Unheated and Heated Batch Methods in Ion Exchange of Clinoptilolite Tevfik Ünaldı

It is well known that natural zeolites consist of aluminia and silica tetrahedra which, bound in a definite way, include crystal structure vacancies, channels and pores [1,2]. About 40 natural zeolites have been identified during the past 200 years; the most widespread are analcime, chabazite, clinoptilolite, erionite, ferrierite, heulandite, laumontite, mordenite, and phillipsite. More than 150 zeolites have been synthesized; the most common are zeolites A, X, Y, and ZMS-5. Clinoptilolite has the structural formula (Na,K)6(Al6Si30O72). 20H2O, characterized by two different rings, which are 8(3,3x4,6 A) T and 10(3,0x7,6 A) T on the abplane, and channels with rings of 8(2,6x4,7 A) T on the bc-plane. As shown in Figure 1, there are exchangeable cations of Na+, K+, Ca2+ and Mg2+ through the channels

ml of deionized water.Five grams of 300-mesh clinoptilolite were poured into each solution and stirred gently, and then put in suspension for 72 hours at room temperature.Subsequently, the clinoptilolite-solution suspension system was filtered.After filtration, the clinoptilolite was washed eight times with deionized water at 98°C and dried at 110°C for 16h. [9,10] ure 1.Cation positions in clinoptilolite. [3]

Heated batch methods
Ion exchange is conducted using a heated/cooled back magnetic stirrer system.A 100 ml solution was prepared by mixing 5 g of 300 mesh clinoptilolite with deionized water kept at 98°C for 2h.
The clinoptilolite-solution suspension system was then filtered.After filtration, the clinoptilolite was washed eight times with deionized water at 98°C and then dried at 110°C for 16 h. [11,12]

Ion exchange
The process of ion exchange occurs between the A Z A cation in solution and B Z B * cation in the zeolite, and can be formulated as follows: ZB A Z A + ZA B Z B *  ZB A Z A * + ZAB Z B where ZA and ZB show the valences of the cations, and A Z B * and B Z B * show the cations in the zeolite structure.
Ion-exchange reactions are stoichiometric, graphical representations of equilibrium concentrations of exchangeable ions in both solutions; the structure of zeolite may be ascertained from ion-exchange isotherms.
Before an ion-exchange isotherm may be obtained, equilibrium of ion exchange must be reached.In zeolites A, X, and Y with low framework densities, the equilibrium of exchange between one valence ions (such as Na + and K + ) is obtained in approximately one week.In zeolite structures with high framework densities, the equilibrium of exchange among high valence ions is obtained in a few months.
After the time of equilibrium is defined, this procedure could be utilized in order to plot an isotherm.Zeolites react with a solution containing ions of both A Z A and B Z B. Although the relative amounts of ions A Z A and B Z B might vary, solutions must have a constant total normality (N).According to the condition of equinormality, total ionic intensities of any solution in the system of the zeolite/solution must be constant before and after ionexchange reactions.
The ionic intensity of any solution is where Ci are the concentrations of opposing ions in ion-grams per liter, and Zi are the valences of opposing ions.
Because of the distribution of A Z A and B Z B between the phases, the solution and solid phases in equilibrium must be analyzed.Thus, a plot of the equivalent fraction of ion in solution (As) versus equivalent fraction of the same ion in zeolite (AZ * ) isotherm may be obtained.
The ion-exchange isotherm indicates the relative preferences of any ion within the zeolite structure.Besides, the separation factor of ion A within the zeolite structure is α = (AZ * /BZ * )(mB/mA) where AZ * ve BZ * are equivalent fractions of ions A and B in zeolite, respectively, and mA and mB are the concentrations of ions in solutions in mole/liter.The total of the equivalent fractions of AZ * and BZ * must equal 1.On the basis of ion selectivity, if α >ZA/ZB , the zeolite prefers A Z A ions; if α=ZA/ZB, the zeolite has no preference; and if α < ZA/ZB , the zeolite prefers B Z B ions. [13,14] 2.4.Ion-exchange rate XRF analyses were conducted on 0.1N-, 0.5N-and 1N-modified forms of solid-phase Naclinoptilolite, and especially the values of exchangeable and other cations differed greatly from values of the natural form (Table 1).The numbers of atoms in the unit cell were calculated with the knowledge that the unit cell includes 72 oxygen atoms.The numbers of atoms calculated and the following formula were used: where Xform = the ion-exchange rate of the forms and the number of atoms in the unit cell of the same form, and Anatural = the number of atoms in the unit cell of the natural form.

Rate of ion selectivity
The rate of ion selectivity, as termed by us, is different from the ''ion selectivity'' of the ionexchanged forms; this rate is calculated from the percentages of ions in the structure, and thus is similar to the rate of ion exchange.For Na + , K + , Ca 2+ and Mg 2+ (exchangeable cations) forms, this quantity may be calculated using αform = [(Aform/Anatural) -1] x100 and for non-exchangeable cations such as Co 3+ , Cd + , Cr 3+ , Ag + forms using αform = Aformx100 where αform is the ion-selectivity rate of the ionic form, and Aform and Anatural are the numbers of atoms in the unit cells of the ion-exchanged form and the natural form, respectively.

Rate of ion exchange
The results of chemical analyses and the numbers of atoms in the unit cells of natural and Na + , K + , Ca 2+ , Mg 2+ , Co 3+ , Cd 2+ , Cr 3+ and Ag + modified forms of clinoptilolite are given in Tables 1 and 2. As shown in the chemical formula of clinoptilolite, the numbers of atoms in the unit cell were calculated with the knowledge that the unit cell includes 72 oxygen atoms (Table 2).Tables 3 and 4 were derived from data given in Table 2. Ion-exchange rates and the ordering of ion-exchange rate for unheated and heated methods applied to Na-clinoptilolite from Bigadiç-Balıkesir (Turkey) are given in Tables 3 and 4.   The order of ion-exchange rate of Na + and K + forms is constant under both unheated and heated conditions.On the other hand, high ion-exchange rates occur under the effects of heating.The ion-exchange rate order of Ca 2+ , Mg 2+ , Co 3+ , Cd 2+ , Cr 3+ and Ag + forms changes upon heating, plus Fe 3+ is leached from the structure.Although iron occurs as Fe 3+ in the general order, Fe 2+ was depleted instead of Fe 3+ because Fe 3+ cannot be depleted from the structure of clinoptilolite.Generally speaking, the ion-exchange process acts more on the surface of clinoptilolite than on its inner sites. [15]nerally, the ion-exchange rate of the cations increases with increase in normality via application of both unheated and heated batch methods.Ion-exchange rates increase via the heating method compared to the unheated one.Forced-ion exchange occurs using the heated batch method as compared to natural ion exhange via the unheated method.The ion exchange of cations is controlled by cation valence, cation radius, ionization potential and the location of cations within pores.
The weak connection of the +1 valence of the Na + cation to the structure of 10T is related to its low ionization potential, resulting in leaching -with a high ion-exchange rate-from its structure (Figure 1).The Mg 2+ cation exhibits low depletion due to its having symmetrical binding to the structure of 8T, despite having the lowest ionization potential.Ca 2+ is more depleted than Mg 2+ because of the unsymmetrical binding of Ca 2+ to the structure, although the ionization potential of Ca 2+ is higher than that of Mg 2+ .The K + cation is strongly bound between the 8T and 10T structures; therefore, it is only slightly depleted.5. Ionic radii and ionization potentials of exchangeable cations. [16]

Table 4 .
Ordering of ion-exchange rate for unheated and heated methods for Na-clinoptilolite from Bigadiç-Balıkesir (Turkey)