Formation of schwertmannite and its transformation to jarosite in the presence of acidophilic iron-oxidizing microorganisms

Schwertmannite [Fe8O8(OH)6SO4] and jarosite [XFe3(SO4)2(OH)6] were synthesized from FeSO4-mineral salts solutions that were inoculated with acidophilic iron-oxidizing bacteria (Acidithiobacillus ferrooxidans). Schwertmannite was transformed to NH4+/H3O+ jarosite upon increasing the aging time (19 to 40 days), temperature (36 to 45 °C), or the ammonium concentration (11.4 to 111.4 mM) in acid (about pH 2.0) media. Thus, in this phase transformation, metastable schwertmannite served as a solid phase precursor for jarosite formation. Scanning electron microscope and chemical analyses showed that the morphology, specific surface area and chemical composition of the jarosites thus produced varied depending on the experimental conditions. Because the surface areas and the molar ratios of Fe:S in the two Fe(III) hydroxysulfates are different, the transformation of schwertmannite to jarosite impacts the biogeochemistry of Fe and S as well as minor and trace metals in acid mine drainage.