Chitosan is derived from chitin, a natural aminopolysaccharide found in the
exoskeleton of crustaceans, insects and some fungi. It is produced via partial
deacetylation of chitin in 40-50% aqueous alkali, sodium hydroxide in most cases
at 120-150°C under heterogenous conditions (Kurita,
2006). Chitosan in various forms have shown effectiveness as a pollutant
adsorbent. Chitosan beads have shown ability to adsorp Cu(II) ions in aqueos
solutions at approximately 100% removal (Ngah and Fatinathan,
2008). Its weak mechanical property requires chemical and physical modifications.
Cross-linking using a cross-linking agent such as glutaraldehyde (Cestari
et al., 2008; Kyzas and Lazaridis, 2009)
is required to reduce the crystallinity of chitosan thus improving its sorption
capacities of reactive and basic dyes.
An important feature of chitosan is the degree of deacetylation. Although the
hydroxyl groups also attract the dye molecules, it is the amine groups that
are the main active groups which influence the polymer s performance.
The degree of deacetylation of chitosan depends on the number of available amine
groups. Thus an increase in the degree of deacetylation generally gives an increase
in sorption capacity for anionic dyes due to the availability of protonated
amine groups (Guibal et al., 2003; Saha
et al., 2005). The presence of protonated amino groups in chitosan
contributes towards better sorption capabilities of chitosan as compared to
chitin by allowing ionic bonding between the amino groups and the dye ions (Wong
et al., 2008). This study focuses on the adsorption of a basic dye
(crystal violet) using different types of chitosan.
MATERIALS AND METHODS
Chitin from crab shells in powder form and crystal violet as the adsorbate
were supplied by Sigma Aldrich Co. (USA). The molecular structure of crystal
violet and chitosan is shown in Fig. 1 and 2,
respectively. Sodium hydroxide, NaOH for deacetylation was supplied by Merck,
|| Molecular structure of crystal violet
|| Molecular structure of chitosan
||Preparation of chitosan with different degree of deacetylation
Chitin from crab shells were sieved into particles of sizes 1 mm for homogeneity.
Chitin was deacetylated to chitosan by using different concentrations of NaOH;
5% (w/v), 30% (w/v) and 50% (w/v) at 100°C for 1 h in a hot water bath.
The product was washed with distilled water followed by 50% methanol and 50%
acetone. They were then oven dried at 90°C for 24 h. Degree of deacetylation
was determined by using an FTIR spectrophotometer by measuring the absorbance
at wavenumbers 1655 cm-1 (A1655) attributed to the amide
groups and the corresponding value of the hydroxyl band at 3450 cm-1
(A3450) (Cha et al., 1997). The corresponding
NaOH concentration and degree of deacetylation are summarized in Table
1. The equation in determining the degree of deacetylation is shown in Eq.
A fixed mass of adsorbent (0.03 g) was weighed into an Erlenmeyer flask with
25 mL of crystal violet solution with predetermined initial concentrations.
The temperature remained at room temperature while the solution pH remain constant.
The flasks were sealed and shaken on an orbital shaker at 100 rpm for 72 h.
The concentration of dye in each sample was determined via UV/Vis at λmax
589 nm. The same procedure was repeated for the different types of deacetylated
chitin (25.54, 27.22 and 35.06%). The amount of dye adsorbed per unit mass of
chitosan (mg g-1) can be calculated as shown in Eq.
where, q is the amount of dye adsorbed per gram adsorbent (mg g-1),
Co is initial dye concentration (mg L-1), Ce
concentration of dye at equilibrium with solid phase (mg L-1), V
volume of working solution used (L) and m mass of sorbent used (g).
RESULTS AND DISCUSSION
In order to optimize the design of the adsorption system, the adsorption data
were analyzed according to the Langmuir, Freundlich and D-R isotherm models.
The most suitable isotherm model for the experimental data for adsorption systems
would be evaluated based on the highest correlation coefficient value, R2
via linear regression analysis. All equilibrium isotherms were used to determine
the adsorption capacity as well as the nature of adsorption in removing crystal
Langmuir isotherm: Adsorption is assumed to occur via monolayer coverage
of the adsorbate at the outer layer of the adsorbent (Wong
et al., 2008). Furthermore, adsorption occurs at fixed number of
definitive localized sites on the surface which can only adsorb only one molecule
known as a monolayer. Furthermore all sites are equivalent and no interaction
can be observed between adsorbed moieties and adjacent molecules. Therefore
this isotherm considers that the energies and enthalpy resulting from adsorption
are the same (Paulino et al., 2007). The linearized
equation is represented by Eq. 3:
where, Ce, is the equilibrium dye concentration in solution (mg
L-1); qe is the equilibrium dye concentration on the adsorbent
(mg g-1); qmax is the monolayer saturation capacity of
the adsorbent (mg g-1) while b is the Langmuir constant related to
energy (L mg-1). A linear plot as according to Eq.
3 yielding a straight line would prove that the system obeys the Langmuir
model. The plots for crystal violet at various DD were strongly linear as shown
in Fig. 3a.
The variation in adsorption properties was not proportional to the degree of
deacetylation as also reported by Piccin et al. (2009).
The amine groups of chitosan would be positively charged thus attracting the
negative charge of the sulphonate groups found in methyl blue via electrostatic
interation (Chiou et al., 2004; Prado
et al., 2004). For basic dye such as crystal violet the adsorption
capacity paled in comparison to the acidic dye. This was due to the positive
charge of the basic dye that creates strong coulombic repulsions between chitosan
and the basic dye that explains for the lower adsorption capacity (Kyzas
and Lazaridis, 2009). The hydroxyl group (-OH) in chitosan could adsorb
the basic dye via covalent and hydrogen bonding as similar to the adsorption
mechanism of cellulose polymers with reactive dyes (Sakkayawong
et al., 2005). The adsorption of the dyes for the three DDs
were favorable as their RL values were between 0 and 1. RL>1
represents unfavourable adsorption, RL = 1 represents linear adsorption
while the adsorption is irreversible if RL = 0.
Freundlich isotherm: The Freundlich model is used to describe non-ideal
sorption onto heterogenous surfaces and multilayer sorption (Rafatullah
et al., 2009). The linearized equation is written as:
where qe is the solid phase sorbate concentration at equilibrium
(mg g-1), Ce is liquid phase sorbate concentration in
equilibrium (mg L-1), Kf is Freundlich constant (L g-1)
and 1/n is the heterogeneity factor as shown in Table 2.
||(a) Langmuir, (b) D-R and (c) Freundlich isotherm plot for
the adsorption of crystal violet onto chitosan, degree of deacetylation
(DD: 25.54%, 27.22% and 35.06%)
||Isotherm constants for the adsorption of crystal violet onto
chitosan at different degree of deacetylation
A plot of ln qe versus ln Ce yields a straight line with
high correlation for crystal violet for all types of chitosan, Fig.
3c. The adsorption capacity for crystal violet decreases with increasing
degree of deacetylation as shown by the Kf value. This is due to
the increased availability of amine groups that are positively charged that
repel against the dye molecules.
The n value denotes the favorability of the adsorption. In this work, the n
value for both dyes being studied is above 1 for all DDs thus the adsorption
intensity is favorable at higher concentrations (Ngah and
Fatinathan, 2008). As the Freundlich isotherm gave a better fit compared
to the Langmuir isotherm, it can be concluded that the surface of the deacetylated
chitin is made up of small heterogenous adsorption patches.
D-R isotherm: Table 2 shows the conversion to liquid
and gaseous products This isotherm is used to determine whether the adsorption
is chemical or physical in nature. The linearized form is given in Eq.
where, β is a constant related to the mean free energy of adsorption per
mole of the adsorbate (mol2/kJ2); qm is the
theoretical saturation capacity (mol g-1); qe is the sorption
capacity at equilibrium (mol g-1); ε is the polanyi potential
(J2/mol2) calculated as in Eq. 5:
where, R is the gas constant (8.314 J/molK) and T is the absolute temperature
(K). Hence by plotting ln qe versus ε2, qm
and β can be obtained (Fig. 3b). The D-R parameters are
listed in Table 2. The correlation for this isotherm is much
higher compared to the Freundlich isotherm. The constant β gives an idea
about the mean free energy E (kJ mol-1) of adsorption per molecule
of the adsorbate when it is transferred to the surface of the solid from infinity
in the solution and the equation is as shown Eq. 6 (Dubey
and Gupta, 2005):
This parameter gives information regarding the nature of the adsorption. If
E is between 8 and 16 kJ mol-1, the adsorption process is chemisorptions
via ion-exchange. If E<8 kJ mol-1, the process is physical in
nature. Based on Table 2, the E value for the three types
of chitosan in adsorbing crystal violet are in the chemisorption process via
In this study, various degrees of deacetylated chitin ranging from 25.54-35.06%
DD were able to adsorb both acid and basic dyes. The adsorption systems were
evaluated based on three isotherm models which were the Langmuir, Freundlich
and D-R models. Increasing the DD improved on the adsorption of methyl blue
but not for crystal violet. The adsorption systems showed good correlation for
all three isotherm models with D-R giving the highest correlation. This proves
that the adsorption systems have both monolayer and heterogenous adsorption
sites. Based on the D-R model, the adsorption of both dyes onto three different
types of chitosan was via ion-exchange.
We are grateful to the School of Science and Technology, Universiti Malaysia
Sabah for the technical and financial support throughout this study.