In situ observations of clay mineral dissolution by AFM : investigation of the dissolution rate and mechanism

点击翻译按钮获取中文摘要

In this study, we examined the dissolution behavior of clay minerals (especially, dioctahedral phyllosilicate minerals) under alkaline conditions at 25-50℃, using AFM with an air/fluid heater system. The main goals of this study were to comprehend the dissolution behavior of clay minerals under alkaline conditions, to determine the reliable dissolution rate, and to reveal the effect of temperature on the dissolution rate.The 2: 1 phyllosilicates, such as smectite and muscovite, dissolution took place only at the edge surfaces that are lesser than a few percent of the total surface area (TSA), while the basal surfaces were completely unreactive. The edge surface area (ESA)-normalized dissolution rate at a certain pH and temperature condition, therefore, has a constant value independent of the size of etch pit or island (particle). In contrast, the TSA-normalized dissolution rates varied with the size of etch pit or island.The activation energy for muscovite dissolution under alkaline conditions was very close to that for montmorillonite and illite dissolution. A model dissolution rate equation, which simultaneously includes the effect of pH and temperature, was deduced from the effect of pH on the activation energy, the rate equation of muscovite dissolution at 25℃, and the Arrhenius equation. The dissolution rates estimated from the model were in good agreement with the experimental rates from 25 to 70℃. The dissolution reaction order with respect to hydroxyl activity (or pH)increases with temperature.