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The removal of heavy metal cations by natural zeolites

Abstract

In this study, the adsorption behavior of natural (clinoptilolite) zeolites with respect to Co2+,Cu2+,Zn2+,and Mn2+ has been studied in order to consider its application to purify metal finishing wastewaters. The batch method has been employed, using metal concentrations in solution ranging from 100 to 400 mg/l. The percentage adsorption and distribution coefficients (Kd) were determined for the adsorption system as a function of sorbate concentration. In the ion exchange evaluation part of the study, it is determined that in every concentra­tion range, adsorption ratios of clinoptilolite metal cations match to Langmuir, Freundlich, and Dubinin–Kaganer–Radushkevich (DKR) adsorption isotherm data, adding to that every cation exchange capacity metals has been calculated. It was found that the adsorption phe­nomena depend on charge density and hydrated ion diameter. According to the equilibrium studies, the selectivity sequence can be given as Co2+ > Cu2+ > Zn2+ > Mn2+. These results show that natural zeolites hold great potential to remove cationic heavy metal species from industrial wastewater.

1. Introduction

Many toxic heavy metals have been discharged into the environment as industrial wastes, causing serious soil and water pollution [1].Pb+2,Cu+2,Fe+3,and Cr+3 are espe­cially common metals that tend to accumulate in organisms, causing numerous diseases and disorders [2].Theyare also common groundwater contaminants at industrial and mili­tary installations. Numerous processes exist for removing dissolved heavy metals, including ion exchange, precipi­tation, phytoextraction, ultrafiltration, reverse osmosis, and electrodialysis [3–6]. The use of alternative low-cost materi­als as potential sorbents for the removal of heavy metals has been emphasized recently.
Activated carbon adsorption is considered to be a partic­ularly competitive and effective process for the removal of heavy metals at trace quantities [7]; however, the use of activated carbon is not suitable in developing countries due to the high costs associated with production and regener­ation of spent carbon [8]. Various treatment processes are available, among which ion exchange is considered to be cost-effective if low-cost ion exchangers such as zeolites are used [9].
Zeolites are naturally occurring hydrated aluminosilicate minerals. They belong to the class of minerals known as “tectosilicates.” Most common natural zeolites are formed by alteration of glass-rich volcanic rocks (tuff) with fresh water in playa lakes or by seawater [10]. The structures of zeolites consist of three-dimensional frameworks of SiO4 and AlO4 tetrahedra. The aluminum ion is small enough to occupy the position in the center of the tetrahedron of four oxygen atoms, and the isomorphous replacement of Si4+ by Al3+ produces a negative charge in the lattice. The net nega­tive charge is balanced by the exchangeable cation (sodium, potassium, or calcium). These cations are exchangeable with certain cations in solutions such as lead, cadmium, zinc, and manganese [11,12]. The fact that zeolite exchangeable ions are relatively innocuous (sodium, calcium, and potas­sium ions) makes them particularly suitable for removing undesirable heavy metal ions from industrial effluent waters. One of the earliest applications of a natural zeolite was in removal and purification of cesium and strontium radioiso­topes [13].
Clinoptilolite is the most abundant natural zeolite and has the chemical formula Na0.1K8.57Ba0.04(Al9.31Si26.83O72)· 19.56H2O [14]. Its characteristic tabular morphology shows an open reticular structure of easy access, formed by open channels of 8-to 10-membered rings [15]. Considerable re­search has been conducted to characterize the chemical, sur­face, and ion-exchange properties of clinoptilolite [15–19]. The ion-exchange capacity of natural zeolite (clinoptilolite) for inorganic cations has been investigated by many au­thors [20,21].
The selectivity series of clinoptilolite in the sodium form was determined by Zamzow et al., as follows: Pb2+ > Cd2+ > Cs+ > Cu2+ > Co2+ > Cr3+ > Zn2+ > Ni2+ > Hg2+ [22]. Jama and Yücel observed that clinoptilolite shows a very high preference for ammonium ions over sodium and calcium but not over potassium [23].Mieret al. described the interactions of Pb2+,Cd2+,and Cr3+ com­peting for ion-exchange sites in natural clinoptilolite [24]. Inglezakis et al. studied the ion exchange of Pb2+,Cu2+ , Fe3+,and Cr3+ on natural clinoptilolite and showed that equilibrium is favorable for Pb2+, unfavorable for Cu2+,and of sigmoid shape for Cr3+ and Fe3+ [25].
As seen from the literature review, zeolites can be used for the removal of some heavy metals from wastewater. The clinoptilolite samples from different regions show differ­ent behavior in ion-exchange processes. In this study, the adsorption properties of the natural zeolite (Clinoptillolite, Western Anatolian) with respect to some heavy metal cations in solution were investigated.

2. Materials and methods

2.1. Zeolite source and conditioning
Samples of zeolite were taken from the Enli Mining Com­pany open pit mine in Manisa-Gördes in Western Anato­lia. The crushed original zeolite was ground and passed through 300 × 600-µm sieves and was dried in an oven at 100 ± 5 ◦C for 24 h. It was characterized by X-ray diffrac­tion (XRD) and chemical analysis. Chemical composition of zeolite samples was determined by the usual analytical methods for silicate materials. Standard wet chemical analy­sis, along with instrumental methods, was adopted [26]. Al2O3,Fe2O3, CaO, and MgO were analyzed with titrimet­ric methods and SiO2 was analyzed with gravimetric meth­ods. Na2Oand K2O were found out by flame photometry. The results of the chemical analyses are presented in Table 1.
Table 1 Chemical composition and physical properties of natural zeolite sample (wt%)

Chemical composition (%) Physical properties
SiO2 69.31 Appearance porosity (%), 41.5
Al2O3 13.11 Appearance density (g/cm3), 2.27
Fe2O3 1.31 Weight of per unit volume (g/cm3), 1.32
CaO 2.07 Water absorption (original) (%), 31.3
MgO 1.13 Water absorption (grinding) (%), 103.7
Na2O 0.52 Oil absorption (g oil/100 g sample), 51
K2O 2.83 Whiteness (%), 68
SO3 0.10 Original bleaching (g sample/g tonsil), 1.95
H2O 6.88 Active bleaching (g sample/g tonsil), 1.92
Si/Al 4.66 pH 7.5

2.2. Reagents
Inorganic chemicals were supplied by Merck as analy­tical-grade reagents and deionized water was used. The metal ions studied were Co(II), Cu(II), Zn(II), and Mn(II). We prepared a synthetic stock solution of copper, man­ganese, and cobalt using their nitrate salts, Cu(NO3)2·3H2O, Mn(NO3)2·4H2O, and Co(NO3)2·6H2O, respectively, in deionized water. The stock solution of Zn(II) was prepared using ZnCl2 salts.
2.3. Batch adsorption studies
The ion exchange of heavy metals on natural zeolite was carried out using the batch method. Batch adsorption exper­iments were conducted using 10 g of adsorbent with 500 ml of solutions containing heavy metal ions of desired concen­trations at constant temperatures (25 ◦C) in 500-ml plastic bottles. The particle size of the sample (clinoptilolite) used was in the range of 63–106 µm. The bottles were shaken in a sieve for 5.5 h and solutions containing heavy metals were filtered through Whatman filter paper (No. 42). The exact concentration of metal ions and filterable metal concentra­tions were determined by AAS (ATI UNICAM 929 atomic absorption spectrophotometer). The flame type was air– acetylene and absorption wavelengths are Cu2+(324.7 nm), Zn2+(213.9 nm), Co2+(240.7 nm), and Mn2+(279.5 nm).
The percent adsorption (%) and distribution ratio (Kd) were calculated using the equations
Ci – Cf
% adsorption =× 100, (1)Cf
where Ci and Cf are the concentrations of the metal ion in initial and final solutions, and respectively, and
amount of metal in adsorbent V
Kd =× ml/g, (2)
amount of metal in solution m
where V is the volume of the solution (ml) and m is the weight of the adsorbent (g).
The percent adsorption and Kd (ml/g) can be correlated by the following equation [27]:
100Kd
% adsorption = . (3)
Kd + V/m

Fig. 1. Adsorption of metal ions by natural zeolite sample as a function of initial concentration: m = 10 g, V = 500 ml, pH 6–7, time 5.5 h.

Fig. 2. Variation of metal ions on natural zeolite as a function of initial concentration: m = 10 g, V = 500 ml, pH 6–7, time 5.5 h.

3. Results and discussion

3.1. Properties of natural zeolites
A chemical analysis of the treated zeolite is presented in Table 1. This study showed that natural zeolite contained a complement of exchangeable sodium, potassium, and cal­cium ions.
The Si/Al ratio is 4.66 (mol/mol) and the correspond­ing ratio of (Na + K)/Ca is 1.85. The chemical composi­tion, the theoretical exchange capacity, and the Si/Al ratio, generally ranging from 4 to 5.5, are typical for clinoptilo­lite [28–30]. Low-silica members are enriched with calcium, whereas high-silica clinoptilolites are enriched with potas­sium, sodium, and magnesium. It was found that the natural zeolite contains approximately 70% clinoptilolite and pla­gioclase, mica, quartz, smectite, and K-feldsparchlorite min­erals by XRD analysis.
3.2. Adsorption of metals on natural zeolite
The adsorption of Cu2+,Co2+,Zn2+,and Mn2+ onto natural zeolite as a function of their concentrations was stud­ied at 30 ◦C by varying the metal concentration from 100 to 400 mg/l while keeping all other parameters constant. The results are shown in Figs. 1 and 2. Percentage adsorption for Cu2+,Co2+,Zn2+,and Mn2+ decreases with increas­ing metal concentration in aqueous solutions. These results indicate that energetically less favorable sites become in­volved with increasing metal concentrations in the aqueous solution. The heavy metal uptake is attributed to different mechanisms of ion-exchange processes as well as to the ad­sorption process. During the ion-exchange process, metal ions had to move through the pores of the zeolite mass, but also through channels of the lattice, and they had to replace exchangeable cations (mainly sodium and calcium). Diffu­sion was faster through the pores and was retarded when the ions moved through the smaller diameter channels. In this case the metal ion uptake could mainly be attributed to ion-exchange reactions in the microporous minerals of the zeolite samples. Fig. 2 illustrates Kd as a function of metal ions concentrations. The Kd values increase with the decreasing concentration of metal ions. In other words, the Kd values increase as dilution of metal ions in solution pro­ceeds.
These results indicate that energetically less favorable sites become involved with increasing metal concentration in the aqueous solution. The maximal exchange levels at­tained were as follows: Cu2+ 66.10%, Co2+ 77.96%, Zn2+ 45.96%, and Mn2+ 19.84%.
3.3. Isotherm models
The sorption data have been subjected to different sorp­tion isotherms, namely, Langmuir, Freundlich, and Dubinin– Kaganer–Radushkevich (DKR). The equilibrium data for metal cations over the concentration range from 100 to 400 mg/lat30 ◦C have been correlated with the Langmuir isotherm [31]:
Ce/Cads = 1/Qb + Ce/Q, (4)
where Ce is the equilibrium concentration of metal in so­lution (M), Cads is the number of metal ions sorbed onto zeolite, and Q and b are Langmuir constants related to sorp­tion capacity and sorption energy, respectively. Maximum sorption capacity (Q) represents monolayer coverage of sor­bent with sorbate and b represents enthalpy of sorption and should vary with temperature. A linear plot is obtained when Ce/Cads is plotted against Ce over the entire concentration range of metal ions investigated. The Langmuir model pa­rameters and the statistical fits of the sorption data to this equation are given in Table 2. The Langmuir model effec­tively described the sorption data with all R2 values >0.95. According to the Q (mmol/kg) parameter, sorption on ze­olite is produced following the sequence Co2+ > Cu2+ > Zn2+ > Mn2+ .
Table 2 Characteristic parameters and determination coefficient of the experimental data according to the Langmuir equation

Metal Q (mmol/kg) b (l/g) R2
Cu2+ 141.12 1.16 0.9775
Co2+ 244.13 1.75 0.9919
Zn2+ 133.85 0.85 0.9803
Mn2+ 76.78 0.33 0.9503

Zeolites, in general, are weakly acidic in nature and sodium-form exchangers are selective for hydrogen (R–Na+ H2O ⇔ RH + Na++ OH-), which leads to high pH val­ues when the exchanger is equilibrated with relatively dilute electrolyte solutions [32], making metal hydroxide precipi­tation feasible. In natural zeolites these metals seem to reach saturation, which means that the metal had filled possible available sites and further adsorption could take place only at new surfaces.
Ion-exchange capacity of heavy metal cations listed in Ta­ble 2 indicates the following selectivity sequence: Co2+ > Cu2+ > Zn2+ > Mn2+. The heavy metal cations are present as hexaaqua complex ions with six surrounding water mole­cules in the solution and they passed the channel of zeolite in this form [23]. Since the adsorption phenomena depend on the charge density of cations, the diameter of hydrate cations is very important. The charges of the metal cation are the same (+2); therefore Mn2+ ions (the biggest diameter) have minimum adsorption, and Co2+ ions (the least diame­ter) have maximum adsorption.
The Langmuir model effectively described the sorption data with all R2 values >0.95, according to the b (mmol/g) parameter. Sorption on natural zeolite is produced follow­ing the sequence Co2+ > Cu2+ > Zn2+ > Mn2+. The high Si/Al ratio of clinoptilolite results in a typical low anionic field that gives rise to good selectivity.
The Freundlich sorption isotherm, one of the most widely used mathematical descriptions, usually fits the experimen­tal data over a wide range of concentrations. This isotherm gives an expression encompassing the surface heterogeneity and the exponential distribution of active sites and their ener­gies. The Freundlich adsorption isotherms were also applied to the removal of Co2+,Cu2+,Zn2+,and Mn2+ on zeolite (Fig. 3),
log Cads = log K + 1/nlog Ce, (5)
where Ce is the equilibrium concentration in mg/land Cads shows that the adsorption seems to follow the Freunlich
isotherm model as well as the Langmuir isotherm. The con­stants K and n were calculated for each cation (Table 3). K is a parameter related to the temperature and n is a char­acteristic constant for the adsorption system under study. Values of n between 2 and 10 shows good adsorption.

Fig. 3. Freundlich plots for metal ions adsorption onto natural zeolite.
Table 3 Freundlich adsorption equations and constants (log k and n) for metal cations on natural zeolite

Metal log k (mg/g) 1/n R2
Cu2+ 0.13 0.30 0.9914
Co2+ 0.28 0.36 0.9390
Zn2+ 0.19 0.43 0.9417
Mn2+ 0.81 0.50 0.9897

The numerical value of 1/n < 1 indicates that adsorp­tion capacity is only slightly suppressed at lower equilibrium concentrations. This isotherm does not predict any saturation of the sorbent by the sorbate; thus infinite surface coverage is predicted mathematically, indicating multilayer adsorption on the surface [33].
The Dubinin–Kaganer–Radushkevich (DKR) has been used to describe the sorption of metal ions on clays. The DKR equation has the form
ln Cads = ln Xm – βε2 , (6)
where Cads is the number of metal ions adsorbed per unit weight of adsorbent (mg/g), Xm is the maximum sorption capacity, β is the activity coefficient related to mean sorption energy, and ε is the Polanyi potential, which is equal to
ε = RT ln(1 + 1/Ce), (7)
where R is the gas constant (kJ mol-1K-1)and T is the tem­perature (K). The saturation limit Xm may represent the total specific micropore volume of the sorbent. The sorption po­tential is independent of the temperature but varies according to the nature of sorbent and sorbate [27]. The slope of the plot of ln Cads versus ε gives β (mol2/J2) and the intercept yields the sorption capacity, Xm (mg/g). The sorption space in the vicinity of a solid surface is characterized by a series of equipotential surfaces having the same sorption potential. This sorption potential is independent of the temperature but varies according to the nature of sorbent and sorbate. The sorption energy can also be worked out using the following relationship:
E = 1/-2β. (8)
The plot of lnCads against ε2 for metal ion sorption on natural zeolite is shown in Fig. 4. The DKR parameters are calculated from the slope of the line in Fig. 4 and listed in Table 4. As shown in Table 4, the E values are 11.95 for Cu2+, 11.03 for Co2+, 9.77 for Zn2+, and 8.81 kJ/mol for Mn2+ on the natural zeolite. They are the orders of an ion-exchange mechanism, in which the sorption energy lies within 8–16 kJ/mol [34,35]. The sorption capacityXm in the DKR equation is found to be 0.22 for Cu2+, 0.50 for Co2+, 0.29 for Zn2+, and 0.15 mmol/kg for Mn2+.

4. Summary

It was considered that the metal cations are formed as high-spin aqua complexes and the hydrate ion diameter is the same as the diameter of aqua-complex ions. The diameters of hydrated ions are given as rMn > rZn > rCu > rCo in the literature [36].
If the ionic radii are compared with the free dimensions of the clinoptilolite channels, it is apparent that all of the unhydrated ions can pass readily through the channels, but since the hydrated ions are approximately the same size as the channel dimensions, they may exchange only with difficulty. It follows that for exchange to take place, at least some of the waters of hydrationmust be stripped from the solvated ions [37].

Fig. 4. DRK plots of metal ions on natural zeolite at constant temperature.
Table 4 Parameter obtained in the DKR equation

DKR Cu2+ Co2+ Zn2+ Mn2+
Xm (mmol/kg) 0.22 0.50 0.29 0.15
ß (mol2/J2) -3.50 × 10-9 -4.11 × 10-9 -5.24 × 10-9 -6.44 × 10-9
Sorption energy (E, kJ/mol) 11.95 11.03 9.77 8.81
Correlation coefficient, R2 0.9774 0.9644 0.9601 0.9817

The DKR and Langmuir equations were used to describe the sorption isotherms of single-solute systems. The sorption energy E determined in the DKR equation (11.95 for Cu2+, 11.03 for Co2+, 9.77 for Zn2+, and 8.81 kJ/mol for Mn2+) revealed the nature of the ion-exchange mechanism in both systems. The sorption isotherms of binary solute systems could be fitted by the competitive Langmuir model, where the Langmuir parameters were directly adopted from those obtained in single-solute systems.
These results show that natural zeolite can be used effectively for the removal of metal cations from wastewater. This naturally occurring material provides a substitute for the use of activated carbon as adsorbent due to its availability and its low cost.

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