A new chitosan biopolymer derivative as metal-complexing agent: Synthesis, characterization, and metal(II) ion adsorption studies

Article abstract of DOI:10.1016/j.carres.2010.06.005

In this study, a new chitosan biopolymer derivative (CTSL) has been synthesized by anchoring a new vanillin-based complexing agent or ligand, namely 4-hydroxy-3-methoxy-5-[(4-methylpiperazin-1-yl)methyl] benzaldehyde, (L) with chitosan (CTS) by means of condensation. The new material was characterized by elemental (CHN), spectral (FTIR and solid state ¹³C NMR), thermal (TG-DTA and DSC), structural (powder XRD), and morphological (SEM) analyses. The CTSL was employed to study the equilibrium adsorption of various metal ions, namely, Mn(II), Fe(II), Co(II), Cu(II), Ni(II), Cd(II), and Pb(II), as functions of pH of the solutions. Its kinetics of adsorption was evaluated utilizing the pseudo first order and pseudo second order equation models and the equilibrium data were analyzed by Langmuir isotherm model. The CTSL shows good adsorption capacity for metal ions studied in the order Cu(II) > Ni(II) > Cd(II) ≥ Co ≥ Mn(II) > Fe(II) > Pb(II) in all studied pH ranges due to the presence of many coordinating moieties present in it.

Full text of DOI:10.1016/j.carres.2010.06.005

Carbohydrate Research 345 (2010) 2013–2022
Contents lists available at ScienceDirect
Carbohydrate Research
journal homepage: www.elsevier.com/locate/carres
A new chitosan biopolymer derivative as metal-complexing agent:
synthesis, characterization, and metal(II) ion adsorption studies
*
K. R. Krishnapriya, M. Kandaswamy
Department of Inorganic Chemistry, University of Madras, Chennai 600 025, India
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study, a new chitosan biopolymer derivative (CTSL) has been synthesized by anchoring a new
vanillin-based complexing agent or ligand, namely 4-hydroxy-3-methoxy-5-[(4-methylpiperazin-
1-yl)methyl] benzaldehyde, (L) with chitosan (CTS) by means of condensation. The new material was
characterized by elemental (CHN), spectral (FTIR and solid state 13C NMR), thermal (TG-DTA and DSC),
structural (powder XRD), and morphological (SEM) analyses. The CTSL was employed to study the equi-
librium adsorption of various metal ions, namely, Mn(II), Fe(II), Co(II), Cu(II), Ni(II), Cd(II), and Pb(II), as
functions of pH of the solutions. Its kinetics of adsorption was evaluated utilizing the pseudo first order
and pseudo second order equation models and the equilibrium data were analyzed by Langmuir isotherm
model. The CTSL shows good adsorption capacity for metal ions studied in the order Cu(II) > Ni(II) > C-
d(II) P Co P Mn(II) > Fe(II) > Pb(II) in all studied pH ranges due to the presence of many coordinating
moieties present in it.
Received 27 March 2010
Received in revised form 2 June 2010
Accepted 11 June 2010
Available online 20 June 2010
Keywords:
Biopolymer
Chitosan
Vanillin
Chitosan derivatives
Adsorption
Metal–polymer complex
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
matrix may improve its capacity for interaction with metallic ions
by complexation, thereby increasing their adsorption proper-
Biosorption has recently received a great deal of attention due
to the low cost of the materials used in these applications and
for the environmentally friendly impact of the treatment of ex-
hausted sorbents. Several types of biomass have been tested for
the recovery of precious metals, including fungal biomass,1,2 algal
biomass but also polymers of biological origin.3 Chitosan (CTS)
(hetero polymer constituted of glucosamine and a fraction of ace-
tylglucosamine residues) is a well-known biopolymer character-
ized by its high sorption properties due to its high nitrogen
content. This biopolymer and its derivatives have great potential
applications in the areas of biotechnology, biomedicine, food ingre-
dients, and cosmetics because of their many useful features such as
hydrophilicity, biocompatibility, biodegradability, anti-bacterial
property, and remarkable affinity for many bio-macromole-
cules.4–8 The presence of amino groups in the polymeric chain of
chitosan leads to the possibility of several chemical modifications,
including the preparation of Schiff bases by reaction with
aldehydes and ketones.9,10 The reaction of chitosan with aromatic
aldehydes in acetic acid to produce the corresponding Schiff bases
has been described by Tirkistani.11 Its attractiveness as an
analytical reagent arises from the fact that they enable simple
and inexpensive determinations of various organic and inorganic
substances.12 The insertion of functional groups in the chitosan
ties.13,14 In this sense the modification of chitosan with aldehydes
to produce Schiff bases may result in a potentially complexing
material for metallic species with potential analytical and environ-
mental applications.15,16
With growing interest in the chemical modification of chito-
san,17–21 a new chitosan derivative, CTSL, was synthesized using
substituted vanillin namely 4-hydroxy-3-methoxy-5-[(4-meth-
ylpiperazin-1-yl)methyl] benzaldehyde (L). The complexing agent
or ligand is anchored to chitosan in order to develop a new adsor-
bent for metal ions. The characterization of the product has been
performed by elemental analysis, FTIR, and solid state 13C NMR
spectroscopy. This material was also employed to study the kinet-
ics and the equilibrium adsorption of various metal ions such as
Mn(II), Fe(II), Co(II), Cu(II), Ni(II), Cd(II), and Pb(II) as functions of
the pH solution.
2. Experimental
2.1. Materials
Chitosan of low molecular weight was purchased from Aldrich
(Cat. Number 44,8869) with a deacetylation percentage in the
range of 75–85%, with Brookfield viscosity 20 cps, and used as re-
ceived. Stock solutions (1000 mg Lꢀ1) of Mn(II), Fe(II), Co(II), Cu(II),
and Ni(II) were prepared by dissolving the appropriate amount of
metal(II) sulfate (analytical grade) in double distilled water and
* Corresponding author. Tel./fax: +91 44 22300488.
E-mail address: mkands@yahoo.com (M. Kandaswamy).
0008-6215/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carres.2010.06.005

2014
K. R. Krishnapriya, M. Kandaswamy / Carbohydrate Research 345 (2010) 2013–2022
were standardized with
a
standard solution of 0.01 mol Lꢀ1
recorded on a Perkin–Elmer RX1 model spectrophotometer on KBr
disks in the wavenumber range 4000–250 cmꢀ1 1H NMR and 13C
EDTA.22 Stock solutions (1000 mg Lꢀ1) of Cd(II) and Pb(II) were
prepared by dissolving the appropriate amount of their metal(II)
nitrates (analytical grade) in double distilled water and their con-
centrations were estimated using Atomic Absorption Spectroscopy.
Working standard solutions of all metal ions were prepared
through the dilution of the respective 1000 mg Lꢀ1 stock solutions
with distilled water. All other chemicals and solvents were of ana-
lytical grade and used as received.
.
NMR spectra were recorded using a model FX-80-Q Fourier
transform nuclear magnetic resonance spectrometer. TG and DTG
analyses were carried out with TG-50 Shimadzu Model Thermo-
gravimetric analyzer with a heating rate of 10 °C minꢀ1, under a
nitrogen atmosphere. Powder X-ray diffraction studies were carried
out using a scientific high-resolution Guinier X-ray Powder diffrac-
tometer using Cu Ka1 radiation with a quartz monochromator.
Micrographs of CTSL before and after metal(II) ion adsorption were
taken by using a JSM-5600LV, JEOL model scanning electron micro-
scope. The metal(II) ion concentration was measured by using a
Perkin Elmer AAnalyst-750 Model atomic absorption spectropho-
tometer (AAS).
2.2. Synthesis of the ligand: 4-hydroxy-3-methoxy-5-[(4-
methylpiperazin-1-yl)methyl] benzaldehyde (L)
Formaldehyde solution (37%) (2 mL 0.025 mol) was added drop-
wise with stirring to N-methylpiperazine (2.0 g, 0.02 mol) in etha-
nol (20 mL) at 60 °C. Then, the stirring was further continued for an
2.6. Adsorption experiments
hour and
a solution of 4-hydroxy-3-methoxy benzaldehyde
2.6.1. Effect of pH on adsorption
(3.04 g, 0.02 mol) dissolved in 60 mL of ethanol was added to it
and refluxed for 18 h. After refluxing, the solvent was removed
by distillation under vacuum using rotary evaporator and the res-
idue was extracted with chloroform. The white colored solid was
recovered after removing the chloroform by distillation. It was
then further purified by silica gel column chromatography using
chloroform-light petroleum ether (bp 60–80 °C, 1:9 v/v) as the elu-
ent. The product was obtained by slow evaporation of the eluted
solvent as white colored crystals. Yield: 4.0 g (76%). mp: 118 °C.
Analytical data for C14H20N2O3; Calcd: C, 63.62; H, 7.63; N, 10.60.
Found: C, 63.60; H, 7.52; N, 10.56. Mass (EI) m/z: 264 (M+). Selected
The adsorption properties of chitosan derivative and the effect
of the pH on adsorption were studied from pH 3 to 10 utilizing var-
ious buffer solutions (KCl/HCl for pH 2 and 3; acetic acid/sodium
acetate for pH 4, 5, and 6; tris(hydroxymethyl) aminomethane/
HCl for pH 7.5 and 8.5; and ammonia/ammonium chloride for pH
9.5 and 10). Aliquots (50.0 mL) of 100 mg Lꢀ1 of metal ion solutions
buffered at different pH values were placed in contact with 50 mg
samples of chitosan derivative for a period of 2 h by agitation. After
4 h the shaking was turned off, and immediately thereafter the
adsorbent material was decanted, and 2 mL of the filtrate was re-
moved and diluted in volumetric flasks to determine the metal
ion concentration by atomic absorption spectrophotometry
(AAS). Each run was duplicated under identical conditions. The
quantity of metal (II) ions adsorbed at different pH values was cal-
culated with the following Eq. 1:23,24
FT-IR data (KBr,
m
cmꢀ1): 3445, 1641, 1592, 1472. 1H NMR (CDCl3, d
ppm): 2.31 (s, NCH3, 3H), 2.60 (br s, N–CH2–CH2–N, 8H), 3.82 (s, N–
CH2-Ar, 2H), 3.94 (s, CH3O, 3H), 7.17 (s, Ar-H, 1H), 7.34 (s, Ar-H, 1H),
9.77 (s, CHO, 1H). 13C NMR (CDCl3, d ppm): 45.7 (N–CH3), 52.3, 54.6
(N–CH2–CH2 –N), 55.9 (N–CH2-Ar), 60.6 (–OCH3), 109.5, 120.6,
125.5, 128.2, 148.6, 153.9 (ArC), 190.6 (CHO). Single Crystal Analy-
sis CCDC No.: 673897.
ðC0 ꢀ CeÞV
Qe
¼
;
ð1Þ
W
where Qe is the equilibrium adsorption capacity of the vanillin-
based chitosan derivative CTSL (mg gꢀ1 adsorbent), V is the volume
of solution (mL), C0 and Ce are the initial and equilibrium concentra-
tion of the solute in (mg Lꢀ1), respectively, and W is the weight of
the sorbent (g).
2.3. Synthesis of the chitosan biopolymer derivative (CTSL)
Chitosan powder 1.0 g (0.0048 mol of glucosamine residue) was
dissolved in 25 mL of 1 wt % acetic acid and diluted with methanol
(150 mL). Then 1.6 g of 4-hydroxy-3-methoxy-5-[(4-methylpipera-
zin-1-yl)methyl] benzaldehyde (L) (0.006 mol) dissolved in chloro-
form (20 mL) was added slowly to it. The mixture was stirred at
room temperature for 16 h, followed by refluxing for 18 h, which
resulted in a brown colored gel. It was then decanted and thor-
oughly washed with chloroform to remove any unreacted L and
then dried in vacuum at 60 °C to give CTSL as a brown colored pow-
der. Yield: 1.87 g (72%). Analytical data for CTSL (C20H29N2O6):
Found: C, 46.52; H, 6.01; N, 6.55. 13C NMR (Solid State, d ppm):
23.1, 59.0, 73.9, 81.9, 102.8, 126.1, 159.0. Selected FT-IR data
The amount of metal ions adsorbed by the adsorbent can also be
calculated as follows:
ðC0 ꢀ CeÞV
qe ¼
;
ð2Þ
WM
where qe is the adsorption capacity of the adsorbent at equilibrium
in mmol metal ion/g adsorbent and M is the atomic weight of the
corresponding metal ion.
2.6.2. Adsorption kinetics
(KBr,
m
cmꢀ1): 3431, 1638, 1404, 1033, 750, 649.
The adsorption kinetics of the new chitosan material was deter-
mined at pH 8.5. A 50 mg of chitosan derivative and 50.0 mL of
Cu(II) ion solutions buffered at 8.5 were shaken using an orbital
shaker at 200 rpm for 2 h. After pre-determined time periods,
2 mL aliquots were removed and the concentrations of Cu(II) ions
in solutions were measured.
2.4. Preparation of metal-CTSL
The metal complexes of the chitosan derivative were prepared
by the addition of 50.0 mL of 100 mg Lꢀ1 of metal ion (Mn(II),
Fe(II), Co(II), Cu(II), Ni(II), Cd(II), and Pb(II)) solutions buffered at
pH 7.5 to 50 mg sample of chitosan derivative (CTSL) for a period
of 4 h by agitation. After 4 h the shaking was turned off and imme-
diately thereafter the adsorbent material was decanted and dried
at 60 °C.
2.6.3. Adsorption equilibrium isotherms
Samples of the chitosan derivative (50.0 mg) were added to
25.0 mL of Cu(II) ion solution in several concentrations and previ-
ously buffered to the optimum adsorption pH. After this procedure,
they were kept under stirring until the adsorption equilibrium was
reached. Aliquots were taken and diluted in order to determine
metal ion concentration by AAS. It was observed that the adsorp-
tion process was fast and reached equilibrium after 2 h and
remained constant for 24 h.
2.5. Instrumentation
Elemental analyses were carried out on a Carlo Erba model 1106
elemental analyzer. Fourier transform infrared spectra (FTIR) were

K. R. Krishnapriya, M. Kandaswamy / Carbohydrate Research 345 (2010) 2013–2022
2015
singlets in the region around 3.80–3.95 ppm are observed due to
methoxy and benzylic protons. Broad singlet is obtained around
2.60 ppm for piperazine N–CH2 and N–CH3 protons of L. The 13C
NMR spectrum of the compound showed peaks in the region
around 45, 52–54, 65, 109–154, and 190 ppm indicating the pres-
ence of N–CH2, Ar-CH2, O–CH3, aromatic(C), and CHO carbons,
respectively.25
3. Results and discussion
3.1. Synthesis and characterization of vanillin-based ligand
The vanillin-based ligand or complexing agent L was prepared
by the Mannich base reaction of 4-hydroxy-3-methoxy benzalde-
hyde (vanillin) with equivalent amounts of formaldehyde and sec-
ondary amine namely N-methyl piperazine as shown in Scheme 1.
The compound was characterized by FTIR and 1H and 13C NMR
spectral techniques. The FT-IR spectra of the compound exhibit a
3.2. X-ray diffraction studies
band around 3410–3420 cmꢀ1 due to the stretching of
group. The band around 1640 cmꢀ1 is observed due to the presence
of (C@O) of aldehyde group of the compound. The band around
1590 cmꢀ1 is appeared due to the presence of
(C@C) stretching
of aromatic group. The band around 1470 cmꢀ1 indicates the
m(–OH)
The molecular formula of L is C14H20N2O3. The compound crys-
tallizes in the monoclinic space group P21/n with one molecule
in the unit cell. The dimension of the unit cell is a = 14.4364(7) Å,
b = 6.0102(3) Å, c = 16.5733(8) Å, b = 107.13(1)°, Z = 4. The molecu-
lar structure was solved by direct methods and refined by full-ma-
trix least-squares procedures. SHELXS97 and SHELXL97 programs are
used for structure solution and structure refinement,26 respec-
tively. The crystal structure of L is as shown in Figure 1 and the
crystal data are given in Table 1. CCDC No.: 673897.
m
m
m
(–O–CH3) bending of methoxy methyl group of the compound.
The 1H NMR spectrum of the compound exhibits a singlet peak
around 9.77 ppm due to the presence of aldehydic proton. The
peak at 7.17 ppm appeared due to the two aromatic protons. The
3.3. Synthesis and characterization of CTSL
CHO
The new chitosan derivative CTSL was obtained by condensa-
tion of chitosan with equivalent amounts (glucosamine units) of
vanillin-based ligand L. The reaction between aldehyde group from
ligand and amine groups from chitosan provides Schiff base forma-
tion in the polymeric structure as shown in Scheme 2. The ligand
introduces coordinating groups into the chitosan, such as phenolic
group, imine group, and piperazine unit, conferring metal ion che-
lation properties to the new adsorbent. Hence, the new adsorbent
has a special characteristic, in that it contains ligand capable of
coordinating to metal ions in solution anchored to them.
CHO
H
N
EtOH
HCHO
+
+
O
N
O
OH
N
CH3
CH3
OH
CH3
N
CH3
(L)
3.3.1. Elemental analysis
The C/N ratio from the elemental analysis data of chitosan and
its new derivative CTSL are found to be 6.05 and 7.10, respectively.
Scheme 1. Synthesis of new vanillin-based ligand (L).
Figure 1. Single crystal ORTEP diagram of L.

2016
K. R. Krishnapriya, M. Kandaswamy / Carbohydrate Research 345 (2010) 2013–2022
Table 1
tion of O–H superimposed to the N–H stretching band and the inter
Crystal data and structure refinement for L
hydrogen bonds of the polysaccharide. The band due to C–H stretch-
ing appears at 2929 cmꢀ1. The band at 1637 cmꢀ1 indicates the
presence of CTS-NHAc (acetyl) units (with >C@O stretching). The
bands at 1144 cmꢀ1 (asymmetric stretching of the C–O–C bridge),
1067, and 1031 cmꢀ1 (skeletal vibrations involving C–O stretching)
are characteristic of chitosan’s saccharide structure.29–32 The FT-IR
spectrum of the chitosan derivative shows an additional peak
around 3420 cmꢀ1 for phenolic –OH group apart from a strong
peak at 3440–3470 cmꢀ1 for CTS, corresponding to the stretching
vibration of –N–H group and –OH group. The splitting of the band
in the region around 1638 cmꢀ1 indicates the presence of imine
(>C@N) group in the derivative. The band at 1067 cmꢀ1 assigned
to the stretching vibration of secondary >C–OH group shifts toward
lower wavelength 1033 cmꢀ1, which may be due to steric effect
resulting from the condensation of ligand to CTS. Further, peaks
around 1400 cmꢀ1 are due to aromatic backbone vibration and a
peak around 1250 cmꢀ1 is due to phenolic C–O group of the ligand
moiety.28,30 The high-resolution solid state 13C NMR is used for
characterization of new adsorbent and the 13C NMR spectra of
CTS and CTSL are shown in Figure 3a and b, respectively. When
compared to CTS, the 13C NMR spectrum of CTSL shows additional
peaks in the region 120–180 ppm which are attributed to the
aromatic carbon atoms present in the chitosan derivative. The
difference in the nature and intensity of peaks in the region
20–110 ppm of CTSL when compared to CTS25,26 indicates that
the additional aliphatic carbon atoms are present in the derivative.
Empirical formula
C14H20N2O3
CCDC No.
673897
264.32
293(2)
0.71073
Monoclinic
P21/n
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
14.4364(7)
6.0102(3)
16.5733(8)
90.00
107.13(1)
90.00
1397.21(12)
4
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
Volume (Å3)
Z
Density (calcd) (Mg/m3)
Absorption coefficient (mmꢀ1
F(0 0 0)
1.257
0.089
568
)
Crystal size (mm)
Index ranges
0.24 ꢁ 0.20 ꢁ 0.20
ꢀ21 6 h 6 21, ꢀ8 6 k 6 8, ꢀ24 6 l 6 24
19354
Reflections collected
Independent reflections
Completeness to 2h
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
4806 [R(int) = 0.0247]
99.8%
Full-matrix least-squares on F2
4806/0/185
1.042
Final R indices [I > 2
R indices (all data)
r
(I)]
R1 = 0.0501, wR2 = 0.1456
R1 = 0.0741, wR2 = 0.1638
0.265 and ꢀ0.193
Largest diff. peak and holes (e Åꢀ3
)
3.3.3. Thermal analysis
The TG-DTA curves of CTS and CTSL are shown in Figure 4a and
b, respectively. The chitosan shows thermal degradation at two
stages, one at 73 °C with a 22% loss and another at 324 °C with a
41% loss in polymer mass. The thermal degradation of CTSL occurs
at two different ranges of temperatures namely 65 °C and 306 °C
with different percentage weight loss of 19% and 37%, respectively.
The first one is due to water elimination which adsorbed physically
to the biopolymer matrix and the second one may be due to the
dehydration of the saccharide rings, depolymerization, and decom-
position of the polymer. The main thermal degradation process of
CTSL takes place at temperature lower than the secondary degra-
dation stage of chitosan indicating that the CTSL polymer is less
stable than the chitosan. It seems that the instability of the CTSL
polymer compared to the chitosan may be due to the decrease in
The degree of substitution (DS) of the ligand, L to –NH2 group on
chitosan is calculated by the following equation:27
aðC=NÞm ꢀ ðC=NÞo
DS ¼
ð3Þ
n
where (C/N)m is the C/N ratio of the modified chitosan derivative, (C/
N)o is the C/N ratio of the original chitosan, and ‘a’ and ‘n’ are the
number of nitrogen and carbon introduced after chitosan modifica-
tion, respectively. The DS obtained for CTSL is 0.54.
3.3.2. FT-IR and solid state 13C NMR analysis
The FT-IR spectra of the CTS and CTSL are shown in Figure 2a
and b, respectively. The FT-IR spectrum of CTS exhibits a strong
peak at 3446 cmꢀ1 which can be assigned to axial stretching vibra-
OH
O
O
HO
CHO
N
OH
O
H
n
C
AcOH:MeOH (v/v, 1:6)
O
HO
O
+
OH
N
CH3
NH2
n
O
CH3
OH
(CTS)
N
N
CH3
(L)
N
CH3
(CTSL)
Scheme 2. Synthesis of new chitosan biopolymer derivative (CTSL).

K. R. Krishnapriya, M. Kandaswamy / Carbohydrate Research 345 (2010) 2013–2022
2017
Figure 2. The FTIR spectra of (a) CTS and (b) CTSL.
the number of primary amino groups after chemical modification
process,29 whereas, the DSC thermogram for chitosan24 showed a
wide endothermic peak at 111 °C and an exothermic peak at
305 °C. The DSC thermograms for the chitosan and its derivative
are given in Figure 5a and b, respectively. The chitosan derivative
exhibits endothermic peaks in the region 72 °C and exothermic
peaks in the region 325 °C. It is well known that for chitosan and
its derivatives the endothermic peaks observed for water vapor
elimination in the polymer matrix and the exothermic peaks ob-
served for the decomposition of the polymer29 and these differ-
ences in thermograms of chitosan and its derivative may be
caused by the chemical modification.32
(0 0 2). The studies indicate that the intensity of peaks at
2h = 10 °C and 20 °C of CTSL decreases also with the broadening of
peaks when compared to free chitosan. This indicates that the crys-
tallinity of modified chitosan derivative decreases. This decrease in
crystallinity of chitosan derivative is attributed to the deformation
of the strong hydrogen bond in the free chitosan molecule and also
due to the substitution of ligand on it. This indicates that CTSL is less
crystalline and more amorphous than free chitosan.32,34
3.3.5. SEM analysis
The difference in structural morphology between chitosan and
CTSL is further supported by the difference in their SEM images.
The SEM images of the surface of CTS and its derivative are shown
in Figure 7a and b, respectively. The chitosan derivative displayed
a more extensive three-dimensional network compared to the
smooth lacunose surface of chitosan and it is attributed to the
condensation of vanillin-based complexing agent on polymer
surface.29,33
3.3.4. Powder X-ray diffraction studies
The powder X-ray diffraction patterns of CTS and CTSL are given
in Figure 6a and b, respectively. The CTS shows the characteristic
sharp peak at 2h = 10 °C, due to the presence of (0 0 1) and (1 0 0)
and that at 2h = 20 °C caused by the presence of (1 0 1) and

2018
K. R. Krishnapriya, M. Kandaswamy / Carbohydrate Research 345 (2010) 2013–2022
we infer that the adsorption capacity of modified chitosan deriva-
tive is affected by the pH. The metal(II) ion adsorption by CTSL in-
creases with the pH of the solution from 3.5 to a maximum value of
6.5 and then decreases with further increase in pH. At acidic pH
3.5–5.5, the lower adsorption of metal ions is attributed to the pro-
tonation of the complexation site (amine), which decreases the
complex formation by the chitosan derivative with metal ion.36,37
Around pH 6.5–7.0 the amino group exists as free amine (without
protonation) and in addition, the deprotonation takes places at
phenolic group present in the chitosan derivative which increases
the metal ion uptake by the adsorbent. The buffer solution,
tris(hydroxymethyl) aminomethane/HCl used for pH 7.5 and 8.5,
can form complexes with the metal ion in order to avoid precipita-
tion of metal hydroxide. This complex formation decreases the me-
tal ion uptake by the CTSL. From the studies, we found that the
binding capacities of the derivative for Mn, Fe, Co, Cu, Ni, Cd, and
Pb were 19.8, 18.4, 22.4, 56.5, 34.6, 46.1, and 51.8 mg/g,
respectively, and these values are higher than the values obtained
for unmodified chitosan.10,17,18,21 The results also indicate that the
new adsorbent has a good adsorption capacity for Cu(II) ions, in the
pH ranges studied, suggesting possible selectivity for this metal ion
and thus following the order of binding capacity in mmol gꢀ1 for
the derivative as Cu(II) > Ni(II) > Cd(II) P Co P Mn(II) > Fe(II) > P-
b(II). This sequence maintained the trend in the well-known Ir-
vin-Williams series.37,38 The CTSL forms more stable complex
with copper(II) ions in aqueous solution than other metal ions
studied and hence shows maximum adsorption toward Cu(II) ions
(see Fig. 9).
3.5.2. Adsorption kinetics
Kinetics of adsorption is one of the most important character-
istics to be responsible for the efficiency of adsorption. Tradition-
ally, the kinetics of interaction at solid/solution interface is
described following the expressions originally given by Lagergren
which are special cases for the general Langmuir rate equation.39
The fitting validity of these models is traditionally checked by
the pseudo first order and pseudo second order models to inter-
pret the experimental data. A good correlation of the kinetics
data explains the adsorption mechanism of the metal ion on
the solid phase.
Figure 3. The solid state 13C NMR spectra of (a) CTS and (b) CTSL.
3.4. Preparation and characterization of metal-CTSL
The pseudo first order equation was represented by Eq. 4:
k1
2:303
The metal complexes of the chitosan derivative were prepared
by the adsorption of one mole of metal(II) ions such as Mn(II),
Fe(II), Co(II), Cu(II), Ni(II), Cd(II), and Pb(II) ions on equimolar chito-
san derivative (glucosamine unit) in aqueous solution (pH 7.5). The
proposed schematic representation of structure for the metal-com-
plexed chitosan derivative [M-(CTSL)] is shown in Scheme 3. The
resulting [M-(CTSL)] is characterized using FTIR spectral analysis
and SEM analysis. The FTIR spectra of [Cu-(CTSL)] when examined
showed adsorption bands at 514 and 462 cmꢀ1 due to stretching
vibration of N–Cu and O–Cu, respectively. Further, new bands near
logðQe ꢀ QtÞ ¼ logQe ꢀ
t
ð4Þ
where k1 (minꢀ1) is the pseudo first order adsorption rate constant,
Qt is the amount adsorbed at time t (min), and Qe denotes the
amount adsorbed at equilibrium, both in mg gꢀ1. The plots of log
(Qe ꢀ Qt) versus t gave the k1 and Qe values.
The pseudo second order equation, based on adsorption capac-
ity at equilibrium, can be expressed by Eq. 5:
t
1
k2Q2
1
¼
þ
t
ð5Þ
600–500 cmꢀ1 indicated the presence of SO4 ions in the resulting
2ꢀ
Qt
Qe
e
copper complex of chitosan derivative.
where k2 (g mgꢀ1 minꢀ1) is the adsorption rate constant of pseudo
second order and Qe can be obtained from the intercept and slope
of the plot of (t/Qt) versus t.
The adsorption kinetics studies for CTSL toward Cu(II) show the
first order and second order rate constants k1 and k2 to be1.8 ꢁ 10ꢀ2
minꢀ1 (Qe, cal = 29.1 mg gꢀ1with R = 0.934) and 1.4 ꢁ 10ꢀ3 g mgꢀ1
minꢀ1 (Qe, cal = 58.1 mg gꢀ1with R = 0.999), respectively. Based on
the obtained correlation coefficients R, and the experimental and
calculated Qe values, the pseudo second order equation was the
model that showed the best fit for the experimental kinetics data
and not the pseudo first order kinetics equation. This indicates that
The SEM images for [Cu-(CTSL)] and [Co-(CTSL)] are shown in
Figure 7c and d, respectively. It is observed that the surface of
the metal-complexed derivatives exhibits more pores when
compared to CTS or its derivative which may be due to the metal
sites coordinated to chitosan derivative.33–35
3.5. Adsorption studies
3.5.1. Effect of pH on adsorption
The effect of pH on adsorption of metal ions by vanillin-based
chitosan derivative CTSL is illustrated in Figure 8. From this study,

K. R. Krishnapriya, M. Kandaswamy / Carbohydrate Research 345 (2010) 2013–2022
2019
a
b
120
110
100
90
0.03
0.02
0.01
0.00
-0.01
57.26°C
94.64°C
80
38.81%
(1.507mg)
70
265.32°C
60
81.23°C
50
26.01%
(1.010mg)
40
Residue:
24.81%
(0.9633mg)
30
352.59°C
20
34.96°C
10
62.90°C
100
0
0
200
300
400
500
600
700
800
Exo Up
Temperature (°C)
Universal V4.1D TA Instruments
Figure 4. The TG-DTA curves of (a) CTS and (b) CTSL.
chemical sorption is the rate-limiting step of adsorption mechanism
and not involving a mass transfer in solution.39
capacity increased with the equilibrium concentration of the metal
ion in solution, progressively reaching saturation of adsorbent. The
following Langmuir equation 6 takes into account adsorption sites
with the same energy, where the formation of a monolayer occurs
on the surface of the adsorbent with saturation of the sites:
3.5.3. Adsorption isotherms
An adsorption isotherm, the relation between the amounts of
solute adsorbed (Qe) and the remaining concentrations in the aque-
ous phase (Ce), is basically important to describe how adsorbate
interacts with adsorbent and so is critical in optimizing the use
of sorbents. Therefore, correlation of equilibrium data by a theoret-
ical equation is necessary to practical operation. In the past, the
Langmuir equation39,40 has been used to correlate isotherm data
for the sorption of metal ions and organic solutes on to raw and
substituted chitosan derivative.
The equilibrium studies were carried out for this derivative to-
ward various metal(II) ions at the pH value of optimum adsorption
(pH 8.5) and with the necessary contact time to reach the adsorp-
tion equilibrium of each metal, it is found that the adsorption
KLCeQm
1 þ KLCe
Qe
¼
ð6Þ
where Qe and Ce are the amount adsorbed (mg gꢀ1) and the adsor-
bate concentration in solution (mg Lꢀ1), respectively, both at equi-
librium. KL (L gꢀ1) is the Langmuir constant and Qm (g mgꢀ1) is
the maximum adsorption capacity for monolayer formation on
adsorbent. The linear form of the Langmuir isotherm, represented
by Eq. 7, is employed to determine the KL and Qm values from linear
and angular coefficients obtained by:
Ce
1
Ce
¼
þ
ð7Þ
Qe KLQm Qm

2020
K. R. Krishnapriya, M. Kandaswamy / Carbohydrate Research 345 (2010) 2013–2022
Figure 5. The DSC thermogram of (a) CTS and (b) CTSL.
Figure 6. The powder X-ray diffraction patterns of (a) CTS and (b) CTSL.

K. R. Krishnapriya, M. Kandaswamy / Carbohydrate Research 345 (2010) 2013–2022
2021
Figure 7. SEM image of (a) CTS, (b) CTSL, (c) Cu-CTSL, and (d) Co-CTSL.
and chitosan derivative with mere vanillin which may be due to
polydentate moiety containing nitrogen and hydroxo donors being
present in it.
OH
O
N
O
HO
3.6. EPR studies
H
n
In order to interpret the complexation of modified chitosan
derivative with Cu(II) ions, EPR studies at room temperature on
the copper-adsorbed chitosan derivative were carried out. The
studies showed an axial EPR spectrum (Fig. 6) for copper-adsorbed
chitosan derivative [M-(CTSL)] typical to mononuclear metal–li-
gand [Cu(II)-(L)] complex at liquid nitrogen temperature41 and
the parameters were found to be gk ðg Þ as 2.26 (2.03) and A as
C
O
O
CH3
N
N
k
?
157.9 for [Cu-(CTSL)]. This is suggestive of a tetragonally distorted
octahedral, square pyramidal, or square-planar geometry having a
CH3
Cu(II)
1
(M(II)-CTSL)
Ni(II)
Co(II)
where,
= M(II) = Metal(II) ion
0.8
0.6
0.4
0.2
0
Mn(II)
Fe(II)
Cd(II)
Pb(II)
Scheme 3. Structure of metal complex of the new chitosan derivative.
The Qm values were calculated for CTSL according to Langmuir
isotherm model. The results obtained from adsorption isotherms
for various metal(II) ions according to the Langmuir model are gi-
ven in Table 2 and it shows good correlation coefficients (R >0.99)
with the experimental data from adsorption equilibrium of metal
ions, suggesting homogeneous adsorption, which means
a
monolayer.39
0
2
4
6
8
10
12
The nature and the number of coordination sites present in the
chitosan polymer influences the adsorption capacity of chitosan
derivative. From the present study, we infer that the anchoring of
chitosan with vanillin-based complexing agent enhances the
adsorption capacities of adsorbent when compared to free chitosan
pH
Figure 8. Effect of pH on metal(II) ion adsorption by CTSL. {[M2+] = 100 mg Lꢀ1
temperature = 25 °C; contact time = 2 h; shaking rate = 200 rpm; adsorbent
;
mass = 50 mg}.

2022
K. R. Krishnapriya, M. Kandaswamy / Carbohydrate Research 345 (2010) 2013–2022
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.carres.2010.06.005.
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K.R.K. acknowledges the Department of Science and Technology
(DST), New Delhi, India, for the financial assistance received
through DST-SERC Project SR/S1/IC-27/2004 sanctioned to M.K.