A Comparative Study of Dye Removal Using Fly Ash Treated by Different Methods

7/28/2019 A Comparative Study of Dye Removal Using Fly Ash Treated by Different Methods

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A comparative study of dye removal using fly ashtreated by different methods

Shaobin Wang *, Y. Boyjoo, A. Choueib

Department of Chemical Engineering, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia

Received 7 October 2004; received in revised form 19 January 2005; accepted 28 January 2005

Available online 9 April 2005

Abstract

The effect of different methods for fly ash treatment using conventional chemical, sonochemical and microwave

method on dye adsorption in aqueous solution was investigated. Three basic dyes, methylene blue, crystal violet and

rhodamine B, are employed for adsorption testing. It is found that fly ash shows different adsorption capacity depend-

ing on type of dyes. Chemical treatment using HCl will increase the adsorption capacity. The adsorption capacity of

HCl treated fly ash varies with the preparation conditions. Microwave treatment is a fast and efficient method while

producing the sample with the highest adsorption capacity. Solution pH and inorganic salts in dye solution can signif-

icantly influence the adsorption. The adsorption data have been analysed using Langmuir, Freundlich and Redlich

Peterson isotherms. The results indicate that the Freundlich and RedlichPeterson models provide the better

correlations with the experimental data.

2005 Elsevier Ltd. All rights reserved.

Keywords: Fly ash; Sonochemical treatment; Microwave heating; Basic dyes; Wastewater; Adsorption

1. Introduction

In recent years, colour stuff discharged from textile

and pulp mills has become one of the important prob-

lems in wastewater treatment. These coloured com-

pounds are not only aesthetically displeasing but also

inhibiting sunlight penetration into the stream and

reducing the photosynthetic reaction. Some dyes are alsotoxic and even carcinogenic. Therefore, it is highly nec-

essary to reduce dye concentration in the wastewater

(Vandevivere et al., 1998). Adsorption has been used

extensively in industrial processes for separation and

purification. In wastewater treatment, activated carbon

is the most popular adsorbent but certain problems with

the high cost and regeneration limit the applications. At

present, there is a growing interest in using other low-

cost adsorbents for adsorption. If a sorbent is inexpen-

sive and ready for use, the adsorption process will be a

promising technology.

Fly ash is one of solid wastes largely produced from

power generation. Currently, its applications are onlylimited to civil engineering including cement and brick

production and as a filling in road works. Research is

therefore needed to develop new alternative environ-

mental friendly applications that can further exploit fly

ash. Recently, various kinds of fly ash have been used

as low-cost sorbents for removal of heavy metals, organ-

ics and dyes from waters (Alemany et al., 1996; Rama-

krishna and Viraraghavan, 1997; Kao et al., 2000;

Banerjee et al., 2003; Janos et al., 2003). Most efforts

0045-6535/$ - see front matter 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.chemosphere.2005.01.091

* Corresponding author. Tel.: +61 8 9266 3776.

E-mail address: [email protected] (S. Wang).

Chemosphere 60 (2005) 14011407

www.elsevier.com/locate/chemosphere
mailto:[email protected]:[email protected]


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are focused on the research of adsorption property.

However, an applicability of the fly ashes for the water

treatment depends strongly on their origin and few

research has been attempted to improve the adsorption

capacity.

Sonochemical (Suslick and Price, 1999; Thompson

and Doraiswamy, 1999) and microwave (Cundy, 1998;Rao et al., 1999) technologies have been applied for

materials synthesis recently and they show much higher

efficiency than traditional methods. We have reported

that chemical treatment of fly ash can improve the

adsorption capacity (Wang et al., 2004). In this paper

we report our further investigation in utilisation of fly

ash treated by different chemical methods for dye

removal from wastewater. We employed different methods,

conventional chemical and heat treatment, sonochemical

treatment and microwave heat treatment, to compare the

effect of the treatment on adsorption behaviours.

2. Experimental

2.1. Adsorbent materials and dyes

A sample of raw fly ash (FA) was collected from the

electrostatic precipitator from a power station in Wes-

tern Australia, where a sub-bituminous black coal is

fired. The chemical compositions of the fly ash are

SiO2 (55%), Al2O3 (29%), Fe2O3 (8.8%), CaO (1.6%)

and MgO (1.0%). Trace elements include As, B, Ba,

Cd, Co, Cr, Cu, Pb, Mn, Ni, and Zn. The samples

(5 g) were treated by 1 M HCl solution (10 ml) under

various heating conditions, otherwise indicated. Two

samples were prepared by treatment of the received

FA at room temperature and 100 C for 24 h, respec-

tively, using a conventional oven, referred to as FA-

HCl-RT and FA-HCl-100. One sample (FA-HCl-S)

was treated with HCl solution in an ultrasonic bath

(40 Hz, 300 W, FXP14M, Unisonics, Australia) for

1 h. Two another samples were obtained by treatingwith HCl solution under microwave heating at 2 and

10 min, respectively (Samung domestic microwave

800 W), which are referred to FA-HCl-M1 and FA-

HCl-M2. After treatment, all samples were filtrated,

washed and dried at 100 C overnight.

Three basic dyes, methylene blue (MB), crystal violet

(CV), and rhodamine B (RB) were selected for adsorp-

tion tests. They were obtained from AJAX Chemical.

Their chemical structures are displayed in Fig. 1. A

stock solution with a concentration at 104 M was pre-

pared and the solutions for adsorption tests were pre-

pared from the stock solution to the desiredconcentrations.

2.2. Characterisation of adsorbents

The XRD patterns of all the adsorbents were deter-

mined with an automated Siemens D500 Bragg-Brent-

ano instrument using Cu Ka radiation at 40 kV and

40 mA over the range (2h) of 570.

The surface areas of samples were determined by N2adsorption under 196 C using Autosorb (Quanta-chrome Corp.). All samples were degassed at 200 C

for 4 h, prior to the adsorption experiments. The BET

surface area was obtained by applying the BET equation

to the adsorption data.

S+

N

N

CH3

CH3

NCH

3

CH3

Cl

NCH

3

CH3

N

CH3

CH3

N+ CH3CH3 Cl

N+(C2H

5)2(C2H5)2N O

COOH

Cl

Methylene Blue

Crystal Violet

Rhodamine B

Fig. 1. Chemical structure of dyes.

1402 S. Wang et al. / Chemosphere 60 (2005) 14011407


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The pH of the various samples were measured as fol-

lows: 0.1 g samples were mixed with 10 ml of distilled

water and were shaken at 30 C for 24 h and then the slurry

was filtered and the pH of the solution was measured

by a pH meter (Radiometer PHM250 ion analyser).

2.3. Adsorption studies

The adsorption was performed by batch experiments.

Solid (0.05 g) in 100 ml of dye solution of varying con-

centration was shaking at 100 rpm for 72 h (Certomat

R shaker from B. Braun) at a constant temperature

(30 1 C). The contact time was determined by a pre-

liminary investigation (Wang et al., 2004). The determi-

nation of dyes was done spectrophotometrically on a

Spectronic 20 Genesis Spectrophotometer (USA) by

measuring absorbance at kmax of 665, 590 and 556 nm

for MB, CV and RB, respectively. To investigate the

effect of pH on adsorption, a series of dye solution wasprepared by adjusting pH over a range of 211 using

1 M HNO3 or NaOH solution. The pH of solutions

was measured with a pH meter (Radiometer PHM250

ion analyser). The effect of inorganic salts on adsorption

was also investigated using KCl.

3. Results and discussion

3.1. Characterisation of the adsorbents

The XRD patterns of some adsorbents treated by dif-

ferent methods are shown in Fig. 2. It is seen that there

are not significant difference for all XRD profiles. But

acid-treated samples present higher intensity of diffrac-

tion peaks. The major phases for all samples are quartz

and mullite. Some minor phases, hematite and magne-

tite, are also existed. Those results suggest that acid-

treatment will not induce bulk phase changes.

The pH of the prepared adsorbents is given in Table

1. As shown that the untreated FA and treated FA sam-

ples display acid property with different strength. Raw

FA exhibits the lowest pH, indicating that it has stron-

ger acid functional groups. After acid treatment, the

pH values of all samples are increased at varying extents.

Microwave acid treatment and sonochemical treatment

results in higher pH than those from conventional heat-

ing treatment. The enhancement of pH is probably due

to destruction of acid functional groups by the strongacid and microwave chemical and sonochemical treat-

ment induce stronger interactions of surface functional

groups with acid.

The results of N2 adsorption on textural properties

also demonstrate that HCl treatment can improve the

surface areas of fly ash samples. This is probably due

to the removal of some soluble inorganic materials in

unburned carbons of fly ash. The SBET of samples by

sonochemical and microwave treatment show a higher

value than conventional chemical treatment.

3.2. Adsorption tests

3.2.1. Effect of FA weight:HCl volume ratio

Fig. 3 presents the effect of ratio of acid volume to

solid weight on methylene blue adsorption. One can

see that the amount of adsorption increases with the

increasing equilibrium concentration. Acid treatment

can significantly influence the adsorption capacity.

Higher ratio of acid volume to solid weight results in a

2

0 20 40 60 80

Intensity(a.u.)

0

10000

20000

30000

FA

FA-HCl-RT

FA-HCl-SFA-HCl-M1

Fig. 2. XRD patterns of various fly ash adsorbents.

Table 1

Physico-chemical properties of the adsorbents

Adsorbent SBET (m2 g1) pH

FA 15.6 4.4

FA-HCl-RT 28.3 6.0

FA-HCl-100 30.1 5.6

FA-HCl-S 30.5 6.9

FA-HCl-M1 35.7 6.8

FA-HCl-M2 28.6 6.3

Ce (M)

0.0 5.0e-6 1.0e-5 1.5e-5 2.0e-5 2.5e-5 3.0e-5 3.5e-5

Qe

(mol/g)

0.0

5.0e-6

1.0e-5

1.5e-5

2.0e-5

2.5e-5

3.0e-5

FA:HCl=5:0

FA:HCl=5:10

FA:HCl=5:20

Fig. 3. Effect of HCl:FA ratio on MB adsorption isotherm;

pH = 5.2.

S. Wang et al. / Chemosphere 60 (2005) 14011407 1403


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higher adsorption capacity with the adsorption capacity

increasing from 1.2 105 to 2.5 105 mol/g when the

ratio is changed from 5:0 to 5:20.

3.2.2. Effect of different methods

Fig. 4 shows a comparison of adsorption by various

adsorbents for MB, CV and RB. As shown that acid

treatment will generally increase the adsorption of dyes

and all the adsorbents demonstrate higher adsorption

to methylene blue than crystal violet and rhodamine B.

The mode of chemical treatment results in differenteffects on adsorption behaviour. Higher temperature

treatment under conventional heating process will defi-

nitely enhance the adsorption. Sonochemical treatment

pH

2 10 12

Amountadsorbed(mol/g)

0

1e-5

2e-5

3e-5

4e-5 MBCV

RB

4 6 8

Fig. 5. Effect of solution pH on dye adsorption. Adsorbent:

FA, [MB]0 = 2.3 105 M, [CV]0 = 2.1 10

5 M, and [RB]0 =

1.8 105 M.

KCl concentration (M)

0.0 0.1 0.2 0.3 0.4 0.5 0.6


0.0

5.0e-6

1.0e-5

1.5e-5

2.0e-5

2.5e-5

CV

MB

RB

Fig. 6. Effect of inorganic salt KCl in dye solution on

adsorption. Dye: MB, adsorbent: FA.

0.00E+00

5.00E-06

1.00E-05

1.50E-05

2.00E-05

FA FA-HCl-RT FA-HCl-100 FA-HCl-M1F A-HCl-M2F A-HCl-S


MB

R-BCV

Fig. 4. Comparison of dye adsorption on various adsorbents:

[MB]0 = 2.7

10

5

M, pH = 5.2; [CV]0 = 2.0

10

5

M, pH =5.9; [RB]0 = 2.4 105 M, pH = 6.4.

Ce (M)

0.0 5.0e-6 1.0e-5 1.5e-5 2.0e-5 2.5e-5

Qe

(mol/g)

0

2e-6

4e-6

6e-6

8e-6

1e-5

Experiment

Langmuir

Freudlich

Redlich-Peterson

(a)

Ce (M)

0.0 5.0e-6 1.0e-5 1.5e-5 2.0e-5 2.5e-5

Qe

(mol/g)

0

2e-6

4e-6

6e-6

8e-6

1e-5

(b)

Ce (M)

0.0 5.0e-6 1.0e-5 1.5e-5 2.0e-5 2.5e-5

Qe

(mol/g)

0

2e-6

4e-6

6e-6

8e-6

1e-5

(c)

Fig. 7. Comparison of RB adsorption isotherms on various

adsorbents: (a) FA; (b) FA-HCl-S; (c) FA-HCl-M1.



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seems to have a better effect than the conventional chem-

ical treatment. Microwave chemical treatment will incur

the best effect on adsorption. For the adsorption of dif-

ferent dyes, it is also seen that the adsorption capacity of

dyes follows the order of MB > CV > RB. The difference

is probably due to the structure of the dyes. From Fig. 1,

we can see that crystal violet and rhodamine B moleculesare much bigger than methylene blue, which will prevent

the molecules entering the smaller pores of adsorbents,

resulting in lower adsorption.

3.2.3. Effect of pH

The pH of the dye solution plays an important role in

the whole adsorption process and particularly on the

adsorption capacity. The variation of adsorption of

three basic dyes on fly ash over a broad range of pH is

shown in Fig. 5. As shown, the adsorption is lower at

pH < 7 and then is increased to higher value at pH > 7

for methylene blue and crystal violet. Quite significantenhancement in the adsorption of dye is reached at

pH = 10 than the pH = 8. This variation is quite similar

to the previous reports (Mohan et al., 2002; Janos et al.,

2003). For rhodamine B, the figure displays that the

adsorption shows a slight decrease at higher pH. This

is due to the presence of an acidic group in the dye which

will dissociate with the increasing pH, giving rise to a

negative charge on the dye molecule. It is known that

ionic dyes upon dissolution release coloured dye anions/

cations into solution. The adsorption of these charged

dye groups onto the adsorbent surface is primarily influ-

enced by the surface charge on the adsorbent which is in

turn influenced by the solution pH. Above the zero point

of charge (pHpzc) the negative charge density on the sur-

face of the fly ash favours the sorption of basic (cationic)

dyes (Janos et al., 2003).

3.2.4. Effect of inorganic salts

The effect of inorganic salt (KCl) on adsorption of

basic dyes is presented in Fig. 6. As seen, the presence

of inorganic salt will significantly influence the adsorp-

tion of MB and CV while it exerts less effect on RB

adsorption. The dye adsorption will increase with the

increasing KCl concentration. This is different from

the investigation reported by Janos et al. (2003). They

tested the effect of inorganic salts (NaCl and CaCl2)

on some acid and basic dye adsorption and found thatthe dye adsorption was not affected. But in their investi-

gation, the highest concentration of salts is only 2 mM,

which is quite different from this investigation. Our

results show that higher concentration of salts will pro-

mote the adsorption of dyes on adsorbents.

3.2.5. Adsorption isotherms

The equilibrium adsorption isotherm is of impor-

tance in the design of adsorption systems. Several iso-

therm equations are available and the three important

isotherms are selected in this study, the Langmuir, Fre-

undlich and RedlichPeterson isotherms.The Langmuir adsorption isotherm assumes that

adsorption takes place at specific homogeneous sites

within the adsorbent and has found successful applica-

tion to many sorption processes of monolayer adsorp-

tion. The Langmuir isotherm can be written in the form

Qe QmKCe

1 KCe1

Qe is the adsorbed amount of the dye, Ce is the equilib-

rium concentration of the dye in solution, Qm is the

monolayer adsorption capacity and Kis the constant re-

lated to the free energy of adsorption.The Freundlich isotherm is an empirical equation

employed to describe heterogeneous systems. The Fre-

undlich equation is

Qe KC1=ne 2

where K and n are Freundlich adsorption isotherm con-

stants, being indicative of the extent of the adsorption

Table 2

Comparison of adsorption isotherm models

Model Parameters R2

Fly ash

Langmuir isotherm Qm = 9.29 106 mol/g K= 1.53 106 M1 0.928

Freundlich isotherm K= 2.10 105 mol/g 1/n = 0.0796 0.994

RedlichPeterson isotherm K= 31.6 l/g a = 1.53 106 M1 b = 0.926 0.964

FA-HCl-S




FA-HCl-M1






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and the degree of non-linearity between solution concen-

tration and adsorption, respectively.

The RedlichPeterson isotherm model combines ele-

ments from both the Langmuir and Freundlichequations,

and the mechanism of adsorption is a hybrid one and does

not follow ideal monolayer adsorption. The Redlich

Peterson equation is widely used as a compromise be-

tween Langmuir and Freundlich systems. It is expressed as

Qe KCe

1 aCbe3

K and a are the RedlichPeterson isotherm constants

and b is the exponent, which lies between 1 and 0.

Fig. 7 shows a comparison of adsorption isotherms

for curve fitting of the experimental results with abovethree adsorption isotherms. The model parameters from

all isotherms obtained from non-linear regression are

presented in Table 2. As seen that the RedlichPeterson

model is better than the Langmuir model in simulation

of the adsorption isotherm and the Freundlich model

will be the best (as evident from correlation coeffi-

cients). This suggests that some heterogeneity in the sur-

face or pores of the fly ash will play a role in dye

adsorption.

Fig. 8 presents the adsorption isotherms of three fly

ash samples, FA, FA-HCl-S and FA-HCl-M1, for

MB, CV and RB adsorption at 30 C. The Freundlich

isotherm is used for the description of adsorption iso-

therms and the parameters are given in Table 3. As

shown that three fly ash samples show the adsorption

capacity in an order of FA-HCl-M1 > FA-HCl-

S > FA. The adsorption of MB, CV and RB on raw

fly ash is around 1.2 105, 8.0 106 and 7.0

106 mol/g, respectively, while the adsorption on FA-

HCl-M1 can reach to 2.0 105, 1.6 105, and 1.0

105 mol/g, respectively. The enhancement in adsorp-

tion can be attributed to the surface modification of

the chemical methods because XRD patterns do not

show significant difference for all the samples. However,

the BET surface area and the pH of solids in water areincreased, suggesting that the more active sites for

adsorption are produced.

4. Conclusion

Various fly ash samples treated by HCl via conven-

tional heating, sonic and microwave heating have been

investigated for removal of basic dyes, methylene blue,

crystal violet and rhodamine B from aqueous solution.

It is found that the raw fly ash generally exhibits high

capacity and HCl treatment will improve the adsorption

(a)

Ce (M)

0.0 5.0e-6 1.0e-5 1.5e-5 2.0e-5 2.5e-5 3.0e-5 3.5e-5

Qe

(mol/g)

0.0

5.0e-6

1.0e-5

1.5e-5

2.0e-5

2.5e-5

FA

FA-HCl-S

FA-HCl-M1

Freundlich model

(b)

Ce (M)

0 5e-6 1e-5 2e-5 2e-5

Qe

(mol/g)

0.0

2.0e-6

4.0e-6

6.0e-6

8.0e-6

1.0e-5

1.2e-5

1.4e-5

1.6e-5

1.8e-5

(c)

Ce (M)

0.0 5.0e-6 1.0e-5 1.5e-5 2.0e-5 2.5e-5

Qe

(mol/g)

0

2e-6

4e-6

6e-6

8e-6

1e-5

Fig. 8. Adsorption isotherms of various fly ash samples at

30

C: (a) MB; (b) CV; (c) RB.

Table 3

Parameters of Freundlich isotherms of various adsorbents

Sample Dyes K (mol/g) 1/n R2

Fly ash MB 4.21 105 0.1181 0.982

CV 3.34 105 0.1228 0.972

RB 2.10 105 0.0796 0.994

FA-HCl-S MB 3.16 105 0.0555 0.963

CV 7.00 105 0.1495 0.949

RB 2.00 105 0.0928 0.990

FA-HCl-M1 MB 3.27 105 0.0462 0.993

CV 1.70 104 0.2132 0.956

RB 1.24 105 0.0440 0.963



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capacity depending on heating method. Acid treatment

will induce changes in surface properties of adsorbent

while does not bring about the changes in bulk phases.

The dye adsorption was also influenced by solution pH

and inorganic salt. Higher pH will generally result in

higher adsorption for MB and CV, but will decrease

slightly the adsorption of RB. Presence of inorganic saltswill promote the adsorption of dyes. Adsorption

isotherm can be fitted by Langmuir, Freundlich, and

RedlichPeterson models, in which the Freundlich and

RedlichPeterson models are the better ones.

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A Comparative Study of Dye Removal Using Fly Ash Treated by Different Methods