Effects of ion specificity on the surface electrical properties of kaolinite and montmorillonite

Abstract

Clay minerals present a widespread problem in mineral processing and dewatering due to their shrinking and swelling characteristics. These minerals complex surface and interlayer interactions with ions in water play a critical role in governing their behavior. In this paper, the surface electrical properties and suspensions pH of kaolinite (a non-swelling, 1:1 type clay) and montmorillonite (a swelling clay, 2:1 type clay) in the presence of alkaline monovalent salts were investigated to identify the interactions between the ions and the minerals from an ion-pairing perspective. For benchmarking, the two key oxides of SiO2 and Al2O3 constituting the clay minerals were also used in the experiments. It was found that the particle zeta potentials and the suspension pH were strongly related to the type and concentration of the salt ions in water. The results show that ion-pairing and hydration energy being specific to salt ion pairs are the two main driving forces in governing the minerals surface charge and the suspension pH. Ions with weak pairing abilities such as NaI and CsF would have a significant effect on changing the electrical properties of clay surfaces. However, unpaired ions affinity to the surface of the mineral can be minimized because of their high hydration energy. Thus, the interaction with clay minerals surface would be weaker for ions with lower surface charge density (e.g., Cs+ and I−) and smaller hydration energy. Our quantification of the experimental results using the electrical double-layer theory also confirms the ion-pairing and hydration energy theory. The outcome of our investigation would be useful for a better understanding of the dewatering of fine clay-rich tailings.