抄録
Allophane as a poorly ordered aluminum silicate is known to have lower tolerance to alkaline condition than layer silicates have, and easily dissolve completely under higher alkali concentrations. However, changes in structure and dissolution mechanism under dilute alkali concentrations are not clear. In this research, we examined interaction of allophane with dilute NaOH solutions (<pH12) with pure nano-ball allophane samples obtained by using less than 0.2μm fraction from inner part of pumice grains. The pumice grains were collected from two sites of Japan, and the separated samples were KyP (Si/Al=0.67) and KnP (Si/A1=0.99). Dissolution kinetics of Si and Al were analyzed by dividing dissolution process as two steps: first rapid dissolution step within 12 h and following slow step after 12 h. At the rapid step, dissolution rate of Al were higher by twice than that of Si for KyP, but for KnP, the both were comparable. The dissolution rate of Si was comparable between the two samples. Si/Al ratio of dissolved materials at the step was smaller than that of untreated sample for KyP, but was greater than that of untreated sample for KnP. These indicate Al of the fundamental allophane structure, imogolite sheet, is susceptible to hydroxyl attack, and in KnP structure, accessorily attached Si prevented hydroxyls to approach the site. Infrared spectra also showed more dissolution of the fundamental sheet of KyP at the rapid step. From the chemical structure of nano-ball allophane, the most susceptible part of the fundamental sheet is considered to be the pore region where many Al are exposed as broken edge.
