Sorption Of Americium Research Articles

Eu(III) and Am(III) adsorption on aluminum (hydr)oxide minerals: surface complexation modeling

Americium is a highly radioactive actinide element found in used nuclear fuel. Its adsorption on aluminum (hydr)oxide minerals is important to study for at least two reasons: (i) aluminum (hydr)oxide minerals are ubiquitous in the subsurface environment and (ii) bentonite clays, which are proposed engineered barriers for the geologic disposal of used nuclear fuel, have the same ≡AlOH sites as aluminum (hydr)oxide minerals. Surface complexation modeling is widely used to interpret the adsorption behavior of heavy metals on mineral surfaces. While americium sorption is understudied, multiple adsorption studies for europium, a chemical analog, are available. In this study we compiled data describing Eu(III) adsorption on three aluminum (hydr)oxide minerals—corundum (α-Al2O3), γ-alumina (γ-Al2O3) and gibbsite (γ-Al(OH)3)—and developed surface complexation models for Eu(III) adsorption on these minerals by employing diffuse double layer (DDL) and charge distribution multisite complexation (CD-MUSIC) electrostatic frameworks. We also developed surface complexation models for Am(III) adsorption on corundum (α-Al2O3) and γ-alumina (γ-Al2O3) by employing a limited number of Am(III) adsorption data sourced from literature. For corundum and γ-alumina, two different adsorbed Eu(III) species, one each for strong and weak sites, were found to be important regardless of which electrostatic framework was used. The formation constant of the weak site species was almost 10,000 times weaker than the formation constant for the corresponding strong site species. For gibbsite, two different adsorbed Eu(III) species formed on the single available site type and were important for the DDL model, whereas the best-fit CD-MUSIC model for Eu(III)-gibbsite system required only one Eu(III) surface species. The Am(III)-corundum model based on the CD-MUSIC framework had the same set of surface species as the Eu(III)-corundum model. However, the log K values of the surface reactions were different. The best-fit Am(III)-corundum model based on the DDL framework had only one site type. Both the CD-MUSIC and the DDL model developed for Am(III)-γ-alumina system only comprised of one site type and the formation constant of the corresponding surface species was ~ 500 times stronger and ~ 700 times weaker than the corresponding Eu(III) species on the weak and the strong sites, respectively. The CD-MUSIC model for corundum and both the DDL and the CD-MUSIC models for γ-alumina predicted the Am(III) adsorption data very well, whereas the DDL model for corundum overpredicted the Am(III) adsorption data. The root mean square of errors of the DDL and CD-MUSIC models developed in this study were smaller than those of two previously-published models describing Am(III)-γ-alumina system, indicating the better predictive capacity of our models. Overall, our results suggest that using Eu(III) as an analog for Am(III) is practical approach for predicting Am(III) adsorption onto well-characterized minerals.Graphical

Sorption and Spatial Distribution of 137Cs, 90Sr and 241Am on Mineral Phases of Fractured Rocks of Nizhnekansky Granitoid Massif

The sorption behavior and spatial microdistribution of Cs-137, Sr-90 and Am-241 onto the surface of a fractured rock sample from the R-11 borehole of the exocontact zone of the Nizhnekansky granitoid massif were studied. The sorption efficiency of the fractured sample increases in the row of Sr < Cs < Am, where americium is the most retained radionuclide. Based on the image processing of radiograms and scanning electron microscopy data, the mineral relative sorption efficiency (RSE) values were determined to quantify the contribution of the mineral phases of the fractured sample to radionuclide retention. It was established that cesium and strontium are predominantly retained in cracks filled with secondary mineral chlorite. Zeolite is a less effective sorbent with respect to cesium and strontium. Americium sorption is uniform over the whole surface of the fractured sample, with close RSE values for all mineral phases (RSE ~1). The behavior of cesium in heterogeneous and monomineral systems of crushed mineral phases of quartz, biotite and zeolite NaA imitating minerals of the fractured rock sample R-11 was determined. It was shown that the fraction of the sorbed cesium in a heterogeneous system of two mineral phases—biotite and quartz—was larger than the sum of sorption values for the same separated mineral phases. Based on the models of radionuclide sorption on illite–smectite minerals, it was able to estimate the depth of the penetration of solution into the fractured rock sample R-11. The variations of penetration depths for solutions of specific radionuclide into the fractured rock sample were established.

Americium Sorption by Microplastics in Aqueous Solutions

The interaction of americium by polyamide (PN6) and polyethylene (PE) microplastics (MPs) has been investigated in de-ionized and seawater samples traced with the Am-241 isotope. The effect of the pH and composition of the aqueous solution (seawater) on the sorption efficiency (Kd) have been studied as function of time. In addition, the americium sorption using radionuclide mixtures (e.g., Am-241 and U-232) and its desorption in the presence of EDTA was investigated by means of bath-type experiments. The experimental data (e.g., Kd values) revealed that the sorption efficiency depends on the MP type and the pH that governs the Am(III) speciation in the solution, and the PN6 surface charge determines the sorption efficiency. Moreover, the desorption of Am(III) from MPs is a time-depended process, and the presence of complexing agents (e.g., EDTA) enhances the desorption efficiency. The Kd values in the studied water samples were relatively close to one another and similar to corresponding values in soil systems, indicating the significant role of microplastics on the behavior of americium in natural aquatic systems.

Combining batch experiments and spectroscopy for realistic surface complexation modelling of the sorption of americium, curium, and europium onto muscovite

For a safe enclosure of contaminants, for instance in deep geological repositories of radioactive waste, any processes retarding metal migration are of paramount importance. This study focusses on the sorption of trivalent actinides (Am, Cm) and lanthanides (Eu) to the surface of muscovite, a mica and main component of most crystalline rocks (granites, granodiorites). Batch sorption experiments quantified the retention regarding parameters like pH (varied between 3 and 9), metal concentration (from 0.5 µM Cm to 10 µM Eu), or solid-to-liquid ratio (0.13 and 5.25 g·L−1). In addition, time-resolved laser fluorescence spectroscopy (TRLFS) using the actinide Cm(III) identified two distinct inner-sphere surface species. Combining both approaches allowed the development of a robust surface complexation model and the determination of stability constants of the spectroscopically identified species of (S-OH)2M3+ (logKo −8.89), (S-O)2M+ (logKo −4.11), and (S-O)2MOH (logKo −10.6), with all values extrapolated to infinite dilution. The inclusion of these stability constants into thermodynamic databases will improve the prognostic accuracy of lanthanide and actinide transport through groundwater channels in soils and crystalline rock systems.

Sorption of americium / europium onto magnetite under saline conditions: Batch experiments, surface complexation modelling and X-ray absorption spectroscopy study

HypothesisThis study investigates the adsorption of americium and its chemical analogue europium on magnetite, which is expected to form as a major long-term steel canister corrosion product under anoxic and highly saline conditions. ExperimentsThe sorption of europium on magnetite (solid/liquid ratio = 0.5 g/L) was investigated batch wise in NaCl brines with ionic strength I = 1 m, 3.5 m, and 6.67 m, as a function of pHm for two europium concentrations (6 × 10−10m, 1.2 × 10−5m). Information on the chemical nature of the surface species was obtained by X-ray absorption spectroscopy (XAS) at the americium L3-edge. FindingsRetention of europium by magnetite of >99.5% was found above pHm 6.4 for all ionic strengths for europium concentration of 6 × 10−10m. No ionic strength effect was observed in this pHm range. At 1.2 × 10−5m europium concentration, 95 ± 4% sorption was found above pHm 7.5 for I = 1 m and above pHm 8.0 for I = 3.5 m and 6.67 m. A small ionic strength effect was observed in this case. X-ray absorption spectroscopy (XAS) results are consistent with the batch sorption experiment outcomes, showing an insignificant effect of ionic strength on the pHm dependent sorption. Results from potentiometric titrations of the solid phase, batch sorption experiments and spectroscopy were interpreted consistently with a charge distribution multi-site (CD-MUSIC) triple layer surface complexation model assuming surface coordination of the metal ion via a tridentate binding mode.

Investigation of Americium Sorption from Model Liquid Radwaste Solutions Using TODGA-Based Solid-Phase Extractant

The purpose of this work is to investigate the sorption of americium from model solutions of liquid radwastes using modified solid-phase extracting agents based on TODGA as an alternative method of additional purification of liquid radwaste by removing transuranium elements by means of alkali precipitation. The kinetic parameters of uranium and americium sorption from highly saline weakly acidic (pH ~3.4) model solutions of liquid radwaste with americium content ~0.04 mg/liter were determined for three modified samples synthesized at VNIIKhT and one analog of a TODGA sample from the AXION Company. The time to equilibrium of all experimental samples in terms of americium is almost the same and equals ~240 min. The equilibrium concentration of uranium is reached in approximately 800 min. The salting-out effect in the extraction of uranium and americium from salt solutions is revealed, though this effect is much weaker for uranium than for americium. The most promising sorbent was determined – sample 7. Its distribution factor is 1.4 times higher than that of the AXION sample and the pair distribution factor Am/U is a factor of 2 higher.

Ionic liquid modified silica gel for the sorption of americium(III) and europium(III) from dilute nitric acid medium

The imidazolium bis(2-ethylhexyl) phosphate moiety was chemically attached on silica gel by chemical modification. The resulting product ([SG-Im]+ [DEHP]−) was characterized by FT-IR spectroscopy, thermogravimetry and elemental analysis. The sorption behavior of Am(III) and Eu(III) on [SG-Im]+ [DEHP]− was studied from dilute nitric acid medium for the separation of Am(III) and Eu(III) from aqueous waste. The effect of time, concentrations of nitric acid and europium in aqueous phase on the distribution coefficient (K d) was studied. The study indicated the possibility of using modified silica for the separation of Eu(III) from Am(III) with high separation factors (>50 at 0.1 M HNO3).

Poly(amidoamine) Dendrimer Functionalized Carbon Nanotube for Efficient Sorption of Trivalent f‐Elements: A Comparison Between 1st And 2nd Generation

AbstractPoly(amidoamine) dendrimer functionalized carbon nano tubes were synthesized, characterized and used for the sorption of americium from aqueous acidic waste solution. The sorption of Am3+ has been found to take place through pseudo second order kinetics with the rate constants of 8.61E‐05 and 5.38E‐05 mg g−1 min−1for 1st and 2nd generation, respectively, followingLangmuir isotherm. The MWCNT‐PAMAMG1 and MWCNT‐PAMAMG2 have high radiolytic stability. The stripping study revealed that EDTA can be effectively used for almost quantitative back extraction of metal ion from both generation of dendrimer. Luminescence investigation revealed the presence of single species with no inner sphere water molecule. The 1st generation dendrimer complex was found to be more asymmetric compared to the 2nd generation. The analysis of Judd Offelt parameters and evaluation of photo‐physical properties were carried out to understand the complexation of Am3+ with these sorbents.

Univariate and multivariate analysis to elucidate the soil properties governing americium sorption in soils

Americium (Am) solid–liquid distribution coefficients (Kd) and desorption percentages were quantified in 20 soils with contrasting edaphic properties by means of batch tests aimed at elucidating the mechanisms and soil properties governing Am sorption. The Kd (Am) values ranged from 103 to 105Lkg−1, whereas the desorption percentages were always <1.2%, thereby demonstrating a strong and irreversible sorption of Am. Univariate and multivariate data analyses were performed with the obtained sorption parameters to identify the soil properties controlling Am sorption behaviour and to propose Kd (Am) prediction models. Univariate Linear Regressions (ULR) and Multiple Linear Regressions (MLR) based on the use of original soil variables and a Principal Component Analysis (PCA) and Partial Least Squares Regression (PLS) based on latent variables revealed that Am sorption was controlled by organic matter, pH, the carbonate and amorphous iron content and the soil specific surface area. The MLR and PLS models explained up to 73% and 70% of the Kd (Am) variability, respectively; thus, they can be used to estimate Kd (Am) values directly from soil properties.

Sorption of americium(III) from multicomponent solutions with sorbents based on dibenzo-18-crown-6 and its derivatives

A sorption of metals from multicomponent nitric acid solutions that simulate process solutions resulting from reprocessing of spent nuclear fuel was studied. The sorbent used was a highly porous polymeric sorbent impregnated with crown ethers. Chemicals used in the work were dibenzo-18-crown-6 (DB18C6) and its alkyl-substituted derivatives: di-tert-butyldibenzo-18-crown-6 (DTBDB18C6) and diisooctyldibenzo-18-crown-6 (DIODB18C6). Sorption coefficients were determined for all elements. With the aim to study the possibility of isolation of 241Am from multicomponent solutions, separation factors for Am and ballast elements were determined. The possibility of application of sorbents impregnated with crown ethers for recovery of 241Am from multicomponent process solutions was shown.

Surface complexation modeling of americium sorption onto volcanic tuff

Results of a surface complexation model (SCM) for americium sorption on volcanic rocks (devitrified and zeolitic tuff) are presented. The model was developed using PHREEQC and based on laboratory data for americium sorption on quartz. Available data for sorption of americium on quartz as a function of pH in dilute groundwater can be modeled with two surface reactions involving an americium sulfate and an americium carbonate complex. It was assumed in applying the model to volcanic rocks from Yucca Mountain, that the surface properties of volcanic rocks can be represented by a quartz surface. Using groundwaters compositionally representative of Yucca Mountain, americium sorption distribution coefficient (Kd, L/Kg) values were calculated as function of pH. These Kd values are close to the experimentally determined Kd values for americium sorption on volcanic rocks, decreasing with increasing pH in the pH range from 7 to 9. The surface complexation constants, derived in this study, allow prediction of sorption of americium in a natural complex system, taking into account the inherent uncertainty associated with geochemical conditions that occur along transport pathways.

Sorption of americium(III) and europium(III) from nitric acid solutions by a novel diglycolamide-grafted silica-based resins: Part 2. Sorption isotherms, column and radiolytic stability studies

Abstract Two novel diglycolamide-grafted silica-based resins (DGSRs) were used for the sorption of trivalent actinides and lanthanides from 3 M nitric acid solutions. The sorption of Am(III) onto the resin containing one diglycolamide (DGA) moiety (DGSR-I) was less efficient than the resin containing two DGA moieties (DGSR-II) with Lagergren first order rate constants of (1.20 ± 0.03) × 10 –3 s –1 and (2.01 ± 0.05) × 10 –3 s –1, respectively. The maximum sorption of Eu(III) (used as a representative of the trivalent lanthanide/actinide ions in radioactive wastes) was 10.4 ± 1.1 mg g –1 and 13.9 ± 1.3 mg g –1 by the DGSR-I and DGSR-II, respectively. The sorption of Eu(III) on the grafted silica resins followed the Langmuir monolayer sorption phenomenon with sorption energies of 16.5 ± 2.05 and 17.8 ± 1.6 kJ mol –1 for DGSR-I and DGSR-II, respectively. Column experiments revealed that the breakthrough capacity of Eu(III) on the DGSR-II column was higher than that on the DGSR-I column. Efficient elution could be achieved with a 0.01 M EDTA solution. The radiolytic stability of the resins was ascertained by exposing the resins to a gamma ray dose up to 1030 kGy resulting in a decrease of the K d values with about 50%; at doses &lt;500 kGy the K d values remained unaffected.

Sorption of americium from low-level liquid wastes by nanocrystalline MnO2

Nanocrystalline MnO2, synthesized by alcoholic hydrolysis of KMnO4, has been studied as a sorbent for removal of americium from low level liquid waste solutions. The synthesized MnO2 was found to have BET surface area of 230 m2 g−1. Am(III) was found to be sorbed by MnO2 quantitatively within 15 min at pH 5. The sorption was found to be more than 90 % at as low a pH as 1.2 and reached to near 100 % at all pH values above pH 3.0 There was no effect of ionic strength (0.01–1.0 M NaCl, CaCl2) on the sorption suggesting the sorption following inner sphere complexation mechanism at all the pH values. Adsorption isotherm studies were carried out using Eu(III) as a chemical analogue of Am(III). These studies showed the isotherm data to follow Langmuir adsorption isotherm.

Application of simplified desorption method to study on sorption of americium(III) on bentonite

To elucidate the sorption behavior of americium(III) on bentonite, which is a mixture of montmorillonite clay, quartz and other minerals, simplified desorption experiments were applied to the solid phases collected after the sorption experiments. The sorption–desorption behavior was examined in the final pH range from 2 to 8. The desorption experiments revealed that most of the Am was sorbed on the montmorillonite moiety of the bentonite. The sorption of Am on montmorillonite was divided into two types: one was the “exchangeable” sorption, in which the sorbed Am was desorbed with a 1 M KCl aqueous solution, and the rest was the “unexchangeable” sorption. The exchangeable sorption was ion exchange of mostly Am3+. The unexchangeable sorption was the strong sorption of Am hydroxides. An accessory iron mineral, pyrite, might be involved in the Am sorption on bentonite at neutral pH.

Americium sorption on smectite-rich natural clay from granitic ground water

Because of their significant retention capability, clay minerals have been proposed as a potential engineered barrier in high level nuclear radioactive waste disposal repositories. Smectite-rich natural clay is being considered as a backfill and buffer material for the Indian repository program. In the present study the sorption of Am by the clay, from granitic ground water, has been investigated. To identify the minerals in the clay controlling the sorption process, the adsorption isotherm of Eu(III), a chemical analogue of Am(III), was determined on montmorillonite–kaolinite clay mixtures having 0–20wt% kaolinite. The effect of experimental parameters, such as, pH, ionic strength, and the presence of cation and anions on Am(III) sorption was further investigated to develop a sorption model for the natural clay. Overlapping adsorption isotherms of Eu(III) obtained for different montmorillonite–kaolinite clay suspensions established montmorillonite as the main sorbent for Eu/Am(III) in the natural clay. Americium(III) sorption increases with pH in three distinct stages: at lower pH values (<4) the sorption is virtually insensitive to pH, then rises sharply (4–7) and subsequently attains a constant value at higher pH values (>8). Decreasing ionic strength increases the sorption at pH<6 indicating the dominant role of ion exchange reactions at lower pH. A surface complexation model, developed for natural clay by including ion exchange site and amphoteric sites present at edges, simulates the sorption profiles at varying pH and ionic strength well and confirms the montmorillonite fraction as the sorbent controlling Am(III) sorption. The presence of Ca(II) as well as anions (Cl−, NO3-) does not affect Am(III) sorption on clay under granitic ground water pH and ionic strength conditions. However, the profile of Am(III) sorption to Ca(II)-equilibrated clay differs from that for Na-equilibrated clay corroborating weaker exchange of Ca(II)–Am(III) in comparison to Na(I)–Am(III). The presence of SO42- in the sorption system lowers Am(III) sorption at lower pH values. Modeling the sorption data indicated the participation of SO42- containing Am surface species. The thermodynamic model developed for sorption onto natural clay was checked for Eu(III) sorption from granitic ground water at pH 6.1. The model simulates the sorption at lower metal ion concentration while there is deviation at higher metal ion concentration. Inclusion of more types of surface sites and the effect of organic material need to be tested to correct the model for this deviation.

Separation of oxidized americium from lanthanides by use of pillared metal(IV) phosphate-phosphonate hybrid materials

Abstract Closing the nuclear fuel cycle in the US poses many challenges, one of which is found in the waste streams, which contain both trivalent lanthanides and actinides. The separation of americium from the raffinate will dramatically reduce the long-term radiotoxicity of the waste. The sorption of americium in both the tri- and pentavalent oxidation states was observed for four M(IV) phosphate-phosphonate ion exchange materials in nitric acid at pH 2. High selectivity was observed for reduced Am(III) with K d values ca. 6×105 mL/g, while the K d values for Am(V) were much lower. A new method of synthesizing and stabilizing AmO2 + to yield a lifetime of at least 24 h in acidic media using a combination of sodium persulfate and calcium hypochlorite will be described.

Sorption of Nuclear Waste Components by Layered Hydrazinium Titanate: a Straightforward Route to Durable Ceramic Forms

ABSTRACTLayered hydrazinium titanate LHT-9, (N2H5)1/2Ti1.87O4 is a new nanohybrid material related to lepidocrocite-type titanates. Unique combination of ion exchange, reductive properties, surface activity due to Brønsted acid sites and occurrence of surface titanyl groups allows exploring LHT-9 for simultaneous uptake of almost all components of liquid nuclear wastes. LHT-9 irreversibly removes technetium, molybdenum, palladium and selenium from their aqueous solutions by specific mechanism of reductive adsorption. For removal of cesium, strontium, transition elements, actinides and lanthanides LHT-9 provides mechanisms of ion exchange and surface complexation. Products of adsorption are nanocrystalline and homogeneous powders loaded with 5 to 15 wt. % of radionuclides and non-radioactive elements. LHT-9 can be applied as ready-to-use precursor for one-step synthesis of durable titanate ceramic waste forms similar to SYNROC. An essential advantage of LHT-9 in comparison with other titanate sorbents (monosodium titanate and peroxo-titanate materials) is the absence of Na in its composition that permits arbitrary tailoring of sorbent properties by simple pre-treatment with the desired elements. Results on sorption of americium, cesium, strontium and lanthanides by LHT-9 are discussed.

Kinetics of plutonium and americium sorption to natural clay

Kinetics of Pu(IV) and Am(III) sorption from natural groundwater to three types of clays were studied at trace concentrations of the elements. Higher Kd values were determined for sorption of Pu than of Am, and no clear dependence of the Kd values and the kinetic coefficients on the composition of the clays can be deduced. Kinetic data evaluated by models for six different control processes indicated a sorption mechanism controlled by Pu or Am diffusion in the inert layer on the surface of the clays. Apart from the kinetics of the elements sorption, time-dependent changes in their bonding nature were also studied using a sequential extraction. It was found that Pu(IV) was predominantly associated with amorphous and crystalline Fe oxides as well as natural organic matter sites on the clays, whereas in the case of Am(III) the exchangeable and carbonate sites played the principal role.

Americium(III) sorption from multicomponent solutions by macrocyclic polyester-based sorbents

Americium sorption by crown-ether-impregnated polymeric sorbents from nitric acid solutions and multicomponent nitrate solutions that model process solutions was studied. Sorption of ballast elements by the unimpregnated Porolas-T support was studied. The sorption coefficients Kd of these elements on Porolas-T do not exceed 0.01. Sorption of the same elements by crown-ether-impregnated sorbents was also studied. Dicyclohexano-18-crown-6 (DCH18C6) and its alkyl derivatives were used. Sorption coefficients were determined for all ballast elements. At the final stage of the study, 241Am sorption coefficients of from multi-component solutions were determined. The data obtained signify the utility of crown-ether-impregnated sorbents for recovering 241Am from multicomponent technological solutions.

Preparation of thorium oxalate–silica sorbent and its application for the sorption of americium from aqueous solutions

A new sorbent, thorium oxalate incorporated in silica gel matrix was prepared. This material was characterized by X-ray, Thermo-gravimetric Analysis, surface area and porosity analysis. The material was obtained in the form of granular particles in the mesh size range of 80–150 American Standard of Testing Materials, yielding good liquid flow, when packed in ion exchange column. This sorbent was investigated for the sorption of americium from various aqueous media such as nitric acid, oxalic acid and sulphuric acid by distribution coefficient studies. Column experiments were carried out to study the practical application of this sorbent for removal of americium from oxalic acid-nitric acid solutions. Elution studies were also carried out for the recovery of americium.