| Birnessite, a nanocrystalline phyllomanganate, is widespread in soil surface environments and oceanic sediments. The fate of many trace metals is influenced by their interactions with the birnessite. Sorption mechanisms of heavy metals at the mineral/water interface are largely controlled by the type and number of sorption sites on the mineral surfaces. However, in the case of layered manganese oxides, with a highly reactive interlayer region, the effects of variation of substructures on their sorption capacities and characteristics are still obscure. Sorption experiments combined with powder X-ray diffraction (XRD), FE-SEM and TEM micrographs, and X-ray photoelectron spectroscopy (XPS) were performed to investigate the sorption characteristics and mechanisms of Pb2+, Cu2+, Zn2+and Cd2+onto birnessites with various Mn average oxidation states (AOSMn). Meanwhile, the chemistry states of Mn and O on the surface of birbessites before adsorption with after adsorption were compared, and the relationship between sorption capacity of cation and the AOSmn in the treated birnessite with Mn2+, Pb2+, Cd2+, Zn2+was analyzed, In addition, the characterization of Zn2+and Cd2+co-adsorption and Zn2+desorption by Cu2+were carried out. The sorption mechanisms of several heavy metals on birnessites with various Mn average oxidation state were also discussed.1. The increase in AOSMn of birnessite caused the considerabe decrease of the sorption capacity of heavy metals. The sorption capacity for any given birnessite followed the order Pb2+>> Cu2+>Zn2+>Cd2+, the sorption capacity of Pb2+ranges from1.6to3.9times greater than those of the other metals, while Cu2+,Zn2+, and Cd2+sorb with similar maxima among them. The large differences between maximum Pb2+sorption and that of the other metals were used to estimate the concentration of low-energy binding sites in the different birnessites, which were assigned roughly to particle edge sites. We found that for the birnessite with exclusively Mn4+the contributions of edge sites and interlayer sites to total Pb2+sorption were approximately equivalent; but as Mn AOS decreases, the contribution of the high-energy binding sites sensibly decreases, probably through the increasing Mn3+at the expense of vacant sites, while that of reactive surface edge sites remains roughly constant. It implied that the increase of the high-energy binding sites was an important cause for the increase in metal adsorption with increasing AOSMn·-On birnessite surface, adsorption of Cu2+, Zn2+, and Cd2+mostly occurred at high-energy binding sites, whereas Pb2+maybe occupy at both low-energy binding sites and high-energy binding sites simultaneously. During Pb2+sorption,～1.0mole of H+is released per adsorbed metal ion into octahedral vacancy, and～1.5mole of H+is released per adsorbed Pb2+into the lateral edge sites.2. The Mn2p2/3spectrum of our four synthetic birnessites demonstrated that the layers were comprised of Mn4+O6octahedra, Mn3+O6octahedra and cation vacancies. Decreased proportion of Mn4+/Mn3+in the layer led to the decrease of AOSMn, and the decrease of the proportion of cation vacancies in the layer from13%to3%. Most of Mn2+sorbed above/below vacant layer site, together with protons, compensated for the layer charge deficit resulting from the vacant sites. The undersaturation and saturation of surface oxygens coordinated with Mn in the birnessites structure, of which about20%were undersaturated oxygens being regarded as strong sorption sites for heavy metals. With the Cd2+and Pb2+sorbed onto the surface of birnessite, the content of undersaturated oxygens reduced. The XPS analyses also indicated that heterogeneous affinity sites in the structure of our four samples. Octahedral vacant sites (denoted as H site) and particle edge sites (denoted as L-I site) were present in HB11, HB12and HB13. However, interlayer site binding may now involve additional complexes expected to form above, the layer Mn3+because of insufficient charge neutralization from coordinating oxygens compared to Mn4+. A vacant octahedron shares edges with two Mn3+,and four Mn4+-octahedra led to the stronger undersaturated degree of O layer, thus the more charge compensation was achieved by the coexistence of heavy metal cation on the two sides of a vacant layer octahedron. One cation in triple-corner (TC) surface complex was located above/below vacant layer sites, another one in triple-edge (TE) surface complex was located above/below empty tridentate cavities. The empty tridentate cavities bordering such a vacant octahedron was another low-energy binding sites (denoted as L-Ⅱ site). L-Ⅱ sites showed lower affinity than vacancies, and was expected to be solely present in HB14 because of appreciable amount of Mn3+.3. After treated with different concentration of Mn2+, Cd2+, Zn2+and Pb2+, all of the samples still remained hexagonal layer symmetry. However, as the birnessite treated by Mn2+, most of Mn2+was oxidized to Mn3+on the surface of birnessite and subsequent some surface Mn3+structurally incorporated into the vacant sites, the substructures of Mn-birnessites were changed. Increasing Mn2+concentration from1.0to2.4mmol·L-1led to the decrease of the proportion of Mn4+/Mn3+in the layer and octahedral vacant sites, and thus the Zn2+adsorption capacity decreased. The AOSMn of birnessites were almost unchanged as the concentration of Cd2+, Zn2+and Pb2+increased, indicative of unchanged vacant sites in the layer. Whereas the heavy metals adsorption capacities decreased due to occupancy of the treating cations above/below adsorption sites. When Zn2+/Cd2+sorb onto the treated birnessites surface, Zn2+/Cd2+either entered into the unoccupied active vacant sites, or competed to replace the interlayer cations, which depended on the affinity of the treating cations. On the surface of the birnessite, most Zn and Cd cations were likely located above and below the same vacant layer sites. As Zn2+sorbed onto Cd-birnessites, a few of Cd2+desorbed by Zn2+, whereas the most of Zn2+desorbed as Cd2+sorbed onto Zn-birnessites. In the first, Zn was octahedrally (ZnⅥ) and tetrahedrally (ZnⅣ) coordinated in the birnessite, and ZnⅥ was easy to be desorbed by Cd2+. In the Second, comparing with Zn2+, Cd+sorption predominantly occurred on birnessite surface due to its lower hydration energy.4, The ratio of H+(released)/Zn2+(adsorbed) reveal that Zn2+binds to vacant sites with two coordination species (ie., ZnⅥ and ZnⅣ) on the birnessite surface. The desorption percentages of Zn2+by Cu2+at different pH values were in the order HB14>HB12> HB13. The ratio of ZnⅥ/ZnⅣ varies not only with the Zn surface coverage, but also with the content of Mn3+in the layer. For HB12and HB13with the lower proportion of Mn3+in the layer, the higher permanent charges resulting from vacant layer sites leads to the higher Zn+adsorption capacity. With increase of Zn adsorption capacity, ZnⅥ/ZnⅣ ration increases, therefore, the Zn2+desorption percentage increases. For HB14with lower permanent charges and Zn+loading, the Zn/Zn'v ration would decrease. However, the higher abundance of OH-bordering Mn3+in the layer of HB14likely favors ZnvI forming, thereby the Zn2+desorption percentages of HB14increases.
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