Influence of mineralogical and morphological properties on the cation exchange behavior of dioctahedral smectites

• Published in: Colloids and surfaces. A, Physicochemical and engineering aspects Vol. 481; pp. 591 - 599
• Main Authors: Delavernhe, L., Steudel, A., Darbha, G.K., Schäfer, T., Schuhmann, R., Wöll, C., Geckeis, H., Emmerich, K.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 20-09-2015
• Subjects: Cation exchange capacity; Edge site density; Particle size distribution; Smectite; Variable charge; Edge site density; Particle size distribution; Cation exchange capacity; Smectite; Variable charge
• ISSN: 0927-7757; 1873-4359
• DOI: 10.1016/j.colsurfa.2015.05.031

•Dioctahedral smectites display structural heterogeneities and variation in size.•Wyoming smectite is significantly different from other dioctahedral smectites.•CEC increases with pH similar for natural and reduced charge montmorillonites (RCM).•RCM are suitable to study edge surface properties of montmorillonites.
Structure and morphology determine the smectites properties. Thereby, edge sites play a key role for the reactivity and colloidal behavior of the smectite. Even so, the edge surface area represents only 1–3% of the total surface area its contribution is up to 10% of the cation exchange capacity (CEC). Na-saturated dioctahedral smectites <0.2μm were characterized in detail with respect to their structure (layer charge, octahedral cation distribution, iron content, and layer charge distribution within tetrahedral and octahedral sheets). Furthermore, reduced charge montmorillonite (RCM) of these materials were studied. Particle dimension was estimated from argon adsorption, calculation of specific surface area and direct observation of single layers by atomic force microscopy (AFM), which also revealed morphology and particle size distribution of single layer. CEC was measured both for Na-saturated samples and RCM using the Cu-trien method over a pH range of 4–9. The Wyoming smectite with a mean layer diameter of 277nm and a low layer charge of 0.26eq/f.u. differs strongly from the others smectites with a mean layer diameter of about 100nm and layer charges between 0.30 and 0.37eq/f.u. Furthermore, the layer stacking is much higher for the Wyoming smectite in powder and RCM. Accordingly the edge properties differ between the four smectites. Based on calculation of edge site density and CEC measurements, we showed that the RCM can be used to study the edge site reactivity of the montmorillonites. The increase of CEC above their pHPZC,edge is equal for Na-saturated montmorillonite and RCM. The present study highlights the diversity in structure and morphology and resulting edge properties of four smectites.