Insight into the magnetic properties of Pb-dopped iron oxide nanoparticles during Fe(III) bio-reduction by Shewanella oneidensis MR-1

Anthropogenic activities have led to a significant accumulation of Pb in the environment, posing a threat to ecosystems. Iron oxides that display magnetic properties are ubiquitous in the environment and Pb partitioning onto these minerals is considered one of the most critical geochemical processes controlling its environmental fate. In anoxic environments, iron oxides undergo redox cycling due to biotic and abiotic routes, resulting in their transformation/dissolution and evolution of their magnetic characteristics. However, there is still a lack of knowledge on the impact of Pb on the dynamic of iron oxides bioreduction. Furthermore, there is little information available regarding the nature of biogenic minerals and their magnetic signatures. Here we incubated Pb-bearing ferrihydrites (Fh_Pb) with various Pb/(Fe + Pb) molar ratios (i.e., 0, 2 and 5%) with the iron-reducing bacterium Shewanella oneidensis MR-1, for 21 days. XRD analyses of the initial Fh_Pb displayed characteristic features of Fh and a decrease of magnetization in the presence of Pb. During the bio-reduction process, Pb doping led to a drop in the rate and extent of reduction. At the end of the incubation period, the analysis of the aqueous solutions revealed a minor proportion of Pb in solution, indicating that a significant proportion of the Pb is sorbed onto the biogenic minerals. Magnetite (95%) and siderite (5%) formed during the bioreduction of Fh, while magnetite (~80%) and goethite (~20%) precipitated in the presence of Pb as revealed by transmission Mössbauer spectroscopy. Furthermore, the size of the magnetite particles decreased from about 11 nm in absence of Pb to 6 nm with 2% of Pb-bearing, while 5% of Pb led to particles too small to be quantified with our TEM measurements. The combined effect of the decrease in particle size of magnetite, substitution of Pb and the precipitation of goethite in the Pb-dopped assay led to a significant decrease of magnetization at room temperature. Overall, this study highlights the effect of Pb on iron oxides bio-reduction and transformation processes and the sensitivity of magnetism to serve as a monitoring method.