The Behavior of Arsenic during the Thermal and Chemical Decomposition of the Ammonium–Arsenic Jarosite

Arsenic, an element of environmental impact, can be incorporated into jarosite–type compounds and remain stabilised within the structure under a wide range of environmental conditions. In this study, a sample of ammonium–arsenic jarosite was synthesised by precipitation in sulphate medium at controlled pH of 1.2–1.8. The behaviour of arsenic during the thermal and chemical decomposition of jarosite was analysed; the degradation in alkaline medium of jarosite was also studied. According to the results, the synthesised jarosite is composed of joined rhombohedral crystals, forming tightly spherical shaped particles, 37–54 μm size. The ammonium jarosite produced possessed a high arsenic concentration; its calculated stoichiometry being (NH4)Fe2.45[(SO4)1.80(AsO4)0.20][(OH)4.15(H20)1.85]. It was found that arsenic is stabilised in the jarosite structure; upon heating, it remains in residual solids above 700°C, whilst in alkaline medium an incongruent dissolution takes place, with the arsenic retained in the solid phase along with iron. These solids, when exposed to high temperatures (1200°C), transform into a type of iron oxide known as hematite, so with arsenic it is retained an iron compound forming a stable compound which withstands high temperatures.


Introduction
In more than 70 countries, drinking water is polluted with arsenic. This is a public health problem affecting more than 150 million people worldwide [1,2,3]. One of its main causes is acid mine drainage (AMD), where arsenic is one of the metalloid contaminants, with concentrations that can reach up to hundreds of mg L −1 . This is the result of the oxidation of arsenic-rich sulphides, such as arsenopyrite and pyrite [2]. Several arsenic removal techniques have been developed, but they have not been entirely efficient due to the instability of the arsenic-containing residues [3].
Natural jarosite-type compounds are usually associated with minerals such as quartz, kaolinite, calcite, orthoclase, gypsum, goethite, hematite, shwermannite and amorphous Fe-oxides [4]. In the hydrometallurgical industry, inorganic compounds are used to eliminate impurities; for instance the synthesis of ammonium jarosite is widely used in zinc smelting to remove impurities such as Fe; in the "jarosite process", ammonium and iron sulfates are added to the leach liquor to precipitate an ammonium jarosite. Due to their different coordination sites, jarosites have the capability to incorporate elements of environmental impact into their structure: substitutions in the cationic position (Pb 2+ , Hg 2+ , Cd 2+ ), or the substitution of sulphate (SO4) 2− by arsenate (AsO4) 3-, phosphate (PO4) 3-or chromate (CrO4) 2− [5]. Dutrizac and Jambor and Dutrizac et al.; studied the behaviour of arsenic in jarosite-type compounds (Na and K), incorporating 4 wt.% AsO4 3− in potassium jarosite and 1 wt.% in sodium jarosite [6,7]. Asta et al. studied the efficiency of jarosite, goethite and schwertmannite in absorbing arsenic from AMD, concluding that greater amounts of As were incorporated by potassium jarosite and schwertmannite [2]. Patiño et al. studied the decomposition kinetics in alkaline medium and subsequent cyanidation of jarosite-type compounds; they determined that the decomposition reaction is carried out in three stages; an induction period, a progressive conversion period and a stabilization zone [8][9][10].
In a recent study on the decomposition of an arsenic-containing jarosite compound in alkaline media [11,12], it was found that arsenic remained in the decomposition product, mainly as a constituent of amorphous iron hydroxide. There are few works on the dissolution and stability of such compounds, making the data insufficient for evaluating the real environmental risk caused by the improper disposal of the generated precipitates [13]. Ammonium jarosite may prove to be an alternative for the removal of arsenic contained in residual waters or in AMD; in both cases lime is added to the water, increasing the pH and favouring the precipitation and sedimentation of contaminant species, including arsenic [3]. Therefore, it is important to analyse the behaviour of the ammonium-arsenic jarosite in alkaline media. Besides, the hydrometallurgical industries, mainly the zinc industry use jarosite precipitation to control iron, sulfates and alkaline metals, amongst others.
This generates a great amount of potentially dangerous residues, because upon their dissolution or decomposition, the arsenic might be released to the environment in bioavailable forms. Studies on the dissolution of natural and synthetic jarosites not containing arsenic had been carried out at different pH conditions. It has been found that those jarosites remained stable at highly acid and oxidizing conditions [14] and that dissolution is faster at alkaline pH than in acidic media; besides, it was observed than dissolution occurs in an erratic manner [12]. Therefore, in this article a study on the reactivity of an ammonium-arsenic jarosite is presented, with the aim to provide information regarding the stability of As in such a compound, at extreme conditions of alkaline pH and temperature. Accordingly, the thermal decomposition behaviour and the stoichiometry of the dissolution reaction in alkaline media of the synthetized jarosite was analysed in order to determine the effect on the liberation of arsenic from the solid residues. Thus, an ammonium-arsenic jarosite was synthesized to prevent interferences due to the presence of associated minerals. The results of this work will be the basis for future studies on the reactivity of jarosite-type compounds containing elements of environmental impact, either natural or of industrial origin.

Materials and Methods
The sample of ammonium-arsenic jarosite was synthesised by a dissolution-precipitation technique, using ACS grade chemical reagents and deionised water. An aqueous solution with 0.25 mol L −1 Fe(SO4)3, 0.30 mol L −1 (NH4)2SO4, 0.03 mol L −1 As2O4 and 0.01 mol L −1 H2SO4 was prepared and placed in a glass reactor set on a hot plate stirrer (Barnstead Thermolyne Super-Nuova) at ≈ 95°C and pH of 1.2-1.8 (Orion 3 Star pH-meter equipped with a Thermo Ross Ultra Sure Flow pH electrode) and 500 min −1 during 24 hours in a reflux system. The resulting sample was rinsed several times with hot water (70°C) to remove residual SO4 2− and Fe 3+ . Finally, the solids were dried at 65°C for 5 hours.
The sample of ammonium-arsenic jarosite was characterised using X-ray diffraction (XRD, Siemens Thermal decomposition of the arsenic ammonium jarosite was carried out in a TGA 851e Mettler-Toledo thermogravimetric analyser, in a flowing air atmosphere (666×10 −3 m 3 s −1 ) with a heating rate of 10 °C min −1 from room temperature to 1200°C.
Decomposition of ammonium-arsenic jarosite was conducted in alkaline medium (NaOH, pH 12.90 and 30°C for 25 min). The decomposition solids were characterized by AAS, ICP, XRD and SEM-EDS. The decomposition solids were exposed to high temperatures (400, 700 and 1100 °C) to observe their evolution towards new crystal phases.

Synthesis and characterization
Comparison between the XRD pattern obtained from the synthesized sample and the ICDD−PDF (International Center for Diffraction Data−Powder Diffraction Files) file confirms that the most intense peaks correspond to the structure of ammonium jarosite ( Figure 1).   The density of ammonium−arsenic jarosite was determined with a picnometer, using distilled water as immersion liquid. The result was 2.72 g cm −3 , which is similar to the values reported in previous studies on jarosite type compounds with arsenic [11,13,15] and ammonium jarosite with silver [10], as shown on Table 2. Particle size distribution shows that the main particle size was retained in meshes 400 and 500, with diameters ranging between 25 and 38 microns, and a retained weight percentage of 57.25 % and 22.42 % respectively.
The infrared spectrum of the ammonium−arsenic jarosite is shown in Figure 3, together with the spectrum corresponding to an ammonium jarosite synthesised at similar conditions. Three main differences can be observed in the spectra: the overlapping peaks at 865 and 833 cm −1 (spectrum (a)), corresponding to the As−O asymmetric stretching vibration mode [19] due to the presence of arsenic as the arsenate ion; the sharpness of the band at 3400 cm −1 and the shoulder appearing at around 560 cm −1 (spectrum (b)), due to stretching (ν) and out−of−plane bending (γ) vibrations, respectively, of the OH ion. As it was espected, the substitution of sulphate ions by arsenate ions leaded to an increase in the OH/H2O ratio in the structure. The band at 1630 cm −1 , corresponding to the bending vibration of water molecules, is very similar in both spectra; however, the results from the TGA analysis, presented in the next section, have shown that the water contents is higher in the ammonium−arsenic jarosite.
The fundamental asymmetric vibrations of the sulphate group are due to the stretching (ν3) and bending (ν4) modes; the symmetric stretching (ν1) and bending (ν2) vibrations could also be observed in the infrared, although as weak or very weak bands [20,21]. Reduction in symmetry in the crystal structure of sulphates such as jarosites will cause the splitting of the asymmetric ν3, SO4 2− bands.
Infrared bands for ammonium jarosite have been observed at 1080 and 1200 cm −1 [22]. Accordingly, the bands at 1070 and 1186 cm −1 are assigned to the S−O and S=O asymmetric ν3 stretching modes and the band at 625 cm −1 is ascribed to the bending ν4 SO4 2− mode [21,23]. The intense band at 990 cm −1 is attributed to an OH − librational mode [21,22]. In addition, the band observed at 1420 cm −1 corresponds to the NH bending mode and the bands observed between 500 and 460 cm −1 are assigned to the Fe−O covalent bonding [21].

Thermal decomposition
The thermogravimetric analysis of the ammonium−arsenic jarosite is shown in Figure 4, together with the corresponding thermogram of an arsenic jarosite synthesised by the same method. As can be seen, the ammonium−arsenic jarosite shows five events of mass loss presents (Figure 4a). therefore, as the only difference between the jarosite samples in Figure 4 is the As contents, these loss could be attributed to the transformation of iron arsenate (FeAsO4) to iron(III) arsenide (FeAs) [24], since this compound has a high melting point, it is very stable and is easy to form in the presence of iron and arsenic. This is in agreement with the EDS analysis of the thermal decomposition residues of ammonium−arsenic jarosite ( Figure 5), that shows the presence of arsenic. In addition, ICP analyzes were performed by the solid decomposition, obtaining a concentration of 20.53 ppm, which corresponds to 2.05 weight percent, indicating that it lost 0.15 percent of the total weight of the arsenic, therefore, mass loss of this element is not complete [25−29].  1.125Fe2O3 + 0.2FeAsO4 → 1.125Fe2O3 + 0.2FeAs + 0.4O2 1.125Fe2O3 + 0.2FeAs (Final residues)

Decomposition in alkaline medium
The decomposition of ammonium−arsenic jarosite in NaOH medium has a short induction period. During this period, the jarosite does not react, and therefore, the SO4 2− and NH4 + concentrations are found at negligible levels in the solution ( Figure 6). The induction period has been previously observed in the decomposition of lab−synthesized jarosites [13], as well as with jarosites from industrial plants, i.e. ammonium jarosite. The concatenation of [OH − ] ions with the particle surface creates active sites until a reaction front is established. The medium ions (OH − and Na + ) diffuse through this reaction front towards the particle. At the same time, NH4 + and SO4 2− ions are released from the particle, diffusing towards the solution. This generates a progressive conversion period, where SO4 2− and NH4 + concentrations progressively increase until reaching stabilization, which indicates that the reaction has concluded.   Figure 8b is the EDS spectrum of the gel halo, where we can observe how the intensity of sulfur diminishes, while iron and arsenic are still distributed throughout the particle. This indicates that ammonium and sulfur have diffused from the core towards the bulk of the solution. An analysis by ICP decomposition solution was conducted after 50 minutes of reaction and a concentration of 1.08 ppm of arsenic was found, which corresponds to 0.108 percent of the total weight of the compound, so most of the arsenic reains in the particle.    The decomposition solids were exposed to high temperatures to observe their evolution into new crystal phases. At 110 °C and 400 °C, amorphous solids were obtained ( Figure 11). However, at temperatures between 700−1200 °C, the solids evolved into a crystal phase that was identified as hematite (Fe2O3), as observed in Figure 11. An analysis by ICP of the solids of the thermal decomposition was conducted of 1100 °C and a concentration of 19.48 ppm of arsenic was found, which corresponds to 1.948 percent of the total weight of the compound, so most of the arsenic reains sorbed in the hematite particles in an amorphous compound.   2) X-ray diffraction confirmed that the synthesized sample is a jarosite type compound with arsenic.
FTIR analysis confirmed the presence of As. TGA analysis confirmed that arsenic is retained at high temperatures (1100 °C).
3) The alkaline decomposition is characterized by an induction period, a progressive conversion period and a stabilization zone. During the progressive conversion period, sulphate and ammonium diffuse from the bulk of the particle into the solution, while arsenic and iron are retained in an amorphous iron hydroxide solid (2.45 Fe(OH)3•0.20 AsO4 3− ).
4) The decomposition solids were exposed to high temperatures. It was observed that starting at 700°C, a new transformation takes place. The result is a new crystal compound that has been identified as hematite. Additionally, the loss of As is not total, even at elevated temperature (890 °C).
5) The ammonium-arsenic jarosite synthesised in this work, even after exposed at elevated temperatures (900°C) and high alkaline conditions (pH=12.9), has proven to retain the highest amount of arsenic than similar compounds reported previously. Hence, this type of jarosites have potential application in the control of arsenic and other elements of environmental significance contained in the acid mine drainage.