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

在这项研究中，我们研究了粘土矿物（特别是二八面体层状硅酸盐矿物）在25-50℃碱性条件下的溶解行为，使用AFM与空气/流体加热系统。本研究的主要目的是了解粘土矿物在碱性条件下的溶解行为，确定可靠的溶解速率，揭示温度对溶解速率的影响。2: 1层状硅酸盐，如蒙脱石和白云母，只在小于总表面积（TSA）百分之几的边缘表面发生溶解，而基面完全不发生反应。因此，在一定pH和温度条件下，边缘表面积（ESA）归一化溶解速率具有恒定值，与蚀刻坑或岛（颗粒）的大小无关。相比之下，tsa标准化溶解速率随蚀刻坑或岛的大小而变化。白云母在碱性条件下的溶解活化能与蒙脱土和伊利石的溶解活化能非常接近。从pH对活化能的影响、25℃下白云母溶解速率方程和Arrhenius方程出发，推导出了同时考虑pH和温度影响的模型溶解速率方程。模型计算的溶解速率与25 ~ 70℃范围内的实验速率吻合较好。溶解反应顺序与羟基活性（或pH值）有关，随温度升高而增加。

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.