Smectite, sodium bentonite

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Sodium bentonite - Sodium bentonite is composed substantially of smectite, usually montmorillonite, with sodium as the major exchangeable cation. As such it is water swellable and will hydrate to form the characteristic colloidal structure. 

Water impedance - Once swollen by contact with water, a layer of smectite will prevent further passage of water. Smectites, particularly sodium bentonite, are therefore widely used to prevent seepage loss from ponds, ditches, reservoirs, and waste disposal areas, to line and waterproof tunnels and die below-grade walls of residential and commercial buildings, and to seal cracks and fissures in rocks and concrete. 

Many nanodays are based on the smectite clay, montmorillonite, a hydrated sodium calcium aluminum magnesium silicate hydroxide, (Na,Ca)(Al, Mg)6(Si40io)3 (OH) wH 2O. M ontmorillonite is found throughout the world in small quantities in its natural geological state. In large deposits, where the mineral is found in greater than 50% concentrations admixed vdth a variety of other minerals, it is known as bentonite. 

Example 5.1 (A constituent description of bentonite clay). Bentonite consists mainly of smectitic clay minerals such as montmorillonite and beidelite, with macro-grains that are mainly quartz and chalcedony. The chemical formulae of water and quartz are H2O and SiOa, respectively, and we assume that the chemical formula of a smectitic clay mineral is Nai/3Al2[Sin/3Ali/3]Oio(OH)2. Note that the sodium ions Na+ exist in an interlayer space of the clay minerals. For simplicity, the porosity is assumed to be n = 0.5, and the composition (i.e., mass fraction) of the solid phase is given as consisting of 50% clay minerals and 50% quartz. The pores are assumed to be filled with pure water. The REV is a 1 cm cube. 

The rock in which montmorillonite and similar minerals are dominant is bentonite. Bentonites that are used industrially are predominantly composed of either sodium montmorillonite or calcium montmorillonite and to a much lesser extent of hectorite. Smectite minerals have significantly different physical and chemical properties, which dictate their industrial utilization to a large degree. 

Bentonites also remove compounds from contaminated water which, at first sight, should not interact strongly with smectites. An example are halogenated hydrocarbons (chloroethanes, m-dichlorobenzene and several bromo hydrocarbons) which are irreversibly bound by sodium montmorillonite. (The adsorption on fumed silica is reversible). The irreversibility of the process indicates chemisorption. Certain aromatic halogeno hydrocarbons (2 and 4-bromotoluene and 1,2 dichlorobenzene) are, at least partially, catalytically transformed into higher molecular weight products [71]. 

Smectite - The smectites are water swellable cl s having a sheet or platelet structure. Smectite is the mineralogical term for this class of clays, which includes montmorillonite, hectorite, and saponite. Montmorillonite clays derive their name from the Montmorillon section of France where this material was first observed and later classified. Most smectites are more commonly known under the geological term bentonite. By convention, a bentonite is understood to be an ore or product with a substantial smectite content. The name bentonite derives from Fort Benton, Wyoming, the site of an important deposit. Lattice substitutions within the smectite clays creates a charge imbalance which is compensated by exchangeable alkali and alkaline earth cations. This contributes to the ability of these clay to swell and impart considerable plasticity in ceramic formulations. When the exchangeable cations are predominately sodium, the individual platelets can separate to produce a colloidal structure in water. 

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