XAS studies of Ni and Zn sorbed to hydrous manganese oxide

Adsorption mechanisms of Zn2+ and Ni2+ on hydrous manganese oxide (HMO) wer e investigated using XAS. Analyses reveal that both metal ions retain their primary hydration shell when sorbed to HMO, which is consistent with physi cal sorption. These local structures are invariant of pH (3.0-7.0) and adso rbate 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 shel l was observed with 6-80 atoms at 3.34 Angstrom for Ni and 8-10 0 atoms at 3.49 Angstrom for Zn. The lack of Ni or Zn contributions in the second shel l eliminates the possibility of surface precipitation or polymerization. Li kewise, the absence of Mn atoms in the second shell suggests outer-sphere a dsorption. Interestingly, the local structure of Ni and Zn sorbed to HMO di d not change with reaction time when as much as 90% of the sorbed contribut ion was due to the slow sorption process,thus supporting intraparticle diff usion as the rate-limiting mechanism. This result demonstrates that adsorpt ion sites located along the micropore walls of HMO are similar to ones loca ted on the external surface. Overall, metals from the same group in the Per iodic Table appear to form similar adsorption complexes with HMO.