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The problem connected to the long-term storage of high-level nuclear waste has initiated an extensive search for suitable host rock formations and backfill materials to guarantee an isolation of hazardous nuclear elements from the biosphere for thousands of years. Besides other alternatives, clays and clayey materials have been in the focus of various sorption studies. With respect to long-term predictions of migration processes, the transuranium elements and fission products are of particular interest. The present review gives a detailed summary of sorption studies conducted during the past decades describing the impact of various geochemical parameters on the immobilization of Am(III) by different clays and clay minerals.

Time-resolved laser fluorescence spectroscopy (TRLFS) is used to investigate the complexation of Cm(III) with methacrylate in NaCl solution as a function of the temperature (20–60 °C), ionic strength (0–2.5 mol/kg) and ligand concentration (0–0.2 mol/kg). The aqueous speciation of Cm(III) is obtained by deconvolution of the experimental emission spectra using pure component spectra of all Cm(III) species involved under the given experimental conditions. By applying the specific ion interaction theory (SIT), the temperature-dependent thermodynamic stability constants (log K0n(T), n = 1,2) are determined by extrapolating the conditional stability constants to zero ionic strength. Additionally, the binary ion interaction coefficients (e(i,j)) for the interaction between Cl− and the complex species [Cm(Meth)n]3−n (n = 1,2) are calculated. The temperature dependency of log K0n in combination with the integrated Van't Hoff equation provides the molal standard reaction enthalpies and entropies (ΔrH0m, ΔrS0m) of the respective complexation reactions confirming that both, the formation of [CmMeth]2+ and [Cm(Meth)2]+ are endothermic and entropy driven. At temperatures ≥70 °C, an additional Cm(III) species is formed that is attributed to the thermically induced polymerization of methacrylate.

The complexation of Cm(III) with malonate is studied by time-resolved laser fluorescence spectroscopy (TRLFS) in the temperature range from 25 to 90 °C. Three complexes ([Cm(Mal)n]3–2n, n = 1, 2, 3) are identified and their molar fractions are determined as a function of the ligand concentration, the ionic strength, and the temperature. A general shift of the chemical equilibrium toward higher complexes with increasing temperature is observed, with the [CmMal3]3– complex forming only at T > 40 °C. The conditional stability constants (log K′n(T)) are calculated and extrapolated to Im = 0 with the specific ion interaction theory (SIT). The log Kn0(T) values increase by 0.25 to 0.5 logarithmic unit in the studied temperature range. The temperature dependency of the log K°n(T) is fitted by the integrated Van’t Hoff equation, yielding the thermodynamic functions ΔrH°m and ΔrS°m. The results show positive reaction enthalpies and entropies for each complexation step. While the ΔrH°n values are constant within th...

The complexation of Am(III) with formate in aqueous solution is studied as a function of the pH value using a combination of extended X-ray absorption fine structure (EXAFS) spectroscopy, iterative transformation factor analysis (ITFA), and quantum chemical calculations. The Am LIII-edge EXAFS spectra are analyzed to determine the molecular structure (coordination numbers; Am–O and Am–C distances) of the formed Am(III)–formate species and to track the shift of the Am(III) speciation with increasing pH. The experimental data are compared to predictions from density functional calculations. The results indicate that formate binds to Am(III) in a monodentate fashion, in agreement with crystal structures of lanthanide formates. Furthermore, the investigations are complemented by thermodynamic speciation calculations to verify further the results obtained.

The complexation of trivalent lanthanides with a commercial polycarboxylate based concrete superplasticizer (Glenium® 51) is investigated using different spectroscopic techniques. Time-resolved laser fluorescence spectroscopy (TRLFS) in combination with a charge neutralization model is used to determine temperature dependent conditional stability constants (logβ′(T)) for the complexation of Eu(III) with Glenium® 51 in 0.1 mol kg−1 NaCl solution in the temperature range of 20–90 °C. Only one complex species is observed, and logβ′(T) (given in kg per mol eq) shows a very slight increase with temperature from 7.5 to 7.9. The related conditional molar reaction enthalpy (ΔrH′m) and entropy (ΔrS′m) obtained using the Van't Hoff equation show that the complexation reaction is slightly endothermic and entropy driven. The thermodynamic investigations are complemented by structural data for complexes formed with Gd(III) or Tb(III) and Glenium® 51 using extended X-ray absorption fine structure (EXAFS) spectroscopy. The results imply a non-chelate coordination of the trivalent metals through approximately three carboxylic functions of the polycarboxylate comb polymer which are attached predominantly in a bidentate fashion to the lanthanide under the given experimental conditions.

The complexation of the trivalent heavy metals Eu(III) and Cm(III) with polyacrylate as a model ligand is investigated by time-resolved laser fluorescence spectroscopy (TRLFS) and subsequent thermodynamic modelling (charge neutralization model). The aqueous speciation of both metals is investigated as a function of temperature (20–60 °C) and ligand concentration (0–0.4 g/kg) in 0.1 mol/kg NaCl solution ([H+]total ≈ 10−4 mol/kg). Only one complex species is observed at all experimental conditions. With increasing temperature the aqueous Eu(III) and Cm(III) speciation is slightly shifted towards the complexes species. This effect is attributed to the stronger deprotonation of the polymer, which is also reflected by an increase of its loading capacity. The complex stability constants, however, show no visible temperature dependency. Log β’(T) ranges between 5.8 and 5.9 for both Eu(III) and Cm(III) over the whole range of experimental conditions.

The distribution coefficients (K d) 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 K d values strongly depend on the oxidation state of the actinide. The K d 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 K d values of 0.04–0.05 m3/kg. The K d 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 K d value of Pu(VI) (13 ± 3 m3/kg) is about one order of magnitude lower than the K d 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.

The interaction between different trivalent lanthanides and a synthetic acrylate based PCE-superplasticizer (52IPEG4.5) is investigated by using a combination of laser- and synchrotron based spectroscopic techniques. Time-resolved laser fluorescence spectroscopy (TRLFS) is used to obtain thermodynamic data (stability constants (log β′(T)), reaction enthalpy (ΔrH) and entropy(ΔrS)) of the complexation reaction of Eu(III) and 52IPEG4.5 as a function of the temperature (20–80 °C) and ligand concentration (

Pore waters of natural clays, which are investigated as potential host rock formations for high-level nuclear waste, are known to contain large amounts of low-molecular weight organic compounds. These small organic ligands might impact the aqueous geochemistry of the stored radionuclides and, thus, their migration behavior. In the present work, the complexation of Cm(III) with formate in aqueous NaCl solution is investigated by time-resolved laser fluorescence spectroscopy (TRLFS) as a function of the ionic strength (0.5–3.0 mol/kg), the ligand concentration (0–0.2 mol/kg) and the temperature (20–90 °C). The Cm(III) speciation is determined by deconvolution of the emission spectra. The obtained distribution of Cm(III) species is used to calculate the conditional stability constants (log K′(T)) at a given temperature and ionic strength which are extrapolated to zero ionic strength by using the specific ion interaction theory (SIT). Thus, the thermodynamic log K0n(T) values for the formation of [Cm(Form)n](3−n)+ (n = 1, 2) and the ion interaction coefficients (e(i,k)) for [Cm(Form)n](3−n)+ (n = 1, 2) with Cl− are obtained. The log K01(T) (2.11 (20 °C)–2.49 (90 °C)) and log K02(T) values (1.17 (30 °C–2.01 (90 °C)) increase continuously with increasing temperature. The log K0n(T) values are used to derive the standard reaction enthalpies and entropies (ΔrH0m, ΔrS0m) of the respective complexation reactions according to the Van’t Hoff equation. In all cases, positive ΔrH0m and ΔrS0m values are obtained. Thus, both complexation steps are endothermic and entropy-driven.

During recent decades, the search for possible repositories for high-level nuclear waste has yielded large amounts of sorption data for actinides on different minerals. Clays and clay minerals are of special interest as potential host-rock formations and backfill materials, by virtue of their good retardation properties. Neptunium (Np) is one of the actinides which is considered in long-term scenarios due to its long-lived nuclide 237Np (t1/2 = 2.1 × 106 y). Because neptunium sorption is heavily dependent on the experimental conditions, comparison of sorption data from different experiments is challenging. Normalizing reported data with respect to the surface area of the sorbent enables conversion of conventional distribution coefficients (Kd) to normalized (Ka) values, which improves comparability among the results of different experiments. The present review gives a detailed summary of sorption data of Np on clays and clay minerals and examines critically the applicability of the Ka approach.

The complexation of acetate with Am(III) is studied as a function of the pH (1–6) by extended X-ray absorption fine-structure (EXAFS) spectroscopy. The molecular structure of the Am(III)–acetate complexes (coordination numbers, oxygen and carbon distances) is determined from the rawk3-weighted AmLIII-edge EXAFS spectra. The results show a continuous shift of Am(III) speciation with increasing pH value towards the complexed species. Furthermore, it is verified that acetate coordinates in a bidentate coordination mode to Am(III) (Am—C distance: 2.82 ± 0.03 Å). The EXAFS data are analyzed by iterative transformation factor analysis to further verify the chemical speciation, which is calculated on the basis of thermodynamic constants, and the used structural model. The experimental results are in very good agreement with the thermodynamic modelling.

The complexation of Cm(III) with human serum transferrin was investigated in a pH range from 3.5 to 11.0 using time-resolved laser fluorescence spectroscopy (TRLFS). At pH ≥ 7.4 Cm(III) is incorporated at the Fe(III) binding site of transferrin whereas at lower pH a partially bound Cm(III) transferrin species is formed. At physiological temperature (310 K) at pH 7.4, about 70% of the partially bound and 30% of the incorporated Cm(III) transferrin species are present in solution. The Cm(III) results obtained by TRLFS are in very good agreement with Am(III) EXAFS results, confirming the incorporation of Am(III) at the Fe(III) binding site at pH 8.5.

The complexation of trivalent actinides (An(III)) with chloride is studied in the temperature range from 25 to 200 °C by spectroscopic methods. Time-resolved laser fluorescence spectroscopy (TRLFS) is applied to determine the thermodynamic data of Cm(III)-Cl(-) complexes, while extended X-ray absorption fine structure spectroscopy (EXAFS) is used to determine the structural data of the respective Am(III) complexes. The experiments are performed in a custom-built high-temperature cell which is modified for the respective spectroscopic technique. The TRLFS results show that at 25 °C the speciation is dominated mainly by the Cm(3+) aquo ion. Only a minor fraction of the CmCl(2+) complex is present in solution. As the temperature increases, the fraction of this species decreases further. Simultaneously, the fraction of the CmCl2(+) complex increases strongly with the temperature. Also, the CmCl3 complex is formed to a minor extent at T160 °C. The conditional stability constant log β'2 is determined as a function of the temperature and extrapolated to zero ionic strength with the specific ion interaction theory approach. The log β°2(T) values increase by more than 3 orders of magnitude in the studied temperature range. The temperature dependency of log β°2 is fitted by the extended van't Hoff equation to determine ΔrH°m, ΔrS°m, and ΔrC°p,m. The EXAFS results support these findings. The results confirm the absence of americium(III) chloride complexes at T = 25 and 90 °C ([Am(III)] = 10(-3) m, [Cl(-)] = 3.0 m), and the spectra are described by 9-10 oxygen atoms at a distance of 2.44-2.48 Å. At T = 200 °C two chloride ligands are present in the inner coordination sphere of Am(III) at a distance of 2.78 Å.

The temperature dependence of the sorption of 8 μM neptunium(V) on Opalinus Clay (OPA, Mont Terri, Switzerland) was investigated between 40 and 80 °C and compared to previous results for 20 °C. The batch experiments were performed under ambient air conditions in synthetic pore water (pH = 7.6, I = 0.4 M) under variation of the solid-to-liquid ratio between 2 and 20 g/L. Under these conditions the sorption of Np(V) on OPA was found to be an endothermic process as the average distribution coefficient (K d ) increases continuously with increasing temperature from 23 ± 4 L/kg at 40 °C to 678 ± 254 L/kg at 80 °C. In the temperature range of 40-80 °C, the plot of log(K d ) vs. 1/T could be fitted to the Van't Hoff equation delivering an apparent sorption enthalpy of 80 ± 16 kJ/mol and an apparent sorption entropy of 278 ± 48 J/K·mol. Since pore water consists of several constituents (mainly NaCl, CaCl 2 , and MgCl 2 ), additional batch experiments were performed at room temperature to investigate the influence of 0.1 and 0.4 M NaClO 4 , NaCl, CaCl 2 , and MgCl 2 electrolytes on the sorption of Np(V) on OPA at pH 7.6. The results showed significant differences between monovalent (Na + ) and divalent (Mg 2 + , Ca 2 + ) electrolytes. While the sorption isotherms in sodium electrolytes (I = 0.4 M) provided K d values of 231 ± 13 (NaClO 4 ) and 224 ± 17 L/kg (NaCl), low distribution coefficients of 54 ± 6 L/kg (MgCl 2 , I = 0.1 M) and less were determined in magnesium and calcium electrolytes. A weak dependence on ionic strength was observed only in the batch experiments using NaClO 4 as electrolyte. In all other electrolytes the influence of ionic strength on the sorption of Np(V) was negligible between I = 0.1 and 0.4 M.

The complexation of Cm(III) with succinate in an aqueous NaCl solution was studied as a function of ionic strength, ligand concentration, and temperature using time-resolved laser fluorescence spectroscopy (TRLFS). After the Cm(III) speciation was determined by peak deconvolution, the temperature-dependent thermodynamic stability constants (log Kn0(T)) were determined for the stepwise formation of [CmSucn]3–2n (n = 1–3) in the temperature range 20–80 °C (n = 3 only when T ≥ 50 °C) using the specific ion interaction theory (SIT). The first and second complexation steps show an endothermic behavior, as the respective standard reaction enthalpies (ΔrHm0) and entropies (ΔrSm0) derived from the integrated van’t Hoff equation are positive. These TRLFS results are complemented by quantum chemical calculations to resolve the molecular structure of the formed Cm(III) complexes. The results show that the formation of a seven-membered chelate ring is the favored conformation of [CmSucn]3−2n (n = 1−3).

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 p CO2 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 K d 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).

The pH dependence (1–7) of Am(III) complexation with lactate in aqueous solution is studied using extended X-ray absorption fine-structure (EXAFS) spectroscopy. Structural data (coordination numbers, Am—O and Am—C distances) of the formed Am(III)–lactate species are determined from the rawk3-weighted AmLIII-edge EXAFS spectra. Between pH 1 and pH 6, Am(III) speciation shifts continuously towards complexed species with increasing pH. At higher pH, the amount of complexed species decreases due to formation of hydroxo species. The coordination numbers and distances (3.41–3.43 Å) of the coordinating carbon atoms clearly point out that lactate is bound `side-on' to Am(III) through both the carboxylic and the α-hydroxy function of lactate. The experimentally determined coordination numbers are compared with speciation calculations on the basis of tabulated thermodynamic stability constants. Both EXAFS data and thermodynamic modelling are in very good agreement. The EXAFS spectra are also analyzed by iterative transformation factor analysis to further verify the determined Am(III) speciation and the used structural model.

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) L(III)-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.

The complexation of Cm(III) with propionate is studied by time resolved laser fluorescence spectroscopy (TRLFS) in saline solutions (NaCl and CaCl2) as a function of ionic strength, ligand concentration, and temperature. The molar fractions of the different Cm(III) species are determined by peak deconvolution of the measured fluorescence spectra. By using the specific ion interaction theory (SIT), the stability constants at zero ionic strength (log K0n) of the first and second complexation steps are determined. The stability constants of the mono- and dipropionate complexes increase continuously with increasing temperature between 20–90 °C in both background electrolytes. The log K0n values are linearly correlated to the reciprocal temperature, indicating ΔrH0m = const. and ΔrC0p,m = 0. Therefore, the thermodynamic constants (ΔrH0m, ΔrS0m, ΔrG0m) for the formation of the [Cm(Prop)]2+ and [Cm(Prop)2]+ complexes are derived from the integrated van't Hoff equation. The results show that both reactions are entropy driven. Furthermore, neither the formation of ternary species including Ca2+ nor a complexing effect of Cl− at elevated temperatures is observed under the chosen experimental conditions. Lastly, the ion–ion interaction coefficients of both complexed species with Cl− are derived for the first time.