Simon Bayle

Maria Rosa Beccia

Christophe Moulin

Pier Lorenzo Solari
email: pierlorenzosolari@synchrotron-soleil.fr

Christophe Den Auwer
email: christophe.denauwer@univ-cotedazur.fr

Pierre Crançon
email: pierrecrançon@cea.fr

Uranium speciation in a chalky soil

Keywords: Uranium, speciation, X-ray absorption spectroscopy, migration, carbonates

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

and rock constituents, mainly mineral carbonates and organic matter.

Second, uranium sorption isotherms were established for calcite, chalk and chalky soil materials along with EXAFS and TRLFS analysis. The presence of at least two adsorption complexes of uranyl onto carbonate materials (calcite) could be inferred from TRLFS. The first uranyl tricarbonate complex has a liebigite type structure and is dominant for low loads on the carbonate surface (< 10 mgU/kg(rock)).

The second uranyl complex is incorporated into the calcite for intermediate (~ 10 to 100 mgU/kg(rock)) to high (high: > 100 mgU/kg(rock)) loads.

Finally, the presence of uranium-humic substance complex in subsurface soil materials was underlined in the EXAFS analysis by the occurrence of both monodentate and bidentate carboxylate (or/and carbonate) functions, and confirmed by sorption isotherms in the presence of humic acid. This observation is of particular interest since humic substances may be mobilized from soil, potentially enhancing uranium migration under colloidal form.

Introduction

Uranium is naturally present at various levels in the earth's crust, but can be transformed by human activities such as mining, agriculture (phosphate fertilizers) and military industries or conflicts (Kosovo, Iraq). [START_REF] Gillon | Modelling the Baseline Geochemistry of Groundwater in a Chalk Aquifer Considering Solid Solutions for Carbonate Phases[END_REF][2][3][4][5][6] Therefore, uranium, transformed into various chemical and physical forms, may be concentrated on a local level in the environment. Generally speaking, this is referred to under the so-called TNORM (Transformed Naturally Occurring Radioactive Material) denomination.

Many studies have examined the health effects, corrosion, speciation and mobility of uranium in soil and subsurface. 3,[5][6][7][8][9][10][11][12][13][14] The behavior of uranium in subsurface environments is strongly related to its geochemical background, which is also driven by water-rock interactions. 15 Under oxic and suboxic conditions, the oxo form of uranium (named U(VI) in the following, and usually called uranyl) is mostly present in subsurface solid materials by sorption, coprecipitation or complexation mechanisms. [START_REF] Gillon | Modelling the Baseline Geochemistry of Groundwater in a Chalk Aquifer Considering Solid Solutions for Carbonate Phases[END_REF]12,[16][17][18][START_REF] Dhoum | Evaluation of Uranium and Arsenic Retention by Soil from a Low Level Radioactive Waste Management Site Using Sequential Extraction[END_REF][START_REF] Gabriel | Reactive Transport of Uranyl in a Goethite Column: An Experimental and Modelling Study[END_REF][START_REF] Welch | Factors Controlling As and U in Shallow Ground Water, Southern Carson Desert, Nevada[END_REF][START_REF] Salbu | Oxidation States of Uranium in DU Particles from Kosovo[END_REF] But reduction to U(IV) may also occur in geochemical or biochemical reducing conditions. [START_REF] Abdelouas | Reduction of UžVI/ to UžIV/ by Indigenous Bacteria in Contaminated Ground Water[END_REF] Nevertheless, speciation in the presence of strong complexing ligands present naturally in the environment like carbonates [START_REF] Dong | Determination of the Formation Constants of Ternary Complexes of Uranyl and Carbonate with Alkaline Earth Metals (Mg 2+ , Ca 2+ , Sr 2+ , and Ba 2+ ) Using Anion Exchange Method[END_REF][START_REF] Meleshyn | Influence of (Calcium-)Uranyl-Carbonate Complexation on U(VI) Sorption on Caand Na-Bentonites[END_REF][START_REF] Zeh | Speciation of Uranium in Gorleben Groundwaters[END_REF] or humic substances, [START_REF] Gillon | Modelling the Baseline Geochemistry of Groundwater in a Chalk Aquifer Considering Solid Solutions for Carbonate Phases[END_REF]3,8,[START_REF] Lenhart | Uranium(VI) Complexation with Citric, Humic and Fulvic Acids[END_REF][START_REF] Crançon | Speciation and Mobility of Uranium(VI) in Humic-Containing Soils[END_REF][START_REF] Bryan | The Effects of Humic Substances on the Transport of Radionuclides: Recent Improvements in the Prediction of Behaviour and the Understanding of Mechanisms[END_REF][START_REF] Moulin | Fate of Actinides in the Presence of Humic Substances under Conditions Relevant to Nuclear Waste Disposal[END_REF][START_REF] Reiller | Impacts of Humic Substances on the Geochemical Behaviour of Radionuclides[END_REF][START_REF] Graham | An Integrated Colloid Fractionation Approach Applied to the Characterisation of Porewater Uranium-Humic Interactions at a Depleted Uranium Contaminated Site[END_REF] redox processes, [START_REF] Chen | Processes Affecting Transport of Uranium in a Suboxic Aquifer[END_REF][34][START_REF] Senko | In -Situ Evidence for Uranium Immobilization and Remobilization[END_REF] as well as sorption to rapid colloidal vehicles, [START_REF] Reiller | Impacts of Humic Substances on the Geochemical Behaviour of Radionuclides[END_REF][START_REF] Artinger | Humic Colloid-Borne Migration of Uranium in Sand Columns[END_REF][START_REF] Crançon | Uranium Facilitated Transport by Water-Dispersible Colloids in Field and Soil Columns[END_REF][START_REF] Mibus | Migration of Uranium(IV)/(VI) in the Presence of Humic Acids in Quartz Sand: A Laboratory Column Study[END_REF][START_REF] Oliver | Depleted Uranium Mobility Across a Weapons Testing Site: Isotopic Investigation of Porewater, Earthworms, and Soils[END_REF] may be responsible for a potential long-range migration of uranium in the environment. This latter process may lead to a significant proportion of uranium in surface water [START_REF] Andersson | Particle Transport of 234U-238U in the Kalix River and in the Baltic Sea[END_REF] or groundwater [START_REF] Zeh | Speciation of Uranium in Gorleben Groundwaters[END_REF]41 transported as suspended particles or colloids. [START_REF] Gillon | Modelling the Baseline Geochemistry of Groundwater in a Chalk Aquifer Considering Solid Solutions for Carbonate Phases[END_REF][START_REF] Harguindeguy | Colloidal Mobilization from Soil and Transport of Uranium in (Sub)-Surface Waters[END_REF][START_REF] Harguindeguy | Isotopic Investigation of the Colloidal Mobility of Depleted Uranium in a Podzolic Soil[END_REF] On the other hand, the migration of uranium in carbonate-rich environments leads to the formation of stable uranylcalcium-tricarbonate complexes that decrease the uranyl adsorption onto natural solid materials, thus potentially enhancing its mobility in soil, sediments and groundwater. [START_REF] Dong | Determination of the Formation Constants of Ternary Complexes of Uranyl and Carbonate with Alkaline Earth Metals (Mg 2+ , Ca 2+ , Sr 2+ , and Ba 2+ ) Using Anion Exchange Method[END_REF][START_REF] Meleshyn | Influence of (Calcium-)Uranyl-Carbonate Complexation on U(VI) Sorption on Caand Na-Bentonites[END_REF][START_REF] Harguindeguy | Isotopic Investigation of the Colloidal Mobility of Depleted Uranium in a Podzolic Soil[END_REF][START_REF] Ho | Adsorption of Uranyl Species from Bicarbonate Solution onto Hematite Particles[END_REF][START_REF] Crançon | Assessing Migration of Uranium through Chalk Substrate: Field Study and Reactive Transport Modelling[END_REF][START_REF] Stewart | Impact of Uranyl-Calcium-Carbonato Complexes on Uranium(VI) Adsorption to Synthetic and Natural Sediments[END_REF][START_REF] Prat | Uranium Speciation in Drinking Water from Drilled Wells in Southern Finland and Its Potential Links to Health Effects[END_REF] In the more specific and restricted framework of military applications, the use of uranium munitions on the battlefields of Kosovo or Iraq has led to uranium concentration in soils above levels classically found in geological materials. 4,[START_REF] Hamilton | Depleted Uranium ž DU/ : A Holistic Consideration of DU and Related Matters[END_REF][49][50] This has also been detected in technical firing ranges. [START_REF] Graham | An Integrated Colloid Fractionation Approach Applied to the Characterisation of Porewater Uranium-Humic Interactions at a Depleted Uranium Contaminated Site[END_REF][50][START_REF] Oliver | Assessing Depleted Uranium (DU) Contamination of Soil, Plants and Earthworms at UK Weapons Testing Sites[END_REF][START_REF] Toque | The Corrosion of Depleted Uranium in Terrestrial and Marine Environments[END_REF] In the case of the war in Kosovo, 4,[START_REF] Oliver | Assessing Depleted Uranium (DU) Contamination of Soil, Plants and Earthworms at UK Weapons Testing Sites[END_REF][53][START_REF] Radenković | Depleted Uranium Mobility and Fractionation in Contaminated Soil (Southern Serbia)[END_REF][START_REF] Sansone | Radioecological Survey at Selected Sites Hit by Depleted Uranium Ammunitions during the 1999 Kosovo Conflict[END_REF] these were initially deposited as large metallic fragments and pellets, small shells, and particles. 3,[9][10][11][START_REF] Salbu | Oxidation States of Uranium in DU Particles from Kosovo[END_REF][START_REF] Lind | Characterisation of DU Particles from Kosovo and Kuwait[END_REF][START_REF] Oliver | Distribution and Partitioning of Depleted Uranium (DU) in Soils at Weapons Test Ranges -Investigations Combining the BCR Extraction Scheme and Isotopic Analysis[END_REF] In these conditions, oxidation over time of the initial U(0) deposits in soil most often leads to mixed U(IV)/U(VI) oxides in various forms and proportions. 3,[START_REF] Salbu | Oxidation States of Uranium in DU Particles from Kosovo[END_REF][START_REF] Haschke | Corrosion of Uranium in Air and Water Vapor: Consequences for Environmental Dispersal[END_REF] At the same time, in oxic conditions, leaching, caused by rainfall of metallic U fragments further altered in UO 2 (s), then produces highly soluble uranyl oxyhydroxide such as metaschoepite ((UO 2 ) 8 O 2 (OH) 12 •10(H 2 O)), 3,[START_REF] Cachoir | Chemical Durability of Uranium Dioxide Stored in Geologic Medium : Uranium Remobilisation[END_REF][START_REF] Mceachern | A Review of the Oxidation of Uranium Dioxide at Temperatures below 400°C[END_REF][START_REF] Schimmack | Long-Term Corrosion and Leaching of Depleted Uranium (DU) in Soil[END_REF][START_REF] Handley-Sidhu | Corrosion and Transport of Depleted Uranium in Sand-Rich Environments[END_REF][START_REF] Finch | The Corrosion of Uraninite under Oxidizing Conditions[END_REF][START_REF] Gorman-Lewis | Review of Uranyl Mineral Solubility Measurements[END_REF] which might then migrate through the soil toward the surface of shallow aquifers. [START_REF] Schimmack | Long-Term Corrosion and Leaching of Depleted Uranium (DU) in Soil[END_REF][START_REF] Handley-Sidhu | Corrosion and Transport of Depleted Uranium in Sand-Rich Environments[END_REF][START_REF] Finch | The Corrosion of Uraninite under Oxidizing Conditions[END_REF][START_REF] Gorman-Lewis | Review of Uranyl Mineral Solubility Measurements[END_REF][START_REF] Bower | Metaschoepite Dissolution in Sediment Column Systems-Implications for Uranium Speciation and Transport[END_REF] This short review underlines the need for speciation data in the open field when U(0) fragments have been deposited for a long time (more than 10 years). Nevertheless, speciation data is required for determination and modeling of migration mechanisms, such as retention on the immobile phases or complexation with mobile ligands such as humic substances. In this paper, we propose to describe the geochemical behavior of metallic uranium deposited in a natural carbonate rendosol (or rendzina, following U.S. classification) in a former military site. These uranium materials had subsequently undergone natural alteration (essentially linked to weathering) for several decades under oxic conditions. [START_REF] Crançon | Assessing Migration of Uranium through Chalk Substrate: Field Study and Reactive Transport Modelling[END_REF][START_REF] Hubert | Transport of Radionuclides in an Unconfined Chalk Aquifer Inferred from U-Series Disequilibria[END_REF] They had also undergone migration processes in the soil, going deeper in the subsurface through Cenomanian chalk substrate. We combine here analytical chemistry (ICP-MS), Xray Absorption Fine Structure spectroscopy (XAFS) at the U L edge and Time Resolved Laser Fluorescence Spectroscopy (TRLFS) in order to generate a full picture of the migration mechanisms in the natural soil.

Materials and Methods

Study site location, geological and geochemical settings

The migration of uranium in deep soil horizons was investigated in a localized deposit of dispersed metallic uranium U(0) nuggets and particles on the surface of a carbonate rendosol (i.e. rendzina), developed on a chalk substrate from the Upper Cretaceous (≈ 100 to 65 million years ago). Details about the study site and the geological and geochemical settings are given elsewhere, as well as preliminary results on the U distribution and aqueous and colloidal speciation in the subsurface and aquatic environment. [START_REF] Crançon | Assessing Migration of Uranium through Chalk Substrate: Field Study and Reactive Transport Modelling[END_REF][START_REF] Hubert | Transport of Radionuclides in an Unconfined Chalk Aquifer Inferred from U-Series Disequilibria[END_REF][START_REF] Bourdon | Weathering Rates from Top to Bottom in a Carbonate Environment[END_REF][START_REF] Renard | Estimating Transmissivity Fields and Their Influence on Flow and Transport: The Case of Champagne Mounts[END_REF][START_REF] Gillon | Modelling the Baseline Geochemistry of Groundwater in a Chalk Aquifer Considering Solid Solutions for Carbonate Phases[END_REF][START_REF] Gillon | Kinetics of Incongruent Dissolution of Carbonates in a Chalk Aquifer Using Reverse Flow Modelling[END_REF][START_REF] Mangeret | Reactive Transport Modelling of Groundwater Chemistry in a Chalk Aquifer at the Watershed Scale[END_REF][START_REF] Claveranne-Lamolère | Investigation of Uranium-Colloid Interactions in Soil by Dual Field-Flow Fractionation/Capillary Electrophoresis Hyphenated with Inductively Coupled Plasma-Mass Spectrometry[END_REF][START_REF] Claveranne-Lamolère | Colloidal Transport of Uranium in Soil: Size Fractionation and Characterization by Field-Flow Fractionation-Multi-Detection[END_REF][START_REF] Cao | Heterogeneous Behaviour of Unconfined Chalk Aquifers Infer from Combination of Groundwater Residence Time, Hydrochemistry and Hydrodynamic Tools[END_REF][START_REF] Cao | Sources and Behavior of Perchlorate in a Shallow Chalk Aquifer under Military (World War I) and Agricultural Influences[END_REF] The vegetation on site is a dry grasslands developed on thin chalky rendzina and calcisols more or less cryoturbated, with low organic carbon content (Total Organic Carbon mass content typically lower than 5%). Table S1 of SI summarizes the essential information about the mineralogical and organic composition of the soil and the chalk substrate at the study site, and their evolution along a vertical soil profile. Soils and colluviums are characterized by a larger noncarbonate fraction as opposed to the chalk units. The soil exhibits three main horizons (Figure 1). The topsoil layer is a thin (thickness ~ 10 cm) Aca horizon, poorly developed, that exhibits a porous and granular structure characterized by an organic matter content of about 5 mass.%. The mineral content is more than 60% of calcium carbonates (mainly calcite, with traces of dolomite), supplemented by a non-carbonate fraction composed of ~35% of silicates and aluminosilicates such as silica, kaolinite, feldspar and micas, and about 3% iron oxy-hydroxide. [START_REF] Crançon | Assessing Migration of Uranium through Chalk Substrate: Field Study and Reactive Transport Modelling[END_REF][START_REF] Gillon | Modelling the Baseline Geochemistry of Groundwater in a Chalk Aquifer Considering Solid Solutions for Carbonate Phases[END_REF][START_REF] Mangeret | Reactive Transport Modelling of Groundwater Chemistry in a Chalk Aquifer at the Watershed Scale[END_REF] The second soil horizon is a transitional silty-clay altered (Sca type) and extends from ~10 to 60-70 cm deep, and is composed of chalk (86%), organic matter (~5%) and of iron oxides. The bedrock is constituted by a cryoturbated chalky substrate that contains more than 95% calcite, less than 5% of aluminosilicates, and an organic matter content about 25 times lower (~0.2 %) than that measured in the topsoil layer. A surface soil (0-20 cm) and a deeper host rock (Campanian chalk, 1-2 m) were collected within the site limits but outside the historical uranium deposits area and are referred to hereinafter as Chalky Soil and Chalk respectively.

Soil and subsoil samples for the determination of the uranium vertical migration profile were collected every 10 cm in a trench dug in the deposits area of the former military site, at increasing depth from the surface to 2 m depth. [START_REF] Knechtenhofer | Fate of Heavy Metals in a Strongly Acidic Shooting-range Soil: Small-scale Metal Distribution and Its Relation to Preferential Water Flow[END_REF] In order to take into account the heterogeneity of uranium distribution in deposit and soil materials, the samples were taken over a horizontal width of 20 cm, crushed, dried, and then homogenized by quartering before being put into 500 ml vials calibrated for gamma spectrometry. Raw samples were air-dried, prescreened by sieving at 5 mm to remove the largest gravel particles and other fragments, crushed gently, and again sieved to a particle size of < 200 µm prior to physicochemical characterizations and adsorption experiments.

Uranium model materials

Altered uranium pellets from the initial deposit. Altered U nuggets were picked manually from the soil's surface inside the field site, then gently washed with distilled water to remove soil particles.

Visually, nuggets can be divided into two main categories: dark-colored and yellowish-colored (Figure 2a). argon atmosphere, in a 5% perchloric acid solution and using degassed purified water. Then, the pH was adjusted to 7 using a 0.1 M NaOH solution. The precipitate formed after a few minutes, was isolated after solution filtration and extensively rinsed using degassed purified water. The composition and structure of metaschoepite was then confirmed by X-ray diffraction (data not shown).

Uranyl-oxo-hydroxide reference (Metaschoepite

U-HA sample. Humic acids Sigma-Aldrich™ in solid form were dissolved in distilled water at atmospheric equilibrium (pCO 2 ≈ 10 -3.5 atm) to obtain a concentration of [HA] = 5 g.L -1 , which was continuously stirred for a period of 24 h. U(VI) was added to 10 mL of the humic acid solution (no undissolved HA was visible) from a stock solution (PlasmaCAL, Standard for ICP, SCP Science, 10000 µg/mL in 4% HNO 3 ) to obtain a final U concentration of [U] = 10 -3 mol.L -1 . Once U was added to the humic acid solution, the pH was adjusted to 7.5-8 using 0.1 M NaOH solution.

U-triscarbonate sample. The uranyl triscarbonate sample (UO 2 (CO 3 ) 3 4-) was prepared by mixing a uranyl stock solution (PlasmaCAL, Standard for ICP, SCP Science, 10000 µg/mL in 4% HNO 3 ) in the presence of an excess of carbonate at 0.5 mol.L -1 obtained by dissolving Na 2 CO 3 . The final U concentration was equal to 4.2.10 -2 mol.L -1 , and the pH was adjusted to 7.5.

Uranyl-calcium tricarbonate sample: mineral

Liebigite reference sample (Ca 2 [UO 2 (CO 3 ) 3 ], 11H 2 O)
was obtained from the mineralogy collection of the Musée National d'Histoire Naturelle (MNHN), Paris, France.

Experimental sorption isotherms

Batch sorption experiments. Adsorption isotherms of U(VI) were determined onto synthetic and natural carbonate substrates : purified calcite in the presence and in the absence of humic acid, natural chalk and chalky soil from the study site.

Batch experiments conducted at 25 °C in an open and oxic atmosphere. All solutions and suspensions were prepared with deionized Milli-Q 18 MΩ cm (Millipore™) water. Purified calcite and Humic Acid were from Sigma-Aldrich™ (detailed properties are given elsewhere [START_REF] Vdović | Electrokinetic Behaviour of Calcite-the Relationship with Other Calcite Properties[END_REF][START_REF] Kim | Characterization of Humic and Fulvic Acids from Gorleben Groundwater[END_REF] ). The specific surface area for carbonate materials (calcite, chalk and chalky soil) were determined by BET method (see

XAFS main settings

All XAFS (X-ray Absorption Fine Structure) analyses were performed on the MARS beamline of SOLEIL synchrotron (Saint-Aubin, France). [START_REF] Sitaud | Characterization of Radioactive Materials Using the MARS Beamline at the Synchrotron SOLEIL[END_REF][START_REF] Llorens | X-Ray Absorption Spectroscopy Investigations on Radioactive Matter Using MARS Beamline at SOLEIL Synchrotron[END_REF] The beamline optics consist essentially of a watercooled double-crystal monochromator (FMB Oxford), which is used for selecting the incident energy of the X-ray beam and for horizontal focusing, and two large water-cooled reflecting mirrors (IRELEC/SESO) that are used for high-energy rejection (the harmonic part) and vertical collimation and focusing of the X-ray beam. All measurements were taken in fluorescence mode using a 13element high purity germanium detector (ORTEC, electronics XIA -XMA). All EXAFS data processing was performed using the ATHENA code. [START_REF] Ravel | HEPHAESTUS: Data Analysis for X-Ray Absorption Spectroscopy Using IFEFFIT[END_REF] The E 0 energy was identified at the maximum of the first derivate around absorption edge. Fourier transform (FT) was performed with a Hanning window. Fourier transform with k 2 weighting was performed between 3.5 and 12 Å -1

EXAFS data acquisition and

for each reference sample. Fittings of the references were performed in R space between 1 and 5 Å using the ARTEMIS code (version DEMETER 0.9.25). [START_REF] Ravel | HEPHAESTUS: Data Analysis for X-Ray Absorption Spectroscopy Using IFEFFIT[END_REF] The same global amplitude factor S 0 2 and the same energy threshold correction factor ΔE 0 were used for every path of the fits. The agreement factor r (%) and the quality factor (QF = χ 2 red ) of the fits were provided directly by DEMETER. Partial structural models were used for the pure U phases to calculate phases and amplitudes using the FEFF6 simulation code integrated in DEMETER.

EXAFS data treatments of soil samples by Linear Combination Fitting (LCF). EXAFS spectra of soil samples from the vertical U migration profile were analyzed by Linear Combination Fitting in a three component-mixing model, using the EXAFS spectra of U-triscarbonate, metaschoepite and U-AH complex as references, respectively. LCF was performed with a mean square routine on EXAFS spectra between 3.5 and 10 Å -1 in k space. Hence, the amplitude of each reference compound in the composition of each soil sample was determined. The sum of the weight of the combination was left free to evolve. Uranium samples preparation. All soil samples were finely crushed in fine powder with an agate stone pestle. A total mass of 5 mg of either U nugget from the polluted soil (nugget #2) or reference synthetic metaschoepite was accordingly crushed then mixed with about 195 mg of polyethylene to limit fluorescence reabsorption during XAFS analysis.

Time-Resolved Laser Fluorescence Spectroscopy main settings

Time-resolved fluorescence spectra of uranyl species in the solid phase were collected at room temperature using a FP920 spectrofluorometer with embedded ICCD (Intensified Charge Couple Device) camera (Edinburgh Instruments™) cooled at -20°C by Peltier effect and coupled to a Nd-YAG (Quantel™ Surelite) laser source delivering an initial beam of 10 mJ energy at 1064 nm (~1.2 eV), with a 10 Hz pulse frequency. For fluorescence spectroscopy, the excitation beam was set to 355 nm (third harmonic, ~3.5 eV). The sample holder and the ICDD camera were oriented respectively at 45° and 90° from the incident axis to avoid direct laser beam transmission. Data acquisition and preprocessing was performed by F900 software (Edinburgh Instruments™). Data post-processing (baseline determination and spectra deconvolution by Gauss functions) is performed using Origin 2002 Pro (Origin Lab™).

Results

Profile of uranium concentration in soil

The concentration of U in a soil profile was determined on a series of discrete samples with 10-cm increments from the surface to a depth of 2 m in a trench dug in the site. Total U content was determined from ICP-MS analysis after complete solubilization of soil and rock materials by hydrofluoric acid. Details of the analysis results are reported in SI, Table S1 and Figure S1. Figure 1 shows the profile of the soil U content compared to a picture and a brief description of the soil's structure. The blue dotted lines represent the mean natural U concentration in the geochemical background of the Campanian chalks (and surrounding soils) (close to 1 ppm) as determined outside the site. [START_REF] Crançon | Assessing Migration of Uranium through Chalk Substrate: Field Study and Reactive Transport Modelling[END_REF] The highest U content is found in the topsoil layer, corresponding to the initial surface deposit. Then, its content in the soil decreases rapidly according to depth, from 0 to 50 cm, constituently with soil organic matter content (Table S1 of SI). Below, the U content decreases asymptotically down to a depth of 2 m, where there is a predominant amount of carbonate minerals in the chalk materials. Throughout the soil's surface, the U deposit appears to be essentially constituted by fine particles of alteration products (lower than ~50 µm in size), as confirmed by electron microscope imaging (Figure S2 of SI). The continuous decrease of U content according to depth corresponds to a migration front that extends beyond 250 cm deep, where U content in chalk remains four times higher than the geochemical background in surrounding rocks. The migration of U towards deeper chalk horizons thus appears to be limited by retention onto geological materials. There is a positive correlation between U and organic matter contents throughout the vertical profile (Figure S1 of SI). This being said, the influence of the chemical composition of the soil matrix on the U speciation and retention, thus its migration regime, needs to be demonstrated.

Batch sorption experiments

Experimental sorption isotherms of U onto pure calcite are reported in Figure 3 (see Table S3 of SI for experimental conditions), and compared to data from the bibliography. [START_REF] Dong | Influence of Calcite and Dissolved Calcium on Uranium(VI) Sorption to a Hanford Subsurface Sediment[END_REF][START_REF] Geipel | Laser and X-Ray Spectroscopic Studies of Uranium-Calcite Interface Phenomena[END_REF][START_REF] Niu | Spectroscopic Studies on U(VI) Incorporation into CaCO3: Effects of Aging Time and U(VI) Concentration[END_REF] All these sorption isotherms use calcite materials from various origins, with various surface properties, solid/liquid ratios (varying from 5 to 100 g.L -1 ) and pH conditions (from 7.0 to 9.1). Nevertheless, isotherm data are globally consistent. The lower the solid/liquid ratio and the calcite grain size, the higher the specific surface area and the sorption of U are. The U affinity for calcite surface is mild, with distribution coefficient K d (see Table S4 of SI) ranging from 3 to 21 L.kg -1 . Sorption patterns on calcite are mainly non-linear, exhibiting typical Freundlich (at low adsorbate concentrations) or Langmuir patterns (for higher adsorbate concentration, and when experimental conditions are closer to critical U concentrations in adsorbate regarding either sorbent surface saturation or U solid phase solubility) with significant correlation coefficients higher than 0.90. The n values between 0.6 and 0.9 reported for the Freundlich isotherms suggest that the adsorption process depends more or less on the surface coverage of the adsorbent and is not strictly proportional to the concentration of the adsorbate in solution. As a consequence, the uranyl adsorption reaction has a global stoichiometry less than 1. [START_REF] Sparks | Sorption Phenomena on Soils[END_REF] Maximal adsorption capacity of the sorbent qm estimated from Langmuir function fitting are typically ranging from 0.02 to 6 µmol.m -2 consistent with previous observation on pure synthetic calcite. [START_REF] Geipel | Laser and X-Ray Spectroscopic Studies of Uranium-Calcite Interface Phenomena[END_REF][START_REF] Elzinga | Spectroscopic Investigation of U(VI) Sorption at the Calcite-Water Interface[END_REF] The experimental sorption isotherms of U onto chalk are also reported in Figure 3, and compared to data from Géhin et al. obtained on the same materials under identical conditions. [START_REF] Géhin | Thermodynamic and Kinetic Properties of U(VI) Sorption on Chalk Substrates[END_REF] U sorption isotherms on chalk are consistent and close in form and properties to the sorption isotherms for uranyl on calcite. U affinity for chalk is lower than that of calcite, with distribution coefficient K d ranging from 0.3 to 10 L.kg -1 . Comparable to calcite, U sorption patterns on chalk are mainly non-linear. The n values reported for Freundlich isotherms are close to 0.3 -0.8, suggesting (as for pure calcite) a multiple speciation and sorption mechanism for U on chalk, dependent on adsorbate concentration in solution, with a global stoichiometry for the sorption reaction lower than 1. Maximal adsorption capacity q m estimated for chalk from the Langmuir equation are lower than that of calcite, and typically range from 0.01 to 0.1 µmol.m -2 (Table S4 of SI).

Finally, sorption isotherms for U on chalky soil are consistent with the sorption isotherms for U on pure calcite and chalk, but with a slightly higher affinity (K d close to 20 L.kg -1 , see Table S4 in SI).

Comparable to those for calcite and chalk, U sorption patterns on chalky soil are non-linear, with n values reported for Freundlich isotherms close to 0.3-1.0. Maximal adsorption capacities qm estimated for chalky soil from the Langmuir equation (about 0.1 -0.7 µmol.m -2 ) are slightly higher than those estimated for chalk. Since the composition of chalky soil mixes calcite (~ 60%) with secondary silicates, oxides and organic phases (~ 10% kaolinite, ~ 10% clays and feldspars, ~ 15% silica, ~ 5% iron and aluminum oxi-hydroxides, ~ 1% organic carbon), this increasing affinity of U for soil materials compared to chalk may be imputed in large part to secondary phases, as classically observed in geological subsurface materials. [START_REF] Gillon | Modelling the Baseline Geochemistry of Groundwater in a Chalk Aquifer Considering Solid Solutions for Carbonate Phases[END_REF]18,92,[START_REF] Catalano | Uranyl Adsorption onto Montmorillonite: Evaluation of Binding Sites and Carbonate Complexation[END_REF] As a complement, the sorption of U onto chalk doped with purified Aldrich humic acid (HA) was studied in batch experiments ([chalk + HA] = 100 g.L-1, with [HA] = 2 mass.%). U affinity for chalk + HA is notably higher than for chalk, with the distribution coefficient estimated Kd close to 26 L.kg-1., and remarkably close to U affinity for the chalky soil. Sorption isotherms for uranyl on chalk+HA and chalky soil are very similar. Thus, complexing organic compounds such as humic acids are likely to play a key role in U retention in soil materials, as already observed by many authors. [START_REF] Gillon | Modelling the Baseline Geochemistry of Groundwater in a Chalk Aquifer Considering Solid Solutions for Carbonate Phases[END_REF][START_REF] Beneš | Adsorption of Uranium on Clay and the Effect of Humic Substances[END_REF][START_REF] Ho | Effect of Humic Acid on Uranium Uptake by Hematite Particles[END_REF][START_REF] Lenhart | Uranium(VI) Sorption to Hematite in the Presence of Humic Acid[END_REF][START_REF] Payne | Uranium Adsorption on Ferrihydrite -Effects of Phosphate and Humic Acid[END_REF][START_REF] Schmeide | Uranium(VI) Sorption onto Phyllite and Selected Minerals in the Presence of Humic Acid[END_REF][START_REF] Wei | Interaction between Uranium and Humic Acid (1): Adsorption Behaviors of U(VI) in Soil Humic Acids[END_REF][100][101]

Uranium speciation determination

EXAFS and TRLFS analysis of the source term deposits and uranyl references

Heterogeneously dispersed particles and fragments constitute the initial U deposit on the soil's surface.

Larger fragments that range from a few millimeters to a few centimeters in size are hereafter called U nuggets. Two different nuggets of about 2 mm in size (nuggets #1 and #2, Figure 2a), were sampled on the site and are fully oxidized displaying the characteristic dark-yellowish color of uranyl hydrates. Despite a color difference, solid phase composition is found to be rather homogeneous between the two nuggets. Dark-yellowish colored pellet (nugget #1) is found to be very close to schoepite -UO 2 (OH) 2 with low crystallinity. The yellowish-colored altered pellet (nugget #2) is found to be composed of an assemblage of various low-crystallinity uranyl hydrates, mainly represented by pseudo schoepite (U(VI)O 3+x .2H 2 O) or metaschoepite ((UO 2 ) 8 O 2 (OH) 12 •10(H 2 O)) (X-ray diffraction data, see X-ray diffractogram of Figure 2b). The EXAFS spectra recorded at the U L II edge for nuggets #1 and #2 are given in Figure 2c, together with their Fourier transform (FT, Figure 2d). Spectra of nuggets #1 and #2 are remarkably similar and can be adjusted with structural parameters close to those of metaschoepite (Table S5 of SI): 2 axial O at 1.82 ± 0.01 Å, 2.4 ± 0.2 equatorial O at 2.32 ± 0.1 Å and 2.4 ± 0.2 equatorial O at 2.5 ± 0.1 Å Å , and about 3 ± 0.5 distant U at 3.8 ± 0.15 Å . This also confirms that U is fully oxidized to oxidation state +VI in these two samples where uranium is present as a mixture of oxyhydroxide complexes. Time Resolved Laser Fluorescence Spectroscopy (TRLFS) spectra for nugget #1 and synthetic metaschoepite are given in Figure S4 of SI, together with respective deconvolution peaks. Table S6 of SI gives corresponding details of spectra and lifetime properties, together with data from the literature about schoepite and metaschoepite. 102 Four major bands can be identified at about 500, 520, 540 and 569 nm that correspond to a fluorescence pattern identified for metaschoepite. 102 Fluorescence lifetime estimated for nugget #1 (131 ± 4 µs) is higher than that estimated for metaschoepite (50 ± 5 µs). The breadth of vibronic bands and the poor resolution of the spectra are consistent with the presence of multi-phase precipitates with various hydration states, resulting in multiple local U environments in the structure of uranyl hydrates, as has been cited by many authors. [START_REF] Geipel | Laser and X-Ray Spectroscopic Studies of Uranium-Calcite Interface Phenomena[END_REF][START_REF] Elzinga | Spectroscopic Investigation of U(VI) Sorption at the Calcite-Water Interface[END_REF]103 A uranyl -humic acid complex in solution (named U-HA) was also prepared as a model of uranyl complexation by an excess of HA. The corresponding EXAFS spectrum is given in Figure S3 of SI.

The spectrum was fitted with 2 axial O at 1.80 ± 0.01 Å, and two distinct U-O equatorial contributions with 2.5 ± 0.1 O at 2.29 ± 0.01 Å and 3.0 ± 0.2 O at 2.44 ± 0.01 Å (see Table S5 of SI). The structural parameters obtained for the U-HA complex are in agreement with data from the literature on uranylcarboxylate species, which exhibit two equatorial distances. [104][105][106] However bidentate {COO} groups can either originate from carboxylate or carbonate groups. In the second case, the uranium would be in the ternary complex form UO 2 (CO 3 ) 2 HA 2-(described by Steudtner et al. 107 under similar conditions).

However, the present EXAFS data do not show the contribution from a distal oxygen of the carbonate(s), which can be explained by (1) the absence of any carbonate in the equatorial plane of uranyl, (2) by a lack of sensitivity of the fit to the presence of a distal oxygen (in case there is only one, for instance).

Finally, the EXAFS spectra of the uranyl tricarbonate sample, in solution (UO 2 (CO 3 ) 3 4-, named Ucarb) recorded at the U L III edge is also given in Figure S3 of SI. The spectrum has been adjusted (see Liebigite structure reported in the literature. [START_REF] Elzinga | Spectroscopic Investigation of U(VI) Sorption at the Calcite-Water Interface[END_REF]108,109 TRLFS analysis of uranium sorbed onto calcite and soil materials.

The normalized fluorescence spectra of uranium sorbed onto calcite, natural chalk and natural chalky soil are given in Figure S5 of SI for an initial dissolved U concentration varying from 1 to 1000 ppm.

Spectral properties and fitting parameters for TRLFS spectra are given in Table S6 of SI. Fluorescence spectra for U sorbed onto calcite, chalk and chalky soil are remarkably similar and are observed to change as the U concentration in the overlying solution, and therefore the U loading at the sorbent surface increases. For the lowest dissolved U concentration (1 ppm), the spectrum is centered at 525 nm and is poorly (but clearly) structured. Five relative maxima, corresponding to five major vibronic bands, are identified respectively at 489, 505, 524, 543 and 570 nm. Assuming this spectrum is assigned to a single component, this pattern observed for a longer-lived species is very close to the one observed for natural liebigite (Figure S6 and other published luminescence data given in Table S6 of SI); and also to U(VI)-substituted synthetic calcite and aragonite in a liebigite-like structure. [START_REF] Elzinga | Spectroscopic Investigation of U(VI) Sorption at the Calcite-Water Interface[END_REF]102,110 This shows that for low U loadings on the carbonate mineral surfaces (including calcite, chalk and chalky soil), U is mainly associated with calcite within a tricarbonate liebigite-type structure, that represents an adsorbed U species, [START_REF] Elzinga | Spectroscopic Investigation of U(VI) Sorption at the Calcite-Water Interface[END_REF] and will be hereafter called "species 1". For U concentrations in overlying solution ranging between 10 to 50 ppm, the fluorescence spectra of U sorbed onto chalk samples exhibit at first a slight blue-shift of the main bands. This suggests a change in the interaction between U(VI) and the calcite surface, leading to the formation of another uranyl tricarbonate-like surface species. [START_REF] Elzinga | Spectroscopic Investigation of U(VI) Sorption at the Calcite-Water Interface[END_REF]111,112 The fluorescence bands of this second U sorbed species (hereafter called "species 2") can be isolated from any spectrum recorded on chalk or soil samples with intermediate U loading by subtracting bands of species 1 from the raw fluorescence spectrum (Figure S7 of SI). Five relative maxima can be identified in the subtraction residuals, respectively at 488, 498, 517, 539 and 565 nm, that are found to be very close to those of U incorporated in calcite, as defined by Elzinga (see Table S6 of SI). [START_REF] Elzinga | Spectroscopic Investigation of U(VI) Sorption at the Calcite-Water Interface[END_REF] Then, for higher dissolved U concentrations (essentially above 100 ppm), the fluorescence pattern becomes more unstructured, centered at about 530 nm, and exhibits a clear red-shift with an apparent smoothing of fluorescence maxima. Uranyl hydroxide precipitates are expected at these concentrations, and indeed the spectra shown in Figure S8 of SI for solutions higher than about 100 ppm are composed of at least two components, the main one being consistent with the luminescence spectra of uranyl hydroxide such as schoepite. [START_REF] Geipel | Laser and X-Ray Spectroscopic Studies of Uranium-Calcite Interface Phenomena[END_REF]102 As in synthetic metaschoepite and nugget #1, the absence of a clear structure in the vibronic bands indicates that the precipitate is probably not a pure phase. 113 It can be noted that in the presence of HA in the soil samples, the precipitation of uranyl hydrates is delayed and is not significantly observed before the soil U content reaches about 500 ppm.

By assigning the global fluorescence signal to a triple component system (species 1 + species 2 + schoepite), all of the spectra recorded for U sorption onto calcite, natural chalk and natural chalky soil given in Figure S5 of SI were able to be convoluted by a linear combination of these three components, with regression coefficients generally above 0.99 (see Figure S8 of SI). Deconvolutions were conducted by keeping a constant, for each component spectrum, the band positions, and the peak widths and relative areas. Deconvolution results clearly outline the dominance of uranyl-carbonate species 1 for low U loadings on the surface of carbonate materials, and of uranyl-carbonate species 2

for high U loading. The red-shift of fluorescence spectra observed for high U concentrations in adsorbate is clearly attributed to the precipitation of uranyl hydrates.

Qualitative approach of the uranium speciation along the vertical soil profile

The XAFS spectra of the soil samples from the study site, from the surface to a depth of 100 cm (at 0, , and that no significant evolution of the oxidation state can be observed from the surface to the lowest points of the vertical profile.

In order to clarify the speciation and its possible evolution down from the surface, EXAFS analysis was also performed on the same soil samples series. The EXAFS spectra of the soil samples at various depths are presented in Figure 4 together with those of three reference materials: i) the metaschoepitelike weathered U nugget taken from the initial deposits (U-nugget #1), ii) an HA uranyl complex in solution (U-HA), and iii) the tri-carbonate uranyl complex in solution (U-carb). It is apparent that the EXAFS spectrum of the surface sample is qualitatively similar to that of the nugget. Going from the subsurface to deeper soil horizons, the XAFS contribution between 6.5 and 9 Å -1 is significantly modified and tends towards a plateau observable on the U-carb reference spectrum. The signal to noise ratio decreases from subsurface to depth because of the decrease of U quantity in the sample. On the corresponding Fourier transforms (FT) shown in Figure S10 of SI, an evolution of the second contribution of the FT between R+ = 1.7 and 2.5 Å is also clearly visible. This second contribution is associated with the equatorial oxygens of the uranyl oxocation, and the modification of the modulus of the FT suggests a rearrangement of these oxygens. Finally, a U-U contribution in R space at R+ = 3.8 Å is clearly observable in subsurface samples above 20 cm. This U-U contribution, characteristic of a condensed phase (as for the spectra of U-nugget #1, #2), and clearly visible for surface soil samples, decreases with depth and is undistinguishable below 30 cm, as can be seen for the deepest sample of the profile at 100 cm.

Normalized TRLFS spectra for U recorded on the same sample series from the soil vertical profile are given in Figure 5. The fluorescence spectra are broad, poorly resolved, and noisy, despite high U concentration in the soil in the upper layers. These observations suggest a complex multiple-species speciation scheme for U, as well as a significant quenching of the fluorescence signal, and the presence of non-fluorescent species such as U-AH complexes. The maximum fluorescence spectra are observed at ~ 520 nm, consistent with results obtained for U sorption experiments on soil materials.

This pattern is very close to the one observed for U sorbed onto calcite, chalk and chalky soil (see Figures S5 and S6 of SI), and can be attributed to a dominant uranyl tricarbonate species. By subtracting bands of species 1 (liebigite-type structure) from the raw sample spectra, five relative maxima can be mainly identified in residuals at 488, 498, 517, 539 respectively and 565 nm, that correspond to species 2 (U incorporated in calcite, as defined by Elzinga). [START_REF] Elzinga | Spectroscopic Investigation of U(VI) Sorption at the Calcite-Water Interface[END_REF] All of the spectra recorded along the soil profile were deconvoluted by a combination of three components (species 1 + species 2 + schoepite), together with a wide Gaussian function for representation of the luminescence baseline (Figure S11 of SI). Deconvolution results clearly outline the presence of at least two uranyl-carbonate species that change in proportion with U(VI) loading on the carbonate mineral surface and, consequently, with depth in the soil profile, are consistent with results obtained for U sorption experiments on soil materials. The presence of relictual uranyl hydrates from the initial deposit can be noticed in the topsoil sample, where U concentration is maximal (~ 6500 ppm).

Quantitative approach of uranium speciation using EXAFS Linear Combination Fitting

In order to quantitatively describe the evolution of U speciation along the vertical profile, a linear combination fitting (LCF) procedure was implemented with EXAFS. The LCF procedure considers the three reference materials (components) described above: U-nugget #1, U-HA and U-carb. For each sample, the total number of components was not set to 100% (as it should be) in order to verify that without this constraint, the total percentage does not substantially deviate from 100%. EXAFS linear combination fits are presented in Figure 4, and the respective repartition of the three components obtained from the LCF procedure is represented in Figure 6. Although the 3 components are not significant at all depths (the significance of the selected components as a function of depth is given in Figure S12 of SI with comparative F-tests), a 3-component system was considered for LCF consistency. First, one can observe that all samples along the soil profile are correctly fitted with a combination of the three components mentioned above. Secondly, we see that at the soil's surface (0 cm), U speciation is mainly constituted by uranyl oxyhydroxides (metaschoepite-like) obtained from the alteration of the U nuggets. One can note that only one component (U-nugget #1) is significant for this fit but the model of 3 components was applied for all samples for overall consistency. The corresponding spectrum was fitted according to this assumption and the fit is presented in Figure S13 and Table S7 of SI. Thirdly, from 10 to 60 cm depth, a mixing between the uranyl oxyhydroxides from the deposits and uranium-humic substances complexes can be observed, with a participation of U carbonate in a liebigite-type structure whose contribution ranges from minor (between 10 and 30 cm) to major (between 40 and 60 cm). Finally, below 70 cm depth, the participation of U-carbonate complexes with liebigite-like structure is clearly dominant. And the spectrum of the deepest sample (100 cm) was also fitted according to this assumption (see Figure S13 and Table S7 of SI) showing the consistency of the LCF result.

Discussion

In the soil profile of the study site, uranium is initially deposited as U metal fragments and particles, called "nuggets". After alteration, the nuggets that were observed have a schoepite-like structure. The total U content decreases sharply, by more than five orders of magnitude from top to bottom of the vertical profile (250 cm). This continuous decrease in depth correlates firstly with the rapid disappearing of particles from the U nuggets from the initial surface deposits, and secondly, with a migration front of U that extends from the soil's surface to beyond 250 cm deep. Despite a long period of alteration in the soil (several decades), a large part of the U thus remains in the soil, suggesting that: i) its corrosion is strongly kinetically-limited in soils, consistent with observations made by many authors); 3,[START_REF] Toque | The Corrosion of Depleted Uranium in Terrestrial and Marine Environments[END_REF][START_REF] Haschke | Corrosion of Uranium in Air and Water Vapor: Consequences for Environmental Dispersal[END_REF][START_REF] Schimmack | Long-Term Corrosion and Leaching of Depleted Uranium (DU) in Soil[END_REF]114,115 and ii) a large part of U is retained by sorption onto the geological materials. In all the samples along the soil profile, U is fully oxidized and appears as uranyl U(VI) species.

Experimental sorption isotherms of U on calcite, purified natural chalk and chalky soil show that U is moderately retained on carbonate, comparable to silicate minerals such as clays. [START_REF] Gillon | Modelling the Baseline Geochemistry of Groundwater in a Chalk Aquifer Considering Solid Solutions for Carbonate Phases[END_REF]18,92 For comparable pH and temperature conditions, lower sorption capacity is observed for chalk compared to calcite. This may be explained by differences in crystal structure for natural calcite in chalk compared to purified calcite, leading to lower surface site density for calcite in chalk (~ 0.01 to 0.1 µmol.m-2), than for purified synthetic calcite (~ 1 to 9 µmol.m-2). [START_REF] Dong | Influence of Calcite and Dissolved Calcium on Uranium(VI) Sorption to a Hanford Subsurface Sediment[END_REF][START_REF] Geipel | Laser and X-Ray Spectroscopic Studies of Uranium-Calcite Interface Phenomena[END_REF] Sorption isotherms on calcite are mainly nonlinear, exhibiting typical Freundlich (at low adsorbate concentrations) or Langmuir patterns (for higher adsorbate concentration). Sorption isotherms and speciation analysis by both EXAFS and TRLFS techniques strongly suggests that U sorption onto chalk and chalky soil follows globally the same speciation scheme as observed for U sorbed onto calcite. The role of secondary phases (essentially organic matter) appears to be predominant at low solid/liquid ratios and/or at high U concentration in adsorbate.

U speciation is observed to evolve from surface to bottom, highlighting several phenomena:

1) The decrease in the contribution of uranyl metaschoepite, as in the first 10 -70 cm of soil, as revealed by EXAFS analysis, attributed to residual weathered fragments and particles of U inherited from the initial deposits. This suggests that the vertical migration of U through the soil is partly realized under micro particle forms. This observation is not surprising, since on the one hand, residual U particles are classically identified in surface soils in comparable contexts (firing test ranges, or battlefields from Kosovo and Kuwait conflicts 9,10,12,[START_REF] Mibus | Migration of Uranium(IV)/(VI) in the Presence of Humic Acids in Quartz Sand: A Laboratory Column Study[END_REF]116 ), and on the other hand, vertical migration of particles is known to occur in soils, enhancing the transport of radioactive fallouts through topsoil horizons. [117][118][119][120][121][122] The speciation of these micro particles could not be studied within the framework of this research because their size (of the order of a few micrometers) is smaller than the lowest size of the synchrotron beam achievable on the MARS beamline.

2) The dominant contribution of the uranium-humic substances complex from 10 to about 60 cm suggests that organic matter is one of the principal complexing agents of U during its migration in the first few decimeters. This is confirmed by sorption isotherms onto chalky soil and onto chalk doped with humic acids which clearly highlights the part taken by humic substances for complexation and immobilization of U in the chalky soil. Nevertheless, these observations are of interest for further investigations on U migration in the subsurface, since humic substances are also known to be easily mobilizable from soils, mobile in soils and aquifers, and can enhance U migration under colloidal form. 36-38,42,43,72,123,124 3) In the first 120 cm of the soil profile, where U content is typically above 10 ppm, the results of EXAFS and TRLFS speciation analysis and interpretations from sorption isotherms are in agreement. The TRLFS analysis indicates a double speciation of U onto carbonate minerals, revealing the presence of at least two complexes for adsorbed U, that change proportion with U loading with a slight rearrangement of oxygen equatorial coordination, consistent with previous interpretations from other authors. [125][126][127] For low U loading on the calcite surface, a bidentate uranyl tricarbonate complex with a liebigite-type structure dominates the speciation. For intermediate to high U loadings, one may propose that the speciation is dominated by a change of conformation in which the uranyl carbonate complex mixes bidentate and monodentate structures and may correspond to U incorporated into trigonal calcite.

A double mechanism shall then be considered for U retention onto carbonate minerals consistent with models developed for sorption and coprecipitation into calcite. It is based on the rearrangement of the uranyl equatorial oxygen coordination in a tricarbonate complex as a function of uranyl environmental conditions. 125-127

TOC Art

Figures

Natural soil characteristics

Modelling of sorption isotherms K d Isotherm

Kd isotherm describes the simplest adsorption process in which the amount of surface adsorbate is proportional to the concentration of adsorbate in the aqueous phase. This linear isotherm model describes an appropriate fit to the adsorption of adsorbate at relatively low concentrations, assuming an homogeneous surface and no maximum sorption capacity for the sorbent (Ayawei 2017). 3 The default stoichiometry (fixed adsorbate on the surface X:adsorbate in solution at equilibrium) in the sorption reaction is assumed to be equal to (1:1), as shown hereafter for the example of uranyl ion :

The sorption concentration on the substrate at equilibrium, (mg.kg -1 ), and the distribution coefficient (L.kg -1 ) were calculated for uranium using the following equations:

Where (mg.kg -1 ) is the amount of the uranium sorbed onto the calcite substrate at equilibrium respectively, (mg.L -1 ) is the initial concentration of uranium in the aqueous solution, (mg.L -1 ) is the equilibrium concentration of uranium in the solution, V (L) is the volume of the solution and m (kg) is the weight of the substrate.

Freundlich Isotherm

The Freundlich equation is an empirical expression for non-linear adsorption equilibrium, widely used in environmental soil chemistry to describe the isothermal variation of gas or solute adsorption (Sparks 2003).

Freundlich isotherm is applicable to adsorption processes that occur on heterogeneous surfaces, and defines the surface heterogeneity and the exponential distribution of active sites and their energies. It can be expressed as:

Where q eq and c eq were defined earlier, K F is the distribution coefficient, and n is a correction factor.

The following equation represents the linear form of the Freundlich a Freundlich isotherm (Boparai, Joseph et al. 2011): (10) The single K F term in the Freundlich equation implies that the energy of adsorption on a surface is independent of surface coverage. Thus, one of the major disadvantages of the Freundlich equation is that it does not predict an adsorption maximum, failing at high adsorbate concentration in the aqueous phase. The value of n can be between 0 and 1, therefore the equation holds well only over a limited range of dissolved adsorbate concentration:

when n = 0, the adsorption is independent of adsorbate concentration in solution,

when n =1, adsorption is directly proportional to adsorbate concentration in solution (equivalent to Kd isotherm).

Langmuir Isotherm

Langmuir isotherm was primarily designed to describe gas-solid phase adsorption, and is also used to qualify and quantify the adsorptive capacity of various adsorbents (Sparks 2003). 4 The Langmuir isotherm accounts for the surface coverage by balancing the relative rates of adsorption and desorption (dynamic equilibrium), and describes a progressively increasing surface occupancy as a function of solute concentration, up until the entire surface area is coated with a monolayer of molecules and no further adsorption can occur (Azizian 2021). 5 The Langmuir equation can be written in the following linear forms: or ( 11), (12) Where q eq and c eq were defined earlier, K L is the distribution coefficient, and q m is Langmuir constant related to adsorption capacity (mg.kg -1 ) of the sorbent, which implies that larger specific surface area and/or pore volume will result in higher adsorption capacity.

One of the most important assumptions of Langmuir isotherm is one-site occupancy adsorption.

Adsorption is proportional to the fraction of the surface of the adsorbent that is open while desorption is proportional to the fraction of the adsorbent surface that is covered (Boparai 2011). The fit was performed in real space, on a window up to R = 4

EXAFS and TRLFS spectra of the uranium model compounds

Uranyl carbonates

Liebigite (nat.) 466 ( 7) 486 ( 10) 506 ( 13) 525 ( 18) 548 ( 17) 565 (33) 0,980 184 This work

Liebigite (nat.) 463 ( 7) 481 ( 10) 502 ( 13) 524 ( 12) 548 ( 17) 574 ( 14) 349 Wang 2008 16 U incorporated in Aragonite 484 (11) 503 ( 13) 525 ( 15) 546 ( 14) 560 ( 45) Elzinga 2004 9 U incorporated in Calcite 497 ( 13) 517 ( 16) 537 (19) With:

- 1 2 Chi square of tested model - 0 2 Chi square of best model.

ν 1 degrees of freedom of tested model fit.

ν 0 degrees of freedom of best model fit.

The  2 values are obtained from ATHENA between k = 3.5 and k = 10.5 from LCF fitting using 1,2 or 3 components ν the degree of freedom is obtained by subtracting the number of variables from the number of independent points (calculated according with the Stern's rule (Stern, 1993 [START_REF] Dhoum | Evaluation of Uranium and Arsenic Retention by Soil from a Low Level Radioactive Waste Management Site Using Sequential Extraction[END_REF] ).

The results show 3 regimes: (1) the surface and the first few cm can be adjusted properly using a 1-component fit (Nugget) compared to a 3-component fit (F value less than 1).

(2) the soil part of the site (from 20 to 50 cm), cannot be adjusted properly using various combinations of a 2-component fit and requires a 3-component fit Nugget + U-HA + U-carb) (F values larger than 1).

(3) the substrate part (above 70 cm) of the site can be adjusted with a 1-comonent fit (U-carb) compared to a 3-component fit (F value less than 1).

The LCF data using 3 comonents for consistency are in full agreement with this analysis as shown in Figure 6.

  processing. U L II edge (20 948 eV) was probed to analyze the L 1 fluorescence line (17 220 eV) in the natural soil samples. This choice is justified by the fact that a strong K edge fluorescence line of strontium at 14165 eV is present close to the main fluorescence line of U at the L III edge (13 614 eV) due to the high concentrations of Sr in the carbonate materials of the study site. Consequently, beam energy calibration was performed at the molybdenum K edge (20 000 eV).

10, 20 ,

 20 30, 40, 50, 60, 80 and 100 cm), were recorded at the U L II edge. The XANES portion of the absorption spectrum is a fingerprint of the U oxidation state and environment. In this region, the shoulder observed at ca. 15 eV after the white line maximum is typical of multiple scattering features along the trans-dioxo form of uranyl. XANES spectra (FigureS9of SI, left) and their first derivative (Figure S9, right) clearly indicate that all the soil samples contain U at formal oxidation state +VI under the form of UO 2 2+

Figure 1 :

 1 Figure 1: Evolution of mass uranium content in ppm (right graph) in soil and chalk along a vertical profile from the study site. Schematic subsoil structure is given on the left, from the topsoil layer to the chalk substratum down to 2.7 m deep.

Figure

  Figure 2abcd: a) Pictures of altered uranium nuggets from the study site: left, dark yellow altered nugget (nugget #1), right, yellowish altered nugget (nugget #2). b) XRD diffractograms of nuggets 1 and 2 (left and right respectively) in comparison with some references of interest. c) EXAFS spectra

Figure 3 .

 3 Figure 3. Experimental adsorption isotherms of uranyl onto solid carbonates substrates (pure calcite, chalk and chalky soil) with corresponding modelling using Kd, Freundlich or Langmuir adjustments. Experimental isotherms data are collected from this work, from older experiments on the same sorbents (Géhin et al., 2016 79 ), and from bibliography on pure Calcite (Dong et al., 2005 87 ; Geipel et al., 1997 88 ).

Figure 4 :

 4 Figure 4: EXAFS spectra (left) and Fourier transform (right) of reference material for uranyl speciation (green: U-nugget #1 in green, brown: U sorbed on HA, and blue: uranyl-tricarbonate complex), together with the series of discrete samples from the vertical soil profile (black to brown faded colored spectra) from surface to 100 cm deep. In red are presented the Linear Combination Fittings of experimental spectra (left), and the real part of the different contributions (right). The integration window of the Fourier transform is of Hanning type and is conducted between k = 3 to 11Å -1 .

Figure 5 :

 5 Figure 5: Compared normalized fluorescence spectra of uranium in a chalky soil vertical profile from the study site, with decreasing U content with depth. Spectra were acquired with a delay of 5 µs with a gate width of 20 µs, by stacking 500 repetitions. The main fluorescence emission lines of uranyl tricarbonate specie 1 (Liebigite-type structure) are reported in dashed lines.

Figure 6 :

 6 Figure 6: Compared evolutions of soil uranium content (right) and normalized uranium speciation along the soil vertical profile (left), as estimated from LCF on EXAFS analysis using a 3-components model.

Figure S1 :

 S1 Figure S1: Mass distribution of total carbonate content (grey and white circles, in mass.%), total organic matter content (black squares, in mass.%)), and uranium (grey diamonds, in ppm) as a function of depth in soil in the study site.

Figure S2 .

 S2 Figure S2. Electron microscope imaging showing the presence of uranium microparticles in the surface soil sample (0 cm, top images) and at 20 cm deep (bottom images) of a vertical profile realized in the chalky soil from the study site. Backscattered electron imaging (on left) shows the presence of multiple U particles lower in size to 50 µm, as well as fluorescence mapping using EDX at the U L- edge (on right).

Figure S3 :

 S3 Figure S3: EXAFS spectra (left) and Fourier transform (right) of U sorbed on HA and Uranyl-carb. In red is presented the fit of the data, and in colored the real part of the different contributions. The integration window of the Fourier transform (in blue) is of Hanning type and is between k = 3 and k = 12.5 Å -1 . The spectra, the fit, and the different contributions of the real part are grouped at the bottom.

Figure S4 :

 S4 Figure S4: Normalized fluorescence spectra of uranium nugget#1 (top, dark curves) and synthetic metaschoepite (bottom, dark grey curves) at room temperature using an excitation wavelength of 355 nm. Spectrum for metaschoepite is recorded using a delay of 20 µs and a gate width of 200 µs. Spectrum for nugget#1 is recorded, using a delay of 100 µs and a gate width of 200 µs. The main fluorescence emission lines obtained from spectra deconvolution are reported above the curves (boxes).

Figure S5 :

 S5 Figure S5: Compared normalized fluorescence spectra of uranium sorbed onto calcite (black curves), onto natural chalk (gray curves), and onto natural chalky soil (light gray curves), for initial dissolved uranium concentration varying from 1 to 1000 ppm. Spectra were acquired with a delay of 100 µs with a gate width of 50 µs, by stacking 500 repetitions.

Figure S6 .

 S6 Figure S6. Normalized fluorescence spectra of uranium sorbed onto Calcite for initial dissolved uranium concentration of 1 ppm. The spectrum deconvolution is achieved considering five major vibronic bands resolved as Gaussian peaks. Peaks positions, reported in boxes, are comparable with published spectral data on U(VI)-substituted synthetic calcite and aragonite, in a Liebigite-like structure (Elzinga 2004 9 , Wang 2005 17 , Wang 2008 16 ). The TRLFS spectrum of a natural uranyl calcium triscarbonate mineral (liebigite) has been recorded with 200 µs delay and exhibits 6 major maxima fairly consistent, with a slight red-shift, with other luminescence data found in literature for liebigite, uranium incorporated in aragonite in a tricarbonate structure, or calcium-uranyl tricarbonate aqueous complex.

Figure S7 :

 S7 Figure S7: Determination of fluorescence bands of U(VI)-carbonate sorbed second specie in residuals (in gray) after subtraction of liebigite cumulative peaks from raw fluorescence spectrum of U(VI) sorbed onto chalk for initial dissolved uranium concentration of 10 ppm (in black). Residual spectrum deconvolution is achieved considering four major bands resolved as gaussian peaks. Peaks positions, reported in boxes, are comparables with published spectral data for U(VI) incorporated in calcite, as described by Elzinga 2004. 9

Figure S8 :

 S8 Figure S8: Normalized fluorescence spectra of uranium sorbed onto calcite (left), chalk (center) and chalky soil (right) for initial dissolved uranium concentration varying from 5 to 100 ppm. Detail of spectra deconvolution is reported considering three components: uranyl carbonate with liebigite-type structure (specie 1, in blue), uranyl incorporated in calcite (specie 2, from Elzinga 2004 9 , in green), natural Schoepite (from Wang 2008 16 , in yellow). Spectra baseline are fitted considering a Gaussian model, and reported in dashed red line. Deconvolution regression coefficient is given below legends, for indication.

Figure S9 :

 S9 Figure S9: XANES spectra recorded at the U L II edge for soil samples at increasing depth in a vertical profile in the study site (left) and corresponding derivatives (zoom on the edge region, right).

Figure S10 :

 S10 Figure S10: Modulus of the Fourier transform (FT) of the EXAFS spectra recorded at the U L II edge for soil samples at increasing depth in a vertical profile in the study site, (left), and real part of the FT (zoom on the first coordination sphere region, right).

Figure S11 :

 S11 Figure S11: Normalized fluorescence spectra of uranium in a chalky soil profile from the study site, for increasing depth and decreasing U content (soil U content is reminded in the top left cornner for each spectrum). Detail of spectra deconvolution is reported considering three components: uranyl carbonate with liebigite-type structure (specie 1, in blue), uranyl incorporated in calcite (specie 2, from Elzinga 2004 9 , in green), natural Schoepite (from Wang 2008 16 , in yellow). Spectra baseline are fitted considering a Gaussian model, and reported in dashed red line. Deconvolution regression coefficient is given below legends, for indication

Figure S13 :

 S13 Figure S13: EXAFS spectra at the U L II edge and Fourier transform of the surface sample (0 cm, top) and deepest sample (100 cm, bottom).

  ). Metaschoepite reference sample was synthetized following the method proposed by Bruno et al.,[START_REF] Bruno | The Solubility of Amorphous and Crystalline Schoepite in Neutral to Alkaline Aqueous Solutions[END_REF] by dissolving uranyl acetate (UO 2 (H 4 C 2 O 2 ) 2 ) under

Table S2

 S2

	of SI). Solid sorbents (calcite, chalky soil, chalk) were put in suspension with distilled water,
	in order to obtain a mass out of volume ratio close to 100 g.L -1 . After 2 weeks of stirring and the
	solution being considered at equilibrium, U(VI) was added in the reactor with an initial concentration
	ranging from ~ 1 to 1 × 10 6 µg. L -1 , in solutions prepared by dilution from parent stock solution of
	uranyl nitrate (1000ppm, Fluka™) with deionized water. After the addition of U,the pH of the
	suspension was adjusted to 7.3 with HCl or NaOH, if necessary. Reactors consisted either of a closed
	50 ml PTFE reaction vessel (for U adsorption on calcite), or on a 2 part Pyrex® glass reaction vessel
	(Ace-Glass Inc, NJ, USA) containing a working volume of 1 L (for U adsorption on chalky soil and
	chalk), and constantly shaken during the duration of the experiment (1 week). The pH of the
	suspensions was recorded every 3 days using solid polymer open junction Xerolyt electrodes (Mettler-
	Toledo, France). The reacted suspensions were sampled after one week of equilibrium at room
	temperature (25°C). The samples were quickly filtered and acidified with 2% HNO 3 before the
	analysis. Concentration of U was measured by ICP-MS (Perkin Elmer ICP-MS Elan DRC-e) with
	expanded uncertainty established at a level of confidence of 95%.
	Sorption Isotherms modelling. Experimental sorption isotherms data were modelled with three
	different functions: Kd, Freundlich and Langmuir isotherms (see details in SI paragraph 2.), in order to
	determinate sorbent sorption capacity from parameter estimation. 80-83

Table S5

 S5

	of SI) with 2 axial O at 1.82 ± 0.01 Å, and 6 equatorial O at 2.45 ± 0.01 Å , corresponding to
	3 bidentate CO 3 2-groups in very good agreement with the atomic environment of uranium in the

Table S1 :

 S1 Summary of the mineralogical and organic composition (given in mass. %) and bulk uranium content (in ppm) of the soil and chalk samples from the study site (modified fromGillon et al.,

	2010). 1
				Mineralogical composition (mass.%)			(ppm)
	Sample	Calcite + Dolomite	Feldspars + Micas	Kaolinite	Silica	Gibbsite Fe-oxides	Matter Organic	U
	Topsoil -colluvium*	59%	11%	17%	6%	0%	3%	4%	1-2
		0	87%	3%	2%	0%	2%	1%	5%	6358
		-10	86%	3%	4%	0%	2%	1%	4%	502
	area)	-20	87%	3%	3%	0%	2%	1%	3%	260
	(contaminated samples profile	-30 -40 -50 -60 -70 -80 -90	88% 89% 91% 93% 95% 95% 96%	3% 3% 2% 2% 2% 2% 1%	4% 4% 3% 1% 1% 1% 0%	0% 0% 0% 0% 0% 0% 0%	2% 2% 2% 2% 2% 2% 2%	1% 1% 1% 1% 1% 0% 0%	3% 2% 1% 0.2% 0.3% 0.3% 0.3%	195 152 120 60 22 16 17
	Soil	-100	94%	2%	0%	0%	2%	1%	0.3%	15
		-200	96%	1%	0%	0%	2%	0%	0.1%	8
		-250	97%	1%	0%	0%	2%	0%	0.1%	4
	Chalk substrate*	96%	2%	0%	0%	1%	0%	0.2%	1

*: references values in the geochemical background outside contaminated area

Table S2 .

 S2 Specific surfaces of solid phases samples used as adsorbent in sorption experiments.

	Sample type	Specific surface (BET) (m².g -1 )	Reference
	Purified Calcite	2.2	Vdovic 2001 2
	Chalk	3.2 ± 0.1	This work
	Chalky soil	5.4 ± 0.3	This work

Table S3 .

 S3 Concentration of the different elements composing the electrolyte

		Concentration (mmol.L -1 )
	HCO 3 -	3.8
	Ca 2+	2.2
	Mg 2+	0.03
	Sr 2+	0.004
	Na +	0.168
	K +	0.024
	Cl -	0.487
	NO 3 -	0.218
	SO 4 2-	0.068
	F -	0.019
	SiO 2 (aq)	0.067

Table S4 .

 S4 Experimental conditions and statistical parameter estimation for uranyl adsorption isotherms on carbonates solid phases from sorption experiments.

	Phase type	Source	M/V (g.L -1 )	Experimental data Sbulk (m².g -1 ) pH T (°C)	[sites]Calcite (µmol/m²)	Type of model	R²	Isotherm fitting parameters RMSE K(d,F,L) (L.kg -1 ) n	[sites]Calcite qm (µmol/m²)
		Dong 2005 7	100	n.m.	8,4 25	n.m.	Freundlich 0,998 0,24	13,0	0,86
		Geipel 1997 8	5	6,8	8,6 25	1,2	Langmuir 0,972 211	0,016	2,1
	Purified Calcite	Elzinga 2004 9 Niu 2019 10 Niu 2019 10	1 10 10	10 n.m. n.m.	8,3 25 6,5 25 8,5 25	9 n.m. n.m.	Freundlich 0,964 216 Langmuir 0,970 56 Freundlich 0,985 120	232,5 0,068 75,0	0,52 0,6	0,4
		This work	100	2,2	7,0 25	n.m.	Freundlich 0,904 6,2	14,5	0,59
		This work	100	2,2	9,1 25	n.m.	Langmuir 0,955	22	0,050	0,60
		Géhin 2016 11	100	3,2	7,3 12	n.m.	Langmuir 0,556 7,2	0,12	0,03
		Géhin 2016 11	100	3,2	7,3 12	n.m.	Freundlich 0,515 7,5	5,1	0,35
		Géhin 2016 11	100	3,2	7,3 12	n.m.	Kd	0,344 9,6	0,33
		This work	200	3,2	7,4 25	n.m.	Langmuir 0,999 0,05	1,2	0,01
	Chalk	This work	200	3,2	7,4 25	n.m.	Freundlich 0,994 0,09	5,4	0,79
		This work	200	3,2	7,4 25	n.m.	Kd	0,978 0,19	6,3
		This work	100	3,2	7,4 25	n.m.	Freundlich 0,969	14	26,5	0,66
		This work	100	3,2	7,4 25	n.m.	Kd	0,970 21,6	9,3
		This work	100	3,2	7,6 25	n.m.	Langmuir 0,777 13,3	0,75	0,1
		Géhin 2016 11	100	5,4	7,3 12	n.m.	Langmuir 0,892 22,3	0,14	0,24
		Géhin 2016 11	100	5,4	7,3 12	n.m.	Freundlich 0,799 31,0	39,0	0,35
		Géhin 2016 11	100	5,4	7,3 12	n.m.	Kd	0,509 56,5	20
	Chalky	This work	200	5,4	7,3 25	n.m.	Langmuir 0,988 1,86	0,02	0,8
	soil	This work	200	5,4	7,3 25	n.m.	Freundlich 0,990 1,86	28,8	1,2
		This work	200	5,4	7,3 25	n.m.	Kd	0,988 1,86	23,1
		This work	100	5,4	7,3 25	n.m.	Freundlich 0,959 174	18,8	1,02
		This work	100	5,4	7,3 25	n.m.	Kd	0,962 157	21,7
	Chalk +	Géhin 2016 11	100	3,2	7,3 12	n.m.	Langmuir 0,937 5,4	0,41	0,15
	Humic	Géhin 2016 11	100	3,2	7,3 12	n.m.	Freundlich 0,929 5,8	31,8	0,7
	Acid	Géhin 2016 11	100	3,2	7,3 12	n.m.	Kd	0,901 7,55	26,1

Table S5 :

 S5 Parameters used for fitting the EXAFS spectra of the reference samples U-nugget #1; Unugget #2, U sorbed on HA and uranyl tricarbonate complex. For comparison, data from the XRD analysis of metaschoepite and Liebigite are added.

					U-C	3*	0.008	2.93 ± 0.01
					U-COdist	6*	0.02	4.02 ± 0.02
					U-Oax	2		1.80 ± 0.01
			15
	Liebigite	2001) (Bernhard	Contrib. U-Oax U-Ocarb U-C U-Ca	N 2 6 3 2	 2 (Å ²) 0.004	R (Å ) 1.82 ± 0.01 2.43 ± 0.01 2.88 ± 0.01 4.02 ± 0.02	e 0 ; S 0 2 ;  red 2 ; R f (%)
					U-Oeq1	2.4 ± 0.2	0.013	2.31 ± 0.01
		1
		Nugget		U-Oeq2	2.4 ± 0.2	0.013	2.50 ± 0.01	4.6; 1.0; 8.6; 0.9
					U-U	3.5 ± 0.3	0.019	3.92 ± 0.02
					U-Oax_MD	4	0.008	3.64
					U-Oax	2	0.003	1.82 ± 0.01
					U-Oeq1	2.4 ± 0.2	0.009	2.33 ± 0.01
		2
		Nugget		U-Oeq2	2.4 ± 0.2	0.009	2.51 ± 0.01	5.6; 1.0; 3.1; 1.1
					U-U	2.4 ± 0.3	0.009	3.90 ± 0.02
					U-Oax_MD	4	0.006	3.64 ± 0.02
	Méta-schoepite	2006) 12 (Froideval	U-Oax U-Oeq1 U-Oeq2 U-U	2 1.9 ± 0.4 1.8 ± 0.4 1.9 ± 0.4	----	1.79 ± 0.01 2.32 ± 0.02 2.51 ± 0.02 3.91 ± 0.02
					U-Oax	1.9 ± 0.2	-	1.77 ± 0.01
	Shoepite	(Walshe	13 2014)	U-Oeq1	3.5 ± 1.0	-	2.32 ± 0.02
					U-Oeq2	2.4 ± 0.6	-	2.51 ± 0.02
					U-Oax	1.7 ± 0.3	-	1.81 ± 0.01
		14
		1996)		U-Oeq1	2.1 ± 0.5	-	2.26 ± 0.02
		(Allen		U-Oeq2	1.9 ± 0.5	-	2.47 ± 0.02
		Shoepite		U-U1 U-U2	1.2 ± 0.4 1.4 ± 0.4	--	3.84 ± 0.02 4.53 ± 0.02
					U-Oax	2*	0.003	1.80 ± 0.01
			acids	U-Omono	2.5 ± 0.1	0.003	2.29 ± 0.01
	U-HA	-Humic	U-Obi	3.0 ± 0.2	0.003	2.44 ± 0.01	6.2; 1.2; 2.1; 0.3
			UO2 2+	U-Cmono	N(Omono)	0.013	3.03 ± 0.02
					U-Cbi	N(Obi)/2	0.013	3.49 ± 0.02
	U-carb	UO2(CO3)	3 4-	U-Oax U-Ocarb	2* 6*	0.003 0.011	1.82 ± 0.01 2.45 ± 0.01	6.9; 1.0; 7.0; 0.3

Table S6 .

 S6 Spectral properties and fitting parameters for TRLFS spectra of reference samples (this work, and literature, white cells), and for U(VI) sorbed onto calcite and chalk (gray cells).

						Timelife
	Reference samples & data :	Spectral maxima, nm (Width, nm)	fit R²	µs	Source
	Uranyl hydrates
	Metaschoepite (syn.)	500 (21) 519 (20) 539 (26) 566 (28) 0,993	50	This work
	Nugget #1	500 (22) 520 (17) 540 (23) 569 (27) 0,997	131	This work
	Metaschoepite	511	527	552		17	Wang 2008 16
	Schoepite	514 (26) 532 (22) 552 (25) 580 (36)		21	Wang 2008 16

Table S7 :

 S7 Best fit parameters of the EXAFS spectra at the U L II edge of the surface sample (0 cm) and deepest sample (100 cm).

		Contribution	N		s 2 (Ų)	R (Å)	E 0 (eV)	s 0 2	 red	2	R factor (%)
		U-Oax	2		0.004	1.82 ± 0.01
	sample Surface	U-Oeq1 U-Oeq2 U-U	2.8 ± 0.2 2.9 ± 0.2 3.0 ± 0.3	0.010 0.010 0.012	2.27 ± 0.01 2.45 ± 0.01 3.91 ± 0.02	5.2	1	7.9	2.3 %
		U-Oax_MD	4		0.008	3.64
		Contribution	N	s 2 (Å ²)		R (Å )	E 0 (eV)	s 0 2	 red 2	R factor (%)
		U-Oax	2	0.002		1.81 ± 0.01
	cm sample	U-Oeq U-C	6* 3*	0.007 0.004		2.44 ± 0.01 2.9 2± 0.02	6.5	0.9	1.9	3.4 %
	100	U-Odist	6*	0.004		4.2 ± 0.03

Acknowledgements

This worked was financed by Commissariat à l'Energie Atomique et aux Energies Alternatives, Synchrotron SOLEIL and Université Côte d'Azur with the contribution of Academy Space Environment, risk and resilience of Université Côte d'Azur (through the UCAJEDI Investments in the Future project managed by the National Research Agency (ANR) with the reference number ANR-15-IDEX-01).

Supporting Information

Supporting Information file is provided with 3 sections: 1. Natural soil characteristics; 2. Modelling of sorption isotherms; 3. EXAFS and TRLFS spectra of the uranium model compounds.

Supporting Information file

Uranium speciation in a chalky soil

Simon Bayle 1,2,3 (ORCID 0000-0003-3851-5459), Maria Rosa Beccia 1 (ORCID 0000-0001-5397-9759), Christophe Moulin 3,4 , Pier Lorenzo Solari 2 *, Christophe Den Auwer 1 * (ORCID 0000-0003-