Sorption Of Np Research Articles

Sorption of Cs, Np, and Pu by the rocks of the Yeniseisky site depending on the temperature and ionic strength of Mg-K-phosphate leaching solutions

Sorption of Cs, Np, and Pu by the rocks of the Yeniseisky site depending on the temperature and ionic strength of Mg-K-phosphate leaching solutions

Sorption behaviour of neptunium in marine and fresh water bottom sediments in Far East area of Russia (Lake Khanka and Amur Bay)

The concentration and sorption behavior of 237Np on the bottom sediments of water bodies in the Far East region of Russia (Lake Khanka and Peter the Great Bay) were studied for the first time. The 237Np concentrations vary from 1.06 × 10−6 to 4.43 × 10−5 mBq g−1 in the bottom sediments of Lake Khanka and from 1.05 × 10−4 to 2.52 × 10−3 mBq g−1 for Amur Bay. The experiment on the adsorption of Np on marine and lake sediment showed that it is sorbed through complexation with silicates (albite, leucite). The Np sorption isotherm on marine sediments is described by the Langmuir equation; the distribution coefficients (Kd) of Np vary from 57 to 588 mL g−1. For lake sediments, the isotherm is described by the Henry equation; the Kd value reaches 935 mL g−1.

Impact of a Biological Chelator, Lanmodulin, on Minor Actinide Aqueous Speciation and Transport in the Environment.

Minor actinides are major contributors to the long-term radiotoxicity of nuclear fuels and other radioactive wastes. In this context, understanding their interactions with natural chelators and minerals is key to evaluating their transport behavior in the environment. The lanmodulin family of metalloproteins is produced by ubiquitous bacteria and Methylorubrum extorquens lanmodulin (LanM) was recently identified as one of nature's most selective chelators for trivalent f-elements. Herein, we investigated the behavior of neptunium, americium, and curium in the presence of LanM, carbonate ions, and common minerals (calcite, montmorillonite, quartz, and kaolinite). We show that LanM's aqueous complexes with Am(III) and Cm(III) remain stable in carbonate-bicarbonate solutions. Furthermore, the sorption of Am(III) to these minerals is strongly impacted by LanM, while Np(V) sorption is not. With calcite, even a submicromolar concentration of LanM leads to a significant reduction in the Am(III) distribution coefficient (Kd, from >104 to ∼102 mL/g at pH 8.5), rendering it even more mobile than Np(V). Thus, LanM-type chelators can potentially increase the mobility of trivalent actinides and lanthanide fission products under environmentally relevant conditions. Monitoring biological chelators, including metalloproteins, and their biogenerators should therefore be considered during the evaluation of radioactive waste repository sites and the risk assessment of contaminated sites.

Unraveling the Np(V) sorption on ZrO2: A batch, spectroscopic and modeling combined approach

The interactions of the long-lived actinide neptunium with the corrosion product zirconia (ZrO2) have to be considered in the safety assessment of a repository for radioactive waste. The sorption of Np(V) on ZrO2 was investigated in the absence of carbonate at the macroscopic and molecular scale. At the macroscopic level, the Np(V) uptake was independent of ionic strength and the isoelectric point of the pristine zirconia was increased, both suggesting the presence of inner-sphere Np(V) surface complexes. The Np(V) sorption isotherms indicated the presence of strong and weak sorption sites. Molecular level information were derived from in situ attenuated total reflection Fourier-transform infrared spectroscopy and extended X-ray absorption fine structure spectroscopy (EXAFS), which confirmed the presence of Np(V) inner-sphere complexes. EXAFS experiments revealed the formation of a bidentate inner-sphere surface complex in the weak sorption site regime. The derived information at the macroscopic and molecular levels were used to parametrize a charge distribution multi-site complexation (CD-MUSIC) model. The derived thermodynamic constants can help to better predict the environmental fate of Np(V) in the context of nuclear waste repository assessments and can also support the appraisal of safety-relevant scenarios for the extended interim storage of spent nuclear fuel.

Np(V) Retention at the Illite du Puy Surface.

In this study, Np(V) retention on Illite du Puy (IdP) was investigated since it is essential for understanding the migration behavior of Np in argillaceous environments. The presence of structural Fe(III) and Fe(II) in IdP was confirmed by Fe K-edge X-ray absorption near-edge structure (XANES) and 57Fe Mössbauer spectroscopy. In batch sorption experiments, a higher Np sorption affinity to IdP was found than to Wyoming smectite or iron-free synthetic montmorillonite. An increase of the relative Np(IV) ratio sorbed onto IdP with decreasing pH was observed by solvent extraction (up to (24 ± 2)% at pH 5, c0(Np) = 10-6 mol/L). Furthermore, up to (33 ± 5)% Np(IV) could be detected in IdP diffusion samples at pH 5. Respective Np M5-edge high-energy resolution (HR-) XANES spectra suggested the presence of Np(IV/V) mixtures and weakened axial bond covalency of the NpO2+ species sorbed onto IdP. Np L3-edge extended X-ray absorption fine structure (EXAFS) analysis showed that significant fractions of Np were coordinated to Fe─O entities at pH 9. This highlights the potential role of Fe(II/III) clay edge sites as a strong Np(V) surface complex partner and points to the partial reduction of sorbed Np(V) to Np(IV) via structural Fe(II).

Sorption of Np, Pu, Am, Sr, Cs on the Mineral Phases of the Rocks of the Nizhnekansky Granitoid Massif under Conditions of Underground Repositories for Radioactive Wastes

Sorption of Np, Pu, Am, Sr, Cs on the Mineral Phases of the Rocks of the Nizhnekansky Granitoid Massif under Conditions of Underground Repositories for Radioactive Wastes

Effect of Ca(II) on U(VI) and Np(VI) retention on Ca-bentonite and clay minerals at hyperalkaline conditions - New insights from batch sorption experiments and luminescence spectroscopy

In deep geological repositories for radioactive waste, interactions of radionuclides with mineral surfaces occur under complex geochemical conditions involving complex solution compositions and high pH resulting from degradation of cementitious geo-engineered barriers. Ca2+ cations have been hypothesized to play an important role as mediators for the retention of U(VI) on Ca-bentonite at (hyper)alkaline conditions, despite the anionic character of both the mineral surface and the aqueous uranyl species. To gain deeper insight into this sorption process, the effect of Ca2+ on U(VI) and Np(VI) retention on alumosilicate minerals has been comprehensively evaluated, using batch sorption experiments and time-resolved laser-induced luminescence spectroscopy (TRLFS). Sorption experiments with Ca2+ or Sr2+ and zeta potential measurements showed that the alkaline earth metals sorb strongly onto Ca-bentonite at pH 8–13, leading to a partial compensation of the negative surface charge, thereby generating potential sorption sites for anionic actinyl species. U(VI) and Np(VI) sorption experiments in the absence and presence of Ca2+ or Sr2+ confirmed that these cations strongly enhance radionuclide retention on kaolinite and muscovite at pH ≥ 10. Concerning the underlying retention mechanisms, site-selective TRLFS provided spectroscopic proof for two dominating U(VI) species at the alumosilicate surfaces: (i) A ternary U(VI) complex, where U(VI) is bound to the surface via bridging Ca cations with the configuration surface ≡ Ca – OH – U(VI) and, (ii) U(VI) sorption into the interlayer space of calcium (aluminum) silicate hydrates (C-(A-)S-H), which form as secondary phases in the presence of Ca due to partial dissolution of alumosilicates under hyperalkaline conditions. Consequently, the present study confirms that alkaline earth elements, which are ubiquitous in geologic systems, enable strong retention of hexavalent actinides on clay minerals under hyperalkaline repository conditions.

Neptunium interaction with microplastics in aqueous solutions

The neptunium sorption by different microplastic materials (MPs) has been studied in laboratory water and seawater samples, which had been previously traced with the Np-237 isotope. The microplastics used were polyethylene terephthalate (PET) and polyamide nylon (PN6), and the investigations included the pH effect and seawater composition on the sorption efficiency. The evaluation of the experimental data (Kd values) revealed that the sorption efficiency does not differ significantly from one microplastic to another and mainly the hydrolysis of Np(V) in alkaline conditions affects the sorption efficiency. In seawater the Kd values decline dramatically because of the increased ionic strength (salinity) and the occurrence of various cations (e.g. Ca2+, Fe2+), which effectively compete Np(V) sorption by the MPs.

Paving the way for examination of coupled redox/solid-liquid interface reactions: 1 ppm Np adsorbed on clay studied by Np M5-edge HR-XANES spectroscopy

The recently emerged actinide (An) M4,5-edge high-energy resolution X-ray absorption near-edge structure (HR-XANES) technique has proven to be very powerful for oxidation state studies of actinides. In this work, for the first time, Np M5-edge HR-XANES was applied to study Np sorption on illite. By improving the experimental conditions, notably by operation of the spectrometer under He atmosphere, it was possible to measure Np M5-edge HR-XANES spectra of a sample with ≈ 1 μg Np/g illite (1 ppm). This is 30–2000 times lower than Np loadings on mineral surfaces usually investigated by X-ray absorption spectroscopy. A newly designed cryogenic configuration enabled sample temperatures of 141.2 ± 1.5 K and successfully prevented beam-induced changes of the Np oxidation state. The described approach paves the way for the examination of coupled redox/solid-liquid interface reactions of actinide ions via An M4,5-edge HR-XANES spectroscopy at low metal ion concentrations, which are of specific relevance for contaminated sites and nuclear waste disposal studies.

Stomata facilitate foliar sorption of silver nanoparticles by Arabidopsis thaliana

Application of nanopesticides may substantially increase surface attachment and internalization of engineered nanoparticles (ENPs) in food crops. This study investigated the role of stomata in the internalization of silver nanoparticles (Ag NPs) using abscisic acid (ABA)-responsive ecotypes (Ler and Col-7) and ABA-insensitive mutants (ost1-2 and scord7) of Arabidopsis thaliana in batch sorption experiments, in combination with microscopic visualization. Compared with those of the ABA-free control, stomatal apertures were significantly smaller for the Ler and Col-7 ecotypes (p ˂ 0.05) but remained unchanged for the ost1-2 and scord7 mutants, after exposure to 10 μM ABA for 1 h. Generally Ag NP sorption to the leaves of the Ler and Col-7 ecotypes treated with 10 μM ABA was lower than that in the ABA-free control, mainly due to ABA-induced stomatal closure. The difference in Ag NP sorption with and without ABA was less pronounced for Col-7 than for Ler, suggesting different sorption behaviors between these two ecotypes. In contrast, there was no significant difference in foliar sorption of Ag NPs by the ost1-2 and scord7 mutants with and without ABA treatment. Ag NPs were widely attached to the Arabidopsis leaf surface, and found at cell membrane, cytoplasm, and plasmodesmata, as revealed by scanning electron microscopy and transmission electron microscopy, respectively. These results highlight the important role of stomata in the internationalization of ENPs in plants and may have broad implications in foliar application of nanopesticides and minimizing contamination of food crops by ENPs.

Sorption of Np(IV) on MX-80 in Ca-Na-Cl Type Reference Water of Crystalline Rock

The pH dependence of sorption distribution coefficient (Kd) of Np(IV) on MX-80 in Ca-Na-Cl type solution with the ionic strength of 0.3 M, which was similar to one of the reference groundwaters in crystalline rock, was experimentally investigated under the reducing conditions. The overall trend of Kd on MX-80 was independent of pH at 5 ≤ pH ≤ 10 but increased as pH increased at pH ≤ 5. The 2-site protolysis non-electrostatic surface complexation and cation exchange model was applied to the experimentally measured pH dependence of Kd and the optimized surface complexation constants of Np(IV) sorption on MX-80 were estimated. The values of surface complexation constants in this work agreed relatively well with those in the Na-Ca-Cl solution previously evaluated, suggesting that compared to Na+, the competition of Ca2+ with Np(IV) for surface complexation on MX-80 was not much strong in Ca-Na-Cl solution. The sorption model well predicted the pH dependence of Kd values but slightly overestimated the sorption at the low pH region.

Mineral-specific heterogeneous neptunium sorption onto geological repository rocks in oxic and anoxic conditions and various temperatures

Neptunium poses challenges for radioactive wastes management due to its long half-life and ingrowth from americium decay, as well as its high mobility in water in its oxidized forms. Np(V)-bearing aqueous solutions were contacted with polished and crushed gneiss and dolerite from the Yeniseisky site, Russia, which is under consideration as a location for an underground repository, both in oxic and anoxic conditions at 20 and 90 °C. Neptunium sorption on crushed gneiss and dolerite in oxic conditions at 20 °C was quite low, but increased substantially in anoxic conditions. Autoradiography on samples treated at 20 °C revealed that sorbed Np was associated mostly with dark colored iron-bearing minerals, and solvent extraction indicated partial reduction of Np(V) to Np(IV). Sorption of Np from aqueous solutions contacted with dolerite and gneiss at 90 °C was in all cases higher than at the lower temperature. Autoradiography of polished rock samples treated in anoxic conditions at 90 °C revealed that Fe-bearing minerals had 3–4 orders of magnitude more Np sorbed on them than the quartz and feldspar grains. X-ray photoelectron spectroscopy indicated this sorbed Np was present as NpO2. This study supports the concept that the geologic conditions present at the planned repository site will retard Np migration.

Sorption of Cs(I) and Np(V) onto Clays from the Ostrozhanskoe Deposit (Belarus)

The sorption behavior of Np(V) and Cs(I) on natural and acid-treated bentonite-like clay of Ostro-zhanskoe deposit (Belarus) was studied in a wide range of radionuclide concentrations and pH values of the suspension. The sorption equilibrium with both radionuclides is attained within the first minutes of the interac¬tion. The sorption behavior on the untreated and treated clay is similar. Based on the experimental data, ther-modynamic modeling of the Cs(I) and Np(V) sorption onto the clays was performed.

Modeling the sorption of Np(V) on Na-montmorillonite – effects of pH, ionic strength and CO2

Abstract Results from batch type experiments were modeled using the 2 SPNE SC/CE model developed by Bradbury and Baeyens. This work focused on the applicability of this model to the sorption of Np(V) on Na-montmorillonite under high saline conditions (0.1–3.0 M NaCl) in the pH range of 2–10 and in the presence of dissolved CO2 (p(CO2) = 10−3.3 atm). Under ambient air conditions two additional surface complexation species had to be taken into account, which are ternary species involving one carbonate ligand (≡SONpO2(CO3)2−, ≡SONpO2(CO3)Na−). The gained set of complexation parameters was successfully tested over a wide range of Np(V) concentrations (10−4 to 10−12 M) under Ar atmosphere and ambient air condition.

Effects of soil organic carbon (SOC) content and accessibility in subsoils on the sorption processes of the model pollutants nonylphenol (4-n-NP) and perfluorooctanoic acid (PFOA)

Subsoils control the release of hydrophobic pollutants to groundwater systems, but the role of subsoil soil organic carbon (SOC) in sorption processes of hydrophobic organic pollutants remains unclear. Thus, this study aimed to understand the role of subsoil SOC in sorption processes of 4-n-nonylphenol (NP) and perfluorooctanoic acid (PFOA) as model pollutants. To characterize the sorption behavior of NP and PFOA, 42 sub- and 54 topsoil samples were used for batch experiments. Differences in NP and PFOA sorption between sub- and topsoil samples and its mechanisms were identified using multiple regression analysis. Generally, the sorption of NP and PFOA was linear in all samples. The sorption of NP to soil samples (logKD = 1.78–3.68) was significantly higher and less variable than that of PFOA (logKD = −0.97–1.44). In topsoils, SOC content had the highest influence on NP and PFOA sorption. For NP, hydrophobic interactions between NP and SOC were identified as the most important sorption mechanism. For PFOA, hydrophobic as well as electrostatic interactions were determined depending on soil pH. In subsoils, the relevance of SOC content for pollutant sorption decreased drastically. For NP, not SOC content but rather SOC quality was relevant in SOC poor subsoils. For PFOA, clay and iron oxide content were found to be relevant for pollutant interactions with the solid phase. Thus, especially in SOC depleted subsoils, the sorption potential for PFOA remained unpredictable.

Linear Free Energy Relationship for Actinide Sorption to Graphene Oxide.

Th(IV) and Np(V) sorption to graphene oxide (GO) was studied as a function of pH from 0-7.5 and analyte concentrations (0.01-1 mg/L for Th(IV) at pH 3 and 0.005-10 mg/L for Np(V) at pH 7). Starting at pH 1, greater than 90% Th(IV) sorption to GO occurred while significant Np(V) sorption to GO started at pH 5. Surface complexation modeling (SCM) using an electrostatic double layer model simultaneously modeled Th(IV) and Np(V) sorption to GO over the pH and the analyte concentration ranges. The SCM indicated that Th(IV) complexation to sulfonate sites dominated at a low pH 0-3 and its complexation to carboxylate sites dominated at a higher pH 3-7.5. In contrast, Np(V) showed a stronger affinity for sulfonate sites than carboxylate sites over the pH and concentration ranges examined in this work. Combining the results from a previous study on Eu(III) and U(VI) sorption to GO, the affinities of actinide/lanthanide sorption to GO was found to follow the trend in actinide/lanthanide ion effective charges (e.g., Th4+ (+4) > UO22+ (+3.2) > Eu3+ (+3) > NpO2+ (+2.2)), which is similar to actinide sorption to iron oxides and clay minerals. Moreover, a linear free energy relationship was observed between the stability constants for actinides and Eu(III) (with the oxidation state from III to VI) complexation to carboxylate sites on GO and the stability constants for their complexation to carbonate.

Np(V) sorption and solubility in high pH calcite systems

Np(V) behaviour in alkaline, calcite containing systems was studied over a range of neptunium concentrations (1.62 × 10−3 μM–1.62 μM) in two synthetic, high pH, cement leachates under a CO2 controlled atmosphere. The cement leachates were representative of conditions expected in an older (pH 10.5, Ca2+) and younger (pH 13.3, Na+, K+, Ca2+) cementitious geological disposal facility. These systems were studied using a combination of batch sorption and solubility experiments, X-ray absorption spectroscopy, and geochemical modelling to describe Np behaviour. Np(V) solubility in calcite equilibrated old and young cement leachates (OCL and YCL) was 9.7 and 0.084 μM, respectively. In the OCL system, this was consistent with a Np(V)O2OH(am) phase controlling solubility. However, this phase did not explain the very low Np(V) solubility observed in the YCL system. This inconsistency was explored further with a range of pH 13.3 solubility experiments with and variable Ca2+(aq) concentrations. These experiments showed that at pH 13.3, Np(V) solubility decreased with increasing Ca2+ concentration confirming that Ca2+ was a critical control on Np solubility in the YCL systems. X-ray absorption near-edge structure spectroscopy on the precipitate from the 42.2 μM Np(V) experiment confirmed that a Np(V) dioxygenyl species was dominant. This was supported by both geochemical and extended X-ray absorption fine structure data, which suggested a calcium containing Np(V) hydroxide phase was controlling solubility. In YCL systems, sorption of Np(V) to calcite was observed across a range of Np concentrations and solid to solution ratios. A combination of both surface complexation and/or precipitation was likely responsible for the observed Np(V) reaction with calcite in these systems. In the OCL sorption experiments, Np(V) sorption to calcite across a range of Np concentrations was dependent on the solid to solution ratio which is consistent with the formation of a mono-nuclear surface complex. All systems demonstrated slow sorption kinetics, with reaction times of weeks needed to reach apparent equilibrium. This could be explained by slow recrystallisation of the calcite surface and/or the presence of Np(V) colloidal species. Overall, these data provide valuable new insights into Np(V) and actinide(V) behaviour in alkaline conditions of relevance to the disposal of intermediate level radioactive wastes.

Np(V) uptake by various clays

In this work, we study different clay samples to examine the effect of structural features of clay minerals (2:1 and 1:1 structures), interlayer cations (Na/Ca/Li forms of montmorillonite), surface area and presence of accessory minerals (iron oxides, calcite, quartz) on Np(V) sorption at trace concentrations. Various techniques, e.g., XRD, XRF, DTG, FTIR, BET absorption and Mossbauer spectroscopy, were used to characterize these clays. The presence of iron oxide as an accessory mineral has a strong effect on Np(V) sorption. Sorption of Np(V) onto different clay minerals with different surface areas, interlayer cations and structures is very similar at a pH range of 4.5–11.5.

Neptunium(V) sorption to vadose zone sediments: Reversible, not readily reducible, and predictable based on Fe-oxide content

Abstract Neptunium (Np) is among the key risk-drivers at the Savannah River Site's (SRS) low-level waste disposal facility located in South Carolina. A series of studies were undertaken to identify and to model the key geochemical processes controlling Np sorption to SRS vadose zone sediments. The approach was to conduct Np(V) laboratory batch and flow studies using two sediments representing end-member depositional facies recovered beneath the disposal facility. Baseline distribution coefficients (Kd values; Np concentration ratio of sediment:porewater) were 9.05 ± 0.61 L kg−1 and 4.26 ± 0.24 L kg−1 for the clayey and sandy end-member sediments, respectively. The addition of natural organic matter (NOM) to the sediment resulted in only a two fold increase in the Kd values, most likely due to the formation of ternary sediment-NOM-Np complexes. None of the reduction treatments (ascorbic acid, dithionite, zero valent iron, hydrogen peroxide, and anaerobic atmosphere), including some long-term (71-day) equilibration experiments, resulted in significant increases in Kd values. This indicated that little to no reduction of Np(V) to the more strongly sorbing Np(IV) occurred. Among the key novel findings in this research was that batch desorption tests and stop-flow stir-cell kinetic experiments indicated that the Np(V) sorption was completely reversible. These observations were used to develop a simple conceptual model describing Np(V) sorption. The conceptual model described NpO2+ reversibly complexation to iron oxide coatings on the sediments. The model was successfully applied without any adjustable parameters to an independent set of experimental data, requiring only the dithionite-extractable Fe concentration from the independent dataset. There is no standard approach for quantifying reactive sorption site concentrations on composite materials (e.g., soils and sediments). In this study we proposed such a method based on an operationally defined fraction of the extractable iron. This parameterization approach was calibrated with two sediments then used to blindly and successfully predict sorption on another end-member soil. The successful modeling approach taken in this work 1) identified key reactions that are or are not influencing the system to develop a simple but appropriate conceptual model and 2) calibrated the fraction of extractable iron required for surface site density determination and used the calibrated model for blind predictions. This modeling approach could be used for other composite materials to allow for comparisons of the fraction of surface reactive extractable metals.

Evaluation of 1st and 2nd generation of poly(amidoamine) dendrimer functionalized carbon nanotubes for the efficient removal of neptunium

Herein, sorption of pentavalent neptunium from aqueous acidic solution was carried out onto 1st and 2nd generation of poly(amidoamine) dendrimer functionalized MWCNTs. The Np(V) sorption kinetics followed pseudo second order kinetics with rate constants of 1.4 × 10−4 and 6.84 × 10−4 g mg−1 min−1 for MWCNT–PAMAMG1 and MWCNT–PAMAMG2, respectively. The sorption of Np(V) onto MWCNT–PAMAMGn proceeded via chemisorption and the data fitted well to Langmuir isotherm model. MWCNT–PAMAMGn was found radiolytically stable up to 500 kGy dose of gamma radiation. Sodium carbonate was found to strip Np effectively from sorbent.