Some Interface Electrochemical Properties of Kaolinite

Abstract

AbstractThe interface electrochemical properties of clay were theoretically analyzed to obtain some relationships among point of zero net charge (PZNC), point of zero net proton charge (PZNPC), intrinsic surface reaction equilibrium constants (K in 1‐pK model, Kinta1 and Kinta2 in 2‐pK model, *KintNa and *Kint in inert electrolyte chemical binding model) and structural negative charge density (σst) of clay, and some interface electrochemical parameters of kaolinite were measured. The following main conclusions were obtained. For clay possessing structural negative charges, the PZNC independent of electrolyte concentration (c) should exist just as amphoteric solid without structural charges such as oxides or hydroxides. A common intersection point (CIP) should appear among the potentiometric (or acid‐base) titration curves at different c and the pH at the CIP should be pHPZNC. A CIP among potentiometric titration curves at different c for kaolinite was observed, and the value of pHPZNC of kaolinite was 2.16. The values of pHPZNPC were decreased with increasing c, which arises from the presence of structural negative charges of kaolinite. In addition, it was observed that a good linear relationship existed between pHPZNPC and lg c. According to the values of pHPZNC and σst measured, the intrinsic surface reaction equilibrium constants, pK and pKinta1 and pKinta2 of 1‐pK and 2‐pK models could be directly calculated for clay, and the values of pK, pKinta1 and pKinta2 for kaolinite were 2.93, 1.90 and 3.97, respectively. These experimental values of pKinta1 and pKinta2 for kaolinite are obviously lower than those optimized with fitting programs in literatures, which maybe arises from the introduction of a type of permanent negatively charged sites in the models of literatures. An interesting result obtained in this study is that the inert electrolyte chemical binding does not exist for kaolinite, which also arises from the presence of structural negative charges.