Structure and reactivity study of biotic and abiotic poorly crystalline manganese oxides

Manganese oxides (Mn-oxides) have environmental and economic significance. The most common Mn-oxides in the environment are fine grained and poorly crystalline, e.g., vernadite. The overall objective of this research was to utilize multiple experimental and theoretical approaches to improve our understanding of the structure and reactivity of biotic and abiotic poorly crystalline Mn-oxides which are analogues to naturally-occurring vernadite.;The crystal structure of three poorly crystalline Mn-oxides including delta-MnO 2, polymeric MnO2 and a biogenic Mn-oxide (BioMnOx), were investigated using atomic pair distribution function (PDF) analysis. Results indicate that these Mn-oxides are layered structures with nearly hexagonal layer symmetry. The best structural model for the simulation of the PDFs is a monoclinic structure in C 2m space group. A high degree of disorder is present in the stacking direction. The formation of poorly crystalline Mn-oxides in the natural environment is largely mediated by Mn-oxidizing microorganisms. The impacts of cations (H+, Ni(II), Na+ and Ca2+) during biotic Mn(II) oxidation on the structure of Mn octahedral layers of biogenic Mn-oxides (BioMnOx) were investigated. Results indicate that H + and Ni(II) enhance vacant site formation; whereas, Na+ and Ca2+ favor formation of Mn(III) and its ordered distribution in Mn octahedral layers. A Ni(II) sorption study on BioMnOx produced at either pH 6, 7 or 8 in CaCl2 solutions demonstrated that Ni(II) sorbs at vacant sites in the interlayer of the BioMnOx and the maximum Ni(II) sorption capacity increases as the BioMnOx formation pH decreases. This relation indicates that the quantity of BioMnOx vacant sites increases as formation conditions become more acidic, which is in good agreement with the structural study.;Density functional theory (DFT) calculations were used to investigate As(V) and As(III) surface complex structures and reaction energies on both Mn(III) and Mn(IV) sites to compare Mn(III) and Mn(IV) reactivity for As(III) oxidation. Results show that Mn(III) sites are less reactive in terms of As(III) oxidation due to their lower affinity for As(III) adsorption, higher potential to be blocked by As(V) complexes, and slower electron transfer rates with adsorbed As(III). Results from this study offer an explanation regarding the experimental observations of Mn(III) accumulation on birnessite and the long residence time of As(III) adsorption complexes on manganite (gamma-MnOOH) during As(III) oxidation.