = 50,000. These K values predict little effect of current and future influent chemistries on U removal by DOWEX, where both uranyl carbonate species and TcO₄^- are removed such that effluent concentrations meet groundwater treatment requirements.« less

Radioactive technetium-99 (Tc) present in waste streams and subsurface plumes at legacy nuclear reprocessing sites worldwide poses potential risks to human health and the environment. This research comparatively evaluated efficiency of zero valent iron (ZVI) toward reductive removal of Tc(VII) in presence of Cr(VI) from NaCl and Na₂SO₄ electrolyte solutions under aerobic conditions. In both electrolytes, anticorrosive Cr(VI) suppressed oxidation of ZVI at elevated concentrations resulting in the delay of initiation of Tc(VII) reduction to Tc(IV)). In the absence of Cr(VI), no such delay was observed in the analogous systems. At low ionic strength, retarded ZVI oxidation inhibited Tc(VII) reductionmore » in part due to the competing thermodynamically more favorable reduction of Cr(VI) to Cr(III); this effect was particularly pronounced in the chloride medium at the ionic strength ? 0.1. Higher ionic strength favored reduction of both Tc(VII) and Cr(VI), which followed a second-order reaction rate in both electrolytes attributed to the more efficient iron oxidation as evident from x-ray diffraction and electron microscopy measurements. Magnetite was the primary iron oxide phase, and its higher fraction in the SO₄^2- solutions facilitated reductive removal of Tc(VII) and Cr(VI). In the Cl- matrix, Cr(VI) promoted further oxidation of magnetite as well as formation of chromite diminishing overall reductive capacity of this system and resulting in less effective removal of Tc(VII) compared to the SO₄^2- solutions.« less