Vermiculite exchange capacity

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-... 

Quite often Al, Fe, and Mg hydroxides partially fill the interlayer position of the derived vermiculites and decrease their exchange capacity and their ability to contract completely to 10 A when heated or when treated with a potassium solution. This material can usually be removed by treating the clay with a solution of sodium citrate (Tamura,1958). As the content of hydroxy interlayer material increases, the expandable clay tends to assume the character of a chlorite. Thus, in the weathering of a mica or illite it is not uncommon to form discrete vermiculite-like, beidellite-like, monf-morillonite-like and chlorite-like layers. These various layers can occur as discrete packets or interstratified in a wide variety of proportions. 

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

Of special significance with respect to their properties as sorbents are the clay minerals (e.g. kaolinite, montmorillonite, vermiculite, illite, chlorite), mainly due to their high exchange capacity. 

Be able to calculate the ion exchange capacity of a smectite or vermiculite clay (meq/100 g clay) from the chemical formula of the clay. 

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 ... 

In the case of vermiculite, the short-chain organic cations do not penetrate into the interlayer space, and are adsorbed only on the external surface of the mineral. This is clearly seen from the analytical measurements, showing that only 0.06 meq/g of Na" " ions is substituted when Na-vermiculite is treated with tetramethylammonium salt solution, the total exchange capacity being equal to 1.5 meq/g. Therefore, the values of specific surface area obtained from water and hexane adsorption isotherms for the initial and [(CH3)4N] modified vermiculite are virtually the same (Table 3). Such a pronounced difference in the... 

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. 

Other important characterization studies are ion-exchange capacity, especially cationic, and specific surface measurement. CEC is an important parameter in clay characterization and soil behavior, because it affects nutrient availability, among other soil properties. Some clay minerals (smectites and vermiculites) have a high... 

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