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Journal of Chemistry
Volume 2019, Article ID 9568360, 15 pages
https://doi.org/10.1155/2019/9568360
Research Article

Dissolution and Solubility of the Synthetic Natroalunite and the Arsenic-Incorporated Natroalunite at pH of 2.00–5.60 and 25–45°C

1College of Environmental Science and Engineering, Guilin University of Technology, Guilin 541004, China
2Guangxi Key Laboratory of Environmental Pollution Control Theory and Technology, Guilin University of Technology, Guilin 541004, China

Correspondence should be addressed to Yinian Zhu; nc.ude.tulg@nainiyuhz and Yanpeng Liang; nc.ude.tulg@gnepnaygnail

Received 16 August 2019; Accepted 16 October 2019; Published 3 November 2019

Academic Editor: Sedat Yurdakal

Copyright © 2019 Yinian Zhu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Arsenic is very harmful to most living organisms. The solubility data of As-containing compounds are significant in geoscience and environmental science. The arsenic-incorporated natroalunite precipitation has been proposed to eliminate arsenic from water, both for industrial practice and remediation of polluted areas. Unfortunately, only few works have been made on partial arsenic incorporation in natroalunite and the thermodynamic data for natroalunite and arsenic-incorporated natroalunite now are still lacking. Moreover, the dissolution mechanisms of arsenic-incorporated natroalunites have never been studied. In the present work, the dissolution of the synthetic natroalunite [Na0.93(H3O)0.61Al2.82(SO4)2(OH)6] and the synthetic arsenic-incorporated natroalunite [Na0.88(H3O)2.44Al2.35(AsO4)0.38(SO4)1.62(OH)6] at 25°C, 35°C, and 45°C was experimentally examined in HNO3 solution (pH of 2.00 and 4.00) and pure water. The characterizations confirmed that the solids showed no recognizable change after dissolution. All dissolutions underwent a pH variation, which was caused by a great depleting of H3O+/OH ions, typically at the reaction beginning. The dissolution in H3O+ medium proved to be near-stoichiometric within the short beginning period, and the dissolved Na+, Al3+, SO42−, and AsO43− concentrations were stoichiometric according to the initial solids and then appeared to be nonstoichiometric with the Na/SO4 mole ratios higher and the Al/SO4 and AsO4/SO4 mole ratios lower than the stoichiometry until the experimental end, indicating that the components were released from solid to solution preferentially after the following order: Na+ (H3O+) > SO42− > AsO43− > Al3+. From the experimental results under the condition of initial pH 2.00 and 25°C, the solubility products [Ksp] and the Gibbs free energies of formation [] were calculated to be 10−81.02±0.33∼10−81.04±0.27 and −4713 ± 2 to −4714 ± 1 kJ/mol for the natroalunite and 10−92.30±0.30∼10−92.41±0.37 and −5078 ± 2 to −5079 ± 2 kJ/mol for the arsenic-incorporated natroalunite, respectively. The thermodynamic quantities, , , , and , were determined to be 462303.43 J/K·mol, 122466.83 J/mol, −1140.39 J/K·mol, and 4280.13 J/K·mol for the natroalunite dissolution reaction at initial pH 2.00 and 25°C and to be 526925.48 J/K·mol, 159674.76 J/mol, −1232.38 J/K·mol, and 1061.12 J/K·mol for the dissolution of the arsenic-incorporated natroalunite at initial pH 2.00 and 25°C, respectively.

1. Introduction

The element arsenic (As) is very harmful to most animals and plants [1, 2]. Its high levels in air, water, and soil can result from a combination of human activities and natural processes [3, 4]. Arsenic is commonly found in mine waters and solid wastes from nonferrous metal mining and metallurgy, and many researchers have made efforts to develop techniques to eliminate and stabilize this toxicant, but it is still a very great challenge for nonferrous metal industries [5, 6]. Information about the processes controlling arsenic in environments is needed to predict its distribution in different phases. The solubility data of As-containing compounds are significant in geoscience and environmental science [7].

The precipitation of metal arsenates is an important process controlling arsenic levels in natural environment and industrial discharges. Acid mine drainage and industrial wastewaters are generally treated by adding low-cost calcium oxide/hydroxide to adjust effluent pH and reduce the arsenic release into natural waters through the formation of low-soluble calcium arsenate precipitates [812], which nevertheless are not stable and may release arsenate ions in some water environments [12]. Scorodite (FeAsO4·2H2O), one of the most frequent arsenate minerals in the oxidizing belt of nonferrous metal deposits, is considered to control the As levels in mining areas and also extensively studied for arsenic removal and residue-storage [13, 14], assuming that it is comparatively insoluble, but in fact, all Fe/Al arsenates are metastable and tend to form hydroxides and release arsenic remarkably into the aqueous environment [1517]. Besides, most acidic mine drainages and metallurgical discharges are sulfate-rich and arsenic-containing wastewaters, for which it is especially interesting to eliminate arsenic as crystalline sulfate-arsenate precipitates such as the Fe(SO4)x(AsO4)1−x(OH)x·(1 − x)H2O compounds with the lower arsenic release in leaching test than FeAsO4·2H2O [18].

Recently, many scholars aim to study the AsO4-for-SO4 replacement in natroalunite [NaAl3(SO4)2(OH)6], a member of the alunite supergroup [AB3(TO4)2(OH,H2O)6] that exists in oxidizing, acidic, and S/Al-rich environments, and it is very common and extremely stable in nature [19, 20]. The ideal natroalunite has a common formula of NaAl3(SO4)2(OH)6, made up of linear tetrahedral-octahedral-tetrahedral (T-O-T) laminas, where the Na+ ion sites with an AO6(OH)6 icosahedra coordination and the Al3+ and SO42− ion sites with slightly distorted AlO6 octahedra (O) and SO4 tetrahedral (T) coordinations, respectively [21]. The A sites are commonly filled by K+, Na+, and H3O+, the B sites by Fe3+ and Al3+, and the TO4 sites by SO42−, PO43−, and AsO43−, respectively. Multiple replacements are possible due to the charge balance by the coupled replacement at different sites, the OH/TO4 protonation, and the cationic vacancy [22, 23]. Therefore, the alunite group minerals may act as a provisional store for the substituted toxic metal(loid)s and then release these toxic metal(loid)s when the minerals dissolve in water [19, 24]. Since so many different ions can substitute into the natroalunite lattice, the arsenic-incorporated natroalunite (As-natroalunite) precipitation has been proposed to eliminate arsenic from water, both for industrial practice and remediation of polluted areas [22, 25].

The incorporation of AsO4 in the SO4 sites of alunite can achieve 15 mol% [23, 26]. Moreover, the alunite supergroup minerals are more stable than the jarosite supergroup minerals since the trivalent aluminum ion is unreducible in the anoxic milieu. Their solubility, stability, and mobility depend not only on the solution pH but also on the degree of isomorphous incorporation in the alunite structure [22]. Lately, the replacement of arsenate for sulfate in natroalunite is examined by some scholars [2730]. Nevertheless, few works have been carried out on the arsenic incorporation in natroalunite. The hydrothermal-precipitated natroalunite could incorporate about 8 mol% AsO4 in its structure and release <1 mg/L As in the leaching tests at pH 4-5 for 6 months [30]. The formation of arsenic-incorporated natroalunites has been also proposed to stabilize the complex arsenic-containing wastes of the nonferrous metal industry [16].

However, the forming conditions of arsenic-incorporated natroalunites within a broad pH range have not been thoroughly examined, and few basic works on the isomorphous replacement have been made [30]. In this paper, pure crystalline natroalunite and pure crystalline arsenic-incorporated natroalunite (As-natroalunite) were synthesized by a simple hydrothermal technique and then characterized using several instruments to investigate the influences of the arsenic incorporation on the structure and morphology of the arsenic-incorporated natroalunite. The dissolution mechanisms of the synthetic natroalunite and the arsenic-incorporated natroalunite were studied at different temperatures and initial pHs. Moreover, the total solution concentrations of sodium, aluminum, sulfate, and arsenate from the dissolution experiments were applied to calculate their solubility product and Gibbs free energy of formation.

2. Experimental Methods

2.1. Solid Preparation and Characterization
2.1.1. Solid Preparation

The arsenic-free natroalunite was hydrothermally prepared after the following reaction with the stoichiometric Na : Al : SO4 mole ratio of 1 : 3 : 2 : Na+ + 3Al3+ + 2SO42− + 6OH = NaAl3(SO4)2(OH)6 [7]. The arsenic-incorporated natroalunite was prepared with a Na+ excess because of the adding of NaOH to adjust solution pH, and aluminum was therefore the controlling constituent in the synthesis of the arsenic-incorporated natroalunite. Firstly, a Na + Al precursor solution was prepared by dissolving 35.9872 g of Al2(SO4)3·18H2O and 2.5567 g of Na2SO4 in 50 mL pure water with stirring at 600 r/min for 30 min. 150 mL pure water or the mixture of 20 mL of 1 M H3AsO4 solution and 130 mL pure water were then added into the Na + Al solution. After this, the resulting mixing slurries were regulated to pH 4.00 with 1M NaOH solution, quickly agitated for 30 min, and finally heated at 200°C for 2∼48 h in 200 mL stainless-steel autoclaves. After cooling, the obtained products were filtered, washed three times with pure water, and dried at 110°C for 24 h.

2.1.2. Characterization

The solid chemical components were determined by dissolving 0.05 g of the synthetic arsenic-free natroalunite or the synthetic arsenic-incorporated natroalunite in 20 mL of 6 mol/L chlorhydric acid and then diluted in 50 mL volumetric flasks with ultrapure water. The total concentrations of sodium, aluminum, arsenic, and sulfur were analyzed using an inductively coupled plasma-optical emission spectrometer (ICP-OES, PerkinElmer Optima 7000 DV). Assuming the structure of S + As = 2, the formula coefficients for the synthetic solids were then calculated. All synthetic natroalunites before and after the dissolution experiment were investigated by an X-ray diffractometer (XRD, X’Pert PRO) operated at 40 kV and 40 mA with Cu Kα radiation over the scanning 2θ range of 10°∼90° at the interval of 0.10°/minute. Besides, the solids were characterized using a Fourier-transform infrared spectrophotometer (FT-IR, Nicolet Nexus 470) over 4000∼400 cm−1 in the KBr pellet. The solid morphologies were examined under a field-emission scanning electron microscope (FE-SEM, Hitachi S-4800).

2.2. Dissolution Experiment

For each of the dissolution experiments, four grams of the synthetic natroalunite or the synthetic arsenic-incorporated natroalunite was weighed and then put in 100 mL polypropylene bottle. And afterwards, 100 mL of nitric acid solution (pH 2.00/4.00) or pure water (pH 5.60) was filled. All bottles were capped and placed in three water baths of different temperatures (25°C, 35°C, and 45°C). The mixed solid solutions were stirred using magnetic stirrers. Their pHs were allowed drifting freely and monitored by measurement at regular time. During the dissolution experiments, the aqueous solutions (5 mL) were collected from each bottle at 22 time intervals (1 h, 3 h, 6 h, 12 h, 1 d, 2 d, 5 d, 10 d, 15 d, 20 d, 30 d, 45 d, 60 d, 75 d, 90 d, 120 d, 150 d, 180 d, 210 d, 240 d, and 300 d), filtered into the 25 mL volumetric flask using 0.22 μm disposable syringe filter and immediately acidified with 0.2% nitric acid solution before storing them refrigerated for analysis. 5 mL of the acidic solution or ultrapure water was supplemented after each solution sampling and their dilution effects were taken into account in the PHREEQC calculation. The total dissolved concentrations of sodium, aluminum, arsenic, and sulfur were also determined using the PerkinElmer ICP-OES instrument. The total dissolved concentrations of sulfur and arsenic were given below as sulfate and arsenate since they are the dominating species under the dissolution condition. The dissolution tests of the samples at 25°C and initial solution pH 2.00 were performed in duplicate to check the reproducibility of the results.

2.3. Thermodynamic Calculation

Speciation-solubility calculation was carried out with the pHs, the temperatures, and the total dissolved concentrations of sodium, aluminum, arsenic, and sulfur from the batch dissolutions. The aqueous activities of Na+, Al3+, AsO43−, SO42−, and OH were calculated using PHREEQC with its minteq.v4.dat database, and the solubility product (Ksp) was computed in terms of the mass-action expression of the stoichiometric dissolution. The minteq.v4.dat database includes all thermodynamic data from the MINTEQA2 database file [3133]. The Debye–Hückel equations were applied in the PHREEQC calculation since all solution ionic strengths in the present experiment were lower than 0.023 mol/L.

3. Results and Discussion

3.1. Solid Characterization
3.1.1. Chemical Composition

The formula coefficients for the synthetic natroalunite and the synthetic arsenic-incorporated natroalunite were estimated from the ICP chemical analysis to be Na0.93(H3O)0.61Al2.82(SO4)2(OH)6 and Na0.88(H3O)2.44Al2.35(AsO4)0.38(SO4)1.62(OH)6, respectively. The precipitates almost always had an apparent Al deficiency in the B3+ sites and an appreciable Na deficiency in the A+ sites (7∼12%) of the natroalunite AB3(TO4)2(OH)6. The latter might reflect an incorporation of H3O+ to form a solid solution that was hardly proved since H3O+ cannot be measured directly using microprobe technique or wet chemistry [34]. Up to 14% of the B3+ sites could be unoccupied, which were also probably charge-balanced by H3O+ [35]. Hereafter, there might be a large degree of changeability in the natroalunite component, which will in turn influence the solution Na : Al :As : S ratio during dissolution.

3.1.2. X-Ray Diffractometric Analysis

The XRD patterns of the synthetic natroalunite and the arsenic-incorporated natroalunite before and after dissolution are shown in Figure 1. For the natroalunite and the arsenic-incorporated natroalunite prepared, the natroalunite-type mineral was the only phase in comparison with the Reference code 01-089-3952 for natroalunite. The reflection peaks of the synthetic arsenic-incorporated natroalunite shifted gradually to a lower angle with the increase in the AsO4-for-SO4 replacement [7]. The cell parameters a and c were determined to be 6.9870 Å and 16.5840 Å for the synthetic natroalunite and 7.0016 Å and 16.7053 Å for the arsenic-incorporated natroalunite, respectively. The cell parameter a of ∼7 Å predominantly depends on the B atomic radius and the cell parameter c of ∼17 Å on the A and T atomic radius. Due to the significantly greater As-O bond length (∼1.57 Å) than the S-O bond length (∼1.45 Å) in the TO4 tetrahedra that is perpendicular to the c axis, the AsO4-for-SO4 replacement would enlarge especially the parameter c of natroalunite [36]. Additionally, the A+ ions in the 12-fold coordination [AO6(OH)6] polyhedra that lie between two TO4-tetrahedra layers can strongly influence the interlayer spacing in the natroalunite crystals. Thus, when significant H3O+ substitution happened together with the AsO4-for-SO4 substitution in the crystal lattice, both substitution effects overlapped [28, 30, 36].

Figure 1: X-ray diffraction patterns of the synthetic natroalunite and the synthetic arsenic-incorporated natroalunite before and after dissolution at 25–45°C for 300 d.
3.1.3. Fourier-Transform Infrared Spectra Analysis

The M-O stretching vibration, the AsO4 vibration, the SO4 vibration, and the OH bending/stretching vibration could be identified in the FT-IR spectra for the natroalunite and the arsenic-incorporated natroalunite before and after dissolution (Figure 2). The stretching vibration bands of the AlO6 octahedra were found to be around 513∼515 and ∼480 cm−1 [24, 30, 37, 38]. The 1026∼1030 cm−1 bands, the 426∼434 cm−1 bands, the ∼1207 and ∼1099 cm−1 bands, and the 663∼665 and 627∼631 cm−1 bands were assigned to the (SO4) symmetric stretching vibration, the (SO4) symmetric stretching vibration, the (SO4) antisymmetric stretching vibration, and the (SO4) vibration, respectively [24, 30]. The 604 cm−1 bands and the 3452∼3460 cm−1 bands belonged to the OH bending vibration and the OH stretching vibration, respectively [24, 30]. The 1637∼1639 cm−1 vibration bands were attributed to the framework deformation of powerful H-bonded H2O molecule [30, 37]. Additionally, the symmetric stretching vibration (AsO4) around 895 cm−1 was recognized for the arsenic-incorporated natroalunite.

Figure 2: Fourier-transform infrared spectra of the synthetic natroalunite and the synthetic arsenic-incorporated natroalunite before and after dissolution at 25–45°C for 300 d.
3.1.4. Scanning Electron Micrograph Analysis

The morphology of the synthetic natroalunite and the arsenic-incorporated natroalunite before and after dissolution at initial pH 2.00 and 25°C is illustrated in Figure 3. The precipitates showed pseudo-octahedral crystal forms (2∼5 μm size) with certain nanosized fine grains, which were also approved to be the natroalunite phase by ICP and XRD analyses. The arsenate-for-sulfate substitution did not change the morphologies of the natroalunite crystals, i.e., the pseudo-octahedra (regular ditrigonal scalenohedra), which looked like spheroid due to the probable precipitation-dissolution process. The trigonal crystals of natroalunite are morphologically rhombohedra with a 90°50′ interfacial angle, which makes them look like cubes [28, 30] or the ditrigonal scalenohedral looks like octahedra in the present work (Figure 3). Nevertheless, even in the case of the extreme dissolution at initial pH 2.00 and 25°C what appeared to be the original crystal boundaries remained well-preserved.

Figure 3: Field-emission scanning electron microscopic images of the synthetic natroalunite and the synthetic arsenic-incorporated natroalunite before and after dissolution at 25°C for 300 d.
3.2. Evolution of Solution pH and the Total Elemental Concentrations during Dissolution

The total dissolved concentrations of sodium, aluminum, arsenic, and sulfur and their mole ratios for the dissolution of the arsenic-free and arsenic-incorporated natroalunites at different initial pHs and temperatures are plotted versus the long-term dissolution time in Figures 4 and 5.

Figure 4: Aqueous evolution during the dissolution of the synthetic natroalunite at 25–45°C for 300 d.
Figure 5: Aqueous evolution during the dissolution of the synthetic arsenic-incorporated natroalunite at 25–45°C for 300 d.

For the dissolution of the synthetic natroalunite and the synthetic arsenic-incorporated natroalunite at initial pH 2.00 and 25°C (Figures 4 and 5), the solution pHs increased from 2.00 to 4.10 and from 2.00 to 4.00 within 120 h of dissolution and then varied between 3.96∼4.34 and 3.93∼4.22, respectively. For the dissolution at initial pH 4.00 and 25°C, the solution pHs increased from 4.00 to 4.61∼5.47 within the first 3 h of interaction and then decreased gradually and finally varied between 4.14∼4.43 after 48 h. For the dissolution at initial pH 5.60 and 25°C, the solution pHs decreased from 5.60 to 4.55 and from 5.60 to 5.13 within 24∼72 h of dissolution and then varied between 4.33∼4.68 and 4.62∼5.21, respectively. It is consistent with the results that the solution pHs declined to pH ∼3.8 for the dissolution of jarosite in the water of pH 4, while in its dissolution at pH 3, the solution pHs rose to about 3.2, which means that jarosite and its dissolution resultants had certain pH buffering capability [25]. Various dissolution mechanisms dominated at high and low pHs, i.e., jarosite dissolution could either increase the pHs of aqueous acidic mediums (initial pHs <3∼4) or decrease the pHs of aqueous subneutral-alkaline mediums due to the H+ depleting as the dissolution reactions proceeded [39]. Two different dissolution mechanisms for jarosite occurred at low (<3.5) and high (>3.5) pHs. Under low pH (<3.5) condition, the solution pHs increased gradually with time, suggesting H+ depleting [40]. This result indicated that the jarosite dissolution at pH <3.5 followed the mechanism that consumed H+ and produced aqueous Fe3+ species. Particularly in the low pH solution with H2SO4, aqueous Fe3+-SO42− complexes would control the aqueous Fe3+ speciation. On the contrary, under high pH (>3.5) condition, the solution pHs declined with time, indicating a OH consumption in the dissolution [40].

The dissolved Na+ concentrations rose quickly at the start and attained the highest values after about 240 h of dissolution. A steady state was approached after 2160 h of interaction. The AsO4-for-SO4 replacement had a great influence on the Na+ release, i.e., the dissolved Na+ concentrations increased significantly with the increase of this substitution. The increase of the initial solution pH was not beneficial for the Na+ release into solution. The temperature showed also an obvious influence on the Na+ release. For the dissolution of the synthetic natroalunite, the Na+ release rates increased in the following order: 25°C > 45°C > 35°C. For the dissolution of the synthetic arsenic-incorporated natroalunite, the Na+ release rates increased in the following order: 35°C > 45°C > 25°C.

The dissolved Al3+ concentrations rose quickly at the start and attained the highest values after about 12 h of dissolution and then declined slowly. After 2160 h of interaction, a steady status was approached. The AsO4-for-SO4 replacement had a great influence on the Al3+ release. On the contrary to the Na+ release, the dissolved Al3+ concentrations decreased significantly with the increase of this substitution. The increase of the initial solution pH could obviously decrease the Al3+ release into solution. The temperature showed also an obvious influence on the Al3+ release. For the dissolution of the synthetic natroalunite, the Al3+ release rates increased in the following order: 25°C > 35°C > 45°C. For the dissolution of the synthetic arsenic-incorporated natroalunite, the Al3+ release rates increased in the following order: 35°C > 45°C > 25°C.

The dissolved SO42− concentrations generally fluctuated within the first 2880 of the reaction and then gradually reached a steady state. The dissolved SO42− concentrations increased significantly with the increase of the AsO4-for-SO4 replacement. For the dissolution of the synthetic natroalunite, the dissolved SO42− concentrations rose quickly at the start and attained the highest values after about 480–1800 h of dissolution and then a steady state was gradually approached. The increase of the initial solution pH was not beneficial for the SO42− release into solution. The SO42− release rates increased in the following order: pH 2.00 > pH 4.00 > pH 5.60 and 25°C ≈ 35°C > 45°C. For the dissolution of the synthetic arsenic-incorporated natroalunite, the SO42− release rates increased in the following order: pH 4.00 > pH 2.00 > pH 5.60 and 25°C > 35°C∼45°C.

The dissolved AsO43− concentrations increased significantly with the increase of the AsO4-for-SO4 replacement. For the dissolution of the arsenic-incorporated natroalunite at initial pH 2.00, the dissolved AsO43− concentrations rose quickly at the start and attained the highest values after about 6–24 h of dissolution. After that, the dissolved AsO43− concentrations decreased gradually and a steady state was gradually approached after 2880 h of dissolution. The temperature increase was beneficial for the AsO43− release into solution. The dissolved AsO43− concentrations for the dissolution at 35°C and 45°C were slightly greater than those at 25°C. For the dissolution at the initial pHs of 4.00 and 5.60, the dissolved AsO43− concentrations increased gradually in the first 720 h and then approached to a steady state.

3.3. Overall Dissolution Reaction and Mechanism

The solution pHs were always greater than 2.00 for the dissolution at initial pH 2.00. The hydrion depleting indicated that the H+ sorption onto negatively charged oxygen ions of the arsenate and sulfate groups of (As-)natroalunite might result in the conversion of AsO43− and SO42− to H2AsO4 and HSO4 on the natroalunite surface and accelerate the dissolution process. Furthermore, the dissolution/exchange coexistence suggested that the hydrion depleting was caused not only by the hydrion adsorption/desorption but by many different reactions on the (As-)natroalunite surface [7]. Therefore, a comprehensive elucidation of the hydrion depleting should include at least the stoichiometric dissolution of (As-)natroalunite and the speciation of dissolved components in solution.

3.3.1. Natroalunite [Na0.93(H3O)0.61Al2.82(SO4)2(OH)6]

Dissolution in the present experiment did not result in the formation of any measurable precipitates based on the XRD and FE-SEM results (Figures 1 and 3), which was in accordance with the saturation index calculation with PHREEQC. The stoichiometric dissolution reaction of the synthetic natroalunite [Na0.93(H3O)0.61Al2.82(SO4)2(OH)6] could be simply expressed by the following dissolution equation:

The reaction could produce 5.39 mol OH for a mole of dissolved natroalunite. Thus, the natroalunite dissolution at lower initial pH could result in a pH increase, suggesting hydrion depleting or OH releasing. This indicated that the natroalunite dissolution at lower pHs followed a reaction mechanism which consumed hydrions and the Al/sulfate complexes should dominate the aqueous speciation of Al3+:

For the dissolution of the synthetic natroalunite [Na0.93(H3O)0.61Al2.82(SO4)2(OH)6] at 25°C and initial pH 2.00 (Figure 4), the dissolved Na+, Al3+, and SO42− concentrations rose quickly and simultaneously in the first 0–3 h of interaction with the aqueous Na/Al mole ratios of 0.30–0.34, the aqueous Al/SO4 mole ratios of 1.44–1.59, and the aqueous Na/SO4 mole ratios of 0.54–0.71, which were in close proximity to the stoichiometric Na/Al, Al/SO4, and Na/SO4 mole ratios of 0.33, 1.41, and 0.47 for the synthetic natroalunite, respectively. This result is similar to the observation in the dissolution experiments of alunite [20], arsenojarosite [41], Na-jarosite [19, 39], and K-jarosite [21, 40], i.e., nearly congruent dissolutions were observed at low pHs, whereas incongruent dissolutions were found at circum-neutral to high pHs. Two different dissolution mechanisms, one for lower pHs and the other for higher pHs, might take place [42].

A shift from congruent to incongruent dissolution could be observed over the longer term [19]. As the time increased, the synthetic natroalunite dissolved nonstoichiometrically and released Na+ preferentially over Al3+ to the aqueous solution, and the aqueous Na/Al and Na/SO4 mole ratios increased and the aqueous Al/SO4 mole ratios decreased rapidly. After 3 h, the aqueous Na+ and Al3+ concentrations as well as the solution pHs increased continuously, while the total aqueous SO42− concentrations decreased rapidly. From 12 h to 2880 h, the pHs approached a steady state (3.96∼4.34), the aqueous Na+ and SO42− concentrations increased, while the aqueous Al3+ concentrations decreased slowly. The cations in the A sites could be preferentially released to solution at the start of dissolution, while the cations in the B sites would be preferentially retained in solid [19]. Alunite dissolved almost congruently for the experiment at pH around 3 (i.e., dissolved Al/K mole ratio ≈3) or incongruently for the experiment at pH ≥ 3.9 (i.e., dissolved Al/K mole ratio <3) [42].

After 2880 h of dissolution, the solution pHs and the total solution Na+, Al3+, and SO42− concentrations tending to reach the steady state with the stoichiometric Na/Al, Al/SO4, and Na/SO4 mole ratios were around 6.54–7.35, 0.24–0.29, and 1.74–2.03, which were significantly higher than the stoichiometric Na/Al, Al/SO4, and Na/SO4 mole ratios of 0.33, 1.41, and 0.47 for the synthetic natroalunite, respectively (Figure 4). This means that Na+ cations preferred to be released from solid to solution relative to Al3+ and SO42− ions and the SO42− anions preferred to be released from solid to solution relative to Al3+ ions, i.e., the components were released from solid to solution preferentially after the following order: Na+ (H3O+) > SO42− > Al3+. Additionally, this differential tendency became more obvious with the increasing dissolution temperature and initial solution pHs. Selective release of Na+ in the A site and SO42− in the T site resulted in higher aqueous Na+ and SO42− concentrations compared to Al3+ deep in the T-O-T structure of natroalunite [21]. Similar selective release of K+ and SO42− was also reported for the jarosite dissolution [21, 43]. Jarosite dissolved near-congruently in the first few days and then incongruently with a decrease of the dissolved Fe3+ concentration and an increase of the dissolved SO42− concentration, which would result in the formation of an Fe-rich residuum [19].

The speciation calculation using PHREEQC showed that for the natroalunite dissolution at initial pH 2.00 and 25°C, the aluminum species appeared in the sequence of AlSO4+ > Al3+ > Al(SO4)2 > Al(OH)2+ > Al(OH)2+ > AlH-SO42+ > Al(OH)30 > Al(OH)4; the sulfate species appea-red in the sequence of SO42− > AlSO4+ > NaSO4 >Al(SO4)2 > HSO4 > AlHSO42+. The concentrations of all sodium, aluminum, and sulfate species attained a steady state at the experimental end.

3.3.2. As-Natroalunite [Na0.88(H3O)2.44Al2.35(AsO4)0.38(SO4)1.62(OH)6]

The stoichiometry of the synthetic As-natroalunite dissolution reaction varied as a function of time, solution pH, and temperature. The speciation of dissolved Al3+, AsO43−, and SO42− can strongly affect the pH-dependence of the dissolution stoichiometry [19]. The stoichiometric dissolution of the synthetic arsenic-incorporated natroalunite [Na0.88(H3O)2.44Al2.35(AsO4)0.38(SO4)1.62(OH)6] can be expressed by the general reaction as follows:

Theoretically, one mole of the arsenic-incorporated natroalunite would release 3.56 mol OH according to this dissolution reaction. Consequently, its dissolution at lower initial pH could result in a pH increase with time, suggesting OH release or H+ depletion. The As-natroalunite dissolution at lower pHs followed the reaction mechanisms that consumed H+ ions, and the aqueous Al/sulfate/arsenate complexes should dominate the aqueous speciation of Al3+ (equations (2), (3), and (5)).

For the dissolution of the synthetic As-natroalunite [Na0.88(H3O)2.44Al2.35(AsO4)0.38(SO4)1.62(OH)6] at 25°C and initial pH 2.00 (Figure 5), the solution Na/Al mole ratios reached 6.84–5.21 in the first hour dissolution and then increased gradually with time; the aqueous AsO4/(AsO4 + SO4) mole ratios increased with time and reached 0.08–0.10 in 6–12 h dissolution and then decreased gradually with time; the aqueous Na/(AsO4 + SO4) mole ratios fluctuated greatly within 5040 h dissolution; the aqueous Al/(AsO4 + SO4) mole ratios decreased generally with time.

The transient peak values in the total dissolved Na+, Al3+, AsO43−, and SO42− concentrations during the mineral dissolution between 3 and 240 h were also reported in earlier researches, which was possibly related to the particle size dispersion [19, 44]. The smaller the particle size of the As-natroalunite, the larger its dissolution rate and solubility. Henceforth, the transient highest concentrations might result from the faster dissolution of finer particles and the following reprecipitation to coarser particles [44].

After 5760 h of dissolution, the solution pHs and the total dissolved Na+, Al3+, AsO43−, and SO42− concentrations became constant and reached the steady state with the stoichiometric Na/Al, and Na/(AsO4 + SO4) mole ratios were around 24.73–28.08 and 2.29–2.39, which were significantly higher than the stoichiometric Na/Al and Na/(AsO4 + SO4) mole ratios of 0.37 and 0.44 for the synthetic As-natroalunite, and the stoichiometric AsO4/(AsO4 + SO4) and Al/(AsO4 + SO4) mole ratios were around 0.02 and 0.08–0.10, which were significantly lower than the stoichiometric AsO4/(AsO4 + SO4) and Al/(AsO4 + SO4) mole ratios of 0.16 and 1.18 for the synthetic As-natroalunite, respectively (Figure 5). This means that Na+ cations preferred to be released from solid to solution relative to Al3+, AsO43−, and SO42− ions, the SO42− anions preferred to be released from solid to solution relative to AsO43− anions, and the AsO43− and SO42− anions preferred to be released from solid to solution relative to Al3+ cations, i.e., the components were released from solid to solution preferentially after the following order: Na+ (H3O+) > SO42− > AsO43− > Al3+, suggesting incongruent dissolution of the (As-)natroalunite and/or formation of AsO4/Al-rich residuals as the alunite dissolution at pH > 5 [20].

K-Cr(VI)-jarosites dissolved congruently in acidic solutions and incongruently in alkaline solutions [45]. The dissolution rate was properly limited by the release of both B or TO4 groups from natroalunite, which would be hindered by the outer Na+ and OH group layers that were energetically stable in the solvated surface [42]. The dissolution was restrained by the Al–O bond breakage in the AB3(TO4)2(OH)6 structure of the alunite supergroup [40, 45]. Al3+ ions were left behind either as a reprecipitated Al-rich phase or as a residue of the alunite octahedral layers in the solid phase [25]. The degree of replacement of other ions into the AB3(TO4)2(OH)6 structure can affect the stability and reactivity of the alunite supergroup minerals [25].

Additionally, this differential tendency became more obvious with the increasing dissolution temperature and initial solution pHs. For the dissolution of the synthetic As-natroalunite [Na0.88(H3O)2.44Al2.35(AsO4)0.38(SO4)1.62(OH)6] at 25°C and initial pH 4.00/5.60 (Figure 5), the solution Na/Al mole ratios were significantly higher and the Na/(AsO4 + SO4), Al/(AsO4 + SO4) and AsO4/(AsO4 + SO4) mole ratios were lower than the values for their dissolution at initial pH 2.00. For the dissolution of the synthetic As-natroalunite [Na0.88(H3O)2.44Al2.35(AsO4)0.38(SO4)1.62(OH)6] at 35°C/45°C and initial pH 2.00 (Figure 5), the aqueous Na/Al and Na/(AsO4 + SO4) mole ratios were lower and the Al/(AsO4 + SO4) and AsO4/(AsO4 + SO4) mole ratios were higher than the values for their dissolution at 25°C, which was similar to the experimental result of the K-Cr(VI)-jarosite dissolution [45]. All the performed dissolutions undertook a pH variation that was related to the great H3O+/OH depleting, extraordinary at the experimental start, i.e., within the short induction period, which was reflected in a drastic pH increase/decrease [45]. The arsenic substitution into jarosite can limit the ability of the OH attachment onto the gain surfaces and cause a H2O attach, which could be related to the electrostatic repulsion of the OH ions at higher pH owing to the AsO4 bonds on the surface; moreover, the increase in the Fe-AsO4 bond number can inhibit the systems’ dependency on H3O+/OH [41]. This phenomenon was not found in the K-Cr-jarosite dissolution, suggesting an incessant dependency of the reaction on the H3O+/OHconcentration [45].

The speciation calculation using PHREEQC showed that for the As-natroalunite dissolution at initial pH 2.00 and 25°C, the sodium species appeared according to the sequence of Na+ > NaSO4 > NaH2AsO40 > NaHAsO4 >NaAsO42−; the aluminum species appeared according to the order of AlSO4+ > Al3+ > Al(SO4)2 > Al(OH)2+ >Al(OH)2+ > AlHSO42+ > AlH2AsO42+ > Al(OH)30 > Al(OH)4 >AlHAsO4+ > AlAsO40; the arsenate species appeared according to the order of H2AsO4 > NaH2AsO40 > H3AsO40 > HAsO42−> AlH2AsO42+ > NaHAsO4 > NaAsO42− > AlHAsO4+ > AsO43−> AlAsO40; the sulfate species appeared according to the order of SO42− > AlSO4+ > NaSO4 > HSO4 > Al(SO4)2 > AlHSO42+. At the dissolution start, the concentrations of all sodium, aluminum, arsenate, and sulfate species except HSO4 and AlHSO42+ increased with the dissolution time and finally attained a steady state. The dissolved sodium and sulfate were principally present as Na+ and SO42− within the entire pH range, correspondingly. Nevertheless, the speciation of dissolved aluminum and arsenate displayed an obvious variation with pHs.

For the dissolution of the synthetic natroalunite and the synthetic As-natroalunite at 25°C and initial pH 2.00 (Figures 4 and 5), all solutions were undersaturated with respect to Al(OH)3 from the PHREEQC calculation, i.e., aluminum (hydr-)oxides were unlikely to precipitate. Instead, it was likely to form some aluminum-rich and arsenic-rich reaction products. Aluminum hydroxides were possibly expected to form only at pH 4.00 and 5.60 because the solutions were oversaturated-saturated with respect to Al(OH)3, although no secondary precipitate on the surface was detected by the XRD and SEM results due to the very small amounts of formation (Figures 1 and 3).

The mechanism for the As-natroalunite dissolution at high pHs shifted from H+ to OH mediated process because of the Al3+ hydrolysis [20]. The saturation calculation with PHREEQC for the dissolution at initial pH 2.00 indicated that all solutions were clearly undersaturated as for all possible aluminum (hydr-)oxides (amorphous aluminum hydroxide [Al(OH)3(a)], gibbsite [Al(OH)3], diaspore [AlOOH], and AlAsO4·2H2O, Al4(OH)10SO4), whose precipitation was thermodynamically unfavorable. In contrast, most of the solutions at initial pH 4.00 and 5.60 were nearly saturated with respect to these aluminum (hydr-)oxides, and some precipitation of these phases could not be absolutely ruled out.

3.4. Determination of Solubility

The solubility product (Ksp) is equal to the ion activity product (IAP) at the dissolution equilibrium, i.e., the Ksp calculation requires that the equilibrium has been attained, which could be simplified using sufficient solid to guarantee that excess (As-)natroalunite remained at equilibrium.

The aqueous activities of Na+, Al3+, AsO43−, SO42−, and OH in the last three equilibrated solutions (5760 h, 6480 h, and 7200 h) were calculated using PHREEQC with the minteq.v4.dat database [31] to determine the solubility products (Ksp) of the synthetic natroalunite and the arsenic-incorporated natroalunite. The solution species Na+, NaAsO42−, NaHAsO4, and NaH2AsO40 were considered in the simulation for the total sodium; Al3+, Al(OH)4, Al(OH)3, Al(OH)2+, AlOH2+, Al2(OH)24+, Al3(OH)45+, Al13O4(OH)247+, AlAsO40, AlHAsO4+, AlH2AsO42+, AlHSO42+, Al(SO4)2, and AlSO4+ for the total aluminum; H3AsO4, AsO43−, HAsO42−, H2AsO4, NaH2AsO40, NaHAsO4, NaAsO42−, AlAsO40, AlHAsO4+, and AlH2AsO42+ for the total arsenate; SO42−, HSO4, AlHSO42+, Al(SO4)2, and AlSO4+ for the total sulfate.

The stoichiometrical dissolution reactions of the synthetic natroalunite [Na0.93(H3O)0.61Al2.82(SO4)2(OH)6] and the synthetic arsenic-incorporated natroalunite [Na0.88(H3O)2.44Al2.35(AsO4)0.38(SO4)1.62(OH)6] can be described with the dissolution reactions (1) and (4), respectively.

Presuming unit activity of the solid is as follows:where {} is the thermodynamic activity and Ksp is the solubility product according to the dissolution reactions (1) and (4).

The Gibbs free energies of formation, [Na0.93(H3O)0.61Al2.82(SO4)2(OH)6] and [Na0.88(H3O)2.44Al2.35(AsO4)0.38(SO4)1.62(OH)6], can also be estimated by the use of the following thermodynamic data in the literature [46, 47], [Na+] = −262 kJ/mol, [Al3+] = −489.4 kJ/mol, [AsO43−] = −647.618 kJ/mol, [SO42−]= −744.6 kJ/mol, [OH] = −157.3 kJ/mol, and [H2O] = −237.18 kJ/mol. The Ksp value is related to the standard free energy of reaction (ΔGro) and can be expressed under the standard condition (298.15 K and 0.101 MPa) by

For (1),

Rearranging,

For (4),

Rearranging,

The calculated solubility products together with the total dissolved concentrations of sodium, aluminum, arsenic, and sulfur including solution pHs in the last three equilibrated solutions (5760 h, 6480 h and 7200 h) are given in Table 1 for the synthetic natroalunite and Table 2 for the synthetic arsenic-incorporated natroalunite.

Table 1: Analytical data and solubility product of the synthetic natroalunite [Na0.93(H3O)0.61Al2.82 (SO4)2(OH)6].
Table 2: Analytical data and solubility product of the arsenic-incorporated natroalunite [Na0.88(H3O)2.44Al2.35(AsO4)0.38(SO4)1.62](OH)6].

From the experimental results under the condition of initial pH 2.00 and 25°C, the solubility product [Ksp] and the Gibbs free energies of formation [Na0.93(H3O)0.61Al2.82(SO4)2(OH)6] had been calculated to be 10−81.02±0.33∼10−81.04±0.27 and −4713 ± 2∼−4714 ± 1 kJ/mol kJ/mol for the natroalunite correspondingly. Few Ksp and data for natroalunite have been reported previously in the literature. For natroalunite (NaAl3(SO4)2(OH)6), the value of −4614.02 kJ/mol was determined [48] from the Ksp value of 1079.7 [49]; besides, the value of −4622.40 kJ/mol was presented [34].

The thermodynamic property of the arsenic-incorporated natroalunite has scarcely been examined previously. In the present work, with the arsenic incorporation increasing from 0% to 19%, the Ksp value for the arsenic-incorporated natroalunite decreased from 10−81.02±0.33∼10−81.04±0.27 to 10−92.30±0.30∼10−92.41±0.37, and the corresponding values from −4713 ± 2 to −4714 ± 1 kJ/mol to −5078 ± 2 to −5079 ± 2 kJ/mol, respectively. Generally, the Ksp and values for the natroalunite and the arsenic-incorporated natroalunite increased with the increasing dissolution temperature. They are also more soluble at initial pH of 2.00 than at initial pH of 4.00 and 5.60.

3.5. Determination of Thermodynamic Data

The solubility products (Ksp) for the synthetic natroalunite and the arsenic-incorporated natroalunite can also be estimated using the following equations [50]:

For the synthetic natroalunite (Natroal-1),

For the arsenic-incorporated natroalunite (As-Natroal-1).

For the arsenic-incorporated natroalunite (As-Natroal-2),

The thermodynamic quantities , , , and for the dissolution of the synthetic natroalunite and the arsenic-incorporated natroalunite at initial pH 2.00 were estimated after equations (17)∼(20) [50] and are given in Table 3:where A, B, and C are the empirical coefficients of equation (13).

Table 3: Thermodynamic data of the natroalunite and the arsenic-incorporated natroalunite in aqueous acidic media (initial pH 2).

The ΔG° values for the dissolution of the synthetic natroalunite and the synthetic arsenic-incorporated natroalunite increased with the temperature increase which showed that their dissolutions were energy expending processes [50]. The positive value of ΔH° indicated also that the dissolution was an endothermic reaction and the solubility increased with the increase in temperature. The higher ΔS° value for the natroalunite dissolution indicated that the order degree caused by SO42− was greater than by AsO43− in aqueous solution, which is related to the different bond distances (S-O 1.473 Å and As-O 1.682 Å) [50, 51]. The smaller SO42− with a low charge can result in greater entropy, and consequently the synthetic natroalunite was more soluble than the arsenic-incorporated natroalunite in aqueous media. The higher ΔCp° value coupled with the higher ΔS° value for the dissolution of the synthetic natroalunite in comparison to the corresponding values for the dissolution of the synthetic arsenic-incorporated natroalunite (Table 3) suggested a significant interaction of SO42− with the aqueous acidic media and a higher solubility of the natroalunite in the aqueous acidic media. But the incorporation of H3O+ into the natroalunite crystal structure with the substitution of SO42− by AsO43− can increase the ΔS° values for the As-natroalunite dissolution and increase their solubility.

4. Conclusions

The lattice parameters were determined to be a = 6.9870 Å and c = 16.5840 Å for the synthetic natroalunite [Na0.93(H3O)0.61Al2.82(SO4)2(OH)6] and a = 7.0016 Å and c = 16.7053 Å for the synthetic arsenic-incorporated natroalunite [Na0.88(H3O)2.44Al2.35(AsO4)0.38(SO4)1.62(OH)6]. The symmetric stretching vibration (AsO4) around 895 cm−1 in the infrared spectra was recognized for the arsenic-incorporated natroalunite. The solids showed regular ditrigonal scalenohedron (pseudo-octahedron) crystal forms (2∼5 μm size) with certain nanosized fine grains. The XRD, FT-IR. and FE-SEM characterization proved that the synthetic natroalunite and the arsenic-incorporated natroalunite showed no detectable changes during dissolution.

All dissolutions underwent a pH variation, which was caused by a great depleting of H3O+/OH ions, typically at the reaction beginning. The dissolution in H3O+ medium proved to be near-stoichiometric within the short beginning period and the dissolved Na+, Al3+, SO42−, and AsO43− concentrations were stoichiometric according to the initial solids and then appeared to be nonstoichiometric with the Na/SO4 mole ratios higher and the Al/SO4 and AsO4/SO4 mole ratios lower than the stoichiometry until the experimental end, indicating that the components were released from solid to solution preferentially after the following order: Na+ (H3O+) > SO42− > AsO43− > Al3+.

From the experimental results under the condition of initial pH 2.00 and 25°C, the solubility product [Ksp] and the Gibbs free energies of formation [] were calculated to be 10−81.02±0.33∼10−81.04±0.27 and −4713 ± 2 to −4714 ± 1 kJ/mol kJ/mol for the natroalunite, 10−92.30±0.30∼10−92.41±0.37 and −5078 ± 2 to −5079 ± 2 kJ/mol for the arsenic-incorporated natroalunite, respectively. Thermodynamic quantities, , , , and were calculated to be 462303.43 J/K·mol, 122466.83 J/mol, −1140.39 J/K·mol, and 4280.13 J/K·mol for the natroalunite dissolution reaction at initial pH 2.00 and 25°C and to be 526925.48 J/K·mol, 159674.76 J/mol, −1232.38 J/K·mol, and 1061.12 J/K·mol for the dissolution reaction of the arsenic-incorporated natroalunite at initial pH 2.00 and 25°C, respectively.

Data Availability

The powder XRD data in XML format, the FT-IR data in XLSX format, and all solution analytical data in XLSX format used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.

Acknowledgments

The authors are grateful for the financial support from the National Natural Science Foundation of China (41763012, 21707024, and 51978188), the Guangxi Science and Technology Planning Project (GuiKe-AD18126018), the Guangxi Natural Science Foundation (2018GXNSFAA050044), and the Special Fund for Guangxi Distinguished Experts.

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