Karin Knapp

Remi Norrfors
email: norrfors@kth.se

Muriel Marsac

Stephanie Bouby

Susanna Heck

Johannes Wold

Thorsten Lützenkirchen

Schäfer

Norrfors

Rémi Marsac

Muriel Bouby

Stephanie Heck

Susanna Wold

Johannes Lützenkirchen

Thorsten Schäfer

Montmorillonite colloids: II. Colloidal size dependency on radionuclide adsorption

Keywords: Montmorillonite colloids, particle size effect, adsorption, radionuclides, nuclear waste disposal

come

Introduction

The deep geological repository concept for high-level nuclear waste includes several barriers (technical, geotechnical and geological) to isolate or retard the potential transport of radionuclides (RNs) towards the biosphere. Compacted bentonite is planned to be the geotechnical barrier surrounding the metal canisters [START_REF] Skb | Design and production of the KBS-3 repository[END_REF]. Bentonite consists mainly of montmorillonite (smectite clay) and is chosen for its favourable swelling capacities, its high cation adsorption capacities and its strong mechanical stability [START_REF] Karnland | Mineralogy and sealing properties of various bentonites and smectite-rich clay materials[END_REF]. In case of canister failure, this geotechnical barrier will retard the RN transport towards the biosphere. However, in contact with groundwater of low ionic strength, the bentonite barrier may release stable montmorillonite colloids [START_REF] Geckeis | Results of the colloid and radionuclide retention experiment (CRR) at the Grimsel Test Site (GTS), Switzerland -impact of reaction kinetics and speciation on radionuclide migration[END_REF][START_REF] Missana | Generation and stability of bentonite colloids at the bentonite/granite interface of a deep geological radioactive waste repository[END_REF][START_REF] Schäfer | Nanoparticles and their influence on radionuclide mobility in deep geological formations[END_REF]. The montmorillonite colloids can be transported through the geosphere via advective water flows [START_REF] Möri | The colloid and radionuclide retardation experiment at the Grimsel Test Site: influence of bentonite colloids on radionuclide migration in a fractured rock[END_REF]. To be able to estimate the colloid facilitated transport of RNs, a large and complex set of parameters has to be considered.

Colloidal transport of montmorillonite through bedrock fractures depends on the ground water flow velocity [START_REF] Zhang | A review of colloid transport in fractured rocks[END_REF]. At high water flow velocity, i.e. in clean bedrock fractures with a low amount of fracture filling material (FFM), a laminar water flow is expected which can separate the colloids according to their mass (for the same specific density), and thus their size. In the opposite, if the fracture contains sufficient amount of FFM, the FFM may be simplified seen as a porous medium, where larger colloids can be transported in the fastest streamlines due to size exclusion effects [START_REF] Bradford | Physical factors affecting the transport and fate of colloids in saturated porous media[END_REF][START_REF] Simpkins | Transport of colloids in saturated porous media: A pore-scale observation of the size exclusion effect and colloid acceleration[END_REF]. Since size separation of montmorillonite colloids may occur in bedrock fractures, their size, morphology, stability and RN adsorption capacities are some of the important parameters for estimating colloid facilitated transport of RNs. These parameters should therefore be included in reactive transport modelling. In a recent study on montmorillonite colloidal stability performed at various ionic strengths [START_REF] Norrfors | Montmorillonite colloids: I. Characterization and stability of dispersions with different size fractions[END_REF], no dependency on the montmorillonite size could be observed. Based on this study, hydrodynamic induced size fractionation and the potential formation of different colloid size fractions will not influence their stability under the same geochemical conditions but some colloids may be able to migrate further due to e.g. steric stabilisation effects by natural occurring dissolved organic matter as fulvic acid (FA).

It is of high importance to examine how the RN adsorption depends on the clay colloid size and thus, the total exposed surface area. Previous studies have demonstrated a size-dependent reactivity of hematite nano-mineral surfaces on Cu 2+ adsorption [START_REF] Madden | Insights for size-dependent reactivity of hematite nanomineral surfaces through Cu2+ sorption[END_REF]. If the RN adsorption is shown to depend on the montmorillonite colloidal size, owing to their exposed lateral surface area, the amount of RNs transported through the bedrock would be over-or underestimated in the reactive transport models used today [START_REF] Vahlund | A direct numerical approach to solving the transport equations for radionuclide transport in fractured rock[END_REF]. Alternatively, if the exposed surface area does not have a large impact on the adsorption capacities, averaged adsorption capacities for montmorillonite can be used, as in the case of cation exchange, which is thought to be independent of colloidal size [START_REF] Stul | Particle-size distribution, cation exchange capacity and charge density of deferrated montmorillonites[END_REF]. Tournassat et al. [START_REF] Tournassat | Nanomorphology of montmorillonite particles: Estimation of the clay edge sorption site density by low-pressure gas adsorption and AFM observations[END_REF] suggested a model where the amount of edge sites can be calculated from the aspect ratio and the density of the clay particles by assuming a site density of silanol and aluminol edge sites per mass clay. Given the same density of all clay particles, the amount of edge sites are proportional to the aspect ratio. This model was further developed by the authors in a previous work [START_REF] Norrfors | Montmorillonite colloids: I. Characterization and stability of dispersions with different size fractions[END_REF] where montmorillonite colloids were successfully separated into fractions with varying mean colloid sizes. These colloids were used in the adsorption study presented in this work.

Accordingly, this work aims to investigate the influence of the exposed lateral surface area of montmorillonite colloids (which is related to their size) on the adsorption of trace concentrations of RNs in a synthetic groundwater (SGW) representing a glacial meltwater type [START_REF] Geckeis | Results of the colloid and radionuclide retention experiment (CRR) at the Grimsel Test Site (GTS), Switzerland -impact of reaction kinetics and speciation on radionuclide migration[END_REF][START_REF] Kunze | The influence of colloid formation in a granite groundwater bentonite porewater mixing zone on radionuclide speciation[END_REF][START_REF] Möri | The colloid and radionuclide retardation experiment at the Grimsel Test Site: influence of bentonite colloids on radionuclide migration in a fractured rock[END_REF]. In this work, the term clay colloids refer to aggregates, consisting of stacks of several clay mineral layers. Seven montmorillonite dispersions with different mean particle sizes, including one containing fulvic acids (FA), were used for adsorption studies with 232 Th(IV), 99 Tc(VII), 233 U(VI), 237 Np(V) and 242 Pu(IV) during a long (up to six months) equilibration time. The 2 sites protolysis non-electrostatic surface complexation and cation exchange (2 SPNE SC/CE) model [START_REF] Bradbury | Predictive sorption modelling of Ni(II), Co(II), Eu(IIII), Th(IV) and U(VI) on MX-80 bentonite and Opalinus Clay: A "bottom-up" approach[END_REF] was used to predict the uptake of RNs. In addition, we attempted to estimate the influence of expected amounts of edge sites, calculated for each particle size, on the RN adsorption process.

Material and Methods

Montmorillonite colloids and fulvic acids

Various dispersions containing size fractionated montmorillonite colloids originating from raw Wyoming MX80 bentonite (M w = 372.6 g/mol [START_REF] Karnland | Mineralogy and sealing properties of various bentonites and smectite-rich clay materials[END_REF]), American Colloid Co., were used in this study. Preparation and characterization of the montmorillonite dispersions are explained in detail in previous work [START_REF] Norrfors | Montmorillonite colloids: I. Characterization and stability of dispersions with different size fractions[END_REF]. A summary of the preparation protocol is presented schematically in Figure 1. Details of centrifugation speeds and times are given in Table 1. In brief, the un-purified MX80 bentonite (10 g/L) was placed into carbonated synthetic groundwater (SGW) and settled during three days. The supernatant was collected and named dispersion S0. The dispersion S0 was further separated into different size fractions by sequential or direct centrifugation. In the sequential (ultra-)centrifugation, S0 was first centrifuged to obtain supernatant S1. Thereafter, S1 was centrifuged to obtain supernatant S2 etc., resulting as well in supernatants S3 and S3.5. For comparison, S0 was also directly ultra-centrifuged (UC) to obtain the dispersion S3.5 UC . It was found that the dispersions S3.5 and S3.5 UC exhibit the same size distribution of colloids despite the different separation protocol. All dispersions have been characterized according their size, chemical composition, mineralogical composition and colloidal concentration in a previous paper, where all details can be found [START_REF] Norrfors | Montmorillonite colloids: I. Characterization and stability of dispersions with different size fractions[END_REF]. Suspension concentration was adjusted to 20 mg/L in the RN batch adsorption experiments.

To be able to estimate the exposed lateral surface and thus the amount of adsorption edge sites in each montmorillonite dispersions, the clay particles were assumed to be disc-shaped [START_REF] Norrfors | Montmorillonite colloids: I. Characterization and stability of dispersions with different size fractions[END_REF]. Then, their mean disc diameter and height were calculated by equations developed by Jennings and Parslow [START_REF] Jennings | Size and thickness measurement of polydisperse clay samples[END_REF][START_REF] Jennings | Particle Size Measurement: The Equivalent Spherical Diameter[END_REF].

For this purpose, the mean colloidal sizes of each dispersion measured by different techniques were used in the equations to obtain the mean disc sizes and heights, see Table 1 and [START_REF] Norrfors | Montmorillonite colloids: I. Characterization and stability of dispersions with different size fractions[END_REF]. These diameters are mean values for the whole size distribution of colloids in the respective dispersions. S0 includes both larger and smaller sized colloids, whereas S3.5 and S3.5 UC only include smaller colloids and were therefore less polydisperse.

The amount of adsorption edge sites was calculated according to the work of White and Zelazny [START_REF] White | Analysis and implications of the edge structure of dioctahedral phyllosilicates[END_REF] and [START_REF] Tournassat | Nanomorphology of montmorillonite particles: Estimation of the clay edge sorption site density by low-pressure gas adsorption and AFM observations[END_REF], assuming a clay density of 2.7 g/cm³. The calculated mean clay particle disc diameters were used to determine their lateral area from which the amount of sites in the clay dispersions could be estimated (in mmol/kg). The estimated amounts of sites are presented in Table 1.

Complementary, one clay suspension was prepared in presence of FA, representing natural organic matter. The fulvic acid (FA-573) has been extracted from a natural groundwater (Gohy573) of the Gorleben site, Germany [START_REF] Wolf | Humic substances in performance assessment of nuclear waste disposal: Actinide and iodine migration in the far-field[END_REF]. A small amount of FA was weighed, dissolved in NaOH and diluted in a separate clay dispersion (10 g raw MX80 bentonite in 1 L SGW). Thereafter, this FA-clay dispersion was ultra-centrifuged during the same time and at the same speed as the dispersion S3.5 UC , and denoted S3.5 UC,FA . The final concentration of FA in the batch adsorption experiment was approximately 1.9 mg/L after dilution of the dispersion S3.5 UC,FA to a concentration of 20 mg/L montmorillonite colloids.

Synthetic Groundwater (SGW)

Synthetic, carbonated groundwater (SGW) is used as background electrolyte to simulate a glacial meltwater of low ionic strength, similar to the granitic groundwater present at the Grimsel Test Site [START_REF] Geckeis | Results of the colloid and radionuclide retention experiment (CRR) at the Grimsel Test Site (GTS), Switzerland -impact of reaction kinetics and speciation on radionuclide migration[END_REF][START_REF] Kunze | The influence of colloid formation in a granite groundwater bentonite porewater mixing zone on radionuclide speciation[END_REF][START_REF] Möri | The colloid and radionuclide retardation experiment at the Grimsel Test Site: influence of bentonite colloids on radionuclide migration in a fractured rock[END_REF]. The SGW is prepared by mixing different salts (NaCl, CaCl 2 , MgCl 2 , NaF, Na 2 SO 4 and NaHCO 3 ), yielding the composition presented in Table 2. The total ionic strength is 1.6 mM and the pH is 8.4 ± 0.1 prior to contact with montmorillonite colloids and radionuclide cocktail. [START_REF] Guillaumont | Update on the chemical thermodynamics of uranium, neptunium, plutonium, americium and technetium[END_REF][START_REF] Neck | Solubility and Hydrolysis Behaviour of Neptunium(V)[END_REF][START_REF] Runde | Solubility and sorption of redox-sensitive radionuclides (Np, Pu) in J-13 water from the Yucca Mountain site: comparison between experiment and theory[END_REF], Tc(VII) [START_REF] Alliot | Speciation of Technetium(IV) in Bicarbonate Media[END_REF], U(VI) and Pu(IV) [START_REF] Huber | Sorption reversibility kinetics in the ternary system radionuclide-bentonite colloids/nanoparticles-granite fracture filling material[END_REF], Th(IV) (Östhols et al., 1994)).

Experimental conditions and procedure

All experiments were performed at room temperature in a glove box under Ar atmosphere (<1 ppm O 2 ). pH was measured with a semi micro Ross electrode (81-03, Orion Co.) in combination with a digital pH-meter (720A, Orion Co.). This set-up was calibrated using four commercial buffer solutions (Merck). The redox potentials in the clay dispersions were measured using an Orion 525A (Eh meter) and a Pt electrode combined with Ag/AgCl reference system (Metrohm). Raw data were converted into Eh vs. standard hydrogen electrode (SHE) by correcting for the potential of the reference electrode. A commercial redox-buffer (+220 mV, Schott instruments) was used for calibration. An equilibration time of 30 min was applied for all Eh measurements. Uncertainties in Eh measurements were ± 50 mV [START_REF] Altmaier | Intercomparison of redox determination methods on designed and near-neutral aqueous systems[END_REF][START_REF] Kirsch | Oxidation State and Local Structure of Plutonium Reacted with Magnetite, Mackinawite, and Chukanovite[END_REF].

Each batch adsorption sample was prepared in duplicate and contained 20 mg/L montmorillonite in a total volume of 50 mL. For monitoring radionuclide adsorption, aliquots of the samples were taken after 72 h (~3 days), 260 h (~2 weeks), 580 h (~1 month) and 3800 h (~6 months). Initially, the top 0.5 mL of the sample was analysed by ICP-MS to obtain the amount of stable montmorillonite colloids in the dispersion. Thereafter, approximately 4 mL of each sample was transferred into ultra-centrifugation vials, sealed by welding inside the glove box and afterwards ultra-centrifuged (Beckman XL-90, rotor type 90Ti) for 1h at 90.000 rpm (centrifugal force of approximately 7•10 5 × g). Subsequently, a volume of 0.5 mL of the ultra-centrifuged supernatants was sampled and analysed by ICP-MS. This ultracentrifugation procedure has been proven to effectively remove montmorillonite colloids and thereby the RNs associated to colloids [START_REF] Altmaier | Solubility and colloid formation of Th(IV) in concentrated NaCl and MgCl2 solution[END_REF]). When the experiment ended (i.e. after 6 months), all samples were characterised by pH = 8.9 ± 0.3 and Eh (SHE) = +230 ± 50 mV.

From the ICP-MS data, the distribution of RNs was calculated and defined as (i) the amount of soluble species RNs in dispersion (%Free), (ii) RNs associated to montmorillonite colloids Identical adsorption results were obtained for these two dispersions, indicating a good reproducibility of the fractionation procedure developed [START_REF] Norrfors | Montmorillonite colloids: I. Characterization and stability of dispersions with different size fractions[END_REF]. K D values for each RN were calculated for the overall RN adsorption as "average K D values", which does not distinguish between suspended (%Colloids) and particulate (settled particles, %Part) clay. The expression for K D is given in equation 1:

𝐾 = [ ] [ ] [ ] • (𝑑𝑚 /𝑘𝑔) (1)
where [𝑅𝑁] and 𝑚 are the total, initial concentrations of RNs and montmorillonite, respectively.

[𝑅𝑁] is the concentration of soluble species of RNs in solution after a given equilibration time. A standard deviation of log (K D (dm 3 /kg)) = 0.5 was applied to all K D values, as suggested by [START_REF] Bradbury | Predictive sorption modelling of Ni(II), Co(II), Eu(IIII), Th(IV) and U(VI) on MX-80 bentonite and Opalinus Clay: A "bottom-up" approach[END_REF], due to experimental uncertainties for strongly adsorbing elements.

Additionally, with the mean amount of adsorption sites estimated for each clay dispersion, a As mentioned above, the montmorillonite dispersions (S0 and S1) containing the largest sized clay particles contain a mixture of colloidal sizes. In these dispersions, the larger clay particles sediment with time, whereas the smaller clay colloids will remain suspended.

Therefore, a better evaluation of RN adsorption onto different colloid sizes could be made in these dispersions if the average K D value was compared to the K D value for the particulates (settled, large particles) in the clay dispersion, i.e. K D,part , defined as:

𝐾 , = [ ] [ ] [ ] [ ] • (𝑑𝑚 /𝑘𝑔) (3)
where [𝑅𝑁] is the amount of RNs associated to stable, suspended clay colloids and 𝑚 the amount of clay particulates.

Geochemical speciation modelling

Predominance diagrams were plotted with PhreePlot [START_REF] Kinniburgh | PhreePlot: Creating graphical output with PHRREQC[END_REF], which contains an embedded version of the geochemical speciation program PHREEQC [START_REF] Parkhurst | User's guide to PHREEQC (version 2)[END_REF]. Stability constants for dissolved species and solubility calculations were included in the NEA database (U/Np/Pu: [START_REF] Guillaumont | Update on the chemical thermodynamics of uranium, neptunium, plutonium, americium and technetium[END_REF], Tc: [START_REF] Rard | Chemical thermodynamics of technetium[END_REF], Th: [START_REF] Rand | Chemical Thermodynamics of Thorium[END_REF]). Fluoride and carbonate complexes for all RNs were considered in the calculations. However, at the present chemical conditions, the fluoride complexes did not play a significant role for the speciation. The formation constants of Pu(OH) 2 (CO 3 ) 2 2-and Np(OH) 2 (CO 3 ) 2 2-selected by THEREDA (Release 2013-08-02) were used in all calculations. The selected constants (in the NEA and THEREDA databases) were consistent with each other. The Davies equation was used for activity coefficient calculations.

Reduction of sulfate and CO 2 was not considered in the calculations.

The cation exchange capacity (CEC) was estimated to 0.87 eq/kg by Marques Fernandes et al.

(Marques [START_REF] Fernandes | U(VI) sorption on montmorillonite in the absence and presence of carbonate: A macroscopic and microscopic study[END_REF]. With the available adsorption constants, preliminary tests with all RNs showed that cation exchange reactions had no significant influence at pH 8.9.

Consequently, cation exchange reactions were neglected in the present study.

To predict adsorption on natural clay rocks, Bradbury and Baeyens [START_REF] Bradbury | Predictive sorption modelling of Ni(II), Co(II), Eu(IIII), Th(IV) and U(VI) on MX-80 bentonite and Opalinus Clay: A "bottom-up" approach[END_REF] applied a linear additive model (LAM). Their "bottom-up" approach describes the overall adsorption of an element to a complex mineral mixture as the sum of the amount adsorbed to each single type of mineral. For bentonite, they considered montmorillonite as the only adsorbent for radionuclides. In the present case, montmorillonite represents 75 wt.% of were not observed [START_REF] Turner | Neptunium(V) sorption on montmorillonite; an experimental and surface complexation modeling study[END_REF]. Adsorption of TcO 4 -to clays can be neglected [START_REF] Guillaumont | Update on the chemical thermodynamics of uranium, neptunium, plutonium, americium and technetium[END_REF][START_REF] Ticknor | The effect of fulvic acid on the sorption of actinides and fission products on granite and selected minerals[END_REF].

Though added in oxidized form, Np(V) might be reduced to Np(IV) at the clay surface in line with work by Marsac et al. [START_REF] Marsac | Neptunium redox speciation at the illite surface[END_REF] in the Np-illite system. Np(IV) adsorbs more strongly to minerals than the oxidized form. Surface complexation constants for Np(IV) are unknown for montmorillonite, but Bradbury and Baeyens [START_REF] Bradbury | Modelling the sorption of Mn(II), Co(II), Ni(II), Zn(II), Cd(II), Eu(III), Am(III), Sn(IV), Th(IV), Np(V) and U(VI) on montmorillonite: Linear free energy relationships and estimates of surface binding constants for some selected heavy metals and actinides[END_REF][START_REF] Bradbury | Sorption modelling on illite. Part II: Actinide sorption and linear free energy relationships[END_REF] proposed linear free energy relationships (LFER) between cation hydrolysis constants (equation 4) and surface complexation constants (equation 5) for illite and montmorillonite within the 2SPNE SC/CE model.

𝑀 + 𝑥𝐻 𝑂 ⇌ 𝑀(𝑂𝐻) ( ) + 𝐻 , giving OH K x (4)
≡ SOH + 𝑀 + (𝑥 -1)𝐻 𝑂 ⇌ ≡ SOM(OH) ( ) ( ) + 𝑥𝐻 , giving S K (x-1) (5)
The LFERs for montmorillonite (equation 6) and illite (equation 7) are: log K ( ), = 8.1 + 0.90 × log K (6) log K ( ), = 7.9 + 0.83 × log K (7)

Uncertainties associated to the parameters of the LFERs were omitted for simplicity.

According to Marsac et al. [START_REF] Marsac | Neptunium redox speciation at the illite surface[END_REF] the formation of ≡SONp(OH) 4 -was required to describe Np(IV) adsorption to illite. The same might be true for montmorillonite, as in the Th(IV) case [START_REF] Bradbury | Modelling the sorption of Mn(II), Co(II), Ni(II), Zn(II), Cd(II), Eu(III), Am(III), Sn(IV), Th(IV), Np(V) and U(VI) on montmorillonite: Linear free energy relationships and estimates of surface binding constants for some selected heavy metals and actinides[END_REF]. Unfortunately, the formation constant of ≡SOAn(OH) 4 -cannot be estimated by a LFER since the formation constant for the corresponding aqueous species (An(OH) 5 -(aq) ) was not available. Instead, the combination of equations 6 and 7 yielded an illite-montmorillonite LFER, which can be applied to Np(IV), using surface complexation constants determined by Marsac et al. [START_REF] Marsac | Neptunium redox speciation at the illite surface[END_REF]:

log K ( ), = 1.08 × log K ( ), -0.46 (8)
The Np(IV) constants were used for Pu(IV) because of their similar hydrolysis constants. By analogy with Np(IV), U(IV) and Tc(IV) surface complexes might form at the montmorillonite surface. However, given the lower redox potentials of the U(VI)/U(IV) and Tc(VII)/Tc(IV) couples compared to Np(V)/Np(IV), reduction of U(VI) or Tc(VII) was not expected at the relatively high Eh of our experiments. Thus, U(IV) and Tc(IV) adsorption to montmorillonite was not further considered.

Bradbury and Baeyens [START_REF] Bradbury | Modelling the sorption of Mn(II), Co(II), Ni(II), Zn(II), Cd(II), Eu(III), Am(III), Sn(IV), Th(IV), Np(V) and U(VI) on montmorillonite: Linear free energy relationships and estimates of surface binding constants for some selected heavy metals and actinides[END_REF] showed that cations with different chemical behavior (related to their oxidation state) do not compete for the same adsorption S1.

Our simulations were based on the estimations and assumptions discussed above. In addition, the impact of carbonates on the adsorption of most RNs onto clays has not been investigated.

For instance, the potential formation of ternary carbonate-RN-surface complexes, as observed for U(IV) (Marques [START_REF] Fernandes | U(VI) sorption on montmorillonite in the absence and presence of carbonate: A macroscopic and microscopic study[END_REF]) cannot be generally taken into account.

Results and Discussion

3.1 Batch adsorption results

Stability of montmorillonite colloids

The montmorillonite colloidal stability was followed via the evolution of the suspended Alconcentration at a given time, relative to the initial Al-concentration. The percentage of stable colloids for the different dispersions after 3 days, 2 weeks, 1 month and 6 months is shown in Figure 3. No significant sedimentation was observed after 3 days in any dispersion except in those containing larger montmorillonite colloids, i.e. S0 and S1. Here, a clear sedimentation was observed at longer times (e.g. only 55% and 82% remain suspended after 6 months for the dispersions S0 and S1, respectively). In previous work [START_REF] Norrfors | Montmorillonite colloids: I. Characterization and stability of dispersions with different size fractions[END_REF], the long term stability of the undiluted clay dispersions were studied. In that study, 43% and 70% of the colloids remained stable in dispersions S0 and S1 after 2 months without shaking [START_REF] Norrfors | Montmorillonite colloids: I. Characterization and stability of dispersions with different size fractions[END_REF]. Similar trends were found here even though the dispersions were diluted to 20 mg/L. Interestingly, the colloidal dispersion obtained after sequential (ultra-) centrifugation (S3.5) and the dispersion obtained from one direct ultra-centrifugation step (S3.5 UC ), have a similar size distribution [START_REF] Norrfors | Montmorillonite colloids: I. Characterization and stability of dispersions with different size fractions[END_REF], showing the same high colloidal stability (Figure 3). [START_REF] Guillaumont | Update on the chemical thermodynamics of uranium, neptunium, plutonium, americium and technetium[END_REF][START_REF] Huber | Sorption reversibility kinetics in the ternary system radionuclide-bentonite colloids/nanoparticles-granite fracture filling material[END_REF][START_REF] Ticknor | The effect of fulvic acid on the sorption of actinides and fission products on granite and selected minerals[END_REF]. ). The concentration of Tc(VII) remains constant in solution up to 6 months for all dispersions (Figure 4b and SIF -2, Figure S1). This supports the above statements that no reduction of technetium or adsorption onto clay colloids occurs within the analytical uncertainties, as expected from the speciation calculations. Slow reduction of Tc(VII) to Tc(IV) in presence of synthetic or Febex montmorillonite colloids has previously been noted in natural groundwater of similar low ionic strength (~1 mM) and pH = 9 after 1 month [START_REF] Huber | Radionuclide desorption kinetics on synthetic Zn/Ni-labeled montmorillonite nanoparticles[END_REF], which was related to the observed decrease in Eh to approximately -200 mV after 1 month.

Since most FA remains free in dispersion (section 2.4), no distinction between Tc-FA complexes and soluble species of Tc could be done and the Tc distribution in S3.5, S3.5 UC,FA and S3.5 UC was identical, as expected (SIF -2, Figure S1).

Uranium adsorption

In the pH-Eh predominance diagram for uranium (Figure 5a) the present experimental conditions (pH = 8.9 ± 0. U(VI) is known to form highly soluble complexes with carbonate at pH 9 [START_REF] Bernhard | Uranyl(VI) carbonate complex formation: Validation of the Ca2UO2(CO3)3(aq.) species[END_REF][START_REF] Bradbury | Predictive sorption modelling of Ni(II), Co(II), Eu(IIII), Th(IV) and U(VI) on MX-80 bentonite and Opalinus Clay: A "bottom-up" approach[END_REF][START_REF] Hartmann | Sorption of radionuclides onto natural clay rocks[END_REF][START_REF] Fernandes | U(VI) sorption on montmorillonite in the absence and presence of carbonate: A macroscopic and microscopic study[END_REF][START_REF] Regenspurg | Removal of uranium(VI) from the aqueous phase by iron(II) minerals in presence of bicarbonate[END_REF]. In previous studies (Marques [START_REF] Fernandes | U(VI) sorption on montmorillonite in the absence and presence of carbonate: A macroscopic and microscopic study[END_REF], a log (K D (dm 3 /kg)) value of 2.7 was presented for U(VI) adsorption onto montmorillonite in 1 mM HCO 3 -and 0.1 M NaClO 4 at pH 9. This corresponds to an uptake of 1% uranium with our low clay concentration (20 mg/L). Our simulation for the SGW also predicts ~1% uptake. In does not strongly adsorb onto montmorillonite at pH 8.9, the results for dispersion S3.5 UC,FA were not expected to differ from those for dispersions in absence of FA, which agrees with the distribution presented (SIF -3, Figure S2).

Interestingly, a significant release of naturally occurring 238 U in the raw MX80 clay was observed in all dispersions (SIF-4, Figure S3). This was a consequence of the presence of carbonates in the SGW. This observation further strengthens the assumption that soluble Ucarbonate complexes were the favoured species under the present chemical conditions. presented in the Supporting Information (SIF -3, Figure S2).

Neptunium adsorption

According to the pH-Eh predominance diagram (Figure 6a), Np should prevail as Np(V) as

NpO 2 CO 3 -in the SGW (pH = 8.9 and Eh = 230 mV) and remain in solution due to weak Np(V) uptake onto montmorillonite [START_REF] Missana | Grimsel Test Site -Investigation Phase V. The CRR Final Project Report Series II: Supporting Laboratory Experiments with Radionuclides and Bentonite Colloids NAGRA Technical Report Series NTB[END_REF][START_REF] Turner | Neptunium(V) sorption on montmorillonite; an experimental and surface complexation modeling study[END_REF][START_REF] Wu | Neptunium(V) Sorption and Diffusion in Opalinus Clay[END_REF]. The Np distribution in the S0 dispersion is presented in Figure 6b for different equilibration times. The results for the other clay dispersions can be found in the Supporting Information (SIF -5, Figure S4). Overall, as expected, no Np uptake on montmorillonite was detected for any clay dispersion and equilibration time.

Recently, surface induced reduction of Np(V) to Np(IV) in the presence of illite has been evidenced [START_REF] Marsac | Neptunium redox speciation at the illite surface[END_REF]. Although Np(V) prevailed in solution, its reduction to Np(IV) was shown to be thermodynamically favoured at the illite surface due to strong Np(IV) adsorption. Structural Fe(II) in illite was suspected to be the redox partner, and Fe(II) is also present in our montmorillonite [START_REF] Holmboe | Influence of γ-radiation on the reactivity of montmorillonite towards H2O2[END_REF]. Therefore, surface induced reduction of Np(V) to Np(IV) might also be relevant in the case of montmorillonite. Figure 6c shows the predicted percentage of Np uptake by montmorillonite as a function of Eh in the SGW at pH 8.9. For Eh < 50 mV, Np(IV) prevails both at the surface and in solution, leading to nearly complete Np uptake by montmorillonite. For Eh > 300 mV, Np(V) prevails both at the surface and in solution and no uptake was predicted. For 50 < Eh < 300 mV, Np(V) prevails in solution whereas Np(IV) prevails at the surface, leading to an intermediate behaviour, which is highly sensitive to changes in Eh. The measured Eh (+230 mV) is shown as a dashed line, with the experimental uncertainty (± 50 mV), as dotted lines in Figure 6c.

The model predicts an uptake of approximately 10% Np at pH 8.9 and Eh (SHE) = +230 mV.

However, given the uncertainties in (i) the experimental Eh value and (ii) Np(IV) surface complexation constants reported in the 2 SPNE SC/CE model [START_REF] Marsac | Neptunium redox speciation at the illite surface[END_REF], the experimental Np uptake was qualitatively consistent with the predictions.

since FA remains predominantly in solution (SIF -5, Figure S4).

Thorium adsorption

The Th speciation in the SGW is shown as a function of pH in Figure 7a. Th was predicted to be present as Th(OH) 4(aq) (60%) and Th(OH) 2 (CO 3 ) 2 2-(40%) at pH 8.9, marked as a vertical line in Figure 7a. Accordingly, the model predicts 83% of Th(IV) uptake onto montmorillonite, corresponding to log (K D (dm 3 /kg)) = 5.4. The distribution of Th in the S0 dispersion between particulates, suspended colloids or soluble species as a function of equilibration time is presented in Figure 7b. The results for the other clay dispersions are presented in the Supporting Information (SIF -6, Figure S5). Thorium was strongly adsorbed in all clay dispersions. The amount of settled Th increases with time in S0 (Figure 7b). This follows the increase in the amount of depositing clay particles, as already mentioned (Figure 3), pointing to the association of a fraction of Th with those unstable, large montmorillonite particulates. For the clay dispersions containing smaller sized colloids (S2→S3.5 UC ), no sedimentation was expected which was in line with the experimental results, especially as small amounts of particulates found in all dispersions could rise from artefacts (e.g. adsorption to walls), as this fraction was not measured directly (but calculated from the total amount Th subtracting the measured amounts of Th in colloidal and aqueous phases).

The K D values calculated for Th in the different clay dispersions are presented as a function of equilibration time in Figure 7c. Data points for samples where the concentration of Th as soluble species were below the detection limit are not shown. Nevertheless, no significant variations in Th uptake was found. Overall, no kinetic effects were observed. Our log (K D (dm 3 /kg)) = 6.4 ± 0.5 were similar to those previously observed. Log (K D (dm 3 /kg)) = 6.4 ± 0.5 and log (K D (dm 3 /kg)) = 5.3 ± 0.5 for batch adsorption experiments were reported by Huber et al. [START_REF] Huber | Sorption reversibility kinetics in the ternary system radionuclide-bentonite colloids/nanoparticles-granite fracture filling material[END_REF][START_REF] Huber | Radionuclide desorption kinetics on synthetic Zn/Ni-labeled montmorillonite nanoparticles[END_REF] and [START_REF] Bouby | Interaction of bentonite colloids with Cs, Eu, Th and U in presence of humic acid: A flow field-flow fractionation study[END_REF], respectively, in a low ionic strength (~1 mM) groundwater at pH = 8.3 -9.5 at carbonate concentrations of ~0.05 mM, comparable to the SGW used in our study. The experimental K D value was slightly higher than expected from modelling (dashed line in Figure 7c).

Nevertheless, the model for Th(IV) uptake was calibrated with the experimental results of [START_REF] Bradbury | Modelling the sorption of Mn(II), Co(II), Ni(II), Zn(II), Cd(II), Eu(III), Am(III), Sn(IV), Th(IV), Np(V) and U(VI) on montmorillonite: Linear free energy relationships and estimates of surface binding constants for some selected heavy metals and actinides[END_REF], involving a measured log (K D (dm 3 /kg)) ≈ 5.5 for Th at pH = 9 in 0.1 M NaClO 4 , i.e. in higher ionic strength and in absence of carbonate.

In the montmorillonite dispersion obtained in presence of FA (S3.5 UC,FA ), the Th uptake onto clay colloids was significantly lower compared to the dispersions in absence of organic matter (S3.5 UC , Figure 7c and SIF -6, Figure S5). Log (K D (dm 3 /kg)) was reduced to 5.3 ± 0.5 and remains constant over time (Figure 7c). FA form complexes with Th(IV) which compete with Th-montmorillonite surface complexes. This K D value corresponded to a decrease in the amount of adsorbed Th by approximately 15% (see histograms in the Supporting Information, SIF -6, Figure S5). Pan et al. [START_REF] Pan | Sorption of Th(IV) on Nabentonite: Effects of pH, ionic strength, humic substances and temperature[END_REF] reported a decrease in log (K D (dm 3 /kg)) from ~6.0 to ~5.4 after addition of FA in absence of carbonate, at pH = 9 and in 50 mM NaCl, with a FA to Na-bentonite ratio of 1:5 compared to 1:10 in our study. [START_REF] Yu | Adsorption of Th(IV) onto Al-pillared rectorite: Effect of pH, ionic strength, temperature, soil humic acid and fulvic acid[END_REF] reported an even larger decrease in log (K D (dm 3 /kg)) value in presence of FA (from ~5.2 to ~3.9) with a FA to bentonite ratio of 1:30 at pH = 9, at slightly higher ionic strength (0.1 M NaNO 3 ). Therefore, the present results were in line with literature data.

Plutonium adsorption

In the predominance pH-Eh diagram for Pu (Figure 8a), the experimental conditions are marked by a black square (pH = 8.9 and Eh = 230 mV). In the SGW, Pu was expected to prevail as Pu(IV) mainly as Pu(OH) 2 (CO 3 ) 2 2-(aq) . Pu(IV) is known to adsorb strongly onto montmorillonite [START_REF] Geckeis | Results of the colloid and radionuclide retention experiment (CRR) at the Grimsel Test Site (GTS), Switzerland -impact of reaction kinetics and speciation on radionuclide migration[END_REF][START_REF] Latrille | Retention of Sn(IV) and Pu(IV) onto four argillites from the Callovo-Oxfordian level at Bure (France) from eight equilibrated sedimentary waters[END_REF][START_REF] Lujanienė | Sorption of Cs, Pu and Am on clay minerals[END_REF]. Even though the total amount of Pu would exceed the solubility limit of Pu(IV) [START_REF] Huber | Sorption reversibility kinetics in the ternary system radionuclide-bentonite colloids/nanoparticles-granite fracture filling material[END_REF], the final aqueous Pu concentration was below the saturation with respect to PuO 2(am,hydr)

because of the strong adsorption of Pu onto montmorillonite. Furthermore, since adsorption is usually a faster process than nucleation and precipitation, precipitation of Pu was not expected [START_REF] Zhao | Sorption and Precipitation of Plutonium in the Presence of Goethite at Elevated Temperatures[END_REF]. An uptake of 97% Pu(IV) was predicted by the model. This was consistent with the simulated Np uptake at Eh < 50 mV (see Figure 6c), where Np(IV) would prevail both at the surface and in solution. A Pu uptake of 97% corresponds to log (K D (dm 3 /kg)) = 6.3 in our experimental conditions.

The distribution of Pu as a function of equilibration time is presented for the clay dispersion S0 in Figure 8b. Pu was largely associated to montmorillonite. The distribution of Pu in the other clay dispersions is presented in the Supporting Information and was comparable to the results found for S0 (SIF -7, Figure S6 and Figure 8b). As for Th, some Pu is associated to particulates present in the clay dispersions with the largest colloids (i.e. S0 and S1) which is seen over time). This was explained by sedimentation of larger montmorillonite particles (see Figure 3). In the dispersion with smaller colloids, no Pu particulates were found (SIF -7, Figure S6). K D values for Pu adsorption to each clay suspension are presented as a function of equilibration time in Figure 8c. Pu uptake does not significantly evolve over the first month, with an average distribution coefficient for all dispersions of log (K D (dm 3 /kg)) = 5.7 ± 0.5.

Although, the Pu(IV) surface complexation constants for the 2 SPNE SC/CE model used in this study were based on several approximations, the predicted log (K D (dm 3 /kg)) = 6.3 (dashed line in Figure 8c) agrees well with the experimental data. High uptake of Pu show that Pu(IV) predominated in the SGW. However, the uptake was slightly lower than for Th (Figure 7c) and the slight increase in uptake of Pu over time (almost insignificant given the error bars) might suggest the presence of a small fraction of Pu(III), as present initially, which took time to oxidize to Pu(IV). After 6 months equilibration time, the uptake of Pu was similar to those reported for Th and those predicted from the model. They indicate a consistent high adsorption of the two tetravalent RNs. Previous studies [START_REF] Geckeis | Results of the colloid and radionuclide retention experiment (CRR) at the Grimsel Test Site (GTS), Switzerland -impact of reaction kinetics and speciation on radionuclide migration[END_REF][START_REF] Huber | Radionuclide desorption kinetics on synthetic Zn/Ni-labeled montmorillonite nanoparticles[END_REF] reported log (K D (dm 3 /kg)) = 5.4 ± 0.5 and log (K D (dm 3 /kg)) = 6.4 ± 0.5 in ground waters of low ionic strength (~1 mM) and pH = 9.5 -9.6 for adsorption of Pu(IV) to montmorillonite. These values were consistent with the present work despite higher Eh in our study (0 to -200 mV in [START_REF] Geckeis | Results of the colloid and radionuclide retention experiment (CRR) at the Grimsel Test Site (GTS), Switzerland -impact of reaction kinetics and speciation on radionuclide migration[END_REF][START_REF] Huber | Radionuclide desorption kinetics on synthetic Zn/Ni-labeled montmorillonite nanoparticles[END_REF]), due to the large stability field of Pu(IV) at pH ≈ 9.

As for Th, we observe a lower Pu uptake onto the clay in the S3.5 UC,FA dispersion since FA was acting as a competing ligand (Figure 8c). Pu adsorption in the presence of FA remained constant at log (K D (dm 3 /kg)) = 5.1 ± 0.5. In previous published work, the reported competitive effect of added FA differs. Thus, Ticknor et al. [START_REF] Ticknor | The effect of fulvic acid on the sorption of actinides and fission products on granite and selected minerals[END_REF] investigated Pu uptake by montmorillonite ([Pu(IV)] = 1.1•10 -10 M, m/V = 25 g/L) in the presence of FA (0 -4.84 mg/L) at pH = 7.7, with 56 mg/L HCO 3 and an ionic strength of ~0.2 M, and observed a maximum decrease in log (K D (dm 3 /kg)) from ~4.8 to ~3.5, even though their FA to montmorillonite ratio was far less than in our study. On the opposite, Boggs et al. [START_REF] Boggs | Plutonium(IV) sorption to montmorillonite in the presence of organic matter[END_REF] did not find any effect of FA addition at pH = 8 in 0.01 M NaCl. In that study, the FA to montmorillonite ratio was higher (m/V = 0.1 g/L, [FA] = 1.5 mg/L, [Pu(IV)] = 10 -10 M). A decrease in Pu adsorption was expected under our conditions as suggested by the Th The present batch adsorption results do not suggest that any of the tested clay colloid dispersions particularly favour adsorption of Th(IV) and Pu(IV), within the analytical uncertainties (see Figures 7c and8c where the uptake of Th and Pu was normalized to the mass of montmorillonite and expressed as log K D according to equation 1). To better compare the amount of adsorbed RNs, the estimated amounts of edge sites were calculated by assuming disc-shaped particles with a given mean particle size [START_REF] Norrfors | Montmorillonite colloids: I. Characterization and stability of dispersions with different size fractions[END_REF]. The estimates for each clay dispersion are included in Table 1, which suggests an increase by a factor of approximately six in the amount of sites (corresponding to both weak and strong sites according to the 2 SPNE SC/CE model) between the dispersions containing all the clay particle sizes (S0) and those more restricted to smaller sizes (S3.5 UC ). Based on these estimations, a factor six difference in the amount of adsorbed RNs could be expected. K D values normalized to the amount of sites (K D,sites ), instead of mass, could be calculated using equation 2. This yielded a maximal theoretical difference of log(6) = 0.8 units between the two dispersions S0 and S3.5. Nevertheless, this value was similar to the experimental uncertainty (± 0.5 units) for log K D measurements, and could explain why no significant difference in adsorption between the clay dispersions could be detected in our study.

To better compare the adsorption onto smaller and larger clay colloids, K D values for the montmorillonite pertaining to the particulate phase (K D,part , for the larger particles) were calculated, using equation 3. This K D,part value can be compared to the K D value of stable, suspended montmorillonite colloids (corresponding to smaller colloids). All clay dispersions were polydisperse and in the dispersions with the largest sized colloids, the smaller sized colloids were present as well. From the amount of settled colloids (Figure 3), compared to the amount of Th in the particulate phase in dispersion S0 after 6 months (Figure 7b), a log (K D,part (dm 3 /kg)) = 6.2 ± 0.5 was obtained. This was similar to log (K D (dm 3 /kg)) = 6.3 ± 0.5 obtained for the smaller, stable dispersion S3.5, within the experimental uncertainties.

Consequently, the amount of adsorbed Th was similar for both clay dispersions and no difference in K D values related to the mean colloidal size in the clay dispersions could be inferred.

To summarize, no significant differences between adsorption of strongly adsorbing RNs onto the smallest and largest sized montmorillonite colloids could be deduced, so that an "average log (K D )" should provide sufficient estimates for RN adsorption to bentonites. This "average log (K D )" can be used in reactive transport modelling with confidence, which simplifies models for RN migration accordingly. To better study a potential effect of colloidal size, one may recommend the use of RNs which adsorb to clays stronger than penta-hexa-and heptavalent RNs but weaker than tetravalent RNs.

While an ultimate statement concerning desorption properties related to the mean colloidal (particle) size for the added RNs cannot be made, an effect did occur for natural 238 U which varied significantly with the clay dispersion properties (see Supporting Information, SIF-4).

Conclusions

In this work, experiments were performed on RN uptake onto size separated montmorillonite colloids. Np(V), Tc(VII) and U(VI) were found not to adsorb onto montmorillonite under the present experimental conditions. Instead, they were present as soluble species, mainly as smallest and the largest mean colloidal sizes, an estimated difference by a factor of six in the amounts of edge sites was calculated. This was too small to be detectable within the experimental uncertainties. Therefore, based on the experimental results for MX-80 presented here, it seems to be valid to implement an "average log K D " for all colloidal sizes in reactive transport modelling codes.

While a variation in the montmorillonite colloidal size does not seem to influence the adsorption behaviour of strongly adsorbing RNs, the question remains if it affects RN desorption and desorption kinetics. This is presently investigated in a separate study.

Figure 1 :

 1 Figure 1: Schematic diagram of bentonite fractionation into various aggregate size fractions by sequential and direct (ultra-)centrifugation.

(

  %Colloid) and (iii) the amount of RNs associated to particulates (i.e. settled particles, %Part). This is illustrated in Figure2. When present initially, 70% of the FA was found free in the dispersion after fractionation. No distinction between soluble RNs species and dissolved FA-RN complexes could be made with this method. The concentration of RNs as soluble species was measured in the supernatant after ultra-centrifugation. Colloidal RN concentration was determined from the difference in concentration before and after ultracentrifugation. Particulate RNs was the remaining RN concentration based on the initial concentration. All results presented are mean values of duplicate samples with its standard deviation. The dispersion S3.5 was proved to have the same size distribution as S3.5 UC .

Figure 2 :

 2 Figure 2: Schematic diagram of the radionuclide (RN) distribution in the batch adsorption samples, as determined from ICP-MS measurements.

K

  D value normalized to the amount of sites (K D,sites ) was calculated. The relationship between the mass-normalized (K D ), the sites-normalized values (K D,sites ) and the amount of sites in one clay dispersion ([Sites]) is given by equation 2: log 𝐾 , (𝑑𝑚 /𝑚𝑜𝑙 𝑜𝑓𝑠𝑜𝑟𝑝𝑡𝑖𝑜𝑛 𝑠𝑖𝑡𝑒𝑠) = log 𝐾 (𝑑𝑚 /𝑘𝑔) -log[𝑆𝑖𝑡𝑒𝑠] (𝑚𝑜𝑙/𝑘𝑔) (2)

  MX80 bentonite. Bradbury and Baeyens developed the 2 Site Protolysis Non Electrostatic Surface Complexation and Cation Exchange (2SPNE SC/CE) model for surface complexation and cation exchange reactions at montmorillonite surfaces[START_REF] Bradbury | Modelling the sorption of Mn(II), Co(II), Ni(II), Zn(II), Cd(II), Eu(III), Am(III), Sn(IV), Th(IV), Np(V) and U(VI) on montmorillonite: Linear free energy relationships and estimates of surface binding constants for some selected heavy metals and actinides[END_REF][START_REF] Bradbury | Modelling sorption data for the actinides Am(III), Np(V) and Pa(V) on montmorillonite[END_REF]. Two types of proton active sites were necessary to describe the titration data, but only one type was assumed to bind cations by the model approach. The cation binding sites consist of weak sites ([≡WOH] = 4•10 -2 mol/kg) as well as less abundant strong sites ([≡SOH] = 2•10 -3 mol/kg) with identical acid-base properties. Bradbury and Baeyens[START_REF] Bradbury | Modelling the sorption of Mn(II), Co(II), Ni(II), Zn(II), Cd(II), Eu(III), Am(III), Sn(IV), Th(IV), Np(V) and U(VI) on montmorillonite: Linear free energy relationships and estimates of surface binding constants for some selected heavy metals and actinides[END_REF][START_REF] Bradbury | Modelling sorption data for the actinides Am(III), Np(V) and Pa(V) on montmorillonite[END_REF] determined surface complexation constants for Np(V), U(VI) and Th(IV) to strong sites of montmorillonite. Due to the low RN concentrations investigated in this study and the lack of RN surface complexation constants for the weak sites, only the strong sites were considered as adsorption sites. In solution, all RNs were predominately present as neutral or negative species due to complexation with OH -/CO 3 2-.Marques-Fernandez et al. (Marques Fernandes et al., 2012) determined thermodynamic constants for the formation of ternary montmorillonite-U(VI)carbonate surface complexes. Ternary montmorillonite-Np(V)-carbonate surface complexes

  sites, and have to be modelled by considering several types of binding sites for the different RNs. The different sites were given the same surface site density. The tetravalent RNs Th(IV), Np(IV) and Pu(IV) were expected to compete for the same strong sites. Applying the concept further, Np(V) and U(VI) have no competitor for their respective sites in our study. The natural occurrence of 238 U, was not taken into account in the calculations, which apply only to the 233 U added to the clay dispersions. The surface complexation reactions and constants used in the modeling is presented in the Supporting Information (SIF -1), Table

Figure 3 :

 3 Figure 3: The amount of stable montmorillonite colloids in the seven clay dispersions (Table 1) as a function of time.

  Figure S1). The concentration of Tc(VII) remains constant in solution up to 6 months for all

Figure 4

 4 Figure 4: a) Predominance pH-Eh diagram for technetium where the chemical conditions in our study (pH = 8.9 and Eh = 230 mV) are indicated by a black square. b) The distribution of Tc, in percentage, between particulates (%Part, blue), stable clay colloids (%Colloid, red) or soluble species (%Free, green) in the S0 dispersion is shown for different equilibration times. Histograms with the Tc distribution for the other clay dispersions are presented in the Supporting Information (SIF-2, Figure S1).

  3 and Eh = 230 ± 50 mV) are indicated by a black square. Due to the presence of carbonate in the SGW, uranium should be mainly present as UO 2 (CO 3 ) 3 4-(aq) .

Figure 5b ,

 5b Figure 5b, the experimental uranium adsorption for dispersion S0 is presented as the distribution of U between particulates, suspended colloids and soluble species as a function of equilibration time. The results for the other clay dispersions are presented in the Supporting Information (SIF -3, Figure S2). No uranium ( 233 U) adsorption onto montmorillonite was detected, within the experimental uncertainties, in any clay dispersions at any equilibration time. Instead, in line with published studies (Marques Fernandes et al., 2012) and model predictions, uranium remains as soluble carbonate complexes in all clay dispersions. Since FA

Figure 5

 5 Figure 5: a) Predominance pH-Eh diagram for uranium in the SGW, where the chemical conditions are indicated by a black square (pH = 8.9 and Eh = 230 mV). b) The distribution of U, in percentage, between particulates (%Part, blue), stable clay colloids (%Colloid, red) or soluble species (%Free, green) in the S0 dispersion, shown for different equilibration times. The results obtained for the other clay dispersions are

Figure 6 :

 6 Figure 6: a) Predominance pH-Eh diagram for Np in the SGW. The black square corresponds to the present experimental conditions (pH = 8.9 and Eh = 230 mV). b) Histograms presenting the distribution of neptunium, in percentage, between particulates (%Part, blue), stable colloids (%Colloid, red) or soluble species (%Free, green) in the S0 dispersion shown for different equilibration times. Histograms with the Np distribution for the other clay dispersions are presented in the Supporting Information (SIF -5, Figure S4). c) Predicted percentage uptake of Np by bentonite in the SGW as a function of Eh at pH 8.9. The experimental Eh (230 ± 50 mV) is shown as dashed lines.

Figure 7

 7 Figure 7: a) Species distribution diagram for thorium as a function of pH in the SGW. The experimental pH = 8.9 is marked as a vertical line. b) Histograms presenting the distribution of Th, in percentage, between particulates (%Part, blue), stable clay colloids (%Colloid, red) or soluble species (%Free, green) in the S0 dispersion, shown for different equilibration times. The results obtained for the other clay dispersion are presented in the Supporting Information (SIF -6, Figure S5). c) Adsorption of Th for all different sized montmorillonite dispersions, expressed as log (K D (dm 3 /kg)) values as a function of equilibration time. The dashed line corresponds to the predicted Th adsorption from modelling.

Figure 8 :

 8 Figure 8: a) Predominance pH-Eh diagram for Pu in the SGW. The experimental conditions are marked by a black square (pH = 8.9 and Eh = 230 mV). b) Histograms presenting the distribution of Pu, in percentage, between particulates (%Part, blue), stable clay colloids (%Colloid, red) or soluble species (%Free, green) in the S0 dispersion, shown for different equilibration times. The results obtained for the other clay dispersions are presented in the Supporting Information (SIF -7, Figure S6). c) Adsorption of Pu for all different sized montmorillonite dispersions, expressed as log (K D (dm 3 /kg)) values as a function of equilibration time. The dashed line corresponds to the predicted Pu adsorption from modelling.

  of colloidal size effect on adsorption of RNs onto montmorillonite

  , respectively. On the contrary, Th(IV) and Pu(IV) strongly adsorb onto montmorillonite. If fulvic acids are present (as here during the fractionation step of the clay colloids and after the dilution of their dispersions), the amount Pu and Th associated with clay was significantly decreased. During the experimental observation period (up to 6 months), larger clay particles settled with time. A part of Th and Pu were adsorbed and remain adsorbed on these larger, unstable montmorillonite particles (i.e. in the particulate phase). All experimental results were in line with model predictions. No difference in adsorption behaviour of Th and Pu onto different sized clay colloids, expressed as mass-normalized K D values, was found. Between the clay dispersions with

Table 1 :

 1 Dispersions used in the batch adsorption experiments, separation protocol performed, mean diameter of the clay discs and amounts of adsorption sites determined[START_REF] Norrfors | Montmorillonite colloids: I. Characterization and stability of dispersions with different size fractions[END_REF].

	Colloidal	Conditions of separation	Mean diameter of	Amount of
	dispersion		clay discs (nm)	adsorption sites
		(C: centrifugation;		(mmol/kg)
		UC: ultracentrifugation)
	S0	3 days sedimentation	1496	11.2
	S1	C: 30 min (S0) at 313 × g	940	17.8
	S2	C: 1 h (S1) at 700 × g	500	33.5

Table 2

 2

	: The chemical composition of the synthetic groundwater (SGW) used as background electrolyte in
	all samples (Norrfors et al., 2015).
		Concentration
	Element	(mM)
	Na +	1.2
	Ca 2+	0.05
	F -	0.1
	Cl -	0.074
	SO 4 2-	0.04
	HCO 3 -	1.0
	pH	8.4 ± 0.1

Acknowledgements

The Swedish Nuclear Fuel and Waste Management Co. (SKB) is gratefully acknowledged for generous financial support. This work is a part of the project CP-BELBaR Fission 2010-1.1.1. This work has been supported by the European FP7 TALISMAN (Transnational Access to Large Infrastructure for a Safe Management of ActiNide) project under the grant agreement # 323300; JRP no TALI-C02-10. The authors gratefully acknowledge C. Walschburger (Institute for Nuclear Waste Disposal) for laboratory work and analytical support.