Preparation and characterization of carboxyl-functionalized chitosan magnetic microspheres and submicrospheres for Pb2+ removal

Colloids and Surfaces A: Physicochem. Eng. Aspects 482 (2015) 353–364

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

Preparation and characterization of carboxyl-functionalized chitosan magnetic microspheres and submicrospheres for Pb2+ removal Yanyan Xu a , Qifeng Dang a,∗ , Chengsheng Liu a,∗ , Jingquan Yan b , Bing Fan c , Jinping Cai a , Jingjing Li d a

College of Marine Life Science, Ocean University of China, 5 Yushan Road, Qingdao 266003, PR China School of Medicine and Pharmacy, Ocean University of China, 5 Yushan Road, Qingdao 266003, PR China c Qingdao Aorun Biotechnology Co., Ltd., Room 602, Century Mansion, 39 Donghaixi Road, Qingdao 266071, PR China d College of Life Science and Bioengineering, Beijing University of Technology, 100 Ping Le Yuan, Chaoyang District, Beijing 100124, PR China b

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• The chitosan magnetic microspheres (EDCMM) and submicrospheres (EDCMSM) were prepared for Pb2+ removal. • Both of them were comprehensively characterized by SEM, TEM, FTIR, BET and VSM techniques. • EDCMSM had more rapid adsorption rate and EDCMM had higher absorbability, while both of them had excellent reusability. • EDCMM and EDCMSM were promising novel adsorbents for the sorption of heavy metal ions.

a r t i c l e

i n f o

Article history: Received 21 March 2015 Received in revised form 9 June 2015 Accepted 12 June 2015 Available online 16 June 2015 Keywords: Carboxylated chitosan Magnetic microsphere Magnetic submicrosphere Characterization Adsorption Pb2+

a b s t r a c t The present work aimed to develop two kinds of novel adsorbents, carboxylated chitosan magnetic microspheres (EDCMM) and submicrospheres (EDCMSM), for comparative study of Pb2+ removal. Several techniques, including SEM, TEM, FTIR, BET and VSM, were applied to characterize the adsorbents. Batches of tests were conducted to investigate the Pb2+ adsorption at different pH values, contact time, initial Pb2+ concentrations and temperatures. EDCMSM with higher specific surface area and a lower content of carboxyl had faster initial absorbing rate but lower adsorption capacity for Pb2+ than that of EDCMM. The adsorption processes for both EDCMM and EDCMSM fitted well with pseudo-secondorder model and were dominated by chemical reaction process. The isotherm data for both adsorbents were well described by the Langmuir model, which suggested monolayer adsorptions. Thermodynamics analysis suggested that the adsorption processes of both EDCMM and EDCMSM were endothermic and spontaneous in nature. Both adsorbents still exhibited good adsorption performances after the fifth adsorption–regeneration cycle. All these results indicated that both EDCMM and EDCMSM might be promising adsorbents for the Pb2+ removal. © 2015 Elsevier B.V. All rights reserved.

∗ Corresponding authors. Tel.: +86 0532 82032586; fax: +86 0532 82032586. E-mail address: [email protected] (C. Liu). http://dx.doi.org/10.1016/j.colsurfa.2015.06.028 0927-7757/© 2015 Elsevier B.V. All rights reserved.

354

Y. Xu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 482 (2015) 353–364

1. Introduction With the rapid development of industries, large amount of metal-contamiated wastewater is discharged to environment and causes threats to human health and ecosystems [1]. Pb2+ is one of the most common heavy metal ions in the polluted water bodies for its widespread use in industrial, especially in battery manufacturing and metal plating [2]. Besides, it is one of the most toxic heavy metal ions to human due to its accumulation in bone, brain, kidney and liver, which may lead to many serious diseases, such as anaemia, encephalopathy, nephritic syndrome and hepatitis [3]. To minimize the pollution of Pb2+ , the United States Environmental Protection Agency (USEPA) has established the maximum contamination level (MCL) of Pb2+ to 0.006 mg/L for the groundwater and surface water [4]. However, the estimate of U.S.EPA shows that more than 300 Superfund sites in the United States are contaminated with lead [5]. Therefore, it is of highly importance to develop methods for Pb2+ removal from wastewater. Adsorption has been widely studied on water purification for its convenient operation, high efficiency, low cost and easy regeneration [2,3,6–8]. Among various mineral and organic adsorbents, chitosan is one of the most popular adsorbents for its wide availability, nontoxicity, biocompatibility, and high chemical activity [9–15]. Chitosan has been studied extensively on the removal of heavy metal ions, such as cadmium, chromium, copper, mercury and lead ion from aqueous solutions [9,16–19]. However, chitosan is unstable in lower pH, which makes it easy to loss in heavy metal solutions and limits its applications in the field of water purification. In order to increase the stability of chitosan in acid solution, some modifications with glutaraldehyde have been done in the published literature [20]. Modifications with glutaraldehyde lead to a reduction of adsorption capacity because the occupation of amine groups and the decrease in the accessibility to internal sites [21]. Thus, it is very meaningful to increase the stability of chitosan by modifications, meanwhile increase the adsorption sites on chitosan. Modifications on adsorbent with active ligands, such as amino, carboxyl and thiol groups have been reported to be useful methods to acquire more active points for large adsorption capacity [6]. Among these active ligands, carboxyl group is an excellent ligand for the strong electrostatic interaction with various heavy metals [6]. Carboxyl-grafted chitosan derivatives have been reported to improve the adsorption capacities of Co(II), Ni(II) and Cu(II) [22–24]. These results demonstrate that the introduction of carboxyl group is beneficial to the heavy metal ions removal. Chitosan and its derivatives can be prepared as powders [25], membranes [26], gels [27] and particles [16] to accommodate different applications. Chitosan particles are widely used in the field of heavy metal removal [28]. The adsorbent with smaller particle is reported to have higher adsorption capacity due to its higher specific surface area [29]. Submicroparticles, with grain sizes ranging from 100 nm to 1.0 ␮m, have shown excellent performance as a potential catalyst for its high surface area [30]. Besides, submicrometer fraction has been reported to be promising solid transducers in ion-selective electrodes for its high surface area, uniform structural size distribution, high hydrophobicity, and simple preparation process [31]. Hence, with high surface area, submicrospheres might have excellent properties in the field of heavy metal removal. However, recovery of adsorbents with small particle by ordinary methods, such as filtration and centrifugation, may result in loss of the adsorbents and secondary pollutions [32]. An efficient improvement is the combination of chitosan with magnetic core. In present study, two novel adsorbents, carboxylated chitosan magnetic microspheres (EDCMM) and submicrospheres (EDCMSM)

were prepared for comparative study of Pb2+ removal. The adsorbents were characterized by scanning electron microscope (SEM), transmission electron microscopy (TEM), fourier transform infrared (FTIR), Brunauer–Emmett–Teller (BET) and vibrating sample magnetometer (VSM). Batches of tests were conducted to investigate the Pb2+ adsorption at different pH values, contact time, temperatures, and initial Pb2+ concentrations. In order to study the adsorption mechanism of EDCMM and EDCMSM, the adsorption kinetics, isotherms and thermodynamics were analyzed. Meanwhile, the influences of particle size and active groups on the adsorption of Pb2+ were compared. The regeneration and reusability of EDCMM and EDCMSM were examined for further application. 2. Experimental 2.1. Materials Chitosan (industrial grade, degree of deacetylation ≥85%, viscosity 12 cps) was purchased from Qingdao Honghai Bio-tech Co., Ltd. Fe3 O4 nanoparticles were provided by Beijing DK nano-technology Co., Ltd. Glutaraldehyde with a concentration of 25% (v/v) and other chemicals used were of analytical grade and commercially obtained from Sinopharm Chemical Reagent Co., Ltd. Distilled water was used throughout. 2.2. Adsorbent preparation 2.2.1. Preparation of chitosan magnetic microspheres (CMM) Chitosan magnetic microspheres were prepared by microemulsion method. Briefly, 100 mL liquid paraffin and 0.7 mL Span-80 were added to 200 mL beaker with mechanical stirring (300 rpm) for 10 min, followed by addition of 20 mL 2% (w/v) citric acid suspension containing 6% (w/v) Fe3 O4 nanoparticles. After 30 min, 2 mL NH4 OH and 2 mL tetraethylorthosilicate (TEOS) were added successively, and then kept constant stirring for 12 h at 300 rpm. The desired SiO2 /Fe3 O4 magnetic cores were obtained after washing with water and freeze-dried. In order to prepare CMM, 1.2 g chitosan was dissolved in 20 mL 2% (v/v) of acetic acid solution to obtain 6 wt% of chitosan solution, then 0.6 g SiO2 /Fe3 O4 magnetic cores were well-distributed in the chitosan solution under stirring (500 rpm) for 20 min. Following that the mixture was dropped into the micro-emulsion system composing of 100 mL liquid paraffin and 0.7 mL Span-80. After stirring for 30 min, 3.5 mL methanal was added to the micro-emulsion system for the precrosslinking of chitosan layer under constant stirring for 2 h. The acquired particles were collected by magnet, washed with water, freeze-dried and then added to 0.1% (v/v) of glutaraldehyde solution for the second crosslinking. After that, the particles were soaked in 0.1 M of HCl for 5 h to free pre-crosslinking sites. The resultant particles were washed with water and freeze-dried for further experiments. 2.2.2. Preparation of chitosan magnetic submicrospheres (CMSM) Chitosan magnetic submicrospheres were prepared by the method of Ren et al. [32] with some modifications. Typically, 10 mL cyclohexane, 2 mL ritonX-100 and 2 mL n-hexanol were added to 50 mL beaker and the solution was stirred at 300 rpm for 20 min. Then, 2% (v/v) of citric acid suspension (600 ␮L, containing 2% (w/v) of Fe3 O4 nanoparticles) was added dropwise. After 30 min, 100 ␮L NH4 OH and 100 ␮L TEOS were added and the system was stirred at 300 rpm for 12 h to prepare SiO2 /Fe3 O4 submicrospheres. Following that 1200 ␮L 2% (w/v) of chitosan solution was added into the above micro-emulsion system with constant stirring for 30 min. SiO2 /Fe3 O4 submicrospheres were wrapped up with the solution of chitosan by the collision and fusion of water droplets. Then the chi-

Y. Xu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 482 (2015) 353–364

tosan layer was pre-crosslinked by adding 10 ␮L methanal to the micro-emulsion system. The precipitate was separated by magnet, rinsed with distilled water, freeze-dried, and added to 0.1% (v/v) of glutaraldehyde solution for a second crosslinking. After that, the particles were fully dispersed in 0.1 M HCl for 5 h to release precrosslinking sites and then freeze-dried for further experiments.

2.2.3. Carboxyl-functionalization of CMM and CMSM (EDTA modified) The surface modifications of CMM and CMSM with EDTA were performed according to the method of Ren et al. [32]. The asprepared CMM or CMSM (0.1 g) was dispersed in 40 mL 0.1 M of EDTA solution and stirred for 24 h at 200 rpm. Then the precipitate was rinsed with distilled water and ethanol for several times and fully dispersed in 6 mL phosphate solution (0.003 M, pH 5, 0.1 M NaCl) by ultrasonic treatment for 10 min. After that, 4 mL 1-ethyl-3(3-dimethylaminopropyl)-carbodiiide (EDAC) solution (25 g/L) was added to the phosphate solution under ultrasonic treatment for 25 min, and then kept machanical stirring for 10 h. The resultant particles, EDCMM or EDCMSM, were washed thoroughly with distilled water and then freeze-dried for the adsorption experiments.

355

2.3. Instrumentation The physico-chemical characteristics of EDCMM and EDCMSM were determined by standard procedures. The points of zero charge (PHPZC ) of EDCMM and EDCMSM were determined by the solid addition method [33]. The morphology and the size of EDCMM were obtained with JSM-840 scanning electron microscope (SEM, JEOL, Japan). The morphology and the size of EDCMSM were acquired by JSM-1200EX transmission electron microscope (TEM, JEOL, Japan) operated at 100 kV. The FTIR spectra were obtained with a Nicolet AVATAR360 instrument (Nicolet Instrument, Thermo Company, Madison, USA) at 25 ◦ C. The specific surface areas were gained by ASAP 2020 Micromeritics instrument and by Brunauer–Emmett–Teller (BET) method. Hysteresis loops were operated under a vibrating sample magnetometer (VSM, Lake Shore Cryotronics, USA).

2.4. Adsorption and desorption experiments 2.4.1. Batches of adsorption experiments In order to acquire the removal capacity of EDCMM and EDCMSM for Pb2+ , batches of adsorption experiments were carried

Fig. 1. TEM images of carboxylated chitosan magnetic submicrospheres (EDCMSM) (a and b) and SEM images of carboxylated chitosan magnetic microspheres (EDCMM) (c and d).

356

Y. Xu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 482 (2015) 353–364

Fig. 2. Magnetization curves of Fe3 O4 , carboxylated chitosan magnetic submicrospheres (EDCMSM) and carboxylated chitosan magnetic microspheres (EDCMM).

out by adding 50 mg EDCMM or EDCMSM and 30 mL Pb2+ solution to 50 mL flask. The contents were shaken in a thermostatic shaker at a speed of 200 rpm and the supernatant was analyzed for Pb2+ concentration. The effect of pH was studied at Pb2+ concentrations of 40 ppm with the pH range from 2 to 7 at 25 ◦ C. The adsorption capacities at different contact time were conducted at Pb2+ concentration of 300 ppm with the pH 6.0 at 25 ◦ C. To explore the effect of initial Pb2+ concentration, the adsorption study was operated at Pb2+ concentration varying from 40 to 400 ppm with the pH 6.0 at 25 ◦ C. For adsorption experiments at different temperatures (10, 15, 25, 40 and 60 ◦ C), the mixtures were conducted at the pH 6.0 with a Pb2+ concentration of 300 ppm. The adsorption capacities (mg/g) of EDCMM and EDCMSM were calculated according to Eq. (1). qe =

(Ci − Ce ) V M

(1)

where Ci (mg/L) is the initial concentrations of Pb2+ ; Ce (mg/L) is the equilibrium concentrations of Pb2+ ; M (g) is the weight of adsorbent; and V (L) is the volume of the solution.

2.4.2. Desorption and regeneration experiments Reusability is one of the important aspects for the practical application of the adsorbent. In order to study the reusability of the adsorbents, the regeneration experiments of EDCMM and EDCMSM were both carried out in five consecutively adsorption/desorption cycles. For each cycle, 30 mL, 100 mg/L of Pb2+ solution was adsorbed by 0.05 g EDCMM or EDCMSM for 120 min to reach adsorption equilibrium. The adsorbent loaded with Pb2+ was desorbed using 0.01 M of EDTA solution, and then washed thoroughly with water for using in next cycle of adsorption.

Fig. 3. FTIR spectra of chitosan (CS), carboxylated chitosan magnetic microspheres (EDCMM) and carboxylated chitosan magnetic submicrospheres (EDCMSM).

Y. Xu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 482 (2015) 353–364

357

Fig. 4. Zero charge points of carboxylated chitosan magnetic microspheres (EDCMM) (a) and carboxylated chitosan magnetic submicrospheres (EDCMSM) (b).

2.5. Statistical analysis All the experiments were carried out in triplicate. The results were presented as means ± standard deviation (SD).

of EDCMM was more regular and uniform than that of EDCMSM. The BET specific surface area of EDCMSM (6.83 m2 /g) was about 10 times as that of EDCMM (0.70 m2 /g), which was caused by the different particle size of EDCMSM and EDCMM [34].

3. Results and discussion 3.1. Characterizations 3.1.1. TEM, SEM and BET analysis The TEM images of EDCMSM and SEM images of EDCMM are given in Fig. 1. Fig. 1(b) and (d) shows that both of the particles had core/shell structure and the thicknesses of shells for EDCMSM and EDCMM were about 10 nm and 10 ␮m, respectively. Fig. 1(a) reveals that the particles size of EDCMSM was approximately 100–400 nm and was assigned to submicroscale. The particles size of EDCMM ranged from 100 ␮m to 200 ␮m (Fig. 1(c)). The results indicated that the percentage of shell of EDCMM matrix was higher than that of EDCMSM matrix. As presented in Fig. 1(a) and (c), the sphericity

3.1.2. Magnetic analysis The magnetic hysteresis loops of Fe3 O4 nanoparticles, EDCMSM and EDCMM are shown in Fig. 2. The saturation magnetization of Fe3 O4 , EDCMSM and EDCMM were 93.056 emu/g, 66.563 emu/g and 14.117 emu/g, respectively. It could be found that the saturation magnetization values of EDCMSM and EDCMM were lower than that of Fe3 O4 . The low saturation magnetization of EDCMSM and EDCMM were related with the different mass percentages of chitosan and SiO2 in adsorbents. It has been reported that the saturation magnetization decreased as the percentage of non-magnetic substances increased in the composite [35]. Despite this difference, both EDCMSM and EDCMM still could be rapidly separated from water by magnet.

358

Y. Xu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 482 (2015) 353–364

Fig. 5. Effect of pH values on removal of Pb2+ (initial Pb2+ concentration: 40 mg/L; adsorbent dose: 0.05 g; contact time: 120 min; temperature: 25 ◦ C).

3.1.3. FTIR spectral analysis The FTIR spectra of chitosan, EDCMM and EDCMSM in the range of 4000–500 cm−1 are given in Fig. 3. In the spectrum of chitosan, the absorption peaks at 1661.67 and 1601.98 cm−1 could be assigned to amide band I and N H bending vibration of amide, respectively. In comparison with the spectrum of chitosan, the evidently enhanced characteristic absorption peak of EDCMM around 1649.58 cm−1 was assigned to the C N bond generated from the cross-linking of chitosan with glutaraldehyde. The peak at 1631.74 cm−1 belonged to the C O stretching vibrations of the COO− , which indicated the successful combination of amino groups on chitosan with EDTA. The enhanced peaks of EDCMM at 578.51 and 1096.19 cm−1 correspond to the Fe O vibration of the Fe3 O4 particles and Si O vibration of Si O Si groups, respectively. The spectrum of EDCMSM had the same characteristic absorption peaks with that of EDCMM, which suggested that the carboxylated chitosan magnetic particles were successfully prepared.

3.2. Adsorption properties of EDCMM and EDCMSM for lead ions 3.2.1. Effect of initial pH on the adsorption The point of zero charge is the pH value at which the quantities of anions equal to cations on the surface, i.e., surface charge is zero. The adsorbent tends to adsorb anions when its surface is positively charged at the range of pH < pHpzc . Contrarily, cations’ adsorption is favorable at pH > pHpzc . Herein, it is important to determine pHpzc of the adsorbent for investigating the adsorption mechanism. In present study, adsorption capacities were researched at the pH range of 2–7. Fig. 4 shows that pHpzc of EDCMM (a) and EDCMSM (b) laid at pH values of 2.7 and 3.6, respectively. The values of pHpzc are in good correlation with the content of carboxyl groups on carbides [36]. The results obtained in present work implied that the number of carboxyl on EDCMM was larger than that of EDCMSM. Fig. 5 shows the removal efficiency of Pb2+ on the resultant composites under different pH solutions with an initial Pb2+ concentration

Fig. 6. Effect of contact time on removal of Pb2+ (initial Pb2+ concentration: 300 mg/L; adsorbent dose: 0.05 g; pH value: 6.0; temperature: 25 ◦ C).

Y. Xu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 482 (2015) 353–364

359

Fig. 7. Linear fitting of experimental data using pseudo-first-order kinetic (a) and pseudo-second-order kinetic (b) models (initial Pb2+ concentration: 300 mg/L; adsorbent dose: 0.05 g; pH value: 6.0; temperature: 25 ◦ C).

of 40 ppm. It was observed that the Pb2+ removal efficiency by EDCMSM increased from 80.71% to 100% (w/w) with the solution pH ranged from 2.0 to 6.0 and decreased to 93% (w/w) when the solution pH was 7.0. And the same trend had been found on the Pb2+ removal efficiency of EDCMM. This adsorption behavior could be explained by surface charge and proton-competitive adsorption. When pH < pHpzc , the surface of adsorbent was positively charged, and electrostatic repulsion would existed between the surface of the adsorbents and the Pb2+ , which inhibited the adsorption of Pb2+ . Contrarily, when pH > pHpzc , the surface of adsorbents was negatively charged and the adsorption of cations (Pb2+ ) was favorable. When the pH increased from 6.0 to 7.0, Pb2+ was prone to form Pb(OH)2 . The interaction between Pb2+ and OH− led to the decreasing of removal efficiency [16]. It could be noticed that the removal efficiency of Pb2+ increased slowly when the pH value was at the pHpzc of the adsorbent. The adsorption behavior could be owned to the reduction in the accessibility to internal sites. The swelling rate of adsorbent decreases at the point of zero charge and causes the

reduction in the accessibility to internal sites [37]. Herein, the pH value of 6.0 was chosen for other absorption studies. 3.2.2. Effect of contact time on the adsorption and the kinetic studies The results shown in Fig. 6 indicated that the adsorption increased rapidly at first, slowed down thereafter, and finally reached to equilibrium. The initial rapid adsorption was due to a large number of vacant adsorption sites on the adsorbent surface, and then it decreased since the adsorption sites were occupied by Pb2+ , until the adsorption attained equilibrium. It was in accord with the point that the metal ions diffused into the pores and were absorbed slowly by the interior surface of adsorbent when almost all the exterior active points were occupied [24]. In the case of EDCMM, the amounts of adsorption reached to 94.45 mg/g and 154.1 mg/g at the contact time of 5 min and 80 min, respectively. However, for EDCMSM, the amounts of adsorption reached to 117.4 mg/g and 133.4 mg/g at the contact time of 5 min and

360

Y. Xu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 482 (2015) 353–364

Table 1 Kinetic model parameters for Pb2+ adsorption on EDCMM and EDCMSM. Adsorbent

EDCMM EDCMSM

Pseudo-first-order model

Pseudo-second-order model

K1 (min−1 )

qe,exp (mg/g)

R1 2

K2 (g/mg min)

qe,cal (mg/g)

R2 2

0.04823 0.05842

156.968 133.659

0.728 0.961

0.0013 0.0071

162.8 134.7

0.997 0.999

EDCMM: carboxylated chitosan magnetic microspheres; EDCMSM: carboxylated chitosan magnetic submicrospheres; initial Pb2+ concentration: 300 mg/L; adsorbent dose: 0.05 g; pH value: 6.0; temperature: 25 ◦ C.

Fig. 8. Effect of initial Pb2+ concentrations on removal of Pb2+ (adsorbent dose: 0.05 g; pH value: 6.0; contact time: 120 min; temperature: 25 ◦ C).

Table 2 Parameters of the Langmuir and Freundlich isotherm models for Pb2+ adsorption on EDCMM and EDCMSM. Adsorbent

EDCMM EDCMSM

Langmuir

Freundilich 2

KL (L/mg)

qm (mg/g)

R1

0.2756 0.32048

164.81 141.83

0.975 0.988

RL

KF

n

R2 2

0.009 0.008

64.52 54.11

5.882 5.376

0.445 0.605

EDCMM: carboxylated chitosan magnetic microspheres; EDCMSM: carboxylated chitosan magnetic submicrospheres; initial Pb2+ concentration: 300 mg/L; adsorbent dose: 0.05 g; pH value: 6.0; contact time: 120 min; temperature: 25 ◦ C.

80 min, respectively. The larger amount of adsorption at 5 min of EDCMSM might be due to the higher external surface area than that of EDCMM. It was shown that the specific surface area of EDCMSM (6.83 m2 /g) was about 10 times as that of EDCMM (0.70 m2 /g). The adsorbent with higher surface area had more high affinity sites for heavy metal ions, which lead to faster adsorption rate and higher adsorption capacity [29]. Meanwhile, the increase of carboxyl groups on adsorbent could also enhance the removal of Pb2+ by electrostatic interaction and chelation [23]. And the less adsorp-

tion capacity at equilibrium of EDCMSM could be connected to the lower content of carboxyl on EDCMSM than that on EDCMM. The adsorption process is dominated by the interaction between Pb2+ and carboxyl groups [38]. The superiorities of quick adsorption and high adsorption capacities of EDCMM and EDCMSM for Pb2+ might be due to the combined effect of specific surface area and carboxyl groups. The kinetics models of pseudo-first-order and pseudo-secondorder were used to analyze the mechanisms of adsorption.

Table 3 Thermodynamic parameters for Pb2+ adsorption on EDCMM and EDCMSM. Adsorbent temperature (K)

EDCMM S (J/K mol)

283 288 298 313 333

46.66

EDCMSM H (KJ/mol)

G (KJ/mol)

4.708

−8.496 −8.730 −9.197 −9.897 −10.830

S (J/K mol)

32.16

H (KJ/mol)

G (KJ/mol)

2.238

−6.862 −7.024 −7.346 −7.828 −8.471

EDCMM: carboxylated chitosan magnetic microspheres; EDCMSM: carboxylated chitosan magnetic submicrospheres; initial Pb2+ concentration: 300 mg/L; adsorbent dose: 0.05 g; pH value: 6.0; contact time: 120 min.

Y. Xu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 482 (2015) 353–364

361

Fig. 9. Linear fitting of experimental data using Langmuir adsorption model (a) and Freundlich adsorption model (b) (adsorbent dose: 0.05 g; pH value: 6.0; contact time: 120 min; temperature: 25 ◦ C).

The pseudo-first-order equation is generally represented as following Eq. (2) [10]. log (qe − qt ) = ln (qe − qt ) = ln(qe ) − k1 t

(2)

The pseudo-second-order model [39] can be expressed by Eq. (3). 1 t t = + 2 qt q k2 qe e

(3)

where qe (mg/g) is the adsorption capacity at equilibrium; qt (mg/g) is the amount of adsorption at time t; k1 (min−1 ) is the equilibrium rate constant of pseudo-first-order kinetics; and k2 (g/mg min) is the equilibrium rate constant of pseudo-second-order kinetics. The pseudo-first-order model is a assumption that the adsorption rate is proportional to the difference between equilibrium capacity and adsorption amount at time t. The pseudo-secondorder model assumes that the adsorption is a pseudo–chemical reaction and the driving force is the difference between equilibrium capacity and the adsorption amount at time t. However, in the

pseudo-second-order model, the adsorption rate is proportional to the square of driving force. The plots of the pseudo-first-order and pseudo-second-order kinetic models are shown in Fig. 7, and the calculated parameters of the kinetics models are given in Table 1. The results demonstrated that the experimental data of both EDCMM and EDCMSM matched better with the pseudo-second-order model than the pseudo-firstorder model. Meanwhile, the values of qe,exp calculated from the pseudo-second-order model were in better agreement with the experimental data than that of the pseudo-first-order model. These kinetics data suggested that adsorption processes of EDCMM and EDCMSM were both dominated by chemical reaction process. 3.2.3. Effect of Pb2+ concentration on the adsorption The adsorption curves with various initial Pb2+ concentration are shown in Fig. 8. The results indicated that the adsorption capacity of both EDCMM and EDCMSM increased with the increase of initial Pb2+ concentrations. The adsorption capacity almost reached to maximum value at the initial Pb2+ concentration of 400 mg/L, and

362

Y. Xu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 482 (2015) 353–364

Fig. 10. Effect of temperatures on Pb2+ removal (initial Pb2+ concentration: 300 mg/L; adsorbent dose: 0.05 g; pH value: 6.0; contact time: 120 min).

Fig. 11. Five consecutive adsorption–desorption cycles of carboxylated chitosan magnetic microspheres (EDCMM) and carboxylated chitosan magnetic submicrospheres (EDCMSM) for Pb2+ (initial Pb2+ concentration: 100 mg/L; adsorbent dose: 0.05 g; pH value: 6.0; contact time: 120 min; temperature: 25 ◦ C).

the Pb2+ adsorption capacity of EDCMM and EDCMSM were 160.7 and 138.9 mg/g, respectively. The Langmuir and Freundlich models were used to evaluate the Pb2+ adsorption properties of EDCMM and EDCMSM. The experimental results are shown in Fig. 9. The Langmuir model is a assumption that the binding sites have the same adsorption energy for each molecule, and there is not any interaction among the adsorbed molecules. The monolayer adsorption takes place on an adsorbent that has a structurally homogeneous surface. The Langmuir model is described by Eq. (4). Ce 1 Ce = + qe KL qm qm

(4)

where qe (mmol/g) is the adsorption capacity of Pb2+ ; Ce (mM) is the equilibrium concentration of Pb2+ ; qm (mmol/g) is the maximum

adsorption capacity of adsorbent; and KL (L/mmol) is the constant of the Langmuir model related to the affinity of binding sites. The Freundlich model is used to describe a kind of adsorption onto heterogeneous surface with a uniform energy distribution. It is not restricted to the formation of the monolayer. The Freundlich model is expressed as Eq. (5). log(qe ) = log(KF ) +

1 log(Ce ) n

(5)

where KF is the Freundlich constants related to adsorption capacity; and n is the Freundlich constants related to adsorption intensity. The obtained parameters of Langmuir and Freundlich isotherm are listed in Table 2. It could be noticed that the Langmuir model matched better with the experimental data of both EDCMM and EDCMSM with the higher correlation coefficient R2 (R2 > 0.97), which suggested a monolayer adsorption. Therefore, the Pb2+

Y. Xu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 482 (2015) 353–364

adsorption onto both EDCMM and EDCMSM could be attributed to homogenous distribution of molecules on the polymeric surface. Furthermore, in order to indicate the favorability of the adsorption process, the values of equilibrium parameter (RL ) were calculated by Eq. (6). RL =

1 1 + KL C0

(6)

where KL (L/mmol) is the Langmuir constant; C0 (mmol/L) is the initial concentration of metal ions; RL indicates the type of the adsorption is irreversible (RL = 0), favorable (0 < RL < 1), linear (RL = 1), or unfavorable (RL > 1) [39]. Table 2 shows that the RL values of EDCMM and EDCMSM were 0.009 and 0.00775, respectively. The results suggested that the adsorption processes of EDCMM and EDCMSM for Pb2+ were both favorable. 3.2.4. Adsorption thermodynamics Fig. 10 illustrates the effect of temperature on Pb2+ removal of EDCMM and EDCMSM. It could be noticed that the Pb2+ adsorption on EDCMM and EDCMSM increased slightly as the temperature increased from 10 to 60 ◦ C. The results indicated that the Pb2+ adsorption on both adsorbents were endothermic processes. It is well known that the increase of temperature would enlarge the equilibrium constant of endothermic reaction. The enlarged equilibrium constants led to the increase of Pb2+ adsorption capacities onto EDCMM and EDCMSM. In addition, thermodynamic parameters of adsorption process such as entropy change (S), enthalpy change (H) and free energy change (G) were calculated by Eqs. (7)–(9), and the results are listed in Table 3. Kb =

qe Ce

ln Kb =

(7) S H − R RT

G = H − TS

363

were quite effective, and both adsorbents had potential application values in industrial-scale practice. 4. Conclusions Two novel adsorbents, EDCMM and EDCMSM, which were confirmed by TEM, SEM and FTIR, were successfully prepared. The VSM measurement showed that both EDCMM and EDCMSM had good magnetic property, which endowed them to be easily separated from water by magnet. Equilibrium adsorption data of EDCMM and EDCMSM followed the pseudo-second-order kinetic model, which suggested that the Pb2+ adsorption onto both adsorbents were dominated by chemical reaction process. The adsorption of Pb2+ onto EDCMM and EDCMSM could be attributed to monolayer adsorption. From the results of adsorption experiments, the EDCMSM had more rapid adsorption rate at the first 5 min than that of EDCMM, due to its higher specific surface area. Moreover, the less adsorption capacity near the adsorption equilibrium of EDCMSM could be attributed to the lower content of carboxyl in EDCMSM than that in EDCMM. The Pb2+ adsorption capacity of both adsorbents could still be kept above 94% of the original adsorption capacity after the fifth cycle, which indicated that the prepared adsorbents had remarkable repeatabilities. The results of present work indicated that EDCMM and EDCMSM might be promising novel adsorbents for Pb2+ removal. Acknowledgements This work was supported by the Science and Technology Development Funds of Qingdao Shinan (2014-13-005-SW & 201414-003-SW), the National Natural Science Foundation of China (NSFC, 31400812) and the Shandong Province Natural Science Foundation (ZR2014CQ052 & ZR2011CZ003).

(8) References (9)

where Kb, is the equilibrium constant; qe (mg/g) is the equilibrium adsorption capacity of Pb2+ ; Ce (mg/L) is the equilibrium concentration of Pb2+ ; R (8.314 J/mol K) is the gas constant; and T (K) is the absolute temperature. Table 3 shows that the values of S of EDCMM and EDCMSM were both positive, which indicated an increase in the randomness at the solid/liquid interface. The positive values of S might be related to the liberation of proton in the interaction between active points and Pb2+ . The similar result has been observed in the sorption of uranium and thorium onto CA/XAD-16, which may be related to the liberation of water of hydration during the adsorption process [40]. The positive values of H implied that the adsorption process of Pb2+ was endothermic in nature. In addition, the negative values of G indicated that the adsorption of Pb2+ was spontaneous process. Table 3 shows that the G values decreased with the increasing temperature, which suggested the adsorption process for Pb2+ was favorable at high temperature. 3.3. Desorption and reusability studies The results of desorption and re-adsorption tests for five cycles are summarized in Fig. 11. The re-adsorption performance of EDCMM and EDCMSM could still maintained approximately 94% and 98% (w/w) of the original Pb2+ removal capacities after the fifth adsorption–regeneration cycle, respectively. The difference of re-adsorption capacities between EDCMM and EDCMSM might be attributed to higher external surface area of EDCMSM. These results indicated that the regenerations of the adsorbents by EDTA solution

[1] W.S.W. Ngah, M.A.K.M. Hanafiah, Removal of heavy metal ions from wastewater by chemically modified plant wastes as adsorbents: a review, Bioresour. Technol. 99 (2008) 3935–3948. [2] D. Bulgariu, L. Bulgariu, Sorption of Pb(II) onto a mixture of algae waste biomass and anion exchanger resin in a packed-bed column, Bioresour. Technol. 129 (2013) 374–380. [3] V.K. Gupta, A. Rastogi, Biosorption of lead(II) from aqueous solutions by non-living algal biomass Oedogonium sp and Nostoc sp – a comparative study, Colloid Surf. B 64 (2008) 170–178. [4] M.A. Barakat, New trends in removing heavy metals from industrial wastewater, Arab J. Chem. 4 (2011) 361–377. [5] K. Chatham-Stephens, J. Caravanos, B. Ericson, P. Landrigan, R. Fuller, The pediatric burden of disease from lead exposure at toxic waste sites in low and middle income countries, Environ. Res. 132 (2014) 379–383. [6] M. Machida, B. Fotoohi, Y. Amamo, L. Mercier, Cadmium(II) and lead(II) adsorption onto hetero-atom functional mesoporous silica and activated carbon, Appl. Surf. Sci. 258 (2012) 7389–7394. [7] A. Mittal, L. Kurup, V.K. Gupta, Use of waste materials – bottom ash and de-oiled soya, as potential adsorbents for the removal of Amaranth from aqueous solutions, J. Hazard. Mater. 117 (2005) 171–178. [8] A. Mittal, J. Mittal, A. Malviya, D. Kaur, V.K. Gupta, Adsorption of hazardous dye crystal violet from wastewater by waste materials, J. Colloid Interface Sci. 343 (2010) 463–473. [9] M. Sprynskyy, B. Buszewski, A.P. Terzyk, J. Namiesnik, Study of the selection mechanism of heavy metal (Pb2+ , Cu2+ , Ni2+ , and Cd2+ ) adsorption on clinoptilolite, J. Colloid Interface Sci. 304 (2006) 21–28. [10] M.S. Chiou, H.Y. Li, Adsorption behavior of reactive dye in aqueous solution on chemical cross-linked chitosan beads, Chemosphere 50 (2003) 1095–1105. [11] E.I. Unuabonah, B.I. Olu-Owolabi, K.O. Adebowale, A.E. Ofomaja, Adsorption of lead and cadmium ions from aqueous solutions by tripolyphosphate-impregnated Kaolinite clay, Colloid Surf. A 292 (2007) 202–211. [12] P.D. Johnson, M.A. Watson, J. Brown, I.A. Jefcoat, Peanut hull pellets as a single use sorbent for the capture of Cu(II) from wastewater, Waste Manage. 22 (2002) 471–480. [13] E. Pehlivan, S. Cetin, B.H. Yanik, Equilibrium studies for the sorption of zinc and copper from aqueous solutions using sugar beet pulp and fly ash, J. Hazard. Mater. 135 (2006) 193–199.

364

Y. Xu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 482 (2015) 353–364

[14] V.K. Gupta, A. Mittal, L. Kurup, J. Mittal, Adsorption of a hazardous dye, erythrosine, over hen feathers, J. Colloid Interface Sci. 304 (2006) 52–57. [15] V.K. Gupta, R. Jain, S. Varshney, Removal of Reactofix golden yellow 3 RFN from aqueous solution using wheat husk – an agricultural waste, J. Hazard. Mater. 142 (2007) 443–448. [16] G.D. Yang, L. Tang, X.X. Lei, G.M. Zeng, Y. Cai, X. Wei, Y.Y. Zhou, S.S. Li, Y. Fang, Y. Zhang, Cd(II) removal from aqueous solution by adsorption on alpha-ketoglutaric acid-modified magnetic chitosan, Appl. Surf. Sci. 292 (2014) 710–716. [17] R.S. Vieira, M.L.M. Oliveira, E. Guibal, E. Rodriguez-Castellon, M.M. Beppu, Copper, mercury and chromium adsorption on natural and crosslinked chitosan films: an XPS investigation of mechanism, Colloid Surf. A 374 (2011) 108–114. [18] M.R. Gandhi, G.N. Kousalya, N. Viswanathan, S. Meenakshi, Sorption behaviour of copper on chemically modified chitosan beads from aqueous solution, Carbohydr. Polym. 83 (2011) 1082–1087. [19] J.C.Y. Ng, W.H. Cheung, G. McKay, Equilibrium studies for the sorption of lead from effluents using chitosan, Chemosphere 52 (2003) 1021–1030. [20] W.S.W. Ngah, S. Ab Ghani, A. Kamari, Adsorption behaviour of Fe(II) and Fe(III) ions in aqueous solution on chitosan and cross-linked chitosan beads, Bioresour. Technol. 96 (2005) 443–450. [21] W.S.W. Ngah, C.S. Endud, R. Mayanar, Removal of copper(II) ions from aqueous solution onto chitosan and cross-linked chitosan beads, React. Funct. Polym. 50 (2002) 181–190. [22] M. Sljivic, I. Smiciklas, I. Plecas, M. Mitric, The influence of equilibration conditions and hydroxyapatite physico-chemical properties onto retention of Cu2+ ions, Chem. Eng. J. 148 (2009) 80–88. [23] G.Z. Kyzas, M. Kostoglou, N.K. Lazaridis, Copper and chromium(VI) removal by chitosan derivatives-equilibrium and kinetic studies, Chem. Eng. J. 152 (2009) 440–448. [24] E. Repo, J.K. Warchol, T.A. Kurniawan, M.E.T. Sillanpaa, Adsorption of Co(II) and Ni(II) by EDTA- and/or DTPA-modified chitosan: kinetic and equilibrium modeling, Chem. Eng. J. 161 (2010) 73–82. [25] T.P. Learoyd, J.L. Burrows, E. French, P.C. Seville, Modified release of beclometasone dipropionate from chitosan-based spray-dried respirable powders, Powder Technol. 187 (2008) 231–238. [26] C. Tang, Y.X. Guan, S.J. Yao, Z.Q. Zhu, Preparation of ibuprofen-loaded chitosan films for oral mucosal drug delivery using supercritical solution impregnation, Int. J. Pharmaceut. 473 (2014) 434–441. [27] S. Kumar, J. Dutta, P.K. Dutta, Preparation and characterization of N-heterocyclic chitosan derivative based gels for biomedical applications, Int. J. Biol. Macromol. 45 (2009) 330–337.

[28] M.S.D. Erosa, T.I.S. Medina, R.N. Mendoza, M.A. Rodriguez, E. Guibal, Cadmium sorption on chitosan sorbents: kinetic and equilibrium studies, Hydrometallurgy 61 (2001) 157–167. [29] A.G. Caporale, P. Punamiya, M. Pigna, A. Violante, D. Sarkar, Effect of particle size of drinking-water treatment residuals on the sorption of arsenic in the presence of competing ions, J. Hazard. Mater. 260 (2013) 644–651. [30] D. Mondal, G. Villemure, G.C. Li, C.J. Song, J.J. Zhang, R. Hui, J.W. Chen, C. Fairbridge, Synthesis, characterization and evaluation of unsupported porous NiS2 sub-micrometer spheres as a potential hydrodesulfurization catalyst, Appl. Catal. A Gen. 450 (2013) 230–236. [31] J.J. Ye, F.H. Li, S.Y. Gan, Y.Y. Jiang, Q.B. An, Q.X. Zhang, L. Niu, Using sp(2)-C dominant porous carbon sub-micrometer spheres as solid transducers in ion-selective electrodes, Electrochem. Commun. 50 (2015) 60–63. [32] Y. Ren, H.A. Abbood, F.B. He, H. Peng, K.X. Huang, Magnetic EDTA-modified chitosan/SiO2 /Fe3 O4 adsorbent: preparation, characterization, and application in heavy metal adsorption, Chem. Eng. J. 226 (2013) 300–311. [33] I.D. Mall, V.C. Srivastava, G.V.A. Kumar, I.M. Mishra, Characterization and utilization of mesoporous fertilizer plant waste carbon for adsorptive removal of dyes from aqueous solution, Colloid Surf. A 278 (2006) 175–187. [34] P.G. Tratnyek, R.L. Johnson, Nanotechnologies for environmental cleanup, Nano Today 1 (2006) 44–48. [35] U. Topal, Factors influencing the remanent properties of hard magnetic barium ferrites: impurity phases and grain sizes, J. Magn. Magn. Mater. 320 (2008) 331–335. [36] L.S. Cerivic, S.K. Milonjic, M.B. Todorovic, M.I. Trtanj, Y.S. Pogozhev, Y. Blagoveschenskii, E.A. Levashov, Point of zero charge of different carbides, Colloid Surf. A 297 (2007) 1–6. [37] X.X. Lei, L. Tang, G.M. Zeng, M.S. Wu, Y.Y. Zhou, Y. Pang, Y.Y. Liu, Z. Li, G.D. Yang, Study on magnetic chitosan microparticles for rapid removal of heavy metals, Adv. Mater Res. Switz. 518–523 (2012) 2844–2848. [38] E. Guibal, Interactions of metal ions with chitosan-based sorbents: a review, Sep. Purif. Technol. 38 (2004) 43–74. [39] Y.M. Hao, M. Chen, Z.B. Hu, Effective removal of Cu(II) ions from aqueous solution by amino-functionalized magnetic nanoparticles, J. Hazard. Mater. 184 (2010) 392–399. [40] A. Rahmani-Sani, A. Hosseini-Bandegharaei, S.H. Hosseini, K. Kharghani, H. Zarei, A. Rastegar, Kinetic, equilibrium and thermodynamic studies on sorption of uranium and thorium from aqueous solutions by a selective impregnated resin containing carminic acid, J. Hazard. Mater. 286 (2015) 152–163.