Ion exchange studies on natural and modified zeolites and the concept of exchange site accessibility

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Abstract

In the present study natural and Na-rich form of clinoptilolite are examined, in respect to ion exchange of Pb2+, Cu2+, Cr3+, and Fe3+. Equilibrium and kinetic studies performed, under the same normality (0.01 N). Equilibrium studies demonstrate that Na+ enrichment of clinoptilolite is beneficial in respect to metal uptake for all metals, except Cr3+, which is shown to have the same equilibrium behavior in both materials. Kinetic study shows that diffusion coefficients are in the range of 0.16 to 9×10−9 cm2/s, and are not always improved in Na-rich form of clinoptilolite. The effect of temperature on diffusion coefficients is also examined, and Arrhenius activation energy is determined to be in the range of 3.02 to 13.9 kcal/mol, for all metals and materials, except Cu2+, which have extremely low activation energy in the natural sample, equal to 0.04 kcal/mol.

Introduction

Heavy metals are well-known toxic substances and therefore their removal from wastewaters is required prior to discharge into receiving waters. Among several methods available, ion exchange is an attractive one; its application is relatively simple and safe, as mild operating conditions are applied. Ion exchange is the exchange of ions between a liquid phase and a porous solid, which may be synthetic or natural (ion exchangers as resins or zeolites). Especially when a low-cost exchanger is used, e.g., zeolites, the method can be cost-effective [1]. Natural zeolites are capable of removing several cations from aqueous solutions by utilizing ion exchange. Clinoptilolite is one of the zeolites that have received extensive attention due to its attractive selectivity for certain heavy metal cations such as lead, cadmium, and nickel [1], [2], [3]. Overall rates of adsorption, desorption, and ion exchange in porous materials are generally controlled by mass transport within the pore network, rather than by the kinetics of sorption or ion exchange itself [4]. The most important kinetic parameter is then the pore or solid diffusion coefficient. Actually, the effective diffusion coefficient is a macroscopic average over a large number of ions (or molecules) in pores of all different sizes, shapes, and directions and at larger and smaller distances from the pore walls, expressing the average ability of the species to make headway in any given direction [5]. Diffusion coefficients of cations in zeolites are generally characterized as “effective” or “average,” since they include complicated phenomena: two basic types of diffusion, e.g., through the zeolite pores and the channels of the lattice, and the exchange of more than one cation.

Similarly, equilibrium in such systems is a complicated phenomenon. In order to construct the isotherm for an ion exchange “couple” on zeolites, a basic assumption has to be made to ensure that cations in the liquid phase are exchanged only with one type of solid-state cations, in most of the cases Na+ ions, and the other cations of the material are in inaccessible sites [6]. Although this is nearly true for homoionic clinoptilolites generated by reacting the material with concentrated pretreatment solutions at high temperatures and for long time periods, generally it is not possible to exchange only one of the clinoptilolite cations [6]. Extended exposure of clinoptilolite samples to concentrated sodium solutions has been found to be ineffective in displacing all the calcium and potassium ions from this zeolite, and the sample may require extensive conditioning over several days, with a high concentration of the selected cation to obtain the homoionic form [7], [8], [9]. For instance, in lead–sodium exchange in homoionic Na-clinoptilolite, equilibrium is ternary in nature, as K+ ions were found in the solution [10].

Furthermore, it is noted that cadmium and lead uptake by clinoptilolite are attributed to different mechanisms of ion exchange as well as to the adsorption process [8]. Another phenomenon during ion exchange is that hydrogen ions are exchanged along with heavy metals such as cadmium, lead, and copper by clinoptilolite [8], [9], [11], [12]. Finally, in the case of Cu2+ exchange on clinoptilolite, metal cations are exchanged and also adsorbed on the surface of the zeolite [13].

Despite the interest in ion exchange with clinoptilolite, few data exist on the rates of heavy metal exchange on this material, in terms of diffusion coefficients. Diffusion coefficients for Pb2+ and Cd2+ in Na-rich and natural clinoptilolite have been measured [3], [14], [15], [16] and for Cu2+ on Na- and Ca-rich clinoptilolite [17]. Kinetics of Pb2+ and Cu2+ in Na-rich clinoptilolite has been reported [13], for Pb2+ and Cd2+ on natural and Na-rich clinoptilolite [18], [19], for Cr3+ ions [20], and for Pb2+ and Cu2+, but without determination of diffusion coefficients [11].

Few data also exist for the equilibrium of heavy metals exchange on this material. Equilibrium studies for Pb2+ and Cu2+ are presented for NH4-rich clinoptilolite [13], for Pb2+ on Na-rich clinoptilolite [6], [21], for Pb2+ on natural and Na-rich clinoptilolite [10], [21], for Pb2+ on natural clinoptilolite and Na-rich clinoptilolite [22], and for Cu2+ on natural clinoptilolite and Ca-rich clinoptilolite [17], [23].

In the present work, ion-exchange behavior of natural and Na-rich clinoptilolite against Pb2+, Cu2+, Fe3+, and Cr3+ is presented in terms of solid diffusion coefficients and equilibrium distribution coefficients for each metal. Also, the effect of temperature on diffusion coefficients of ion exchange is investigated, in terms of Arrhenius activation energy. Apart from the new data on the specified ion-exchange system (distribution and diffusion coefficients and activation energy), the concept of exchange site accessibility is introduced in order to explain the relationship between the equilibrium and kinetic results.

Section snippets

Materials

The mineral used was collected from a deposit in the northern part of Greece. It was ground and then sieved to different fractions, of which 1.18–1.4 mm was used in the study. The chemical composition of the material was obtained through SEM/EDS measurements using the Jeol Model JSM-6100 scanning microscope. The following samples were used in this study:

  • (a)

    Natural sample (as-received clinoptilolite).

  • (b)

    Modified (Na-rich) sample. This sample was prepared in a 70-cm-long column with diameter 2.09 cm,

Materials

In Table 1 the chemical composition of the material is presented (% w/w).

The concentration of exchangeable cations in the solid phase is measured to be 1.67±0.06 meq/g, by conversion of the material in near-homoionic Na+ form, a method described in detail by Inglezakis et al. [15]. The Si/Al ratio is 4.29 (mol/mol) and the corresponding ratio of Na+K/Ca is 1. Chemical composition and the ratio Si/Al, generally ranging from 4 to 5.5, are typical for clinoptilolite [2], [21]. Modified

Analysis of kinetic and equilibrium results

Generally it is true that the preferred counterion is taken up by the exchanger at a higher relative rate [5]. Diffusion coefficients are determined using the relative rates of approaching equilibrium U(t), rather than the absolute rate of exchange in terms of number of ions exchanged per unit time, and from this point of view they are a measure of this rate, expressing the ease of movement in zeolite structure. Relative exchange rate is proportional to the diffusion coefficient where the

Summary

Equilibrium studies demonstrate that Na+ enrichment of clinoptilolite is beneficial with respect to metal uptake for all metals, except Cr3+, which is shown to have the same equilibrium behavior in both materials. According to the distribution coefficients, the selectivity series for the natural sample is Pb2+>Cr3+>Fe3+>Cu2+. The selectivity series for the modified sample is similar, following the order Pb2+>Cr3+≅Fe3+>Cu2+, up to (X) approximately equal to 0.5, where the selectivity is changing

Acknowledgements

We are grateful to E. Thoma, of the Special technical personnel of the Metallurgical Engineering Department of NTUA, for the SEM/EDS analysis.

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