Reaction Sequence of Nickel(II) with Kaolinite: Mineral Dissolution and Surface Complexation and Precipitation
1998
Eick, Matthew J. | Fendorf, Scott E.
The dissolution kinetics of Ni(II) sorbed to kaolinite at pH 7 were examined as a function of initial aqueous Ni(II) concentrations; the local molecular structure of the Ni surface complexes were determined using extended x-ray absorption fine structure (EXAFS) spectroscopy. The dissolution of kaolinite was nonstoichiometric with a preferential release of Si over Al. The dissolution rate, RSᵢ, increased with an increase in the aqueous concentration of Ni. A rate law was developed representing a fractional-order dissolution reaction with respect to the surface concentration of Ni. Absorption spectra were obtained from kaolinite samples washed with 0.10 M NaNO₃ adjusted to pH 3.4 to remove amorphous Al-hydroxide surface deposits or adsorbed Al and from kaolinite that was untreated. For all samples, EXAFS results revealed the presence of multinuclear Ni surface complexes with a similar, but not identical, local structural environment to pure crystalline Ni(OH)₂. The Ni-Ni bond distances were shorter (3.06–3.11 Å) than in Ni(OH)₂(s) (3.13 Å) and increased with an increase in surface coverage. Additionally, Al was present in all but the highest surface coverage as a second neighbor backscatterer. At the lower surface coverages (Γ ≤ 0.406 µmol m⁻²), Al is present at 2.69 Å, indicating the formation of a bidentate Ni surface complex. With increased surface coverage, the d(Ni-Al) increases to 2.96 Å, which is believed to result from a Ni,Al-hydroxide precipitate. No significant structural differences were found between kaolinite pretreatments. Based on these results, a hypothesized mechanism for Ni sorption on kaolinite is proposed.
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