# Ferrous iron oxidation in the presence of antimonate at neutral pH: Mutual effects on iron mineral products and antimony sequestration

L. Wegner, E.D. Burton, C. Mccammon, A.C. Scheinost, B. Planer-Friedrich, S. Peiffer, K. Hockmann

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L. Wegner, E.D. Burton, C. Mccammon, A.C. Scheinost, B. Planer-Friedrich, et al. Ferrous iron oxidation in the presence of antimonate at neutral pH: Mutual effects on iron mineral products and antimony sequestration. Journal of Hazardous Materials, 2025, 488, pp.137380. 10.1016/j.jhazmat.2025.137380. hal-05099721

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# Ferrous iron oxidation in the presence of antimonate at neutral pH: Mutual effects on iron mineral products and antimony sequestration

L. Wegner<sup>a,*</sup>, E.D. Burton<sup>b</sup>, C. McCammon<sup>c</sup>, A.C. Scheinost<sup>d,e</sup>, B. Planer-Friedrich<sup>f,1</sup>, S. Peiffer<sup>a</sup>, K. Hockmann<sup>g</sup>

<figure><img src="image_3.png" /></figure>

## HIGHLIGHTS

* Neutral pH Fe(II) oxidation at low molar Sb(V):Fe ratios produced lepidocrocite.

* A molar Sb(V):Fe ratio greater than 0.1 induced feroxyhyte formation.

* Sb(V) was incorporated into Fe oxides via Sb(V)-for-Fe(III) substitution.

* Sb(V) incorporation reached up to 16.2 mol% in the Fe(III) precipitates.

## GRAPHICAL ABSTRACT

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

## ARTICLE INFO

### Keywords:

EXAFS

Feroxyhyte

Lepidocrocite

Mössbauer spectroscopy

Sb

## ABSTRACT

Antimony is a priority pollutant, whose mobility in redox-dynamic environments may be controlled by interactions with Fe(III) hydroxide minerals that form via Fe(II) oxidation. In this study, we examined the Fe(III) hydroxide precipitates and associated mechanisms of Sb(V) sequestration that result from Fe(II) oxidation in the presence of Sb(V) under neutral pH conditions. To achieve this aim, oxidation experiments were carried out in O₂-saturated, Fe(II)-bearing solutions (buffered at pH 7) over a range of environmentally relevant Sb(V) concentrations (equivalent to Sb(V):Fe(II) molar ratios of 0, 0.01, 0.04, 0.1 and 0.25). Under these experimental conditions, Fe(II) oxidation occurred rapidly (within 20 minutes) causing associated removal of Sb(V) from solution via coprecipitation with the resulting Fe(III) hydroxides. At low Sb(V):Fe(II) ratios (< 0.1), lepidocrocite was the only Fe(III) mineral product of Fe(II) oxidation, whereas higher ratios resulted in formation of feroxyhyte. Both lepidocrocite and feroxyhyte retained Sb(V) within their crystal structure via Sb(V)-for-Fe(III) substitution. This mechanism of Sb(V) retention largely protected the solid-phase Sb(V) from release

<footnote><sup>a</sup> University of Bayreuth, Bayreuth Center of Ecology and Environmental Research (BayCEER), Department of Hydrology, Bayreuth 95440, Germany</footnote>

<footnote><sup>b</sup> Southern Cross University, Faculty of Science & Engineering, Lismore, NSW 2480, Australia</footnote>

<footnote><sup>c</sup> University of Bayreuth, Bayerisches Geoinstitut, Bayreuth 95440, Germany</footnote>

<footnote><sup>d</sup> The Rossendorf Beamline, European Synchrotron Radiation Facility, Grenoble 38043, France</footnote>

<footnote><sup>e</sup> Helmholtz Zentrum Dresden Rossendorf, Institute for Resource Ecology, Dresden 01328, Germany</footnote>

<footnote><sup>f</sup> University of Bayreuth, Bayreuth Center of Ecology and Environmental Research (BayCEER), Environmental Geochemistry, Bayreuth 95440, Germany</footnote>

<footnote><sup>g</sup> University of Freiburg, Institute of Earth and Environmental Sciences, Applied Geochemistry, Freiburg 79104, Germany</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: Laura.Wegner@uni-bayreuth.de (L. Wegner).</footnote>

<footnote n="1">¹ We acknowledge with respect the involvement of Britta Planer-Friedrich, who sadly passed away during the submission of this manuscript.</footnote>
processes. Collectively, these results highlight the coupled role that interactions between Sb geochemistry and the Earth's near-surface Fe cycle can play in controlling both Fe(III) hydroxide mineralogy and Sb mobility.

# 1. Introduction

Antimony (Sb) is a suspected carcinogen that is listed as a priority pollutant by the US Environmental Protection Agency and the United Nations [55,56]. Despite its potentially hazardous nature, Sb is used in numerous products in our daily lives, including plastics, batteries, and glassware [16]. Antimony also plays an important role in a wide range of emerging green technologies, such as next-generation batteries for electric transport [35,4] or catalytic converters for the production of fuels and other value-added products from greenhouse gasses [26,36, 53].

Due to rapid growth in industrial use of Sb, the global mine production of Sb has approximately tripled in the past 30 years [54]. This has resulted in a steadily growing legacy of environmental Sb contamination, which is often associated with an uncontrolled release of Sb into aquatic ecosystems. For example, Sb concentrations in surface and groundwater as high as 10's to 100's of mg L⁻¹ have been reported in the vicinity of mining and ore-processing sites in Europe [19], North America [46], Asia [17], and Australia [2]. In contrast, natural Sb background concentrations typically do not exceed a few µg L⁻¹ [14].

In natural waters, Sb occurs mainly in the +III and +V redox states, with Sb(V), speciating mainly as Sb(OH)₆ in the absence of complexing agents, being predominant under oxic to slightly reducing conditions [8, 15,20,38,41]. The environmental behavior of Sb(V) is strongly influenced by interactions with Fe(III) hydroxides (a term used here to collectively refer to Fe(III) oxides, hydroxides, and oxyhydroxides) [33, 38,50,6]. Oxidation of soluble aqueous Fe(II) to poorly soluble Fe(III) is a well-known pathway for the formation of Fe(III) hydroxide minerals [11]. This pathway commonly occurs in situations where anoxic Fe (II)-bearing water is exposed to oxic conditions, such as during discharge of subsurface water from underground mine workings to well-oxygenated surface zones [10,9]. A recent study showed that in such solutions, Sb(III) and Fe(II) can be co-oxidized to Sb(V) and Fe(III), respectively [60].

Despite the clear environmental relevance of such situations, the behavior of Sb(V) during the initial Fe(III) hydroxide formation via aqueous Fe(II) oxidation has received surprisingly little attention. Results from incubation experiments with contaminated soil indicate that Sb(V) may be removed from the aqueous phase upon Fe(II) oxidation [22,38]; presumably via Sb(V) uptake by newly-formed Fe(III) hydroxides. In addition, recent research on the formation of schwertmannite (a sulfate-bearing Fe(III) hydroxide) via aqueous Fe(II) oxidation under strongly acidic pH conditions has revealed that Sb(V) itself can affect the final Fe(III) hydroxide mineralogy, with high Sb(V) concentrations acting, for example, to inhibit schwertmannite formation [47]. However, at present, the effect of Sb(V) in potentially shaping the mineralogy of Fe(III) hydroxides that are produced via Fe(II) oxidation under near-neutral pH remains largely unexplored. Likewise, the mechanisms that control Sb(V) uptake by Fe(III) hydroxides that are formed via Fe(II) oxidation under such conditions have not been systematically explored. This represents a significant knowledge gap, especially considering that Fe(III) hydroxide precipitation in the presence of Sb(III) has previously been found to lead to substantial Sb immobilization [59].

The aim of this study was to examine the Fe(III) hydroxide mineral products and associated mechanisms of Sb(V) sequestration that result from Fe(II) oxidation in the presence of Sb(V) under near-neutral pH conditions. To achieve this aim, experiments were carried out in oxygen-saturated Fe(II)-bearing solutions that contained a range of environmentally relevant Sb(V) concentrations (equivalent to initial aqueous Sb (V):Fe(II) molar ratios of 0, 0.01, 0.04, 0.1 and 0.25). The oxidation experiments were buffered at pH 7, as Sb-contaminated mining waters

are often buffered at near-neutral pH, due to the presence of abundant carbonate minerals [13,42]. After Fe(II) oxidation was complete, samples of the resulting Fe(III) hydroxides were characterized using X-ray diffractometry (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), Mössbauer spectroscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy and wet chemical extraction techniques.

# 2. Methods

## 2.1. General methods

All glassware was soaked in 2% HCl overnight and rinsed with ultrapure water (MilliQ) three times prior to use. All reagents were analytical grade. Solutions were prepared using MilliQ water. An ambient temperature of 21 ± 1 °C was maintained for all reactions.

## 2.2. Fe(II) oxidation experiment

For the Fe(II) oxidation experiment, four treatments with molar Sb: Fe ratios of 0.01, 0.04, 0.10, and 0.25 (treatments Sb-0.01, Sb-0.04, Sb-0.1, and Sb-0.25), as well as an Sb-free control treatment (Sb-0), were prepared in triplicate. These Sb:Fe ratios were prepared in 4 L of a background electrolyte solution containing 100 mmol L⁻¹ KCl, with 50 mmol L⁻¹ 3-(N-morpholino)-propane sulfonic acid (MOPS) as a pH buffer (adjusted to pH 7 using 6 M KOH). It should be noted that sulfonate-containing buffers, such as MOPS, have been widely used in previous studies of Sb and Fe behaviour [21,23,24,7,8], and have been shown to have no significant interaction with metal ions in solution due to steric reasons [12,28]. Aliquots of a 10 mmol L⁻¹ Sb(V) stock solution, prepared from KSb(OH)₆, were added to achieve aqueous Sb(V) concentrations of 0.01, 0.04, 0.10, or 0.25 mmol L⁻¹. The Sb concentrations were chosen to represent concentrations frequently found in the vicinity of mining sites [17,3]. The oxygen supply was provided by stirring the solution vigorously on a magnetic stirrer at 1500 rpm and bubbling ambient air through it using an air pump.

The Fe(II) oxidation reaction was initiated by adding 1 mmol L⁻¹ FeCl₂ dissolved in 0.01 M HCl to the aerated solution. 10 mL subsamples were collected from the solution with a syringe immediately before the start of the experiment, as well as at defined intervals (2, 4, 6, 8, 10, 15, 20, 25, 30, 40, 50, and 60 minutes) following the addition of the Fe(II) solution. The samples were passed through 0.2 µm cellulose acetate filters (Minisart NML, Sartorius), stabilized with 200 µL concentrated HCl, and stored in a refrigerator. After the last sampling, the precipitates were allowed to settle for 1 hour and then separated from the liquid by repeated centrifugation and decantation (4 min at 4000 rpm each). Since remains of the electrolyte solution had been shown to cause interferences in the XRD patterns, the background electrolyte was removed by washing with methanol [8]. The precipitates were stored at -80 °C until they were freeze-dried (Alpha 1-4 LDplus, Christ) and ground for solid-phase analyses.

## 2.3. Analyses

The pH and dissolved oxygen concentrations were monitored with a HACH 40D multi-Meter equipped with a luminescent dissolved oxygen probe (IntelliCAL LDO101, Hach) and a gel-filled pH electrode (IntelliCAL PHC101, Hach). This data can be found in the Supporting Information (SI Figure S1).

Dissolved Fe(II) concentrations in the stabilized samples were measured photometrically using the phenanthroline method [1] within
1 h after the experiment. The detection limit (DL), determined as the mean of 3 replicates + 3 * standard deviation, was 1 mg L⁻¹. Determination of total Sb and Fe concentrations in stabilized aqueous samples and extracts was conducted by inductively coupled plasma mass spectroscopy (ICP-MS, Perkin-Elmer, ELAN-DRCe, DL: 0.005 mg L⁻¹).

X-ray diffractometric analyses were conducted using a Bruker D4 Endeavor instrument with a Co X-ray source (λ = 0.178890 nm), scanning from 5 to 90 °2θ with a 0.02 °2θ step-size and a 2 s count-time, with powder samples mounted on sample holders using ethanol. For transmission electron microscopy (TEM), a Hitachi HF5000 Cs-STEM/TEM instrument operated at 200 kV was used for imaging. For selected area electron diffraction (SAED), a Gatan Digital Micrograph was employed to acquire images on a Gatan Oneview camera in 4kx4k pixel collection mode. The sums of four patterns per treatment were collected to increase result quality. Scanning electron microscopy samples (SEM) were mounted on Si wafers, coated with platinum, and analyzed using a Zeiss Ultra plus instrument with an Inlens SE-Detector (Everhart Thornley).

⁵⁷Mössbauer spectra of solid-phase samples and synthetic feroxyhyte (δ'-FeOOH, synthesized according to [51]) were obtained in transmission mode on a constant-acceleration Mössbauer spectrometer equipped with a nominal 1.85 GBq ⁵⁷Co/Rh γ-radiation source. The velocity scale was calibrated relative to a 25 µm thick α-Fe foil at room temperature. Dried powders (~55 mg) were loaded into plastic sample holders (1.13 cm²) and placed in a continuous-flow cryostat with the sample in N₂ vapor for measurement at 80 K, whereas the ⁵⁷Co/Rh source remained at room temperature. Spectra were analyzed using the Recoil [32] and MossA software [44] by applying Voigt-based fitting routines.

X-ray Absorption Near-Edge Structure (XANES) and Extended X-ray Absorption Fine-Structure (EXAFS) spectra were acquired at the Rosendorf Beamline (BM20) at the European Synchrotron Radiation Facility (ESRF), Grenoble, France [49]. XANES and EXAFS spectra were collected at the Sb K-edge (30,491 eV) in transmission mode. The energy of the liquid N₂-cooled Si(111) double-crystal monochromator (DCM) was calibrated using an Sb foil. During the measurement, the samples were kept at 15 K with a closed-cycle He cryostat (CryoVac) to avoid photon-induced changes of oxidation state and to reduce thermal disorder [31]. Least-square shell-fit analysis of Sb K-edge EXAFS spectra was performed in Artemis [48], as described previously [7]. Details on the data collection and processing are available in SI Section 2. Wavelet transform (WT) analysis was used to resolve the centres of the backscattering wave functions in both k-space (Å⁻¹) and R-space (Å), thus offering additional information to discriminate different atoms within one atomic shell, which cannot be identified by shell-by-shell fitting [18].

The total Sb concentration in the precipitates was determined by digesting 50 mg of freeze-dried precipitates in 6 M HCl within 48 h [30]. The operationally-defined surface-bound fraction of Sb was assessed via a 1 M KH₂PO₄ extraction [29], and a repeated desorption experiment was used to examine the pool of easily remobilized Sb. Experimental details on the extraction experiments and their effectiveness are given in SI Section 3. The percentage of Sb(V)-for-Fe(III) substitution was calculated from the extraction assay using SI Equation 1.

# 3. Results and discussion

## 3.1. Trends in aqueous Fe and Sb concentrations

Aqueous Fe(II) was removed from solution in all treatments within approximately 20 min (Fig. 1). This occurred together with the formation of red-orange Fe(III) hydroxide particles and parallel decreases in aqueous Sb concentrations. These trends are consistent with the Sb uptake by Fe(III) hydroxide phase(s) that formed via Fe(II) oxidation. While almost all aqueous Sb was removed by such uptake in the Sb-0.01 treatment, approximately 20–30 % of the initial aqueous Sb remained in solution for the Sb-0.04, Sb-0.1 and Sb-0.25 treatments (SI Figure S4).

<figure><img src="image_5.png" /><figcaption>Fig. 1. Dissolved Fe(II) and Sb concentrations during Fe(II) oxidation at molar Sb:Fe ratios of 0.25, 0.1, 0.04, and 0.01. Error bars indicate the standard deviation of triplicates.</figcaption></figure>

The final aqueous Sb concentrations at the completion of the oxidation experiment equate to solid-phase Sb concentrations in the Fe(III) hydroxide phases of approximately 75 ± 3, 225 ± 12, 547 ± 79 and 1400 ± 19 mmol·Sb g⁻¹ in the Sb-0.01, Sb-0.04, Sb-0.1 and Sb-0.25 treatments, respectively (Table 1).

## 3.2. Mineralogy and solid-phase Fe characterization

X-ray diffractometry revealed the presence of lepidocrocite (γ-FeOOH) and/or feroxyhyte in solids collected at the completion of the Fe(II) oxidation experiment. Specifically, in the Sb-0 treatment, all XRD peaks could be attributed to lepidocrocite (Fig. 2a), with SEM showing the presence of plate-like lepidocrocite crystals with sharp edges (Fig. 3a). In this treatment (Sb-0), lepidocrocite formation was expected,

Table 1

Characteristics of the solid-phase products after Fe(II) oxidation at molar Sb:Fe ratios of 0, 0.01, 0.04, 0.1, and 0.25 (denoted as Sb-0, Sb-0.01, Sb-0.04, Sb-0.1 and Sb-0.25, respectively). Fe(III) substitution was calculated from extraction data using Equation 1 in SI.

<table><thead><tr><th>Treatment</th><th>Sb solid phase concentration (mmol g<sup>-1</sup>)</th><th>Fe(III) substitution (mol%)</th><th>d-spacing of (020) plane (Å)</th></tr></thead><tbody><tr><td>Sb-0</td><td>&lt; 0.2</td><td>0.0</td><td>6.3</td></tr><tr><td>Sb-0.01</td><td>75 ± 3</td><td>1.0</td><td>6.8</td></tr><tr><td>Sb-0.04</td><td>225 ± 12</td><td>3.4</td><td>8.3</td></tr><tr><td>Sb-0.1</td><td>547 ± 79</td><td>8.2</td><td>8.3</td></tr><tr><td>Sb-0.25</td><td>1400 ± 19</td><td>16.2</td><td>-</td></tr></tbody></table>
<figure><img src="image_6.png" /><figcaption>**Fig. 2.** a) X-ray powder diffractograms and (b) 80 K Mössbauer spectra of solid-phase products after Fe(II) oxidation at molar Sb:Fe ratios of 0, 0.01, 0.04, 0.1, and 0.25 (denoted as Sb-0, Sb-0.01, Sb-0.04, Sb-0.1 and Sb-0.25, respectively. In (a), the labels “L” and “Fx” denote lepidocrocite and feroxyhyte, respectively.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 3.** Microscopic analyses of selected solid-phase products after Fe(II) oxidation: a) SEM image of lepidocrocite platelets formed at an Sb:Fe ratio of 0.01; b) SEM image showing the close association between distorted lepidocrocite platelets and feroxyhyte aggregates formed at an Sb:Fe ratio of 0.1; c) SEM image and d) corresponding TEM-SAED pattern of feroxyhyte aggregates precipitated at an Sb:Fe ratio of 0.25. Additional TEM images of the other treatments are provided in the SI.</figcaption></figure>

as lepidocrocite is well known to be the primary product of aqueous Fe (II) oxidation under similar circum-neutral pH conditions [34,52,60].

The XRD patterns indicate that lepidocrocite was the only mineral phase present in solids from the Sb-0.01 and Sb-0.04 treatments (Fig. 2a). However, in these treatments, an increase in lepidocrocite's XRD peak widths and a decrease in peak intensities was observed (relative to the Sb-0 treatment), with this effect being more pronounced

for lepidocrocite which had formed at higher Sb concentrations. As such, the presence of Sb appears to have also caused a clear expansion along the lepidocrocite b-axis, which is evident from shifts in the position of the (020) peak in the XRD patterns for the Sb-0.01, Sb-0.04 and Sb-0.1 treatments relative to the Sb-0 treatment (Fig. 2a). This shift is associated with an increase in d-spacing from 6.3 Å to 8.3 Å (Table 1) in the lepidocrocite structure.
<caption>**Table 2**</caption>

80 K Mössbauer parameters of feroxyhyte (Fx), lepidocrocite (L) and the solid-phase products after Fe(II) oxidation at molar Sb:Fe ratios of 0, 0.01, 0.04, 0.1, and 0.25.

<table><thead><tr><th>Sample</th><th>Species</th><th>CS (mm s<sup>-1</sup>)<sup>a</sup></th><th>ΔE<sub>0</sub> or ε (mm s<sup>-1</sup>)<sup>b</sup></th><th>B<sub>hf</sub> (T)<sup>c</sup></th><th>R.A.<sup>d</sup></th></tr></thead><tbody><tr><td>Feroxyhyte<sup>e</sup></td><td>Fx</td><td>0.48 ± 0.01</td><td>-0.06 ± 0.01</td><td>46.4 ± 0.6</td><td>100 %</td></tr><tr><td>Lepidocrocite<sup>f</sup></td><td>L</td><td>0.49 ± 0.05</td><td>0.65 ± 0.05</td><td>-</td><td>100 %</td></tr><tr><td>Sb-0</td><td>L</td><td>0.49 ± 0.01</td><td>0.64 ± 0.01</td><td>-</td><td>100 %</td></tr><tr><td>Sb-0.01</td><td>L</td><td>0.48 ± 0.01</td><td>0.64 ± 0.01</td><td>-</td><td>100 %</td></tr><tr><td>Sb-0.04</td><td>L</td><td>0.48 ± 0.01</td><td>0.65 ± 0.01</td><td>-</td><td>100 %</td></tr><tr><td>Sb-0.1</td><td>L</td><td>0.48 ± 0.01</td><td>0.66 ± 0.01</td><td>-</td><td>33 ± 2 %</td></tr><tr><td></td><td>Fx</td><td>0.48 ± 0.02</td><td>-0.04 ± 0.01</td><td>43.5 ± 0.1</td><td>67 ± 2 %</td></tr><tr><td>Sb-0.25</td><td>Fx</td><td>0.51 ± 0.01</td><td>-0.03 ± 0.01</td><td>33.1 ± 0.5</td><td>100 %</td></tr></tbody></table>

<sup>a</sup>Center shift (relative to α-Fe<sup>0</sup>). <sup>b</sup>Quadrupole splitting (paramagnetic) or quadrupole shift (magnetic). <sup>c</sup>Hyperfine magnetic field. <sup>d</sup>Relative abundance. <sup>e</sup>Synthesized according to Schwertmann and Cornell [51] and verified via XRD; <sup>f</sup>According to previously published values [25].

In contrast, solids from the Sb-0.1 treatment display additional XRD peaks at 41.5, 47.5, 63.4, and 75.4 °2θ, while the intensities of the XRD peaks of lepidocrocite further decreased (Fig. 2a). The positions of these XRD peaks matched those previously reported for feroxyhyte [43]. At even higher Sb concentrations, i.e. in the Sb-0.25 treatment, feroxyhyte was the only mineral product that was detectable via XRD. Transmission electron microscopy with SAED confirmed feroxyhyte to be the only mineral phase present in solids from the Sb-0.25 treatment (Fig. 3c and d, SI Figure S5). Examination via SEM revealed that feroxyhyte in the Sb-0.25 treatment existed as very fine crystallites that were aggregated into nanometer-scale cloud-like clusters (Fig. 3c). Feroxyhyte is a rare short-range order FeOOH polymorph that occurs in a range of environmental settings, including those characterized by rapid Fe(II) oxidation [11,5,58,57].

<sup>57</sup>Fe Mössbauer spectra of precipitates collected at 80 K from the Sb-0, Sb-0.01, and Sb-0.04 treatments exhibited a paramagnetic doublet with hyperfine values that are consistent with lepidocrocite (Fig. 2b, Table 2) [25]. Precipitates from the Sb-0.1 treatment presented as a sextet (67 % of the total spectral area), in addition to the lepidocrocite doublet (33 % of the total spectral area), while in the Sb-0.25 treatment, the Mössbauer spectrum consisted solely of a sextet. Fitting of the hyperfine values for the sextets revealed center shift (CS) and quadrupole splitting (ΔEQ) values that are similar to synthetic feroxyhyte (Table 2, and SI Figure S6, S7).

The <sup>57</sup>Fe Mössbauer spectroscopy results show that the magnetic hyperfine field (Bhf) for feroxyhyte that formed during the Fe(II) oxidation experiment were reduced relative to the Bhf of pure feroxyhyte. Decreases in the Bhf value can be caused by the substitution of diamagnetic ions into the Fe(III) hydroxide crystal structure [40]. In the context of the present study, the Bhf decreases may therefore reflect substantial levels of substitution of Fe(III) by diamagnetic Sb in feroxyhyte. This suggests that the removal of Sb(V) from solution in the Sb-0.1 and Sb-0.25 likely involved a significant degree of Sb(V) coprecipitation with Fe(III) during feroxyhyte formation.

Collectively, the XRD, TEM, and Mössbauer spectroscopy results clearly show that aqueous Fe(II) oxidation in the presence of low levels of Sb(V) led to lepidocrocite formation, whereas Fe(II) oxidation in the presence of higher levels of Sb(V) instead led to feroxyhyte formation. Hence, it appears that Sb(V) acts as a mineral switch during near-neutral pH Fe(II) oxidation, shifting the mineralogy of the resulting Fe(III) hydroxide from lepidocrocite to feroxyhyte. To the best of our knowledge, the present study is the first to identify this effect of Sb(V) in steering the mineralogy of Fe(III) hydroxides that are produced via aeration of Fe(II)-bearing solutions. However, this finding echoes earlier research in anoxic systems on ferrihydrite transformation in the presence of Sb(V), which also found that high levels of Sb(V) promoted the formation of feroxyhyte [21,6,8]. Our results also concur with an earlier study in which feroxyhyte was found in Sb-rich sediments in the vicinity of an Sb mine [27].

### 3.3. Solid-phase Sb characterization

Linear combination fitting (LCF) of the Sb K-edge XANES spectra for solids collected from the oxidation experiment confirmed the presence of Sb(V) with negligible solid-phase Sb(III) (SI Figure S8). Modelling of the Sb K-edge EXAFS spectra revealed Sb coordination by a first shell comprising 4.6 – 6.7 O atoms at an inter-atomic distance of 1.98 Å (Fig. 4, Table 3). This is consistent with an octahedral coordination of Sb (V) by O, thereby supporting the results of our LCF analyses of the XANES spectra [50].

In the Sb-0.01 treatment, subsequent peaks in the Fourier transform were well described by Sb-Fe scattering paths at 3.13 Å and 4.03 Å with CNs of 4 and 1.6, respectively. Similar fitting results (Sb-Fe paths at 3.15 Å and 3.99 Å with CNs of 5.5 and 1.4, respectively) were found for the Sb-0.04 treatment (Table 3). Inter-atomic Sb-Fe distances of 3.13 Å and 4.03 Å are indicative of edge-sharing and single corner-sharing between adjacent SbO<sub>6</sub> and FeO<sub>6</sub> octahedra [37,39]. Linkages of this nature (with comparable CNs) are a defining feature of the local coordination environment around Fe atoms in lepidocrocite. This suggests

<figure><img src="image_8.png" /><figcaption>**Fig. 4.** (a) Sb K-edge EXAFS spectra and (b) Fourier-transform magnitudes of the Fe(III) hydroxide products formed at molar Sb:Fe ratios of 0.01, 0.4, 0.1, and 0.25 (denoted as Sb-0.01, Sb-0.04, Sb-0.1, and Sb-0.25, respectively). Solid lines represent experimental data and grey dotted lines are shell fits. Approximate positions of peaks in (b) due to the backscattering contributions of Sb and Fe in various coordination are indicated (DC = double-corner sharing; E = edge sharing within a single octahedral chain; E' = edge sharing between two octahedral chains; SC = single-corner sharing).</figcaption></figure>
<caption>**Table 3**</caption>

Antimony K-edge EXAFS shell-fit results of the Fe oxide products at molar Sb:Fe ratios of 0.01, 0.4, 0.1, and 0.25. CN, R, ΔE₀ and σ² denote the coordination number, inter-atomic distance, energy shift and Debye-Waller factor, respectively.ª

<table><thead><tr><th>Sample</th><th>Shell</th><th>CN (-)<sup>b</sup></th><th>R (Å)</th><th>ΔE<sub>0</sub> (eV)</th><th>σ<sup>2</sup> (Å<sup>2</sup>)<sup>d</sup></th><th>R-factor<sup>e</sup></th></tr></thead><tbody><tr><td rowspan="3">Sb-0.01</td><td>Sb-O</td><td>6.7 ± 1.0</td><td>1.98 ± 0.00</td><td rowspan="3">2.2 ± 1.2</td><td>0.004 ± 0.000</td><td rowspan="3">0.012</td></tr><tr><td>Sb-Fe<sub>1</sub></td><td>4.0 ± 0.9</td><td>3.13 ± 0.01</td><td>0.004 ± 0.001</td></tr><tr><td>Sb-Fe<sub>2</sub></td><td>1.6 ± 0.7</td><td>4.03 ± 0.02</td><td>0.004 ± 0.001</td></tr><tr><td rowspan="3">Sb-0.04</td><td>Sb-O</td><td>5.6 ± 0.6</td><td>1.99 ± 0.00</td><td rowspan="3">4.1 ± 0.7</td><td>0.003 ± 0.000</td><td rowspan="3">0.004</td></tr><tr><td>Sb-Fe<sub>1</sub></td><td>5.5 ± 0.9</td><td>3.15 ± 0.01</td><td>0.007 ± 0.001</td></tr><tr><td>Sb-Fe<sub>2</sub></td><td>1.4 ± 0.6</td><td>3.99 ± 0.02</td><td>0.007 ± 0.001</td></tr><tr><td rowspan="3">Sb-0.1</td><td>Sb-O</td><td>5.8 ± 0.7</td><td>1.99 ± 0.00</td><td rowspan="3">3.1 ± 1.0</td><td>0.003 ± 0.000</td><td rowspan="3">0.006</td></tr><tr><td>Sb-Fe<sub>1</sub></td><td>3.1 ± 1.1</td><td>3.12 ± 0.01</td><td>0.008 ± 0.003</td></tr><tr><td>Sb-Fe<sub>2</sub></td><td>3.2 ± 1.1</td><td>3.57 ± 0.01</td><td>0.008 ± 0.003</td></tr><tr><td rowspan="5">Sb-0.25</td><td>Sb-O</td><td>4.6 ± 0.6</td><td>1.98 ± 0.00</td><td rowspan="5">4.0 ± 1.10</td><td>0.002 ± 0.000</td><td rowspan="5">0.003</td></tr><tr><td>Sb-Sb<sub>1</sub></td><td>1.9 ± 0.6</td><td>3.09 ± 0.02</td><td>0.002 ± 0.003</td></tr><tr><td>Sb-Fe<sub>1</sub></td><td>2.0 ± 1.0</td><td>3.14 ± 0.03</td><td>0.002 ± 0.002</td></tr><tr><td>Sb-Sb<sub>2</sub></td><td>2.7 ± 0.7</td><td>3.47 ± 0.03</td><td>0.002 ± 0.002</td></tr><tr><td>Sb-Fe<sub>2</sub></td><td>4.3 ± 1.4</td><td>3.53 ± 0.03</td><td>0.002 ± 0.002</td></tr></tbody></table>

that Sb(V) was retained by solids produced in the Sb-0.01 and Sb-0.04 treatment through incorporation into the lepidocrocite structure via Sb(V)-for-Fe(III) substitution. This finding agrees with the above-mentioned expansion of the lepidocrocite crystals along the b-axis, which was likely a consequence of the loss of H⁺ from the lepidocrocite structure. The heterovalent substitution of Fe(III) by Sb(V) introduced additional positive charge into the lepidocrocite structure [59], which may have been compensated for by a release of H⁺ ions. The incorporation of Sb(V) is thus likely to have weakened the hydrogen bonding between the Fe(III)O₃OH₃ layers, thereby leading to the observed increase in d-spacing from 6.3 to 8.3 Å (SI Figure S9, Table 1).

In agreement with the findings from the Sb-0.01 treatment, the Sb K-edge EXAFS data for solids produced in the Sb-0.1 treatment were also well described by two Sb-Fe paths (Table 3). The shorter of these two paths occurred at 3.12 Å, which is consistent with edge-sharing between SbO₆ and FeO₆ octahedra (as also found for the Sb-0.01 and Sb-0.04 treatments) [37,39,50]. The longer of the two Sb-Fe paths in the Sb-0.1 treatments was fit by an inter-atomic distance of 3.57 Å, which is consistent with double corner-sharing between adjacent SbO₆ and FeO₆ octahedra. This contrasts with the presence of single-corner sharing in the Sb-0.01 and Sb-0.04 treatments, but agrees with the XRD and Mössbauer spectroscopic results in that the local coordination environment in feroxyhyte is characterized by the presence of both edge and double-corner sharing linkages between FeO₆ octahedra [37,39]. This observation, together with relatively large CNs of ~3 for edge and double-corner sharing linkages between SbO₆ and FeO₆ in the Sb-0.1 treatment – compared with the expected CNs of 1–2 for adsorption complexes on the Fe(III) hydroxide surface – suggests that Sb(V) was retained within the feroxyhyte structure through Sb(V)-for-Fe(III) substitution [21,50].

Similar to the Sb-0.1 treatment, modelling of the Sb K-edge EXAFS data for the Sb-0.25 treatment also indicates the presence of two Sb-Fe paths (Table 3). These paths are characterized by inter-atomic distances of 3.14 Å and 3.52 Å with CNs of 2 and 4, respectively. As also observed for the Sb-0.1 treatment, these distances and CNs align with edge and double-corner sharing between SbO₆ and FeO₆ octahedra within the structure of the feroxyhyte that formed in the Sb-0.25 treatment. In addition to these Sb-Fe paths, solids produced in the Sb-0.25 treatment also contained additional Sb-Sb backscattering paths at inter-atomic distances of 3.09 Å and 3.47 Å (Table 3). The presence of Sb-Sb backscattering is supported by Morlet wavelet transform (WT) analysis for the 0.25 treatment, which (unlike the other treatments) shows a distinct feature at k > 12 Å⁻¹ (SI Figure S2). The presence of these Sb-Sb paths, in addition to the more abundant Sb-Fe paths, likely reflects the relatively Sb-rich nature of the feroxyhyte that formed in the

Sb-0.25 treatment (whereby this feroxyhyte was characterized by 16.2 mol% Sb(V)-for-Fe(III) substitution; Table 1).

Overall, the Sb K-edge EXAFS results show that Sb(V) was incorporated into the crystal structure of both lepidocrocite and feroxyhyte by heterovalent Sb(V)-for-Fe(III) substitution. Such substitution is feasible because Sb(V) and Fe(III) have comparable ion sizes (Sb(V): 0.60 Å; Fe(III): 0.55 Å) when octahedrally coordinated with O [6]. Substitution of Sb(V) into the structure of feroxyhyte has been previously described by Hockmann et al. [21] in a study examining the effect of Fe(II)-induced ferrihydrite transformation on Sb behavior. However, the present study provides direct evidence that Sb(V)-for-Fe(III) substitution can also occur in lepidocrocite. From the present results (Table 1), the upper limit for such substitution in lepidocrocite appears to be between 3.4 and 8.2 mol% (given lepidocrocite was the only Fe(III) phase present at 3.4 mol% substitution, whereas lepidocrocite was largely replaced by feroxyhyte at the higher 8.2 mol% substitution). At higher mol% levels, lepidocrocite formation was inhibited in favor of the formation of feroxyhyte, which can evidently accommodate larger degrees of Sb (V)-for-Fe(III) substitution (Table 1).

The results of the present study are of particular significance from an environmental pollution perspective as incorporation of Sb(V) into the crystal structure of Fe(III) hydroxide minerals may be a relatively secure immobilization mechanism for Sb(V) in Sb-contaminated environments. This is because incorporated Sb(V) species are expected to be largely protected from desorption that may induce changes in aqueous-phase properties such as pH, ionic strength, and competitive ions. To test this hypothesis, we subjected the Sb(V)-coprecipitated lepidocrocite and feroxyhyte from the Fe(II) oxidation experiments to extraction with a 1 M KH₂PO₄ solutions. The results show that only ~2% to ~6% of total solid phase Sb(V) was recovered by the 1 M KH₂PO₄ extraction (SI Figure S3), while ~75% recovery would be expected if Sb(V) was surface-adsorbed (SI Section 3). Likewise, we also found that < 1% of solid-phase Sb(V) was remobilized via the desorption assay described in the SI section. This very low recovery during both KH₂PO₄ extraction and the desorption assay support the concept that Sb(V) that is incorporated into the lepidocrocite or feroxyhyte structure is largely protected from short-term desorption processes.

## 4. Conclusion

This study has provided an in-depth systematic examination of the Fe (III) hydroxide mineral products and associated mechanisms of Sb(V) sequestration that result from near-neutral pH Fe(II) oxidation in the presence of Sb(V). The following key conclusions can be drawn from this study:

<footnote>ªFit uncertainties are given for the last significant figure. ᵇThe amplitude reduction factor was set to 0.9 [62], and the uncertainty associated with this estimate has been included in the error of CN. ᶜThe energy shift was constrained to a single value for all shells for each spectrum. ᵈFor each spectrum, one fitted value for the Debye-Waller factor was allowed for near-neighbour oxygens, and another one for metal(loid) scatterers. ᶠR-factor = Σᵢ(dataᵢ-fitᵢ)²/Σᵢ dataᵢ.</footnote>
* Under our experimental conditions, oxidation of Fe(II) from solutions with molar Sb(V):Fe(II) ratios of less than 0.1 resulted in the formation of lepidocrocite. This is consistent with a large number of previous studies on Fe(II) oxidation in near-neutral pH environments.

* In contrast, at initial aqueous Sb(V):Fe(II) ratios greater than 0.1, oxidation of aqueous Fe(II) resulted in the formation of feroxyhyte. Although feroxyhyte is a relatively rare Fe(III) hydroxide in most surface environments, this finding suggests that it may occur more commonly in Sb(V)-rich waters that experience aqueous Fe(II) oxidation.

* The lepidocrocite and feroxyhyte that are formed via Fe(II) oxidation remove aqueous Sb(V) from solution by incorporating Sb(V) into their crystal structures via Sb(V)-for-Fe(III) substitution during mineral precipitation.

* The present study provides compelling evidence that lepidocrocite can retain Sb(V) via this heterovalent substitution mechanism (with our results suggesting that such substitution can occur at levels of up to somewhere between ~3.4 and ~8.2 mol%).

* Feroxyhyte that forms via Fe(II) oxidation can accommodate relatively large amounts of Sb(V)-for-Fe(III) substitution, with the highest level of substitution observed in the present study amounting to 16.2 mol%.

* Sb(V) that is incorporated into the structure of lepidocrocite and feroxyhyte appears to be largely protected from desorption processes, thereby suggesting formation of these Sb(V)-bearing Fe(III) hydroxides may securely immobilize Sb(V) in Sb-contaminated environments.

Even though this study lacks the complexity of natural systems, these findings expand our understanding of interactions between Sb geochemistry and the Earth's near-surface Fe cycle and emphasize the role that such interactions can play in controlling both Fe(III) hydroxide mineralogy and Sb mobility. Given, for example, that organic ligands can influence Fe(II) oxidation kinetics [61], and that co-occurring ions like arsenic may compete with Sb for binding sites during oxidation [45] or alter the properties of resulting precipitates [59], future research should build on these insights by investigating the impact of environmental matrices on Sb sequestration during Fe(II) oxidation.

## Environmental implication

Antimony is a hazardous material, whose mobility in redox-dynamic environments may be controlled by interactions with Fe(III) hydroxide minerals that form via Fe(II) oxidation. This study examined the Fe(III) hydroxide mineral products and associated mechanisms of Sb(V) sequestration that result from Fe(II) oxidation in the presence of Sb(V). The outcomes enhance our fundamental understanding of Sb's environmental behavior by providing new insights into Sb(V) incorporation into Fe(III) hydroxides, while showing that such incorporation impacts the mineralogy of Fe(III) hydroxides that form via Fe(II) oxidation.

## CRediT authorship contribution statement

**Britta Planer-Friedrich:** Writing – review & editing. **Stefan Peiffer:** Writing – review & editing. **Kerstin Hockmann:** Writing – review & editing, Supervision, Project administration, Methodology, Funding acquisition, Conceptualization. **Laura Wegner:** Writing – original draft, Investigation, Formal analysis, Data curation, Conceptualization. **Edward Burton:** Writing – review & editing, Supervision, Project administration, Funding acquisition, Conceptualization. **Catherine McCammon:** Writing – review & editing, Formal analysis. **Andreas C. Scheinost:** Writing – review & editing, Formal analysis.

## 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

Funding from the Germany Academic Exchange Service (DAAD-PPP Australia grant no. 57446537), the Australian Research Council (grant no. FT200100449) and by the Female Professor Programme of Germany's Federal Ministry of Education and Research (BMBF), the Equal Opportunities Department at University of Bayreuth as well as the Heinrich Böll Foundation are gratefully acknowledged. We acknowledge the European Synchrotron Radiation Facility (ESRF) for provision of synchrotron radiation facilities and Dr. Graeme Auchterlonie for the collection of the TEM data at the University of Queensland, Australia. Laboratory assistance was given by Roz Hagan, Jutta Eckert, Martina Rohr, and Stefan Will. This research was supported by the Environmental Analysis Laboratory (EAL), which is a Southern Cross University NATA accredited research support facility.

## Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jhazmat.2025.137380.

## Data availability

Data will be made available on request.

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