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Dissolution and Solubility of the Synthetic Natroalunite and the Arsenic-Incorporated Natroalunite at pH of 2.00-5.60 and 25-45[degrees]C.

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 [8-12], which nevertheless are not stable and may release arsenate ions in some water environments [12]. Scorodite (FeAs[O.sub.4] x 2[H.sub.2]O), 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 [15-17]. 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[(S[O.sub.4]).sub.x][(As[O.sub.4]).sub.1-x][(OH).sub.x] x(1 - x)[H.sub.2]O compounds with the lower arsenic release in leaching test than FeAs[O.sub.4] x 2[H.sub.2]O [18].

Recently, many scholars aim to study the As[O.sub.4]-for-S[O.sub.4] replacement in natroalunite [Na[Al.sub.3][(S[O.sub.4]).sub.2][(OH).sub.6]], a member of the alunite supergroup [A[B.sub.3][(T[O.sub.4]).sub.2][(OH,[H.sub.2]O).sub.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 Na[Al.sub.3][(S[O.sub.4]).sub.2][(OH).sub.6], made up of linear tetrahedral-octahedral-tetrahedral (T-O-T) laminas, where the [Na.sup.+] ion sites with an A[O.sub.6][(OH).sub.6] icosahedra coordination and the [Al.sup.3+] and S[O.sub.4.sup.2-] ion sites with slightly distorted Al[O.sub.6] octahedra (O) and S[O.sub.4] tetrahedral (T) coordinations, respectively [21]. The A sites are commonly filled by [K.sub.+], [Na.sup.+], and [H.sub.3][O.sup.+], the B sites by [Fe.sup.3+] and [Al.sup.3+], and the T[O.sub.4] sites by S[O.sub.4.sup.2-], P[O.sub.4.sup.3-], and As[O.sub.4.sup.3-], respectively. Multiple replacements are possible due to the charge balance by the coupled replacement at different sites, the OH/T[O.sub.4] 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 (Asnatroalunite) precipitation has been proposed to eliminate arsenic from water, both for industrial practice and remediation of polluted areas [22, 25].

The incorporation of As[O.sub.4] in the S[O.sub.4] 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 [27-30]. Nevertheless, few works have been carried out on the arsenic incorporation in natroalunite. The hydrothermal-precipitated natroalunite could incorporate about 8 mol% As[O.sub.4] in its structure and release <1mg/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:S[O.sub.4] mole ratio of 1:3:2: [Na.sup.+] + 3[Al.sup.3+] + 2S[O.sub.4.sup.2-] + 6O[H.sup.-] = Na[Al.sub.3][(S[O.sub.4]).sub.2][(OH).sub.6] [7]. The arsenic-incorporated natroalunite was prepared with a [Na.sup.+] 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 [Al.sub.2][(S[O.sub.4]).sub.3] x 18[H.sub.2]O and 2.5567 g of [Na.sub.2]S[O.sub.4] 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 [H.sub.3]As[O.sub.4] 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[degrees]C for 2~48h in 200 mL stainless-steel autoclaves. After cooling, the obtained products were filtered, washed three times with pure water, and dried at 110[degrees]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 plasmaoptical emission spectrometer (ICP-OES, Perkin-Elmer 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 K[alpha] radiation over the scanning 2[theta] range of 10[degrees]~90[degrees] at the interval of 0.10[degrees]/minute. Besides, the solids were characterized using a Fourier-transform infrared spectrophotometer (FT-IR, Nicolet Nexus 470) over 4000~400 [cm.sup.-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[degrees]C, 35[degrees]C, and 45[degrees]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 [micro]m 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[degrees]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.sup.+], [Al.sup.3+], As[O.sub.4.sup.3-], S[O.sub.4.sup.2-], and O[H.sup.-] were calculated using PHREEQC with its minteq.v4.dat database, and the solubility product ([K.sub.sp]) 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 [31-33]. The Debye-Huckel 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 [Na.sub.0.93][([H.sub.3]O).sub.0.61][Al.sub.2.82][(S[O.sub.4]).sub.2][(OH).sub.6] and [Na.sub.0.88][([H.sub.3]O).sub.2.44][Al.sub.2.35][(As[O.sub.4]).sub.0.38][(S[O.sub.4]).sub.1.62][(OH).sub.6], respectively.

The precipitates almost always had an apparent Al deficiency in the [B.sup.3+] sites and an appreciable Na deficiency in the A+ sites (7~12%) of the natroalunite A[B.sub.3][(T[O.sub.4]).sup.2][(OH).sub.6]. The latter might reflect an incorporation of [H.sub.3][O.sup.+] to form a solid solution that was hardly proved since [H.sub.3][O.sup.+] cannot be measured directly using microprobe technique or wet chemistry [34]. Up to 14% of the [B.sup.3+] sites could be unoccupied, which were also probably charge-balanced by [H.sub.3][O.sup.+] [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 As[O.sub.4]-for-S[O.sub.4] replacement [7]. The cell parameters a and c were determined to be 6.9870 [Angstrom] and 16.5840 [Angstrom] for the synthetic natroalunite and 7.0016 [Angstrom] and 16.7053 [Angstrom] for the arsenic-incorporated natroalunite, respectively. The cell parameter a of ~7 [Angstrom] predominantly depends on the B atomic radius and the cell parameter c of ~17 [Angstrom] on the A and T atomic radius. Due to the significantly greater As-O bond length (~1.57 [Angstrom]) than the S-O bond length (~1.45 [Angstrom]) in the T[O.sub.4] tetrahedra that is perpendicular to the c axis, the As[O.sub.4]-for-S[O.sub.4] replacement would enlarge especially the parameter c of natroalunite [36]. Additionally, the [A.sup.+] ions in the 12-fold coordination [A[O.sub.6][(OH).sub.6]] polyhedra that lie between two T[O.sub.4]-tetrahedra layers can strongly influence the interlayer spacing in the natroalunite crystals. Thus, when significant [H.sub.3][O.sup.+] substitution happened together with the As[O.sub.4]-for-S[O.sub.4] substitution in the crystal lattice, both substitution effects overlapped [28, 30, 36].

3.1.3. Fourier-Transform Infrared Spectra Analysis. The M-O stretching vibration, the As[O.sub.4] vibration, the S[O.sub.4] 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 Al[O.sub.6] octahedra were found to be around 513~515 and -480 [cm.sup.-1] [24, 30, 37, 38]. The 1026-1030 [cm.sup.-1] bands, the 426-434 [cm.sup.-1] bands, the -1207 and -1099 [cm.sup.-1] bands, and the 663-665 and 627-631 [cm.sup.-1] bands were assigned to the [v.sub.1](S[O.sub.4]) symmetric stretching vibration, the [v.sub.2](S[O.sub.4]) symmetric stretching vibration, the [v.sub.3](S[O.sub.4]) antisymmetric stretching vibration, and the [v.sub.4](S[O.sub.4]) vibration, respectively [24, 30]. The 604[cm.sup.-1] bands and the 3452-3460 [cm.sup.-1] bands belonged to the OH" bending vibration and the O[H.sup.-] stretching vibration, respectively [24, 30]. The 1637-1639 [cm.sup.-1] vibration bands were attributed to the framework deformation of powerful H-bonded [H.sub.2]O molecule [30, 37]. Additionally, the symmetric stretching vibration [v.sub.1](As[O.sub.4]) around 895 cm was recognized for the arsenic-incorporated natroalunite.

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[degrees]C is illustrated in Figure 3. The precipitates showed pseudo-octahedral crystal forms (2~5 [micro]m 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[degrees]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[degrees]C what appeared to be the original crystal boundaries remained well-preserved.

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.

For the dissolution of the synthetic natroalunite and the synthetic arsenic-incorporated natroalunite at initial pH 2.00 and 25[degrees]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[degrees]C, the solution pHs increased from 4.00 to 4.61~5.47 within the first 3h 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[degrees]C, the solution pHs decreased from 5.60 to 4.55 and from 5.60 to 5.13 within 24~72h 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.sup.+] depleting [40]. This result indicated that the jarosite dissolution at pH <3.5 followed the mechanism that consumed H+ and produced aqueous [Fe.sup.3+] species. Particularly in the low pH solution with [H.sub.2]S[O.sub.4], aqueous [Fe.sup.3+]-S[O.sub.4.sup.2-] complexes would control the aqueous [Fe.sup.3+] 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.sup.+] 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 As[O.sub.4]-for-S[O.sub.4] replacement had a great influence on the [Na.sup.+] release, i.e., the dissolved [Na.sup.+] concentrations increased significantly with the increase of this substitution. The increase of the initial solution pH was not beneficial for the [Na.sup.+] release into solution. The temperature showed also an obvious influence on the [Na.sup.+] release. For the dissolution of the synthetic natroalunite, the [Na.sup.+] release rates increased in the following order: 25[degrees]C > 45[degrees]C > 35[degrees]C. For the dissolution of the synthetic arsenic-incorporated natroalunite, the [Na.sup.+] release rates increased in the following order: 35[degrees]C > 45[degrees]C > 25[degrees]C.

The dissolved [Al.sup.3+] 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 As[O.sub.4]-for-S[O.sub.4] replacement had a great influence on the [Al.sup.3+] release. On the contrary to the [Na.sup.+] release, the dissolved [Al.sup.3+] concentrations decreased significantly with the increase of this substitution. The increase of the initial solution pH could obviously decrease the [Al.sup.3+] release into solution. The temperature showed also an obvious influence on the [Al.sup.3+] release. For the dissolution of the synthetic natroalunite, the [Al.sup.3+] release rates increased in the following order: 25[degrees]C > 35[degrees]C > 45[degrees]C. For the dissolution of the synthetic arsenic-incorporated natroalunite, the [Al.sup.3+] release rates increased in the following order: 35[degrees]C > 45[degrees]C > 25[degrees]C.

The dissolved S[O.sub.4.sup.2-] concentrations generally fluctuated within the first 2880 of the reaction and then gradually reached a steady state. The dissolved S[O.sub.4.sup.2-] concentrations increased significantly with the increase of the As[O.sub.4]-for-S[O.sub.4] replacement. For the dissolution of the synthetic natroalunite, the dissolved S[O.sub.4.sup.2-] 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 S[O.sub.4.sup.2-] release into solution. The S[O.sub.4.sup.2-] release rates increased in the following order: pH 2.00 > pH 4.00 > pH 5.60 and 25[degrees]C = 35[degrees]C > 45[degrees]C. For the dissolution of the synthetic arsenic-incorporated natroalunite, the S[O.sub.4.sup.2-] release rates increased in the following order: pH 4.00 > pH 2.00 > pH 5.60 and 25[degrees]C > 35[degrees]C~45[degrees]C.

The dissolved As[O.sub.4.sup.3-] concentrations increased significantly with the increase of the As[O.sub.4]-for-S[O.sub.4] replacement. For the dissolution of the arsenic-incorporated natroalunite at initial pH 2.00, the dissolved As[O.sub.4.sup.3-] concentrations rose quickly at the start and attained the highest values after about 6-24 h of dissolution. After that, the dissolved As[O.sub.4.sup.3-] concentrations decreased gradually and a steady state was gradually approached after 2880 h of dissolution. The temperature increase was beneficial for the As[O.sub.4.sup.3-] release into solution. The dissolved As[O.sub.4.sup.3-] concentrations for the dissolution at 35[degrees]C and 45[degrees]C were slightly greater than those at 25[degrees]C. For the dissolution at the initial pHs of 4.00 and 5.60, the dissolved As[O.sub.4.sup.3-] 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 As[O.sub.4.sup.3-] and S[O.sub.4.sup.2-] to [H.sub.2]As[O.sub.4.sup.-] and HS[O.sub.4.sup.-] 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 [[Na.sub.0.93][([H.sub.3]O).sub.0.61][Al.sub.2.82][(S[O.sub.4]).sub.2][(OH).sub.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 [[Na.sub.0.93][([H.sub.3]O).sub.0.61][Al.sub.2.82][(S[O.sub.4]).sub.2][(OH).sub.6]] could be simply expressed by the following dissolution equation:

[mathematical expression not reproducible] (1)

The reaction could produce 5.39 mol O[H.sup.-] for a mole of dissolved natroalunite. Thus, the natroalunite dissolution at lower initial pH could result in a pH increase, suggesting hydrion depleting or O[H.sup.-] 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 [Al.sup.3+]:

[Al.sup.3+] + nO[H.sup.-] = Al[(OH).sub.n.sup.(3-n)+], n = 1~4 (2)

[H.sup.+] + S[O.sub.4.sup.2-] = HS[O.sub.4.sup.-] (3)

For the dissolution of the synthetic natroalunite [[Na.sub.0.93][([H.sub.3]O).sub.0.61][Al.sub.2.82][(S[O.sub.4]).sub.2][(OH).sub.6]] at 25[degrees]C and initial pH 2.00 (Figure 4), the dissolved [Na.sup.+], [Al.sup.3+], and S[O.sub.4.sup.2-] 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 A1/s[O.sub.4] mole ratios of 1.44-1.59, and the aqueous Na/S[O.sub.4] mole ratios of 0.54-0.71, which were in close proximity to the stoichiometric Na/Al, Al/S[O.sub.4], and Na/S[O.sub.4] 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.sup.+] preferentially over [Al.sup.3+] to the aqueous solution, and the aqueous Na/Al and Na/S[O.sub.4] mole ratios increased and the aqueous Al/S[O.sub.4] mole ratios decreased rapidly. After 3 h, the aqueous [Na.sup.+] and [Al.sup.3+] concentrations as well as the solution pHs increased continuously, while the total aqueous S[O.sub.4.sup.2-] concentrations decreased rapidly. From 12 h to 2880 h, the pHs approached a steady state (3.96~4.34), the aqueous [Na.sup.+] and S[O.sub.4.sup.2-] concentrations increased, while the aqueous [Al.sup.3+] 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 [approximately equal to] 3) or incongruently for the experiment at pH [greater than or equal to] 3.9 (i.e., dissolved Al/K mole ratio <3) [42].

After 2880 h of dissolution, the solution pHs and the total solution [Na.sup.+], [Al.sup.3+], and S[O.sub.4.sup.2-] concentrations tending to reach the steady state with the stoichiometric Na/Al, Al/ S[O.sub.4], and Na/S[O.sub.4] 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/S[O.sub.4], and Na/S[O.sub.4] mole ratios of 0.33, 1.41, and 0.47 for the synthetic natroalunite, respectively (Figure 4). This means that [Na.sup.+] cations preferred to be released from solid to solution relative to [Al.sup.3+] and S[O.sub.4.sup.2-] ions and the S[O.sub.4.sup.2-] anions preferred to be released from solid to solution relative to [Al.sup.3+] ions, i.e., the components were released from solid to solution preferentially after the following order: [Na.sup.+] ([H.sub.3][O.sup.+]) > S[O.sub.4.sup.2-] > [Al.sup.3+]. Additionally, this differential tendency became more obvious with the increasing dissolution temperature and initial solution pHs. Selective release of [Na.sup.+] in the A site and S[O.sub.4.sup.2-] in the T site resulted in higher aqueous [Na.sup.+] and S[O.sub.4.sup.2-] concentrations compared to [Al.sup.3+] deep in the T-O-T structure of natroalunite [21]. Similar selective release of [K.sup.+] and S[O.sub.4.sup.2-] 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 [Fe.sup.3+] concentration and an increase of the dissolved S[O.sub.4.sup.2-] 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[degrees]C, the aluminum species appeared in the sequence of AlS[O.sub.4.sup.+] > [Al.sup.3+] > Al[(S[O.sub.4]).sub.2.sup.-] > Al[(OH).sup.2+] > Al[(OH).sub.2.sup.+] > AlH- S[O.sub.4.sup.2+] > Al[(OH).sub.3.sup.0] > Al[(OH).sub.4.sup.-]; the sulfate species appeared in the sequence of S[O.sub.4.sup.2-] > AlS[O.sub.4.sup.+] > NaS[O.sub.4.sup.-] > Al[(S[O.sub.4]).sub.2.sup.-] > HS[O.sub.4.sup.-] > AlHS[O.sub.4.sup.2+]. The concentrations of all sodium, aluminum, and sulfate species attained a steady state at the experimental end.

3.3.2. As-Natroalunite [[Na.sub.0.88][([H.sub.3]O).sub.2.44][Al.sub.2.35][(As[O.sub.4]).sub.0.38] [(S[O.sub.4]).sub.1.62][(OH).sub.6]]. The stoichiometry of the synthetic Asnatroalunite dissolution reaction varied as a function of time, solution pH, and temperature. The speciation of dissolved [Al.sup.3+], As[O.sub.4.sup.3-], and S[O.sub.4.sup.2-] can strongly affect the pH-dependence of the dissolution stoichiometry [19]. The stoichiometric dissolution of the synthetic arsenic-incorporated natroalunite [[Na.sub.0.88][([H.sub.3]O).sub.2.44][Al.sub.2.35][(As[O.sub.4]).sub.0.38] [(S[O.sub.4]).sub.1.62][(OH).sub.6]] can be expressed by the general reaction as follows:

[mathematical expression not reproducible] (4)

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

n[H.sup.+] + As[O.sub.4.sup.3-] = [H.sub.n]As[O.sub.4.sup.(3-n)-], n = 1~3 (5)

For the dissolution of the synthetic As-natroalunite [[Na.sub.0.88][([H.sub.3]O).sub.2.44][Al.sub.2.35][(As[O.sub.4]).sub.0.38][(S[O.sub.4]).sub.1.62][(OH).sub.6]] at 25[degrees]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 As[O.sub.4]/ (As[O.sub.4] + S[O.sub.4]) 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/(As[O.sub.4] + S[O.sub.4]) mole ratios fluctuated greatly within 5040 h dissolution; the aqueous Al/ (As[O.sub.4] + S[O.sub.4]) mole ratios decreased generally with time.

The transient peak values in the total dissolved [Na.sup.+], [Al.sup.3+], As[O.sub.4.sup.3-], and S[O.sub.4.sup.2-] 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 Asnatroalunite, 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.sup.+], [Al.sup.3+], As[O.sub.4.sup.3-], and S[O.sub.4.sup.2-] concentrations became constant and reached the steady state with the stoichiometric Na/Al, and Na/(As[O.sub.4] + S[O.sub.4]) mole ratios were around 24.73-28.08 and 2.29-2.39, which were significantly higher than the stoichiometric Na/Al and Na/ (As[O.sub.4] + S[O.sub.4]) mole ratios of 0.37 and 0.44 for the synthetic As-natroalunite, and the stoichiometric As[O.sub.4]/(As[O.sub.4] + S[O.sub.4]) and Al/(As[O.sub.4] + S[O.sub.4]) mole ratios were around 0.02 and 0.08-0.10, which were significantly lower than the stoichiometric As[O.sub.4]/(As[O.sub.4] + S[O.sub.4]) and Al/(As[O.sub.4] + S[O.sub.4]) mole ratios of 0.16 and 1.18 for the synthetic As-natroalunite, respectively (Figure 5). This means that [Na.sup.+] cations preferred to be released from solid to solution relative to [Al.sup.3+], As[O.sub.4.sup.3-], and S[O.sub.4.sup.2-] ions, the S[O.sub.4.sup.2-] anions preferred to be released from solid to solution relative to As[O.sub.4.sup.3-] anions, and the As[O.sub.4.sup.3-] and S[O.sub.4.sup.2-] anions preferred to be released from solid to solution relative to [Al.sup.3+] cations, i.e., the components were released from solid to solution preferentially after the following order: [Na.sup.+] ([H.sub.3][O.sup.+]) > S[O.sub.4.sup.2-] > As[O.sub.4.sup.3-] > [Al.sup.3+], suggesting incongruent dissolution of the (As-)natroalunite and/or formation of As[O.sub.4]/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 T[O.sub.4] groups from natroalunite, which would be hindered by the outer [Na.sup.+] and O[H.sup.-] group layers that were energetically stable in the solvated surface [42]. The dissolution was restrained by the Al-O bond breakage in the A[B.sub.3][(T[O.sub.4]).sub.2][(OH).sub.6] structure of the alunite supergroup [40,45]. [Al.sup.3+] 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 A[B.sub.3][(T[O.sub.4]).sub.2][(OH).sub.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 [[Na.sub.0.88][([H.sub.3]O).sub.2.44][Al.sub.2.35][(As[O.sub.4]).sub.0.38][(S[O.sub.4]).sub.1.62][(OH).sub.6]] at 25[degrees]C and initial pH 4.00/5.60 (Figure 5), the solution Na/Al mole ratios were significantly higher and the Na/ (As[O.sub.4] + S[O.sub.4]), Al/(As[O.sub.4] + S[O.sub.4]) and As[O.sub.4]/(As[O.sub.4] + S[O.sub.4]) mole ratios were lower than the values for their dissolution at initial pH 2.00. For the dissolution of the synthetic As-natroalunite [[Na.sub.0.88][([H.sub.3]O).sub.2.44][Al.sub.2.35][(As[O.sub.4]).sub.0.38][(S[O.sub.4]).sub.1.62][(OH).sub.6]] at 35[degrees]C/45 C and initial pH 2.00 (Figure 5), the aqueous Na/Al and Na/ (As[O.sub.4] + S[O.sub.4]) mole ratios were lower and the Al/(As[O.sub.4] + S[O.sub.4]) and As[O.sub.4]/(As[O.sub.4] + S[O.sub.4]) mole ratios were higher than the values for their dissolution at 25[degrees]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 [H.sub.3][O.sup.+]/O[H.sup.-] 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 [H.sub.2]O attach, which could be related to the electrostatic repulsion of the OH" ions at higher pH owing to the As[O.sub.4] bonds on the surface; moreover, the increase in the Fe-As[O.sub.4] bond number can inhibit the systems' dependency on [H.sub.3][O.sup.+]/O[H.sup.-] [41]. This phenomenon was not found in the K-Cr-jarosite dissolution, suggesting an incessant dependency of the reaction on the [H.sub.3][O.sup.+]/O[H.sup.-]concentration [45].

The speciation calculation using PHREEQC showed that for the As-natroalunite dissolution at initial pH 2.00 and 25[degrees]C, the sodium species appeared according to the sequence of [Na.sup.+] > NaS[O.sub.4.sup.-] > Na[H.sub.2]As[O.sub.4.sup.0] > NaHAs[O.sub.4.sup.-] > NaAs[O.sub.4.sup.-2]; the aluminum species appeared according to the order of AlS[O.sub.4.sup.+] > [Al.sup.3+] > Al[(S[O.sub.4]).sub.2.sup.-] > Al[(OH).sup.2+] > Al[(OH).sub.2.sup.+] > AlHS[O.sub.4.sup.2+] > Al[H.sub.2]As[O.sub.4.sup.2+] > Al[(OH).sub.3.sup.0] > Al[(OH).sub.4.sup.- ] > AlHAs[O.sub.4.sup.+] > AlAs[O.sub.4.sup.0]; the arsenate species appeared according to the order of [H.sub.2]As[O.sub.4.sup.-] > Na[H.sub.2]As[O.sub.4.sup.0] > [H.sub.3]As[O.sub.4]0 > HAs[O.sub.4.sup.2-] > Al[H.sub.2]As[O.sub.4.sup.2+] > NaHAs[O.sub.4.sup.-] > NaAs[O.sub.4.sup.2-] > AlHAs[O.sub.4.sup.+] > As[O.sub.4.sup.3-] > AlAs[O.sub.4.sup.0]; the sulfate species appeared according to the order of S[O.sub.4.sup.2-] > AlS[O.sub.4.sup.+] > NaS[O.sub.4.sup.-] > HS[O.sub.4.sup.-] > Al[(S[O.sub.4]).sub.2.sup.-] > AlHS[O.sub.4.sup.2+]. At the dissolution start, the concentrations of all sodium, aluminum, arsenate, and sulfate species except HS[O.sub.4.sup.-] and AlHS[O.sub.4.sup.2+] increased with the dissolution time and finally attained a steady state. The dissolved sodium and sulfate were principally present as [Na.sup.+] and S[O.sub.4.sup.2-] 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[degrees]C and initial pH 2.00 (Figures 4 and 5), all solutions were undersaturated with respect to Al[(OH).sub.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).sub.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.sup.+] to O[H.sup.-] mediated process because of the [Al.sup.3+] 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).sub.3(a)]], gibbsite [Al[(OH).sub.3]], diaspore [AlOOH], and AlAs[O.sub.4] x 2[H.sub.2]O, [Al.sub.4][(OH).sub.10]S[O.sub.4]), 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 ([K.sub.sp]) 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.sup.+], [Al.sup.3+], As[O.sub.4.sup.3-], S[O.sub.4.sup.2-], and O[H.sup.-] 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 ([K.sub.sp]) of the synthetic natroalunite and the arsenic-incorporated natroalunite. The solution species [Na.sup.+], NaAs[O.sub.4.sup.2-], NaHAs[O.sub.4.sup.-], and Na[H.sub.2]As[O.sub.4.sup.0] were considered in the simulation for the total sodium; [Al.sup.3+], Al[(OH).sub.4.sup.-] Al[(OH).sub.3], Al[(OH).sub.2.sup.+], AlO[H.sup.2+], [Al.sub.2][(OH).sub.2.sup.4+] , [Al.sub.3][(OH).sub.4.sup.5+], [Al.sub.13][O.sub.4][(OH).sub.24.sup.7+], AlAs[O.sub.4.sup.0], AlHAs[O.sub.4.sup.+], Al[H.sub.2]As[O.sub.4.sup.2+], AlHS[O.sub.4.sup.2+], Al[(S[O.sub.4]).sub.2.sup.-], and AlS[O.sub.4.sup.+] for the total aluminum; [H.sub.3]As[O.sub.4], As[O.sub.4.sup.3-], HAs[O.sub.4.sup.2-], [H.sub.2]As[O.sub.4.sup.-], Na[H.sub.2] As[O.sub.4.sup.0], NaHAs[O.sub.4.sup.-], NaAs[O.sub.4.sup.2-], AlAs[O.sub.4.sup.0], AlHAs[O.sub.4.sup.+], and Al[H.sub.2]As[O.sub.4.sup.2+] for the total arsenate; S[O.sub.4.sup.2-], HS[O.sub.4.sup.-], AlHS[O.sub.4.sup.2+], Al[(S[O.sub.4]).sub.2.sup.-], and AlS[O.sub.4.sup.+] for the total sulfate.

The stoichiometrical dissolution reactions of the synthetic natroalunite [[Na.sub.0.93][([H.sub.3]O).sub.0.61][Al.sub.2.82][(S[O.sub.4]).sub.2][(OH).sub.6]] and the synthetic arsenic-incorporated natroalunite [[Na.sub.0.88][([H.sub.3]O).sub.2.44] [Al.sub.2.35][(As[O.sub.4]).sub.0.38][(S[O.sub.4]).sub.1.62][(OH).sub.6]] can be described with the dissolution reactions (1) and (4), respectively.

Presuming unit activity of the solid is as follows:

[K.sub.sp] = [{[Na.sup.+]}.sup.0.93][{[Al.sup.3+]}.sup.2.82][{S[O.sub.4.sup.2-]}.sup.2] [{O[H.sup.-]}.sup.5.39] (6)

or [K.sub.sp] = [{[Na.sup.+]}.sup.0.88] [{[Al.sup.3+]}.sup.2.35] [{As[O.sub.4.sup.3-}.sup.0.38] [{S[O.sub.4.sup.2- ]}.sup.1.62][{O[H.sup.-]}.sup.3.56] (7)

where {} is the thermodynamic activity and [K.sub.sp] is the solubility product according to the dissolution reactions (1) and (4).

The Gibbs free energies of formation, [DELTA][G.sup.o.sub.f] [[Na.sub.0.93] [([H.sub.3]O).sub.0.61][Al.sub.2.82][(S[O.sub.4]).sub.2][(OH).sub.6]] and [DELTA][G.sup.o.sub.f] [[Na.sub.0.88][([H.sub.3]O).sub.2.44][Al.sub.2.35] [(As[O.sub.4]).sub.038][(S[O.sub.4]).sub.1.62][(OH).sub.6]], can also be estimated by the use of the following thermodynamic data in the literature [46, 47], [DELTA][G.sup.o.sub.f] [[Na.sup.+]] = -262 kJ/mol, [DELTA][G.sup.o.sub.f] [[Al.sup.3+]] = -489.4 kJ/mol, [DELTA][G.sup.o.sub.f] [As[O.sub.4.sup.3-]] = -647.618 kJ/mol, [DELTA][G.sup.o.sub.f] [S[O.sub.4.sup.2-]] = -744.6kJ/mol, [DELTA][G.sup.o.sub.f] [O[H.sup.-]] = -157.3 kJ/mol, and [DELTA][G.sup.o.sub.f] [[H.sub.2]O] = -237.18 kJ/mol. The [K.sub.sp] value is related to the standard free energy of reaction ([DELTA][G.sup.o.sub.r]) and can be expressed under the standard condition (298.15 K and 0.101 MPa) by

[DELTA][G.sup.o.sub.r] = -5.708 log [K.sub.sp]. (8)

For (1),

[mathematical expression not reproducible] (9)

Rearranging,

[mathematical expression not reproducible] (10)

For (4),

[mathematical expression not reproducible] (11)

Rearranging,

[mathematical expression not reproducible] (12)

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.

From the experimental results under the condition of initial pH 2.00 and 25[degrees]C, the solubility product [[K.sub.sp]] and the Gibbs free energies of formation [DELTA][G.sup.o.sub.f] [[Na.sub.0.93][([H.sub.3]O).sub.0.61][Al.sub.2.82] [(S[O.sub.4]).sub.2][(OH).sub.6]] had been calculated to be [10.sup.-81.02 [+ or -] 0.33] ~[10.sup.-81.04 [+ or -] 0.27] and -4713 [+ or -] 2~ -4714 [+ or -] 1 kJ/mol kJ/mol for the natroalunite correspondingly. Few [K.sub.sp] and [DELTA][G.sup.o.sub.f] data for natroalunite have been reported previously in the literature. For natroalunite (Na[Al.sub.3][(S[O.sub.4]).sub.2][(OH).sub.6]), the [DELTA][G.sup.o.sub.f] value of -4614.02 kJ/mol was determined [48] from the [K.sub.sp] value of [10.sup.79.7] [49]; besides, the AGf 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 [K.sub.sp] value for the arsenic-incorporated natroalunite decreased from [10.sup.-81.02[+or -]0.33] ~[10.sup.-81.04 [+ or -] 0.27] to [10.sup.-92.30 [+ or -] 0.30] ~[10.sup.-92.41 [+ or -] 0.37], and the corresponding [DELTA][G.sup.o.sub.f] values from -4713 [+ or -] 2 to -4714 [+ or -] 1 kJ/mol to - 5078 [+ or -] 2 to -5079 [+ or -] 2 kJ/mol, respectively. Generally, the [K.sub.sp] and [DELTA][G.sup.o.sub.f] 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 ([K.sub.sp]) for the synthetic natroalunite and the arsenic-incorporated natroalunite can also be estimated using the following equations [50]:

-log [K.sub.sp] = A/T + B + CT, (13)

For the synthetic natroalunite (Natroal-1),

-log [K.sub.sp] = 60221.6658/T + 506.6639 - 0.7502T, (14)

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

-log [K.sub.sp] = 8175.9700/T + 175.2114 - 0.1860T, (15)

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

-log [K.sub.sp] = 42272.1180/T - 127.3042 + 0.2916T. (16)

The thermodynamic quantities [DELTA][G.sup.o], [DELTA][H.sup.o], [DELTA][S.sup.o], and [DELTA][C.sup.o.sub.p] 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:

[DELTA][G.sup.o] = 2.3026R(A + BT + C[T.sub.2]), (17)

[DELTA][H.sup.o] = 2.3026R(A - C[T.sub.2]), (18)

[DELTA][S.sup.o] = -2.3026R (B + 2CT), (19)

[DELTA][C.sup.o.sub.p] = -2.3026R(CT), (20)

where A, B, and C are the empirical coefficients of equation (13).

The [DELTA][G.sup.o] 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 [DELTA][H.sup.o] indicated also that the dissolution was an endothermic reaction and the solubility increased with the increase in temperature. The higher [DELTA][S.sup.o] value for the natroalunite dissolution indicated that the order degree caused by S[O.sub.4.sup.2-] was greater than by As[O.sub.4.sup.3-] in aqueous solution, which is related to the different bond distances (S-O 1.473 [Angstrom] and As-O 1.682 [Angstrom]) [50, 51]. The smaller S[O.sub.4.sup.2-] 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 [DELTA][C.sup.o.sub.p] value coupled with the higher [DELTA][S.sup.o] 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 S[O.sub.4.sup.2-] with the aqueous acidic media and a higher solubility of the natroalunite in the aqueous acidic media. But the incorporation of [H.sub.3][O.sup.+] into the natroalunite crystal structure with the substitution of S[O.sub.4.sup.2-] by As[O.sub.4.sup.3-] can increase the [DELTA][S.sup.o] values for the As-natroalunite dissolution and increase their solubility.

4. Conclusions

The lattice parameters were determined to be a = 6.9870 [Angstrom] and c = 16.5840 [Angstrom] for the synthetic natroalunite [[Na.sub.0.93] [([H.sub.3]O).sub.0.61][Al.sub.2.82][(S[O.sub.4]).sub.2][(OH).sub.6]] and a = 7.0016 [Angstrom] and c = 16.7053 [Angstrom] for the synthetic arsenic-incorporated natroalunite [[Na.sub.0.88][([H.sub.3]O).sub.2.44][Al.sub.2.35][(As[O.sub.4]).sub.0.38][(S[O.sub.4]).sub.1.62][(OH).sub.6]]. The symmetric stretching vibration [v.sub.1](As[O.sub.4]) around 895 [cm.sup.-1] in the infrared spectra was recognized for the arsenic-incorporated natroalunite. The solids showed regular ditrigonal scalenohedron (pseudo-octahedron) crystal forms (2~5 [micro]m 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 [H.sub.3][O.sup.+]/O[H.sup.-] ions, typically at the reaction beginning. The dissolution in [H.sub.3][O.sup.+] medium proved to be near-stoichiometric within the short beginning period and the dissolved [Na.sup.+], [Al.sup.3+], S[O.sub.4.sup.2-], and As[O.sub.4.sup.3-] concentrations were stoichiometric according to the initial solids and then appeared to be nonstoichiometric with the Na/S[O.sub.4] mole ratios higher and the Al/S[O.sub.4] and As[O.sub.4]/S[O.sub.4] 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.sup.+] ([H.sub.3][O.sup.+]) > S[O.sub.4.sup.2-] > As[O.sub.4.sup.3-] > [Al.sup.3+].

From the experimental results under the condition of initial pH 2.00 and 25[degrees]C, the solubility product [[K.sub.sp]] and the Gibbs free energies of formation [AGf] were calculated to be [10.sub.-81.02 [+ or -] 0.33] ~[10.sup.-81.04 [+ or -] 0.27] and -4713 [+ or -] 2 to -4714 [+ or -] 1 kJ/mol kJ/mol for the natroalunite, [10.sup.-92.30 [+ or -] 0.30] ~[10-92.41 [+ or -] 0.37] and - 5078 [+ or -] 2 to -5079 [+ or -] 2 kJ/mol for the arsenic-incorporated natroalunite, respectively. Thermodynamic quantities, [DELTA][G.sup.o], [DELTA][H.sup.o], [DELTA][S.sup.o], and [DELTA][C.sup.o.sub.p] were calculated to be 462303.43 J/K x mol, 122466.83 J/mol, -1140.39 J/K x mol, and 4280.13 J/K x mol for the natroalunite dissolution reaction at initial pH 2.00 and 25[degrees]C and to be 526925.48 J/K x mol, 159674.76 J/mol, -1232.38 J/K x mol, and 1061.12 J/K x mol for the dissolution reaction of the arsenic-incorporated natroalunite at initial pH 2.00 and 25[degrees]C, respectively.

https://doi.org/10.1155/2019/9568360

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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Yinian Zhu [ID], (1) Huiling Xuan, (1) Yanpeng Liang [ID], (2) Qiming Yan [ID], (1) Zongqiang Zhu [ID], (3) Zhangnan Jiang, (1) Lihao Zhang [ID], (2) Huili Liu [ID], (1) and Shen Tang (2)

(1) College of Environmental Science and Engineering, Guilin University of Technology, Guilin 541004, China

(2) Guangxi Key Laboratory of Environmental Pollution Control Theory and Technology, Guilin University of Technology, Guilin 541004, China

Correspondence should be addressed to Yinian Zhu; zhuyinian@glut.edu.cn and Yanpeng Liang; liangyanpeng@glut.edu.cn

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

Academic Editor: Sedat Yurdakal

Caption: Figure 1: X-ray diffraction patterns of the synthetic natroalunite and the synthetic arsenic-incorporated natroalunite before and after dissolution at 25-45[degrees]C for 300 d.

Caption: Figure 2: Fourier-transform infrared spectra of the synthetic natroalunite and the synthetic arsenic- incorporated natroalunite before and after dissolution at 25-45[degrees]C for 300 d.

Caption: Figure 3: Field-emission scanning electron microscopic images of the synthetic natroalunite and the synthetic arsenic-incorporated natroalunite before and after dissolution at 25[degrees]C for 300 d.

Caption: Figure 4: Aqueous evolution during the dissolution of the synthetic natroalunite at 25-45[degrees]C for 300 d.

Caption: Figure 5: Aqueous evolution during the dissolution of the synthetic arsenic-incorporated natroalunite at 25- 45[degrees]C for 300 d.
Table 1: Analytical data and solubility product of the synthetic
Natroalunite [[Na.sub.0.93][([H.sub.3]O).sub.0.61][Al.sub.2.82]
[(S[O.sub.4]).sub.2][(OH).sub.6]].

                                     Analytical data (mmol/L)
Temp           Initial    Time (h)
([degrees]C)      pH                  pH      Na      Al

                            5760     4.08   9.961    1.448
25               2.00       6480     4.16   9.813    1.355
                            7200     4.06   9.902    1.406
                            5760     4.05   10.022   1.512
25               2.00       6480     4.17   10.054   1.376
                            7200     4.07   10.013   1.444
                            5760     4.38   6.494    0.100
25               4.00       6480     4.51   6.438    0.100
                            7200     4.42   6.699    0.101
                            5760     4.35   6.444    0.126
25               5.60       6480     4.57   6.392    0.128
                            7200     4.39   6.398    0.127
                            5760     4.03   13.332   1.099
35               2.00       6480     4.12   13.389   0.970
                            7200     4.01   13.330   1.019
                            5760     4.15   11.520   0.554
45               2.00       6480     4.24   11.620   0.604
                            7200     4.13   11.549   0.586

               Analytical data (mmol/L)
Temp                                      log [K.sub.sp]
([degrees]C)   As[O.sub.4]   S[O.sub.4]

                  0.000        4.991         - 81.10
25                0.000        5.132         - 80.77
                  0.000        5.086         - 81.25
                  0.000        4.990         - 81.20
25                0.000        5.157         - 80.69
                  0.000        5.061         - 81.16
                  0.000        3.918         - 82.93
25                0.000        3.859         - 82.23
                  0.000        3.848         - 82.69
                  0.000        4.101         - 82.82
25                0.000        3.998         - 81.63
                  0.000        4.004         - 82.59
                  0.000        5.253         - 80.13
35                0.000        5.323         - 79.81
                  0.000        5.256         - 80.33
                  0.000        3.889         - 78.91
45                0.000        4.039         - 78.33
                  0.000        3.990         - 78.96

Temp           Average log [K.sub.sp]   [DELTA][G.sup.o.sub.f]
([degrees]C)                                   (kJ/mol)

                                                - 4714
25             - 81.04 [+ or -] 0.27            - 4712
                                                - 4715
                                                - 4714
25             - 81.02 [+ or -] 0.33            - 4712
                                                - 4714
                                                 -4724
25              -82.62 [+ or -] 0.39             -4720
                                                 -4723
                                                 -4724
25              -82.34 [+ or -] 0.71            - 4717
                                                 -4722

35              -80.09 [+ or -] 0.28

45              -78.73 [+ or -] 0.40

Temp           Average [DELTA][G.sup.o.sub.f] (kJ/mol)
([degrees]C)

25                         -4714 [+ or -] 1

25                         -4713 [+ or -] 2

25                         -4723 [+ or -] 2

25                         -4721 [+ or -] 4

35

45

Table 2: Analytical data and solubility product of the
arsenic-incorporated natroalunite [[Na.sub.0.88] [([H.sub.3]
O).sub.2.44][Al.sub.2.35][(As[O.sub.4]).sub.0.38]
[(S[O.sub.4]).sub.1.62]][(OH).sub.6]].

                                        Analytical data (mmol/L)
Temp           Initial pH    Time (h)
([degrees]C)                             pH       Na        Al

                               5760     3.98    16.477     0.666
25                2.00         6480     4.09    16.614     0.592
                               7200     4.00    16.573     0.590
                               5760     3.95    16.377     0.673
25                2.00         6480     4.08    16.531     0.596
                               7200     3.96    16.466     0.630
                               5760     4.88    13.771     0.015
25                4.00         6480     4.99    13.860     0.017
                               7200     4.87    13.832     0.018
                               5760     4.65     7.703     0.016
25                5.60         6480     4.73     7.760     0.016
                               7200     4.62     7.601     0.016
                               5760     3.70    11.048     1.011
35                2.00         6480     3.80    11.340     0.988
                               7200     3.74    11.392     0.990
                               5760     3.61    10.770     0.886
45                2.00         6480     3.74    11.057     0.835
                               7200     3.61    11.011     0.819

               Analytical data (mmol/L)
Temp                                      log [K.sub.sp]
([degrees]C)   As[O.sub.4]   S[O.sub.4]

                  0.161        6.744          -92.42
25                0.151        7.110          -92.00
                  0.154        7.026          -92.47
                  0.160        6.936          -92.59
25                0.151        7.007          -92.04
                  0.155        6.950          -92.60
                  0.106        7.634          -92.28
25                0.109        7.602          -91.67
                  0.102        7.603          -92.15
                  0.112        4.322          -93.21
25                0.111        4.238          -92.90
                  0.108        4.394          -93.38
                  0.204        4.746          -91.62
35                0.219        4.835          -91.10
                  0.214        4.846          -91.42
                  0.194        4.598          -90.53
45                0.190        4.653          -89.92
                  0.196        4.684          -90.62

Temp           Average log [K.sub.sp]   [DELTA][G.sup.o.sub.f]
([degrees]C)                                   (kJ/mol)

                                                -5079
25              92.30 [+ or -] 0.30             -5077
                                                -5079
                                                -5080
25              92.41 [+ or -] 0.37             -5077
                                                -5080
                                                -5078
25              92.04 [+ or -] 0.37             -5075
                                                -5078
                                                -5084
25              93.16 [+ or -] 0.26             -5082
                                                -5085

35              91.38 [+ or -] 0.28

45              90.36 [+ or -] 0.44

Temp           Average [DELTA][G.sup.o.sub.f] (kJ/mol)
([degrees]C)

25                         -507 8 [+ or -]  2

25                         -5079 [+ or -] 2

25                         -5077 [+ or -] 2

25                         -5083 [+ or -] 2

35

45

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

Sample               Temp        log [K.sub.sp]   [DELTA][G.sup.o]
                  ([degrees]C)                      (J/K x mol)

                       25           - 81.03          462303.43
Natroalunite           35           - 80.09          472271.06
                       45           - 78.73          479366.12
                       25           - 92.35          526925.48
As-natroalunite        35           - 91.38          538893.25
                       45           - 90.36          550148.86

Sample               Temp        [DELTA][H.sup.o]   [DELTA][S.sup.o]
                  ([degrees]C)       (J/mol)          (J/K x mol)

                       25           122466.83           -1140.39
Natroalunite           35           209505.66           -853.13
                       45           299417.07           -565.88
                       25           159674.76           -1232.38
As-natroalunite        35           181253.18           -1161.17
                       45           203543.76           -1089.95

Sample               Temp        [DELTA][C.sup.o.sub.p] (J/K x mol)
                  ([degrees]C)

                       25                      4280.13
Natroalunite           35                      4423.76
                       45                      4567.38
                       25                      1061.12
As-natroalunite        35                      1096.72
                       45                      1132.33
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Title Annotation:Research Article
Author:Xuan, Yinian Zhu Huiling; Liang, Yanpeng; Yan, Qiming; Zhu, Zongqiang; Jiang, Zhangnan; Zhang, Lihao
Publication:Journal of Chemistry
Date:Nov 1, 2019
Words:12022
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