Uptake Equilibria and Mechanisms of Heavy Metal Ions on

Oct 21, 2004 - In this study, the uptake equilibria and mechanisms of heavy metal ions Pb(II), Cu(II), Cd(II), and Zn(II) on microporous titanosilicat...
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Ind. Eng. Chem. Res. 2004, 43, 7900-7906

Uptake Equilibria and Mechanisms of Heavy Metal Ions on Microporous Titanosilicate ETS-10 L. Lv, G. Tsoi, and X. S. Zhao* Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge, Singapore 119260, Singapore

In this study, the uptake equilibria and mechanisms of heavy metal ions Pb(II), Cu(II), Cd(II), and Zn(II) on microporous titanosilicate ETS-10 were attempted in the absence and presence of background electrolyte NaNO3 of different concentrations in batch systems. The uptake properties of the metals were found to be related to the zeta (ζ) potential profile of ETS-10 and the characteristics of the heavy metals in terms of their hydrolysis, hydrated ionic radii, and hydration energies. The ionic strength of NaNO3 and the pH were observed to have a significant impact on the equilibrium uptake capacity due to the competition of Na+ and proton and the variation of surface electrochemical properties of ETS-10 adsorbent. The desorption data of counterions indicated that ion exchange is mainly responsible for the uptake of heavy metals Cu(II), Cd(II), and Zn(II), while other mechanisms such as adsorption and complexation may be cooperative in addition to ion exchange in the case of Pb(II) uptake on ETS-10 solid. Introduction Heavy metals such as lead (Pb), copper (Cu), cadmium (Cd), and zinc (Zn) and the associations with their transport and distribution are of serious concern for human health and the environment. These metals are present in many natural systems and industrial waste streams and must be controlled to an acceptable level according to environmental regulations worldwide. Many treatment processes such as chemical precipitation,1 adsorption,2 reverse osmosis,3 electrodialysis,4 ultrafiltration,5,6 and ion exchange7,8 are currently available for the removal of heavy metal cations from aqueous solutions. An ion exchange process based on zeolites has been considered to be a cost-effective approach for waste decontamination.9 In the past few decades, both natural and synthetic zeolites have been studied for heavy metal removal.9-12 With increasingly stringent environmental regulations for heavy metals present in drinking water and wastewater, novel zeolite ion exchangers with fast ion exchange rates and high uptake capacities are desired. ETS-10, first synthesized by Engelhard in 1989,13 is a microporous zeolitic titanosilicate with the basic anhydrous formula Na1.5K0.5TiSi5O13.14 Unlike conventional aluminosilicate zeolites, which are chemically constituted from SiO4 and AlO4 tetrahedra giving rise to one negative charge for each AlO4 tetrahedron balanced by an ion-exchangeable cation like Na+, the framework of ETS-10 is composed of SiO4 tetrahedra and TiO6 octahedra. The presence of each tetravalent Ti atom in an octahedron generates two negative charges, which are balanced by alkali metal ions Na+ and K+.14,15 It is obvious that ETS-10 is a suitable ion exchanger for divalent heavy metal cations. Indeed, it has been observed that ETS-10 displays an extremely fast adsorption rate toward heavy metal ions.16,17 For example, the uptake equilibrium of Pb(II) on ETS-10 * To whom correspondence should be addressed. Tel.: (65)68744727. Fax: (65)-67791936. E-mail: [email protected].

can be reached within 5 s.17 With such a fast uptake rate, Kuznicki et al.16 have suggested that ETS-10 is a promising water filter for treating drinking water. In addition to the fast-kinetics property, it has also been observed that ETS-10 possesses the highest equilibrium uptake capacity of Pb(II) ions among all zeolite materials.17 A continuation of our previous studies is reported in the present paper,17,18 with the aim of developing a better ion exchanger for heavy metal removal. In this work, the adsorption equilibria of heavy metal ions Pb(II), Cu(II), Cd(II), and Zn(II) on ETS-10 in the absence or presence of electrolyte NaNO3 of different concentrations are experimentally examined and theoretically analyzed. The influence of pH and ionic strength on the adsorption properties of the metal ions as well as on the ζ- potentials of ETS-10 adsorbent is investigated and discussed. Furthermore, possible uptake mechanisms of the heavy metal ions are proposed. Experimental Section Synthesis and Characterization of ETS-10. The ETS-10 sample used in this study was synthesized by using TiF4 as a titanium source, following the procedures described by Yang and co-workers.19 The final product was characterized by using the powder X-ray diffraction (XRD) technique on a Shimadzu XRD-6000 diffractometer (Cu KR radiation) at 40 kV and 30 mA, Fourier transform infrared (FTIR) spectroscopy on a Bio-Rad spectrometer (KBr methods), Raman spectroscopy on a Bruker FRA 106/S FT-Raman spectrometer, and scanning electron microscopy (SEM) on a JEOL JSM-5600LV. Aqueous Solutions of Heavy Metals. The aqueous solutions of various concentrations of the heavy metal ions were prepared by using lead(II) nitrate (GR grade, Merck), copper(II) nitrate trihydrate (GR grade, Rideldel-Hae¨n), cadmium(II) nitrate tetrahydrate (GR grade, Merck), zinc(II) nitrate hexahydrate (98%, Arcos), and ultrapure water. Measurement of the ζ Potential. Suspensions containing 0.1 g/L ETS-10 solid with ionic strengths of

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0, 0.01, and 0.1 M NaNO3 were respectively stirred for 12 h at room temperature, followed by oscillation in an ultrasonic bath for 10 min. The pH of each suspension was adjusted from 2 to 7 by using 0.1 M HNO3 solution. Then the suspension was left for 1 h without stirring to let the solid settle. The upper supernatant was used for the measurement of ζ-potential on a Brookhaven Zeta-plus 4 instrument. An average value was taken from eight measurements. Measurement of Cation Exchange Capacity (CEC). A typical measurement of CEC was carried out as follows.20 ETS-10 was dried at 100 °C overnight before use. An amount of 250 mg of the dried sample was added to 25 mL of 0.2 M barium acetate-acetic acid buffer solution in a 50 mL tube. The tube was then shaken for 1 h, followed by centrifugation to separate the solid from the liquid. After the solid was washed with 25 mL of ultrapure water, 25 mL of 1 M NH4OAc solution was added under shaking for 5 min. After centrifugation, the upper solution was transferred into a 100 mL volumetric flask. A second portion of 25 mL of 1 M NH4OAc was added to the bottom solid under shaking, followed by centrifugation for 10 min. The upper clear solution was combined with the previous one in the 100 mL volumetric flask. The CEC was determined by measuring the concentration of barium ions in the flask. Batch Mode Adsorption. All batch adsorption experiments were carried out in a 50 mL conical flask. The working solutions were prepared by diluting a given volume of a stock solution to obtain different initial concentrations (Ci). About 150 mg of ETS-10 solid was placed in the flask containing 30 mL of a heavy metal working solution. To avoid precipitation of heavy metals on the surface of ETS-10 and excessive competition by H+ ions, the pH value of the solution was periodically measured and maintained at 5 by using nitric acid (69.5%, ultrapure, Fluka) except for those experiments for studying the pH effect. The mixture was agitated at 298 K and 180 rpm on a GFL 3017 orbital shaker for 1 h, which was sufficient to reach adsorption equilibrium. The equilibrium concentrations (Ce) of the heavy metal ion and the desorbed counterions were measured by using an inductively coupled plasma atomic emission spectrometer (ICP-AES) on a Perkin-Elmer ICP Optima 3000DV. The amount of heavy metal adsorbed, qe (mmol/g), was calculated using

qe ) VL(Ci - Ce)/m

(1)

where VL is the volume of solution (mL) and m is the mass of ETS-10 sample (mg). To study the influence of pH on the uptake efficiency of the heavy metals, similar experiments were carried out for all metals at an initial concentration (Ci) of 3.5 mmol/L. The pH of the adsorption system was adjusted in the range of 2.5-6.0 by using nitric acid. Blank experiments were also conducted. To study the influence of electrolyte on the uptake of the heavy metal ions, batch experiments were conducted in the presence of 0.01 and 0.1 M NaNO3 electrolyte. The desorbed amounts of Na+ and K+ ions were also measured and calculated from eq 1 by using the their concentration differences in the solutions before and after adsorption. Blank experiments were conducted.

Figure 1. Experimental uptake isotherms of Pb(II), Cu(II), Cd(II), and Zn(II) in the absence (a) and presence of (b) 0.01 M and (c) 0.1 M NaNO3 electrolyte at pH 5 and 298 K.

Results Characterization of the ETS-10 Sample. The detailed characterization data of the ETS-10 sample can be found elsewhere.17 XRD, FTIR, Raman, and SEM data demonstrate that the sample is a pure ETS-10 phase. Due to octahedral titanium [TiO6]2- and relatively high amounts of titanium in their structure (TiO2/ SiO2 ) 0.2), ETS-10 has high cation exchange capacity (CEC) (∼1.62 mequiv/g ETS-10 anhydrous) on the basis of the desorbed amount of barium in NH4OAc solution. Uptake Equilibrium of Heavy Metals on ETS-10. The uptake equilibrium curves of heavy metal ions Pb(II), Cu(II), Cd(II), and Zn(II) on ETS-10 in the absence of electrolyte NaNO3 at pH 5 and 298 K are shown in Figure 1a. It can be seen that the heavy metal ions displayed different adsorption equilibrium properties on ETS-10. Pb(II) exhibited the highest maximum adsorption capacity, while Zn(II) possessed the lowest maximum adsorption amount. It is also seen from Figure 1a that the adsorptions of Pb(II) and Cd(II) attained the maximum uptake at an extremely low equilibrium concentration, showing that ETS-10 has a high affinity toward these two metal ions in aqueous solution. For the adsorptions of Cu(II) and Zn(II), the isotherm slopes at the low equilibrium concentration region are smaller than that of the Pb(II) and Cd(II) isotherms, implying a weaker affinity of ETS-10 toward Cu(II) and Zn(II) ions than toward Pb(II) and Cd(II)

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Table 1. Langmuir and Freundlich Isotherm Parameters of Heavy Metal Ion Adsorption on ETS-10 in the Absence and Presence of 0.01 and 0.1 M NaNO3 Electrolyte at pH 5 and 298 K NaNO3 Langmuir isotherm Freundlich concn (M) qm metal b r2 ∆g 1/n KF r2 ∆g Pb(II) Cu(II) Cd(II) Zn(II) Pb(II) Cu(II) Cd(II) Zn(II) Pb(II) Cu(II) Cd(II) Zn(II)

0 0 0 0 0.01 0.01 0.01 0.01 0.1 0.1 0.1 0.1

0.840 0.453 0.404 0.236 0.823 0.418 0.364 0.176 0.738 0.126 0.289 0.065

408 9.26 395 10.6 162 3.66 129 4.35 141 0.762 10.7 2.03

0.989 0.999 0.975 0.996 0.969 0.997 0.998 0.991 0.994 0.978 0.998 0.975

ions. The overall maximum adsorption capacity of the four heavy metal ions in the absence of NaNO3 follows Pb(II) > Cu(II) = Cd(II) > Zn(II). Because of the presence of electrolytes in many contaminated waste streams, the uptake behaviors of the heavy metal ions in the presence of electrolyte NaNO3 of concentrations 0.01 and 0.1 M were examined, and their adsorption isotherms are shown in Figure 1b,c. It is seen that in the presence of 0.01 M NaNO3 electrolyte the adsorption behaviors of the four heavy metal ions on ETS-10 are essentially similar to that in the absence of NaNO3, indicating a minor effect of lowconcentration NaNO3. However, when the concentration of NaNO3 was increased to 0.1 M (see Figure 1c), the adsorption equilibrium properties were significantly altered, particularly for the isotherms of Cu(II) and Zn(II). It seems that the uptake processes of Pb(II) and Cd(II) on ETS-10 can bear a relatively high concentration of background electrolyte, while the uptakes of Cu(II) and Zn(II) are greatly affected by a relatively high concentration of electrolyte. The Langmuir equation has been employed in many works to describe ion exchange data,21-23 although it was originally derived for describing monolayer adsorption. The experimental uptake equilibrium data were fitted to both the Langmuir and Freundlich equations. The Langmuir isotherm can be written as

qe )

qmbCe 1 + bCe

(2)

where Ce (mmol/L) is the equilibrium concentration of a heavy metal ion, qe (mmol/g) is the amount adsorbed at equilibrium per gram of ETS-10 solid, qm is the maximum adsorption amount of the heavy metal ion, and b is a constant in relation to the energy of adsorption. The Freundlich isotherm is written as

qe ) KFCe1/n

(3)

where KF and n are the two constants of the Freundlich equation, which are generally temperature dependent. Both equations were used here as empirical formulas for extracting important parameters. Linear regression of the two isotherms using a leastsquares method allowed us to obtain the constants of the two equations, which are, together with the correlation coefficients (r2), summarized in Table 1. It is seen that the values of r2 of the Langmuir model are higher than those for Freundlich equation for the four heavy metal ions, indicating that the Langmuir isotherm can

8.24 5.59 11.5 5.24 11.4 23.8 15.6 10.6 12.0 32.0 6.42 16.2

0.137 0.239 0.179 0.261 0.137 0.325 0.224 0.318 0.195 0.587 0.315 0.461

0.738 0.321 0.366 0.180 0.747 0.241 0.329 0.114 0.713 0.045 0.223 0.034

0.913 0.915 0.891 0.908 0.853 0.948 0.918 0.958 0.975 0.930 0.960 0.944

24.3 18.4 36.9 26.0 86.4 43.2 32.0 17.1 13.7 41.9 18.2 33.2

better describe the uptake equilibria of the heavy metal ions on ETS-10. To determine an appropriate adsorption isotherm equation to be used for describing the experimental data, a normalized standard deviation (∆g) was calculated using the following equation:

x

∆g (%) ) 100

∑[(qeexp - qecal)/qeexp]2 N-1

(4)

where the superscripts “exp” and “cal” refer to the experimental and calculated values of equilibrium adsorption capacity; N is the number of experimental runs. The values of ∆g are included in Table 1 as well. It is seen that the ∆g values for the adsorptions of the heavy metal ions on ETS-10 fitted to the Langmuir model are much lower than those fitted to the Freundlich model regardless of the presence or absence of electrolyte NaNO3. This observation is consistent with the linear regression data discussed earlier. The Langmuir isotherm constant (b) can be used to indicate the affinity of ETS-10 toward the heavy metal ions. It is seen from Table 1 that the order of the affinity constant (b) for the four heavy metal ions in the absence or presence of 0.01 M NaNO3 followed Pb(II) = Cd(II) . Zn(II) = Cu(II), while it became Pb(II) . Cd(II) > Zn(II) > Cu(II) in the presence of 0.1 M NaNO3. Overall, with the increase in the concentrations of background electrolyte NaNO3, the constant (b) for the heavy metal ions was decreased. It should be noted that the order of the maximum adsorption capacities of the heavy metal ions did not follow the order of Langmuir constant b. ζ-Potential of ETS-10. To elucidate the adsorption behaviors observed above, ζ-potentials of ETS-10 solid in the aqueous solutions of different pH and ionic strengths were measured and are shown in Figure 2. It can be seen that both pH and NaNO3 electrolyte have an impact on the ζ-potential of ETS-10. In the presence of 0.1 M NaNO3, the isoelectric point (iep) is about pH 3.3. When pH was increased to be above 3.3, the ζ-potential was decreased to negative values. Further increasing pH led to a constant ζ-potential of about -10 mV over the pH range of 4-7. However, in the absence or presence of 0.01 M NaNO3, ETS-10 exhibited a negatively charged surface in the pH range of 2-7. It is also seen from Figure 2 that at pH 5, the pH value that was employed in the adsorption equilibrium measurements, the surface charges of ETS-10 were more negative in the presence of 0.01 M NaNO3 than those in the presence of 0.1 M NaNO3.

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Figure 2. Variation of ζ-potential of ETS-10 in solution versus pH. Error bars are included. Figure 4. Amounts of heavy metal ions adsorbed on ETS-10 solid and of alkali metal ions desorbed in blank experiments and in adsorption experiments under different concentrations of NaNO3 electrolyte. Cross-hatched bars, amounts adsorbed of heavy metals; diagonally lined bars, amounts desorbed of alkali metal ions in blank experiments; vertically lined bars, amounts desorbed of alkali metal ions in adsorption experiments. (a) Pb(II), (b) Cu(II), (c) Cd(II), and (d) Zn(II). Error bars are included.

Figure 3. Effect of pH on uptake capacity (qe) of heavy metal ions on ETS-10 and desorbed amount of alkali metal ions (qd) at 298 K (C0 ) 3.5 mmol/L; batch factor ) 30 mL/150 mg).

Effect of pH on Heavy Metal Uptake. The experimental data for the uptake of the four heavy metal ions on ETS-10 in the pH range of 2.5-6.0 in the absence of electrolyte are shown in Figure 3. It can be seen that the relative amount adsorbed of Pb(II) on ETS-10 was insignificantly affected by pH in comparison with those of the rest of the heavy metal ions assayed. Increasing pH resulted in a sharp increase in the adsorption amounts of the heavy metal ions, in particular for Cd(II) and Cu(II). At pH 2.5, the adsorbed amount of Pb(II) was still relatively high, whereas the adsorbed amounts for the other metals were very low. At pH 6, with an initial heavy metal concentration of 3.5 mmol/L, the amounts adsorbed of Pb(II), Cu(II), Cd(II), and Zn(II) were measured to be 0.7, 0.6, 0.6, and 0.3 mmol/g, respectively. The overall relative variation of the amount of Pb(II) adsorbed in the pH range of 2-6 is 40%. However, for the other metals, it is above 500%. Further increase in pH was not attempted because of the possibility of precipitation of the metal ions at pH >6.24 The dotted line in Figure 3 gives the profile of the desorbed amount of alkali metal ions Na+ and K+ in blank experiments as a function of pH. It can be seen that about 1.75 mmol/g alkali metal ions were displaced at pH 2. The desorbed amount gradually declined with increasing pH of the solution. At pH 6, only about 0.6 mmol/g alkali metal ions was detected. These observations indicate that protons in solution can also exchange with the alkali metal ions. This would result in competitive uptake of protons with the heavy metal ions for the ion exchange sites. Because the concentration of proton

ions is decreased as pH is increased, it is easy to understand the decrease in the amount of alkali metal ions desorbed with increasing pH. Effect of Electrolyte on Heavy Metal Uptake. Figure 4 shows the amounts of heavy metal adsorbed and the amounts of counterions desorbed under different concentrations of electrolyte NaNO3. As discussed above, the desorption of alkali metal ions is related to the ion exchange of protons with the alkali metal ions. In the absence or presence of background electrolyte NaNO3, the desorbed amounts of counterions in the blank experiments were around 1.0 mmol/g. In the adsorption experiments of heavy metal ions Cu(II), Cd(II), and Zn(II), the values of M+/Me(II), where Me(II) represents Cd(II), Cu(II), or Zn(II), were all larger than 2, while for the adsorption of Pb(II), M+/Pb(II) was approximately in the range of 1.0-1.2. These observations are an indication of different uptake mechanisms of the heavy metal ions on ETS-10 adsorbent. It was also observed that the desorbed amount of K+ in solution was about one-third that of Na+, indicating an equal ion exchange opportunity of the two alkali metal ions with the heavy metal ions or proton, agreeing well with the formula of ETS-10, i.e., Na1.5K0.5TiSi5O13.14 Effect of Initial Ionic Concentration on Distribution Coefficient (KD). The distribution coefficient, KD (mL/g), has been used in the literature25,26 to indicate the adsorption affinity of a solid adsorbent toward a solute. In this work, the values of KD were used to help understand the affinities of ETS-10 for the heavy metal ions. KD is defined as

KD )

A h Ai - Ae V ) A Ae m

(5)

where A h and A are the activities of the heavy metal ions adsorbed on the surface of ETS-10 and in aqueous solution, respectively. Subscripts “i” and “e” denote the initial and equilibrium states, respectively. In dilute solutions, initial concentration (Ci) and equilibrium concentration (Ce) can be used to replace Ai and Ae,

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Figure 5. Distribution coefficients (KD) of (a) Pb(II), (b) Cu(II), (c) Cd(II), and (d) Zn(II) as a function of initial concentration Ci (mmol/L) in the absence and presence of NaNO3 electrolyte of different concentrations at pH 5 and 298 K. Table 2. Some Physical Parameters of Heavy Metal Ions in Solution24,28

metal

ionic radius (nm)

hydrated radius (nm)

hydration energy (kJ/mol)

solubility of hydroxides (pKs)

lead copper cadmium zinc

0.133 0.071 0.097 0.074

0.401 0.419 0.426 0.430

-1481 -2100 -1807 -2046

16.7 18.8 14.4 14.8

respectively. V/m (mL/g) is the ratio of the liquid volume over the mass of ETS-10 solid, namely the batch factor. The impact of initial concentrations of the heavy metal ions on their distribution coefficients at pH 5 and 298 K is shown in Figure 5. This figure also illustrates the effect of background electrolyte on distribution coefficients. It can be seen that the distribution coefficients of the heavy metal ions were decreased in different ways as the initial ionic concentration was increased. Further increase in initial concentrations led to a constant KD. It is also seen that the presence of electrolyte NaNO3 decreased the distribution coefficients for the heavy metal ions. Such a decrease became more remarkable as the concentration of NaNO3 was increased. Discussion The uptake properties of the heavy metal ions on ETS-10 are complicated. The process could be cumulatively affected by many factors, such as the ionic and hydrated ionic radius of the heavy metal ions, solubility of their hydroxides, their hydration energies, and surface complexation. In general, the larger the ionic radius is, the greater its affinity to ETS-10 solid is.27 In the absence of background electrolyte NaNO3, the order of the affinity constants (b) of ETS-10 toward the four heavy metals (see Figure 1a and Table 1) approximately follows the sequence of their ionic radii (see Table 2), which determine their hydration energies. It is known that ETS-10 compromises 12-membered oxygen ring windows with an opening size of 0.49 × 0.76 nm, which are straight along [100] and [010] directions and crooked along the direction of disorder.29 However, the hydrated

heavy metal ions all have a diameter larger than the pore-opening size of ETS-10. It is believed that partial dehydration of the hydrated heavy metal ions must have occurred in order to adsorb on the surface of ETS-10. Thus, hydrated Pb(II) and Cd (II) ions, which have a lower hydration energy than Cu(II) and Zn(II), have a higher affinity for ETS-10. However, the exceptionally high affinity of Cd(II) on ETS-10 indicates that the ionic radius or hydration energy is not the only crucial factor determining the uptake affinities of heavy metals; others such as charge neutralization efficiency would be cooperative as well.30 The highest maximum uptake capacity of Pb(II) on ETS-10 among the heavy metal ions is also due to its lowest hydration energy, which allows a high concentration of dehydrated Pb(II) ions to enter the channels of ETS-10. Another possible reason is complexation of Pb(II), which will be discussed later. It should also be noted that the effect of the formation of copper hydroxide on the observed adsorption amount cannot be ruled out as Cu has the largest pKs values among the heavy metals (see Table 2). Ionic strength has a key impact on the uptake of the heavy metals on ETS-10. In the presence of 0.01 and 0.1 M NaNO3, the affinity constants (b) and the maximum uptake capacities (qm) of the four heavy metal ions were decreased with the increase in the ionic strength (see Table 1). This can be explained in terms of two aspects. First, competition of Na+ ions with the heavy metal ions for the ion exchange sites of ETS-10 resulted in the observed decrease in the maximum uptake capacities with increasing electrolyte NaNO3 concentration. Second, the presence of NaNO3 had an effect on the surface electrochemical properties of ETS-10, and this effect became stronger as the concentration of NaNO3 was increased (see Figure 2). The presence of a tetravalent Ti in an octahedron in the framework of ETS-10 results in two negative charges.14,15 Therefore, ETS-10 can be considered as a permanently charged material. In addition, ETS-10 presents stacking disorder along the c crystallographic axis, together with lattice defects, including Ti-OH and Si-OH groups.31,32 Thus, in aqueous solution without the presence of electrolyte NaNO3, the following processes may occur on the surface of ETS-10: H+

OH-

T-OH2+ 798 T-OH 798 T-O- + H2O

(6)

where “T” represents Ti or Si in the framework of ETS10. In the absence of any electrolyte, at a relatively low pH value, surface defects are protonated, resulting in the reduction of negative charges. At a relatively high pH value, OH- ion can take one proton from the surface hydroxyl groups of ETS-10, resulting in a more negative ζ-potential of ETS-10 particles as can be seen from Figure 2. At pH 5, which was the pH value employed in the equilibrium experiments, the measured ζ-potential of ETS-10 in the absence of NaNO3 was about -45 mV. In the presence of electrolyte NaNO3, the negativity of the ζ-potential was lowered; thus the surface charges of ETS-10 became less negative as the concentration of NaNO3 was increased. The surface charges of ETS-10 in the system can be described using the theory of electric double layer (EDL), which is defined as33

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1/κ )

x

kT

(7)

∑

1000NAe2

cizi2

where 1/κ is the thickness of the EDL, NA is Avogadro’s number, ci and zi denote the molar concentration and the valence of the ith ion species in the bulk solution,  is the static permittivity, e is an electron charge, k is the Boltzmann constant, and T is the absolute temperature. Equation 7 shows that, for a given heavy metal ion, the concentration of ionic strength contributes significantly to the compression of the thickness of EDL and consequently causes a reduction in the ζ-potential of ETS-10, which in turn weakens the affinity of ETS10 for the heavy metal ions. Therefore, the Langmuir constants and the maximum uptake capacities were observed to decrease markedly with the increase in the concentration of background electrolyte NaNO3. The decrease in the distribution coefficients KD (see Figure 5) as the ionic strength of NaNO3 was increased can be explained similarly. Apart from the above reasons, an increase in the initial concentration of the heavy metal ions (Ci) has also resulted in a decrease in the distribution coefficients (see Figure 5), since the adsorption sites on the internal pore surface of ETS-10 would become more and more crowded by the increase in the amount of metal ions adsorbed as the initial ionic strength of the heavy metals was increased. On the basis of the desorption data of the counterions in blank and adsorption experiments, ion exchange was believed to be predominant in the uptake course of the heavy metal ions Cu(II), Cd(II), and Zn(II) on ETS-10. Theoretically, for an ideal ion exchange mechanism, 1 mol of bivalent heavy metal ion will replace 2 mol of M+, i.e., M+/Me(II) ) 2. However, at pH 5, proton in the adsorption systems could have competed with the metal cations for the ion exchange sites of ETS-10. Furthermore, under the experimental conditions, ETS10 can easily hydrolyze. Therefore, the following mechanisms can be envisioned in the batch adsorption systems as

S-M2 + Me2+ T S-Me + 2M+

(8)

S-M2 + 2H2O T S-H2 + 2M+ + 2OH-

(9)

2+

S-H2 + Me

+

T S-Me + 2H

(10)

where “S” refers to ETS-10 solid. In accordance with the above proposed mechanisms, the continuous addition of hydrogen ions to keep a constant pH during equilibrium measurements promoted the forward reaction of (9) and the backward reaction of (10), resulting in the displacement of around 1.0 mmol/g of the counterions and a decrease in the amounts adsorbed of the heavy metal ions. This is also revealed by the experimental data in Figure 3, which shows that the amounts adsorbed of the four heavy metals were decreased as the pH was decreased because of the increased concentration of proton, which competed with the heavy metal ions for the surface adsorption sites of ETS-10. The observed significant difference of the ratios between M+ over Pb(II) and M+ over the rest of metals (see Figure 4) can be explained by the experimental data in Figure 3, which indicates that the order of the competitive ability of the four heavy metal

ions and protons for the counterions Na+ and K+ follows Pb(II) > H+ > Cu(II) > Cd(II) > Zn(II) > Na+/K+. Thus, in the cases of adsorptions of Cu(II), Cd(II), and Zn(II), competition of protons for the counterions Na+ and K+ is strong, resulting in the observed high ratios of M+/ Me(II). In particular, it may be believed that the increased amounts desorbed of alkali metal ions in adsorption experiments in comparison to those in blank experiments are primarily attributed to the uptake of heavy metal ions, and they are almost 2 times higher than the amounts adsorbed of heavy metals in the absence or presence of 0.01 M NaNO3, indicating the occurrence of ion exchange. However, obvious deviation can be seen under the NaNO3 concentration of 0.1 M, which is probably due to the relative error in the measurements of high ionic concentrations of Na+. In the case of adsorption of Pb(II), first, the experimentally observed ratio of M+/Pb(II) lower than the theoretical value (2) is due to the higher adsorption affinity of ETS10 toward Pb(II) than toward proton. Second, the slightly higher desorbed amount of the counterions in adsorption experiments than in blank experiments implies that other uptake mechanisms as proposed in reactions 11 and 12, in addition to ion exchange mechanism, may be dominantly involved in the course of Pb(II) uptake.

S-O- + Me2+ + H2O f S-OMe(OH) + H+ S-O- + Me2+ f S-OMe+

(11) (12)

The lattice defects in ETS-10 framework also provide the adsorption sites for the heavy metal ions. Especially, more defects are formed when ETS-10 is synthesized from TiF4 due to the mineralizing role of F- ions.18 Under the experimental conditions, the defects may be present as Ti-O- and Si-O- groups. The oxygen atoms in these groups have lone pairs of electrons, which can bind a proton or a metal ion through sharing a pair of electrons to form a complex. Pb(II) ion may have a higher tendency to complex with the oxygen atoms than Cu(II), Cd(II), and Zn(II), partially resulting in the observed much lower ratio of M+/Pb(II) than the ratios of M+ over the rest of the heavy metal ions. Conclusions In batch systems, the maximum uptake capacities of heavy metal ions Pb(II), Cu(II), Cd(II), and Zn(II) without and with 0.01 M NaNO3 follow the order of Pb(II) > Cu(II) = Cd(II) > Zn(II), while it becomes Pb(II) > Cd(II) > Cu(II) > Zn(II) in the presence of 0.1 M NaNO3. The order of Langmuir affinity constant b of the heavy metal ions toward ETS-10 is related to the ζ-potential profiles of ETS-10 in aqueous solutions and the physicochemical characteristics of the heavy metal ions, including ionic radius and hydration energy. The order of maximum adsorption capacity is also dependent on the hydration energy of the heavy metal ions assayed and the effective interaction with the lattice oxygen. The effect of electrolyte NaNO3 on the adsorption equilibrium properties of the heavy metals on ETS-10 is determined by its concentration. In addition to the ion exchange mechanism, other mechanisms such as adsorption or complexation may be involved in the uptake process of the heavy metal ions on ETS-10, in particular for Pb(II). The competitive

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Received for review March 13, 2004 Revised manuscript received August 31, 2004 Accepted September 11, 2004 IE0498044