Journal of the Taiwan Institute of Chemical Engineers 70 (2017) 32–41

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Mesoporous zeolite–activated carbon composite from oil palm ash as an effective adsorbent for methylene blue W.A. Khanday a, F. Marrakchi a,b, M. Asif c, B.H. Hameed a,∗ a

School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia Laboratoire d’Electrochimie et Environnement, Ecole Nationale d’Ingénieurs de Sfax, Université de Sfax, BP 1173, 3038 Sfax, Tunisia c Chemical Engineering Department, College of Engineering, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia b

a r t i c l e

i n f o

Article history: Received 13 May 2016 Revised 24 September 2016 Accepted 17 October 2016 Available online 3 November 2016 Keywords: Activated carbon Adsorption Composite Methylene blue Oil palm ash Zeolite

a b s t r a c t A mesoporous high-surface-area zeolite–activated carbon (Z–AC) composite was prepared by chemically facilitated NaOH activation and hydrothermal treatment with oil palm ash as substrate. The prepared Z– AC composite was characterized by X-ray diffraction, Fourier transform infrared spectroscopy, BET surface area and pore structural analysis, and scanning electron microscopy. The adsorption performance of Z–AC for methylene blue (MB) removal was examined using a batch method. The effects of initial dye concentration (25–400 mg/L), temperature (30 °C–50 °C), and pH (3–13) on the adsorption of MB on Z–AC were studied. Pseudo-second-order kinetics was found to describe the adsorption process better than pseudofirst-order kinetics. Freundlich and Langmuir isotherms applied on the adsorption data reveal that data best fitted Freundlich model. The maximum adsorption capacity values of the Z–AC composite for MB were 143.47, 199.6, and 285.71 mg/g at 30, 40, and 50 °C, respectively. These results show that the Z–AC composite could provide basis for more low-cost composites to be used as adsorbents for dye removal. © 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction Paint, plastic, paper, tannery, and textile industries use dyes to a large extent. Annual production of dyes is estimated to exceed 70 0,0 0 0 tons, approximately 15% of which is being discharged into wastewater [1]. These dyes at even trace levels constitute the most important and dangerous source of environmental pollution [2]. Deterioration in water quality occurs because of dyes by spreading odor and color to water and thus hindering the penetration of sun light, ultimately influencing the process of photosynthesis of aquatic organisms [3]. In addition, most dyes are toxic, and some are even carcinogenic. Dyes lasting long lifetimes in water bodies contaminate food chains, thus resulting in adverse effects on the health of animals and human beings [4]. Therefore, the removal of dye from water reservoirs requires considerable attention. Numerous techniques, such as oxidation [5], catalytic degradation [6], and biodegradation [7], have been recently developed for dye removal. These techniques are time-consuming and involve high operation costs. However, for tackling dye pollution, adsorption is a powerful technique because of its insensitivity to toxic

∗

Corresponding author. Fax: +60 45941013. E-mail address: [email protected] (B.H. Hameed).

substances, low cost, and facile operation. Investigations on the removal of dyes by adsorption are focused on utilizing readily available and cheap materials, such as fruit peels [8,9], sludge [10], and husk [11]. Composite adsorbents are effectively used at present for dye removal. A biodegradable magnetic composite microsphere is used effectively for removal of cationic dyes [12]. Polyacrylonitrile–polyamidoamine composite nanofibers [13], polyacrylamide/activated carbon hydrogels [14], magnetic ferrite nanoparticle–alginate composites [15], graphene-modified magnetic polypyrrole nanocomposites [16], and TiO2 –zeolite nanocomposites [17] have been effectively used for methylene blue (MB) removal. Activated carbon (AC) is broadly employed to remove wastewater pollutants. However, commercial AC is expensive. The synthesis of AC from various cheap agricultural wastes has been given particular attention. Oil palm ash is created from palm oil mills by the burning of oil palm shell and fiber used as fuel in boiler to generate steam. Malaysia, the world’s biggest palm oil supplier, operates more than 200 palm oil mills and thus disposes off tons of ash annually without any commercial gain [18]. The literature [19] reveals that oil palm ash contains approximately 40% SiO2 and 6% Al2 O3 , which are essential constituents for zeolite formation, and 5.5% unburnt carbon, which can be activated, thus facilitating the synthesis of zeolite–activated carbon composite.

http://dx.doi.org/10.1016/j.jtice.2016.10.029 1876-1070/© 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

W.A. Khanday et al. / Journal of the Taiwan Institute of Chemical Engineers 70 (2017) 32–41

We aim to convert SiO2 and Al2 O3 components of oil palm ash into zeolite along with simultaneous activation of unburnt carbon present to produce zeolite–AC (Z–AC) composite. Considering the advantage of using activated carbons and zeolites in adsorption processes [20,21], the Z–AC composite can become as a low-cost adsorbent in the removal of dyes and other pollutants from waste water. Composite synthesis was carried out to enhance the surface area and increase dye removal capacity. The Z–AC composite was prepared by chemically facilitated NaOH activation, followed by hydrothermal treatment with oil palm ash as substrate. MB dye was considered as model adsorbate to evaluate the adsorption efficiency of the prepared Z–AC composite. We examined the effects of numerous parameters, such as initial dye concentration, pH, and contact time, on adsorption capacity. The obtained adsorption data were described by exploring isotherm and kinetic models. 2. Materials and methods 2.1. Adsorbate In the current work, a cationic dye, MB was selected as the model adsorbate. MB is extremely difficult to degrade under natural environmental conditions. MB was supplied by Sigma-Aldrich (M), Malaysia, and was used without purification. An appropriate quantity of MB was dissolved in double-distilled water obtained from a USF ELGA water treatment system to prepare stock solutions. Working solutions of the desired concentration (25– 400 mg/L) were prepared by consecutive dilutions of the standard stock solution.

33

Table 1 Chemical composition of oil palm ash. Compound

Weight (%)

SiO2 Al2 O3 MgO K2 O CaO C P2 O5 Fe2 O3 Ignition loss Others

49.63 4.34 6.01 9.65 5.52 8.57 1.98 3.20 9.50 1.60

potassium bromide disc sample method of investigation. FT-IR was carried out both before adsorption and after adsorption. The surface physical properties of Z–AC composite were analyzed by Autosorb I (Quantachrome Corporation, USA) by using N2 as adsorbate at −196 °C. External surface area, mesopore volume, and mesopore surface area were obtained from surface area analysis technique by t-plot method. Barrett–Joyner–Halenda method was employed to deduce pore size distribution and average pore width. Field-emission scanning electron microscopy (FE-SEM, LEO SUPRA 35VP) was used to study the morphology of the Z–AC composite and its textural structure. Energy-dispersive spectroscopy (EDS, X-ray Microanalysis System Ametek EDAX-GENESIS 20 0 0) was employed for elemental analysis. 2.4. Adsorption studies

2.2. Preparation of Z–AC composite For the current work, oil palm ash was obtained from a local palm oil mill in Penang, Malaysia, and used without purification. All reagents and chemicals of high analytical grade quality were obtained from Sigma-Aldrich (M), Malaysia, and used as collected. Z–AC composite was prepared from oil palm ash in two steps, namely, fusion followed by hydrothermal treatment [22]. Ten grams of oil palm ash was mixed with NaOH (98% purity, from Sigma-Aldrich) at a ratio of 1:3 by mass and then heated at 800 °C for 90 min in N2 atmosphere. NaOH was used in activation treatment because it not only results in production of activated carbon but also assists the hydrothermal synthesis of zeolites. After NaOH fusion treatment, the material obtained was grinded and mixed with kaolin at the ratio of 1:0.2 to add aluminum oxide and then suspended in deionized water followed by stirring and aging overnight. The mixture was stored in a Teflonlined autoclave and heated for 8 h at 100 °C. After hydrothermal treatment, the product was filtered and washed with deionized water to bring the pH to approximately 7.

A 0.2 g portion of Z–AC composite and 200 mL MB solutions with varying initial dye concentrations (25–400 mg/L) were placed in stoppered Erlenmeyer flask (250 mL) for performing batch adsorption experiments. Solutions were continuously agitated at 120 rpm in isothermal water bath shaker for 30 h at 30 °C. At each predetermined time interval, the MB final concentration was determined until the process attained equilibrium. For studying the effect of solution pH, 100 mL of MB solution at 100 mg/L initial concentration was taken in a pH range of 3–13 adjusted by using dilute NaOH or HCl (0.1 M). A UV/Visible double-beam spectrophotometer from Shimadzu, Japan, was employed for analyzing MB concentration by absorbance measurements at 668 nm. The equilibrium adsorption amount, qe (mg/g), was determined by:

qe =

( C 0 − C e )V W

(1)

where Ce (mg/L) and C0 (mg/L) respectively represent the equilibrium and initial concentration of MB, Z–AC composite adsorbent mass is denoted by W (g), and solution volume is V (L). 3. Result and discussion

2.3. Characterization of Z–AC composite 3.1. Adsorbent characterization A 25 g portion of dry oil palm ash was grinded to fine powder and then heated to determine the loss on ignition (LOI) prior to chemical composition analysis. An X-ray Fluorescence Rigaku RIX 30 0 0 (XRF) spectrometer was used to determine the chemical composition of oil palm ash. The powder X-ray diffraction (XRD) pattern of the Z–AC composite was recorded in the 2θ ranging between 5° and 50° with 2°/min scanning rate and using SIEMENS XRD D50 0 0 equipped ˚ at 30 mA and 40 kV. with Kα Cu radiation (λ = 1.54056 A) Surface functional groups were characterized by a Perkin–Elmer Spectrum GX infrared spectrometer with the wave number ranging from 40 0 0–40 0 cm−1 and the resolution of 4 cm−1 by using

Oil palm ash was found to contain a high percentage of SiO2 along with some other metal oxides. Percentage composition of oil palm ash is given in Table 1. The XRD pattern of Z–AC composite shows both crystalline and amorphous phases. Crystalline and amorphous phases are attributed to zeolite and AC, respectively. The main crystalline peaks detected in XRD pattern (Fig. 1) at 2θ of 7.92°–8.80° and 23.10°– 24.40° are characteristic to zeolite and is in good agreement with that reported in the literature [23,24]. The FT-IR transmission spectra for Z–AC composite before and after adsorption are shown in Fig. 2. The FT-IR spectra show a

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W.A. Khanday et al. / Journal of the Taiwan Institute of Chemical Engineers 70 (2017) 32–41

Fig. 1. XRD pattern of the Z–AC composite.

90 80

MB Z-AC

70 1640

Transmittance (%)

60

Z-AC

3700 - 3300

706

50 40

550 1640

30 1508 1050

20

706

3700 - 3300

10 0 4000

1050

3500

3000

2500

2000

1500

1000

550

500

-1

Wavenumber (Cm ) Fig. 2. FTIR pattern of Z–AC (before adsorption) and MB Z–AC (after adsorption of MB; C0 = 200 mg/L).

band at 1640 cm−1 , which is attributed to scissor vibrations arising owing to proton vibrations in the water molecule. The bands at 1050 and 706 cm−1 represent the asymmetric and symmetric stretching vibrations, which respectively correspond to the SiO4 or AlO4 structure. The band at 550 cm−1 is assigned to the deformation vibration of the Al–O–Si group. The broad peak from 3400– 3700 cm−1 has been associated with the stretching of hydroxyl groups of the zeolitic structure [25]. Comparing the spectra of the Z–AC composite with MB Z–AC composite, we can see that some of the peaks disappeared or shifted. Shifting of peaks in the region

from 3400 cm−1 to 3700 cm−1 (OH stretch) and disappearance of peak at around 1508 cm−1 (NH deformation) are observed. The detected changes in the spectrum confirm that the functional groups present on the Z–AC composite surfaces are possibly involved in the sorption process. The pore size distributions and N2 adsorption–desorption isotherm of the Z–AC composite at −196 °C are displayed in Fig. 3. The Z–AC composite presented a type IV isotherm (IUPAC system), which indicates the existence of a mesoporous nature [26]. The textural parameters of the Z–AC composite are summarized

W.A. Khanday et al. / Journal of the Taiwan Institute of Chemical Engineers 70 (2017) 32–41

35

350

Adsorption Desorption

250

200 Incremental Pore Volume (cm³/g)

Quantity Adsorbed (cm³/g STP)

300

150

100

50

0.008

0.006

0.004

0.002

0.000 0

0 0.0

0.2

0.4

10

20

30

Pore Diameter (Å)

0.6

0.8

40

50

60

1.0

Relative Pressure (P/Po) Fig. 3. Nitrogen adsorption–desorption isotherm and pore size distribution of the Z–AC composite obtained at 77 K.

Table 2 Surface physical characteristics of the Z–AC composite. Surface physical parameters

Value

BET surface area (m2 /g) Langmuir surface area (m2 /g) External surface area (m2 /g) Mesopore surface area (m2 /g) Mesopore volume (cm3 /g) Average pore diameter (nm)

615.406 666.418 393.429 269.872 0.575 3.048

a

in Table 2. Average pore diameter confirms the mesoporous texture with pore diameter range between 20 and 40 A˚ (2 and 4 nm). SEM images of the Z–AC composite under 10 0 0× and 15,0 0 0× magnification are shown in Fig. 4(a) and (b), respectively. The SEM micrographs showed that the Z–AC composite exhibits a porous and loose texture. Zeolite and AC portions are visible under higher magnification. Pores exhibit mesoporous range and are uniform in size and distribution. 3.2. Selection of Z–AC composite as adsorbent MB removal by Z–AC was compared with that of non-activated oil palm ash, oil palm ash zeolite, and activated oil palm ash. The Z–AC composite showed extremely higher percentage removal compared with others (Fig. 5) and is thus considered the adsorbent for all remaining studies.

b Zeolite

Activated carbon

3.2. Effects of initial concentrations and contact time on adsorption MB adsorption was examined at various time intervals with initial concentrations of 25–400 mg/L at 30 °C, and the obtained result obtained is presented in Fig. 6. A rapid increase in adsorption was initially observed, followed by a gradual increase until equilibrium. At initial concentrations of 25 and 50 mg/L, equilibrium is attained faster compared with high initial concentrations. This finding may be due to the relatively high availability of active

Fig. 4. (a) Low-magnification SEM image (10 0 0×) and (b) high-magnification SEM image (15,0 0 0×) of the Z–AC composite.

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W.A. Khanday et al. / Journal of the Taiwan Institute of Chemical Engineers 70 (2017) 32–41

100

% MB removal

80

60

40

20

0 Un-activated palm ash Palm ash zeolite

Activated palm ash

Z-AC composite

Fig. 5. Adsorption uptake of MB onto various adsorbents from oil palm ash (V = 100 mL, W = 0.10 g, initial MB dye concentration = 100 mg/L, shaking speed = 140 rpm, and temperature = 30 °C).

180 160

25 mg/g 150 mg/g 400 mg/g

50 mg/g 200 mg/g

400

800

100 mg/g 300 mg/g

140

qt (mg/g)

120 100 80 60 40 20 0 0

200

600

1000

1200

1400

1600

1800

Time (min) Fig. 6. Effect of the initial concentration on MB adsorption onto the Z–AC composite (V = 200 mL, W = 0.20 g, shaking speed of 140 rpm, and temperature = 30 °C).

sites at low concentrations and for lesser amount of MB dye molecules. The adsorption uptake of the Z–AC composite for MB increased from 24.91–138.15 mg/g at 30 °C with the increase in initial dye concentration from 25–400 mg/L. This finding indicates higher adsorption capacities at higher initial concentration, which may be due to the pressure gradient that generates significant driving forces [27].

3.3. Effect of solution pH on adsorption The adsorption between the surface and adsorbate depends on pH because speciation, degree of ionization, and the surface charge of adsorbate are affected by solution pH The effect of solution pH was examined in the pH range of 3–13 at a temperature of 30 °C and an initial dye concentration 100 mg/L, on the uptake of MB

W.A. Khanday et al. / Journal of the Taiwan Institute of Chemical Engineers 70 (2017) 32–41

37

100 90 80

qe (mg/g)

70 60 50 40 30 20 10 0

2

4

6

8

10

12

14

pH Fig. 7. Effect of solution pH on MB adsorption onto the Z–AC composite (V = 100 mL, W = 0.10 g, initial MB dye concentration = 100 mg/L, shaking speed = 140 rpm, and temperature = 30 °C).

onto the Z–AC composite is shown in Fig. 7. The adsorption of MB was extremely low between pH 3 and pH 5 but increased linearly up to a certain pH and then remained constant. Given that FT-IR studies reveal that the OH group (active site) is involved in adsorption, less adsorption uptake at lower pH is due to the competition of protons with MB for binding to these active sites. In more basic medium, the Z–AC composite can rapidly interact with MB because of the more negative, which results in high adsorption. Similar reports are available in literature [28]. 3.4. Kinetic models For the practical design of an adsorption system, knowledge about potential controlling steps and the mechanism of adsorption, which is provided by adsorption kinetics, is essential. The kinetic data in the present study were modeled using pseudo-first-order (PFO) and pseudo-second-order (PSO) kinetic models. Lagergren’s nonlinear equation of PFO model [29] is written as:



qt = qe 1 − e−k1 t



(3)

where qe (mg/g) is the adsorption capacity of MB at equilibrium, qt (mg/g) is the adsorption capacity at time t (min), and k1 (/min) is the velocity constant of first-order kinetics. Ho’s nonlinear equation of the PSO kinetic model [30] is written as:

qt =

q2e k2 t 1 + qe k2 t

(4)

and its linear form is given as:

t 1 1 = + t qt qe k2 q2e



p

RMSE =

1

(q−qm )2 p

(6)

where q is the experimental adsorption capacity, qm is model prediction adsorption capacity, and p denotes number of data points. The kinetic parameters along with residual RMSE and the coefficient (R2 ) values are presented in Table 3. The PSO kinetic model was found to possess larger coefficients of determination (R2 ) and lower RMSE values compared with PFO. The best fitting of experimental data on PSO kinetic model depict that the adsorption rate of MB onto the Z–AC composite does not depend on the dye concentration in solution but depends on the availability of the adsorption sites. A similar mechanism has been suggested for the adsorption of MB over an AC composite [26]. 3.5. Mechanism of adsorption

(2)

and its linear form is given as:

ln(qe − qt ) = lnqe − k1 t

calculated to evaluate the error of kinetic models. RMSE was calculated according to the following expression:

(5)

where k2 (g/mg/min) is the constant known as velocity or rate constant for second-order kinetics. Fitting was carried by plotting t/qt against t and ln(qe −qt ) against t at 30 °C (figures not shown) for both PSO and PFO models. Root mean square errors (RMSE) were

The adsorption mechanism may proceed in several steps, which include the adsorption of the dye on the sorbent surface, intraparticle diffusion, and diffusion of the dye through the boundary layer. Among these steps, the rate-determining step is often the slowest step for adsorption in batch reactor systems. PFO and PSO kinetic models were not able to explain the contribution of intraparticle diffusion to the adsorption mechanism; hence, intraparticle diffusion model was introduced by Weber and Morris [31] and expressed by the equation:

qt =k pt 1/2 +C

(7)

where kp (mg/g/min1/2 ) is the intraparticle diffusion rate constant and C is the constant depicting the boundary layer effects. According to this model, the plot of qt versus t1/2 should be a straight line if intraparticle diffusion plays a role in the adsorption process; if this line crosses the origin, then intraparticle diffusion is the ratedetermining step. However, as shown in Fig. 8, lines are straight but do not pass through the origin, suggesting that intraparticle diffusion is not the sole rate-determining step [32]. The kp values

38

W.A. Khanday et al. / Journal of the Taiwan Institute of Chemical Engineers 70 (2017) 32–41 Table 3 Kinetic nonlinear model parameters for MB adsorption onto the Z–AC composite (V = 200 mL, W = 0.20 g, shaking speed = 140 rpm, temperature = 30 °C). Pseudo-first-order kinetic model

Pseudo-second-order kinetic model

MB conc. (mg/L)

qe , exp (mg/g)

qe , cal (mg/g)

k1 (1/min)×103

R2

RMSE

qe , cal (mg/g)

k2 (g/mg/min) × 104

R2

RMSE

25 50 100 150 200 300 400

24.91 47.95 75.80 89.60 99.00 120.80 138.15

24.28 47.18 70.08 88.80 95.07 117.17 134.85

4.36 3.76 2.45 2.40 1.98 1.41 1.44

0.99 0.99 0.96 0.98 0.98 0.94 0.96

1.47 1.51 2.08 1.64 1.24 1.66 1.39

27.14 50.61 78.07 92.10 105.63 127.88 151.93

4.81 4.43 3.63 3.37 3.61 3.39 3.01

0.99 0.99 0.98 0.98 0.99 0.98 0.98

0.08 0.10 0.17 0.15 0.12 0.14 0.14

100 90 80

25 ppm

50 ppm

100 ppm

200 ppm

300 ppm

400 ppm

150 ppm

qt (mg/g)

70 60 50 40 30 20 10 0 4

6

8

10

12

t1/2

14

16

18

20

(min1/2)

Fig. 8. Intraparticle diffusion plots for the adsorption of methylene blue onto the Z–AC composite (V = 200 mL, W = 0.20 g, shaking speed = 140 rpm, and temperature = 30 °C). Table 4 Intra-particle diffusion parameters for methylene blue adsorption onto the Z–AC composite. MB conc. (mg/L)

qe , exp (mg/g)

qe , cal (mg/g)

kp (mg/g h1/2 )

R2

RMSE

25 50 100 200 300 400

24.91 47.95 75.80 89.60 99.00 120.80

26.83 49.31 79.13 123.28 146.45 153.32

0.56 0.73 0.96 11.26 19.51 23.37

0.99 0.99 0.96 0.94 0.95 0.98

0.12 0.14 0.23 0.36 0.29 0.17

obtained from the slope of qt versus t1/2 plots along with R2 and RMSE values are given in Table 4. 3.5.1. Thermodynamic studies The effect of temperature on MB adsorption onto the Z–AC composite was studied by varying the temperature over a range of 30–50 °C. An increase in MB adsorption was observed with increased temperature, showing that adsorption onto the Z–AC composite is favorable at high temperature and thus suggesting that the adsorption process is endothermic. This situation is better explained by thermodynamic adsorption parameters, such as enthalpy change (H), entropy change (S), and Gibb’s free energy change (G), which are easily determined by the change in equilibrium constant K with temperature. The value of K is obtained as:

K=

C1 C2

(8)

where C1 is the quantity of MB adsorbed per unit mass of the Z–AC composite and C2 is the aqueous phase concentration of MB. From

the slope and intercept of Van’t Hoff plot thermodynamic parameters like, such as standard enthalpy change (H°) and Standard Entropy change (S°) were determined. In the Van’t Hoff plot, ln K is plotted against 1/T in the following equation.

ln k =

S o R

−

H o RT

(9)

The standard Gibb’s free energy change (G°) was calculated by using the following equation

Go = −RT lnK

(10)

The thermodynamic parameters for MB adsorption onto the Z– AC composite are given in Table 5. The obtained G° values were negative, and H° values were positive, confirming that the adsorption of MB onto the Z–AC composite is spontaneous and endothermic.

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39

Table 5 Thermodynamic parameters of MB adsorption onto the Z–AC composite. Initial concentration (mg/L)

25 50 100 200 300 400

H° (KJ/mol)

S° (KJ/K/mol)

6.23 10.69 30.95 69.32 84.47 71.23

0.05 0.07 0.15 0.27 0.31 0.24

G° (KJ/mol) 30 °C

40 °C

50 °C

−8.63 −10.16 −11.16 −12.63 −9.17 −7.56

−9.19 −12.28 −14.22 −16.47 −11.44 −9.06

−9.87 −12.73 −15.03 −17.81 −14.81 −9.73

Table 6 Langmuir and Freundlich isotherm parameters for the adsorption of MB onto the Z–AC composite. Temperature (°C)

30 40 50

Langmuir isotherm model

Freundlich isotherm model 2

qmax (mg/g)

KL (L/mg)

R

143.47 199.60 285.71

0.04 0.04 0. 05

0.95 0.95 0.94

RMSE

S.D.

KF (mg/g)(L/mg)1/ n

1/n

R2

RMSE

S.D.

0.09 0.05 0.03

0.17 0.10 0.06

36.75 33.82 45.39

0.22 0.31 0.33

0.97 0.99 0.97

0.05 0.02 0.03

0.04 0.03 0.06

Table 7 Comparison of monolayer adsorption of MB onto various adsorbents. Adsorbents

Temperature (°C)

Maximum monolayer adsorption capacities (mg/g)

References

Z-AC composite from oil palm ash Natural zeolite Zeolite NaA Zeolite fly ash (ZFA) Chitosan modified zeolite Biopolymer oak sawdust composite Montmorillonite/CoFe2 O4 composite GO/CA composite TiO2 /SiO2 /Fe3 O4 composite

50 60 30 NA 25 22 NA 25 25

285.71 29.18 64.80 14.30 37.04 38.46 97.75 181.81 147.00

This work [36] [37] [38] [39] [40] [41] [42] [43]

3.6. Adsorption isotherms Isotherms are helpful in explaining adsorbent–adsorbate relationship and adsorbate molecule distribution at equilibrium in the liquid and solid phases. Moreover, the adsorption capacities of the adsorbent are predicted by isotherm analysis. Freundlich and Langmuir isotherms were modeled to simulate the obtained data of MB adsorption onto the Z–AC composite for the initial concentration range of 25–400 mg/L. The Langmuir model [33] assumes that adsorption occurs in a monolayer and describes the adsorbent surface as homogeneous with identical surface sites. Its nonlinear form is expressed as:

qe =

qm KL Ce 1+KL Ce

(11)

and one of its linear forms is expressed as:

Ce Ce 1 = + qe qm KL qm

(12)

where KL (L/mg) is an adsorption constant that is associated with free energy, qm (mg/g) denotes the maximum adsorption capacities, and Ce (mg/L) denotes the equilibrium concentrations of MB. Freundlich isotherm [34] explains the multilayer adsorption process and presumes a heterogeneous adsorbent surface. Its nonlinear form is expressed as: 1

qe = KFCen

(13)

The logarithmic linear form is expressed as:

lnqe = lnKF +

1 lnCe n

(14)

where n and KF (mg/g)(L/mg)1/ n are Freundlich parameters in relation to the adsorption intensity and adsorption capacity, respectively. The isotherm models at 30 °C are fitted in line with linear

form of Eqs. (12) and (14) for the adsorption of MB onto the Z–AC composite (figures not shown). RMSE analysis is conducted identically to Eq. (6) in kinetic modeling. Standard deviation (S.D.) was calculated to determine validity of models by using the following expression:



S.D. =

[(q − qm )/q] n−1

(15)

where qm and q refer to the calculated and experimental data and n is the total number of data points. Freundlich and Langmuir parameters, such as KF , 1/n, KL , and qm , which are determined from the plots by using intercepts and slopes, as well as RMSE, S.D., and correlation coefficients (R2 ) and values are listed in Table 6. Low RMSE and S.D. values, as well as high R2 values for the MB adsorption on the Z–AC composite at different temperature show that the adsorption data fitted closely to both Langmuir and the Freundlich models; however, these values also reveal that the data better fitted the Freundlich model compared with the Langmuir model, thus indicating multilayer adsorption and supporting a physisorption mechanism [35]. Langmuir monolayer adsorption capacities were 143.47, 199.60, and 285.71 mg/g at 30 °C, 40 °C, and 50 °C, respectively. Various adsorbents, such as zeolites and other composites, and their maximum monolayer adsorption capacities for MB uptake are listed in Table 7. The comparison clearly shows the maximum adsorption capacity of MB in the present study is superior to that in previous results [36–43]. 3.7. Reusability Desorption experiments were performed to evaluate the possibility of reusability and regeneration of the Z–AC composite as

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W.A. Khanday et al. / Journal of the Taiwan Institute of Chemical Engineers 70 (2017) 32–41

100

% MB removal

80

60

40

20

0 1

2

3

4

Cycles of adsorption Fig. 9. Adsorption efficiency of the Z–AC composite for MB in various reuse cycles.

an adsorbent. MB desorption was carried out by a simple solvent washing technique, in which the Z–AC composite was washed with acidic ethanol for recovery for the next use. The effect of four consecutive adsorption–desorption cycles was studied, and the results are displayed in Fig. 9. Only a slight decrease in removal efficiency was observed after four cycles of adsorption–desorption process, thereby confirming the reusability of the Z–AC composite for MB removal. Owing to the high adsorption capability, good desorption property, and particular cost-effectiveness, the Z–AC composite can be used for commercial applications. 4. Conclusions The suitability of oil palm ash for the synthesis of a high surface area, mesoporous zeolite–activated carbon composite by NaOH activation and hydrothermal treatment was demonstrated in the present study. The effects of initial dye concentration (25– 400 mg/L), temperature (30 °C–50 °C), and pH (3–13) on the adsorption of MB on Z–AC were studied successfully. Kinetic and equilibrium data for the adsorption of MB were examined by using the nonlinear forms of various models. The acquired result indicates that PSO better describes the adsorption process. Experimental data were found to fit both Langmuir and Freundlich isotherms well, but the Freundlich model was superior according to R2 , RMSE, and S.D. values. The monolayer adsorption capacity values of the Z–AC composite for MB were 143.47, 199.6, and 285.71 mg/g at 30 °C, 40 °C, and 50 °C, respectively. Acknowledgments The first author acknowledges the award of Universiti Sains Malaysia postdoctoral fellowship in aid for research. The authors extend their appreciation to the International Scientific Partnership Program ISPP at King Saud University for funding this research through ISPP# 0042. References [1] Sun H, Cao L, Lu L. Magnetite/reduced graphene oxide nanocomposites: one step solvothermal synthesis and use as a novel platform for removal of dye pollutants. Nano Res 2011;4:550–62.

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