Occurrence of biogenic schwertmannite in sludge bioleaching environments and its adverse effect on solubilization of sludge-borne metals

Secondary minerals such as Fe(OH)3 and jarosite are often hypothesized to be formed in sludge during bioleaching processes involving Acidithiobacillus ferrooxidans and be responsible for the lower solubilization efficiencies of sludge-borne metals. Schwertmannite is a ubiquitous mineral formed in acid mine drainage (AMD) impacted environments. However, the authors report for the first time the occurrence of schwertmannite as the sole secondary Fe mineral in a dissolved organic matter (DOM)-rich tannery sludge bioleaching system and its scavenging effect on soluble metals in bioleached sludge. The tannery sludge bioleach solution with pH not, vert, similar3 was mainly rich in Fe, Cr(III), image and Acidithiobacillus ferrooxidans, but also contained high concentrations of DOM (300–400 mg C L−1). The ocherous precipitate that formed rapidly within 40 h in the sludge bioleach solution during incubation was identified to be pure schwertmannite by XRD and FTIR. Results of SEM coupled with EDS analyses showed that the schwertmannite was present as spherical particles with a diameter of about 1 μm and its chemical formula could be expressed as Fe8O8(OH)4.6(SO4)1.7. Newly formed schwertmannite contained about 2.43% Cr(III) derived from already solubilized sludge-borne Cr(III) by bioleaching, suggesting a strong adverse influence of schwertmannite formation on solubilization efficiency of metals from sludge. Biogenic Fe mineral synthesis experiments varying the concentrations of DOM and metals indicated that the occurrence of schwertmannite as a monominerallic phase in the sludge bioleaching environment was predominantly attributed to the presence of insufficient concentrations of monovalent cations and the high content of DOM. Moreover, the short reaction time, high image concentration and low pH value in this system also facilitated the existence of monominerallic schwertmannite.