Water Qual. Res. J. Can. 2009 · Volume 44, No. 2, 295–306
Copyright © 2009, CAWQ
Experimental Study on the Elimination of Colour and Organic
Matter from Wastewater Using an Inexpensive Biomaterial, Chitosan
Poly R. Modak, Kripa S. Singh,* and Dennis A. Connor
Departments of Civil and Chemical Engineering, University of New Brunswick
Fredericton, NB, E3B 5A3, Canada
Chitosan has been investigated as an inexpensive, biologically derived adsorbent and/or primary coagulant for two reactive
azo dyes in textile wastewater. At natural pH, complete elimination of colour was achieved from 0.1 g/L aqueous solutions
of the textile dyes Procion Orange MX-2R and Procion Red MX-5B with a dose of 6 g of chitosan per litre of dye solution.
However, when pH was lowered (to 4.8 and 5.5 respectively), a dose of only 1 g of chitosan per litre was necessary to eliminate
colour and drastically reduce TOC (total organic carbon) and chemical oxygen demand for the same concentration of dyes.
This allowed about an 80% reduction in sludge volume production. Addition of sodium phosphate dibasic and potassium
sulfate improved the dye removal at higher pH. Colour removal decreased significantly with or without added salts as pH
was adjusted above 7. Equilibrium adsorption experiments showed that both dye solutions follow the Freundlich isotherm,
but not the Langmuir isotherm. Kinetics measurements show a better fit to the pseudosecond-order Lagergren model than
to the first-order Lagergren model. Brunauer-Emmett-Teller, or BET, surface area analysis and scanning electron microscope
micrographs were included for better understanding of the nature of the chitosan surface with and without adsorbed dye.
Chitosan appears to be a natural, clean and excellent product for the adsorption of Procion Orange MX-2R and Procion Red
MX-5B in mildly acidic conditions.
Key words: reactive azo dyes, chitosan, adsorbent, primary coagulant, colour, organic matter
Introduction
used extensively in textile industries where the favourable
characteristics of bright, water-fast colour and simple
application techniques are combined with low energy
consumption (Aksu and Tezer 2005). Brightly coloured,
widely used, water soluble, reactive azo dyes are the
most problematic compared with other forms since they
tend to be unaffected by conventional treatment systems
(Willmott et al. 1998). Therefore, these reactive azo dyes
were chosen for the present study.
During the past three decades several methods
(physicochemical and biological) have been developed
for the removal of dyes, including biological and chemical
oxidation, chemical coagulation, sonication, electrolysis,
biodegradation, advanced oxidation, photocatalysis,
and adsorption, few of which have been accepted by
industries due to the economics and difficulties of use
(Anjaneyulu et al. 2005). Many studies have been reported
for the treatment of azo-dye-containing wastewater
using anaerobic and aerobic biological treatments
(Pagga and Brown 1986; Manu and Chaudhari 2002).
The biological methods are considered economical.
However, conventional aerobic biological processes used
in the textile industry have been found unsuccessful in
removing dyes beyond the adsorbing capacity of the
biomass (Bahorsky 1998). The decolourization of azo
dyes by microorganisms invariably starts by reductive
cleavage of azo bonds under anaerobic conditions
(Carliell et al. 1995; Beydilli et al. 1998). The reductive
cleavage of azo bonds is responsible for the formation
of aromatic amines of dye-related structures that are not
Wastewater generated by dye production industries and
many other industries is not only aesthetically undesirable
but also has an adverse impact on the environment
(Forgacs et al. 2004). Effluent from the dyeing and
finishing processes in the textile industry are known to
contain high levels of colour and surfactant, high chemical
oxygen demand (COD), and moderate biological oxygen
demand (Golob et al. 2005; Balasubramanian et al.
2007). It also obstructs the penetration of light, which is
required for the survival of many aquatic species.
Dyestuffs are divided into classes according to their
chemical composition and applications (Golob and
Ojstrsek 2005). There are many structural varieties, such
as acidic-, basic-, disperse-, azo-, and anthraquinonebased, and metal complex dyes. The most common is
the azo type used in the textile industry, constituting 60
to 70% of all dyestuffs produced (Brown and Devito
1993). Reactive azo dyes are typically azo-based (R1 - N
= N - R2) chromophores combined with different types
of reactive groups such as vinyl sulfone, chlorotriazine,
trichloropyrimidine, and difluorochloropyrimidine.
These dyes are mainly used for dyeing cellulosic fibres,
such as cotton and rayon, but are also used for silk, wool,
nylon, and leather (Yang and McGarrahan 2005). They
differ from all other classes in that they bind to textile
fibres such as cotton through covalent bonds. They are
* Corresponding author: [email protected]
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Modak et al.
further degraded under anaerobic conditions and tend to
accumulate to toxic levels (Carliell et al. 1995; Gottlieb
et al. 2003). It has been postulated that those aromatic
amine derivatives may be cytotoxic or carcinogenic
(Brown and Devito 1993; Gottlieb et al. 2003). From
an environmental point of view, the structure of reactive
azo dye is of great concern since some of the dyes and
their degradation products are cytotoxic or carcinogenic
and, consequently, their treatment cannot depend on
biodegradation alone (Balasubramanian et al. 2007).
Apart from being aesthetically displeasing, coloured
water is now thought to be mutagenic, carcinogenic,
and toxic. Keeping these factors in mind, a laboratoryscale study was performed by the authors using common
reactive azo dyes.
Effluents discharged from dyeing processes cause
considerable environmental pollution, are toxic to some
aquatic organisms, and are of serious health risk to human
beings. Hence, it is necessary to develop methods that are
efficient for dye/colour elimination from the wastewater,
producing superior quality effluent while being cost
effective and especially environmental friendly.
The physicochemical treatment of wastewater remains
the most efficient process for the elimination of dyes
studied here. Physicochemical processes are most widely
used today for wastewater treatment and are the most
appropriate especially when conventional treatments by
biological (anaerobic/aerobic) processes are not successful
due to the complex molecular structure of reactive azo
dyes; the stability of their molecular structures renders
them resistant to biological degradation (Robinson et al.
2001). Due to their toxicity and recalcitrance, these dyes
can be hazardous to the environment though present
at low concentrations. Among several current chemical
processes used for coloured wastewater treatment,
adsorption is recognized as one of the best processes if
designed correctly. It can produce better quality effluent
and remove different types of colouring material (Crini
2006). Moreover, adsorption processes remove the
dye molecule completely, and there is no possibility of
breaking down dye molecules, producing intermediate
by-products and leaving fragments in the wastewater.
Many studies have reported on the possibility of using
activated carbon (McKay 1983; Yang and Al-Duri 2001),
zeolite (Armagan et al. 2004), clay (Sethuraman, and
Raymahashay 1975), peat (Allen and McKay 1987), fly
ash (Gupta et al. 1990), and others like bagasse pith,
straw, rice husk, corncob, and sewage sludge (Crini
2006) as adsorbents. However, the adsorption capacity
of the adsorbents is not very large, so large amounts of
adsorbent are needed. Adsorption using activated carbon
appears to be the best prospect for colour removal from
wastewater. However, in spite of its good efficiency, this
adsorbent is expensive for good quality material and
difficult to regenerate after use (Gong et al. 2005). This
has led many researchers to search for more inexpensive
adsorbents.
Natural biopolymers like chitin and chitosan (both
derived from crab shell) are becoming increasingly
popular because they are inexpensive, easily available,
environmentally friendly, hydrophilic, biocompatible,
biodegradable, and not toxic to microorganisms.
Recently, there has been a resurgence of interest in
natural biopolymers for wastewater treatment (Yoshida
et al. 1993; Stefancich and Delben 1994; Chiou and Li
2003; Crini 2005; Uzun 2006).
The present study mainly focuses on the evaluation
of chitosan’s ability to eliminate colour and organic
matter from wastewater. Among biopolymers, chitosan
has exhibited promising performance in removing
pollutants effectively from water and wastewater (Varma
et al. 2004; Guibal et al. 2006). Chitosan is a linear
polysaccharide based on a glucosamino unit and so
contains many amino and hydroxyl functional groups.
It is obtained by deacetylation of the naturally occurring
chitin, well known for its outstanding biodegradability
and biocompatibility. It is one of the most widely
available natural biopolymers (Kumar 2000). There are
some reports on removal of reactive dyes using chitosan
(Chiou and Li 2003; Sakkayawong et al. 2005; Uzun
2006). Among all the studies on the removal of dyes,
there is no systematic work focused on the elimination
of Procion Orange and Procion Red dyes by chitosan.
This paper is mainly focused on monoazo, highly watersoluble reactive Procion MX dyes, since the toxicity
of Procion dyes are reported by Gottlieb et al. (2003)
and Wong et al. (2006), and they are frequently used by
industries for dyeing cellulose, nylon, silk, and wool.
In the present study, a series of batch experiments
were conducted to evaluate the performance of chitosan
as an adsorbent for removal of reactive azo dyes from
wastewater. The objective of this research was to study
the feasibility of using chitosan as an adsorbent and/or
primary coagulant for the complete removal of these
reactive dyes (Procion Orange MX-2R and Procion
Red MX-5B) from the synthetic dye wastewater. The
variables studied are the chitosan dose, pH, contact
time, and inorganic salts dose to determine the optimum
conditions for the maximum removal of these dyestuffs
by chitosan from the synthetic dye wastewater.
Materials and Methods
Materials
Chitosan (derived from crab shell) used in this study
was obtained from Sigma-Aldrich, Canada (C 3646). It
was used as received without further purification. The
molecular structure of chitosan is shown in Fig. 1. Some
physicochemical properties of chitosan are summarized
in Table 1. The experiments were carried out using two
commercially available reactive dyes, Procion Orange
MX-2R (POMX-2R) and Procion Red MX-5B (PRMX5B) purchased from G & S dye and Accessories Limited,
Canada. Information on their chemical structures,
molecular weights, and absorbance wavelengths are
given in Table 2. Analytical grade sulfuric acid (H2SO4)
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Removal of Colour and Organics Using Chitosan
and sodium hydroxide (NaOH) were used to adjust
the pH. Sodium biphosphate dibasic (Na2HPO4) and
potassium sulfate (K2SO4) from Fisher Scientific were
used to investigate the influence of these salts on colour
removal efficiency.
Methodology
Batch experiments were carried out for evaluating the
performance of chitosan as an adsorbent and/or primary
coagulant for the removal of colour and organic matter
from water containing dyes. This synthetic wastewater
(distilled water plus dye) has been created in order to
simulate the reality of the effluent generally discharged
from the textile industry. The study was performed in
the Water and Environmental Laboratory at the Civil
Engineering Department, University of New Brunswick,
Fredericton, Canada. Different series of batch experiments
were conducted. The methodology involved a rapid mixing
period, a slow mixing period, and a settling period. The
experimental setup of this study consisted of a six-beaker
jar test apparatus (compact laboratory mixers, CLM6,
EC Engineering, Canada). This apparatus included six
square beakers, six mixers, a speed controller, and a
timer as shown in Fig 2. Each beaker contained 500 ml
of the 0.1 g/L synthetic dye solution with different added
masses of chitosan powder, and the beakers were agitated
simultaneously. The experiments were followed by a
rapid mixing at 200 rpm for 2 min, a slow mixing at 90
rpm for 20 min, and a settling period of 30 min. Portions
Fig.1. The structure of the chitosan monomer.
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Modak et al.
Analytical
of the supernatant (10 mL) solutions were then taken out
and analyzed at 30 and 60 min intervals. The residual
dye solutions were filtered using microfiber membrane
filter paper (0.45-μm porosity, Fisher Scientific, U.S.A.)
before measuring the dye concentration. All experiments
were conducted at room temperature.
The batch experiments in this study were performed
in two stages. In the first part of this study, the effect of
chitosan on dye removal was studied at natural pH. That
is, the concentration (dose) of chitosan was varied from
0.5 to 14 g/L but no pH adjustment was made. In the
second part of this experimental study, a concentration
of chitosan at 1 g/L was selected in order to minimize
the quantity of sludge, pH was varied from 4 to 10, and
settling time was varied from 0.5 to 6 h. Then, using the
same chitosan concentration (1 g/L) at optimal pHs, the
effect of different concentrations of inorganic salts was
also tested (150 mg/L SO4- + 10 mg PO4- in each dye
solution and 400 mg/L SO4- + 40 mg PO4- in each dye
solution).
Dye concentration, pH, turbidity, total organic
carbon (TOC), COD, sludge volume percentage, and total
suspended solids (TSS) measurements were performed
before and after treatment of the synthetic wastewater
by chitosan. The measurements of these parameters were
accomplished in three replicates in order to evaluate the
repeatability of the test method.
The adsorption isotherm experiments were done
using 40-ml glass vials loaded with 35 ml of solutions of
dyes in distilled water (1.00 g/L) and six different masses of
chitosan (0.04 to 0.25 g). Vials were mounted horizontally
in an orbital shaker at 20°C and 150 rpm for 357
hours. After filtration through 0.45-micron membranes,
solutions were diluted where necessary and measured for
concentration with a UV spectrophotometer.
The kinetics experiments were conducted at room
temperature using the jar-test apparatus described earlier.
One litre of dye solution (50 mg/L for POMX-2R and 63
mg/L for PRMX-5B) was stirred at 250 rpm with a baffle
bar installed. Chitosan (0.400 g) was added at zero time
and then samples of 3 ml were taken at various times (5,
10, 15, 20, 30, 60, 120, 185, 361, and 661 minutes for
POMX-2R and 6,12, 20, 25, 30, 45, 60, 120, 292, and
390 minutes for PRMX-5B). After centrifugation (4,000
rpm for 2 minutes), the supernatant was measured using
the spectrophotometer.
Absorbance of the sample solutions was measured
to determine the final dye concentration using an
automatic ultraviolet-visible spectrophotometer (UV-VIS
GENESYS 6, Thermo Electron Corporation, U.S.A.). The
wavelengths selected for the absorbance measurement
in this experiment were 436 and 541 nm for POMX2R and PRMX-5B, respectively. A standard curve was
plotted for each dye to show the relationship between
dye concentration and absorbance and so determine the
final dye concentration in each water sample.
pH of the dye and dye-chitosan solutions was
measured during the mixing period and after settling
using a digital pH meter (Accumet AB15, Fisher
Scientific, U.S.A.). COD, TOC, turbidity, sludge volume,
and TSS were measured following the Standard Methods
for the Examination of Water and Wastewater (APHA et
al. 2005). TOC was measured using a TOC-VCPH, Total
Organic Carbon Analyzer from Shimadzu, Japan.
The surface morphology of chitosan before and
after adsorption of dye molecules was observed using a
scanning electron microscope (SEM) (JOEL JSM-6400,
Japan). The samples were prepared by drying in an oven
for 12 hours before introduction to the vacuum. Samples
were placed in glass vials with ethanol, and dispersed
using an ultrasonic bath before pipetting onto an
aluminum stub. Samples were then gold sputter coated
for observation in the SEM. Images were collected using
the software package dPict32.
Brunauer-Emmett-Teller (BET) surface area was
measured from the N2 adsorption isotherm with
Autosorb-1, Quantachrome Automated Gas Sorption
(Quantachrome Corp, U.S.A.). A sample of 0.873 g of
chitosan was used for this experiment. The sample was
heated at 110°C overnight before measuring the BET
surface area.
Results and Discussion
Effect of Chitosan Dose on Dye Removal
The synthetic water was treated with eight different doses
of chitosan. The initial content of dye in synthetic water
was 0.1 g/L. Dye content was analyzed before and after
treatment. Figure 3 shows the effect of chitosan dose on
the percent removal of dyes (POMX-2R and PRMX-5B).
It is apparent that the percent removal of dye increased
linearly at first and gradually approached a plateau
as a function of increasing dose. Chitosan has amine
functional groups which are partially protonated and
attract anions to bind and bridge (Guibal et al. 2006).
This factor caused the anionic dye molecules to bind and
bridge with chitosan. Therefore, chitosan a positively
charged biopolymer could adsorb dye molecules by a
charge neutralization mechanism.
In the present experimental condition, the optimal
dose of chitosan was found to be 6 g/L, which was
Fig. 2. Schematic diagram of the experimental setup.
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Removal of Colour and Organics Using Chitosan
the anionic dye molecules. In batch experiments, the
effect of the initial solution pH on the percent removal
of the dyes using 1 g/L of chitosan for POMX-2R and
PRMX-5B was studied by varying the pH from 4 to 10.
The results are shown in Fig.4. It shows that the percent
removal of dye increases significantly with decreasing
pH. Complete colour removals were achieved at pH
4.8 and 5.5 for POMX-2R and PRMX-5B, respectively.
This suggests that the maximum number of ammonium
(–NH3+) groups of chitosan were available to interact
with the anionic dye molecules at pHs of 4.8 and 5.5
for POMX-2R and PRMX-5B, respectively, and thereby
increased the percent removal of the dyes with the help of
electrostatic interaction. A decline in the percent of dye
removal above pH 7.0 was observed. At pH 10, the percent
of dye removal was 25.4 and 34% for POMX-2R and
PRMX-5B, respectively. This is due to the deprotonation
of the ammonium (–NH3+) groups of chitosan as well as
the electrostatic repulsion between hydroxyl ions (–OH)
of the solution and anionic dye molecules.
The above observations show that the major
adsorption/interaction site of chitosan is the amino
group –NH2, which is easily protonated to form –NH3+
in acidic solution. Many authors have proposed that the
strong electrostatic interaction between the –NH3+ of
chitosan and dye anions can be used to explain the high
adsorption capacity of anionic dyes onto the chitosan
(Chiou and Li 2003). The explanation of dye-chitosan
interaction conforms to our experimental data shown in
Fig. 4. It is assumed that the adsorption/interaction of
anionic dyes onto chitosan occurs through a combination
of electrostatic forces (van der Waals forces of interaction
and hydrogen bonding), with the electrostatic forces
only having a significant effect at lower pHs (Blackburn
2004). Therefore, the electrostatic interaction between
chitosan and dye molecules can be considered as one of
the dominant phenomena in the present experimental
conditions.
8
Fig. 3. Effect of chitosan dose on colour removal and
natural pH of the solution ( : dye removal (%) of POMX2R; Δ: dye removal (%) of PRMX-5B; •: pH of POMX-2R;
: pH of PRMX-5B).
sufficient for the almost total elimination of colour from
both dye effluents. This experiment was carried out at
natural pH of the mixture of aqueous dye solution with
chitosan. The original pH of the 0.1 g/L dye solutions of
POMX-2R and PRMX-5B without chitosan was 5 and 6,
respectively. The phenol groups in their structures made
the dyes rather acidic. The amino group in the structure
of chitosan is charged positively when in the acidic
dye solutions, and a strong chemical affinity is formed
between the positively charged amino groups on chitosan
and the sulfonate groups in the structures of POMX-2R
and PRMX-5B. During the mixing period, the pH of the
dye-chitosan solution was found to be increased from 7.3
to 8.5 for both dyes as shown in the Fig. 3. This can be due
to the shift in the amine-ammonium equilibrium caused
by the formation of dye-ammonium ion pairs. This shift
would increase the pH by consuming the acidity.
Although total elimination of dye (0.1 g/L) could be
achieved by 6 g/L of chitosan, for industrial applications
it is necessary to optimise the process by means of
reasonable consumption of chitosan and quantity of
produced sludge. To make the method more economical
and to produce less sludge, 1 g/L of chitosan was selected
for the next experiments to observe the influence of other
parameters. The preliminary screening test showed that 1
g/L was sufficient to get almost 100% removal of colour
while the maximum amino groups of chitosan are in a
protonated state (–NH3+) at reduced pH.
Effect of pH on Dye Removal and Sludge Volume
Percentage
pH is one of the most important environmental factors
that influences both the dye binding sites of the chitosan
surface and the solution chemistry of dyes. The reactive
dyes release coloured anions in solution. The protonation
of the amino groups of chitosan is strongly influenced by
pH and makes chitosan positively charged and, thereby,
electrostatic attractive interactions lead to sorption with
8
Fig. 4. Colour removal of reactive azo dyes at a 1 g/L chitosan dose as a function of pH ( : POMX-2R; Δ: PRMX-5B).
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Modak et al.
Removal of Organic Matter and Turbidity
Table 3 summarizes the results obtained for the
treatment of dye-containing wastewater with 1 g/L of
chitosan under optimum experimental conditions, i.e.,
pH controlled to 4.8 and 5.5 for POMX-2R and PRMX5B, respectively. The COD and TOC of the wastewater
were found to be decreased by approximately 85% for
POMX-2R and approximately 75% for PRMX-5B. The
values of turbidity of the synthetic water before and
after treatment were 0.17 and 1.5 NTU for POMX2R, 0.22 and 3.7 NTU for PRMX-5B, respectively. The
values of turbidity were increased after the treatment by
chitosan since some dye-adsorbed chitosan particles were
remaining in the treated supernatants.
The present investigation suggested that the
adsorption method using chitosan can be used as a
main process for treatment of wastewater containing
POMX-2R and PRMX-5B generated by dye-synthesizing
industries or textile industries. Chitosan was found highly
efficient for the elimination of colour. This biopolymer
also showed a high capacity to remove organic matter
from the contaminated water (dye solution). Chitosan
treatment produced acceptable values of COD, TOC,
TSS, and turbidity in the treated water.
Fig. 5. The volume of sludge produced by
: chitosan;
: chitosan and POMX-2R;
: chitosan and PRMX-5B.
It has to be emphasised that for complete elimination
of dye with chitosan at natural pH, a large volume of
sludge was produced, which can be seen in Fig. 5. The
results of the present work showed that using 1 g/L of
chitosan at an acidic pH leads to significantly less sludge
in comparison with that used for the complete elimination
of dyes at normal pH. The reduction in sludge volume
percentage was 80% for both of the dyes, and that is
significant from an industrial and economical point
of view.
Effect of Inorganic Salts on Dye Removal
Effect of Time on Dye and TSS Removal
The effect of inorganic salts on the sorption of dyes is an
important parameter since dyes are found in solutions
of high salt concentrations of sulfates and phosphates in
textile wastewater effluents. The dye removal efficiency
was increased by the addition of these inorganic salts
at relatively higher pH (around 6.0 to 6.5) as shown
in Fig. 7a and 7b. It was found that in the presence of
inorganic salts, approximately 99% colour removal
was achieved at pH 6.0 to 6.5, whereas it was around
90% at pH 6.5 without the addition of inorganic salts
for both dyes, as shown in Fig. 4. It is perhaps due to
the fact that chitosan has hydroxyl (–OH) groups which
are capable of chelating Na+ and K+ ions. Since these
chelated units exert a partially positive charge, they may
attract negatively charged dye ions electrostatically. The
addition of salts such as Na2HPO4 and K2SO4 increases
the number of Na+ and K+ chelated units and thereby
promotes the electrostatic interaction between chitosan
and dye molecules. Therefore, chelation might be playing
a dominant role in enhancing the dye removal efficiency
in the presence of inorganic salts.
The effect of contact time was studied by varying the
settling period from 0.5, 1, 2, 4, and 6 h in order to
observe the percent removal of the dye as well as the
amount of TSS with the lapse of contact time as shown in
Fig. 6. It was found that there was no significant change
in percent removal of dye with the lapse of time from 0.5
to 6 h. Dye removal was complete after a half an hour
settling period. The TSS concentration declined with the
increase of settling period; however, 1 h was sufficient to
get a typically acceptable value (0.02 g/L) of TSS for a
safe discharge of the effluent.
Surface Area and Surface Morphology
The BET surface area of the chitosan used in this study was
measured from N2 adsorption isotherms. The values were
computed using Quantachrome AUTOSORB software. It
was found that the BET surface area of the chitosan was
2.6 m2/g. The pore size distribution graph showed that
the chitosan had mesopores and macropores, although
8
Fig. 6. Color removal and TSS concentration at various
contact times ( : removal of POMX-2R dye; Δ: removal
of PRMX-5B dye; : TSS in POMX-2R solution; : TSS in
PRMX-5B solution).
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Removal of Colour and Organics Using Chitosan
(a)
(b)
8
8
8
Fig. 7. Influence of inorganic salt on:
(a) dye removal as a function of pH (POMX-2R) ( : in the presence of 150 mg/L SO42– ions and 10 mg/L PO43– ions; :
in the presence of 400 mg/L SO42– ions and 40 mg/L PO43– ions; and
(b) colour removal as a function of pH (PRMX-5B) (Δ: in the presence of 150 mg/L SO42– ions and 10 mg/L PO43– ions; ◊:
in the presence of 400 mg/L SO42– ions and 40 mg/L PO43– ions).
dye molecules were adsorbed by chitosan onto its pores
and surface, which developed a smooth layer of dye
molecules. Therefore, both surface diffusion and pore
diffusion might have been involved in this adsorption
process.
most of the pores were in the mesopore range (25 to 99
Å). It is evident from the BET results that the surface
of chitosan possesses huge numbers of mesopores and
macropores. Therefore, it is plausible that dye molecules
are diffused along the surface as well as within the pore
volumes.
Adsorption Isotherm Results
To further understand the adsorption mechanism of
dye molecules onto the chitosan, the surface morphology
of chitosan and chitosan after adsorption were observed
using a SEM. Figures 8a and 8b show the SEM images of
the chitosan. Chitosan exhibited heterogeneous bumpy
surface morphology as can be seen in the images. It has a
patchy type of particle size distribution and its pores are
scattered and uneven in size. Its BET surface confirmed
that the chitosan exhibited mesopores and some
macropores as well. Figures 8c and 8d show significant
changes of the structure and appearance on chitosan
after adsorption. The SEM observations indicated that
most of the bumpy surface and pore areas were covered
with a mud-like substance and turned into a smooth
surface. This was due to the adsorbed dye molecules
on the chitosan surface. Thus, these images proved that
For a more thorough characterization of the adsorption
of these dyes onto chitosan, equilibrium measurements of
dye concentration were made. Adsorption capacity (q) was
calculated as the ratio of the mass of dye adsorbed to the
mass of chitosan, and was plotted versus the equilibrium
concentrations (Ce) (Fig. 9). When the logarithms of
these data were plotted, linear regression showed good
correlation, which indicated that the adsorptions of both
dyes on chitosan follow the Freundlich isotherm (q =
KfCe1/n) (Freundlich 1907).
The intensity parameter, n, is greater than 1 for
conditions of favourable performance, and the capacity
factor, Kf, is larger for better performance. For PRMX-5B
and POMX-2R, the n values are both favourable, but
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Modak et al.
POMX-2R is better. Three other azo dyes as indicated in
Table 4 showed intensity parameters in the same range,
but one was higher than the normal range of 0.3 to 1.67
(Aksu and Tezer 2005).
The capacity factors, Kf, for these dyes are all of
the same order of magnitude, but POMX-2R showed
the highest value thereby indicating that it is the most
reactive sorbate. Maximum adsorption capacity,
qmax, was calculated when the initial value of the dye
concentration was applied to the isotherm equation. This
was calculated for the dyes used in the present study. The
qmax value is somewhat higher for PRMX-5B, showing
it to be more economically feasible. These parameters
showed contrary indications of the relative ability of
chitosan to adsorb these two Procion dyes. However,
these parameters can be used to choose an adsorbent
and determine the amounts of adsorbent needed when
designing a system to achieve a particular effluent dye
concentration from a given wastewater concentration.
Adsorption of POMX-2R and PRMX-5B onto chitosan
could not be said to follow the Langmuir isotherm, which
assumes monolayer adsorption to a surface (Langmuir
1918). Plots in the linear form showed lower correlation
than did the Freundlich model for both dyes. The values
of the correlation coefficient were 0.352 and 0.828 for
PRMX-5B and POMX-2R, respectively.
(a)
(b)
(c)
(d)
Kinetics
The rate of adsorption of dyes onto chitosan was
evaluated. Measurements of dye concentrations at various
times during mixing with chitosan were plotted for both
dyes (Fig. 10). When plotted according to the kinetic
models, the data for both dyes were found to conform
Fig. 8 SEM images of chitosan (Sigma-Aldrich, C3646) (a) and (b), and SEM images of chitosan with adsorbed POMX-2R
(c) and PRMX-5B (d).
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Removal of Colour and Organics Using Chitosan
Fig. 10. Disappearance of dyes PRMX-5B (Δ: initially 63
mg/L) and POMX-2R ( : initially 50 mg/L) being stirred at
250 rpm with 0.400 g of chitosan and no pH adjustment.
8
Fig. 9. Adsorption isotherms of PRMX-5B and POMX-2R
on Chitosan with no pH adjustment.
better to the second-order Lagergren model (McKay and
Ho 1999), than to the first-order model (Lagergren 1898)
for which the R2 values were significantly lower at 0.96
and 0.93 for POMX-2R and PRMX-5B, respectively.
The linear form of first- and pseudosecond-order models
are presented as follows:
should be noted that Aksu and Tezer (2005) also found
these quantities to be very dependent on the initial dye
concentration.
Adsorption Mechanism
By considering the adsorption of reactive azo dyes onto
the surface of chitosan, different mechanism may be
involved. The possible mechanisms of the adsorption
process of chitosan and reactive dyes are discussed.
It is well known that reactive azo dyes can react
with fabric molecules (HO–cellulose) and water through
either the chlorotriazine or vinyl sulfonyl group, or both
(Juang et al. 1997). The glucopyranose unit of cellulose
fibre consists of one primary hydroxyl group (–CH2OH)
and two secondary hydroxyl groups (–OH). In the dyeing
process, the HO–cellulose is deprotonated under caustic
conditions and transforms to a cellulose ion (Cell–O–),
which is the nucleophile for the dye-fibre reaction
(Blackburn and Burkinshaw 2003):
First-order Lagergren, integrated linear form:
log (qe – q) = log qe – k1,ad/2.303t
Second-order Lagergren, integrated linear form:
t/q = 1/k2,adqe2 + 1/qet
where q = the amount adsorbed dye on chitosan at time
t, k1,ad = the first-order constant, qe = the equilibrium
sorption capacity, and k2,ad = the second-order rate
constant.
Table 5 gives the resulting values of equilibrium
adsorption capacity (qe), a comparison for qe found
experimentally, and the rate constant of second-order
kinetics (k2,ad) for both POMX-2R and PRMX-5B. Table
5 also provides comparison with other researchers’
results on the adsorption of three vinyl sulfone azo dyes
on dried algae (Aksu and Tezer 2005). The rate constants
calculated in this study were lower than any of those for
vinyl sulfone dyes on algae. The qe values of the dyes in
the present study are significantly higher. However, it
Cell–OH + OH– = Cell–O–
The cellulose ion forms a covalent bond on the reactive
dye, after which the chloride groups of reactive dye are
released into the solution.
The affinity between the reactive dye and chitosan
in alkaline conditions can be explained by the chemistry
303
Modak et al.
of the dyeing process. The glucosamino unit of chitosan
contains an amino group (–NH2) and a hydroxyl group
(–OH) which could interact with dye molecules by
covalent bonding in the same interaction mechanism as
seen for cellulose (Sakkayawong et al. 2005) when the
interaction/sorption takes place in alkaline conditions.
The interaction of the dye (bearing several sulfonate
groups) with the amino groups of chitosan occurs by a
charge neutralization mechanism. At acidic pH, more
protons (H+) are available to protonate the amino groups
of the chitosan molecules as indicated in the following
reaction:
complexity of aqueous solution arises from the large
number of variables involved. These include the pH of
the solution, ionic strength, and solute-solute interaction.
Furthermore, the difference between the physisorption
and chemisorption processes on the surface are not easily
understood. Some mechanisms could be explained based
on the surface morphology of the chitosan.
It is well known that interparticle diffusion
mechanisms are involved in the adsorption process,
including: (1) diffusion within the pore volume, known
as pore diffusion, and (2) diffusion along the surface of
the pores known as surface diffusion. Pore diffusion and
surface diffusion occur in parallel within the adsorbent
particles. At a particular lower temperature, surface
diffusion is more dominant. The BET surface of chitosan
exhibited a huge number of mesopores and macropores.
Therefore, it is plausible that dye molecules are diffused
along the surface as well as within the pore volumes.
where R’ = the remainder of the chitosan molecule. In
aqueous solutions, the reactive dye was dissolved and
the sulfonate groups (DSO3 Na) of the reactive dye were
dissociated, producing anions (DSO3– ):
Conclusions
This study revealed the potential of chitosan as an
adsorbent for the total elimination of Procion MX
reactive azo dyes from wastewater. Chitosan was
found efficient in removing colour and organic matter
through the following possible mechanisms: electrostatic
interaction, covalent bonding, H bonding, and van der
Waals forces. At natural pH, a positive impact on colour
removal efficiency was observed with an increase in the
chitosan dose up to 6 g/L, which was required to achieve
complete removal of 0.1 g of dye per litre after 30 min
of settling time. When pH was adjusted to 4.8 and 5.5,
the removals of POMX-2R and PRMX-5B dyes (0.1 g/L)
were found to be complete, with less sludge produced, at
a chitosan dose of only 1 g/L and a settling period of 30
min. Protonation of the amino groups of chitosan played
a major role in the removal of colour and organic matter
at lower chitosan doses under acidic conditions. The
adsorption efficiency of chitosan decreased significantly
under alkaline conditions due to the deprotonation of the
amino groups of chitosan. The adsorption efficiency was
also substantially influenced by the presence of inorganic
salts. In addition, the surface morphology study showed
that chitosan exhibited a bumpy surface and huge
numbers of pores. Therefore, the dye molecules might
The interaction of dye molecules with active sites of
chitosan was due to the increasing electrostatic attraction
between the two counter ions (Wong et al. 2004):
The difference in the degree of interaction could be
attributed to the specific chemical structure of each dye.
Moreover, the van der Waals forces of attraction are
always present between molecules. Since the chitosan
molecules are near to the dye molecules, these attractive
forces might be one of the possible mechanisms. Dipoledipole hydrogen bonding interaction between the
hydroxyl group of chitosan and the azo group of the dye
molecule, and Yoshida H-bonding between the hydroxyl
group of chitosan and the aromatic residue in the dye
are the possible mechanisms according to Blackburn
(Blackburn 2004).
The above-described mechanisms/interactions can be
important in practice, and sometimes all of them operate
simultaneously, but often one of them is dominant. The
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Removal of Colour and Organics Using Chitosan
have been diffused along the surface as well as within the
pore volumes.
Overall, this study concluded that chitosan can be
used efficiently for removal of colour from wastewater
containing these reactive dyes with 75 to 85% reductions
of COD and TOC.
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Acknowledgments
The authors would like to acknowledge the financial
support provided to the first author by the Government
of New Brunswick through their Work Ability program.
The financial support for this research project was
provided by the NSERC and AIF research grants of the
second author.
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Received: 14 December 2006; accepted: 4 February 2009.
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