XAS studies of Ni and Zn sorbed to hydrous manganese oxide

Adsorption mechanisms of Zn²⁺ and Ni²⁺ on hydrous manganese oxide (HMO) were investigated using XAS. Analyses reveal that both metal ions retain their primary hydration shell when sorbed to HMO, which is consistent with physical sorption. These local structures are invariant of pH (3.0-7.0) and adsorbate loading (10^-4 - 10^-2 mol (g of HMO)^-1), suggesting one average type of adsorption mechanism. In addition to the first shell, a second shell was observed with 6-8 O atoms at 3.34 Å for Ni and 8-10 O atoms at 3.49 Å for Zn. The lack of Ni or Zn contributions in the second shell eliminates the possibility of surface precipitation or polymerization. Likewise, the absence of Mn atoms in the second shell suggests outer-sphere adsorption. Interestingly, the local structure of Ni and Zn sorbed to HMO did not change with reaction time when as much as 90% of the sorbed contribution was due to the slow sorption process, thus supporting intraparticle diffusion as the rate-limiting mechanism. This result demonstrates that adsorption sites located along the micropore walls of HMO are similar to ones located on the external surface. Overall, metals from the same group in the Periodic Table appear to form similar adsorption complexes with HMO.