Temperature-dependent luminescence spectroscopic investigations of uranyl(vi) complexation with the halides F- and Cl.

In the present study we have investigated the complexation of uranyl(vi) with chloride and fluoride using luminescence spectroscopy (TRLFS, time-resolved laser-induced fluorescence spectroscopy). At 25 °C (298.15 K), in the presence of 0-0.175 M fluoride, the first single-component emission spectra for all four uranyl(vi)-fluoride complexes, i.e. UO2F+, UO2F2, UO2F3-, and UO2F42- could be extracted. Based on the aqueous speciation derived from the TRLFS data, log K* values at I = 1 M were calculated for all these complexes and extrapolated to infinite dilution using the SIT approach. In the case of chloride, however, quenching of the uranyl(vi)-luminescence hampered the experiments. Thus, uranyl(vi)-complexation was studied with TRLFS at liquid nitrogen temperatures. Samples were prepared at 25 °C (298.15 K) with chloride concentrations ranging from 0 to 1.0 M followed by instantaneous freezing and subsequent luminescence spectroscopic measurements at -120 °C (153.15 K). This allowed for the determination of the first luminescence spectra for the UO2Cl+ complex with the TRLFS method. The chloride quench reaction was further studied in the temperature range 1-45 °C (274.15-318.15 K) using Stern-Volmer analysis. By applying the Arrhenius and the Eyring equations we obtained the first thermodynamic parameters for the dynamic quench process, i.e. the activation energy (Ea = 55.0 ± 12.9 kJ mol-1), enthalpy (ΔH‡ = 52.5 ± 13.0 kJ mol-1), and entropy (ΔS‡ = 103.9 ± 42.8 J mol-1 K-1).

Complexation of Trivalent Lanthanides (Eu) and Actinides (Cm) with Aqueous Phosphates at Elevated Temperatures.

In this study, the complexation of Eu(III) and Cm(III) with aqueous phosphates was investigated using laser-induced luminescence spectroscopy. Experiments at 25 °C and different ionic strengths (0.6-3.1 mol·L-1 NaClO4) established the formation of EuH2PO42+ and CmH2PO42+. From the conditional stability constants, the respective values at infinite dilution as well as the ε(Me(H2PO4)2+;ClO4-) (Me = Eu or Cm) ion interaction coefficients (using the specific ion interaction theory - SIT) were derived. Further experiments (at constant ionic strength of 1.1 mol·L-1) showed that upon increasing the temperature (25-80 °C), the formation of both EuH2PO42+ and CmH2PO42+ was favored. Using the van't Hoff equation, the molal enthalpy Δ R H m° and molal entropy Δ R S m° of these reactions were derived, corroborating an endothermic and entropy driven complexation process. This work contributes to a better understanding of the coordination chemistry of both trivalent lanthanides and actinides with phosphate ions.

The specific sorption of Np(V) on the corundum (α-Al2O3) surface in the presence of trivalent lanthanides Eu(III) and Gd(III): A batch sorption and XAS study.

The sorption of pentavalent neptunium, Np(V), on corundum (α-Al2O3) was investigated in the absence and presence of trivalent europium or gadolinium as a competing element under CO2-free conditions. The objective of this study was to investigate how a trivalent metal ion with a higher charge than that of the neptunyl(V) ion would affect the sorption of Np(V) when allowed to adsorb on the mineral surface before the addition of Np(V). Batch sorption experiments conducted as a function of pH (pH-edges) and as a function of Np(V) concentration (isotherms) in the absence and presence of 1×10(-5)M Eu(III) showed no sign of Eu being able to block Np sorption sites. Surface complexation modelling using the diffuse double layer model was applied to the batch data to obtain surface complexation constants for the formed Np(V) complexes on corundum. To account for potential changes occurring in the coordination environment of the neptunium ion in the presence of a trivalent lanthanide, X-ray absorption spectroscopy (XAS) measurements were carried out on the samples containing only Np(V) and Np(V)+Gd(III). The results reveal the presence of a bidentate Np(V) edge-sharing complex on the corundum surface in the absence of Gd(III), while the coordination environment of Np(V) on the corundum surface could be changed when Gd(III) is added to the sample before the sorption of Np(V).

A proton NMR study on the specific sorption of yttrium(III) and europium(III) on gamma-alumina [γ-Al2O3].

The specific sorption of Eu(III) and Y(III) on γ-alumina was investigated with solid-state (1)H NMR. Solution pH was shown to influence the recorded (1)H NMR spectra of γ-alumina, and thus, metal ion-containing samples were prepared under constant pH conditions, 8.00±0.05. The metal ion concentration in the samples was varied between 6.58×10(-7) M - 3.95×10(-4) M in case of Y(3+) and 6.58×10(-8) M - 1.32×10(-4) M in case of Eu(3+). The mineral concentration was kept constant at 4 g/l. After separation of the liquid phase, the samples were dried under vacuum to remove physisorbed water from the mineral surface. However, even after 48 h of drying at 150°C and 20 mTorr, water was still detected in the proton spectra as two distinct peaks with chemical shifts at 1.3 and 0.9 ppm. The europium addition to the γ-alumina samples induced significant spectral changes in comparison with yttrium-containing samples. These changes were attributed to the paramagnetism of europium rather than to complexation reactions occurring on the mineral surface. Proton spectra obtained for yttrium samples were therefore used to detect the spectral changes induced by the sorption reaction itself. The results revealed a large distribution of protons being removed from the mineral surface upon yttrium complexation. Removed protons were assigned to both bridging surface hydroxyls such as (Al(VI))(2)-OH as well as to terminal hydroxyls, e.g., of type Al(VI)-OH. Acidic protons belonging to (Al(VI))(3)-OH groups were not observed to participate in the surface reaction.

Sorption of Cm(III) and Gd(III) onto gibbsite, alpha-Al(OH)(3): A batch and TRLFS study.

Gd(III) and Cm(III) sorption onto a pure aluminum hydroxide, gibbsite (alpha-Al(OH)(3)), is studied by batch experiments and time-resolved laser fluorescence spectroscopy (TRLFS). The experiments are conducted under argon atmosphere to exclude the influence of atmospheric CO(2) on solution and surface speciation. Batch experiments are done in two different electrolytes 0.1 M NaClO(4) and 0.1/0.01 M NaCl at a constant gibbsite concentration of 2.2 g/L. Gadolinium concentrations are varied from 6.4x10(-9) to 6.4x10(-5) M. pH-dependent sorption is found to be congruent at Gd(III) concentrations up to 6.4x10(-7) M and a shift of the pH edge to higher pH values is observed for higher metal ion concentrations. Type of background electrolyte anion and ionic strength do not affect the metal ion sorption. The spectroscopic investigations are performed with Cm(III) and gibbsite concentrations of 2x10(-7) M and 0.5 g/L, respectively. From the strongly red-shifted emission spectra two different inner-sphere surface complexes can be identified. A third species appearing at pH 6-11 is assigned to a coprecipitated or incorporated Cm(III) species. This incorporated species is most likely formed as a consequence of the applied experimental procedure. By continuously increasing the pH from 4 we move from high to low gibbsite solubility domains. As a result, aluminum hydroxide precipitates from oversaturated solutions, either covering already adsorbed curium or forming a Al/Cm(OH)(3) coprecipitate. Fluorescence lifetimes for the surface-bound Cm(III) complexes and the incorporated species are at 140-150 and 180-200 micros, respectively. Emission bands of the Cm(III) gibbsite surface complexes appear at comparable wavelengths as reported for Cm(III) species bound to aluminum oxides, e.g., gamma-Al(2)O(3); however, lifetimes are longer. This could presumably arise from either shorter binding distances of the Cm to Al-O sites or a coordination to more surface sites.