Effects of Co and Ni co-doping on the structure and reactivity of hexagonal birnessite

Natural hexagonal birnessites are enriched in various transition metals (TMs). Many studies have examined the effects of single metal doping on the structures and properties of birnessites, but none focused on the simultaneous interaction mechanism of coprecipitation of two different TMs with birnessite. In this work Co and Ni co-doped hexagonal birnessites were synthesized and characterized by powder X-ray diffraction (XRD), elemental analysis, field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS) spectroscopy to investigate the effects of co-doping on the structure and reactivity of birnessite and the crystal chemistry of Co and Ni. These co-doped birnessites have lower crystallinity, i.e., fewer manganese layers stacking in the c* direction, larger specific surface areas (SSAs) and increased Mn average oxidation states (AOSs) than the undoped birnessite, and Co exists in a valence of + 3. Co, Ni and Mn K-edge extended X-ray absorption fine structure spectroscopy (EXAFS) spectra demonstrate an increase in edge-sharing Ni–Me (Me = Ni, Co and Mn) distances in birnessite layers with the increase of the contents of dopants while Mn–Me distances first decrease and then increase while those of Co–Me pairs are nearly constant, coupled with first a decrease and then increase of the in-plane unit-cell parameter b. The effect of co-doping on the amounts of structural Mn and K+, numbers of [MnO6] layers stacked in c* axis, and SSAs, is larger than the effects of doping with Co alone, but less than singly Ni doping. In birnessites doped with both Co and Ni, ~ 74–79% of the total Co and ~ 23–39% of the total Ni are present within the manganese layers. Compared with the spatial distribution of TM in singly doped birnessites, the coexistence of Ni hinders the incorporation of Co into the layers during birnessite crystallization; however, coprecipitation with Co has little effects, neither hindrance nor promotion, on the insertion of Ni into the layers. These results provide insight into the interaction mechanism between coexisting Co, Ni within layered Mn oxides. It further helps us to interpret the geochemical characteristics of multi-metal incorporation into natural Mn oxides and their effects on the structures and physicochemical properties of these minerals.