The influence of montmorillonite on the mineralogical and magnetic changes in magnetite and maghemite during microbial iron reduction

Magnetic iron minerals, such as magnetite (Fe3O4) and maghemite (γ-Fe2O3), are widely distributed in sediments, holding significant potential for reconstructing paleoenvironments through their magnetic properties. However, the impact of microbial iron reduction on the physicochemical and magnetic characteristics of Fe(III)-containing magnetic minerals has recently been recognized as a crucial but often overlooked factor in paleoenvironmental reconstructions. Although magnetic iron minerals are frequently associated with clay minerals in sedimentary settings, our understanding of the influence of clay minerals on the microbial reduction of Fe(III) in magnetic minerals remains limited. To address this knowledge gap, a series of bioreduction experiments were conducted in a non-growth medium, in which lactate acted as the electron donor, magnetite or maghemite nanoparticles (NPs) served as the electron acceptor, and a typical iron-reducing bacterium, Shewanella oneidensis strain MR-1, acted as the reaction mediator, in the presence or absence of montmorillonite (STx-1b). Our results demonstrated that the addition of montmorillonite STx-1b significantly enhanced the dispersion of magnetite and maghemite NPs, thereby increasing the final extent of microbial iron reduction. Moreover, the degree of microbial iron reduction impacted the crystallography and magnetic properties of magnetite and maghemite. Upon bioreduction, the lattice constant, magnetic susceptibility, and the 10-K magnetic coercivity of these magnetic iron minerals exhibited an increase, accompanied by the presence of more structural defects. Specifically, maghemite-magnetite solid solutions were formed in the maghemite-amended biosystems. These findings highlight the influence of clay minerals in natural settings during microbial iron reduction on the magnetic properties of magnetic iron minerals used for paleoenvironmental reconstructions.