Dissolution and chemical changes of allophane and imogolite by dilute nitric acid solutions.

摘要

The effect of acid rain on chemical changes of soil clays, especially amorphous or poorly ordered clay minerals such as allophane and imogolite was studied. Clay materials and minerals that are ubiquitous in the soil environment were treated with adilute nitric acid solution, 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 andSi dissolved by the acid solution were much greater from allophane and imogolite than from montmorillonite, kaolinite 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. Imogolite was more soluble than allophane, releasing larger amounts of Al and Si in the solution. The lower tolerance of imogolite against the acid may bedue to its structural weakness and large surface area based on its tubular morphology. 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 that the acid destroyed structural sites rich in Al, leaving condensed SiO4 tetrahedra in the structures of allophane and imogolite. The allophane sample with a low Si/Al ratio was less resistant to acid compared to allophane with a high Si/Al ratio.