Bioreduction of Hematite Nanoparticles by Shewanella oneidensis MR-1

A dissertation is presented on the bioreduction of hematite (α-Fe2O3) nanoparticles. The study shows that an alternative extracellular electron transfer mechanism other than the classical 'direct-contact' mechanism may be simultaneously employed by Shewanella oneidensis MR-1 during solid-phase metal reduction. This conclusion is supported by analysis of the bioreduction kinetics of hematite nanoparticles coupled with microscopic investigations of cell-mineral interactions. The reduction kinetics of metal-oxide nanoparticles were examined to determine how S. oneidensis utilizes these environmentally-relevant solid-phase electron acceptors. Nanoparticles involved in geochemical reactions show different properties relative to larger particles of the same phase, and their reactivity is predicted to change as a function of size. To demonstrate these size-dependent effects, the surface area normalized reduction rates of hematite nanoparticles by S. oneidensis MR-1 with lactate as the sole electron donor were measured. As evident from whole cell TEM analysis, the mode of nanoparticle adhesion to cells is different between the more aggregated, pseudo-hexagonal to irregular shaped 11 nm, 12 nm, 99 nm and the less aggregated 30 nm and 43 nm rhombohedral particles. The 11 nm, 12 nm and 99 nm particles show less cell contact and coverage than the 30 nm and 43 nm particles but still show significant rates of reduction. This leads to the provisional speculation that S. oneidensis MR-1 employs a pathway of indirect electron transfer in conjunction with the direct-contact pathway, and the relative importance of the mechanism employed depends upon aggregation level and the shape of the particles or crystal faces exposed. In accord with the proposed increase in electronic band-gap for hematite nanoparticles, the smallest particles (11 nm) exhibit one order of magnitude decrease in reduction when compared with larger (99 nm) particles, and the 12 nm rates fall in between these two. This effect may also be due to the passivation of the mineral and cell surfaces by Fe(II), or decreasing solubility due to decrease in size.