Adsorption Characteristics of Modified Wheat Husk for the Removal of a Toxic Dye, Methylene Blue,...

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1JOURNAL OF HAZARDOUS, TOXIC, AND RADIOACTIVE WASTE

Adsorption characteristics of modified wheat husk for the removal of a toxic dye,

methylene blue (MB) from aqueous solutions

Sushmita Banerjee1*, Mahesh C. Chattopadhyaya2, Uma3, Yogesh Chandra Sharma4

ABSTRACT

Adsorption characteristics of modified wheat husk for the removal of a toxic cationic dye,

methylene blue from aqueous solutions have been investigated. The adsorbent, wheat

husk, was characterized by Fourier Transform Infrared Spectra (FTIR), and Scanning

Electron Microscopy (SEM) for its functional group and surface characteristics. The

removal decreased by increasing temperature from 303 to 373 K. The removal was found

to be pH dependent and it decreased from 96.2% to 40.7% by varying the pH of the

solution from 4.5 to 9.5. It was observed that 93.4 % removal of methylene blue was

1Research Scholar, Department of Chemistry, University of Allahabad,

Allahabad 211 002, India.

2Professor, Department of Chemistry, University of Allahabad, Allahabad 211 002, India.

3WOS A, Department of Applied Chemistry, Indian Institute of Technology (Banaras Hindu University) Varanasi, Varanasi 221005, India.

4 Professor, Department of Applied Chemistry, Indian Institute of Technology (Banaras Hindu University) Varanasi, Varanasi 221005, India.

*Corresponding author

E Mail: [email protected], Tel No +91 5322462393

Journal of Hazardous, Toxic, and Radioactive Waste. Submitted December 6, 2012; accepted February 21, 2013; posted ahead of print February 23, 2013. doi:10.1061/(ASCE)HZ.2153-5515.0000191

Copyright 2013 by the American Society of Civil Engineers

J. Hazard. Toxic Radioact. Waste

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achieved at an initial dye concentration of 13.37x10-2 mol/L at 303K. Equilibrium

adsorption data for the removal of methylene blue were analyzed by Langmuir, Freundlich

and Tempkin isotherm models. Values of thermodynamic parameters viz. ΔG°, ΔH°, and

ΔS° were determined. Negative value of ΔH° confirmed exothermic nature of removal of

methylene blue by adsorption on modified wheat husk.

Keywords: Adsorption, isotherm, kinetics, methylene blue, wheat husk

Introduction

The release of dyestuff effluents from different industries like textile, pulp and paper,

leather, paint and colouring industries and especially from the textile industry poses

serious environmental problems (Mane et al, 2007). It is difficult to get rid of these dyes

for several reasons due to their stability. The complex structure and high molecular weight

of the dyes makes their degradation by light or by chemical and biological treatment

difficult. Generally, basic dyes are the brightest class of dyes which are used in carpet and

textile industries. Colour even at very low concentrations in effluents is undesirable.

Dyes absorb and reflect sunlight entering water and thus interfere with the growth of

bacteria and hinder photosynthesis by aquatic plants (Oliveira et al., 2008; Qada et al.,

2006; Vargas et al., 2011; Gong et al., 2007). The dye containing effluents from the

industry often cause serious environmental problems that can be mutagenic or

carcinogenic and require pre-treatment for colour removal prior to disposal into aquatic

systems (Alvarez et al., 2005; Petrova et al., 2010). The treatment technologies like

coagulation and flocculation, reverse osmosis, photo degradation, membrane separation,

biodegradation ion-exchange and adsorption are most often used for the treatment of dye

containing wastewater. Among these methods, adsorption is simple and requires low

maintenance and is the most widely used single method for the removal of dyes from




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aqueous solutions and effluents (Djilani et al., 2012; Al-Degs et. al., 2008; Al-Degs,

2007). Adsorption is highly efficient for the removal of metals, organic matter and dyes.

Activated carbon is one of the adsorbents having maximum use but it is an expensive

material and makes the treatment process costly. It cannot be used for large scale

treatment of effluents by developing nations and, therefore, the search of inexpensive

adsorbent materials has been an area of concern for the scientific workers. Various

adsorbents such as commercial activated carbon, fly ash, natural clay, timber waste, agro

waste, coconut coir, rice husk, cotton stalk etc. (Sharma et al.,2009; Sharma and Uma,

2010; Deng et al.,2009; Han et al.,2007; Bulut and Aydın,2006; Annadurai et al.,2002;

Han et al.,2008) have been applied for the removal of dyes from aqueous solutions. This

further shows versatility of the adsorption process (Karagöz et al., 2008; Sharma et al.,

2011; Budinova et al., 2009; Girgis et al., 2009; Singh et al., 2012 ).

Present work addresses the application of a low cost material, wheat husk for

the removal of methylene blue(MB) from aqueous solutions. In order to enhance the

removal efficiency, the wheat husk was modified by simple methods prior to application.

The equilibrium data was subjected to Langmuir, Freundlich and Tempkin isotherm

models. Kinetic and thermodynamic studies for the removal of MB were also carried out.

The work reported is novel reporting new data using wheat husk as an adsorbent. A

number of cellulosic materials have been used for the removal of dyes but reports on the

application of wheat husk as adsorbent are vary rae. Further, the kinetics of the removal

of the dye also incorporates mass transfer analysis which is not reported in any of the

related work.

Materials and methods

Preparation of the adsorbent




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After collection, wheat husk was sieved to get fine powder and was dried in a hot air oven.

The fine powder was treated with 1% perchloric acid in the ratio of 1:5 (saw dust:

perchloric acid, w/v) at 50 °C for 4 h to remove lignin based ‘color materials’. The wheat

husk was filtered out, washed with distilled water and kept in oven for 24 h at 110 °C. The

dried material was taken out of the oven and was stored in an airtight container for

further use.

Methylene blue, obtained from Merck (Mumbai, India limited) having 99.0% purity was

used for the preparation of stock solution. The stock solution (267.44 mol/L) was prepared

by dissolving accurately weighed quantity of the dye in double distilled water, and

subsequently diluted when needed. The chemical structure of MB is shown in Fig. 1.

Figure 1 here

The modified adsorbent was characterized by FTIR and SEM for its surface analysis,

composition and phase determination. FTIR analysis was carried out by IR spectrometer

(Varian 1600 FT-IR Scimitar Series). The BET-N2 specific surface (m2/g) of the material

was determined by a computer controlled automated porosimeter (Micromeritics ASAP

2020, V302G single port).

Batch adsorption experiments

The studies on removal of methylene blue by modified adsorbent were carried out on a

thermo stated water bath shaker. Stock solution was used for preparation of working

solutions of different concentrations of the dye by further dilution. For batch adsorption

experiments, 50 mL solution of methylene blue of desired concentrations was taken in

reagent bottles by adding desired amount of adsorbent. The content was then agitated at

different rotations on thermostat water bath shaker. After equilibrium, the adsorbent was

separated from the aqueous solutions by centrifugation by a centrifuge (Remi 24, New




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Delhi, India). The residual concentration of MB (methylene blue) in the aliquot was

determined by Spectronic 20 Spectrophotometer ( Baush & Lomb, USA).

The removal (%) and the amount of methylene blue adsorbed per unit mass of the

adsorbent were determined by following expressions:

% Removal of MB = ∗ 100 (1)

q = ∗ V (2)

where Ci and Ce (mol/L) are the initial and equilibrium concentrations respectively of

methylene blue, qe (mg/g) is the amount adsorbed per unit mass of the adsorbent, W(g/L)

is the amount of adsorbent per unit volume and V (L) is the volume of solution.

Results and discussion

Characterization of adsorbent

The IR spectra of wheat husk were measured on a Fourier Transform Infrared

Spectrophotometer (Varian 1600 FT-IR Scimitar Series) to elucidate the functional group

present on the surface of adsorbent (Fig. 2). For measuring IR spectra, modified wheat

husk (MWH) was encapsulated in 400 mg of KBr. The spectra were recorded on a FTIR

within the range of 500 – 6000 cm-1 (Baccar et al., 2009; Ahmad et al., 2007). MWH

exhibited O-H stretching band in the spectrum in 3200-3400 cm-1 range.

Figure 2 here

SEM image (Fig. 3) of MWH confirmed amorphous nature of modified wheat husk. The

image also shows that the surface of the adsorbent has enough roughness with many

humps indicating that the adsorbent is a proper material for adsorption of dyes (Dural et

al., 2011; Kumar et al., 2006).




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Figure 3 here

The chemical characterization of wheat husk was carried out and has been given in Table

1. The bulk density and average particle size of wheat husk were determined and were

found to be 0.37±0.06 gcm-3 and 79.3 ± 7.8% angstroms. The BET specific surface area of

the adsorbent was found to be 81.75 m2/g (Table 1).

Table 1 here

Effect of contact time and initial concentration of methylene blue

Contact time and initial concentration are important parameters for adsorption studies

(Vadivelan and Kumar, 2005; Ncibi et al., 2007). Effect of contact time and initial

concentration on the removal of MB by MWH has been given in Fig. 4. The experiments

were carried out at the initial concentrations namely 13.37x10-2, 20.06x10-2, 26.74x10-2

and 33.43x10-2 mol/L respectively.

Figure 4 here

This figure clearly shows that the removal is rapid in initial stages, increases gradually

and finally becomes almost constant after reaching equilibrium. The time of equilibrium in

the present system was found to be 50 min indicating the process of removal to be fast.

The graphs are smooth and clear showing that the major part of the dye is removed by

adsorption. The removal increased from 34.5 to 93.4 % by decreasing initial dye

concentration from 33.43x10-2 to 13.37x10-2 mol/L. The time profile for the removal of

methylene blue is single, smooth, and continuous leading to saturation, and this profile

suggests possible formation of monolayer coverage of methylene blue on the surface of

the adsorbent during removal process.




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Effect of adsorbent dose on the removal of methylene blue by adsorption on modified wheat husk The effect of adsorbent dose on the removal of methylene blue was studied by taking

10, 20 and 25 g/L of the adsorbent at an initial MB concentration of 13.37x10-2 mol/L at

303K (Fig. 5). The removal of MB increased from 79.80 to 93.4 % by increasing dose

from 10 to 25 g/L. Increasing trend of removal of methylene blue at higher dose of

adsorbent may be attributed to the availability of larger number of active sites for the MB

species in solution (Cengiz et al., 2012). As the adsorbent dosages increase, the surface

sites available for methylene blue are also increased and consequently better adsorption

takes place.

Figure 5 here

Kinetic modeling for the removal of methylene blue from aqueous solutions

Kinetics of any process is necessary as it provides valuable information about reaction

mechanisms of adsorption. The kinetics of adsorption of methylene blue is analyzed by

using two widely applied kinetic models namely Lagergren first-order and pseudo second-

order kinetic models. Pseudo first order and pseudo second order equations can be

expressed as follows (Lagergren, 1898; Ho, 2004):

Pseudo first order equation:

log(q − q ) = logq −.

t (3)

Pseudo second order equation:

= + t (4)

ℎ = (5)

where kad is the rate constant (min-1) of first order equation, qe and qt are the amounts of

methylene blue adsorbed (mgg-1) at equilibrium and at any time t, respectively. k2

(g/mg/min) is the rate constant of pseudo–second order kinetic equation and h is known as




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initial sorption rate. The values of kad were determined from the slope of linear plots of

‘log (qe-qt) vs t’ (Fig.6a) and the value of qe and k2 were determined by the slope and

intercepts of the straight line of the plots ‘t/qt vs t’, respectively (Fig. 6b). The straight line

plots of the figures indicate applicability of the two models for the process of removal of

MB by the selected modified adsorbent. The values of the rate constants of the process of

removal were determined by the graphs of figures 6a and 6b and have been given in Table

2.

Figures 6 a and 6b here

It is clear from Table 2 that values of R2 for the present system are greater for pseudo

second order as compared with that of first order kinetic model. It shows that the process

of removal of MB is governed by a second order kinetics.

Mass Transfer Study

The feasibility of transfer of mass from bulk to the adsorbent surface was studied by using

‘mass transfer model’ reported Sharma et al.(Sharma, 2001). The transfer of mass was

quantified by determining the value of coefficient of mass transfer, βl (cms−1) for the

removal of methylene blue by adsorption on MWH. The model is expressed as follows:

ln − = ln − β . S . t (6)

m = (7)

and Ss was determined by following relation:

S =Ϭ

(8)

where W(g/L) is weight of the adsorbent, V(L) volume of the solution containing the

adsorbate, dp (cm) is diameter of adsorbent particle, ρp (g cm−3) density of adsorbent, and




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‘εp’, a dimensionless parameter is the porosity of the adsorbent. ‘k’ is the Langmuir

constant (L mol−1), m is the mass of adsorbent (g/l), and Ss is the specific surface area of

adsorbent (m2 g−1). A linear graphical relation between ‘ln[(Ct/Co) −1/(1+ mk)] versus t’

indicated that the data fits the proposed model (figure 7). The values of mass transfer

coefficient calculated from the graphs of Figure 7 have been given in Table 3.

Figure 7 here

Effect of pH on the removal of methylene blue by adsorption on modified wheat husk

In adsorption process, solution pH plays an important role (Liu et al., 2001) mainly by

affecting the binding sites of the adsorbents. Removal of methylene blue was greatly

influenced by solution pH. To investigate the effect of pH on removal process, solution

pH was varied between 4.5 to 9.5 while keeping other conditions constant. The pH of the

solutions was adjusted with 0.1 M NaOH or 0.1 M HCl. The maximum removal of

methylene blue by modified wheat husk was found to be higher in higher range of pH at

13.37x10-2 mol/L initial concentration. The removal of methylene blue decreased from

40.7 to 96.2 % by increasing pH from 4.5 to 9.5 (Fig. 8). It is anticipated that at lower

values of pH, H+ ions will compete with dye cations resulting in lower removal.

The removal of dye at different pH values can also be explained on the basis of pHzpc. pHzpc

of the adsorbent, MWH, was determined and found to be 6.27. Above this pH, the

adsorbent surface will have a negative charge which will result in increased uptake of the

dye due to electrostatic forces of attraction between adsorbent and the dye. This can be

clearly understood through favourable removal of the dye at higher values of pH (than

pHzpc). At a pH< pHzpc, the adsorbent surface will acquire a positive charge leading to

favourable removal of pollutant species furnishing anions in the solution.

Figure 8 here




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Effect of temperature

Temperature has significant effect on adsorption process. Mostly adsorption processes are

exothermic in nature, but some cases of endothermic adsorption are also reported

(Ravikumar, 2007). In the present investigations, effect of temperature on the removal of

MB was studied at 303, 313, and 323 K respectively. As the solution temperature is

increased from 303 to 323K, removal was found to decrease from 93.4 to 59.8% (Fig. 9).

It revealed that the process of removal is exothermic in nature. At elevated temperatures,

the solubility of the dye increases and it tends to be there in the bulk; also, the escaping

tendency of the dye molecules gets enhanced at higher temperatures. These two factors

result in net decrease in removal of dye.

Figure 9 here

Equilibrium study

Adsorption equilibrium parameters show the nature of adsorbate-adsorbent interaction.

The data obtained on the removal of methylene blue by adsorption on MWH was tested on

Langmuir, Freundlich and Tempkin models.

Langmuir adsorption isotherm

The Langmuir’s isotherm model assumes that the adsorption takes place on a uniform

surface and is governed by a monolayer deposition of the adsorbate species. The linear

form of Langmuir’s isotherm model is given by the following equation (Langmuir, 1916;

Malash and El-Khaiary, 2010; Almeida et al.,2009):

= + (8)




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where Ce (mol/L), qe (mg/g) are the concentrations of adsorbate and amount of adsorbate

adsorbed at equilibrium, respectively. Qo (mg/g) and b (Lmol-1) are the terms related to

capacity and energy of adsorption, respectively. The data has been plotted as ‘Ce/qe vs Ce’

(Fig. 10). The values of the Langmuir’s constants, Qo and b were determined by slopes

and intercepts of the Fig. 10 and have been given in Table 3. The graphs of Fig. 10 are

straight and smooth indicating that the process of removal of MB by MWH is governed by

a monolayer coverage.

Figure 10 here

Freundlich adsorption isotherm

The linear form of Freundlich model is expressed as follows (Freundlich 1906):

q = K C (9)

The logarithmic form of the equation is expressed as follows;

ln q = ln K + ln C (10)

where Kf is the Freundlich constant denoting adsorption capacity (mg/g ) and 1/n is the

empirical constant indicating adsorption intensity (L/mg) . Value of ‘n’ gives an indication

how favourable the adsorption process is for selected system (Aksu and Donmez,2003).

qe is the amount of adsorbate adsorbed by a unit mass of adsorbent at equilibrium (mg/g )

and Ce is the residual concentration of solute (mol/L) remaining in the solution (mol/L).

The equilibrium data does not fit Freundlich isotherm model for this system (figure not

shown).

The values of Kf and 1/n are calculated from the slopes and intercepts of the plots of ‘ln

Ce vs ln qe’ (Figure not shown). The Freundlich constants ‘Kf’ and ‘n’ for the system were




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calculated and are given in Table 4. Linear plots of the figure confirm the applicability of

Freundlich isotherm model to the system.

Tempkin isotherm

Tempkin isotherm has generally been applied in the following form was used in order to

analyze insight on whether removal of MB by modified wheat husk is due to physical or

chemical interactions (Hadi, 2010; Kumar et al., 2010):

q = lnAC (11)

The linearized expression of Tempkin isotherm can be expressed as follows:

q = B lnA + B lnC (12)

where B1 = RT/b, A is the equilibrium binding constant(L/mol), T is the absolute

temperature (K), R is the gas constant (8.314 J/mol K), and B1 is related to the heat of

adsorption. The values of isotherm constants viz. B1 and A can be calculated from the

slope and the intercept of the plot ‘qe vs ln Ce’ (Figure not shown).

The value of constants of different isotherms Langmuir, Freundlich and Tempkin and

their adsorption capacities decreased by increasing temperature from 303 to 323K. On

comparing the R2 value for the isotherm models, it becomes clear that that Langmuir

isotherm model is most applicable in present system.

Thermodynamic study

The thermodynamic parameters such as change in standard free energy (∆Go), enthalpy

(∆Ho), and entropy (∆So) were undertaken to understand the process of removal of

methylene blue by adsorption on the MWH. Values of these parameters were calculated at

303, 313, and 323K using the following equations (Wu et al., 2012; Dogan et al., 2007):




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∆G = −RTlnK (13)

K = (14)

where Kc is equilibrium constant, Cac and Ce is equilibrium concentration of methylene

blue(mol/L) on the adsorbent and equilibrium concentration of dye in the solution

respectively (mol/L), T is absolute temperature (K) and R is gas constant (1.987cal/

mol/K).

K1 and K2 are the equilibrium concentrations at T1 and T2 respectively:

∆H = R ln (15)

∆S = (∆ ∆ ) (16)

The values of the thermodynamic parameters determined for the present system are given

in Table 5. For the removal of methylene blue on MWH, the values of ∆Go were found to

be negative which indicate that the process of removal of methylene blue is spontaneous

at all the temperatures. Value of enthalpy change, ∆Ho, was negative for this system which

further confirms exothermic nature of the process of adsorption. Negative values of

entropy confirm the possibility of favourable adsorption. The values of ΔGo also suggest

that the adsorption is physical in nature.

Conclusions

MWH, the adsorbent used in present studies has been modified by simple method. Wheat

being staple food globally, the precursor, wheat husk is available in plenty in all parts of

the world. FTIR and SEM of modified wheat husk show the functional groups and the

morphology of the adsorbent surface. The adsorbent shows significant removal of




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methylene blue from aqueous solutions and higher (%) removal has been obtained at low

concentration ranges. Removal of methylene blue was significantly influenced by pH of

the solutions. Removal was higher at higher pH values. It was observed that by increasing

the temperature, removal decreases which confirms exothermic nature of removal process.

The kinetics of process of removal of methylene blue was analysed by pseudo first and

second order kinetic model and observed that removal process is governed by pseudo

second order kinetic model. The kinetic and equilibrium modelling parameters can be used

to design treatment plants for the treatment of dye laden wastewaters. Langmuir isotherm

model was found to be best for the removal of methylene blue by adsorption on modified

wheat husk. Thermodynamic parameters revealed that the removal process is spontaneous.

Negative value of enthalpy change also confirms the exothermic nature of this system.

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Captions of figures

Fig. 1. Chemical structure of methylene blue

Fig. 2. FTIR of modified wheat husk

Fig. 3. SEM of the modified wheat husk

Fig. 4. Effect of initial concentration on removal of methylene blue by adsorption on modified

wheat husk

Fig. 5. Effect of adsorbent dose on removal (% ) of methylene blue by adsorption on modified

wheat husk

Fig. 6a. Pseudo first order plot for the removal of methylene blue by adsorption on modified

wheat husk

Fig. 6b. Pseudo second order plot for the removal of methylene blue by adsorption on modified

wheat husk

Fig.7. Plot of mass transfer study for the removal of methylene blue by adsorption on modified

wheat husk

Fig. 8. Effect of pH on % removal of methylene blue by adsorption on modified wheat husk

Fig. 9. Effect of temperature on % removal of methylene blue by adsorption on modified wheat

husk

Fig. 10. Langmuir Isotherm plot for removal of methylene blue by adsorption on modified wheat

husk




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Table 1. Major characteristics of modified wheat husk

Particle size (μm) 700

Density (gcm-3) 0.37±0.06

Porosity (%) 79.3 ± 7.8

BET-N2 specific surface area (m2g-1) 81.75

Accepted Manuscript Not Copyedited




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Table 2. Values of rate constants of pseudo first order and pseudo-second order constants

for removal of methylene blue by adsorption on modified wheat husk at different

temperatures.

T(K) kad(×10-2 min-1) R2 k2(×10-2 gmg-1min-1) R2

303 1.30 0.90 1.18 0.99 313 1.50 0.93 3.10 0.98 323 2.02 0.98 5.04 0.98





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Table 3. Values of coefficient of mass transfer at different temperatures for the removal of

methylene blue from aqueous solution by adsorption on modified wheat husk

Temperature(K) βl(x10-5 cms-1) 303 1.42

313 1.49

323 1.73





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Table 4. Values of isotherm parameters for the removal of methylene blue from aqueous

solutions by adsorption on modified wheat husk at different temperatures

Isotherms Temperature(K) Parameters Langmuir Qo (mg/g) b(l/mg)

303 4.23 2.69 313 4.04 5.48 323 3.78 0.87

Freundlich Kf(l/g)

1/n (l/g)

303 0.75 0.02 313 0.39 0.13 323 0.25 0.17

Tempkin B1 (mg/g) A (L/mg)

303 79.94 0.07 313 22.69 0.55 323 10.88 0.58





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Table 5. Values of various thermodynamic parameters for the removal of methylene blue

from aqueous solutions by modified wheat husk

Temperature (K) ∆Go (kcal/mol) ∆Ho (kcal/mol) ∆So( kcal mol−1K−1)

303

313

323

-1.36

-1.59

-1.76

-5.51

-3.87

-5.37





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Adsorption Characteristics of Modified Wheat Husk for the Removal of a Toxic Dye, Methylene Blue,...

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