Effects of Si-bearing minerals on the nature of secondary iron mineral products from lepidocrocite bioreduction

Lepidocrocite (γ-FeOOH) bioreduction by Shewanella putrefaciens was examined in systems containing three kinds of quartz silica sand with solid contents ranging from 5 to 70 wt.%. The results notably demonstrated that the formation routes of secondary iron minerals from microbial reduction of γ-FeOOH in silica mineral assemblages are dependent on quartz sand loading and particle size distribution. Biomineralization experiments with a concentration range of aqueous silica showed that a lower Si concentration (less than 2 mM) led to magnetite or magnetite/chukanovite, while higher concentrations resulted in the formation of green rust (GR). Dissolution of very fine quartz particles may have promoted the formation of biogenic GR instead of magnetite. This behavior could be explained by the sorption of dissolved silicate on the lepidocrocite surface, which might limit the bioreduction rate and promote GR formation. As observed with the chemically synthesized GR, the interactions of dissolved or colloidal Si with the lateral faces of the GR crystals could stabilize the GR structure, thus preventing its transformation into other products. Furthermore, complex physical and chemical distribution at the surfaces and within heteroaggregates of mixed mineral phases may affect the diffusion processes of species and therefore the biomineralization mechanism.

► The GR bioformation in silica mineral assemblages are dependent on silica sand loading and particle size distribution.
► The interactions of silica species with the external faces of GR stabilize the structure of both abiotic and biogenic GR.
► The sorption of dissolved silicate or colloidal silica preserves GR and prevents its transformation into magnetite.
► The sorption of dissolved silicate on lepidocrocite surface may limit the bioreduction rate and promote GR formation.
► The heteroaggregatation may affect the diffusion processes of species, and therefore the GR biomineralization mechanism.