Kinetic analysis of the thermal decomposition of a synthetic mercury jarosite

Jarosites are widely used in the hydrometallurgical industry of zinc to eliminate iron and other impurities contained in the concentrates. However, these compounds can also incorporate elements of significant environmental concern such as Tl ⁺ , Hg ²⁺ , Pb ²⁺ , Cd ²⁺ , Cr(VI), and As(V). In this work, the characterization of a synthetic mercury jarosite and its thermal decomposition kinetics are reported. XRD and FTIR analyses confirm that a mercury jarosite-Hg _0.40 (H ₃ O) _0.2 ]Fe _2.71 (SO ₄ ) _2.17 (OH) _4.79 (H ₂ O) _0.44 -was successfully synthesized. Four mass loss events were observed by thermogravimetric analysis at 290 °C, 365 °C, 543 °C, and 665 °C. The third event corresponds to mercury decomposition into mercury oxide, whilst the forth is related to the jarosite to hematite transformation determined by X-ray diffraction starting at around 600 °C. According to the kinetic parameters (activation energy and frequency factor) of the thermal decomposition process, the fourth stage required the highest energy (Ea = 234.7 kJ-mol ^-1 ), which corresponds to elimination of sulfur and oxygen from the jarosite lattice. Results show that jarosite-type compounds have the capability to incorporate heavy metals into their structure, retaining them even at high temperatures. Therefore, they can be used as a remediation strategy for heavy metals, such as mercury and others elements of environmental concern.