SURFACE-CHARGE CHARACTERISTICS OF HYDROXYALUMINOSILICATE AND HYDROXYALUMINUM MONTMORILLONITE COMPLEXES

The electric charge and surface characteristics of montmorillonite (Mt ) complexed with hydroxy-Al (HyA) or hydroxyaluminosilicate (HAS) ions were studied in the pH range of 4 to 8. The HAS or HyA ions were prep ared from solutions containing HyA ions and orthosilicic acid at an Al concentration of 4 mmol L-1; Si/Al atomic ratios of 0, 0.25, 0.5, and 1.0; and NaOH/Al molar ratios of 1.0, 2.0, and 2.5. At pH 4 to 6, the negative charge of Mt was drastically reduced by interlayer formation with HAS or HyA ions. At pH 7 to 8, the magnitude of negative charge of RAS-Mt or HyA-Mt complexes was close to that of Mt, suggesting that HAS or HvA ions were excluded from the interlayer spaces of Mt. A sig nificant positive charge was developed on RAS-Mt complexes formed at N aOH/Al molar ratios of 2.0 and 2.5. The fixation of HAS or HyA ions on Mt caused a significant reduction in total and internal surface areas but a slight increase in external surface area. Our results reveal th at the complexation of Mt with HAS or HyA ions changed the electric ch arge and surface characteristics of Mt, and suggest that RAS and HyA i ons fixed on expansible 2:1 layer silicates contribute as active Al co mponents to humus accumulation and phosphate retention in acidic and n onallophanic Andisols and related soils. Our data also indicate that t he fixation of RAS and HyA ions on Mt can influence the levels of solu tion Al species that cause some toxicological problems in terrestrial and aquatic environments.