REACTION SEQUENCE OF NICKEL(II) WITH KAOLINITE - MINERAL DISSOLUTION AND SURFACE COMPLEXATION AND PRECIPITATION

The dissolution kinetics of NI(II) sorbed to kaolinite at pH 7 were ex amined as a function of initial aqueous NI(II) concentrations; the loc al molecular structure of the Ni surface complexes were determined usi ng extended x-ray absorption fine structure (EXAFS) spectroscopy. The dissolution of kaolinite was nonstoichiometric with a preferential rel ease of Si over Al. The dissolution rate, R-sl, increased with an incr ease in the aqueous concentration of Ni. A rate law was developed repr esenting a fractional-order dissolution reaction with respect to the s urface concentration of Ni. Absorption spectra were obtained from kaol inite samples washed with 0.10 M NaNO3 adjusted to pH 3.4 to remove am orphous Al-hydroxide surface deposits or adsorbed Al and from kaolinit e that was untreated. For ail samples, EXAFS results revealed the pres ence of multinuclear Ni surface complexes with a similar, but not iden tical, local structural environment to pure crystalline Ni(OH)(2). The Ni-Ni bond distances were shorter (3.06-3.11 Angstrom) than in Ni(OH) (2)(s) (3.13 Angstrom) and increased with an increase in surface cover age. Additionally, Al was present in all but the highest surface cover age as a second neighbor backscatterer. At the lower surface coverages (Gamma less than or equal to 0.406 mu mol m(-2)), Al is present at 2. 69 Angstrom, indicating the formation of a bidentate Ni surface comple x. With increased surface coverage, the d(Ni-Al) increases to 2.96 Ang strom, which is believed to result from a Ni,Al-hydroxide precipitate. No significant structural differences were found between kaolinite pr etreatments. Based on these results, a hypothesized mechanism for Ni s orption on kaolinite is proposed.