adsorption of acid dyes

8/6/2019 Adsorption of Acid Dyes

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Presented in the Separation Sessions at Chemeca 2006, the 34th Australasian Chemical and Process Engineering

Conference, Auckland, New Zealand, 17 20 September 2006. Organised by the University of Auckland and the Society

of Chemical Engineers New Zealand (SCENZ).

*Corresponding author.

Adsorption of acid dyes by bamboo derived activated carbon

L.S. Chan, W.H. Cheung, G. McKay*Department of Chemical Engineering, Hong Kong University of Science and Technology,

Clear Water Bay, Kowloon, Hong Kong

Tel. +852 2358-8412; Fax +852 2358-0054; email: [email protected]

Received 15 November 2006; accepted 8 February 2007

Abstract

Bamboo, indigenous to Hong Kong and China, is widely used as scaffolding in construction and buildingprojects. However, over 50,000 tonnes of bamboo scaffolding waste is disposed as landfill waste each year.Nevertheless, these wastes can be used as raw materials for the production of a range of high value added activatedcarbons. The bamboo cane can be heated (charred) at a high temperature in the presence of selected activationchemicals to produce activated carbons for various applications e.g. adsorbents, catalysts or catalyst supports. In

the present study, activated carbons produced by thermal activation of bamboo with phosphoric acid were used foradsorption of acid dyes. Two acid dyes with different molecular sizes were used, namely Acid Yellow 117 (AY117)and Acid Blue 25 (AB25). It was found that dye with smaller molecular size, AB 25, was readily adsorbed onto thecarbon while the larger size dye, AY117, showed little adsorption. It is possible to tailor-make the carbon for theadsorption of dye mixtures in industrial applications, especially textile dyeing. Furthermore, experimental resultswere fitted to equilibrium isotherm models, Langmuir, Freundlich and Redlich-Peterson.

Keywords: Activated carbon; Adsorption; Acid dyes; Bamboo

1. Introduction

The discharge of effluents containing toxicmaterials such as metal ions and dyestuffs from awide range of industries electroplating, micro-

electronics, metal forming, paper, textiles, chemi-

cals is of concern to the public, industry andgovernment alike. The main treatment processesinclude: chemical precipitation, membrane sepa-ration, adsorption and ion exchange. Adsorptionis one of the most effective in addressing the strin-

Desalination 218 (2008) 304312

0011-9164/08/$ See front matter 2008 Published by Elsevier B.V.doi:10.1016/j.desal.2007.02.026


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L.S. Chan et al. / Desalination 218 (2008) 304312 305

gent requirements for pollution abatement. Nu-merous adsorbents including inorganic, agricul-tural and shell-fish by-products have been con-sidered for the adsorption [15].The use of acti-vated carbons however, has been widely favouredbecause of their high adsorption capacities andamphoteric properties which enables their adsorp-tion of both cationic and anionic effluents [6,7].

A challenge in the field of activated carbonproduction is to produce specific materials with agiven properties including pore size distributionand surface area from low cost precursors and atlow temperature. In recent years, considerableresearches have been focused on low cost alter-native materials for the production of active car-

bons from agricultural wastes such as fruit stones,oil-palm shell and bagasse [811].Bamboo is a tropical plant and is indigenous

to Southern Asia, including China, Hong Kong,Thailand and Vietnam. It has a rapid growth rateand consumes little energy (0.5 MJ/kg). It is asustainable product and is widely used in HongKongs construction industry as scaffolding. Over50,000 tonnes of bamboo scaffolding each yearis dumped as construction waste from HongKongs building projects. Bamboo waste can be

used as a raw material for the production of a rangeof activated carbons and carbon chars due to itshigh carbon content (44%). The bamboo can becarbonized in a furnace at high temperature in theabsence of oxygen to produce carbon chars. Thechars can be further treated using various chemi-cals and over a range of temperatures to producea selection of carbons for various uses [12,13].Bamboo-based activated carbons can be used asa potential commercially available activated car-

bon for the treatment of gaseous pollutants, liq-uid pollutants in industrial effluents and in drink-ing water filtration applications.

In the present study, activated carbon fromwaste bamboo scaffolding by low temperaturechemical activation is prepared. In addition, dyeadsorption (Acid Blue 25, AB25 and Acid Yel-low 117, AY117) is conducted on the produced

carbon and compared with a commercially avail-able carbon (Calgon F400).

2. Experimental

2.1. Chemical activation of bamboo

The received waste bamboo scaffolding waswashed with water and reduced in size by ham-mer milling prior to experiment. A particle sizerange of 500710 m was used throughout thepresent study. This raw material has been pre-treated by transfer into alumina containers, soak-ing and saturating with ortho-phosphoric acid(H3PO4) at different acid to bamboo ratio (Xp).The mixture has been stirred thoroughly to en-

sure homogenous mixing of the bamboo andH3PO4. Then, the samples were subjected to a two-step heating process firstly at 150C and then600C in a furnace under flowing nitrogen for arange of time. After heating, samples were cooled,washed and dried for further analysis and charac-terisation.

2.2. Carbon characterisation test

Chemical activated carbons were characterised

by BET surface area, pore size distribution anddye adsorption equilibrium capacity. The appar-ent surface area of the activated carbon was de-termined from N2 adsorption at 77 K in a Quantra-chrome Autosorb 1-CLP. Total surface areas werecalculated using the BET equation [14]. The mo-lecular area of the nitrogen adsorbate was takenas 16.2 2. The total pore volumes were calcu-lated by converting the nitrogen gas adsorbed ata relative pressure 0.98 to the volume of liquid

adsorbate.

2.3. Dye adsorption test

The acid dye adsorption test was used to de-termine the adsorption capacity of the productsusing two acid dyes, Acid Yellow 117, AY117 andAcid Blue 25, AB25. A fixed mass of activated


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306 L.S. Chan et al. / Desalination 218 (2008) 304312

carbon, 0.020 g was weighed into 120 mL coni-cal flasks and brought into contact with 50 mL ofdye solution with predetermined initial dye con-centrations. The flasks were sealed and agitatedcontinuously at 200 rpm in the thermostatic shakerbath and maintained at a temperature of 25C 1C until equilibrium was reached. At time t= 0and equilibrium, the dye concentrations of thesolutions were measured by Varian Cary 1E UV-Vis Spectrophotometer. These data were used tocalculate the adsorption capacity, q

e, of the adsor-

bent. The adsorption capacities (qe) of the each

activated carbon were determined by:

( )0 /e eq C C V m= (1)

where qe = the dye concentration on the adsor-bent at equilibrium (mmol/g) C

0= the initial dye

concentration in the liquid phase (mmol of dye/L),C

e= the liquid-phase dye concentration at equi-

librium (mmol of dye/L), V= the total volume ofdye-activated carbon mixture (L), m = mass ofadsorbent used (g). Finally, the adsorption capac-ity, q

e, was plotted against the equilibrium con-

centration, Ce.

Table 1Physical properties of the activated carbon produced frombamboo

Sample Surfacearea (m2/g)

Total porevolume (cc/g)

Microporevolume (cc/g)

HSA 1869 1.044 1.019LSA 758 0.423 0.418F400 792 0.496 0.485

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0 20 40 60 80 100 120

Pore Diameter (A)

D

v(d)(cc/A/g)

Fig. 1. Pore size distribution of the HSA carbon.

3. Results and discussion

3.1. Carbon characterisation

Two activated carbons of high and low sur-face area were produced, namely, HSA and LSA.The BET surface area and pore volumes are shownin Table 1 for both carbons. Pore size distributionfor HSA is shown in Fig. 1.

As can be seen from Table 1 and Fig. 1, HSAis a microporous carbon having a high microporevolume and surface area. Its maximum is observedat less than 20 , i.e. the microporous region.


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3.2. Acid dye adsorption

Figs. 2 and 3 show the plots ofqeagainst C

e

for both dyes on to the bamboo carbons and F400.

The AB25 adsorption capacity for HSA carbon is

nearly three times higher than that of F400. A simi-

lar adsorption capacity is observed for AY117

between HSA and F400. On the other hand, the

Fig. 2. Sorption plot of AB25 on bamboo carbons and F400.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 0.05 0.1 0.15 0.2 0.25 0.3

Ce (mmol/L)

qe

(mmol/g)

AB25 HSA Carbon

AB25 LSA Carbon

AB25 on F400

Fig. 3. Sorption plot of AY117 on bamboo carbons and F400.

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 0.05 0.1 0.15 0.2 0.25 0.3

Ce (mmol/L)

qe

(mmol/g)

AY117 on HSA Carbon

AY117 on LSA Carbon

AY117 on F400

LSA carbon shows poor adsorption for both acid

dyes. The removal of the acid dyes can be related

to the carbons porosity characteristics which de-

termine the accessibility of the dye molecules.

Since AB25 (MW 416.4) is a small molecule com-

pared to AY117 (MW 848), its adsorption would

mainly take place in the micropores. By compar-

ing the physical properties of the produced car-


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bon, HSA has very high surface area and micro-

pore volume compared to the other two carbons.

Thus, it would provide the higher capacity for

AB25. On the other hand, AY117 is too large to

be adsorbed by the micropores. Although LSA is

also a microporous carbon, its surface area is rela-

tively lower. The adsorption of AB25 depends on

the surface area of the carbon as well. Further-

more, it is possible to use HSA for the removal/

separation of small dye molecules in a mix-sized

dye solution.

3.3. Equilibrium isotherm modelling

The experimental data were fitted into Lang-

muir, Freundlich, RedlichPeterson equations to

determine which isotherm gives the best correla-

tion to experimental data.

3.3.1. Langmuir isotherm

Langmuir [15] proposed a theory to describe

the adsorption of gas molecules onto metal sur-

faces. The Langmuir adsorption isotherm has

found successful application to many other real

sorption processes of monolayer adsorption.

Langmuirs model of adsorption depends on the

assumption that intermolecular forces decrease

rapidly with distance and consequently predicts

the existence of monolayer coverage of the ad-

sorbate at the outer surface of the adsorbent. The

isotherm equation further assumes that adsorption

takes place at specific homogeneous sites within

the adsorbent. It is then assumed that once a dye

molecule occupies a site, no further adsorption

can take place at that site. Moreover, the Langmuir

equation is based on the assumption of a structur-

ally homogeneous adsorbent where all sorption

sites are identical and energetically equivalent.

Theoretically, the sorbent has a finite capacity for

the sorbate. Therefore, a saturation value is

reached beyond which no further sorption can take

place. The saturated or monolayer (as Ct )

capacity can be represented by the expression:

1

L e

e

L e

K Cq

a C=

+(2)

where qeis the solid phase sorbate concentration

at equilibrium (mg/g), Ce

is the aqueous phase

sorbate concentration at equilibrium (mg/L), KL

is Langmuir isotherm constant (L/g), aL

is Lang-

muir isotherm constant (L/mg) and are shown in

Table 2. Fig. 4 shows the Langmuir plot of the

acid dyecarbon system.

Fig. 4. Langmuir plots of the dyecarbon system.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 0.05 0.1 0.15 0.2 0.25 0.3Ce (mmol/L)

qe

(mmol/g)

AB25 HSA Carbon AB25 LSA Carbon

AY117 on HAS Carbon AY117 on LSA Carbon

AB25 on F400 AY117 on F400


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3.3.2. Freundlich isotherm

The Freundlich [16] equation is an empiricalequation employed to describe heterogeneous

systems, in which it is characterized by the het-

erogeneity factor 1/n. Hence, the empirical equa-

tion can be written:

1

ne F eq K C= (3)

where qe

is solid phase sorbate concentration in

equilibrium (mg/g), Ceis liquid phase sorbate con-

centration in equilibrium (mg/L), KFis Freundlich

Table 2

Langmuir constants for the dyecarbon system

KL(L/g) aL(1/mM) R2

AB25

HSA 687 404 0.930LSA 1.34 5.23 0.420

F400 345 498 0.982

AY117

HSA 304 2701 0.316

LSA 0.139 3.17 0.972

F400 10.9 97.7 0.910

Table 3

Freundlich constants for the dyecarbon system

KF(L/g) 1/n R2

AB25

HSA 1.95 0.09 0.968LSA 0.21 0.29 0.455

F400 0.82 0.16 0.870

AY117

HSA 0.14 0.12 0.433

LSA 0.04 0.53 0.977

F400 0.18 0.27 0.980

Fig. 5. Freundlich plots of the dyecarbon system.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 0.05 0.1 0.15 0.2 0.25 0.3

Ce (mmol/L)

qe

(mmol/g)

AB25 HSA Carbon AB25 LSA Carbon


AB25 on F400 AY117 on F400

constant (L/g) and 1/n is the heterogeneity factor

and is shown in Table 3.

This isotherm is another form of the Langmuirapproach for adsorption on an amorphous sur-

face. The amount adsorbed material is the sum-

mation of adsorption on all sites. The Freundlich

isotherm describes reversible adsorption and is not

restricted to the formation of the monolayer. The

Freundlich equation predicts that the dye concen-

trations on the adsorbent will increase so long as

there is an increase in the dye concentration in

the liquid. Fig. 5 shows the Freundlich plot of the

dyecarbon system.


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3.3.3. RedlichPeterson isotherm

Redlich and Peterson [17] incorporated three

parameters into an empirical isotherm equation.

The RedlichPeterson isotherm model combines

elements from both the Langmuir and Freundlichequations and the mechanism of adsorption is a

hybrid one and does not follow ideal monolayer

adsorption.

1

R e

e

R e

K Cq

a C

=+

(4)

where qe

is solid phase sorbate concentration in

equilibrium (mmol/g), Ce

is liquid phase sorbate

concentration in equilibrium (mM), KR

is Redlich

Peterson isotherm constant (L/g), aR

is Redlich

Peterson isotherm constant (mM1/) and is theexponent which lies between 1 and 0. The appli-

cation of this equation has been discussed else-

where and its limiting behaviour is summarised

here:

when = 1

1

R e

e

R e

K Cq

a C

=

+(5)

Fig. 6. RedlichPeterson plots of the dyecarbon system.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 0.05 0.1 0.15 0.2 0.25 0.3

Ce(mmol/L)

qe

(mmol/g) AB25 HSA Carbon AB25 LSA Carbon


AB25 on F400 AY117 on F400

It becomes a Langmuir equation.

when = 0

1

R e

e

R

K C

q a= + (6)

i.e. the Henrys law equation.

The parameters of Eq. (4) were determined by

minimising the distance between the experimen-

tal data points and the theoretical model predic-

tions with the solver add-in function of the Micro-

soft Excel. Table 4 shows the RedlichPeterson

parameters for the dyecarbon system. Fig. 6

shows the plots of the dyecarbon system.

By comparing the three equilibrium isotherm

models, none of the models fit the adsorption of

AY117 on HSA and AB25 on LSA well. For F400,

the RedlichPeterson model fits both dye adsorp-

tion behaviour well. On closer examination, for

AB25, Langmuir model can be used to describe

the system adequately, as in the RedlichPeterson model is very close to unity while in

AY117 system, its adsorption behaviour is towards

Freundlich. For the bamboo carbons, the adsorp-

tions of both dyes do not follow the Langmuir


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Table 4

RedlichPeterson parameters for the dyecarbon system

model. The RedlichPeterson model describes the

dyebamboo carbon system reasonably well. Thissuggests a reasonable fixed value for the sorption

activation energy, which could correspond to the

chelation bond energy between the dye ion and

surface of the carbon, most likely with a lone pair

of electrons on the carbon surface.

However, for AY117 with LSA and F400, the

high correlation of the Freundlich model suggests

more than one mechanism and that a degree of

heterogeneity is possible for ionic species in-

volved in the solution and on the carbon surface.

4. Conclusions

The preparation and investigation of high BET

surface area activated carbons from scrap con-

struction bamboo by low temperature chemical

activation has been demonstrated.

The high surface area carbon shows nearly

three times higher adsorption capacity for small

dye molecule, AB25, than the commercial car-

bon, F400. For AY117, it has similar capacity asF400. However, the low surface area carbon shows

poor adsorption for both dyes. Both surface area

and porosity of the carbon have played an impor-

tant role in the adsorption of the dyes.

The RedlichPeterson isotherm model can be

used to describe the adsorption of the acid dye onto

both bamboo carbon and F400 reasonably well.

KL(L/g) aR(1/mM) R2

AB25

HSA 2113 1111 0.93 0.977LSA 20953 96554 0.71 0.455

F400 377 529 0.98 0.985

AY117

HSA 1029 7510 0.90 0.434

LSA 0.369 8.948 0.59 0.978

F400 94.01 558.5 0.76 0.981

The high adsorption capacity of the smaller

dye molecule has suggested the potential of the

high BET surface area microporous activated car-

bon as good adsorbent for separation of mix-sized

coloured pollutants.

Acknowledgements

The authors would like to acknowledge the

support of the Research Grant Council of Hong

Kong SAR, the Innovation and Technology Fund

of Hong Kong SAR, Hong Kong University of

Science and Technology and Green Island Inter-

national Ltd.

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adsorption of acid dyes

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