The presence of natural and industrial jarosite type-compounds in the environment could have important implications in the mobility of potentially toxic elements such as lead, mercury, arsenic, chromium, among others. Understanding the dissolution reactions of jarosite-type compounds is notably important for an environmental assessment (for water and soil), since some of these elements could either return to the environment or work as temporary deposits of these species, thus would reduce their immediate environmental impact. This work reports the effects of temperature, pH, particle diameter and Cr(VI) content on the initial dissolution rates of K-Cr(VI)-jarosites (KFe3[(SO4)2 − X(CrO4)X](OH)6). Temperature (T) was the variable with the strongest effect, followed by pH in acid/alkaline medium (H3O+/OH−). It was found that the substitution of CrO4 2−in Y-site and the substitution of H3O+ in M-site do not modify the dissolution rates. The model that describes the dissolution process is the unreacted core kinetic model, with the chemical reaction on the unreacted core surface. The dissolution in acid medium was congruent, while in alkaline media was incongruent. In both reaction media, there is a release of K+, SO4 2− and CrO4 2− from the KFe3[(SO4)2 − X(CrO4)X](OH)6 structure, although the latter is rapidly absorbed by the solid residues of Fe(OH)3 in alkaline medium dissolutions. The dissolution of KFe3[(SO4)2 − X(CrO4)X](OH)6 exhibited good stability in a wide range of pH and T conditions corresponding to the calculated parameters of reaction order n, activation energy E A and dissolution rate constants for each kinetic stages of induction and progressive conversion. The kinetic analysis related to the reaction orders and calculated activation energies confirmed that extreme pH and T conditions are necessary to obtain considerably high dissolution rates. Extreme pH conditions (acidic or alkaline) cause the preferential release of K+, SO4 2− and CrO4 2− from the KFe3[(SO4)2 − X(CrO4)X](OH)6 structure, although CrO4 2− is quickly adsorbed by Fe(OH)3 solid residues. The precipitation of phases such as KFe3[(SO4)2 − X(CrO4)X](OH)6, and the absorption of Cr(VI) after dissolution can play an important role as retention mechanisms of Cr(VI) in nature.

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, AsO₄ ^3- 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.

A sample of potassium arsenojarosite was synthesized and thoroughly characterized. The obtained sample is a solid solution of potassium arsenojarosite, whose approximate formula is [K0.75(H3O)0.25 ]Fe1.84[(SO4)1.82(AsO 4)0.18][(OH)2.34(H 2O)3.66]. The decomposition process in alkaline medium was studied in the induction and progressive conversion periods, and the reaction order and activation energy were determined for each case. Under the used experimental conditions, results are consistent with the spherical particle model with decreasing core and chemical control. In both processes, four partial models and two global models were developed in order to describe their basic behavior. The models were validated, and it was proved that they favorably describe the decomposition process in alkaline medium.

In Zimapan, Mexico abundant mineral species have been identified. However, a significant number of these precious metals appear to be related to complex sulfides, as is the case of silver (Ag) in sulfosalts. In this work, a kinetic study of the formation of Ag complexes in the AgAsS₂-S₂O₃²⁻-O₂-NaOH system from a Zn concentrate found in the Zimapan mining district was conducted. The kinetic model application on Ag leaching shows a linear adjustment in the valuation of the particle size effect. The rate of formation of Ag complexes is dependent of temperature, leaching agent concentration and OH⁻ concentration. Furthermore, graphical verification of the kinetic models indicates that the kinetics of the formation of Ag complexes correspond to the shrinking core model with chemical controls as the controlling stage. The formation of Ag complexes was confirmed by Fourier transform infrared (FTIR) spectroscopy.

Metallic elements of higher economic value, occurring in the mineralogy of Zimapán, are Pb, Zn, Cu, and Fe; said elements are sold as concentrates, which, even after processing, generally include significant concentrations of Mo, Cd, Sb, Ag, and As that can be recovered through different leaching methods. In this work, the influence of temperature in the complexation of silver contained in a concentrate of Zn using the technology of thiosulfate with oxygen injection was studied. Chemical and mineralogical characterization of the mineral concentrate from the state of Hidalgo, Mexico confirmed the existence of silver contained in a sulfide of silver arsenic (AgAsS₂) by X-ray Diffraction (XRD). The results obtained by Atomic Absorption Spectrophotometry (AAS) reported abundant metallic contents (% w/w) (48% Zn, 10.63% Fe, 1.97% Cu, 0.84% Pb, 0.78% As, and 0.25% Ag). These results corroborate the presence of metallic sulfides such as pyrite, chalcopyrite, and wurtzite; this last species was identified as the matrix of the concentrate by X-ray Diffraction (XRD) and Scanning Electron Microscopy-Energy-Dispersive X-ray Spectroscopy (SEM-EDS). Pourbaix diagrams were constructed for the AgAsS₂–S₂O₃²⁻–O₂ system at different temperatures, which allowed the chemical reaction of leaching to be established, in addition to determining Eh-pH conditions in which to obtain silver in solution. The highest recoveries of the precious metal (97% Ag) were obtained at a temperature of 333 K and [S₂O₃²⁻] = 0.5 M. The formation of silver dithiosulfate complex (Ag(S₂O₃)₂³⁻) was confirmed by the characterization of the leach liquors obtained from the experiments performed in the temperature range of 298 to 333 K using Fourier transform infrared spectroscopy (FTIR).