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 parametera 0 decreased whereas c0 increased with the synthesis temperature, indicating that H3O+ substitution for K+ in the jarosite structure was favored at low temperatures.