Sodium-exchanged bentonite

The most important industrial example of cation exchange is the preparation of sodium-montmorillonite/bentonite from calcium bentonite. As seen in Table 2.2, calcium ions have greater affinity to the layer charge than sodium ions, so the calcium-sodium cation exchange must be performed in the presence of carbonate ions. It means that calcium-montmorillonite/bentonite is suspended in sodium carbonate solution. Calcium ions precipitate with carbonate ions, so sodium ions can occupy the interlayer space. This process is known as soda activation of bentonite. The disadvantage of soda activation is that sodium-montmorillonite is contaminated with calcium carbonate. 

The crystallographic surface area of bentonite clay is about 750 m2/ g (17), although the actual value that is obtained depends on the exchange cation. Quirk (18) has reported the solid-liquid surface area of bentonites with various alkali and alkaline earth exchange cations dispersed in aqueous solution. The sodium-exchanged form of bentonite has a measured surface area of 700 m2/g, which is close to the theoretical... 

Ion-exchanged bentonite - This is calcium bentonite diat has been dry blended or hydrotreated widi sodiiun carbonate to increase the availability of exchangeable sodium ions sufficiently to enable the cl to behave like sodium bentonite. This is also called sodiiun-exchanged bentonite. 

The clay mineral bentonite (sodium montmorillonite) has an excellent ion exchange and adsorption capacity. Films can be applied to electrode surfaces from colloidal clay solutions by simple dip or spin coating that become electroactive after incorporation of electroactive cations or metal particles 136-143)... 

Sodium bentonite with a cation exchange capacity (CEC) of 75 meq/100 g of clay, supplied by Commercial Minerals Ltd., Australia, was used as starting clay material, to prepare samples for SCD and surfactant treatments. Besides, sodium montmorillonite (Kunipia G), from Kunimine Industrial Company, Japan, was used as the starting clay for samples of pore opening modification. CEC of this clay is 100 meq/100 g of clay. 

In these experiments a commercially available bentonite, marketed under the name Colclay A90 (Ankerpoort, Geertruidenberg, The Netherlands) was used. It is a sodium-montmorillonite with a third of the exchange complex occupied by calcium. 5.0 g of the air-dried powdered bentonite was weighed into a stainless steel mould with an ID of 50 mm between two porous stones of the same diameter. Then the clay was subjected to a compaction pressure of 20.3 MPa for 30 minutes. After compaction, the mould was placed in a bowl of NaCl-solution for five days in which the clay became saturated and swollen. Thus samples were obtained with thickness of 3.8 and 2.8 mm respectively and a diameter of 50 mm. 

Let us summarize here this part of the study, since it is published elsewhere (25). First, it is important to ascertain the fast exchange of the sodium counterions between the bulk solution and the vicinity of the platelets. This is done by checking the effect of temperature on relaxation rates. For instance, for a sample of the Ecca Gum BP bentonite (44.2 mg/mL water) the sodium-23... 

Sposito, G., K. M. Holtzclaw, L. Charlet, C. Jouany, and A. L. Page. 1983. Sodium-calcium and sodium-magnesium exchange on Wyoming bentonite in perchlorite and chloride background ionic media. Soil Sci. Soc. Am. J. 47 51-56. 

French town (Montmorillon) where it was first discovered. In the USA, bentonite is principally mined in Wyoming - hence the term Wyoming clay . The type of bentonite, the source, and its purity influence its properties (Marchal et al. 1995). Bentonite is a complex hydrated aluminum silicate with exchangeable cationic components (Al, Fe, Mg) Si40io (0H)2 (Na+, Ca++). The most commonly used bentonite form in enology is the sodium bentonite. Sodium bentonite has enhanced protein binding capabilities over calcium bentonite. 

The component (A1 or Si) which is in excess for cancrinite formation remains in solution. The spikes in the and N03"-N02 curves in Figure 4 at about 15 min are probably the result of initial ion exchange of the sodium salts with bentonite. 

Bentonites recommended for treating wine have variable chemical compositions, unrelated to their enological properties. Rarely used in their natural state, they are usually activated with sulfuric acid or alkaline salts. In view of their strong ion exchange capacity, it is possible to load bentonites with H+, Na+ or Ca ions, to form acid, calcium or sodium bentonites. 

Filtrol, which had an estimated capacity of 100 to 150 tons per day and a bit over 25% of the market, used halloysite or bentonite treated with sulfuric acid to leach out some of the alumina. This was treated with ammonium hydroxide to produce a hydrated alumina, to which was added some of the leached clay, forming a gelatinous mixture of clay and hydrous alumina. The remainder of the leached clay reacts with sodium silicate and is heated to effect crystallization. After base exchange, the resulting zeolite is mixed with the clay-alumina matrix and spray dried to form microspheres (85). 

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