Cation exchange capacity vermiculite

This fact may explain the superiority of montmorillonite over vermiculite as an adsorbent for organocations (3, 4). Complicating this description, however, is the fact that a sample of any particular layer silicate can have layer charge properties which vary widely from one platelet to another (j>). By measuring the c-axis spacings, cation exchange capacity, water retention, and other properties of layer silicates, one obtains the "average" behavior of the mineral surfaces. 

Foster (1963) established empirically that the cation exchange capacity of vermiculites could be calculated by multiplying the positive charges carried by the interlayer cations by 200. The calculated range of cation exchange capacities of the macroscopic vermiculite she studied ranged from approximately 80 to 200 mequiv/ 100 g. Nearly half the samples have a C.E.C. less than 140 and more than a third have values less than 120. Because it was not known what proportions of the Mg to assign to the interlayer position and what proportion to the octahedral sheet, these calculated values can only be considered minimum values. Macroscopic vermiculites most com-... 

Note CEC = cation exchange capacity HISM = hydroxyinterlayered smectite HIV = hydroxyinterlayered vermiculite Kf = Freundlich metal distribution coefficients LSB = lime stabilized biosolids = total aluminosilicates. 

A vermiculite clay has the structural formula K.Mgo.sKMgaoFe sFeJ sXAl, sSi65)02o(OH)4. Calculate its cation-exchange capacity in meq/IOO g and its surface-site density in mol/g. Assuming the surface area of the clay is 30 mVg, what is its surface-site density in nm and /umol/m ... 

Vermiculites. Vermiculites are 2 1 expanding minerals with a structure similar to micas (Table 7-4). They are considered to be derived from the alteration of micas (Douglas, 1977). Cation exchange capacity is high, as is the surface area. Potassium or NHj in solution tends to be strongly fixed by vermiculites. Upon fixation of these ions, the CEC decreases and the properties of vermiculite become like those of mica. In acid soil, hydroxy aluminum polymers can be fixed in the interlayer position to form an "island-like" structure (Jackson,... 

Chemical Properties. An important chemical property of clays, which directly affects fines migration is the cation exchange capacity (CEC) (6-9). CEC is a measure of the capacity of a clay to exchange cations. It is usually reported in units of milliequivalents per 100 g of clay (meq/100 g). The CEC depends on the concentration of exchangeable cations in the diffuse Gouy-Chapman layer (see later). This concentration depends on the total particle charge, which may vary with pH. Unless stated otherwise, the reported values of CEC are measured at neutral pH. CEC values (meq/lOOg) of common clay minerals are as follows smectites, 80-150 vermiculites, 120-200 illites, 10-40 kaolinite, 1-10 and chlorite, <10 (10). 

Data sources Bolt 1979, Brummer 1986, Kabata-Pendias and Pendias 2001, Tan 1998, Schmitt and Sticher 1991, Sparks 1995. Dioctahedral and trioctahedral vermiculites. Amorphous Al-oxides. Amorphous Fe-oxides.CEC, cation exchange capacity. 

Vermiculite is a hydrous, silicate mineral, which exfoliates greatly when heated sufficiently. The structure of vermiculite consists of 2 tetrahedral sheets for every one octahedral sheet. It has medium shrink-swell capacity with limited expansion. The cation exchange capacity is high in the range of 100-150 meq/100 g. The structure of typical vermiculite contains a central octahedrally-coordinated layer of Mg ions, which lies between two inwardly pointing sheets of silicate tetrahedra. These silicate layers are normally separated by two sheets of interlayer water molecules. Complete removal of water molecules leads to 9.02 A lattice. These layers are electrically neutral and interlayer cations occupy only about one-third of the available sites. The cohesion between the layers is typically weak [10]. 

Explain why the cation exchange capacity of vermiculite is much greater than that of kaolinite. 

As a consequence, vermiculite has the greatest cation exchange capacity among all of flie phyllosihcates, at 100 to 260 meq/100 g. 

Fine-grained micas are present in nearly all soils. Frequently, they are the most abundant component of the clay fraction. In many soils, micas are the principal source of native potassium for plants. On weathering, micas are altered to vermiculite and sometimes to smectitelike minerals and, as such, significantly increase the cation exchange capacity and affect the relative selectivity by soils for various exchangeable cations. 

With these reservations in mind, the approximate range of cation exchange capacities of the vermiculite group can be written as 120 to 200 meq/100 g air-dry Mg-vermiculite. A more satisfactory basis would be to record the exchange capacity as meq/100 g interlayer-water-free and interlayer-cation-free mineral (Walker [1965]), and on this basis, the vermiculites range approximately from 140 to 240 meq. Values of cation exchange capacity below those quoted above have been reported, but their validity is in considerable doubt. These low values have invariably been obtained, not from pure vermicuUtes, but from mixed-layer minerals such as hydrobiotite or chlorite-vermiculite, a correction being applied for the proportion of non-vermiculite layers estimated to be present. The error involved in such corrections is considerable. 

---