Alkaline Decomposition of Solid Solution of Ammonium-Sodium Jarosite with Arsenic

Abstract

In this study, a solid solution of arsenical ammonium-sodium jarosite, with the approximate formula [(NH₄)_0.72 Na_0.06(H₃O)_0.21]Fe_3(2.52)(SO₄)_1.85 (AsO₄)_0.15 [(OH)_4.41 (H₂O)_1.59], was synthesized. The precipitate particle main size was 38 µm, with spherical morphology. A decomposition reaction in alkaline media was carried out; this decomposition comprised three stages: an induction period, a progressive conversion and a stabilization period. The process was controlled by the chemical reaction, and the progressive conversion period was consistent with the shrinking core model. The dissolution reactions of the compound in NaOH media were characterized by an ash layer formed by solid residues made of Fe(OH)₃ with adsorbed arsenate surrounding an unreacted core through which Na⁺, NH₄⁺ and SO₄²⁻ ions diffused into the solution. At the same time, OH⁻ ions diffused from the solution into the reaction front until the core disappeared, indicating the end of the reaction.

1. Introduction

Arsenic is classified as a metalloid, as it presents metallic and non-metallic properties, as well as a wide range of chemical reactivity [1]. The toxicity of this element depends mainly on the kind of bond—organic arsenic compounds are less toxic than inorganic arsenic compounds, while species with As (III) are essentially less toxic than As (V) compounds [2,3]. Arsenic is an impurity in metallurgical minerals and concentrates, and usually enters solution as AsO₄³⁻ during feeding of hydrometallurgical processes [4].

Arsenic contamination of water is a problem in different countries [5]. The strictest regulations in Australia, the U.S. and Europe have set 10 µg/L as the highest acceptable level of contamination by arsenic in drinking water. Contaminated streams are often treated with both physicochemical and biological processes which are effective. However, the sediment formed as a bioproduct of these technologies can create a waste problem [6].

Arsenic is not easy to remove, but it can be transformed into an insoluble and stable compound in the environment by combining with other elements, such as Fe [7]. Heavy elements, particularly arsenic, can be removed by applying jarosite technology. This type of compound, which may come from hydrometallurgical circuits and acid mine drainage, can be incorporated into jarosite-type compounds, reducing the contamination of hydrologic streams by arsenic and other elements. Jarosites are minerals that belong in the alunite supergroup. Precipitation of these compounds, whose general formula is (MFe₃(SO₄)₂(OH)₆, where M = K⁺, Na⁺, NH₄⁺, Rb⁺, Ag⁺, ½Hg²⁺, H₃O⁺ or ½Pb²⁺, is used in the metallurgical industry for control of Fe, SO₄², alkaline elements, and other impurities in solutions of processes [8].

Dutrizac and Jambor [9,10] conducted a study of the incorporation of arsenic in jarosites, in chloride and sulfate media. More recent studies [11,12] have confirmed that there is a relationship between iron and arsenate during the nucleation of solid materials, such as potassium jarosite with arsenic; this causes retention of arsenic during weathering of minerals with this metalloid. Reyes et al. [13] synthesized natrojarosite with arsenic and managed to incorporate 1% AsO₄ inside its structure. They conducted a study on the alkaline reactivity in Ca(OH)₂ and NaOH media where the induction period and particle size were independent, finding it increased exponentially as temperature decreased. The decomposition of arsenical sodium jarosite is characterized by the formation of an iron hydroxide gel with adsorbed arsenate, the removal of sulfate and sodium ions from the lattice, and their diffusion into solution.

Research on the alkaline reactivity of argentian jarosite in Ca(OH)₂ and NaOH media has been conducted by studying the effects of temperature (293–333 K), particle size and OH⁻ and CN⁻ concentration. The stoichiometry of the reaction, dependence of the induction periods, activation energy and orders of reaction were determined, and the stability of these compounds was studied under different conditions [14,15,16].

The aim of this investigation was to study the kinetics of the alkaline decomposition in NaOH media of a solid solution of ammonium-sodium jarosite with arsenic to provide information on the stability, in extreme conditions of alkalinity and temperature, of this compound. The most used method to mitigate acid effluents is increase in pH by the addition of a source of alkalinity, such as caustic soda (NaOH). Understanding the dissolution reactions of phases commonly present in acid mine drainages (AMD), such as jarosite-type compounds, is very important for environmental assessment, since elements such as As(V) could return to the environment after the mitigation process with a chemical-neutralizing agent.

2. Materials and Methods

2.1. Synthesis and Characterization of NH₄-Na Jarosite with Arsenic

Based on studies reported by Dutrizac and Jambor [9] and Patiño and Reyes [17], the synthesis of a solid solution of ammonium-sodium jarosite with arsenic was undertaken. Synthesis conditions are shown in

Table 1. Parameters and conditions for the synthesis of NH₄-Na jarosite with arsenic.

Reagents and Parameters	Conditions
Na2HAsO4·7H2O	0.03 mol/L
(NH4)2SO4	0.3 mol/L
Fe2 (SO4)3	0.25 mol/L
pH	1.2–1.8
T	367 K (94 °C)
stirring	500 min−1
T	24 h

.

The substances used were of reagent-grade. On the heating plate with automatic temperature control, a liter of the synthesis solution was placed in a glass reactor with a coupled spiral condenser (Super-Nuova/Thermo Fisher Scientific, Waltham, MA, USA) and mechanical stirring with a 3-blade propeller (IKA-RW16 basic, IKA Werke GmbH & Co. KG, Staufen, Germany). The duration of the synthesis reaction was 24 h at 367 K (94 °C) and 500 min⁻¹ mechanical stirring. The obtained precipitate was vacuum-filtered with Whatman # 40 filter paper and washed with deionized hot water (338 K), removing the excess iron and SO₄²⁻. The precipitate was dried at 333 K (60 °C) for 5 h in a Felisa F-291 stove.

The precipitate was characterized by X-ray diffraction-XRD (Bruker D-800, Bruker, Billerica, MA, USA) using Cu Kα radiation (1.54056 Å), and scanning electron microscopy, SEM (JEOL JSM-670 equipped with an energy-dispersive X-ray detector, EDS, INCA, JEOL Ltd., Tokyo, Japan). The infrared spectrum was obtained with a PerkinElmer FT-IR System 2000 Fourier transform infrared spectrophotometer (FT-IR, PerkinElmer, Waltham, MA, USA) with a spectral resolution of 4 cm⁻¹ using the potassium bromide pellet technique (KBr). Atomic absorption spectroscopy, AAS (PerkinElmer Analyst 200), inductively coupled plasma optical emission spectrometry, ICP (PerkinElmer Optima-300XL), dichromatometry to determine Fe, and gravimetry to determine SO₄²⁻ as BaSO₄ were conducted for elemental analysis. The N content was determined in a PerkinElmer 2400 Series II elemental analyzer (combustion 1233 K reduction 913 K) [18]. The thermogravimetric analysis of the precipitate was conducted by a TGA 851e Mettler-Toledo, with a heating of 10 K/min using pure argon with a flow of 667 × 10⁻³ m³/s. Particle size was measured using a series of Tyler sieves (USA standard testing sieve, ASTM-11 specifications) with mesh diameters: 150, 90, 75, 53, 44, 38 and 25 µm.

2.2. Decomposition Experiments in NaOH Media

These experiments were conducted in a 0.5 L Pyrex reactor set on a heating plate with magnetic stirring (500 min⁻¹) and automatic temperature control to keep the particles in suspension. The pH was constantly measured with a pH Orion 3 Star pH meter with a Thermo Ross Ultra Sure Flow pH electrode (Thermo Fisher Scientific, Waltham, MA, USA). In all the experiments, the pH was kept constant by adding controlled amounts of NaOH 1.0 mol/L. Initial volumes of 0.5 L, 0.2 g of jarosite were used in all the decomposition experiments. The effects of T, [OH⁻], and d₀ were assessed by keeping two parameters constant and changing the other parameter for each experiment, as shown in

Table 2. Experimental conditions for the decomposition reactions of NH₄-Na jarosite with arsenic in OH⁻ media.

Parameter	[OH−] Effect	T Effect	d0 Effect
T/K	303 K (30 °C)	293−363 K (20−70 °C)	303 K (30 °C)
[OH−1]/mol/L	0.1−0.003	0.05	0.05
d0/µm	38	38	150−25
t/min	0−230	0−35	0−30

. The decomposition experiments in NaOH media were performed under similar conditions to those reported by Reyes et al., Flores et al. and Patiño et al. [13,17,19].

In other studies on the decomposition of jarosite compounds with and without arsenic, it was found that the reaction was characterized by the release of Na⁺ and SO₄²⁻ ions from the jarosite structure to the solution [13,14,15,16,17,19]. Therefore, samples of the solution were taken at different times (≈5 mL) to monitor the progress of the reaction according to the conditions of each experiment. Sulfur was determined by ICP. In addition, it was possible to monitor the progress of the reaction by gravimetrically determining SO₄²⁻ as barium sulfate [18]. Through mass balance, alterations in the reaction volume, due to sample taking and reagent addition, were corrected.

3. Results

Synthesis of Jarosite and Characterization

An elemental analysis of the obtained precipitates during synthesis was performed.

Table 3. Chemical composition of the obtained jarosite.

Element/Specie	W (%)	Analysis Method
Fe	29.4	Volumetry
SO4	38.9	Gravimetry
As/AsO4	1.72/3.19	ICP
Na	0.240	ICP
N/NH4	2.11/2.71	Elemental Analysis
OH− + H3O+ + H2O	25.6	Difference

presents the chemical composition of the obtained ammonium-sodium jarosite with arsenic. By difference the H₃O⁺ + OH⁻ + H₂O content was inferred.

Based on the formula MFe₃(SO₄)₂(OH)₆, with the elemental analysis results and considering the molar relationships of Na⁺ + NH₄⁺ + H₃O = 1, SO₄²⁻ + AsO₄³⁻ = 2, OH⁻ + H₂O = 6, and by considering that the charge difference between SO₄²⁻ and AsO₄³⁻ was compensated by the conversion of OH⁻ into H₂O [20,21], it was possible to establish an approximate formula for the obtained product:

[(NH₄)_0.72Na_0.06(H₃O)_0.21]Fe_2.52[(SO₄)_1.85(AsO₄)_0.15][(OH)_4.41(H₂O)_1.59].

The ammonium-sodium jarosite with arsenic, based on the conditions shown in Table 1, yielded a brownish yellow precipitate. SEM characterization (secondary electrons, SE) showed that the precipitate was made up of spheroidal aggregates of size ranging between 10 and 40 µm (Figure 1a). In the semi-detailed image of a particle of the synthesized ammonium-sodium jarosite with arsenic, a spherical tendency could be discerned (Figure 1b); the particle was made up of rhombohedral microcrystals with sizes ranging between 2 and 6 µm (Figure 1c).

The obtained ammonium-sodium jarosite with arsenic presented a similar morphology and microstructure to those reported for arsenical potassium jarosite and sodium jarosite with arsenic [13,19]. Assessment of the obtained X-ray diffraction spectrum was conducted using Match Version 3. (Crystal Impact, Dr. H. Putz and Dr. K. Brandenburg GbR, Kreuzherrrenstr, Bonn, Germany), which contains the International Center for Diffraction Data-Power Diffraction Files (ICDD-PDF). The intensity of the peaks shown in Figure 2 suggested the presence of crystallographic planes typical of ammonium jarosite, albeit slightly shifted to the left due to the unit cell’s expansion caused by the incorporation of the arsenate ion (AsO₄³⁻) [22]. No other compounds were identified, as there were no additional intense peaks.

For infrared spectroscopy analyses, the spectrum (400–4000 cm⁻¹) was obtained in transmission mode after the preparation of the KBr pellet (Figure 3). The intense absorption in the region between 2600 and 3800 cm⁻¹ was attributed to the lengthening of the bond O–H (ν_OH). Due to increase in the bond energy of hydrogen with the mineral structure, this band changes to lower frequencies for Pb-jarosite (3352 cm⁻¹), Pb-As-jarosite (3343 cm⁻¹), Na-jarosite (3365 cm⁻¹), and increases for K-jarosite (3385 cm⁻¹) [18]. The band observed at 1640 cm⁻¹ was attributed to the deformation of the bond H–O–H. The band shown at 625 cm⁻¹ was attributed to the v₄ bond mode of SO₄²⁻. Next, three intense absorption bands were observed near 1195, 1075 and 987 cm⁻¹, which were attributed (respectively) to the ν₃ (double) and ν₁ vibrations of the sulfates. The spectral band at 1640 cm⁻¹ corresponded to the flexion movement of water molecules. The band at 1420 cm⁻¹ belonged to the bond N–H. The band at 828 cm⁻¹ established the presence of the functional group As−O. The signal at 503 cm⁻¹ was attributed to the vibration of the octahedral coordination of FeO₆.

The thermogravimetric analysis is shown in Figure 4; it indicated that the thermal decomposition of ammonium-sodium jarosite with arsenic occurred in six stages, with mass losses from room temperature up to 1073K (800 °C). The first stage started at 518 K (245 °C) with a 2.4% mass loss corresponding to the evaporation of excess moisture absorbed in the sample. Dehydroxylation occurred between 521K (248 °C) and 693K (420 °C). Ammonium evaporated between 633 K (360 °C) and 713 K (440 °C), with a mass loss of 5.3%. The loss of SO₄ and O occurred between 808 K (535 °C) and 963 (690 °C), with a total weight loss of 48.7%. In a temperature range between 853 K (580 °C) and 893 K (620 °C), there was a slight mass loss, which corresponded to AsO₄.

Dehydration 373–518 K (100–245 °C)

[(NH₄)_0.72Na_0.06 (H₃O)_0.21Fe_2.52[(SO₄)_1.85 (AsO₄)_0.15][(OH)_4.41(H₂O)_1.59] →
[(NH₄)_0.72Na_0.06]Fe_2.52 (SO₄)_1.85 (AsO₄)_0.15(OH)_4.20 + 2.01H₂O

(1)

Dehydroxylation 518–633 K (245–360 °C) and ammonium loss 633–713 K (360–440 °C)

[(NH₄)_0.72Na_0.06Fe_2.52(SO₄)_1.85(AsO₄)_0.15(OH)_4.20] → Na_0.06Fe_2.52O_1.74(SO₄)_1.85(AsO₄)_0.15 + 0.72NH₃ + 2.46H₂O

(2)

Desulfation 808–963 K (535–690 °C)

[Na_0.06Fe_2.52O_1.74 (SO₄)_1.85 (AsO₄)_0.15] → 1.09Fe₂O₃ + 0.28Fe (SO₄)₃ + 1.01SO₂ + 0.15AsO₄ + 0.06NaFe (SO₄)₂

(3)

Previous studies have reported that, when subjected to high temperatures after undergoing alkaline decomposition, jarosite-type compounds transform into a crystal compound identified as hematite with an amorphous oxide [23,24,25]. NH₄₋Na jarosite with arsenic was stable up to 518 K (245 °C), after which dehydroxylation followed by ammonium loss (633–713 K) and subsequent desulfation (808 K) took place. Arsenate remained stable in the structure up to 853 K (580 °C), when it started to volatilize. This was, in fact, very convenient, as the aim of this study was the incorporation and inertization of arsenic in environments with extreme temperatures.

Three syntheses, each with a yield of 30 g of ammonium-sodium jarosite with arsenic were performed. In addition, with a pycnometer, using water as the immersion liquid, the density was determined; the result was 2.75 g cm³. This density was very similar to that reported in other studies on jarosite-type compounds [16,19]. Particle size distribution results showed that the main sizes were retained in the meshes with diameters of between 25 and 38 μm, with a retained weight of 48.5% and 33.1%, respectively (

Table 4. Particle size distribution conducted by wet sieving of the synthesized product (weight %).

Mesh Number	Diameter µm	Weigh t/g	Weight %
+100	150	1.98	7.28
−100 + 170	90	0.43	1.58
−170 + 200	75	0.07	0.26
−200 + 270	53	0.28	1.03
−270 + 325	44	0.72	2.65
−325 + 400	38	13.20	48.50
−400 + 500	25	9.00	33.08
−500	-	1.53	5.62
Total		27.21	100

). According to the sieving results, most of the precipitate obtained was very fine. This could be related to the total reaction time of the synthesis. It would be possible to increase the particle diameter with an increase in the reaction time, however, this would lead to redissolution of the small jarosite particles formed due to increase in pH during the reaction, and therefore the yield of the reaction would have turned out to be very low. For this reason, it was decided to enhance the yield rather than increasing the size of the particle in the precipitate.

4. Discussion

4.1. Controlling Stage and Kinetic Model

The decomposition solution was analyzed at different time intervals to determine the controlling stage that describes the decomposition of ammonium-sodium jarosite with arsenic in NaOH media. A kinetic model was also determined, and the solid residues were characterized by SEM-EDS whereby an image of a partially decomposed particle was produced (Figure 5), showing a reaction front, unreacted core, and an ash layer of solid byproducts (Figure 5a). EDS analyses showed that in the core Na, S, Fe, O and As were present, meaning that these had remained unreacted; Fe, O and As could be identified in the ash layer (Figure 5c). Figure 5d (X-ray mapping results) showed that Fe and O were present throughout particles, while S could only be seen in the core, indicating that they had diffused into the solution. Although NH₄ and Na were not detected in the mapping analyses due to their low concentration, the described behavior indicated that these species had also diffused into the solution. According to the results of the SEM-EDS analysis of partially decomposed particles, in this process arsenic was adsorbed as AsO₄³⁻ by the solid residues made of Fe(OH)₃. The As adsorbed in the iron hydroxide was not released into the solution at times after the total decomposition of ammonium-sodium jarosite with As in alkaline media. This would reduce the bioavailability of arsenic in natural environments with alkaline pH, such as seawater and underground water.

In the curve shown in Figure 6, an induction period followed by a progressive conversion period and a stabilization zone can be observed. In the induction period, only traces of SO₄²⁻ and Na⁺ ions were found in the solution and the color and morphology of the solids remained unchanged. In this stage, difficulty in the diffusion of OH⁻ ions into the surface of the particles of ammonium-sodium jarosite with arsenic and the formation of active sites until the establishing of a reaction front was observed. The end of the induction period was indicated by a color shift in the particles, going from a brownish yellow to orange; at the same time, concentrations of SO₄²⁻ and Na⁺ increased progressively (progressive conversion period) until reaching a stabilization zone, which marked the end of the decomposition process.

Figure 7 shows XRD diffractograms at different reaction times and after total decomposition, corresponding to the data in Figure 6. Solid residues were identified as amorphous Fe(OH)₃, since they did not generate any crystal phases under the decomposition conditions studied for this paper.

The kinetic equation where the controlling stage that describes the process corresponding to the chemical reaction, is:

$$f_1=1-{\left(1-X\right)}^{1/3}=k_{exp}t$$

(4)

The kinetic equation that describes the diffusion through the ash layer as the stage controlling the process is:

$$f_2=1-{\left(1-X\right)}^{2/3}+2\left(1-X\right)=k_{exp}t$$

(5)

where X is a dimensionless number. Physically, it is the amount of decomposed ammonium-sodium jarosite with arsenic that has reacted; t is time and k_exp is the experimental rate constant [26,27].

For ammonium-sodium jarosite with arsenic, the assessment using Equation (4), which is more consistent with the linear requirement, corresponds to chemical control (Figure 8). From Figure 8, the slope of the straight lines represents the k_exp value and their intersection with the axis time represents the induction time (θ). Therefore, the model that best describes the alkaline decomposition process of ammonium-sodium jarosite with arsenic after the induction period is the shrinking core model with chemical control. The k_exp for a chemical control is defined as:

$$k_{exp}=\frac{2V_m{k}_q{C}_A^n}{d_0}$$

(6)

where V_m is the molar volume, k_q is the chemical rate constant of the reaction, C_A is the reactant concentration ([OH⁻]), n is the order of reaction, and d₀ is the initial particle diameter. The molar volume was obtained by means of the molecular weight of the compound (461.29 g/mol) and the calculated density (2.75 g/cm³), resulting in V_m = 167.74 × 10⁻⁶ m³/mol.

4.2. Stoichiometry

Based on the SEM-EDS results (Figure 5), and on the monitoring of the reaction progress of each performed experiment, it was possible to suggest the stoichiometry of the decomposition reaction of ammonium-sodium jarosite with arsenic in NaOH media. The reaction was monitored by determining sulfur in the solution through ICP. Under the conditions mentioned above, the stoichiometry of the process can be expressed as follows:

[(NH₄)_0.72Na_0.064(H₃O)_0.21]Fe_2.52[(SO₄)_1.85(AsO₄)_0.1][(OH)_4.41(H₂O)_1.59]_(sol) + 3.36OH⁻

0.72(NH₄⁺)_(aq) + 0.064Na⁺_(aq) + 1.85SO₄²⁻_(aq) + 2.52Fe(OH)₃•0.15AsO₄^3-_(gel) + 2.01H₂O_(liq)

(7)

4.3. Experimental Data

The experimental data of the alkaline decomposition are summarized in

Table 5. Decomposition of ammonium-sodium jarosite with arsenic in NaOH media. Conversion period: E_a = 51.6 kJ /mol and n = 0.94. Induction period: E_a = 65.0 kJ/mol and n = 1.22.

pH	[NaOH] mol/L	[OH−] mol/L	T K	d0 µm	θ min	kexp min−1	tr a min
11.46	0.003	0.0042	303	38	40.16	0.0045	240
11.61	0.005	0.0059	303	38	23.18	0.0057	210
11.79	0.007	0.0089	303	38	16.00	0.0087	190
12.05	0.010	0.01636	303	38	9.34	0.0138	90
12.39	0.030	0.0358	303	38	2.41	0.0362	35
12.55	0.050	0.0517	303	38	1.60	0.0544	25
12.69	0.070	0.0714	303	38	1.19	0.0627	22
12.85	0.100	0.1032	303	38	0.91	0.0755	20
13.26	0.05	0.1250	293	38	3.65	0.0300	50
13.06	0.05	0.1161	298	38	2.45	0.0414	40
12.86	0.05	0.1056	303	38	1.86	0.0608	30
12.75	0.05	0.1161	308	38	1.08	0.0787	25
12.65	0.05	0.1282	313	38	0.75	0.0971	20
12.27	0.05	0.0732	318	38	0.49	0.0998	18
12.16	0.05	0.0767	323	38	0.34	0.1484	16
11.97	0.05	0.0657	328	38	0.20	0.153	14
11.80	0.05	0.0583	333	38	0.16	0.1805	12
11.65	0.05	0.0535	338	38	0.12	0.2581	10
11.53	0.05	0.0519	342	38	0.08	0.3189	8
12.85	0.05	0.1032	303	90	2.11	0.0328	30
12.87	0.05	0.1081	303	53	1.82	0.0496	30
12.86	0.05	0.1056	303	45	2.05	0.0566	30
12.85	0.05	0.1032	303	38	1.87	0.0638	30
12.84	0.05	0.1009	303	25	1.83	0.0867	30

^a Total reaction time of the experiment.

, representing the [OH⁻], particle size (d₀) and temperature (T), as well as the induction time (θ) and k_exp which were evaluated. In each experiment, the [OH⁻] was determined by considering the ionization constant of water (k_w), pH of the solution and the working temperature [28].

A characteristic of the decomposition curves was the presence of an induction period that decreased as temperature rose and vice versa. In all the dissolution experiments, the dissolution reaction was strongly dependent on temperature and [OH⁻]. For the particle size effect, the induction time was almost the same. For k_exp the opposite happened—when temperature and [OH⁻] increased, the value of the constant also drastically increased.

4.4. Induction Period

In NaOH media, the induction period (θ) increased as [OH⁻] diminished and tended towards zero at high concentrations of [OH⁻]. When representing the logarithm of 1/θ against the logarithm of [OH⁻], an order of reaction n = 1.22 was obtained. Figure 9a shows the dependence of the induction period with [OH⁻]. The Arrhenius equation determines the dependence of k_q with temperature (Equation (8)).

k_q = k₀ e^(−Ea/R)

(8)

4.5. Progressive Conversion Period

Figure 10 illustrates the dependence of k_exp on the initial particle diameter d₀. The particle diameter was inversely proportional to the experimental constant (k_exp α 1/d₀). The model that best describes the alkaline decomposition process of ammonium-sodium jarosite with arsenic after the progressive conversion period is the shrinking core model with chemical control.

The order of reaction obtained with respect to the NaOH concentration was n = 0.94. for [OH⁻] ≥ 3.00 × 10⁻³ mol/L (Figure 11a). The reaction rate had a strong dependence on NaOH concentration under these conditions. With the k_exp values from each of the experiments on the temperature effect, the Arrhenius plot (Figure 11b) for the progressive conversion period was obtained. The value of the obtained slope was −6.2033 in NaOH media. These values correspond to −E_a/R, and consequently the value of the activation energy was E_a = 51.6 kJ/mol. This value confirms that the decomposition process in NaOH media of the NH₄₋Na jarosite with arsenic was controlled by the chemical reaction. All the kinetic parameters obtained for both media are summarized in

Table 6. Kinetic parameters calculated for the decomposition of ammonium-sodium jarosite with arsenic in NaOH media.

NaOH
Kinetic Parameter	Induction	Progressive Conversion
Ea/kJ/mol	65.0	51.6
n	1.22	0.94
k0	9.77 × 1010	3.44 × 106

.

The energy dependence calculated for the induction period was considerably higher than the dependence of the progressive conversion period (see Table 6). The energy consumption was much higher due to the difficulty in chemical adsorption and in the subsequent establishment of a reaction front of OH⁻ ions on the superficial cationic centers, which were very stable. The incorporation of arsenic in the structure of the ammonium-sodium jarosite, reduced the efficiency of the OH⁻ attack on the particle surface. This may have been due to electrostatic repulsion towards the hydroxyls created for the arsenate bonds in the surface at elevated pH; furthermore, increase in the number of Fe-AsO₄³⁻ bonds inhibits the dependence on [OH⁻] in these systems [29]. The arsenic adsorbed in iron hydroxide is not released into solution, with alkaline pH reducing its bioavailability in natural environments, such as seawater and underground waters [17].

The kinetic expression for the induction period for de jarosite decomposition is:

$$\frac{1}{\theta }=\frac{r_0}{V_m}·9.7684\times {10}^{10}·e^{-\mathrm{65,030}/RT}{\left[O{H}^{-}\right]}^{1.22}$$

(9)

For the progressive conversion period the kinetic expression for NH₄-Na jarosite decomposition in NaOH media is:

$$1-{\left(1-X\right)}^{1/3}=3.4395\times {10}^6·e^{-\mathrm{51,580}/RT}{\left[O{H}^{-}\right]}^{0.94}t$$

(10)

5. Implications

Results of the elemental analysis of ammonium-sodium jarosite with arsenic showed an AsO₄³⁻ incorporation of 3.19 W%, which was three times more than the sodium jarosite synthesized by Reyes et al. [13] and Patiño et al. [17] (1.0% WAsO₄³⁻), and lower compared to the jarosite with arsenic synthesized by Patiño et al. [30] (3.8 W%). The obtained precipitates were of a brownish-yellow color, with a main size of 38 µm (48 W% retained) and a rhombohedral morphology.

According to the ICP results shown for the monitoring of each of the decomposition reactions, arsenic was not found in liquids, and was retained in the residual solid, which was made of iron hydroxide, according to the SEM-EDS analysis results of partially decomposed particles. This is very convenient, because after the decomposition of ammonium-sodium jarosite with arsenic in NaOH media, the arsenic appeared to be adsorbed in iron hydroxide and was not released to the solutions, which would reduce the bioavailability in natural environments of the studied media.

The reaction front, an ash layer of iron hydroxide with adsorbed arsenate, and an unreacted core of jarosite with arsenic, were the main characteristics of the progressive conversion period. The experimental data were consistent with chemical control in the shrinking core model. The decomposition of ammonium-sodium jarosite with arsenic in NaOH media had an order of reaction of n = 0.94 for [OH⁻] ≥ 3 × 10⁻³ mol/L, which showed that, after formation of the reaction front, the consumption of OH⁻ ions decreased considerably. Additionally, low concentrations of OH⁻ slowed down the start of the decomposition reaction. For the induction period, the order of reaction was n = 1.22, which indicated that there was higher consumption of OH⁻ ions to support the reaction at the beginning of the decomposition process. The arsenate bonds in the structure of ammonium-sodium jarosite with arsenic generated an electrostatic repulsion towards the hydroxyls under highly alkaline conditions, which limited the OH⁻ attack on the particle surface; in addition, the increase in AsO₄³⁻ bonds inhibited the dependence on [OH⁻].

The activation energy of the reaction in NaOH media for the progressive conversion period was 51.6 kJ/mol, and 65.0 kJ/mol for the induction period. To start the decomposition process, high concentrations of OH⁻ and temperature were needed. If these variables have low values, the start of the reaction becomes kinetically slow, with long induction times. Furthermore, arsenic was retained in the residual solids, even at times after the total decomposition of this jarosite.

Finally, in a comparison with other synthesized jarosites with arsenic, the order of reaction of ammonium-sodium jarosite with arsenic was 0.94, the dependence on OH⁻ being slightly higher than of the Na jarosite with arsenic synthesized by Reyes et al. [13] (n = 0.70), and considerably below that of the potassium jarosite with arsenic synthesized by Patiño et. al. [30] (n = 1.86). The apparent activation energy of ammonium-sodium jarosite with arsenic was E_a = 51.6 kJ/mol, which was lower than the jarosite with arsenic synthesized by Reyes et al. [13], E_a = 57.1 kJ/mol and the jarosite studied by Patiño et al. [30], E_a = 60.3 kJ/mol.

6. Conclusions

The synthesis of ammonium-sodium jarosite with arsenic was achieved, incorporating 3.19 W% of AsO₄. The precipitate was of a brownish yellow color, with particle sizes ranging from 10 to 50 µm and the morphology of the aggregates was of a spherical type. Through chemical analysis, the approximate formula of the synthesized compound was determined to be:

[(NH₄)_0.72 Na_0.06 (H₃O)_0.21]Fe_2.52[(SO₄)_1.85(AsO₄)_0.15][(OH)_4.41(H₂O)_1.59]

Thermogravimetric study indicated that the ammonium-sodium jarosite with arsenic was stable up to 518 K (245 °C), after which dehydroxylation took place, followed by sulfate and ammonium loss. Arsenate was stable in the structure up to 883 K (610 °C), when it began to volatilize.

The decomposition curves of ammonium-sodium jarosite with arsenic showed an induction period and then a progressive conversion period. When temperature and OH⁻ concentration increased, the induction period decreased considerably. The progressive conversion period appeared after the formation of a reaction front; an ash layer of iron hydroxide with adsorbed arsenate formed around an unreacted core of ammonium-sodium jarosite with arsenic. The experimental data are consistent with the shrinking core kinetic model with chemical control. When the temperature and OH⁻ concentration increased, the induction period decreased exponentially and was independent of particle size under the conditions studied.

The induction period depended on high concentrations of the reaction media and high activation energy, indicating that, for the decomposition reaction to start, high [OH⁻] and T conditions were necessary; conversely, if the value of these variables was low, the start of the reaction became kinetically slow (long induction times).

Finally, it is possible to consider the ammonium-sodium jarosite with arsenic compound as an alternative in the removal of arsenic from water, since, even after decomposition, the arsenic is retained in the residual solid.