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


RESULTS
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.


CONCLUSIONS
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.
Instituto de Metalurgia, Universidad Autónoma de San Luis Potosí, Av. Sierra Leona 550, Lomas 2a Sección, C.P. 78210 San Luis Potosí, S.L.P. México.
Iván A
IA
Reyes
Área Académica de Ciencias de la Tierra y Materiales, Universidad Autónoma del Estado de Hidalgo, Carretera Pachuca-Tulancingo km 4.5, C.P. 42081 Mineral de la Reforma, Hgo. México.
Ister
I
Mireles
Ingeniería en Energía, Universidad Politécnica Metropolitana de Hidalgo, Boulevard acceso a Tolcayuca 1009, Ex-Hacienda San Javier, C.P. 43860 Tolcayuca, Hgo. México.
Francisco
F
Patiño
Facultad de Química, Universidad Nacional Autónoma de México, Ciudad Universitaria, C.P. 04510 Mexico, D.F. México.
Thangarasu
T
Pandiyan
Área de Electromecánica Industrial, Universidad Tecnológica de Tulancingo, Camino a Ahuehuetitla 301, Las Presas, C.P. 43642 Tulancingo, Hgo. México.
Mizraim U
MU
Flores
Unidad Profesional Adolfo López Mateos, Escuela Superior de Ingeniería Química e Industrias Extractivas, Instituto Politécnico Nacional, C.P. 07738 Mexico, D.F. Mexico.
Elia G
EG
Palacios
Instituto de Metalurgia, Universidad Autónoma de San Luis Potosí, Av. Sierra Leona 550, Lomas 2a Sección, C.P. 78210 San Luis Potosí, S.L.P. México.
Emmanuel J
EJ
Gutiérrez
Área Académica de Ciencias de la Tierra y Materiales, Universidad Autónoma del Estado de Hidalgo, Carretera Pachuca-Tulancingo km 4.5, C.P. 42081 Mineral de la Reforma, Hgo. México.
Martín
M
Reyes
Chromate analog; Dissolution rates; Jarosite-type compounds; Kinetic analysis; Stability