Adsorption of Cs+ on biogenic birnessite was investigated and compared with (ad)sorption of other heavy metals and also with (ad)sorption on chemically synthesized birnessite. The adsorption density of Cs+ on biogenic birnessite was smaller than that on chemically synthesized birnessite; however, the (ad)sorption densities of Co2+ and Ni2+ on biogenic birnessite were larger than on chemically synthesized birnessite. These phenomena can be interpreted by considering the distinctive nano-structure of biogenic birnessite, which is not only poorly crystalline and includes organic matter but also according to EXAFS contains a greater proportion of vacant central metal sites than is found in synthetic birnessite. Adsorbed Cs+ ions on both birnessites were mainly in the form of outer sphere complexes, which cannot easily occupy vacant central metal sites in biogenic birnessite. Biogenic birnessite has a greater specific surface area and more fine pores than synthetic birnessite, but these factors do not necessarily lead to more adsorption sites for Cs+ ions.
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