CHANGE IN SURFACE CHARGE PROPERTIES OF NANO-BALL ALLOPHANE AS INFLUENCED BY SULFATE ADSORPTION

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Change in surface charge of nano-ball allophane after sulfate adsorption was studied. Cation exchange capacity (CEC) was found to increase with increasing pH as well as after sulfate adsorption, with KnP (allophane sample with high Si/Al ratio) having higher CEC value as compared to KyP (low Si/AI ratio). The increase in CEC after adsorption for both samples was attributed to the charge carried by the anion. However, plotting the increase in CEC vs. the amount of sulfate adsorbed showed a slope of much less than 1, which showed that most of the sulfate adsorption occurred as bidentate reaction. This can be attributed to' the adsorption mechanism with sulfate being adsorbed through ligand exchange aside from anion exchange reaction. Further, the adsorption of sulfate on allophane enhanced the deprotonation of the near silanol group, thus providing a new negative charge, thus an increase in the CEC. Molecular orbital calculation using MOPAC program affirmed these experimental results. The amount of positive charge or AEC on the other hand was found to decrease after sulfate adsorption for both KyP and KnP samples. The decrease in AEC was much lower than the sulfate adsorption, although more sulfate than AEC was adsorbed. This implies that some AEC site remains after much sulfate adsorption, providing nutrients such as Cl^- and NO_3^- for plants.
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