Biogenic Magnetite Nanoparticles: Development and Optimization for Potential Applications

Abstract

The bioproduction of magnetite (Fe3O4) nanoparticles was demonstrated through the reduction of amorphous Fe(III)-oxyhydroxide starting materials by the dissimilatory iron reducing bacterium Geobacter sulfurreducens in an environmentally benign method. Magnetite nanoparticles have magnetic characteristics coupled with a high surface area to volume ratio and biogenically produced magnetite often has a highly reactive surface Fe(II) layer. Through the work described in this thesis, the properties of magnetite nanoparticles were manipulated in several different ways. The control of particle size was achieved through the adjustment of the total amount of bacteria (biomass) introduced at the start of the Fe(III)-oxyhydroxide reduction process. High concentrations of bacteria led to the formation of small (~10 nm) nanoparticles whereas low concentrations led to larger (~50 nm) particle formation. Additional mineral phases were formed, with goethite and siderite observed for very low and very high bacterial concentrations respectively. The change in particle size and additional mineral phases formed were attributed to the rate and extent of Fe(II) formation, linked to changes in biomass loadings, with high biomass releasing high concentrations of Fe(II) and low biomass releasing low concentrations of Fe(II).The control of magnetic properties was achieved by the incorporation of transition metal dopants including zinc and cobalt into the crystal structure of the magnetite, producing nanoparticles of the form MxFe3-xO4, (M=Zn or Co). The different dopants substitute into the crystal structure in different locations (as determined through X-ray absorption and Mӧssbauer spectroscopies). Zinc has a preference for replacing Fe(III) in tetrahedral coordination, resulting in a decrease in the anti-ferromagnetic component between octahedral and tetrahedral lattice sites, leading to an increase in saturation magnetization of the material (>50 %) compared to stoichiometric magnetite. Cobalt has a strong affinity to replace Fe(II) in octahedral coordination which results in an increase in the measured coercivity without significantly decreasing the saturation magnetization.The biotechnological potential of biogenic magnetite was also investigated through an appraisal of Fe(II)-mediated chromate (Cr(VI)) remediation. Decreasing particle size (i.e. increasing surface area to volume ratio) led to an enhanced ability to reduce highly toxic Cr(VI) to non-toxic Cr(III). In separate experiments, cobalt doping in magnetite also significantly increased the effectiveness of the nanomaterial for use in magnetic hyperthermia treatments, which could ultimately be used for cancer therapy. Finally, the scalability of biogenic magnetite production was shown. Geobacter sulfurreducens growth in batch culture and subsequent iron transformation stages were significantly increased in scale by factors of 500× and 1000× respectively. This could pave the way for future commercial production of biogenic magnetite for use in many different applications.

Date of Award	1 Aug 2012
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Jonathan Lloyd (Supervisor) & Richard Pattrick (Supervisor)