Document Type : Research Paper
Former MSc. student, Tarbiat Modares University, Iran
Assistant Professor, Tarbiat Modares University, Iran
Soil colloid particles influence soil chemical and physical properties by their surface electrical charge. With respect to the importance of this issue from both agricultural and environmental points of view, charging behavior of a kaolinite sample was studied in the pH range of 4 to 10 and at three background electrolyte concentrations, including 0.005, 0.05, 0.5 M NaCl, by potentiometric titration. Using a combination of mass and electrolyte titrations, kaolinite point of zero charge (PZC) was determined to be at pH=7.6. Increase in pH from the initial value of 4 caused at first a gradual decrease in positive surface charge and then a gradual increase in negative surface charge, indicating a net positive surface charge at pH<PZC and a net negative surface charge at pH>PZC. Increase in ionic strength reduced positive surface charge at pH<PZC and enhanced negative surface charge at pH>PZC. The adverse effect of ionic strength on the surface charge at pH<PZC was ascribed to the competition between proton and sodium ions for adsorption on the kaolinite planer surfaces, where some permanent negative charge exist.