EFFECT OF MOLYBDATE ADSORPTION ON SOME SURFACE PROPERTIES OF NANO-BALL ALLOPHANE

Abstract

Cation exchange capacity (CEC) of nano-ball allophane measured at same equilibrium pH increased with molybdate adsorption at initial molybdate concentrations of 0.1 and 1.6mM. The increase in CEC was attributed either to deprotonation of silanol group near the adsorption site and/or negative charge carried by the molybdate. Great differences in the amounts of molybdate adsorption between the two initial molybdate concentrations (0.1mM and 1.6mM) was not reflected in the change in CEC. This may be due to polymerization of molybdate at higher concentrations (more than 0.2mM), therefore, increase in CEC was ascribed to adsorption of monomeric molybdate species. Increase in CEC was greater for allophane samples with lower Si/Al ratios than for allophane sample with higher Si/Al ratio, in agreement with the trend of adsorption. Ab inito molecular orbital calculations indicated that Bonsted acidity of silanol group of allophane near adsorption site increased with the molybdate adsorption. The increase in acidity together with free Mo-OH or Mo-O^- groups of molybdate adsorbed in monodentate form contributes the increase in CEC with the molybdate adsorption.