A study on the dissolution rates of K-Cr(VI)-jarosites: kinetic analysis and implications

Abstract

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.