# Towards a better knowledge of U(VI) speciation in weakly alkaline solution through an in-depth study of U(VI) intrinsic colloids

Hélène Barbier, Diane Rébiscoul, Christin Krämer, Xavier Le Goff, Thomas Zemb, Stephanie Szenknect

► To cite this version:

Hélène Barbier, Diane Rébiscoul, Christin Krämer, Xavier Le Goff, Thomas Zemb, et al.. Towards a better knowledge of U(VI) speciation in weakly alkaline solution through an in-depth study of U(VI) intrinsic colloids. Chemosphere, 2025, 371, pp.144029. 10.1016/j.chemosphere.2024.144029. hal-05049936

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# Towards a better knowledge of U(VI) speciation in weakly alkaline solution through an in-depth study of U(VI) intrinsic colloids

Hélène Barbier¹, Diane Rébiscoul¹*, Christin Krämer¹, Thomas Zemb¹, Stephanie Szenknect¹*

¹ ICSM, Univ Montpellier, CNRS, CEA, ENSCM, Site de Marcoule, Bagnols sur Cèze, 30207, France.

*corresponding authors: diane.rebiscoul@cea.fr; stephanie.szenknect@cea.fr

## ABSTRACT

The formation of U(VI) intrinsic colloids has a non-negligible impact on the dissemination of actinides in the environment. It is therefore essential to better identify their nature, formation conditions, and stability domains. These specific points are especially important since the behavior of these elements in environment is generally estimated by geochemical transport modeling. This modeling relies on the accurate prediction of their speciation in various situations based on thermodynamic databases that have to be continuously updated. In this work, we identified and characterized the U(VI) intrinsic colloids that grow in model solutions having simplified composition that consists of NaCl and NaCl with Na₂SiO₃ electrolytes at pH comprised between 7 and 10. Using a simple, reliable and systematic method based on geochemical simulations, elemental analyses by ICP-AES and *in situ* Small and Wide Angle X-ray Scattering characterizations, several characteristics of U(VI) colloids such as their sizes, volumes, concentration and formula were determined and used to calculate the distribution ratio of U(VI) between intrinsic colloids and solution. This distribution ratio is crucial to enhance predictive
1 geochemical calculation, especially to predict the fraction of U(VI) present as colloids that may
2 migrate into natural waters.

3 **KEYWORDS:** U(VI) intrinsic colloids, model solution, Small and wide-angle X-ray scattering,
4 prenucleation cluster, distribution ratio

5 **1. INTRODUCTION**

6 Contamination of the environment by actinides results from military operation and activities
7 related to the nuclear fuel life cycle, from uranium mining to radioactive waste materials disposal
8 (Silva and Nitsche, 1995; Walther and Denecke, 2013). Potential dissemination of these elements
9 in the geosphere is mainly due to their transport in surface water and groundwater (Silva and
10 Nitsche, 1995; Walther and Denecke, 2013). Mobility in environment and bioavailability of
11 actinides are strongly limited by their sorption to the surface of soil and aquifer constituents, or
12 their precipitation in stable solid phases (Choppin, 2003; Maher et al., 2013; Poinssot and Geckeis,
13 2012; Silva and Nitsche, 1995). All of these geochemical processes depend on the chemical form
14 of actinides (Maher et al., 2013; Poinssot and Geckeis, 2012; Silva and Nitsche, 1995).

15 Whereas tetravalent actinides such as U, Th, Np and Pu in oxidation state (IV) are not considered
16 to be strongly mobile in the environment (Grenthe et al., 2020; Husar et al., 2015; Neck and Kim,
17 2001; Neill et al., 2018; OECD, 2008), the same elements in form of actinyl ions in oxidation state
18 (V) or (VI), which form under aerobic conditions, are more mobile (Choppin, 2003; Maher et al.,
19 2013). In this form, the actinide mobility depends on the pH as well as on the presence of
20 complexing anions (sulfates, silicates, carbonates, chlorides ...) and others "competitor" cations
21 at high concentration such as Ca, K, Na, Mg (Maher et al., 2013; Neill et al., 2018; Romanchuk et
22 al., 2020; Silva and Nitsche, 1995). As an example, under alkaline, oxidizing conditions and in
presence of calcium, U(VI) tends to form colloids (species above 1 nm size), which may be transported in natural porous media (Bots et al., 2014; Ding et al., 2016; Shi et al., 2021). Under these conditions, the nature of these colloids formed is more akin to hydrated, poorly crystallized uranates. These metastable uranates could act as reaction intermediates between polynuclear uranium (VI) hydroxide species (containing two to less than ten uranyl moieties) and oxide-type crystalline phases. Other studies have also highlighted the existence of actinide colloids and their migration into the hydrogeosphere in the vicinity of contaminated sites (Chawla et al., 2010; Flury et al., 2002; Liu et al., 2013; Romanchuk et al., 2020; Utsunomiya et al., 2009; Walther and Denecke, 2013).

Thus, the existence of soluble polynuclear species in groundwater as well as intrinsic actinide-based colloids may therefore have a non-negligible impact on the transport of actinides in the environment (Haliena et al., 2016; Powell et al., 2011; Strzelecki et al., 2020). It is therefore essential to identify their nature, formation conditions, and environmental persistence (Abdel-Fattah et al., 2013). These specific properties are crucial since the behavior of these elements in environment is generally estimated by geochemical transport modeling. This modeling relies on the accurate prediction of their speciation in various situations based on thermodynamic databases that have to be continuously updated (Giffaut et al., 2014; Grenthe et al., 2020; Grivé et al., 2015; Reiller and Descostes, 2020; Vercouter et al., 2015). These databases include some formation constants of few actinide polynuclear species such as uranyl hydroxocarbonates (Bots et al., 2014), Th and Pu hydroxydes (Bundschuh et al., 2000; Cho et al., 2005; Neck et al., 2002; Zänker and Hennig, 2014) but do not include many of the others. Indeed, thermodynamic data for polynuclear clusters or colloids are not available in these standard compilations. This limits the accurate prediction of actinide speciation, which requires not only a knowledge of the soluble actinide
species under typical geochemical conditions, but also the stable colloidal species containing potentially tens or even hundreds of actinide atoms. To reach this goal, the actinide colloid concentration as well as the number of actinide atoms per colloid have to be determined. To our best knowledge, this has neither been done before. Here for the first time, we propose a simple, reliable and systematic experimental method for accessing these key properties for U(VI) intrinsic colloids in simplified solutions.

In this work, we aim to explore the evolution and properties of U(VI) colloids that might form under geochemical conditions relevant for radioactive waste disposal, uranium mining sites, or unsaturated zone of sites contaminated by former military activities or nuclear accidents. As silicate species are ubiquitous in the geosphere, the formation of U(VI) colloids was studied at first in simplified model solutions containing only NaCl and NaCl/Na₂SiO₃ at pH varying between 7 and 10. A systematic approach based on geochemical simulations, elemental analyses by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) and *in situ* Small and Wide Angle X-ray Scattering (SWAXS) characterizations were used to probe key properties of U(VI) colloids such as their size, volume, concentration and stoichiometry. The determination of these properties made it possible to calculate the U(VI) distribution coefficient between intrinsic colloids and solution.

## 2. MATERIALS AND METHODS

### 2.1 Preparation of model solutions

200 mL of two model solutions were prepared under air. The first one, referred as NC, was prepared by dissolving NaCl salt (Merck, ACS reagent) in fresh Milli-Q-water (Millipore, 18.2 MΩ) to obtain a 10⁻² M solution. For the second one, referred as NCS, NaCl and Na₂SiO₃·5 H₂O
1 (Merck, ≥95.0 %) salts were dissolved in fresh Milli-Q-water (Millipore, 18.2 MΩ). The chloride
2 and silicate concentrations in the NCS solution were 1.10⁻² M and 2.10⁻³ M, respectively. The Si
3 concentration was chosen to be lower than the calculated solubility of SiO₂(am) at pH = 7, room
4 temperature and under air (pO₂ = 0.2 and pCO₂ = 320 ppm). A U(VI) stock solution was prepared
5 by dissolving UO₂(NO₃)₂ (Sigma-Aldrich) in 2 M HNO₃ solution (Merck, 69 % Suprapur HNO₃).
6 The concentration of U(VI) in the stock solution was analyzed by ICP-OES (Thermo Fisher
7 Scientific – iCAP 7000 Series) and was 0.2 M. Then, 1 mL of the U(VI) stock solution was added
8 to the 200 mL of each model solution. Two groups of four 60 mL Savillex® PFA reactors were
9 filled with 50 mL of each model solution (NC or NCS). Then, the pH of each solution was adjusted
10 to 7, 8, 9 or 10 by adding droplets of 1 M NaOH (Merck, ACS reagent) solution under stirring
11 with the help of a pH electrode (Mettler, Inlab®-Pro). Thereafter, the eight PFA reactors were
12 placed on an orbital shaker for the total duration of the experiment. The pH of the solutions was
13 regularly measured using a pH-meter (Mettler, SevenDirect) calibrated using pH buffers at pH
14 4.01, 7.00, 10.0.

## 2.2 Analysis of model solutions

16 In order to follow the evolution of the concentration of U(VI) in solution, the solutions were
17 regularly sampled and analyzed by ICP-OES. The elemental Si concentration was determined at
18 the beginning and at the end of the experiment in the four NCS reactors. A volume of 2.5 mL of
19 each solution was sampled and centrifuged at 4500 rpm for 5 min. After centrifugation, 0.5 mL of
20 supernatant were directly added to 4.5 mL of 0.2 M HNO₃ solution. This first acidified solution
21 was referred as “C”. At the same time, additional 1.5 mL of supernatant were centrifuged again at
22 4500 rpm for 5 to 10 min in Vivaspin® tubes equipped with a 10 kDa membrane. This membrane
23 nominally retains particles with radii larger than ca. 2 nm (with the precise cut-off being dependent
on the surface potential of the filtered particles). The chemical inertness of the membrane used for ultra-filtration was previously tested using a 10⁻⁵ M UO₂(NO₃)₂ solution at pH 7. ICP-OES analysis of the filtrate revealed U retention of less than 1 mol.%. Subsequently, 1.5 ml of ultrafiltrated solution were added to 3.5 mL of 0.2 M HNO₃ solution (Merck, 69 % Suprapur HNO₃). This last acidified solution was referred as “UF”. C and UF acidified solutions were analyzed using ICP-OES. Beforehand, the ICP-OES was calibrated using several standard solutions prepared by dilution of U, and Si elemental solutions certified at 1000 ppm (SCP Science) in 0.2 M HNO₃ solution. Three to four of the recommended emission wavelengths were selected for each element. Three replicates for each measurement were performed. The detection limit of the analytical technique was 5 x 10⁻⁷ M and 4 x 10⁻⁶ M for U and Si, respectively.

The distribution of U between soluble and colloidal forms was deduced from the analysis of the C and UF samples using the following equation:

$$ \%_{coll} = \left( 1 - \frac{[U]_{UF}}{[U]_{C}} \right) \times 100 (1) $$

where [U]UF (M) and [U]C (M) are the elemental concentration in U measured in the UF and C fractions, respectively.

## 2.3 Data analysis of small and wide angle X-ray scattering (SWAXS)

SWAXS analysis was used to characterize the colloidal particles in solutions. The solutions before and after ultrafiltration were introduced in 3 mm quartz capillary, then SWAXS analyses were carried out in transmission geometry with a Xenocs setup equipped with a Mo anode (λ = 0.71 Å) and a MAR345 2D image plate detector. This wavelength gives access to a scattering range comprised between a few angstroms to 30 nm (0.2 nm⁻¹ < q < 27 nm⁻¹). The collimation was ensured by a Fox2D multilayer mirror and by a set of scatterless slits delimitating the beam to a
square section (0.8 mm side length at the sample position). The distance from the sample to the
detector was calibrated using silver behenate powder and was about 750 mm. Azimuthal averaging
of 2D-data recorded by a MAR345 image plate detector was performed using PySAXS software
("pySAXS," n.d.). To determine the scattered intensity $I(q)$ allowing an access to quantitative
information, raw data were treated considering the electronic background of the detector, the
empty cell subtraction and the intensity calibration using polyethylene as described in Text S1 in
Supporting Information. The scattered intensity $I(q)$ was obtained on an absolute scale (cm$^{-1}$) and
was expressed versus the magnitude of the scattering vector $q = (4\pi \sin\theta)/\lambda$, where $\theta$ was the
scattering angle with an experimental resolution of $\Delta q/q = 0.02$ (Zemb et al., 2003).

To obtain quantitative information about the composition and structure of the colloids, the
simplified scattering intensity expression valid for monodisperse species was used (2):

$$I(q) = N_p \Delta S L D^2 V_p^2 P(q) S(q) \quad (2)$$

where $N_p$ is the density of particles in solution (cm$^{-3}$), $V_p$ is the volume of the particle (cm$^3$), $\Delta S L D$ is the difference of scattering length density between the solution and the particle (cm$^{-2}$), $P(q)$ and $S(q)$ are the form and structure factors, respectively. In the case of diluted solution (as in the present study), the particles do not interact and thus, $S(q) = 1$ at any $q$ value.

Even when the morphology of the colloid is unknown, the gyration radius of the average colloid, $R_g$, can be determined using the Guinier approximation (Rice, 1956) in the low $q$ region, a model free-parameter valid for isotropic diluted system, i.e. when scattering objects are randomly oriented and scatter independently (3).

$$I = I(0)e^{-\left(\frac{q^2 R_g^2}{3}\right)} \quad (3)$$
1 where $I(0)$ (cm$^{-1}$) is the extrapolated intensity at $q = 0$. In addition, the calculation of invariant $Q$

2 was used to determine the volume of a colloid, $V_p$ from the integration of $Iq^2(q)$ in the probed $q$

3 range using the Kratky plot to obtain graphically the integration presented in equation (4).

$$4 \qquad Q = \int_{0}^{\infty} I(q) \cdot q^2 \cdot dq \quad (4)$$

5 Because $Q$ (cm$^{-1}$.nm$^{-1}$) depends solely on the difference of scattering length density between

6 the colloids and the solution ($\Delta SLD$) and on the volume fraction of colloids $\varphi$ (5), it is possible to

7 write equation (6) (Brumberger, 1995) for dilute media:

$$8 \qquad Q = 2\pi^2\varphi(1 - \varphi)\Delta SLD^2 \quad (5)$$

$$9 \qquad Q \approx 2\pi^2 V_p N_p \Delta SLD^2 \quad (6)$$

10 Combining scattering at zero angle and the invariant, $N_p$ can be calculated from $I(0)$ with

11 equation (7):

$$12 \qquad I(0) = V_p^2 N_p \Delta SLD^2 \quad (7)$$

13 Using (6) and (7), $V_p$ can be simply determined without knowing precisely the colloid

14 composition from (8):

$$15 \qquad V_p = \frac{2\pi^2 I(0)}{Q} \quad (8)$$

## 2.4 High Resolution Electron Microscopy

17 High Resolution Transmission Electron Microscopy (HRTEM) images were obtained using

18 JEOL 2200 FS equipped with the field emission gun (FEG) working at an accelerating voltage of
1 200 kV. The samples for electron microscopy were prepared by dropping ethanol dispersions of
2 the samples onto 400 mesh holey carbon-coated copper grids and then evaporating the solvent.

3 **2.5 Geochemical calculations**

4 Geochemical calculations were performed using the PHREEQC software version 3.7.3
5 (Parkhust and Appelo, 2013) associated with the Thermochimie V10 database (Giffaut et al., 2014;
6 Grivé et al., 2015). U(VI) speciation and saturation index with respect to the phases included in
7 the database were determined in the various model solutions at 25°C by considering the initial
8 elemental concentrations in Na, Cl, Si and U, the pH, and the equilibria with air (pO2 = 0.2 and
9 pCO2 = 320 ppm). The pH of the solutions were fixed to the measured value by adding 1 M NaOH,
10 as in the experiments. Then, the saturation index of the solution with respect to the phases included
11 in the database SI = log (Ion activity product / Ks) (with Ks the standard solubility constant), were also
12 calculated at equilibrium. For NCS solution simulation, the solubility constant of the boltwoodite
13 phase selected by the OECD-NEA TDB project (Grenthe et al., 2020) was added to the
14 Thermochimie database:

$$
15 \quad \text{Na(UO}_2\text{)(SiO}_3\text{OH)} \cdot n\text{H}_2\text{O} + 3 \text{H}^+ \rightleftharpoons \text{Na}^+ + \text{UO}_2^{2+} + \text{H}_4\text{(SiO}_4\text{)} + n \text{H}_2\text{O} \quad \log K_s^\circ = 5.810 (9).
$$

16 This value was recalculated by averaging the two standard solubility constants proposed earlier
17 by Shvareva et al. (Shvareva et al., 2011) and Ilton et al. (Ilton et al., 2006).

18
19
20 **3. RESULTS AND DISCUSSION**

20 **3.1 Uranium speciation in model solutions**

21 Simulations performed with Phreeqc, presented Fig. 1 (a) and (b), indicate that U(VI) speciation
22 in the bulk liquid coexisting with crystalline products is dominated by uranyl carbonate species in
1 the two model solutions, mainly (UO₂)₂(CO₃)(OH)₃⁻ at pH 7 and 8 and (UO₂)(CO₃)₃⁴⁻ at pH 9 and
2 10. The similarity of the U(VI) speciation in the NC and NCS solutions can be explained by the
3 low impact of the presence of silicate ions on U(VI) speciation. Indeed, in the thermodynamic
4 database, the formation of only one uranyl silicate species is considered through the following
5 equilibrium:

$$UO_2^{2+} + H_4SiO_4 \rightleftharpoons UO_2SiO(OH)_3^{+} + H^{+} \quad \log K^{\circ} = -1.84 \ (10).$$

7 In addition, the total carbonate ion concentration strongly increases with the pH of the solution
8 in equilibrium with pCO₂ = 320 ppm, from ≈10⁻⁴ M (at pH = 7) to 10⁻¹ M (at pH = 10). The fact
9 that the complexation constants of uranyl with carbonates are higher than the hydrolysis constants
10 of uranyl explains the predominance of uranyl carbonate complexes at pH 9 and 10, where the
11 concentration of carbonate ions is very high.
<figure><img src="image_2.png" /><figcaption>**Fig. 1.** Geochemical calculation performed with $[U]_{tot} = 10^{-3}$ M, at 25°C, at equilibrium with $p_{O2} = 0.2$ and $p_{CO2} = 320$ ppm, pH fixed by adding 1 M NaOH. U(VI) speciation in (a) NC ($[Cl]_{tot} = 10^{-2}$ M) and (b) NCS ($[Cl]_{tot} = 10^{-2}$ M and $[Si]_{tot} = 2 \times 10^{-3}$ M) model solutions calculated with PhreeqC and the Thermochimie (Giffaut et al., 2014) database and variations of the saturation indexes of the (c) NC ($[Cl]_{tot} = 10^{-2}$ M, a) and (d) NCS ($[Cl]_{tot} = 10^{-2}$ M and $[Si]_{tot} = 2 \times 10^{-3}$ M, b) solutions with pH.</figcaption></figure>

The saturation index of the model solutions with respect to selected phases in the thermodynamic database are presented in Fig. 1 (c) and (d). Irrespective of pH, the simulations show that the model solutions are oversaturated with respect to one uranyl-bearing phase at least. The comparison of the results plotted in Fig. 1 (c) and (d) illustrates the impact of the presence of
silicate ions on the nature of the most stable phase. Without silicate in solution, Na-compreignacite, Na₂(UO₂)₆O₄(OH)₆·7H₂O should precipitate below pH 8.5, while at higher pH clarkeite, Na(UO₂)O(OH) should form. We note that the solubility constant of Na-compreignacite included in the Thermochimie database and extracted from the work of Gorman-Lewis et al. (Gorman-Lewis et al., 2008), was not considered by the OECD-NEA TDB project (Grenthe et al., 2020). In the presence of 2.10⁻³ M of silicates, Na-compreignacite should precipitate at pH < 8.5. Between pH 8.5 and 9.5, boltwoodite, Na(UO₂)(SiO₃OH)·nH₂O is the phase controlling uranium solubility and above pH 9.5, the NCS solution becomes slightly undersaturated with respect to uranyl phases. Below pH 8.5, the NCS solution is also strongly oversaturated with respect to soddyite (UO₂)₂SiO₄·2H₂O. It is worth noting that two solubility constants are considered in the Thermochimie V10 database for the same equilibrium:

$$ (UO_2)_2SiO_4 \cdot 2H_2O + 4 H^+ \rightleftharpoons 2 UO_2^{2+} + H_4(SiO_4) + 2 H_2O \quad (11). $$

The value of the solubility constant of soddyite (synt1) (log K_s° = 3.90) reported by Pérez et al. (Péerez et al., 1996), was not selected in the second update of the NEA TDB (Grenthe et al., 2020), whereas the corresponding soddyite data reported by Gorman-Lewis et al. (synt2) (log K_s° = 6.43) (Gorman-Lewis et al., 2007) was selected and recalculated by considering available U(VI) data. In addition, the NEA TDB project also mentions that there is a wide scatter of log K_s° values reported in the literature for this phase. The two values present in the Thermochimie V10 database were probably selected as two extrema to illustrate the variability in the soddyite solubility constants that were reported in the literature. The solubility constant of the clarkeite phase was also taken from the work of Gorman-Lewis et al. (Gorman-Lewis et al., 2008):

$$ \mathrm{Na(UO_2)O(OH)} + 3 \mathrm{H}^+ \rightleftharpoons \mathrm{Na}^+ + \mathrm{UO_2}^{2+} + 2 \mathrm{H_2O} \quad \log \mathrm{K_s°} = 9.40 (-0.9/+0.6) \quad (11). $$
This work was also mentioned in the NEA second update on the chemical thermodynamics of uranium (Grenthe et al., 2020), but this was neither selected, nor discussed. Giammar and Hering, (Giammar and Hering, 2002), also report a solubility constant for a “clarkeite-like” phase formed as a secondary phase during the dissolution of soddyite crystals. These authors mention that soddyite and clarkeite can coexist between pH = 8.55 and 9.35. However, considering Na as a spectator cation, these authors did not consider the potential formation of Na-compreignacite, for the determination of U(VI) solubility. The value obtained by Giammar and Hering is not significantly different from that determined by Gorman-Lewis et al. (Gorman-Lewis et al., 2008), i. e. $log K_s^o = 9.02$. This reinforces the reliability of the selected value for this phase.

These calculations showed that the field of predominance of uranyl phases strongly depends on the pH, and under alkaline conditions (typically for pH higher than 8.5) on the activity of the silicate ions. However, the nature of the phase formed at the timescale of experiment can also be driven by its formation kinetics.

## 3.2 U(VI) behavior in model solutions

The evolution of U(VI) concentration after centrifugation C and ultrafiltration UF in the model solutions are presented on Fig. 2 (a) and (b).
<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Solution analysis results. Evolutions of the U(VI) concentration in (a) NC and (b) NCS solutions at pH 7, 8, 9 and 10 after centrifugation (C, closed symbols) and ultrafiltration (UF, open symbols) and U colloidal fractions as a function of the initial pH of NC and NCS solutions determined at equilibrium (error bars correspond to the standard deviation of the values determined at the concentration plateau). The pH measured at the end of the experiment is also displayed on the graph (c).</figcaption></figure>

As shown on Fig. 2 (a) and (b), [U]~C~ remains constant and close to the initial uranium concentration of the model solutions. As previously presented in Fig. 1 (c) and (d), although the solutions were all supersaturated with respect to an uranyl-bearing phase, this result highlights that no significant precipitation occurs at the timescale of the experiments. Similar results were also obtained by Bots et al. (Bots et al., 2014) in hyperalkaline solutions. In contrast, [U]~UF~ is lower
than the initial uranium concentration in all experiments. In NC model solutions, [U]UF decreases

in less than 2 hours and then reaches a constant value. The higher was the pH, the lower the [U]UF

concentration at equilibrium. For the NCS solutions, [U]UF concentration decreases significantly

during the first two hours, then reaches a constant level that is much lower than in NC solutions.

The differences between [U]C and [U]UF can be attributed to the removal by ultrafiltration of the

solutions using 10 KDa membrane of U-based colloidal species exceeding 2 nm in typical size.

From the differences, the percentages of U present in colloidal fractions were calculated at

equilibrium as a function of the pH and are presented on Fig. 2 (c).

The obtained results show the presence of U(VI)-bearing colloids in all the solutions. The U(VI)

colloidal fraction in NC solutions increases from 50 to 84 mol. % with the increasing initial pH.

The formation of this colloidal phase is accompanied by a corresponding decrease in the pH of the

solution, except at pH 7 (where an increase of pH is observed). This phenomenon is attributed to

the incorporation of hydroxyl groups into U(VI) colloids for 8< pH< 10. In NCS solutions, the

colloidal fraction dominates U(VI) speciation and contains more than 95 mol. % of U(VI)

introduced in the systems irrespective of the initial pH. Moreover, the pH of the model solutions

is not significantly modified by the formation of the U colloids. These observations suggest that

different species and mechanisms lead to the formation of U colloids, the nature of which depends

on the composition and pH of the initial solution.

Table S1 (see Supplementary Information) summarizes the elemental concentrations obtained at

equilibrium and pH in the different model solutions. At the end of the NCS experiment, a slight

decrease in the Si concentration can be observed in the UF fraction irrespective of the pH. This

indicates that the colloidal fraction formed in NCS solutions contained both, U and Si. The

corresponding saturation indexes calculated with Phreeqc are also reported in Table S1. These
calculations show that NC UF solutions are still oversaturated with respect to Na-compreignacite at pH<sup>eq</sup> 7.5 to 8.3. However, at pH<sup>eq</sup> 9.6, geochemical simulation shows that the NC solution at equilibrium is undersaturated with respect to this phase, although the colloidal fraction of U is predominant. Based on this result, two hypotheses can be formulated: (i), the uranyl phase which controls the soluble U(VI) concentration is not in the thermodynamic database or (ii) the value of the solubility product of the crystalline phase are different from those of the colloidal phase(s). The saturation indexes calculated for NCS UF solutions are still oversaturated with respect to several uranyl phases at pH<sup>eq</sup> 7.0 and 7.5. At more alkaline pH, the simulations show that the solutions are undersaturated with respect to all phases in the database, but close to equilibrium with respect to boltwoodite and Na-compreignacite at pH<sup>eq</sup> 9.3. In any of the cases, the precipitation of a solid phase is not observed and U(VI) colloidal particles are predominant. Shi et al. (Shi et al., 2021) also observed the formation of U(VI) intrinsic colloids in natural groundwater containing 10<sup>-3</sup> M of Si at pH 9.8. U(VI) colloids were found as nanometric needle-shape crystals, identified as boltwoodite. At pH 6.5, Shi et al. also reported the presence of stable U(VI) intrinsic colloids, but their amorphous nature made it impossible to characterize the nature of the phase formed. Similarly, it is possible that the composition of the colloidal particles in the NCS solutions evolves with the pH in the range investigated.

### 3.3 Characterization of the colloidal fraction

To go deeper in the characterization of the colloids and their evolution, SWAXS analyses were performed on model solutions. To ensure that the colloids were the source of the SWAXS signal, solutions were measured before and after UF (see Supporting Information, Text S2 and Fig. S1). The results reveal a decrease of scattering intensity at low *q* after UF. This decrease is consistent with a removal of colloids from the solution. In addition, to detect a possible solid phase,
1 transmission and scattering intensities were systematically measured at the bottom of the
2 capillaries and close to the air-liquid interface.

3 *NC solution*

4 The evolution of the SWAXS patterns over 70 hours and after 3 months of ageing in the model
5 solutions are presented in Fig. 3 (a) et (b) respectively. During the first 70 hours, the scattering
6 intensities are similar whatever the location of the analyzed volume (Text S3 and Fig. S2)
7 demonstrating the homogeneity of the solutions within the capillaries.
NC

(a) 0 to 70 hours

<figure><img src="image_4.png" /></figure>

NCS

(d) 0 to 3 months

<figure><img src="image_5.png" /></figure>

(b) 3 months

<figure><img src="image_6.png" /></figure>

(c) HRTEM pH 9

<figure><img src="image_7.png" /></figure>
**Fig. 3.** (a) Evolutions of the SWAXS patterns of the NC solutions at pH 7, 8, 9 and 10 during the first 70 hours of ripening. (b) SWAXS patterns of the NC solutions after 3 months of ageing at pH 7, 8, 9 and 10 at the bottom of the capillaries and close to the air-liquid interface with the associated zoom of the wide-angle scattering displayed as a function of the distance *d* (*d* = $\frac{2\pi}{q}$). (c) HRTEM images (1) and (2) of 2 zones of the solid phase obtained in NC solution at pH 9 after 3 months of ageing, associated electron diffraction pattern of image (4) and FFT of a nanocrystallite on image (4). (d) SWAXS patterns of the NCS solutions at pH 7, 8, 9 and 10 during the 3 months of ripening.

During the early stages of ageing, i.e. from 0 to 70 hours, the scattering intensities evolve as a function of time and pH. At pH 7, the high intensity at low *q* indicates the presence of colloids in the solution after just 1 h. After 17 h, no scattering at low *q* was evident, which indicates the disappearance of the primary colloids. At higher pH, the evolution of the scattering intensity is more complex, possibly due to the dissolution of primary colloids formed during the first few hours and/or the assembly of the primary colloids as already observed for the nucleation and growth of gypsum (Stawski et al., 2016) and calcium carbonate (Avaro et al., 2023; Bolze et al., 2002). Using the method previously described in the Materials and Methods section, $R_g$ and $V_p$ were calculated from the scattering intensities obtained from the early SWAXS measurements, i.e. after 1 h, 3 h, 5 h and 7 h for NC solutions at pH$^{ini}$ 7, 8, 9 and 10 respectively. The Guinier and Kratky plots were used to calculate the values presented in Table 1 and displayed in Fig. S3. All calculated $R_g$ values are found to be higher than the radius of uranyl carbonate species (Bernhard et al., 2001; Gückel et al., 2013) reported in the literature. In addition, a decrease of $R_g$ and $V_p$ of the colloids with increasing of pH and ageing time can be observed (Table 1).
1 **Table 1.** Characteristics of colloids formed in NC solutions at pH<sup>ini</sup> 7, 8, 9, 10 obtained from

2 SWAXS analysis.

<table><thead><tr><th>pH</th><th>7</th><th>8</th><th>9</th><th>10</th></tr></thead><tbody><tr><td>Ageing time / time of measurement</td><td>1h</td><td>3h</td><td>5h</td><td>7h</td></tr><tr><td>R<sub>g</sub> (nm)</td><td>3.0</td><td>2.5</td><td>2.5</td><td>1.9</td></tr><tr><td>V<sub>p</sub> (nm<sup>3</sup>)</td><td>128</td><td>46</td><td>52</td><td>37</td></tr></tbody></table>

3

After 3 months, a precipitate was observed at the bottom of the NC capillaries (schematically represented in Fig. 3(b)). This observation is confirmed by the SWAXS patterns obtained at the bottom of the capillaries that show a rise of the scattering intensity at low *q* and the presence of sharp peaks at high *q*, both increasing in intensity with pH. These peaks are characteristic of an orthorhombic phase, which may be Na-compreignacite Na₂(UO₂)₆O₄(OH)₆, 7H₂O (ICDD 33-1049 synthetic) (Hubbard and O'Connor, 2002) and/or schoepite (UO₂)₈O₂(OH)₁₂·12H₂O ICDD 13-407) (Hubbard and O'Connor, 2002) (Text S4 and Table S2 in Supporting Information). The formation of these phases incorporating OH moieties is consistent with the pH decrease observed after 200 h. This result is also in accordance with geochemical calculation indicating SI > 0 for Na-compreignacite (Fig. 1 (c)). In addition, HRTEM analyses of the solid phase obtained at pH 9 (Fig. 3 (c1)) highlight the presence of polycrystalline nanoparticles of around 25 nm in diameter, which is close to the average size of 30 nm calculated from the SWAXS data using the Scherrer equation (Text S5 and Fig. S4 in Supporting Information). Larger particles with dimensions of a few hundred nanometers can also be observed (Fig. 3 (c2)). These particles are composed of nanocrystallites of typically 3 nm in diameter (Fig. 3 (c3) and (c4), Fig. S5 in Supporting Information), randomly oriented as evidenced by the electron diffraction pattern (Fig. 3 (c4)). The
analysis of single nanocrystallite shows an interreticular distance of 3.2 Å characteristics of an orthorhombic phase as illustrated by the FFT of the HRTEM image (Fig. 3 (c4)). All of these observations are in accordance with the simulation of the SWAXS data of the solid phase formed at pH 9 (Text S6 and Fig. S6 in Supporting Information). In addition, assuming that colloids are spheres, the sizes of the small nanocrystallites constituting the solid phase are smaller than the primary colloids formed at early aging times: R(5 h)=3.2 nm vs R(3 months) =1.5 nm. This may be due to the partial or complete dissolution of the primary colloids to form the observed small nanocrystallites.

### NCS solution

Whereas SWAXS patterns of NC solutions evolved during their ageing, the SWAXS patterns of NCS solutions presented in Fig. 3(d) remain unchanged after 3 months, regardless of the pH and the location of the probed volume. As previously, $R_g$ and $V_p$ of the colloids formed in NCS solutions were determined from the scattering intensities (Fig. S7 (b) and (c) in Supporting Information) and are gathered in Table 2. Irrespective of the pH, $R_g$ and $V_p$ values are similar and higher than the size/volume of uranyl carbonates species (Bernhard et al., 2001; Gückel et al., 2013) reported in the literature.

<table><caption>**Table 2.** Characteristics of colloids formed in NCS solutions at pH<sup>ini</sup> 7, 8, 9, 10 obtained from SWAXS analysis, and prenucleation clusters stoichiometry.</caption><thead><tr><th>pH</th><th>7</th><th>8</th><th>9</th><th>10</th></tr></thead><tbody><tr><td>U/Si</td><td>2.5</td><td>1.8</td><td>1.1</td><td>1.4</td></tr><tr><td>R<sub>g</sub> (nm)</td><td>1.6</td><td>1.6</td><td>1.6</td><td>1.8</td></tr><tr><td>V<sub>p</sub> (nm<sup>3</sup>)</td><td>39</td><td>33</td><td>37</td><td>41</td></tr></tbody></table>
<table><tr><td>N<sub>p</sub> (colloids.L<sup>-1</sup>)</td><td>2.07.10<sup>18</sup></td><td>2.47.10<sup>18</sup></td><td>2.76.10<sup>18</sup></td><td>2.29.10<sup>18</sup></td></tr><tr><td>ρ<sub>mp</sub> (g.cm<sup>-3</sup>)</td><td>3.26</td><td>3.23</td><td>3.16</td><td>3.19</td></tr><tr><td>U atoms/colloid</td><td>249</td><td>201</td><td>197</td><td>233</td></tr><tr><td>[UO<sub>2</sub><sup>2+</sup>]<sub>col</sub><sup>eq</sup> (.nm<sup>-3</sup>)</td><td>6.4</td><td>6.1</td><td>5.4</td><td>5.7</td></tr><tr><td>Si atoms/colloid</td><td>99</td><td>110</td><td>181</td><td>169</td></tr><tr><td>[SiO<sub>4</sub><sup>4-</sup>]<sub>col</sub><sup>eq</sup> (.nm<sup>-3</sup>)</td><td>2.5</td><td>3.3</td><td>4.9</td><td>4.1</td></tr><tr><td>Porosity of the colloid</td><td>0.50</td><td>0.49</td><td>0.45</td><td>0.47</td></tr><tr><td>Prenucleation cluster formula</td><td>[(UO<sub>2</sub>)<sub>2.5</sub>SiO<sub>4</sub>·6.6H<sub>2</sub>O]<sup>+</sup></td><td>[(UO<sub>2</sub>)<sub>1.8</sub>SiO<sub>4</sub>·4.9H<sub>2</sub>O]<sup>-0.4</sup></td><td>[(UO<sub>2</sub>)<sub>1.1</sub>SiO<sub>4</sub>·3.1H<sub>2</sub>O]<sup>-1.8</sup></td><td>[(UO<sub>2</sub>)<sub>1.4</sub>SiO<sub>4</sub>·3.8H<sub>2</sub>O]<sup>-1.2</sup></td></tr></table>

Taking into account the decrease of Si concentrations in solution (Table S1), several assumptions can be made regarding the nature of the colloids. First, colloids may be composed of silica with U(VI) species adsorbed at the surface. To test this hypothesis, SWAXS data were fitted assuming that colloids are spherical SiO₂ nanoparticles with adsorbed U(VI) species. The results of the fit highlight that Si concentration in the colloids are about one order of magnitude higher than that calculated from solution analysis (Text S7 and Fig. S8 in Supporting information). Thus, the objects observed in solution cannot be SiO₂ colloids. Second, the colloids may adopt a core-shell structure, with a clarkeite or Na-compreignacite core (similar to NC solutions) and a silica shell. Under reducing conditions, such core-shell structure was evidenced by Neill et al. (Neill et al., 2018) in alkaline silicate solutions of U(IV). Nevertheless, there is no hydroxyl consumption since the pH remains constant during the experiment. Thus, the formation of these two phases that incorporate hydroxyl moieties in their structure is unlikely. The same conclusion applies for the formation of boltwoodite (Na(UO₂)(SiO₃OH)·nH₂O) expected from geochemical calculations (Fig. 1 (d)). Thus, even if the ratio U/Si in the colloidal fraction is close to 1 at pH 9 and 10, 22
suggesting the potential formation of boltwoodite, this phase was not considered in the following calculations since it integrates hydroxyl in its structure. Considering the variation of the U/Si ratio (from 1.1 to 2.5), the pH-independence of the colloidal fraction (Fig. 2) and the SI of the phases that may form in these solutions (Fig. 1 (d)), we assumed that U(VI) intrinsic colloids are composed of soddyite-like prenucleation clusters, [(UO₂)ₓSiO₄]ₓ⁻² (with x = U/Si obtained from solution analysis, see Supporting Information Text S8, Table S3).

To go deeper in the colloid description, and to precise the formula of the prenucleation clusters, the density of colloids in the probed volume, $N_p$ (in colloid.L⁻¹) (Table 2) was calculated taking into account the concentration of U and Si in the colloidal phase obtained from ICP-OES analyses (Table S3). This method is described in the Supporting Information, Text S9. From $N_p$, the density of colloids $\rho_{mp}$ (in g.cm⁻³) and atomic density of U and Si in colloids were deduced (Table 2). The data gathered in Table 2 show that 2.10¹⁸ to 3.10¹⁸ colloids are formed per liter of solution with no trend between $N_p$ and pH. The number of Si atoms per colloid, which corresponds to the number of prenucleation clusters, [(UO₂)ₓSiO₄]ₓ⁻², are ranging from 100 to 180 depending on pH. Each colloidal particle therefore corresponds to an aggregate of a large number of [(UO₂)ₓSiO₄]ₓ⁻² units. To compare, these values are lower than the number of (UO₂)₂SiO₄ ranging from 150 and 190 if these colloids consist in pure Soddyite (values calculated considering the cell volume of soddyite). Taking into account the number and the molecular volume of UO₂²⁺ and SiO₄⁴⁻ and the volume of a colloid $V_p$, a colloid porosity of 0.5 was calculated regardless of pH. Assuming that the pore volume is occupied by water molecules, the prenucleation cluster hydration rate was estimated (Table 2, prenucleation cluster formula). This calculation is detailed in Supporting Information Text S10.
## 3.4 Estimation of the distribution coefficient of U(VI) between colloidal phase and NCS solution

Even if the formation mechanism of the colloids in NCS solutions is unknown, the determination of the precise concentrations of U(VI) and Si in the colloidal phase (Table 2) and in solution allows to simply calculate a distribution ratio $K_{dx}$ even if the system did not reach an equilibrium (12) (Rice et al., 1993).

$$K_{dx} = \frac{[X]_{col}^{eq}}{[X]_{sol}^{eq}} \quad (12)$$

with $X = U$ or $Si$. The results obtained are presented in Table 3.

<table><caption>**Table 3.** Distribution ratios $K_{dx}$ with $X = UO_2^{2+}$ or $SiO_4^{4-}$ obtained at pH 7, 8, 9 and 10 considering the NCS solutions at the equilibrium.</caption><thead><tr><th>pH</th><th>7</th><th>8</th><th>9</th><th>10</th></tr></thead><tbody><tr><td>K<sub>dUO<sub>2</sub><sup>2+</sup></sub></td><td>1.5.10<sup>5</sup></td><td>5.2.10<sup>4</sup></td><td>3.3.10<sup>5</sup></td><td>3.1.10<sup>5</sup></td></tr><tr><td>K<sub>dsio<sub>4</sub><sup>4-</sup></sub></td><td>3.2.10<sup>3</sup></td><td>4.2.10<sup>3</sup></td><td>9.2.10<sup>3</sup></td><td>6.7.10<sup>3</sup></td></tr></tbody></table>

These values could be included in thermodynamic database to calculate the speciation of species in similar experimental conditions considering the colloidal phase.

## 4. CONCLUSION

In this work, we develop a simple, reliable and systematic method based on geochemical simulations, elemental analyses by ICP-OES and *in situ* SWAXS characterizations to analyze U(VI) intrinsic colloids in simple model solutions of NaCl and NaCl with Na₂SiO₃ at various pH from 7 to 10. From these data, key properties as the concentration of U(VI) intrinsic colloids, their volume, their density and composition have been determined and followed in long-term
experiments. Depending on the solution composition, the evolution of U(VI) colloids is different.
In NaCl solution (NC) the colloids incorporated hydroxides and sodium ions, are not stable and
age to form a solid phase, the Na-compreignacite. This bulk material is made of crystallites of few
nanometers to tens of nanometers. Adding Na₂SiO₃ to the former solution (NCS) leads to
drastically different results. Even after several months, the U(VI) colloids remain stable. This
observation shows that the colloids formed in the two model solutions are different and that silicate
ions bind to uranyl to form nanometer-sized colloids that are stable over the long term, thus
potentially mobile in natural waters. The Fig. 4 summarizes the mechanism of evolution of U(VI)
colloids in NC and NCS solutions, respectively. Knowing the U(VI) concentration in the colloidal
phase, we were able to calculate a distribution coefficient. This coefficient can be included in
thermodynamic databases to calculate in similar environmental conditions, an accurate U(VI)
speciation that takes into account the predominant colloidal form.

<figure><img src="image_8.png" /><figcaption>Fig. 4. Schematic drawing illustrating the evolution of U(VI) colloids in NC and NCS solutions.</figcaption></figure>
To make progress in the prediction of the mobility of U(VI) in environment, the charge of these U(VI) colloids has to be determined. Indeed, these charged U(VI) colloids cannot be viewed as a solid phase but consist of flocculates (if reversible) or coagulated collection of highly hydrated prenucleation clusters (if not reversible). The various charges generated by the different number of cations and anions in the colloids can lead to various adsorption constants on mineral surface. Characterizing the surface charge and modeling the charge distribution at the colloid surface are therefore essential to forecast their behavior at the vicinity of any mineral surface.

To end, the simple, reliable and systematic methodology presented in this paper could also be used to characterize actinide pseudo-colloids that form when manufactured nanoparticles are used as adsorbants in remediation strategies (Liu et al, 2022) or to quantify the adsorption of actinide colloids on various natural adsorbants (Liu et al, 2024).

12

13 Acknowledgements

The authors thank the French research program NEEDS (CNRS, ANDRA, BRGM, CEA, EDF, Framatome, IRSN, Orano) for the financial support of this work (ESPAS project) and John Bartlett and Laxmi Bodapati for language editing of the manuscript.

17 Appendix A. Supplementary data

18
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