Characterization of jarosites produced by chemical synthesis over a temperature gradient from 2 to 40 °C

The purpose of the study was to characterize ferric iron precipitates formed in batch experiments over a temperature range of 2 to 40 °C from ferric sulfate and mineral salts in sulfuric acid. With the exception of elevated levels of K+ to promote jarosite [KFe3(SO4)2(OH)6] formation, the solution composition was intended to simulate fully oxidized bioleaching solutions. Iron-oxidizing acidophilic bacteria were not included in the system. The solutions were held for 10 days in a thermal gradient incubator with a 1.65 °C temperature interval followed by analyses of the residual solutions and precipitated solid phases. The loss of dissolved Fe3+ through solid-phase precipitation increased with temperature, consistent with the expected temperature-dependency of the rate of precipitation. Hydronian jarosites [(K,H3O)Fe3(SO4)2(OH)6] were the main phases identified in all samples by powder X-ray diffraction (XRD) analysis, but a K and SO4 enriched phase similar to Maus' salt [K5Fe3(SO4)6(OH)2·8H2O] was also detected in some samples, mostly those formed at sub-ambient temperatures (< 25 °C). Background areas from XRD powder patterns decreased in a linear fashion with increasing temperature of synthesis, suggesting improved crystallinity of the solid phase at the expense of an X-ray amorphous fraction. Spacing differences (Δ2Θ) between the intense (113) and (021) peaks confirmed the hydronian character of the jarosite phase. The jarosite lattice parameter a0 decreased whereas c0 increased with the synthesis temperature, indicating that H3O+ substitution for K+ in the jarosite structure was favored at low temperatures.