Alkaline decomposition of synthetic jarosite with arsenic

Abstract

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.

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

“…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. Table 2.…”

“…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].…”

Kinetic Analysis of the Decomposition Reaction of the Mercury Jarosite in NaOH Medium

“…There are kinetic stability studies of jarosite−type compounds containing toxic metals such as arsenic, chromium and lead in alkaline medium such as NaOH and Ca(OH) 2 . These studies determine that the decomposition of jarosite−type compounds is closely related to the reaction conditions, mainly the temperature and the pH of the reaction medium . The contribution of this work is to document the behavior of the mercury contained in a synthetic jarosite in sodium hydroxide medium, by examining the reaction nature, providing information on the kinetic model, the controlling stage of the decomposition process and the reaction stoichiometry.…”

Kinetic Analysis of the Decomposition of the KFe3(SO4)2-x(CrO4)x(OH)6Jarosite Solid Solution in Ca(OH)2Medium

“…In this case, there is no proper diffusion of OH − ions into the interface between the ash layer and the unreacted core. 38,46 X-ray diffraction analysis shows the gradual decomposition of the synthetic solid, which becomes amorphous at a reaction time of 140 min and a concentration of Ca(OH) = 1.4 g L −1 are shown in Figure 6. The residual solid is most probably composed of iron hydroxide with weakly adsorbed CrO 4 2− .…”

“…45,46 If the rate of the chemical reaction is slow compared to that of the transport of matter, equation 5 defines the behavior of the reaction:…”