Halloysite exchange capacity

Halloysite has a chemical composition similar to kaolinite, but with a higher water content. The layers of halloysite are like those in kaolinite, but they are stacked with highly random displacements parallel to the layers, as opposed to the regular stacking found in kaolinite. The interlayer distance is greater in halloysite, allowing for the presence of a sheet of water molecules. A small ion-exchange capacity is measurable in kaolinite and halloysite minerals, which arises from a small amount of iso-morphous replacement of Si4+ or Al3+ in the framework 234). 

Dehydrated halloysites have C.E.C. in the range of 6—10 mequiv./lOO g (Van der Marel, 1958 Garrett and Walker, 1959). Garrett and Walker have shown that the exchangable cations are located on the external surfaces of the crystals and not in the interlayer position of halloysite. Until it is possible to obtain accurate chemical analyses of the kaolinite minerals, it will be difficult to determine their exchange capacity and the source of the charge. 

Garrett, W.G. and Walker, G.F., 1959. The cation exchange capacity of hydrated halloysite and the formation of halloysite salt complexes. Clay Miner., 4 75-80. 

There are inconsistencies in the model for the calculation of activity products for the "clays. Exchangeable cations are disregarded for the low exchange capacity kaolinite, halloysite, chlorite, and moderate capacity illite. For certain expansible layer silicates and two zeolites, the logjo of the activity of selected cations is added into the sum of the activity products. 

Because of the interlayer water sheet in hydrated halloysite, intercalation (introduction between the imit cells) of chemicals can occur. This also results in a slightly higher cation exchange capacity for hydrated halloysite (5 to 40 meq/100 g) than for kaolinite (3 to 15 meq/100 g). Halloysite also may be more affected by chemicals than kaolinite. 

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). 

Even higher loading capacity can be achieved when porous fillers with hollow cellular stmcture are loaded with organic and/or inorganic inhibitors (Schmidt, 2002). The hollow cellular stmcture material may be represented by zeolite or halloysite nanotubes. Zeolite particles are attractive carriers also because the cations in their stmcture are rather loosely held and can readily be exchanged for the inhibiting cations in the contact solution (Eckler and Ferrara, 1988). 

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