Removal of dyes from wastewaters by adsorption on pillared clays

Removal of dyes from wastewaters by adsorption on pillared clays

Chemical Engineering Journal 168 (2011) 1032–1040 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsev...

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Chemical Engineering Journal 168 (2011) 1032–1040

Contents lists available at ScienceDirect

Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Removal of dyes from wastewaters by adsorption on pillared clays A. Gil ∗ , F.C.C. Assis, S. Albeniz, S.A. Korili Department of Applied Chemistry, Building Los Acebos, Public University of Navarra, Campus of Arrosadia, E-31006 Pamplona, Spain

a r t i c l e

i n f o

Article history: Received 28 October 2010 Received in revised form 17 January 2011 Accepted 18 January 2011 Keywords: Pillared clay adsorbents Dye adsorption Organic pollutants Acid orange 7 Basic blue 9

a b s t r a c t Two pillared clays are synthesized by intercalation of solutions of aluminium and zirconium and evaluated as adsorbents for the removal of Orange II and Methylene Blue from aqueous solutions. The contact time to attain equilibrium for maximum adsorption was found to be 300 min. Both clays were found to have the same adsorption capacity when Orange II was used as adsorbent, whereas the adsorption capacity of Zr-PILC was higher than that of Al-PILC for Methylene Blue. Variations in the diffusion into micropores can explain the results found. The adsorption kinetics of dyes has been studied in terms of pseudo-first- and -second-order kinetics, and the Freundlich, Langmuir and Sips isotherm models have also been applied to the equilibrium adsorption data. The addition of NaCl has been found to increase the adsorption capacities of the two pillared clays for Orange II. © 2011 Elsevier B.V. All rights reserved.

1. Introduction There is a current need to treat dye-containing effluents prior to their discharge as these compounds and their degradation products can be toxic and carcinogenic, even at low concentrations. Adsorption is an attractive method for the removal of contaminants from effluents since, if the adsorption system is designed correctly; it will result in a high quality treated effluent. In comparison with other processes for the treatment of polluted aqueous effluents, the adsorption process allows flexibility in terms of both design and operation and produces pollutant-free effluents that are suitable for reuse. Additionally, as the adsorption is sometimes reversible, the sorbent can be regenerated, thereby resulting in significant cost savings. Although activated carbon remains the most widely used adsorbent, its relatively high cost restricts its use somewhat [1]. However, in addition to cost, adsorptive properties and availability are also key criteria when it comes to choosing an adsorbent for pollutant removal, thereby encouraging research into materials that are both efficient and cheap. Indeed, several authors have reported the use of adsorbents such as activated silica, chitosan, peat, wood, waste, red mud, fly ash and clays [2]. Furthermore, in light of their low cost, abundance, high sorption properties and potential for ion-exchange, clay minerals are interesting materials for use as adsorbents. In recent years, there has been a growing interest in the use of clay minerals as adsorbents for inorganic and organic compounds, with interactions between dyes and clay particles receiving particular attention [3–10].

∗ Corresponding author. Tel.: +34 948 169602; fax: +34 948 169602. E-mail address: [email protected] (A. Gil). 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.01.078

Pillared InterLayered Clays (PILCs) are porous materials that can be obtained by the intercalation of soils, thereby creating high value added materials from natural solids. However, these materials have been more widely studied in terms of their preparation, textural characterization and catalytic properties [11–13] than their possible application as adsorbents. Pillared clays present a lower structural regularity than zeolites but higher than other classes of adsorbent materials such as activated carbons [14]. Somewhat surprisingly, the application of pillared clays to environmental pollution control in terms of organic compound removal [15–21] in general, and dyes [22–24] in particular, from aqueous media has received little attention. Srinivasan and Fogler [16], for example, evaluated the uptake of benzo(a)pyrene and chlorophenols by a pillared clay in order to study the effect of the presence of cetyl pyridinium in the pillared clay structure on the adsorption of those organic pollutants. More recently, the adsorption of chlorinated phenols from aqueous solution by surfactant-modified pillared clays was reported by Michot and Pinnavaia [17]. Indeed, these authors found that the presence of surfactant molecules creates new adsorption sites, thereby improving the adsorption capacity of the clay studied. Adsorption of the herbicide Diuron on pillared clays has been investigated by Bouras et al. [21], who concluded that such clays can be considered to be powerful competitors to activated carbon for the treatment of aqueous industrial effluents. In light of the above, we decided to assess the effectiveness of pillared clays in removing dyes from aqueous solutions. The pore structure and surface chemistry of the adsorbent have the greatest influence on the adsorption process [25], whereas the pore size distribution affects the efficiency and selectivity of adsorption. Several studies have been conducted to illustrate the relationship between the pore size distribution of adsorbents and

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Table 1 General characteristics of the dyes used. Characteristic

Orange II

Methylene Blue SO3 Na N

Structure

N N (H3C)2N

S Cl

N(CH3)2

OH

C.I. name C.I. number Chemical formula Molecular weight (g/mol) Molecular dimensions (nm) pKa

Acid orange 7 15510 C16 H11 N2 NaO4 S 350.33 1.36 × 0.73 × 0.23 [37] pK1 = 1.0; pK2 = 11.4 [35]

their adsorption capacity [26–31]. Thus, Ebie et al. [28] studied the effect of pore size distribution on the competitive adsorption of organic micropollutants and Natural Organic Matter (NOM) on activated carbon and found that NOM competed with the micropollutants for adsorption sites and blocked their passage to micropores, where micropollutants are preferentially adsorbed. Although adsorption mainly occurs on the micropores of the adsorbent, Hsieh and Teng [27] studied the influence of mesopore volume on the adsorption of phenol, iodine and tannic acid. These authors found that mesopores facilitate the adsorption of the adsorbates in the internal and narrow micropores since the adsorptive capacity increased with mesopore volume for similar surface areas and micropore volumes. Moreover, the influence of mesopore volume on adsorptive capacity was more pronounced for larger adsorbates than for smaller ones. The objective of this paper is twofold: to evaluate the efficiency of two pillared clays as adsorbents of two reactive dyes, namely Orange II and Methylene Blue, and to study the effect of pore structure on the adsorption behaviour, and to test various kinetic and equilibrium models for the adsorption of these dyes on the clays in their single component systems.

2. Experimental procedure 2.1. Preparation of the adsorbents The clay mineral used in this work, a purified natural montmorillonite from Tsukinuno, was supplied by The Clay Science Society of Japan. The clay was intercalated with aluminium and zirconium hydroxy-polycations following a previously described standard procedure [32,33]. In the case of aluminium, the intercalating solution was prepared by slow addition of a 1.50 mol/dm3 solution of NaOH (Aldrich, 97%) to a 0.50 mol/dm3 solution of AlCl3 ·6H2 O (Merck, 97%), with vigorous stirring, to give an OH− /Al3+ molar ratio of 2.0. The hydrolysed solution was allowed to age for 48 h at 323 K whilst stirring continuously. An Al/clay ratio of 10 mmolAl /dm3 gclay was used in the intercalation process. The clay suspensions were kept in contact with the solution for 24 h at room temperature, then centrifuged and washed. In the case of zirconium, a solution of zirconium(IV) (15–16 wt.%) in acetic acid (Sigma–Aldrich) was used as the intercalating solution. This solution was kept in contact with the montmorillonite for 2 h at room temperature whilst maintaining a ratio of 20 mmolZr /gclay . After this time, the clay suspension was centrifuged and washed. Finally, the intercalated solids were dried for 16 h in air at 373 K and then calcined for 4 h at 773 K in order to obtain the pillared clays. The resulting solids are designated as Al-PILC and Zr-PILC.

Basic blue 9 52015 C16 H18 ClN3 S 319.85 1.43 × 0.61 × 0.4 [36] >14 [38]

2.2. Characterization techniques The point of zero charge for the adsorbents was determined from mass titrations [34]. Thus, varying amounts of adsorbent materials in the concentration range 0–100 g/dm3 were put into contact with a 0.01 mol/dm3 KCl solution and stirred for 24 h until the equilibrium pH was reached. The pHPZC is the pH at which a plateau is achieved when plotting the equilibrium pH versus adsorbent concentration. Textural analyses were carried out by nitrogen (Air Liquide, 99.999%) adsorption at 77 K using a static volumetric apparatus (Micromeritics ASAP 2010 adsorption analyser). Prior to analysis, 0.1 g of each sample was degassed for 24 h at 473 K at a pressure lower than 0.133 Pa. The basal spacings of the clays were identified by XRD analysis using a Siemens D-5000 Powder Diffractometer equipped with a ˚ nickel-filtered Cu K␣ radiation source ( = 1.5405 A). 2.3. Adsorption procedure The dyes used in this work were Orange II Sodium Salt (Aldrich, 85%) and Methylene Blue (Merck). The general characteristics of these two dyes [35–38] are presented in Table 1. Two stock solutions of Orange II and Methylene Blue (30 mg/dm3 ) were prepared. In a typical experiment, 0.1 g of the adsorbent was placed in a pyrex test tube containing 10 cm3 of dye solution and the mixture stirred for the desired time. The solution was then separated from the adsorbent by centrifugation for 10 min at 1500 rpm. The dye concentration in the resulting supernatant was determined by UV spectroscopy at 486 nm for Orange II and 663 nm for Methylene Blue. A Perkin Elmer Lambda 3B spectrophotometer was used for these analyses. Samples were measured in duplicate and the average value used in the subsequent analysis. The amount of dye adsorbed on the clay (qt ) was calculated as the change in the aqueous-phase concentration from the initial value, according to the following equation: qt =

V (C0 − Ct ) m

(1)

where C0 is the initial concentration (mg/dm3 ), Ct the concentration (mg/dm3 ) at contact time t (min), V the solution volume (dm3 ), and m the amount of clay added (g). Aqueous solutions of Orange II and Methylene Blue were prepared from their salts and diluted to the required initial concentrations of between 2 and 300 mg/dm3 . In order to determine the equilibrium adsorption capacity of the clay mineral samples, 10 cm3 of adsorbate solution was added to a pyrex test tube containing 0.1 g of clay. All experimental mixtures were stirred

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mechanically at a constant speed of 50 rpm for 24 h. The solutions were separated from the adsorbent by centrifugation for 10 min at 1500 rpm. The dye concentration was subsequently determined by UV–vis spectrometry at a wavelength corresponding to the maximum absorbance. Samples were measured in duplicate and the average value used in subsequent analysis. The amount adsorbed on the clays (qe ) was calculated from the initial and final concentrations according to the following equation: qe =

V (C0 − Ce ) m

(2)

where C0 is the initial concentration (mg/dm3 ), Ce the equilibrium concentration (mg/dm3 ), V the volume of the solution (dm3 ) and m the amount of clay added (g). The effect of sodium chloride (Sigma, 99.5%) on dye adsorption was investigated following the same procedure described above. All experiments were performed with three concentrations of sodium chloride (0, 0.1 and 0.01 mol/dm3 ) in the adsorption solution. 3. Theory 3.1. Kinetic modelling Although several models have been used for batch reactors to describe the transport of adsorbates inside adsorbent particles [39], the mathematical complexity of these models makes them inconvenient for practical use. Simple and explicit relationships between the adsorption performance and operating conditions are therefore preferable. In this respect, lumped kinetic models, which show how the spatially averaged solid phase concentration (qt ) changes with adsorption time, are much simpler and easier to apply for practical operations. Models in this category include pseudo-first- and -second-order rate equations and the intraparticle diffusion model. The pseudo-first- and -second-order models assume that the difference between the average solid phase concentration (qt ) and the equilibrium concentration (qe ) is the driving force for adsorption and that the overall adsorption rate is proportional to this driving force. Both equations have been widely applied to explain the experimental results obtained for aqueous pollutants such as dyes and metal ions [40–43]. The Lagergren [44] pseudo-first-order equation was used to fit the experimental results obtained herein dq = k1 (qe − q) dt

(3)

Integrating Eq. (3) for the boundary conditions t = 0 to t = t and q = 0 to q = qt gives qt = qe [1 − exp(−k1 t)]

(4)

where k1 (1/min) is the first-order rate constant, qt (mg/g) the amount of organic molecules adsorbed at time t (min), and qe (mg/g) the equilibrium adsorption capacity. The adsorption data were also analyzed in terms of a pseudosecond-order mechanism [45]. dq = k2 (qe − q)2 dt

(5)

Integrating Eq. (5) for the boundary conditions t = 0 to t = t and q = 0 to q = qt gives qt =

k2 q2e t 1 + k2 qe t

(6)

where qe (mg/g) is the equilibrium adsorption capacity and k2 (g mg/min) the rate constant of the pseudo-second-order adsorption. The intraparticle diffusion model assumes that intraparticle diffusion is the rate-controlling step, which is generally the case for

well-mixed solutions. The intraparticle diffusion model is a singleresistance model in nature and can be derived from Fick’s second law by assuming that the intraparticle diffusivity (D) is constant and that the uptake of adsorbate by the adsorbent is small in relation to the total quantity of adsorbate present in the solution. The mathematical expression for this model is: qt = k3 t 0.5

(7)

where k3 (mg/g min0.5 ) is the intraparticle diffusion rate constant. The intraparticle diffusion model has been applied to various adsorption systems previously [46,47]. 3.2. Equilibrium isotherm modelling Equilibrium isotherms describe how the adsorbent interacts with the adsorbate. Indeed, the correlation of experimental results with the adsorption model can help to explain the adsorption mechanism and the heterogeneity of the adsorbent surface. It is also of importance in the practical design and operation of adsorption systems. In order to optimize the design of a sorption system to remove dyes from effluents, it is important to establish the most appropriate correlation for the equilibrium curves. The adsorption equilibrium data were therefore fitted to the Freundlich, Langmuir and Sips isotherm models. 3.2.1. Freundlich isotherm This model has been widely applied in heterogeneous adsorption systems, especially those involving organic compounds and highly interactive species on activated carbons. It gives a representation of the adsorption equilibrium between an adsorbent in solution and the surface of the adsorbent, using a multi-site adsorption isotherm for heterogeneous surfaces that can be expressed as: qe = kF Ce1/mF

(8)

where qe (mg/g) is the equilibrium amount of adsorbent adsorbed by the solid, Ce (mg/dm3 ) the equilibrium concentration of adsorbate, and kF and mF empirical constants, and is therefore indicative of the extent of adsorption and its effectiveness. The constant mF gives an indication of the degree of heterogeneity in the distribution of energetic centres and is related to the magnitude of the adsorption driving force. Thus, high mF values for isotherms indicate relative uniformity at the surface, whereas low values suggest high adsorption at low concentrations in solution. Furthermore, low values of mF indicate the existence of a higher proportion of active sites with high energy. 3.2.2. Langmuir isotherm The Langmuir theory describes the monolayer coverage of adsorbate on a homogeneous adsorbent surface. The adsorption isotherm is based on the assumption that sorption takes place at specific homogeneous sites within the adsorbent and that once an adsorbate molecule occupies a site, no further adsorption can take place at that site. The saturation monolayer can be represented by the following expression: qe =

qL kL Ce 1 + kL Ce

(9)

where qe (mg/g) is the equilibrium amount of adsorbate adsorbed by the solid, Ce (mg/dm3 ) the equilibrium concentration of adsorbate, and qL (mg/g) and kL (dm3 /mg) Langmuir constants representing the monolayer adsorption capacity and the energy adsorption, respectively.

A. Gil et al. / Chemical Engineering Journal 168 (2011) 1032–1040

100

A

5

ΔVp/dp (cm STP/g·nm) · 10

100

3

3

Volume adsorbed (cm STP/ g)

150

1035

50

80 Al-PILC

60

Zr-PILC

40 20 0

0 0

0.2

0.4

0.6

0.8

1

1

10

Relative pressure (p/pº) 0.50

Fig. 1. Experimental isotherm for the adsorption of nitrogen at 77 K. () Montmorillonite, () Al-PILC and () Zr-PILC.

0.67 nm

J(x) (mmol STP/g·nm)

3.2.3. Sips isotherm The Sips model is an improvement of the Freundlich and Langmuir equations which proposes that the equilibrium data follow the Freundlich model at low initial solute concentrations and the Langmuir one at high solute concentrations. The equation can be expressed as: qS kS CemS qe = 1 + kS CemS

100

Pore width (nm)

B

0.40

Al-PILC

0.30

Zr-PILC 0.58 nm

0.20 0.10

(10) 0.00

where qe (mg/g) is the equilibrium amount of adsorbate adsorbed by the solid, Ce (mg/dm3 ) the equilibrium concentration of adsorbate, qS (mg/g) and kS (dm3 /mg) the Sips constants representing the monolayer adsorption capacity and the energy of adsorption, respectively, and mS is an empirical constant.

0.4

0.8

1.2

1.6

Pore width (nm) Fig. 2. Mesopore (A) and micropore (B) size distributions derived using the Barrett–Joyner–Halenda method and the Jaroniec–Gadkaree–Choma model. () Al-PILC and () Zr-PILC.

4. Results and discussion 4.1. Characterization of the adsorbents The interactions between adsorbates and adsorbents control the adsorption of organic molecules on the surface of materials. Likewise, the physical characteristics of the adsorbent allow the access of molecules to the pores inside the porous material. The accessible adsorbent surface is therefore especially important in the adsorption processes. The nitrogen adsorption isotherms of the materials are presented in Fig. 1, from which it can be seen that these isotherms are of type I + II in the Brunauer, Deming, Deming and Teller (BDDT) classification [48]. The most important differences between the nitrogen-adsorption isotherms of the montmorillonite and the pillared clays can be seen at relative pressures lower than 0.10. The textural properties of the clays (see Table 2) suggest that the intercalation and pillaring processes result in an important increase in surface area and pore volume with respect to the starting montmorillonite. These results are in accordance with the basal spacings found by XRD (Table 2). The Langmuir surface area (SLang ) was calculated from the adsorption data over the relative pressure range 0.01–0.05 using cross-sectional area for the nitrogen molecule of 0.162 nm2 [48]. The total pore volume (Vp ) was estimated from the

amount of nitrogen adsorbed at a relative pressure of 0.99, assuming the density of the nitrogen condensed in the pores to be equal to that of liquid nitrogen at 77 K (0.81 g/cm3 ) [48]. The external surface area (Sext ) and micropore volume (V␮p ) were estimated using the t-plot method [48]. The pillared clays studied in this work exhibit much higher specific surface areas (272 and 356 m2 /g for Al- and Zr-PILC, respectively) than that of the natural clay (9 m2 /g). A more detailed description of the microporous and mesoporous regions of the clays was obtained from the pore size distributions. The micropore size distributions, as derived using the model proposed by Jaroniec–Gadkaree–Choma [12], were used to examine the micropore characteristics of the materials studied (see Fig. 2). It can be seen from this figure that a first maximum occurs at 0.58 nm in both distributions, with a second maximum at a pore diameter of 0.67 nm. An important increase in the volume adsorbed by the micropores, from 0.002 cm3 /g for the natural clay to 0.128 cm3 /g for Zr-PILC, takes place as a result of the pillaring process (see Table 2). The mesoporous region was characterized using the Barrett–Joyner–Halenda (BJH) method [49]. The Halsey equation [49] was used to calculate the statistical thickness of adsorbed nitrogen assuming a nitrogen molecule cross-sectional area of 0.162 nm2 [48]. A comparison of the mesopore volumes (Vp ) for

Table 2 Specific surface areas, pore volumes, points of zero charge (pHPZC ) and basal spacings (d(0 0 1)) for the adsorbents.

Montmorillonite Al-PILC Zr-PILC

SLang (m2 /g)

Sext (m2 /g)

Vp (cm3 /g)

V␮p (cm3 /g)

Vmp (cm3 /g)

pHPZC

d(0 0 1) (nm)

9 272 356

3 13 23

0.047 0.136 0.186

0.002 0.097 0.128

0.020 0.027 0.044

9.9 4.4 4.1

1.05 1.64 1.91

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A. Gil et al. / Chemical Engineering Journal 168 (2011) 1032–1040

2.0

3

Amount adsorbed (mg/g)

1.5

Volume adsorbed (cm STP/ g)

150

A

1.0

0.5

Al-PILC Zr-PILC pseudo-first-order pseudo-second-order

(a)

0

300

600

900

1200

1500

time (min)

(c)

100

Al-PILC

50

Zr-PILC

0 -5

10

0.0

(b)

0.0001

0.001

0.01

0.1

1

Relative pressure (p/pº) Fig. 4. Experimental isotherm for the adsorption of nitrogen at 77 K. (a) Ultramicropore, (b) micropore and (c) supermicropore regions. () Al-PILC and () Zr-PILC.

50

Amount adsorbed (mg/g)

B 40

Al-PILC

30

Zr-PILC pseudo-first-order

20

pseudo-second-order

10

0 0

300

600

900

1200

1500

time (min) Fig. 3. Kinetic adsorption data for Orange II (A) and Methylene Blue (B) on Al-PILC () and Zr-PILC (). T = 298 K, C0 = 30 mg/dm3 , [NaCl] = 0.1 mol/dm3 , pH = 7. The lines represent a pseudo-first-order model ( ) and a pseudo-second-order model (- - -).

pores in the range 1.5–50 nm, as determined by the BJH analysis, can also be found in Table 2. The pillaring processes do not influence the mesopore size range, although an increase in the external surface area is observed. The chemical characteristics of the adsorbents affect the attractive or repulsive interactions between the adsorbent surface and the adsorbate. pHPZC is an important chemical characteristic of adsorbents as it determines the net charge of the adsorbent surface in the solution. The pHPZC values determined for the clays studied herein are summarized in Table 2. The results show that the pillaring process clearly modifies the surface chemical characteristics but that the characteristics of the two clays are the same, or very similar, after this process. 4.2. Adsorption experiments Adsorption kinetic studies are key to evaluating the effectiveness of an adsorbent. An evaluation of the amount of adsorbed molecules (qt ) as a function of adsorption time is presented in Fig. 3. The adsorption kinetics for the two PILCs and the two organic molecules only differ in terms of the equilibrium capacity achieved for the two materials. Both pillared clays have a higher adsorption capacity than the starting montmorillonite, probably because the former have higher specific surface areas and pore volumes than the latter (see Table 2). The adsorption of dye molecules is restricted by narrower pores, therefore the presence of high surface areas and wider pores always

results in a high dye adsorption capacity [50]. These findings confirm that the pore structure of the adsorbents has an important influence on the adsorption capacity of reactive dyes. It can also be seen that both pillared clays have similar adsorption capacities for Orange II as the adsorbent, whereas the adsorption capacity of Zr-PILC is higher than that of Al-PILC in the case of Methylene Blue. The differences observed in Fig. 3 could be due to the fact that dye adsorption is not sensitive to the same type of porosity. The nitrogen adsorption results presented in Fig. 4 can be used to study the possible influence of the samples’ microstructure on this behaviour. In this respect, Sing reported [51] that two stages of micropore filling with nitrogen can be related to different micropore size ranges. Thus, an initial process (p/p◦ < 0.005) takes place in pores with a size (<0.8 nm) comparable to that of the nitrogen molecule, known as ultramicropores, with a second one occurring at p/p◦ values between 0.005 and 0.2. In this range of relative pressures, the filling of two types of micropores, namely micropores (0.8–1.4 nm) and supermicropores (1.4–2.0 nm) [52], at relative pressures in the ranges 0.005–0.08 and 0.08–0.2, respectively, can also be considered. The range of pores found herein (Fig. 4) clearly shows that the micropores are responsible for the differences between the two samples. Li et al. [30] have reported that the adsorption of solute molecules can take place in pores with a diameter 1.3–1.8-times that of the solute. Therefore, if we consider that the dye molecules most likely diffuse into the porous structure of the adsorbent lengthwise and we assume a value of 1.8 times the molecular diameter, this implies a minimum pore adsorbing diameter of 1.31 for Orange II and 1.10 nm for Methylene Blue (see the molecular dimensions provided in Table 1). Consequently, restricted diffusion into the micropores can explain the results presented above. The surface chemistry of the adsorbents is also an important factor that controls the adsorption of adsorbate. Thus, adsorbents have a net negative charge on their surface when the equilibrium pH of the solution is higher than their pHPZC , whereas they have a net positive charge when the equilibrium pH in the solution is lower than their pHPZC . Al- and Zr-PILC therefore have an overall negative charge as the equilibrium pH of 7 is higher than their pHPZC values, thus meaning that electrostatic attraction between the negatively charged surface and the positively charged dye molecules could favour adsorption. This is the case for Methylene Blue, whereas Orange II, which is an anionic dye, will experience repulsion with the negatively charged surface of the pillared clays. However, the experimental results show that there is a strong interaction between the surface of the clay and the dye (see Fig. 3), thereby suggesting that the effect of Van der Waals and ␲–␲ inter-

A. Gil et al. / Chemical Engineering Journal 168 (2011) 1032–1040

Al-PILC

First-order k1 (1/min) 2 R Second-order k2 (g/mg min) 2 R Intraparticle k3 (mg/g min0.5 ) R  k 3 (mg/g min0.5 ) R

Zr-PILC

Orange II

Methylene Blue

Orange II

Methylene Blue

0.030 0.45 0.59

0.012 66 1

0.103 0.40 1

0.019 147 0.86

0.040 0.15 0.74

0.0017 43 1

0.111 0.13 0.49

0.41 0.97 0.22 0.93

0.054 0.98 0.0062 0.93

0.064 0.97 0.0095 0.62

0.00087 92 0.92

2.0

A Amount adsorbed (mg/g)

Table 3 Pseudo-first- and -second-order and intraparticle rate parameters for Orange II and Methylene Blue adsorption by Al-PILC and Zr-PILC. T = 298 K, C0 = 30 mg/dm3 , [NaCl] = 0.1 mol/dm3 , pH = 7.

1037

1.5

1.0

0.5 Al-PILC Zr-PILC

2.31 0.98 0.31 0.90

0.0 0

10

20

time

30 0.5

40

50

0.5

(min )

50

B Amount adsorbed (mg/g)

actions cannot be neglected and, indeed, that they appear to be dominant [53,54]. The adsorption kinetic parameters were estimated by nonlinear regression (see Table 3) and the regression coefficients for the pseudo-first- and -second order equations for the adsorption of Methylene Blue found to be higher than 0.86. A comparison of these results with the experimental suggests that the secondorder model is more reasonable than the first-order one. In a theoretical analysis of the general equation, Azizian [44] found that the sorption process obeys pseudo-first-order kinetics at high initial adsorbate concentrations but pseudo-second-order kinetics at lower initial concentrations. If the adsorption process follows the intraparticle diffusion model, a plot of qt against t0.5 should be linear and pass through the origin. In contrast, an initial external mass transfer or chemical reaction means that the slope will be linear but will not pass through the origin [55,56]. As the plot is often multilinear for many adsorption systems (see Fig. 5), it is common to segment it into two or more straight lines and to assume that different adsorption mechanisms control the step represented by each straight line [46]. Such a behaviour indicates that the rate constants in the kinetic models may vary with the solid phase concentration (qt ). Two such linear sections can clearly be seen in Fig. 5. The slope of the linear portion (see Table 3) indicates the rate of the adsorption process. The results show that the diffusion rates decrease upon increasing the contact time. As the dye molecules diffuse into the inner structure of the adsorbents, the number of pores available for dif-

40

30 Al-PILC Zr-PILC

20

10

0 0

10

20

time

30 0.5

40

50

0.5

(min )

Fig. 5. Intraparticle diffusion model for the adsorption of Orange II (A) and Methylene Blue (B) on Al-PILC () and Zr-PILC ().

fusion decreases. This reduces the free path of the molecules in the pore and means that some molecules may also be blocked. In accordance with the porous structure presented for these materials in Figs. 2 and 4, where the presence of micropores is mainly observed, the first linear region can be related to adsorption into the micropores and the second one to adsorption on the external surface.

Table 4 Freundlich, Langmuir and Sips parameters for Orange II adsorption by Al-PILC and Zr-PILC. T = 298 K, pH = 7. Al-PILC [NaCl] = 0 mol/dm3 Freundlich kF mF 2 R Langmuir qL (mg/g) kL (dm3 /mg) 2 R Sips qS (mg/g) kS (dm3 /mg) mS 2 R

Zr-PILC [NaCl] = 0.01 mol/dm3

[NaCl] = 0.1 mol/dm3

[NaCl] = 0 mol/dm3

[NaCl] = 0.01 mol/dm3

[NaCl] = 0.1 mol/dm3

0.044 1.38 0.20 0.97

0.12 1.40 0.48 0.991

0.24 1.43 0.21 0.994

0.039 1.34 0.21 0.97

0.12 1.40 0.48 0.991

0.25 1.38 3.5 0.996

4.1 0.0047 0.15 0.98

8.9 0.0061 0.39 0.992

8.5 0.017 0.13 0.996

4.3 0.0043 0.16 0.98

8.9 0.0061 0.39 0.993

38 0.0024 1.8 0.998

9.8 0.017 0.94 0.12 0.996

2.80 0.0023 1.3 0.15 0.98

2.9 0.003 1.2 0.15 0.98

11 0.006 0.92 0.38 0.993

11 0.006 0.92 0.38 0.993

39 0.0024 1.0 1.81 0.998

1038

40

10

Al-PILC

Amount adsorbed (mg/g)

A

A. Gil et al. / Chemical Engineering Journal 168 (2011) 1032–1040

Amount adsorbed (mg/g)

3

NaCl = 0.1 mol/dm

8

3

NaCl = 0.01 mol/dm 3

NaCl = 0 mol/dm

6

4

30

Zr-PILC 20

10

2

Al-PILC 0

0 0

50

100

150

200

250

300

0

B Amount adsorbed (mg/g)

150

200

250

300

Fig. 7. Experimental (symbols) and model (lines, Freundlich) isotherms for the equilibrium adsorption data of Methylene Blue on Al-PILC () and Zr-PILC (). [NaCl] = 0.1 mol/dm3 , T = 298 K, pH = 7.

Zr-PILC

3

NaCl = 0.1 mol/dm

Table 5 Freundlich and Langmuir parameters for Methylene Blue adsorption by Al-PILC and Zr-PILC. [NaCl] = 0.1 mol/dm3 , T = 298 K, pH = 7.

3

8

NaCl = 0.01 mol/dm 3

NaCl = 0 mol/dm 6

Freundlich qF mF 2 R Langmuir qL (mg/g) kL (dm3 /mg) 2 R

4

2

0 50

100

Equilibrium concentration (mg/dm )

10

0

50

3

3

Equilibrium concentration (mg/dm )

100

150

200

250

Al-PILC

Zr-PILC

2.2 2.6 14 0.96

7.0 3.6 230 0.90

21 0.017 29 0.91

27 0.21 527 0.76

300

3

Equilibrium concentration (mg/dm ) Fig. 6. Experimental (symbols) and model (lines, Freundlich) isotherms for the equilibrium adsorption data of Orange II on Al-PILC (A) and Zr-PILC (B). T = 298 K, pH = 7.

The equilibrium liquid phase concentration (Ce ) for adsorbents was measured, their adsorption capacity (qe ) calculated and the results expressed as plots of solid-phase against liquid-phase dye concentration (see Figs. 6 and 7). Both clays were found to have the same adsorption capacity for Orange II, whereas Zr-PILC has a higher adsorption capacity than Al-PILC for Methylene Blue. The shapes of the isotherms indicate L3-type behaviour according to the Giles and Smith classification [57]. Furthermore, they are indicative of a high affinity between the sorbent surface and the reactive dye molecules. Most adsorption isotherms reported for reactive dyes are of the L2 or L3 type [10,43,58], which are usually associated with ionic solute adsorption (e.g. metal cations and ionic dyes) together with weak competition with the solvent molecules. The best fitting results were obtained with the Sips isotherm model. The parameters were subsequently estimated by non-linear regression (see Tables 4 and 5) and the regression coefficients for the Freundlich, Langmuir and Sips equations found to be higher than 0.76. The effect of NaCl on adsorption varied and depended on the interaction between NaCl and the adsorbents/adsorbates. As shown in Fig. 6, the adsorption capacity of the adsorbents increased almost threefold in the presence of NaCl concentrations of between 0 and 0.1 mol/dm3 . Several possibilities have been presented previously to explain the effect of ionic strength on the adsorption process. The electrostatic interaction between the dye molecules and the adsorbent surfaces can be either attractive or repulsive, therefore

the adsorption capacities should increase when the interactions are repulsive and decrease when they are attractive [59,60]. As discussed previously, Orange II is an anionic dye and therefore experiences repulsion with the negatively charged surface of the pillared clays. However, added salts can neutralize the negatively charged dye molecules and the negatively charged surface of clays, thereby enabling them to adsorb more dye molecules. Other forces may, however, also affect the adsorption process [44,58]. In this context, the interactions between an acid-activated pillared clay and several organic cations, including dyes, have been studied by Mishael et al. [61], who found that adsorption was irreversible but saturated at levels below the cation exchange capacity (CEC) of the clay when the dyes were adsorbed from low ionic strength solutions. This mode of adsorption was interpreted in terms of interlayer adsorption with steric hindrance in the pillared galleries, a similar situation to that found in this work. In contrast, adsorption levels well beyond the CEC of the clay could be reached when the dyes were adsorbed from high ionic strength solutions. This was interpreted in terms of a second adsorption mode involving the formation of molecular aggregates [62] on the outer surface of the clay. 5. Conclusions This work shows that pillared clays could be considered as potential low cost adsorbents for dye removal from aqueous solutions. The kinetic study revealed that stirring for 300 min is sufficient for the dye/pillared clay system to reach equilibrium. The pillared clays have the same adsorption capacity for Orange II, whereas the

A. Gil et al. / Chemical Engineering Journal 168 (2011) 1032–1040

adsorption capacity of Zr-PILC for Methylene Blue is higher than that for Al-PILC. This result can be explained by supposing that dye adsorption is not sensitive to the same type of porosity. Thus, micropores in the range 0.8–1.4 nm are the main difference between the two pillared clays, therefore restricted diffusion into such pores can explain the results found. The surface chemistry also affects dye adsorption. The pillaring processes clearly modify the surface chemical characteristics as the two adsorbents show an overall negative charge in the adsorption experiments. The adsorption capacity of both adsorbents increases significantly with increasing NaCl concentration. This result is consistent with the formation of dye aggregates and the presence of electrostatic interactions between the dyes and the pillared clay surfaces, as well as other forces or interactions. Acknowledgements This work was supported by the Spanish Ministry of Science and Innovation (MICINN) through project MAT2010-21177-C02-01. F.C.C. Assis acknowledges financial support from Government of Navarra through a PhD fellowship. References [1] G. Newcombe, Adsorption from aqueous solutions: water purification, in: E.J. Bottani, J.M. Tascón (Eds.), Adsorption by Carbons, Elsevier, Amsterdam, 2008. [2] G. Crini, Non-conventional low-cost adsorbents for dye removal: a review, Bioresour. Technol. 97 (2006) 1061–1085. [3] G. Rytwo, S. Nir, L. Margulies, A model for adsorption of divalent organic cations to montmorillonite, J. Colloid Interface Sci. 181 (1996) 551–560. [4] K.R. Ramakrishna, T. Viraraghavan, Dye removal using low cost adsorbents, Water Sci. Technol. 36 (1997) 189–196. [5] G. Rytwo, S. Nir, M. Crespin, L. Margulies, Adsorption and interactions of methyl green with montmorillonite and sepiolite, J. Colloid Interface Sci. 222 (2000) 12–19. [6] G. Rytwo, D. Tropp, C. Serban, Adsorption of diquat, paraquat and methyl green on sepiolite: experimental results and model calculation, Appl. Clay Sci. 20 (2002) 273–282. [7] M.G. Neumann, F. Gessner, C.C. Schmitt, R. Sartori, Influence of the layer charge and clay particle size on the interactions between the cationic dye methylene blue and clays in an aqueous suspension, J. Colloid Interface Sci. 255 (2002) 254–259. [8] D. Ghosh, K.G. Bhattacharyya, Adsorption of methylene blue on kaolinite, Appl. Clay Sci. 20 (2002) 295–300. [9] A.G. Espantaleón, J.A. Nieto, M. Fernández, A. Marsal, Use of activated clays in the removal of dyes and surfactants from tannery waste waters, Appl. Clay Sci. 24 (2003) 105–110. [10] Y. El Mouzdahir, A. Elmchaouri, A. Mahboub, A. Gil, S.A. Korili, Adsorption of Methylene Blue from aqueous solutions on a Moroccan clay, J. Chem. Eng. Data 52 (2007) 1621–1625. [11] A. Gil, L.M. Gandia, M.A. Vicente, Recent advances in the synthesis and catalytic applications of pillared clays, Catal. Rev. Sci. Eng. 42 (2000) 145–212. [12] A. Gil, S.A. Korili, M.A. Vicente, Recent advances in the control and characterization of the porous structure of pillared clay catalysts, Catal. Rev. Sci. Eng. 50 (2008) 153–221. [13] A. Gil, S.A. Korili, R. Trujillano, M.A. Vicente, Pillared Clays and Related Catalysts, Springer, New York, 2010. [14] A. Gil, Analysis of the micropore structure of various microporous materials from nitrogen adsorption at 77 K, Adsorption 4 (1998) 197–206. [15] T.F. Nolan, K.R. Srinivasan, H.S. Fogler, Dioxin sorption by hydroxy-aluminumtreated clays, Clays Clay Miner. 37 (1989) 487–494. [16] K.R. Srinivasan, H.S. Fogler, Use of inorgano-organo-clays in the removal of priority pollutants from industrial wastewaters: adsorption of benzo(a)pyrene and chlorophenols from aqueous solutions, Clays Clay Miner. 38 (1990) 287–293. [17] L.J. Michot, T.J. Pinnavaia, Adsorption of chlorinated phenols from aqueous solution by surfactant-modified pillared clays, Clays Clay Miner. 39 (1991) 634–641. [18] R. Mokaya, W. Jones, M.E. Davies, M.E. Whittle, Preparation of alumina-pillared acid-activated clays and their use as chlorophyll adsorbents, J. Mater. Chem. 3 (1993) 381–387. [19] Th.G. Danis, T.A. Albanis, D.E. Petrakis, P.J. Pomonis, Removal of chlorinated phenols from aqueous solutions by adsorption on alumina pillared clays and mesoporous alumina aluminum phosphates, Water Res. 12 (1998) 295–302. [20] W. Mathes, G. Kahr, Sorption of organic compounds by Al and Zr-hydroxyintercalated and pillared bentonite, Clays Clay Miner. 48 (2000) 593–602. [21] O. Bouras, J.-C. Bollinger, M. Baudu, H. Khalaf, Adsorption of diuron and its degradation products from aqueous solution by surfactant-modified pillared clays, Appl. Clay Sci. 37 (2007) 240–250.

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