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The texture and mineralogy of solid phases resulting from biogeochemical metal reduction of U(VI)-FeOOH slurries was investigated over a period of four years. Solid-phase reaction products were analyzed with EXAFS, TEM, and XRD following fermentative reduction of uranium-loaded ferric hydroxide precursors with 0.01 and 0.05 cation mole fraction (CMF) U by cultures of Thermoanaerobacter sp. strain TOR-39. Only minor changes could be distinguished between 3 and 51 months for most slurries. Magnetite, goethite, uraninite, and minor akaganéite were present after 3 months at both U-CMFs. Akaganéite was not detected by XRD after 3 months, but was still observed by TEM after 50 months. Increasing uranium in the starting slurries led to a greater proportion of oxidized iron in the solid-phase products. Euhedral goethite and subhedral to euhedral magnetite were observed at all times. Uraninite was observed in clusters of <10 nm particles without any particular relationship to the iron minerals. HRTEM imaging indicated that even the smallest uraninite particles were well crystallized, with textures that remained consistent throughout the duration of experiments. X-ray absorption spectra after 3 months indicated 100% and 96.4% U(IV) in 0.01 and 0.05 CMF U slurries, respectively. EXAFS spectra were consistent with uraninite at both uranium levels, plus additional non-uraninite U(IV) for 0.05 CMF U. One 0.05 CMF U culture slurry was found to have a lower pH and a more oxidized final iron mineral assemblage; in this case uraninite was not observed by XRD, but large (101 nm average diameter) rounded uraninite grains were observed by TEM. These grains were observed in chains or aggregates often connected by necks, in textures suggestive of biological influence. HRTEM demonstrated each grain was composed of poorly oriented, primary, 2-5 nm uraninite crystallites. Uraninite crystal growth occurred by nanoparticle aggregation, but ripening was not observed even though incubation temperatures were held at 65 °C for 20 days. Thus, previous studies of biogenic nanoparticulate uraninite short-term reactivity are likely to be representative of systems aged over a period of years. [ABSTRACT FROM AUTHOR]

Production of both nano-sized particles of crystalline pure phase magnetite and magnetite substituted with Co, Ni, Cr, Mn, Zn or the rare earths for some of the Fe has been demonstrated using microbial processes. This microbial production of magnetic nanoparticles can be achieved in large quantities and at low cost. In these experiments, over 1 kg (wet weight) of Zn-substituted magnetite (nominal composition of ZnFeO) was recovered from 30 l fermentations. Transmission electron microscopy (TEM) was used to confirm that the extracellular magnetites exhibited good mono-dispersity. TEM results also showed a highly reproducible particle size and corroborated average crystallite size (ACS) of 13.1 ± 0.8 nm determined through X-ray diffraction ( N = 7) at a 99% confidence level. Based on scale-up experiments performed using a 35-l reactor, the increase in ACS reproducibility may be attributed to a combination of factors including an increase of electron donor input, availability of divalent substitution metal ions and fewer ferrous ions in the case of substituted magnetite, and increased reactor volume overcoming differences in each batch. Commercial nanometer sized magnetite (25–50 nm) may cost $500/kg. However, microbial processes are potentially capable of producing 5–90 nm pure or substituted magnetites at a fraction of the cost of traditional chemical synthesis. While there are numerous approaches for the synthesis of nanoparticles, bacterial fermentation of magnetite or metal-substituted magnetite may represent an advantageous manufacturing technology with respect to yield, reproducibility and scalable synthesis with low costs at low energy input. [ABSTRACT FROM AUTHOR]

The potentially toxic effects of soluble lanthanide (L) ions, although microbially induced mineralization can facilitate the formation of tractable materials, has been one factor preventing the more widespread use of L-ions in biotechnology. Here, we propose a new mixed-L precursor method as compared to the traditional direct addition technique. L (Nd, Gd, Tb, Ho and Er)-substituted magnetites, L y Fe3 − y O4 were microbially produced using L-mixed precursors, L x Fe1 − x OOH, where x = 0.01–0.2. By combining lanthanides into the akaganeite precursor phase, we were able to mitigate some of the toxicity, enabling the microbial formation of L-substituted magnetites using a metal reducing bacterium, Thermoanaerobacter sp. TOR-39. The employment of L-mixed precursors enabled the microbial formation of L-substituted magnetite, nominal composition up to L0.06Fe2.94O4, with at least tenfold higher L-concentration than could be obtained when the lanthanides were added as soluble salts. This mixed-precursor method can be used to extend the application of microbially produced L-substituted magnetite, while also mitigating their toxicity. [ABSTRACT FROM AUTHOR]

Magnetic nanoparticles, which are unique because of both structural and functional elements, have various novel applications. The popularity and practicality of nanoparticle materials create a need for a synthesis method that produces quality particles in sizable quantities. This paper describes such a method, one that uses bacterial synthesis to create nanoparticles of magnetite. The thermophilic bacterial strain Thermoanaerobacter ethanolicus TOR-39 was incubated under anaerobic conditions at 65 °C for two weeks in aqueous solution containing Fe ions from a magnetite precursor (akaganeite). Magnetite particles formed outside of bacterial cells. We verified particle size and morphology by using dynamic light scattering, X-ray diffraction, and transmission electron microscopy. Average crystallite size was 45 nm. We characterized the magnetic properties by using a superconducting quantum interference device magnetometer; a saturation magnetization of 77 emu/g was observed at 5 K. These results are comparable to those for chemically synthesized magnetite nanoparticles. [ABSTRACT FROM AUTHOR]

Abstract: The magnetic susceptibility (κ RT) and saturation magnetization (M S) of microbially synthesized magnetites were systematically examined. Transition metal (Cr, Mn, Co, Ni and Zn)- and lanthanide (Nd, Gd, Tb, Ho and Er)-substituted magnetites were microbially synthesized by the incubation of transition metal (TM)- and lanthanide (L)-mixed magnetite precursors with either thermophilic (TOR-39) or psychrotolerant (PV-4) metal-reducing bacteria (MRB). Zinc incorporated congruently into both the precursor and substituted magnetite, while Ni and Er predominantly did not. Microbially synthesized Mn- and Zn-substituted magnetites had higher κ RT than pure biomagnetite depending on bacterial species and they exhibited a maximum κ RT at 0.2 cationic mole fraction (CMF). Other TMs’ substitution linearly decreased the κ RT with increasing substitution amount. Based on the M S values of TM- and L-substituted magnetite at 0.1 and 0.02 CMF, respectively, Zn (90.7emu/g for TOR-39 and 93.2emu/g for PV-4)- and Mn (88.3emu/g by PV-4)-substituted magnetite exhibited higher M S than standard chemical magnetite (84.7emu/g) or pure biomagnetite without metal substitution (76.6emu/g for TOR-39 and 80.3emu/g for PV-4). Lanthanides tended to decrease M S, with Gd- and Ho-substituted magnetites having the highest magnetization. The higher magnetization of microbially synthesized TM-substituted magnetites by the psychrotroph, PV-4 may be explained by the magnetite formation taking place at low temperatures slowing mechanics, which may alter the magnetic properties compared to the thermophile, through suppression of the random distribution of substituted cations. [Copyright &y& Elsevier]

A marine psychrotolerant, dissimilatory Fe(III)-reducing bacterium, Shewanella sp. strain PV-4, from the microbial mat at a hydrothermal vent of Loihi Seamount in the Pacific Ocean has been further characterized, with emphases on metal reduction and iron biomineralization. The strain is able to reduce metals such as Fe(III), Co(III), Cr(VI), Mn(IV), and U(VI) as electron acceptors while using lactate, formate, pyruvate, or hydrogen as an electron donor. Growth during iron reduction occurred over the pH range of 7.0 to 8.9, a sodium chloride range of 0.05 to 5%, and a temperature range of 0 to 37°C, with an optimum growth temperature of 18°C. Unlike mesophilic dissimilatory Fe(III)-reducing bacteria, which produce mostly superparamagnetic magnetite (<35 nm), this psychrotolerant bacterium produces well-formed single-domain magnetite (>35 nm) at temperatures from 18 to 37°C. The genome size of this strain is about 4.5 Mb. Strain PV-4 is sensitive to a variety of commonly used antibiotics except ampicillin and can acquire exogenous DNA (plasmid pCM157) through conjugation. [ABSTRACT FROM AUTHOR]