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: kemckayg@ust.hk
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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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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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
AY117 on HAS Carbon AY117 on 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
AY117 on HAS Carbon AY117 on 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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