Zn sorption to biogenic bixbyite-like Mn2O3 produced by Bacillus CUA isolated from soil: XAFS study with constraints on sorption mechanism

Although most reported biogenic Mn oxides are hexagonal birnessites, other types of biogenic Mn oxides also commonly occur in the environment. Compared to hexagonal birnessites, the sorption characteristics and the underlying mechanism of adsorption of heavy-metal ions to those of the other biogenic Mn oxides are still rarely addressed. A strain of Mn-oxidizing bacteria isolated from Claypani-Udic Argosols was identified as Bacillus with 16S rRNA sequencing analysis. The bacterial Mn(II) oxidation product is a poorly crystallized bixbyite-like Mn2O3 (alpha-Mn2O3). The maximum adsorption capacities of Zn(II) onto the biogenic Mn oxide at pH 6.00 and pH 4.00 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 MnO6 octahedra obtained through multiple-FEFF fitting of Mn EXAFS data. At a relatively low Zn2+ loading (100 mmol/kg, pH 6.00), Zn2+ adsorbed onto the biogenic Mn oxide with two types of tetrahedrally coordinated complexes, i.e. (1) coordinated with one regular/distorted MnO6 octahedron as a monodentate-mononuclear complex and (2) with two MnO6 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, pH 4.00; 635 mmol/kg, pH 6.00), two types of octahedrally coordinated complexes are constrained, i.e. (1) coordinated with one regular/distorted MnO6 octahedron as a monodentate-mononuclear complex and (2) with one regular MnO6 octahedron as a bidentate mononuclear complex. This research extends further understanding on the formation of biogenic Mn oxides in the environment and the adsorption mechanism of heavy metals onto low-valence Mn oxides with distorted structures. The application of low valence biogenic Mn oxides to efficiently remove heavy metals from water is also shown to be promising. (C) 2014 Elsevier B.V. All rights reserved