Removal of 137Cs from aqueous solutions using different cationic forms of a natural zeolite: clinoptilolite

Separation and Purification Technology 28 (2002) 103– 116 www.elsevier.com/locate/seppur

Removal of 137Cs from aqueous solutions using different cationic forms of a natural zeolite: clinoptilolite Abdelrahim Abusafa, Hayrettin Yu¨cel * Chemical Engineering Department, Middle East Technical Uni6ersity, Ankara 06531, Turkey Received 14 February 2001; received in revised form 25 February 2001; accepted 10 March 2002

Abstract Distribution coefficients of cesium on natural and cation-enriched (Na+, K+, NH4 + and Ca + 2) forms of clinoptilolite were measured by batch, radioactive tracer technique. The measurements were carried out for an initial cesium concentration range of 10 − 6 –10 − 1 mol/dm3 and at temperatures of 25, 40, 60 and 80 °C. Experimental isotherms evaluated from distribution coefficients were fit to Langmuir, Freundlich and Dubinin-Radushkevich (D-R)models. Of the models tested, D-R model was found to represent the isotherms better in a wider range of concentrations than either Langmuir or Freundlich model. Breakthrough behavior of cesium on natural and cation-enriched forms of clinoptilolite for a particular set of conditions were also determined in a small size column. Column parameters were evaluated using mass transfer zone concept. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Clinoptilolite; Cesium; Sorption isotherm; Dubinin-Radushkevich isotherm

1. Introduction 137

Cs is one of the most abundant radionuclides in nuclear fission products that are routinely or accidentally released [1 – 3]. It has a relatively long half-life of about 30 years and is considered as one of the most hazardous radiotoxic elements for the environment. One of the most effective methods for the treatment and disposal of radioactive wastes has been based on ion-exchange using inorganic ion-exchangers [1]. The desirable characteristics of high exchange capacity and favor* Corresponding author. Tel.: +90-312-210-26-35; fax: + 90-312-210-12-64. E-mail address: [email protected] (H. Yu¨cel).

able selectivity for some radioisotopes have made certain zeolites quite useful for the treatment of radioactive wastes. Thermal, mechanical and radiation stability are further advantages of these materials. A zeolite can also be incorporated into a cement matrix and easily stored in stainless drums which is a common practice for long term storage of radioactive waste. Radioactive wastetreatment processes utilizing zeolites have been developed and used successfully since the introduction of atomic energy installations [1]. Natural zeolites which have been considered for radioactive waste-treatment include mordenite, erionite, chabazite and clinoptilolite [2–9]. Among these, clinoptilolite has received much attention due to its widespread occurrence and high selectivity

1383-5866/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 3 - 5 8 6 6 ( 0 2 ) 0 0 0 4 2 - 4

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for137Cs, and 90Sr and was demonstrated to be effective in removing these radioisotopes from process waste waters [9]. Its metabolic similarity to K favors cesium’s uptake by plants after radiocontamination of agricultural lands as Cernobyl accident of 1986 has dramatically shown. Intake of radiocontaminated food, tea and milk is one of the main pathways for radiation exposure [10,11]. In recent years, studies on natural zeolites as potential, low cost countermeasure amendments in an attempt to reduce transfer of radionuclides including137Cs from soil to plant in radio contaminated soils are gaining importance [12,13]. Due to their high cation-exchange capacities and selectivities for Cs, Ba and Sr as demonstrated first by Ames [3] clinoptilolite and mordenite containing rocks can retard the migration of radionuclides occurring in solution as simple cations and thus are considered as potential hosts for radioactive waste disposal [14– 17]. All these applications which involve zeolites require a better understanding of behavior of different cationic forms of zeolites for sorption of trace concentration of radioisotopes. Since zeolites can be important sources and sinks of thermal energy during dehydration/rehydration and when exposed to thermal effects of radiation, sorption measurements at different temperatures may also be appropriate [18]. Clinoptilolite is a high-silica member of heulandite group of natural zeolites and occurs in abundant and easily mined, sedimentary deposits in many parts of the world [19]. However, the composition, purity and mineralogical characteristics of clinoptilolite may vary widely from one deposit to another and even within the same deposit [1]. The unit cell formula of a natural zeolite can be given as (Li, Na, K)a (Mg, Ca, Sr, Ba)d [Al(a + 2d) Sin − (a + 2d) O2n ]mH2O The unit cell of clinoptilolite is usually characterized on the basis of 72 oxygen atoms (n =36) and, 24 water molecules (m = 24). Clinoptilolite and heulandite are monoclinic zeolite minerals characterized by relatively large,

intersecting open channels of ten- and eight-membered tetrahedral rings [20,21]. The large tenmembered elliptical A ring with approximate dimensions 0.79× 0.35 nm2 and smaller eightmembered B ring with approximate dimensions 0.44×30 nm2 confine channels parallel to c axis. Type C channels parallel to a axis are also formed by eight-membered rings and connect A and B channels [21,22]. Negative charge of the clinoptilolite which comes from the tetrahedrally coordinated aluminum has been balanced by exchangeable cations. Like most zeolitic ion-exchangers, one of the characteristic features of clinoptilolite is the association of the framework charge with various types of cationic sites. The most common cations in clinoptilolite and heulandite are Na+, K+, Ca2 + , and Mg2 + . For a particular zeolite the selectivity and rate of ion-exchange is expected to be significantly influenced by the type, number and location of cations. Cation locations of dehydrated clinoptilolite for the most common naturally occurring cations have been determined [21]. Of the four major cation positions M(1)–M(4), Na and Ca appear to occupy the M(1) and M(2) sites, located at the intersections of the A and B channels, respectively with channel C. Mg and K were observed to occupy the M(4) and M(3) sites which are located at the symmetry center of the ten-membered ring channel A and eight-membered ring channel C, respectively. A unit cell contains four combined M(1) and M(2) sites, four M(3) sites and two M(4) sites. M(1)–M(3) and M(1)– M(4) are stated to be forbidden pairs and thus, in general can not be simultaneously occupied in a cell [21,22].

2. Experimental

2.1. Materials, treatments and characterization The clinoptilolite sample used in this study was from a sedimentary deposit in Bigadic¸ basin located in Western Anatolia. Western Anatolia is rich in clinoptilolite formations [23,24]. Bigadic¸ basin has one of the most important clinoptilolite occurences in Turkey with an estimated reserve of 2 billion tons [25]. The zeolite in the deposit is

A. Abusafa, H. Yu¨ cel / Separation and Purification Technology 28 (2002) 103–116 Table 1 Physical properties of clinoptilolite Apparent density, g/cm3 Solid density, g/cm3 Total pore volume, cm3/g Average pore diameter, mm BET surface area, m2/g

105

sium, magnesium, and sodium as exchangeable cations and it was desirable to prepare homoionic forms prior to Cs exchange. For this purpose, samples were treated with aqueous chloride salt solutions containing 1 mol/dm3 of Na, K, Ca, or NH4 at 60 °C for 24 h in a shaker. The solution to zeolite ratio was taken as 100 cm3 solution: g zeolite and each treatment was repeated for seven times. The cation-exchanged samples were washed with de-ionized water until a negative chloride ion test with 1% silver nitrate solution, oven dried at 50 °C and kept over saturated calcium nitrate solution to ensure a constant moisture content of samples prior to chemical analyses and exchange experiments. Comparisons of the X-ray diffraction patterns of the original and the ion-exchanged samples showed that the crystallinity of the samples were basically maintained. The original and cation-enriched forms of samples were analyzed after Na2CO3 fusion for SiO2, Al2O3, and Fe2O3 by wet chemical methods [29]. Alkaline and alkaline earths were determined after volatilazation of SiO2 with HF4 + H2SO4 by flame atomic absorption spectrophotometry and flame photometry. The water content of the samples were determined by vacuum thermogravimetry using an electrobalance (Cahn RG). Chemical analyses of the original clinoptilolite sample and its Na+, K+, Ca2 + , and NH4 + forms are given in Table 2. Unit cell compositions based on 72 oxygen are also calculated for convenience in comparisons and presented in Table 3. It should be noted that despite the exhaustive treatments used to force the clinoptilolite into its respective homoionic forms, it was not possible to achieve complete exchange for all forms. Some investigators in previous studies working with

1.3889 2.0006 0.2216 0.050 17.5

previously classified as Ca-rich clinoptilolite based on its thermal stability and silica/alumina ratio [24,25]. Ammonium exchange behavior of the samples from the same region has been extensively studied and well documented in literature [25 –27]. Its sorption properties for gases like CO2, SO2, and H2S were also explored [28]. A representative clinoptilolite sample with a clinoptilolite content of about 95% as determined by XRD and carbon dioxide adsorption capacities was used in the experiments. SEM photomicrographs showed a strongly welded structure and classical morphology of individual tabular crystals (coffin shaped) could only be seen in small cavities which are distributed randomly in the matrix of the raw material. The sample was crushed and sieved to obtain a size fraction of 60/80 mesh (ASTM E-11). Some important physical properties of the clinoptilolite sample were measured and are presented in Table 1. Apparent density, pore volume and mean pore size (\ about 6 nm) were determined by mercury porosimeter (model 9310, Micromeritics), True density was measured by a helium pycnometer (model 1320, Micromeritics) and Brunauer-Emmet-Teller (BET) area was determined by an automated N2 adsorption equipment (ASAP 2000, Micromeritics). The original sample contained calcium, potasTable 2 Chemical composition of clinoptilolite samples (weight%)

CLI Na–CLI K–CLI Ca–CLI NH4–CLI

K2 O

Na2O

MgO

CaO

Fe2O3

Al2O3

SiO2

H2O

2.90 1.18 8.69 1.88 4.36

0.45 4.55 0.65 0.40 0.36

1.17 0.75 0.60 1.47 0.44

3.46 0.28 0.20 4.05 0.20

0.62 0.66 0.66 1.12 0.82

12.84 11.68 10.83 11.02 10.69

63.98 68.05 66.52 67.73 70.01

15.37 10.72 10.52 11.42 10.20

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Table 3 Unit cell composition of the clinoptilolite samples

CLI Na–CLI K–CLI Ca–CLI NH4–CLI

K

Na

Mg

Ca

Fe

Al

Si

2.01 0.69 5.09 1.10 2.55

0.39 4.04 0.57 0.36 0.32

0.57 0.56 0.40 1.00 0.29

2.10 0.16 0.12 2.30 0.11

0.01 0.22 0.22 0.39 0.28

6.92 6.30 5.85 5.96 5.77

29.43 31.21 30.51 31.06 32.12

clinoptilolites of identical [26] and different origins [14,30,31] also noted partial exchange in similar treatments. The cations mostly involved in the exchange process are Ca and Na. K and Mg present in the parent material are reluctant to exchange with other cations, therefore Na, Ca, and NH4 exchanged clinoptilolite samples contained significant amounts of the original K and Mg. This behavior of K and Mg is probably related to their specific location and coordination to other atoms in the framework structure as noted before. Some of the non-exchangeable K and Mg may also be associated with impurities in the sample.

2.2. Procedures 2.2.1. Batch experiments In order to obtain distribution coefficients of cesium for original and Na, K, Ca, and NH4 enriched clinoptilolites, batch equilibrium experiments were carried out. In all runs, the zeolite to solution ratio was taken as 1:50 g/cm3. The initial concentrations of Cs were varied in the range from 10 − 6 to 10 − 1 mol/dm3. Radiochemically pure and carrier free 137Cs as cesium chloride was used as tracer for cesium solutions. Sufficient 137 Cs was added to ensure, after equilibration, that there was enough activity present in solution to allow it to be measured with good statistical accuracy All measurements were carried out in duplicates. Two aliquots of the original solution were kept aside for later counting with the equilibrium solution aliquots. The clinoptilolite samples and cesium solutions (5 cm3) spiked with 137 Cs were contacted in screw-cap polycarbonate centrifuge tubes with mixing on a thermostat

shaker (Julabu SW 20C). At the end of a contacting period of about 48 h which was much longer than equilibration times determined for typical conditions by kinetic experiments (see Fig. 1 in Section 3), the liquid and solid phases were separated by a centrifuge (SED 30) at about 5000 rpm and then 2 ml sample was taken from the supernatant for activity measurements. The samples were counted on the gamma detection system (EG-ORTEC Multichannel Analyzer) using a well type Nal(TI) detector (Teledyne isotopes). The output of the analyzer was processed using Maestro II software. The mean values of activity measurements for duplicate samples were used in the calculations. The reproducibility of the activity measurements were good and in no case the difference between duplicate measurements exceeded the statistical error in counting. Distribution coefficients and final equilibrium concentration of cesium were calculated from the mean activity data as given below:

Fig. 1. Exchange kinetics for Cs+ “Na+ giving Cs loading on Na– CLI as a function of time for different initial concentrations of cesium.

A. Abusafa, H. Yu¨ cel / Separation and Purification Technology 28 (2002) 103–116

Fig. 2. Distribution coefficient of cesium for different cationenriched forms of clinoptilolite at 25 °C.

107

2.2.2. Column experiments The fixed-bed ion-exchange experiments were carried out in a column made of a glass tube of 0.5 cm in diameter and 10 cm in height with a jacket for water circulation. About 1 g of each original and cation-enriched forms of clinoptilolite had been placed in the column. The particles had a mesh size of 60/80 mesh and packed height was 7.2 cm. The bed porosity was determined to be 40%. The temperature of the column was kept constant by circulating water at 25 °C through jacket of the column.137Cs spiked CsCl solution containing 1 meq/l Cs was passed downward through the column at a constant flow rate of 5 ml/min by the help of a peristaltic pump (Watson Marlow, model 505 Du). Effluents from the column outlet were collected periodically by a fraction collector (Intee 5512) for concentration measurements. Concentrations were determined by measuring g-activity of 2 ml of aliquots of effuent solution at predetermined throughput volumes (  50 ml) in the same radiation detection system used for batch experiments.

3. Results and discussion

3.1. Batch experiments: distribution coefficients

Fig. 3. Distribution coefficient of cesium for different cationenriched forms of clinoptilolite at 40 °C.

Rd = Concentration in zeolite/ concentration in solution = [(A0 − Af)/Af]V/m Cf = C0Af/A0

(1) (2)

where, Rd =distribution coefficient, ml/g; V = volume of solution, ml; A0 =initial activity of solution, cpm/ml; Af =final activity of solution, cpm/ ml; m =mass of exchanger (hydrated), g; C0 = initial Cs concentration, meq/mL; Cf =final Cs concentration, meq/ml.

Fig. 1 shows the results of some preliminary kinetic runs given as cesium loading on clinoptilolite q(meq/g) with respect to time for several initial concentrations of cesium. The results indicate that equilibrium is established in about 4 h. The distribution coefficients calculated using this data did not differ by more than 1% over a time interval 4–72 h. This also gives an idea about the reproducibility of measurements. Distribution coefficients, Rd as a function of initial cesium concentration, C0 ( before exchange started) for different cation-enriched forms of clinoptilolite and at different temperatures are presented in Figs. 2–5. The data are replotted in Figs. 6–9 to illustrate the effect of temperature on composition dependent distribution coefficients for each cation-enriched form of clinoptilolite. The use of initial cesium concentration rather than equilibrium concentration as abscissa in

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these figures allow a better comparison to be made on the efficiency of different cationic forms for sorption of cesium under identical initial composition of solutions. Equivalent plots involving equilibrium concentrations would have similar trends as these curves. To explain the different behaviors of distribution coefficients observed, it may be convenient to divide the concentration range into two regions: relatively low concentration region (C0 10 − 4 mol/dm3) and a relatively high concentration region (C0 10 − 4 mol/dm3). Fig. 6. Distribution coefficient of cesium of on Na – CLI at different temperatures.

Fig. 4. Distribution coefficient of cesium for different cationenriched forms of clinoptilolite at 60 °C.

Fig. 5. Distribution coefficient of cesium for different cationenriched forms of clinoptilolite at 80 °C.

Fig. 7. Distribution coefficient of cesium of on K – CLI at different temperatures.

Fig. 8. Distribution coefficient of cesium of on Ca – CLI at different temperatures.

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In the relatively high concentration region all cation-enriched forms of clinoptilolite behave similarly and Rd decreases as initial cesium concentration increases, a pattern that points to site heterogeneity. Of course, such a result is not unexpected in view of the presence of multi-sites for different cations of clinoptilolite as described above. In this region, distribution coefficients increases in the order Rd,Na – CLI \Rd,NH4 – CLI \ Rd,K – CLI. A simple model for cation selectivity was introduced by Eisenman [32] for glass electrodes and extended to zeolites by Sherry [33]. This model assumes that the preference of the exchanger for one cation over another depends on whether the difference in their hydration free energies or their Coulombic energies of interaction with the fixed anionic exchange sites predominates. In Eisenman’s model the changes in water content and entropy of the ions are neglected. On the basis of this theory, a diagram of free energy of exchange of alkali metal ions for Na+ as a function of anionic field strength can be calcutated. A certain field corresponds to a certain selectivity series. Heulandite and clinoptilolite are low field strength zeolites (the Si/Al ratio is relatively high) for which the theory predicts a selectivity series of type I (Cs+ \Rb+ \K+ \Na+ \Li+) which has been confirmed experimentally [3– 5] for clinoptilolite. In the high concentration range, it is observed

109

that the selectivity sequence in terms of distribution coefficients is in accordance with Eisenman’s model and previous experimental data. However, in the low concentration region, different sequences which also depends on temperature is observed. For instance, in this region Rd,NH4 – CL \ Rd,Na – CLI at 25, 40, and 60 °C, but at 80 °C Rd,NH4 – CLI B Rd,Na – CLI. It appears that NH4 –CLI provides a more favorable environment to cesium than other cationic forms at temperatures less than about 60 °C. Previous studies in literature may provide some support for this hypothesis. Yang and Armbruster [34] compared crystal structures and cation/water locations of K+ and NH4 + exchanged heulandite by single-crystal Xray diffraction. Important differences in the cation distributions and consequently distributions of channel H2O molecules were observed between these forms. Furthermore, they found that cell dimensions of these cationic forms are significantly different leading to a cell volume of 2127 A3 for NH4-exchanged versus 2103 A3 for K-exchanged heulandite. This observation is in accordance with the fact that NH4 + requires more space due to its hydrogen bonding ability than K+ and less H2O molecules can be accommodated in NH4 + -exchanged heulandite (19 H2O p.f.u.) compared to the K+-exchanged one (16 H2O p.f.u). Smyth et al. [35] determined the crystal structure of a natural clinoptilolite sample containing mainly sodium, potassium and calcium

Fig. 9. Distribution coefficient of cesium of on NH4 – CLI at different temperatures.

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and a Cs-exchanged sample by XRD. They observed that there is not a simple one-for-one cation-exchange taking place, but a complete rearrangement of exchangeable cation sites and waters of hydration in the exchange process. Furthermore, they found that Cs-exchanged sample contained fewer water molecules than natural sample implying that a small amount of water is lost from the zeolite concomitant with the exchange. Tarasevich et al. [36] evaluated thermodynamic constants for Li, NH4, K, Rb, and Cs exchanged into previously Na exchanged clinoptilolite using calorimetric techniques. Their results including heat of immersion of water on Cs loaded clinoptilolite indicated that Cs cations occupy a variety of crystallographic positions not only different in type of rings (ten- and eight-) but also positions in a ring. These authors argue that the exchange enthalpies control the observed equilibrium constant. They reported a value of −23.7 kJ/mol for Cs+ “Na+exchange and they suggest that large cations like Cs interact with more O atoms on the channel surface. On the basis of these studies it may be argued that Cs and NH4 occupy structurally similar locations in clinoptilolite and heulandite which are different from the other cations present in structure and contain less amount of water molecules because of their larger size. It is possible that a slightly distorted 8-membered channel in NH4 – CLI allow for the initially coming Cs ions an easier access for the most favorable sites and a closer interaction with the framework oxygen atoms. This would explain the very large distribution coefficients of cesium for NH4 – CLI at the lowest concentrations and it’s gradual decrease as less energetically favorable sites are occupied as loading of cesium on clinoptilolite increases. For sufficiently low concentrations, one would expect Rd become independent of composition and total ionic concentration (Henry’s law region). This condition is approximately fulfilled by Cs+ “Ca2 + and Cs+ “K+ exchanges in the low concentration region in the temperature range studied. In contrast, Cs+ “Na+ and Cs+ “ NH4 + systems behave quite peculiarly in the low concentration range. For the former system Rd increases as concentration increases at all temper-

atures. For the latter Rd decreases with concentration at temperatures 25, 40 and 60 °C and it increases at 80 °C. The effect of temperature on distribution coefficient appears to be more pronounced in the low concentration range. For Cs“ Na exchange Rd decreases slightly with increasing temperature in the range 25–40 °C then increases in the range 40–80 °C which is more pronounced toward the lowest end of concentration range (Fig. 6). For Cs+ “ Ca2 + exchange, distribution coefficient, Rd decreases monotonously as temperature increases (Fig. 8). This indicates the exothermic nature of the exchange process for this system. For Cs+ “ NH4 + exchange Rd dcreases with temperature at 25, 40 and 60 °C and decreases dramatically at 80 °C (Fig. 9). The reason for this dramatic decrease is not known at this stage. Tomazovic and Ceranic [37] investigated thermal properties of the NH4-clinoptilolite in detail using several techniques and on the basis of their studies one would not expect, at the temperature employed (80 °C), thermal decomposition of NH4 + to ammonia gas leaving H-clinoptilolite behind. Whether this dramatic decrease in Rd is due to a partial collapse in already distorted channels of NH4 + –CLI requires further work for justification. Several common sorption isotherm models including Langmuir, Freundlich, and DubininRadushkevich (D-R) were considered to fit the data for sorption of cesium on different cationic forms of clinoptilolite. These isotherm models which were originally derived mainly for gas-solid systems have also been used successfully to represent sorption of solutes in liquid– solid systems [38–42]. Among the models tested D-R model was found to give a relatively better fit to the experimental equilibrium data in a wider range of concentrations for all cationic forms of clinoptilolite compared to other models. This was reflected in the higher values of statistically important parameter, linear correlation coefficient (LCC) of the linearized form of this model compared to other two models. It must be emphasized that a good fit of data to an isotherm model is not a sufficient test for its validity and furthermore the interpretation of isotherms for liquid–solid system is expected to be more difficult compared to

A. Abusafa, H. Yu¨ cel / Separation and Purification Technology 28 (2002) 103–116 Table 4 Parameters of D-R isotherm model T (°C)

q* (meq/g)

K×10−9 (mol2/J2)

LCC

Na–CLI

25 40 60 80

1.70 2.90 3.00 2.80

4.0 4.0 4.0 3.0

0.950 0.980 0.980 0.985

K–CLI

25 40 60 80

1.36 1.32 1.31 1.37

4.0 4.0 3.0 4.0

0.994 0.997 0.997 0.998

Ca–CLI

25 40 60 80

1.85 1.63 1.76 2.50

4.0 4.0 4.0 4.0

0.990 0.995 0.996 0.997

NH4–CLI

25 40 60 80

1.84 1.69 1.67 1.66

4.0 4.0 3.0 3.0

0.996 0.997 0.998 0.995

gas-solid systems. But still, in addition to its practical usefulness, an isotherm model which represents equilibrium data well might give some insight into the nature of sorption. In this study, the Langmuir plot which assumes uniform sites on solids was found to be applicable over only a rather limited segments of equilibrium cesium concentration probably because of energetically heterogeneous nature of sorption sites of clinoptilolite. Freundlich model describes a sorption isotherm from an ideal solution by an energetically heterogeneous set of sorption sites with the sorption energy varying exponentially [40]. It was found that Freundlich isotherm fit the cesium data well for some segments in the intermediate concentrations region for some cationic forms but become less satisfactory compared to D-R when a wide range of concentrations is considered. Sokolowska and Szczypa [39] have shown the relationship between the D-R equation and other type of isotherms. D-R isotherm which assumes a Gaussian distribution of energy sites is given by equation: q = q*exp(− KE 2)

(3)

where, E= RT In(1 + 1/C), Polanyi potential, J/

111

mol; C= equilibrium concentration of cesium in solution, meq/g; q= equilibrium cesium concentration on clinoptilolite, meq/g; R= gas constant, J/Kmol; T=temperature, K; K= constant related to the ion sorption energy, mo12/J2; q*=maximum cation-exchange capacity, meq/g. This equation can be linearized by logarithmic transformation as: ln q= ln q*−KE 2

(4) 2

A plot of ln q vs. E should give a straight line which allows the estimation of q* and K from the intercept and slope of this line. Linearized forms of D-R plots of the cesium exchange on different cation-enriched forms of clinoptilolite are given in Figs. 13 and 14. Values of q* and K evaluated from the intercepts and slopes of these curves are presented in Table 4. If a very small subregion of the sorption surface is assumed to be uniform in structure and energetically homogeneous and Langmuir isotherm is chosen as the local isotherm, then the mean sorption energy, E is given by [43,44] E= (2K) − 1/2

(5)

The magnitude of E is useful for estimating the type of sorption reaction occurring. In this study, mean sorption energies for cesium sorption on different cation-enriched forms of clinoptilolite was found to vary within a narrow range, 11.2– 12.9 kJ/mol which are in the range of values, 8– 16 kJ/mol reported by Helfferich [45] for ionexchange reactions.

3.2. Breakthrough cur6es: column performance Breakthrough curves of original and cation-enriched forms of clinoptilolite for the conditions stated previously are shown in Fig. 14. The breakthrough characteristics for the different cationic forms of clinoptilolite follow the trends observed with batch distribution coefficients. Original clinoptilolite and Ca–CLI which have the lowest distribution coefficients showed a much earlier breakthrough compared to Na–CLI, K –CLI and NH4 –CLI. For the quantitative interpretation of breakthrough behavior of samples one of the simplest

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112

methods, mass transfer zone (MTZ) concept was applied [46,47]. In this method it is assumed that the exchange process occurs in a restricted region of the bed which is called MTZ, lying between an equilibrium bed zone and unused bed zone. In constant pattern conditions, MTZ of constant height, hz is part of the bed in which the concentration changes from CB to CE where CB is breakthrough concentration and CE is the exhaustion concentration. If tz is the time required for the MTZ to move its own height, after the zone has been established and tE is the time required for MTZ to establish itself and move out of the bed, then height of MTZ is given by the relationship





tz h z = U z tz = h tE −tF

(6)

where hz = height of the MTZ; h =height of the bed; tF = time interval between breakthrough and exhaustion of the bed; tZ and tE are given by the following relationships. V −VB tz = E U0A

(7)

V tE = E U0A

(8)

where VB =effluent volume collected up to the breakthrough point; VE =effluent volume collected up to exhaustion of the bed; U0 =the superficial velocity; A= crosssectional area of the bed. The relationship between tF and tz is given approximately by equation tF = (1− f )tz

(9)

where f is fractional residual capacity of exchange Table 5 Column performance for cesium uptake Form

QB (meq/g)

QE (meq/g)

hz (cm)

p

Na–CLI NH4–CLI K–CLI Ca–CLI CLI

1.40 1.22 1.10 0.41 0.44

1.52 1.50 1.46 0.98 0.79

0.2 2.3 4.3 10.5 9.0

0.92 0.81 0.75 0.42 0.62

zone which can be evaluated from experimental breakthrough curves as

&

f=

VE

(C0 − C)dV

VB

C0(VE − VB)

(10)

The total (equilibrium) exchange capacity per unit amount of zeolite, QE can be calculated by evaluation of the area above the S-curve up to the exhaustion.

&

QE =

VE

(C0 − C)dV

0

zhA

(11)

where z=packed bed density; A= cross-sectional area of bed. The breakthrough capacity of the bed per unit amount of zeolite, QB may be calculated from the relationship

&

QB =

VB

(C0 − C)dV

0

zhA

(12)

Then, the degree of saturation or column efficiency, p can be calculated as: p=

QB QE

(13)

Column parameters for original and cation-enriched forms of clinoptilolite obtained by using Eq. (6)–Eq. (13) are presented in Table 5. It is observed that equilibrium capacities of Na-, NH4, and K-clinoptilolites calculated by Eq. (11) are very close and around 1.5 meq/g hydrated zeolite. However, these values are significantly lower than the ultimate capacity calculated from aluminum content of samples (varies in the range 2.2–2.4 meq/g). There are also significant discrepancies between capacity values determined by the fit of D-R equation to the equilibrium data (Table 4) and those determined by column runs (Table 5) for similar conditions. This is mainly due to deviation of experimental data from the linearized form of D-R model for high concentrations of cesium (Figs. 10–13) Since the capacity values are calculated from the intersection of the straight line portion with the ordinate they are subject to

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113

Fig. 10. D-R plot of cesium sorption on Na – CLI.

Fig. 11. D-R plot of cesium sorption on K – CLI.

considerable uncertainty. Highest column efficiency (92%) was obtained for Na– CLI. This is reflected by its smallest MTZ height (0.2 cm). Column efficiencies of NH4 and K forms (81 and 75%, respectively) are somewhat lower than that of Na form but can still be considered as satisfactory. With 42% column efficiency which is also lower than that of original clinoptilolite (Ca and K rich zeolite) Ca enriched clinoptilolite has the poorest column performance. This is probably

due to unfavorable exchange kinetics in addition to relatively low distribution coefficents as demonstrated by batch experiments. Of course this manifests itself as the longest MTZ height ( 10 cm) among all cationic forms studied.

4. Conclusions Distribution coefficients of cesium on different

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cationic forms of clinoptilolite were determined by batch, radioactive tracer technique in a wide concentration range. All cationic forms have decreas-

Fig. 12. D-R plot of cesium sorption on Ca –CLI.

ing values of distribution coefficients in the relatively high concentration range reflecting the heterogeneous nature of host clinoptilolite for incoming/outgoing cations. In this region, the sequence of distibution coefficients are basically in accordance with the thermodynamic sequence determined previously. However, in the trace concentration region, some cationic forms of clinoptilolite samples behave quite peculiarly. For example, in trace region, ammonium enriched zeolite has the higher distribution coefficients for cesium than sodium enriched clinoptilolite at temperatures less than about 60 °C. We hypothesize that this is due to specific cation locations for different cations and also their influence on the crystal structure. Obviously, more experimental and thereotical work are necessary to test this hypothesis. Column studies confirm batch measurements and all cationic forms except Ca enriched form show satisfactory breakthrough capacities. Relatively poor performance of Ca form is thought to be due both low distribution coefficients and unfavorable exchange kinetics.

Acknowledgements

Fig. 13. D-R plot of cesium sorption on NH4 –CLI.

This work is supported by TUBITAK under project no Misag-88 and METU under project no AFP 97-03-04-01. We are grateful to Dr Gu¨ ngo¨ r Gu¨ ndu¨ z and Dr Inci Go¨ kmen for their valuable criticisms and help throughout the study.

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Fig. 14. Breakthrough curves of different cation-enriched forms of clinoptilolite(60/80 mesh) for the removal of 137Cs at Influent Cs Concentration of 1.0 meq/l, and at 25 °C.

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