Kinetic Analysis of the Decomposition Reaction of the Mercury Jarosite in NaOH Medium

This work presents the kinetic study of the decomposition in NaOH medium of mercury jarosite whose approximate formula is [Hg_0.39(H₃O)_0.22]Fe_2.71(SO₄)_2.17(OH)_4.79(H₂O)_2.09. The reaction progress takes place on the surface of the compound with diffusion of the hydroxyl ions (OH⁻) from the solution to the particle surface moving the reaction front toward the interior of the particle, with the release of ions SO₄ ²⁻ and Hg²⁺ from the core to the reaction medium. The decomposition curve can be described by three kinetics stages: an induction period followed by a progressive conversion period and ending the reaction in the stabilization zone. The results of X−ray diffraction showed that as the decomposition reaction progresses the partially decomposed solids lost its crystallinity ending as amorphous solids. For the induction period, the reaction order (n) was 0.52 for [OH⁻] < 0.0187 mol L⁻¹ and when [OH⁻] > 0.0187 mol L⁻¹ n = 1.48, whereas the calculated activation energy (E_a) was 81.7 kJ mol⁻¹. For the progressive conversion period n = 0.99 for [OH⁻] > 0.0057 mol L⁻¹ and for lower concentrations n ≈ 0, with E_a = 56.9 kJ mol⁻¹, confirming that the decomposition process is controlled by the chemical reaction. Based on the calculated kinetic parameters, the partial and global kinetic expressions of the decomposition process were proposed. These models were compared with the experimental results, and it was favorably proven and described the decomposition process of the mercury jarosite in alkaline medium.