CATION-EXCHANGE CAPACITY OF ZEOLITES
Zeolites are crystalline, hydrated aluminosilicates of alkali and alkaline earth metals, having infinite, three-dimensional atomic structures. They are further characterized by the ability to lose and gain water reversibly and to exchange certain constituent atoms, also without major change of atomic structure. Along with quartz and feldspar minerals, zeolites are three-dimensional frameworks of silicate (SiO4) tetrahedra in which all four corner oxygens of each tetrahedron are shared with adjacent tetrahedra. If each tetrahedron in the framework contains silicon as its central atom, the overall structure is electrically neutral, as is quartz (SiO)2). In zeolite structures, some of the quadri-charged silicon is replaced by triply-charged aluminum, giving rise to a deficiency of positive charge. The charge is balanced by the presence of singly-and doubly-charged atoms, such as sodium (Na+), potassium (K+), calcium (Ca2+), and magnesium (Mg2+), elsewhere in the structure. The empirical formula of a zeolite is of the type:
M2/nO Al2O3 xSiO2 yH2O,
where M is any alkali or alkaline earth atom, n is the charge on that atom, x is a number from 2 to 10, and y is a number from 2 to 7. The chemical formula for clinoptilolite, a common natural zeolite is:
Atoms or cations (i.e., charged metal atoms) within the second set of parentheses are known as structural atoms, because with oxygen they make up the rigid framework of the structure. Those within the first set of parentheses are known as exchangeable ions, because they can be replaced (exchanged) more or less easily with other cations in aqueous solution, without affecting the aluminosilicate framework. This phenomenon is known as ion exchange, or more commonly cation exchange. The exchange process involves replacing one singly-charged exchangeable atom in the zeolite by one singly-charged atom from the solution or replacing two singly-charged exchangeable atoms in the zeolite by one doubly-charged atom from the solution. The magnitude of such cation exchange in a given zeolite is known as its cation-exchange capacity (CEC) and is commonly measured in terms of moles of exchangeable cation per gram (or 100 grams) of zeolite or in terms of equivalents of exchangeable cations per gram (or 100 grams) of zeolite.
A mole of an atom or cation is its molecular or atomic weight written in terms of grains (i.e., gram-molecular weight); thus, a mole of sodium (Na) weighs 22.99 grams, and a mole of calcium (Ca) weights 40.08 g. Because the number of cations in solution that can replace exchangeable cations in a zeolite depend on the charge on the species being exchanged, i.e., 2 Na+ cations can be exchanged for one Ca2+ cation or one Ca2+ can be exchanged for 2 K+cations, expressing the cation-exchange capacity of a given zeolite in terms of moles requires that the nature of the exchanging cations be stated for each exchange reaction. Expressing CEC is terms of equivalents (or milliequivalents) eliminates the need for such cumbersome expressions, because the number of equivalents being exchanged is the same regardless of the charge of the cation To calculate equivalent weight (or gram-equivalent weight) of a given cation, the gram-molecular weight (mole) must be divided by the charge on the cation. Thus, the gram-equivalent weight of calcium (Ca2+) is 40.08/2= 20.04, half the gram-molecular weight. Hence, one mole of Ca2+ (40.08 g) would equal two equivalents of Ca2+(20.04 g), but one mole of Na+ (22.99 g) would still be equal to one equivalent of Na+ (22.99 g).
To calculate the CEC of a given zeolite, one must know the chemical formula of the zeolite. For example, using the formula of clinoptilolite listed above, note that for each formula unit, 3 Na + 3 K are exchangeable or that 6 equivalents of cations are exchangeable for each formula unit. The weight of a formula unit can be calculated by adding up the atomic weights of the constituent atoms. For the above formula, this amounts to 2774.37 g. Thus, this particular clinoptilolite would have a cation-exchange capacity of 6 equivalents per 2774.37 gram, or, recalculating, 0.00216 equivalents per gram, or 216 equivalents per 100 grams or 2.16 milliequivalents (meq) per gram. If we were to exchange the singly-charged Na and K in this formula with doubly-charged Ca, the resultant formula would be written:
Note that only half as many double-charged calcium ions are needed to balance the number of singly charged sodium and potassium ions. Calculating the CEC of the Ca-exchanged clinoptilolite yields the same value in terms of meq/g (actually very slightly more because the molecular weight of 4 calciums is slightly less than the molecular weight of 4 sodiums+ 4 potassium.)
Using CEC expressed in terms of milliequivalents (mcci) per gram (or 100 g) makes it easy to compare bow much of any cation can be exchanged by a particular zeolite, without having toworry about the charge on the cation involved. The calculated CECs of common zeolites, based on theoretical formulae, follow:
It is often desirable to express cation-exchange capacity in terms of weight percent (e.g., how many grams of ammonium ion (NH4+) could be exchanged onto 1 gram of clinoptilolite, or how many grams of copper could be removed from solution by 1 gram of chabazite). If the CEC of the zeolite (or zeolitic ore, for that matter) is known (in terms of meq/g), this is an easy calculation. The number of milliequivalents of the given cation (i.e., NH4+ or Cu2+) must be converted to grams by multiplying by the gram-equivalent weight of the cation. For singly-charged cations, this is the same as multiplying by the gram-molecular weight, but for double charged cations (e.g., Cu2+), the number of meq/g must be multiplied by half the gram-molecular weight (i.e., converting gram-equivalent weight to gram-molecular weight). Hence, 1 g of a sample of clinoptilolite-rich ore having a measured CEC of 2.00 meq/g could load (exchange) 3.67 wt. % (g/100 g) NH4+; likewise, 1 g of a chabazite-rich ore (measured CEC = 3.70 meq/g) could exchange 11.75 wt.4 % (g/100 g) Cu. The calculation is simple; merely multiply the known CEC of the zeolitic material by the milliequivalent weight (i.e., 1/1000 of the equivalent weight) of the atomic species involved. The following table lists the number of grams of various atoms taken up by one gram of zeolitic materials having the indicated CECs.
This information and these tables were prepared by F.A. Mumpton, Chairman of the International Committee on Natural Zeolites.
Hence, a zeolitic material having a measured CEC of 2.25 would theoretically be able to take up (exchange) 0.233 g of lead (Pb2+ from solution) per gram of material.
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