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Polymer Bulletin ISSN 0170-0839 Polym. Bull.DOI 10.1007/s00289-014-1172-8

Studies on the sorption capacity for Pb(II)and Hg(II) of citralidene chitosan

P. Alikutty, V. M. Abdul Mujeeb,M. A. Zubair, K. Muraleedharan &P. Mujeeb Rahman

1 23

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ORI GIN AL PA PER

Studies on the sorption capacity for Pb(II) and Hg(II)of citralidene chitosan

P. Alikutty • V. M. Abdul Mujeeb • M. A. Zubair •

K. Muraleedharan • P. Mujeeb Rahman

Received: 26 July 2013 / Revised: 19 November 2013 / Accepted: 8 April 2014

� Springer-Verlag Berlin Heidelberg 2014

Abstract Citralidene chitosan, prepared by condensation of citral and chitosan,

was characterized by infrared spectroscopy, scanning electron microscopy and

differential scanning calorimetry and evaluated for its Pb(II) and Hg(II) sorption

capacity. The contact time for Pb(II) and Hg(II) sorption was found to be 5 and 4 h,

respectively. The sorption data best fitted to pseudo second-order equation. The

equilibrium sorption data were found to be best fitted to Langmuir model. The

studies revealed that the citralidene chitosan has different binding sites and the

sorption was spontaneous and exothermic. Citralidene chitosan was found to be an

efficient and cheap sorbent for Pb(II) and Hg(II).

Keywords Citralidene chitosan � Langmuir model � Pseudo second-order

equation � Sorption capacity

Introduction

Chitosan, the linear and partly acetylated (1-4)-2-amino-2-deoxy b-D glucan, is a

nontoxic and biodegradable biopolymer produced by alkaline N-deacetylation of

marine chitin, the most abundant natural polymer after cellulose. Chitin is present in

the exoskeleton of crustaceans such as the crabs, prawns and shrimps, and in the

cuticles of insects and the cell walls of most fungi, and is a cheap resource available

in seafood industries [1]. Chitosan is soluble in acid pH range but insoluble in the

neutral or alkaline media [2].

P. Alikutty � V. M. Abdul Mujeeb � K. Muraleedharan (&) � P. Mujeeb Rahman

Depatment of Chemistry, University of Calicut, Malappuram 673635, India

e-mail: kmuralika@gmail.com

M. A. Zubair

Depatment of Chemistry, PSMO College, Tirurangadi, Kerala, India

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DOI 10.1007/s00289-014-1172-8

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As a cheap natural and renewable resource, chitosan and its derivatives posses

unique properties such as biocompatibility, biodegradability and film forming

ability and has many applications in biomedicine, agriculture, environmental

protection and biotechnology. Chitin and chitosan derivatives have immense

potential for purification of water and waste water [3]. Chitosan-based sorbents have

exhibited relatively high sorption capacities for heavy metals due to their high

nitrogen content and porosity [4–6].

Citral is 3,7-dimethyl-2,6-octa dienal, an open-chain terpene aldehyde and

present in the oil of lemon grass (70–80 %), orange, lemon and citronella. It is a

mixture of two stereo isomers: geranial (trans) and neral (cis). Citral is used in

cosmetics, scents and aroma therapy and as a flavouring agent.

Heavy metals are not biodegradable and tend to accumulate in living organisms,

causing various diseases and disorders. Lead and mercury are the oldest metals

known to man and may enter the environment at any point during mining, refining,

geological erosion, manufacturing processes and through industrial wastes. Lead

interferes with the metabolism and action of essential metals particularly Ca, Fe and

Zn. It accumulates in the bone and is harmful mainly through its neurological

effects. Mercury is one of the most toxic metals. Its toxicity is related to the capacity

of its compounds to bioconcentrate in organisms and to biomagnify through food

chain. It affects the gastrointestinal mucous membrane, kidney and nervous system

[7, 8]. Chitosan has been widely used for the removal of heavy metals from neutral

and alkaline solutions. Various functional groups present in chitosan molecule (like

OH and NH2) can co-ordinate and chelate metal cations from aqueous effluents.

Sorption can be enhanced by physical and chemical modification of the polymer.

OH and NH2 functions can be utilized for chemical modification, by esterification,

condensation and other chemical reactions. Since chitosan is soluble in acidic

media, attempts have been made by various researchers to chemically modify

chitosan so that the derivatives can be used in acidic effluents as sorbents. Several

sorption studies had been carried out on physically and chemically modified

chitosan. Pauline et al. [9] have studied the removal of Pb(II) and Ni(II) using

chitosan derived from silkworm chrysalides. Ng et al. [10] carried out equilibrium

studies for the sorption of Pb(II) from effluents. Yawo et al. [11] studied the sorption

capacity of chitosan cellulose beads for Cu(II), Fe(III) and Ni(II). Krishnapriya and

Kandaswamy [12] studied on the metal complexing ability of chitosan derivatives.

Muniyappan and Meenakshi [13] prepared silica gel–chitosan composites and used

it for the removal of Cu(II) and Pb(II). Wan et al. described in their review the

adsorption behaviour of metal ions on several physical and chemical modifications

of chitosan [14]. Miretzky et al. have reviewed sorption behaviour of Hg(II) ions on

several chemically modified chitosan [15]. Amit and Milka [3] in their review

described the behaviour of several metal ions on chitosan derivatives. In the present

study, we have prepared a Schiff base from chitosan and citral, Citralidene chitosan

(CIT-chitosan), with a view to check its potential for metal cation removal from

aqueous solutions. The amino group at C-2 position of chitosan can condense with

aldehyde group to provide this chemical modification. Both chitosan and citral are

naturally occurring and biodegradable. The use of Schiff base as sorbent also has

environmental significance [16, 17]. The CIT-chitosan was characterized by

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scanning electron microscopy (SEM), FTIR and differential scanning calorimetry

(DSC) and evaluated for its Pb(II) and Hg(II) sorption capacity. We have conducted

sorption experiments for the optimization of parameters like contact time, sorbent

dose and pH, and also carried out the kinetic and equilibrium studies. The

experimental data were applied to Langmuir and Freundlich models and kinetic and

thermodynamic parameters have been evaluated to asses various aspects of metal

sorption mechanism [18–20].

Experimental

Materials

Chitosan flakes (Brookfield viscosity [200,000 cps) with 85 % degree of

deacetylation and citral were purchased from Sigma Aldrich Co; USA and used

as such. Lead acetate, mercury acetate, methanol and acetic acid were Merck (India)

grade. All other chemicals used were of analytical grade (Assay [99.9 %). All

reagents were prepared in deionized water.

Methods

Instrumental

The FTIR spectra of the samples were recorded by diluting in KBr pellets using a

Jasco-made Fourier Transform IR Spectrometer (Model: Jasco FTIR 4108). The

SEM images of chitosan and the CIT-chitosan were taken using a Hitachi-made

field emission scanning electron microscope (Model: Hitachi SU—6600 FESEM).

The DSC experiments were carried out in a Perkin Elmer-made (Model: DSC 4000)

instrument in the temperature range 30–375 �C under an atmosphere of flowing

nitrogen. The operational characteristics of the DSC system are flow rate of

nitrogen: 20 mL min-1; sample size: 5 mg; heating rate: 10 �C min-1 and sample

pan: aluminium. The pH measurements were carried out in a Systronics-made pH

meter. The aqueous Pb(II) ion concentration was analyzed with a fast sequential

atomic absorption spectrometer (AAS) (Model: AA240FS) using air–acetylene

flame at 217 nm with a slit width of 1 nm. The Hg(II) concentration was analyzed

with a Shimadzu-made AAS instrument (Model: AA-6300) with slit width of

0.7 nm and wave length of 253.7 nm.

Preparation of citralidene chitosan

The Schiff base was prepared by the condensation reaction of chitosan and citral.

2 g of chitosan (particle size *250 lm) was dissolved in 50 mL of 5 % acetic acid

(v/v), 1.8 mL citral dissolved in 50 mL of methanol was added to the viscous

chitosan solution, stirred for 8 h at room temperature using a magnetic stirrer, kept

overnight, filtered and washed with methanol. Removed methanol and acetic acid by

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vacuum distillation of the reaction mixture and dried at 50 �C for 24 h. The Schiff

base, citralidene chitosan, obtained were powdered and stored in a vacuum

desiccator.

Sorption experiments

The powdered citralidine chitosan (CIT-chitosan) was evaluated for Pb(II) and

Hg(II) sorption. Stock solutions of Pb(II) and Hg(II) (500 mg L-1) were prepared

from lead(II) acetate and mercury(II) acetate using deionized water. All other

concentrations were prepared from these solutions by dilution. Optimization of

parameters like contact time, sorbent dose and pH for sorption of Pb(II) and Hg(II)

was carried out. The effect of contact time on sorption capacity of the CIT-chitosan

was studied in the range of 1–8 h at initial concentrations of 400 mg L-1 of Pb(II)

at a pH value of 4 and Hg(II) at a pH value of 6 with sorbent dose of 25 mg. The

effect of sorbent dose on sorption capacity was studied by varying mass of the CIT-

chitosan from 25 to 100 mg at an initial Pb(II) and Hg(II) concentrations of

400 mg L-1 (25 mL). For pH studies, 25 mL of 400 mg L-1 Pb(II) solution with a

sorbent dose of 25 mg was stirred for 5 h and Hg(II) for 4 h. The pH was changed

from 1 to 7 by adding concentrated HNO3 and NaOH, and was determined using a

pH meter. All sorption experiments were carried out by stirring 25 mL of Pb(II)

solution containing 25 mg of CIT-chitosan, taken in a 125-mL stoppered bottle

made of borosilicate glass at a pH value of 4 for 5 h using a magnetic stirrer at

moderate speed. For mercury sorption, 25 mL of Hg(II) solution at a pH value of 6

was stirred for 4 h with 25 mg of the sorbent. Each experiment was duplicated

under identical conditions.

For kinetic studies, 400 mg L-1 of Pb(II) and Hg(II) solutions were stirred

separately for 8 h at room temperature (30 �C). 1 mL of the sample was withdrawn

in each hour, diluted and concentrations were determined (qt). The equilibrium

studies were conducted separately for Pb(II) and Hg(II) solutions of different initial

concentrations ranging from 100 to 500 mg L-1. All solutions were diluted properly

to put down in the working range of AAS. Thermodynamic parameters were

determined by conducting the sorption experiments with 400 mg L-1 Pb(II) and

Hg(II) solutions at four different temperatures, viz. 30, 40, 50 and 60 �C.

The amount of sorption at equilibrium qe (mg g-1) was obtained from the equation

qe ¼ Ci � Ceð ÞV½ �=W ð1Þ

where Ci is the initial metal ion concentration in the aqueous phase in mg L-1, V is

the volume of the solution (L) and W is the weight of the CIT-chitosan used (g).

Results and discussion

Characterization of citralidene chitosan

In the Schiff base formation, the amino group of chitosan condenses with the

aldehyde group of citral. The biopolymer Schiff base formed was brownish yellow

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in colour, stable in air and insoluble in common organic solvents such as benzene,

methanol, DMF and DMSO. It was insoluble in mineral acids such as HCl and

HNO3 and in organic acids like acetic acid which shows the absence of free amino

group. The FTIR spectroscopy and SEM were used to confirm the structure of the

Schiff base. Figure 1 shows the FTIR spectra of chitosan (a), CIT-chitosan (b) and

CIT-chitosan loaded with Pb(II) (c) and Hg(II) (d). The band around 3,415 cm-1

corresponding to OH and NH stretching vibration in chitosan was shifted to higher

frequency in the spectra of CIT-chitosan. Both the spectra exhibit the absorption

peaks around 1,153, 1,100, 1,020 and 896 cm-1 which can be assigned to

saccharide moiety. In the FTIR spectra of the CIT-chitosan, new absorption peaks

appear at 1,648.84 and 1,612.2 cm-1. The former represent the C=N stretching

vibration of imine group and the latter C=C stretching vibration of citral moiety.

The band at 1,426.1 cm-1 corresponding to C–N axial deform has shifted to

1,451.17 cm-1 which also indicate the formation of C=N bond in the Schiff base.

The FTIR spectra of Pb(II) loaded CIT-chitosan indicated the binding of Pb(II)

through oxygen of CH–OH. The peak at 1,377.89 cm-1 is due to CH bending of

CH–OH group in the Schiff base shifted to higher frequency (1,383.6 cm-1) in the

Pb(II) loaded CIT-chitosan. The shifting of the band 3,423.03 cm-1 corresponding

to OH and NH2 has also shifted to higher frequency (3,428.81 cm-1) indicating the

binding through N and O. However, oxygen atoms are not involved in binding

60

80

100

4000 3500 3000 2500 2000 1500 1000 500

60

80

100

4000 3500 3000 2500 2000 1500 1000 500

aT

rasm

ittan

ce %

c

b

Tra

smitt

ance

%

Wavenumber (cm-1)

d

Wavenumber (cm-1)

Fig. 1 FTIR spectra of chitosan (a), CIT-chotosan (b), CIT-chitosan loaded with Pb(II) (c) and CIT-chitosan loaded with Hg(II) (d)

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Hg(II) ions. The change in frequency of C=C indicates the involvement of pi-bond

in binding Pb(II)and Hg(II) ions. The shifting of the band at 1,451.7 cm-1 in the

Schiff base to higher frequency indicates the involvement of C=N in binding metal

ions [13, 21, 22].

The SEM images of chitosan (a) and the CIT-chitosan (b) are shown in the Fig. 2,

which shows that the surface morphology of the CIT-chitosan is different from that

of chitosan. The CIT-chitosan was more porous and smooth and is capable of

accommodating metal ions.

Chitosan and the CIT-chitosan were subjected to DSC studies under nitrogen

atmosphere in the temperature range 30–375 �C at a heating rate of 10 �C min-1.

The DSC curves of chitosan and CIT-chitosan are shown in Fig. 3; two major

thermal events were observed-first an endotherm (respectively at 89.7 and 78 �C)

followed by an exotherm (respectively at 306.2 and 290.6 �C). The DH values for

the endotherms of chitosan and CIT-chitosan are respectively 394.4 and 296.6 J g-1

and -296.3 and -224.1 J g-1 for the exotherms. The endothermic peaks were

related to the evaporation of absorbed gases present in the sample. Polysaccharides

usually have a strong affinity for water and in the solid state it has a disordered

structure so that it can be easily hydrated [23]. Comparison of peaks shows that

differences in peak area (and hence DH) and position of peak temperatures of

endotherms indicate that chitosan and CIT-chitosan differ in their water holding

capacity which reflects physical and molecular changes during Schiff base

formation. DH values are higher for chitosan because it contains free amino

groups. The second thermal events registered in DSC were exothermic in nature

which is connected to the degradation of the polymer. The peak in CIT-chitosan was

shifted to lower temperature which was attributed to decrease in thermal stability of

Schiff base. Differences in exothermic transitions occurred due to difference in

chemical and structural characteristics in chitosan and Schiff base.

Effect of contact time

Sorption capacity of Schiff base was determined by varying the contact time from 0

to 8 h. Effect of contact time for 400 mg L-1 of Pb(II)/Hg(II) on 0.025 g CIT-

Fig. 2 SEM images of chitosan (a) and CIT-chotosan (b)

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chitosan at 30 �C shows that sorption capacity increased sharply during the first

hour, after that a slow increase was observed until reached saturation in 5 h for

Pb(II) and 4 h for Hg(II) sorption (Fig. 4). The sorption capacity of the Schiff base

in 5 h was 237.5 mg g-1 for Pb(II) and 294.1 mg g-1 in 4 h for Hg(II).

Effect of sorbent dose

Sorption experiments by varying adsorbent dose from 0.025 to 0.1 g show that

sorption capacity decreased as weight of the sorbent increased. The sorbent dose

100 200 300 400

20

24

28

12

16

20

24

28

b

T (C)

a

- H

eat f

low

(µW

)

Fig. 3 DSC curve of chitosan (a) and CIT-chitosan (b)

0 2 4 6 8240

260

280

300

160

200

240

Hg(II)

Sor

ptio

n C

apac

ity (

mg

g-1)

Contact time (h)

Pb(II)

Fig. 4 Effect of contact time for 400 mg L-1 M(II) on 0.025 g CIT-chitosan at 303 K

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study on sorption capacity of CIT-chitosan from 400 mg L-1 M(II) solution at

30 �C shows that maximum sorption occurred were [237.5 and 294.51 mg g-1

respectively for Pb(II) and Hg(II)] when sorbent dose was 0.025 g. Figure 5 shows

this variation.

Effect of pH

The metal ion removal from aqueous solutions is very much dependent on solution

pH. Effluents from industries and urban discharges are having very high or low pH.

So pH study on removal of metal ions is significant. We evaluated the sorption

capacity of Schiff base for the sorption of Pb(II) and Hg(II) at different pH values

ranging from 1 to 7 at 303 K. Maximum sorption observed was at a pH value of 4

for Pb(II) (237.5 mg g-1) and at a pH value of 6 for Hg(II) (294.51 mg g-1). Very

low pH restricts the number of binding sites for sorption. All the sorption

experiments were carried out at these optimum pH values. The abnormal values of

sorption capacity observed above a pH value of 7 were attributed to the probable

reaction of metal ions with NaOH. So application of CIT-chitosan for the sorption

of Pb(II) and Hg(II) was limited to acidic medium. Dependence of sorption capacity

on pH is shown in Fig. 6.

Sorption kinetics

The kinetics of sorption on CIT-chitosan indicated a rapid initial binding followed

by a slow increase until a state of equilibrium was reached in 5 h for Pb(II) and 4 h

for Hg(II) sorption. Sorption data were best fitted to a pseudo second-order kinetic

model [24].

0.02 0.04 0.06 0.08 0.1075

150

225

300

80

160

240

Hg(II)

Sor

ptio

n C

apac

ity (

mg

g-1)

Amount of sorbent (g)

Pb(II)

Fig. 5 Effect of sorbent dose on sorption capacity of CIT-chitosan from 400 mg L-1 M(II) solution at303 K

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t

qt

¼ 1

k2q2e

þ t

qe

ð2Þ

where k2 is the pseudo second-order rate constant (g mg-1 h-1), qe and qt are the

amount of metal ion sorbed (mg g-1) at equilibrium and at time t respectively. A

linear least-squares plot of tqt

against t gave a straight line with slope 1qe

and k2 as

evaluated from the intercept. Figure 7 shows the pseudo second-order plot for the

sorption of Pb(II) and Hg(II) on CIT-chitosan. The rate constant was found to be

0.06944 g mg-1 h-1 for Pb(II) and 0.007392 g mg-1 h-1 for Hg(II). (Lagergren

first-order kinetic model was also applied to the experimental data but fitted with

lower regression values, the first-order rate constant being 1.9191 and 1.6632 h-1

for Pb(II) and Hg(II), respectively.)

Sorption isotherm

The two most commonly used isotherms namely Freundlich and Langmuir

isotherms have been adopted to quantify the sorption capacity of CIT-chitosan

for Pb(II) and Hg(II) sorption, the data were fitted to both the isotherms. The

logarithmic form of Freundlich equation is given in Eq. (3)

log qe ¼ log Kf þ1

nlog Ce ð3Þ

where qe is the amount of metal ion sorbed in mg per gram of sorbent, Ce (mg L-1)

is the equilibrium concentration of metal ion solution, Kf (mg g-1 L1n) is Freundlich

constant that gives a measure of sorbent capacity and 1/n gives a measure of

intensity of sorption. Freundlich sorption isotherm (log Ce vs log qe) for Pb(II) and

1 2 3 4 5 6 7

200

240

280

100

150

200

250

Hg(II)

Sor

ptio

n C

apac

ity (

mg

g-1)

pH

Pb(II)

Fig. 6 Influence of pH on the sorption capacity of CIT-chitosan for M(II) at 303 K. Initial M(II)concentration: 400 mg L-1

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Hg(II) sorption on CIT-chitosan is shown in Fig. 8. Kf and 1/n are represented by the

intercept and slope of the plot and were found to be 151.4 mg g-1 L1n and 0.25

respectively for Pb(II) and 81.28 mg g-1 L1n and 0.266 for Hg(II). The fitting of the

experimental data with Freundlich model indicates the heterogeneity of Schiff base

surface and the presence of different binding sites. The Hg(II) sorption equilibrium

can be best explained by Freundlich model.

The linearized Langmuir isotherm equation is shown below

Ce

qe

¼ 1

Q � bþ1

QCe ð4Þ

where qe is the amount of solute sorbed (mg g-1) at equilibrium and Ce is the

equilibrium concentration (mg L-1), the values of the empirical constants Q and b

denote monolayer sorption capacity and energy of sorption respectively and were

calculated from the slope and intercept of the plot between Ce and Ce/qe (Fig. 9).

The Langmuir fitting indicates the monolayer adsorption. From the plot, Q, the

maximum monolayer sorption capacity was found to be 250 mg g-1 and Langmuir

constant b = 0.0222 L mg-1 for Pb(II) and the corresponding values for Hg(II) are

333.33 mg g-1, and 0.075 L mg-1. Langmuir equilibrium parameter (dimension-

less constant separation factor) RL has also been determined.

RL ¼ 1= 1þ bð ÞC0½ � ð5Þ

where b is the Langmuir constant and C0 is the initial concentration [25]. RL values

lie between 0 and 1 for all the five initial concentrations which indicates that CIT-

chitosan is a favourable adsorbent for both the metals. The present study reveals that

the sorption capacity of CIT-chitosan for Pb(ll) and Hg(II) sorption were higher than

that on some other forms of chitosan and derivatives [3, 5, 14, 15].

0 1 2 3 4 5 6 7 8 90.00

0.01

0.02

0.03

0.01

0.02

0.03

Hg(II)

t / q

t

t (h)

Pb(II)

t / q

t

Fig. 7 Pseudo second-order plot for the sorption of Pb(II) and Hg(II) on CIT-chitosan

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Thermodynamic parameters

Van’t Hoff equation (6) could be used to evaluate the thermodynamic parameters

DS0; DH0 and DG0 and mechanism of adsorption.

log Kc ¼DS0

2:303R� DH0

2:303RTð6Þ

0.4 0.8 1.2 1.6 2.0 2.4

2.1

2.4

2.71.8 2.1 2.4 2.7

2.28

2.34

2.40

Hg(II)

log

q e

log ce

Pb(II)

log

q e

Fig. 8 Freundlich sorption isotherm for Pb(II) and Hg(II) sorption on CIT-chitosan

0 30 60 90 120 150 1800.0

0.2

0.4

0.60 100 200 300 400 500

0.4

0.8

1.2

1.6

2.0

Hg(II)

Ce

/ qe

Ce

/ qe

Ce

Pb(II)

Fig. 9 Langmuir sorption isotherm plot for Pb(II) and Hg(II) on CIT-chitosan

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The value of DG0 was evaluated using Eq. (7)

DG0 ¼ �2:303RT log Kc ð7Þ

where R is the gas constant, T the temperature in Kelvin and Kc is the equilibrium

constant. Kc was determined using the equation

Kc ¼CA

Ce

ð8Þ

where CA (mg L-1) is the amount of solute adsorbed by adsorbent at equilibrium

concentration (g L-1)

A plot of 1T

vs log Kc is linear (Fig. 10). DH0 was calculated from slope and DS0

from the intercept [26]. The values are shown in Table 1. The negative values for

free energy and enthalpy change indicate that sorption were spontaneous and

exothermic.

0.0030 0.0031 0.0032 0.0033

2.7

3.0

3.3

3.6

1.6

1.7

1.8

Hg(II)

log

k c

T -1 (K-1)

Pb(II)

log

k c

Fig. 10 Variation of equilibrium constant with absolute temperature for Pb(II) and Hg(II) sorption

Table 1 Thermodynamic parameters for the sorption of Pb(II) and Hg(II) on CIT-chitosan

T/K DG DH/kJ mol-1 DS/J mol-1 K-1

Pb(II) Hg(II) Pb(II) Hg(II) Pb(II) Hg(II)

303 -10.443 -19.991 -11.6171 -48.5652 -4.3176 -94.3289

313 -10.027 -19.549

323 -10.268 -18.307

333 -10.236 -17.103

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Conclusions

A Schiff base of chitosan with citral (CIT-chitosan) was prepared and characterized

by FTIR, scanning electron microscopy and differential scanning calorimetry. The

CIT-chitosan was evaluated for its Pb(II) and Hg(II) sorption and it was found that

the sorption was influenced by the pH of the solution; maximum sorption was

observed in acid pH range. The sorption kinetics was found to follow pseudo

second-order kinetics with a rate constant of 0.06944 and 0.007392 g mg-1 h-1,

respectively for Pb(II) and Hg(II). Sorption data fitted to both Freundlich and

Langmuir isotherms and we observed that Langmuir model gives the best fit.

Maximum sorption capacity was found to be 250 mg g-1 for Pb(II) and

333.33 mg g-1 for Hg(II). The values were higher than that for some other forms

of chitosan and chitosan modifications. Isotherm studies indicated that the Schiff

base has different binding sites having different binding affinities. The evaluation of

thermodynamic parameters indicated that the sorption was spontaneous and

exothermic. Since CIT-chitosan is stable in acid medium, metal sorption from acid

effluents can be effectively performed where chitosan cannot be used. The Schiff

base from chitosan and citral was found to be an efficient and cheap sorbent for

Pb(II) and Hg(II).

Acknowledgments One of us (PA) acknowledges the University Grants Commission (UGC), New

Delhi, India for the award of a Teacher Fellowship.

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