The effect of pH on the adsorption of phenol and chlorophenols onto natural zeolite

Colloids and Surfaces A: Physicochem. Eng. Aspects 334 (2009) 92–99

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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

The effect of pH on the adsorption of phenol and chlorophenols onto natural zeolite Rushdi I. Yousef a,∗ , Bassam El-Eswed b a b

Department of Chemistry, Faculty of Arts and Sciences, Petra University, P.O. Box 961343, Amman 11196, Jordan Zarka University College, Al-Balqa Applied University, P.O. Box 313, Zarka, Jordan

a r t i c l e

i n f o

Article history: Received 17 July 2008 Received in revised form 16 September 2008 Accepted 2 October 2008 Available online 17 October 2008 Keywords: Natural zeolite Phenol Chlorophenols Adsorption pH effect Two-site Langmuir model

a b s t r a c t The adsorption isotherms of phenol, 2-chlorophenol, 4-chlorophenol, 2,4-dichlorophenol, and 3,5dichlorophenol onto natural zeolite were obtained at pH values of 4.0, 6.0, pKa of phenols, and 10.5. A simple two-site Langmuir model was used to fit the experimental data where two types of interaction were quantified by the model. The first is the pH independent interaction of phenols with hydrophobic sites of zeolite. The second is the pH-dependent phenolate complexation with hydrophilic sites of zeolite (metal ions). The adsorption was enhanced with increasing pH values due to the increase of phenolates complexation with metal ions on zeolite surface. The number of hydrophobic sites of zeolite available for phenols (Qm1 ) was found to be greater than that of the hydrophilic sites (Qm2 ). On the other hand, the affinity constants of the hydrophilic sites (K2 ) were found to be much greater than those of the hydrophobic sites (K1 ). Moreover, the adsorption process of phenols onto zeolite was found to be independent on the ionic strength. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Chlorinated organic compounds are a major source of pollution. These pollutants are now receiving great concern due to ecological and public health concern [1]. Benzene refining plants, oil refineries, coke plants, and plants, which are processing phenols to plastic, all discharge harmful effluents of phenols [2]. Phenols may occur in domestic and industrial wastewater, natural water, and potable water supplies [3]. During the chlorination of water and sewage, phenol is readily transformed into chlorophenols. Over the last century, much research has taken place in the area of adsorption of phenols on activated carbons, which were proved to be very effective in removal of organic pollutants [4–6]. Removal of phenols from water using zeolite has not been studied extensively [4,7–10]. Okolo et al. investigated the interaction of phenol/chlorophenols with Na-Y synthetic zeolites and found that the adsorption capacity of phenol equals 0.8 mmol/g [4]. By dosing phenols vapor into Na-X zeolite and using 1 H and 29 Si MAS NMR spectroscopy, Beutel et al. [9] showed that hydrogen bonding of phenol to the oxygen atoms of zeolite is not the only interaction

∗ Corresponding author. Tel.: +962 6 5715546; fax: +962 6 5715579. E-mail addresses: [email protected] (R.I. Yousef), [email protected] (B. El-Eswed). 0927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2008.10.004

between phenol and Na-X zeolite. Of the same importance is the interaction of aromatic ring of phenol with either Na+ cations or oxygen atoms of the zeolite supercages. Khalid et al. [10] showed that the adsorption capacity of phenol onto HFAU zeolites increases in the range 0.1–0.6 mmol/g with an increase of Si/Al ratio from 5 to 500. Khalid et al. concluded that the Si/Al ratio, rather than the pore size of zeolite, has the predominant effect on the adsorption capacity of phenol. To the best of our knowledge, there is no data available in literature about the effect of pH on the adsorption of phenols onto natural zeolite. Okolo et al. [4] studied the adsorption isotherms of phenol and chlorophenols on synthetic zeolite without the addition of any buffer to avoid complications. Following our investigations on the adsorption behavior of phenol/chlorophenols on Jordanian zeolites [11,12], the present work aims to study the effect of pH on the adsorption of phenols onto natural zeolite. 2. Experimental 2.1. Zeolitic tuff preparation Natural Jordanian zeolitic tuff (from Jabal Aritayn) was crushed using Jaw crusher (LECO), homogenized, and sieved to different particle size portions. A 38.0 g tuff sample of the particle size 500–1000 ␮m was washed (using magnetic stirrer, IKMAG RH Janke

R.I. Yousef, B. El-Eswed / Colloids and Surfaces A: Physicochem. Eng. Aspects 334 (2009) 92–99

Nomenclature Ph 2-CP 4-CP 2,4-DCP 3,5-DCP MB Q

Qm

Qm1 Qm2 K K1 K2 Ka C

Cphenol Cphenolate CH+ pHpzc  CA , CB F A S CSA N q

phenol 2-chlorophenol 4-chlorophenol 2,4-dichlorophenol 3,5-dichlorophenol methylene blue amount of MB adsorbed (mmol MB/g zeolite) or amount of phenol adsorbed (mmol phenol/g zeolite) monolayer adsorption capacity (mmol MB/g zeolite) or monolayer adsorption capacity (mmol phenol/g zeolite) monolayer adsorption capacity of the first sites (mmol phenol/g zeolite) monolayer adsorption capacity of the second sites (mmol phenol/g zeolite) affinity constant (L/mmol MB) or affinity constant (L/mmol phenol) affinity constant for the first sites (L/mmol phenol) affinity constant for the second sites (L/mmol phenol) acid dissociation constant equilibrium solution concentration of MB (mmol/L) or equilibrium solution concentration of phenol (mmol/L) equilibrium concentration of phenol (mmol/L) equilibrium concentration of phenolate ion (mmol/L) concentration of H+ at equilibrium. point of zero charge surface charge (C m−2 ) total concentrations of acid and base added (mol/L) Faraday constant (96,485 C/mol) zeolitic tuff concentration (g/L) specific surface area (m2 /g) cross-sectional area occupied by one molecule (Å2 ) Avogadro’s number (mol−1 ) surface coverage (mmol phenol/g zeolite)

and Kunkel) twice with 2 L distilled water (each for 4 h) to remove any dissolved salts. After filtration, the washed sample was dried overnight in an oven (Hamoudeh Thermo Lab Industries, HO 900D) at 200 ◦ C (weight loss = 6.1%). The dry sample was kept in a desiccator over anhydrous CaCl2 until use in characterization and adsorption experiments. 2.2. Characterization of Jordanian zeolitic tuff The X-ray diffraction (XRD) analysis was carried out (using X-ray diffractometer-6000, Shimadzu) to identify the dominant crystalline phases in the zeolitic tuff. The XRD patterns were measured from 5◦ to 80◦ 2 at a scan rate of 2◦ /min. The crystalline phases were identified by detecting and analyzing the positions of the peaks using the software package supplied with the instrument. The X-ray fluorescence (XRF) analysis of zeolitic tuff was carried out using Diano-2023 instrument to determine the chemical composition of the zeolitic tuff. The point of zero charge (pHpzc ) was determined according to Basaldella et al. [13] and Yean et al. [14] by potentiometric titration. The titration was performed on 0.1 g sample of zeolitic tuff to which 100.0 mL of 0.1 M NaCl was added as a supporting electrolyte. Two

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samples were prepared, the first was titrated with 0.01 M HCl and the second was titrated with 0.01 M NaOH. The specific surface area of zeolitic tuff was determined by the method of methylene blue (MB) described by Hang and Brindly [15]. The adsorption capacity of MB was obtained from batch adsorption experiments. A series of 50 mL bottles were employed. Each bottle was filled with 100 mL of MB (Acros Organics, pure) solution of varying concentrations (8 × 10−6 to 4 × 10−5 M, 10 standards) and 0.02 g of zeolitic tuff. The stoppered bottles were shaken at 25 ◦ C and 320 rpm for 48 h. A 3.0 mL portion of each solution was withdrawn after 48 h and centrifuged. The MB concentrations were determined spectrophotometrically using UV–vis spectrophotometer (Spectroscan-80DV) at  = 665 nm. 2.3. Adsorption of phenols on zeolitic tuff Adsorption isotherms were studied using batch method in the pH range from 4.0 to 10.5 for phenol (Ph), 2-chlorophenol (2-CP), 4-chlorophenol (4-CP), 2,4-dichlorophenol (2,4-DCP), and 3,5-dichlorophenol (3,5-DCP) (all from Fluka). All experiments were conducted at 25 ◦ C. Phenols solutions were prepared in 0.1 M NaCl solution except those prepared in ionic strength-dependent experiments (Section 2.3.2). The pH of the solutions was adjusted using 0.05 M HCl/0.05 M NaOH solutions using Metrohm 744 pH meter. The phenols concentrations were determined spectrophotometrically using UV–vis spectrophotometer (Jassco, V-530). The absorbance values for Ph, 2-CP, 4-CP, 2,4-DCP, and 3,5-DCP were measured at  = 270.0, 280.0, 280.0, 285.0, and 284.5 nm, respectively. 2.3.1. pH-dependent experiments Batch adsorption of phenols onto zeolite was done in the pH range of 4–10.5. A series of 250 mL-stoppered Erlenmeyer flasks were employed. Each flask was filled with 50 mL of adsorbate solutions of varying concentrations (10, 20, 30, 40, 50, 60, 70, 80, 90, and 100 ppm of Ph, 2-CP, 4-CP, 2,4-DCP, and 3,5-DCP) and 0.100 g of zeolitic tuff at pH 4 and 0.1 ionic strength. The flasks were placed in a shaker (GFL, model 3005) run at 150 rpm for 22 h. A 200 ␮L portion was withdrawn after 22 h and diluted to 10 mL by adding 0.1 M NaCl solution adjusted to the same pH of calibration standards. Then concentrations of phenols solutions were measured using the UV–vis spectrophotometer. A relative standard deviation (R.S.D.) of the measurements of equilibrium concentrations was found to be 10–15%. In order to study the effect of pH on the adsorption of phenols, the same batch adsorption experiment was repeated at pH 6, pKa of phenols, and 10.5. The pKa values for Ph, 2-CP, 4-CP, 2,4-DCP, and 3,5-DCP are shown in Table 2. 2.3.2. Ionic strength-dependent experiments The same procedure described in pH-dependent experiments (Section 2.3.1) was done on 50 ppm 2-CP solution using different diluent’s concentrations, viz. 1.0, 0.1, 0.01, and 0.001 M NaCl solutions. All of these solutions were adjusted to pH 4.0. 3. Results and discussion 3.1. Characterization of Jordanian zeolitic tuff The XRD pattern of Jordanian zeolitic tuff is shown in Fig. 1. The XRD pattern showed the characteristic peaks of phillipsite. The chemical composition of Jordanian zeolitic tuff obtained from XRF analysis is given in Table 1. The Si/Al ratio calculated from these data was found as 2.6. The pH versus surface charge () of zeolitic tuff is plotted in Fig. 2. The surface charge () was calculated as a function

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R.I. Yousef, B. El-Eswed / Colloids and Surfaces A: Physicochem. Eng. Aspects 334 (2009) 92–99

Fig. 1. The XRD pattern of Jordanian zeolitic tuff.

Table 1 Chemical composition of Jordanian zeolitic tuff obtained from XRF analysis. Compound

wt%

SiO2 Al2 O3 Fe2 O3 MnO MgO CaO TiO2 K2 O Na2 O P2 O5 L.O.I.

42.01 14.10 11.34 0.33 10.33 8.42 2.15 0.93 1.92 0.43 8.04

Fig. 3. Adsorption isotherm for methylene blue (MB) onto Jordanian zeolitic tuff according to Eq. (2).

where CSA is the cross-sectional area occupied by MB molecule (130 Å2 ) and N is the Avogadro’s number (mol−1 ). The specific surface area of Jordanian zeolitic tuff was found to be 49 m2 /g. It is worth to mention that MB molecules occur nearly completely in the monomeric form in solutions of concentrations in the range (8 × 10−6 to 4 × 10−5 , Section 2.2) used in the present study [16]. 3.2. Effect of pH

of pH from the potentiometric titration according to Eq. (1) [13,14].  (C m−2 ) = (CA − CB + [OH− ] − [H+ ])

F AS

(1)

where F is the Faraday constant (96,485 C/mol), CA and CB are the total concentrations of acid and base added (mol/L), [H+ ] is the proton concentration given by 10−pH /H+ , [OH− ] is the OH− concentration given by 10−(pKw −pH) /OH− , A is the zeolitic tuff concentration (g/L) and S is the specific surface area of zeolitic tuff (m2 /g). The point of zero charge pHpzc is defined as the pH at which  = 0, and was found to be 6.9 for the zeolitic tuff as shown in Fig. 2. The adsorption data of MB onto zeolitic tuff (Fig. 3) was analyzed according to Langmuir equation (Eq. (2)). Q = Qm

KC 1 + KC

(2)

where Q is the amount of MB adsorbed (mmol MB/g zeolite), Qm is the monolayer adsorption capacity (mmol MB/g zeolite), K is the affinity constant (L/mmol MB), and C is the equilibrium solution concentration of MB (mmol/L). The value of Qm obtained was 0.0621 mmol MB/g zeolite. Depending on this value the specific surface area (S) of zeolite can be calculated from Eq. (3) [15,16]. S = Qm CSA N

Fig. 2. A plot of pH versus surface charge of zeolitic tuff (pHpzc = 6.9).

(3)

The present work studied the adsorption of Ph, 2-CP, 4-CP, 2,4DCP, and 3,5-DCP onto Jordanian zeolitic tuff in solutions buffered to pH 4.0, 6.0, pKa of phenols, and 10.5. The pH values were chosen so that two values were below the pKa value of the phenols (4.0 and 6.0), one at the pKa value of the phenols, and one at a pH higher than that of the pKa value (10.5). Thus, the effect of dissociation of phenols on adsorption was investigated. Furthermore, these pH values were below and above the pHpzc of Jordanian zeolitic tuff (6.9, Section 3.1) where the charge of zeolite surface is positive and negative, respectively. 3.2.1. Single-site Langmuir model The adsorption isotherms of phenols were analyzed according to Langmuir equation (Eq. (2)), where Q is the amount of phenol adsorbed (mmol phenol/g zeolite), Qm is the monolayer adsorption capacity (mmol phenol/g zeolite), K is the affinity constant (L/mmol phenol), and C is the equilibrium solution concentration of phenol (mmol/L). The Langmuir adsorption parameters and the adsorption isotherms of phenols onto natural zeolite at different pH values are given in Table 2 and Figs. 4 and 5 (experimental points), respectively. Two general features can be drawn. The first is that the adsorption capacity (Qm ) increases with increasing pH, especially in the case of unsubstituted and monosubstituted phenols. The second is that the phenol shows the highest adsorption capacity (Table 2). In the case of 2-CP, 4-CP, and 2,4DCP, the affinity constant (K) increased at higher pH values, but in the case of other phenols no systematic trend was observed (Table 2). The adsorption capacities (Qm ) of phenol and 2-chlorophenol onto Jordanian zeolitic tuff (Table 2) were found lower than those reported [4] for the same adsorbates onto Na-Y synthetic zeolite, which equal to 0.8 and 0.9 mmol/g, respectively. Although the Si/Al ratio of Jordanian zeolitic tuff (2.6, Section 3.1) is much lower than that of synthetic HFAU zeolite (20) used as adsorbent for phenol, the Qm value of phenol adsorption onto Jordanian zeolite (Table 2) was found close to that of synthetic zeolite (Qm ∼ 0.4 mmol/g) [10]. If phenol molecules behave like MB molecules, which are assumed to be adsorbed on the surface of zeolite, then it is possible to estimate the expected Qm values of phenols from

pKa a

Log total solubility (molality)a

Cross-section (Å)b

Single-site Langmuir model pH

Qm (mmol/g)

K (L/mmol)

R2

Qm estimatedc (mmol/g)

Two-site Langmuir model Qm1 (mmol/g)

Qm2 (mmol/g)

K1 (L/mmol)

K2 (L/mmol)

Sum of square residuals

Ph

9.9

−0.027

4.3

4.0 6.0 9.9 10.5

0.350 0.447 0.403 0.539

26.5 20.1 53.5 13.9

0.953 0.980 0.932 0.959

0.437

0.451

0.078

10.8

37.6 × 103

0.027

2-CP

8.5

−0.743

5.0

4.0 6.0 8.5 10.5

0.273 0.323 0.343 0.301

23.6 51.5 60.1 63.7

0.936 0.951 0.948 0.954

0.323

0.262

0.091

22.3

37.6 × 103

0.038

4-CP

9.2

−0.688

5.0

4.0 6.0 9.2 10.5

0.205 0.354 0.342 0.390

15.8 25.9 146 40.0

0.974 0.820 0.963 0.856

0.323

0.200

0.156

21.3

37.6 × 103

0.045

2,4-DCP

7.8

−1.468

–

4.0 6.0 7.8 10.5

0.294 0.328 0.292 0.291

27.7 30.0 31.4 61.8

0.912 0.907 0.932 0.966

–

0.265

0.025

31.6

37.6 × 103

0.016

3,5-DCP

8.3

−1.343

5.3

4.0 6.0 8.3 10.5

0.245

43.8

0.981

0.287

0.240

0.055

38.1

37.6 × 103

0.014

a b c d

d

0.343 0.267

Ref. [19]. Ref. [8]. Predicted from Eq. (4) depending on cross-sectional area of phenols given in this table. Not determined.

d

28.9 140

d

0.967 0.975

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Table 2 Fitting parameters of adsorption isotherms of phenol and chlorophenols onto Jordanian zeolitic tuff according to single-site Langmuir model (Eq. (2)) and two-site Langmuir model (Eq. (8)).

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Fig. 5. Adsorption isotherms of 2,4-dichlorophenol (2,4-DCP) and 3,5dichlorophenol (3,5-DCP) onto zeolite at pH 4.0, 6.0, pKa of phenols, and 10.5. Points are experimental and line is calculated (Eq. (8) and parameters from Table 2).

model may be best formulated by adopting the mechanism of the interaction between phenols and zeolite.

Fig. 4. Adsorption isotherms of phenol (Ph), 2-chlorophenol (2-CP), and 4chlorophenol (4-CP) onto zeolite at pH 4.0, 6.0, pKa of phenols, and 10.5. Points are experimental and line is calculated (Eq. (8) and parameters from Table 2).

the Qm value of MB (0.0621 mmol/g zeolite, Section 3.1) using Eq. (4). Qm (phenols) =

 CSA of MB  CSA of phenols

Qm (MB)

(4)

where the cross-sectional area (CSA) of MB = 130 Å2 [15]. The values of cross-sectional area (CSA) of phenols given in Table 2 were calculated from the square of cross-sectional diameters of phenols [8]. By substitution of these values in Eq. (4), the estimated Qm for phenols was obtained (Table 2). These estimated values were found close to those obtained experimentally by fitting the data using Langmuir equation (Table 2). Depending on these calculations, we can conclude that the adsorption capacity of phenols decreases with an increase of size of phenol. However, Langmuir model gives conditional adsorption parameters that are dependent on pH and thus provides no explanation for the effect of pH. In order to get insight about the effect of pH on the adsorption of phenol and chlorophenols onto natural zeolite, a necessity arises to formulate a model for such adsorption data. This

3.2.2. Interaction of phenols with zeolite The factors that affect phenols–zeolite interaction are solubility of phenols in water, repulsion between phenolate anions and negatively charged zeolite, hydrogen bonding between hydroxyl groups of phenols and zeolite surface, possible encapsulation of phenols into the pores, and complexation of phenolate anions with zeolite surface metals (Si, Al, Fe, Ti, Mn, Na, K, Ca, Mg, etc.). Solubility of phenols in water. It is well known that the solubility of chlorophenols in water increases with increasing pH. If the solubility is the limiting factor for adsorption process, phenols adsorption must be reduced with increasing pH. Since the reverse was observed in the present work, the solubility effect should not be considered by modeling pH effect on adsorption. Repulsion between phenolate anions and negatively charged zeolite surface. At pH values ≥ pKa value of phenols, the phenols will posses negative charges. Also, at pH values greater that the pHpzc of zeolitic tuff (6.9), the zeolite surface will be negatively charged. Thus, repulsion between phenols and zeolite surface will arise at high pH values. Since adsorption showed an increase with increasing pH, the repulsion between the phenolate anions and negatively charged zeolite surface is not a limiting factor for adsorption process. Hydrogen bonding between hydroxyl groups of phenols and surface silanol/aluminol sites of zeolite. This type of interaction is also expected to enhance the adsorption as the pH decreases because of protonation of phenols as well as the silanol (pKa = 5)/aluminol (pKa = 10) groups [17]. Again this contradicts the observed behavior. Encapsulation of phenols into the zeolitic pores. The reported diameter of phillipsite (the major constituent of Jordanian zeolitic

R.I. Yousef, B. El-Eswed / Colloids and Surfaces A: Physicochem. Eng. Aspects 334 (2009) 92–99

tuff, Section 3.1) channels ranges from 3.2 to 4.3 Å [18]. The crosssectional diameter for phenols (Table 2) is greater than or equal to the diameter of phillipsite channels. Since the values given in Table 2 do not include van der Waals radii, the actual diameters of phenols are somewhat larger. Thus, phenols are not able to access the zeolite pores. Furthermore, the above estimation of Qm values for phenols from the Qm value of MB (Section 3.2.1) confirmed that phenols were adsorbed on the surface rather than in the pores. This conclusion is supported by the work of Khalid et al. [10]. Complexation of phenolate anions with metal ions on the zeolite surface. This type of complexation may occur via a kind of charge transfer from phenolate anions to empty d-orbitals of metals (Si, Al, Fe, Ti, Mn) on the surface of zeolite. Okolo et al. [4] suggested that the benzene ring (␲ electron) rather than the hydroxyl substituent of phenols interacts with synthetic zeolite surface (without pH and ionic strength adjustment). Okolo used this mode of interaction, which weakens with the increase of substitution of chlorine on benzene ring to explain why phenol has higher adsorption capacity than chlorinated phenols towards synthetic zeolite. The complexation of phenolate anions with metals on the surface of zeolite is expected to be regarded as pH increases. This is due to the enhancement of the electron density of phenol rings, which is in agreement with the observed behavior in the present work. Since this type of interaction explains the observed increase of adsorption with increase of pH and since it explains the highest adsorption capacity of phenol, it will be included in modeling the effect of pH on adsorption of phenols onto zeolite. Attempts were made to fit the pH-dependent data simultaneously for each phenol using Eqs. (5) and (6), which are derived from single-site Langmuir equation. Q = Qm

KCphenolate

(5)

1 + KCphenolate

Taking into account the acid dissociation constant of phenol (Ka = Cphenolate CH+ /Cphenol ), the Cphenolate in Eq. (5) can be substituted with Cphenol to obtain Eq. (6). Q = Qm

K Ka (Cphenol /CH+ )

(6)

1 + K Ka (Cphenol /CH+ )

However, these attempts were without success. Consequently, twosite Langmuir model was developed. 3.2.3. Two-site Langmuir model Previous studies on phenol–zeolite interaction indicate that the amount of water molecules adsorbed onto synthetic zeolites increases linearly with an increase of the framework aluminum (decrease of Si/Al ratio) [10]. On the other hand, the adsorption capacity of hydrophobic phenol increases linearly with an increase of the siliceous part (increase of Si/Al ratio) [8,10]. These observations lead to the idea that zeolite has two types of sites. The first is hydrophilic, which is due to the presence of cations associated with aluminate framework. The second is hydrophobic, which is due to highly siliceous parts of the framework of zeolite [8]. Thus, in the derivation of two-site Langmuir model (Eq. (7)), we assumed that there are two types of interaction of phenols with zeolite. The first is the interaction of phenols with hydrophobic sites of zeolite. The second is phenolate complexation with the hydrophilic sites of zeolite (metal ions: Al, Fe, Ti, Na, K, Ca, Mg). Q = Qm1

K1 Cphenol 1 + K1 Cphenol

+ Qm2

K2 Cphenolate 1 + K2 Cphenolate

(7)

where Q is the amount of phenol adsorbed (mmol phenol/g zeolite). The Qm1 and Qm2 are monolayer adsorption capacities (mmol phenol/g zeolite) of the first and second sites, respectively. The

97

K1 and K2 are the affinity constants (L/mmol phenol) for the first and the second sites, respectively. The Cphenol and Cphenolate are the equilibrium concentrations (mmol/L) of phenol and phenolate, respectively. Using the acid dissociation expression of phenols (Ka = Cphenolate CH+ /Cphenol ), Eq. (7) becomes Q = Qm1

K1 Cphenol 1 + K1 Cphenol



+

Qm2 K2 Ka (Cphenol /CH+ ) 1 + K2 Ka (Cphenol /CH+ )



(8)

Eq. (8) was used to fit the pH-dependent data for each phenol (i.e. the data of one of the phenols at pH 4.0, 6.0, pKa , and 10.5) by means of the solver of Microsoft Excel software. Minimization of the four parametric equation was done using the constrains that the four parameters are greater than zero and by using Qm values obtained from single-site Langmuir model (Table 2) as initial guess for Qm1 values. The results of Qm1 , Qm2 , K1 , and K2 that give the least sum of square residuals (difference between experimental and calculated Q) were adapted (Table 2). The resultant fitting curves are shown in Figs. 4 and 5. Eq. (8) was found to explain successfully the effect of pH on the adsorption of phenols onto zeolite. The effect of pH on the value of the calculated second term (stands for hydrophilic sites of zeolite) of Eq. (8) is shown in Fig. 6 (right). The effect of pH is significant from pH 4 to 6, less significant from pH 6 to pKa , and not significant from pKa to pH 10.5. On the other hand, the first term (stands for hydrophobic sites of zeolite) of Eq. (8) was found to be independent of pH and so the value of the first term is constant at all pH values for the same phenol (Fig. 6, left). The results of fitting the data (Table 2) showed that the number of hydrophobic sites of zeolite available for phenols (Qm1 ) is higher than the number of hydrophilic sites ready for phenolate complexation (Qm2 ). On the other hand, the affinity constant of phenolate towards hydrophilic sites (K2 ) is much higher than the affinity of phenols towards hydrophobic sites (K1 ). Attempts to fit the data using fixed Qm1 and Qm2 values for all phenols were failed. This means that zeolite sites are not fixed with respect to different phenols. This is due to the fact that phenols have different occupation of zeolite surface depending on their crosssectional area. As shown in Table 2, the Qm1 value of unsubstituted phenol is higher than that of substituted phenols, indicating the importance of the size of phenols. The size factor is also important in determining Qm2 where the unsubstituted phenol, 2-, and 4-chlorophenols have higher Qm2 values than disubstituted phenols. Similar trend was observed by Okolo et al. [4] for adsorption of phenol on Na-Y synthetic zeolite where the Qm value of phenol was found to be greater than that of monochlorophenols. The above calculations based on cross-sectional area of phenols and methylene blue (Section 3.2.1) can account for this trend where unsubstituted phenol is predicted to have the highest adsorption capacity on zeolite, while disubstituted phenols have the lowest. The affinity constant K1 for phenols interaction with hydrophobic sites of zeolite follows the trend: dichlorophenols > monochlorophenols > unsubstituted phenol. This may indicate that some kind of charge transfer be involved from oxygen donors in zeolite to electron deficient phenol ring. The ␲-acceptor ability of phenols increases with increasing the number of chloro groups on the phenol. Thus, the most substituted phenols have the highest K1 . It is worth to mention that the affinity constants for phenolate complexation by zeolite surface (K2 ) were not fixed to the value of 37.6 × 103 (Table 2), but they were found almost the same for all phenols. The relative contribution of the second term (pH-dependent term) of Eq. (8) to the total amount of adsorption is given in Table 3. It is clear that the contribution of the second term is minimum at

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Fig. 6. The effect of pH on the calculated values of the first (left) and the second (right) terms of Eq. (8).

pH 4.0, but it increases significantly with an increase of pH. This is expected because the number of phenolate groups increases with pH and so their complexation with metal ions on the zeolite surface becomes significant. Furthermore, Table 3 shows that the contribu-

tion of the second term decreases with increasing surface coverage (q, mmol phenol/g zeolite). This is due to that the number of surface metal ions, that are available for phenolate complexation, is not sufficient for high concentrations of phenolates.

Table 3 Percentage contribution of the second term of Eq. (8) to the total amount of adsorption.a .

3.3. Effect of ionic strength

Ph pH = 4.0 pH = 6.0 pH = pKa pH = 10.5

0.1–0.3 7–12 88–22 80–19

2-CP

4-CP

2,4-DCP

2–9 60–30 85–35 67–38

1–7 48–45 97–54 100–54

2–5 47–11 32–13 78–11

3,5-DCP 1–7

The effect of changing ionic strength on the adsorption process was studied for the adsorption of 2-chlorophenol onto zeolite at pH 4.0. The results are shown in Table 4, where no significant effect of

b

70–23 74–25

a The lower limit of the range is for the lowest surface coverage (q) and the upper limit is for the highest surface coverage values. The values were calculated by dividing the value of the second term of Eq. (8) by the value of the total Q after substitution of the fitting parameters (Qm1 , Qm2 , K1 , and K2 ) and the equilibrium concentrations (C). b Not determined.

Table 4 Effect of ionic strength on the equilibrium concentration of 2-chlorophenol (2-CP) on zeolite at pH 4.0. Ionic strength (M of NaCl)

Equilibrium concentration (ppm)

1.0

0.1

0.01

0.001

5.1

5.1

4.4

5.3

R.I. Yousef, B. El-Eswed / Colloids and Surfaces A: Physicochem. Eng. Aspects 334 (2009) 92–99

ionic strength was observed. This is expected from Eq. (8) where the only variable that is affected by ionic strength is H+ and since the concentration of H+ is drawn from pH measurements, so it is activity of H+ . 4. Conclusions The proposed model for the effect of pH on the adsorption of phenol/chlorophenols onto zeolite gives satisfactory fitting of the experimental data. Two types of interaction were quantified by the model. The first is the pH independent interaction of aromatic ring of phenols with the hydrophobic sites of zeolite. The second is the pH-dependent phenolate complexation with metal ions on the hydrophilic sites of zeolite surface. The number of sites of the first type, viz. hydrophobic sites, is greater than those of the second type. The adsorption increases with increasing pH, due to the increase of phenolates complexation with metal ions on zeolite surface. The size of phenol determines their order of adsorption capacity onto the surface of zeolite, where phenol and monochlorophenols revealed higher adsorptivity than dichlorophenols. References [1] K. Kumar, R.L. Pennington, T.T. Zmuda, Capture or destroy toxic air pollutants, Environ. Eng. 100 (1993) 12. [2] R.A. Meyer, D.K. Dittrick, Environmental Pollution and Cleanup, John Wiley and Sons, New York, 2000. ´ V.V. Zˇ ivanovic, ´ A kinetic method for the determination of phenol, J. [3] S. Mitic, Serb. Chem. Soc. 67 (2002) 661. [4] B. Okolo, C. Park, M.A. Keane, Interaction of phenol and chlorophenols with activated carbon and synthetic zeolites in aqueous media, J. Colloid Interface Sci. 226 (2000) 308.

99

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