# Speciation of uranium and radium during the treatment of acidic waters from legacy-mine (Le Cellier, France)

Pascal E. Reiller, Pierre l'Hermite, Valérie Plagnes, Anne Jost, Benoit Reilé, Martine Leermakers, Michaël Descostes

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Pascal E. Reiller, Pierre l'Hermite, Valérie Plagnes, Anne Jost, Benoit Reilé, et al. Speciation of uran- nium and radium during the treatment of acidic waters from legacy-mine (Le Cellier, France). Journal of Environmental Chemical Engineering, 2025, 13 (3), pp.116289. 10.1016/j.jece.2025.116289. cea- 05012309

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Speciation of Uranium and Radium During the Treatment of
Acidic Waters from Legacy-Mine (Le Cellier, France).

Pascal E. Reiller,<fnref n="1" /> Pierre L’Hermite,<fnref n="2" />,<fnref n="3" /> Valérie Plagnes,<fnref n="2" /> Anne Jost,<fnref n="2" /> Benoit Reilé,<fnref n="4" /> Martine Leermakers,<fnref n="5" /> Michaël Descostes<fnref n="3" />,<fnref n="6" />

email : pascal.reiller@cea.fr

<footnote n="1">¹ Université Paris-Saclay, CEA, Service de Physico-Chimie (SPC), F-91191 Gif-sur-Yvette
CEDEX, France.</footnote>

<footnote n="2">² Sorbonne Université, CNRS, EPHE, METIS, F-75005 Paris, France.</footnote>

<footnote n="3">³ ORANO Mining, Environmental R&D Dpt., 125 avenue de Paris, F-92320 Chatillon, France.</footnote>

<footnote n="4">⁴ Cabinet Reilé, F-25290 Ornans, France</footnote>

<footnote n="5">⁵ Archeology, Environmental Changes & Geo-Chemistry (AMGC), Vrije Universiteit Brussels
(VUB), Pleinlaan 2, 1050 Brussels, Belgium</footnote>

<footnote n="6">⁶ Centre de Géosciences, MINES Paris, PSL Research University, Paris, France.</footnote>
**Abstract**

The knowledge of aqueous speciation of uranium and radium in mining context is important
for the modelling strategies based on reactive transport. The use of thermochemical databases
allows accessing to theoretical speciation when the water compositions are known. If the usual
concentration of radium in natural or anthropized waters is too low to have access to speciation
experimentally, this can be achieved for uranium(VI), e.g. using time-resolved laser-induced
fluorescence spectroscopy (TRLFS). In this work, theoretical radium and uranium inorganic
and organic speciation were calculated using the water compositions collected in the legacy-
mine site of Le Cellier (Lozère, France) currently under monitoring after its closure and
decommissioning, and database file extracted from the Prodata database for the PhreeqC and
Orchestra codes. We also have measured TRLFS uranium spectra, which allows monitoring the
uranium(VI) evolution from sulphate-like UO₂(SO₄)₂²⁻ complexes, at the beginning of the
treatment, to characteristic CaₐUO₂(CO₃)₃⁽⁴⁻²n⁾ complexes towards the end of the treatment,
with an expected decrease of the total uranium concentration. Inorganic thermodynamic
calculations are in excellent agreement with the spectroscopic attributions all along the
treatment, whatever the speciation code used. The influence of natural organic matter, as part
of the dissolved organic carbon outside of the legacy-mine perimeter, cannot be ruled out but
is not straightforward to ascertain. The effluent seems only to disturb slightly the uranium
speciation in the local stream from UO₂CO₃(aq) to CaₐUO₂(CO₃)₃⁽⁴⁻²n⁾ due to calcium increase.

**1. Introduction**

The amount of uranium in the environment is a justified matter of concern [1,2,3,4,5,6,7].
Understanding speciation of aqueous uranium and its daughter radium in mining context is then
crucial for optimizing mine water treatment and managing the environmental impact over time
[8,9,10]. Identifying the aqueous forms of these elements provides valuable information for
developing modelling strategies based on reactive transport [11,12,13].

In mining context, radium speciation is influenced by the activity of SO₄²⁻ ion [14] and alkaline
earth sulphate minerals [15,16,17,18,19]. Radium mobility is dependent on interactions with
clays [20,21,22], metal oxides [23,24,25], and sulphate minerals [15,16,17,18,19,26,27,28,29],
which have significant Ra retention/fixation properties.

Uranium mobility is dependent on speciation. Under surface environmental conditions, which are usually oxidizing, uranium(VI) concentrations is controlled by the solubility of oxides, (oxo)hydroxydes, and silicates [30,31,32]. Uranium(VI) speciation is dependent on the inorganic ligands present in surface waters: OH⁻, CO₃²⁻, SO₄²⁻, HnPO₄(3-n)-, F⁻, Si(OH)₄(aq)— see reference [14] and references therein.

Finally, natural organic matter (NOM) from dissolved organic carbon (DOC) in waters, can be
a strong ligand of uranium [33,34]. The Non-Ideal Competitive Adsorption-Donnan (NICA-
Donnan) model [35,36,37] can efficiently represent the effect of pH, ionic strength, competition
between metals, and total metal concentration on the repartition of uranyl(VI) between the
different binding modes in NOM, i.e. non-specific (Donnan), low affinity and high affinity for
protons sites. Milne *et al*. [38,39] have proposed generic NICA-Donnan parameters for a variety
of cations, particularly for UO₂²⁺. These parameters have been used successfully to model
laboratory as well as on site experiments—see e.g. references [33] and[40] page 47 and
references therein.

However, retention processes limit uranium mobility. They may be linked to the neoformation
of insoluble uranyl phosphate minerals, observed as meteoric alteration products of uranium
deposits [41,42,43] or as biomineralization linked to microbial activity [3,44]. In sulphated
mine tailings, under arid conditions where evaporation becomes significant, uranium(VI) can
also be immobilized under the form of sulphate minerals [11,45]. Uranium retention is often exacerbated by the presence of natural phases that have significant adsorption properties, e.g. clays and metal oxides [46,47,48,49]. Therefore, determining uranium(VI) and radium mobility requires exhaustive and detailed characterization of water chemistry, which is complemented by characterization of mineralogy [50,51].

These results are then constrained by speciation calculations based on thermodynamic databases, as illustrated in previous works [8,14], the choice of which is a key factor as it can lead to different results and interpretations [14,31,52,53]. This approach can be constrained by indirect measurements of uranium speciation by specific sensors such as diffusive gradients in thin films [54,55,56,57,58], or directly by time-resolved laser-induced fluorescence spectroscopy (TRLFS) [1,13,59,60,61,62,63,64].

TRLFS has shown a great advantage in studying both spectrophysics and chemistry of uranium(VI). In a general manner, complexation of UO₂²⁺ is inducing a red-shift of the luminescence bands, variation in band spacing (Δv̄), and increase of decay time, as for the formation of UO₂(SO₄)ₙ²⁻²n complexes [65,66]. The formation of UO₂(CO₃)ₙ²⁻²n complexes has been more difficult due to their weak luminescence properties [67,68,69]. Since the first evidence of the formation of calcium(II)triscarbonatouranyl(VI) species from a waste mining pile [70], it is known that UO₂(CO₃)₃⁴⁻, and derived alkaline earth (Ae) species, are showing a blue-shift of the luminescence bands relative to UO₂²⁺ [71]—luminescence properties of other UO₂(CO₃)ₙ(²⁻²n), with n = {1;2}, remain unknown to the best of our knowledge. An interpretation of this blue-shift was proposed from ab initio studies [72]. Thermodynamic data and stoichiometry of AeₙUO₂(CO₃)₃(⁴⁻²n)⁻ are now well established— references [73,74] and references therein. In environmental studies, these species have been evidenced spectroscopically in various environments [1,62,70,75,76,77], particularly in carbonated mine waters [13,59,64,70,75]. It has been noted that the increase in CaCO₃ alkalinity was decreasing
uranium(VI) toxicity [1,78,79,80], onto which the $Ae_nUO_2(CO_3)_3^{(4-2n)-}$ complexes are suspected to have a weak influence. The differences of the speciation in sulphate [60,61,75] and carbonate media [13,64,70,75] in mining areas was evidenced, but never on the same site.

Unfortunately, contrary to lanthanide/actinide(III)-NOM [81,82], the formation of uranium(VI)-NOM complexes is inducing a strong luminescence quenching [83], and no characteristic TRLFS spectrum of any U(VI)-NOM complex has been proposed so far at room temperature. The only available data were obtained at 153 K [84].

Here, we propose to test a combined theoretical/experimental speciation approach on a former mining site: the Le Cellier mine in France. On this legacy-mine, now closed and decommissioned, former tailings from acid heap leaching treatment generates acidic sulphated effluents that are treated by limestone drains before discharge into the environment. This passive treatment offers the unique possibility to sample the waters at different steps in order to study the speciation from a sulphated to a carbonated environment on the same mining site.

After sampling, we will confront the theoretical speciation of waters sampled all through the treatment process from the waste piles (sulphated) to the outlet (carbonated), and finally to the stream nearby. A comparison to the TRLFS spectra and theoretical speciation upstream and downstream of the legacy-mining site will be done to ascertain the speciation.

## 2. Materials and methods

### 2.1. *Sampling site*

The legacy-mining site of Le Cellier (Lozère, France) has been described elsewhere [85, 86, 87]. The uranium extraction activities on the site were stopped in 1990, and heap-leaching tailings are still stored on site in piles with a solid cover preventing any dispersion and limiting the exhalation of radon. A passive and economically effective treatment strategy of the acidic waters from the site before discharge in the Fouillouse stream has been proposed in order to
follow environmental guideline imposed by local authorities. In addition to the layout of the
site [85,86,87], a schematic representation of the water system and sampling points is shown in
Figure SI-1 in the electronic supplementary information. Several outlets from piles are directed
to the limestone drains to increase both pH and alkalinity of the waters. The increase in pH
leads to the precipitation of metal (oxy)hydroxides, such as iron and aluminium, which aids in
reducing uranium concentrations due to their adsorption properties [88,89,90]. Other outlets are
regrouped in a collection basin together with the limestone drains outlets. Depending on the
water properties, a further treatment can be done through a buffer basin and/or an eventual soda
treatment. All waters are passed through a decantation laguna before the final outlet or
discharge in the Fouillouse stream. Waters were collected from several sample points as
indicated in Figure SI-1.

**2.2. Water sampling and analysis**

The general water sampling strategy has been described elsewhere [8]. Water samples were
filtered at 0.45 µm and conditioned immediately after the sampling on site. The filtered samples
were subsampled for analysis of major anions (F⁻, Cl⁻, Br⁻, SO₄²⁻), nutrients (NH₄⁺, NO₃⁻, PO₄³⁻),
Si(OH)₄(aq), major cations (Na⁺, K⁺, Mg²⁺, Ca²⁺), trace elements (Ag, Al, As, Ba, Cu, Fe, Mo,
Pb, Ra, Sr, Ti, V, U), dissolved inorganic (DIC) and dissolved organic carbon (DOC), and
alkalinity. The samples were either stored unacidified (for anions, nutrients, alkalinity and DIC)
or acidified either using HNO₃ (cations and trace elements) or H₃PO₄ for DOC. Major cations
and trace elements were analysed by inductively coupled sector field mass spectrometry ICP-
SF-MS (Thermo Element II); anions by ion chromatography (Metrohm Professional 850 IC),
nutrients by colorimetry using a segmented flow analyser (QuAATro Analyser), DIC and DOC
with a Shimadzu TOC-L analyser, and alkalinity by titration (Mettler Toledo T5 autotitrator).
The pH, dissolved oxygen, and temperature values were recorded in situ once the stabilization
of the physico-chemical parameters had been reached. More details regarding the sampling,
conditioning and analytical methodologies are available elsewhere [91]. Samples dedicated for TRLFS were shipped and analysed in the Laboratory for the Analytical, Nuclear, Isotopic, and Elementary studies from the CEA.

## 2.3. Time-resolved laser-induced fluorescence spectroscopy

The TRLFS set-up was described elsewhere [64, 92, 93, 94, 95]. The frequency-tripled (355 nm) of an Nd-YAG (Surelite II, Continuum) pulsed laser (5 ns pulse, 170 mJ per pulse at 10 Hz) was adjusted to the desired frequency by an optical parametrical oscillator (OPO, Horizon, Continuum, USA). The mainly used excitation wavelength $\lambda_{exc} = 450$ nm, was the best compromise between the uranium(VI) quantum yield and efficiency of the OPO. Some spectra were also recorded at $\lambda_{exc} = 425$ nm [64], and at $\lambda_{exc} = 470$ nm. The laser beam was focused in a fluorescence quartz cell (QS-101, Suprasil, Hellma Analytics). The energy ($E_{nominal}$ ca. 25 mJ) was monitored using a Joule-metre (RJP-734, Laser Probe, Inc.). In view of the large range of uranium concentration of the samples, the incident laser energy was eventually lowered using a diaphragm to avoid the saturation of the detector. The luminescence was collected at 90° from the incident laser beam and focused in an ACTON spectrometer, where only the 300 lines mm$^{-1}$ grid was used. Luminescence spectra were recorded using a CCD camera (ANDOR, UK), at a delay $D$ after the laser pulse, during an aperture time $W$.

Details on calculation of luminescence decay times and decomposition of spectra are recalled in the ESI.

## 2.4. Speciation calculation

Speciation were calculated using PhreeqC 3.4.0.12927 [96,97] or Orchestra [98] software (object2019), and using extraction files from the Prodata 1.5.3.1 database [14,99]—available on demand at prodata@cea.fr. NICA-Donnan generic parameters from Milne *et al.* [38,39] were used in Orchestra calculations (NicaDonnan.txt database file).
This database extraction files are used in the environmental monitoring programs of Orano
Mining Environmental R&D Dpt. [9,11,12,13,19,20,26,27,28,100], and also used by non-
members of these programs [4,58,101,102].

The most important thermochemical data on the formed complexes are regrouped in Table SI-1.
In view of the importance of ionic strength on the speciation of uranium, particularly for
Ae_nUO_2(CO_3)_3^{(4-2n)-} [73], the specific ion-interaction theory (SIT) was used to account for the
ionic strength correction following the mostly accepted thermodynamic constants and functions
from the Nuclear Energy Agency from the Organization for the Economic Co-operation and
Development (NEA-OECD) [103], on which the thermodynamic constants in Prodata are
mostly based. The formation constants and SIT parameters for Ae_nUO_2(CO_3)_3^{(4-2n)-} complexes
are from NaCl medium in Shang and Reiller [92,94]—differences with the NEA-OECD
selection [103] can be estimated in Reiller [73].

**3. Results and discussions**

**3.1. Water compositions**

The water compositions are reported in Table SI-2; the DOC concentrations are regrouped in Table SI-3. The waters in the Fouillouse stream exhibit a low mineralization and a pH ca. 7 both upstream (CEL-01 and CEL-PONT) and downstream of the mining site (CEL-02). Uranium concentrations measured upstream of the site are of the order of nanomole per litre— 1.9 10⁻⁸ and 1.5 10⁻⁹ M at CEL-PONT and CEL-01 respectively. Concentrations of ²²⁶Ra are in the order of 3 10⁻¹⁵ M (0.03 Bq/L). The moderately acidic (pH ca. 5) to acidic (pH ca. 3) waters resulting from the leaching of mine tailings (CEL-35, CEL-31, CEL-32, P-CEL-51, CEL-13, CEL-15, CEL-25, CEL-25bis) are, as expected, more mineralized and exhibit a Ca-Mg-SO₄ facies consistent with concentrations controlled by the solubility of sulphate bearing minerals typical of this geochemical context, e.g. gypsum, barite, jarosite... [8,11,16,19,45,50]—
saturation indices in Table SI-7. The mine waters before treatment (CEL-51) are the most concentrated, with SO₄ concentrations up to 16 mM, and have much higher U concentrations (1.4 10⁻⁵ M) than the stream water, while ²²⁶Ra concentrations show a less marked increase (1.4 10⁻¹⁴ M, i.e. 0.115 Bq/L) in line with a different geochemical behaviour— reference [100] and references therein. CEL-DINT sample is from a former flooded open peat mine, which can eventually be affected by leaching of mine tailings.

Neutralization of pH in the limestone drains, followed by decantation, raises the pH values ca. 6.5 (CEL-52) and finally ca. 7 at the discharge (CEL-03). This is accompanied by an increase in total inorganic carbon (1.2 10⁻³ M) while Ca and SO₄ concentrations remain constant—saturation indices of calcite and gypsum in Table SI-10.

A decrease in U concentration is observed (1.9 10⁻⁶ M), while ²²⁶Ra concentration is little affected and remains very low (1.4 10⁻¹⁴ M, i.e. 0.12 Bq/L). Finally, in the mixing zone (CEL-02), downstream of the discharge, the water chemistry remains little changed, with a pH of 7.5 compared with 7.0 upstream, and slightly higher concentrations of SO₄ (9.2 10⁻⁴ M) and Ca (6.4 10⁻⁴ M). The concentrations of U and ²²⁶Ra are 1.2 10⁻⁷ and 5.6 10⁻¹⁵ M (0.05 Bq/L), respectively, reflecting a limited increase from the natural geochemical background.

As it can be seen in Table SI-2, the dissolved O₂ concentrations in the solutions are close to the saturation limit, which indicate oxic waters. Hence, the speciation of uranium will be calculated related to the dissolved oxygen, assuring that uranium(VI) is controlling the calculated speciation.

## 3.2. Theoretical speciation of radium

From the analysis in Reiller and Descostes [14], completed by other reprocessed data [2, 5, 104, 105], it was awaited that the major influence on radium speciation should be the SO₄²⁻ activity. Using the solution compositions in Table SI-2, the evolution of radium
speciation vs. a(SO₄²⁻) in Figure 1a and Figure SI-2 makes clear that the same conclusion can be drawn for the samples from this particular site. It can also be seen in Figure 1a—and plotting mole percentage of species vs. a(HSO₄⁻) a(H⁺) in Figure SI-3 to amplify the differences between samples—that: (i) the samples taken before the limestone drains from Pile 1 (CEL-35), Pile LBD (CEL-31, CEL32), and Pile 2 (PCEL51) are showing the larger proportion of RaSO₄(aq); (ii) the samples from Pile 3 (CEL13, CEL15, and CEL-25 and CEL-25bis) seem more dominated by Ra²⁺ (Figure SI-3a); (iii) the samples after the limestone drains (CEL-52, CEL-DINT, and CEL-MRD) and the outlet CEL-03 are showing both Ra²⁺ and RaSO₄(aq); and (iv) the samples in the Fouillouse stream are clearly dominated by Ra²⁺ (Figure SI-3b).

It can also be seen on Figure 1b that the dissolved radium concentration remains very low all through the treatment process—represented by the variation of the activity product a(HSO₄⁻) a(H⁺). It can also be seen that the radium concentration before (CEL-01, CEL-PONT) and after the outlet (CEL-02) are not significantly different, and that the contribution to the radium content in the Fouillouse stream is minimal.

<figure><img src="image_2.png" /><figcaption>**Figure 1.** Repartition of radium species using the Prodata 1.5.3.1 database extraction file sorted in the order of log₁₀ a(SO₄²⁻) from Le Cellier site (a), and evolution of radium concentration all through the treatment process vs. the activity product a(HSO₄⁻) a(H⁺) (b).</figcaption></figure>
### 3.3. Theoretical speciation of uranium

The calculated speciation using the water composition in Table SI-2 are reported in Table SI-5 and Table SI-8.

#### 3.3.1. General trends

The concentration of uranium all along the treatment can be usefully plotted as a function of activity products representative of the sulphate-impacted waters, $a(SO_4^{2-}) a(H^+)^2$ (Figure 2a), and after limestone treatment, $a(Ca^{2+}) a(HCO_3^-)^2$ (Figure 2b). It must be noted that carbonate concentrations for sulphate-impacted solutions in Figure 2b were calculated at equilibrium with atmospheric $CO_2(g)$. In Figure 2a, the decrease of uranium concentration with the decrease of $a(SO_4^{2-}) a(H^+)^2$ is clearly shown both for sulphate-impacted before treatment, and for carbonate-dominated samples after treatment. In Figure 2b, the progressive decrease of the uranium concentration with the decrease of $a(Ca^{2+}) a(HCO_3^-)^2$, close to upstream concentration in the Fouillouse stream, is evidenced. The low impact of sulphate on CEL-DINT sample—from the flooded open pit mine—can be seen as both $a(SO_4^{2-}) a(H^+)^2$ and $a(Ca^{2+}) a(HCO_3^-)^2$ values are in the range of other carbonate-impacted waters (Figure 2).
<figure><img src="image_3.png" /><figcaption>**Figure 2.** Evolutions of $c_U$ vs. $\log_{10}(a(\text{SO}_4^{2-}) a(\text{H}^+)^2)$ (a) and $\log_{10}(a(\text{Ca}^{2+}) a(\text{HCO}_3^-)^2)$ (b) for sulphate-impacted (yellow squares), carbonate-impacted (orange diamonds), and Fouillouse waters (blue circles).</figcaption></figure>

### 3.3.2. Sulphate-impacted samples

The speciation of sulphate-impacted waters is presented in Table SI-5. As both pH values and sulphate concentrations are varying, the speciation is presented in Figure 3 and detailed vs. $a(\text{SO}_4^{2-})$ (Figure SI-4a) and $a(\text{SO}_4^{2-}) a(\text{H}^+)^2$ (Figure SI-4b). It can be seen that the importance of $\text{UO}_2(\text{SO}_4)_n^{2-n}$ complexes is increasing with the activity product values. At the lowest values, $\text{UO}_2^{2+}$ and $\text{UO}_2\text{SO}_4(\text{aq})$ are sharing predominance [10,14]. The sulphate complexes $\text{UO}_2\text{SO}_4(\text{aq})$ and $\text{UO}_2(\text{SO}_4)_2^{2-}$ are dominating the speciation at the highest values [10,14]—with $\text{UO}_2(\text{SO}_4)_3^{4-}$ less than 5%
<figure><img src="image_4.png" /><figcaption>Figure 3. Repartition of species sorted by increasing a(SO₄²⁻) in the sulphate-impacted samples.</figcaption></figure>

### 3.3.3. Carbonate-dominated samples

#### 3.3.3.1. Inorganic speciation

The speciation of carbonated waters are regrouped in Table SI-8. Regarding the temperature of the water samples (Table SI-2), it must be recalled here that the ΔrH°m and ΔrS°m values of some uranyl species have not been selected in the NEA-OECD selections [103], e.g. UO₂(OH)₂(aq), UO₂(SO₄)₃⁴⁻, (UO₂)₂CO₃(OH)₃⁻, UO₂CO₃Fₙ(2-n), and AenUO₂(CO₃)₃(4-2n)⁻ with Ae = {Mg²⁺, Sr²⁺, Ba²⁺}, which is inducing a further uncertainty in the calculation results. Shock et al., [106] proposed estimations of some hydroxo complexes of uranyl(VI) based on the first NEA-OECD selection [107], which were corrected afterwards [103] and should be regarded with caution. Similarly to the sulphate-impacted waters, the variation of pH, and Ca and carbonate
concentrations are represented vs. a(Ca²⁺) a(HCO₃⁻)² in Figure 4 and Figure SI-5. It is noteworthy that as the concentration of Mg is always low, then the x ratio,

$$x = \frac{a(\text{Mg}^{2+}) + a(\text{Ca}^{2+})}{a(\text{Mg}^{2+})} \quad (1)$$

is always lower than 0.5 (Table SI-9), and MgUO₂(CO₃)₃²⁻ is always minor [73,94].

As evidenced in Shang et al., [64], at the lowest value of a(Ca²⁺) a(HCO₃⁻)²—or lowest a(Ca²⁺) a(H⁺)² and a(HCO₃⁻) a(H⁺)—, UO₂CO₃(aq) dominates the speciation with the (UO₂)₂CO₃(OH)₃⁻ dimer as second major species—the ratio between UO₂CO₃(aq) and (UO₂)₂CO₃(OH)₃⁻ was dependant on the total U concentration as awaited from the formation of the dimer [14]. The concentration of CaₙUO₂(CO₃)₃⁽⁴⁻²n⁾ complexes increased with a(Ca²⁺) a(HCO₃⁻)², with other AeₙUO₂(CO₃)₃⁽⁴⁻²n⁾ complexes being a minority. The relatively low concentration of sodium in the waters, or low a(Na⁺) a(H⁺)⁻¹, is ensuring the dominance of Ca₂UO₂(CO₃)₃(aq) over CaUO₂(CO₃)₃²⁻ at the highest a(Ca²⁺) a(HCO₃⁻)² values [73] (Figure SI-6).

Finally, the repartition of the uranium concentrations in the samples vs. a(SO₄²⁻) a(H⁺)² (Figure 2a) and a(Ca²⁺) a(HCO₃⁻)² (Figure 2b), and the evolution of a(SO₄²⁻) a(H⁺)² vs. a(Ca²⁺) a(HCO₃⁻)² (Figure 5, and Figure SI-7) allows distinguishing between low pH from higher pH sulphate-impacted samples (yellow symbols), and higher carbonate-impacted samples inside the Le Cellier site (orange symbols) from those in the Fouillouse stream (blue symbols). The slight change in calculated inorganic speciation between CEL-01/CEL-PONT— mainly as UO₂CO₃(aq)—and CEL-02—mainly as CaₙUO₂(CO₃)₃⁽⁴⁻²n⁾—is explained by the higher a(Ca²⁺) a(HCO₃⁻)² value caused by the outlet CEL-03.
<figure><img src="image_5.png" /><figcaption>Figure 4. Repartition of species sorted by increasing a(Ca²⁺) a(HCO₃⁻)² in carbonate-dominated samples.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Figure 5. Evolution of log₁₀(a(SO₄²⁻) a(H⁺)²) vs. log₁₀(a(Ca²⁺) a(HCO₃⁻)²) for the different samples: sulphate-impacted (yellow square), carbonate-impacted (orange diamonds), and samples from the Fouillouse stream (blue circles).</figcaption></figure>
### 3.3.3.2. Accounting for potential complexation by dissolved organic carbon

The concentration of DOC of the samples are reported in Table SI-3. The nature of this organic carbon is not known. Nevertheless, in view of the high short-lived fluorescence signal, and sensitivity to the excitation wavelength evidenced in another legacy mining site [64], the possible interaction of uranium, and other metals, with natural organic matter proxies like humic substances could be tested. Assuming that NOM is composed of humic acids, that ca. 50% of humic acids weight is composed of carbon [108], and considering the conservative hypothesis that 100% of the humic acid's functionality is accessible, a speciation calculation in Figure SI-8 can be done using Orchestra software using the provided objects2019.txt definitions of NICA-Donnan model [98], the Prodata 1.5.3.1 extraction file, and the NICA-Donnan generic humic acid parameters [38,39]. The samples are sorted by increasing total uranium concentration, as it is an important factor for metal-NOM complexation [33,36]. It seems that the samples from the Fouillouse stream (CEL-01, CEL-PONT, and CEL-02) could either be dominated (CEL-01 and CEL-PONT) or contain significant amount (CEL-02) of uranium-NOM complexes—mainly on the low affinity for proton site HA₁ compared to high affinity for proton sites HA₂. The downstream sample CEL-02 could be significantly (ca. 34% in total) in interaction with NOM. The sample from the flooded open pit CEL-DINT could contain 23% of HA complexes and 33% of AenUO₂(CO₃)₃⁽⁴⁻²n⁾⁻.

### 3.4. Classification of samples from TRLFS spectra

Once the theoretical inorganic and organic speciation is known, the verification of TRLFS of the samples can be done. The analysed samples can be readily sorted into four families: (i) 2 samples from the Fouillouse stream that are not showing any detectable uranium(VI) fluorescence (CEL-01, CEL-02); (ii) 4 samples with spectra showing a blue-shift relative to UO₂²⁺ (CEL-03, CEL-52, and CEL-MDR) located after the limestone drain or from the flooded open pit mine (CEL-DINT); (iii) 9 samples with spectra showing comparable red-shifts relative
to UO₂²⁺ (CEL-13, CEL-15, CEL-C25, CEL-25(bis), CEL-35, CEL-32, CEL-51, P-CEL-51, and CEL-31), out of which 8 are showing a non-exponential decay; (iv) 1 sample with characteristics that do not follow any of the two previous characteristic uranium spectra families (CEL-PONT).

As observed otherwise, the blue-shifted spectra from family (ii) are revealing the presence of AenUO₂(CO₃)₃⁽⁴⁻²n⁾- complexes [64,70,75,92,93,94,95]. The samples from family (iii) seem to be close to the characteristics of UO₂SO₄(aq) [60,61,65,66].

### 3.4.1. Samples with no detectable uranium(VI) fluorescence

CEL-01 and CEL-02 were sampled in the Fouillouse stream (Figure SI-1), upstream of the mining site, and downstream of the treatment outlet, respectively. Fluorescence spectra obtained at λexc = 425, 450, and 470 nm at D = 25 ns (Figure SI-9a and Figure SI-10a for CEL-01 and CEL-02, respectively), are not characteristic of uranium(VI). The red shift from 425 to 470 nm is suggesting the fluorescence of a mixture of organic components [64]. Even using longer delay (Figure SI-9b and Figure SI-10b for CEL-01 and CEL-02, respectively), as done by Shang et al., [64], does not reveal a characteristic pattern of uranium(VI) fluorescence, with a comparable red shift with λexc. This means that under these conditions, uranium cannot be detected by TRLFS, either because of a too low total concentration, or due to a strong fluorescence quenching from organic compounds [83]. At slightly higher concentration, Shang et al., [64] were able to detect uranium even with noticeable organic carbon concentration, which were also showing red shift with decreasing λexc from 425 to 450 nm. The second hypothesis here seems more likely. The speciation accounting for NOM complexes (Figure SI-8) could explain the difficulties in detecting uranium luminescence due to the strong quenching effect.
Finally, it seems difficult to conclude firmly about the speciation of uranium in the family (i) sample, but the presence of significant proportion of uranium-NOM complexes cannot be ruled out. Specific studies should be led to clarify this point.

### 3.4.2. Hypsochromic shits

As already shown [64], the luminescence of the samples from family (ii) was dominated at short delay by an ill-resolved fluorescence signal. The characteristic uranium(VI) luminescence bands were better resolved with increasing delay. The non-exponential decays of the samples are shown in Figure SI-12, Figure SI-14, and Figure SI-16. The area-normalized blue-shifted spectra relative to $UO_2^{2+}$ are shown in Figure 6 with offsets for the sake of clarity. The common $\lambda$ values of the characteristic bands of $A_eUO_2(CO_3)_3^{(4-2n)-}$ complexes are clearly shown (see Figure SI-11, Figure SI-13, Figure SI-15, and Figure SI-17, and Table SI-11, Table SI-13, Table SI-15, and Table SI-16), which suggest a comparable speciation. The slight humps at $\lambda > 510$ nm for the CEL-DINT sample indicates a further contribution, which is likely due to the remnant of an organic short-lived component, which could be in agreement with the speciation calculation considering NOM interactions.

These spectra can with no doubt be affected to $Ca_nUO_2(CO_3)_3^{(4-2n)-}$ as shown elsewhere [1, 13, 64]. The firm assignment to either $Ca_nUO_2(CO_3)_3^{(4-2n)-}$ complex is not possible under the experimental conditions used [72]. The almost identity of the CEL-03, CEL-52, CEL-MRD, and CEL-DINT spectra (Figure 6) is in line with the almost identical inorganic speciation (Figure 4 and Table SI-8).
<figure><img src="image_7.png" /><figcaption>Figure 6. Comparison of the area-normalized (and offset) spectra of blue-shifted samples (family ii) CEL-03 (green line), CEL-DINT (red line), CEL-52 (blue line), and CEL-MRD (purple line) at $\lambda_{exc}$ = 450 nm, D ca. 100 ns, W = 1000 ns, 1000 accumulations.</figcaption></figure>

### 3.4.3. Bathochromic shift

#### 3.4.3.1. Sulphate like family

The area-normalized fluorescence spectra of the family (iii) are presented altogether in Figure 7. The decomposition in Gaussian components are presented in Figure SI-18, Figure SI-19, Figure SI-21, Figure SI-23, Figure SI-25, Figure SI-27, Figure SI-29, Figure SI-31, and Figure SI-33. Decomposition parameters and maximum values of Gaussian bands are presented in Table SI-19 to Table SI-36. The luminescence decay analyses are presented in Figure SI-20, Figure SI-22, Figure SI-24, Figure SI-26, Figure SI-28, Figure SI-30, Figure SI-32, and Figure SI-34. The energy-normalized spectra are shown in Figure SI-35.

From Figure 7 is seems evident that the differences between the spectra are minimal. The decompositions at D = 500 ns seem to confirm that the $\lambda_{max}$ values and $\Delta\bar{\nu}$ values are very near
from one another and very close to values observed otherwise for laboratory [65,66] or in-field
[60,61] studies.

This confirms that the TRLFS spectra were mainly due to UO₂SO₄(aq) with minor participation
of other complexes. A comparable evolution was noted otherwise [60,61,75]—see speciation
recalculated using Prodata 1.5.3.1 in Figure SI-41a,b. This was also suggested elsewhere
[7,109,110,111,112]—though sometimes in a wider a(SO₄²⁻) range—and can be recalculated
using Prodata 1.5.3.1 (Figure SI-41a,b).

<figure><img src="image_8.png" /><figcaption>Figure 7. Area-normalized fluorescence spectra of the samples with comparable red-shifts (family iii) suggesting the presence of UO₂(SO₄)₂²⁻ complexes.</figcaption></figure>

3.4.3.2. CEL-PONT

The spectrum and decomposition in Gaussian bands of the CEL-PONT sample at *D* = 500 ns is shown in Figure SI-36. The luminescence decay with its non-exponential decomposition is shown in Figure SI-37. The difference in spectrum between both component at *D* = 500 ns and *D* = 8500 ns (Figure SI-38) is not evident. To the best of our knowledge, no comparable
spectrum in the literature was obtained in this pH range. The detection of carbonatouranyl(VI) complexes has long been proved difficult in TRLFS [67,69]. It was not possible to decipher between the possible complexes in solution either from the decomposition or from the decay analysis.

The inorganic speciation of the CEL-PONT sample was composed of UO₂CO₃(aq) (49%) and (UO₂)₂CO₃(OH)₃⁻ (31%) (Table SI-8). This confirms that even if the solution is carbonate dominated, the calcium content was not sufficient to stabilize the CaₐUO₂(CO₃)₃⁽⁴⁻²n⁾⁻ complexes. In view of the difficulties encountered by past authors to obtain luminescence of carbonate complexes of uranyl [67,69] the luminescence of UO₂CO₃(aq) may seem unlikely under these conditions. The 99% of calculated uranium-NOM complexes in CEL-PONT (Figure SI-8) seems much too high in view of the uranium luminescence signal observed, and at odds with CEL-01. Further works may be needed to clarify this point.

**3.5. Comparison of the samples fluorescence after attribution**

The comparison of spectra representative of the identified families is presented in Figure 8. It is clear that the spectrum of CEL-PONT is completely different from the two other families, notwithstanding that its speciation is dominated by carbonate. The a(Ca²⁺) a(HCO₃⁻)² and a(HCO₃⁻) a(H⁺) values are nearly respectively one, and half an order of magnitude lower, than e.g. CEL-02 (Figure 4a, Figure 5) caused by the influence of the outlet (CEL-03). It seems that all the peaks are more red-shifted than the spectra belonging to the family (ii) dominated by sulphate. The two first luminescence peaks are mostly hidden into the third one. Attribution of luminescence transitions is difficult at this stage and a full study of the possible complexes involved seems to be desirable, but unfortunately out of the scope of this study.

The total uranium concentration of the samples are plotted in Figure SI-39 evidencing the CEL-01 and CEL-02 samples that cannot be detected by TRLFS. The fluorescence intensities
at λ<sub>em</sub> = 492 nm and total areas of the family (ii) sulphate-like spectra are shown in Figure SI-35. The logarithmic values of F<sub>492 nm</sub> and area for samples in Figure 7 and Figure SI-35 are described as straight lines vs. log<sub>10</sub>c<sub>U</sub> on Figure SI-40. CEL-15 and CEL-25—as well as CEL-25bis—from Pile 3 show close concentrations and fluorescence signals of uranium; CEL-13 seems to show a slightly higher c<sub>U</sub> value, but nevertheless in the same order of magnitude. The other samples, i.e. CEL-32 from Pile 1, CEL-35 from Pile LBD, P-CEL-51 from Pile 2, and CEL-51—sampled after the mixing of Pile LBD and Pile 2—, show comparable c<sub>U</sub> and fluorescence values. Interestingly, CEL-31 and CEL-32 do not show the same fluorescence intensity, when they are issued from very near sampling points.

<figure><img src="image_9.png" /><figcaption>**Figure 8.** Comparison of the area-normalized luminescence of spectra representative of family (i) (after treatment, CEL-03, blue line), family (ii) (before treatment, CEL-31, orange line), and atypical family (iii) (CEL-PONT, green line).</figcaption></figure>
4. Conclusions

Waters sampled from French legacy-mining site of Le Cellier—where a dedicated passive water
treatment is devoted to manage the acidic and sulphated mining waters—offered the unique
opportunity to study the speciation of radium and uranium by theoretical speciation, and to
monitor by TRLFS for uranium, from the pile tailings to the outlet to the Fouillouse stream.
The radium speciation evolves from RaSO₄(aq) to Ra²⁺, and the contribution of the outlet to the
radium concentration in the Fouillouse stream is minimal. For uranium, the efficacy of the water
treatment, monitored through the formation of UO₂(SO₄)₂⁻ and
CaₐUO₂(CO₃)₃⁽⁴⁻²n)⁻ complexes by TRLFS, was in agreement with the theoretical calculations.
As expected, the formation of the UO₂(SO₄)₂⁻ complexes is driven by the sulphate
concentration, or SO₄²⁻ activity, and acidity released from the sulphate minerals from the
tailings. The formation of the CaₐUO₂(CO₃)₃⁽⁴⁻²n)⁻ is driven by the Ca²⁺ and HCO₃⁻ activities
induced by the limestone drains. Speciation and TRLFS spectra in the Le Cellier site are in
excellent agreement. From speciation calculation, it seems that the calcium concentration in the
outlet changes the speciation in the Fouillouse stream from UO₂CO₃(aq) upstream to
CaₐUO₂(CO₃)₃⁽⁴⁻²n)⁻ downstream.

The use of TRLFS as a speciation technique was limited by (i) the low concentration of uranium
in the Fouillouse stream, and/or (ii) by the luminescence quenching from natural organic matter.
Hence, it has not been possible for the time being to attribute univocally a speciation by TRLFS
in the Fouillouse stream, either upstream or downstream the treatment outlet, due to a strong
luminescence quenching, which is suspected to be from a natural organic origin. The formation
of natural organic complexes of uranium in the Fouillouse stream cannot be ruled out, but seems
unlikely in the waters from inside the Le Cellier site. Further works would be needed to clarify
this point. Notwithstanding this drawback, the coupled approach is well adapted to the mining
sites.
Finally, as the speciation of the Le Cellier site is now well characterized, and thermodynamic data from the Prodata extraction datafiles seem reliable, as recently seen independently otherwise [101,102,113]. It is then possible to build exercises of reactive transport to anticipate the eventual migration of uranium and radium within legacy-mining sites under monitoring.

## Acknowledgments

This work was done in the framework of the ORANO-CEA NOPRA program. All staff at ORANO Mining/Après-Mines France Department is acknowledged for their strong support.

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