# Thermodynamic formation constants of the calcium(II)triscarbonatoactinylate(VI) (U, Np, and Pu) complexes using capillary electrophoresis ICP-MS

Ruopei Sun, Erwan Dupuis, Pascal E. Reiller

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Ruopei Sun, Erwan Dupuis, Pascal E. Reiller. Thermodynamic formation constants of the calcium(II)triscarbonatoactinylate(VI) (U, Np, and Pu) complexes using capillary electrophoresis ICP-MS. Inorganic Chemistry, 2025, 64 (50), pp.24963-24977. 10.1021/acs.inorgchem.5c04940. cea-05440989

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Thermodynamic formation constants of the
calcium(II)triscarbonatoactinylate(VI) (U, Np, and
Pu) complexes using capillary electrophoresis ICP-
MS

Ruopei Sun, Erwan Dupuis, and Pascal E. Reiller*

Université Paris-Saclay, CEA, Service de Physico-Chimie, Gif-sur-Yvette 91191, France

pascal.reiller@cea.fr

Inorganic Chemistry 64 (50), 24963-24977

<https://doi.org/10.1021/acs.inorgchem.5c04940>

**ABSTRACT** The formation constants of Ca*n*AnO₂(CO₃)₃*(4-2n)- (An = U, Np, and Pu) are determined in 0.1 mol/kg water NaClO₄ medium at 25°C via the coupling of capillary electrophoresis and inductively coupled plasma mass spectroscopy (CE-ICP-MS) using polyetheretherketone (PEEK) capillaries. Prior to the calcium titration experiments the stabilization of Np(VI) and Pu(VI) under the experimental conditions, and their complexation behavior with carbonate are studied. The data selection from limited available studies and the impact of minor AnO₂(CO₃)ₘ²⁻²m species are also discussed. Experimental results indicate that for the three actinides,
$AnO_2(CO_3)_m^{2-2m}$ complexes form $CaAnO_2(CO_3)_3^{2-}$ complexes first and subsequently $Ca_2AnO_2(CO_3)_3(aq)$ complexes, as already shown for U(VI). The cumulative stability constants at infinite dilution and at 25°C are $log_{10}\beta^°(CaUO_2(CO_3)_3^{2-}) = 27.01 ± 0.11$, $log_{10}\beta^°(Ca_2UO_2(CO_3)_3(aq)) = 30.13 ± 0.16$, $log_{10}\beta^°(CaNpO_2(CO_3)_3^{2-}) = 25.14 ± 0.28$, $log_{10}\beta^°(Ca_2NpO_2(CO_3)_3(aq)) = 28.64 ± 0.24$, $log_{10}\beta^°(CaPuO_2(CO_3)_3^{2-}) = 25.85 ± 0.69$ and $log_{10}\beta^°(Ca_2PuO_2(CO_3)_3(aq)) = 29.15 ± 0.66$. Theoretical calculations using the obtained data also suggest that the formation of $Ca_nAnO_2(CO_3)_3^{(4-2n)-}$ complexes favors the stabilization of Np(VI) and Pu(VI) in calcium and carbonate-rich oxidant environments.

## INTRODUCTION

The environmental chemistry of actinides (An) is a major concern of the long-term storage and management of nuclear waste. Among the constituents of nuclear waste, uranium, neptunium, and plutonium are particularly problematic due to the half-lives of some of their isotopes being superior to 1000 years. Deep geological disposal strategies are under study or even implemented by many countries to ensure the long-term containment of either spent nuclear fuel or high-level radioactive waste. Nevertheless, in the event of canister corrosion and waste failure, actinides can be dissolved through leaching by natural waters, and consequently enter the environment with the migration of contaminated water.

Under oxidizing conditions, U(VI), Np(VI), and Pu(VI) can be present as hexavalent actinyl species $AnO_2^{2+}$, which are soluble and mobile under a wide range of aqueous conditions by forming strong complexes with various inorganic and organic ligands, particularly with the carbonate ion $CO_3^{2-}$.¹⁻³ Among the derived carbonato compounds, calcium(II)triscabonatoactinylate(VI) $Ca_nUO_2(CO_3)_3^{(4-2n)-}$ complexes have gathered interest over
the years since their identification in the seepage water of a German uranium mine in the 1990s.⁴
Considering the abundance and ubiquity of carbonate in the natural environment, AnO₂(CO₃)ₙ²⁻²n
complexes are expected to be the dominant forms of dissolved An(VI) in carbonate-bearing waters,
as limestone-hosted ground waters and seawater.⁵,⁶ The formation of CaₙUO₂(CO₃)₃⁽⁴⁻²n)⁻ had been
confirmed to have a suppressing effect on uranium adsorption on sediments,⁷⁻⁹ and clay minerals
as bentonite¹⁰ and montmorillonite.¹¹ Given the similarity between the U(VI), Np(VI), and Pu(VI)
species, it is possible that Np(VI) and Pu(VI) share a comparable behavior with U(VI) in an
aqueous environment rich in carbonate and calcium.¹² Inasmuch, besides CaₙUO₂(CO₃)₃⁽⁴⁻²n)-
complexes, the first complex of Pu(VI), CaPuO₂(CO₃)₃²⁻ has been experimentally evidenced by Jo
et al.¹³ Nonetheless, the existence of the second species cannot be excluded, as the technique used
by the authors, visible-NIR absorption spectroscopy, required a high concentration of Pu (0.39
mM), which consequently increased the concentration of carbonate (17 mM) and limited the
amount of soluble calcium (0–0.72 mM), as pointed out by Reiller.¹² Hence, the formation
behavior of Ca₂PuO₂(CO₃)₃(aq) and other CaₙAnO₂(CO₃)₃⁽⁴⁻²n)⁻ species remains unclear.

In a previous study¹⁴ we have successfully determined the formation constants of
CaₙUO₂(CO₃)₃⁽⁴⁻²n)⁻ by the coupling of capillary electrophoresis (CE) and inductively coupled
plasma mass spectrometry (ICP-MS), in excellent agreement with the literature.²,¹⁵⁻²¹
Polyetheretherketone (PEEK) capillaries were chosen to conduct the experiments to avoid the
strong interaction between uranium(VI) and silica.²²,²³

In this study, we propose to use the same methodology to elucidate the formation behavior of the
Ca<sub>n</sub>AnO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>(<sup>4-2n</sup>-) complexes (An = U, Np or Pu), in a radiologically controlled laboratory<sup>24,25</sup>
using a comparable CE apparatus. With this hyphenation one can work under trace level of
actinides, i.e., 3 &times; 10<sup>-9</sup> mol/L for U and 5 &times; 10<sup>-10</sup> mol/L for Pu according to Ambard et al.<sup>26</sup> Such
low concentration may allow observing the species that were not observed formerly due to
concentration limitations of NIR Visible spectroscopy. Complexation constants of the
Ca<sub>n</sub>AnO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>(<sup>4-2n</sup>-) complexes will be determined all together with U(VI) allowing comparison
with the former determinations using CE-ICP-MS and other techniques. The obtained constants
and evolutions, as well as their impact on Np(VI) and Pu(VI) environmental chemistry will be
discussed.

## EXPERIMENTAL

**Control of pH values.** As in previous studies,<sup>14-16,27</sup> the pH determinations were carried out at room temperature. The pH meter (SevenEasy S20, Mettler Toledo) was calibrated regularly with four pH buffers (1.68, 4.01, 6.87, and 9.18, SI Analytics) to guarantee the accuracy of measurements. Instead of reading the values directly, the potential values were plotted against the pH values of buffers. By performing linear regression and calculating the 95% confidence interval, the pH values were recalculated from the potential of each sample. This procedure allowed controlling the uncertainty of the pH values below 0.05 pH unit. The pH values of samples and electrolytes were regularly checked before performing the electrophoresis.

**Electrolytes.** The electrolyte solutions were prepared by dissolving an appropriate amount of NaClO<sub>4</sub> (Sigma-Aldrich, ≥ 98%) in Millipore deionized water (Alpha-Q, 18.2 MΩ.cm) to have an ionic strength of 0.1 mol/kg<sub>water</sub>—molality scale afterward noted mol/kg<sub>w</sub>. In order to maintain the hexavalent oxidation state throughout the experiments, 12.5% NaClO (Carlo Erba, RPE for analysis) was diluted in the electrolyte to have a final concentration of *ca.* 10 mmol/kg<sub>w</sub>. The added amount of NaHCO<sub>3</sub> (Sigma-Aldrich, ACS reagent) solution (0.01, 0.05, and 0.1 mol/kg<sub>w</sub>) was calculated from the atmospheric equilibrium at the desired pH values using the thermodynamic data from Table 1. Afterwards, the samples were left open for 2 hours to reach equilibrium with
the atmosphere. Finally, desired amount of 0.2 mol/kg<sub>w</sub> Ca(ClO<sub>4</sub>)<sub>2</sub> (Acros Organics, ACS reagent) solution was added to have a series of electrolytes at varying Ca concentration. All the solutions were prepared by weighing.

**Sample preparation.** The U(VI) stock solution was prepared by dissolving U<sub>3</sub>O<sub>8</sub> in 37% hydrochloric acid (SCP Science, PlasmaPurePlus) during one week. The uranium concentration of this solution was measured by ICP-MS. A sub-stock solution of 5 × 10<sup>-5</sup> mol/kg<sub>w</sub> was prepared by diluting an appropriate amount of U(VI) stock solution in a 0.1 mol/kg<sub>w</sub> NaClO<sub>4</sub> solution. This sub-stock solution was further diluted in the corresponding electrolyte to have a final concentration of 5 × 10<sup>-7</sup> mol/kg<sub>w</sub> of uranium.

Np and Pu stock solutions were prepared from in-house solutions of Np and Pu in concentrated nitric acid ([HNO<sub>3</sub>] ≥ 1M). To oxidize Np and Pu to their +VI oxidation state, aliquots of the in-house solutions were dried and subsequently fumed with 1 mL of concentrated HClO<sub>4</sub> on a hot plate set to 200°C. This fuming process was repeated 3 times to stabilize the oxidation state of Np and Pu to +VI. Np and Pu were added to the electrolyte in a similar manner of U to obtain the same final concentration, 5 × 10<sup>-7</sup> mol/kg<sub>w</sub>.

5 × 10<sup>-5</sup> mol/kg<sub>w</sub> of Cs<sup>+</sup> (Fisher Bioreagents, 99.999%) was added to monitor the performance of the electrophoreses. Ga-NOTA (Ga(NO<sub>3</sub>)<sub>3</sub>·9H<sub>2</sub>O, Fluka, puriss. and 2,2',2"-(1,4,7-triazacyclononane-1,4,7-triyl)triacetic acid (NOTA), CheMatch), which was shown to be an excellent electroosmotic marker in the preceding studies<sup>14,25</sup> was spiked at 1.28 × 10<sup>-5</sup> mol/kg<sub>w</sub> in the sample. It contained an excess of gallium to ensure NOTA would not complex the actinides. Finally, an appropriate amount of 0.2 mol/kg<sub>w</sub> Ca(ClO<sub>4</sub>)<sub>2</sub> (Acros Organics, ACS reagent) solution was added to have a series of samples matching the Ca concentration of the electrolytes.
**Electrophoresis.** A custom CE system was employed to perform the electrophoresis, as previously described.²⁴ In order to be able to perform electrophoreses with negative voltage, an Agilent 7100CE system, located outside of the glovebox, was used as a high-voltage generator only. Pressure was applied to the system using a pressure-based flow controller (MFCS Series, Fluigent). PEEK capillaries (i.d. 100 µm, o.d. 360 µm, IDEX Health & Science) were cut to 105 cm and conditioned for 10 min with 2% HNO₃, 10 min with deionized water, and 10 min with 0.1 M NaOH (Merck, > 99%) before use. Before and between each run of electrophoresis, the capillary was rinsed for 2 min with 2% HNO₃, 2 min with deionized water, and 2 min with the electrolyte. The samples were injected hydrodynamically into capillary for 7 s at 50 mbar, and the electrophoresis was performed at room temperature (25°C), at a voltage comprised between -8 and +8 kV and a pressure comprised between 15 and 50 mbar.

The CE instrument was coupled with a single quadrupole ICP-MS (Xseries II, Thermo Scientific) using a MiraMist CE nebulizer (Burgener Research). The argon flow rate for nebulization was set to 1 mL/min. 2% HNO₃ was used as make-up liquid at a flow rate of 10 µL/min, with the aid of the pressure-based flow controller. The value and stability of the CE current was checked for each CE run.

## THEORETICAL

**Speciation calculation.** The speciation calculations were done using PHREEQC (version 3.5.0) and the Thermochimie 12a database extraction file for Davies²⁸ correction for ionic strength (https://www.thermochimie-tdb.com/).²⁹ The U, Np, and Pu thermodynamic constants included are taken from the latest update (except NpO₂(CO₃)ₘ²⁻²m complexes) of the thermodynamic database (TDB) project from the Nuclear Energy Agency (NEA) of the Organization for Economic Cooperation and Development (OECD), afterwards termed by TDB-NEA.² The thermodynamic
constants of NpO₂(CO₃)ₘ²⁻²m were either taken directly for NpO₂(CO₃)(aq) and NpO₂(CO₃)₂²⁻, or calculated from the Gibbs energies of formation of NpO₂(CO₃)₃⁴⁻, NpO₂²⁺, and CO₃²⁻ taken from the 2001 and 2003 update versions¹,³—only the constant for the oxidation of NpO₂(CO₃)₃⁵⁻ to NpO₂(CO₃)₃⁴⁺ is selected. This data selection will be discussed in the following section (vide post).

**Calculation of electrophoretic mobility and stability constants.** The electrophoretic mobility of species during a CE run can be calculated as below, taking into account the electroosmotic effect:

$$ \mu = \frac{L^2}{V} \left( \frac{1}{t} - \frac{1}{t_{\text{eof}}} \right) \qquad (1) $$

where μ is the electrophoretic mobility (m² V⁻¹ s⁻¹) of the peak, L is the length of the capillary (m), V is the applied voltage (Volt), t and tₑof (s) are the migration time of the peak associated to the complex, and migration time of the neutral marker, respectively.

As mentioned in the previous study,¹⁴ based on the kinetics of the complexation, the behavior of the metal ligand complexes can be categorized in three situations:³⁰ (i) labile complexes, if the metal-ligand half-life t₁/₂ is less than 10⁻³ s, and the ligand is present within the electrolyte solution, then the species cannot be separated due to their rapid kinetic of ligand exchange, and appear as one single peak in the electropherogram; (ii) stable complexes, if the t₁/₂ is higher than 10³ s, then the rate of ligand exchange is not fast enough during the electrophoresis, each species appears as a single peak; and (iii) the intermediate case suggests a potential disequilibrium, and the experimental verification of the complex stability is recommended. The behavior of CaₙUO₂(CO₃)₃⁽⁴⁻²n⁾- complexes was experimentally proved to be the labile case as only one peak was observed in the electropherograms in our previous study.¹⁴ In analogy, the behaviors of possible CaₙNpO₂(CO₃)₃⁽⁴⁻²n⁾- and CaₙPuO₂(CO₃)₃⁽⁴⁻²n⁾- were assumed to be the same,¹²,¹³ which has to be verified in the experiments. Based on the assumption that all the species would appear
as one single peak, the observed global electrophoretic mobility of Ca_nAnO_2(CO_3)_3(4-2n)- should be
a combination of all the species with their molar fraction α respectively, and can be expressed as
follows:

$$
\mu = \sum_{i=0}^{n} \alpha_{Ca_iAnO_2(CO_3)_3(4-2i)-} \times \mu_{Ca_iAnO_2(CO_3)_3(4-2i)-} \quad (2)
$$

Continuous focus on the thermodynamic behavior of Ca_nUO_2(CO_3)_3(4-2n)- complexes (n = 0, 1, 2)
has proved that their formation could reduce the minor U(VI) soluble species such as
(UO_2)_p(CO_3)_m(OH)_n^{2p-2m-n} in the pH range 7.8-10.²,³,³¹ However, due to the limited knowledge on
Ca_nNpO_2(CO_3)_3(4-2n)- and Ca_nPuO_2(CO_3)_3(4-2n)-, the possible minor species have to be taken into
account in the calculation. Based on the literature,¹⁻³,³¹ in equilibrium with atmospheric CO_2(g)
from pH 7.8 to 10, the most significant minor species, i.e., carbonato complexes, are NpO_2(CO_3)_2²⁻
for Np(VI), and PuO_2(CO_3)(aq) and PuO_2(CO_3)_2²⁻ for Pu(VI). Considering An in trace compared
to Ca, and accounting for the carbonato minor species, the possible complexation and their
cumulative formation constants in the Ca_nAnO_2(CO_3)_m(2+2n-2m) samples can be summarized as:

$$
\mathrm{AnO_2^{2+} + mCO_3^{2-} \rightleftharpoons AnO_2(CO_3)_m^{2-2m}} \qquad \beta_{0m} = \frac{[AnO_2(CO_3)_m^{2-2m}]}{[CO_3^{2-}]^m [AnO_2^{2+}]}
$$

$$
n\text{Ca}^{2+} + \text{AnO}_2(\text{CO}_3)_3^{4-} \rightleftharpoons \text{Ca}_n\text{AnO}_2(\text{CO}_3)_3(4-2n)- \quad \beta_{n3} = \frac{[\text{Ca}_n\text{AnO}_2(\text{CO}_3)_3(4-2n)-]}{[\text{Ca}^{2+}]^n [\text{AnO}_2(\text{CO}_3)_3^{4-}]} \quad (4)
$$

where square brackets represent concentration on the molality scale (mol/kgw), with n = {0, 1, 2}
and m = {1, 2, 3}.

For Ca_nPuO_2(CO_3)_m^{2+2n-2m}, i.e., PuO_2(CO_3)(aq), PuO_2(CO_3)_2^{2-}, PuO_2(CO_3)_3^{4-}, CaPuO_2(CO_3)_3^{2-}, and Ca_2PuO_2(CO_3)_3(aq) (equation 2) can be expressed as follows, by applying equation (3) and (4),
$$ \mu_{\text{Pu(VI)}} = \frac{\mu_{01} + [\text{CO}_3]^{2-}\mu_{02}\frac{\beta_{02}}{\beta_{01}} + [\text{CO}_3]^{2-}\mu_{03}\frac{\beta_{03}}{\beta_{01}} + [\text{CO}_3]^{2-}\frac{\beta_{03}}{\beta_{01}}[\text{Ca}^{2+}]\mu_{13}\beta_{13} + [\text{CO}_3]^{2-}\frac{\beta_{03}}{\beta_{01}}[\text{Ca}^{2+}]^2\mu_{23}\beta_{23}}{1 + [\text{CO}_3]^{2-}\frac{\beta_{02}}{\beta_{01}} + [\text{CO}_3]^{2-}\frac{\beta_{03}}{\beta_{01}} + [\text{CO}_3]^{2-}\frac{\beta_{03}}{\beta_{01}}[\text{Ca}^{2+}]\beta_{13} + [\text{CO}_3]^{2-}\frac{\beta_{03}}{\beta_{01}}[\text{Ca}^{2+}]^2\beta_{23}} \quad (5) $$

where $\mu_{\text{Pu(VI)}}$ is the observed global electrophoretic mobility of each sample, $\mu_{\text{nm}}$ and $\beta_{\text{nm}}$ represent the electrophoretic mobility and formation constant of each $\text{Ca}_n\text{PuO}_2(\text{CO}_3)_m^{2+2n-2m}$ species, respectively. The unit of concentration is on the molality scale (mol/kg$_w$).

In a similar manner for $\text{Ca}_n\text{NpO}_2(\text{CO}_3)_m^{2+2n-2m}$, i.e., $\text{NpO}_2(\text{CO}_3)_2^{2-}$, $\text{NpO}_2(\text{CO}_3)_3^{4-}$, $\text{CaNpO}_2(\text{CO}_3)_3^{2-}$, and $\text{Ca}_2\text{NpO}_2(\text{CO}_3)_3(\text{aq})$:

$$ \mu_{\text{Np(VI)}} = \frac{\mu_{02} + [\text{CO}_3]^{2-}\mu_{03}\frac{\beta_{03}}{\beta_{02}} + [\text{CO}_3]^{2-}\frac{\beta_{03}}{\beta_{02}}[\text{Ca}^{2+}]\mu_{13}\beta_{13} + [\text{CO}_3]^{2-}\frac{\beta_{03}}{\beta_{02}}[\text{Ca}^{2+}]^2\mu_{23}\beta_{23}}{1 + [\text{CO}_3]^{2-}\frac{\beta_{03}}{\beta_{02}} + [\text{CO}_3]^{2-}\frac{\beta_{03}}{\beta_{02}} + [\text{CO}_3]^{2-}\frac{\beta_{03}}{\beta_{02}}[\text{Ca}^{2+}]\beta_{13} + [\text{CO}_3]^{2-}\frac{\beta_{03}}{\beta_{02}}[\text{Ca}^{2+}]^2\beta_{23}} \quad (6) $$

These formulae were incorporated in OriginPro 2018 (OriginLab, USA) to calculate the constants by fitting the electrophoretic mobility of samples against their $\text{Ca}^{2+}$ concentration.

It must be stressed here that the uncertainties on the different $\log_{10}\beta$ from the NEA-TDB selection have not been propagated in the calculus of the adjusted constants.

**Ionic strength correction.** Since one could never prepare the hypothetical solution at zero ionic strength, formation constants of complexes are always determined at a certain constant ionic strength with a background electrolyte, depending on the complexity of the system studied. The constants determined are conditional constants that are valid only for the conditions in which they have been determined. To extrapolate to standard conditions, Davies$^{28}$ developed an empirical equation, based on Debye and Hückel relations,$^{32}$ to calculate the activity coefficients of electrolytes in combination with an ion-association approach:

$$ \log_{10} \gamma_i = -A Z_i^2 \left( \frac{\sqrt{I_m}}{1 + \sqrt{I_m}} - 0.3 I_m \right) \quad (7) $$
where $γ_i$ is the activity coefficient, A is the Debye-Hückel limiting slope²—depending on temperature and pressure—, $z_i$ is the charge of the particular ion i, and $I_m$ is the ionic strength of the solution on the molality scale.

Consequently, the formation constant of ML_j type complexes at infinite dilution can be written as:

$$ \log_{10} K_j^\circ = \Delta\log_{10}\gamma + \log_{10}K_j \quad (8) $$

in which $\Delta\log_{10}\gamma_i = \log_{10}\gamma(\text{ML}_j) - \log_{10}\gamma(\text{M}) - j\log_{10}\gamma(\text{L})$.

Using the Davies equation (Equation 7), $\Delta\log_{10}\gamma$ can be calculated as:

$$ \Delta\log_{10}\gamma_i = -A\Delta z_i^2 \left( \frac{\sqrt{I_m}}{1+\sqrt{I_m}} - 0.3 I_m \right) \quad (9) $$

where $\Delta z_i^2 = z(\text{ML}_j)^2 - z(\text{M})^2 - j z(\text{L})^2$.

For Ca_nUO₂(CO₃)₃⁽⁴⁻²n⁾ complexes, Davies equation works fairly well up to ionic strength of 0.1 mol/kg_w for single electrolyte systems.¹² Specific ion interaction theory (SIT) is broadly employed for higher ionic strength.²,¹² The case of a species M towards a counter ion X from the background electrolyte is then expressed as follows:

$$ \Delta\log_{10}\gamma_M = -Az_i^2 \frac{\sqrt{I_m}}{1 + 1.5\sqrt{I_m}} + \sum_i(\nu_M \varepsilon(M,X) m_X) \quad (9) $$

where $v_m$ is the stoichiometric parameter of the species—positive for production and negative for consumption, $\varepsilon(M,X)$ is the specific ion interaction coefficient between M and X, $m_X$ is the molality of X.
RESULTS AND DISCUSSION

**Speciation calculation.** As discussed in the introduction, the concentration of An(VI) was set to $5 \times 10^{-7}$ mol/kg<sub>w</sub> in order to enhance the concentration difference between calcium and actinides for a better investigation of the formation of CaAnO₂(CO₃)₃²⁻ and Ca₂AnO₂(CO₃)₃(aq). Nonetheless, at this level of concentration, the speciation of CaₐAnO₂(CO₃)₃⁽⁴⁻²n⁾⁻ complexes cannot be directly confirmed by spectroscopic methods as in preceding researches,¹³,¹⁵ Comparing to stable U(VI) in oxidizing carbonate-bearing aqueous condition, Np and Pu can exist under more than one oxidation state.²⁰ Accordingly, the electrophoretic behavior of the oxidation states of Np and Pu must be studied in the first place, in absence of a spectroscopic confirmation of speciation.

Consequently, 5 samples were prepared at pH 9.76, 9.08, 8.76, 8.44, and 7.96. An excess of carbonate (ca. 100 mM) was added to force the speciation of An(VI) to be AnO₂(CO₃)₃⁴⁻ all over the pH range for a better comparison. The electropherograms of these samples are presented in Figure 1. Beside the Ga-NOTA peaks (neutral marker), three main An peaks can be observed following the mobility order U > Np > Pu, i.e., following the effective ionic radius.³³ Another less negative peak of Np can be found in all the electropherograms in Figure 1 but Figure 1d, and another negative peak of Pu in Figure 1e as an overlapping peak, which can be confirmed by reversing the voltage (Figure 1f).

From the speciation calculation for Np and Pu with 100 mM of carbonate under oxidizing condition, two oxidation states are possible. The An(VI), which possesses a higher affinity to carbonate tends to be more negative by forming AnO₂(CO₃)₃⁴⁻ across the pH range, while the An(V), which possesses a lower affinity towards carbonate, would have its global charge decreasing with the decrease of pH values. The dominant species of Np(V) at pH 9.76, NpO₂(CO₃)₂³⁻, gradually converts to NpO₂(CO₃)⁻ with decreasing pH values. Similarly, Pu(VI)
presents as more negatively charged species $PuO_2(CO_3)_3^{5-}$ and $PuO_2(CO_3)_2^{3-}$ at higher pH, and
exists as $PuO_2(CO_3)^-$ at lower pH values.

Experimentally, we observed the main peaks of Np and Pu remaining at the same position with U(VI), in good accordance with the calculation; these two peaks can reasonably be attributed to Np(VI) and Pu(VI). The other peak of Np, shifted towards less negative mobility with the decrease of pH, suggests it corresponds to Np(V). This hypothesis will be further studied in the next part of the study with the addition of Tris (tris(hydroxymethyl)-aminomethane) as a reducing buffer.
<figure><img src="image_2.png" /><figcaption>**Figure 1.** Electropherograms of samples at pH 7.96 (a), 8.44 (b), 8.76 (c), 9.08 (d), 9.76 (e) under -8 kV and 9.76 under +8 kV (f). c(NaClO) = 6.5 mM, [carbonate] = 100 mM, [An] = 5 × 10⁻⁷ mol/kg<sub>w</sub>, [NOTA] = 1.28 × 10⁻⁵ mol/kg<sub>w</sub>, ratio Ga: NOTA = 1:0.95, T = 25°C. Voltage = -8 kV. PEEK capillary i.d. 100 µm, o.d. 360 µm, L = 105 cm.</figcaption></figure>

**Effect of carbonate and calcium on the redox of An(V)/An(VI).** To better understand the behavior of An(VI) under our conditions, the effect of carbonate and calcium were also investigated. The pH value was fixed *ca.* 7.9 in the first place to have a relatively higher solubility of calcite (CaCO₃), and limit the ratio of An minor species as An(OH)ₙ²⁻ complexes. In Figure 2, the concentration of carbonate was decreased from 78.4 to 15.3 mM. It can be observed that the relative proportion of Np(V) peak increases with the decrease of carbonate, and becomes less negative due to its weaker affinity for carbonate, while the three An(VI) peaks remained almost
unchanged. This suggests that the complexation with CO₃²⁻ favors the stabilization of Np(VI) and Pu(VI). The attribution of Pu(V) and Np(V) was further confirmed by a sample spiked with 10.3 mM of reducing buffer, Tris. It is noteworthy that in Figure 2e and Figure 2f, under the same pH = 7.9 and carbonate concentration 15.3 mM with the presence of Tris, the assumed Np(VI) and Pu(VI) peaks are shifted to the assumed Np(V) and Pu(V) peaks, respectively, while U(VI) remained at the similar position.

Similar behavior can be identified in the case of Ca in Figure 3. The intensity of Np(V) declines rapidly with the Ca concentration, the peak is eventually disappearing, indicating that the formation of CaₐAnO₂(CO₃)₃⁽⁴⁻²n)⁻ complexes enables the stabilization of Np(VI) and Pu(VI). It is also worth noting that while An(VI) peaks become less negative—in agreement with the complexation from AnO₂(CO₃)₃⁻ to CaₐAnO₂(CO₃)₃⁽⁴⁻²n)⁻—Np(V) remained roughly at the same position. Due to the relatively high concentration of carbonate (20.3 mM), the observed mobility of An(VI) reached the plateau around 0.4 mM of Ca, limited by the solubility of calcite (CaCO₃)—from Plummer and Busemberg³⁴ in Thermochimie 12a. From the above results, a key phenomenon established the basis of our following experiments: An(V) and An(VI) species are labile complexes and two independent peaks, although they may appear as an overlapping peak.
<figure><img src="image_3.png" /><figcaption>**Figure 2.** Electropherograms of samples with different concentration of carbonate: 78.4 mM (a), 51.5 mM (b), 25.7 mM (c), 20.3 mM (d), 15.3 mM (e) and 15.3 mM carbonate with 10.2 mM Tris (f). c(NaClO) = 6.5 mM, pH = 7.9, [An] ≈ 5 × 10⁻⁷ mol/kg<sub>w</sub>, [NOTA] = 1.28 × 10⁻⁵ mol/kg<sub>w</sub>, ratio Ga : NOTA = 1 : 0.95, T = 25°C. voltage = -8 kV, PEEK capillary i.d. 100 µm, o.d. 360 µm, L = 105 cm.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Figure 3.** Electropherograms of samples with different nominal Ca concentration: 0 mM (a), 0.1 mM (b), 0.2 mM (c), 0.4 mM (d) and 0.8 mM (e). c(NaClO) = 6.5 mM, [carbonate] = 20.3 mM, pH = 7.9, [An] ≈ 5 × 10⁻⁷ mol/kg<sub>w</sub> (U blue, Np green, and Pu red), [NOTA] = 1.28 × 10⁻⁵ mol/kg<sub>w</sub>, ratio Ga : NOTA = 1 : 0.95, T = 25°C. voltage = -8 kV, PEEK capillary i.d. 100 µm, o.d. 360 µm, L = 105 cm.</figcaption></figure>

**Data selection and the electrophoretic mobility of AnO₂(CO₃)ₘ²⁻²m complexes.** To obtain the equilibrium constants βₙ₃ of CaₙAnO₂(CO₃)₃⁽⁴⁻²n⁾- complexes using Equation 5 and 6, the parameters μ₀m and β₀m from AnO₂(CO₃)ₘ²⁻²m complexes, which are the main coexisting minor species, must be evaluated. Available data of these species are summarized in Table 1. Apart from NpO₂(CO₃)ₘ²⁻²m, there is a general consensus on the data of UO₂(CO₃)ₘ²⁻²m and PuO₂(CO₃)ₘ²⁻²m.
Several formation constants were proposed for $NpO_2(CO_3)_m^{2-2m}$ complexes, but these values are still under discussion. In the first selection from the TDB-NEA, Lemire *et al.*¹ selected the $log_{10}\beta_m^°$ values from solubility measurements of $NpO_2CO_3(s)$ in 3 M $NaClO_4$.³⁵ It should be remarked that the values were determined from an overly sparse dataset, as highlighted by both the original authors³⁵ and the TDB-NEA reviews.¹,³ Recently, in the latter update of the TDB-NEA, Grenthe *et al.*² noted that the aforementioned authors overlooked the study from Kato *et al.*³⁶, which provided a larger dataset and key information for assessing the thermodynamic properties of aqueous $NpO_2(CO_3)_m^{2-2m}$ complexes and the solubility product of $NpO_2CO_3(cr)$. Nevertheless, the latter selection presents a significant limitation for our study: for the species $NpO_2(CO_3)_2^{2-}$, an upper limit $log_{10}\beta_{2°} \le 15.0$ was proposed rather than a selected value; and the $log_{10}\beta_{1°}$ and $log_{10}\beta_{3°}$ values were modified accordingly. This upper limit was not considered as selected in the latter review,² and reported for information only. As the absence of $log_{10}\beta_{2°}$ may introduce deviations into our calculations, the data from Vitorge *et al.*³⁵ and Lemire *et al.*¹ were nonetheless selected in our study for the fitting of results.
<table><caption>**Table 1.** Main stability constants at 25°C and I_m = 0 used in this work</caption><thead><tr><th>Reaction</th><th>log<sub>10</sub>K°</th><th>Reference</th></tr></thead><tbody><tr><td>CO<sub>2</sub>(g) + H<sub>2</sub>O ⇌ CO<sub>3</sub><sup>2-</sup> + 2H<sup>+</sup></td><td>-18.15 ± 0.06</td><td>Grenthe, et al.<sup>2</sup></td></tr><tr><td>UO<sub>2</sub><sup>2+</sup> + CO<sub>3</sub><sup>2-</sup> ⇌ UO<sub>2</sub>CO<sub>3</sub>(aq)</td><td>9.94 ± 0.03</td><td>Guillaumont et al.<sup>3</sup></td></tr><tr><td>UO<sub>2</sub><sup>2+</sup> + 2 CO<sub>3</sub><sup>2-</sup> ⇌ UO<sub>2</sub>(CO<sub>3</sub>)<sub>2</sub><sup>2-</sup></td><td>16.61 ± 0.09</td><td>Guillaumont et al.<sup>3</sup></td></tr><tr><td>UO<sub>2</sub><sup>2+</sup> + 3 CO<sub>3</sub><sup>2-</sup> ⇌ UO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub><sup>4-</sup></td><td>21.84 ± 0.04</td><td>Guillaumont et al.<sup>3</sup></td></tr><tr><td>NpO<sub>2</sub><sup>2+</sup> + CO<sub>3</sub><sup>2-</sup> ⇌ NpO<sub>2</sub>CO<sub>3</sub>(aq)</td><td>9.32 ± 0.61</td><td>Lemire et al.<sup>1</sup><br>Vitorge et al.<sup>35</sup></td></tr><tr><td>NpO<sub>2</sub><sup>2+</sup> + 2 CO<sub>3</sub><sup>2-</sup> ⇌ NpO<sub>2</sub>(CO<sub>3</sub>)<sub>2</sub><sup>2-</sup></td><td>16.52 ± 0.73</td><td>Lemire et al.<sup>1</sup><br>Vitorge et al.<sup>35</sup></td></tr><tr><td>NpO<sub>2</sub><sup>2+</sup> + 3 CO<sub>3</sub><sup>2-</sup> ⇌ NpO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub><sup>4-</sup></td><td>19.37 ± 0.19</td><td>Lemire et al.<sup>1</sup><br>Vitorge et al.<sup>35</sup></td></tr><tr><td>NpO<sub>2</sub><sup>2+</sup> + CO<sub>3</sub><sup>2-</sup> ⇌ NpO<sub>2</sub>CO<sub>3</sub>(aq)</td><td>9.86 ± 0.10</td><td>Grenthe et al.<sup>2</sup><br>Kato et al.<sup>36</sup></td></tr><tr><td>NpO<sub>2</sub><sup>2+</sup> + 2 CO<sub>3</sub><sup>2-</sup> ⇌ NpO<sub>2</sub>(CO<sub>3</sub>)<sub>2</sub><sup>2-</sup></td><td>≤ 15.0</td><td>Grenthe et al.<sup>2</sup><br>Kato et al.<sup>36</sup></td></tr><tr><td>NpO<sub>2</sub><sup>2+</sup> + 3 CO<sub>3</sub><sup>2-</sup> ⇌ NpO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub><sup>4-</sup></td><td>19.90 ± 0.50</td><td>Grenthe et al.<sup>2</sup><br>Kato et al.<sup>36</sup></td></tr><tr><td>PuO<sub>2</sub><sup>2+</sup> + CO<sub>3</sub><sup>2-</sup> ⇌ PuO<sub>2</sub>CO<sub>3</sub>(aq)</td><td>9.50 ± 0.50</td><td>Guillaumont et al.<sup>3</sup></td></tr><tr><td>PuO<sub>2</sub><sup>2+</sup> + 2 CO<sub>3</sub><sup>2-</sup> ⇌ PuO<sub>2</sub>(CO<sub>3</sub>)<sub>2</sub><sup>2-</sup></td><td>14.70 ± 0.50</td><td>Guillaumont et al.<sup>3</sup></td></tr><tr><td>PuO<sub>2</sub><sup>2+</sup> + 3 CO<sub>3</sub><sup>2-</sup> ⇌ PuO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub><sup>4-</sup></td><td>18.00 ± 0.50</td><td>Guillaumont et al.<sup>3</sup></td></tr><tr><td>NpO<sub>2</sub><sup>2+</sup> + e<sup>-</sup> ⇌ NpO<sub>2</sub><sup>+</sup></td><td>19.59 ± 0.07</td><td>Guillaumont et al.<sup>3</sup></td></tr><tr><td>PuO<sub>2</sub><sup>2+</sup> + e<sup>-</sup> ⇌ PuO<sub>2</sub><sup>+</sup></td><td>15.82 ± 0.09</td><td>Guillaumont et al.<sup>3</sup></td></tr><tr><td>NpO<sub>2</sub><sup>2+</sup> + e<sup>-</sup> ⇌ NpO<sub>2</sub><sup>+</sup></td><td>19.59 ± 0.07</td><td>Guillaumont et al.<sup>3</sup></td></tr><tr><td>PuO<sub>2</sub><sup>2+</sup> + e<sup>-</sup> ⇌ PuO<sub>2</sub><sup>+</sup></td><td>15.82 ± 0.09</td><td>Guillaumont et al.<sup>3</sup></td></tr><tr><td>UO<sub>2</sub><sup>2+</sup> + H<sub>2</sub>O ⇌ UO<sub>2</sub>(OH)<sup>+</sup> + H<sup>+</sup></td><td>-5.25 ± 0.24</td><td>Guillaumont et al.<sup>3</sup></td></tr><tr><td>UO<sub>2</sub><sup>2+</sup> + 2 H<sub>2</sub>O ⇌ UO<sub>2</sub>(OH)<sub>2</sub> + 2 H<sup>+</sup></td><td>-12.15 ± 0.07</td><td>Guillaumont et al.<sup>3</sup></td></tr><tr><td>UO<sub>2</sub><sup>2+</sup> + 3 H<sub>2</sub>O ⇌ UO<sub>2</sub>(OH)<sub>3</sub><sup>-</sup> + 3 H<sup>+</sup></td><td>-20.25 ± 0.42</td><td>Guillaumont et al.<sup>3</sup></td></tr><tr><td>UO<sub>2</sub><sup>2+</sup> + 4 H<sub>2</sub>O ⇌ UO<sub>2</sub>(OH)<sub>4</sub><sup>2-</sup> + 4 H<sup>+</sup></td><td>-32.40 ± 0.68</td><td>Guillaumont et al.<sup>3</sup></td></tr><tr><td>2 UO<sub>2</sub><sup>2+</sup> + CO<sub>3</sub><sup>2-</sup> + 3 H<sub>2</sub>O ⇌ (UO<sub>2</sub>)<sub>2</sub>(CO<sub>3</sub>)(OH)<sub>3</sub><sup>-</sup> + 3 H<sup>+</sup></td><td>-0.86 ± 0.51</td><td>Guillaumont et al.<sup>3</sup></td></tr><tr><td>NpO<sub>2</sub><sup>2+</sup> + H<sub>2</sub>O ⇌ NpO<sub>2</sub>(OH)<sup>+</sup> + H<sup>+</sup></td><td>-5.10 ± 0.40</td><td>Lemire et al.<sup>1</sup></td></tr><tr><td>NpO<sub>2</sub><sup>2+</sup> + 2 H<sub>2</sub>O ⇌ NpO<sub>2</sub>(OH)<sub>2</sub> + 2 H<sup>+</sup></td><td>-12.21 ± 1.50</td><td>Lemire et al.<sup>1</sup></td></tr><tr><td>NpO<sub>2</sub><sup>2+</sup> + 3 H<sub>2</sub>O ⇌ NpO<sub>2</sub>(OH)<sub>3</sub><sup>-</sup> + 3 H<sup>+</sup></td><td>-21.20 ± 1.10</td><td>Grenthe, et al.<sup>2</sup></td></tr><tr><td>NpO<sub>2</sub><sup>2+</sup> + 4 H<sub>2</sub>O ⇌ NpO<sub>2</sub>(OH)<sub>4</sub><sup>2-</sup> + 4 H<sup>+</sup></td><td>-32.00 ± 1.10</td><td>Grenthe, et al.<sup>2</sup></td></tr><tr><td>NpO<sub>2</sub><sup>2+</sup> + CO<sub>3</sub><sup>2-</sup> + 2 H<sub>2</sub>O ⇌ NpO<sub>2</sub>(CO<sub>3</sub>)(OH)<sub>2</sub><sup>2-</sup> + 2 H<sup>+</sup></td><td>-7.69 ± 1.30</td><td>Grenthe, et al.<sup>2</sup></td></tr><tr><td>2 NpO<sub>2</sub><sup>2+</sup> + CO<sub>3</sub><sup>2-</sup> + 3 H<sub>2</sub>O ⇌ (NpO<sub>2</sub>)<sub>2</sub>CO<sub>3</sub>(OH)<sub>3</sub><sup>-</sup> + 3 H<sup>+</sup></td><td>-2.87 ± 1.64</td><td>Lemire et al.<sup>1</sup></td></tr><tr><td>2 UO<sub>2</sub><sup>2+</sup> + NpO<sub>2</sub><sup>2+</sup> + 6 CO<sub>3</sub><sup>2-</sup> ⇌ (UO<sub>2</sub>)<sub>2</sub>(NpO<sub>2</sub>)(CO<sub>3</sub>)<sub>6</sub><sup>6-</sup></td><td>-53.59 ± 2.70</td><td>Lemire et al.<sup>1</sup></td></tr><tr><td>PuO<sub>2</sub><sup>2+</sup> + H<sub>2</sub>O ⇌ PuO<sub>2</sub>(OH)<sup>+</sup> + H<sup>+</sup></td><td>-5.50 ± 0.50</td><td>Lemire et al.<sup>1</sup></td></tr><tr><td>PuO<sub>2</sub><sup>2+</sup> + 2 H<sub>2</sub>O ⇌ PuO<sub>2</sub>(OH)<sub>2</sub> + 2 H<sup>+</sup></td><td>-13.20 ± 1.50</td><td>Lemire et al.<sup>1</sup></td></tr><tr><td>PuO<sub>2</sub><sup>2+</sup> + 3 H<sub>2</sub>O ⇌ PuO<sub>2</sub>(OH)<sub>3</sub><sup>-</sup> + 3 H<sup>+</sup></td><td>-24.00 ± 1.60</td><td>Grenthe, et al.<sup>2</sup></td></tr><tr><td>2 PuO<sub>2</sub><sup>2+</sup> + 2 H<sub>2</sub>O ⇌ (PuO<sub>2</sub>)<sub>2</sub>(OH)<sub>2</sub><sup>2+</sup> + 2 H<sup>+</sup></td><td>-7.50 ± 1.50</td><td>Lemire et al.<sup>1</sup></td></tr><tr><td>3 PuO<sub>2</sub><sup>2+</sup> + 6 CO<sub>3</sub><sup>2-</sup> ⇌ (PuO<sub>2</sub>)<sub>3</sub>(CO<sub>3</sub>)<sub>6</sub><sup>6-</sup></td><td>51.00 ± 2.50</td><td>Grenthe, et al.<sup>2</sup></td></tr></tbody></table>
Two An(VI) samples at pH 7.78 and 9.95 were prepared to measure the electrophoretic mobility of AnO₂(CO₃)₂²⁻ species. The ionic strength was fixed to 0.1 mol/kg_w NaClO₄ and the samples were in equilibrium with atmospheric CO₂(g). Based on the data selected in Table 1, at pH = 9.95 the three An(VI) are present as AnO₂(CO₃)₃⁴⁻, which allowed their electrophoretic mobility values to be readily measured. On the other hand, based on the calculation using PhreeqC, at pH = 7.78 Np(VI) should be present predominantly as NpO₂(CO₃)₂²⁻ (ca. 83%) and ca. 11% of other minor charged species, while Pu(VI) appears as a mixture of PuO₂CO₃(aq) (ca. 64%), PuO₂(CO₃)₂²⁻ (ca. 22.5%) and ca. 7% of other charged minor species. In a previous study, it was demonstrated that assuming the electrophoretic mobility of a neutral species as nil did not change the results.¹⁴ In this study, the electrophoretic mobility of neutral species was thus fixed to zero. Considering the difficulty of determine the electrophoretic mobility of each minor species and the fact that the overall mobility is mainly contributed by NpO₂(CO₃)₂²⁻ and PuO₂(CO₃)₂²⁻, we adapted the approximation of using the observed mobility of Np sample at pH 7.78 as the electrophoretic mobility of NpO₂(CO₃)₂²⁻. For the case of Pu, as the percentage of PuO₂(CO₃)₂²⁻ is significantly smaller, a fraction factor of 4.4 was applied based on Equation 2. The results are summarized in Table 2. The corrected value of PuO₂(CO₃)₂²⁻ may be unusual, however, as only one non-Ca-dependent term contains it, mathematically this parameter impacts only the values around [Ca²⁺] = 0 and has little effect on the fitting results, it was adopted nonetheless.

<table><caption>**Table 2.** Measured electrophoretic mobility of An(VI)-CO₃ species</caption><thead><tr><th>Species</th><th>Electrophoretic mobility<br>(10<sup>-5</sup> cm<sup>2</sup> V<sup>-1</sup> s<sup>-1</sup>)</th></tr></thead><tbody><tr><td>UO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>⁴⁻</td><td>-34.83</td></tr><tr><td>NpO<sub>2</sub>CO<sub>3</sub>(aq)</td><td>0 (assumed)</td></tr><tr><td>NpO<sub>2</sub>(CO<sub>3</sub>)<sub>2</sub>²⁻</td><td>-19.07</td></tr><tr><td>NpO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>⁴⁻</td><td>-34.22</td></tr><tr><td>PuO<sub>2</sub>CO<sub>3</sub>(aq)</td><td>0 (assumed)</td></tr><tr><td>PuO<sub>2</sub>(CO<sub>3</sub>)<sub>2</sub>²⁻</td><td>-47.93 (-10.87 before approximation)</td></tr><tr><td>PuO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>⁴⁻</td><td>-33.20</td></tr></tbody></table>
**Determination of Ca<sub>n</sub>AnO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>(<sup>4-2n</sup>) complexes.** After the selection of β<sub>0m</sub> and determination of μ<sub>0m</sub>, a series of Ca-An(VI)-CO<sub>3</sub> samples with increasing Ca concentration (from 0.5 mmol/kg<sub>w</sub> to 8 mmol/kg<sub>w</sub>) were prepared at 0.1 mol/kg<sub>w</sub> NaClO<sub>4</sub> to determine their thermodynamic constants. The pH was fixed at 7.78 and the samples were in equilibrium with atmospheric CO<sub>2</sub>(g) to fix the variable [CO<sub>3</sub><sup>2-</sup>] in Equation 5 and 6. This pH value was chosen as a compromise between maximizing the Ca<sup>2+</sup> solubility and minimizing the interference of An minor species. To cancel out the effect of An retention on PEEK capillaries, as already observed and described in the previous study with U(VI),<sup>14</sup> at least 3 pairs of electrophoreses were conducted for each sample: the first with positive voltage; and the second with negative voltage to calculate the half-sum values. This retention tends to become stronger with successive runs of electrophoresis, the capillary was replaced once the results of a new pair gave a standard error more than 15%. The measured electrophoretic mobility values of the actinides in each sample and their electropherograms are presented in the Figures S1-S8. It is worth noting that the ratio of An(V)/An(VI) is different under negative and positive modes: a greater proportion of Np(VI) can be always observed under positive mode than negative mode, and this is more evident for Pu, that Pu(V) was never observed under positive mode. This phenomenon results from the nature of electrophoresis itself, that under high negative voltage, the cathode, which is in contact directly with the electrolyte and the injected sample, is a reducing environment. Vice versa, the An(V) are in an oxidizing environment under positive mode. As Pu(V)/Pu(VI) is more sensitive to the change of redox than Np(V)/Np(VI),<sup>1</sup> its ratio varies more drastically between the two mode, while Np(V)/Np(VI) appears always as a mixture.

Figure 4 presents the observed electrophoretic mobility of these samples. One could note that the electrophoretic mobility of each An(VI) becomes gradually less negative with the increasing Ca
concentration, which is in accordance with the complexation from AnO₂(CO₃)₃⁴⁻ to CaAnO₂(CO₃)₃²⁻, and eventually Ca₂AnO₂(CO₃)₃(aq). It could be observed that the Pu(VI) and Np(VI) mobility series seem less negative than the U(VI) series.

As U(VI) is always under one oxidation state under our condition—one single peak in the electropherograms—the mobility values of the U(VI) series show excellent consistency with low uncertainty. The Np(VI) and Pu(VI) mobility series are showing higher uncertainty, primarily originating from the peak decomposition process as illustrated in Figure S1 of the Electronic Supplemental Information. As observed and discussed earlier, the complexation with carbonate favors the stabilization of An(VI). With the low concentration of carbonate at pH 7.78 in equilibrium with atmospheric CO₂(g), Np and Pu were present as a mixture of An(V) and An(VI) in most of the samples. Additionally, the applicable voltage was limited by the 0.1 mol/kg_w NaClO₄ ionic strength to avoid Joule heating, which hindered the effective separation of the two peaks as detailed in Sun et al.¹⁴
<figure><img src="image_5.png" /><figcaption>**Figure 4.** Observed electrophoretic mobility of Ca-An(VI)-CO₃ samples with different Ca concentration and their fitting results. General results of the three actinides series (a), Fitting results of U(VI) (b), Np(VI) (c) and Pu(VI) (d), respectively. c(NaClO₄) = 0.102 mol/kg<sub>w</sub>, c(NaClO) = 11.2 mmol/kg<sub>w</sub>, [carbonate] in equilibrium with atmospheric CO₂, pH = 7.78, [An] ≈ 5 × 10⁻⁷ mol/kg<sub>w</sub>, [NOTA] = 1.28 × 10⁻⁵ mol/kg<sub>w</sub>, ratio Ga : NOTA = 1 : 0.95, T = 25°C. Voltage = ± 4 kV, PEEK capillary i.d. 100 µm, o.d. 360 µm, L =105 cm.</figcaption></figure>

**Fitting and discussion of the results.** Figure 4b, Figure 4c and Figure 4d present the fitting results of the three An(VI) series by applying Equation 5 and Equation 6, and the data in Table 1 and Table 2. The obtained mobility values and constants for the different species at 25°C are summarized in Table 3 and compared with existing data in Table 4. First, Ca<sub>n</sub>UO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>(4-2n)-
complexes are exhibiting good consistency with previous results,¹²,¹⁵,¹⁶ and particularly with our
previous study using the same strategy,¹⁴ the extrapolations using either Davies equation or
ε(Ca_nUO₂(CO₃)₃⁽⁴-2n⁾, Na⁺ClO₄⁻) value from Shang et al.¹⁶ are giving comparable values. The
values from Grenthe et al.² were selected mostly from Endrizzi & Rao¹⁹ extrapolated from 0.5
mol/kg_w NaCl using in analogy ε(UO₂(CO₃)₃²⁻, Na⁺) = -(0.02 ± 0.09) kg_w mol⁻¹ and
ε(Ca₂UO₂(CO₃)₃, Na⁺Cl⁻) = 0. These analogies were proven inappropriate in other studies.¹²,¹⁵,¹⁶
The affinity of AnO₂(CO₃)₃⁴⁻ species show an affinity towards Ca as Pu > Np > U. which is the
opposite of the order observed for the formation of AnO₂(CO₃)₃m²-2m complexes. This trend of
affinity can be further confirmed by the speciation calculation using the obtained data as shown in
Figure S9, that at pH 9 and atmospheric CO₂(g), the fraction of non-complexed species
AnO₂(CO₃)₃⁴⁻ is 31% for uranium, 9% for neptunium and less than 1% for plutonium.

To date only few experimental works have been conducted for the other Ca_nAnO₂(CO₃)₃⁽⁴-2n⁾-
complexes: especially no works have been reported for Np(VI), yet.

Reiller¹² summarized and analyzed available U, Np, and Pu thermodynamic data from complexes
in literature, which helps in providing an estimation of Ca_nAnO₂(CO₃)₃⁽⁴-2n⁾⁻ for Np(VI) and Pu(VI)
using linear free energy relationships (LFERs). The uncertainties were no better than 1.1 and 1.6
log units at 1σ for Np(VI) and Pu(VI), respectively. The estimated values for Ca_nNpO₂(CO₃)₃⁽⁴-2n⁾⁻
are in general agreement with our experimental determination given the high uncertainty for the
estimations. It can also be noted that the Np(VI) constants estimations from Reiller¹² were close
to the U(VI) experimental values, when our Np(VI) experimental values are slightly higher but
comparable at 2σ.
Jo et al.¹³ were the first to report experimentally the complexation between Ca²⁺ and PuO₂(CO₃)₃⁴⁻ via UV/NIR absorption spectroscopy. Only CaPuO₂(CO₃)₃²⁻ was observed, because this technique requires a relatively high concentration of the target element—[Pu(VI)] = 0.39 mM compared to [CO₃²⁻] = 17 mM and [Ca²⁺] = 0.72 mM—, which may have limited the formation of Ca₂PuO₂(CO₃)₃(aq).¹² The value for log₁₀K°(CaPuO₂(CO₃)₃²⁻) = 6.05 ± 0.50 from Jo et al.¹³ is 2 orders of magnitude lower compared to our data log₁₀K°(CaPuO₂(CO₃)₃²⁻) = 7.85 ± 0.47. Contrary to Jo et al.¹³ our experiment allows proposing log₁₀K°(Ca₂PuO₂(CO₃)₃) = 11.15 ± 0.43. The differences between the two studies are the total concentration of Pu, Ca, and equilibrium with CO₂(g)—avoiding calcite precipitation in our case. We see no apparent reason that can help reasoning the difference between the two studies. Jo et al.¹³ carefully verified the Pu redox speciation through spectroscopy, and we checked the mobility of the An(V) and An(VI) species. Further works are then needed to clarify this point. The estimated value from Reiller¹² for CaPuO₂(CO₃)₃²⁻ is showing discrepancy with our experimentally determined data—only accounting for 2σ for the estimated data can resolve it. Nonetheless, the formation constants of Ca₂PuO₂(CO₃)₃(aq) are reported here for the first time.
<table><caption>**Table 3.** Fitting results of the three An(VI) series.</caption><thead><tr><th>Series</th><th>Reactions and species</th><th>log<sub>10</sub>K</th><th>log<sub>10</sub>K°</th><th>Mobility<br>(10<sup>-5</sup> cm<sup>2</sup> V<sup>-1</sup> s<sup>-1</sup>)</th><th>r<sup>2</sup></th></tr></thead><tbody><tr><td rowspan="10">U</td><td>UO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub><sup>4+</sup></td><td></td><td></td><td>-34.83 (fixed)</td><td rowspan="10">0.9971</td></tr><tr><td>Ca<sup>2+</sup> + UO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub><sup>4+</sup> ⇌</td><td rowspan="4">3.41 ± 0.10</td><td>5.12 ± 0.10<sup>a</sup></td><td rowspan="4">-20.83 ± 1.69</td></tr><tr><td>CaUO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub><sup>2-</sup></td><td>5.17 ± 0.10<sup>b</sup></td></tr><tr><td>Ca<sup>2+</sup> + UO<sub>2</sub><sup>2+</sup> + 3CO<sub>3</sub><sup>2-</sup> ⇌</td><td>26.96 ± 0.11<sup>a,c</sup></td></tr><tr><td>CaUO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub><sup>2-</sup></td><td>27.01 ± 0.11<sup>b,c</sup></td></tr><tr><td>2Ca<sup>2+</sup> + UO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub><sup>4+</sup> ⇌</td><td rowspan="6">5.66 ± 0.16</td><td>8.23 ± 0.16<sup>a</sup></td><td rowspan="6">0 (assumed)</td></tr><tr><td>Ca<sub>2</sub>UO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>(aq)</td><td>8.29 ± 0.16<sup>b</sup></td></tr><tr><td>Ca<sup>2+</sup> + CaUO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub><sup>4+</sup> ⇌</td><td>3.11 ± 0.13<sup>a</sup></td></tr><tr><td>Ca<sub>2</sub>UO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>(aq)</td><td>3.12 ± 0.13<sup>b</sup></td></tr><tr><td>2Ca<sup>2+</sup> + UO<sub>2</sub><sup>2+</sup> + 3CO<sub>3</sub><sup>2-</sup> ⇌</td><td>30.07 ± 0.16<sup>a,c</sup></td></tr><tr><td>Ca<sub>2</sub>UO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>(aq)</td><td>30.13 ± 0.16<sup>b,c</sup></td></tr><tr><td rowspan="10">Np</td><td>NpO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub><sup>4+</sup></td><td></td><td></td><td>-34.22 (fixed)</td><td rowspan="10">0.9853</td></tr><tr><td>Ca<sup>2+</sup> + NpO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub><sup>4+</sup> ⇌</td><td rowspan="4">4.05 ± 0.27</td><td>5.76 ± 0.27<sup>a</sup></td><td rowspan="4">-17.83 ± 6.69</td></tr><tr><td>CaNpO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub><sup>2-</sup></td><td>5.77 ± 0.27<sup>b</sup></td></tr><tr><td>Ca<sup>2+</sup> + NpO<sub>2</sub><sup>2+</sup> + 3CO<sub>3</sub><sup>2-</sup> ⇌</td><td>25.13 ± 0.28<sup>a,d</sup></td></tr><tr><td>CaNpO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub><sup>2-</sup></td><td>25.14 ± 0.28<sup>b,d</sup></td></tr><tr><td>2Ca<sup>2+</sup> + NpO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub><sup>4+</sup> ⇌</td><td rowspan="6">6.68 ± 0.22</td><td>9.25 ± 0.22<sup>a</sup></td><td rowspan="6">0 (assumed)</td></tr><tr><td>Ca<sub>2</sub>NpO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>(aq)</td><td>9.27 ± 0.22<sup>b</sup></td></tr><tr><td>Ca<sup>2+</sup> + CaNpO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub><sup>4+</sup> ⇌</td><td>3.49 ± 0.35<sup>a</sup></td></tr><tr><td>Ca<sub>2</sub>NpO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>(aq)</td><td>3.50 ± 0.21<sup>b</sup></td></tr><tr><td>2Ca<sup>2+</sup> + NpO<sub>2</sub><sup>2+</sup> + 3CO<sub>3</sub><sup>2-</sup> ⇌</td><td>28.62 ± 0.24<sup>a,d</sup></td></tr><tr><td>Ca<sub>2</sub>NpO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>(aq)</td><td>28.64 ± 0.24<sup>b,d</sup></td></tr><tr><td rowspan="10">Pu</td><td>PuO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub><sup>4+</sup></td><td></td><td></td><td>-33.20 (fixed)</td><td rowspan="10">0.9797</td></tr><tr><td>Ca<sup>2+</sup> + PuO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub><sup>4+</sup> ⇌</td><td rowspan="4">6.14 ± 0.47</td><td>7.85 ± 0.47<sup>a</sup></td><td rowspan="4">-23.43 ± 1.35</td></tr><tr><td>CaPuO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub><sup>2-</sup></td><td>7.88 ± 0.47<sup>b</sup></td></tr><tr><td>Ca<sup>2+</sup> + PuO<sub>2</sub><sup>2+</sup> + 3CO<sub>3</sub><sup>2-</sup> ⇌</td><td>25.85 ± 0.69<sup>a,c</sup></td></tr><tr><td>CaUO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub><sup>2-</sup></td><td>25.88 ± 0.69<sup>b,c</sup></td></tr><tr><td>2Ca<sup>2+</sup> + PuO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub><sup>4+</sup> ⇌</td><td rowspan="6">8.58 ± 0.43</td><td>11.15 ± 0.43<sup>a</sup></td><td rowspan="6">0 (assumed)</td></tr><tr><td>Ca<sub>2</sub>PuO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>(aq)</td><td>11.19 ± 0.43<sup>b</sup></td></tr><tr><td>Ca<sup>2+</sup> + CaPuO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub><sup>4+</sup> ⇌</td><td>3.30 ± 0.64<sup>a</sup></td></tr><tr><td>Ca<sub>2</sub>PuO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>(aq)</td><td>3.31 ± 0.64<sup>b</sup></td></tr><tr><td>2Ca<sup>2+</sup> + PuO<sub>2</sub><sup>2+</sup> + 3CO<sub>3</sub><sup>2-</sup> ⇌</td><td>29.15 ± 0.66<sup>a,c</sup></td></tr><tr><td>Ca<sub>2</sub>PuO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>(aq)</td><td>29.19 ± 0.66<sup>b,c</sup></td></tr></tbody></table>

<footnote>a calculation using Davies²⁸ equation; b calculation using SIT ε(Ca<sub>n</sub>UO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>(4-2n)-, Na<sup>+</sup>ClO<sub>4</sub><sup>-</sup>) from Shang *et al.*¹⁶;</footnote>

<footnote>c calculated using log<sub>10</sub>β°<sub>3</sub> (Table 1) from Grenthe, et al. ²; d calculated using log<sub>10</sub>β°<sub>3</sub> (Table 1) from Lemire *et al.*¹</footnote>
<table><caption>Table 4. Existing stability constants of Ca-An(VI)-CO₃ complexes.</caption><thead><tr><th></th><th>AnO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub><sup>4+</sup> +<br>Ca<sup>2+</sup> ⇌<br>CaAnO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub><sup>2-</sup></th><th>AnO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub><sup>4+</sup> + 2<br>Ca<sup>2+</sup> ⇌<br>Ca<sub>2</sub>AnO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>(aq)</th><th>Ionic strength<br>correction</th><th>Methodology</th><th>Reference</th></tr></thead><tbody><tr><td rowspan="10">log<sub>10</sub>K°, U</td><td>5.12 ± 0.10</td><td>8.23 ± 0.16</td><td>Davies</td><td>CE-ICP-MS</td><td>This work</td></tr><tr><td>5.17 ± 0.10</td><td>8.29 ± 0.16</td><td>SIT (ε from<sup>16</sup>)</td><td>CE-ICP-MS</td><td>This work</td></tr><tr><td>5.28 ± 0.39</td><td>8.46 ± 0.67</td><td>Davies</td><td>CE-ICP-MS</td><td><sup>14</sup></td></tr><tr><td>5.36 ± 0.01</td><td>8.65 ± 0.02</td><td>SIT, NaCl</td><td>TRLFS</td><td><sup>15</sup></td></tr><tr><td>5.42 ± 0.04</td><td>8.69 ± 0.06</td><td>SIT, NaClO<sub>4</sub></td><td>TRLFS</td><td><sup>16</sup></td></tr><tr><td>5.43 ± 0.03</td><td>8.87 ± 0.03</td><td>SIT</td><td>TRLFS</td><td><sup>27</sup></td></tr><tr><td>3.8 ± 0.25</td><td>8.95 ± 0.25</td><td>Davies</td><td>TRLFS</td><td><sup>17</sup></td></tr><tr><td>5.34 ± 0.04</td><td>8.86 ± 0.04</td><td>Davies</td><td>Anion<br>exchange</td><td><sup>18</sup></td></tr><tr><td>5.43 ± 0.14</td><td>7.97 ± 0.19</td><td>SIT</td><td>TRLFS</td><td><sup>21</sup></td></tr><tr><td>5.15 ± 0.06</td><td>9.00 ± 0.06</td><td>SIT, analogy<br>with<br>ε(UO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub><sup>2-</sup>,<br>Na<sup>+</sup>) and<br>ε(Ca<sub>2</sub>UO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>, Na<sup>+</sup>Cl<sup>-</sup>) = 0</td><td>Ion selective<br>electrode<br>potentiometry</td><td><sup>19</sup></td></tr><tr><td rowspan="3">log<sub>10</sub>K°, Np</td><td>5.18 ± 0.15</td><td>8.70 ± 0.16</td><td>SIT</td><td>TRLFS</td><td>Jo et al.<sup>37</sup></td></tr><tr><td>5.43 ± 0.04</td><td>7.96 ± 0.20</td><td>Davies</td><td>Anion<br>exchange</td><td>Maia et al.<sup>38</sup></td></tr><tr><td>5.16 ± 0.20</td><td>8.96 ± 0.40</td><td>SIT, NaCl</td><td>Data selection</td><td><sup>2</sup> based on <sup>19</sup></td></tr><tr><td rowspan="2">log<sub>10</sub>K°, Pu</td><td>5.25 ± 0.08</td><td>8.58 ± 0.14</td><td>Davies</td><td>Data selection</td><td><sup>12</sup></td></tr><tr><td>5.76 ± 0.27</td><td>9.25 ± 0.22</td><td>Davies</td><td>CE-ICP-MS<br>LFER</td><td>This work</td></tr><tr><td rowspan="2"></td><td>5.2 ± 1.1</td><td>8.5 ± 1.1</td><td>Davies</td><td>CE-ICP-MS</td><td><sup>12</sup></td></tr><tr><td>7.85 ± 0.47</td><td>11.15 ± 0.43</td><td>Davies</td><td>UV/Vis<br>absorption<br>spectroscopy</td><td>This work</td></tr><tr><td></td><td>6.05 ± 0.50</td><td>Not observed</td><td>Davies</td><td>LFER</td><td>Jo et al.<sup>13</sup></td></tr><tr><td></td><td>5.2 ± 1.6</td><td>8.5 ± 1.6</td><td></td><td></td><td>Reiller<sup>12</sup></td></tr><tr><td></td><td>Ca<sup>2+</sup> + AnO<sub>2</sub><sup>2+</sup> +<br>3CO<sub>3</sub><sup>2-</sup> ⇌<br>CaUO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub><sup>2-</sup></td><td>2Ca<sup>2+</sup> + AnO<sub>2</sub><sup>2+</sup> +<br>3CO<sub>3</sub><sup>2-</sup> ⇌<br>Ca<sub>2</sub>UO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>(aq)</td><td></td><td></td><td></td></tr><tr><td rowspan="5">log<sub>10</sub>β°, U</td><td>26.96 ± 0.11</td><td>30.07 ± 0.16</td><td>Davies</td><td></td><td>This work</td></tr><tr><td>27.01 ± 0.11</td><td>30.13 ± 0.16</td><td>SIT (ε from<sup>16</sup>)</td><td></td><td>This work</td></tr><tr><td>27.26 ± 0.06</td><td>30.53 ± 0.07</td><td>SIT, NaClO<sub>4</sub></td><td>Data selection</td><td><sup>16</sup></td></tr><tr><td>27.00 ± 0.20</td><td>30.80 ± 0.40</td><td>SIT, NaCl</td><td>Data selection</td><td><sup>2</sup> based on <sup>19</sup></td></tr><tr><td>27.09 ± 0.40</td><td>30.42 ± 0.15</td><td>Davies</td><td></td><td><sup>12</sup></td></tr><tr><td rowspan="2">log<sub>10</sub>β°, Np</td><td>25.13 ± 0.28</td><td>28.62 ± 0.24</td><td></td><td></td><td>This work,<br>referring to<br>NpO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub><sup>4-</sup><br>in Lemire et<br>al.<sup>1</sup></td></tr><tr><td>25.66 ± 0.57</td><td>29.15 ± 0.55</td><td>Davies</td><td></td><td>This work<br>referring to<br>NpO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub><sup>4-</sup><br>in Grenthe et<br>al.<sup>2</sup></td></tr><tr><td>log<sub>10</sub>β°, Pu</td><td>25.85 ± 0.69</td><td>29.15 ± 0.66</td><td>Davies</td><td></td><td>This work</td></tr></tbody></table>
**Simulation of the charge evolution and comparison of the data.** One could note that the AnO₂²⁺ ions exhibit different affinity towards carbonate as summarized in Table 1. As the decrease of ion radius (UO₂²⁺ > NpO₂²⁺ > PuO₂²⁺) the formation constants of AnO₂(CO₃)ₘ²⁻²m complexes also decrease. This results in a somewhat different initial speciation, i.e., [Ca] = 0, of An(VI) under mildly alkaline pH and may lead to distinct evolutions of their speciation with changes in Ca content. To gain a general understanding of the possible effect of CaₙAnO₂(CO₃)₃⁽⁴⁻²n⁾-complexation on the An(VI) speciation, the evolution of global charge during a Ca²⁺ titration at pH 7.78 was calculated based on the existing thermodynamic data of CaₙAnO₂(CO₃)₃⁽⁴⁻²n⁾-complexes and compared with the data obtained in this work. The curves are presented in Figure 5.

In the case of U(VI), which has been extensively investigated, at pH 7.78 the speciation is mainly negative species UO₂(CO₃)₃⁴⁻ (ca. 52%), UO₂(CO₃)₂²⁻ (ca. 20%) and (UO₂)₂(CO₃)(OH)₃⁻ (ca. 24% of total U). The formation of CaₙAnO₂(CO₃)₃⁽⁴⁻²n⁾, resulting in a less negative overall charge, appears always as an increasing curve. Both the experimental results in Figure 4a and the calculation with data obtained in this work (Figure 5b) are consistent with our previous study (Figure 5a).¹⁴

However, given the weaker affinity of Np(VI) and Pu(VI) toward carbonate, the initial speciation is dominated by NpO₂(CO₃)₂²⁻ (ca. 84%) for Np(VI) and appears as a mixture of PuO₂CO₃(aq) (ca. 65%) and PuO₂(CO₃)₂²⁻ (22.5%), with a higher proportion of the first complex for Pu(VI). If we adopt the estimation of Reiller,¹² i.e. the formation constants of other CaₙAnO₂(CO₃)₃⁽⁴⁻²n⁾-complexes are of the same order of magnitude as U(VI), in the case of Np(VI) the complexation should induce only slight changes of charge in the early phase and lead to a decreasing total negative charge with increasing Ca content (NpO₂(CO₃)₂²⁻ → CaNpO₂(CO₃)₃²⁻ →
$Ca_2NpO_2(CO_3)_3$). The curve of Pu(VI) should show a more flattened tendency than Np(VI) due to its higher proportion of $PuO_2CO_3(aq)$. It should be emphasized that for Pu(VI), if only one complex $CaPuO_2(CO_3)_2^{2-}$ occurs as observed by Jo *et al.*,¹³ the complexation of Pu(VI) by $Ca^{2+}$ should lead to a decreasing curve.

Experimentally Np(VI) and Pu(VI) appeared as two increasing curves (Figure 4a). For Np(VI) the increasing tendency is consistent with the data of Reiller¹² (Figure 5a), and the calculation with fitted data gives an increasing curve with higher slope since they are slightly higher than those of Reiller.¹² The case of Pu(VI), however, is the contrary of the data from Jo *et al.*¹³ The increasing tendency indicates that the second complex $Ca_2PuO_2(CO_3)_3(aq)$ was formed as well as $Ca_2UO_2(CO_3)_3(aq)$ and $Ca_2NpO_2(CO_3)_3(aq)$. Compared to the flat curve calculated from the data of Reiller,¹² the increasing tendency suggests that the formation constants of $Ca_nPuO_2(CO_3)_3^{(4-2n)-}$ are notably higher than those of U(VI) and Np(VI). The detailed speciation calculation of the three series is presented in the Figure S10.
<figure><img src="image_6.png" /><figcaption>**Figure 5.** Simulation of the charge evolution with increasing Ca content using different data. By applying the data from Sun et al.¹⁴, Jo et al.¹³ and Reiller¹² (a); by applying the obtained data in this work (b). Thermochimie 12a database by changing the $NpO_2(CO_3)_m^{2-2m}$ data from Grenthe et al.² back to Lemire et al.¹, Davies format. pH 7.78, carbonate in equilibrium with atmospheric $CO_2(g)$, $[An] = 5 \times 10^{-7}$ mol/kgw and the oxidation states are fixed at +VI.</figcaption></figure>

**Speciation calculations with the obtained data.** The obtained data were applied to calculate the possible effect of $Ca_nAnO_2(CO_3)_3^{(4-2n)-}$ complexation on An speciation from pH 6 to 10 in equilibrium with atmospheric $CO_2(g)$ and $O_2(g)$. As presented in Figure 6, with the presence of $5 \times 10^{-3}$ mol/kgw of $[Ca^{2+}]$ the speciation of Np is rapidly dominated by $Ca_nNpO_2(CO_3)_3^{(4-2n)-}$ complexes from around pH 8.0, and it was primarily $Ca_2NpO_2(CO_3)_3(aq)$ until pH 9.0. $CaNpO_2(CO_3)_3^{2-}$ and $NpO_2(CO_3)_3^{4-}$ begin to increase from pH 9.0 due to the precipitation of calcite at higher pH values. Similar behavior can be observed in the case of Pu, the $Ca_nPuO_2(CO_3)_3^{(4-2n)-}$ predominance extend over a broader pH range than Np, as the formation constants of $Ca_nPuO_2(CO_3)_3^{(4-2n)-}$ are significantly higher than those of $Ca_nNpO_2(CO_3)_3^{(4-2n)-}$. Therefore, even with the precipitation of calcite the speciation is still dominated by $CaPuO_2(CO_3)_3^{2-}$ rather than $PuO_2(CO_3)_3^{4-}$ within the pH range we studied. One can further confirm the stabilization of An(VI)
by the Ca complexation by comparing the results with those in absence of Ca²⁺ (short-dashed lines). One may infer that in oxidant aqueous environment with higher Ca content the complexation facilitates the stabilization of An(VI) and may enhance their environmental mobility.

Figure 7 presents the calculated Pourbaix diagrams (pH – E<sub>h</sub>) of Np and Pu from pH 5 to 9.5 with 5 × 10<sup>-3</sup> mol/kg<sub>w</sub> [Ca<sup>2+</sup>], in equilibrium with atmospheric CO<sub>2</sub>(g). Ca<sub>n</sub>AnO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>(4-2n)- complexes are the dominant species of An(VI) under oxidizing and mildly alkaline conditions, and CaAnO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>2- becomes the dominant species instead of Ca<sub>2</sub>AnO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>(aq) at higher pH values due to the precipitation of calcite.

<figure><img src="image_7.png" /></figure>

**Figure 6.** Speciation calculation of Np (a) and Pu (b) by applying the obtained data in this work using Thermochimie 12a database and implementing the data obtained and those of NpO<sub>2</sub>(CO<sub>3</sub>)<sub>m</sub>2- from Grenthe *et al.*<sup>2</sup> back to Lemire *et al.*,<sup>1</sup> Solid lines for the species with the presence of Ca<sup>2+</sup> and short-dashed lines for the corresponding species without Ca<sup>2+</sup>. c(NaCl) = 0.1 mol/kg<sub>w</sub>, CO<sub>2</sub> and O<sub>2</sub> in equilibrium with atmosphere (pCO<sub>2</sub> = -3.4 and pO<sub>2</sub> = -0.679), [An] = 5 × 10<sup>-7</sup> mol/kg<sub>w</sub>, [Ca] = 5 × 10<sup>-3</sup> mol/kg<sub>w</sub>.
<figure><img src="image_8.png" /><figcaption>**Figure 7.** Pourbaix diagrams (pH - Eh) of aqueous Np (a) and Pu (b) species. Calculated by Phreeplot software using Thermochimie 12a database and implementing the data obtained in this work and those of $NpO_2(CO_3)_m^{2-2m}$ from Grenthe *et al.*² back to Lemire *et al.*¹ [An] = $5 \times 10^{-7}$ mol/kg, $[Ca^{2+}] = 5 \times 10^{-3}$ mol/kg, c(NaCl) = 0.1 mol/kg, $O_2$ and $CO_2$ in equilibrium with atmosphere (p$CO_2$ = -3.4 and p$O_2$ = -0.679), T = 25°C.</figcaption></figure>

**Affinity difference between $AnO_2^{2+}-CO_3^{2-}$ and $Ca^{2+}-AnO_2(CO_3)_3^{4-}$.** The differences of affinity between $AnO_2-CO_3$ and $Ca_nAnO_2(CO_3)_3^{(4-2n)-}$ can be plotted through the evolution of the stepwise constants,

$$AnO_2(CO_3)_{m-1}^{2-2(m-1)} + mCO_3^{2-} \rightleftharpoons AmO_2(CO_3)_m^{2m-} \quad K^\circ_{0m} \quad (11)$$

$$Ca^{2+} + Ca_{n-1}AnO_2(CO_3)_3^{(4-2(n-1))-} \rightleftharpoons Ca_nAnO_2(CO_3)_3^{(4-2n)-} \quad K^\circ_{n3} \quad (12)$$

vs. the inverse of the effective radius with a +VI coordination³³ in Figure 8. The decrease of $CO_3^{2-}$ affinity for $AnO_2(CO_3)_{m-1}^{2-2(m-1)}$ with $1/R_{VI}$ is shown in Figure 8a for all the stepwise constants $log_{10}K^\circ_m$. Conversely, the increase of $Ca^{2+}$ affinity for $Ca_{n-1}AnO_2(CO_3)_3^{(4-2(n-1))-}$ is more apparent for $K^\circ_{n1}$ than for $K^\circ_{n2}$. The increase in cumulative $log_{10}\beta^{\circ 23} = log_{10}K^{\circ 13} + log_{10}K^{\circ 23}$ is then more due to the first stepwise $log_{10}K^{\circ 13}$ value.
It is then possible to calculate the grand cumulative constants $log_{10}\beta^{\circ}_{1,n3} = log_{10}\beta^{\circ}_{n3} + log_{10}\beta^{\circ}_{03}$:

$$nCa^{2+} + AnO_2^{2+} + 3CO_3^{2-} \rightleftharpoons Ca_nAnO_2(CO_3)_3^{(4-2n)-} \quad \beta^{\circ}_{1n3} \quad (13)$$

Due to the higher $log_{10}\beta^{\circ}_{03}(UO_2(CO_3)_3^{4-})$ value compared to the other $AnO_2(CO_3)_3^{4-}$ complexes, the grand cumulative constants are increasing in the order $NpO_2^{2+} \approx PuO_2^{2+} < UO_2^{2+}$ (Table 3).

<figure><img src="image_9.png" /><figcaption>**Figure 8.** Variations of the stepwise constants of formation of the $AnO_2(CO_3)_m^{2-2m}$ calculated from Guillaumont *et al.*³ (a), and $Ca_nAnO_2(CO_3)_3^{(4-2n)-}$ complexes in this study (b).</figcaption></figure>

## CONCLUSIONS

The complexation between $AnO_2(CO_3)_3^{4-}$ complexes and $Ca^{2+}$ was investigated by CE-ICP-MS using PEEK capillaries. In addition to the confirmation of the formation of $CaPuO_2(CO_3)_3^{2-}$¹³ the formation $Ca_nNpO_2(CO_3)_3^{(4-2n)-}$ and $Ca_2PuO_2(CO_3)_3(aq)$ were proposed for the first time. Given that the An(V)/An(VI) speciation cannot be directly confirmed with CE-ICP-MS—as it would be in spectrophotometry—, preliminary experiments were conducted to understand the behaviors of An(V) and An(VI) during electrophoresis under our conditions. The complexation with $CO_3^{2-}$ and $Ca^{2+}$ were also studied, and the possible evolution of charge with increasing $Ca^{2+}$ concentration
was discussed based on existing data. For the NpO₂(CO₃)ₘ²⁻²m constants, the original selection of NEA-TDB¹ was preferred in front of the more recent update,² in view of the mobility results. More studies on the NpO₂(CO₃)ₘ²⁻²m constants is then necessary. Experimentally, we observed similar behaviors as was expected, i.e., the total charge of An(VI) became less negative with the increase of [Ca²⁺], in agreement with the complexation sequence from AnO₂(CO₃)ₘ²⁻²m to CaAnO₂(CO₃)₃²⁻, and Ca₂AnO₂(CO₃)₃(aq). The obtained stability constants for U(VI) and Np(VI) were consistent with reported data. Nevertheless, PuO₂(CO₃)ₘ²⁻²m complexes showed a stronger affinity toward Ca²⁺ than Np(VI) and U(VI). Theoretical speciation calculation suggests these complexations favor the stabilization of An(VI) and may eventually enhance their mobility in oxidant carbonate- and calcium-rich waters.

## AUTHOR CONTRIBUTIONS

Ruopei Sun: Investigation, Data curation, Visualization, Writing – original draft. Erwan Dupuis: Supervision, Conceptualization, Validation, Writing – review & editing. Pascal E. Reiller: Supervision, Funding Acquisition, Project administration, Conceptualization, Validation, Writing – review & editing.

## SUPPORTING INFORMATION

Supporting information contains 10 Figures and 4 Tables: Figure S1 to S8, electropherograms with increasing [Ca] for different samples; Figure S9 and S10 speciation of actinyl(VI) using determined log₁₀K values. Table S1 to S3, An(VI) electrophoretic mobility values with increasing [Ca]; Table S4 Gibbs energies of reactions and formations of the species.
ACKNOWLEDGEMENT

This work was financed by ONDRAF-NIRAS (contract CCHO 2021-0440/00/00). Dr Hélène Isnard is acknowledged for her strong support in general and her help during the preparation of actinides stock solutions in particular. The whole ACTINEO team is acknowledged for their strong support.

REFERENCES

1 Lemire, R., Fuger, J., Nitsche, H., Rand, M., Rydberg, J., Sullivan, J., Ullman, W., Vitorge, P., Spahiu, K. & Wanner, H. *Chemical Thermodynamics 4. Chemical Thermodynamics of Neptunium and Plutonium*. Vol. 4 (OECD, Elsevier, Amsterdam, 2001).

2 Grenthe, I., Gaona, X., Plyasunov, A. V., Rao, L., Runde, W. H., Grambow, B., Koning, R. J. M., Smith, A. L. & Moore, E. E. *Chemical Thermodynamics 14. Second Update on the Chemical Thermodynamics of Uranium, Neptunium, Plutonium, Americium and Technetium*. Vol. 14 (OECD Nuclear Energy Agency Data Bank, Eds., OECD Publications, 2020).

3 Guillaumont, R. & Mompean, F. J. *Update on the chemical thermodynamics of uranium, neptunium, plutonium, americium and technetium*. Vol. 5 (Elsevier Amsterdam, 2003).

4 Bernhard, G., Geipel, G., Brendler, V. & Nitsche, H. Speciation of uranium in seepage waters of a mine tailing pile studied by time-resolved laser-induced fluorescence spectroscopy (TRLFS). *Radiochim. Acta* **74**, 87-91, doi:10.1524/ract.1996.74.special-issue.87 (1996).

5 Clark, D. L., Hobart, D. E. & Neu, M. P. Actinide Carbonte Complexes and Their Importance in Actinide Environmental Chemistry. *Chemical Reviews* **95**, 25-48, doi:10.1021/cr00033a002 (1995).

6 Choppin, G. R. Actinide speciation in the environment. *Journal of Radioanalytical and Nuclear Chemistry* **273**, 695-703, doi:10.1007/s10967-007-0933-3 (2007).

7 Stewart, B. D., Mayes, M. A. & Fendorf, S. Impact of Uranyl–Calcium–Carbonato Complexes on Uranium(VI) Adsorption to Synthetic and Natural Sediments. *Environmental Science & Technology* **44**, 928-934, doi:10.1021/es902194x (2010).

8 Zheng, Z., Tokunaga, T. K. & Wan, J. Influence of Calcium Carbonate on U(VI) Sorption to Soils. *Environmental Science & Technology* **37**, 5603-5608, doi:10.1021/es0304897 (2003).

9 Dong, W., Ball, W. P., Liu, C., Wang, Z., Stone, A. T., Bai, J. & Zachara, J. M. Influence of Calcite and Dissolved Calcium on Uranium(VI) Sorption to a Hanford Subsurface Sediment. *Environmental Science & Technology* **39**, 7949-7955, doi:10.1021/es0505088 (2005).
10 Meleshyn, A., Azeroual, M., Reeck, T., Houben, G., Riebe, B. & Bunnenberg, C. Influence of (Calcium-)Uranyl-Carbonate Complexation on U(VI) Sorption on Ca- and Na-Bentonites. *Environmental Science & Technology* **43**, 4896-4901, doi:10.1021/es900123s (2009).

11 Stockmann, M., Fritsch, K., Bok, F., Fernandes, M. M., Baeyens, B., Steudtner, R., Müller, K., Nebelung, C., Brendler, V., Stumpf, T. & Schmeide, K. New insights into U(VI) sorption onto montmorillonite from batch sorption and spectroscopic studies at increased ionic strength. *Science of The Total Environment* **806**, 150653, doi:10.1016/j.scitotenv.2021.150653 (2022).

12 Reiller, P. E. Predominance of the alkaline earth(II) triscarbonatoactinyl(VI) complexes in different geochemical contexts: Review of existing data and estimation of potentially unidentified species. *Chemosphere* **350**, 141049, doi:10.1016/j.chemosphere.2023.141049 (2024).

13 Jo, Y., Cho, H.-R. & Yun, J.-I. Visible-NIR absorption spectroscopy study of the formation of ternary plutonyl(VI) carbonate complexes. *Dalton Trans.* **49**, 11605-11612, doi:10.1039/d0dt01982h (2020).

14 Sun, R., Dupuis, E., Aupiais, J. & Reiller, P. E. Study of Ca<sub>n</sub>UO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>(<sup>4-2n</sup>-) complexes using CE-ICP-MS with polyetheretherketone (PEEK) capillaries. *Dalton Trans.* **54**, 719-727, doi:10.1039/D4DT02781G (2025).

15 Shang, C. & Reiller, P. E. Determination of formation constants and specific ion interaction coefficients for Ca<sub>n</sub>UO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>(<sup>4-2n</sup>-) complexes in NaCl solution by time-resolved laser-induced luminescence spectroscopy. *Dalton Trans.* **49**, 466-481, doi:10.1039/C9DT03543E (2020).

16 Shang, C., Reiller, P. E. & Vercouter, T. Spectroluminescence measurements of stability constants of Ca<sub>n</sub>UO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>(<sup>4-2n</sup>-) complexes in NaClO<sub>4</sub> medium and investigation of interaction effects. *Dalton Trans.* **49**, 15443-15460, doi:10.1039/d0dt03164j (2020).

17 Bernhard, G., Geipel, G., Reich, T., Brendler, V., Amayri, S. & Nitsche, H. Uranyl(VI) carbonate complex formation: validation of the Ca<sub>2</sub>UO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>(aq.) species. *Radiochim. Acta* **89**, 511-518, doi:10.1524/ract.2001.89.8.511 (2001).

18 Dong, W. & Brooks, S. C. Determination of the Formation Constants of Ternary Complexes of Uranyl and Carbonate with Alkaline Earth Metals (Mg<sup>2+</sup>, Ca<sup>2+</sup>, Sr<sup>2+</sup>, and Ba<sup>2+</sup>) Using Anion Exchange Method. *Environmental Science & Technology* **40**, 4689-4695, doi:10.1021/es0606327 (2006).

19 Endrizzi, F. & Rao, L. Chemical speciation of uranium(VI) in marine environments: complexation of calcium and magnesium ions with [(UO<sub>2</sub>)(CO<sub>3</sub>)<sub>3</sub><sup>4-</sup>] and the effect on the extraction of uranium from seawater. *Chem.-Eur. J.* **20**, 14499-14506, doi:10.1002/chem.201403262 (2014).

20 Reiller, P. E. & Descostes, M. Development and application of the thermodynamic database PRODATA dedicated to the monitoring of mining activities from exploration to remediation. *Chemosphere* **251**, 126301, doi:10.1016/j.chemosphere.2020.126301 (2020).

21 Lee, J.-Y. & Yun, J.-I. Formation of ternary CaUO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub><sup>2-</sup> and Ca<sub>2</sub>UO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>(aq) complexes under neutral to weakly alkaline conditions. *Dalton Trans.* **42**, 9862-9869, doi:10.1039/c3dt50863c (2013).
22 Prikryl, J. D., Jain, A., Turner, D. R. & Pabalan, R. T. Uranium VI sorption behavior on silicate mineral mixtures. *Journal of Contaminant Hydrology* **47**, 241-253, doi:10.1016/S0169-7722(00)00153-4 (2001).

23 Štamberg, K., Venkatesan, K. A. & Vasudeva Rao, P. R. Surface complexation modeling of uranyl ion sorption on mesoporous silica. *Colloids and Surfaces A: Physicochemical and Engineering Aspects* **221**, 149-162, doi:10.1016/S0927-7757(03)00139-0 (2003).

24 Martelat, B., Isnard, H., Vio, L., Dupuis, E., Cornet, T., Nonell, A. & Chartier, F. Precise U and Pu Isotope Ratio Measurements in Nuclear Samples by Hyphenating Capillary Electrophoresis and MC-ICPMS. *Analytical Chemistry* **90**, 8622-8628, doi:10.1021/acs.analchem.8b01884 (2018).

25 Dupuis, E., Isnard, H. & Chartier, F. Separation and isotope ratio measurements of actinides and lanthanides in spent nuclear fuel samples by CE-MC-ICP-MS. *J. Anal. Atom. Spectrom.* **37**, 2340-2350, doi:10.1039/d2ja00265e (2022).

26 Ambard, C., Delorme, A., Baglan, N., Aupiais, J., Pointurier, F. & Madic, C. Interfacing capillary electrophoresis with inductively coupled plasma mass spectrometry for redox speciation of plutonium. *Radiochim. Acta* **93**, 665-673, doi:10.1524/ract.2005.93.11.665 (2005).

27 Shang, C. & Reiller, P. E. Effect of temperature on the complexation of triscarbonatouranyl(vi) with calcium and magnesium in NaCl aqueous solution. *Dalton Trans.* **50**, 17165-17180, doi:10.1039/D1DT03204F (2021).

28 Davies, C. W. & Shedlovsky, T. Ion association. *Journal of the Electrochemical Society* **111**, 85Ca (1964).

29 Madé, B., Bower, W., Brassinnes, S., Duro, L., Blanc, P., Lassin, A., Harvey, L. & Begg, J. D. Recent developments in ThermoChimie – A thermodynamic database used in radioactive waste management. *Appl. Geochem.* **180**, doi:10.1016/j.apgeochem.2024.106273 (2025).

30 Sonke, J. E. & Salters, V. J. M. Disequilibrium effects in metal speciation by capillary electrophoresis inductively coupled plasma mass spectrometry (CE-ICP-MS); theory, simulations and experiments. *Analyst* **129**, 731-738, doi:10.1039/B407162J (2004).

31 Grenthe, I., Fuger, J., Konings, R. J., Lemire, R. J., Muller, A. B., Nguyen-Trung, C. & Wanner, H. *Chemical thermodynamics of uranium*. Vol. 1 (Elsevier Amsterdam, 1992).

32 Debye, V. P. Zur theorie der electrolyte. *Phyikalishce Zeitschrift.*, 185-206 (1923).

33 Shannon, R. D. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. *Acta Crystallogr., Sect. A* **32**, 751-767, doi:10.1107/S0567739476001551 (1976).

34 Plummer, L. N. & Busenberg, E. The solubilities of calcite, aragonite and vaterite in CO₂-H₂O solutions between 0 and 90°C, and an evaluation of the aqueous model for the system CaCO₃-CO₂-H₂O. *Geochim. Cosmochim. Acta* **46**, 1011-1040, doi:10.1016/0016-7037(82)90056-4 (1982).

35 Vitorge, P. & Capdevila, H. Np (V) and Np (VI) in bicarbonate/carbonate aqueous solutions. Report No. CEA-R-5793, (CEA Saclay, 1998).
https://inis.iaea.org/records/e48wq-xe437.

36 Kato, Y., Kimura, T., Yoshida, Z. & Nitani, N. Carbonate Complexation of Neptunyl(VI) Ion. *Radiochim. Acta* **82**, 63-68, doi:10.1524/ract.1998.82.special-issue.63 (1998).
37 Jo, Y., Kirishima, A., Kimuro, S., Kim, H.-K. & Yun, J.-I. Formation of CaUO₂(CO₃)₃²⁻ and Ca₂UO₂(CO₃)₃(aq) complexes at variable temperatures (10–70 °C). *Dalton Trans.* **48**, 6942-6950, doi:10.1039/C9DT01174A (2019).

38 Maia, F. M. S., Ribet, S., Bailly, C., Grivé, M., Madé, B. & Montavon, G. Evaluation of thermodynamic data for aqueous Ca-U(VI)-CO₃ species under conditions characteristic of geological clay formation. *Appl. Geochem.* **124**, 104844, doi:10.1016/j.apgeochem.2020.104844 (2021).