Synthesis and topology of the reaction of mercury jarosite in NaOH medium

In this work, we propose the slow addition of reagents to a Fe₂(SO₄)₃·nH₂O solution contained in a one-liter glass reactor, which in turn was immersed in oil in order to improve heat transfer. Hg(NO₃)₂·H₂O was slowly added using different temperature, pH and stirring conditions. The obtained precipitates were vacuum filtered and rinsed with hot distilled water to remove the iron, sulfate and nitrate excess. A total of 11 syntheses were performed, and the obtained products were characterized by dichromatometry, gravimetry, Atomic Absorption Spectroscopy (AAS), X-ray diffraction (XRD) and Scanning Electron Microscopy-Energy Dispersive X-ray Spectroscopy (SEM-EDS), as well as Inductively Coupled Plasma Spectrometry (ICP). Of the syntheses we performed, synthesis number four is the one that presents optimal experimental conditions in order to obtain mercury jarosite: 0.27 mol L^–1 Fe₂(SO₄)₃·nH₂O, 0.58 mol L^–1 Hg(NO₃)₂·H₂O, 93 °C (366 K) and 400 rpm mechanical stirring for 24 h. The synthesized sample has the approximate formula Hg_0.4(H₃O)_0.2Fe_2.71(SO₄)_2.17(OH)_4.79(H₂O)_2.12; 9.59 grams of this compound were obtained. The study on the topology of the reaction of mercury jarosite in alkaline medium (NaOH) was conducted under the following conditions: 0.05 mol L^–1 NaOH, pH 12.70, T 30 °C (303 K), d₀ 38 µm and 500 rpm magnetic stirring. SEM-EDS results indicate that the OH^– ions diffuse from the bulk of the solution, through the ash layer, and towards the particle, while the (SO₄)^2– and Hg²⁺ ions diffuse from an unreacted core towards the solution, which suggests that the kinetic model that best describes the dissolution is that of constant size spherical particles and unreacted core.