Characterization And Surface Chemical Properties Of Al-and Fe-Doped H-Birnessites

Manganese oxide are ubiquitous and most reactive minerals in geological environments. Natural Mn oxides are usually enriched in various metals. The oxides of Mn, Al, Fe often intergrow at small-scale in the soils and they affect each other’s geochemical behaviors by adsorption and isomorphous substitution. The research of Al isomorphous substitution for Fe oxide and Fe isomorphous substitution for Al oxide is in progress, while there’re less report about how A1and Fe influence Mn oxide’s structure and quality in the natural environment. The acid Birnessite is a kind of layered manganese oxide minerals which has weak crystalline, small particles and high activity. With metal ions doping into the the structure, birnessite’s micromorphology and quality such as adsorption and oxidation change significantly. It’s yet unclear how A1and Fe cations affect acid birnessite.So in this paper, acid birnessite doped with Al/Fe were synthetized through adding Aluminum chloride or ferric chloride when manganese chloride was oxidized by Hydrochloric acid. The structure and properties of doped birnessites were investigated by powder XRD, FESEM, TEM, FTIR, XPS, XAS and chemical composition analyses. The performance of these doped birnessites on adsorption of heavy metals (Pb2+/Zn2+), anion (PO43-/AsO43-) and oxidation of As(III), Cr(III) were investigated batch experiments.The main results are as follows.1.In experimental conditions, A1is doped only to6.7%, while Fe can be doped easily to21%,but impurity appeared when doping Fe.2.Powder XRD shows that, the basic layer structure did not change after Al/Fe doping. But the doped birnessites had weaker crystallinity compared to the undoped one, leading to larger specific surface areas. And the PZC of the doped birnessites increase significantly.3.A1-and Fe-doped H-birnessites show great changes in the unit cell parameters, volume and crystal densities. The unit cell parameter b of the Al-doped H-birnessites is gradually decreased while of the Fe-doped H-birnessites is gradually increased. The unit cell parameters a and c of two series both decreased.4.The XPS analysis indicate that the Aluminum exists as Al3+, and the Ferric exists as Fe3+. Analysis of Mn2p3/2shows that the contents of high valence Mn on the surface of Al-doped H-birnessites gradually increased with the increase of the amount of doping elements, while the Fe-doped H-birnessites are opposite. Fit of Ols shows that the content of hydroxyl groups on the Al-doped H-birbessites surface also gradually decreased and the Fe-doped H-birnessites surface increased.5.Mn K-edge XANES and EXAFS spectra demonstrate that doping had little effect on the basic layer structures and local Mn environments. The manganese average oxidation state (Mn AOS) of Al-doped H-birnessites gradually increased while Fe-doped H-birnessites gradually decreased. With incorporating into the birnessite structures, Al(Ⅲ) substituted for layer Mn(Ⅲ) while Fe(Ⅲ) substituted for interlayer Mn(IV).6.These Al-and Fe-doped H-birnessites have enhanced adsorption capacity for Pb2+/PO43-/AsO43-,and improved oxidation ability towards As(III), while the adsorption capacity for Zn2+and oxidation ability towards Cr(Ⅲ) decreased.