•Biosorption of 47 elements by exopolymer nanofibrils excreted from Leptothrix cells are tested.•The sorbed elements distribute almost homogeneously throughout the fibrillar aggregate.•The nanofibril matrix sorbing the elements is nearly amorphous.•The sorbed elements plausibly are bound to the matrix by ionic binding, especially via OH.Leptothrix species, aquatic Fe-oxidizing bacteria, excrete nano-scaled exopolymer fibrils. Once excreted, the fibrils weave together and coalesce to form extracellular, microtubular, immature sheaths encasing catenulate cells of Leptothrix. The immature sheaths, composed of aggregated nanofibrils with a homogeneous-looking matrix, attract and bind aqueous-phase inorganics, especially Fe, P, and Si, to form seemingly solid, mature sheaths of a hybrid organic–inorganic nature. To verify our assumption that the organic skeleton of the sheaths might sorb a broad range of other metallic and nonmetallic elements, we examined the sorption potential of chemically and enzymatically prepared protein-free organic sheath remnants for 47 available elements. The sheath remnants were found by XRF to sorb each of the 47 elements, although their sorption degree varied among the elements: >35% atomic percentages for Ti, Y, Zr, Ru, Rh, Ag, and Au. Electron microscopy, energy dispersive x-ray spectroscopy, electron and x-ray diffractions, and Fourier transform infrared spectroscopy analyses of sheath remnants that had sorbed Ag, Cu, and Pt revealed that (i) the sheath remnants comprised a 5–10 nm thick aggregation of fibrils, (ii) the test elements were distributed almost homogeneously throughout the fibrillar aggregate, (iii) the nanofibril matrix sorbing the elements was nearly amorphous, and (iv) these elements plausibly were bound to the matrix by ionic binding, especially via OH. The present results show that the constitutive protein-free exopolymer nanofibrils of the sheaths can contribute to creating novel filtering materials for recovering and recycling useful and/or hazardous elements from the environment.Download high-res image (312KB)Download full-size image

Amorphous Fe3+-based oxide nanoparticles produced by Leptothrix ochracea, aquatic bacteria living worldwide, show a potential as an Fe3+/Fe0 conversion anode material for lithium-ion batteries. The presence of minor components, Si and P, in the original nanoparticles leads to a specific electrode architecture with Fe-based electrochemical centers embedded in a Si, P-based amorphous matrix.Keywords: anode material; bacterial iron oxides; iron-oxidizing bacteria; lithium-ion batteries; nanoparticles;

Microporous and mesoporous silica derived from biogenous iron oxide is an attractive catalyst for various organic reactions. Biogenous iron oxide contains structural silicon, and amorphous silica remains after iron oxide is dissolved in concentrated hydrochloric acid. The amorphous silica containing slight amounts of iron (Si/Fe = ∼150) is composed of ∼6-nm-diameter granular particles. The amorphous silica has a large surface area of 540 m2/g with micropores (1.4 nm) and mesopores (<3 nm). By using pyridine vapor as a probe molecule to evaluate the active sites in the amorphous silica, it was found that this material has strong Brønsted and Lewis acid sites. When the catalytic performance of this material was evaluated for reactions including the ring opening of epoxides and Friedel–Crafts-type alkylations, which are known to be catalyzed by acid catalysts, this material showed yields higher than those obtained with common silica materials.Keywords: acid catalysts; amorphous silica; biogenous iron oxide; Brønsted acid; iron-oxidizing bacteria; Lewis acid;

We prepared nano–micrometer-architectural acidic silica from a natural amorphous iron oxide with structural silicon which is a product of the iron-oxidizing bacterium Leptothrix ochracea. The starting material was heat-treated at 500 °C in a H2 gas flow leading to segregation of α-Fe crystalline particles and then dissolved in 1 M hydrochloric acid to remove the α-Fe particles, giving a gray-colored precipitate. It was determined to be amorphous silica containing some amount of iron (Si/Fe = ∼60). The amorphous silica maintains the nano–microstructure of the starting material—∼1-μm-diameter micrometer-tubules consisting of inner globular and outer fibrillar structures several tens of nanometer in size—and has many large pores which are most probably formed as a result of segregation of the α-Fe particles on the micrometer-tubule wall. The smallest particle size of the amorphous silica is ∼10 nm, and it has a large surface area of 550 m2/g with micropores (0.7 nm). By using pyridine vapor as a probe molecule to evaluate the active sites in the amorphous silica, we found that it has relatively strong Brønsted and Lewis acidic centers that do not desorb pyridine, even upon evacuation at 400 °C. The acidity of this new silica material was confirmed through representative two catalytic reactions: ring-opening reaction and Friedel–Crafts-type reaction, both of which are known to require acid catalysts.Keywords: acid catalysts; amorphous silica; biogenous iron oxides; Brønsted acid; iron-oxidizing bacteria; Lewis acid;

By heating an amorphous iron oxide produced by Leptothrix ochracea, an iron-oxidizing bacterium species, at 600–1100 °C in air for 2 h, vivid red-colored powdered materials including α-Fe2O3 (hematite) and amorphous silicate with high thermostability were prepared which offer potential for use as overglaze enamels on porcelain. The precise color tone of the materials greatly depends on the heat-treatment temperature. The most strikingly beautiful sample, heat-treated at 800 °C, is light yellowish-red in color (L* = 47.3, a* = 34.1, and b* = 34.6), has a unique microstructure, and does not fade in color even with reheating at 800 °C, which is the firing temperature for overglaze enamel on porcelain. The sample primarily consists of crystalline hematite particles ∼40 nm in diameter with slightly longer axis unit-cell parameters than those of pure hematite. The particles are covered with amorphous silicate phase ∼5 nm in thickness and are intricately interconnected into microtubules with an average diameter of 1.26 μm. The attractive color of this material is due to the following structural features: small particle size (∼40 nm), nanocomposite of hematite and amorphous silicate, and a microtubule structure that inhibits aggregation of individual hematite particles and microtubules.Graphical abstractHighlights► An iron oxide of bacterial origin was heat treated in air. ► The product is a vivid-yellowish-red powder with high thermostability. ► The powder is an aggregate of microtubule structures. ► The microtubule is composed of nanocomposites of α-Fe2O3/amorphous-silicate.

Iron oxide produced by iron-oxidizing bacteria, Leptothrix ochracea, (biogenous iron oxide: BIO) was used as a support for immobilized palladium catalysts with organic cross-linkers. Palladium immobilized on BIO bearing imidazolium chloride delivered the desired biaryl products in sufficient yields in the Suzuki–Miyaura coupling reactions under solvent-free conditions and could be reused several times without significant loss of catalytic activity. It is shown that BIO can be exploited as a useful support for immobilization of palladium and the BIO-immobilized palladium catalyst effectively promotes the solvent-free Suzuki–Miyaura coupling reactions.

In nature, there are various iron oxides produced by the water-habitant bacterial group called “iron-oxidizing bacteria”. These iron oxides have been studied mainly from biological and geochemical perspectives. Today, attempts are made to use such iron oxides as novel functional materials in several applications. However, their quantitative structural characteristics are still unclear. We studied the structure of iron oxide of microtubular form consisting of amorphous nanoparticles formed by an iron-oxidizing bacterium, Leptothrix ochracea, using a combination of high-energy X-ray diffraction and reverse Monte Carlo simulation. We found that its structure consists of a framework of corner- and edge-sharing distorted FeO6 octahedral units, while SiO4 tetrahedral units are isolated in the framework. The results reveal the atomic arrangement of iron oxide of bacterial origin, which is essential for investigating its potential as a functional material.Highlights► The amorphous structure of bacterial iron oxide was investigated. ► The structure was simulated by high-energy X-ray diffraction and reverse Monte Carlo simulation. ► The structure was constructed of a framework of corner- and edge-sharing distorted FeO6 octahedral units. ► SiO4 tetrahedral units were distributed isolatedly in the framework of FeO6 octahedral units.

Leptothrix species in aquatic environments produce uniquely shaped hollow microtubules composed of aquatic inorganic and bacterium-derived organic hybrids. Our group termed this biologically derived iron oxide as “biogenous iron oxide (BIOX)”. The artificial synthesis of most industrial iron oxides requires massive energy and is costly while BIOX from natural environments is energy and cost effective. The BIOX microtubules could potentially be used as novel industrial functional resources for catalysts, adsorbents and pigments, among others if effective and efficient applications are developed. For these purposes, a reproducible system to regulate bacteria and their BIOX productivity must be established to supply a sufficient amount of BIOX upon industrial demand. However, the bacterial species and the mechanism of BIOX microtubule formation are currently poorly understood. In this study, a novel Leptothrix sp. strain designated OUMS1 was successfully isolated from ocherous deposits in groundwater by testing various culture media and conditions. Morphological and physiological characters and elemental composition were compared with those of the known strain L. cholodnii SP-6 and the differences between these two strains were shown. The successful isolation of OUMS1 led us to establish a basic system to accumulate biological knowledge of Leptothrix and to promote the understanding of the mechanism of microtubule formation. Additional geochemical studies of the OUMS1-related microstructures are expected provide an attractive approach to study the broad industrial application of bacteria-derived iron oxides.

The polymeric complex method has been employed to investigate the effects of substitution by lanthanum into hematite powder on structures, morphology and color tone properties. X-ray diffraction measurements evidenced the existence of Fe2 − xLaxO3 at 1000 °C–1200 °C by shifting peaks to lower angles. At higher temperature, lattice parameters and cell volume were found to expand due to the incorporation of La3+ ion into the hematite in the range 301.98–303.87 Å3. Crystalline size decreased when increasing the amount of La3+ ion in the reaction process. Suppression of particle growth was observed by the Field Emission Scanning Electron Microscopy studies at higher temperatures. Brighter yellowish-red color than that of pure hematite was obtained and it grew into deeper color tone rapidly by increasing the amount of La3+ ion into the hematite.