The decomposition of the solid solution of potassium jarosite with chromium(VI) in Ca(OH) 2 media was studied in the present work. According to experimental results, the incorporation of CrO 4 2− into the crystal structure of jarosite resulted in a solid solution with the following approximate formula: [K 0.86 , and E a = 51.56 kJ mol −1 . The CrO 4 2− diffusion is slower compared to that of sulfate, a slight amount of chromate is adsorbed in the layer of the solid residue consisting on Fe(OH) 3 . Finally, the equations that satisfactorily describe the reaction process were established from the data obtained.

BackgroundThe 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.ResultsThis 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 CrO42−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+, SO42− and CrO42− 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 EA and dissolution rate constants for each kinetic stages of induction and progressive conversion.ConclusionsThe 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+, SO42− and CrO42− from the KFe3[(SO4)2 − X(CrO4)X](OH)6 structure, although CrO42− 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.

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