| Due to the increasing development of industry, heavy metal pollution, especially on soil, is becoming more and more seriously threatening the whole ecosystem. Birnessite is one kind of manganese oxide wich exists widely and has greatest activity. It can easily gather other transition metals, such as Co and Ni, making isomorphous substitution take place, thus making great contribution to their geochemistry behavior. In acid birnessite, Mn4+mainly exists in the MnO6layer, whereas Mn3+at the vacancies. The research on the cobalt-doped acid birnessite indicated that the cobalt exists as Co3+by instituting Mn4+in the MnO6layer and can improve birnessite’s ability for adsorpotion of Pb2+and Zn2+as well as oxidation of As(III). Ni exists as Ni2+in birnessite and mainly constitutes [NiO6] octahedron, most inhabit around octahedral vacancies, only a little settle in the MnO6layer by representing Mn3+. Doping nickel decreases the amount of MnO6layers, and weakens birnessite’s adsorption ability for heavy metals. Due to these two different kinds of influence of Co and Ni, it is still unknown about the influence on birnessite’s crystal structure, micro morphology, physical and chemical characteristics by double doping of them. What’s more, reseaching on double doping can help us understand natural existing status of manganese oxides in environment.Cobalt and nickel double-doped birnessite were synthesized by refluxing process at atmospheric pressure with different molar ratios of (Co+Ni)/Mn. As-prepared materials were characterized by powder X-ray diffraction, chemical analysis, SSA analysis, FE-SEM analysis, FTIR analysis, XPS analysis and other technologies to investigate the effects of cobalt and nickel doping on the physical and chemical properties of birnessite and its adsorption and oxidation ability to heavy metals. The main results are as follows.1. The double induction of cobalt and nickel didn’t change birnessite’s layer crystal structure and micro-morphologies. Like birnessit, doped minerals also glomerate into flower shape. However, Co and Ni doping, especially Ni, decreased the thickness of MnO6octahedral layer. 2. As total amount of doping and ratio of nickel increased, the crystallinity degree of birnessite reduced, Mn AOS lowered a little bit and then grew in, SSA enlarged; hydroxyl oxygen on crystal surface increased. Pure birnessite consisted of a certain amount of MnO6octahedral vacancies; however, they reduced greatly after cobalt and nickel doping.3. In the crytal structure of birnessite and Co and Ni double doped birnessite, Mn mainly exists as Mn4+, Co as Co (Ⅲ), and Ni as Ni (Ⅱ). Nickel mainly existed between MnO6octahedral layers, however cobalt mostly stayed in the layers.4. The order of amounts of heavy metal ions adsorbed by Pure birnessite and doping ones was the same:Pb2+>Zn2+. However, doping cobalt and nickel had weakened birnessite’s adsorption capacity for heavy metal ions. This may owe to the reduction of adsorption sites which induced by reduction of MnO6octahedral vacancies. Similar to pure birnessite, cobalt and nickel containing birnessites’maximum adsorption capacity for Pb2+is approximately2000mmol-kg-1; but doping has greatly abated birnessites’ability to adsorb, and maximum adsorption capacity for Zn2+decreased from1225mmol-kg-1to600mmol-kg-1. Besides, the incorption of cobalt and nickel also reduced the oxidation capacity of birnessite towards As (Ⅲ), however the initial oxidation rate increased. |
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