Behavior and mechanism of Ni(II) uptake on MnO2 by a combination of macroscopic and EXAFS investigation

The effects of pH, ionic strength, competing ions and initial metal concentrations on the uptake behavior and mechanism of radioactive Ni(II) onto MnO2 was investigated using a combination of classical macroscopic methods and the extended X-ray absorption fine structure (EXAFS) spectroscopy technique. The results indicated that the uptake of Ni(II) on MnO2 is obviously dependent on pH but independent of ionic strength, which suggested that the uptake of Ni(II) onto MnO2 is attributed to an inner-sphere surface complex rather than an outer-sphere surface complex. EXAFS analysis shows that the hydrated Ni(II) is adsorbed through six-fold coordination with an average Ni-O interatomic distance of 2.04 ± 0.01 A. It can be inferred from the EXAFS analysis that the inner-sphere surface complex of Ni(II) onto MnO2 is involved in both edge-sharing and corner-sharing linkages. Both the macroscopic uptake data and the molecular level evidence of Ni(II) surface speciation at the MnO2-water interfaces should be factored into better prediction of the bioavailability and mobility of Ni(II) in soil and water environment. (author)