| Mn oxides are environmentally ubiquitous with multiple Mn oxidation states and usually exist as fine granular, spherical or film-like particles distributed in soils and sediments. As an most important source of reactive mineral surfaces, natural highly active sorbents and oxidants in the environment, Mn oxides widely participate in a variety of adsorption and/or redox reactions. These reactions control the concentration, speciation, behavior and bioavailability of many heavy-metal ions and organic pollutants in the environment. The formation of Mn oxides in the environment is generally considered to be closely related to the microbial activity. Up to now, the evidence of biogenic manganese oxide was mainly from marine and lacustrine Mn-oxidizing microorganisms. Microbially mediated Mn(II) oxidation processes have been studied mainly using three phylogenetically different bacteria and and a variety of fungi. Accordingly, our knowledge of the adsorption and oxidation of metal cations by biogenic Mn oxides and the transformation of biogenic Mn oxides into other Mn oxide minerals is mainly based on the Mn oxides produced by these microorganisms. However, the primary products of Mn(II) oxidation by the aforementioned microorganisms(bacteria and fungi) are exclusively nanoparticulate, poorly-crystalline hexagonal birnessites with the presence of Mn mainly as Mn(IV). Considering the diverse mechanisms of biooxidation of Mn(II), other types of biogenic Mn oxides could also be produced microbially in the environment. In our study, a strain of Mn-oxidizing bacteria isolated from Claypani-Udic Argosols was identified as Bacillus with 16 S r RNA sequencing analysis. An array of techniques: powder X-ray diffraction(XRD), specific surface area(SSA), field emission scanning electron microscopy(FESEM),X-ray photoelectron spectroscopy(XPS), X-ray absorption near edge structure spectroscopy(XANES) and extended X-ray absorption fine structure spectroscopy(EXAFS) were used to investigate the structure and properties of the Mn-oxidizing production of the Mn-oxidizing bacteria. And extended X-ray absorption fine structure spectroscopy(EXAFS) was used to investigated the coordination mechanisms of Zn2+ and Cu2+, as a model of heavy metals, adsorbed onto the bixbyite-phase low valence biogenic Mn oxide, which with regular and distorted Mn O6 octahedra alternately arranged in the structure. The mainly results were listed as following:1. 8 Mn-oxidizing strains were isolated from soils and Fe-Mn nodules of different areas of Hubei and Shandong province, using Leptothrix discophora medium as the basic culture medium. The Mn-oxidizing rate/concentration of the bacteria isolated from Fe-Mn nodules of Wuhan is highest among all the 8 Mn-oxidizing strains. The average Mn-oxidizing rate/concentration of this bacteria was 45.6% when cultured 12 d in Leptothrix discophora medium. The bacteria was indentifies as Bacillus megaterium with 16 S r DNA sequencing analysis. The result of gram stain shows that it’s a gram positive.2.The structure and properties of the Mn-oxidizing production of the Mn-oxidizing bacteria Bacillus CUA was investigated using XRD, SSA, FESEM, XPS, XANESand EXAFS. The results show that Mn(II) oxidation product is a poorly-crystalline bixbyite(α-Mn2O3). It is composed of uniform aggregates of granular particles with sizes of 25~50 nm. The specific surface area of the biogenic Mn oxide was 49 m2/g, which is greater than of the bixbyite synthesized chemically. XPS result shows that the bulk-averaged Mn oxidation state(AOS) of biogenic Mn oxides is 2.75, which is very close to the AOS fitting result of 2.73 using XANES. The Mn K-edge EXAFS of biogenic Mn oxide was fitted using a Multiple-FEFF fitting method and the result show that the six Mn-O distances of the regular Mn1O6 octahedra in the biogenic Mn oxide are 2.095 ?, while the Mn–O distances in the distorted Mn2O6 octahedra have three different pairs at 1.903 ?, 2.178 ? and 2.350 ?.3. The adsorption isotherm of Zn(II) on bixbyite-phase low valence biogenic Mn at p H6.0 and p H4.0 were in line with Langmuir adsorption law. The maximum adsorption capacities of Zn(II) onto the biogenic Mn oxide at p H6.0 and p H4.0 were 663 mmol/kg and 629 mmol/kg, respectively. The complex structure of adsorbed Zn2+ was constrained using Zn EXAFS analysis, combined with structural parameters of the biogenic Mn oxide with alternately arranged regular and distorted Mn O6 octahedra obtained through Multiple-FEFF fitting of Mn EXAFS data. At a relatively low Zn2+ loading(100 mmol/kg, p H6.0), Zn2+ adsorbed onto the biogenic Mn oxide with two types of tetrahedrally coordinated complexes, i.e.(1) coordinated with one regular/distorted Mn O6 octahedron as a monodentate-mononuclear complex and(2) with two Mn O6 octahedra(two regular, two distorted or a regular and a distorted) as a bidentate-binuclear complex. While, at a relatively high Zn2+ loading(556 mmol/kg, p H4.0; 635 mmol/kg, p H6.0), two types of octahedrally coordinated complexes are constrained, i.e.(1) coordinated with one regular/distorted Mn O6 octahedron as a monodentate-mononuclear complex and(2) with one regular Mn O6 octahedron as a bidentate mononuclear complex.4. The maximum adsorption capacity of Cu(II) onto this biogenic Mn oxide at p H6.0 was 796 mmol/kg. The complex structure of adsorbed Cu(II) was constrained using Cu extended X-ray absorption fine structure(EXAFS) analysis, combined with structural parameters of the biogenic Mn oxide with alternately arranged regular and distorted Mn O6 octahedra obtained through multiple-FEFF fitting of Mn EXAFS data. The sorbed Cu(II) was found to coordinate with the biogenic Mn oxide particle edges as inner-sphere complexes. At a relatively low Cu2+ loading(233 mmol/kg, p H6.0), Cu(II) adsorbed onto the biogenic Mn oxide with two types of coordinated complexes, i.e.,(1) coordinated with one regular/distorted Mn O6 octahedron as a monodentate-mononuclear complex and(2) with two adjacent Mn O6 octahedra as a bidentate-binuclear complex. While, at a relatively high Cu2+ loading(787 mmol/kg, p H6.0), only one type of coordinated complex was constrained, the adsorbed Cu(II) coordinated with one regular/distorted Mn O6 octahedron as a monodentate-mononuclear complex. |
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