Pascal E Reiller
email: pascal.reiller@cea.fr

Thomas Vercouter
email: thomas.vercouter@cea.fr

Lara Duro

Christian Ekberg

Thermodynamic Data Provided Through the FUNMIG Project: Analyses and Prospective

We are highlighting some of the needs for good quality thermodynamic data in radioactive waste management. A presentation of the thermodynamic data produced within the 6 th framework integrated project FUNMIG (Fundamental processes of radionuclide migration) and how these works have helped in filling in relevant thermodynamic data gaps is given. The manuscript does not intend to be a complete review of thermodynamic data, but a short overview of the aqueous complexation of americium (III), lanthanides (III), uranium (VI), and thorium (IV) by sulphates, silicates, carbonates and phosphates. The work presented is based on the latest developments published in the literature and specifically addressed within the IP FUNMIG.

Introduction

The quality of thermodynamic data is one issue of relevance in different scientific and technical activities. Calculation of reaction yields, design of industrial processes, the development of protocols and processes in the pharmaceutical industry, mining activities, heating and cooling systems are some examples of activities where thermodynamic data are applied. The global nuclear cycle is not an exception where quality-assured thermodynamic data are needed [START_REF] Voigt | Quality assurance in thermodynamic databases for performance assessment studies in waste disposal[END_REF]. Many economic and intellectual resources have been invested in the determination of good quality thermodynamic data of the elements of interest for nuclear activities. From the preparation of the nuclear fuel itself, to the evolution of the different elements of a nuclear power plant, up to the final end of the spent nuclear fuel cycle, and products derived from its treatment after the discharge from the nuclear power plant reactor, research activities need good quality thermodynamic data. In the last years, nuclear waste management has been one of the disciplines where a larger quantity of efforts has been devoted to this type of research. Many nuclear waste management national agencies and related organisations have developed their own thermodynamic database for the elements of interest for nuclear waste management (Yui, 2001;[START_REF] Hummel | Nagra/PSI chemical thermodynamic data base 01/01[END_REF][START_REF] Bion | BASSIST: An applied thermodynamic database for radionuclide chemistry[END_REF][START_REF] Duro | Update of a thermodynamic data-base for radionuclides to assist solubility limits calculation for performance assessment[END_REF].

The primary waste form resulting from nuclear energy production is spent nuclear fuel (SNF), but some national programs have chosen reprocessing. The most accepted concept for storage of either SNF or wastes from the reprocessing is currently its deep burial at several hundreds of meters underneath the surface in a geologically stable suitable host-rock, and isolated from the geological media by several engineering barriers based on material whose main function is to prevent the contact of the water either with the spent fuel or with the wastes (ANDRA, 2005a;[START_REF] Andra | Dossier 2005 -Argile. Tome Évolution phénoménologique du stockage géologique[END_REF]. The use of good quality thermodynamic data is therefore an important tool to evaluate the possible release of these radionuclides into the repository system, that is the near-field, and into the undisturbed geosphere, that is the far-field.

A typical composition of a sample of SNF after 38 years of its discharge from the reactor (inventory) is shown in Figure 1 [START_REF] Nirex | Radionuclide content for a range of irradiated fuels[END_REF]. Only those elements accounting for more of the 0.01% of the mass of the SNF are represented in the plot, from where the high amount of thermodynamic data of interest for the nuclear waste managers can be deduced. Many of the elements present in the waste, like actinide (An) elements, are not very common in other physico-chemical studies, so are the thermodynamic functions and data for these elements. [START_REF] Nirex | Radionuclide content for a range of irradiated fuels[END_REF].

Since the beginning of the nineties, the Nuclear Energy Agency of the Organisation for the Economic Co-operation and Development (NEA-OECD) has commissioned panels of experts to provide reliable and internally consistent compilations of thermodynamic formation data (ΔfG°m, ΔfH°m, S°m, and C°p,m), reaction data and stability constants (ΔrG°m, ΔrH°m, ΔrS°m, log10K°) [START_REF] Wanner | France. performance analysis of HLW repository in Japan[END_REF][START_REF] Grenthe | Modelling in Aquatic Chemistry[END_REF] for the nuclides involved in the nuclear cycle, i.e. uranium [START_REF] Grenthe | Chemical Thermodynamics 1: Chemical Thermodynamics of Uranium[END_REF], americium [START_REF] Silva | Chemical Thermodynamics 2: Chemical Thermodynamics of Americium with an Appendix on Chemical Thermodynamics of Uranium (Grenthe[END_REF], technetium [START_REF] Rard | Chemical Thermodynamics 3: Chemical Thermodynamics of Technecium[END_REF], neptunium and plutonium [START_REF] Lemire | Chemical Thermodynamics 4: Chemical Thermodynamics of Neptunium and Plutonium[END_REF], selenium [START_REF] Olin | Chemical Thermodynamics 7: Chemical Thermodynamics of Selenium[END_REF], nickel [START_REF] Gamsjäger | Chemical Thermodynamics 6. Chemical Thermodynamics of Nickel[END_REF], zirconium [START_REF] Brown | Chemical Thermodynamics 8. Chemical Thermodynamics of Zirconium[END_REF], thorium (Rand et al., 4/41 2009) as well as organic complexes of some of theses metal ions [START_REF] Hummel | Chemical Thermodynamics 9: Chemical Thermodynamics of Compounds and Complexes of U, Np[END_REF].

Important activity is devoted to updates with new data [START_REF] Silva | Chemical Thermodynamics 2: Chemical Thermodynamics of Americium with an Appendix on Chemical Thermodynamics of Uranium (Grenthe[END_REF][START_REF] Guillaumont | Chemical Thermodynamics 5: Update on the Chemical Thermodynamics of Uranium, Neptunium, Americium and Technetium[END_REF]. Much more than a catalogue of thermodynamic data and constants, these Thermochemical Data Bases publications (NEA-TDB reviews) provide an extensive and comprehensive scientific review of the information available in the scientific literature. The reviews are based both on data obtained from solution experiments as well as from calorimetric measurements and preferably supported by pieces of evidence of structure of the formed complexes whenever possible.

Considering the high level of expertise involved and the desired high quality of the selected thermodynamic data, these data selections inherently have inevitable justified gaps. In some cases, the raw data available in the literature are subject to critics or are not sufficiently documented as to be used by the expert panel. One illustrative example can be sought of in the first selection of thermodynamic data published for uranium [START_REF] Grenthe | Chemical Thermodynamics 1: Chemical Thermodynamics of Uranium[END_REF] where only an upper value for the formation of UO2(OH)2(aq) was given. The same occurred with the selection of thermodynamic data for plutonium [START_REF] Lemire | Chemical Thermodynamics 4: Chemical Thermodynamics of Neptunium and Plutonium[END_REF], where no cumulative formation constants for the Pu(IV) hydroxocomplexes (Pu(OH)n 4-n , with 2 ≤ n ≤ 4) were selected. Finally, one can also think about the solubility of AmPO4(s), which can be assessed using the corresponding Gibbs energy of the dissolution reaction, ΔrG°m, given by [START_REF] Silva | Chemical Thermodynamics 2: Chemical Thermodynamics of Americium with an Appendix on Chemical Thermodynamics of Uranium (Grenthe[END_REF]. But as no selected log10K° value was present in the selection, substantial differences may appear in theoretical speciation calculations [START_REF] Bion | Influence of thermodynamic database on the modelisation of americium (III) speciation in a simulated biological medium[END_REF]. Subsequent updates of the thermodynamic data selection based on new experimental data -in the case of UO2(OH)2(aq) -or deeper analyses of existing literature -in the case of Pu(OH)4(aq) -were able to partially fill in these gaps [START_REF] Silva | Chemical Thermodynamics 2: Chemical Thermodynamics of Americium with an Appendix on Chemical Thermodynamics of Uranium (Grenthe[END_REF][START_REF] Guillaumont | Chemical Thermodynamics 5: Update on the Chemical Thermodynamics of Uranium, Neptunium, Americium and Technetium[END_REF]. Nevertheless, one may note that there is still a need of experimental program in this field and data were recently reported [START_REF] Neck | Solubility and redox reactions of Pu(IV) hydrous oxide: Evidence for the formation of PuO2+x(s, hyd)[END_REF][START_REF] Yun | Investigation of the hydrolysis of plutonium(IV) by a combination of spectroscopy and redox potential measurements[END_REF].

The construction of applied thermodynamic databases of good quality is a task that will probably never end, although in the last years marked advances have been made thanks to the combination of different analytical techniques, molecular simulations and strategies for estimation of gaps in the available thermodynamic data. Management of uncertainties is also a matter of concern. Nevertheless, it is out of the scope of this paper and the reader is strongly encouraged to refer to expert views in the domain [START_REF] Ekberg | Sensitivity analysis and simulation uncertainties in predictive geochemical modelling[END_REF][START_REF] Denison | The effects of database parameter uncertainty on uranium(VI) equilibrium calculations[END_REF].

Given the quality criteria of the review in [START_REF] Silva | Chemical Thermodynamics 2: Chemical Thermodynamics of Americium with an Appendix on Chemical Thermodynamics of Uranium (Grenthe[END_REF], only few data sets were available for extrapolation to zero ionic strength, and the authors called for more experimental data at other ionic strength values. In [START_REF] Guillaumont | Chemical Thermodynamics 5: Update on the Chemical Thermodynamics of Uranium, Neptunium, Americium and Technetium[END_REF], spectroscopic data on Cm(III) were included in the data selection, assuming that the differences in comportment between americium and curium were smaller than the experimental uncertainties, and that spectroscopic data were less influenced by activity variations. The main differences are actually caused by relying on Cm(III) for evaluating Am(III) data, and some of them will be discussed in the present paper.

In natural media the main inorganic ligands are hydroxyl, carbonate, sulphate, silicate and phosphate. Contrary to the other ligands, phosphate is often not present in appreciable amount in deep groundwaters, except in ores or in living organism [START_REF] Purvis | Uranium biosorption by the lichen Trapelia involuta at a uranium mine[END_REF][START_REF] Quiroz | Binding of thorium(IV) to carboxylate, phosphate and sulfate functional groups from marine exopolymeric substances (EPS)[END_REF], as in biofilms in granite environments. These ligands are accounted for in the critical reviews together with their auxiliary data. As an illustration of their importance, the theoretical repartition of soluble species of americium (III) using the last update of NEA selection [START_REF] Guillaumont | Chemical Thermodynamics 5: Update on the Chemical Thermodynamics of Uranium, Neptunium, Americium and Technetium[END_REF] can be calculated using PHREEQC in an argillaceous water proposed by [START_REF] Gaucher | Modelling the porewater chemistry of the Callovian-Oxfordian formation at a regional scale[END_REF] (Figure 2). Variation of carbonate concentration with pH is controlled via the solubility of Dolomite (Ca,Mg)(CO3)2. Given the number of possible complexes between pH 7 and 8.5, it is evident that accurate thermodynamic data are needed.

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Furthermore, as a consequence of the aforementioned quality criteria of data and stoichiometry selection, one has to be assured that all the possible complexes are taken into account in theoretical calculation exercises. [START_REF] Gaucher | Modelling the porewater chemistry of the Callovian-Oxfordian formation at a regional scale[END_REF], calculated using the data from [START_REF] Guillaumont | Chemical Thermodynamics 5: Update on the Chemical Thermodynamics of Uranium, Neptunium, Americium and Technetium[END_REF].

Furthermore, the hydroxo, carbonate, and sulphate complexation of americium (III) was revised between the former [START_REF] Silva | Chemical Thermodynamics 2: Chemical Thermodynamics of Americium with an Appendix on Chemical Thermodynamics of Uranium (Grenthe[END_REF] and the latter selection [START_REF] Guillaumont | Chemical Thermodynamics 5: Update on the Chemical Thermodynamics of Uranium, Neptunium, Americium and Technetium[END_REF] of NEA data. The visual results of the proportion of the hydroxo and carbonato complexes are reported in Figure 3 for the two selections at I = 1 mol/L using the specific ion interaction theory [START_REF] Guillaumont | Chemical Thermodynamics 5: Update on the Chemical Thermodynamics of Uranium, Neptunium, Americium and Technetium[END_REF]. Here the main difference lies in the decrease of the selected values for first and second hydrolysis constants. The high ionic strength used in Figure 3 helps in visualising the highly charged Am(CO3)3 3-complex and its influence on the repartition of species. In the case of sulphate, selected values of complexation constants have also been changed. This may have a significant impact on the speciation calculation, especially when sulphate is present at moderate concentration like in some groundwaters equilibrated with clays or if oxidation of pyrite occurs [START_REF] Degueldre | Study of the pore water chemistry through an argillaceous formation: A paleohydrochemical approach[END_REF][START_REF] Beaucaire | Groundwater characterisation and modelling of water-rock interaction in an argillaceous formation (Tournemire, France)[END_REF][START_REF] Gaucher | A robust model for pore-water chemistry of clayrock[END_REF]. -3.46 atm, between values selected in Silva et al. (1995) (left) and [START_REF] Guillaumont | Chemical Thermodynamics 5: Update on the Chemical Thermodynamics of Uranium, Neptunium, Americium and Technetium[END_REF] (right).

Within the FUNMIG program, the first research and technological development component (RTDC-1) was dedicated to provide fundamental process knowledge and the required data for processes with comparably well established conceptual understanding. This included filling gaps or reducing apparent inconsistencies between thermodynamic data, which were the tasks of one work-package involving two teams: the Laboratory for the Speciation of

Radionuclides and Molecules at the Commissariat à l'Energie Atomique (CEA, France) and the Department of Chemical and Biological Engineering at the Chalmers University of Technology (CTH, Sweden). From the expertise of these groups, and the other related programs within FUNMIG, it was proposed to focus the works on the speciation of trivalent lanthanides and actinides, and hexavalent uranium for the former group and on the phosphate complexation of tetravalent thorium for the latter. Some elements studied were selected as proxies for other elements of interest but more difficult to handle in laboratory assays; europium (III) and thorium (IV) were used as analogues for the other trivalent actinides -i.e. Am(III) and Cm(III) -and tetravalent actinides -i.e. U(IV), Np(IV), and Pu(IV) -, respectively.

Furthermore, the effect of temperature on the stability of some complexes was one of the issues of interest. Depending on current national projects and scenarios, the temperature of interest for the deep geological disposal of nuclear waste ranges from 0 °C -deep permafrost penetration -to more than 100 °C -accidental scenarios -. Within this range, effects of temperature are not always negligible, thus there was a need of values for the formation enthalpies, entropies, and heat capacities of some species, which were not always available in the literature. Therefore, the effect of temperature was investigated for the complexation between hexavalent uranium and sulphate on the one hand, and between tetravalent thorium and phosphate on the other hand.

The topics of the work were: i) the analyses of previous data on americium and curium carbonates Am(CO3)n 3-2n and Cm(CO3)n 3-2n [START_REF] Vercouter | Complexes aqueux de lanthanides (III) et actinides (III) avec les ions carbonates et sulfates. Etude thermodynamique par spectrofluorimétrie Laser résolue en temps et spectrométrie de masse à ionisation électrospray[END_REF]2005a), ii) europium carbonates Eu(CO3)n 3-2n [START_REF] Vercouter | Complexes aqueux de lanthanides (III) et actinides (III) avec les ions carbonates et sulfates. Etude thermodynamique par spectrofluorimétrie Laser résolue en temps et spectrométrie de masse à ionisation électrospray[END_REF]Vercouter et al., 2005c), iii) dedicated experiments on thorium phosphates Th(H2PO4)n 4-2n (Ekberg et al., 2010), iv) dedicated experiments on uranium(VI) sulphates UO2(SO4)n 2-2n [START_REF] Vercouter | Stoichiometries and thermodynamic stabilities for aqueous sulfate complexes of U(VI)[END_REF], and analysis of previous trivalent lanthanide sulphates Ln(SO4)n 3-2n (Vercouter et al., 2005b), v) dedicated experiments on trivalent actinides and europium silicates (Vercouter et al., 2009a;2009c).

We will present and discuss hereafter an overview of these works. 9/41 2. Data provided through FUNMIG

The carbonate system

The complexation of radionuclides by carbonate has been issue of study in a huge amount of works in the literature. Testimony of these efforts are the publications previously mentioned of the NEA-TDB reviews, as well as the many specific experimental works, which have been mainly conducted at room temperature. Nevertheless, not so many of these experiments were performed at temperatures above 20-25 °C, although higher temperatures can help stabilizing solid phases, which is a prerequisite when performing solubility measurements [START_REF] Giffaut | Influence des ions chlorures sur la chimie des actinides, Influence de la radiolyse et de la température[END_REF]. In a large number of examples solubility data were used to propose different stoichiometries of the aqueous complexes in equilibrium with the solid phases, and to obtain their stability values. Because the speciation may involve a number of species and due to the scattering of solubility measurements that is usually not negligible, the interpretation model of the data is often not unique, and has to be carefully tested. For instance, a very long discussion in the open literature was maintained during the last decades, e.g., on the stability of the thorium carbonate complexes. A consensus seems to have been reached in [START_REF] Rand | Chemical Thermodynamics 11: Chemical Thermodynamics of Thorium[END_REF].

The use of solubility experiments to establish stoichiometry and stability of aqueous complexes may present some limitations that cannot be solved if complementary analytical techniques are not used. Some of the limitations associated to solubility experiments result from the very low solubility of most actinides solid phases under geological conditionsespecially for An(III) and An(IV) -. In addition, actinide and lanthanide solid phases exhibit a tendency to form colloidal particles [START_REF] Altmaier | Solubility and colloid formation of Th(IV) in concentrated NaCl and MgCl2 solution[END_REF][START_REF] Wang | Observation of aqueous Cm(III)/Eu(III) and UO2 2+ nanoparticulates at concentrations approaching solubility limit by laser-induced fluorescence spectroscopy[END_REF] in the metastability domain around the solubility limits (see Figure 7.2 in [START_REF] Stumm | Aquatic chemistry, chemical equilibria and rates in natural waters, Third Edition[END_REF]. This requires the use of adequate techniques for particle detection [START_REF] Walther | Comparison of colloid investigations by single particle analytical techniques: A case study on thorium-oxyhydroxides[END_REF], 10/41 and often complicates the validation of solubility data interpretation models with equilibrium reactions.

A second limitation of solubility experiments is that in many cases the characterization of the solubility-controlling solid phase is ambiguous and even the solid phases suffer of transformations during the experiment that preclude a reliable understanding of the processes occurring in the set up -the accessible parameter is in fact the product of the solubility product Ks and the complexation constant βn -. Consequently, the univocal determination of the aqueous complexes in equilibrium with the solid phase(s), which is based upon slope analyses of solubility curves, requires testing different models with different sets of equilibriums. This may lead to a too large amount of unknown parameters as to elucidate the stoichiometry of the species existing in the system.

The solubility analyses of actinide and lanthanide carbonate phases is a good illustration of these issues. As stated in NEA reviews [START_REF] Silva | Chemical Thermodynamics 2: Chemical Thermodynamics of Americium with an Appendix on Chemical Thermodynamics of Uranium (Grenthe[END_REF] "Am(III) solubility studies in carbonate solutions are associated with ambiguities and uncertainties arising from possible solid phase transformations and/or alterations". Most of the solubility data used in deriving NEA selected thermodynamic parameters on americium carbonate phases had been obtain under ambient conditions (20 ≤ T °C ≤ 25, Table 12-8, p369-370 in [START_REF] Guillaumont | Chemical Thermodynamics 5: Update on the Chemical Thermodynamics of Uranium, Neptunium, Americium and Technetium[END_REF], and higher temperature were not considered. Increasing temperature usually favours a better crystallisation of the solids and probably reduces the scattering of the solubility measurements. Solubility measurements of solid americium carbonate phases had been performed at different temperatures between 10 and 70°C [START_REF] Giffaut | Influence des ions chlorures sur la chimie des actinides, Influence de la radiolyse et de la température[END_REF]. Whereas the NEA expert panel did not considered these data because of the lack of unambiguous solid phase characterisation, these data have been used to determine thermodynamic data of chemical reactions involving Am(III) at 25°C and also for their temperature dependence [START_REF] Vercouter | Complexes aqueux de lanthanides (III) et actinides (III) avec les ions carbonates et sulfates. Etude thermodynamique par spectrofluorimétrie Laser résolue en temps et spectrométrie de masse à ionisation électrospray[END_REF]Vercouter et al., 2005a).

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Hereafter we report the main information and conclusions, and the new thermodynamic values that have been determined in [START_REF] Vercouter | Complexes aqueux de lanthanides (III) et actinides (III) avec les ions carbonates et sulfates. Etude thermodynamique par spectrofluorimétrie Laser résolue en temps et spectrométrie de masse à ionisation électrospray[END_REF] and Vercouter et al. (2005a) from the reinterpretation of the batch experiments at different temperatures in 4 mol/L NaCl in [START_REF] Giffaut | Influence des ions chlorures sur la chimie des actinides, Influence de la radiolyse et de la température[END_REF]. First, the bicarbonate and carbonate concentrations in the batch solutions were corrected for the possible modification of the CO2(g) partial pressure in equilibrium with the solutions by contact with air during the experiments that lasted several weeks. Because of the lack of XRD analyses, the nature of the solubility-controlling phase can only be determined from the shape of the solubility curves described by the data points. Consequently, different interpretations were tested, based on the nature of the solubility-controlling solid phase and its possible changes during the experiments in order to explain the observed variation of Am(III) solubility at each temperature. The temperature of the batch solutions was varied step by step from 70°C down to 20°C (phase 1), and then up to 70°C again (phase 2). The data at 30 and 70 °C illustrate the results obtained during the two phases of the temperature change (Figure 4).

The Am(III) concentrations measured in phase 1 and phase 2 describe similar solubility curves, which supports that equilibrium was achieved, and that the solid phases had not significantly been altered by the temperature cycle, except maybe for measurements at [CO3 2-] > 0.1 mol/L at 70 °C comparing the results from the phase 1 and 2 (Figure 4b). Two main interpretation models were considered: i) a control of the solubility either by Am2(CO3)3(s) or NaAm(CO3)2(s) depending on the carbonate content; ii) a control of the solubility by AmOHCO3(s). According to the representation of data shown in Figure 4 a control of solubility by Am2(CO3)3(s) and NaAm(CO3)2(s) cannot be ruled out compared to the former interpretation of a control by AmOHCO3(s) [START_REF] Vercouter | Complexes aqueux de lanthanides (III) et actinides (III) avec les ions carbonates et sulfates. Etude thermodynamique par spectrofluorimétrie Laser résolue en temps et spectrométrie de masse à ionisation électrospray[END_REF]in Supp. Info. of Vercouter et al., 2005a). From this alternative interpretation, another set of thermodynamic parameters have been proposed for both complexation and dissolution reactions. The AmCO3 + , Am(CO3)2 -, and 12/41 Am(CO3)3 3-stoichiometries have been confirmed as in the original works, but their stepwise formation constants, enthalpy, and entropy were significantly different.

-12 A good agreement is obtained with the Cm(III) data in Table 1 measured by TRLFS at high Na + total concentration between 10°C and 70 °C (Vercouter et al., 2005a). From the TRLFS spectra, only one contribution was evidenced for -1.0 ≤ log10[CO3 2-] ≤ 0.3, and the modification of spectrum was only occurring for lower [CO3 2-] values. The stepwise constant proposed for equilibrium Cm(CO3)2 -+ CO3 2-⇄ Cm(CO3)3 3- 13/41 was log10K3(3 mol/L) = 2.01 ± 0.05, with an extrapolation to zero ionic strength using the specific interaction theory (SIT, [START_REF] Grenthe | Chemical Thermodynamics 1: Chemical Thermodynamics of Uranium[END_REF] of log10K°3 = 0.9 ± 0.1. This value is similar to log10K°3 = 0.9 ± 0.2 obtained for Am(III) from the re-interpreted solubility data in 4 mol/L NaCl (Table 1). Hence, the value of the cumulative constant for the equilibrium Cm 3+ + 3CO3 2-⇄ Cm(CO3)3 3- is proposed to be log10β°3 = 13.84 ± 0.64 instead of 15 ± 1 selected in [START_REF] Guillaumont | Chemical Thermodynamics 5: Update on the Chemical Thermodynamics of Uranium, Neptunium, Americium and Technetium[END_REF].

-11 -10 -9 -8 -7 -6 -5 -4 -6 -5 -4 -3 -2 -1 0 1 -12 -11 -10 -9 -8 -7 -6 -5 -4 -6 -5 -4 -3 -2 -1 0 1 log 10 [CO 3 2- ] Am 2 (CO 3 ) 3 (s) N a A m ( C O 3 ) 2 ( s ) AmOHCO 3 (s) (b) log 10 [CO 3 2- ] log 10 [Am] + A log 10 [Am] Am 2 (CO 3 ) 3 (s) N a A m ( C O 3 ) 2 ( s ) AmOHCO 3 (s) (a) log 10 [Am] + A log 10 [Am] -12 -11 -10 -9 -8 -7 -6 -5 -4 -6 -5 -4 -3 -2 -1 0 1 -12 -11 -10 -9 -8 -7 -6 -5 -4 -6 -5 -4 -3 -2 -1 0 1 log 10 [CO 3 2- ] Am 2 (CO 3 ) 3 (s) N a A m ( C O 3 ) 2 ( s ) AmOHCO 3 (s) (b) log 10 [CO 3 2- ] log 10 [Am] + A log 10 [Am] Am 2 (CO 3 ) 3 (s) N a A m ( C O 3 ) 2 ( s ) AmOHCO 3 (s) (a) log 10 [Am] + A log 10 [Am]
Table 1. Thermodynamic data at 25 °C extrapolated to I = 0, and for 4 mol/L NaCl (Am) [START_REF] Giffaut | Influence des ions chlorures sur la chimie des actinides, Influence de la radiolyse et de la température[END_REF] reinterpreted in (Vercouter et al., 2005a), and for 3 mol/L NaClO4 (Cm) (Vercouter et al., 2005a). 4.9±0.5 (++) 3.8±0.1 -21.5±0.6 -5.7±12.1 -1340±560 53±29 Am(CO3)2 -+ CO3 2-⇄ Am(CO3)3 3-0.9±0.2 (*) 1.2±0.3 (**) 2.9±0.5 (+) 2.1±0.8 (++) 2.0±0.1 -11.5±0.6 10±10 72±23

I = 3 mol/L NaClO4 Cm(CO3)2 -+ CO3 2-⇄ Cm(CO3)3 3- 0.9±0.1 (*)
2.01±0.05 -11.5±0.3 12.2±4.4 79±16

(*) Extrapolation to zero ionic strength of log10K(25°C) values derived from data at different temperatures, using SIT parameter from [START_REF] Silva | Chemical Thermodynamics 2: Chemical Thermodynamics of Americium with an Appendix on Chemical Thermodynamics of Uranium (Grenthe[END_REF].

(**) Extrapolation to zero ionic strength using the values obtained in 3 mol/L NaClO4, 0.1 mol/L and 4 mol/L NaCl at 22±2°C [START_REF] Vercouter | Complexes aqueux de lanthanides (III) et actinides (III) avec les ions carbonates et sulfates. Etude thermodynamique par spectrofluorimétrie Laser résolue en temps et spectrométrie de masse à ionisation électrospray[END_REF]). Stepwise formation constants derived from americium (III) solubility data have been fitted

with the SIT formula (Table 1). The log10K2° values are in fairly good agreement with the values determined from solubility studies at ambient temperature [START_REF] Vercouter | Complexes aqueux de lanthanides (III) et actinides (III) avec les ions carbonates et sulfates. Etude thermodynamique par spectrofluorimétrie Laser résolue en temps et spectrométrie de masse à ionisation électrospray[END_REF], as well as with values selected in NEA-TDB reviews (Table 1). However, the log10K3° value differs significantly from the NEA selection. The possible reasons of such discrepancy are discussed 14/41 in more details in Vercouter et al. (2005a). In particular, [START_REF] Guillaumont | Chemical Thermodynamics 5: Update on the Chemical Thermodynamics of Uranium, Neptunium, Americium and Technetium[END_REF] selected Am(CO3)3 3-as the highest carbonato complex of Am(III), but they also discussed values from TRLFS experiments on Cm(III) interpreted with Cm(CO3)4 5-as the limiting complex (Fanghänel et al., 1998;[START_REF] Ekberg | Sensitivity analysis and simulation uncertainties in predictive geochemical modelling[END_REF]. Solubility data on Am(III) and Eu(III), analysed with straight lines whose slopes are related to the stoichiometries of the aqueous complexes at equilibrium, support the stability of tricarbonato complexes as limiting complexes [START_REF] Vercouter | Complexes aqueux de lanthanides (III) et actinides (III) avec les ions carbonates et sulfates. Etude thermodynamique par spectrofluorimétrie Laser résolue en temps et spectrométrie de masse à ionisation électrospray[END_REF]Vercouter et al., 2005a;Vercouter et al., 2005c). For instance, the slope analysis of the solubility curves of Am(III) measured at different temperatures does not evidence the presence of Am(CO3)4 5-(Figure 4), in agreement with the NEA review panel's statement for Am(III). The same conclusion was made for Eu(III) for which only maximum possible values of log10K4 could be determined (Vercouter et al., 2005c).

A repartition of Am(III) species using the present value of log10K3° is shown in Figure 5. It appears clearly that under this hypothesis the tricarbonato americium (III) complex exists in a more restricted domain than predicted with the NEA selected values (see Figure 3b). -3.46 atm, with the selected values of [START_REF] Guillaumont | Chemical Thermodynamics 5: Update on the Chemical Thermodynamics of Uranium, Neptunium, Americium and Technetium[END_REF], except log10K°3 = 0.9 for the stepwise formation of Am(CO3)3 3-from Vercouter et al. (2005a).

15/41

Consequently, TRLFS data on Cm(III) have been interpreted considering an analogous behaviour, on the contrary to what was proposed by Fanghänel et al. (1998;[START_REF] Ekberg | Sensitivity analysis and simulation uncertainties in predictive geochemical modelling[END_REF]. Conflicting interpretations of TRLFS data for Cm(III) between Fanghänel et al. (1998;[START_REF] Ekberg | Sensitivity analysis and simulation uncertainties in predictive geochemical modelling[END_REF] and Vercouter et al. (2005a), may come from differences in the fitting hypotheses, i.e.

stoichiometries of complexes, or from additional effects related to the high ionic strength of the solution, which induces variations of activity coefficients, especially for highly charged complexes. This discrepancies may be resolved by focusing on the aqueous complexes that would be stable in concentrated carbonate solutions, by examining relevant studies on Am(III), Cm(III), and Ln(III). The stability of the limiting carbonate complexes M(CO3)3 3-or M(CO3)4 5-, in the f-block series for lanthanides and actinides is a question of debate, and both stoichiometries have been considered in experimental studies. Some authors proposed the formation of (Eu or Cm)(CO3)4 3-, which are more or less in the middle of the f-series, from solubility studies [START_REF] Faucherre | Préparation à l'état solide et structure en solution des carbonates complexes des lanthanides[END_REF], from spectroscopic observations (Fanghänel et al., 1998), or from solvent extraction interpretation [START_REF] Rao | Studies on stability-constants of europium(III) carbonate complexes and application of SIT and ion-pairing models[END_REF]. (about 2) seemed to be consistent with the formation of Eu(CO3)3 3-, but the inherent uncertainty of this analysis (± 0.5 [START_REF] Kimura | Determination of the hydration number of Cm(III) in various aqueous solutions[END_REF] did not permit to fully ascertain the stoichiometry. The solubility curve determined as a function of the CO3 2-concentration was actually a straight line in the molar concentration range -2 ≤ log10[CO3 2-] ≤ 0, whose slope was consistent with n = 3, that is with a dominance of Eu(CO3)3 3-in solution.

According to previous solubility measurements of KEu(CO3)2•5H2O(s), the complex Eu(CO3)4 5- was apparently stable in the molar concentration range -2 ≤ log10[CO3 2-] ≤ 0 and in K + media, supposedly through the stabilization of the highly charged complex by ion-pairing [START_REF] Faucherre | Préparation à l'état solide et structure en solution des carbonates complexes des lanthanides[END_REF]. However, a more likely explanation would be that the solid-solution system did not reach the equilibrium after the 24 h contacting time, leading to an apparent slope of the solubility curve (Vercouter et al., 2005c;[START_REF] Philippini | Evidence of different stoichiometries for the limiting carbonate complexes across the lanthanide(III) series: A capillary electrophoresis-mass spectrometry study[END_REF]. Interestingly, other solubility measurements for Ce(III) support the stoichiometry Ce(CO3)4 5-in concentrated carbonate solutions [START_REF] Ferri | Studies on metal carbonate equilibria.5. The cerium(III) carbonate complexes in aqueous perchlorate media[END_REF]. Recently, [START_REF] Philippini | Evidence of different stoichiometries for the limiting carbonate complexes across the lanthanide(III) series: A capillary electrophoresis-mass spectrometry study[END_REF]2010) investigated the stability of limiting carbonate complexes of Ln(III) by solubility method and TRLFS [START_REF] Philippini | Evidence of different stoichiometries for the limiting carbonate complexes across the lanthanide(III) series[END_REF] and by capillary electrophoresis hyphenated to ICP-MS [START_REF] Philippini | Evidence of different stoichiometries for the limiting carbonate complexes across the lanthanide(III) series: A capillary electrophoresis-mass spectrometry study[END_REF], and confirmed that both Ln(CO3)3 3-and Ln(CO3)4 5-could be stable: The tetra-carbonato complex could only be significantly formed with sufficiently big Ln 3+ ions, i.e. in the beginning of the lanthanide series, in agreement with the solubility results on Ce(III) [START_REF] Ferri | Studies on metal carbonate equilibria.5. The cerium(III) carbonate complexes in aqueous perchlorate media[END_REF].

Through these studies, it appears that the stoichiometry of the limiting carbonate complexes mainly depends on the size of the metal ion as observed along the Ln(III) series, which suggest to be cautious with the use of analogies in these particular cases: For Am(III) and Cm(III), the limiting complex would be a tricarbonato complex by analogy with Eu(III) and a tetracarbonato complex by analogy to Nd(III). The difference in the stabilisation energy between the two stoichiometries is weak, and close to experimental or data interpretation uncertainties.

The phosphate system

The review of the panel of experts of the OECD NEA-TDB project in charge of thorium thermodynamic data [START_REF] Rand | Chemical Thermodynamics 11: Chemical Thermodynamics of Thorium[END_REF] was finally issued after the end of the FUNMIG project. In the aforementioned review, a thorough revision of the available experimental data on the complexation between Th(IV) and phosphate is presented. The authors comment on the different data available in the literature, obtained either by liquid-liquid extraction or by solubility measurements. As already mentioned, solubility experiments present important drawbacks related to the ambiguous characterization of the solid phase, so that the existing data were not selected as sufficiently reliable by the review panel. A publication is under preparation on the EXAFS characterization of gel formation at higher Th and phosphate concentrations (Ekberg et al., in prep), what also complicates interpretation of experimental data if not considered. [START_REF] Rand | Chemical Thermodynamics 11: Chemical Thermodynamics of Thorium[END_REF] comment on the ambiguity of the data presented by [START_REF] Fourest | Solubility of thorium phosphate-diphosphate[END_REF], which is one of the few works that present data at circumneutral and alkaline pH ranges. Östhols (1995) also measured solubility data, in this case of a presumably well characterized ThO2(cr), although under the used experimental conditions the authors proposed that the aqueous speciation seemed to be dominated by hydroxo-phosphate complexes and therefore the stability of the phosphate complexes could not be obtained. Östhols (1995) also pointed out that the fit of the experimental data was rather poor and that the ThO2 surface contained phosphor after the solubility experiment. Although more than 10 experimental studies are reviewed, the review panel selected data from [START_REF] Zebroski | Thorium complexes with chloride, fluoride, nitrate, phosphate and sulfate[END_REF] as the most extensive research work on the complex formation in the Th(IV)-phosphate system [START_REF] Rand | Chemical Thermodynamics 11: Chemical Thermodynamics of Thorium[END_REF]. The corresponding selection of data from [START_REF] Zebroski | Thorium complexes with chloride, fluoride, nitrate, phosphate and sulfate[END_REF], performed at important H + activity, i.e., 0.25 ≤ a H + mol/L ≤ 2 and I = 0.2 mol/L (Na,H)ClO4, leads to the stability constants of the different complexes formed between Th 4+ and H3PO4(aq) shown in Table 2.

Th 4+ + HPO4 2-⇄ ThHPO4 2+ log10β1 (1 M) ≈ 9 log10β°1 ≈ 12 (extrapolated * ) Th 4+ + 2HPO4 2-⇄ Th(HPO4)2(aq) log10β1 (1 M) =17±3.5 log10β°2 = 21.5±3.5 (extrapolated * ) Th 4+ + 3HPO4 2-⇄ Th(HPO4)3 2- log10β1 (1 M) = 23.7 ± 0.2 log10β°3 = 28.3 ± 0.3 (extrapolated * )
* Extrapolation to zero ionic strength of log10β(1 M) values using the ion specific interaction coefficients values in [START_REF] Rand | Chemical Thermodynamics 11: Chemical Thermodynamics of Thorium[END_REF] for ε(Na + ,HPO4 2-) = -0.15 ± 0.06, and ε(Th 4+ ,ClO4 -) = 0.7 ± 0.1. The values of ε(ThHPO4 2+ , ClO4 -) = 0.4 ± 0.1 kg/mol is taken from ε(Th(H2PO4)2 2+ , ClO4 -) in [START_REF] Rand | Chemical Thermodynamics 11: Chemical Thermodynamics of Thorium[END_REF], and ε(Na + , Th(HPO4)3 2-) = -0.1 ± 0.2 kg/mol from ε(Na + , Th(OH)2(CO3)2 2-) and ε(Na + , Th(OH)4CO3 2-) in [START_REF] Rand | Chemical Thermodynamics 11: Chemical Thermodynamics of Thorium[END_REF], respectively. Auxiliary data on phosphate hydrolysis are from [START_REF] Rand | Chemical Thermodynamics 11: Chemical Thermodynamics of Thorium[END_REF].

This speciation scheme leads to the aqueous distribution of species shown in Figure 6a, using the SIT, for total phosphate concentration of 10 -4 mol PO4/L, from where it can be seen that In many cases, studies of extraction and distribution between two immiscible liquids are performed by batch techniques. After separation, samples are taken in each individual phase and analyzed for their metal content. This method can induce important difficulties to the process in case of multisampling and for this reason [START_REF] Ekberg | Complexation of thorium with phosphate[END_REF] used the AKUFVE technique. This technique consists on having two phases (usually an aqueous solution and an organic solution) in equilibrium with each other while each phase at the same time is continuously available for on-line analysis [START_REF] Rydberg | Solvent extraction studies by AKUFVE method.1. Principle and general problems[END_REF]. From the analyses, it was concluded that the phosphate species participating in the complexation with Th(IV) was HPO4 2-at pH 8 under the experimental conditions. Therefore this result complements the thermodynamic data selected by [START_REF] Rand | Chemical Thermodynamics 11: Chemical Thermodynamics of Thorium[END_REF] and provides parameters needed to calculate the speciation of Th(IV) and phosphate at circumneutral pH values. 20/41 [START_REF] Ekberg | Complexation of thorium with phosphate[END_REF] evaluated stability constants of the 1:1, 1:2 and 1:3 complexes formed in 1 mol/L (Na,H)ClO4 at 15, 25 and 35 °C and at pH 7, 8 and 8.7 with enthalpy and entropy determined with the obtained stability constants for the 1:2 and 1:3 complexes.

The stability constants obtained by these authors at 1 mol/L (Na,H)ClO4 ionic strength, extrapolated to infinite dilution by the SIT are shown in Table 2. The first constant log10β°1 is only an approximation as the experimental set-up did not permit to obtain low enough competing ligand concentration. Therefore, this constant was rather determined by extrapolation of log10β3 and log10β2.

Extrapolation using SIT to log10β° has been done with the specific interaction coefficient selected in [START_REF] Rand | Chemical Thermodynamics 11: Chemical Thermodynamics of Thorium[END_REF] for H + , Th 4+ , and HPO4 2-, i.e. ε(H + , ClO4 -) = 0.14 ± 0.02 kg/mol, ε(Th 4+ , ClO4 -) = 0.7 ± 0.1 kg/mol, ε(Na + , HPO4 2-) = -0.15 ± 0.06 kg/mol. The values of specific ion interaction coefficient are estimated via analogy to be ε(ThHPO4 2+ , ClO4 -) = 0.4 ± 0.1 kg/mol -from Th(H2PO4)2 2+ in [START_REF] Rand | Chemical Thermodynamics 11: Chemical Thermodynamics of Thorium[END_REF] -, and ε(Na + , Th(HPO4)3 2-) = -0.1 ± 0.2 kg/mol -from Th(OH)2(CO3)2 2-and Th(OH)4CO3 2-in [START_REF] Rand | Chemical Thermodynamics 11: Chemical Thermodynamics of Thorium[END_REF], also in agreement with ε(Na + , Th(SO4)3 2-) = -0.091 ± 0.038 -, respectively.

The resulting speciation is shown in Figure 6b, using the SIT, as function of pH at 0.1 mol/L (NaClO4) and [PO4]total = 10 -4 mol/L. Under these conditions, the phosphate complexes proposed by [START_REF] Ekberg | Complexation of thorium with phosphate[END_REF] would dominate over the hydroxo complexes of Th(IV) in a larger pH range.

It can be seen that the speciation changes completely. The adequacy or reliability of the constants obtained by [START_REF] Ekberg | Complexation of thorium with phosphate[END_REF] cannot be tested in front of analogue elements data given that no data for tetravalent actinide aqueous phosphate are available in the literature under circumneutral pH conditions, as pointed out by [START_REF] Rand | Chemical Thermodynamics 11: Chemical Thermodynamics of Thorium[END_REF]. The importance of ThHPO4 2+ can also be questioned as its log10β°1 value is only estimation. constants at I = 3 mol/L [START_REF] Vercouter | Stoichiometries and thermodynamic stabilities for aqueous sulfate complexes of U(VI)[END_REF]. Although it looks consistent with the NEA selected value, it is not obvious that the simple SIT equation can be applied to a concentrated sulphate medium [START_REF] Ciavatta | On the complex formation equilibria between dioxouranium(VI) and sulfate ions[END_REF].

The enthalpy of the reaction for the stepwise constant of the third complex was determined in [START_REF] Vercouter | Stoichiometries and thermodynamic stabilities for aqueous sulfate complexes of U(VI)[END_REF]. A value of ΔrH°m = 11 ± 8 kJ/mol was obtained, showing that the coordination of uranyl by a third sulphate ligand is less endothermic than the coordination by the second sulphate, which is in turn less endothermic than the coordination by the first sulphate ligand. This is a usual trend for actinide complex formation with inorganic anions.

The enthalpy values suggest the possibility of sulphate anions acting as monodentate and bidentate ligands towards UO2 2+ in the third complex, as it happens in the case of the uranyl acetate [START_REF] Vercouter | Stoichiometries and thermodynamic stabilities for aqueous sulfate complexes of U(VI)[END_REF].

Lanthanide/Actinide-Sulphate system

The complexation of lanthanides (III) by sulphate anions Ln 3+ + nSO4 2-⇄ Ln(SO4)n 3-2n was also studied in the framework of a clayey interstitial water (Vercouter et al., 2005b).

Similarly to the carbonate system, differences in Am(III) sulphate selected data in a former NEA review [START_REF] Silva | Chemical Thermodynamics 2: Chemical Thermodynamics of Americium with an Appendix on Chemical Thermodynamics of Uranium (Grenthe[END_REF] and its update [START_REF] Guillaumont | Chemical Thermodynamics 5: Update on the Chemical Thermodynamics of Uranium, Neptunium, Americium and Technetium[END_REF]. In [START_REF] Guillaumont | Chemical Thermodynamics 5: Update on the Chemical Thermodynamics of Uranium, Neptunium, Americium and Technetium[END_REF], it is estimated that ion-exchange and solvent extraction data, which formed the basis of the former selection, are probing different phenomena as compared to spectroscopic techniques, due to the existence of both outer-and inner-sphere complexes. This argument can be questioned and was discussed in depth in Vercouter et al. (2005b). Nevertheless, the update review has preferentially used data on Cm(III) in NaCl [START_REF] Paviet | Thermodynamics of curium(III) in concentrated electrolyte solutions: Formation of sulfate complexes in NaCl/Na2SO4 solutions[END_REF][START_REF] Neck | Aquatische Chemie und Thermodynamische Modellierung von Trivalenten Actiniden[END_REF] for monosulphato and disulphato complexes of americium(III).

The sulphate complexation of the analogous Eu(III) was therefore investigated by TRLFS at I ≤ 2.2 mol/kg (Na2SO4, NaClO4), and the sulphate complexation of La(III) was also checked by nano-electrospray ionization mass spectroscopy at low ionic strength (HNO3, H2SO4). Since the TRLFS analysis provided a more complete set of results, only these data are reported here.

The authors tested different modelling of their data, and concluded that the best fit was obtained when considering two inner sphere complexes EuSO4 + and Eu(SO4)2 -, leading to log10β°1 = 3.78 ± 0.06 and log10K°2 = 1.5 ± 0.2, and hence log10β°2 = 5.3 ± 0.2. These values compare well with other experimental values for Eu(III) and also with the Am(III) data selected in [START_REF] Silva | Chemical Thermodynamics 2: Chemical Thermodynamics of Americium with an Appendix on Chemical Thermodynamics of Uranium (Grenthe[END_REF], i.e., log10β°1 = 3.85 ± 0.03 and log10β°2 = 5.4 ± 0.7, suggesting that the analogy between Eu(III) and Am(III) is relevant. These data predict a higher stability of the complexes compared with that predicted with the selected data in [START_REF] Guillaumont | Chemical Thermodynamics 5: Update on the Chemical Thermodynamics of Uranium, Neptunium, Americium and Technetium[END_REF] based on log10β°1 = 3.30 ± 0.15 and log10β°2 = 3.70 ± 0.15. These differences can be seen in Figure 7a,b at varying sulphate concentration in acidic medium (I = 3 mol/L), using the SIT. The differences are mainly due to the stability of the disulphato americium(III) complex. In a water composition more representative of an argillaceous water [START_REF] Gaucher | A robust model for pore-water chemistry of clayrock[END_REF], the differences in AmSO4 + concentrations can be viewed in Figure 7c,d where the proportion in monosulphato americium (III) complex is halved between the former review [START_REF] Silva | Chemical Thermodynamics 2: Chemical Thermodynamics of Americium with an Appendix on Chemical Thermodynamics of Uranium (Grenthe[END_REF] and the latter [START_REF] Guillaumont | Chemical Thermodynamics 5: Update on the Chemical Thermodynamics of Uranium, Neptunium, Americium and Technetium[END_REF]. The data obtained within FUNMIG tends to be in favour of the former selection, which supports the analogy between Eu(III) and Am(III). The analogy between Am(III) with Cm(III) was already apparent in [START_REF] Silva | Chemical Thermodynamics 2: Chemical Thermodynamics of Americium with an Appendix on Chemical Thermodynamics of Uranium (Grenthe[END_REF] from ion-exchange and solvent extraction studies, and it should also be noted that these values of complexation constants are also very similar to what was selected for Pu(III) [START_REF] Lemire | Chemical Thermodynamics 4: Chemical Thermodynamics of Neptunium and Plutonium[END_REF][START_REF] Guillaumont | Chemical Thermodynamics 5: Update on the Chemical Thermodynamics of Uranium, Neptunium, Americium and Technetium[END_REF]. (Gaucher et al., 2009) (c, d, down).

The silicate system

The silicate system was studied within the FUNMIG project in order to settle difficulties of retention experiments interpretation in clay media. One of the outcomes of this work was a review of the literature data (Vercouter et al., 2009c) where some of the main inconsistencies and gaps were identified. Among others, the authors remarked the difficulty in studying the AmOSi(OH)3 + HCO3 -⇄ AmCO3 + + Si(OH)4(aq) log10K° = 0.85 AmOSi(OH)3 2+ + 2H2O ⇄ Am(OH)2 + + Si(OH)4(aq) + H + log10K° = -13.42

At the considered ionic strength, for A or B values higher than K[AmOSi(OH)3 2+ /AmCO3 + ] and K[AmOSi(OH)3 2+ /Am(OH)2 + ], respectively, the silicate species dominates. For A or B values lower than the K values, the carbonato or hydroxo species dominate. Noteworthy is the fact that using the log10K°[Cm 3+ /Cm(CO3)3 3-] proposed by Vercouter et al. (2005a) would draw the Am(CO3)2 -/Am(CO3)3 3-border out of the limit of Figure 8b.

From the previous Figures it seems clear that the complexation of Am(III), and analogous trivalent elements, with silicate may influence the mobility and interactions in, for example, the vicinity of the clay buffer material installed in the repository to buffer water intrusion. (2003) in the carbonate-silicate domain at pH 7 (a, left) and in the carbonate-pH domain (b, right) for waters equilibrated with quartz (plain line, [START_REF] Guillaumont | Chemical Thermodynamics 5: Update on the Chemical Thermodynamics of Uranium, Neptunium, Americium and Technetium[END_REF], and Mg-Montmorillonite-Na (dotted line, [START_REF] Blanc | Thermochimie: Estimation des entropies, capacités calorifiques et volumes molaires des phyllosilicates deshydratés et hydratés[END_REF], at I = 0.1 mol/L NaClO4.

Am(CO3)n 2-n .

28/41

One of the made during the experimental program of the project was the use of novel techniques to establish structure and elucidate different structural rearrangements in solution. One example of this advance is the use of electrospray ionisation mass spectrometry (ESI-MS). ESI-MS is based on a soft ionization process that keeps ions largely intact, and generates a mass spectrum from which species can be identified by the mass-to-charge ratio of the parent complex ion. This technique has recently succeeded in detecting silicate oligomeric species formed in aqueous solutions containing structure-directing templates for the synthesis of zeolites or mesoporous structures [START_REF] Bussian | Speciation in solution: Silicate oligomers in aqueous solutions detected by mass spectrometry[END_REF][START_REF] Eggers | Mass spectrometric investigation of small silicate polyhedra in solution[END_REF][START_REF] Pelster | Detailed study on the use of electrospray mass spectrometry to investigate speciation in concentrated silicate solutions[END_REF]. Vercouter et al. (2009a) studied the complexation of Eu(III) with orthosilicic acid by TRLFS in acidic conditions. Although the solutions were undersaturated with respect to amorphous silica, where monomeric Si(OH)4 is expected to be the only Si species, the data could not be interpreted only considering the EuOSi(OH)3 2+ complex, and suggested the presence of polymeric or colloidal particles interacting with the Eu 3+ ions. These observations were similar to the ones reported in the case of Cm(III) solutions in similar conditions [START_REF] Panak | Complexation of Cm(III) with aqueous silicic acid[END_REF]. The negative mode of ESI-MS was used to check the presence of oligomeric silicates in the solutions. The ionization process was considered to be soft enough to limit dehydroxylation reactions during the formation of the spray. Figure 9 shows an ESI mass spectrum of a 10 -3 mol/L silicate solution at pH 5, where the different peaks are attributed to the different fragments indicated in the inserts. A number of oligomeric silicate species were identified, which may be related to their presence in the aqueous solution. When increasing the pH of the aqueous solutions, the peak intensities of the monomeric silicates decrease whereas the peak intensities of the oligomeric species increase. Such a trend is also expected in aqueous solutions, which supports that ESI-MS can effectively probe the distribution of silicate species in solution. But, these oligomeric species are not supposed to be prominent predicted, and the obtained by [START_REF] Thakur | Polymerization study of o-Si(OH)4 and complexation with Am(III), Eu(III) and Cm(III)[END_REF] for Eu, Cm, and Am are significantly higher than the ones obtained by following the recommendation in [START_REF] Guillaumont | Chemical Thermodynamics 5: Update on the Chemical Thermodynamics of Uranium, Neptunium, Americium and Technetium[END_REF].

The resulting range of possible values for this log10β°1 is thus from -2.2 (adjusting to [START_REF] Panak | Complexation of Cm(III) with aqueous silicic acid[END_REF] to -1.0 (adjusting to [START_REF] Thakur | Polymerization study of o-Si(OH)4 and complexation with Am(III), Eu(III) and Cm(III)[END_REF]. The NEA selected value appears to be a reasonable mean of these estimations. To account for this distribution, it would then be necessary to increase the log10β°1 uncertainty from 0.18 to at least 0.5. [START_REF] Wadsak | Formation of americium(III) complexes with aqueous silicic acid[END_REF][START_REF] Thakur | Polymerization study of o-Si(OH)4 and complexation with Am(III), Eu(III) and Cm(III)[END_REF], Cm(III) (square: [START_REF] Steinle | Komplexierung von Cm (III) mit monokieselsäure[END_REF][START_REF] Panak | Complexation of Cm(III) with aqueous silicic acid[END_REF][START_REF] Pathak | Thermodynamic study of metal silicate complexation in perchlorate media[END_REF], and Eu(III) (circle [START_REF] Jensen | Complexation of europium(III) by aqueous orthosilicic acid[END_REF][START_REF] Pathak | Thermodynamic study of metal silicate complexation in perchlorate media[END_REF][START_REF] Thakur | Polymerization study of o-Si(OH)4 and complexation with Am(III), Eu(III) and Cm(III)[END_REF] as a function of the experimental ionic strengths. The theoretical curve from [START_REF] Guillaumont | Chemical Thermodynamics 5: Update on the Chemical Thermodynamics of Uranium, Neptunium, Americium and Technetium[END_REF] is represented with the corresponding uncertainties (dashed line).

Conclusions, perspectives and prospective

The contribution of FUNMIG to deepen in the knowledge of thermodynamic properties of radionuclides was focused on the complexation of lanthanides and actinides by carbonate, sulphate, and silicate. Some of the values selected in NEA-TDB reviews have been confirmed while others are proposed to be modified. These new values can be used to calculate the aqueous speciation of actinides under conditions relevant for the long-term storage of radioactive wastes.

Americium was one of the actinides whose thermodynamic data was more extensively studied within the project. In Figure 11 the predominance diagram, of the americium species at pH 7 and total sulphate concentration between 14 and 40 mM is obtained using Phreeplot. These values are representing the lowest value for which AmSO4 + is prominant over Am 3+ , and the solubility limit in clay environment [START_REF] Gaucher | A robust model for pore-water chemistry of clayrock[END_REF]. Sulphate concentration can be significantly lower, e.g. between 0.01 up to 20 mM [START_REF] Degueldre | Study of the pore water chemistry through an argillaceous formation: A paleohydrochemical approach[END_REF][START_REF] Beaucaire | Groundwater characterisation and modelling of water-rock interaction in an argillaceous formation (Tournemire, France)[END_REF]. It can be seen that, depending on the total silicate concentration, the situation varies.

At pH 7, -2 ≤ log10[ total CO3] ≤ -3 and -5 ≤ log10[Si(OH)4(aq)] ≤ -4, i.e. in equilibrium with a clayey environment, and I = 0.1 mol/L, the coexistence of at least three different monocomplexes is predicted. If the silicate concentration is controlled by SiO2(am), i.e., log10[Si(OH)4(aq)] = -2.7 [START_REF] Gunnarsson | Amorphous silica solubility and the thermodynamic properties of H4SiO4° in the range of 0° to 350° C at Psat[END_REF], then the americium speciation should even be more controlled by the silicate system. In addition, it seems that oligomeric silicates may be present in rather acidic media, as observed in the framework of the project by ESI-MS. These oligomeric silicates seem to influence the speciation of actinides(III) [START_REF] Panak | Complexation of Cm(III) with aqueous silicic acid[END_REF].

32/41 [Si(OH)4(aq)] and total carbonate. Thermodynamic data from [START_REF] Guillaumont | Chemical Thermodynamics 5: Update on the Chemical Thermodynamics of Uranium, Neptunium, Americium and Technetium[END_REF], except for Am(SO4)n 3-2n from [START_REF] Silva | Chemical Thermodynamics 2: Chemical Thermodynamics of Americium with an Appendix on Chemical Thermodynamics of Uranium (Grenthe[END_REF].

Only the carbonate, sulphate, and silicate complexes are considered in this exercise. Other inorganic complexes, as phosphate, or natural or anthropogenic organic complexes potentially affecting the current speciation were not taken into consideration. It is noteworthy that very little information are known for the temperature dependence of the stability of AmCO3 + and AmOSi(OH)3 2+ complexes although they are relevant stoichiometries as shown in Figure 11.

The in the thorium phosphate system indicates the formation of this type of complexes for pH values higher than the ones in the previous literature data, and allows the calculation of the effect of phosphate in the behaviour of thorium in pH of interest for nuclear waste management studies. The impact that the formation of these species may have on the solubility of thorium at pH 8 and I = 0.1 mol NaClO4/L is shown in Figure 12. It can be seen that a concentration of phosphate higher that 10 -4 is needed to have a measurable effect confirming the assumption of Östhols (1995). Enthalpies of reaction have also been estimated.

The results obtained from the study of the complexation of uranyl by sulphate at different temperatures permitted the estimation of ΔrH of the three UO2(SO4)n 2-2n complexes. 2 on the solubility at pH 8 and I = 0.1 mol NaClO4 /L of ThO2 (am,hyd,aged) (dotted) and ThO2(am,hyd,fresh) (plain) from [START_REF] Rand | Chemical Thermodynamics 11: Chemical Thermodynamics of Thorium[END_REF].

In these works, the combination of different techniques, liquid-liquid extraction, solubility measurements, TRLFS, and also ESI-MS, has allowed to complete thermodynamic data bases with experimental determinations. Some promising results have been obtained, in some cases

Figure 1 .

 1 Figure 1. Example of inventory of a SNF[START_REF] Nirex | Radionuclide content for a range of irradiated fuels[END_REF].

Figure 2 .

 2 Figure 2. Percent of soluble species of americium(III) in equilibrium with CaMg(CO3)2 in an

Figure 3 .

 3 Figure 3. Differences induced in the speciation diagram of trace Am(III) at I = 1 mol/L, and pCO2

Figure 4 .

 4 Figure 4. Solubility of Am(III), [Am], in bicarbonate and carbonate solutions with 4 mol/L NaCl

  (+) [START_REF] Silva | Chemical Thermodynamics 2: Chemical Thermodynamics of Americium with an Appendix on Chemical Thermodynamics of Uranium (Grenthe[END_REF] (++) [START_REF] Guillaumont | Chemical Thermodynamics 5: Update on the Chemical Thermodynamics of Uranium, Neptunium, Americium and Technetium[END_REF]

Figure 5 .

 5 Figure 5. Repartition of species for trace concentration of Am(III) considering I = 1 mol/L, and

Vercouter

  et al. (2005c) studied the equilibrium NaEu(CO3)2•5H2O(s) + (n-2) CO3 2-⇄ Eu(CO3)n 3-2n + Na + + 5H2O by solubility experiments in NaClO4 and NaCl, and by TRLFS. The latter techniques indicated that only one carbonate complex was present at [Na + ]total = 3 mol/L, and that only dissociation of this complex occurs when decreasing [Na + ] and [CO3 2-]. From the Eu(III) fluorescence decay time analysis, the number of remaining water molecules in the first hydration sphere

Figure 6

 6 Figure 6 Solution speciation of the thorium phosphate system at I = 0.1 mol/L (NaClO4), [PO4]tot =

Figure 7 .

 7 Figure7. Differences in the repartition of Am(III) species using Am(SO4)n3-2n constants from(a, c,

Figure 8 .

 8 Figure 8. Predominance diagram of americium(III) aqueous species from Guillaumont et al.

Figure 10 .

 10 Figure 10. Values of the formation constants of the monosilicate complexes of Am(III) (triangle:

Figure 11 .

 11 Figure 11. Predominance diagram of traces of americium(III) at pH 7, total sulphate

Figure 12 .

 12 Figure12. Effect of the Th(HPO4)n4-2n complexes proposed in[START_REF] Ekberg | Complexation of thorium with phosphate[END_REF]

Table 2 .

 2 Formation constants for the different Th-phosphate complexes selected in[START_REF] Rand | Chemical Thermodynamics 11: Chemical Thermodynamics of Thorium[END_REF] and proposed in[START_REF] Ekberg | Complexation of thorium with phosphate[END_REF].

	Equilibrium	log10β°
		Rand et al. (2009)
	Th 4+ + H3PO4 ⇄ ThH3PO4 4+	1.89 ± 0.31
	Th 4+ + H3PO4 ⇄ ThH2PO4 3+ + H +	3.45 ± 0.32
	Th 4+ + 2H3PO4 ⇄ Th(H2PO4)2 2+ + 2H +	6.20 ± 0.32
	Th 4+ + 2H3PO4 ⇄ Th(H3PO4)(H2PO4) 3+ + H + 5.42 ± 0.32
		Ekberg et al. (2010)

2.3.1. Uranium(VI)-Sulphate [START_REF] Vercouter | Stoichiometries and thermodynamic stabilities for aqueous sulfate complexes of U(VI)[END_REF] studied the uranium(VI) sulphate system. They used TRLFS to record the spectra of several solutions containing U(VI)-sulphate complexes. From the analyses of the solutions at 0.1 mol/L Na + ionic medium at 25 °C, stability constants of the two first sulphato complexes of U(VI) were determined and found to be close to the data selected by [START_REF] Guillaumont | Chemical Thermodynamics 5: Update on the Chemical Thermodynamics of Uranium, Neptunium, Americium and Technetium[END_REF] (see Table 3).

The temperature dependence of the reactions was determined from the results of the fluorescence spectra at 6 different temperatures in the interval 10-75 °C. No dependence on the enthalpy of reaction with temperature was assumed, so that rCp = 0. the formation of UO2SO4(aq) was found to be even more endothermic by about 10 kJ/mol compared with the data selected in [START_REF] Guillaumont | Chemical Thermodynamics 5: Update on the Chemical Thermodynamics of Uranium, Neptunium, Americium and Technetium[END_REF]. The formation enthalpy for UO2(SO4)2 2-was found to be similar (Table 3).

Table 3. Formation constants for the uranium(VI)-sulphate complexes

Equilibrium

Thermodynamic data and constants A review of the calorimetric data in the literature showed that discrepancies do exist, especially for the first complexation reaction (Vercouter et al., 2009b). The ΔrH for the monosulphato complex formation proposed in et al. (2008) was however in good agreement with the calorimetric determination by [START_REF] Bailey | Heats of dilution and thermodynamics of dissociation of uranyl and vanadyl sulfates[END_REF] after reinterpretation with the log10β°1 recommended value in [START_REF] Guillaumont | Chemical Thermodynamics 5: Update on the Chemical Thermodynamics of Uranium, Neptunium, Americium and Technetium[END_REF]. Recently, [START_REF] Tian | Spectrophotometric and calorimetric studies of U(VI) complexation with sulfate at (25 to 70) °C[END_REF] carried out calorimetric and spectroscopic measurements at I = 1.0 mol/L that resulted in additional values: ΔrHm cal = 17.7 ± 0.3 and ΔrHm spec = 16.1 ± 1.7 kJ/mol for the formation of UO2SO4(aq), and ΔrHm cal = 25.5 ± 0.8 and ΔrHm spec = 26.9 ± 1.2 kJ/mol for the stepwise formation of UO2(SO4)2 2-. Significant differences with the previous sets can also be noticed (Table 3), which probably emphasizes the difficulty of getting accurate values for the formation of such moderate complexes. The ΔrH°m of the stepwise reactions of sulphate complexation are positive, indicating that the reactions are endothermic and that the main contribution to the enthalpy is the dehydration energy. [START_REF] Vercouter | Stoichiometries and thermodynamic stabilities for aqueous sulfate complexes of U(VI)[END_REF] evidenced the formation of the trisulphato uranyl(VI) complex at high sulphate concentration in 3 mol/L Na + ionic medium. Under these conditions, only pieces of evidence of the third complex were found, while the spectroscopic signatures of the other two complexes could not be clearly obtained. A value of the stepwise constant in 3 mol/L Na + medium was determined for the following equilibrium.

The extrapolated log10β3° was not proposed, because the corresponding SIT parameters are unknown and can be hardly estimated. The authors judged that the extrapolation would incorporate more errors in the calculated magnitude than the analytical error itself.

The value selected by [START_REF] Guillaumont | Chemical Thermodynamics 5: Update on the Chemical Thermodynamics of Uranium, Neptunium, Americium and Technetium[END_REF] for the reaction UO2 2+ + 3SO4 2-⇄ UO2(SO4)3 4- log10β3° = 3.02 ± 0.38 was adopted from the work of [START_REF] Geipel | Uranium(VI) sulfate complexation studied by time-resolved laser-induced fluorescence spectroscopy (TRLFS)[END_REF] as it was the only data available in the literature. The extrapolation to infinite dilution with SIT was made in Guillaumont et al.

system in a that the polymerization of silicic acid is avoided. Most experimental data in the literature has been obtained at acidic pH values, where the observation of the complexes results easier, although it becomes evident that the polymerization of silicic acid occurs even in acidic solutions for concentrations of Si above 2 10 -3 mol/L. In any case, pieces of evidence for the formation of monosilicic complexes of Am(III), Cm(III) and Eu(III) occur, although polymerization of silicates could explain the discrepancies in the published stability constants of more than one order of magnitude. The selected data in [START_REF] Guillaumont | Chemical Thermodynamics 5: Update on the Chemical Thermodynamics of Uranium, Neptunium, Americium and Technetium[END_REF] is log10K° = -1.68 ± 0.18 for the following reaction

The interaction of lanthanides and trivalent actinides with silicates can be of interest in different aspects of the evaluation of the migration of these elements. An illustration of the relative relevance of carbonate versus silicate complexes of Am(III), referring to [START_REF] Guillaumont | Chemical Thermodynamics 5: Update on the Chemical Thermodynamics of Uranium, Neptunium, Americium and Technetium[END_REF], is shown in Figure 8a, using Phreeplot [START_REF] Kinniburgh | Creating graphical output with PHREEQC[END_REF]) and embedded PHREEQC. The dominant aqueous species of Am(III) are represented as a function of silicate and carbonate concentrations at pH 7. It is clear from this representation that either AmCO3 + or AmOSi(OH)3 2+ dominates the speciation depending on the carbonate and silicate content of the water. Using Phreeplot and PHREEQC, in Figure 8b it is assumed that the solution is equilibrated either with quartz or with Mg-Montmorillonite-Na using solubility product from [START_REF] Guillaumont | Chemical Thermodynamics 5: Update on the Chemical Thermodynamics of Uranium, Neptunium, Americium and Technetium[END_REF] and [START_REF] Blanc | Thermochimie: Estimation des entropies, capacités calorifiques et volumes molaires des phyllosilicates deshydratés et hydratés[END_REF], respectively. It implies that the concentration of Si in solution ranges from 10 -5 to 10 -4 M. The concentration range is coherent with actual values in clays [START_REF] Degueldre | Study of the pore water chemistry through an argillaceous formation: A paleohydrochemical approach[END_REF][START_REF] De Craen | Geochemistry of Boom Clay pore water at the Mol site[END_REF][START_REF] Gaucher | Modelling the porewater chemistry of the Callovian-Oxfordian formation at a regional scale[END_REF][START_REF] Beaucaire | Groundwater characterisation and modelling of water-rock interaction in an argillaceous formation (Tournemire, France)[END_REF]. In this case, the parameters A = [Si(OH)4(aq)]/[HCO3 -] and B = [Si(OH)4(aq)].[H + ] control the predominance of one or the other species. They are to be compared to the following equilibrium constants calculated from [START_REF] Guillaumont | Chemical Thermodynamics 5: Update on the Chemical Thermodynamics of Uranium, Neptunium, Americium and Technetium[END_REF].

under conditions, and H5Si2O7 -should only represent 5% at pH 10 at equilibrium with quartz from thermodynamic data [START_REF] Guillaumont | Chemical Thermodynamics 5: Update on the Chemical Thermodynamics of Uranium, Neptunium, Americium and Technetium[END_REF]. The actual presence of the oligomeric species in low silicate and slightly acidic media is a question of interest since it may have biased the thermodynamic constants of metal-silicate complexes. Production of new experimental data accounting for oligomers formation is highly recommended. This point is especially relevant given that oligomeric species are usually neglected in speciation modelling of radionuclides in naturally relevant solutions, so that the detection of oligomeric silicate compounds at low concentration is an analytical challenge. (Vercouter et al., 2009a).

In their review of the stability of trivalent actinides silicate complexes and lanthanides, Vercouter et al. (2009c) compared the formation constants measured for Eu(III) [START_REF] Jensen | Complexation of europium(III) by aqueous orthosilicic acid[END_REF][START_REF] Pathak | Thermodynamic study of metal silicate complexation in perchlorate media[END_REF][START_REF] Thakur | Polymerization study of o-Si(OH)4 and complexation with Am(III), Eu(III) and Cm(III)[END_REF], Am(III) [START_REF] Wadsak | Formation of americium(III) complexes with aqueous silicic acid[END_REF][START_REF] Thakur | Polymerization study of o-Si(OH)4 and complexation with Am(III), Eu(III) and Cm(III)[END_REF] and Cm(III) [START_REF] Steinle | Komplexierung von Cm (III) mit monokieselsäure[END_REF][START_REF] Panak | Complexation of Cm(III) with aqueous silicic acid[END_REF][START_REF] Pathak | Thermodynamic study of metal silicate complexation in perchlorate media[END_REF][START_REF] Thakur | Polymerization study of o-Si(OH)4 and complexation with Am(III), Eu(III) and Cm(III)[END_REF] to assess possible discrepancies of the data existing in the literature (see Figure 10). The authors remarked that the values recommended in [START_REF] Guillaumont | Chemical Thermodynamics 5: Update on the Chemical Thermodynamics of Uranium, Neptunium, Americium and Technetium[END_REF] for the stability constants of Am(III) and Cm(III) agreed with only some experimental values. The experimental value for Cm(III) in [START_REF] Panak | Complexation of Cm(III) with aqueous silicic acid[END_REF] is lower than the one