Grey degraded soils of four representative profiles in Viet Nam were examined for particle-size distribution and mineralogical composition. They were different in parent material, landform and land use, but three common characteristic horizons were identified in the field: the light grey surface horizon, the eluviated A2 or AB horizon, and the compacted B horizon accumulated with plinthite or laterite. In the particle-size distribution, the upper horizons were dominated by sand particles with the texture of sand, loamy sand or sandy loam. The clay content was nearly equal to or below 10%, but increased with depth and reached to the level of 30 to 50% in the lower horizons. In the clay fraction, kaolin mineral was found to be predominant. Quartz was another dominant mineral. These results indicate the occurrence of more or less intensive leaching and/or decomposition of clay particles in the upper horizons of grey degraded soils.

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Thioacetamide molecules were intercalated into the interlayer space of Cd (II)-montmorillonite by solid-solid reactions at room temperature. The reactions were confirmed by powder X-ray diffraction, infrared spectroscopy, CHN analysis, and thermal analysis of the products. The basal spacings of the products with the molar ratios of thioacetamide to interlayer Cd (II) cation of 3: 1 and 6: 1 were ca. 1.5 and 1.8 nm, respectively. The basal spacings decreased gradually to ca. 1.3 nm when the products were stored in air at room temperature for long periods (i.e. several months). The intensities of the IR bands due to thioacetamide and the organic contents of the products were reduced correspondingly, suggesting that thioacetamide molecules were partly decomposed to give CdS and the decomposition accompanied the deintercalation and sublimation of thioacetamide from the interlayer space.

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Complexes of Na-montmorillonite with three amino acids: lysine, arginine and histidine were prepared at different pH values. The basal spacings of the complexes were assessed by X-ray diffractometry at controlled condition of relative humidity. For those complexes, the basal spacing unchanged with relative humidity with the exception of those at high pH (10 and 13). For these samples the change in the basal spacing with relative humidity was the same as that seen for montmorillonite alone. At high pH (>10) lysine, arginine and histidine were apparently not intercalated into montmorillonite. These observations may be explained in terms of the pH-dependent charge characteristics (dissociation) of the amino acids used. Lysine, arginine and histidine have an amino group in both the main and side chain of the molecules. At low pH, both amino groups will be positively charged. At high pH, amino groups will hardly have no charge, though a carboxyl group will almost charged negatively. Since interlayer complex formation is dependent on molecular charge, the basal spacing of montmorillonite-amino acid complexes is sensitive to variations in relative humidity.

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The adsorption of sulfate and nitrate on two allophane samples, with high and low Si/Al ratio, was found to decrease with increasing pH. The highest adsorption of sulfate and nitrate was obtained at initial pH 3 for both the allophane samples. No adsorption of nitrate was observed at equilibrium pH above 7, however, sulfate was still adsorbed. The strong adsorption of sulfate at pH about below 7 could be attributed to ligand exchange reaction, whereas nitrate was non-specifically adsorbed throughout the pH examined. Allophane sample with low Si/Al ratio or KyP adsorbed more sulfate and nitrate than with the high Si/Al ratio sample or KnP. This is because KyP has high aluminol groups (Al-OH, Al-OH₂) per unit mass than KnP. The aluminol groups are believed to be responsible for the adsorption of sulfate and nitrate on nano-ball allophane. Both sulfate and nitrate adsorption results showed a better fitness with the Freundlich adsorption equation with sulfate having a higher k and n values as compared to nitrate which implies that sulfate was strongly adsorbed on nano-ball allophane as compared to nitrate.

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Smectite was formed from some igneous rocks, such as granodiorite, granite, andesite and basalt by reaction with NaOH solution at 100°C under atmospheric pressure. The proper concentration of NaOH solution for formation of smectite is 2-3 M, and the amount of starting sample less than 0.5 g in 100 ml solution is the strict condition.

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An energy map for 1-layer kaolin polytypes was developed, plotting internal energies calculated from results of Molecular Dynamics (MD) simulations. The energy map indicates stabilities of various kaolin 1-layer polytypes by the internal energies as a function of the interlayer shift.
The interlayer shifts were described on the basis of a pseudo-hexagonal lattice which contains an asymmetric unit of kaolinite structure. The MD simulations were carried out at grid points on a (001) plane of pseudo-hexagonal lattice with increments of 1/12 along each axis. In order to remain interlayer configurations in each structure, we adopted a constrainthat the positions of non-hydrogen atoms had been fixed. The present energy map shows two energetic minima, which correspond to structures of kaolinite and its enantiomorph, and has no minimum at (1/3, 1/3) in fractional coordinates of a projection onto (001) plane of pseudohexagonal lattice, which had been clearly shown in the electrostatic potential map by previous workers.
We also tried to follow trajectories of interlayer shifts during all-atom relaxations by MD simulations (NPT ensemble, 293 K, 0.1 MPa). The trajectories of interlayer shifts from different initial structures converged to five spots on the energy map, which include the two energetic minima. Average internal energies have lower values at the two minima on the energy map than at other three spots which trapped some trajectories. More long simulation time or higher temperature may complete relaxations of the trapped structures.
Both of the two approaches, the energy map and following trajectories, support that the kaolinite and its enantiomorph are the most stable structures as 1-layer kaolin polytype.

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Copper and zinc ions were strongly attracted to dissociated silanol groups at the inner surface of hollow spherical nano-ball allophane particle. Examination of adsorption isotherms of copper on allophane by the Langmuir theory indicated that the copper adsorption increased with the increasing bulk solution pH, and was higher for the allophane sample with a higher Si/Al ratio. The copper adsorption was greater than zinc adsorption previously reported with the same samples, and in both cases, solution pH decreased with the adsorptions. The copper and zinc adsorption on 0 atoms of the silanol groups are so strong that some of Si was released probably as Si-Cu compounds. Molecular orbital calculations with MOPAC AM1 basis set indicated that the copper and zinc ions (Cu²⁺, CuOH⁺, Zn²⁺ and ZnOH⁺) could adsorb not only on the dissociated silanol groups, but also with the undissociated silanol groups. Bond distances between the heavy metal atoms and 0 atom of the silanol groups were shorter for copper than for zinc, in agreement with experimental results. The calculations also indicated that, when the heavy metal ions interacted with undissociated silanol groups, dissociation reaction of the silanol groups accelerated. These are due to bonding formation between heavy metal and 0 atom of the silanol groups, and coincide with the observed pH decrease.

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