Accepted Manuscript

An improved synthesis of chitosan bead for Pb(II) adsorption

Yangcheng Lu, Jing He, Guangsheng Luo

PII: S1385-8947(13)00556-1

DOI: http://dx.doi.org/10.1016/j.cej.2013.04.078

Reference: CEJ 10693

To appear in: Chemical Engineering Journal

Received Date: 24 December 2012

Revised Date: 14 April 2013

Accepted Date: 18 April 2013

Please cite this article as: Y. Lu, J. He, G. Luo, An improved synthesis of chitosan bead for Pb(II) adsorption,

Chemical Engineering Journal (2013), doi: http://dx.doi.org/10.1016/j.cej.2013.04.078

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1

An improved synthesis of chitosan bead for Pb(II) adsorption 1

Yangcheng Lu1, Jing He, Guangsheng Luo 2

State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua 3

University, Beijing 100084, China 4

ABSTRCT 5

In this study, an improved synthesis method of preparation of Pb(II) imprinted 6

chitosan (Pb(II)-CS) bead with uniform size and porous morphology was proposed to 7

combine the microfluidic technique with crosslinking solidification. The infrared 8

spectrum indicated the reversible chelation of Pb(II) with amino group played important 9

role in amino group protection and Pb(II) adsorption. The adsorption capacity of 10

Pb(II)-CS bead reached 79.2 mg/g for Pb(II) by optimizing preparation conditions and 11

the desired adsorption time was just less than 30 min. The adsorption performance of 12

Pb(II) on Pb(II)-CS matched with the pseudo-second-order kinetic model and the 13

Langmuir isotherm model well. The recycled use of the Pb(II)-CS bead demonstrated 14

little change in the adsorption capacity. The Pb(II)-CS bead might be used as an 15

effective adsorbent for Pb(II) removal from water. 16

Keywords: Pb(II)-CS bead, Pb(II) adsorption, microfluidic, crosslinking solidification, 17

ion-imprinting 18

1 Corresponding author. Tel./fax: +86 10 62773017

E-mail address: luyc@tsinghua.edu.cn (Y.C. Lu)

2

19

1. Introduction 20

Heavy metals are common environmental pollutants and pose significant threats to 21

human health. Lead is a common heavy metal contaminant. If inhaled or swallowed it 22

can cause mental deficiency, brain damage, anaemia as well as behavioral problems [1]. 23

Among the various wastewater treatment techniques, biosorption of heavy metals is a 24

promising alternative due to its high selectivity, easy handling, lower operating costs, 25

high efficiency in removing very low levels of heavy metals from dilute solutions, 26

reduced quantity of chemical or biological sludge, and regeneratability of biosorbents. 27

Chitosan, produced by alkaline deacetylation of chitin, is the second-most abundant 28

polysaccharide in nature. Because of the presence of amine groups, chitosan has 29

chelating ability towards a number of metal ions [2]. Flake and powder forms of 30

chitosan are not suitable for using as adsorbents due to their low surface areas and none 31

porosity. This can be avoided by casting chitosan beads with high porosity and large 32

surface area, together with crosslinking to make the beads insoluble in acidic media [3]. 33

In the traditional method, the chitosan solution was sprayed [1] or injected [4] as 34

droplets into a dilute sodium hydroxide solution for shaping and solidifying chitosan 35

beads, and then the chitosan beads were crosslinked with glutaraldehyde, 36

epichlorohydrin or ethyleneglycol glycidyl ether. Although the crosslinking enhances 37

the resistance of chitosan against acid, alkali and chemicals, it can reduce the adsorption 38

capacity. Methods were tried to blend chitosan with other polymers to enhance the 39

3

mechanical strength of the chitosan bead [5,6] so as to overcome the problem of amino 40

group occupation and introduce some new functional groups. While, it led to dense 41

inner-structure and poor adsorption kinetics. Ion-imprinting method is an alternative to 42

prevent amino group excessively crosslinking. Various metal ions have been used as 43

templates to synthesize ion imprinted chitosan bead to improve the adsorption capacity 44

for imprinted metals [7-9]. 45

Until now, the most frequent preparation of ion imprinted chitosan bead includes 46

alkaline neutralization following by cross-linking and ion stripping. The chitosan 47

aggregates would uncontrollably form by alkaline neutralization and result in two 48

consequent shortcomings. Firstly, the inner structure of chitosan bead was dense and not 49

preferred for mass transfer. Secondly, the chelation conformation of amino group and 50

metal ion met significant change unfavorable for metal ions adsorption in water. 51

In this work, an improved preparation of ion imprinted chitosan bead was proposed 52

to mitigate the change of polymeric structure by one-step crosslinking solidification. 53

Owing to the requirements of size control and uniform crosslinking, microfluidic 54

technique, widely reported for producing monodisperse bubbles, droplets and beads 55

recently [10-12] was used to generate aqueous microdroplets containing chitosan. An 56

inertial solvent was selected as the crosslinker carrier and the continuous phase. Pb(II), 57

glutaraldehyde and n-octanol were selected as the template ion, the crosslinker and the 58

continuous phase, respectively. Pb(II)-CS bead was prepared and characterized. 59

Furthermore, determinations on capacity and kinetics of Pb(II) adsorption in aqueous 60

4

solution were carried out and some expected advantages were revealed. 61

2. Materials and Methods 62

2.1 Chemicals and reagents 63

Chitosan (CS, deacetylation 85 % and MW 5.0×104 g/mol) was obtained from 64

Yuhuan Ocean Biochemical Co. Ltd. (Zhejiang, China). Acetic acid, Span80, n-octanol, 65

Glutaraldehyde and EDTA were purchased from VAS Chemical Co. Ltd. (Tianjin, 66

China), and PbCl2 and CuCl2 from Sinopharm Chemical Reagent Co. Ltd. (Beijing, 67

China). All chemicals were of analytical grade and used without further purification. 68

0.27 g of PbCl2 was dissolved in 2.0 L water as a stock solution (100 mg/L). 69

2.2 Preparation of crosslinked Pb(II)-CS beads 70

Preparation of Pb(II)-CS bead included two sequent steps. The first was to obtain 71

water droplets containing chitosan and Pb(II) in a microfluidic device. The second step 72

was to convert droplets into beads by chemical crosslinking in a solidification bath. 73

The microfluidic device, fabricated by end mill method reported elsewhere [13], 74

was composed of four pieces of polytetrafluoroethylene (PTFE) plates. One plate of 32 75

mm × 20 mm × 16 mm contained the inlet and the outlet of the continuous phase 76

solution. One plate of 32 mm × 20 mm × 12 mm contained the inlet of dispersed 77

phase solution. One plate of 32 mm × 20 mm × 1 mm served as the dispersion 78

medium, on which two 200 μm (diameter) sieved pores were located along the flow 79

5

direction. One plate of 32 mm × 20 mm × 1 mm contained a 14 mm × 2 mm 80

through groove as the main channel. As shown in Figure 1, the dispersed phase and the 81

continuous phase were injected into the microfluidic device using two plunger pumps 82

(Beijing satellite manufacturing factory, China). Droplets were generated by 83

crossflowing rupture and then collected in the solidification bath with stirring. 84

2.0 g of CS and 1.0 g of PbCl2 were dissolved in 100 mL of 1 %wt acetic acid 85

aqueous solution as the dispersed phase. 16 g of Span80 dissolved in 800 g of n-octanol 86

was used as the continuous phase. 50 %wt glutaraldehyde aqueous solution and 87

n-octanol were mixed in 1:1 (w/w) ratio by stirring overnight, and then settled to 88

separate out the upper phase as the solidification bath (with addition of 2 %wt Span80 89

before use). The flow rates of the dispersed phase and the continuous phase were 2 90

mL/min and 60 mL/min, respectively. In the solidification bath, droplets were gradually 91

solidified to form beads after 30 min at room temperature. Next these beads were 92

filtered off, washed three times with acetone and water separately, stripped by 0.05 M 93

EDTA aqueous solution and water in sequence, and frozen dried for further 94

determinations. 95

2.3 Adsorption experiment and regeneration 96

Batch experiments were carried out for adsorption studies. In details, 0.02 g of dry 97

Pb(II)-CS beads and 50 mL of PbCl2 solution were added in a conical flask with stopper, 98

and then shaken at 200 rpm for predetermined time intervals. All the operations were 99

6

conducted in a thermostat at constant temperature. The concentration of Pb(II) in 100

aqueous solution was measured by atomic absorption spectrophotometer. Pb(II) 101

adsorption capacity of Pb(II)-CS bead was calculated by the following equation [14]: 102

0 f(C -C ) Vq=

1000 m

××

(Equation 1) 103

where q is Pb(II) adsorption capacity (mg/g), C0 and Cf are the initial and final 104

concentration (mg/L) of Pb(II). V is the volume of solution (mL) and m the weight of 105

Pb(II)-CS beads (g, dry-basis). 106

Furthermore, the effect of pH on Pb(II) adsorption of Pb(II)-CS beads was examined. 107

The initial pH of Pb2+ stock solution was 5.33. 0.1 mol/L HCl and 0.1 mol/L NaOH 108

were used to adjust pH. 109

So as to investigate the reusability of Pb(II)-CS bead, multi-cycle 110

adsorption-desorption experiments were carried out as well. In the adsorption step, 111

0.02g of dry Pb(II)-CS beads were added into PbCl2 solution (50 mL, 50 ppm). In the 112

desorption step, Pb(II)-CS beads were filtered out first, and then washed three times 113

with sufficient 0.05 M EDTA aqueous solution and deionized water in sequence. In 114

following, the next adsorption step was conducted. 115

3. Results and discussion 116

3.1 Preparation of Pb(II)-CS bead 117

The synthesis of Pb(II)-CS was illustrated by the following four steps, as shown in 118

7

Figure 2(a): (1) Pb(II) was chelated with CS by amino group (–NH2) in the acetic acid 119

aqueous solution; (2) monodisperse microdroplets (Figure 2(b)) containing Pb(II) and 120

chitosan were generated by a microfluidic device; (3) Glutaraldehyde in the continuous 121

phase was quickly delivered into microdroplets by mass transfer and triggered 122

crosslinking solidification to form beads; (4) The chelated Pb(II) was removed by 123

EDTA and imprinted sites were exposed. 124

3.2. Characterization of Pb(II)-CS 125

Figures 3a and 3b show scanning electron microscopy (SEM) images of Pb(II)-CS 126

beads. As shown, chitosan beads were almost spherical with an average diameter of 80 127

μm. The coefficient of variance around 11% revealed good mono-dispersibility. Highly 128

developed macropores were found on the surface, and many wrinkles were observed in 129

the normal section. As expected, Pb(II)-CS beads were endowed with good uniformity 130

on size and morphology by using microfluidic technique; porous structure was 131

preserved to some extent in the improved synthesis. 132

The FTIR spectra of the samples are shown in Figure 3c, where 1, 2, 3, and 4 are 133

chitosan powder, Pb(II)-CS before stripping, Pb(II)-CS as prepared, Pb(II)-CS after 134

adsorption, respectively. The broad and strong band ranging from 3200 to 3600 cm-1 135

may attribute to the overlapping of O-H stretching vibration at higher wavenumber and 136

N-H stretching vibration at lower wavenumber. The peak at wavenumber of 1635 cm-1 137

can be assigned to the N-H bending vibration. Comparing sample 1 and sample 2, the 138

8

band ranging from 3200 to 3600 cm-1 shifts to the left and the peak at wavenumber of 139

1635 cm-1 disappears in the spectrum of sample 2 due to the chelation of Pb(II) and 140

–NH2. Comparing sample 2 and sample 3, the peak at wavenumber of 1635 cm-1 141

appears again in the spectrum of sample 3 indicating Pb(II) as template may be removed 142

from Pb(II)-CS bead by EDTA stripping. However, the peak at wavenumber of 1635 143

cm-1 in the spectrum of sample 3 seems weaker than that in the spectrum of sample 1, 144

because –NH2 reacts with glutaraldehyde more or less. The comparison of sample 3 and 145

sample 4 is quite similar with that of sample 1 and sample 2. In general, we suggested 146

that amino group (–NH2) played an important role in Pb(II) adsorption of Pb(II)-CS 147

beads. 148

3.3 Determination of preparation parameters 149

Solidification time 150

The solidification time, referred to the period of beads being in the solidification 151

bath, had direct influence on the degree of crosslinking. Less-crosslinked beads might 152

have more structural deficiency and poor mechanical stability. On the other hand, 153

over-crosslinked beads might present low adsorption capacity. So the solidification time 154

needs optimization. 155

In this section, all the Pb(II)-CS beads were prepared under the same conditions 156

with the exception of the solidification time. In every adsorption experiment, 0.08 g of 157

beads were added to 100 mL aqueous solution containing 50 mg/L Pb(II) for 30 min 158

9

adsorption (Adsorption equilibrium was confirmed to be reached after 30 min 159

adsorption in following). The results are shown in Figure 4a. The adsorption capacity 160

attained a maximum value of 41.3 mg/g when the solidification time was 30 min. 161

Therefore 30 min was selected as the solidification time in the following experiments. 162

Concentration of Pb(II) as template 163

The preparation of Pb(II)-CS bead was featured by the addition of Pb(II) as template in 164

the chitosan solution. While, redundant addition of Pb(II) might increase cost and 165

address the pressure of waste treatment. In this section, all the Pb(II)-CS beads were 166

prepared under the same conditions with the exception for Pb(II) concentration in the 167

chitosan solution. The results of adsorption experiments using these beads are shown in 168

Figure 4b. Initially, the adsorption capacity increases steeply with the addition of Pb(II). 169

With the increasing of Pb(II) concentration, this trend gradually weakens until a 170

platform is reached when Pb(II) concentration is 10.0 g/L or more. In the experiments, 171

the adsorption capacity attained a maximum value at Pb (II) concentration of 12 g/L, 172

quite close to that at Pb(II) concentration of 10.0 g/L. In such cases, most of active 173

amino groups in chitosan might have been chelated with Pb(II) and protected. Since the 174

surplus addition of Pb(II) had no significant improvement of adsorption performance, 175

10.0 g/L was selected for the concentration of Pb(II) as template in the following 176

experiments. 177

Concentration of chitosan 178

10

Chitosan solutions with various concentrations were used to prepare Pb(II)-CS 179

beads in this section. Results are shown in Figure 4c. The 2 %wt chitosan solution 180

responded to the highest adsorption capacity. Since the amount of -NH2 will increase 181

with the increasing of the concentration of chitosan, it was an explanation that the 182

adsorption capacity increased with the increasing of the concentration of chitosan 183

solution. However, high concentration of chitosan also resulted in dense inner structure 184

of beads. The accessibility of amino groups in the kernel of beads might determine the 185

adsorption capacity. Therefore, the optimal concentration of chitosan in this section, 2 186

%wt, was selected in the following experiments. 187

3.4 Adsorption kinetics 188

Equilibrium time 189

A serial of adsorption experiments were carried out under the same conditions with 190

the exception of adsorption time. The time profile of adsorption capacity is shown in 191

Figure 5a. The adsorption capacity increased steeply until it reached a platform after 30 192

min adsorption. 30 min was regarded as the equilibrium time of adsorption in terms of 193

thermodynamics studies. The initial fast adsorption was due to the availability of many 194

adsorption sites close to the surface [15]. With the proceeding of adsorption, the 195

available sites decreased and the adsorption rate decreased [16]. Therefore, the rate of 196

adsorption, concerned in engineering [ 1 7], was dependent on the density and 197

distribution of available adsorption sites in Pb(II)-CS beads. 198

11

To our knowledge, the equilibrium time (30 min) in this work was much shorter 199

than other chitosan-based beads reported for Pb(II) adsorption, such as chitosan/PVA 200

(poly(vinyl alcohol)) hydrogel beads (500 min) [5], chitosan-coated sand(CCS) (4 h) 201

[18], and chitosan-based hydrogel graft-copolymerized with methylenebisacrylamide 202

and poly(acrylic acid) (1500 min) [19], etc. More comparisons of these chitosan beads 203

in Pb(II) adsorption performance are listed in Table 3. The fast adsorption rate in this 204

work was due to the highly developed macropores on the surface of beads. 205

Kinetic model 206

Two different kinetic models were commonly applied to evaluate the adsorption 207

kinetics of Pb(II) [20]. They are described by the pseudo-first-order equation (Equation 208

2) and the pseudo-second-order equation (Equation 3), respectively 209

( )e t e 1ln q q ln q k t− = − (Equation 2)

210

2t 2 e e

t 1 1t

q k q q

⎛ ⎞= + ⎜ ⎟

⎝ ⎠ (Equation 3) 211

where qe (mg/g) and qt (mg/g) are the adsorption capacities at equilibrium and at time t 212

(min), respectively. k1 and k2 was calculated from the slope of the linear plot of 213

( )e tln q q− versus t and the intercept of the linear plot of t

t

q versus t (Figure 5b), 214

respectively. qe was calculated from the intercept of plot e tln(q -q ) vs. t or the slope of 215

t

t

q vs. t [7]. The correlation coefficients (R2) and rate constants at various 216

concentrations of Pb(II) are listed in Table 1. The results indicated that 217

12

pseudo-second-order kinetic model was matched better than the pseudo-first-order 218

kinetic model and the adsorption process might be a chemical reaction controlled 219

process. 220

3.5 Adsorption isotherms 221

Effects of pH 222

Figure 6a represents the effect of pH on Pb(II) adsorption with Pb(II)-CS beads. 223

The adsorption capacity of Pb(II) increases with pH increasing generally, from about 2 224

mg/g at pH 0.77 comes to 79.2 mg/g at pH 5.33. In the experiments, Pb(II)-CS beads 225

always kept fine integrity in appearance, even at pH 0.77, since the effective 226

crosslinking had been accomplished in preparation. 227

The low adsorption capacity at low pH might be because most of amino groups 228

were protonated [21]. On the other hand, the combination or precipitation between Pb2+ 229

and hydroxide might occur instead of adsorption at high pH. So, near neutral pH was 230

preferred to Pb(II) adsorption by Pb(II)-CS beads. 231

Effects of initial ion concentration 232

The effect of initial metal concentration on the equilibrium adsorption capacity of 233

the beads was studied and the results are shown in Figure 6b. As Pb(II) concentration 234

increasing from 10 mg/L to 50 mg/L, the adsorption capacity of Pb(II)-CS beads 235

increased from 45.8 mg/g to 79.2 mg/g. Further increasing of Pb(II) concentration had 236

little effect on the equilibrium adsorption capacity. Such a trend indicated that the 237

13

adsorption of Pb(II) on Pb(II)-CS beads might be fit to the Langmuir isotherm model. 238

Adsorption isotherm 239

Herein, two isotherms - Freundlich (Equation 4) and Langmuir (Equation 5) were 240

used to analyze the equilibrium experimental data [22]. 241

e F e

1ln q ln K ln C

n= + (Equation 4) 242

e e

e m m

C C1

q q b q= + (Equation 5) 243

where Ce is the Pb(II) concentration (mg/L) at equilibrium, qe is the adsorption capacity 244

at equilibrium (mg/g), KF (L/mg) is the Freundlich constant, 1/n is the heterogeneity 245

factor, b (L/mg) and qm (mg/g) are the Langmuir coefficients, representing the 246

adsorption equilibrium constant and the monolayer capacity, respectively. 247

The Freundlich isotherm assumes that the adsorption occurs on a heterogeneous 248

surface by multilayer adsorption and the amount of adsorbate adsorbed increases 249

infinitely with the increasing of concentration [23]. While, the Langmuir model assumes 250

monolayer adsorption on homogenous surface where the binding sites have equal 251

affinity and energy, and the interaction between the adsorbed species can be neglected 252

[24]. The constants and correlation coefficients of two models are listed in Table 2. The 253

results show that the adsorption capacity decreased with the increase of temperature, 254

indicating that low temperature was favor for Pb(II) adsorption of Pb(II)-CS beads. It 255

implied that the adsorptive forces between the active sites of the adsorbent, adsorbate 256

species and the adjacent molecules of the adsorbed phase were weakened with 257

14

temperature rising [25]. In addition, the adsorption for Pb(II) of Pb(II)-CS beads was 258

better fitted to the Langmuir isotherm model (linear plot was shown in Figure 6c) with 259

R2 higher than that of Freundlich isotherm model, indicating that the adsorption of Pb2+ 260

was mainly monolayer. The calibrated monolayer capacity for Pb(II) of Pb(II)-CS beads 261

was 73.5 mg/g, which was close to the optimal experimental result shown in Figure 6b 262

(about 79.2 mg/g). 263

Moreover, the adsorption capacity in present work was comparatively high. As 264

listed in Table 3, the indexes were 0.95 mg/g for chitosan/PVA (poly(vinyl alcohol)) 265

hydrogel beads [5], 12.32 mg/g for chitosan-coated sand [18], and 22.7 0mg/g for 266

Pb(II)-IIP on nano-TiO2 matrix [26]. 267

Thermodynamic analysis 268

The thermodynamic parameters (Gibbs free energy change ΔGo, the enthalpy 269

change ΔHo and the entropy change ΔSo) can be calculated according to the following 270

equations [27]: 271

o oH ΔSln K

RT R

Δ= − + (Equation 6) 272

o o oG H T SΔ = Δ − Δ (Equation 7) 273

where R is the universal gas constant, T the absolute temperature, and K the equilibrium 274

constant (K=qe/Ce). The values of ΔHo and ΔSo were calculated from the slope and 275

intercept of plot of lnK versus 1/T. The values of all these thermodynamic parameters 276

are listed in Table 4. 277

15

The negative ΔGo and enthalpy indicated Pb(II) adsorption on Pb(II)-CS beads was 278

a spontaneous and exothermic process. In addition, the negative ΔSo implied that the 279

disorder of the system decreased during adsorption process. 280

Desorption and reuse 281

The reuse of the Pb(II)-CS beads was examined by determining the equilibrium 282

adsorption capacities of the beads in multi-cycle adsorption-desorption. The experiment 283

results (Table 5) show that the adsorption capacities of the beads had no significant 284

difference between the first run and the seventh run. In evidence, the stability of these 285

Pb(II)-CS beads was quite well. 286

4. Conclusions 287

In this work, microfluidics technique and crosslinking solidification were combined 288

to prepare Pb(II)-CS beads with uniform size and porous morphology successfully. The 289

infrared spectra verified –NH2 could chelate with Pb(II) reversibly to provide protection 290

in preparation and Pb(II) adsorption in using. The saturated adsorption capacity of 291

Pb(II)-CS bead reached 79.2 mg/g for Pb(II) by optimizing preparation conditions and 292

the equilibrium time was just less than 30 min. The adsorption of Pb(II) with Pb(II)-CS 293

was found to be a spontaneous and exothermic process, and match with the 294

pseudo-second-order kinetic model and the Langmuir isotherm model well. As potential 295

biosorbents for Pb(II) removal from water, the Pb(II)-CS beads showed a good stability 296

16

in terms of adsorption capacity during the multi-cycle adsorption-desorption process. 297

Acknowledgements 298

We gratefully acknowledge the support of the National Natural Science Foundation of 299

China (20876084, 21036002, 21176136) and National Science and Technology Support 300

Program of China (2011BAC06B01) on this work. 301

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[26] C. Li, J. Gao, J. Pan, Z. Zhang, Y. Yan, Synthesis, characterization, and adsorption 371

performance of Pb(II)-imprinted polymer in nano-TiO2 matrix, J. Environ. Sci.21 372

(2009) 1722-1729. 373

[27] J.C.Y. Ng, W. Cheung, G. McKay, Equilibrium studies of the sorption of Cu(II) 374

ions onto chitosan, J. Colloid. Interface Sci. 255 (2002) 64-74.375

20

376

Figure Captions 377

378

Figure 1. Scheme of experiment set-up. 379

Figure 2. (a) Preparation of Pb(II)-CS beads; (b) microdroplets. 380

Figure 3. Characterizations of Pb(II)-CS beads. SEM images: a (surface) and b (interior). FTIR 381

spectra: c (1. chitosan powder; 2. Pb(II)-CS before stripping; 3. Pb(II)-CS as prepared; 4. Pb(II)-CS 382

after adsorption). 383

Figure 4．Comparisons on equilibrium adsorption capacity of Pb(II)-CS beads with various 384

preparation conditions: (a) Effect of the solidification time; (b) Effect of the concentration of Pb(II) 385

as template; (c) Effect of the concentration of chitosan solution; Adsorption experimental parameters: 386

C0 = 50 ppm, T = 25 oC. Reference parameters in preparation: solidification time, 30 min; Pb(II) 387

concentration, 10 g/L; concentration of chitosan, 2 %wt. 388

Figure 5．(a) The time profiles of adsorption capacity of Pb(II)-CS beads. Adsorption experimental 389

parameters: T = 25 oC. The lines in (b) represent calibration results according to the 390

pseudo-first-order equation. 391

Figure 6．The dependence of adsorption capacity of Pb(II)-CS beads on pH (a), initial concentration 392

(b) and temperature (c). Reference adsorption experimental parameters: C0 = 50 ppm, T = 25 oC. The 393

lines in (c) represent calibration results according to the Langmuir Equation.394

21

395

Tables 396

397

Table 1 398

Kinetics constants, correlation coefficients (R2) for Pb(II) adsorption on Pb(II)-CS beads based on 399

different models. T=25 oC. 400

Pseudo-first-order kinetics Pseudo-second-order kinetics

Concentration, mg/L

k1, min-1 R2 k2, g mg-1min-1 qe, mg g -1 R2

50 0.181 0.9387 0.00354 90.9 0.9955

100 0.165 0.9788 0.00439 84.7 0.9981

200 0.242 0.9376 0.00179 100 0.9734

22

401

Table 2 402

Freundlich and Langmuir constants and correlation coefficients (R2) for Pb(II) adsorption on 403

Pb(II)-CS beads. 404

Freundlich constants Langmuir constants

Temperature, oC

KF, L/mg N R2 qm, mg/g b, L/mg R2

25 30.444 4.616 0.9989 73.5294 0.2173 0.997

45 22.151 4.167 0.9862 59.1716 0.1883 0.9974

55 18.464 3.917 0.9252 51.813 0.1880 0.9968

23

405

Table 3 406

Comparisons of different chitosan based beads for Pb(II) adsorption 407

Species

Equilibrium

time

(min)

Adsorption

capacity

(mg/g)

Reference

Pb(II)-CS beads 30 79.2 This article

chitosan/PVA (poly(vinyl alcohol))

hydrogel beads

500 0.95 [7]

chitosan-coated sand (CCS) 2400 12.32 [18]

chitosan-based hydrogels,

graft with methylenebisacrylamide and

poly(acrylic acid)

1500

95

[23]

Pb(II)-IIP on nano-TiO2 matrix 2400 22.7 [26]

24

408

Table 4 409

Values of thermodynamic parameters for Pb(II) adsorption on Pb(II)-CS beads 410

Metal ion

ΔHo

(kJ mol-1)

ΔSo

(J mol-1 K-1)

Temperature

(oC)

ΔGo

(kJ mol-1)

R2

25 -4.81

45 -4.09 Pb(II) -15.57 -36.1

55 -3.73

0.9915

25

411

Table 5 412

Adsorption capacities of Pb(II)-CS beads during the multi-cycle adsorption-desorption process. C0 = 413

50 ppm, T = 25 oC. 414

Runs 1 2 3 4 5 6 7

qe, mg/g 79.2 79.2 79.1 79.1 78.9 78.7 78.6

26

415

Figures 416

417

Figure 1 418

419

27

420

Figure 2 421

422

(a) 423

424

(b) 425

426

28

427

Figure 3 428

429

(a) 430

431

(b) 432

4000 3000 2000 1000

tran

smitt

ance

(%

)

wavenumbers (cm-1)

1

2

3

41635

3477

3462

3483

3446

1635

433

29

(c) 434

Figure 4 435

0 30 60 90 1200

10

20

30

40

50

q, m

g/g

Solidification time, min

0 3 6 9 12

0

20

40

60

80

Concentration of Pb(II), mg/L

q,m

g/g

436

(a) (b) 437

1 2 30

20

40

60

q, m

g/g

Concentration of chitosan, wt%

438

(c) 439

30

440

Figure 5 441

442

0 20 40 60

0

20

40

60

80

q, m

g/g

t, min

C0=50ppm

4 8 12 16

0.08

0.12

0.16

0.20

C0=50ppm

C0=100ppm

C0=200ppm

t/qt

t, min

443

(a) (b) 444

31

445

Figure 6 446

1 2 3 4 5 6

0

20

40

60

80

q e, mg/

g

pH

0 50 100 150 200

50

60

70

80

q e, mg/

g

Initial concentration, mg/L

447

(a) (b) 448

10 20 30 40

0.2

0.4

0.6

0.8 25OC 45OC 55OC

Ce/q

e, g/L

Ce, mg/L

449

(c) 450

32

� Porous ion imprinted chitosan beads with uniform size were obtained. 451

� Active –NH2 for Pb2+ adsorption was reserved after crosslinking solidification. 452

� The adsorption capacity for Pb2+ kept at 76.8 mg/g during repeated use. 453

� The adsorption equilibrium was reached within 30 min. 454

455