# Sb(III) and Sb(V) retention by aptian sands: Quantitative insights into adsorption and heterogeneous redox reactions controlled by iron-containing minerals

Esra Orucoglu, Sylvain Grangeon, Nicolas C.M. Marty, Nicolas Maubec, Myriam Agnel, Benoit Madé, Mathieu Debure

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Esra Orucoglu, Sylvain Grangeon, Nicolas C.M. Marty, Nicolas Maubec, Myriam Agnel, et al., Sb(III) and Sb(V) retention by aptian sands: Quantitative insights into adsorption and heterogeneous redox reactions controlled by iron-containing minerals. Applied Clay Science, 2025, 277, pp.107974. 10.1016/j.clay.2025.107974. hal-0522293

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Research Paper

# Sb(III) and Sb(V) retention by aptian sands: Quantitative insights into adsorption and heterogeneous redox reactions controlled by iron-containing minerals☆

Esra Orucoglu<sup>a,∗</sup>, Sylvain Grangeon<sup>a</sup>, Nicolas Marty<sup>a</sup>, Nicolas Maubec<sup>a</sup>, Myriam Agnel<sup>b</sup>, Benoit Madé<sup>c</sup>, Mathieu Debure<sup>a</sup>

## ARTICLE INFO

**Keywords:**

Sorption
Aptian sands
Fe-oxides
Glauconite
Sb(III)
Sb(V)
Heterogeneous redox reaction

## ABSTRACT

The retention of Sb(III) and Sb(V) by Aptian sands, which contain quartz and glauconite, was investigated as a function of Sb concentration and with 7 d and 30 d contact time. Two samples, collected along a vertical profile, were used for the experiments. They originated from the capillary fringe (S<sub>CFr</sub>) and saturated (S<sub>Sat</sub>) horizons and were respectively representative of oxidizing and reductive redox conditions. The Aptian sands had a lower retention capacity for Sb(V) than for Sb(III). Sb(V) distribution coefficients (R<sub>d</sub>) ranged between 10 ± 3 L·kg<sup>-1</sup> and 13 ± 4 L·kg<sup>-1</sup> in S<sub>Sat</sub> and between 11 ± 3 L·kg<sup>-1</sup> and 19 ± 4 L·kg<sup>-1</sup> in S<sub>CFr</sub>, after 7 d interaction. These values almost doubled when the interaction time was increased to 30 d and, in all conditions, Sb(V) was the only species detected in the aqueous solution. The evolution of Sb(III) R<sub>d</sub> as a function of Sb equilibrium concentration and time was more complex. When contact time was 7 d, R<sub>d</sub> first increased from 68 ± 7 L·kg<sup>-1</sup> ([Sb]<sub>eq</sub> = 2.2·10<sup>-8</sup> M) to 547 ± 33 L·kg<sup>-1</sup> ([Sb]<sub>eq</sub> = 1.4·10<sup>-7</sup> M) and then decreased down to 323 ± 21 L·kg<sup>-1</sup> ([Sb]<sub>eq</sub> = 1.1·10<sup>-6</sup> M) for S<sub>Sat</sub> after 7 d of interaction. Sb(III) retention by S<sub>CFr</sub> was slightly lower: R<sub>d</sub> increased from 15 ± 4 ([Sb]<sub>eq</sub> = 3.4·10<sup>-8</sup> M) to 174 ± 13 L·kg<sup>-1</sup> ([Sb]<sub>eq</sub> = 7.7·10<sup>-7</sup> M) and then decreased down to 137 ± 10 L·kg<sup>-1</sup> ([Sb]<sub>eq</sub> = 2.3·10<sup>-6</sup> M). While Sb(V) was the only species detected in the aqueous solution at lower concentrations, Sb(III) was increasingly present when added Sb(III) concentration increased, hence pointing out to heterogeneous redox interaction between Sb(III) and the solid phase that could be successfully modeled.

Overall, this study demonstrated that Sb(III) and Sb(V) retention depended on the speciation of added Sb, on the redox reactivity of the sand samples, and on contact time. Such results are a step forward to our capacity to model, and hence quantitatively predict, Sb mobility in the natural environment.

## 1. Introduction

Antimony (Sb) is an element of concern due to its toxicity and widespread industrial applications (Rakshit et al., 2011; Fan et al., 2013; Mitsunobu et al., 2013; Ungureanu et al., 2015; Inam et al., 2018; He et al., 2019; Bagherifam et al., 2021). Its toxicity and mobility in the environment are directly influenced by its redox state, itself governed by factors as diverse as the solution composition, including redox potential (Eh) and pH, and the interactions with mineral surfaces and organic matter (Herath et al., 2017, Li et al., 2018, He et al., 2019, Bolan et al.,

2022). While Sb has four possible oxidation numbers (-III, 0, +III, +V), Sb(III) and Sb(V) are the most common in the environment (Rakshit et al., 2011; Biver et al., 2011; Fan et al., 2013; Long et al., 2020; Bolan et al., 2022). Sb(III) compounds are more toxic but less soluble than those of Sb(V) (Mitsunobu et al., 2008, 2010; Li et al., 2018; Bolan et al., 2022). Sb can also form organic complexes; yet, inorganic species are more frequent and generally more toxic (Wilson et al., 2010; Li et al., 2018; He et al., 2019).

Due to the speciation control on Sb mobility and toxicity, a sound description of its geochemistry is crucial to the prediction of its fate in

<footnote><sup>a</sup> BRGM, Orléans, France</footnote>

<footnote><sup>b</sup> Andra, Centre de Meuse/Haute-Marne, Bure, France</footnote>

<footnote><sup>c</sup> Andra, R&D Division, DISTEC/DRG/DCG, Châtenay-Malabry, France</footnote>

<footnote>* This article is part of a Special issue entitled: 'Clay Conf. Hannover 2024' published in Applied Clay Science.</footnote>

<footnote>* Corresponding author.
E-mail address: E.Orucoglu@brgm.fr (E. Orucoglu).</footnote>
the environment. However, it is frequently observed that Sb speciation, and in particular oxidation state, is at thermodynamic disequilibrium with solution redox conditions (Filella et al., 2002; Fan et al., 2013; Fawcett et al., 2015; Ungureanu et al., 2015), hence pointing out kinetic and heterogeneous redox reaction effects, namely biotic and abiotic redox reactions with mineral surfaces (Hockmann et al., 2014, Zhang et al., 2022b). Sb(III) is oxidized to Sb(V) by Mn(III/IV) and Fe(III) minerals, reactive oxygen species, sulfate-reducing bacteria and microorganisms. Sb(V) is reduced to Sb(III) by Fe(II)/Fe(III) minerals, dissolved sulfur species and microorganisms (Herath et al., 2017, Zhang et al., 2022b). More specifically, Belzile et al. (2001) investigated Sb(III) oxidation by natural and synthetic Fe and Mn (oxy)hydroxides (hereafter referred to as “hydroxides”) and observed that 0.26 µmol of Sb(III) could be oxidized to Sb(V) by 100 mg of Mn hydroxides at pH 7.5 within 2 d. In contrast, only 0.19 µmol of Sb(III) could be oxidized by 100 mg of Fe hydroxides at pH 7.1. However, the lower reactivity of Fe hydroxides as compared to Mn hydroxides under laboratory conditions cannot be straightforwardly generalized to actual environmental conditions, since it was observed that the oxidation rate depended on the synthetic versus natural origin of the hydroxides, with the synthetic ones having a higher reactivity. This was interpreted as the result of a difference of crystallinity between natural and synthetic phases and of the presence of structurally incorporated or adsorbed foreign ions. No significant effect of light on oxidation rate was observed, which contrasts with Fan et al. (2014) who reported that Sb(III) adsorbed on goethite can be photo-oxidized to Sb(V). In addition to Fe and Mn hydroxides, clay minerals are expected to play a significant role in Sb geochemistry, due not only to their capacity to adsorb anion by surface complexation at their edges, but also because of their redox properties resulting from the frequent occurrence of structural Fe(II)/Fe(III). Fe can be incorporated in clays either in the tetrahedral or octahedral sheets of clay layers though isomorphic substitution, with a resulting total Fe content of 1–30 wt%, and with a very large degree of variability in the Fe(II)/Fe(III) ratio (Amonette, 2002; Zhang et al., 2022b). As a result, clays reduce elements such as Cr(VI), Tc(VII), and Hg(II) (Taylor et al., 2000; Bishop et al., 2011; O'Loughlin et al., 2020; Zhang et al., 2022b) and oxidize inorganic (As, Sb) as well as organic (triclosan, polycyclic aromatic hydrocarbons, tetracycline, organic dyes) species (Liyanapatirana et al., 2010; Gu et al., 2011; Ilgen et al., 2012, 2017; Jia et al., 2014; Fan et al., 2023).

For the specific case of clay/Sb interaction, Ilgen et al. (2012) investigated Sb(III) oxidation and Sb(V) reduction by kaolinite (a Fe-free clay) and nontronite (a Fe-rich clay), in the presence or absence of aqueous Fe(II). When nontronite contained only Fe(III), no Sb(III) oxidation was observed. In contrast, when nontronite contained 0.2 Fe (II)/Fe(II + III), substantial oxidation was detected possibly because of Fe(III) within a Fe(II)–O–Fe(III) system being more reactive than Fe(III)–O–Fe(III) in oxidizing Sb(III), and in particular in presence of redox-active Fe(II)–O–Fe(III) at clay edge sites (Liu et al., 2022). Oppositely, Sb(V) reduction was not observed, regardless of the nontronite structural Fe(II)/Fe(II + III) ratio or of sorbed Fe(II).

To conclude with Sb redox reactions, it must be noted that reactive oxygen species generated by organic matter or microorganisms can also oxidize Sb(III) (He et al., 2019; Zhang et al., 2022b). Soil organic matter can be an electron donor, catalyze the redox cycle through electron exchange and act as a competitor ligand and increase the mobility of Sb (Verbeeck et al., 2020).

Another important mechanism affecting Sb mobility in soils is sorption. Minerals and organic matter can sorb Sb to a greater or lesser extent, depending on environmental conditions such as Eh, pH, ionic strength, or presence of competitive anions, for example. Several studies have focused on the sorption of Sb by Fe and Mn hydroxides (Thanabalasingam and Pickering, 1990; Tighe et al., 2005; Leuz et al., 2006; Mitsunobu et al., 2010; Vithanage et al., 2013; Qi and Pichler, 2016; Essington and Stewart, 2018; Yang et al., 2019; Burton et al., 2020; Ferrari et al., 2024) while few studies exist concerning Al

hydroxides (Thanabalasingam and Pickering, 1990; Rakshit et al., 2011; Ilgen and Trainor, 2012; Essington and Stewart, 2016), clay minerals (Xi et al., 2011, 2014; Ilgen and Trainor, 2012; Rakshit et al., 2015; Dousova et al., 2018; Zhang et al., 2022a) iron oxide coated sand (Cai et al., 2015) and organic matter (Pilarski et al., 1995; Tighe et al., 2005).

This study aims at contributing to the understanding of Sb fate in the environment. For this purpose, the Sb retention capacities of Aptian sands were investigated. Aptian sands are of interest for practical purposes, because a low and medium-level waste storage facility operated by Andra is constructed over a ~ 10 m thick sandy superficial aquifer belonging to the Upper Aptian underlaid by dozen of meters (up to 80 m) of thick impermeable clay (Amédro et al., 1995; Altmann and Chupeau, 1997; Amédro and Matrion, 2014; Lerouge et al., 2018, 2023). In this storage site, Sb is a radioelement of interest (Andre-Jehan and Molinas, 1993). The Aptian sands are also of interest for fundamental research purposes because they contain glauconite, a Fe-rich phyllosilicate (López-Quirós et al., 2020) that is frequently observed in sedimentary rocks (McRae, 1972), but for which little sorption (i.e., adsorption and heterogeneous redox reaction) data exist despite its broad interest (Debure et al., 2020). Two samples were collected in the Aptian sands, one in the critical zone, and more specifically in the capillary fringe zones ($S_{CFr}$) and one in the saturated zone ($S_{Sat}$) (i.e. below the water table). These two sand samples were fully characterized for their geochemistry and mineralogy, to quantify the effect of the composition of the solid phase on Sb retention. Both Sb(III) and Sb(V) sorption experiments were conducted and quantitatively interpreted by taking into account both sorption and heterogeneous redox reactions. Both explicit considerations of the different reaction mechanisms and the study of glauconite-rich samples make that this study contributes to a better understanding of Sb geochemistry and geochemical behavior in glauconite-rich solids (rocks, sediments, soils).

## 2. Materials and methods

### 2.1. Groundwater composition and synthetic groundwater preparation

Two groundwater samples were taken from two different piezometers to determine the chemical composition of the Aptian sands pore water, in the vicinity of the Aube disposal facility (“CSA”, France), operated by Andra (French National Agency for Radioactive Waste Management). Major and trace element concentrations, alkalinity and pH measurements were conducted with inductively coupled plasma atomic emission spectroscopy (ICP-AES), ICP-MS, ionic chromatography, titration and pH electrode. Since the quantity of water recovered from site sampling was too low to allow performing all laboratory measurements and to ensure that all experiments were performed with the same initial solution composition, it was decided to perform geochemical calculation to allow for the reproduction of porewater composition from mixing of laboratory salts to develop a representative “synthetic” porewater. Calculations were performed with Phreeqc 3.7.3 software and the ThermoChimie database (version 12a) (Madé et al., 2025) and it was found that pore water composition of Aptian sand could be satisfyingly modeled by mixing, in a glovebox under 3 %CO₂/ 97 % N₂ atmosphere, 0.35 mM SiO₂, 80 µM KCl, 27 µM FeCl₂·4H₂O, 0.5 mM Ca(OH)₂, 90 µM Mg(OH)₂ and 0.14 mM Na₂SO₄ chemicals and 0.14 mM HNO₃, 3.5 µM H₃PO₄ and 55 µM HCl acids into ultra-pure water (18.2 MΩ·cm). The actual porewater composition taken from piezometers on the CSA site and that obtained in the laboratory are compared in supplementary data. Table S1–1. All experiments reported in this manuscript were performed using this synthetic porewater composition.

### 2.2. Collection and characterization of Aptian sand samples

Sand samples were collected from capillary fringe zone ($S_{CFr}$), and saturated zone ($S_{Sat}$) in the CSA. While the saturated zone is the zone below water table, the capillary zone is the transition zone between the
unsaturated and the saturation zones where physical, chemical and biological reactions are active due to the concomitant presence of solid, water and gas phases (Zuo et al., 2023). To avoid any sample alteration, in particular for the sample S<sub>Sat</sub>, which was under anoxic and possibly reducing conditions, samples were sealed in airtight aluminum bags immediately after collection. Once in the laboratory, samples were stored and used in an anoxic (pO<sub>2</sub> < 10<sup>-6</sup> bar) N<sub>2</sub>-CO<sub>2</sub> glovebox, with the ratio of N<sub>2</sub> and CO<sub>2</sub> in the gas phase being tuned to match the equilibrium porewater composition (see above). Solid samples (S<sub>CFr</sub>, and S<sub>Sat</sub>) were dried, grinded to an equivalent diameter < 80 µm and homogenized in the glovebox.

The chemical composition of the solids was quantified using X-ray fluorescence (Al, Ca, Fe, K, Mg, Mn, Na, P, Si and Ti) and inductively coupled plasma atomic emission spectroscopy (trace elements: e.g. Sr, Ba, Zn, Zr, Cu, B, Sb), after total dissolution in an acidic medium. Crystalline phases and phyllosilicates (clay minerals, micas) were identified and quantified using X-ray diffraction (XRD), using a Bruker D8 Advance Da Vinci diffractometer equipped with a LynxEye XE-T detector and a Cu anode (λ = 1.5418 Å). In such anodes, the minor contribution from W (λ = 0.7106 Å) is unavoidable and led to the presence of low-intensity reflections that were not considered during data modeling. Data were collected in continuous scan mode and averaged every 0.03°2θ CuKα. Oriented mounts were prepared from the <2 µm size fraction and diffraction patterns were recorded under different saturation conditions with the following order: air-dried, NaCl, MgCl<sub>2</sub>, LiCl and Ethylene Glycol (EG) saturation, to identify phyllosiliates. For this experiment, data were collected in the 2.5–35°2θ CuKα interval, with a total collection time of 3.5 h. Powder samples collected over the 4–90°2θ CuKα interval, with a total collection time of 4 h, were used for quantification, using the Profex software, which is a graphical user interface to the BGMN software (Doebelin and Kleeberg, 2015).

Because Fe and Mn oxides could play a role in Sb geochemistry even at trace concentration that could not be quantified with XRD (see below), a sequential extraction procedure was applied to quantify amorphous and crystalline Fe and Mn phases, using a modified version of the Tessier protocol (Tessier et al., 1979; Guedron et al., 2009). In this method, 2 g solid samples were first contacted with 20 mL 1 M MgCl<sub>2</sub> to quantify exchangeable cations. After 1 h interaction, solid and liquid phases were separated by centrifugation. In the second step, carbonate minerals were removed from the solids by contacting them for 5 h with 20 mL of 1 M acetate (NaOAc) buffered to pH 5 with acetic acid (HOAc). Then amorphous Fe and Mn oxides were extracted through a 24 h interaction with 20 mL ascorbate. Finally, crystalline Fe and Mn oxides were dissolved from the centrifuged samples with a 5 h interaction with 20 mL of 0.04 M hydroxylamine hydrochloride (NH<sub>2</sub>OH·HCl) in 25 % (v/v) HOAc solution at 80 °C. Sequential extraction was repeated three times for each solid sample and the results are presented as mean and standard error values. Fe concentration in the extracts was quantified using UV-VIS at 510 nm wavelength using an Agilent Technologies Cary UV–Vis spectrophotometer. Both acetate buffer, hydroxylamine and 1 % Phenanthroline 1–10 were used in various amounts to quantify both total Fe and Fe(II) but both results were close, suggesting that aqueous Fe was Fe(II). Mn measurements were done using an ICP-MS.

Fourier-transform infrared spectroscopy (FTIR) analyses were con-
ducted to investigate the presence of amorphous silica. FTIR spectra of
the two samples were obtained using a Bruker Tensor 27 FTIR spec-
trometer having a middle-infrared (MIR) source and a KBr beam splitter.
Each sample was scanned 30 times in the 4000–400 cm<sup>-1</sup> spectral range
with a resolution of 4 cm<sup>-1</sup>. The pellet discs (diameter equal to 13 mm)
were prepared with a fine-grinded mixture of 0.5 mg sample and 150 mg
KBr. Before analyses, both the S<sub>CFr</sub> and S<sub>Sat</sub> sand samples were heated at
50 °C in the glove box for 1 d to minimize water adsorption.

A JSM-IT800 scanning electron microscope (SEM) equipped with an
energy-dispersive X-ray spectrometer (EDS/EDX) has been used to get
complementary information about the structure of the primary and
secondary minerals. The sand samples were embedded in epoxy resins

and left for polymerization overnight, in an oven under vacuum condi-
tion. They were then polished and carbon coated for SEM-EDX analysis
just before measurement. Total organic carbon was determined by py-
rolysis technique.

2.3. Sb leaching experiments

To quantify the “natural geochemical background” of Sb (i.e, the
aqueous Sb concentration spontaneously released from the solid at solid
to liquid ratio and contact times identical to those of sorption experi-
ments) and to allow quantitatively accounting for the contribution of
naturally occurring Sb in the calculation of Sb retention, leaching ex-
periments were performed in the glovebox (97 %N₂/3 % CO₂ condi-
tions). 12 aliquots of both S<sub>CFr</sub> and S<sub>Sat</sub> samples were equilibrated with
synthetic groundwater solution in 50 mL polypropylene copolymer
(PPCo) tubes, with 7 d or 30 d contact time, at different solid to liquid
ratios (5, 10, 20, 50, 100 and 200 g·L⁻¹) for each sample. All experi-
ments were duplicated. Additionally, two blank samples containing only
synthetic groundwater were prepared for each contact time. Samples
were agitated in a horizontal shaker. Once the target contact time was
reached, the suspensions were separated with centrifugation using a
Sigma 2–16 P. Vivaspin 20 PES 3 kDa tubes were used and centrifuga-
tion was performed for 30 min at 7800 rpm (6461 g). Sb concentration
in the filtered solution was analyzed with inductively coupled plasma-
mass spectrometry (ICP-MS).

2.4. Sb retention experiments

The Sb(III) and Sb(V) retention capacity of S<sub>CFr</sub> and S<sub>Sat</sub> samples were investigated in the glovebox to avoid any contact with atmospheric O<sub>2</sub> and to keep CO<sub>2</sub> concentration at equilibrium with the solution, itself at equilibrium with the solid. For each Sb oxidation number, two sorption isotherms differing by the contact time (7- or 30-d) were prepared using synthetic groundwater as the aqueous matrix. In each isotherm, consisting of 10 duplicated samples (50 mL PPCo centrifuge tubes kept under horizontal agitation), Sb concentration ranged between 4·10<sup>-8</sup> and 1.1·10<sup>-5</sup> M, solid to liquid ratio was 0.02 kg·L<sup>-1</sup>, and pH was 6.5 ± 0.2 pH. Due to the low Sb solubility, and although preliminary thermodynamic calculations were done to assess the Sb concentration below the solubility of any Sb phase, an additional sample was prepared, with Sb concentration equal to the maximum concentration used in the batch experiments but with no solid sample to check for potential homogenous nucleation and precipitation of a Sb solid phase. After the desired contact time was reached, the suspensions were centrifuged using a Sigma 2-16 P (6461 g for 30 min) and Vivaspin 20 PES 3 kDa tubes. After separation, the filtered solutions were analyzed for their pH in the glovebox and kept there until Sb concentration and speciation analyses. These latter were conducted using respectively ICP-MS and liquid chromatography-inductively coupled plasma-mass spectrometry (LC-ICP-MS) coupling.

Leaching experiments at various solid to liquid ratios were per-
formed to determine the concentration of Sb spontaneously released in
solution by the tubes used for the experiment and by the S<sub>CFr</sub> and S<sub>Sat</sub>
samples (section S2 in supplementary information). Briefly, Sb con-
centration in the presence of S<sub>CFr</sub> or S<sub>Sat</sub> sand samples that were placed in
PPCo tubes and conditions identical to those of the sorption experiment
(same solid to liquid ratio, same solution) was less than 2·10<sup>-8</sup> mol L<sup>-1</sup>,
and was taken into consideration in the quantification of Sb retention
(Fig. S2.1).

2.5. Quantification of Sb retention and data modeling

Concentration of adsorbed Sb, (C<sub>ads</sub> in mol·kg<sup>-1</sup>) and distribution coefficient (R<sub>d</sub> in L·kg<sup>-1</sup>) were calculated from the initial Sb concentration (C<sub>init</sub> in mol·L<sup>-1</sup>), the Sb concentration leached from sands and PPCo tubes (C<sub>leached</sub> in mol·L<sup>-1</sup>), the Sb concentration that remains in
the solution ($C_{eq}$ in mol·L⁻¹) and the solid concentration ($R_{SL}$ in kg·L⁻¹).

$$C_{ads} = \frac{C_{init} + C_{leached} - C_{eq}}{R_{SL}} \quad (1)$$

$$R_d = \frac{C_{init} + C_{leached} - C_{eq}}{C_{eq} \times R_{SL}} \quad (2)$$

The error bands were calculated as follows (Tournassat et al., 2013):

$$uC_{ads} = \frac{\sqrt{uC_{tot}^2 + uC_{eq}^2}}{R_{SL}}; \Delta C_{ads} = k \times uC_{ads} \quad (3)$$

$$uR_d = \sqrt{\left(\frac{uC_{tot}}{R_{SL} \cdot C_{eq}}\right)^2 + \left(\frac{C_{tot} \cdot uC_{eq}}{R_{SL} \cdot C_{eq}^2}\right)^2}; \Delta R_d = k \times uR_d \quad (4)$$

Where $\Delta$ values are the considered error bands, $k$ is the coverage factor (taken at a value of $k = 2$ for a confidence level of 95.5 %, assuming normally distributed values, Danzer, 2007), $uC_{tot}$ and $uC_{eq}$ are the uncertainties associated with the total concentration and the equilibrium concentration respectively (2 % uncertainty was considered in the measured values). Uncertainty on $R_{SL}$ was neglected.

Sb data modeling was performed with the PhreeqC software (Parkhurst and Appelo, 2013). Sb(V) sorption data, for which no evidence for heterogenous reaction could be found, (see below) was modeled with a parsimonious model inspired from previous works performed on metal sorption by clay minerals (Tournassat et al., 2013; Cherif et al., 2017; Orucoglu et al., 2018), which assumed sorption by Fe oxides and made use of previously published sorption parameters (Dzombak and Morel, 1990, Van Geen et al., 1994, Ali and Dzombak, 1996, Appelo et al., 2002, Dixit and Hering, 2006, Mathur and Dzombak, 2006). In the case of Sb(III), aqueous data provided evidence for heterogeneous redox reactions (see below). As far as is known, such reactions cannot be handled in a mechanistic way (i.e. modification of the crystal structure of the sorbent due to interaction with Sb) by thermodynamic modeling codes such as PhreeqC without significant numerical developments in which new parameters that could not be constrained from the literature or the present study would need to be introduced (e.g., new solubility constants and/or solid solution models, etc.). To circumvent this issue, a model was developed in which Sb(III) was oxidized by Fe(III) in solution (to mimic oxidation at the surface of the solid), using the most soluble Fe solid from the database as the aqueous Fe(III) source (Ferrihydrite) and with its quantity being adjusted to match experimental data (see the corresponding discussion in the following). In addition, Sb(III) and Sb(V) were both allowed to sorb to the solid with a constant $K_d$ value, with the Sb(V) $K_d$ value being guided by that experimentally measured in the present study, and the Sb (III) $K_d$ being adjusted. PhreeqC does not directly include the option of using $K_d$, because it uses more realistic approaches to model sorption (surface complexation, ion exchange). In the case of Sb complexation to the surface of a mineral surface, the reaction is:

$$\equiv X + Sb_{(eq)} \equiv XSb \quad (5)$$

Where $\equiv X$ is the surface complexation site at the surface of the mineral, $Sb_{(eq)}$ is aqueous Sb, and $\equiv XSb$ is Sb adsorbed to the surface site X. The corresponding equilibrium reaction, for a non-electrostatic model and assuming that the activity of Sb$_{(eq)}$ is 1, is:

$$K_{eq} = \frac{[\equiv XSb]}{([\equiv X] - [\equiv XSb]) [Sb_{(eq)}]} \quad (6)$$

Defining the number of sorption sites to a very large value so that $[\equiv XSb]$ is negligible as compared to $[\equiv X]$, it comes that:

$$K_{eq} = \frac{[\equiv XSb]}{([\equiv X]) [Sb_{(eq)}]} \quad (7)$$

Using the definition of the $R_d$ value (Eq. (3)), reminding that $[\equiv XSb]$

is the concentration of sorbed Sb, and omitting Sb speciation in solution, it comes that:

$$K_{eq} = \frac{R_d}{[\equiv X]} \times \frac{S}{L} \quad (8)$$

### 3. Results and discussion

#### 3.1. Mineralogical composition of S<sub>CFr</sub> and S<sub>Sat</sub> sand samples

The chemical composition of S<sub>CFr</sub> and S<sub>Sat</sub> samples was dominated by Si (97.0 and 94.5 wt% SiO₂, respectively – Table 1). The remaining major elements were Fe (1.19 and 1.74 wt% Fe₂O₃), Al (1.48 and 1.53 wt% Al₂O₃), and K (0.74 and 0.99 wt% K₂O). Other elements were present at concentrations (< 0.25 wt%). The higher Si content in S<sub>CFr</sub> as compared to S<sub>Sat</sub> samples together with a lower content of all other major elements suggests that the zone in the S<sub>CFr</sub> samples was subject to long-term leaching as a result of fluctuations of the water table. This was further supported by the analysis of trace element content: all elements but Se, U, and V were significantly depleted in S<sub>CFr</sub> samples. The higher abundance of Se, U, and V was compatible with a higher amount of Fe oxides in S<sub>CFr</sub> as compared to S<sub>Sat</sub> samples (despite a lower amount of total Fe) as these elements have high affinity for Fe oxides (Taylor and Giles, 1970; Liu et al., 2002; Um et al., 2008; Grangeon et al., 2023).

Consistent with the chemical analyses, the powder XRD patterns of both samples were dominated by quartz and minor amounts of feldspars and phyllosilicates (Fig. 1A). According to the Rietveld refinement analysis, the mineralogy of both samples was similar; about 95 wt% was quartz and the remaining part of the sands was made of microcline (~2 wt%), mica and (or) 10 Å clay mineral (~2 wt%), and kaolinite (≤1 wt%). Complementary FTIR measurements showed the presence, in the spectra of both samples, of two absorption bands at 796 and 778 cm⁻¹

<caption>**Table 1**</caption>

Chemical composition of the two sand samples. Major elements expressed as equivalent oxides. Note that the sum of elements is not strictly equal to 100 % due to the sum of uncertainties on each element analyzed. La, Nb, Y, Bi, Cd, Hg, Sb, Sn, As, Co and Be also analyzed but below the detection limit in both samples. Detection limits: 20 mg·kg⁻¹ for all elements but Hg (0.025 mg·kg⁻¹), Cd and Be (2 mg·kg⁻¹), As and Co (5 mg·kg⁻¹), and Bi and Sb (10 mg·kg⁻¹).

<table><thead><tr><th rowspan="2">Element (Oxides eq., wt%)</th><th colspan="2">Sample</th></tr><tr><th>S<sub>CFr</sub></th><th>S<sub>Sat</sub></th></tr></thead><tbody><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>1.48</td><td>1.53</td></tr><tr><td>CaO</td><td>0.10</td><td>&lt;0.10</td></tr><tr><td>Fe<sub>2</sub>O<sub>3</sub></td><td>1.19</td><td>1.74</td></tr><tr><td>K<sub>2</sub>O</td><td>0.74</td><td>0.99</td></tr><tr><td>MgO</td><td>&lt;0.20</td><td>0.23</td></tr><tr><td>MnO</td><td>&lt;0.02</td><td>&lt;0.02</td></tr><tr><td>Na<sub>2</sub>O</td><td>&lt;0.20</td><td>&lt;0.20</td></tr><tr><td>P<sub>2</sub>O<sub>5</sub></td><td>&lt;0.05</td><td>&lt;0.05</td></tr><tr><td>SiO<sub>2</sub></td><td>97.0</td><td>94.5</td></tr><tr><td>TiO<sub>2</sub></td><td>0.08</td><td>0.12</td></tr><tr><td colspan="3">Trace elements (mg·kg<sup>-1</sup>)</td></tr><tr><td>Sr</td><td>12</td><td>542</td></tr><tr><td>Ba</td><td>23</td><td>337</td></tr><tr><td>Zn</td><td>14</td><td>243</td></tr><tr><td>Zr</td><td>22</td><td>83</td></tr><tr><td>Cu</td><td>&lt;5</td><td>78</td></tr><tr><td>Cr</td><td>&lt;10</td><td>60</td></tr><tr><td>V</td><td>26</td><td>51</td></tr><tr><td>B</td><td>16</td><td>48</td></tr><tr><td>Li</td><td>&lt;10</td><td>42</td></tr><tr><td>Ce</td><td>14</td><td>36</td></tr><tr><td>Ni</td><td>10</td><td>32</td></tr><tr><td>Pb</td><td>&lt;10</td><td>22</td></tr><tr><td>W</td><td>&lt;10</td><td>16</td></tr><tr><td>Mo</td><td>&lt;5</td><td>10</td></tr><tr><td>CN</td><td>0.29</td><td>0.91</td></tr><tr><td>U</td><td>0.8</td><td>0.6</td></tr><tr><td>Se</td><td>0.7</td><td>&lt;0.5</td></tr></tbody></table>
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** (A): X-ray powder diffraction patterns of the sand samples ($S_{CFr}$ and $S_{Sat}$) and Rietveld refinements of diffraction data ($S_{CFr}$-R and $S_{Sat}$-R). B and C: Detail of the low-angle part, after equilibration at 11 % relative humidity using LiCl (solid line) and after ethylene-glycol saturation (dashed line). Q: Quartz, K: Kaolinite; G: Glauconite; M: Microcline; W: quartz reflection at tungsten wavelength.</figcaption></figure>

typical for quartz (Fig. S3.1, Table S3.1). The intensity ratio of the absorption bands at 796 and 1079 cm⁻¹ (0.40 and 0.38 in $S_{CFr}$ and $S_{Sat}$ respectively) in the baseline corrected spectra allowed determining that the quartz had high degree of crystallinity (Farcas and Touzé, 2001; Saikia et al., 2008; Ellerbrock et al., 2024). Coherent with previous studies performed on Aptian sands (Le Cointe, 2011), rare minerals were observed with SEM, namely ilmenite, rutile, barite, arsenopyrite, monazite, and sulfurs (supplementary data Figs. S3.2, S3–3, and S3.4). However, their presence in trace amounts together with the fact that they are often found included in a mineral matrix makes that their contribution to the overall geochemistry of the system was certainly negligible. Total organic carbon content was below the detection limit of 0.1 wt%.

To gain further insights into the nature of the phyllosilicates, XRD patterns of oriented samples were recorded under low relative humidity conditions (11 %, controlled using saturated LiCl salt), after saturation with ethylene glycol (Fig. 1B, Fig. 1C), and at intermediate relative humidity levels (supplementary data Fig. S3.5). While reflections attributed to kaolinite (K), microcline (M) and quartz (Q) remained unchanged (Fig. 1B,C), the reflection at 10 Å (8.84°2θ CuKα) was modified: its intensity decreased, and it shifted to 9.97 Å (8.87°2θ CuKα). In addition, in the XRD pattern of $S_{CFr}$, a new reflection appeared at 17 Å (5.17°2θ CuKα). Such behavior is typical for the evolution of the 001 reflection of glauconite (López-Quirós et al., 2020). The potential presence of illite, which could contribute to the reflection at 10 Å, is very unlikely since the weak intensity of the 002 reflection at 5 Å (17.7°2θ CuKα), as observed here (Fig. 1), is a robust indicator to distinguish glauconite from illite (Moore and Reynolds, 1997). Furthermore, a high-resolution specific data acquisition in the 50–70°2θ CuKα could evidence the presence of a 06 reflection at 1.52 Å in the powder patterns, hence allowing the unambiguous identification of glauconite (Singh et al., 2023). In the XRD pattern of $S_{Sat}$, the reflection at 5.17°2θ CuKα was of extremely low intensity and could not be used alone as an indicator for the presence of glauconite, but since the 001 to 002 ratio and

the position of the 06 were identical to those of $S_{CFr}$, it could be concluded that $S_{Sat}$ also contained glauconite.

Glauconite is a mixed-layer mineral, built of Fe-rich mica-like layers and smectite-like layers, the latter providing glauconite with the ability to shrink-swell. The total Fe content of glauconite is generally reported to range between 10 and 30 wt% $Fe_2O_3$, while the $Fe^{3+}/Fe^{2+}$ ratio ranges from 1 to 90 (Thompson and Hower, 1975). In the present case, since the reflection at 17 Å (5.17°2θ CuKα) was broad and of low intensity, it could be speculated that glauconite present in $S_{CFr}$ and $S_{Sat}$ samples had low content of smectite layers and was hence a highly evolved glauconite, close to the structure of a true mica. This was further investigated using SEM, with which it could be observed that glauconite formed typical pellet structures (Fig. 2A-B). Analyses with EDX in the SEM could evidence that the chemical composition of glauconite was little variable (n = 7; Table S3.2) (Baldermann et al., 2012) and that the structural formula was:

$K_{0.75(3)}Ca_{0.02(3)}(Si_{0.70(4)}Al_{0.30(4)})[Mg_{0.30(1)}Al_{0.49(5)}Fe_{0.19(2)}Fe_{1.02(5)}]O_{10}(OH)_2$, where species in brackets form the tetrahedral sheet, those in square brackets the octahedral sheet, those on the left of the structural formula the interlayer, and where the numbers under brackets in site occupancy is the uncertainty on the last digit.

Furthermore, the high $K^+$ occupancy of 0.75(3) showed, coherent with XRD, that glauconite was highly evolved and close to an ideal mica (Thompson and Hower, 1975).

SEM was also used to investigate the nature of the feldspar detected by XRD. From chemical analyses (n = 7; Fig. S3.6 and Table S3.3), it could be computed that its chemical formula was $Na_{0.08(4)}K_{0.91(2)}Al_{1.03(1)}Si_{2.98(1)}O_8$, and hence that the feldspar in $S_{CFr}$ and $S_{Sat}$ samples was very close to an ideal microcline.

## 3.2. Fe speciation in $S_{CFr}$ and $S_{Sat}$

Since Fe-rich phases are known to be (redox-)reactive towards Sb, Fe speciation in $S_{CFr}$ and $S_{Sat}$ was studied specifically. As discussed above,
<figure><img src="image_5.png" /><figcaption>Fig. 2. SEM images of glauconite showing the typical pellet structure A: in $S_{Sat}$, B: in $S_{CFr}$. Same scale in both images.</figcaption></figure>

total Fe concentration, expressed as $Fe_2O_3$, was 1.48 wt% and 1.53 wt% in $S_{CFr}$ and $S_{Sat}$ samples, respectively. Fe(II) concentration, expressed as FeO, was 0.07 wt% in $S_{CFr}$ while it was below the detection limit of 0.05 wt% in $S_{Sat}$. Given that total Fe concentration was similar in both samples and that mineralogy was also similar, it is speculated that FeO in $S_{Sat}$ was probably close to 0.05 wt%. From $Fe_2O_3$ and FeO concentration in $S_{CFr}$, it could be calculated that the Fe(II)/Fe(III) ratio in this sample was 10% (atomic ratio), close to the ratio of 18% deduced from the structural formula of glauconite, coherent with this mineral being the main Fe-bearing phase. Such conclusion was further supported by the fact that, using glauconite structural formula and assuming its abundance to be 1–3 wt% (according to XRD quantification), glauconite contributed to 0.45–1.37 wt% of samples $Fe_2O_3$ (absolute value). This value was commensurable with total sample $Fe_2O_3$ (1.19–1.74 wt% Table 1), confirming that glauconite was the main Fe-bearing mineral. However, because calculated $Fe_2O_3$ content was slightly lower than total $Fe_2O_3$ content and because the Fe(II)/Fe(III) ratio in glauconite was higher than in the bulk sample, the presence of Fe(III) oxides, undetected by XRD, was likely. To test this hypothesis, sequential extractions were performed (.

Table 2). 82 and 91 wt% of Fe were not extracted in $S_{CFr}$ and $S_{Sat}$ samples respectively. This residual Fe probably corresponded to glauconite and other accessory minerals listed above. No exchangeable Fe could be detected, in agreement with the absence of appreciable amounts of swelling clays. Between 0.5 and 1.1% of total Fe was extracted by the acetate/acetic acid attack and, since no carbonate minerals could be detected by XRD, this could be related to desorption of Fe adsorbed at particle surfaces. The ascorbate and hydroxylamine extraction steps, which targeted respectively amorphous and crystalline oxides, led to Fe extraction yields of 3.8% and 13.5% for $S_{CFr}$ sand and 1.9% and 6.1% for $S_{Sat}$. Hence, it could be concluded that both $S_{CFr}$ and $S_{Sat}$ samples contained low yet significant concentrations of amorphous and crystalline Fe oxides, with a higher concentration in $S_{CFr}$ (despite a lower total Fe content), in agreement with the likely presence of $O_2$ in this soil horizon, at least seasonally.

<caption>**Table 2**</caption>

Total Fe (wt%) extracted from sand samples at different steps of the sequential extraction.

<table><thead><tr><th>Extraction Yield (wt%) of Fe</th><th colspan="2">Sample</th></tr><tr><th>Sequential extraction steps</th><th>S<sub>CFr</sub></th><th>S<sub>Sat</sub></th></tr></thead><tbody><tr><td>MgCl<sub>2</sub></td><td>nd</td><td>nd</td></tr><tr><td>Acetate/Acetic Acid</td><td>0.45</td><td>1.07</td></tr><tr><td>Ascorbate</td><td>3.83</td><td>1.93</td></tr><tr><td>Hydroxyl Amine</td><td>13.46</td><td>6.08</td></tr></tbody></table>

Note: nd stands for “not detected”.

### 3.3. Groundwater composition and synthetic porewater preparation

The geochemical composition of the groundwater samples taken from two different piezometers representative of porewater which could be under the influence of the facilities (groundwater S1) or non-affected by the facilities (groundwater S2) are presented in Table 3. Both water samples were slightly acidic (pH ~6) with low alkalinity (0.69 and 0.95 mM) and low ionic strength, with concentrations of the major elements below 1 mM. The main cations were Ca (0.32–0.51 mM) and Na (0.28–0.36 mM). Concerning anions, $Cl$, $NO_3^-$ and $SO_4^{2-}$ were dominant (0.08–0.23 mM). Si concentration was lower (0.09–0.35 mM). Amongst all trace elements analyzed, only B, Li and Sr had concentrations higher than 1 µM (respectively 1.34–4.64 µM, 0.82–5.43 µM, and 1.11–1.83 µM). In both solutions, the concentration of Be, Cd, Cr, Mo, Sb and U was below 0.01 µM and that of Pb, Ti, Sn, Bi, Zr, Cs and Hg was below the detection limit of the instruments used.

To ensure experimental reproducibility (systematic use of the same water composition for all sorption experiments), it was chosen to

<caption>**Table 3**</caption>

Geochemical characterization of Aptian sands groundwater (piezometers S1 and S2).

<table><thead><tr><th rowspan="2">Parameter</th><th colspan="2">Sample</th></tr><tr><th>S1</th><th>S2</th></tr></thead><tbody><tr><td>pH</td><td>6.3</td><td>5.9</td></tr><tr><td>Major elements (mM)</td><td></td><td></td></tr><tr><td>Ca</td><td>0.32</td><td>0.54</td></tr><tr><td>K</td><td>0.13</td><td>0.07</td></tr><tr><td>Na</td><td>0.28</td><td>0.36</td></tr><tr><td>Mg</td><td>0.05</td><td>0.13</td></tr><tr><td>Fe</td><td>0.0004</td><td>0.0274</td></tr><tr><td>Si</td><td>0.09</td><td>0.35</td></tr><tr><td>F</td><td>0.02</td><td>0.01</td></tr><tr><td>Cl</td><td>0.1</td><td>0.19</td></tr><tr><td>NO<sub>3</sub></td><td>0.12</td><td>0.14</td></tr><tr><td>SO<sub>4</sub></td><td>0.09</td><td>0.23</td></tr><tr><td>PO<sub>4</sub></td><td>0.004</td><td>0.004</td></tr><tr><td>Alkalinity</td><td>0.69</td><td>0.95</td></tr><tr><td>Trace elements (µM)</td><td></td><td></td></tr><tr><td>B</td><td>1.34</td><td>4.64</td></tr><tr><td>Sr</td><td>1.11</td><td>1.83</td></tr><tr><td>Li</td><td>0.82</td><td>5.43</td></tr><tr><td>Br</td><td>0.17</td><td>0.56</td></tr><tr><td>Zn</td><td>0.05</td><td>0.35</td></tr><tr><td>Ni</td><td>0.05</td><td>0.09</td></tr><tr><td>Ba</td><td>0.04</td><td>0.03</td></tr><tr><td>Se</td><td>0.03</td><td>0.01</td></tr><tr><td>I</td><td>0.03</td><td>0.04</td></tr><tr><td>As</td><td>0.02</td><td>0.09</td></tr><tr><td>Co</td><td>0.02</td><td>0.06</td></tr><tr><td>Mn</td><td>0.02</td><td>1.07</td></tr><tr><td>Cu</td><td>0.01</td><td>0.01</td></tr></tbody></table>
produce synthetic pore water having a composition within the range of variability of natural groundwater composition (Table S1.1).

### 3.4. Sb(V) and Sb(III) retention by Aptian sands

Sb(V) $R_d$, after 7 d and 30 d interaction with $S_{CFr}$ and $S_{Sat}$, were almost constant as a function of aqueous Sb concentration, exception made of data at the lowest Sb concentration, especially after 7 d contact time (Fig. 3A and C). This was attributed to a pH effect since these samples had significantly lower pH values (Fig. S4.1). Omitting these data points, in 7 d of contact time experiments, the $R_d$ values varied between $10 \pm 3.4 \text{ L·kg}^{-1}$ and $13 \pm 4 \text{ L·kg}^{-1}$ for $S_{Sat}$ sands and between $11 \pm 3 \text{ L·kg}^{-1}$ and $19 \pm 4 \text{ L·kg}^{-1}$ for $S_{CFr}$ sands, respectively. After 30 d contact time, $R_d$ approximately doubled to reach $12 \pm 3$ to $25 \pm 4 \text{ L·kg}^{-1}$ and $24 \pm 2$ to $36 \pm 2 \text{ L·kg}^{-1}$ for $S_{Sat}$ and $S_{CFr}$ samples, respectively.

In all experiments in which Sb(V) was introduced, Sb(V) was the only Sb oxidation number detected in the aqueous phase (Fig. 3B and D). Hence, it was concluded that Sb(V) sorption by $S_{CFr}$ and $S_{Sat}$ sands proceeded by adsorption and did not involve any heterogeneous (surface-induced) redox reaction.

This hypothesis of Sb(V) retention being due to adsorption allowed building safely a thermodynamic surface complexation model. Because Fe oxides were present in the sample and because they are reactive towards Sb, the choice was made to build a parsimonious model (Table S4.1), assuming only sorption by Fe oxides. This allowed reproducing quantitatively sorption data, and hence supported the idea of Sb (V) sorption by $S_{CFr}$ and $S_{Sat}$ sands being controlled by Fe oxides, despite these latter being present at trace concentration (Fig. 4). Furthermore, geochemical calculations indicated that $Sb(OH)_6$ was the dominant species under the experimental conditions, hence limiting the

<figure><img src="image_6.png" /><figcaption>Fig. 4. Modeling of Sb(V) sorption on $S_{CFr}$ and $S_{Sat}$ sands after 30 d. Symbols: data points; lines: model.</figcaption></figure>

interaction between Sb(V) and negatively charged clay layers (Frasca et al., 2014, Miller et al., 2015, Tournassat et al., 2015). Consequently, glauconite probably has a minor contribution to Sb(V) retention.

The Sb(III) $R_d$ for $S_{Sat}$ were higher than those obtained with $S_{CFr}$ over the whole range of aqueous Sb(III) aqueous concentration tested and no

<figure><img src="image_7.png" /><figcaption>Fig. 3. Sb(V) retention capacities of $S_{Sat}$ and $S_{CFr}$ sands (A,C) and Sb speciation (B,D) after 7- and 30 d contact time: In (A) Filled circle:Sb(V)- $S_{Sat}$-30d, circle:Sb(V)- $S_{Sat}$-7d, in (B) full circle:Sb(V)- $S_{CFr}$-30d, open circle:Sb(V)- $S_{CFr}$-7d; in (C and D) full circle:Sb(V)-30 d, open circle:Sb(V)-7d, full square:Sb(III)-30 d, open square:Sb(III)- 7d.</figcaption></figure>
clear kinetic effect (difference between 7 d and 30 d sorption values) could be observed (Fig. 5). More importantly, retention capacity of the sands as a function of Sb aqueous concentration (“[Sb]eq”) had a “bell curve” shape. For S<sub>Sat</sub>, after 7 d of interaction, R<sub>d</sub> first increased from 68 ± 7 L·kg<sup>-1</sup> ([Sb]eq = 2.2·10<sup>-8</sup> M) to 547 ± 33 L·kg<sup>-1</sup> for S<sub>Sat</sub> ([Sb]eq = 1.4·10<sup>-7</sup> M) and then decreased down to 323 ± 21 L·kg<sup>-1</sup> ([Sb]eq = 1.1·10<sup>-6</sup> M). For S<sub>CFr</sub>, R<sub>d</sub> increased from 15 ± 4 ([Sb]eq = 3.4·10<sup>-8</sup> M) to 174 ± 13 L·kg<sup>-1</sup> ([Sb]eq = 7.7·10<sup>-7</sup> M) and then decreased down to 137 ± 10 L·kg<sup>-1</sup> ([Sb]eq = 2.3·10<sup>-6</sup> M).

While the second part of the curve (R<sub>d</sub> decrease with increasing [Sb (III)]<sub>eq</sub> could be explained by sorption site saturation (Tournassat et al., 2013), the first part (R<sub>d</sub> increase with increasing [Sb(III)]<sub>eq</sub>) suggested a more complex phenomenon. The evolution of Sb speciation in solution as a function of [Sb(III)]<sub>eq</sub> provided clues concerning the retention mechanism. Despite Sb(III) being introduced, only Sb(V) was detected up to [Sb]eq = 1.3·10<sup>-7</sup> mol L<sup>-1</sup> in S<sub>CFr</sub> and [Sb]eq = 4.3·10<sup>-8</sup> mol L<sup>-1</sup> in S<sub>Sat</sub> after 30 d interaction, subsequently its proportion linearly decreased with [Sb]eq to the benefit of Sb(III). Hence, at [Sb]eq < 5·10<sup>-8</sup> mol L<sup>-1</sup>, all Sb(III) added to the system was probably oxidized to Sb(V), and it was certainly sorption of Sb(V) that contributed to the observed “Sb(III)” retention. Such a hypothesis was supported by the close R<sub>d</sub> values of “Sb(III)” at the lowest Sb concentration and those observed in the Sb(V) retention experiments (see above). At [Sb]eq > 5·10<sup>-8</sup> mol L<sup>-1</sup>, both Sb(III) and Sb(V) coexisted, and it could be speculated that the increase of R<sub>d</sub>, which was concomitant to the increase in Sb(III)/Sb(V), was due to Sb(III) having a higher affinity for the solid phase than Sb(V). This possible reaction scheme was tested using a simple and empirical geochemical model that aimed at testing if the evolution of both Sb retention and Sb(III)/Sb(V) as a function of [Sb]eq could be reproduced with a combination of Sb(III) oxidation and Sb(III)

and Sb(V) sorption (Fig. 6). For this purpose, the Sb(V) R<sub>d</sub> was chosen to be equal to 10 L kg<sup>-1</sup> as observed in the Sb(V) sorption experiment (Fig. 3), Sb(III) R<sub>d</sub> was adjusted to match mean R<sub>d</sub> observed at high [Sb]eq (Fig. 5), and the quantity of reactive Fe(III) responsible for Sb(III) oxidation was adjusted to match both the increase in R<sub>d</sub> (which manifests itself by an inflexion point in the C<sub>ads</sub> vs C<sub>eq</sub> representation) and the point at which Sb(V) was detected. Despite its simplicity, this model satisfyingly reproduced the quantity of Sb retained by the solid phase and Sb(III)/Sb(V) as a function of [Sb]eq (Fig. 6).

The Sb(III) R<sub>d</sub> and Fe content of the mineral participating in Sb(III) oxidation to Sb(V), expressed as Fe<sub>2</sub>O<sub>3</sub>, were respectively 400 L kg<sup>-1</sup> and 1 µg g<sup>-1</sup> for S<sub>CFr</sub> sand and 2000 L kg<sup>-1</sup> and 0.4 µg g<sup>-1</sup> for S<sub>Sat</sub> sand. It must be emphasized here that R<sub>d</sub> values are those of Sb(III) sorption by the redox-active Fe-bearing mineral(s), not by the bulk sample, and that the possible nature of the Fe-bearing phase(s) is discussed in the following. Successful data modeling required considering the presence of a minor amount (4–10 %) of Sb(V) in the Sb(III) solution added to the system, possibly due to salt oxidation before introduction in the glove box. The lower amount of Fe(III) available for oxidation in S<sub>Sat</sub> as compared to S<sub>CFr</sub> was, again, coherent with the presence, at least seasonally, of O<sub>2</sub> in S<sub>CFr</sub> and hence of possibly more oxidized Fe at mineral surfaces. The low amount of Fe involved in reduction as compared to the content of Fe-bearing phases (amorphous and crystalline Fe oxides or glauconite) in both samples suggests that the surface of Fe-bearing phase(s) involved in Sb(III) oxidation were passivated after reaction, possibly because of Sb(V) remaining sorbed or incorporated, or because Fe(II) produced during Sb(III) oxidation remained at the surface of the Fe-bearing phase(s). The low Sb concentration on the solid phase after sorption prevented the use of spectroscopic methods that are traditionally used to determine the nature of the sorbing phase and (or)

<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Sb(III) retention capacities of S<sub>Sat</sub> and S<sub>CFr</sub> sands (A,C) and Sb speciation (B,D) after 7- and 30 d contact time: In (A) full square:Sb(III)- S<sub>Sat</sub>-30d, open square: Sb(III)- S<sub>Sat</sub>-7d, in (B) full square:Sb(III)- S<sub>CFr</sub>-30d, open square:Sb(III)- S<sub>CFr</sub>-7d; in (C and D) full circle:Sb(V)-30 d, open circle:Sb(V)-7d, full square:Sb(III)-30 d, open square:Sb(III)- 7d.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Modeling of Sb(III) sorption on S<sub>CFr</sub> and S<sub>Sat</sub> sand samples after 30 d of contact time. Symbols: data points; lines: model. (A): Sorbed Sb as a function of equilibrium Sb. Full circle: S<sub>Sat</sub>, open circle: S<sub>CFr</sub>. (B) and (C): relative abundances of Sb(III) and Sb(V) as a function of equilibrium Sb(Sb<sub>tot</sub> stands for “total aqueous Sb”, i.e., Sb(<b>III</b>)<sub>eq</sub> + Sb(V)<sub>eq</sub>. Full symbols: Sb(V). Open symbols: Sb(III). Solid and dotted lines are modeled Sb(V) and Sb(III), respectively.</figcaption></figure>

the sorption site, such as X-ray absorption spectrometry for example.

**4. Implications for Sb geochemistry**

This work contributed to a better understanding of Sb reactivity and hence mobility in glauconite-rich formations. It could be observed that sorption kinetics are negligible at the time scale investigated here (week to month) and hence point out fast mechanisms of interaction between Sb and mineral surfaces. Despite the samples being essentially composed of quartz, this latter had no measurable reactivity towards Sb in the experimental conditions tested. In contrast, Fe-bearing phases, despite being present at low concentrations, certainly control Sb sorption. Sb(V) retention could be quantitatively modeled assuming the sole contribution of Fe oxides and using a parsimonious surface complexation model that had been previously applied successfully, which highlights that this model developed by Dzombak and Morel (1990) and Mathur and Dzombak (2006) in the specific context of Sb sorption by goethite and ferrihydrite apply to a large range of models and natural sand samples. The mechanism of Sb(III) retention was more complex. Coherent with previous studies (Belzile et al., 2001; Ilgen et al., 2012; Fan et al., 2014), it could be observed that Sb(III) is oxidized to Sb(V), possibly at the surface of Fe oxides through a heterogeneous redox reaction. However, the potential role of glauconite in Sb(III) oxidation could not be ruled out for two reasons. First, while no literature studies pertaining to the redox properties of glauconite towards inorganic species could be found, oxidation of organic species was evidenced (Van Delft et al., 1990), and it can reasonably be assumed that glauconite is probably also redox-active towards inorganic species. Second, from the amount of Sb(III) oxidized to Sb(V), the amount of reactive Fe(III) was 0.5 10<sup>-8</sup>.1.25 10<sup>-7</sup> mol L<sup>-1</sup>. Assuming that the iron oxide particles present in the samples had a spherical shape, 1 µm diameter, a goethite structure, and an abundance of 0.15 wt% (i.e., 10 % of the about 1.5 wt% Fe<sub>2</sub>O<sub>3</sub>, as suggested by chemical analyses discussed above), the quantity of Fe(III) exposed on the external surfaces of iron oxides was ~9 10<sup>-8</sup> mol L<sup>-1</sup>, commensurable with the amount of reactive Fe(III) deduced from the quantity of Sb(III) oxidized. The sole consideration of external surfaces and not of the total Fe(III) content was based on previous works which showed that the surface of Mn oxides is passivated by precipitates after Sb(III) oxidation (Sun et al., 2018). Such calculation could be interpreted as evidence for Fe oxides being responsible for Sb(III) oxidation. However, performing the same calculation for glauconite, assuming that it represented 2 wt% of the samples, had the structural formula described above, had a typical size of 10 µm diameter in the layer plane and 1 µm perpendicular, and that redox reaction occurs on the edges of phyllosilicates (Ilgen and Trainor, 2012), the amount of Fe(III) that

would be available for redox reaction is ~2.8 10<sup>-7</sup> mol L<sup>-1</sup>, i.e. very close to both the amount of Fe(III) calculated for goethite and of the amount of redox-active Fe(III) in the samples. Hence, mass-balance calculations cannot help distinguishing if the mineral responsible for Sb(III) oxidation to Sb(V) is glauconite, a Fe oxide, or a combination of both. Further work is required to clarify this issue and more generally to obtain a mechanistic model for glauconite reactivity. By analogy with montmorillonite (Bradbury and Baeyens, 2005; Tournassat et al., 2013; Orucoglu et al., 2018), such a model would be important to quantify, and hence allow modeling and predicting, the fate of elements in the environment, with fundamental and applied purposes, for example predicting the geochemical fate of elements in the environment (Hao et al., 1987), increasing the quality of agricultural soils (Dasi et al., 2024), or in the context of (radioactive) waste storage (Bruneel et al., 2020, 2021). While some detailed experimental and modeling work has been done on the sorption of non-redox elements (e.g. Smith et al. (1996)), a unified sorption model is lacking. In addition, several clues point out to a potential capacity of glauconite to oxidize redox-sensitive elements, namely the present work on Sb(III), but also evidence for glauconite redox properties with regards to organics (Van Delft et al., 1990) and for illite (with which glauconite is structurally close) redox reactivity towards redox-sensitive elements (e.g. As(III) or Hg(II)) (Manning and Goldberg, 1997; Debure et al., 2018, 2020). Changes in the redox state of elements have large implications in predicting element mobility, since the oxidation number of a given chemical element can have a large influence on its solubility (e.g. U, As, Cr) (Mandiwana et al., 2007, Sharp et al., 2011, Debure et al., 2018, Chevrier et al., 2025) and must hence be adequately taken into account when estimating the geochemical fate of a given element. However, determining the redox reactivity of solids is generally complicated. In the present case, this problem was exacerbated by the low Sb solubility that resulted in Sb-sorbed solids (Aptian sands) having low Sb concentration. This virtually precluded the use of spectroscopic methods to probe the Sb sorbing phases and in particular those in which all Sb had been subject to heterogeneous redox reaction (i.e., low Sb(III) initial concentration), and hence precluded a quantitative mechanistic understanding of the heterogeneous redox reaction taking place (e.g. determination of the crystallographic site on which this reaction took place). One way of circumventing this problem would be to use pure minerals (i.e. pure ferrihydrite, pure glauconite) to study Sb(III) heterogeneous redox reaction, assess the contribution of each phase and build models that could be transposed to natural solids.
### CRediT authorship contribution statement

**Esra Orucoglu:** Writing – review & editing, Writing – original draft, Validation, Project administration, Methodology, Investigation, Formal analysis, Conceptualization.
**Sylvain Grangeon:** Writing – review & editing, Writing – original draft, Validation, Supervision, Methodology, Investigation, Formal analysis, Conceptualization.
**Nicolas Marty:** Writing – review & editing, Validation, Investigation, Formal analysis, Conceptualization.
**Nicolas Maubec:** Writing – review & editing, Methodology, Investigation, Formal analysis, Conceptualization.
**Myriam Agnel:** Writing – review & editing, Validation, Supervision, Resources, Project administration, Funding acquisition, Conceptualization.
**Benoît Madé:** Writing – review & editing, Validation, Supervision, Resources, Project administration, Funding acquisition, Conceptualization.
**Mathieu Debure:** Writing – review & editing, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Conceptualization.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

This work was mainly supported by the French National Agency for Radioactive Waste Management (Andra) as part of the Andra-BRGM scientific partnership. We gratefully acknowledge G. Wille for performing SEM and EDX studies, N. Lafaurie for conducting LC-ICP-MS measurements and S. Touzelet for his technical assistance. We also thank the three anonymous reviewers for their comments and suggestions which improved the quality of this study.

### Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.clay.2025.107974.

### Data availability

I have shared the link to additional information about our study at the Attach file step

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