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(特别是双八面体层状硅酸盐矿物)在25-50℃碱性条件下的溶解行为。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，并揭示温度对溶解速率的影响。蒙脱石和白云母等2：1的层状硅酸盐只在小于总表面积(TSA)的边缘表面发生溶解，而基面完全没有反应。因此，在一定的pH和温度条件下，边缘表面面积(ESA)的归一化溶解速率有一个恒定值，与蚀坑或岛(颗粒)的大小无关。在碱性条件下，白云母的溶解活化能与蒙脱石和伊利石的溶解活化能非常接近。根据pH对活化能的影响、25℃时白云母的溶解速率方程和阿累尼乌斯方程，推导出同时考虑pH和温度影响的模型溶解速率方程。在25~70℃范围内，模型估算的溶出度与实验值吻合较好。与羟基活度(或pH)有关的溶解反应级数随温度升高而增大。

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.