NEW CONCEPTS FOR CHANGE IN CHARGE CHARACTERISTICS OF ALLOPHANE WITH PHOSPHATE ADSORPTION

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Observed changes in the amounts of negative and positive charges of allophane with phosphate adsorption (Na H₂PO₄ in NaCl with total [Na]=10 mM) were each divided into two components. One is due to increase in solution pH, variable charge characteristics, and the other is net effect caused by the adsorbed phosphate. The net effect increased negative charge and decreased positive charge of allophane. To explain the net effect, we assumed that OH^- was initially released from allophanephosphate adsorption complex just the same amount as the adsorbed H₂PO₄^-. This situation is comparable with that when the same amount of NaOH was added to original allophane. Then, we compared the reaction of the OH^- with functional groups, Si-OH, Al-OH₂⁺ and P-OH, between the two situations. The comparison indicated that the functional groups of the adsorption complex had more abilities to react with the OH^- than those of the original allophane. Inductive effect of the adsorbed phosphate which accelerated deprotonation reaction of inherent functional groups of allophane, Si-OH and Al-OH₂⁺, was suggested.