ADSORPTION MECHANISMS OF COPPER AND ZINC ON NANO-BALL ALLOPHANE

Copper and zinc ions were strongly attracted to dissociated silanol groups at the inner surface of hollow spherical nano-ball allophane particle. Examination of adsorption isotherms of copper on allophane by the Langmuir theory indicated that the copper adsorption increased with the increasing bulk solution pH, and was higher for the allophane sample with a higher Si/Al ratio. The copper adsorption was greater than zinc adsorption previously reported with the same samples, and in both cases, solution pH decreased with the adsorptions. The copper and zinc adsorption on 0 atoms of the silanol groups are so strong that some of Si was released probably as Si-Cu compounds. Molecular orbital calculations with MOPAC AM1 basis set indicated that the copper and zinc ions (Cu2+, CuOH+, Zn2+ and ZnOH+) could adsorb not only on the dissociated silanol groups, but also with the undissociated silanol groups. Bond distances between the heavy metal atoms and 0 atom of the silanol groups were shorter for copper than for zinc, in agreement with experimental results. The calculations also indicated that, when the heavy metal ions interacted with undissociated silanol groups, dissociation reaction of the silanol groups accelerated. These are due to bonding formation between heavy metal and 0 atom of the silanol groups, and coincide with the observed pH decrease.