Effect of Acid Species and Co-Existing Anions on the Dissolution of Al and Si from Allophane by the Treatment of Diluted Acid Solutions.
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It is not so clear that how soil clays are affected by the acid rain which comes down into soil environment. Especially, effect of acid rain on allophane, the main clay material present in volcanic ash soil, has not been known. Influence of acid rain on allophane is expected to be serious in damage in comparison with crystalline clay minerals in soil, because of its amorphous or poorly ordered nature in the chemical structure. The present study has been concerned with the effect of acid species and co-existing anions on the dissolution of aluminium (Al) and silicon (Si) from allophane by the treatment of diluted sulfuric or nitric acid solutions which are acid components in natural acid rain. There was remarkable difference in the dissolution of Al and Si between the acid species. Sulfuric acid solution had more ability than nitric acid solution to dissolve Al and Si from allophane. This indicates that not only hydrogen ion but also sulfate ion is responsible for the dissolution reaction. The amount of Al dissolved was greater than that of Si by the treatment of both acid solutions. And, the degreeof dissolution was higher in allophane with low Si/Al ratio than in that with high ratio. It was found that the co-existence of sulfate ion (added as sodium sulfate) in the both acid solutions accelerated the dissolution of Al and Si. The presence of a large quantity of nitrate ion (added as sodium nitrate) in sulfuric acid solution inhibited the dissolution. Such inhibitory action by nitrate ion was not recognized for nitric acid solution. Molecular orbital calculation showed that adsorption and bonding of sulfate ion to aluminol group (Al-OH) in the chemical structure of allophane reduced the bond order between Al and oxygen (0) atoms in the structure. Therefore, the calculation proved that the mechanism, for facilitating Al and Si dissolutions by sulfate ion, involved in weakening of chemical combination of Al and 0 in the structure and enhancement in solubility of allophane by acid solution.
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