オルトケイ酸との反応によるSi/Al比の低いアロフェンの表面酸性変化

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Allophane formed in weathered volcanic ash and pumice has a morphology of aggregates composed of fine hollow spherules with diameters of 3.5 to 5.0 nm. The chemical compositions of the wall of hollow spherules vary in Si/Al atomic ratio from 1 : 2 to 2 : 2. There is a regular dependency in physicochemical properties such as structural stability against heating and grinding and surface properties depending upon the Si/Al ratio of allophane. The ratio has been regarded as a factor affecting the nature and properties of allophane. However, it is not known why the ratio of Si and Al making up the structure governs the properties. In this study, allophane having a low Si/Al ratio were enriched with Si, and changes in the surface acidity as the result of Si enrichment were measured ; the mechanism of change being analyzed by means of the molecular orbital method. An allophane sample (Si/Al=1.15:2) was separated from pumice, and treated with diluted solutions of silicic acid (8×10^<-4> and 16×10^<-4> mol L^&lr;-1>). The Si/Al ratios for the samples after the treatments were 1.55 : 2 and 1.82 : 2. Measured surface acidity was higher in intensity for the samples after treatment than before treatment. Infrared spectoscopy proved that Si atoms added to the chemical structure of allophane having a low Si/Al ratio were present in the structure in the state of condensed SiO_4 tetrahedra. A molecular orbital calculation suggests that the intensification of acidity resulting from Si enrichment was ascribable to electron-level interactions of silanol groups (Si-OH) with alminol groups (Al-OH) located near holes in the walls of allophane hollow spherules and with the Si-OH of condensed SiO_4 tetrahedra newly formed in the structure after treatment.
社団法人日本土壌肥料学会の論文
1997-10-05

オルトケイ酸との反応によるSi/Al比の低いアロフェンの表面酸性変化

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