NET SURFACE-CHARGE CHARACTERISTICS OF ALLOPHANE AND SYNTHETIC ALUMINOSILICATES

Potentiometric titration (PT) and electrophoretic mobility (EM) in KCl and NaCl solutions were used to determine the surface charge of soil allophane, several inorganic soil fractions coated with iron oxide, an d synthetic non crystalline tau-alumina, hydrous iron oxide and two al uminosilicates with different SiO2/Al2O3 ratio (prepared by coprecipit ation and hydrolysis). There is almost no cation effect (Na+ or K+) on point of zero net proton charge (PZNPC), point of zero salt effect (P ZSE), and isoelectric point (IEP) with the exception of coprecipitated aluminosilicate. The IEP determined from EM measurements, the PZNPC a nd the PZSE both determined through PT, are coincident (within experim ental errors) only for pure oxides. Iron oxide free samples and synthe tic aluminosilicates show a significant PZSE shifting in comparison wi th PZNPC. PZSE values are directly related to (Al2O3 + Fe2O3)/(Al2O3 Fe2O3 + SiO2) ratio, which depends on composition.