Dissolution and chemical changes of allophane and imogolite by dilute nitric acid solutions
1999
Henmi, I. (Matsuyama Sinonome Junior Coll., Ehime (Japan)) | Matsue, N. | Henmi, T.
The present study was concerned with clarification of the effect of acid rain on chemical changes of soil clays, especially amorphous or poorly ordered clay minerals such as allophane and imogolite. Clay materials and minerals that are ubiquitous in the soil environment were treated with a dilute nitric acid solution, which is one of the main components of acid rain. There was a remarkable difference in the dissolution of Al and Si Between amorphous or poorly ordered clay materials and crystalline layer silicate clay minerals. The amounts of Al and Si dissolved by the aid solution were much greater from allophane and imogolite than from montmorillonite, kaolimite and halloysite. Almost no change in structure was observed for the layer silicate clay minerals. This indicates that amorphous or poorly ordered clay materials are weak against acid rain in comparison with crystalline layer silicates. A difference was recognized in the solubility of allophane and imogolite in the acid solution. Imogolite was greater in solubility, releasing larger amounts of Al and Si in the solution than allophane. The lower tolerance of imogolite against the acid may be due to its structural weakness and large surface area based on its tubular morphology. Molecular orbital calculation showed that nano-ball-shaped allophane with a diameter of 5 nm was more stable in structure in terms of total energy than nano-tube-shaped imogolite with a diameter of 2 nm. IR spectra showed the acid caused more destruction of structural sites rich in Al, leaving condensed SiO4 tetrahedral in the structures of allophane and imogolite. As for allophane, the allophane sample with a low Si/Al ratio was less resistant the acid as compared to the sample with a high Si/Al ratio
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