adsorption characteristics of methylene blue on some ... · studies on adsorption kinetics showed...

International Journal of Applied Engineering Research ISSN 0973-4562 Volume 13, Number 13 (2018) pp. 11112-11122

© Research India Publications. http://www.ripublication.com

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Adsorption Characteristics of Methylene Blue on Some Special Modified

Clays

Omer L. Uyanika*, Necla Bektasb, and Nurseli Uyanikb

aBahcesehir University, Faculty of Engineering and Natural Sciences, Department of Molecular Biology and Genetics, 34349 Besiktas, Istanbul, Turkey.

bIstanbul Technical University, Department of Chemistry, 34469 Maslak, Istanbul, Turkey. *Corresponding author

Abstract

The objective of this research was to study the adsorption

behavior of methylene blue dye onto the surface of

organomodified sodium montmorillonites. Various

organoclays prepared by modifying sodium montmorillonite

(NaMMT) were used as adsorbents to remove methylene blue

(MB), a cationic dye, from aqueous solution. The modifiers

used were octadecyl ammonium chloride (ODACl), benzyl

dimethyl hexadecyl ammonium chloride (BDMHACl), and

glycidyl trimethyl ammonium chloride (GTMACl). In the

adsorption experiments, the concentrations of MB in solution

at various instants during adsorption were determined using

UV-VIS spectroscopy at λmax = 663 nm. Adsorption capacities

were observed to increase with increasing initial dye

concentration, to decrease with increasing adsorbent mass, and

to decrease with increasing temperature. Studies on adsorption

kinetics showed that the adsorption fitted pseudo-second-order

kinetic model. Thermodynamic parameters for adsorption,

ΔG°, ΔH°, and ΔS° were also computed and these results

indicated that the adsorption is spontaneous within the

temperature range studied and is exothermic. The equilibrium

data at constant temperature were applied for Langmuir and

Freundlich isotherm models and the data were well described

by the Langmuir model. As a result of this study, it was

observed that, being an organoclay, NaMMT modified with

GTMACl has a higher adsorption capacity due to its high

inorganic content.

Keywords: Organoclay, adsorption, adsorption capacity

INTRODUCTION

Dyeing is a very important step in various industries including

textile, plastics, paper, and leather. Due to the consumption of

large quantities of water, significant amount of colored

wastewater is generated by these industries [1]. Color

influences the perception of water quality negatively because

an even very small concentration of dyes in water is visible and

undesirable [2]. The strong color of wastewaters from dyeing

processes causes a reduction in light penetration, which, in turn,

significantly affects photosynthetic activity in aquatic life [3].

Moreover, many of these dyes are quite toxic and are known to

be carcinogenic and hence they are seriously hazardous to

aquatic living organisms [4].

Color removal technologies have been classified into three

main groups, namely, biological treatments, chemical methods,

and physical methods [5]. Biological treatment is widely used

in the decolorization of wastewaters. There have been various

studies on aerobic (in the presence of oxygen) treatments [6-8],

on anaerobic (without oxygen) treatments [6,9] and on

combined aerobic-anaerobic treatment [10,11]. Although

biological treatment is generally more economical than

chemical and physical methods, its application has some

drawbacks such as the need for large land area, restricted

flexibility in the design and operation of the treatment systems,

and sensitivity to toxicity of some chemicals [12]. On the other

hand, although many of the dye molecules are biodegraded by

such treatments, some of them are quite resistant to

biodegradation because of their complex structures [13].

Among the chemical methods for dye wastewater treatment,

one of them is coagulation and flocculation which is conducted

by adding Ca2+, Al3+, or Fe3+ ions [14,15] or polymeric

nanocomposite coagulants [16] to the wastewater. Another

group of chemical methods is oxidation in which oxidizing

agents such as chlorine in the form of sodium hypochlorite or

calcium hypochlorite [17], hydrogen peroxide [18], Fenton’s

reagent, a solution of hydrogen peroxide and an iron catalyst

[19,20], and ozone [21] have been used. The application of

chemical methods is often discouraged due to their high cost

and disposal problems resulting from accumulation of

concentrated sludge. Besides these, there is also the possibility

that some toxic compounds may form as a result of unavoidable

side reactions during decolorization of dye wastewater by

chemical treatment.

Physical methods for color removal include adsorption and

membrane filtration techniques such as nanofiltration,

electrodialysis, and reverse osmosis. In membrane filtration

techniques short lifetimes of membranes due to their fouling

causes their frequent periodic replacements. Thus, such

applications are not always economically feasible [5]. Liquid

phase adsorption in which the substance to be removed (the

adsorbate) is accumulated at the interface between liquid and

solid (the adsorbent), has been the most widely used method for

decolorization [22].The main advantages of adsorption are low

initial cost, simplicity of design, ease of operation, and

insensitivity to toxic adsorbates. Also, unlike chemical

methods, harmful substances are not produced during

adsorption [5].

The most popular adsorbent for dye removal from wastewater

has been activated carbon due to its great capacity to adsorb

dyes [23]. This great capacity results from its porous structure

that leads to a large surface area per unit mass ranging from 500

to 2000 m2/g [24]. Although activated carbon is effective for



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the adsorption of various dyes from wastewaters, it poses some

disadvantages [25]. The most important of these disadvantages

is its high cost. Also, activated carbon becomes exhausted after

its use in wastewater treatment and thus its adsorption

capability diminishes drastically. This necessitates

regeneration of activated carbon for further use which, in turn,

adds cost since regeneration methods are quite expensive. On

the other hand, activated carbon is ineffective against disperse

and vat dyes. These disadvantages have forced researchers to

search for low-cost adsorbents.

A good alternative to activated carbon is clay minerals. Natural

clay minerals are known by humans since the early days of

civilization. They are considered to be suitable adsorbents

because of their low cost, abundance in most parts of the world,

and high adsorption capacities. Clays possess layered structure

and they always have exchangeable ions on their surface. This

allows adsorption of cations or anions in the pollutant to the

clay surface without influencing the structure of clay mineral

[25,26]. Good adsorption properties of clays may also be

attributed to their high surface area per unit mass and high

porosity [27]. An increasing interest has been observed in

utilizing clay minerals such as montmorillonite, kaolinite,

perlite, sepiolite, bentonite as adsorbents for dye molecules.

Various aspects of adsorption (including adsorption capacities,

adsorption kinetics, adsorption thermodynamics, and

adsorption isotherms) of dyes on clay minerals have been

studied [28-32].

It has been observed by various researchers that the adsorption

capacities of clay minerals can be improved by surface

modification. The adsorption of azo-reactive dyes on sepiolite

surfaces which was modified with quaternary amines was

investigated and the adsorption capacity was observed to be

considerably greater than that of unmodified sepiolite [33]. The

adsorption of an acidic dye (Acid Blue 193) onto Na-bentonite

and dodecyltrimethyl ammonium bromide-modified bentonite

(DTMA-bentonite) was studied and the surface properties of

Na-bentonite were observed to be greatly improved by surface

modification due to the increased surface area caused by the

large organic groups in the structure of the modifier [34]. A

group of researchers investigated the effect of cold N2 gas

plasma surface modification on the adsorption performance of

a cationic dye, methylene blue, onto bentonite clay and they

observed that this modification increased the adsorption

capacity of bentonite [35]. Bentonite was also modified by a

cationic surfactant, hexadecyltrimethyl ammonium chloride for

the removal of three cationic dyes namely methylene blue,

crystal violet, and Rhodamine B, from aqueous solution and it

was observed that modification increased the adsorption

efficiency of bentonite due to its increased surface area and thus

increased number of dye molecules that can be bound to the

clay surface [36]. Some clays and zeolites were modified by an

ionic liquid, 1-hexadecyl-3-methylimidazolium chloride to

investigate the adsorption of chloramphenicol (a

pharmaceutical) from water onto their surfaces and the results

indicated that adsorption capacity of chloramphenicol was

higher on modified clays (especially on montmorillonite) and

zeolites than on unmodified ones which is probably due to an

increase in surface hydrophobicity [37].

In this study, sodium montmorillonite (NaMMT) was modified

by octadecyl ammonium chloride (ODACl), benzyl dimethyl

hexadecyl ammonium chloride (BDMHACl), and glycidyl

trimethyl ammonium chloride (GTMACl). The XRD and TGA

characterizations of these organomodified MMT samples were

made and these samples were used as adsorbents in adsorption

studies of methylene blue (MB) dye from aqueous solution.

The adsorption experiments were conducted by using UV-VIS

spectroscopy at λmax= 663 nm. This study included the

investigation of the effects of initial dye concentration,

temperature, and amount of adsorbent on adsorption capacity.

The kinetics and thermodynamic parameters and the adsorption

isotherm representing the equilibrium were also determined.

EXPERIMENTAL

Materials

NaMMT (Nanofil 757) used for the preparation of organoclay

was received from Southern Clay Products. The empirical

formula of NaMMT is Na0.2Ca0.1Al2Si4O10(OH)2(H2O)10. It

was a highly purified natural NaMMT with a cation exchange

capacity of 80 meq/100 g. It had a medium particle size (< 10

μm) and a bulk density of approximately 2.6 g/cm3. The

modification agents used were ODACl (Merck), BDMHACl

(Fluka), and GTMACl (Merck). The chemical structures of the

modification agents are shown in Figure 1. MB (a cationic dye),

was the adsorbate. The chemical structure of MB is shown in

Figure 2. All other reagents (Merck) were used without further

purification.

(a) (b)

(c)

Figure 1. The chemical structures of (a) ODACl; (b)

BDMHACl; (c) GTMACl

Figure 2. The chemical structure of MB



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Equipment

The X-ray diffraction (XRD) patterns of NaMMT and the

modified MMT samples were recorded using the Shimadzu

LabX XRD-6000 diffractometer Cu Kα (λ = 1.5405 Å)

radiation. The samples were scanned from 2θ = 2 – 40°.

Thermal characterization of all samples were carried out by

using Seiko Exstar 6200 TG/DTA thermal gravimetric analyzer

from 30 ºC to 1000 ºC at a heating rate of 10 ºC/min. During

analysis, nitrogen was used as carrier gas with a constant flow

rate of 150 mL/min.

In adsorption experiments, concentration of the adsorbate in

aqueous solution was determined by using Perkin Elmer

Lambda 25 Model double beam UV-VIS Spectrophotometer.

Its wavelength range is between 190-1100 nm. The wavelength

for maximum absorbance was determined and then all the

adsorption experiments were carried out at that wavelength. In

order to determine the wavelength corresponding to maximum

absorbance, 5, 10, 20, 40, 60, 80, and 90 mg/L aqueous

solutions of MB were prepared by properly diluting the solution

that has an MB concentration of 100 mg/L. The absorption

spectra for these solutions were obtained using UV-VIS

spectrophotometer in the wavelength range from 400 to 700

nm. It has been observed that the wavelength corresponding to

maximum absorbance is 663 nm. Then the calibration equation

between absorbance and MB concentration at this wavelength

was obtained.

Modification procedure

ODACl (50 mmol) was dissolved in 100 mL of distilled water

in an acidic medium and heated up to 80°C. The solution was

mixed with a suspension composed of 20 g clay in 400 mL hot

water. The mixture was vigorously stirred for 1 h at 80°C. The

white powder that was formed was filtered and the precipitate

was washed with fresh hot water. After stirring for one more

hour, the filtration and washing process was repeated once

more. The final precipitate was dried in an oven and then

ground. This modification procedure was also repeated with

BDMHACl and GTMACl for the preparation of other

organoclays.

Adsorption experiments

For adsorption experiments, aqueous solutions having 40, 60,

80, and 100 mg/L MB concentrations were prepared. All

experiments were conducted at a pH of 5.40. A sample of MB

solution of known concentration was put into a flask together

with organoclay. The mixture was stirred at 180 rpm at constant

temperature. Equilibrium for the adsorption of MB on

organoclays was reached in approximately three hours and the

mixture was filtered at the end of this period. The concentration

of MB in the solution was determined by using the calibration

equation obtained from UV-VIS spectrophotometer

measurements at 663 nm.

The adsorption capacity, 𝑞𝑡, at time t, which is defined as

milligrams of MB adsorbed per gram of adsorbent, is calculated

from

𝑞𝑡 = 𝑉 (𝐶𝑡− 𝐶0)

𝑎 (1)

where V is the volume of the solution in L, Ct is the

concentration (in mg/L) of MB at time t, C0 is the initial

concentration (in mg/L) of MB, and a is the mass of adsorbent

in g.

RESULTS AND DISCUSSION

The X-ray diffraction (XRD) analysis

The XRD image of unmodified clay (NaMMT) is given in

Figure 3. From the diffraction values, the distance between

layers (d001) in the unmodified clay was calculated as 12.3 Å

using the Bragg equation. The XRD images of the

organomodified clays are given in Figure 4. The XRD results

of the unmodified and organomodified samples are given in

Table 1. The d001 values for ODACl MMT, BDMHACl MMT

and GTMACl MMT samples were obtained as 31.5 Å, 24.2 Å

and 19.4 Å, respectively. Due to its low organic content

GTMACl MMT has smaller interlayer separation than the other

two organomodified MMTs.

Table 1. XRD results for the NaMMT and organomodified

samples

Sample Description 2 d001 (Å)

NaMMT 7.19 12.3

ODACl MMT 2.80 31.5

BDMHACl MMT 3.65 24.2

GTMACl MMT 4.55 19.4

Figure 3. XRD image of NaMMT.

Figure 4. XRD images of (a) ODACl MMT; (b)

BDMHDACl; (c) GTMACl



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Thermal characterization

TGA curves for NaMMT and organomodified clays are given

in Figure 5. The curve for NaMMT showed that the thermal

decomposition of unmodified clay occurred in two steps. In the

first step, the weight loss between 30 – 120°C is 7 % and it is

due to the removal of physically bound water molecules on the

surface of NaMMT and between the silicate layers. The second

step corresponds to the weight loss of 5 % between 575 –

675°C. This decomposition is attributed to the dehydroxilation

of – OH groups in the structure. On the other hand, the residue

at 1000°C is observed to be 86 % by weight.

Figure 5. TGA curves of NaMMT and organomodified clays

Figure 6. The variation of adsorption capacity with time for ODACl MMT

sample at various initial MB concentrations. (Mass of organoclay = 0.05 g, T = 30 °C, pH = 5.40)

Figure 7. The variation of adsorption capacity with time for BDMHDACl MMT

sample at various initial MB concentrations. (Mass of organoclay = 0.05 g, T = 30 °C, pH = 5.40).



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Figure 8. The variation of adsorption capacity with time for GTMACl MMT

sample at various initial MB concentrations. (Mass of organoclay = 0.05 g, T = 30 °C, pH = 5.40).

TGA curves for organomodified MMTs shows that the thermal

decomposition of almost all organomodified MMTs also

occurred in two steps. Considering the first decomposition step,

the MMT sample modified with GTMACl lost 6 % of its weight

between 30 – 185 °C due to the removal of physically bound

water molecules on the surface and between the silicate layers

like in unmodified MMT, although the weight losses for MMT

samples modified with ODACl and BDMHACl are 2 % and 1

%, respectively, within the same temperature range. On the

other hand, the second step in thermal decomposition occurred

within different temperature ranges and with different percent

weight losses for the three organomodified MMTs. The MMT

modified with GTMACl lost 9 % of its weight between 260 –

630 °C, the MMT modified with ODACl lost 25 % of its weight

between 340 – 420 °C, and the MMT modified with

BDMHACl lost 32 % of its weight between 240 – 420 °C. If

the residues at 1000 °C are compared, the results are 85 %, 70

%, and 65 % by weight for MMTs modified with GTMACl,

ODACl, and BDMHACl, respectively, indicating that

interactions of ODACl and BDMHACl by MMT surface are

greater than that of GTMACl. This result is consistent with that

obtained from XRD analysis such that interlayer separation in

GTMACl is smaller than those of other organomodified

MMTs. The results of TGA are shown in Table 2.

Table 2. TGA results for the NaMMT and organomodified

sample

Sample

description

Temperature ranges

of degradation (°C)

Weight

loss (wt%)

Residue at

1000 °C

(wt%)

NaMMT 30 – 120

575 – 675

7

5

86

ODACl

MMT

30 – 200

340 – 420

2

25

70

BDMHACl

MMT

30 – 120

240 – 420

1

32

65

GTMACl

MMT

30 – 185

260 – 630

6

9

85

Effect of initial dye concentration on adsorption capacity

The adsorption capacities for the three modified

montmorillonite samples (ODACl MMT, BDMHACl MMT,

and GTMACl MMT) as a function of time at various initial MB

concentrations (40, 60, 80, and 100 mg/L) are shown in Figures

6, 7, and 8, respectively. The equilibrium adsorption capacity

increases from 26 to 168 mg/g for ODACl MMT sample

(Figure 6), from 29 to 207 mg/g for BDMHDACl MMT sample

(Figure 7), and from 140 to 212 mg/g for GTMACl MMT

sample (Figure 8) as initial MB concentration is changed from

40 to 100 mg/L, indicating that for each organoclay, the

adsorption capacity increases with increasing methylene blue

concentration. These plots also indicate that the time to

equilibrium does not change significantly with changing initial

MB concentration for a given organoclay sample. For any

sample, the increase in the equilibrium adsorption capacity with

increasing initial dye concentration can be attributed to the

greater driving force resulting from greater concentration

gradient between MB in the solution and on the adsorbent

surface. Comparison of the equilibrium adsorption capacities

showed that GTMACl MMT sample has the highest

equilibrium adsorption capacity among the three

organomodified samples due to its highest inorganic content.

Effect of adsorbent mass on adsorption capacity

The adsorption capacities for the three modified

montmorillonite samples (ODACl MMT, BDMHACl MMT,

and GTMACl MMT) as a function of time at various amounts

of adsorbent (0.01, 0.03, and 0.05 g) are shown in Figures 9,

10, and 11, respectively. In the experiments carried out with

ODACl MMT, the mass of MB adsorbed at equilibrium

increases from 1.06 mg (for 0.01 g adsorbent) to 1.31 mg (for

0.05 g adsorbent) while the equilibrium adsorption capacity

decreases from 106 mg/g (for 0.01 g adsorbent) to 26.2 mg/g

(for 0.05 g adsorbent) (Figure 9). A similar behavior is

observed in the experiments carried out with BDMHACl MMT

and GTMACl MMT samples. The mass of MB adsorbed at



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equilibrium increases from 1.04 mg (for 0.01 g adsorbent) to

1.45 mg (for 0.05 g adsorbent) while the equilibrium adsorption

capacity decreases from 104 mg/g (for 0.01 g adsorbent) to 29

mg/g (for 0.05 g adsorbent) for BDMHACl MMT samples

(Figure 10) and the mass of MB adsorbed at equilibrium

increases from 2.2 mg (for 0.01 g adsorbent) to 7.0 mg (for 0.05

g adsorbent) while the equilibrium adsorption capacity

decreases from 220 mg/g (for 0.01 g adsorbent) to 140 mg/g

(for 0.05 g adsorbent) for GTMACl MMT samples (Figure 11).

It is observed that the adsorption capacity decreases as the

adsorbent mass increases. Thus, using excessive amounts of

adsorbent in the adsorption of dyestuff in wastewaters is not

economical.

Figure 9. The variation of adsorption capacity with time for

ODACl MMT sample at various masses of adsorbent. (Initial

dye concentration = 40 mg/L, T = 30 °C, pH = 5.40)


BDMHDACl MMT sample at various masses of adsorbent.

(Initial dye concentration = 40 mg/L, T = 30 °C, pH = 5.40)


GTMACl MMT sample at various masses of adsorbent. (Initial

dye concentration = 40 mg/L, T = 30 °C, pH = 5.40)

Effect of temperature on adsorption capacity

The effect of temperature on adsorption capacity was

investigated on a GTMACl MMT sample since GTMACl

MMT has higher adsorption capacity than ODACl MMT and

BDMHACl MMT. Experiments were conducted at 30 °C,

40°C, and 50 °C, using 0.05 g GTMACl MMT and an aqueous

solution having an MB concentration of 40 mg/L and a pH of

5.40. The adsorption capacity of 0.05 g GTMACl MMT sample

as a function of time at the specified temperatures is shown in

Figure 12. It is observed that the highest equilibrium adsorption

capacity appeared to be 140 mg/g at 30 °C. The adsorption

capacity decreases as temperature increases. This can be

attributed to the decrease in the tendency of the MB molecules

to be adsorbed onto the surface as a result of an increase in

kinetic energies due to increased temperature.

Figure 12. The variation of adsorption capacity with time for a

GTMACl MMT sample at various temperatures. (Mass of

GTMACl MMT sample = 0.05 g, initial dye concentration = 40

mg/L, pH = 5.40)

Adsorption kinetics studies

Kinetic models have been developed to explain adsorption

mechanism and they have been used to determine the rate of

adsorption process. The applicability of the results of

adsorption experiments to pseudo-first-order and pseudo-

second-order kinetic models have been investigated. The



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differential rate equation for the pseudo-first-order model first

presented by Lagergren was expressed as follows [38]:

𝑑 𝑞𝑡

𝑑 𝑡= 𝑘1 (𝑞𝑒 − 𝑞𝑡) (2)

where qt and qe are the amounts of dye per gram of adsorbent

(mg/g) adsorbed on the clay at time t and at equilibrium,

respectively, and k1 is the first-order rate constant (min-1).

Separating the variables and integrating with the boundary

conditions, at t = 0, qt = 0 and at t = t, qt = qt gives

ln(𝑞𝑒 − 𝑞𝑡) = ln 𝑞𝑒 − 𝑘1 𝑡 (3)

If pseudo-first-order kinetics was applicable, the plot of ln (qe – qt) against t would give a straight line and k1 and qe would be

calculated from the slope and intercept of this line.

The pseudo-second-order differential rate equation is as

follows [39]:

𝑑 𝑞𝑡

𝑑 𝑡= 𝑘2 (𝑞𝑒 − 𝑞𝑡)2 (4)

where k2 is the pseudo-second-order rate constant (g. mg-1. min-

1), and qt and qe have their usual meanings. Separating the

variables and integrating with the boundary conditions, at t = 0, qt = 0 and at t = t, qt = qt gives

1

𝑞𝑒− 𝑞𝑡=

1

𝑞𝑒+ 𝑘2 𝑡 (5)

Equation 5 is rearranged into the following form:

𝑡

𝑞𝑡=

1

𝑘2𝑞𝑒2 +

𝑡

𝑞𝑒 (6)

If pseudo-second-order kinetics was applicable, the plot of t/qt

against t would give a straight line and k2 and qe would be

calculated from the slope and intercept of this line.

Results of adsorption experiments obtained from ODACl

MMT, BDMHACl MMT, and GTMACl MMT samples were

tested for integrated pseudo-first-order and pseudo-second-

order expressions. The results fitted pseudo-second-order

kinetic model. As a representative graph, t/qt versus t plots for

GTMACl MMT at 30 °C, 40 °C, and 50 °C (at constant mass

of adsorbent, initial dye concentration, and pH) are shown

(Figure 13). A straight line was obtained for each temperature

and the equilibrium adsorption capacity, qe, and the adsorption

rate constant, k2, were obtained from the slope and intercept of

each straight line. The values of the kinetic parameters (qe and

k2) at 30 °C for the three modified clay samples and the

correlation coefficients, r2, for the straight lines are given in

Table 3. Pseudo-second-order kinetics was reported also for the

adsorption of rhodamine B on sodium montmorillonite [40], of

methylene blue on sodium montmorillonite [29], of neutral red

on halloysite nanotubes [41], and of methyl violet on halloysite

nanotubes [42].

Figure 13. The applicability of the experimental results to the

integrated pseudo-second-order rate expression for the

adsorption of MB on GTMACl MMT at various temperatures.

(Mass of GTMACl MMT sample = 0.05 g, initial dye

concentration = 40 mg/L, pH = 5.40).

Table 3. Kinetic parameters (k2 and qe) for the adsorption of

MB (with an initial MB concentration of 40 mg/L) on three

different modified clays at 30 °C.

Type of clay k2 (g/mg min) qe (mg/g) r2

GTMACl MMT 6.83 x 10-4 147.1 0.996

BDMHDACl MMT 1.07 x 10-4 32.4 0.974

ODACl MMT 5.05 x 10-4 33.8 0.976

The effect of temperature on the kinetic parameters was

investigated for the adsorption of MB on GTMACl MMT

sample (the one having the highest equilibrium adsorption

capacity). From the slopes and intercepts of the three straight

lines (representing three different temperatures) in Figure 13,

qe and k2 at each temperature were determined. The results of

these computations are given in Table 4 together with the r2

values.

Table 4. Kinetic parameters (k2 and qe) for the adsorption of

MB (with an initial MB concentration of 40 mg/L) on

GTMACl MMT at three different temperatures.

T (°C) k2 (g/mg min) qe (mg/g) r2

30 6.83 x 10-4 147.1 0.996

40 2.43 x 10-3 120.5 0.999

50 7.42 x 10-3 106.4 0.999

The activation energy for the adsorption of MB on GTMACl

MMT was determined by applying the data in Table 4 to

Arrhenius equation

𝑘2 = 𝐴 exp (− 𝐸𝑎

𝑅 𝑇) (7)

where Ea is the activation energy (kJ mol-1) for adsorption, A is

the temperature-independent preexponential (g mg-1 min-1), R

is the gas constant (8.314 x 10-3 kJ mol-1 K-1), and T is the

temperature of the solution (K). Equation 7 is linearized into

the following form:

ln 𝑘2 = ln 𝐴 − 𝐸𝑎

𝑅 𝑇 (8)



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From Equation 8, a plot of ln k2 versus 1/T gives a straight line

with slope – Ea /R. Applying the data in Table 4 to Equation 8

gives the straight line in Figure 14. The activation energy was

calculated from the slope of this straight line, as Ea = 37 kJ mol-

1 for the adsorption of MB on GTMACl MMT. The magnitude

of activation energy is an indication of the type of adsorption,

i.e., whether it is chemisorption (40–800 kJ/mol) or

physisorption (5–40 kJ/mol) [43]. The calculated value of Ea

suggests physisorption for the adsorption of MB on GTMACl

MMT.

Figure 14. Arrhenius plot (ln k2 versus 1/T) for the adsorption

of MB on GTMACl MMT (with an initial MB concentration

of 40 mg/L).

Adsorption thermodynamics studies

Adsorption of MB on the adsorbent can be expressed as

MB (in solution) → MB (adsorbed)

For the determination of thermodynamic parameters for this

process, namely standard free energy change (ΔG°), standard

entropy change (ΔS°), and standard enthalpy change (ΔH°),

first the relationship among the three is written:

∆𝐺0 = ∆𝐻0 − 𝑇 ∆𝑆0 (9)

ΔG° can also be expressed as

∆𝐺0 = −𝑅 𝑇 ln 𝐾𝑐 (10)

where, R is the gas constant, T is the temperature of the solution

in Kelvin, and Kc is the equilibrium constant at temperature T,

for the adsorption of MB on the adsorbent, i.e.,

𝐾𝑐 = 𝐶𝑎

𝐶𝑒 (11)

where ca is the equilibrium concentration (mg/L) of MB on the

adsorbent and ce is the equilibrium concentration (mg/L) of MB

in the solution. Equations 9 and 10 can be combined and

rearranged as

ln 𝐾𝑐 = − ∆ 𝐻0

𝑅 𝑇+

∆ 𝑆0

𝑅 (12)

Thermodynamic parameters were calculated for GTMACl

MMT sample since this sample has higher equilibrium

adsorption capacity than BDMHACl MMT and ODACl MMT

samples. ce and ca values were obtained from the data in Figure

12 at 30 °C, 40 °C, and 50 °C as follows: at each temperature,

ce was calculated using the calibration equation obtained from

UV/VIS absorbance measurements and ca was calculated as the

difference between c0 (initial concentration of MB) and ce. Then

ce and ca values were substituted into Equation 11 to calculate

Kc at each temperature. ΔG° was calculated at each temperature

using Equation 10. From Equation 12, the plot of ln Kc versus

1/T (van’t Hoff plot) gives a straight line with slope - ΔH°/R

(Figure 15). ΔH° was calculated from the slope of this straight

line assuming that ΔH° is independent of temperature within

the temperature interval investigated. Finally, ΔS° at each

temperature was calculated from Equation 9. The equilibrium

constants and thermodynamic parameters for the adsorption of

MB on GTMACl MMT are given in Table 5.

Figure 15. Van’t Hoff plot (ln Kc versus 1/T) for the

adsorption of MB on GTMACl MMT.

Table 5. Equilibrium constants and thermodynamic

parameters for the adsorption of methylene blue on GTMACl

MMT.

T (°C) Kc ΔG0 (kJ/mol) ΔH0 (kJ/mol) ΔS0 (J/K mol)

30 2.26 - 2.05

- 27.6

- 84.3

40 1.49 - 1.03 - 84.9

50 1.15 - 0.38 - 84.3

It is observed from the table that the calculated values of

standard free energy change, ΔG°, are negative, pointing a

spontaneous adsorption. The magnitude of ΔG° values varied

from – 2.05 to – 0.38 kJ mol-1 as the temperature increased from

30 °C to 50 °C. These values fall between – 20 and 0 kJ mol-1,

that is suggested as the range for physical adsorption [34].

Furthermore, the absolute value of ΔG° decreased with

increasing temperature, indicating that the degree of

spontaneity decreases as temperature is raised. Adsorption of

MB on modified clay would be nonspontaneous at sufficiently

high temperatures. The standard entropy changes, ΔS°, for

adsorption are negative. This decrease in entropy is due to the

more ordered state of MB molecules on the solid surface than

in the solution. The standard enthalpy change, ΔH°, was

calculated to be – 27.6 kJ mol-1. This means that adsorption



11120

process is exothermic which is supported by an increase in

equilibrium adsorption capacity with decreasing temperature

(Figure 12). It was suggested that the adsorption is physical in

nature if the absolute magnitude of ΔH° is lower than 40 kJ mol-

1 [34]. This is another indication of the physical nature of

adsorption of MB on modified montmorillonites.

Adsorption isotherms

Adsorption isotherms indicate the equilibrium relationship at

constant temperature, between the concentration of the

adsorbate in solution and the quantity of adsorbate adsorbed per

unit mass of adsorbent. Among the various adsorption

isotherms, Langmuir and Freundlich models are the two most

commonly applied ones.

The basic assumption of Langmuir model is the monolayer

adsorption of the adsorbate molecules on a homogeneous

surface [44]. It was also assumed that there are no interactions

between the adsorbed molecules. The linearized form of

Langmuir model is as follows [42]:

𝑐𝑒

𝑞𝑒=

𝑐𝑒

𝑞𝑚+

1

𝐾𝐿 𝑞𝑚 (13)

where ce is the concentration of adsorbate at equilibrium

(mg/L), qe is the mass of adsorbate adsorbed by the adsorbent

at equilibrium (mg/g), qm is a constant indicating the theoretical

(maximum) adsorption capacity corresponding to monolayer

coverage (mg/g), and KL is a constant related to energy of

adsorption (L/mg).

Freundlich model assumes that the sites on the surface of the

adsorbent have different binding energies; the stronger binding

sites are occupied first and the binding strength decreases with

increasing degree of site occupation [45]. The linear form of

the Freundlich model is as follows [42]:

ln 𝑞𝑒 = ln 𝐾𝐹 + (1

𝑛) ln 𝑐𝑒 (14)

where qe is the mass of adsorbate adsorbed by the adsorbent at

equilibrium (mg/g), ce is the concentration of adsorbate at

equilibrium (mg/L), KF is a constant related to adsorption

capacity, and n is a constant related to adsorption intensity.

The experimental equilibrium data was analyzed by the

Langmuir and Freundlich models. Using the data obtained for

the adsorption of MB on GTMACl at 30°C at four different

initial MB concentrations, Langmuir model was checked by

plotting ce/qe versus ce (Figure 16) and Freundlich model was

checked by plotting ln qe versus ln ce (Figure 17). Langmuir

plot resulted in a straight line with a correlation coefficient of

0.990 and Freundlich plot resulted in a straight line with a

correlation coefficient of 0.895. Hence, it was concluded that

the equilibrium data are fitted satisfactorily with Langmuir

isotherm model, i.e., a monolayer adsorption. From the slope

of the straight line in Figure 16, the theoretical (maximum)

Langmuir adsorption capacity, qm was found to be 260 mg/g.

Figure 16. Langmuir plot from the data obtained for the

adsorption of MB on GTMACl MMT at four different initial

MB concentrations. (T = 30 °C).

Figure 17. Freundlich plot from the data obtained for the

adsorption of MB on GTMACl MMT at four different initial

MB concentrations. (T = 30 °C).

CONCLUSIONS

In this study, the modifiers containing large organic groups

(ODACl, BDMHACl, and GTMACl) were used for the

modification of MMT to increase the surface area for

adsorption and hence its adsorbing capacity for the organic dye

MB. The GTMACl MMT sample has the highest equilibrium

adsorption capacity at all selected initial MB concentrations for

a constant mass of adsorbent. XRD and TGA results showed

that GTMACl MMT has a higher inorganic content than the

other two organomodified MMTs, yet besides its high

equilibrium capacity that is comparable with that of unmodified

MMT it will be compatible with organic substances due to its

organic content and thus GTMACl MMT can be used as a new

nanocomposite adsorbent.

It was also observed that the equilibrium adsorption capacity

increases with increasing MB concentration for a given mass of

adsorbent and decreases with increasing mass of adsorbent for

a constant initial concentration of MB. The effect of

temperature on adsorption capacity was investigated using

GTMACl MMT sample, the one that has the highest

equilibrium adsorption capacity. The equilibrium adsorption

capacity decreased with increasing temperature probably



11121

because high kinetic energies of the adsorbate molecules

reduced their tendencies to be adsorbed onto the surface.

Adsorption kinetics data fitted pseudo-second-order kinetic

model. Thermodynamic parameters were also evaluated and

the negative values of ΔG° indicated the spontaneous nature of

adsorption within the temperature range studied and the

negative value of ΔH° indicated the exothermic nature of

adsorption. Finally, having negative values of ΔS° is in

agreement with the fact that MB adsorption occurred via a

decrease in the disorder.

As a final statement, it may be said that the organoclay prepared

by modifying MMT with GTMACl, which has smaller affinity

to the clay surface than ODACl and BDMHACl, is a better

adsorbent for MB. This organoclay that is compatible with

organic materials and having a high adsorption capacity may be

a promising nanocomposite adsorbent for the treatment of

wastewaters.

ACKNOWLEDGEMENT

The authors wish to thank Southern Clay Products for

supplying Nanofil 757 used. This research was supported by

Istanbul Technical University Grant No. 32575.

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