Samer Amayri

Uranyl(VI) carbonate complex formation: Validation of the Ca2UO2(CO3)3(aq.) species

We recently discovered a neutral dicalcium uranyl tricarbonate complex, Ca2UO2(CO3)3(aq.), in uranium mining related waters [1]. We are now reporting a further validation of the stoichiometry and the formation constant of this complex using two analytical approaches with time-resolved laser-induced fluorescence spectroscopy (TRLFS) species detection: i) titration of a non-fluorescent uranyl tricarbonate complex solution with calcium ions, and quantitative determination of the produced fluorescent calcium complex via TRLFS; and ii) variation of the calcium concentration in the complex by competitive calcium complexation with EDTA4-. Slope analysis of the log (fluorescence intensity) versus log[Ca2+] with both methods have shown that two calcium ions are bound to form the complex Ca2UO2(CO3)3(aq.). The formation constants determined from the two independent methods are: i) logβ°213=30.45±0.35 and ii) logβ°213=30.77±0.25. A bathochrome shift of 0.35 nm between the UO2(CO3)34- complex and the Ca2UO2(CO3)3(aq.) complex is observed in the laser-induced photoacoustic spectrum (LIPAS), giving additional evidence for the formation of the calcium uranyl carbonate complex. EXAFS spectra at the LII and LIII-edges of uranium in uranyl carbonate solutions with and without calcium do not differ significantly. A somewhat better fit to the EXAFS of the Ca2UO2(CO3)3(aq.) complex is obtained by including the U-Ca shell. From the similarities between the EXAFS of the Ca2UO2(CO3)3(aq.) species in solution and the natural mineral liebigite, we conclude that the calcium atoms are likely to be in the same positions both in the solution complex and in the solid. This complex influences considerably the speciation of uranium in the pH region from 6 to 10 in calcium-rich uranium-mining-related waters.

Mixed complexes of alkaline earth uranyl carbonates: a laser-induced time-resolved fluorescence spectroscopic study.

The interaction of the alkaline earth ions Mg(2+), Sr(2+) and Ba(2+) with the uranyl tricarbonato complex has been studied by time-resolved laser-induced fluorescence spectroscopy. In contrast to the non-luminescent uranyl tricarbonato complex at ambient temperature the formed products show luminescence properties. These have been used to determine the stoichiometry and complex stabilities of the formed compounds. As the alkaline earth elements are located in an outer shell of the complex the influence of the type of the alkaline earth element on the stability constant is not very drastic. The stability constants range from log beta113 degrees = 26.07+/-0.13 to log beta113 degrees = 26.93+/-0.25 for the first reaction step and from log beta213 degrees = 29.73+/-0.47 to log beta213 degrees = 30.79+/-0.29 for the overall complex formation with two alkaline earth ions.

Sorption of neptunium(V) on Opalinus Clay under aerobic/anaerobic conditions

Abstract The interaction between neptunium(V) and a natural argillaceous rock (Opalinus Clay (OPA), Mont Terri, Switzerland) has been investigated in batch sorption experiments by varying pH (6–10), Np(V) concentration (10−12–10−4 M), solid-to-liquid ratio (2–20 g/L), and partial pressure of CO2 (10−3.5 and 10−2.3 atm) under aerobic/anaerobic conditions in saturated calcite solution. All batch experiments were carried out using well characterized aerobic and anaerobic dry powders of OPA. The results show a great influence of pH on Np(V) sorption. Under aerobic conditions sorption increases with increasing pH until maximum sorption is reached between pH 8–9. At pH>9 sorption decreases due to the formation of negatively charged Np(V)-carbonate complexes. By increasing pCO2 from 10−3.5 to 10−2.3 atm, the sorption edge is shifted ≈0.5 units to lower pH values. Under anaerobic conditions stronger sorption of 8×10−6 M Np(V) was found, possibly due to partial reduction of Np(V) to Np(IV). The sorption of 8×10−6 M Np(V) under aerobic conditions at pH 8.2 in saturated calcite solution increases continuously with increasing solid-to-liquid ratio of OPA in the range of 2–20 g/L with a constant Kd value of 126±13 L/kg. The sorption isotherm was measured over seven orders of magnitude in Np(V) concentration using 239Np as tracer. The sorption isotherm could be divided in a part of linear sorption behaviour between 10−13–10−9 M Np(V) and non-linear behaviour in the range of 10−9–10−4 M Np(V).

Application of XAFS Spectroscopy to Actinide Environmental Science

The use of XAFS Spectroscopy and related synchrotron radiation techniques for the molecular‐level speciation of environmental contaminants including actinides has led to an improved understanding of the fundamental chemical and biological processes determining their behavior in complex systems. Several recent applications of XAFS spectroscopy to actinides in model systems and more complex environmental samples are reviewed to highlight the impact these studies have on our knowledge about the bioavailability of actinides, the development of remediation strategies, and predictive models for risk assessment. XAFS studies of actinide ion sorption at solid/aqueous solution interfaces are presented in greater detail. Representative examples include XAFS studies in combination with batch‐type experiments of U(VI), Np(V), Pu(III), and Pu(IV) sorption on kaolinite.

Speciation of Np(V) uptake by Opalinus Clay using synchrotron microbeam techniques

Synchrotron-based X-ray absorption spectroscopy has been used to determine the chemical speciation of Np sorbed on Opalinus Clay (OPA, Mont Terri, Switzerland), a natural argillaceous rock revealing a micro-scale heterogeneity. Different sorption and diffusion samples with Np(V) were prepared for spatially resolved molecular-level investigations. Thin sections of OPA contacted with Np(V) solution under aerobic and anaerobic conditions as well as a diffusion sample were analysed spatially resolved. Micro-X-ray fluorescence (μ-XRF) mapping has been used to determine the elemental distributions of Np, Fe and Ca. Regions of high Np concentration were subsequently investigated by micro-X-ray absorption fine structure spectroscopy to determine the oxidation state of Np. Further, micro-X-ray diffraction (μ-XRD) was employed to gain knowledge about reactive crystalline mineral phases in the vicinity of Np enrichments. One thin section was also analysed by electron microprobe to determine the elemental distributions of the lighter elements (especially Si and Al), which represent the main elements of OPA. The results show that in most samples, Np spots with considerable amounts of Np(IV) could be found even when the experiments were carried out in air. In some cases, almost pure Np(IV) LIII-edge X-ray absorption near-edge structure spectra were recorded. In the case of the anaerobic sample, the μ-XRF mapping showed a clear correlation between Np and Fe, indicating that the reduction of Np(V) is caused by an iron(II)-containing mineral which could be identified by μ-XRD as pyrite. These spatially resolved investigations were complemented by extended X-ray absorption fine structure measurements of powder samples from batch experiments under aerobic and anaerobic conditions to determine the structural parameters of the near-neighbour environment of sorbed Np.

Neptunium(V) sorption on kaolinite

Abstract The sorption behavior of neptunium(V) onto the clay mineral kaolinite was studied in batch experiments under different experimental conditions: [Np(V)]=7 × 10−12–8 × 10−6 M, solid-to-liquid ratio 2–20 g L−1, I=0.1 and 0.01 M NaClO4, pH=6–10, ambient air and Ar atmosphere. The short-lived isotope 239Np (T1/2=2.36 d) was used instead of 237Np (T1/2=2.14 × 106 a) to study the sorption behavior of Np(V) at environmentally-relevant concentrations, i.e., 7 × 1012 M Np. In addition, 239Np(V) served as tracer to measure sorption isotherms over six orders of magnitude in Np concentration (4.8 × 10−12–1.0 × 10−4 M). The results show that Np(V) sorption on kaolinite is strongly influenced by pH, CO2, and ionic strength. The sorption of 8 × 10−6 M Np(V) at pH 9.0, and ionic strength of 0.1 M NaClO4 was proportional to the solid-to-liquid ratio of kaolinite in the range of 2–10 g L−1. In the absence of CO2, the Np(V) uptake increased continuously with increasing pH value up to 97% at pH 10. Under ambient CO2, the sorption of Np decreased above pH 8 up to zero at pH 10. An increase of Np(V) concentration from 7 × 10−12 to 8 × 10−6 M resulted in a shift of the sorption pH edge by up to one pH unit to higher pH values. The ionic strength influenced the Np(V) sorption onto kaolinite only in the presence of ambient CO2. Under Ar atmosphere the sorption of Np(V) was independent from ionic strength, indicating the formation of inner-sphere complexes of Np(V) with kaolinite. Time-dependent batch experiments at pH 9.0 under ambient CO2 showed that the sorption of Np(V) on kaolinite is fast and fully reversible over six orders in Np(V) concentration.

Sensitive redox speciation of neptunium by CE-ICP-MS.

Capillary electrophoresis (CE) was used to separate the neptunium oxidation states Np(IV) and Np(V), which are the only oxidation states of Np that are stable under environmental conditions. The CE setup was coupled to an inductively coupled plasma mass spectrometer (Agilent 7500ce) using a Mira Mist CE nebulizer and a Scott-type spray chamber. The combination of the separation capacity of CE with the detection sensitivity of inductively coupled plasma mass spectrometry (ICP-MS) allows identification and quantification of Np(IV) and Np(V) at the trace levels expected in the far field of a nuclear waste repository. Limits of detection of 1 × 10-9 and 5 × 10-10 mol L-1 for Np(IV) and Np(V), respectively, were achieved, with a linear range from 10-9 to 10-6 mol L-1. The method was applied to study the redox speciation of the Np remaining in solution after interaction of 5 × 10-7 mol L-1 Np(V) with Opalinus Clay. Under mildly oxidizing conditions, a Np sorption of 31% was found, with all the Np remaining in solution being Np(V). A second sorption experiment performed in the presence of Fe2+ led to complete sorption of the Np onto the clay. After desorption with HClO4, a mixture of Np(IV) and Np(V) was found in solution by CE–ICP–MS, indicating that some of the sorbed Np had been reduced to Np(IV) by Fe2+.

Actinide Sorption Studies Using the Isotopes 237Np and 239Np

The sorption of Np(V) on γ-Al2O3 and the reference clay mineral kaolinite was studied in batch experiments in the presence and absence of ambient CO2 with 0.1 M NaClO4 as background electrolyte. The short-lived isotope 239Np (t1/2 = 2.36 d) was used instead of 237Np (t1/2 = 2.14 × 106 a) to study the sorption behaviour of Np(V) at environmentally-relevant concentrations, i.e., 7 pM Np. In addition, 239Np served as tracer to measure sorption isotherms over six orders of magnitude in neptunium concentration. γ-Al2O3 served as a reference for clay minerals like kaolinite to investigate the interaction of Np(V) with aluminol groups, which are crucial binding sites of clays.

Neptunium(V) sorption onto gibbsite

Abstract The sorption behavior of neptunium(V) on gibbsite (γ-Al(OH)3) was studied by batch experiments in the presence/absence of ambient CO2 as a function of solid-to-liquid ratio, pH, ionic strength, and Np(V) concentration. The sorption of 8×10−6 M Np(V) at ionic strength of 0.1 M NaClO4 was proportional to the solid-to-liquid ratio of gibbsite with a Kd value of ∼45±6 mL/g at pH 9.0. The sorption behavior of Np(V) on gibbsite was strongly affected by ambient CO2 and pH in the pH range of 6.0–10.0. In the presence of CO2 at ionic strengths of 0.01 and 0.1 M NaClO4, the maximum Np(V) uptake was at pH 8.5 for 8×10−6 and 7×10−12 M Np(V). In the absence of CO2 at the same ionic strengths, the Np(V) uptake increased with increasing pH up to ∼95% at pH 10.0. The sorption edges of Np(V) shifted by approximately one unit toward higher pH while increasing the concentration from 7×10−12 to 8×10-−6 M Np(V) and the amount of Np(V) sorbed at pH 8.5 decreased by ca. 40% under ambient air conditions. Ionic strength had no effect on the sorption of Np(V) onto gibbsite whether CO2 was present or not, indicating the formation of inner-sphere surface complexes between Np(V) and γ-Al(OH)3.

Distribution coefficients for the sorption of Th, U, Np, Pu, and Am on Opalinus Clay

Abstract The distribution coefficients (Kd) are determined for the sorption of Th(IV), U(VI), Np(V), Pu(III/IV/VI), and Am(III) in the system natural Opalinus Clay (OPA) from Mont Terri, Switzerland, and synthetic OPA pore water at pH7.6 as a function of solid-to-liquid ratio and element concentration. A linear sorption behavior under the chosen experimental conditions was obtained. The Kd values strongly depend on the oxidation state of the actinide. The Kd values for the tri- and tetravalent actinides are in the range of 29–159 m3/kg. U and Np in the oxidation states six and five, respectively, are weakly sorbed on OPA with Kd values of 0.04–0.05 m3/kg. The Kd values for the redox-stable Am(III) and Th(IV) are in the same range as for Pu(III) and Pu(IV) which are redox sensitive. The Kd value of Pu(VI) (13 ± 3 m3/kg) is about one order of magnitude lower than the Kd values of Pu(III) and Pu(IV), but still more than two orders of magnitude higher compared to the values obtained for U(VI) and Np(V). This discrepancy is attributed to the partial reduction of Pu(VI) to Pu(IV) during sorption.