Charge Characteristics of Nano-Ball Allophane as Affected by Zinc Adsorption
Effect of Zn adsorption on the change in charge characteristics of two nano-ball allophane samples with different Si/Al ratio at various pH values was studied. The Cation Exchange Capacity (CEC) values of the two nano-ball allophane samples were likely to decrease whereas Anion Exchange Capacity (AEC) increased after Zn adsorption at initial concentration of 0.18 mM. The change in charge characteristics of the allophane samples was controlled by equilibrium pH, amounts of Zn adsorption and Zn species in the solution. The decrease in CEC values after Zn adsorption was due to neutralization reactions between cationic Zn with SiO¯ functional groups. The decrease in CEC values was found to be higher in case of allophane sample with higher Si/Al ratio (KnP) than that with lower Si/Al ratio counterpart (KyP), due to its higher adsorptive capacity for Zn species. The slight increase in the AEC values after Zn adsorption was probably due to in part to the initial H+ ions released into the bulk solution. Molecular orbital calculation indicated that the adsorbed Zn species has a ability to accelerate the deprotonation of undissociated SiOH groups near the adsorption sites by an inductive effect.
Cited References Fulltext