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Desalination 220 (2008) 96–107

Adsorption kinetics of naphthalene onto organo-sepiolite from aqueous solutions Özer Gök, A. Safa Özcan*, Adnan Özcan Department of Chemistry, Faculty of Science, Anadolu University, Yunusemre Campus, 26470 Eskis¸ ehir, Turkey Tel. +90-222 3350580 ext. 5781; Fax +90-222 3204910; email: [email protected] Received 20 December 2006; accepted 3 January 2007

Abstract In this study, the adsorption kinetics of naphthalene onto organically modified-sepiolite was investigated by means of the effects of pH, contact time, adsorbent dosage and temperature. The modification of natural sepiolite was accomplished with a cationic surfactant, which is namely dodecyltrimethylammonium (DTMA) bromide. The surface characterization both natural- and modified-sepiolite were carried out by using FTIR method to observe the intercalation of DTMA between the sepiolite layers. The elemental and thermal analyses were also performed to understand the modification. The optimum pH values and the equilibrium contact time for the adsorption of naphthalene onto DTMA–sepiolite were found as 6 and 75 min, respectively. The kinetic parameters of the adsorption process were calculated from experimental data. According to these parameters, adsorption process follows the pseudo-second-order kinetic model with the high correlation coefficients (r2 = 0.999). The obtained results show that modified-sepiolite is reasonably effective adsorbent for the removal of organic contaminants, which are an important source for the environmental pollutants. Keywords: Naphthalene; Adsorption; Clay; Organo-sepiolite; Kinetics; Environmental pollution

1. Introduction Various toxic chemicals such as polycyclic aromatic hydrocarbons, heavy metals, dyes, solvents have been discharged to the environment as industrial wastes, causing serious water, air and soil pollutions and they were also threaten the *Corresponding author.

human health. Polycyclic aromatic compounds have been widely studied and a big concern is paid to environmental subjects and due to their potential carcinogenic, mutagenic or both. They are mainly emitted from combustion processes including engine exhaust, industrial processes, natural gas, domestic heating systems, barbecue, smoke, incomplete combustion of fossil fuels, volcanic eruptions and forest fires [1–4].

Presented at the conference on Desalination and the Environment. Sponsored by the European Desalination Society and Center for Research and Technology Hellas (CERTH), Sani Resort, Halkidiki, Greece, April 22–25, 2007. 0011-9164/08/$– See front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.desal.2007.01.025

Ö. Gök et al. / Desalination 220 (2008) 96–107

The removal of these kinds of pollutants from environment cannot be accomplished by using traditional methods. It is now extensively recognized that adsorption provide a feasible, effective method for the removal of pollutants from wastewaters [1,5]. Activated carbon is the most commonly used adsorbent in the adsorption process, due to its high adsorption capacity, high surface area and high degree of surface reactivity, whereas the regenaration was difficult and expensive [6]. The research is now focused on new, efficient, low-cost and easily obtainable natural materials, e.g., clay materials [7]. The wide usefulness of these kinds adsorbents is a result of their high specific surface area, high chemical and mechanical stability, and variety of surface and structural properties. The pore structure and chemical properties generally determine the adsorption ability of the clays [8]. Sepiolite is a natural clay mineral and a fibrous hydrated magnesium silicate. It has a unit cell formula (Si12)(Mg8)(O30)(OH)4(OH)2 · 8H2O and a general structure formed by an alternation of blocks and tunnels that grow up in the fiber direction. Each block consists of two tetrahedral silica sheets enclosing a central magnesia sheet. However, the silica sheets are discontinued and inversion of these silica sheets gives rise to tunnels in the structure. These characteristics of sepiolite make it a powerful adsorbent for organic molecules. In addition, the isomorphous substitution of Al3+ for Si4+ in the tetrahedral sheets of the lattice of sepiolite forms negatively charged adsorption sites. Such sites are occupied by exchangeable cations (Na+, Ca2+, etc.) that compensate for the electrical charge at the sepiolite surface, resulting hydrophilic environment at the surface [9–11]. The natural form of the clay shows relatively ineffective as an adsorbent for neutral organic contaminants such as polycyclic aromatic hydrocarbons. The adsorbent properties of sepiolite for neutral polycyclic aromatic hydrocarbons can be

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greatly improved by replacing the natural inorganic interlayer cations (Na+ or Ca2+) with large organic cations of the long chain alkyl hydrocarbons. These modified clays are called as organoclays. The organophilic properties of the modified clay result in part from the reduced degree of the hydration of the organic cations as compared to the natural inorganic exchangeable cations. Such organoclays have been played an effective role the removal of the various neutral organic contaminants including naphthalene or the others from aqueous solutions [12–15]. A limited studies on the adsorption of naphthalene have been carried out by using porous silica gel [16], synthetic activated carbon [17,18], TPMA-, HDTMA- and BDTDA-modified montmorillonite [19], HDTMA-modified kaolinite and halloysite [20], HDTMA-modified Na-montmorillonite [21] and zeolite [22], but none of them has been investigated the adsorption of naphthalene onto surfactant modifiedsepiolite. Adsorption kinetics is the basic requirements for the design of the adsorption systems. The equilibrium kinetic data a specific adsorbate/ adsorbent system can be obtained successfully by experimentally, a time interval procedure for the adsorption systems design [23]. The aim of this work is to investigate, experimentally, the potential of organo-sepiolite to adsorb naphthalene, which is a nonpolar neutral polycyclic aromatic hydrocarbon, as a model compound. Various kinetic models including the Lagergren first-order, pseudo-second-order, Elovich and intraparticle diffusion were applied to the experimental data to evaluate the adsorption mechanism of naphthalene onto organo-sepiolite. The effects of pH, contact time, adsorbent dosage and temperature were studied. In addition, the kinetic parameters were also calculated to determine the rate constants and which kinetic models give the best correlation to the experimental data.

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2. Materials and methods

2.3. Preparation of DTMA-modified sepiolite

2.1. Materials

The Na-exchanged form of sepiolite was prepared by stirring samples for 24 h with a 1 M aqueous solution of NaCl. Excess NaCl and other exchangeable cations were removed from the exchanged sepiolite by filtering and it was washed several times with deionized water until a negative chloride test was obtained with 0.1 M AgNO3. The Na-saturated sepiolite (30 g) was dispersed in 0.5 dm3 of distilled water and DTMA– sepiolite prepared by adding the surfactant at twice the cation exchange capacity of the sepiolite and stirred for 24 h. After treatment, sepiolite was washed with deionized water until free of salts and a negative chloride test had been obtained with 0.1 M AgNO3 [25]. It was then dried, and sieved through a 63 μm size sieve and samples collected from under the sieve and dried in an oven at 65°C for 24 h before to use for the naphthalene adsorption studies.

Naphthalene was obtained from Merck and was used as the representative compound for the experiments in this study. The adsorbent, which is sepiolite, used in this work was provided from Eskis¸ ehir (Turkey). It was crushed, ground, sieved through a 63 μm size sieve and samples were collected from under the sieve and dried in an oven at 110°C for 2 h prior to use. Natural sepiolite was characterized with respect to its cation exchange capacity (CEC) by the methylene blue method [24] and it was found as 544.3 mmol kg–1. The BET surface area of natural sepiolite was determined from N2 adsorption isotherm with a Surface Area Analyzer (Quantachrome Instruments, Nova 2200e) and the result was 152 m2 g−1. 2.2. Material characterization The chemical analysis of natural sepiolite was conducted using an energy dispersive X-ray spectrometer (EDX-LINK ISIS 300) attached to a scanning electron microscope (SEM-Cam Scan S4). The crystalline phases present in sepiolite were determined via X-ray diffractometry (XRD-Rigaku Rint 2000) using Cu Kα radiation. FTIR spectra were recorded (KBr) on a Jasco FT/IR-300E Model Fourier transform infrared spectrometer to observe surface modification. The elemental analysis (Vario EL III Elemental Analyzer, Hanau, Germany) of DTMA–sepiolite was performed to determine C/N ratio in DTMA– sepiolite. Thermal analysis (Setaram, Labsys TG-DTA Model) was done to observe the intercalation of surfactant onto clay layers. The analyses for natural sepiolite, pure surfactant and DTMA-modified sepiolite were carried out in the temperature range 25–1000, 25–600, 25–1000°C, respectively at a heating rate of 10°C min–1.

2.4. Adsorption experiments The pH experiments were conducted by mixing 50 cm3 of a 10 mg dm−3 aqueous naphthalene solution with 1 g dm−3 of DTMA– sepiolite concentration at 20°C and at various pH values in the range 1–8. The solution pH was carefully adjusted by adding small amount of HCl or NaOH solution and measured using a pH meter (Fisher Accumet AB15), while naphthalene solutions contained in 100 mL Erlenmeyer flasks closed with glass stoppers to avoid evaporation were stirred using a mechanical magnetic stirrer. The blank experiments were also carried out to observe the effect of vaporization of naphthalene. The amount of vaporization during the experiments was subtracted from the experimental data. Once the optimum pH had been attained as 6, kinetic studies were conducted at this pH value with


increasing periods of time at 10, 15 and 20°C, until no more naphthalene was removed from the aqueous phase and equilibrium had been achieved. After such time (75 min), the samples were filtered and the equilibrium concentrations ascertained by spectrophotometer (Shimadzu UV-2101PC) at the respective lmax value, which is 219 nm for naphthalene. The amount of naphthalene adsorbed onto DTMA–sepiolite was determined by the difference between the initial and remaining concentrations of naphthalene solution. The adsorption capacity (qe) was determined by using the following equation taking into the concentration differences of the solution at the beginning and the equilibrium accounts

qe =

(Ci − Ce ) V m

(1)

where Ci and Ce are the initial and the equilibrium naphthalene concentrations (mg dm−3), V the volume of solution (dm3) and m is the amount of adsorbent used (g).

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3. Results and discussion 3.1. Chemical composition of sepiolite X-ray diffraction (XRD) analysis together with the chemical analysis of (29.3% SiO2, 12.4% MgO, 9.09% CaO and 34.99% CaCO3) indicates that sepiolite and dolomite are the major components along with traces of Al, K and Fe oxides in the form of impurities. 3.2. FTIR analysis FTIR spectra of natural sepiolite and DTMAmodified sepiolite were illustrated in Fig. 1. The band at 3687 cm−1 that corresponds to stretching (nOH) vibrations of hydroxyl groups (belong to Mg3OH) attached to octahedral Mg2+ ions located in the interior blocks of natural sepiolite and DTMA-modified sepiolite [26,27]. The band at 3621 cm−1 indicates that assigned to H–O–H stretching vibrations of water molecules weakly hydrogen bonded to the Si–O surface in both of the samples. The broad band at 3419 cm−1, observed each sample, is due to H–O–H vibrations of adsorbed water.

Fig. 1. FTIR spectra of (a) natural sepiolite and (b) DTMA–sepiolite.

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A pair of bands at 2854 and 2928 cm−1 was only observed with DTMA-modified sepiolite can be assigned to the symmetric and asymmetric stretching vibrations of the methylene groups and their bending vibrations between 1380 and 1456 cm−1, supporting the intercalation of surfactant (DTMA) molecules between the silica layers, whereas these stretching bands are not observed in the natural sepiolite [28]. The band at 1660 cm−1 corresponds to the –OH deformation of water, because the –OH stretching band at 3419 cm−1 suggests the presence of some interlamellar water [29] and the stretching vibrations obtained at 1630, 1613 and 1456 cm−1 could be characteristics of reversibly adsorbed carbonate on the oxide surfaces [30] for both of the samples and this result was well agreed with XRD results that sepiolite contents of dolomite. The Si–O coordination bands at 1210, 1089 and 981 cm−1 are observed as a result of the Si–O vibrations. The deep band at 1018 cm−1 represents the stretching of Si–O in the Si–O–Si groups of the tetrahedral sheet and the band at 882 cm−1 conducted with bending vibration of carbonate and two peaks at around 690 cm−1 represent the bending vibration of Mg3OH for both of samples. The bands at 503 and 474 cm−1 due to Si–O–Al (octahedral) and Si–O–Si bending vibrations respectively, natural sepiolite and DTMA-modified sepiolite [31]. 3.3. Elemental and thermal analysis The ratio of C/N for DTMA–sepiolite from elemental analysis results is 12.40 and the calculated value of C/N ratio is 12.86. These results confirm that the intercalation of DTMA molecules between sepiolite layers occurs. They are also consistent with above FTIR analysis results. Thermal analysis of the organoclay complexes offered information about the thermal reactions, properties, and stability of the complexes, the amount and properties of the adsorbed water in

the organoclays, and the bonding between the organic species and the clay [32]. The thermal analysis curves of the natural sepiolite, pure surfactant and surfactant (DTMA)-modified sepiolite were illustrated in Fig. 2a–c. The intensity of the differential thermogravimetric (DTG) peaks for DTMA–sepiolite (organoclay) in the temperature region 20–100°C was lower than for unmodified natural sepiolite due to the hydrophobicities of this sample as it can be seen from Fig. 2a and c. The DTG peak between 300 and 400°C, centered at 365°C, was observed in DTMA-modified sepiolite and it confirms the bonding of the surfactant to sepiolite surface but this peak is not observed in the natural sepiolite. 3.4. Effect of pH The effect of pH on the removal of naphthalene onto DTMA–sepiolite from aqueous solution was illustrated in Fig. 3. As can be seen from Fig. 3, the maximum naphthalene removal was observed at around neutral pH values. It was observed that the adsorption procedure is highly dependent on the pH of the solution, which affects the surface charge of the adsorbent and the degree of ionization of adsorbate. Naphthalene adsorption is decreased at low pH value (1.5), but it increases with increasing pH value up to around 3 and there is a little increase in the adsorption and thus further experiments were carried out at pH 6. Due to the neutral properties of naphthalene, it is expected that its adsorption was the highest value at pH 6. 3.5. Effect of adsorbent dosage on naphthalene uptake The results of the experiments with varying adsorbent dosage are presented in Fig. 4. With an increase in the adsorbent dosage from 0.2 to 1.0 g dm−3, the percentage of naphthalene


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Fig. 2. Differential thermogravimetric (DTG) curves of (a) natural sepiolite, (b) pure surfactant and (c) surfactant (DTMA)-modified sepiolite.

uptake increases from 21.56% to 29.50%, as the number of possible binding sites are increased, whereas the adsorption capacity of DTMA– sepiolite for naphthalene decreased from 10.78 3.5 3.0

to 2.950 mg g−1. Various reasons have been suggested to explain the decreased adsorption capacity at increasing adsorbent concentration including availability of solute, electrostatic interactions and interference between binding sites [33]. The adsorbent dosage of DTMAmodified sepiolite for further adsorption experiments was selected as 1.0 g dm−3.

q (mg g–1)

2.5

3.6. Adsorption kinetic considerations

2.0 1.5 1.0 0.5 0.0

0

1

2

3

4 pH

5

6

7

8

Fig. 3. Effect of pH for the adsorption of naphthalene onto DTMA–sepiolite at 20°C.

The influence of contact time on the amount of naphthalene adsorbed was investigated at various temperatures as shown in Fig. 5. It is seen that the amount of adsorption increased with increasing in contact time. Maximum adsorption was observed at 75 min, beyond which there was almost no further increase in the adsorption. This was therefore fixed as the equilibrium contact time.

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Four kinetic models equations, i.e. the Lagergren first-order equation, pseudo-second-order equation, Elovich equation and intraparticle diffusion equation, were considered to interpret the experimental data. The Lagergren first-order rate expression [34] is given as

35 30

Uptake %

25 20 15 10

ln ( q1 − qt ) = ln q1 − k1t

5 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 m (g dm−3)

Fig. 4. Effect of adsorbent dosage for the adsorption of naphthalene onto DTMA–sepiolite at 20°C.

(2)

The pseudo-second-order kinetic model equation [35] is expressed as

t 1 1 t = + 2 qt q2 k2 q2

(3)

The Elovich equation is generally expressed as follows [36]: The equilibrium adsorption capacity of naphthalene onto DTMA–sepiolite was found to increase with increasing temperature from 10 to 20°C (Fig. 5), indicating that naphthalene adsorption on the adsorbent was favored at higher temperatures. This effect suggests that an explanation of the adsorption mechanism associated with the removal of naphthalene onto DTMA–sepiolite involves a temperature dependent process.

dqt = a exp ( − b qt ) dt

(4)

To simplify the Elovich equation, Chien and Clayton [37] assumed abt >> 1 and by applying the boundary conditions qt = 0 at t = 0 and qt = qt at t = t Eq. (4) becomes [38]

qt =

1 1 ln(ab ) + ln t b b

(5)

The intraparticle diffusion equation [39] can be written by following:

qt = k pt 1/ 2 + C

3.0

q (mg g−1)

2.5 2.0 1.5 1.0 10°C 15°C 20°C

0.5 0

0

20

40

60

80 100 t (min)

120

140

160

Fig. 5. Effect of contact time for the adsorption of naphthalene onto DTMA–sepiolite at various temperatures.

(6)

where q1 and qt are the amounts of naphthalene adsorbed on the adsorbent at equilibrium and at various times t (mg g−1), k1 is the rate constant of the Lagergren first-order model for the adsorption process (min−1), q2 is the maximum adsorption capacity (mg g−1) for the pseudo-second-order model, k2 is the rate constant of the pseudosecond-order model for the adsorption process (g mg−1 min−1), a is the initial adsorption rate (mg g−1 min−1), b is the desorption constant (g mg−1) for Elovich equation during any one experiment, C is the intercept, and kp is the

Ö. Gök et al. / Desalination 220 (2008) 96–107 3.0

1

2.8

20°C 30°C 40°C

0

2.6 qt (mg g−1)

–1 ln(q1 – qt)

103

–2

2.4 2.2 2.0 1.8

–3

1.6 –4 –5

0

20

40

60

80 100 t (min)

120

140

160

Fig. 6. Lagergren first-order kinetic plots for the adsorption of naphthalene onto DTMA–sepiolite at various temperatures. −1

−1/2

intraparticle diffusion rate constant (mg g min ). The straight-line plots of ln(q1 − qt) vs. t for the Lagergren first-order model (Fig. 6), t/qt against t for the pseudo-second-order model (Fig. 7) and the plots of qt vs. ln(t) (Fig. 8) for the Elovich model for the adsorption of naphthalene onto DTMA–sepiolite have been drawn to obtain the rate parameters. The kinetic parameters of naphthalene under different conditions were calculated from these 80

60 t/qt (min g mg–1)

10°C 15°C 20°C

1.4

40

20

0

10°C 15°C 20°C 0

20

40

60

80 100 t (min)

120 140

160

180

Fig. 7. Pseudo-second-order kinetic plots for the adsorption of naphthalene onto DTMA–sepiolite at various temperatures.

1.2 6.0

6.5

7.0

7.5

ln t

8.0

8.5

9.0

9.5

Fig. 8. Elovich kinetic plots for the adsorption of naphthalene onto DTMA–sepiolite at various temperatures.

plots and are given in Table 1. The correlation coefficients (r12 ), for the Lagergren first-order kinetic model are between 0.878 and 0.979, the correlation coefficients (r22 ) , for the pseudosecond-order kinetic model are 0.999 and the correlation coefficients (rE2 ) , for the Elovich kinetic model are between 0.904 and 0.941. They are probable, therefore, that this adsorption system is not followed by the Lagergren first-order or Elovich kinetic models, it is fitted the pseudosecond-order kinetic model. The calculated q2 values agree with experimental q values, and also, the correlation coefficients for the pseudosecond-order kinetic plots were very high. The pseudo-second-order rate constants increase from 6.71 × 10–2 to 1.23 × 10–1 g mg−1 min−1 with an increase in the solution temperatures from 10 to 20°C (Table 1), indicating that the adsorption of naphthalene onto DTMA– sepiolite is rate-controlled. The Lagergren first-order, pseudo-secondorder and Elovich models cannot identify the diffusion mechanism and the kinetic results were then subjected to analyze by the intraparticle diffusion model and it may be the rate-controlling step. If this does occur, then the plot of uptake, qt, vs. square root of time, t1/2, should be linear and if it passes through the origin then the intraparticle

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Table 1 Kinetic parameters for the adsorption of naphthalene onto DTMA–sepiolite at various temperatures t (°C) 10

15

Lagergren first-order k1 (min−1) q1 (mg g−1) r12

5.44 × 10–2 1.847 0.922

4.58 × 10–2 1.242 0.878

4.78 × 10–2 0.942 0.979

Pseudo-second-order k2 (g mg−1 min−1) q2 (mg g−1) r22

6.71 × 10–2 2.329 0.999

8.50 × 10–2 2.677 0.999

1.23 × 10–1 2.857 0.999

Elovich a (mg g−1 min−1) b (g mg−1) rE2

5.54 × 10–2 3.125 0.904

1.520 4.092 0.941

Intraparticle diffusion kp (mg g−1 min−1/2) C rp2

0.150 0.987 0.963

0.105 1.675 0.996

diffusion will be the sole rate-limiting process [11,40–42]. In the present study, it was found that the plots of qt vs. t1/2 exhibited an initial linear portion followed by a plateau which occurred after 75 min for DTMA–sepiolite (Fig. 9). The 3.0 2.8

qt (mg g−1)

2.6 2.4 2.2 2.0 1.8 10°C 15°C 20°C

1.6 1.4 1.2

2

3

4

5 6 t1/2 (min1/2)

7

8

9

Fig. 9. Intraparticle diffusion plots for the adsorption of naphthalene onto DTMA–sepiolite at various temperatures.

20

86.16 5.314 0.928 8.66 × 10–2 2.072 0.981

initial curved portion of the plots seems to be due to the boundary layer adsorption and the linear portion to the intraparticle diffusion, with the plateau corresponding to the equilibrium [42]. However, neither plot passed through the origin. This indicates that although intraparticle diffusion was involved in the adsorption process, it was not the rate-controlling step. Values of the intraparticle diffusion constant, kp, were obtained from the slopes of the linear portions of the plots and are listed in Table 1. The correlation coefficients for the intraparticle diffusion model (rp2 ) were between 0.963 and 0.996. These values indicate that the adsorption of naphthalene onto DTMA–sepiolite may be followed by the intraparticle diffusion up to 75 min. In this study, the maximum adsorption capacity of DTMA–sepiolite obtained for naphthalene was 2.857 mg g–1 from the pseudo-secondorder kinetic model. It was comparable to data in the literature [19] with HDTMA–montmorillonite (0.342 mg g–1), BDTDA–montmorillonite


(3.888 mg g–1) and TPMA–montmorillonite (5.141 mg g–1). As it can be seen from above results, DTMA–sepiolite is a reasonable adsorbent for naphthalene adsorption.

[2]

4. Conclusions Natural sepiolite used in this study was firstly modified by a surfactant, which is dodecyltrimethyl ammonium (DTMA) bromide, in order to obtain organo-sepiolite. The adsorption kinetic mechanism of naphthalene onto DTMA–sepiolite was then investigated. The following results were obtained: (1) The surface characterization of DTMA– sepiolite was examined by using FTIR, elemental and thermal analysis methods. (2) The adsorption capacity of DTMA–sepiolite increases with increasing temperature. This result indicates that the adsorption process was endothermic in nature. (3) The pseudo-second-order kinetic model agrees very well with the dynamic behavior for the adsorption of naphthalene onto DTMA– sepiolite at the various temperatures. However, the evidence is provided that the adsorption of naphthalene onto DTMA–sepiolite is a complex process, so it cannot be sufficiently described by a single kinetic model throughout the whole kinetic process. For instance, intraparticle diffusion (up to 75 min) played an important role, but it was not the main ratedetermining step during the adsorption. (4) A maximum of 2.857 mg g−1 from the pseudosecond-order kinetic equation for naphthalene removal could be achieved at pH 6 and 20°C. (5) The obtained reasonable results indicate that DTMA–sepiolite can be a promising adsorbent for the removal of polycyclic aromatic hydrocarbons such as naphthalene.

[3]

[4]

[5]

[6]

[7]

[8]

[9]

[10] [11]

[12]

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