A New Method for the Selective Removal of Cadmium and Zinc Ions from Aqueous Solution by Modified Clinoptilolite

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1 487 A New Method for the Selective Removal of Cadmium and Zinc Ions from Aqueous Solution by Modified Clinoptilolite Saeed Hajialigol 1, M.A. Taher 1 and A. Malekpour 2* (1) Chemistry Department, Science Faculty, Shahid Bahonar University, Kerman, Iran. (2) Jaber Ibn Hayyan Research Laboratory, Atomic Energy Organization of Iran, Tehran, Iran (Received 6 August 26; accepted 12 October 26) ABSTRACT: Natural and modified clinoptilolite were used to remove zinc and cadmium ions from aqueous solution. The raw material was characterized by XRD and XRF analysis. Clinoptilolite was modified with benzyldimethyltetradecylammonium chloride (BDTA) to increase the adsorption of neothorin [2-(2-arsenophenylazo)chromotropic acid disodium salt, C 1 H 11 AsN 2 Na 2 O 11 S 2 ]. All experiments were undertaken using a continuous method. The ultimate goal of these studies was the selective removal of trace amounts of Cd(II) and Zn(II) ions from aqueous solution using a modified form of clinoptilolite. The results obtained showed that Cd(II) and Zn(II) ions were adsorbed quantitatively onto modified clinoptilolite over the respective ph ranges of and The influence on the adsorption process of various parameters such as the ionic concentration, the flow rate, the particle size, the ph value and the presence of other cations was studied to obtain the optimum conditions. Although clinoptilolite and its surfactant-modified form were not capable of the selective adsorption of the cations studied, on the basis of the results obtained it was possible to selectively remove Zn(II) and Cd(II) ions from aqueous solution by modified clinoptilolite using a two-step process, i.e. initial treatment with BDTA followed by treatment with neothorin. INTRODUCTION The release of heavy metals into the environment is a potential threat to water and soil quality as well as to plant, animal and human health (Wingenfelder et al. 25). Zinc is essential for humans and deficiency states with clinical abnormalities have been identified (Caussy et al. 23). This element plays an essential role in cellular systems and enzymes (Shahwan et al. 25). Zinc is commonly found in contaminated soils, sediments, and wastewater and groundwater streams. Even though zinc is an essential requirement for a healthy body, too much zinc can be harmful. Excessive absorption of zinc can also suppress copper and iron absorption. On the other hand, the free zinc ion in solution is highly toxic to plants, invertebrates, and even vertebrate fish. Environmental studies of cadmium as a cumulative toxic metal have increased in number recently. Kidney, liver, lung and pancreas are the main human organs for the accumulation of cadmium. Cadmium toxicity may be manifested by a variety of syndromes and effects including *Author to whom all correspondence should be addressed. amalek@entc.org.ir.

2 488 S. Hajialigol et al./adsorption Science & Technology Vol. 24 No renal dysfunction, hypertension, hepatic injury, lung damage and teratogenic effects (Taher et al. 25). Numerous processes have been established for the removal of heavy metal ions from aqueous solution. Of these, ion exchange, precipitation, phytoextraction, ultrafiltration, reverse osmosis and electrodialysis have been employed to the greatest extent. The importance of finding alternative lowcost materials for the removal of heavy metals has been emphasized recently (Erdem et al. 24). Various treatment processes are available with ion exchange being considered as cost-effective, especially if low-cost ion-exchangers such as zeolites are used (Bailey et al. 1999). Zeolites are naturally occurring hydrated aluminosilicate minerals. They belong to a class of minerals known as tectosilicates. Most common natural zeolites are formed by the modification of glass-rich volcanic rocks (tuff) by treatment with fresh water in playa lakes or by seawater (Badillo-Almaraz et al. 23). Zeolite structures consist of three-dimensional frameworks of SiO 4 and AlO 4 tetrahedra. The aluminium ion is small enough to occupy the position in the centre of the tetrahedron of four oxygen atoms, and the isomorphous replacement of Si 4+ by Al 3+ produces a negative charge in the lattice (Erdem et al. 24). Clinoptilolite is the most abundant natural zeolite and has the chemical formula (Na 2,K 2,Ca) 3 Al 6 Si 3 O 72 24H 2 O (Breck 1974). Its characteristic tabular morphology formed by open channels of 8- and 1-membered rings provides an open reticular structure with easy access (Mondale et al. 1995). Considerable research has been devoted to characterizing the chemical, surface and ion-exchange properties of clinoptilolite (Mondale et al. 1995; Ackley and Yang 1992). The ion-exchange capacity of natural zeolite (clinoptilolite) towards inorganic cations has been investigated by many authors (Haggerty and Bowman 1994; Joshi and Mohan 1983; Blanchard et al. 1984). As seen from the literature, zeolites can be used for the removal of some heavy metal ions from wastewater. Clinoptilolite samples from different regions show different behaviours in ion-exchange processes. The object of the present work was to evaluate the adsorption of trace amounts of Cd(II) and Zn(II) ions from wastewater using a modified form of clinoptilolite (with BDTA and neothorin). Thus, the optimum conditions necessary to achieve the maximum removal of these cations has been determined and the best contact time, particle size and flow rate established. The influence of ph on the adsorption of Zn(II) and Cd(II) ions as well as the effect of some interfering species has also been evaluated. EXPERIMENTAL The natural zeolitic tuff (clinoptilolite) examined in this work originated from the Semnan region in the centre of Iran. The structure of the as-synthesized material was determined by X-ray diffraction (XRD) methods which were undertaken using a D8 Advance Bruker powder diffractometer employing Cu Kα (λ = 1.54 Å) radiation. The clinoptilolite tuff was ground and sieved to a size range of mm. Prior to use in experiments, the zeolite was treated with 1 ml of a 1 M solution of a sodium acetate/acetic acid buffer (ph 5) and then rinsed with water to remove any residual carbonate. The pretreated minerals were dried at 333 K overnight and stored in polyethylene containers. The ammonium ion has a high affinity for clinoptilolite and so it can be used to replace other cations. Thus, H-clinoptilolite was prepared from the ammonium form of clinoptilolite by calcination at 653 K for 2 h. The quaternary amine, BDTA (C 23 H 42 ClN 2H 2 O), of 98% purity as purchased from Merck, was used for modifying the zeolite surface. Such modification was performed as follows: 25 ml of a BDTA solution (5 mmol/l) was added to clinoptilolite, the resulting suspension shaken at 298 K for 24 h and the adsorbent obtained dried at K.

3 Selective Removal of Cd(II) and Zn(II) Ions from Aqueous Solution by Modified Clinoptilolite 489 AVarian model SpectrAA 22 atomic absorption spectrometer was used for measuring Zn(II) and Cd(II) ion concentrations employing an air/acetylene flame. The instrumental parameters were adjusted to the manufacturer s recommendations (see Table 1). Standard Cd(II) and Zn(II) ion solutions were prepared in distilled water from Cd(NO 3 4H 2 O and Zn(NO 3 4H 2 O (Merck). A.1 mol/l solution of neothorin was similarly prepared in distilled water, whilst a buffer solution of ph was prepared by mixing the appropriate ratio of a.1 mol/l solution of acetic acid and a.1 mol/l solution of sodium acetate. Various metal cations were used to study their interference in the adsorption process. The final modified form of clinoptilolite was prepared according to the following procedure: 1. g of the BDTA-modified clinoptilolite was added to a glass column (5 mm height 5 mm i.d.) equipped with a Teflon tap. Then 4 ml of a.1 mol/l solution of neothorin in distilled water was passed through the column at a flow rate of 1 ml/min. Before sample loading, the column was preconditioned by passing a buffer solution through it. Sample solutions of Cd(II) ions (1.5 mg/l) and Zn(II) ions (.5 mg/l) were prepared and their ph values adjusted by adding appropriate amounts of buffer solution. Then, 5 ml of each solution was added individually to the neothorin-loaded column. Adsorption of Cd(II) and Zn(II) ions on the column was determined using the atomic absorption spectrometer. The corresponding distribution coefficients deduced from the column experiments were then calculated using the equation: K d = ( Ci Cf) C where C i and C f are the initial and final concentrations of the solution (mg/l), V is the volume of the initial solution (ml) and m is the mass of zeolite employed (g). f V m (1) RESULTS AND DISCUSSION X-Ray diffraction showed that clinoptilolite was the only zeolitic material present in the sample (Figure 1), the main mineralogical impurities being quartz, albite and calcite. Elemental analysis (see data in Table 2) showed that the raw material had an Si/Al ratio of The column experiments were undertaken using 5 ml of wastewater solutions which included 7.5 µg Cd(II) and 2.5 µg Zn(II) ions (1.5 mg/l and.5 mg/l, respectively). The initial experimental results are shown in Table 3. In addition, in the column method, the influence of the sample flow TABLE 1. Instrument Settings Setting Element Cd Zn Wavelength (nm Lamp current (ma) 4 5 Slit width (nm).5 1 Acetylene flow rate (l/min) Air flow rate (l/min)

4 49 S. Hajialigol et al./adsorption Science & Technology Vol. 24 No Intensity (a.u.) θ (degrees) Figure 1. XRD spectrum of natural clinoptilolite and comparison with database reference. TABLE 2. Elemental Analysis of Natural Clinoptilolite Chemical constituent Amount (wt%) SiO Al 2 O CaO 2.32 Na 2 O 1.87 MgO 1.31 TiO 2.21 Fe 2 O Loss on ignition 9.78 rates were investigated and the zeolite particle sizes were optimized to achieve the maximum adsorbed amount of neothorin and a suitable flow rate. Under the best conditions,.4 mmol neothorin was taken up by each gram of the surfactant form of clinoptilolite. The surfactant actually plays an intermediate role in the adsorption of neothorin. Thus, when neothorin solution is brought into contact with the natural form of clinoptilolite, it is not adsorbed on the zeolite surface. However, after modification with surfactant (BDTA), a noticeable increase was

5 Selective Removal of Cd(II) and Zn(II) Ions from Aqueous Solution by Modified Clinoptilolite 491 TABLE 3. Initial Experimental Results Property Cation Cd(II) Zn(II) Initial concentration (mg/l) Final concentration (mg/l) Adsorption (mg cation/g zeolite) K d (ml/g) observed in the adsorption of neothorin. This behaviour was followed by observing the UV spectrum of neothorin solution (shown in Figure 2) before and after passage through the zeolite column, when a decrease was observed in the neothorin concentration in the presence of the surfactant-modified form of clinoptilolite. Initial experiments with natural clinoptilolite showed very little adsorption of neothorin on the zeolite surface, but after modification with BDTA this increased up to a value of.4 mmol/g (~ 5-times increase). Although BDTA and other surfactant molecules are incapable of entering the channels and pores in clinoptilolite, one side of this large molecule (polar side) can be exchanged with ionic sites on the clinoptilolite surface (Haggerty and Bowman 1994). After such modification of clinoptilolite with BDTA, the external surface of the zeolite becomes organophilic and thereby capable of adsorbing neothorin as a selective ligand for Cd(II) and Zn(II) ions. Influence of clinoptilolite particle size on the adsorption of Cd(II) and Zn(II) ions To study the effect of particle size, the surface-treated zeolite sample was sieved to effect the separation of particles in the following size ranges: mm; mm; Absorbance (a.u.) Wavelength (nm) Figure 2. UV spectrum of neothorin.

6 492 S. Hajialigol et al./adsorption Science & Technology Vol. 24 No mm; mm; and <.11 mm. The adsorption of ligand (neothorin) and sample onto the adsorbent was high when the particle size was <.11 mm, but the flow rate was fairly low under these circumstances. In contrast, the extent of ligand adsorption was low when the particle size was greater than.125 mm. As a consequence, a particle size within the range mm was selected as the preferred value for subsequent experiments. Influence of ph on the adsorption of Cd(II) and Zn(II) ions onto surfactant-modified clinoptilolite The effects of ph on the adsorption of Cd(II) and Zn(II) ions onto clinoptilolite are shown in Figures 3 and 4. With both ions, adsorption was carried out at different ph values keeping other variables constant and thereby enabling the optimum ph value for adsorption to be established. On the basis Distribution coefficient, K d (ml/g zeolite) ph Figure 3. Effect of ph on the adsorption of Cd(II) ions onto surfactant-modified clinoptilolite. Distribution coefficient, K d (ml/g zeolite) ph Figure 4. Effect of ph on the adsorption of Zn(II) ions onto surfactant-modified clinoptilolite.

7 Selective Removal of Cd(II) and Zn(II) Ions from Aqueous Solution by Modified Clinoptilolite 493 of the results obtained, it was found that Cd(II) and Zn(II) ions were adsorbed quantitatively at ph values in the ranges and , respectively. This indicates that the adsorption of both cations could be effect over the range At ph values below 4., the concentration of hydroxonium ions in the solution would be high and prevent any adsorption onto the active sites on the substrate surface. Similarly, in basic solutions (ph > 7.), the concentration of OH ions would be high, leading to precipitation of the cations as their corresponding hydroxides rather than their adsorption onto the adsorbent surface. In addition, at high ph values, the formation of a stable complex would lead to a decrease in any possible adsorption of Zn(II) ions onto the adsorbent. Influence of flow rate on the adsorption of Cd(II) and Zn(II) ions onto surfactant-modified clinoptilolite The effect of sample flow rate was also investigated, with the latter being varied from.25 ml/min to 3. ml/min. The results obtained with Cd(II) and Zn(II) ions, respectively, are depicted in Figures 5 and 6. These suggest that the optimum extent of adsorption in both cases occurred at 1. ml/min and this was adopted for both ions in subsequent experiments. Sorption capacity of surfactant-modified clinoptilolite towards Cd(II) and Zn(II) ions The sorption capacity of clinoptilolite modified with neothorin towards both ions studied was evaluated by obtaining the relevant breakthrough curves. For such experiments, two columns containing 1. g of the modified zeolite were used and different volumes of Cd(II) and Zn(II) ion solutions were passed through each column until the corresponding ion was detected in the flow solution emerging from the column. Modified clinoptilolite showed sorption capacities of.45 mg Cd(II) ion/g adsorbent and.475 mg Zn(II) ion/g adsorbent, respectively. The breakthrough curves for the two ions from their respective columns are depicted in Figures 7 and 8. Optimization of the parameters investigated on the basis of the above results led to an increase in the distribution coefficient, K d, from ml/g zeolite to ml/g zeolite for Cd(II) ions and from ml/g zeolite to ml/g zeolite for Zn(II) ions Distribution coefficient, K d (ml sample/g zeolite) Sample flow rate [ml Cd(ll) ion solution/min] Figure 5. Effect of flow rate on the adsorption of Cd(II) ions onto surfactant-modified clinoptilolite.

8 494 S. Hajialigol et al./adsorption Science & Technology Vol. 24 No Distribution cofficient, K d (ml sample/g zeolite) Sample flow rate [ml Zn(ll) ion solution/min] Figure 6. Effect of flow rate on the adsorption of Zn(II) ions onto surfactant-modified clinoptilolite..14 Cd(ll) ion conc. (mg/l) Volume of solution (ml) Figure 7. Breakthrough curve for Cd(II) ions from surfactant-modified clinoptilolite Zn(ll) ion conc. (mg/l) Volume of solution (ml) Figure 8. Breakthrough curve for Zn(II) ions from surfactant-modified clinoptilolite.

9 Selective Removal of Cd(II) and Zn(II) Ions from Aqueous Solution by Modified Clinoptilolite 495 TABLE 4. Effect of Ion Interference on Cd(II) and Zn(II) Ion Adsorption Ion Salt Tolerance limit/mg ion Zn(II) Cd(II) Mn 2+ Mn(NO Co 3+ Co(NO 3 ) Ce 3+ Ce(NO 3 ) 3 5 Ni 2+ Ni(NO Cu 2+ Cu(NO Cr 3+ Cr(NO 3 ) Pb 2+ Pb(NO Mg 2+ Mg(NO Al 3+ AlCl Bi 3+ Bi(NO 3 ) Influence of interfering ions on the adsorption processes for Cd(II) and Zn(II) ions The influence of ion interference was investigated by using aqueous solutions of various metal ions which were added individually to Cd(II) and Zn(II) ion samples. Such samples contained initial concentration of 7.5 µg Cd(II) ion and 2.5 µg Zn(II) ion, respectively. The corresponding mixtures were then passed through individual columns packed with surfactant-modified clinoptilolite. The results obtained (as listed in Table 4) indicate that the adsorption processes for both ions studied were free from interference by added ions. CONCLUSIONS Because of the importance of generating a good adsorbent for toxic species, studies have been made of the use of a low-cost adsorbent for the selective removal of Zn(II) and Cd(II) ions from aqueous solutions. It has been shown that clinoptilolite modified by BDTA and followed by the quantitative adsorption of neothorin provides a good adsorbent for this purpose. The main advantages of this procedure are as follows: 1. Natural clinoptilolite is very cheap and abundant. 2. A simple system which could be used as a continuous procedure was established for effecting the removal of such ions. 3. Neothorin is highly selective towards the adsorption of Cd(II) and Zn(II) ions. However, its adsorption onto clinoptilolite could only be effected after the mineral had been initially treated with BDTA. 4. Spectrophotometry could not be employed directly for the determination of metal ion concentrations in the presence of neothorin since many metal ion/neothorin complexes absorb at similar wavelengths. However, this problem can be readily solved by using atomic absorption spectroscopy employing an air/acetylene flame.

10 496 S. Hajialigol et al./adsorption Science & Technology Vol. 24 No The results obtained showed that modification of natural clinoptilolite with BDTA and neothorin led to the formation of a new adsorbent which is suitable for the removal of Zn(II) and Cd(II) ions from aqueous solutions. REFERENCES Ackley, M.W. and Yang, R. (1992) Zeolites 12, 78. Badillo-Almaraz, V., Trociller, P. and Davila-Rangel, I. (23) Nucl. Instrum. Methods 21, 424. Bailey, S.E., Olin, T.J., Bricka, R.M. and Adrian, D.D. (1999) Water Res. 33, Blanchard, G., Maunaye, M. and Martin, G. (1984) Water Res. 18, 151. Breck, W. (1974) Zeolite Molecular Sieves, John Wiley & Sons Inc., New York. Caussy, D., Gochfeld, M., Ggurzau, E., Negau, C. and Ruedel, H. (23) Ecotoxicol. Environ. Saf. 56, 45. Erdem, E., Karapinar, N. and Donat, R. (24) J. Colloid Interface Sci. 28, 39. Haggerty, G.M. and Bowman, R.S. (1994) Environ. Sci. Technol. 28, 452. Joshi, M.S. and Mohan, R.P. (1983) J. Colloid Interface Sci. 95, 131. Mondale, K.D., Carland, R.M. and Aplan, F.F. (1995) Miner. Eng. 8, 535. Shahwan, T., Zunbul, B., Eroglu, A.E. and Yilmaz, S. (25) Appl. Clay Sci. 3, 29. Taher, M.A., Mostafavi, A., Afzali, D., Rezaeipour, E. and Khayatzadeh Mahani, M. (25) Anal. Sci. 21, 383. Wingenfelder, U., Nowak, B., Furrer, G. and Schulin, R. (25) Water Res. 39, 3287.