Reaction Sequence of Nickel(II) with Kaolinite: Mineral Dissolution and Surface Complexation and Precipitation

Abstract

AbstractThe 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.