Card-house microstructure of purified sodium montmorillonite gels evidenced by filtration properties at different pH

Abstract

The effect of pH on the static filtration properties of purified sodium montmorillonite dispersions depends on the applied pressure. At 1.5 × 10⁵ Pa, the double-layer repulsion resists the applied pressure at natural pH, which is close to neutral pH, and at basic pH. At acidic pH, where the cake is the thinnest and the least permeable, the edge-to-face attractions act as hinges. At natural pH, the thickest and the most permeable cake is obtained. It retains less water than the acidic one because of the absence of edge-to-face contact between particles. These interpretations are confirmed by transmission electron microscopy photographs. At basic pH, the cake retains more water than at lower pH, but it has an intermediate permeability, possibly because of adsorption of water on the negatively charged edges of the clay mineral layers. At 5.7 × 10⁵ Pa, the permeability of the cakes obtained decreases as the pH increases, which probably means the breakdown of the double-layer repulsion at natural pH and the breakdown of the “repulsive network” at basic pH. The results also show that the water retention is the highest in the acidic cake and the lowest in the neutral cake, which probably means that the edge-toface attractions in the acidic cake resist the pressure more strongly than the face-to-face repulsions in the cake at the natural pH. The X-ray diffraction patterns of the acidic cake do not show a basal reflection, whereas the other cakes show interparticular distances ranging from about 13 to about 39 Å (natural cake) and from about 19 to about 33 Å (basic cake).