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Adsorption of quinoline (pK(sub a) = 4.92) and background electrolyte (LiCl) onto specimen kaolinite was measured as a function of surface-bound humic acid (HA) concentration (foc = 0-0.5%), pH (3-10), and ionic strength (1-10 mM). Complexation of HA on the kaolinite surface (4.5 mg C kg(super -1)) reduced the point of zero net charge (pznc) for kaolinite by more than one pH unit and resulted in a significant increase in negative surface charge. Maximum sorption of quinoline occurred near its pK(sub a) for all sorbents. Below the pK(sub a) quinoline sorption increases with increasing pH and decreasing proton competition. Above the pK(sub a), sorption is reduced in parallel with (but offset to a higher pH from) the ionized fraction. Competition with Li+ for surface sites is apparent from diminished quinoline adsorption with increasing ionic strength, but sorption of the ionized form of quinoline is always favored and kaolinite exhibits selectivity for cationic quinoline over Li+ (K(sub exc) = 65 at pH 5). However, increasing foc diminishes quinoline sorption and selectivity (K(sub exc) = 32 at pH 5) and increases sorption reversibility relative to uncoated kaolinite. Humic acid along exhibits lower selectivity for quinoline (K(sub exc = 4 at pH 5). The results indicate that mineral-sorbed humic stubstances can diminish retention of cationic quinoline despite an increase in overall cation-exchange capacity.

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, [R.sub.Si], 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 NaN[O.sub.3] 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).sub.2]. The Ni-Ni bond distances were shorter (3.06-3.11 [Angstrom]) than in Ni[(OH).sub.2](s) (3.13 [Angstrom]) 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 ([Gamma] [less than or equal to] 0.406 [micro]mol [m.sup.-2]), Al is present at 2.69 [Angstrom], indicating the formation of a bidentate Ni surface complex. With increased surface coverage, the d(Ni-Al) increases to 2.96 [Angstrom], 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.

Highly weathered kaolinitic soils in Brazil often have adequate levels of K and Mg to support plant growth. The source of K and Mg in these soils and their relationship with the dominant mineral, kaolinite, is addressed in this article. Crystallographic, chemical, and morphological properties of kaolinite, from selected highly weathered Brazilian soils, were investigated by x-ray diffraction (XRD), analytical electron microscopy (AEM), chemical, and thermal methods. Kaolinite properties showed significant variations such as d(001) spacing ranging between 0.713 to 0.728 nm, width at half height (WHH) between 0.30 to 0.97 [degrees] 2 [Theta], and dehydroxylation temperature between 489 to 518 [degrees] C. Kaolinite in the clay fraction has relatively poor crystal order with a mean crystallinity index value(CI) of 12.7. The dominant forms of the clay-fraction kaolinite were elongated and rounded, with relatively lower proportions of hexagonal particles. The silt-fraction kaolinite showed a tendency to form subspherical large aggregates with high stability. The average [Fe.sub.2][O.sub.3] level in the kaolinite of the clay fraction (19.1 g [kg.sup.-1]) was higher than that obtained for the silt fraction (6.6 g [kg.sup.-1]). The smaller kaolinite particles of the clay fraction showed a lower degree of crystal order, higher K and Mg levels, and lower dehydroxylation temperatures. From the strong relationship between the asymmetry index (AI) of the (001) diffraction line and the level of K in kaolinite from the younger soils, we believe that both K and Mg in kaolinite are part of residual micaceous layers interleaved in kaolinite crystals.

The mineralogical and chemical characteristics of saprolites and soils developed from granulitic, monzonitic, and charnockitic gneisses and mylonites of the Blue Ridge Mountains and western Piedmont foothills of central Virginia, were studied. Micromorphological, x-ray diffraction (XRD), scanning electron microscopy (SEM), and microprobe analysis showed that the mineralogical and geochemical evolution of biotite in profiles developed on gneisses is consistent with pseudomorphic weathering of mica to kaolinite and halloysite, with or without a mica-vermiculite intermediate phase. On mylonitic rocks, saprolites and soils also contain muitimineral pseudomorphs after biotite, in which gibbsite crystals eventually fill the whole volume. There is evidence of topotaxial formation of halloysite after biotite. Although gibbsite is present at the first stages of the weathering of biotite, it is not clear if gibbsite forms directly from the mica or if it is a weathering product of kaolinite and/or halloysite. The source of aluminium may also be located outside the original biotite crystal. These multimineral assemblages suggest that microenvironments of weathering are controlling the formation of secondary products from the parent biotite rather than the so called anti-gibbsite effect, at least at this scale of investigation. It is suggested that the mylonitic fabric and subvertical foliation planes of these rocks are responsible for this mineralogical and geochemical evolution.