Characterization of Ni-rich hexagonal birnessite and its geochemical effects on aqueous Pb²⁺/Zn²⁺ and As(III)

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• 作者：Hui Yin ; Wenfeng Tan ; Lirong Zheng ; Haojie Cui ; Guohong Qiu ; Fan Liu ; Xionghan Feng
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2012
• 出版时间：15 September, 2012
• 年：2012
• 卷：93
• 期：Complete
• 页码：47-62
• 全文大小：1061 K

文摘

Hexagonal birnessite is the most ubiquitous manganese oxide in geological environments. It is often highly enriched in trace metal ions such as Ni and plays an important role in metal(loids) geochemistry. Nanostructured birnessites containing different amounts of Ni were synthesized by addition of Ni²⁺ to initial reactants. Powder X-ray diffraction (XRD), element analysis, field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), X-ray absorption spectroscopy (XAS) and isothermal adsorption and oxidation of metal(loids) were carried out to investigate the effects of Ni doping on the substructure and physicochemical properties of birnessite, and Ni crystal chemistry in birnessite. These Ni-rich birnessites have Ni contents as high as 2.99 % (Ni5) and 6.08 % (Ni10) in weight. EXAFS results show that Ni5 has 23.7 % of the total Ni (0.71 wt. % ) and Ni10 has 34.5 % of the total Ni (2.10 wt. % ) in Mn octahedral layer with the remaining Ni located at vacancies and edge sites. The Ni-rich birnessites have weaker crystallinity and thermal stability, fewer layers stacked along the c axis, ¡«1.5-2.7 times larger surfaces areas, and a higher Mn average oxidation numbers (AONs) compared to the birnessite without Ni. Additionally, the doping of Ni during birnessite crystallization enhances the formation of vacancies in the layer; however, adsorption capacities for Pb²⁺ and Zn²⁺ by these Ni-rich birnessites are reduced, mainly because of vacancies and edge sites occupation by a large amount of Ni. The Ni-rich birnessites exhibit much higher oxidation capability and can completely oxidize As(III) in solution at rapid initial reaction rates under the experimental condition. The results indicate that incorporation of Ni into the natural birnessite in ferromanganese nodules may be achieved both by direct coprecipitation with Mn to build the layers and migration over time from adsorbed Ni on the surface into the layer structure. It is also implied that Ni doping in birnessite has great impact on the geochemical behaviors of heavy metals, either in adsorption or oxidation reactions.