Surface Charge Characteristics of Hydroxyaluminosilicate- and Hydroxyaluminum-Montmorillonite Complexes

Abstract

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 prepared 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 HAS-Mt or HyA-Mt complexes was close to that of Mt, suggesting that HAS or HyA ions were excluded from the interlayer spaces of Mt. A significant positive charge was developed on HAS-Mt complexes formed at NaOH/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 that the complexation of Mt with HAS or HyA ions changed the electric charge and surface characteristics of Mt, and suggest that HAS and HyA ions fixed on expansible 2:1 layer silicates contribute as active Al components to humus accumulation and phosphate retention in acidic and nonallophanic Andisols and related soils. Our data also indicate that the fixation of HAS and HyA ions on Mt can influence the levels of solution Al species that cause some toxicological problems in terrestrial and aquatic environments.