Dissolution, Stability and Solubility of Tooeleite [Fe 6 (AsO 3 ) 4 (SO 4 )(OH) 4 · 4H 2 O] at 25–45 ◦ C and pH 2–12

: Tooeleite [Fe 6 (AsO 3 ) 4 (SO 4 )(OH) 4 · 4H 2 O] was synthesized and characterized to investigate its possible immobilization for arsenic in acidic and alkali environments by a long-term dissolution of 330 d. The synthetic tooeleite was platy crystallites of ~1 µ m across, giving the lattice parameters of a = 6.4758 Å, b = 19.3737 Å and c = 8.9170 Å. For the tooeleite dissolution, the dissolved arsenic concentration showed the lowest value of 427.3~435.8 mg / L As at initial pH 12 (ﬁnal pH 5.54). The constituents were dissolved preferentially in the sequence of SO 42 − > AsO 33 − > Fe 3 + in the aqueous medium at initial pH 2–12. The dissolved iron, arsenite and sulfate existed mainly as FeSO 4 + / Fe 3 + , H 3 AsO 30 and SO 42 − at initial pH 2, and in the form of Fe(OH) 30 / Fe(OH) 2 + , H 3 AsO 30 and SO 42 − at initial pH 12, respectively. The tooeleite dissolution was characterized by the preferential releases of SO 42 − anions from solid surface into aqueous medium, which was fundamentally controlled by the Fe-O / OH bond breakages and the outer OH − group layers. From the data of the dissolution at 25 ◦ C and initial pH 2 for 270–330 d, the ion-activity product [log - IAP], which equaled the solubility product [ K sp ] at the dissolution equilibrium, and the Gibbs free energy of formation [ ∆ G fo ] were estimated as − 200.28 ± 0.01 and − 5180.54 ± 0.07 kJ / mol for the synthetic tooeleite, respectively.


Introduction
As arsenic is found widely in Earth's crust and is one of the chemicals of greatest health concern, inorganic arsenic compounds were classified by the International Agency for Research on Cancer in Group 1 (carcinogenic to humans) [1]. The extremely toxic arsenic is common in wastes from the mining-metallurgical industry for non-ferrous and precious metals. It can be released into the environment and, finally, threaten human beings [1,2]. The elimination of the most toxic inorganic As(III) is more difficult owing to its higher solubility and mobility than As(V) [3,4]. It is still a practical challenge to eliminate trivalent arsenic effectively from contaminated waters with very high arsenic concentration and low pH [5].
As-bearing minerals are important in the dissolution-precipitation equilibria and geochemical cycling of arsenic [6,7]. It is difficult to assess the arsenic contamination scale exactly [8]. The weathering of the mining tailings of metallic sulfide ores can lead to the formations of acid mine drainages (AMDs), containing very high contents of iron, sulfate and toxic metal(loid)s including As, Pb, Zn, Cd, Co, Cu, Hg, Mo, Ni, Ru, Sb, Se, Sn, Te, Bi, etc. [9][10][11]. The oxidation of ferrous ions and the progressive neutralization Moreover, there are many controversies in the literature about the dissolution process, solubility and stability of tooeleite, further research is needed on the co-incorporation and release of arsenic in/from the mineral and its long-term stability [25,27].
This work aimed to synthesize crystalline tooeleite by a simple hydrothermal method. Different instruments are then applied to inspect the structure and morphology of the obtained tooeleite. Its dissolution mechanism, solubility and long-term stability at diverse solution pHs and temperatures are examined, and simultaneously its suitability as storage materials for arsenic is also discussed.

Synthesis
The synthesis of tooeleite was completed in the same manner with slight modifications after the pre-experimental result as that used for measuring the heat capacity of tooeleite by relaxation calorimetry [37]. Firstly, 248.65 mL of ultrapure water were mixed with 1.35 mL 98% H 2 SO 4 to prepare a sulfuric acid solution in a polyethylene bottle, into which 5.00 g of As 2 O 3 was then added. The resulting slurry was heated in a 90 • C waterbath with a constant stirring at 600 rpm for 4h and then air-cooled. The undissolved solids were removed by filtration. After that, 10.50 g of Fe 2 (SO 4 ) 3 ·9H 2 O were added, and the resulting slurry was mixed and heated in a 90 • C water bath with a constant stirring at 600 rpm for 1 h. After air-cooling to room temperature, the mixed solution was adjusted to pH = 3.00 with 10 mol/L NaOH solution and then was heated in a 90 • C water bath with a constant stirring at 600 rpm for another 1 h. Finally, the suspension solution was air-cooled and separated using vacuum filtration. The obtained precipitates were rinsed 3 times using 50 mL ultrapure water and oven-dried at 110 • C for 24 h. The color of the tooeleite precipitate obtained in our work was yellow, like that observed for the mineral tooeleite in literature [32].

Characterization
The bulk elemental composition of the synthetic tooeleite was determined by digesting 50 mg of the prepared tooeleite in 20 mL of 6 M HCl solution, which was then diluted to 50 mL with ultrapure water. The iron, arsenic and sulfur concentrations were analyzed using an inductively coupled plasma-optical emission spectrometer (ICP-OES, Optima 7000DV, Perkin-Elmer Ltd., Waltham, MA, USA) or an atomic absorption spectrometer (AAS, PinAAcle 900T, Perkin-Elmer Ltd., Waltham, MA, USA) when the concentrations of iron or arsenic were less than the ICP-OES detection limits. The contents of the crystal water were then calculated based on the thermogravimetric analysis (TGA), which was completed using a thermal analyzer (TA, STA 409, Netzsch-Gerätebau GmbH, Selb, Germany) from room temperature to 1130 • C at 10 • C/min in a 20 mL/min N 2 atmosphere. The synthesis for tooeleite was repeated ten times to obtain enough products for the following tests, and all of them were characterized by an X-ray diffractometer (XRD, X'Pert PRO, PANalytical B.V., Almelo, the Netherlands) with Cu-Kα radiation of 1.540598 Å at 40 kV/40 mA and recognized by comparing with the reference for tooeleite (00-044-1468) from the International Centre for Diffraction Data (ICDD) to check the reproducibility in the synthesis procedure. The functional groups and the morphology of the tooeleite were observed using a Fourier transform infrared spectrophotometer (FT-IR, Nicolet Nexus 470, Thermo Fisher Scientific Inc., Waltham, MA, USA) over the 400~4000 cm −1 range and a field emission scanning electron microscope (FE-SEM, S-4800, Hitachi Ltd., Tokyo, Japan), respectively.

Dissolution Experiments
In each dissolution test, five grams of the synthetic tooeleite were weighed into a 100 mL polythene bottle, in which 100 mL of HNO 3 or NaOH solution of different pHs (pH 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 12) were then added. The capped bottle was placed in a thermostatic water bath (25 • C) and agitated by a magnetic stirring bar. The pH drifted freely and was recorded periodically. The experiments studying the effect of the temperature (25 • C, 35 • C or 45 • C) were conducted at initial pH 2. From each flask, 5 mL of the solution were collected periodically at 23 intervals form 1 h to 7920 h (330 d), filtered into a volumetric flask and instantly stabilized with 0.2% HNO 3 to 25 mL. After each sampling, 5 mL of HNO 3 or NaOH solution that had been adjusted to the pH measured at sampling were replenished into the bottle to hold a relatively constant solid/solution ratio. The variation of the solution components due to this sampling-replenishing was amended using the mass balance. The total iron, arsenic and sulfur concentrations were determined by the ICP-OES or AAS instrument. After 330 d dissolution, the residual solid of tooeleite was taken out from the bottle and characterized using XRD, FT-IR and FE-SEM as previously described to examine the possible variation of the mineral properties during dissolution. The dissolution experiment at different temperatures was made in triplicate (25 • C, initial pH = 2.00) or duplicate (35 • C and 45 • C, initial pH = 2.00) to check the repeatability.

Thermodynamic Calculation
The elemental speciation calculation for the tooeleite dissolution was conducted using the PHREEQC program (Version 3.6.2, U.S. Geological Survey, Denver, CO, USA) [38]. The aqueous activities of Fe 3+ , AsO 3 3− , SO 4 2− and OH − were first computed and then the ion-activity product [log -IAP] was determined according to its definition, which was equal to the solubility product [log -K sp ] for the synthetic tooeleite at the dissolution equilibrium. The minteq.v4.dat database was recompiled based on the MINTEQA2 database (Version 3.0, U.S. Environmental Protection Agency, Athens, GA, USA) [39], including the thermodynamic data of all aqueous species and solid phases for the speciation simulation, in which the Debye-Hückel equation was chosen automatically because the aqueous ionic strength in the present work was <0.07889 mol/L.

XRD
The XRD spectra of the synthetic tooeleite before and after 330 d dissolution are illustrated in Figure 1. The structural characterization was made by Rietveld refinements using the MDI Jade and the PANalytical HighScore Plus programs. The single crystal data of tooeleite (ICDD Powder Diffraction File as No. 00-044-1468) were applied as initial structural models [28]. The most intense peaks of all samples were principally in the identical positions with the similar intensities and matched very well the peaks for tooeleite [28] and the structural analysis showed that only the phase tooeleite was in the solid product with the space group Pbc 57 and the orthorhombic structure. The most intense peaks (d obs , I obs , hkl) in the powder XRD spectrum of the synthetic tooeleite were 9.69(76)020, 4.48 (13) (16)313, giving the cell parameters of a = 6.4758 Å, b = 19.3737 Å and c = 8.9170 Å, which were close to a = 6.4160 Å, b = 19.4500 Å and c = 8.9410 Å for tooeleite [28]. The XRD spectra of tooeleite, collected after 330 days of dissolution, did not show any evidence of other mineral phases even if it could not be excluded that new phases with low abundance and/or poor crystallinity presented eventually. Similar results were also reported in the literature for the hydroxyapatite dissolution in simple aqueous solutions [40]. The 2800~3700 cm −1 spectra of the OH stretching were characterized by the bands at 3549, 3466, 3406~3415, 3225~3242 and 3184~3199 cm −1 . The bands at 3184~3242 cm −1 and 3406~3549 cm −1 were ascribed to the OH stretching vibrations of the adsorbed H2O and the crystal water in the tooeleite structure, respectively, which confirmed the presence of strong H-bonds in the crystal structure. The strong bands at 1637~1643 cm −1 were easily assigned to the bending of the sorbed H2O, i.e., the framework deformation vibration of H2O [5].
AsO3 3− had a planar triangular shape and its fundamental vibrations in water had been studied by many researchers [27,43], which yet showed some differences in the related band positions. The characteristic bands at 752 and 680 cm −1 were ascribed to the v1 and v3 vibrations of AsO3 3− , respectively [43]. The bands of the v1 and v3 vibrations were found at 690 and 672 cm −1 [44]. The v1 and v3 vibration bands of AsO3 3− were found to exist at 653 and 631 cm −1 [45]. The v1 and v3 vibrations of AsO3 3− were recorded at 772 and 696 cm −1 for the synthetic tooeleite, respectively [27]. In this work, the arsenite IR spectra of tooeleite in 687~773 cm −1 were observed as illustrated in Figure 2. The moderate bands at 771~773 cm −1 and the weaker strong bands at 687~682 cm −1 were ascribed to the antisymmetric stretching vibration (v1) and the As-O stretching vibration (v3) of AsO3 3− , respectively. The bands at 511~513 cm −1 were assigned to the Fe-O-As vibration [5,46].

FT-IR
The FT-IR spectra were recorded for the synthetic tooeleite before and after dissolution for 330 d ( Figure 2) and interpreted based on the literature data [5,22,41]. No obvious variation was detected in the FT-IR spectra after dissolution. All FT-IR spectra of the synthetic tooeleite presented the bending or stretching vibrations of AsO 3 3− , SO 4 2− and OH − as reported previously [42], but the split stretching vibration of the FeO 6 octahedra [41] was not observed in this work. The 2800~3700 cm −1 spectra of the OH stretching were characterized by the bands at 3549, 3466, 3406~3415, 3225~3242 and 3184~3199 cm −1 . The bands at 3184~3242 cm −1 and 3406~3549 cm −1 were ascribed to the OH stretching vibrations of the adsorbed H 2 O and the crystal water in the tooeleite structure, respectively, which confirmed the presence of strong H-bonds in the crystal structure. The strong bands at 1637~1643 cm −1 were easily assigned to the bending of the sorbed H 2 O, i.e., the framework deformation vibration of H 2 O [5].
Minerals 2020, 10, x FOR PEER REVIEW 6 of 16 The very low intense bands at 870-872 cm −1 were detected for the tooeleite after dissolution, which were ascribed to the v1 symmetric stretching vibration of AsO4 3− and thought as a spectroscopic evidence for the oxidization of AsO3 3− to AsO4 3− , which was yet not confirmed by the XPS and XRD analysis [26].

FE-SEM
The morphologies of the synthetic tooeleite that was recognized by XRD were investigated using FE-SEM ( Figure 3). The pure tooeleite consisted of platy crystallites of ~1 μm across, which aggregated together and exhibited a reticulated flower structure, which was in agreement with some previous research [6,7,27,28,35,37]. No significant morphological variation of the synthetic tooeleite was observed after dissolution at initial pH 2~12 and 25~45 °C for 330 d (Figure 3).

Evolution of the Aqueous Solutions and Dissolution Mechanism
The dissolution of the synthetic tooeleite is normally described by the following reaction (Equation (1)).
Theoretically, 1.00 mol of the synthetic tooeleite could liberate 4.00 mol OH − after Equation (1). Consequently, the dissolution in strong acidic solution could cause a solution pH rising, indicating the H + consumption, or the dissolution in strong alkali solution could cause a solution pH reduction, indicating the OH + consumption, and the aqueous complexations could control the speciation reactions of all released constituents (Equations (2)-(4)).
The characteristic bands at 752 and 680 cm −1 were ascribed to the v 1 and v 3 vibrations of AsO 3 3− , respectively [43]. The bands of the v 1 and v 3 vibrations were found at 690 and 672 cm −1 [44]. The v 1 and v 3 vibration bands of AsO 3 3− were found to exist at 653 and 631 cm −1 [45]. The v 1 and v 3 vibrations of AsO 3 3− were recorded at 772 and 696 cm −1 for the synthetic tooeleite, respectively [27]. In this work, the arsenite IR spectra of tooeleite in 687~773 cm −1 were observed as illustrated in Figure 2.
The moderate bands at 771~773 cm −1 and the weaker strong bands at 687~682 cm −1 were ascribed to the antisymmetric stretching vibration (v 1 ) and the As-O stretching vibration (v 3 ) of AsO 3 3− , respectively.
The bands at 511~513 cm −1 were assigned to the Fe-O-As vibration [5,46]. The very low intense bands at 870-872 cm −1 were detected for the tooeleite after dissolution, which were ascribed to the v 1 symmetric stretching vibration of AsO 4 3− and thought as a spectroscopic evidence for the oxidization of AsO 3 3− to AsO 4 3− , which was yet not confirmed by the XPS and XRD analysis [26].

FE-SEM
The morphologies of the synthetic tooeleite that was recognized by XRD were investigated using FE-SEM ( Figure 3). The pure tooeleite consisted of platy crystallites of~1 µm across, which aggregated together and exhibited a reticulated flower structure, which was in agreement with some previous research [6,7,27,28,35,37]. No significant morphological variation of the synthetic tooeleite was observed after dissolution at initial pH 2~12 and 25~45 • C for 330 d (Figure 3). rose up to 2.44~3.36 in the early 1 h of dissolution and then fluctuated and gradually decreased to 2.33~3.30 after 6480 h dissolution. For the dissolution at initial pH 4.00~12.00, the solution pH decreased gradually in the early 4300 h and then reached the steady state. The final solution pHs at the end of the experiment increased from 3.54 to 5.54 with the increasing initial pH from 4.00 to 12.00 (Figure 4n). The final solution pHs for the dissolution at initial pH 2.00 decreased with the increasing temperature, i.e., from 2.31~2.33 at 25 °C, 2.27~2.28 at 35 °C to 2.21~2.23 at 45 °C (Figure 4 a-c).

Evolution of the Aqueous Solutions and Dissolution Mechanism
The dissolution of the synthetic tooeleite is normally described by the following reaction (Equation (1)).
For the dissolution of tooeleite at 25 • C and initial pH 2.00~3.00 (Figure 4a,d), the aqueous pH rose up to 2.44~3.36 in the early 1 h of dissolution and then fluctuated and gradually decreased to 2.33~3.30 after 6480 h dissolution. For the dissolution at initial pH 4.00~12.00, the solution pH decreased gradually in the early 4300 h and then reached the steady state. The final solution pHs at the end of the experiment increased from 3.54 to 5.54 with the increasing initial pH from 4.00 to 12.00 (Figure 4n). The final solution pHs for the dissolution at initial pH 2.00 decreased with the increasing temperature, i.e., from 2.31~2.33 at 25 • C, 2.27~2.28 at 35 • C to 2.21~2.23 at 45 • C (Figure 4 a-c).
The dissolved Fe 3+ concentration increased rapidly up to 0.6285 mmol/L in the early 1 h and then decreased to 0.3277 mmol/L from 1 h to 240 h; after that increased gradually to a steady state of 0.5747~0.5770 mmol/L after 6480 h of dissolution at initial 25 • C and pH 2.00 (Figure 4a). For the dissolution at 25 • C and initial pH 12.00, the dissolved Fe 3+ concentration increased rapidly up to 0.003449 mmol/L in the early 1 h and then fluctuated with a general decreasing trend to a steady state of 0.000702~0.000718 mmol/L after 6480 h (Figure 4m). The final dissolved Fe 3+ concentration decreased with the increasing initial pH from 2.00 to 12.00 with two sharp decreases from 2.00 to 3.00 and from 11.00 to 12.00 (Figure 4n). The final dissolved Fe 3+ concentration for the dissolution at initial pH 2.00 decreased with the increasing temperature, i.e., from 0. The dissolved Fe 3+ concentration increased rapidly up to 0.6285 mmol/L in the early 1 h and then decreased to 0.3277 mmol/L from 1 h to 240 h; after that increased gradually to a steady state of 0.5747~0.5770 mmol/L after 6480 h of dissolution at initial 25°C and pH 2.00 (Figure 4a). For the dissolution at 25°C and initial pH 12.00, the dissolved Fe 3+ concentration increased rapidly up to 0.003449 mmol/L in the early 1 h and then fluctuated with a general decreasing trend to a steady state of 0.000702~0.000718 mmol/L after 6480 h (Figure 4m). The final dissolved Fe 3+ concentration decreased with the increasing initial pH from 2.00 to 12.00 with two sharp decreases from 2.00 to 3.00 and from 11.00 to 12.00 (Figure 4n). The final dissolved Fe 3+ concentration for the dissolution at initial pH 2.00 decreased with the increasing temperature, i.e., from 0.5747~0.5770 mmol/L at 25 °C, 0.4324~0.4339 mmol/L at 35 °C to 0.4186~0.4234 mmol/L at 45 °C (Figure 4a-c).
The dissolved AsO3 3− concentration increased quickly up to the peak values of 9.37~13.36 mmol/L in the early 72~480 h and then slightly decreased to a steady state of 5.78~6.70 mmol/L after 6480 h of dissolution (Figure 4a-m). For the tooeleite dissolution, the dissolved arsenic concentrations It is concluded that all constituents were preferentially dissolved from solid into solution in the sequence of SO 4  . It was possible to form iron-rich precipitates. Although the XRD spectra exhibited that no other minerals than tooeleite existed, it could not be confirmed here that they did not exist in smaller quantity, which was under the detection limit (Figure 1). Two different mechanisms for the tooeleite dissolution happened at low and high pHs. At initial pH < 3, the aqueous pH increased gradually with time, suggesting a hydrion-consuming (Equation (5)); on contrary, at initial pH > 3, the aqueous pH decreased progressively with time, showing a hydroxyl-consumption (Equation (6)).  (Table S1, Supplementary Material). The dissolution was fundamentally controlled by the breakages of Fe-O/OH bonds in the lattice structure of tooeleite and restrained by the outer OH − layers, which was also found in the alunite dissolution previously [47]. Tooeleite dissolved congruently at initial pH 2 and incongruent when initial pH >3 [29].
and AsO3/(AsO3+SO4) mole ratios on the tooeleite surface increased from 1.01~1.16 and 0.76~0.80 to 1.01~1.24 and 0.76~0.81 after 330 d dissolution at 25 °C and initial pH 2.00, respectively (Table S1, Supplementary Material). The dissolution was fundamentally controlled by the breakages of Fe-O/OH bonds in the lattice structure of tooeleite and restrained by the outer OH − layers, which was also found in the alunite dissolution previously [47]. Tooeleite dissolved congruently at initial pH 2 and incongruent when initial pH >3 [29].

Solubility Calculation
The aqueous activities of Fe 3+ , AsO3 3− , SO4 2− and OH − in the final steady state (6480 h, 7200 h and 7920 h) were first computed using the PHREEQC program with its built-in minteq.v4.dat database [38], which was supplemented with the thermodynamic data of some aqueous metal-arsenite species [19,48], and then the ion-activity product [logˍIAP] was computed according to its definition. At the dissolution equilibrium, the saturation index for the synthetic tooeleite [Fe6(AsO3)4(SO4)(OH)4·4H2O]

Solubility Calculation
The aqueous activities of Fe 3+ , AsO 3 3− , SO 4 2− and OH − in the final steady state (6480 h, 7200 h and 7920 h) were first computed using the PHREEQC program with its built-in minteq.v4.dat database [38], which was supplemented with the thermodynamic data of some aqueous metal-arsenite species [19,48], and then the ion-activity product [log -IAP -IAP] was computed according to its definition. At the dissolution equilibrium, the saturation index for the synthetic tooeleite [Fe 6 (AsO 3 ) 4 (8).
∆G r o = −5.708 logK sp (8) For Equation (1), Rearranging, Because the relatively higher solid/water ratio (5 g/100 mL) was applied in the dissolution experiment, only <8% of the solid dissolved into water, i.e., the bulk constituent of the synthetic tooeleite showed no significant change after 7920 h of dissolution. The analytical data of the dissolution at 25 • C and initial pH 2 for 270 d (6480 h), 7200 h (300 d) and 7920 h (330 d) together with the calculated thermodynamic properties for the synthetic tooeleite are given in Table 2.
The dissolution tests were carried out until the differences in the ion activity products [IAPs] determined from the last three samples were within the analytical uncertainty of ±0.13 log units. The dissolution system reached equilibrium and all solutions could be considered to be saturated with tooeleite [51]. The ion-activity product [log -IAP], which equaled the solubility product [log -K sp ] at equilibrium, and the free energy of formation [ [29]. The result of this work was not in good agreement with the solubility product of log -K sp ≈ 23 for tooeleite [10,31], which was estimated from the residual iron and arsenic analysis in the synthetic experiment [28]. However, it was very close to the thermodynamic properties of synthetic tooeleite that were measured by the calorimetry technique [37]. The ∆G f o and log -K sp values were calculated to be −5396.3 ± 9.3 kJ/mol and −17.25 ± 1.80 for the reaction Equation (11), respectively [37]; and the log -K sp was re-calculated to be −238.09 for the reaction Equation (1).
Tooeleite was stable only at high arsenite and sulfate concentrations and formed under the oxidation of Fe 2+ to Fe 3+ and the firm conservation of the trivalent oxidation state of arsenic [37].

Conclusions
For the dissolution of the synthetic tooeleite [Fe 6 (AsO 3 ) 4 (SO 4 )(OH) 4 ·4H 2 O], the dissolved arsenic concentrations exhibited a minimum of 427.3~435.8 mg/L As at 25 • C and initial pH 12.00 with the final pH 5.54. The constituents were dissolved preferentially in the sequence of SO 4 2− > AsO 3 3− > Fe 3+ at 25 • C and initial pH 2.00~12.00. The dissolved iron, arsenite and sulfate were present mainly as FeSO 4 + /Fe 3+ , H 3 AsO 3 0 and SO 4 2− at 25 • C and initial pH 2.00, and as Fe(OH) 3 0 /Fe(OH) 2 + , H 3 AsO 3 0 and SO 4 2− at 25 • C and initial pH 12.00, respectively. The tooeleite dissolution was characterized by the preferential release of SO 4 2− anions from solid surface into aqueous medium, which was controlled by the Fe-O/OH bond breakages and hindered by the outer OH − group layers. From the data of the dissolution at 25 • C and initial pH 2.00 for 6480~7920 h, the ion-activity product [log -IAP], which was very close to the solubility product [log -K sp ], and the Gibbs free energy of formation [∆G f o ] were computed to be −200.28 ± 0.01 and −5180.54 ± 0.07 kJ/mol for the synthetic tooeleite, respectively.

Supplementary Materials:
The following are available online at http://www.mdpi.com/2075-163X/10/10/921/s1, Figure S1: Change of the mole ratios between the solution components in the tooeleite dissolution at 25 • C and initial pH 2 or 12, Table S1: SEM-EDS analysis results of tooeleite before and after dissolution.