Bioreduction of Pb-substituted ferrihydrite: impact on the nature of the biogenic minerals formed, the speciation and bioavailability of Pb
Résumé
Levels of metals in terrestrial environments are an increasing concern, due to their threat to human health and ecosystem quality (Panagos et al., 2013). While metals risk assessment studies focused on their total concentration, research are currently oriented toward the evaluation of their speciation and bioavailability. Ferrihydrites (Fh), due to their large surface area and reactive surface properties, are important carrier phases in soils on which metals can be adsorbed or coprecipitated with (Tiberg, 2016). However, anaerobic transformation of such oxides in presence of dissimilatory iron reducing (DIR) bacteria like Shewanella oneidensis sp., may result in metals redistribution between the soil solution and the new (bio)formed Fe minerals (Frierdich et al., 2011). Further, the presence of metals in Fh may influence the nature of the (bio)formed minerals. To further our understanding, pure and Pb-substituted Fh (Pb/(Pb+FeIII) molar ratios of 2 and 5 %) were synthesized by coprecipitation (Schwertmann and Cornell, 2000). These Fh were incubated separately in anaerobic conditions with Shewanella oneidensis MR-1 cells in an appropriate liquid medium, for: i) 21 days (prolonged bioreduction), and ii) two 7-day periods separated by a 7-day aerobic oxidation (successive redox cycles). The nature of the (bio)formed minerals was assessed using XRD and Mössbauer spectroscopy. The bioreduction extent was measured thanks to the ferrozine method and UV-visible spectroscopy, and the partitioning of Pb between the liquid and solid phases was assessed using AAS. Finally, Pb bioavailable content in solution was determined using whole cell biosensors. Magnetite (M) was the main mineral obtained after the prolonged bioreduction (97 %), and increasing proportions of goethite (Gh) formed with Pb substitution (22 and 35 % for systems made with Pb/(Pb+FeIII) molar ratios of 2 and 5 % respectively). Gh and lepidocrocite were found to form in shorter bioreduction periods (1st anaerobic period of the successive redox cycles). The aerobic oxidation converts all the (bio)formed minerals to a poorly crystallized phase except in the highest Pb substituted system. Following the 2nd anaerobic period, M and Gh formed, while no change occurred in the highest Pb substituted system. Only minor amounts of the Pb introduced in the systems within the Fh used were found in the liquid phase and were not bioavailable. Most of the Pb introduced was associated to the (bio)formed minerals, whatever the nature of those minerals. Our results show the complex succession of (bio)formed minerals during bioreduction, the impact of a Pb substitution within ferrihydrite on the nature of the (bio)formed minerals and how these minerals act as efficient carrier phases of Pb.
