Settling and Flocculation Value of Sodium‐Montmorillonite Particles in Aqueous Media

Abstract

AbstractThe effect of pH, particle size, and clay concentration on the flocculation value (FV) and the settling rate of Na‐montmorillonite in dilute suspension was studied in solution of various NaCl concentrations. Flocculation values increased with increasing pH, being 10, 13, 31, and 44 mmol dm−3 of NaCl at pHs 5, 7.5, 8.5, and 9.8, respectively. The final gel volume of the samples at all pHs was high (0.35 < gel volume < 0.49 m3 kg−1), and the corresponding thickness of the water film associated with the clay surface was approximately 500 nm. The high gel volume indicates that a three‐dimensional “card‐house” structure was formed. The presence of a negative charge on the broken edges at pH 9.8, the high FV (44 mmol dm−3 NaCl) and the high gel volume suggest that an open structure with a face‐to‐face association predominates. Face‐to‐face association between two platelets may take place at surfaces having either a lower or higher specific charge density than the average montmorillonite value of 0.117C m−2. At pHs equal or below the point of zero charge of the edges, edge‐to‐edge or edge‐to‐face association may occur. The time that it takes to establish a boundary between the gel and the supernatant depends on the size of the clay particles. The smaller the particles, the longer is the settling time. The settling rate depends also on the clay concentration. The boundary between the gel and the clear solution was observed after 1 and 4 d for clay concentrations of 0.1% and 0.05%, respectively, but at higher clay concentrations (0.2 and 0.5%) the suspensions remained stable for more than 7 d.