Sorption speciation of nickel(II) onto Ca-montmorillonite: batch, EXAFS techniques and modeling

The sorption speciation of Ni(II) on Ca-montmorillonite was evaluated using a combination of batch experiments, extended X-ray absorption fine structure (EXAFS) spectroscopy and modeling. The pH and temperature at the aqueous-montmorillonite interface affects both the extent of Ni(II) sorption as well as the local atomic structure of the adsorbed Ni(II) ions. At 0.001 mol L(-1) Ca(NO(3))(2) and low pH, the study reveals that the majority of Ni(II) is adsorbed in the interlayers of Ca-montmorillonite coordinated by six water molecules in an octahedron as an outer-sphere complex. At higher pH, inner-sphere surface complexes are formed. The Ni-Si/Al distances (R(Ni-Al) = 3.00 ?, R(Ni-Si1) = 3.10 ? and R(Ni-Si2) = 3.26 ?) determined by EXAFS confirm the formation of mononuclear complexes located at the edges of Ca-montmorillonite platelets at pH 7.5 and 8.5. At pH 10.0, the Ni-Ni/Si distances (R(Ni-Ni) = 3.07 ? and R(Ni-Si) = 3.26 ?) indicates the formation of Ni-phyllosilicate precipitates. A rise in temperature promotes inner-sphere complexation, which in turn leads to an increase in Ni(II) sorption on Ca-montmorillonite. Sorption edges are fitted excellently by surface complexation model (SCM) with the aid of surface species determined from EXAFS spectroscopy.