CHARGE CHARACTERISTICS MODIFICATION MECHANISMS OF NANO-BALL ALLOPHANE UPON ORTHOSILICIC ACID ADSORPTION

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Mechanism of change in charge development after orthosilicic acid adsorption on two low Si/Al and one high Si/Al nano-ball allophane samples were studied. The change in charge characteristics was found to be governed by equilibrium pH, and amounts and form of the acid adsorbed. Increase in cation exchange capacity CEC) after adsorption correlated highly with adsorption below the pK_1 of the acid. Increases in CEC and decreases in anion exchange capacity were marginal below equilibrium pH 8 due to the adsorption of the neutral species, H_4SiO_4, of the acid. Molecular orbital calculations indicated polymerization of the silanol group by the neutral species of the acid at pH≤8 as being more feasible than adsorption at the aluminol site confirming the high Si/Al sample to be a SiO_4 tetrahedral product of the low Si/Al samples. At pH >8, change in CEC was greater in the low Si/Al samples than their higher Si/Al counterpart as a result of their higher preference for the anion species, H_3SiO_4 of the acid. Increase in CEC, which was astronomical at pH≥pK1 of the acid, was attributed to a permanent charge imparted onto the mineral by adsorption of the anion species of the acid at the Al-OH_2 site.
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