Kinetic analysis of the decomposition of the KFe3 (SO4 )2-x (CrO4 )x (OH)6 jarosite solid solution in Ca(OH)2 medium

Abstract

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 CrO4 2-into the crystal structure of jarosite resulted in a solid solution with the following approximate formula: [K0.86 (H3 O)0.14 ]Fe2.67 [(SO4 )1.23 (CrO4 )0.77 ][(OH)5.01 (H2 O)0.99 ]. The experimental data describe a reaction based on the shrinking core model with chemical control for spherical particles. A reaction order of n = 0.67 and an activation energy (Ea ) of 63.75 kJ mol-1 were obtained in the induction period (tind ). The progressive conversion period is characterized by the diffusion of K+ , SO4 2-and CrO4 2-ions into the solution. In this stage, n = 1.99 with respect to OH- , and Ea = 51.56 kJ mol-1 . The CrO4 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.