Adsorption of methylene blue onto spinel magnesium aluminate nanoparticles: Adsorption isotherms, kinetic and thermodynamic studies

Accepted Manuscript Adsorption of methylene blue onto spinel magnesium aluminate nanoparticles: Adsorption isotherms, kinetic and thermodynamic studies Bushra Ismail, Syed Tajammul Hussain, Sohaib Akram PII: DOI: Reference:

S1385-8947(13)00074-0 http://dx.doi.org/10.1016/j.cej.2013.01.034 CEJ 10270

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

10 October 2012 8 January 2013 9 January 2013

Please cite this article as: B. Ismail, S.T. Hussain, S. Akram, Adsorption of methylene blue onto spinel magnesium aluminate nanoparticles: Adsorption isotherms, kinetic and thermodynamic studies, Chemical Engineering Journal (2013), doi: http://dx.doi.org/10.1016/j.cej.2013.01.034

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1

Adsorption of methylene blue onto spinel magnesium aluminate

2

nanoparticles: Adsorption isotherms, kinetic and thermodynamic studies

3

Bushra Ismaila,*, Syed Tajammul Hussainb, Sohaib Akramb

4

a

5

b

Department of Chemistry, COMSATS Institute of Information Technology, Abbottabad 22060, Pakistan Nanoscience and Catalysis Division, National Centre for Physics, Quaid-e-Azam University Campus, Islamabad

6 7 8 9 10 11 12 13 14 15 16 17 18 19

* Corresponding author:

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Email address: [email protected], Phone: +92 992 383 591, Fax: +92 992 383 441

1

1

Adsorption of methylene blue onto spinel magnesium aluminate

2

nanoparticles: Adsorption isotherms, kinetic and thermodynamic studies

3

Bushra Ismaila,*, Syed Tajammul Hussainb, Sohaib Akramb

4

a

5

b

6

Abstract:

7

Magnesium aluminate spinel has been synthesized at nanoscale by coprecipitation method. The

8

synthesized material was characterized for the phase composition, quantitative description,

9

surface morphology, surface area, pore volume and pore diameter. X-ray diffraction analysis

Department of Chemistry, COMSATS Institute of Information Technology, Abbottabad 22060, Pakistan Nanoscience and Catalysis Division, National Centre for Physics, Quaid-e-Azam University Campus, Islamabad

10

confirmed the formation of the cubic spinel phase with no peaks corresponding to the impurity

11

phases. The Scherrer crystallite size calculated from line broadening is 20 nm. The pore surface

12

area was found to be 343m2/g by N2 vapor adsorption experiments and material was

13

characterized as mesoporous on the basis of pore diameter analysis. The scanning electron

14

micrograph shows the agglomeration of particles while the nanosize is confirmed by the

15

transmission electron micrograph. The porous nature of the material was then explored for

16

carrying out the adsorption of methylene blue solutions on to the surface. Adsorption studies

17

were carried out with 0.1g of the material and the effect of pH and shaking times were studied

18

and both were found to influence the adsorption. Adsorption data was also fitted to Temkin,

19

Freundlich and the Langmuir adsorption models in order to study the mechanism of adsorption

20

by interpreting the calculated parameters like, heat of adsorption (BT), binding energy (KT),

21

degree of adsorption (KF), heterogeneous factor (n), and energy of adsorption (KL), binding

22

forces (aL), and the separation factor (RL). The pseudo-second order rate constant (Kp-2), initial

23

sorption rate (Srate) and the half adsorption time (t1/2) were also calculated and explained to 2

1

clarify the mechanism of adsorption onto MgAl2O4 surface. Gibbs free energy was also

2

calculated from the adsorption data at room temperature.

3

Key words: Mesoporous oxide; Water treatment; Adsorption; Isotherms; Kinetics

4

1. Introduction:

5

Many industries such as textile, chemicals, refineries, leather, plastic, paper, etc., use different

6

kinds of dye stuffs in various processing steps. The concentration of the dye stuffs in the waste

7

waters is variable depending upon the type of industry, e.g. wastewaters from the first stage of

8

dyeing in the leather industry has values ranging from 1024 to 4553 mg L-1 whereas, the second

9

dyeing stage has 1980 to 5083 mg L-1 concentrations of residual dye [1]. Generally, 30-40 % of

10

these dyes remain in the waste waters coming from these industries. These dyes are chemically

11

and photolytically stable and the complex aromatic structures of these dyestuffs hinder in the

12

natural bio-degradation process resulting in the turbidity as well as foul odors of these waste

13

waters [2]. That is why; the color removal from waste waters has attracted the attention of the

14

researchers worldwide, because not only these compounds themselves but their hydrolysis in the

15

untreated waste waters produce other toxic compounds which pose threat to aquatic life [3].

16

Many physical and chemical methods have been used for the color removal from waste waters.

17

Traditionally used methods include coagulation, sedimentation and activated sludge, etc. More

18

advanced methods include ozonation, membrane separation, electrochemical and ultrasonic

19

techniques, photocatalysis, adsorption, etc. [4]. The adsorption process is preferred as it is

20

environmentally friendly and cost effective. It has high treatment efficiency and the selection of

21

adsorbent plays very important role in determining its cost effectiveness [5].

3

1

The search for the efficient and low cost adsorbents is still underway. A large number of high

2

surface area adsorbents have been used since long. These include activated carbon, jute fiber

3

carbon, cedar sawdust and crushed brick, un-burnt carbon, bentonite, garlic peel, modified

4

expanded graphite power, starch, aluminosilicates, silica gel, zeolites, titania, etc., [6-9]. Many

5

new and promising adsorbents e.g. graphene and carbon nanotubes have also been reported [10]

6

during the past few years.

7

The nanoporous oxide materials are mostly hydrophilic and polar in nature. Based on the pore

8

sizes these can be categorized either as microporous (pore diameter less than 2 nm) or

9

mesoporous (pore diameter in the range of 2-50 nm), whereas their % porosities range from 30-

10

60 %. These have high mechanical strength, good thermal stability and are resistant to the

11

chemical attacks. They also have long life times and have moderate costs [11-13]. Apart from the

12

better chemical/thermal stability and the excellent mechanical strength, the adsorption capacity

13

and the selectivity are the important criteria for a good adsorbent which in turn depend upon the

14

specific surface area, surface nature, pore size and pore size distribution. The large surface area

15

requires the small crystallite sizes of the material which in turn is dependent upon the synthesis

16

method used [14].

17

A brief recent literature survey is presented below for the color removal using oxide materials.

18

Adsorption of reactive red dye was studied on the pretreated Fe3O4 nanoparticles by Wang et al

19

[15]. The particle size was 5-10 µm and the adsorption-desorption equilibrium was achieved in

20

60 min and the experimental adsorption capacity (qe) was 17.33 mg/g for the theoretical value of

21

20 mg/g. Graphene oxide was used for the removal of methylene blue [16] and the equilibrium

22

was established in 250 min and qe was 243 mg/g. Apart from bare oxide materials, the composite

23

materials have also been used. CuFe2O4/sawdust composites were used for the removal of

4

1

cyanine acid red and the equilibrium was reached in 15 min [7]. Fe3O4/zeolite composites were

2

used for the removal of reactive orange and indigo carmine and the equilibrium reached in 400

3

min while the qe was calculated to be 1.1 and 0.58 mg/g for the respective dyes [12]. Similarly,

4

Fe3O4/maize cob composites were used for the removal of methylen blue and the equilibrium

5

reached in 10 min [17]. Mn3O4/silica composite materials were used for the removal of

6

methylene blue and equilibrium achieved in 30 min and the percent removal was 82.17 % [18].

7

Trimetallic oxides e.g. magnesium aluminate spinel MgAl2O4 has been used in diverse

8

applications such as refractory material, catalyst or catalyst support, humidity sensors,

9

microwave dielectric and ceramic capacitor, structural material in fusion reactors, etc, [19].

10

Magnesium aluminum spinel (MAS) (Mg2+)[Al3+, Al3+]O4 is a normal spinel in which the

11

divalent Mg2+ ions occupy tetrahedral sites, represented by parenthesis, and the trivalent Al3+

12

ions occupy octahedral sites, represented by square brackets [20]. Magnesium aluminate has a

13

high melting point (2135 ºC), low density (3.58 g/cm3), excellent strength at extremely high

14

temperatures (Knoop hardness of 1150 kg/mm2, flexural strength of 140 MPa and Young’

15

modulus of 190 GPa) and good resistance against chemical attacks.

16

Though, a tremendous amount of literature can be found on this material but the emphasis had

17

been mostly on the discovery of the better synthesis method and it has not been tested for any

18

applications except as a humidity sensor [21]. The properties mentioned above give a clear clue

19

for the possibility of exploration of its characteristics as an adsorbent material.

20

Keeping the mentioned discussion in the mind, the present study aims at (a) the synthesis of

21

magnesium aluminate material at nanoscale by coprecipitation method, (b) to test the adsorption

22

characteristics of the synthesized material using methylene blue as adsorbate by fitting of the

23

adsorption data to various isotherm models (Temkin, Freundlich, BET, Langmuir) and (c) to

5

1

study the mechanism of adsorption by fitting the data to various kinetic models and study of

2

thermodynamic properties. The synthesis at nanoscale would result in the large surface area

3

required for a good adsorbent and fitting of the adsorption isotherms and kinetic models would

4

help in determining the mechanism of adsorption.

5

Methylene blue has been selected for the present study. The removal of MB from wastewaters is

6

an environmental issue and has launched the extensive research efforts in this regard. It is a

7

heterocyclic aromatic compound which is heavily used in the textile industry and it is also

8

present in the effluents of waste waters coming from other industries. The acute exposure to

9

methylene blue dye may cause some harmful effects such as increased heart rate, shock,

10

vomiting, jaundice, and tissue necrosis in humans [3].

11 12

2. Experimental:

13

2.1 Synthesis of the material

14

The starting materials were Mg(NO3)2.6H2O (Merck, 99.9%), Al(NO3)3.9H2O (Merck, 95.0%),

15

aqueous NH3 (Reidel, 33.0%) and were used as supplied. In coprecipitation method using

16

ammonia [22], 5.128g of magnesium nitrate and 15.005g of aluminum nitrate were dissolved

17

separately in 100mL deionized water (DI) and added simultaneously into a flask containing

18

200mL of DI water. Ammonia solution (2M) was added as a precipitating agent and the pH of

19

the solution was maintained at 9. The solution was stirred for 1 h and aged at room temperature

20

overnight and precipitates were then washed and dried at 393K for 12hrs and annealed at 1223K

21

for 8hrs.

22

2.2 Characterization

6

1

The compound formation, phase purity and crystallinity of the prepared spinel materials

2

were identified by powder X-ray diffraction technique using (PANalytical 3040/60 X` Pert PRO)

3

with Cu Kα radiation source over a range of 10 - 85◦ at a scan speed 1s/step. Diffraction peaks

4

were used to identify the structure of samples by matching their observed patterns with the

5

standard pattern of magnesium aluminate spinel (ICSD ref. code No. 00-021-1152, a = 8.08 Å,

6

Vcell = 528 Å3). Lattice parameter, cell volume, X-ray density and the crystallite size were also

7

calculated using the following equations and match well with the standard pattern. Lattice

8

parameter “a”, unit cell volume “Vcell”, Scherrer crystallite size “D” and X-ray density “ρx-ray”

9

have been calculated using the following relations (1-4):

[

]

1/ 2

10

a = d 2 (h 2 + k 2 + l 2 )

11

Vcell = a 3

12

D=

13

ρ x − ray =

14

where d is value of d-spacing of lines in XRD pattern, hkl are corresponding indices to each line

15

in the pattern, β is the broadening of diffraction line measured at half width of its maximum

16

intensity, λ is the X-ray wavelength and is equal to 1.542 A°, θ the Bragg’s angle and K the

17

constant which is equal to 0.9 for cubic system, Z is the number of molecules per formula unit (Z

18

= 8 for spinel system), M is the molar mass, h is the height of pellet, Vcell and NA have their usual

19

meanings [22]. BET (Brunauer-Emmet-Teller) specific surface area studies were carried out on

20

Qunatachrome NovaWin 2 using N2 vapor adsorption. Surface morphology was studied using

kλ β cosθ B ZM Vcell N A

(1)

(2)

(3)

(4)

7

1

Philips CM 200 transmission electron microscope (TEM). Energy dispersive X-ray spectroscope

2

EDS coupled with scanning electron microscope (JEOL, JSM5910) is used for determining the

3

chemical composition of the synthesized material.

4

2.3 Adsorption studies

5

A stock solution of 100 mg/L of methylene blue was prepared. The necessary dilutions were

6

carried out in 25 ml of distilled water in order to prepare 1, 3, 5, 7, 9 and 11 mg/L solutions. The

7

quantity of the powder added was 0.1 g in each flask and the all the flasks were shaken for an

8

optimized time of 160 minutes at room temperature and the absorption spectrum of the filtrate

9

solutions were studied at the λmax of the dye which is 665 nm which is measured by using Perkin

10

Elmer UV/VIS Lambda 25 spectrophotometer. The pH of the solution was found to be 7. The

11

effect of pH was studied for 0.1 g MgAl2O4 in 25 ml of 3mg/L solution of dye using respective

12

buffer solutions for maintaining the pH at 2-6 and shaking the solutions for 160 min. The effect

13

of shaking time was also studied for the same 25 ml of 3 mg/L dye concentration and adding 0.1

14

g of the nanoparticles and shaking time was varied from 10 min to 160 min. The buffer solutions

15

are made by the standard methods in order to maintain the pH at the required value. Buffers of

16

pH 2, 4 were used as received from Fluka, while buffer of pH 3 was made by a mixture of

17

0.1mol/L of potassium hydrogen Phthalate and HCl, a buffer of pH 5 was made by a mixture of

18

0.1 mol/L of potassium hydrogen Phthalate and NaOH, and the buffer of pH 6 was made from a

19

mixture of 0.1 mol/L of potassium dihydrogen phosphate and NaOH [23].

20

3 Results and Discussions:

21

3.1 Characterization of the synthesized material

22

The important characterizations for the adsorbent material include the phase composition,

23

quantitative description, surface morphology, surface area, pore volume and pore diameter

8

1

analyses. The phase composition of the synthesized material was studied using X-ray diffraction

2

analysis. The measurements were performed on the well ground powdered samples

3

approximately 1gm in weight. The obtained pattern is shown in Figure 1 and the corresponding

4

hkl values are matched with the standard pattern of magnesium aluminate spinel (ICSD ref. code

5

No. 00-021-1152, a = 8.08 Å, Vcell = 528 Å3). The sharp and high intensity peaks are obtained

6

with hkl values of (111), (220), (311), (400), (422), (511), (440), (622) and (444). For the

7

comparison, the d-spacing and the 2theta values of the synthesized sample are compared with the

8

standard patterns of MgAl2O4 (ICSD ref. code No. 00-021-1152), Al2O3 (ICSD ref. 00-001-

9

1243) and MgO (ICSD ref. 00-001-1235) as shown in Table 1. As seen from 2theta and d-

10

spacing values in Table 1, it is concluded that the obtained pattern matches well with the

11

standard pattern of magnesium aluminate spinel. The absence of any extra peaks in the pattern

12

indicates that the synthesized sample exhibits a single-phase structure. The line broadening is

13

corrected for instrumental broadening using the relation β = (β 2 exp − β 2 inst )2 , where βexp is the

14

broadening of the sample, βinst is the broadening for the MgAl2O4 composed of large particles

15

>500 nm. The structural parameters are calculated using all the indexed peaks and then

16

averaging out for each parameter. The half widths are given by the instrument itself and are not

17

self calculated. The calculated values of lattice parameter, cell volume and X-ray density are 8.09

18

Å, 530 Å3 and 3.57 g/cm3 respectively, as given in the Table 2. These values match well with the

19

standard pattern of magnesium aluminate spinel. The Scherrer crystallite size calculated from the

20

line broadening has a value of 20 nm.

21

The surface area of the material is important along with the phase composition studies, and is

22

determined by the N2 vapor adsorption by BET method. The surface area of the material as

23

obtained from N2 adsorption experiments is 121 m2/g. The pore volume and the pore diameter

1

9

1

were 1.02 cc/g and the 121 Å respectively, and the surface area of the pores was 343 m2/g. Based

2

on the pore diameter the material is characterized as the mesoporous material (2nm < pore

3

diameter < 50 nm). The surface morphology as seen under scanning electron micrograph is

4

shown in Figure 2. The material is the porous and smaller particles joined together to form large

5

agglomerates. The inset of Figure 2 shows the TEM micrograph and the particle size is seen to

6

be in the nanometer range. The energy dispersive X-ray utility coupled with scanning electron

7

microscope was used for the determination of the chemical composition. The quantitative

8

description of the synthesized material is given in Table 2. The mol % values match with the

9

nominal compositions within the limits of experimental errors.

10

3.2 Adsorption studies

11

The adsorption studies were carried out on the synthesized material using methylene blue. The

12

adsorption studies can be categorized into three sections: (a) Effect of pH and shaking time (b)

13

fitting of data to adsorption isotherms and (c) kinetics and thermodynamic studies.

14

3.2.1

Effect of pH and shaking time

15

A Figure 3 shows the effect of pH on the adsorption of the dye at the λmax of the dye which is

16

665 nm. The adsorption is found to be pH dependant and it increases with the increase in pH and

17

is found to be maximum at neutral pH. Figure 4 shows % adsorption and the absorbance at λmax

18

of the dye in the filtrate as a function of pH and it is found that absorbance is lowest and %

19

adsorption is maximum for the neutral pH, indicating the suitability of the adsorption at neutral

20

or mild acidic conditions. The effect of pH can be explained by considering both the nature of

21

the adsorbent and the adsorbate as a function of pH as shown in graphical abstract. As far as

22

MgAl2O4 is concerned, a surface reaction creates M-OH type groups when oxide powder is

23

dispersed in water (step 1 in graphical abstract). Depending on the pH value, these M-OH groups 10

1

dissociate. A positively charged surface M-OH2+ is present at acidic pH due to the presence of

2

more H+ ions in the solution at the acidic pH (step II in graphical abstract). While at the basic pH

3

negatively charged M-O- surface exists (step III in graphical abstract). The structure of MB is

4

shown in the inset of Figure 5. Methylene blue is a basic dye and it gets protonated at lower pH

5

(step IV in graphical abstract). The PHzpc of magnesium aluminate spinel is 11.8 [24] meaning

6

that the spinel will be positively charged below this pH. A competition exists between protonated

7

MB and H+ ions at a lower pH and also an electrostatic repulsion is found between the positively

8

charged surface of magnesium aluminate and the protonated MB. Both these factors contribute

9

towards lower adsorption of MB at lower pH as H+ ions are preferably adsorbed at the oxide

10

surfaces [24]. That is why; the adsorption of MB is favored at higher pH. This favorable

11

adsorption at higher pH requires no pH maintenance in the batch reactors which normally

12

operate at pH of 7-9. Figure 5 shows the absorbance of the dye solution after the experimental

13

times of 0-160 min. The absorbance is lowest for the maximum shaking time used in the present

14

study i.e. 160 min. The increase in shaking time allows more adsorption with the maximum

15

adsorption at a shaking time of 160 min. The percent adsorption or removal rate (φ) can be

16

calculated from the equation below:

17

Φ=

Co − Ct X 100% Co

(5)

18

where Co is the initial concentration, Ct is the concentration after time t. The calculated values

19

for removal rate for 10, 40, 80 and 160 min are 40, 72, 76 and 80% respectively. The adsorption-

20

desorption equilibrium seems to be achieved after 40 minutes but the removal rate constantly

21

increased to 80 % for the time of 160 minutes, that is why the shaking time was kept at 160 min.

11

1

The adsorption capacity, qads (mg/g) can be determined by using the initial concentration Co

2

(mg/L), concentration at equilibrium Ceq (mg/L), volume of the solution, V (L) and the mass of

3

the nanoparticles, m (g) by using the relation below:

4

qads =

(C

o

− Ceq ) m

(6)

V

5

The adsorption capacity is also plotted against shaking time in Figure 6 and it is found to

6

increase sharply with time till it becomes almost constant. This rapid uptake initially is due to the

7

concentration gradient that is created between solute and the solvent and with the time it

8

decreases causing a slower increase in the amounts adsorbed [25]. In the same way, the empty

9

sites on the adsorbent get filled with the time and the adsorption desorption equilibrium is

10

attained.

11

3.2.2

Adsorption isotherms

12

Figure 7 shows the adsorption isotherm and it is seen that there is an initial increase in the

13

adsorption and then the adsorption attains the almost constant value at higher concentrations. A

14

sharp increase in adsorption capacity is also indicative of the high affinity of magnesium

15

aluminate surface for the MB. The adsorption rate calculated for the respective concentrations is

16

88, 41, 32, 24, 23 and 23%. The adsorption data obtained was fitted to various isotherm models

17

in order to find out the nature of adsorption. The linear form of Temkin equation is given below

18

[26]:

19

qads = BT ln KT + BT ln Ceq

(7)

20

The Temkin isotherm equation assumes that the heat of adsorption of all the molecules in the

21

layer decreases linearly with coverage due to adsorbent–adsorbate interactions, and that the

22

adsorption is characterized by a uniform distribution of the binding energies, up to some 12

1

maximum binding energy. Plot of qads (mg/g) vs ln Ceq (mg/L) gives a straight line with a slope

2

of BT (mg/g) and the intercept of BT lnKT (L/g) as shown in Figure 8, where, BT is the heat of

3

adsorption and its value calculated from the graph is 0.1845 (mg/g) and the KT is the binding

4

energy of adsorbent and adsorbate and its value calculated from the graph is 2.43 (L/g) as given

5

in Table 3.

6

The linear form of Freundlich isotherm is given below [18]:

7

ln qads = ln K F +

1 ln Ceq n

(8)

8 9

Plot of ln qads (mg/g) Vs ln Ceq (mg/L) will give 1/n as slope and ln KF (mg/g) as an intercept

10

and its value calculated from the graph is 0.646 (mg/g) from Figure 9. Where KF is a constant

11

showing degree of adsorption and n is the heterogeneous factor and is related to intensity of

12

adsorption and its value is (1.49). The value of n describes the adsorption characteristics as

13

follow:

14

n = 1, no interaction between the adsorbed species means a homogenous adsorption

15

n > 1, favorable adsorption

16

n < 1, unfavorable adsorption

17

The value of n is greater than unity meaning that the amount adsorbed increases less rapidly than

18

the increase in the concentration and is favorable and the formation of new sites takes place and

19

there is an interaction between the adsorbed molecules. As seen from R2 value, the Table 3, the

20

Freundlich model best fits the experimental data. This model is used for the heterogeneous

21

surface energy systems and for describing a multilayer adsorption system with interactions

22

between adsorbed molecules.

23

The linear form of Langmuir model is given below 13

1

Ceq qads

=

Ceq 1 + aL KL KL

(9)

2 3

Langmuir theory has as a basic assumption that adsorption occurs at specific homogeneous sites

4

inside the adsorbent and that once a dye molecule occupies a site, no additional adsorption can

5

occur there. Plot of Ceq/qads (g/L) vs Ceq (mg/L) gives a straight line as shown in Figure 10 with

6

a slope of aL/KL and the intercept equal to 1/KLn, where aL = Binding forces b/w adsorbate and

7

adsorbent and its value calculated from the graph is 0.185 (L/mg), KL = Energy of adsorption and

8

its value is 0.175 (L/mg). The adsorption capacity Ccap is 0.945 mg/g and the value for the

9

separation factor RL is 0.401-0.905 (g/L) and both are calculated from equations 10, and 11

10

respectively.

11

Ccap =

12

RL =

13

The value of RL describes the adsorption characteristics as follows:

KL aL

(10)

1 1 + K L Ceq

(11)

14

RL =1 linear

15

RL = 0 irreversible

16

RL > 1 unfavorable

17

0 < RL< 1 favorable

18

Based on the values of RL the adsorption process is said to be favorable.

19

The linear form of BET equation is given below: Ceq

20

Co ⎛ C ⎞ qads ⎜⎜1 − eq ⎟⎟ Co ⎠ ⎝

=

1 c − 1 Ceq + xm C xm C Co

(12)

14

1

The data did not fit to BET model indicating that the adsorption follows chemisorption.

2 3

3.2.3

kinetics and thermodynamic studies

4

Kinetic studies on the data obtained were also carried out in order to explain the adsorption

5

process over the mesoporous surfaces. The equilibrium time and the mechanism of action of

6

adsorbent for the removal of dye from water are determined by studying the adsorption kinetics.

7

The obtained data was fitted to the Lagergren pseudo first order, chemisorptions pseudo second

8

order, intraparticle diffusion model and liquid film diffusion models. In the case of the first order

9

reaction, rate of adsorption is dependent on the concentration gradient, while, in the second order

10

reaction rate, it is equal to the square of the concentration gradient.

11

Linear forms of the pseudo first order and the pseudo second order are given below.

12

ln

13

1 t t = + 2 qt k p −2 qe qt

14

The data did not fit to the first order kinetics while a straight line is obtained as shown in Figure

15

11 that confirms the pseudo-second order chemisorptions mechanism for the dye removal. The

16

pseudo second order rate constant can be determined from the intercept of the plot and the initial

17

sorption rate and the half adsorption times are calculated from the equations below and are given

18

in Table 3.

qe = ln qe − k p −1t qt

(13)

(14)

15

1

S rate = k p −2 qe2

2

(15) t 1 =

3

(16)

2

1

k p − 2 qe

4

Various steps are involved in the adsorption process. The most important ones are (a) the

5

transport of solute molecules from aqueous phase to the surface of adsorbent, (b) the diffusion of

6

solute molecules into the interior of the pores. The liquid film diffusion model is applied to the

7

first case and the intraparticle diffusion model is applied to the second case. The respective

8

equations are given below:

9

⎛ q ⎞ ln⎜⎜1 − t ⎟⎟ = − k fd t ⎝ qe ⎠

(17)

10

qt = K id t 1 + I

(18)

11

Where I (mgg-1) shows the boundary layer effect, the more the thickness of the boundary layer

12

the more will be the value of I. The data best fits to the intraparticle diffusion model as shown in

13

Figure 12 with the value for Kid equal to 0.0663 mgg-1min-1/2. If the plot goes through the origin,

14

then the intraparticle diffusion is the rate limiting step. Two separate regions are abserved in

15

Figure 11, the first straight line is due to the macropore diffusion in which the surface sites are

16

utilized initially, while the second line is due to the micropore diffusion in which the diffusion of

17

the dye from the surface film to the micopores takes place.

18

The Gibbs free energy is calculated from the following equation:

19

ΔG = − RT ln K

2

(19)

16

1

The calculated value is -1.08KJ/mole and indicates the spontaneity of the process and a favorable

2

interaction of the MB towards adsorbent.

3 4

Conclusion:

5

A ceramic oxide mesoporous material of MgAl2O4 has been successfully tested as a possible

6

adsorbent material. This material has high chemical stability, high thermal stability and higher

7

strength hence, longer life times as opposed to the traditionally used carbon, glass or polymeric

8

materials. The material was successfully prepared by coprecipitation method adding the

9

economic parameter to the effectiveness of the studied material as an adsorbent. Adsorption-

10

desorption equilibrium is reached in 160 min and the high adsorption at neutral pH validates the

11

use of the material in the treatement plants which normally operate at the pH of 7-9. The

12

adsorption data was fitted to various adsorption isotherms and the Freundlich equation was found

13

to be the best fit to the experimental data. The pseudo-second order rate equation was followed

14

indicating the chemisorptions mechanism onto the surface.

15

References:

16 17 18 19 20 21

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adsorption by tannery solid waste, Chem. Eng. J. 183 (2012) 30-38. 2. M. Toor, B. Jin, Adsorption characteristics, isotherm, kinetics, and diffusion of modified

natural bentonite for removing diazo dye, Chem. Eng. J. 187 (2012) 79-88. 3. J. Ma, Y. Jia, Y. Jing, Y. Yao, J. Sun, Kinetics and thermodynamics of methylene blue

adsorption by cobalt-hectorite composite, Dyes Pigments 93 (2012) 1441-1446.

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4. J. Huang, Y. Cao, Z. Liu, Z. Deng, W. Wang, Application of titanate nanoflowers for dye

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removal: A comparative study with titanate nanotubes and nanowires, Chem. Eng. J. 191

3

(2012) 38-44.

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5. S. Elemen, E. Perrin, A. Kumbasar, S. Yapar, Modeling the adsorption of textile dye on

5

organoclay using an artificial neural network, Dyes Pigments 95 (2012) 102-111.

6

6. D. K. Mahmoud, M.A.M. Salleh, W. Azlin, W.A. Karim, A. Idris, Z.Z. Abidin, Batch

7

adsorption of basic dye using acid treated kenaf fibre char: Equilibrium, kinetic and

8

thermodynamic studies, Chem. Eng. J. 181-182 (2012) 449- 457.

9 10 11 12 13

7. S. Hashemian, M. Salimi, Nano composite a potential low cost adsorbent for removal of

cyanine acid, Chem. Eng. J. 188 (2012) 57-63. 8. P.R. Chang, D. Qian, D.P. Anderson, X. Ma, Preparation and properties of the succinic

ester of porous starch, Carbohydrate Polymers 88 (2012) 604-608. 9. T.S. Anirudhan, A.R. Tharun, Preparation and adsorption properties of a novel

14

interpenetrating polymer network (IPN) containing carboxyl groups for basic dye from

15

aqueous media, Chem. Eng. J. 181-182 (2012) 761-769.

16

10. T. Liua, Y. Li, Q. Du, J. Sun, Y. Jiao, G. Yang, Z. Wang, Y. Xia, W. Zhang, K. Wang, H.

17

Zhuc, D. Wu, Adsorption of methylene blue from aqueous solution by graphene, Colloids

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Surfaces B: Biointerfaces 90 (2012) 197-203.

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11. K.S. Stefansk, M. Nowacka, A.K. Radzimska, T. Jesionowski, Preparation of hybrid

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pigments via adsorption of selected food dyes onto inorganic oxides based on anatase

21

titanium dioxide, Dyes Pigments 94 (2012) 338-348.

18

1

12. D. A. Fungaro, M. Yamaur, T.E.M. Carvalho, Adsorption of anionic dyes from aqueous

2

solution on zeolite from fly ash-iron oxide magnetic nanocomposite, J. At. Mol. Sci. 2

3

(2011) 305-316.

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13. N. Buvaneswari, C. Kannan, Adsorption of cationic and anionic organic dyes from

aqueous solution using silica. J. Environ. Sci Eng. 52 (2010) 361-366. 14. O.O. Kehinde, T.A. Oluwatoyin, O.O. Aderonke, Comparative analysis of the

7

efficiencies of two low cosadsorbents in the removal of Cr(VI) and Ni(II) from aqueous

8

solution, Afr. J. Environ. Sci. Tech. 3 (2009) 360-369.

9

15. H. Wang, Y. Shen, C. Shen, Y. Wen, H. Li, Enhanced adsorption of dye on magnetic

10

Fe3O4 via HCl-assisted sonication pretreatment, Desalination 284 (2012) 122-127.

11

16. Y. Li, Q. Dua, T. Liua, X. Peng, J. Wang, J. Sun, Y. Wang, S. Wu, Z. Wang, Y. Xia, L.

12

Xia, Comparative study of methylene blue dye adsorption onto activated carbon,

13

graphene oxide, and carbon nanotubes, Chem. Eng. Res. Design (2012) Article in press

14

17. K. A. Tan, N. Morad, T.T. Teng, I. Norli, P. Panneerselvam, Removal of cationic dye by

15

magnetic nanoparticle (Fe3O4) impregnated onto activated maize cob powder and kinetic

16

study of dye waste adsorption, APCBEE Procedia 1 (2012) 83-89.

17

18. J.H. Park, I. Jang, B. Kwon, S.C. Jang, S. G. Oh, Formation of manganese oxide shells on

18

silica spheres with various crystal structures using surfactants for the degradation of

19

methylene blue dye, Mater. Res. Bull. 48 (2013) 469-475.

20

19. M.J. Iqbal, B. Ismail, C. Rentenberger, H. Ipser, Modification of the physical properties

21

of semiconducting MgAl2O4 by doping with a binary mixture of Co and Zn ions , Mater

22

Res Bull 46 (2011) 2271-2277.

19

1

20. M.J. Iqbal, B. Ismail, Correlation between structural and electrical properties of

2

Mg1−2xZnxNixAl2O4 (x = 0.0–0.5) ceramic nanomaterials synthesized by a urea assisted

3

microwave combustion method, J. Alloys Compd. 504 (2010) 440-445.

4

21. A. Laobuthee, S. Wongkasemjit, E. Traversa, R.M. Laine, MgAl2O4 spinel powders from

5

oxide one pot synthesis (OOPS) process for ceramic humidity sensors, J. Eur. Ceram.

6

Soc. 20 (2000) 91-97.

7

3+

3+

22. M.J. Iqbal, B. Ismail, Electric, dielectric and magnetic characteristics of Cr , Mn

and

8

Fe3+ substituted MgAl2O4: Effect of pH and annealing temperature., J. Alloys Compd,

9

472 (2009) 434-440.

10 11 12 13 14 15 16

23. M. J. Iqbal, M. N. Ashiq, Adsorption of dyes from aqueous solutions on activated

charcoal, J. Hazardous Mater. B139 (2007) 57-66. 24. T. Kadosh, Y. Cohen, Y. Talmon, W.D. Kaplan, In situ characterization of spinel, J. Am.

Ceram. Soc. 95 (2012) 3103-3108. 25. K.Y. Foo, B.H. Hameed, Insights into the modeling of adsorption isotherm systems,

Chem. Eng. J. 156 (2010) 2-10. 26. M.A. Malana, R.B. Qureshi, M.N. Ashiq, Adsorption studies of arsenic on nano

17

aluminium doped manganese copper ferrite polymer (MA, VA, AA) composite: Kinetics

18

and mechanism, Chem. Eng. J. 172 (2011) 721-727.

19

20

1

List of Figures

2

Figure No.1: X-Ray diffraction pattern of the synthesized magnesium aluminate spinel

3

Figure No.2: Scanning electron micrograph of the magnesium aluminate spinel: Inset:

4

Transmission electron micrograph of the synthesized material

5

Figure No.3: Plots of absorbance against wavelength (λ) at pH values of 2-7

6

Figure No.4: Plot of % adsorption and absorbance at λmax of MB for different pH values

7

Figure No.5: Absorbance against wavelength (λ) at various shaking times (t) from 0-160 min

8

Figure No.6: Plot of adsorption capacity (qads) against the shaking time (t)

9

Figure No.7: Plot of adsorption capacity (qads) against equilibrium concentration (Ceq) of the dye

10

Figure No.8: Fitting of the adsorption data to the Temkin isotherm model

11

Figure No.9: Fitting of the adsorption data to the Freundlich isotherm model

12

Figure No.10: Fitting of the adsorption data to the Langmuir isotherm model

13

Figure No.11: Fitting of the adsorption data to the pseudo- second order kinetic model

14

Figure No.12: Fitting of the adsorption data to the intraparticle diffusion model

21

(311) (400)

(111)

20

1

(440)

(511)

(220)

30

40 50 Position [°2Theta]

2 3

(622) (444)

(422)

Figure 1

4

22

60

70

80

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Figure 2

15

23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Figure 3

17

24

1 2

Figure 4

25

1

N

2 3 4

N

S+

N

Cl-

5 6 7 8 9 10 11

Figure 5

12

26

1 2 3 4

Figure 6

5

27

1

Figure 7

2 3

28

1

Figure 8

2 3

29

1 2 3

Figure 9

4 5

30

1 2 3

Figure 10

4 5

31

1 2 3

Figure 11

4

32

1 2 3

Figure 12

4

33

1

Table No. 1: Comparison of experimental d-spacing and 2theta values with the standard patterns

2

of MgAl2O4, MgO and Al2O3

3

Experimental

MgAl2O4 (00-021-1152)

MgO (00-001-1235)

Al2O3 (00-001-1243)

2theta

d-spacing

2theta

d-spacing

2theta

d-spacing

2theta

d-spacing

18.789 31.530 36.967 44.959 55.693 59.737 65.203 76.986 83.373

4.6338 2.8371 2.4341 2.0022 1.6505 1.5474 1.4177 1.2386 1.1583

19.029 31.272 36.853 44.833 55.660 59.371 65.243 77.326 82.645

4.6600 2.8580 2.4370 2.0200 1.6500 1.5554 1.4289 1.2330 1.1666

37.121 43.038 62.260 74.679 79.079

2.4200 2.1000 1.4900 1.2700 1.2100

35.165 37.934 43.437 57.955 60.026 61.799 66.763 77.549 85.678

2.5500 2.3700 2.0800 1.5900 1.5400 1.5000 1.4000 1.2300 1.1400

4 5 6 7 8 9 10 11 12 13

34

1

Table No. 2: Calculated structural parameters and the chemical composition of the synthesized

2

material Structural parameters

Chemical composition

Standard*

Exp.

Lattice parameter, a, Å

8.08

8.09

Mg

14.285 15.124

Cell volume, Vcell, Å3

528

530

Al

28.571 27.381

X-ray density, ρX-ray, gcm-3

3.58

3.58

O

57.142 60.494

Scherrer crystallite size, Ds, nm

-

20

Theo. Mol %

Calc. Mol %

3 4

* Standard pattern of magnesium aluminate (ICSD ref. code No. 00-021-1152)

5 6 7 8 9 10 11 12 13 14

35

1 2

Table No. 3: Parameters calculated by fitting the data to various adsorption isotherm models and pseudo-second order kinetic model Temkin model parameters

KT (L/g) 2.43

BT (mg/g) R2 0.1845 0.951 Freundlich model parameters

KF (mg/g) 0.646

n R2 1.49 0.986 Langmuir model parameters

KL (L/mg) 0.175

aL (L/mg) RL (g/L) 0.185 0.401-0.905 Pseudo second order kinetic model parameters

Kp-2 (g/mg. min) 0.17

Srate (mg/g. min) 0.07

3 4 5 6 7

36

t1/2 (min) 0.114

R2 0.946 R2 0.999

1

Graphical abstract:

O-

H2O

OH

MgAl2O4

MgAl2O4

O-

OH

Low H

OH

O-

O-

High H+

OH II

OH2+ OH2+

MgAl2O4 OH2+

H2+ +

High H

H2+

H2+ Methylene blue

MB+ IV

H2+

H2+

37

O-

MgAl2O4 III

OH

I

+

OH2+

OH2+

1 2 3 4 5 6

Highlights:

• • • • •

Nanosized MgAl2O4 synthesized by coprecipitation method Mesoporous material having large surface area was obtained Adsorption favored at neutral pH Chemisorption process is involved Multilayer adsorption occurs with interactions between the adsorbed molecules

7

38