Biotransformation of lepidocrocite in the presence of quinones and flavins

This study investigated the bioreduction of lepidocrocite (γ-FeIIIOOH) and its mineral transformation in the presence of exogenous (quinones) and endogenous (flavins) electron transfer mediators (ETMs) at low concentrations of the ETMs and bacterial cells (Shewanella putrefaciens CN32). It is very important to investigate the bioreduction of lepidocrocite in the presence of different ETMs because biotransformation of Fe(III)-containing minerals can be stimulated by ETMs and affect fate and transport of contaminants in contaminated environments. In the absence of phosphate, green rust formation was observed with fast Fe(II) production rate (0.44–0.56 mM d−1) during the bioreduction of lepidocrocite with exogenous ETMs, while goethite formed at slow Fe(II) production rate (0.24–0.29 mM d−1) with endogenous ETMs. In the presence of phosphate, formation of green rust and vivianite was observed with fast Fe(II) production rate (0.54–0.74 mM d−1) during the bioreduction of lepidocrocite with exogenous ETMs, while vivianite formed at moderate Fe(II) production rate (0.36–0.40 mM d−1) with endogenous ETMs. Vivianite formed in all experimental cases with phosphate in a broad range of Fe(II) production rates (0.23–0.74 mM d−1). Our results (1) suggest that exogenous and endogenous ETMs can significantly but differently affect the biotransformation of lepidocrocite, especially at low concentrations of the ETMs and bacterial cells, (2) highlight the importance of Fe(II) production rate to determine the formation of specific biogenic minerals, (3) provide additional evidence that phosphate can significantly affect the bioreduction rate and the mineral transformation, and (4) help to understand the basic knowledge about complex interactions among microbial cell, soil mineral, and ETM in natural environments and engineered systems.