Adsorption characteristics, isotherm, kinetics, and diffusion of modified natural bentonite for removing the 2,4,5-trichlorophenol

CLAY-02891; No of Pages 7 Applied Clay Science xxx (2014) xxx–xxx

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Research paper

Adsorption characteristics, isotherm, kinetics, and diffusion of modified natural bentonite for removing the 2,4,5-trichlorophenol H. Zaghouane-Boudiaf ⁎, Mokhtar Boutahala, Sousna Sahnoun, Chafia Tiar, Fatima Gomri Laboratoire de Génie des Procédés Chimiques (LGPC), Département de Génie des Procédés, Faculté de Technologie, Université Ferhat Abbas, Sétif 19000, Algeria

a r t i c l e

i n f o

Article history: Received 6 September 2012 Received in revised form 14 January 2013 Accepted 24 December 2013 Available online xxxx Keywords: Bentonite Sulfuric acid Surfactant Adsorption 2,4,5-trichlorophenol

a b s t r a c t Adsorption of the 2,4,5 trichlorophenol (TCP) from aqueous solution onto the surface of organo-bentonites was investigated spectrophotometrically. Natural bentonite was activated with sulfuric acid at 90 °C and exchanged with a set of 4 alkyltrimethylammonium bromides (alkyl = C12, C14, C16 and C18) to evaluate the effect of carbon chain length on the TCP adsorption. X-ray diffraction was used to study the change in the structural properties of the samples. The basal spacing of the activated-bentonite (AB) increased from 13.4 to 21.5 Å by intercalation of the cationic surfactants in the interlayer space of the clays. The intercalated cationic surfactants were characterized by Fourier transform infrared spectroscopy (FTIR). The surface areas of organo-bentonites were found to be much lower than that of AB. The contact time on the adsorption process was studied and the adsorption of TCP onto organo-bentonites followed pseudo-second-order kinetics. Adsorption isotherms were established and found to correlate with the Langmuir model with correlation coefficient of 0.998. Adsorption capacity of organobentonite increased with increasing the alkyl chain length. Results showed that TCP strongly interacted with AB exchanged with octadecyltrimethylammonium bromide (C18). © 2014 Elsevier B.V. All rights reserved.

1. Introduction Natural bentonites are argillaceous materials that have been widely used to remove toxic metal ions, dyes, chlorophenols and drugs (Al-Anber, 2010; Baghel et al., 2006; Bhattacharyya and Gupta, 2007; Putra et al., 2009; Zheng et al., 2008; Zhi-rong and Shao-qi, 2010). Due to their high specific surface area, high cation exchange capacity (CEC), their chemical and physical stability, they can be effectively employed as adsorbents for many wastewater pollutants. Their adsorption capacity toward organic molecules is possible but very low. To improve adsorption of organic pollutants, many researchers have focused on the surface modifications by exchanging interlayer inorganic cations (e.g. Na+, K+, Ca2+) with organic cations such as quaternary alkylammonium. Modification of the bentonite with surfactant molecules changes the properties of the bentonite from hydrophilic to hydrophobic and organophilic. It also increases the basal spacing of the layers. Thus, organophilic bentonite can have various applications, especially as adsorbents for a great variety of organic pollutants (Gładysz-Płaska et al., 2012; Kaufhold et al., 2007; Yilmaz and Yapar, 2004; Zaghouane-Boudiaf and Boutahala, 2011a; Zhu and Zhu, 2008). Another modification of bentonite is the treatment of clay minerals with concentrated inorganic acids usually at high temperature. This process is known as acid activation. The most significant mechanism in the activation of natural bentonite is cation exchange by H+ ions

(Babaki et al., 2008). These transformations in the bentonite give rise to significant changes in the cation exchange capacity (CEC), chemical and mineralogical characteristics of the bentonite (Komadel and Madejová, 2006). Acid treatments of the clay minerals retain the layered morphology and improve the adsorption properties of the clay by increasing the number of active sites (Min-Yu and Su-Hsia, 2006). Depending on the materials used and the modification procedure, the cost of adsorbent is different. In previous works, adsorption of the 2,4,5-trichlorophenol (TCP) was studied onto Algerian bentonite converted to Na-montmorillonite, activated with sulfuric acid (ZaghouaneBoudiaf and Boutahala, 2011b), onto Na-montmorillonite exchanged with hexadecyltrimethylammonium (MtC16) and acid-organomontmorillonite (AMt-C16) (Zaghouane-Boudiaf and Boutahala, 2011a). Contrary to the previous works, the modification procedure used in this study is simple. The mineral was not converted into homoionic form; this factor decreases the cost of the production of adsorbents. Natural bentonite was activated with sulfuric acid and exchanged with four surfactants: dodecyl (C12), tetradecyl (C14), hexadecyl (C16) and octadecyl (C18) trimethylammonium bromide at 100% CEC. To test the obtained organo-samples, adsorption of the 2,4,5-trichlorophenol (TCP) was investigated. 2. Experimental 2.1. Materials

⁎ Corresponding author. Tel./fax: +213 36 96 55 38. E-mail address: [email protected] (H. Zaghouane-Boudiaf).

The bentonite used in this study was from Hammam Boughrara (West Algeria). Its chemical composition was found to be as

0169-1317/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.clay.2013.12.030

Please cite this article as: Zaghouane-Boudiaf, H., et al., Adsorption characteristics, isotherm, kinetics, and diffusion of modified natural bentonite for removing the 2,4,5-trichlorophenol, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2013.12.030

2

H. Zaghouane-Boudiaf et al. / Applied Clay Science xxx (2014) xxx–xxx

follows: 69.4% SiO2, 1.1% MgO, 14.7% Al2O3, 0.8% K2O, 0.3% CaO, 1.2% Fe 2 O 3 , 0.5% Na 2 O, 0.2% TiO 2 , 0.05% As and 11% loss of ignition (Khalaf et al., 1997). The cation exchange capacity (CEC) was 0.90 meq/g (Aliouane et al., 2002). Dodecyl, tetradecyl, hexadecyl and octadecyl-trimethylammonium bromide (respectively: C12, C14, C16 and C18), sulfuric acid and the 2,4,5-trichlorophenol (TCP) were acquired from Sigma-Aldrich Chemicals (99% of purity) and used without any purification for prepared solutions. 2,4,5trichlorophenol has a molecular weight of 197.45 g/mol and linear formula: Cl3C6H2OH. Its solubility in water at 20 °C is 900 mg/L and it has a pKa between 6.7 and 6.94. 2.2. Preparation of samples The raw bentonite was treated under mechanical stirring with 1 M H2SO4 solution at 90 °C for 4 h. The mass ratio of the bentonite to the acid solution was 1:1. The activated solid was washed with distilled water until SO 24 − free and dried at 80 °C. The derivate is an acidactivated bentonite noted AB. C12, C14, C16 and C18 in amount equivalent to 100% of the CEC of AB were dissolved separately in 1 L of distilled water at 80 °C and stirred for 3 h. A total of 10 g AB was added to each 1 L surfactant solution. The dispersions were stirred for 3 h at 80 °C. The separated samples were washed with distilled water several times, until the supernatant solution was free of bromide ions. The results were oven-dried at 80 °C until the water was completely evaporated. The derivates are ABC12, ABC14, ABC16 and ABC18.

reproducibility. The TCP uptake by organo-bentonites was calculated by the following equation: qt ¼

ðC0 ‐Ct Þ:V m

1

where qt is the amount of adsorbed TCP at time t (mg/g); V the volume of solution (L),C0 and Ct are the initial and at time t TCP concentration respectively (mg/L) and m is the weight of adsorbent (g). 2.5. Adsorption isotherms A constant volume of TCP solution (50 mL) with varying initial concentrations (10–250 mg/L) was mixed with a constant amount of 20 mg. The dispersions were shaken at a temperature of 23 ± 1 °C, under an agitation speed (which was optimized) of 100 rpm. The dispersions were maintained at a constant pH 4 over 120 min. The TCP qe loading (in mg per unit weight of sample) was obtained using the following equation: qe ¼

ðC0 −Ce Þ:V m

2

where C0 and Ce (mg/L) are initial and equilibrium TCP concentration respectively; V (L) is the volume of the solution, and m (g) is the adsorbent mass.

2.3. Characterization of prepared samples

3. Results and discussion

To study the change in the structural properties of the samples, the distance between the layers of the modified clays was determined by X-ray diffraction analysis using a Bruker D8 advance diffractometer operating at 40 kV and 30 mA with CuKα radiation (λ = 0.15406 nm). Radial scans were recorded in the reflection scanning mode from 2θ = 2–80°. Bragg's law n λ = 2d sinθ was used to compute the d001 of the examined samples. Nitrogen gas adsorption–desorption isotherms were measured using a Quanta Chrome Autosorb-1 instrument at 77 K. The measurements were made after degassing under vacuum at 180 °C for 6 h. The specific surface area (SB.E.T) and the pore size were calculated respectively by the B.E.T and B.J.H method (Barrett et al., 1951; Brunauer et al., 1938). The total pore volume was calculated from the maximum amount of nitrogen gas adsorption at partial pressure (P/P0) = 0.95. FT-IR study was carried out using FTIR 8400S Shimadzu having a standard mid-IR DTGS detector. FT-IR spectra were recorded, in the range of 400–4000 cm − 1 with KBr pellet technique. Batch equilibrium experiments were used to estimate point of zero charge (pHpzc) of the prepared samples. Initial pH values (pHi) of 50 cm3 of NaCl 0.01 M aqueous solutions were adjusted in pH range of 2–12 using 0.1 M of HCl or NaOH. Then, 0.15 g of adsorbent was added to each sample. The dispersions were stirred for 48 h at ambient temperature, then filtered and the final pH of the solutions (pHf) was determined. The point of zero charge was found from a plot of (pHi − pHf) versus pHi (figure not showed).

3.1. XRD study

2.4. Kinetic studies Adsorption experiments were carried out in a batch equilibrium mode. The effect of initial TCP concentration of 150 mg/L was investigated. An amount of organo-bentonites (50 mg) was dispersed in 50 mL of TCP solution and stirred with an agitation speed of 100 rpm. After each time of contact, the pH of solution was measured (no significant change of the pH was observed), then a sample was removed and centrifuged. The amount of TCP adsorbed was derived from the initial and final concentrations of TCP in the liquid phases measured with UV at 290 nm. All experiments were run in triplicate to ensure

The XRD pattern of raw bentonite (Fig. 1, RB) showed the d001 reflection at 2theta = 6.64. The corresponding interlayer spacing is 1.33 nm. This corresponds to mono- and bi-layers of water molecules in the interlayer space (Caillere et al., 1982; Lee and Kim, 2002). The characteristic diffraction peaks of raw bentonite are respectively 4.5, 2.55, 1.69 and 1.49 Å. Impurities like quartz (Q), cristobalite (C) and feldspar (F) are observed in the spectra. Fig. 1 (AB) showed the intensity of most of the XRD peaks of bentonite which decreased sharply after acid treatment. Obviously, the dissolution of the smectite phase resulted in a successive dissolution of the octahedral sheet and the release of Al, Fe and Mg. In addition, Al was removed from the tetrahedral sheet, which changed the electron density in the crystal structure and this affected reflection intensity (Steudel et al., 2009). Appearance of a new peak of 1.384 nm (2theta = 6.38°) indicated interlamellar expansion Q Q M

M

13.30Å

C

M

M

Q M

F

RB 13.84Å

AB 14.25Å

ABC12

17.84Å

ABC14

20.03Å

ABC16

21.50Å

ABC18 0

10

20

30

40

50

60

2θ θ (degrees) Fig. 1. XRD patterns of the raw bentonite and prepared samples (Q = quartz; M = montmorillonite; C = cristobalite; F = feldspar).

Please cite this article as: Zaghouane-Boudiaf, H., et al., Adsorption characteristics, isotherm, kinetics, and diffusion of modified natural bentonite for removing the 2,4,5-trichlorophenol, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2013.12.030

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3.2. FTIR study FTIR spectroscopy is very sensitive to modification of the clay structure upon acid treatment and intercalated surfactants. Upon acid treatment, as protons penetrate into the clay layers and attack the OH groups, the resulting dehydroxylation connected with successive dissolution of the central atoms can be readily followed by changes in the characteristic absorption bands, attributed to vibrations of OH groups and/or octahedral cations. In order to obtain complementary evidence for the intercalation of hydrogen and surfactant ions into the silicate lattice, FTIR spectra were recorded in the region of 400–4000 cm−1. Fig. 2 showed the bands at 3420 and 1636 cm−1 due to H\O\H stretching and bending vibrations of adsorbed water respectively (Yuehong et al., 2010). In all treated bentonites spectra, the intensities of these bands decreased substantially compared with those of raw bentonite. They decrease more in all organophilic bentonites indicating appearance of organophilicity in the samples. When the surfactants intercalated into the gallery of bentonite, the water bound directly to the hydrated cations was removed with the replacement of the hydrated cations by surfactant cations. Simultaneously, the surface property of bentonite is

modified from hydrophilic to hydrophobic surface. H2O is not easily adsorbed on the organo-bentonite (Yuehong et al., 2010). In all FTIR spectra (Fig. 2), intensity of hydroxyl stretching bands at 3618 cm−1 also reduces after the acid treatment. It is due to the removal of octahedral cations causing the loss of water and the hydroxyl groups linked to them (Min-Yu and Su-Hsia, 2006). The AlMgOH bands at 845–847 cm− 1 disappeared after acid treatment in contrast to the AlAlOH band at 916–920 cm− 1. This latter disappeared completely after 20 h of acid treatment. The leaching of the octahedral cations increased the peak area of the Si\O bands at 520–467 cm − 1 (Steudel et al., 2009). Fig. 2 showed also intense bands at 2921 and 2853 cm − 1 . These bands are attributed to the antisymmetric (νas(CH2)) and symmetric \CH2 stretching (νs (CH2 )) modes. The νas (CH2) and νs (CH2) of the used pure surfactants (C12, C14, C16 and C18) locate at ca. 2919 and 2850 cm− 1 respectively. When they were intercalated into montmorillonite, a significant frequency shift was observed: νas(CH2) for ABC12 and ABC14 is 2931 cm− 1. For ABC16 and ABC18 the νas(CH2) is 2921 cm−1 indicating more gauche conformational molecules introduced into alkyl chain with the decrease of alkyl chain length. The long alkyl chain within the gallery prefers to form a solid-like molecular environment which increases the degree of the hydrophobic properties of samples, whereas the short alkyl chain forms liquid-like molecular environment (Sarkar et al., 2011; Yuehong et al., 2010). This deduction is confirmed in the next by adsorption study of the TCP onto organo-bentonites. Other bands appeared on the spectra of the organo-bentonites at 1475 and at 722 cm−1. These two bands are attributed to the scissoring and rocking vibration of methylene group (CH2), respectively. The positions of these two bands remained constant with respect to increasing carbon number in alkyl chain and were singlet in nature (Sarkar et al., 2011).

8

N2 adsorption amount (mmol/g)

with slight reduction of intensity and broadening of the peaks. The value of W.H.P. (width of the peak at half maximum) of the XRD peak for RB is lower than the W.H.P. of the XRD peak for AB. The increase in value W.H.P. of the d001 XRD peak showed that the crystallinity of AB decreases by acid treatment (Zaghouane-Boudiaf and Boutahala, 2011c). Activation also decreased the intensity of the d060 reflection (partial dissolution of Fe3+). The same result was found by Erena et al. (2008). In addition, as shown in Fig. 1, after modification of AB cationic surfactants, the d001 peak in AB shifts to 14.25, 17.84, 20.03 and 21.50 Å in ABC12, ABC14, ABC16 and ABC18, respectively. This indicated that the surfactants dodecyl (C12), tetradecyl (C14), hexadecyl (C16) and octadecyl (C18) trimethylammonium ions were entered in the interlayer space of the acid-bentonite by ion exchange. The effect of alkyl chain length and loading on the basal spacing of organoclays and arrangement of intercalated surfactants was investigated by other authors (Heinz et al., 2007; Lagaly, 1981). We observed that the basal spacing of the organo-bentonites increased as the alkyl chain length of the surfactant increased. Organo-bentonites (ABC12, ABC14) intercalated with alkyl chain surfactants C12 and C14 showed basal reflections of 14.25 Å and 17.84 Å which indicated respectively a lateral monolayer and pseudotrilayer arrangement. Organo-bentonites (ABC16, ABC18) intercalated with C16 and C18, the basal spacings of 20.03 and 21.50 Å indicated a pseudotrilayer arrangement (Zhu et al., 2003).

3

AB

7 6 5 4 3 2

RB 1 0 5

3618

1636

3420

1038

845

520

798

467 722

RB 2853

1475

AB

2931

ABC12

2931

ABC14 ABC16

2921

ABC18

2921

4000

3500

N2 adsorption amount (mmol/g)

916

ABC12 ABC14

4

3

ABC16

2

1

ABC18 0

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm ) Fig. 2. FTIR spectra of the raw bentonite and prepared samples (4000–400 cm−1).

0,0

0,2

0,4

0,6

0,8

1,0

P/P0 Fig. 3. N2 adsorption–desorption isotherms on the raw bentonite and prepared samples.

Please cite this article as: Zaghouane-Boudiaf, H., et al., Adsorption characteristics, isotherm, kinetics, and diffusion of modified natural bentonite for removing the 2,4,5-trichlorophenol, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2013.12.030

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3.3. Surface area Fig. 3 presented the N2 adsorption–desorption isotherms of the raw, acid-activated and organo-bentonites. As shown, the N2 adsorption capacity of the raw bentonite increased significantly after acid-activation process. It creates new pores which increase specific surface area, surface acidity through the replacement of cations Al3 +, Fe3+ and Ca2+ (from the structure) with H+ and formation of amorphous silica phase (Kooli et al., 2005). During clay activation with mineral acids, the acid dissolves part of Al2O3 as well as MgO from the lattice, which leads to opening of the crystal lattice and an increase in internal surface area. After exchange with surfactant solutions, the N2 adsorption capacity of the organo-bentonites decreases and it was decreased more with the increase of alkyl chain length. In this case, the organic cations may block the access of nitrogen molecules to the adsorption sites and the pore network. All the samples showed typical type IV isotherm characteristics with adsorption hysteresis. The hysteresis loops are of type H4, often observed with microporous adsorbents with sheets bound together. Between these sheets may occur capillary condensation. This hysteresis loop occurred at a higher relative pressure (p/po N 0.7), which indicated that the samples have a wide pore size distribution and different pore shapes. Based on the assumption of cylindrical pores, the specific surface area (SBET) was calculated by the BET equation; the total pore volume (VT) was evaluated by converting the adsorption volume of nitrogen at relative pressure 0.95 to equivalent liquid volume of the adsorbate, while the average pore size (r) was estimated by the equation: r = (4VT/SBET). The BET surface area, pore volume and pore size of the samples are summarized in Table 1. As shown, after acid treatment, the AB shows a more developed porosity than bentonite RB. BET surface area and pore volume of RB increased from 45 m2/g and 0.068 cm3/g to 282 m2/g and 0.25 cm3/g respectively. Table 1 shows also that, BET surface area and pore volume of AB decreased from 282 m2/g and 0.25 cm3/g to 1.1 m2/g and 0.00 cm3/g respectively, indicating that surfactants with large molecular size occupied part of the interlayer space resulting in inaccessibility of the internal surface to nitrogen molecules and the blocking of the pores in the organobentonites (Kooli et al., 2005; Seki and Yurdakoç, 2005). All the samples contained small mesopores with an average diameter of 46 b d b 74 Å. 3.4. The determination of pHpzc Effect of pH can be described on the basis on the point of zero charge (pHpzc), the point where the net charge of adsorbent is zero (Mohan et al., 2011). When solution pH b pHpzc, adsorbents will react as a positive surface and as a negative surface when solution pH N pHpzc. The experimental determination of pHpzc of samples AB, ABC12, ABC14, ABC16 and ABC18 has been determined respectively as: 4.5; 6.0; 6.5; 6.5 and 6.8.

for adsorption of TCP onto organo-bentonites was examined and the results are in Fig. 4. This figure showed rapid adsorption of TCP within the first 10 min for all samples indicating high affinity between the TCP molecules and the organophilic surface of bentonites. Fig. 4 showed also that the greatest adsorption capacity for the same concentration of 150 mg/L is attributed to the ABC18. In organo-bentonites hydrophobicity and organophilicity increased with increasing the alkyl chain. In this case ABC18 is the most hydrophobic and most organophilic samples. Pseudo-first and pseudo-second order models were employed to correlate the kinetics data. The pseudo-first-order kinetics adsorption model was suggested by Lagergren (Yao et al., 2011) for the sorption of solid/liquid systems and can be expressed in integrated and linear form using the following equation: ln ðqe −qt Þ ¼ ln qe −k1 t

3

where k1 is the rate constant of adsorption (min−1), qe and qt are the adsorption loading of TCP (mg/g) at equilibrium and at time t (min), respectively. If the pseudo-first-order kinetics is applicable, a plot of ln(qe − qt) versus t should provide a straight line from which k1 and predicted qe can be determined from the slope and intercept of the plot, respectively (figure not showed). The pseudo-second-order kinetics model (Ho, 2006) is expressed as: t 1 1 ¼ þ t qt k2 q2e qe

4

where k2 (g/mg.min) is the rate constant of pseudo-second-order adsorption. Plotting t/qt against t (Fig. 5), a straight line is obtained and the rate constant k2 as well as qe can be calculated. The pseudosecond order model is based on the assumption that the rate limiting step may be chemisorption which involves valence forces by sharing or electron exchange between the adsorbent and the adsorbate (Foo and Hameed (2012a)). The maximum adsorption capacities qe, k1, k2 and the correlation coefficient R2, calculated from the pseudo-first and pseudo-second order model are shown in Table 2. The maximum adsorption capacities qe calculated from the pseudo-second order are in accordance with the experimental values. The R2 values for the pseudo-second order are higher than that of the pseudo-first order model. These imply that the adsorption obeys a pseudo-second order model. The applicability of the pseudo-second-order kinetic model was confirmed also by the low value of normalized standard deviation as shown in Table 2. 120 100

3.5. Kinetic modeling

Table 1 Porosity structure of raw and prepared bentonites. AB

qt(mg/g)

80

The rate of adsorption is highly important in the design and evaluation of adsorbents in removing the TCP from the solution. The kinetics

60 40

Properties

RB

ABC12

ABC14

ABC16

ABC18

BET surface area (m2/g) External surface area (m2/g) Micropore surface area (m2/g) Total pore volume (cm3/g) Micropore volume (cm3/g) Mesopore volume (cm3/g) % of micropore % of mesopore Average pore size (Å)

45 282 161 20 125 92 25 157 69 0.068 0.25 0.16 0.013 0.10 0.01 0.055 0.15 0.15 19 40 6 81 60 94 62 46 46

101 83 17 0.13 0.02 0.11 15 85 50

23 25 n.d. 0.21 n.d. n.d. n.d. n.d. 74

1.1 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

ABC12 ABC14 ABC16 ABC18

20 0 0

20

40

60

80

100

120

Time (min) Fig. 4. Effect of contact time on the adsorption of TCP onto organo-bentonites (pH = 4, C0 = 150 mg/L, madsorbent = 50 mg, Vsol = 50 mL, agit. speed = 100 rpm, Temp. = 23 ± 1 °C).

Please cite this article as: Zaghouane-Boudiaf, H., et al., Adsorption characteristics, isotherm, kinetics, and diffusion of modified natural bentonite for removing the 2,4,5-trichlorophenol, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2013.12.030

H. Zaghouane-Boudiaf et al. / Applied Clay Science xxx (2014) xxx–xxx

0,35

5

120

ABC12 ABC14 ABC16 ABC18

0,30

100 80

qt(mg/g)

0,25

t/qt

0,20 0,15

60 40

ABC12 ABC14 ABC16 ABC18

0,10 20 0,05 0 0,00

0

2

4

6

8

10

12

14

0

16

2

4

Time (min)

To identify the diffusion mechanism, the intraparticle diffusion model based on the theory proposed by Weber and Morris (1962) was investigated to the adsorption of the TCP onto organo-bentonites. The model is expressed by the following equation: 0:5

þ Ci

5

where ki (mg/g min0.5) is the rate parameter of stage i, and obtained from the slope of the straight line of qt versus t0.5 (Fig. 6). Ci, the intercept of stage i, gives an idea about the thickness of boundary layer. If intraparticle diffusion occurs, qt versus t0.5 will be linear and if the plot passes through the origin, then the rate limiting process is only due to the intraparticle diffusion. Fig. 6 shows that plots of qt against t0.5 consist of two separate linear regions. It has been suggested that the first one can be attributed to the instantaneous adsorption or external surface adsorption, the second to intraparticle diffusion. Referring to Fig. 6, the first stage was completed within the first 3 min and the second stage of intraparticle diffusion control was then attained. The adsorption rate ki was initially faster and then slowed down when the time increased. Fig. 6 showed also that the linear line did not pass through the origin. It shows that intraparticle diffusion was not the only rate limiting mechanism in the adsorption process (Wu et al., 2009). Some other mechanism along with intraparticle diffusion is also involved. It may be concluded that surface adsorption and intraparticle diffusion were concurrently operating during the TCP and organo-bentonite interactions (Zaghouane-Boudiaf and Boutahala (2011a)). The kinetic data were further analyzed using the Boyd model (Boyd et al., 1947) expressed as: Bt ¼ −0:4977− ln ð1− FÞ

The adsorption isotherms describe how the adsorbate molecules are distributed between the liquid–solid phases when the system reaches the equilibrium (Hameed et al., 2008). In the endeavor to explore novel adsorbents in accessing an ideal adsorption system, it is important to establish the most appropriate correlation for the equilibrium curves. The equation parameters of these equilibrium models reveal the sorption mechanism, surface properties and affinity of the adsorbent. The adsorption data obtained were analyzed with the Langmuir and Freundlich isotherm equations. The Langmuir and Freundlich parameters were calculated from the slope and intercept of the linear plot of Ce/qe versus Ce and lnqe versus lnce, respectively (figures not showed). The obtained results are in Table 3. Due to the inherent bias resulting from linearization, alternative isotherm parameters were determined by non-linear regression (Foo and Hameed, 2010). For non-linear regression (Fig. 8), a trial and error procedure was developed to determine the isotherm parameters by maximizing the respective coefficient of determination between

ABC12 ABC14 ABC16 ABC18

4

Bt

7

Table 2 Kinetic models parameters obtained in adsorption of 2,4,5-TCP onto organo-bentonites. Pseudo-first-order

ABC12 ABC14 ABC16 ABC18

12

3.6. Adsorption isotherm studies

6

qe : qt

10

Pore diffusion is the rate-limiting step if the plot Bt versus t passes through the origin. Conversely, the adsorption process is film diffusion controlled. As illustrated from the curve as shown in Fig. 7, the linear curves did not pass through the origin, and the points were scattered around the plots, thus ascertained that the adsorption of TCP onto organo-bentonites was governed by film diffusion controlled mechanism (Foo and Hameed, 2012a).

where Bt is the mathematical function of F and F represents the fraction of solute adsorbed at time, t (min), given by: F¼

8

Fig. 6. Plots of intraparticle diffusion model for the adsorption of TCP onto organobentonites.

Fig. 5. Pseudo-second order plots of TCP adsorption onto organo-bentonites.

qt ¼ k i t

6

Time0.5

3

2

Pseudo-second-order

qe,exp

qe,cal

k1

R2

qe,cal

k2

R2

Δq (%)

48.4 78 105 119

5 11 13 14

0.066 0.026 0.049 0.048

0.806 0.391 0.022 0.740

48.4 78.1 105.3 119.3

0.070 0.027 0.019 0.016

0.999 0.999 0.999 0.999

0.19 1.83 1.68 0.90

qe (mg.g−1), k1 (min−1), k2 (g (mg min)−1).

1

0

3

6

9

12

15

Time (min) Fig. 7. Plots of Boyd model for the adsorption of TCP onto organo-bentonites.

Please cite this article as: Zaghouane-Boudiaf, H., et al., Adsorption characteristics, isotherm, kinetics, and diffusion of modified natural bentonite for removing the 2,4,5-trichlorophenol, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2013.12.030

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Table 3 Langmuir and Freundlich adsorption isotherm constants. Adsorbents

Langmuir isotherm

Freundlich isotherm

Linear fit

ABC12 ABC14 ABC16 ABC18

Non-linear fit 2

Linear fit 2

Non-linear fit 2

qm

KL

R

qm

KL

R

KF

1/n

R

KF

1/n

R2

87.2 170.0 170.0 202.8

0.007 0.01 0.01 0.045

0.985 0.963 0.993 0.992

84.6 176.4 160.4 200.6

0.008 0.057 0.009 0.045

0.998 0.995 0.984 0.996

1.44 3.2 21.11 12.43

0.71 0.72 0.43 0.63

0.992 0.990 0.978 0.934

1.60 3.5 26.0 22.8

0.70 0.70 0.37 0.45

0.993 0.994 0.978 0.972

qm (mg/g); KL (L/mg); KF (mg(1 − 1/n). L1/n/g).

experimental data and the isotherms. This provides a mathematically rigorous method for determining isotherm parameters using the original form of isotherm equation (Foo and Hameed, 2012b). The best fitting isotherm was tested by determination of the non-linear regression, and the parameters of the isotherms have been obtained. It well known that Langmuir model assumes that the adsorption is a process which occurs at a homogeneous surface, in which the molecules form a monolayer of adsorbate on the surface of the material. Adsorption of each sorbate molecule onto the surface has equal sorption activation energy (Langmuir, 1916). The Freundlich isotherm is an empirical equation that assumes that the adsorption surface becomes heterogeneous during the course of the adsorption process (Freundlich, 1906). The expressions of the Langmuir (Eq. (8)) and Freundlich (Eq. (9)) models are respectively: qe ¼

qm KL Ce 1 þ KL Ce

8

1 = qe ¼ K F Ce n

9

where qe (mg/g) is the amount of TCP adsorbed at equilibrium, Ce is the equilibrium TCP concentration in solution (mg/L), qm and KL are the Langmuir monolayer capacity of the adsorbent (mg/g) and adsorption constant (L/mg) respectively, KF (mg/g) and 1/n are the Freundlich constants characteristic of the system, indicative of the relative adsorption capacity and the intensity of the adsorption respectively. The detailed parameters of these different forms of isotherm equations were listed also in Table 3. The most correlation coefficients (R2) exceed 0.9; suggesting that all models fit the experimental results well, with the nonlinear Langmuir model fitting the experimental data better than the others. This result indicated that adsorption sites of the organobentonites are uniform having the same adsorption energy and a monolayer adsorption takes place. Fig. 8 shows that the retention of TCP on AB is very low (≃0 mg/g). The monolayer adsorption capacities of organo200

Experimental points AB ABC12 ABC14 ABC16 ABC18 Freundlich fit Langmuir fit

160

qe(mg/g)

120

80

40

0 0

40

80

120

160

Ce(mg/L) Fig. 8. Equilibrium isotherms for adsorption of TCP onto acid and organo-bentonites.

bentonites obtained with the Langmuir model are 84.6; 160.4; 176.4 and 200.6 mg/g. This improvement was attributed to the increase in the organophilic character and interlayer spacing due to intercalation of surfactant more particularly by octadecyltrimethylammonium ions. Hydrophobicity and organophilicity increased with increasing the alkyl chain, thus ABC18 becomes more hydrophobic and more organophilic than the other samples and enhanced the adsorption. In all samples, hydrophobic interactions of the TCP should be involved with both alkylammonium ions and the remaining non-covered portion of siloxane surface (Schoonheydt and Johnston, 2006). 4. Conclusion Preparations of new materials were carried out through the acid treatment of the raw bentonite and intercalation of alkylammonium with different chain. Acid treatment increases the basal spacing (d001) from 13.3 to 13.84 Å. The d001 increases more from 13.84 to 21.50 with the increase of the alkyl chain of the surfactant. The Fourier transform infrared spectroscopy confirms the intercalation of the surfactant by the appearance of new bands in the organo-bentonites specter. The BET surface area increases after acid treatment, but decreases by intercalation of the surfactant in the interlayer spacing. Adsorption of the 2,4,5 TCP onto the organo-bentonites shows that the process follows the pseudo-second-order kinetics model. The experimental equilibrium data are better described by the non-linear Langmuir isotherm model. The maximum monolayer adsorption capacity is 200.6 mg/g. The asprepared organo-bentonites are found to be effective adsorbents for the removal of TCP from aqueous solutions indicating a promising potential of these low cost adsorbent materials for water treatment. References Al-Anber, M.A., 2010. Removal of high-level Fe3+ from aqueous solution using natural inorganic materials: bentonite (NB) and quartz (NQ). Desalination 250, 885–891. Aliouane, N., Hammouche, A., De Doncker, R.W., Telli, L., Boutahala, M., Brahimi, B., 2002. Investigation of hydration and protonic conductivity of H-montmorillonite. Solid State Ionics 148, 103–110. Babaki, H., Salem, A., Jafarizad, A., 2008. Kinetic model for the isothermal activation of bentonite by sulfuric acid. Mater. Chem. Phys. 108, 263–268. Baghel, A., Singh, B., Pandey, P., Dhaked, R.K., Gupta, A.K., Ganeshan, K., Sekhar, K., 2006. Adsorptive removal of water poisons from contaminated water by adsorbents. J. Hazard. Mater. B137, 396–400. Barrett, E.P., Joyner, L.G., Halenda, H.P., 1951. The determination of pore volume and area distributions in porous substance. I. Computations from nitrogen isotherms. J. Am. Chem. Soc. 73, 373–380. Bhattacharyya, K.G., Gupta, S.S., 2007. Adsorptive accumulation of Cd(II), Co(II), Cu(II), Pb(II), and Ni(II) from water on montmorillonite: influence of acid activation. J. Colloid Interface Sci. 310, 411–424. Boyd, G.E., Adamson, A.W., Myers Jr., L.S., 1947. The exchange adsorption of ions from aqueous solutions by organic zeolites II: kinetics. J. Am. Chem. Soc. 69, 2836–2848. Brunauer, S., Emmet, P.H., Teller, E., 1938. Adsorption of gases in multimolecular layers. J. Am. Chem. Soc. 60, 309–319. Caillere, C., Henin, S., Rautureau, M., 1982. Minéralogie des argiles, Structure et propriétés physico-chimiquessecond ed. Ed. Masson. Erena, E., Afsin, B., Onal, Y., 2008. Removal of lead ions by acid activated and manganese oxide coated bentonite. J. Hazard. Mater. 161, 677–685. Foo, K.Y., Hameed, B.H., 2010. Insights into the modeling of adsorption isotherm systems. Chem. Eng. J. 156, 2–10. Foo, K.Y., Hameed, B.H., 2012a. Textural porosity, surface chemistry and adsorptive properties of durian shell derived activated carbon prepared by microwave assisted NaOH activation. Chem. Eng. J. 187, 53–62.

Please cite this article as: Zaghouane-Boudiaf, H., et al., Adsorption characteristics, isotherm, kinetics, and diffusion of modified natural bentonite for removing the 2,4,5-trichlorophenol, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2013.12.030

H. Zaghouane-Boudiaf et al. / Applied Clay Science xxx (2014) xxx–xxx Foo, K.Y., Hameed, B.H., 2012b. Factors affecting the carbon yield and adsorption capability of the mangosteen peel activated carbon prepared by microwave assisted K2CO3 activation. Chem. Eng. J. 180, 66–74. Freundlich, H.M.F., 1906. Über die adsorption in lösungen. Z. Phys. Chem. 57, 385–470. Gładysz-Płaska, A., Majdan, M., Pikus, S., Sternik, D., 2012. Simultaneous adsorption of chromium (VI) and phenol on natural red clay modified by HDTMA. Chem. Eng. J. 179, 140–150. Hameed, B.H., Tan, I.A.W., Ahmad, A.L., 2008. Adsorption isotherm, kinetic modeling and mechanism of 2,4,6-trichlorophenol on coconut husk-based activated carbon. Chem. Eng. J. 144, 235–244. Heinz, H., Vaia, R.A., Krishnamoorti, R., Farmer, B.L., 2007. Self-assembly of alkylammonium chains on montmorillonite: effect of chain length, head group structure, and cation exchange capacity. Chem. Mater. 19, 59–68. Ho, Y., 2006. Review of second-order models for adsorption systems. J. Hazard. Mater. 136, 681–689. Kaufhold, S., Pohlmann-Lortz, M., Dohrmann, R., Nüesch, R., 2007. About the possible upgrade of bentonite with respect to iodide retention capacity. Appl. Clay Sci. 35, 39–46. Khalaf, H., Bouras, O., Perrichon, V., 1997. Synthesis and characterization of Al-pillared and cationic surfactant modified Algerian bentonite. Microporous Mater. 8, 141–150. Komadel, P., Madejová, J., 2006. Chapter 7.1: acid activation of clay minerals. Develop. Clay Sci. 1, 263–287. Kooli, F., Khimyak, Y.Z., Alshahateet, S.F., Chen, F., 2005. Effect of the acid activation levels of montmorillonite clay on the cetyltrimethylammonium cations adsorption. Langmuir 21, 8717–8723. Lagaly, G., 1981. Characterization of clays by organic compounds. Clay Miner. 16, 1–21. Langmuir, I., 1916. The constitution and fundamental properties of solids and liquids. J. Am. Chem. Soc. 38, 2221–2295. Lee, S.Y., Kim, S.J., 2002. Expansion of smectite by hexadecyltrimethylammonium. Clay Miner. 50, 435–445. Min-Yu, T., Su-Hsia, L., 2006. Removal of basic dye from water onto pristine and HCl activated montmorillonite in fixed beds. Desalination 194, 156–165. Mohan, D., Sarswat, A., Singh, V.K., Franco, M.A., Pittman Jr., C.U., 2011. Development of magnetic activated carbon from almond shells for trinitrophenol removal from water. Chem. Eng. J. 172, 1111–1125. Putra, E.K., Pranowo, R., Sunarso, J., Indraswati, N., Ismadji, S., 2009. Performance of activated carbon and bentonite for adsorption of amoxicillin from wastewater: mechanisms, isotherms and kinetics. Water Res. 43, 2419–2430. Sarkar, M., Dana, K., Ghatak, S., 2011. Evolution of molecular structure and conformation of n-alkylammonium intercalated iron rich bentonites. J. Mol. Struct. 1005, 161–166.

7

Schoonheydt, R.A., Johnston, C.T., 2006. Surface and interface chemistry of clay minerals. In: Bergaya, F., Theng, B.K.G., Lagaly, G. (Eds.), Handbook of Clay Science. I. Developments in Clay Science. Elsevier, Amsterdam, pp. 87–113. Seki, Y., Yurdakoç, K., 2005. Paraquat adsorption onto clays and organoclays from aqueous solution. J. Colloid Interface Sci. 287, 1–5. Steudel, S., Batenburg, L.F., Fischer, H.R., Weidler, P.G., Emmerich, K., 2009. Alteration of swelling clay minerals by acid activation. Appl. Clay Sci. 44, 105–115. Weber, W.J., Morris, J.C., 1962. Proc. Int. Conf. Water pollution symposium vol. 2, Pergamon, Oxford, pp. 231–266. Wu, F.C., Tseng, R.L., Juang, R.S., 2009. Initial behavior of intraparticle diffuse on model used in the description of adsorption kinetics. Chem. Eng. J. 153, 1–8. Yao, Yunjin, Bing, He, Feifei, Xu, Xiaofeng, Chen, 2011. Equilibrium and kinetic studies of methyl orange adsorption on multiwalled carbon nanotubes. Chem. Eng. J. 170, 82–89. Yilmaz, N., Yapar, S., 2004. Adsorption properties of tetradecyl- and hexadecyl trimethylammonium-bentonites. Appl. Clay Sci. 27 (2004), 223–228. Yuehong, Ma, Jianxi, Zhu, Hongping, He, Peng, Yuan, Wei, Shen, Dong, Liu, 2010. Infrared investigation of organo-montmorillonites prepared from different surfactants. Spectrochim. Acta, Part A 76, 122–129. Zaghouane-Boudiaf, H., Boutahala, M., 2011a. Adsorption of 2,4,5-trichlorophenol by organo-montmorillonites from aqueous solutions: kinetics and equilibrium studies. Chem. Eng. J. 170, 120–126. Zaghouane-Boudiaf, H., Boutahala, M., 2011b. Kinetic analysis of 2,4,5-trichlorophenol adsorption onto acid-activated montmorillonite from aqueous solution. Int. J. Miner. Process. 100, 72–78. Zaghouane-Boudiaf, H., Boutahala, M., 2011c. Preparation and characterization of organo montmorillonites. Application in adsorption of the 2,4,5-trichlorophenol from aqueous solution. Adv. Powder Technol. 22, 735–740. Zheng, Hong, Wang, Yang, Zheng, Yan, Zhang, Hongmei, Liang, Shuping, Long, Mei, 2008. Equilibrium, kinetic and thermodynamic studies on the sorption of 4-hydroxyphenol on Cr-bentonite. Chem. Eng. J. 143, 117–123. Zhi-rong, Liu, Shao-qi, Zhou, 2010. Adsorption of copper and nickel on Na-bentonite. Process Safety. Env. Protect. 88, 62–66. Zhu, Lifang, Zhu, Runliang, 2008. Surface structure of CTMA+ modified bentonite and their sorptive characteristics towards organic compounds. Colloids Surfaces A: Physicochem. Eng. Aspects 320, 19–24. Zhu, J.X., He, H.P., Guo, J.G., Yang, D., Xie, X.D., 2003. Arrangement models of alkylammonium cations in the interlayer of HDTMA+ pillared montmorillonites. Chin. Sci. Bull. 48, 368–372.

Please cite this article as: Zaghouane-Boudiaf, H., et al., Adsorption characteristics, isotherm, kinetics, and diffusion of modified natural bentonite for removing the 2,4,5-trichlorophenol, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2013.12.030