ADSORPTION OF SOME LOW MOLECULAR WEIGHT ORGANIC ACIDS ON NANO-BALL ALLOPHANE

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Aluminol groups such as Al-OH or Al-OH_2 are shown to have an important role in adsorbing low molecular organic compounds, acetic, oxalic and citric acid, on nano-ball allophane samples. The amount of the three organic compounds adsorbed was higher for KyP sample with Si/Al=0.67 than KnP sample with Si/Al=0.99 due to much more aluminol content of the KyP sample. Langmuir and Freundlich adsorption equations were used to describe the adsorption of the organic compounds. At lower pH, the amount of organic compounds adsorbed by nano-ball was as follow: oxalate > citrate > acetate, and at higher pH, citrate > oxalate > acetate. For all cases, the Langmuir and Freundlich porameters tended to decrease with increasing solution pH. Adsorption of acetate by allophane at pH 4-6 fitted with Langmuir model and at pH 8-10 fitted with Freundlich model. For adsorption of oxalate, only at pH 4 suited with the Langmuir model. All the adsorption data of citrate fitted with the Freundlich model. From the observed changes in solution pH with adsorption, reaction mechanism between the three organic compounds and allophane was proposed as follow: a) Deprotonation (release of H^+), this was reaction between acetate or citrate and allophane at low pH; b) Dehydroxylation (release of OH^-), this was reaction between acetate or citrate and allophane at high pH and reaction of oxalate. At lower pH, Al was easier to be released than Si from KyP, but at higher pH, Si was easier to be released than Al. For KnP, release of Si was much more than Al at all pHs, because the KnP sample contains a lot of condensed Si tetrahedra which is bonded weakly on the fundamental allophane structure.
日本粘土学会の論文
1999-11-01

ADSORPTION OF SOME LOW MOLECULAR WEIGHT ORGANIC ACIDS ON NANO-BALL ALLOPHANE

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