Biosynthesis of schwertmannite by Acidithiobacillus ferrooxidans cell suspensions under different pH condition

Oxidation of FeSO4 solution with initial pH in the range of 1.40–3.51 by Acidithiobacillus ferrooxidans LX5 cell at 26 °C and subsequent precipitation of resulting Fe(III) were investigated in the present study. Results showed that the oxidation rate of Fe(II) was around 1.2–3.9 mmol l− 1 h− 1. X-ray diffraction (XRD) indicated that the formed precipitates were composed of natrojarosite with schwertmannite when the initial pH was 3.51, while only schwertmannite was produced when initial pH was in the range of 1.60–3.44 and no precipitate occurred when initial pH ≤ 1.40. Scanning electron microscope (SEM) analyses showed that precipitates formed in solution with initial pH 3.51 were spherical particles of about 0.4 µm in diameter and had a smooth surface, whereas precipitates in solution with initial pH ≤ 3.44 were spherical particles of approximately 1.0 µm in diameter, having specific sea-urchin morphology. Specific surface area of the precipitates varied from 3.42 to 23.45 m2 g− 1. X-ray fluorescence analyses revealed that schwertmannite formed in solution with initial pH in the range of 2.00–3.44 had similar elemental composition and could be expressed as Fe8O8(OH)4.42(SO4)1.79, whereas Fe8O8(OH)4.36(SO4)1.82 and Fe8O8(OH)4.29(SO4)1.86 as its chemical formula when the initial pH was 1.80 and 1.60, respectively.