Alkaline decomposition of synthetic jarosite with arsenic
© Patiño et al.; licensee BioMed Central Ltd. 2013
Received: 12 June 2012
Accepted: 1 April 2013
Published: 8 April 2013
The widespread use of jarosite-type compounds to eliminate impurities in the hydrometallurgical industry is due to their capability to incorporate several elements into their structures. Some of these elements are of environmental importance (Pb2+, Cr6+, As5+, Cd2+, Hg2+). For the present paper, AsO43- was incorporated into the lattice of synthetic jarosite in order to carry out a reactivity study. Alkaline decomposition is characterized by removal of sulfate and potassium ions from the lattice and formation of a gel consisting of iron hydroxides with absorbed arsenate. Decomposition curves show an induction period followed by a conversion period. The induction period is independent of particle size and exponentially decreases with temperature. The conversion period is characterized by formation of a hydroxide halo that surrounds an unreacted jarosite core. During the conversion period in NaOH media for [OH-] > 8 × 10-3 mol L-1, the process showed a reaction order of 1.86, and an apparent activation energy of 60.3 kJ mol-1 was obtained. On the other hand, during the conversion period in Ca(OH)2 media for [OH-] > 1.90 × 10-2 mol L-1, the reaction order was 1.15, and an apparent activation energy of 74.4 kJ mol-1 was obtained. The results are consistent with the spherical particle model with decreasing core and chemical control.
Arsenic has been found in underground waters in several countries around the world at levels that surpass those established by the World Health Organization (10 μg L-1) . This issue affects more than 70 countries, where the health of approximately 150 million people is at risk. Of the total, 110 million people live in south and south east Asia , and the rest are found in countries such as Argentina, Mexico, Chile, Peru, United States, Brazil and Canada [3, 4].
In the last decade, several alternatives have been suggested in order to reduce the concentration of arsenic in potable water: coagulation-flocculation, ion exchange, membrane filtration, precipitation with alum, ozone oxidation, precipitation with iron, lime softening, adsorption with minerals, and activated alumina among others. However, only a few of these have proved to be effective . Arsenic cannot be easily eliminated, but it can be combined with other elements, such as iron, and turned into an insoluble compound that is stable in the environment .
Jarosites are used to eliminate iron in the zinc mining industry, and their structure can incorporate impurities, such as As and Pb. Dutrizac et al. [7, 8] have studied the incorporation of arsenic in the structure of Na and K jarosites. They achieved incorporation of 4% AsO4 in synthetic jarosite and 1% in synthetic natrojarosite. Asta et al.  also synthesized jarosites and goethites that, when exposed to acidic pH, showed the capability of absorbing arsenic. Studies on the dissolution of natural and synthetic jarosites not containing arsenic have been carried out at different pH conditions, and it has been found that dissolution is faster at an alkaline pH compared to acidic pH; besides, dissolution occurs in an erratic manner [10, 11]. Smith et al.  synthesized lead- and lead-arsenic jarosites, which underwent dissolution at pH 2 and 8, but kinetic study of decomposition was not conducted. Savage et al.  synthesized jarosites with arsenic, varying the potassium arsenate concentration in order to study the substitution of arsenic for sulfur inside the unit cell; they concluded that arsenate expands the unit cell, since the bond distance between arsenic and oxygen is longer than that between sulfur and oxygen. However, a study on the reactivity of these compounds was not conducted. Patiño et al. [14–18] have thoroughly studied the reactivity of argentian jarosites in alkaline media, where cyanidation was also carried out in order to recover the silver contained in those compounds. Recently, Reyes et al.  have synthesized synthetic natrojarosite with arsenic, incorporating 1% AsO4 in its structure; they studied the alkaline reactivity in NaOH and Ca(OH)2, and they found that the decomposition is strongly related to the reaction conditions, such as temperature, pH and particle size .
For this work, a sample of jarosite with arsenic was synthesized, incorporating ≈ 4% AsO4 in its structure. Next, a kinetic study of alkaline decomposition was carried out to obtain the apparent activation energy and reaction order, and to compare these data with the results obtained by Reyes et al.  in order to determine the differences in the reaction rates between both synthetic jarosites with arsenic.
Materials and experimental procedure
The alkaline decomposition experiment was carried out in a conventional thermostated glass kettle with magnetic stirring. The pH of the solution was constantly measured for each experiment with an Orion 3 Star pH-meter equipped with a Thermo Ross Ultra Sure Flow pH electrode, and the experiments were planned so that the reactant concentrations would be constantly adjusted. For the decomposition in NaOH and Ca(OH)2 media, both of ACS reagent grade, J. T. Baker, 0.4 g of synthetic jarosite with arsenic were used in an initial volume of 1 L. pH was kept constant in both media by adding small amounts of NaOH or Ca(OH)2 during the experiment. OH- concentration was calculated according to the pH of the solution, and the ionization constant of water was calculated according to the working temperature .
The use of NaOH was to subject the synthetic jarosite with arsenic to extreme conditions of alkalinity. Besides, decomposition reactions in this media are free from interference kinetics (according to Patiño et al.) [14–17]. On the other hand, Ca(OH)2 is widely used in the industry as means to control pH, as well as in different environmental remediation processes (water treatment, soil treatment, etc.)
As previously noted, alkaline decomposition is characterized by the release of sulfate and potassium ions from the lattice, as well as by their quick diffusion towards the solution. In NaOH and Ca(OH)2 media, reaction progress was followed by taking 5 mL samples that were analyzed for potassium by atomic absorption spectrometry AAS (Perkin Elmer Analyst 200) and for sulfur by inductively coupled plasma ICP (Perkin Elmer Optima-3000XL).
The alterations due to sampling and addition of the reagent were corrected by mass balance. Of the three techniques (ICP, AAS, gravimetry), AAS was selected to follow the alkaline decomposition kinetics of the synthetic jarosite with arsenic. As already mentioned, several experiments were carried out to observe the evolution of the solids at different conversion values. These solids were characterized by chemical analyses, XRD, SEM-EDS and AAE.
For the stoichiometric study of the reaction, synthetic jarosite with arsenic samples were treated with NaOH and Ca(OH)2 for long periods of time. Later, sulfate and potassium in the solution were analyzed, and the residues were characterized by XRD, EDS and AAE.
Results and discussion
Decomposition of the synthetic jarosite with arsenic in NaOH and Ca(OH)2
K extraction at different time periods (NaOH, Ca(OH) 2 and Ca(OH) 2 attacked with HNO 3 )
Ca(OH)2attacked with HNO3
K % Extraction
K % Extraction
K % Extraction
The alkaline decomposition kinetics was followed by AAS analysis of potassium. The first characteristic of the kinetic curves is the presence of an induction period (θ) that rises as temperature decreases and grows as [OH-] diminishes. This behavior has also been noticed in previous studies, e.g. synthetic argentian natrojarosite, synthetic argentian jarosite and synthetic natrojarosite with arsenic [16, 18, 19].
The solids underwent EDS analysis during the induction period, and there were no morphological changes or formation of a solid layer detectable at practical levels of resolution of the EDS technique (≈ 0.1 μm). This leads to the conclusion that the induction period is a superficial phenomenon at molecular thickness levels related to adsorption processes, whose detailed mechanism requires techniques that were not available for this study. At any rate, this could be the period during which active sites are created until the reaction front is established. It was proved that superficially attacking the synthetic jarosite with arsenic in HNO3 (a way to create defects or to remove submicron films) minimizes the induction period (θ). This was confirmed by conducting an experiment under the same conditions of decomposition in Ca(OH)2; results are shown in Figure 3. It can be noticed that the induction period diminishes, while keeping the same experimental rate constant. However, in this study, the induction period has not been eliminated, and its dependence on OH- concentration, temperature and particle size have been determined. EDS testing of the solids after the induction period (θ) indicates the presence of a reaction front containing a non-reacting core of synthetic jarosite with arsenic and a gel halo of iron hydroxide with adsorbed arsenate (see Figure 5A). The presence of As in the solid residues was confirmed by AAE. It was observed that 100% of the arsenic remains in the residual solids in the decomposition experiments at 30°C. For long residence times (780 min) and high reaction temperatures (70°C), about 95% of the initial arsenic was retained in the solid, which reveals that at these conditions there is a slight diffusion of As toward the solution. Since the reaction occurs only at the interface, the kinetics has been described by separating the induction period from the conversion period.
Observation of a high number of cases indicates that the kinetic model of unreacted core gives a fairly accurate description of the decomposition of jarosite type compounds [14–19]. The controlling stage in a heterogeneous reaction is that which presents the highest resistance. In the spherical particle model with decreasing core two stages can be slow: matter transport through the ash layer (halo), or the chemical reaction at the interface between the unreacted core and the ash halo . The potential control of the decomposition process by transport in the bulk of the solution during the conversion period was assessed by comparing the decomposition times obtained under diffusive control. The determined values indicate that the decomposition times are fast, which supports the conclusion that the process is not controlled by mass transfer [21–23].
Consequently, it was observed that ion diffusion through the decomposition gel was quick. This can be noticed in Figure 5B, where the presence of iron and arsenic is current over the particle, while potassium and sulfur are only visible in the core, which indicates that these two ions have diffused into solution. It can also be noticed that the distribution of Fe is not regular throughout the particle. Since only Fe and As are present in the halo, the proportion of iron is much higher than in the core, which remains unreacted.
Where X = reacted fraction; K q = chemical constant; C A = reactant concentration; V m = molar volume of the solid; r 0 = initial radius; n = reaction order.
Data are consistent with the selected model (Figure 3B). The values of the experimental rate constants K exp were obtained by linear regression of equation (2) with a regression coefficient of r > 0.99. The values of the induction times (θ) were obtained by considering the intersections between the regression straight line and the time axis.
Dependence of the induction period
Dependence of the conversion period
Comparison of the reaction orders (n) and apparent activation energy (Ea) of the synthetic jarosite with arsenic and synthetic natrojarosite with arsenic in NaOH and Ca(OH) 2 media respectively
Activation energy, Ea
Synthetic jarosite with arsenic a
>8.00 × 10 -3
>1.90 × 10 -2
Synthetic natrojarosite with arsenic b
>3.84 × 10 -3
>2.21 × 10 -2
Synthetic jarosite c
>2.70 × 10 -2
>3.50 × 10 -3
According to these results, under the experimental conditions used for this work, the synthetic jarosite with arsenic has a higher energy dependence compared to the synthetic natrojarosite with arsenic; this difference in energy dependence becomes even more evident when compared to the values obtained in the alkaline decomposition of the synthetic jarosite without arsenic.
The elevated apparent activation energy of this compound is related to increased incorporation of arsenate (4 times higher compared to synthetic natrojarosite with arsenic ); this causes the lattice to expand , and as consequence, the amount of energy required for alkaline decomposition becomes much higher.
A comparison with other decomposition studies on synthetic jarosites without arsenic at similar conditions shows that the energy dependence is higher for ammonium jarosite  and argentian jarosite , and lower for plumbojarosite  and natrojarosite . Regarding the concentration effect, there are no definite criteria by which a comparison can be made. This is because reagents diffuse in different ways on the particle surface. However, since the behavior of these compounds does not differ much, the natural compounds show a behavior similar to that of the studied synthetic compounds.
The decomposition of the synthetic jarosite with arsenic in alkaline media presents an induction period, where there are no changes on the surface of the jarosite. It is followed by a conversion period, until total decomposition of the synthetic jarosite with arsenic is reached.
During the conversion period, sulfate and potassium ions diffused into the solution, remaining an X-ray amorphous iron hydroxide gel with adsorbed arsenate. In the alkaline decomposition in NaOH media for [OH-] 1.0 × 10-4 at 6.40 × 10-3 mol L-1, 1/θ was proportional to OH0.23. For [OH-] > 6.40 × 10-3 mol L-1, 1/θ was proportional to OH2.65, and apparent activation energy of 84.66 kJ mol-1 was obtained.
During the conversion period, for [OH-] > 8.00 × 10-3 mol L-1, the concentration order was proportional to [OH-]1.86. For [OH-] < 8.00 × 10-3 mol L-1 the effect is zero, and apparent activation energy of 60.3 kJ mol-1 was obtained. In the decomposition in Ca(OH)2 media for [OH-] 1.7 × 10-3 at 3.12 × 10-2 mol L-1, 1/θ was proportional to [OH-]0.24; for [OH-] > 3.12 × 10-2 mol L-1, θ was independent from OH- concentration, and an apparent activation energy of 88.2 kJ mol-1 was obtained.
During the conversion period, for [OH-] > 1.90 × 10-2, the reaction order was 1.15. For [OH-] < 1.90 × 10-2 mol L-1the reaction order was zero and apparent activation energy of 74.4 kJ mol-1 was obtained. Under the studied conditions, the obtained activation energies are consistent with the spherical particle model with decreasing core and chemical control.
The energy dependence calculated for the synthetic jarosite with arsenic is higher than the calculated for the synthetic natrojarosite with arsenic under similar decomposition conditions; therefore it is more difficult for the alkaline decomposition of the synthetic jarosite with arsenic to start.
We thank the Universidad Autónoma del Estado de Hidalgo (Mexico) for letting us use their extractive metallurgy laboratory. We would also like to thank the Institute of Metallurgical Research of the U.M.S.N.H. (Mexico) for permitting us to perform the SEM-EDS and XRD studies.
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